Heat Transfer Exercises

Heat Transfer Exercises tài liệu, giáo án, bài giảng , luận văn, luận án, đồ án, bài tập lớn về tất cả các lĩnh vực kinh...

Heat Transfer: Exercises Chris Long; Naser Sayma Download free books at Chris Long & Naser Sayma Heat Transfer: Exercises Download free eBooks at bookboon.com Heat Transfer: Exercises © 2010 Chris Long, Naser Sayma & Ventus Publishing ApS ISBN 978-87-7681-433-5 Download free eBooks at bookboon.com Heat Transfer: Exercises Contents Contents Preface Introduction Conduction 11 Convection 35 Radiation 60 Heat Exchangers 79 www.sylvania.com We not reinvent the wheel we reinvent light Fascinating lighting offers an ininite spectrum of possibilities: Innovative technologies and new markets provide both opportunities and challenges An environment in which your expertise is in high demand Enjoy the supportive working atmosphere within our global group and beneit from international career paths Implement sustainable ideas in close cooperation with other specialists and contribute to inluencing our future Come and join us in reinventing light every day Light is OSRAM Click on the ad to read more Download free eBooks at bookboon.com Preface Heat Transfer: Exercises Preface Worked examples are a necessary element to any textbook in the sciences, because they reinforce the theory (i.e the principles, concepts and methods) Once the theory has been understood, well chosen examples can be used, with modification, as a template to solve more complex, or similar problems This work book contains examples and full solutions to go with the text of our e-book (Heat Transfer, by Long and Sayma) The subject matter corresponds to the five chapters of our book: Introduction to Heat Transfer, Conduction, Convection, Heat Exchangers and Radiation They have been carefully chosen with the above statement in mind Whilst compiling these examples we were very much aware of the need to make them relevant to mechanical engineering students Consequently many of the problems are taken from questions that have or may arise in a typical design process The level of difficulty ranges from the very simple to challenging Where appropriate, comments have been added which will hopefully allow the reader to occasionally learn something extra We hope you benefit from following the solutions and would welcome your comments Christopher Long Naser Sayma Brighton, UK, February 2010 Download free eBooks at bookboon.com Introduction Heat Transfer: Exercises Introduction Example 1.1 The wall of a house, m wide and m high is made from 0.3 m thick brick with k  0.6 W / m K The surface temperature on the inside of the wall is 16oC and that on the outside is 6oC Find the heat flux through the wall and the total heat loss through it Solution: For one-dimensional steady state conduction: q  k q dT k   Ti  To  dx L 16  6  20 W / m Q  qA  20  6    840 W The minus sign indicates heat flux from inside to outside Download free eBooks at bookboon.com Introduction Heat Transfer: Exercises Example 1.2 A 20 mm diameter copper pipe is used to carry heated water, the external surface of the pipe is subjected to a convective heat transfer coefficient of h  W / m K , find the heat loss by convection per metre length of the pipe when the external surface temperature is 80oC and the surroundings are at 20oC Assuming black body radiation what is the heat loss by radiation? Solution qconv  h Ts  T f   680  20  360 W / m For metre length of the pipe: Qconv  q conv A  q conv  2 r  360     0.01  22.6 W / m For radiation, assuming black body behaviour:  q rad   Ts4  T f4   q rad  5.67  10 8 353  293  q rad  462 W / m For metre length of the pipe Qrad  q rad A  462     0.01  29.1 W / m A value of h = W/m2 K is representative of free convection from a tube of this diameter The heat loss by (black-body) radiation is seen to be comparable to that by convection Download free eBooks at bookboon.com Introduction Heat Transfer: Exercises Example 1.3 A plate 0.3 m long and 0.1 m wide, with a thickness of 12 mm is made from stainless steel ( k  16 W / m K ), the top surface is exposed to an airstream of temperature 20oC In an experiment, the plate is heated by an electrical heater (also 0.3 m by 0.1 m) positioned on the underside of the plate and the temperature of the plate adjacent to the heater is maintained at 100oC A voltmeter and ammeter are connected to the heater and these read 200 V and 0.25 A, respectively Assuming that the plate is perfectly insulated on all sides except the top surface, what is the convective heat transfer coefficient? Solution Heat flux equals power supplied to electric heater divided by the exposed surface area: q V I V I 200  0.25    1666.7 W / m A W L  This will equal the conducted heat through the plate: k T2  T1  t 1666.7  0.012   98.75C qt T1  T2   100  16 k q (371.75 K) The conducted heat will be transferred by convection and radiation at the surface:  q  hT1  T f    T14  T f4 h  q   T14  T f4 T  Tf     1666.7  5.67  10 371.75 8 371.75  293  293   12.7 W / m Download free eBooks at bookboon.com K Introduction Heat Transfer: Exercises Example 1.4 An electronic component dissipates 0.38 Watts through a heat sink by convection and radiation (black body) into surrounds at 20oC What is the surface temperature of the heat sink if the convective heat transfer coefficient is W/m2 K, and the heat sink has an effective area of 0.001 m2 ? Solution q  Q  hTs  T    Ts4  T4 A   0.38  6Ts  293  5.67  10 3 Ts4  293 0.001  5.67  10 8 Ts4  6Ts  2555.9  This equation needs to be solved numerically Newton-Raphson’s method will be used here: f  5.67  10 8 Ts4  6Ts  2555.9 df  22.68  10 8 Ts3  dTs T n 1 s 5.67  10 8 Ts4  6Ts  2555.9 T  T  22.68Ts3   df     dTs  n s f n s Start iterations with Ts0  300 K Ts1  300  5.67  10 8  300   300  2555.9  324.46 K 22.68  300  5.67  10 8  324.46   324.46  2555.9  323 K T  324.46  22.68  324.46  s Download free eBooks at bookboon.com Introduction Heat Transfer: Exercises The difference between the last two iterations is small, so: Ts0  323 K  50C The value of 300 K as a temperature to begin the iteration has no particular significance other than being above the ambient temperature 360° thinking Discover the truth at www.deloitte.ca/careers © Deloitte & Touche LLP and affiliated entities 10 Click on the ad to read more Download free eBooks at bookboon.com ...Chris Long & Naser Sayma Heat Transfer: Exercises Download free eBooks at bookboon.com Heat Transfer: Exercises © 2010 Chris Long, Naser Sayma & Ventus Publishing... text of our e-book (Heat Transfer, by Long and Sayma) The subject matter corresponds to the five chapters of our book: Introduction to Heat Transfer, Conduction, Convection, Heat Exchangers and... sign indicates heat flux from inside to outside Download free eBooks at bookboon.com Introduction Heat Transfer: Exercises Example 1.2 A 20 mm diameter copper pipe is used to carry heated water,