Analyzing thermal comfort conditions through numerical simulation for a machs office

Trang 1 MINISTRY OF EDUCATION AND TRAINING HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY AND EDUCATION GRADUATION PROJECTMAJOR: THERMAL ENGINEERING TECHNOLOGYHo Chi Minh City, July 2023ANALY

GENERAL INTRODUCTION

Introduction of air conditioning system

Since ancient times, people have known to light fires to keep warm in winter and use fans or find cool caves to avoid the heat of summer

In 1845, American doctor John Gorrie built the first pneumatic air conditioner to air-condition his private hospital The event marks an important historical milestone for world air conditioning technology

Since the early years of the 20th century, many works to control temperature and humidity for human consumption appeared in the US and some countries around the world These works are mainly to control the temperature but have not reached the completion and meet the necessary technical requirements

Inventor and engineer Willis Haviland Carrier appeared at the right time to bring the air conditioning industry in the US in particular and the world in general to flourish It was he who defined air conditioning as a combination of heating, cooling, humidifying, dehumidifying, filtering and washing air, automatically maintaining constant air state control for all comfort needs and technology

1.1.2 Purpose of air conditioning and technical requirements

The purpose of the air conditioning and ventilation system is to create comfort and fresh air environment for users as well as to cool down the equipment and machinery Creating a fresh air environment according to the parameters of temperature, humidity, air convection, dust filtration and control of pollutants is of paramount importance In parallel with the above conditions, the installation of the air conditioning system must ensure that it does not create a large amount of noise and vibration inside the building Special attention should be paid to noise and vibration control of air conditioning systems and areas where low noise levels are required

Air conditioning systems are usually installed in the following areas: offices, shops, apartment buildings, and public services, etc

The ventilation system is usually installed in the following areas: basement, kitchen, toilet, technical room, etc.

The mean of air conditioners

Economic development is always associated with the development of science and technology Today, air conditioning technology is constantly developing to meet the needs of human life and production

Basic parameters affecting from the environment to humans:

 Relative humidity of the air

 Concentration of harmful substances in the air

Influence of the environment on human

The internal temperature of the human body is always stable at 37 o C During movement and work, people always emit a certain amount of heat into the surrounding air Therefore, when the ambient air temperature changes, it will affect the heat transfer process

3 from the human body to the environment When the ambient temperature is too high or too low, it will cause discomfort to people and affect the process of living and working of people

Air conditioners can overcome this, for each specific case the air conditioning system is a means of creating an environment with a temperature of 24 o C to 28 o C, which is a comfortable and comfortable environment for human activities

1.3.2 The effect of relative humidity

The relative humidity of the air is a decisive factor in the rate of evaporation and the loss of moisture from the human body to the environment (in the form of sweat)

If the relative humidity of the surrounding air decreases to a certain amount, the amount of moisture escaping from the human body easily evaporates into the air, that is, the body radiates heat to the surrounding air better and vice versa If the relative humidity is too large, it will limit the body's moisture drainage process, and the sweat secreted and evaporated will be deposited on the skin causing discomfort Normally, when the temperature is around 24 o C to 27 o C, for people to feel comfortable, the relative humidity of the air is about 60% to 65%

1.3.3 Influence from the concentration of harmful substance in the air

Air-conditioned space is a relatively enclosed space in which people can live or work

In addition to pollution caused by objective factors such as smoke, toxic substances available in the air, people and their activities are also one of the main causes of air pollution in the space that needs to be cooled Causes of pollution caused by humans: Due to breathing, smoking, odors emitted by the human body arising in the process of living and production This is also the main cause of reducing O2, increasing CO2 The amount of CO2, causing people to feel stuffy and uncomfortable

Some popular air conditioning system

Using Central Air Conditioning System (Chiller) is located at the machine room to supply cold-water for the whole building by pump to the AHU, FCU Central air conditioning options include many different types such as:

Central air conditioning with cold-water: Central air conditioning only produce cold water and supply to heat exchangers located in rooms by pumping system

Central air conditioning with the air: Central air conditioning produce cold air and provide it to function rooms by means of a ductwork system In addition, central air conditioning is also classified according to two main cooling methods for air conditioners: water cooled and air cooled

Regarding the structure of the VRV system, there are two units: outdoor unit and indoor unit, indoor unit with evaporator and fan The allowable length and height between outdoor unit and indoor unit is very large (about 100 m in length and 50 m in height), the height between indoor unit can reach 15 m

Spot air conditioner system consists of single spot units installed for individual air- conditioned areas There are 2 types of air conditioners for homes and offices : One-way air conditioners and two-way air conditioners

 For the spot air conditioner system, the fresh air supply to the room is usually directly supplied by the blower, so the air is not treated with dust, moisture and often creates a high temperature difference between the supplemental supply airflow Pulse and cold supply air flow of the indoor unit, causing discomfort to people in the air-conditioned room

 Large power consumption coefficient, high operating cost

 Durability and service life are not high (about 5-6 years)

Figure 1.4: Spot air conditioner system

Figure 1.5: Spot air conditioner system (two-way)

Figure 1.6: Spot air conditioner system type Multi

Project overview

Address: 364 Ung Van Khiem Street, 25 Ward, Binh Thanh District

 The number of floors: 8 floors with a ground floor, 1 mezzanine, 5 office floor and a terrace

 Direction of building: Curtain wall directs Ung Van Khiem Street and North side

Figure 1.8: Plan of 2 nd floor

1.5.2 Parameters of the work according to the calculation conditions in the design

 Outside air condition (estimate following QCVN 02:2009 BXD)

Average highest temperature of hottest month in summer: 34.6 o C

Table 2.1: Average highest air temperature month and year

Table 1.2: Average relative air humidity month and year

1.5.3 VRV air conditioning system with ceiling mounted ducts

VRV (Variable Refrigerant Volume) air conditioning system is an advanced technology in the field of air conditioning, developed by Daikin company VRV is a trademark of Daikin, but the term "VRV" is often used to refer to air conditioning systems with similar operating principles from different manufacturers

The VRV air conditioning system uses a central controller to adjust the capacity and temperature of the individual chillers to provide cooling for different areas of the building These indoor units have the ability to automatically adjust the capacity according to the actual needs of each area, saving energy and providing optimal comfort

VRT Smart significantly reduces energy by optimizing power according to thermal load, especially during low load operation The interior environment is comfortably maintained

Daikin's VRV central air conditioning system is used to develop large-scale air conditioning systems on a single refrigerant system, thereby reducing the space required for air conditioning units Even with a 20-story building, all outdoor units can be installed on the terrace

Refrigerant cooling of the board: Daikin's exclusive refrigerant cooling capability helps maintain cooling capacity even at high outdoor temperatures

Dual redundancy operation: Room outdoor unit and backup compressor ensure continuous operation

Strong anti-corrosion models: Strong anti-corrosion models provide durable operation in wet and coastal areas In addition, the outdoor unit can be installed at a distance of 0m from the coast

The capacity is suitable for office spaces to commercial buildings, the power range can be extended to 60 HP

Usually, quiet compared to other commercial air conditioners

Simpler installation than traditional system

Refrigerant used is environmentally friendly compared to Spot air conditioners Disadvantages:

Pipes and connections must be arranged and installed before the indoor unit is put into operation It is important that the installation location and various jobs are done professionally for the system to work well in the long run as a small error can be difficult to track down due to a lot of work in the process setting

Difficult to handle when a leak occurs

High cost compared to conventional air conditioners

Ducted air conditioning system: Ducted air conditioning system is a type of air conditioning system installed on the ceiling and connected to ducts to distribute cold air to spaces in a building or industrial space

Uniform Air Distribution: Ducted ceiling air conditioning systems are designed to distribute air evenly and efficiently throughout spaces Ducts connecting from the indoor unit to the air grill that have been hung on the ceiling help distribute cold or hot air evenly throughout the space

Saving space: Because the ducted ceiling air conditioning system is installed on the ceiling, it does not take up space as well as usable space This saves space and allows optimal use of space in the building or other areas for other purposes

High aesthetics: The air duct system and duct net in the duct ceiling air conditioning system are designed delicately and aesthetically, helping to integration with the interior architecture of the space Air grills have small ventilation holes and do not affect the aesthetics of the space much

Diverse adjustability: The ducted ceiling air conditioning system allows flexible adjustment of temperature, fan speed and air direction for each specific space This helps to create an environment that is comfortable and adjustable to the needs of the user

1.5.4 Heat sources acting on the building

Since the building with windows is located on the west side, it is subjected to maximum radiation in the afternoon and is partly reduced through the use of blind

Electronic devices also cause significant amounts of heat with computers, printers, coffee makers and other electronic devices

Sensible and latent heat of a person is also of interest because the space is relatively large 160m 2 so the standard number of people can be up to 20 people

Most of the surrounding is not blocked by other buildings, so the heat transfer through the wall is relatively large

CALCULATING AND CHECKING THE COOLING LOAD FOR THE PROJECT

Calculate heat and humidity sources

2.1.1 Radiant heat transfer through the glass Q 11

Determined by the formula: = [4.1, page 123, [1]]

Including Q’11: Radiation transfers heat instantly through the window

With: nt – Instantaneous effect factor

F– Surface area of window with steel frame, m 2

Rt – Radiates heat of the sun through the glass door into the room, W/m 2 e – Correction factor, determine by the formula: e = εc.εds.εmm.εkh.εm.εr

Include: εc – Influence coefficient of sea level, determine by the formula: ε = 1 + 0.023 [4.3, page 124, [1]]

Altitude above sea level at Ho Chi Minh city is 18m, consider the room located on the 2nd floor with the height 6.8m so H = 18+6.8= 24.8 m ε = 1 + 0.023 = 1.0006

This coefficient is almost equal to 1 so we choose c = 1 for all other cases

13 εds – The factor that takes into account the effect of the difference between the dew point temperature of the observed air and the dew point temperature of the air above sea level is 20 o C, determine by the formula: ε = 1 − (! " # ) 0.13 [4.4, page 124, [1]]

Look up the graph I-d at tN = 34.6 o C and φN = 72% we have ts = 28.8 o C, deduce: ε = 1 −(28.8 − 20)

10 × 0.13 = 0.8856 εmm – Cloud effect coefficient, choose the largest case εmm = 1 εkh – The frame influence factor, based on construction documents using aluminum frames εkh = 1.17 εm – Glazing coefficient, depending on the color and type of glazing, based on construction documentation using clear glass, flat with thickness 8mm (Because we can’t look for this parameter of this kind of glass, so we chose the value of basic glass with thickness 6mm in document [1]) εm = 0.94 εr – Solar factor taking into account the effect of glazing is different from background and curtain glazing r = 1 and RT replaced by heat radiated into the room other than the basic glass *K

(page 124, document [1]), from there, we can calculate the correction factor e as follows: e = 1 x 0.8856 x 1 x 1.17 x 0.94 x 1 = 0.97

RN: Solar radiation coming from outside the glass, W/m2 αk, τk, pk, αm, τm, pm – Absorption coefficient, Penetration, Reflection of glass and curtain, look up the table 4.3, table 4.4 page 132, document [1] and [2]:

Ho Chi Minh city has the coordinate10 o 38’ then look up the table 4.2 page 131, [1], combine with above formula we have the table below:

Table 2.1: Value of solar radiation through the glazed window

* Calculate for an office of 2 nd floor:

Office on the 2 nd floor is subject to direct radiation from the sun because almost the windows locate at West Wall, based on the table above:

Figure 2.1: Dimension of glass window

The office has 7 windows in the west wall, so the surface area of window with steel frame: 0.9x2.55x7 = 16.065 m 2

The density per average area gs is determined by the formula:

G’- Mass of wall with outer face directly exposed to solar radiation and floor exposed to earth, kg

G’’- Mass of walls whose outer surfaces are not directly exposed to solar radiation and whose floors are not above the ground, kg

According to the wall and foundation structure of the building, we have:

Mass 1 m 2 perforated brick wall 0.2 m: 1350× 0.2 = 270 (kg/m 2 )

Mass 1 m 2 armoured concrete floor 0.3 m: 2400 × 0.3 = 720 (kg/m 2 )

- The 2 nd floor office has the following measured parameters:

- Area of outer wall exposed to solar radiation: 24.803x2.5 = 62.008 m 2

- Area of the inner wall that is not directly exposed to solar radiation:

Mass of wall with external surface exposed to radiation G’:

The mass of the floor that is not hold at the ground and the wall whose outer surface is not exposed to solar radiation G’’:

160.67 = 843.807 =9/> ?@AAB With value gs = 843.807 kg/ m 2 look up the table 4.6 page 134, document [1]

2.1.2 Sensible heat transmitted through the roof due to radiation and temperature difference Q 21

An air-conditioned room located between floors in an air-conditioned building means that the above is also an air-conditioned room, then Δt = 0 và Q21 = 0

The above case is an unconditioned space, we have the formula:

Q21 - Heat flow into the space to be conditioned due to the heat accumulation of the roof structures and due to the temperature difference of the air between the outside and the inside, (W) k - Coefficient of heat transfer through the roof, depends on the structure of the roof material, (W/m 2 K), look up the table 4.9 and 4.15 [page 140 and 145, [1]] Δt td - Equivalent temperature difference, ( 0 C)

+ Case 1: The top is the terrace which direct contact with the outside space:

N = 0,42 - Absorption coefficient of solar radiation of some types of roof surface, look up the table 4.10 [page141, [1]]

R T = 789 (W/m 2 ) - Amount of solar radiation penetrating through the roof, look up the table 4.2 [page 131, [1]] α N = 20 (W/m 2 K) – Coefficient of heat release outside the substrate when in direct contact with the air outside, according to [page 142, [1]]

+ Case 2: For the upper roof is the non-air-conditioned space Δt = 0.5x(tN – tT) = 0.5x(34.6 – 24) = 5.3 0 C

*Calculated for 2nd floor office: Because the 2 nd floor is closed with a suspended ceiling, it is considered an unconditioned space

X Y , X [ - The heat transfer coefficients of the outer and inner surfaces of the enclosure, respectively, W/m 2 K ;

Because the ceiling is indirect contact with the outside air, so: αN = 10 W/m 2 K, αT = 10 W/m 2 K [ page142,[1]]

- Thickness of material i-th layer, m, \] ^ _` = 0.01>;

 i - Thermal conductivity coefficient of the i-th layer material in cladding structures, W/m.K,  ceiling = 0.41 W/m.K;

Oa R a.aO a.bO R Oa O = 4.457 W/m 2 K Ceiling area: F = 160.67 m 2

Substitute into the formula, we get: Q 21” = 4.457 x 160.67 x 5.3 = 3779.536 (W)

So, the total heat transfer through the roof in the 2 nd floor office is:

Q 21 = Q 21” = 3779.536 (W) 2.1.3 Heat transfer through the structure Q 22

Heat transfer through the wall Q22 also includes 2 components:

Due to the temperature difference between outdoor and indoor Δt= tN – tT

Due to the solar radiation hitting the wall, for example, the wall is facing east, west , however, when calculating this amount of heat, it is considered as zero

Heat transfer through the wall is also calculated by the expression:

Q2i - Heat transfer through walls, doors, glass, etc, W; ki - Corresponding heat transfer coefficient of wall, door, glass, W/m 2 K;

Fi – Surface area of wall, door, glass respectively, m 2

 Heat transfer through the wall Q 22w :

Fw – wall surface area, m 2 Δt – Temperature difference, o C kw – The coefficient of heat transfer through the wall, determined by the formula:

5 c d , + ∑ eg f f + d - d , + ∑ + d - (h/7 8 i) With: αN – The coefficient of heat dissipation outside the wall; When the wall is in direct contact with the outside air αN = 20 W/m 2 K; When the wall is indirect contact with the outside air αN = 10 W/m 2 K [[1], page142] αT – The coefficient of heat dissipation inside the house; αT = 10 W/m 2 K

R– Thermal conductivity of the i-th material layer of the wall structure, m 2 K/W δi – Thickness of mortar and brick; According to the project documents we have: δgrout = 0.01 m; δhollow-brick = 0.2 m λi – Thermal conductivity coefficient; According to appendix A – QCVN 09:2013/BXD: λgrout = 0.93 W/m.K; λhollow-brick = 0.88 W/m.K

The coefficient of heat transfer through a 200mm thick brick wall direct contact the outside air is:

The coefficient of heat transfer through the 200mm thick brick wall in contact with the corridor is:

* Calculated for office of 2 nd floor

Area of walls in direct contact with outdoor space: 62.008 m 2

Area of the wall in contact with the corridor: 7 m 2

Temperature difference in room and outdoor: Δl = t N – tT= 34.6 – 24 = 10.6 o C

Temperature difference in room and corridor: Δl = 0.5x(tN – tT ) = 0.5x(34.6 – 24) = 5.3 o C

From there, calculate the amount of heat transmitted through the room wall:

Heat transfer through the door is calculated according to the following formula:

Fd – Door area, m 2 Δt – Temperature difference, o C

+ For doors that open to the outdoors: Δt = tN – tT = 34.6 – 24 = 10.6 o C

+ For doors opening into non-air-conditioned spaces or corridors: Δt = 0.5x(tN – tT) = 0.5x(34.6 – 24) = 5.3 o C kd – Door heat transfer coefficient, defined according to table 4.12 [[1], page 144] Because it is aluminum frame door with 8mm tempered glass, so:

Figure 2.3: Dimension of the door

The door opens to the corridor so Δt = 5.3 0 C

Heat transfer through glazed window determine by the formula:

Fg – Surface area of glass, m 2 Δt – Temperature difference, Δt = tN – tT = 34.6 – 24 = 10.6 o C kg – The coefficient of heat transfer through the glass, because it is familiar a type of windows glass, so kk = 4.817 (W/m 2 K)

*Calculated for office of 2 nd floor:

This office has a curtain wall at the north face, so

2.1.4 Heat transfer through the floor Q 23

Heat transfer through the floor determine by the formula:

Include: kf – Heat transfer coefficient through the floor, W/m 2 K

Ff – Floor area m 2 Δt – Temperature difference

The floor is on the ground so:

Because the concrete floor thick 300mm with a 20mm thickness mortar layer, granite tiles are at the top

The floor is located on the basement, adjacent to the non-air-conditioned space, so:

+ For the floor between air conditioning space: Q23 = 0

*Calculated for office of 2 nd floor:

Because below room in 1 st floor has a gypsum ceiling, so the 300mm thickness concrete floor with a mortar layer over 20mm located on the 2nd floor that was considered stay on none the air conditioning space ∆t = 5.3 0 C

 Q 23 = 2.41 x 5.3 x 160.67 = 2052.238 W 2.1.5 Heat generated by the light Q 31

Heat generated by the light bulb calculated by the formula:

Include: nl – Immediate effect factor, following table 4.8 [[1], page 136], we have g s >700 and using hour is up to 10 hours, so: nl=0.87 nd – Simultaneous impact factor, because this is an office project, so we choose nd0.85 [page 146, [1]]

Q - The total heat for lighting (W).With florescence lamp Q= ∑ 1.25xN (W)

N - The total capacity of the light bulb Because the project use 28 Paragon panel light 600x600 with 40W capacity

* Calculate for office of 2 nd floor

Heat generated by device calculated by the formula:

Include: Ni Electrical capacity of the device (W)

Because of limitations in determining the number and capacity of electrical equipment used at the project, our team will make a rough estimate of the equipment for this project:

Sensible and latent heat generated by people calculated by the formula:

Q4h – Sensible heat generated by people, determined by the formula:

24 n – The number of people in the air-conditioned room nd– Coefficient of non-simultaneous effect choose nd = 0,9 [ page 148, [1]] qh – Sensible heat generated by people, W/person

Q4a – Latent heat generated by people, determined by the formula:

With: qa – Latent heat generated by a people, W/person

Determined by table 4.18, [1] at 24 o C which is the temperature in air conditioning space We can calculate the sensible and latent heat radiating from a person that is 70W/ person and 60W/person, respectively Number of people n determined by TCVN 5687 –

2010 standard, TCVN 4470 – 2012, standard AS 1668.2 – 1991 and base on project data

According to room space and appendix F, TCVN 5687-2010, we have 8-10 m 2 /person, so 2 nd floor office with 160.67 m 2 has 16-20 people (choose 20 people)

Sensible and latent heat of a person is 70W/ person and 60W/person Deduce: Sensible heat generated by people:

Latent heat generated by people:

Deduce: Q 4 = Q 4h + Q 4a $60 (W) 2.1.8 Heat of fresh air Q hN và Q aN

Air-conditioned rooms must always be provided with a fresh amount of wind to ensure sufficient oxygen needed for the occupants of the room Fresh air has outdoor status

N with enthalpy IN, temperature tN and moisture dN larger than indoor air, so when brought

25 into the room, fresh air will give off a current amount of sensible heat QhN and latent heat

Including: dN, dT - Moisture (g/kg) n - Number of people in air-conditioned space, calculated Q4 l - Air flow to be supplied to a person in a second (l/s.person), refer to the standards TCVN 5687 – 2010, TCVN 4470 – 2012 and standard AS 1668.2 – 1991

Check graph I-d at outdoor temperature 34.6 o C, 72% and room temperature 24 o C, 60% The moisture value: dN = 25.32 (g/kg dry air) dT = 11.19 (g/kg dry air)

* Calculated for the office of the 2nd floor

Number of people calculated in Q4, n = 20 person

Air flow need to be supply to a person l = 25m 3 /h person = 7 l/s.person follow the standard TCVN 5687-2010

=> Q N = Q hN + Q aN = 1780.8 + 5934.6 = 7715.4 (W) 2.1.9 Heat released by permeation of air Q 5

Due to the difference in temperature and pressure between the outdoor and the inside air-conditioned room, it is inevitable that the outside air will leak into the air-conditioned

26 room through the door or chink in the door when the door is opened The amount of air that leaks in will bring in some heat into the room and the heat released will be calculated by the following formula:

Q5h – Heat generated by permeation of air, determined by the formula:

Q5a – Latent heat of air entering the room, determined by the formula:

Q 5a = 0.84 x (dN – d T ) x V x ξ, (W) [4.24, page 151, [1]] ξ – Experience factor, depends on the volume of the room V(m 3 ) determined according to the table 4.20, page 151, document [1]

* Calculated for office in 2 nd floor:

Since the room has a volume less than 500m 3 so ξ = 0,7

Flow rate of permeable air: s = t 3

2.1.10 Check for dew on the wall

When there is a temperature difference between indoor and outdoor, a temperature field will appear on the wall, including the glass door The temperature on the hot wall surface should not be lower than the dew point temperature If it is equal to or less than the dew point temperature on the wall, dew will occur The phenomenon of dew on the wall causes heat loss to increase, the refrigeration load required to increase, but also causes a

27 loss of beauty due to mold To prevent dew, the actual heat transfer coefficient kt of the wall must be less than the maximum heat transfer coefficient kmax calculated by the following expression:

, − - h/7 8 i Include: αN W/m 2 K: When the outer surface of the wall is in direct contact with the outdoor air tN, tT – calculated temperature of outdoor and indoor air

+ Inside Temperature: tT = 24°C tsN – Outside dew point temperature, tsN = 28.8°C determine by tN and φN So: k ƒ„… = 20 ×34.6 − 28.8

34.6 − 24 = 10.94 W/m K Heat transfer coefficient through doors, glass, floor, walls are all less than value kmax = 10.94 W/m 2 K Therefore, the office does not have a dew phenomenon

Calculating for the office on the 2nd floor, we get the following heat load parameters:

Radiant heat transfer through the glass Q11 3913.851 W

Heat transmitted through the roof by radiation and temperature difference Q21

Heat transfer through the structure Q22 3028.423 W

Sensible heat through the floor Q23 2052.238 W

Heat released by the light Q31 1035.3W

Sensible heat is generated by people Q4h 1260 W

Latent heat is generated by people Q4a 1200 W

Sensible heat is carried by the fresh air QhN 1780.8 W

Latent heat is carried by the fresh air QaN 5934.6 W

Sensible heat of permeation of air Q5h 1162.37 W

Latent heat of permeation of air Q5a 3337.29 W

Selection of air conditioning diagrams

Establishing an air conditioning diagram is to establish the air treatment process on the humidification graph after calculating the sensible and latent heat, then calculate the required capacity of the air handling equipment, which forms the basis for the selection of system types, devices and equipment layouts of the system

Through the survey, preliminary assessment of the project characteristics combined with the data in the design documents, we choose the single stage circulating air conditioner

29 option as the most appropriate It ensures the technical requirements and at the same time ensures the economy of the entire project

Working principle: The air in the room (T) is partially taken to mix with the fresh outside air (N) to form mixed air (H) The air mixture passing through the indoor unit is cooled down to the state (O≡V) and supplied to the room to exchange moist heat with the room air The process goes on continuously creating a cycle

Figure 2.4: Diagram of one-stage recirculation principle

Calculation of air conditioner diagram

 Calculation for 2 nd floor office

2.3.1 Room Sensible Heat Factor (RSHF) ɛ hf

The sensible heat factor represents the process ray of self-transforming air in the cold chamber V-T

Room Sensible Heat Factor hf calculated by the formula: ˆ ‰Š = ‰Š

Qhf – Total sensible heat of the room (no consist of sensible heat of fresh air), W

Qaf – Total latent heat of the room (no consist of latent heat of fresh air),W

Based on the results calculated above:

2.3.2 Grand sensible Heat Factor (GSHF) ɛ ht

The grand sensible heat factor is the inclination of the process line from the mixing point to the inlet point This is the process of cooling and dehumidifying the air in the indoor unit after mixing fresh air and recirculating wind We consider the amount of heat due to leakage as an additional component that does not depend on the active supply of fresh air, so it is necessary to add the sensible and latent heat of fresh air to calculate the heat coefficients ˆ ‰ = ‰

Qh – The total sensible heat includes the sensible heat of the fresh air, W

Qa – The total latent heat includes the latent heat of the fresh air,W

Based on the results calculated above, we have:

The bypass factor ɛbf is the ratio between the amount of air that passes through the indoor unit but does not exchange moist heat with the unit to the total amount of air blowing through the indoor unit.

The bypass factor ɛbf depends on many factors, the most important of which are the moist heat exchanger surface, the number of rows of tubes, and the air speed Based on the table 4.22 page 162 [1], we can choose the Bypass factor ɛbf = 0,1

2.3.4 Effective Sensible Heat Factor (ESHF) ɛ hef

Is the ratio between the effective heat of the room and the total effective heat of the room: ˆ ‰ Š = ‰ Š

Qhef - Effective Room Sensible Heat ERSH

Qaef – Effective Room Latent Heat ERLH

So, the ERSH and ERLH:

Draw a diagram of an air conditioner

We need to define the following parameters:

Determine the angular point G: tG = 24 o C, φG = 50%

Determine the points T and N on the graph based on the available initial parameters:

T: the state of the air in the room: tT = 24 o C, φT = 60%

N: the state of the fresh air: tN = 34.6 o C, φN = 72%

On the heat coefficient scale currently located to the right of the moist graph, draw lines ɛhf= 0.95, ɛht=0.71, ɛhef= 0,91 go through the point G

From point T draw a line parallel to ɛhef – G cut φ = 100% at point S is the dew point of the device

From point S draw a line parallel to ɛht – G confluent NT at point H is mixing point

From point T draw a line parallel to ɛhf – G confluent SH at point O≡V is the point after coil

Figure 2.5: Diagram of air conditioning on Psychometric Chart

Calculate FCU capacity

From the above graph we can determine the enthalpy value of the points:

Table 2.3: Parameters of junction points

( o C) (%) (g/kg dry air) (kJ/kg)

∆tVT = tT – tV = 24 – 16.4 = 7.6 < 10 => Satisfy FCU hygienic conditions capacity

The volumetric flow of air also can be determined by the formula:

Qhef – Effective heat of the room, W tT, ts – Room temperature and dew point temperature, o C

It follows that the capacity of FCU:

G – Mass flow rate of air through the coil, kg/s

Comment: There is a difference between the manual calculation of 35.2 kW and the resulting refrigerated load of the building 42 kW

Calculating the velocity through the air grills

An office uses 12 supply and 6 return air grills (following the catalogue air grills of DaFa company where supplier air grill for the project)

Velocity at the face of the air grills is calculated by the formula:

” = 4 Include: v – Velocity at the air grills, m/s

Q – Flow rate through the air grills, m 3 /s

Because fresh air flow needs to be provided for 20 people according to the standard TCVN 5687:2010 is 25 m 3 /h person => L = 25x20 = 500m 3 /h =0.14 m 3 /s The air flow from the return room is 2.404m 3 /s Because the leakage air flow is quite small 0.342 m 3 /h so this can be ignored

We have the volumetric flow rate at the mixing point of each FCU:

So the volumetric flow rate per supply air grill is: 0.848 ÷ 4 = 0.212 > ’ /N and return air grill is:

S – Area of air grill, m 2 Because the building used Dafa's air diffuser, the surface size is 600x600 with an area of free surface 60% and return air grill is single layer grill which dimension is 600x600 with an area of free surface is 75%

The velocity at the supply air grill is: – = x× x× x 8 8 = j/ ~/|

The velocity at the return air grill is: – = }

CREATING GEOMETRY USING REVIT 2021 SOFTWARE

Geometric description

The air conditioning system used for the 2 nd floor office uses a ceiling mounted FCU with a duct connection As shown in the figure, fresh air is supplied to the FCUs by light blue pipes

The simulated office area has an area of 160.67 m 2 , designed to cut the 46 o angle of the Northwest wall The height of the 2 nd floor room measured according to the cross- sectional drawing of the work is 3.2 (m), the height from the floor bottom to the suspended ceiling is 2.5 (m) Select the supply and return air grills with dimensions of 600x600(mm).

Create geometry

Before performing the simulation, our team has to create the geometry including air extrusion, architectural extrusion, equipment extrusion, human extrusion using 3D Revit

Based on the construction drawing parameters, our team performs the following steps in turn to produce a geometric model:

Step 1: Export 2D drawings from Autocad 2D 2018 software to Revit 3D 2021 software

From the Revit 2021 interface, we select Insert on the toolbar and then select Import CAD to insert the 2D plan file from Autocad 2018

Step 2: Initialize the covering architectural extrusion for the office

Our team started creating architectural extrusions for the office including covering structures (walls, columns, beams, floor, suspended ceiling, windows, glass ) with the Ribbon toolbar

Figure 3.2: Architectural block after completion

After finishing creating the covering architectural extrusion, our group proceeded to create a template for the objects that need to be simulated including:

 Supply air grill and return air grill extrusions

 Desk (include: PC case, keyboard, screen, desk) extrusions

Step 3: Use Create tool to create Air extrusion

In the Create tool, select Extrusion to create the height of the block, in the Properties interface, enter the Extrusion End as 2500 (mm) (the height from the 2 nd floor to the suspended ceiling)

In order to facilitate the simulation, our team simplified the device and human extrusions Use the Create tool to create other extrusions

Step 4: Creating Supply Air Grill and Return Air Grill extrusions

The total number of air grills installed for the room includes: 6 return air vents (Return Air Grill) and 12 supply air vents (Supply Air Grill) In there:

 12 air diffuser connected to 12 supply box of air

 Supply Air grills size: 600x600 mm

 6 return air grills connected to 6 return box of air

 Return Air grills size: 600x600mm

 The number of people in the room currently considered is 20 people The average height of Vietnamese people is 1m65

 Created in a sitting people with 1.3m high, 0.5m wide

Step 6: Creating working desk extrusions

The number of desks (including: desks, PC cases, monitors, keyboards) is 20 sets The details are as follows:

Step 7: Creating the Water dispenser extrusion

 Electric water heater size: 310 x 300 x 970mm

Step 8: Creating the Coffee machine extrusion

Step 9: Creating the Printer extrusions

 Quantity: 28 sets of similar sized LEDs

Step 11: Creating the table extrusion

Step 12: Synthesize extrusions to simulate

Equipment extrusions, person and air extrusions are synthesized as shown below:

OVERVIEW OF CFD AND ANSYS SOFTWARE

Introduction to CFD and Ansys software

ANSYS (Analysis Systems) is a complete package of finite element analysis (FEA) software for simulation, industrial design calculations, has been and is being used worldwide in most of the industries engineering fields: structure, heat, flow, electricity, electromagnetism, interactions between media, between physical systems

Computational fluid dynamics or CFD is a branch of fluid mechanics that uses the analysis of systems involving fluid flow, heat transfer, and related phenomena such as chemical reactions using machine-based simulations count This technique is very effective and spans many areas of industrial and non-industrial applications

4.1.3 The formation and development of CFD

 From the 1960s onwards, the aerospace industry integrated CFD techniques into the design, R&D and production of aircraft and jet engines

 Recently, methods have been applied to the internal design of internal combustion engines, combustion chambers of gas turbines and furnaces

 The ultimate aim of developments in the field of CFD is to provide equivalent capabilities to other CAE (computer-aided engineering) tools such as stress analysis code CFDs have been involved in the broader industry community since the 1990s

The application of CFD in HVAC system

HVAC, fully known as Heating, Ventilation, and Air Conditioning, is a system that creates thermal comfort and ensures indoor air quality in all seasons of the year Typically, HVAC design calculations assume “ideal flow” (air is mixed perfectly in a very small amount of time), and manuals and estimators can be used quantity to make the calculation process fast and simple The result of this approach can lead to improper calculation of HVAC equipment, thereby increasing operating costs and reducing energy efficiency Therefore, it is still necessary to have a tool to verify the design before actual construction

To verify the design, engineers can either make experimental models or run CFD simulations under real operating conditions In which, CFD simulation is a tool that has been and is being used popularly by a number of large corporations such as SAMSUNG,

LG, DAIKIN or Ramboll because of its values and benefits

4.2.1 CFD simulation helps improve ventilation of negative pressure rooms

Air is a high-speed transport agent, making airborne infectious diseases such as acute respiratory infections spread quickly and dangerously In stagnant environments, droplets produced by various respiratory activities can travel great distances up to 6 meters at 50 m/s (sneeze), 2 meters at 10 m/s (sneezes), and 2 meters at 10 m/s s (cough) and 1 meter at 1 m/s (breath) This distance may increase or deform if the surrounding air is moving

In the case of an acute respiratory infection patient requiring hospitalization, the person is placed in a controlled facility called an isolation room Unlike other isolation rooms, the airborne infection isolation room (AIIR) is also known as a negative pressure (or pressure) room This is where computational fluid dynamics tools (CFD) plays an essential role

Experimental assessment of room pressure and airflow modeling can be difficult at the early stages of hospital layout design Using CFDs allows HVAC engineers to virtualize airflow patterns and use them to provide information on further design optimization that needs to be compliant with ASHRAE Standard-170 (Facilities Ventilation) guidelines Department of Health Care)

Figure 4.2: Arrangement of negative pressure room

Figure 4.3: Result of simulation for AIIR

The CFD simulation results show that the airborne infection isolation room becomes positive pressure relative to the corridor due to the entrance of air from the window This air acts as a medium to carry infected air from the room to the hallway There is still more than 10% of the infection concentration two meters away from the patient This would make the design unsafe for medical staff and patients in other spaces

A fan delivering 550 CFM is introduced into the hallway To avoid wind effects on the new system, the windows will be sealed After modifying the geometry to incorporate these adjustments simulation was performed

The new results obtained above show that able to reduce the contamination concentration to almost zero with the configuration, making it safe for medical staff caring for the patient The project above is a simple illustration of how CFD can be applied to the negative pressure room design process, helping to identify design flaws early in the process

CFD simulation execution process

Figure 4.5: The process of performing the CFD simulation problem

Detailed implementation of CFD simulation for the project

Step 1: After creating a basic 3D model in Revit 2021, we perform the steps to export to a file with “.sat’’ and convert to Ansys software:

- Choose File  Export  Cad Formats  ACIS (SAT)

Step 2: Opening Workbench (Ansys) 19.2, create Geometry

- At the left edge of the main interface, we hold down the left mouse button and drag (1) “Geometry’’ from Component systems into Project Schematic

- Next double click on (2) Geometry  (3) Import Geometry (4) Browse to open file “.sat” had saved at step 1

After finishing, we have a model as shown below

Step 3: Classification of architectures and device parts (including people)

Since the overlay and device architecture components will be decomposed when exporting to Ansys, a step of classifying the architectures and devices must be performed

Step 4: Suppress the architecture parts and keep the device parts and air part for convenience of simulation Click to select Suppress body to compress the parts without simulation

Figure 4.8: Architecture parts are suppressed

Step 5: Boolean with Unite (Combine the air part with window parts because we need to check a heat radiation which is located from the west direction)

In Ribbon toolbar choose (1) Create, next choose (2) Boolean

+ In Operation, choose (3) Unite to combine parts

+ In Tool bodies, choose Air and window parts

Figure 4.9: Boolean with Unite parts

Figure 4.10: An unite air part

Step 6: Boolean with Subtract (Use the unite air parts have just create in step 5 to eliminate a device parts in side)

In Ribbon toolbar choose Create, next choose Boolean

+ In Operation, choose (1) Subtract to eliminate parts

+ In Target bodies, choose Air parts

+ In Tool bodies, choose Devices, Lights, SAG, RAG, Door parts

Figure 4.11: Boolean with Subtract parts

After Generate, we have an Subtract air parts below shown:

Figure 4.12: Final Air part will be simulated

Then we create Named selections for the surfaces of heat-generating devices and people, heat-absorbing surfaces that heat transfer through walls, glass, windows, floors,

51 suspended ceilings, corner walls and create Named selections for the Supply Air and

Return Air as Inlets and Outlets

Step 7: Name selection for the faces which will be set parameters

+ Pick the surfaces and name

Figure 4.13: Name selection for West and East Wall

Figure 4.14: Name selection for Curtain Wall

Figure 4.15: Name selection for Suspended ceiling, Floor and Corridor Wall

Figure 4.16: Name selection for Led Panel

Figure 4.17: Name selection for Windows

Figure 4.18: Name selection for Door

Figure 4.19: Name selection for Supply Air Grill

Figure 4.20: Name selection for Return Air Grill

Figure 4.21: Name selection for People and Devices

- After finishing creating the Geometry, go back to the main interface

- In left side of interface Workbench, we push Mesh from Component Systems to Project Schematic after that double click to symbol of Mesh

- In Mesh choose Geometry (1)Air Fluid/Solid(2)Fluid

Figure 4.23: Set the fluid state for air block

Double click on Mesh in Project, we setup:

Figure 4.24: Setup the parameters for meshing

 Mesh Defeaturing choose Yes (Mesh Auto Correction)

 Capture Curvature choose Yes (Automatic mesh correction on curved surfaces)

 Capture Proximity choose Yes (Mesh smoothing)

Figure 4.25: Advance setup to have better quality mesh

 Check Mesh Quality choose Yes (Automatically check the mesh quality)

 Smoothing choose High (Mesh smoothness)

 Skewness (Mesh element deflection) (Important)

Concept of Skewness and Aspect Ratio:

Reference to Ansys Appendix A Mesh Quality Introduction to Ansys Meshing

(2010), this document recommends the following mesh quality:

Skewness: Deviation is one of the main indicators of mesh quality In interpolated regions, heavily skewed cells can reduce the accuracy of the numerical solution Rectangles and triangles are appreciated for offsets and conversely oblique rectangles and parallelograms are not

 Skewness parameters are evaluated through the equations:

 Here is the equation for the deviation analysis of the triangle and quadrilateral:

 Equilibrium Deviation: ăâêôơš9@˜ —=˜™š˜NN = max Ê ÔžŸƠ#Ô]

->ôđ is the largest angle

->ơš is the smallest angle

-˜ is the angle with respect to the equidistant (Means 90 for rectangle, 60 for triangle)

The standard Skewness must be close to 0 that it is better

Figure 4.26: Skewness mesh quality rating scale

Aspect Ratio: The aspect ratio is the ratio of the longest side length to the shortest side length The aspect ratio applies to triangular, tetrahedral, rectangular, and hexagonal sections and is defined differently for each element The aspect ratio can also be used to determine how close faces or cells are

Ansys recommends a maximum aspect ratio of 40, but it depends on the flow characteristics, so in some cases 50 may be acceptable Some empirical simulation problems for some cases, the maximum value can be < 14 is a good mesh

Orthogonal Quality: is also known as orthogonal quality, its evaluation range is from 0 to 1, values near 0 are worst and values near 1 are said to be good, here orthogonal quality is understood as counting perpendicular between the vectors of the faces in the divided mesh

Figure 4.29: Maximum Aspect Ratio value

Evaluate the results of the mesh

From the 3 standard parameters above, we have evaluation:

- Maximum skewness = 0.83309, this value around 0.8-0.95 so result is acceptable

- Maximum aspect ratio = 13.342< 14, satisfy the requirement

- Min orthogonal quality =0.16691, this value around 0.15-0.2 so result is acceptable

- In Statistics, Elements is 11.339.514 (Because to save time and simplify the simulation, our team takes this meshing result)

Figure 4.30: The number of Elements

Figure 4.31: Meshing finish with Tetrahedron Mesh

Tetrahedron Mesh is often used for CFD simulation with the 3D model which has a complicated shape

After completing the Mesh step we return to the main interface, then drag Fluent from Component Systems and double click

 Options choose Double Precision (round 2 digits after comma)

 Processing Options choose Parallel and fill number of cores in CPU’s computer

According to Versteeg, An introduction to CFD Finite volume method, page 20:

For compressible flows equation is often rearranged to give an equation for the enthalpy The specific enthalpy h and the specific total enthalpy h 0 of a fluid are defined: ¯(°± a ) ¯• + ²ơ³(´ℎ ê) = ²ơ³(= 9Bô² ả) + ¯œ ¯• + Ê ¯(ãá ¯ ạạ ) ạ + ¯ºãá ¯ ằạ ẳ ằ + ¯(ãá ¯ ẵạ ) ẵ + ¯ºắá ạằ ẳ ¯ ạ + ¯ºắá ¯ ằằ ẳ ằ + ¯ºắá ¯ ẵằ ẳ ẵ + ¯(¿á ¯ ạẵ ) ạ ¯º¿á ằẵ ẳ ¯ ằ + ¯(¿á ¯ ẵẵ ) ẵ Đ + —ℎ From Setup interface, choose General,

 Scale: Convert Units to Milimeter

 Check: To make sure that face area statistics of model always a positive numbers

 Report Quality: To report information of Orthogonal quality

 Solver  Type choose Pressure-Based (used for incompressible and lightly compressed flows) and Velocity Formulation choose Absolute

 Time Transient ( variation over time)

 Gravity The model is built with the bottom surface lying on the OXY plane, so the gravity direction is in the Z axis

 Choose the model's energy equations and viscous

 Turn on the Energy Equation because the problem needs to investigate heat transfer in space

 In Viscous choose Model is k-epsilon (simulation of turbulent fluid and k- epsilon with wall functions offer very low computational cost with acceptable errors)

 Near-Wall Treatment choose Standard Wall Functions (To get results close to the wall as standard)

Figure 4.34: Turn on Energy Equation

Thus far we have not defined the specific energy E of a fluid Often the energy of a fluid is defined as the sum of internal (thermal) energy i, kinetic energy (u 2 + v 2 + w2) and gravitational potential energy

The energy equation is: À ÁÂ Á• = −²ơ³(´ê) + Ê ¯(ãá ạạ ) ¯ ạ + ¯ºãá ằạ ẳ ¯ ằ + ¯(ãá ẵạ ) ¯ ẵ + ¯ºắá ạằ ẳ ¯ ạ + ¯ºắá ằằ ẳ ¯ ằ + ¯ºắá ẵằ ẳ ¯ ẵ + ¯(¿á ạẵ ) ¯ ạ + ¯º¿á ằẵ ẳ ¯ ằ + ¯(¿á ¯ ẵẵ ) ẵ Đ + ²ơ³(= 9Bô² ả) + — Â

According to Versteeg, An introduction to CFD Finite volume method, page 22:

In a Newtonian fluid the viscous stresses are proportional to the rates of deformation The three-dimensional form of Newton’s law of viscosity for compressible flows involves two constants of proportionality: the first (dynamic) viscosity, à, to relate stresses to linear deformations, and the second viscosity, λ, to relate stresses to the volumetric deformation The nine viscous stress components, of which six are independent, are: Ã ƠƠ = 2ÄÅê Åđ+ ặ ²ơ³ ê Ã ÇÇ = 2ÄÅ³ Åẩ+ ặ ²ơ³ ê Ã = 2ÄÅ™ Åẫ + ặ ²ơ³ ê The k-ε model equations: Ê = = / , @ = Ë O/Ì Í

Specific Heat: 1006,43 J/kg.K Thermal conductivity: 0,0242 W/m.K Viscosity: 1,789.10 5 kg/m.s

Specific Heat: 871 J/kg.K Thermal conductivity: 202,4 W/m.K

Specific Heat: 840 J/kg.K Thermal conductivity: 1.55 W/m.K

6 Solid Skin (person) Density: 1000 kg/m 3

Declare the parameters of the material is a basic condition for the software to calculate the heat transfer through the materials, thereby giving accurate simulation results

Figure 4.41: Properties of Person skin

Figure 4.43: Properties of Hollow Brick

Table 4.2: Parameters of boundary conditions

No Name selections Types of boundaries

(Supply Air Grill) Velocity inlet 12 Velocity: 0.98 m/s

2 Surface inlet Wall 12 Material: Aluminum

4 Surface outlet Wall 6 Material: Aluminum

5 Curtain wall Wall 1 Thermal conditions: Convection

Free stream temperature: 34.6 o C Wall Thickness: 8 mm

6 Windows Wall 1 Thermal conditions: Radiation

Emissivity: 0.92 External temperature: 34.6 o C Wall Thickness: 8 mm Material: Glass

7 Corridor wall Wall 1 Thermal conditions: Convection

8 Directsun wall Wall 1 Thermal conditions: Radiation

External temperature: 34.6 o C Wall Thickness: 200 mm Material: Brick

9 Floor Wall 1 Thermal conditions: Convection

External temperature29.3 o C Wall Thickness: 300 mm Material: Concrete

10 Suspended ceiling Wall 1 Thermal conditions: Convection

11 Led panel Wall 28 Thermal conditions: Heat Flux

12 Printer Wall 2 Thermal conditions: Heat Flux

13 PC Wall 20 Thermal conditions: Heat Flux

14 Person Wall 20 Thermal conditions: Heat Flux

Heat Flux: 80 (W/m 2 ) Material: PU (Person Unit)

15 Water dispenser Wall 1 Thermal conditions: Heat Flux

16 Coffee machine Wall 1 Thermal conditions: Heat Flux

17 Table Wall 1 Thermal conditions: Temperature

18 Table office Wall 1 Thermal conditions: Temperature

19 Door Wall 1 Thermal conditions: Convection

In Boundary Conditions, we start setting parameters for the Name Selections boundary names We set it up step by step as follows:

Figure 4.44: Setting boundary condition for Inlet

Figure 4.45: Setting boundary condition for Outlet

Figure 4.46: Thermal conditions of Curtain wall

Figure 4.47: Thermal conditions of Door

Figure 4.48: Thermal conditions of Windows

Figure 4.49: Thermal conditions of Corridor wall

Figure 4.50: Thermal conditions of Directsun wall

Figure 4.51: Thermal conditions of Floor

Figure 4.52: Thermal conditions of Suspended ceiling

Figure 4.53: Thermal conditions of Led panel

Figure 4.54: Thermal conditions of Printer

Figure 4.55: Thermal conditions of PC

Figure 4.56: Thermal conditions of Person skin

Figure 4.57: Thermal conditions of Water dispenser

Figure 4.58: Thermal conditions of Coffee machine

In this section we will implement methods for simulation In the Scheme box, select the SIMPLEC method to simplify the simulation process In the Spatial Discretization section, set the Second Order Upwind options as shown… This choice makes the simulation process take a long time but helps to increase the convergence of the model

Select the Hybrid Initializations method and click Initialize to initialize 10 initial values Because Hybrid initialization should be preferred and usually it accelerates the overall computation Standard initialization is just filling the filed properties with constant values, while hybrid initialization solves a number of iterations (10) of a simplified

76 equation system and thereby gets usually a better guess for the flow variables, in particular for the pressure field

- Number of Time Steps: Run at 150 Time steps Because of limitation of time

After the Ansys has completed the Iteration, we check the convergence of the model by the following:

In (1)Results  (2)Reports  (3)Fluxes  (4)Mass Flow Rate  (5)Inlet, Outlet  (6)Compute

The mass flow Rate difference between Inlet and Outlet is approximately 0 kg/s, achieving mass flow equilibrium condition

We proceed to check the results in Results

Create Results in Project Schematic Then we continue to transfer data from the Setup section to the Results section

Figure 4.64: Convert data to Results

In Results interface, we create parameters such as Contour, Plane, Volume Rendering,

ANALYSIS OF SIMULATION RESULTS

Expressing the Results

Creating the Volume Rendering that show the general temperature in the room

Create 3 cross-sections and planes which stay nearly the people to investigate states of air such as temperature, velocity

Figure 5.2: Temperature plane near the people

Create streamline show the velocity movement of air from the SAG to RAG

Figure 5.4: The distribution of air flow following vector

Comparison with Thermal Comfort Standard

Checking the Thermal Comfort Standard in VATD (Vertical Air Temperature Difference) by comparing the temperature difference between a seated person's head and ankles

Figure 5.5: Allowable Vertical Air Temperature Difference Between Head and Ankles

The Vertical Air Temperature Difference will be surveyed based on 2 straight lines created from the position next to the person, at the height of 0.1m and 1.1m respectively (0.1 is the distance from the floor to the ankle, 1.1 is the distance from the floor to the head)

Figure 5.6: The percent dissatisfied depending on air temperature difference between head and ankles

According to ASHRAE - 55 standard for vertical air temperature difference Thermal stratification results in a difference in air temperature at head and ankles that can cause thermal discomfort This section specifies the allowable difference between head air temperature and ankle air temperature

Creating the chart to make sure the temperature difference between Head and Ankles of 3 planes

Figure 5.7: Temperature difference between Head and Ankles in Plane 1

Evaluation: The temperature difference maximum of the sitting people between head and ankles in 3 Plane is 1.5 o C compared to ASHRAE - 55 standard that required vertical air temperature difference VATD < 3 o C, so satisfy specified thermal comfort requirements

Besides that, according to Floor Surface Temperature Standard (ASHRAE-55/

2017) When representative occupants are seated with feet in contact with the floor, floor surface temperatures within the occupied zone shall be 19℃ to 29℃

Figure 5.14: Floor temperature near people is range of 24 – 28.7 o C

According to Appendix A–Design standards for air conditioners (TCVN 5687:2010), the suitable velocity for the space where have a light labor, is from 0.8 m/s to

1 m/s Look at the Velocity category is showed at the Plane 1, the highest air speed is near a Suply Air Grill with about 1 m/s and decreases as it goes away We see that the speed meets the design standards TCVN 5687:2010

Figure 5.15: Velocity at Supply Air Grill in plane 1

General Evaluate: Thermal comfort conditions for occupants in the space are met

Nevertheless, CO2 levels are also a point of concern in enclosed spaces Currently, the room space is only allowed to exhaust air through the opening and closing of doors and windows, so suppose opening and exiting happen little or rarely during work, then we need an exhaust system By creating an exhaust air system with a suitable flow for office space according to the ventilation standard TCVN 5687:2010, it helps to release the amount of CO2 emitted by people during respiration and brings a comfortable for people However, this exhaust air intake can increase the power consumption to handle the amount of heat in the exhaust air into the room Realizing that the amount of heat transferred from the curtain wall increases temperature for people sitting near the area, it is recommended to install additional curtains to reduce the heat transfer coefficient and reduce the impact on the thermal comfort conditions of people sitting near curtain wall

Solution

The design of an additional exhaust vent helps to remove CO2, but this may change the thermal comfort conditions, so the second simulation we will investigate the change when there is a fresh air system with the purpose of removing CO2

VCO2 – CO2 exhaust air from people (m 3 /h.person) β - Allowed CO2 concentration, % Volume Choosing β = 0.15 a - CO2 concentration in ambient air, % Volume

Because CO2 flow depends on the intensity of activity Since the office belongs to light activity, we have VCO2 = 0.03 m 3 /h person

Table 5.1: Necessary amount of CO 2 exhaust ventilation for 1 person

The MACH 2 nd floor’s office has 20 people, so we have a CO2 exhaust air flow:

We use a single grill with a size of 600x600 placed at the corner of the coffee table to ensure aesthetics and absorb odors as well as reduce CO2 for the office

Velocity at the face of the air grills is calculated by the formula: ẻ = 4 Inside: v – Velocity at the air grill, m/s

Q – Air flow through the air grill, m 3 /s

Fresh air flow provided is 0.14 m 3 /s

The return air flow from the room is: 2.126 – 0.14 = 1.986 m 3 /s

We have the volumetric flow rate at the mixing point of each devices is: ¦ ÕR

So we have the air flow of each supply air grills is: 0.709 ÷ 4 = 0.18 > ’ /N and also the air flow of each return air grills is: ¦ Õ

The velocity of supply air grill: – = x× x× x / = /k ~/|

The velocity of return air grill: – = kk

Results of solution

Figure 5.16: The movement of room air

Investigate the temperature difference between the head and ankles at 3 initial Plane:

Figure 5.17: Temperature difference between Head and Ankles in Plane 1 with solution

Figure 5.23:Temperature difference between Head and Ankles in Plane 7 with solution

Figure 5.24: Floor temperature near people is range of 24 - 28.8 o C

Evaluate: When installing an additional CO2 exhaust ventilation system, a part of the air discharged out leads to an increase in the surrounding temperature of about 1 degree However, the temperature difference between the head and the ankles is still within the allowable range of the regulator thermal comfort and floor temperature remain within the allowable range However, a significant benefit is that the amount of CO2 in the room is solved while ensuring comfortable thermal conditions for people

CONCLUSION AND RECOMMENDATION

Conclusion

From simulation results, we can survey and evaluate standards related to room temperature Besides, it is possible to consider thermal comfort standards such as the temperature difference between the head and ankles of the occupants, the standard of floor temperature and at the same time place the exhaust vent to reduce the CO2 concentration in the room

However, this study still has certain limitations Due to the limited time of exposure to the new research method, the in-depth knowledge of Ansys simulation software is not much, so it is inevitable that the results will be short of results Therefore, the group needs the evaluation and comments of the lecturers to improve the quality of this research topic.

Recommendation

Research on the optimization of thermal comfort for people in general and the distribution of temperature and velocity contours in air-conditioned rooms in particular in Vietnam and around the world is still very new And in our country, the demand for using air conditioning systems is very large and very necessary, this is also a positive signal in the field of air conditioning design Therefore, the study of temperature field optimization is very necessary at present and in the future

The team found that there are still some limitations in the current design:

About office design: Due to some objective issues, the team could not directly approach the office outside, but only received parameters, architectural documents, and drawings through specialized software

About software: Because the team designed the architectural blocks and simulation equipment on Revit 3D software, it was a bit difficult to convert data to Ansys (because the software is not optimized for the connection with Revit) Leads to data errors or loss of blocks, discrete blocks when exporting the model to Ansys software

The process of launching the simulation in the Solution section depends on the computer configuration, for low-configuration machines it will run the simulation with a

95 longer time and give inaccurate results, so this leads to asynchrony in exporting results among group members

The research team has several recommendations:

Research on thermal comfort factors will improve the distribution of the temperature field in the conditioned room Rearrange the sitting position of the worker Indoor environment conditions need to be care more, especially the amount of CO2 and moisture

In addition, although simulation is a very useful method, it is still necessary to create realistic and experimental models to get the best results

[1] Nguyen Duc Loi (2019), “Giáo Trình Hệ Thống Điều Hòa Không Khí”, Vietnam Education Publishing House, Ha Noi, 339 pages

[2] Nguyen Duc Loi (2011), “Hướng Dẫn Thiết Kế Hệ Thống Điều Hòa Không Khí”, Vietnam Education Publishing House, Ha Noi, 467 pages

[5] TCVN 5687-2010: Ventilation _ Air conditioning _ Design standards

[6] Ansys, Inc, Ansys Fluent tutorial Guide, January 2018

[10] Baoqing Deng, Yujie Zhang, Fei Long, A superficial density method to describe the diffuser boundary condition in CFD simulation of indoor airflow, 2018

[11] Catalin George Popovici, HVAC system functionality simulation using ANSYS- Fluent, Bucharest, Romania, 26-28 October 2016

[12] Catalin-George Popovici, Valeriu Sebastian Huditeanu, Numerical simulation of HVAC system functionality in a sociocultural building, Tirgu-Mures, Romania, 8-9 October 2015

[13] Cuuduongthancong.com, Psychometric Chart Carrier pdf

[14] George Pichurov, Peter Stankov, Detelin Markov, “HVAC control based on CFD analysis of room airflow”, 2006

[15] https://www.advantech.vn/mo-phong-cfd-giup-cai-thien-he-thong-thong-gio-cua- phong-ap-luc-am-25

[16] https://vi.wikipedia.org/wiki/Willis_Carrier

[17] https://www.cungcapmaybom.vn/uu-nhuoc-diem-cua-he-thong-water-chiller

[18] https://hvacdesign.vn/blogs/cam-nang-kien-thuc/vrv-la-gi-vrf-la-gi-nguyen-ly- hoat-dong

[19] https://sunhouse.com.vn/dieu-hoa-1-cuc-khac-gi-dieu-hoa-2-cuc

Tiêu đề	Analyzing Thermal Comfort Conditions Through Numerical Simulation For A Mach’s Office
Tác giả	Quan Thanh Truong, Pham Thanh Danh, Nguyen Lam Chi Thanh
Người hướng dẫn	Doan Minh Hung, Dr
Trường học	Ho Chi Minh City University of Technology and Education
Chuyên ngành	Thermal Engineering Technology
Thể loại	graduation project
Năm xuất bản	2023
Thành phố	Ho Chi Minh City

Định dạng
Số trang	123
Dung lượng	4,19 MB