Nghiên cứu đặc tính truyền nhiệt của thiết bị bay hơi ống micro với môi chất r134a

Nghiên cứu đặc tính truyền nhiệt của thiết bị bay hơi ống micro với môi chất r134a Nghiên cứu đặc tính truyền nhiệt của thiết bị bay hơi ống micro với môi chất r134a Nghiên cứu đặc tính truyền nhiệt của thiết bị bay hơi ống micro với môi chất r134a Nghiên cứu đặc tính truyền nhiệt của thiết bị bay hơi ống micro với môi chất r134a Nghiên cứu đặc tính truyền nhiệt của thiết bị bay hơi ống micro với môi chất r134a

TĨM TẮT Đặc tính truyền nhiệt thiết bị bay ống micro sử dụng môi chất R134a nghiên cứu thực nghiệm Thiết bị bay ống micro làm ống đồng cánh nhơm với kích thước (L*W*H) 225*43*200mm; có 10 pass, pass có ống đồng có đường kính thủy lực 0,82mm Nghiên cứu thực nghiệm dựa thay đổi lưu lượng không khí qua thiết bị bay từ 19 đến 47,5 l/s với thông số khác không thay đổi sử dụng hệ thống thực nghiệm Bên cạnh đó, kết nghiên cứu so sánh với thiết bị bay FNA với kích thước diện tích trao đổi nhiệt Trong nghiên cứu này, thơng số như: nhiệt độ, độ ẩm, mật độ dòng nhiệt, suất lạnh nghiên cứu so sánh hai thiết bị bay ống micro thiết bị bay FNA Kết cho thấy rằng, nhiệt độ mơi trường 30oC, mật độ dịng nhiệt thiết bị bay tăng tăng lưu lượng không khí qua thiết bị bay Mật độ dịng nhiệt thiết bị bay ống micro lớn mật độ dòng nhiệt thiết bị bay FNA 1,32 lần, điều kiện thực nghiệm với lưu lượng 47,5 l/s Ngoài ra, suất lạnh thiết bị bay tăng tăng lưu lượng khơng khí qua thiết bị bay Năng suất lạnh tối đa thiết bị bay ống micro 840W Công suất lạnh thiết bị bay ống micro cao 1.4 lần công suất lạnh thiết bị bay FNA lưu lượng khơng khí 47,5 l/s nhiệt độ khơng khí đầu vào 30oC ABSTRACT The heat transfer characteristics of a microtube evaporator use refrigerant R134a were studied experimentally This evaporator was made from copper tubes and aluminum foils with overall dimensions (L*W*H) of 225*43*200mm The evaporator has 10 passes, each pass including copper pipes (with hydraulic diameter of 0.82mm) and 83 aluminum foils (with the thickness of 0.3 mm); the distance between the two foils is 2.6 mm The experiment was conducted based on the change of air volume flow of micro tube evaporator from 19 đến 47.5 l/s with other experimental conditions unchanged Beside, the results were compared with the FNA evaporator with the same overall dimensions and heat exchange area In this study, parameters such as temperature, humidity, heat flux, cooling capacity were compared between micro tube evaporators and FNA evaporator The results show that, at the same ambient temperature of 30oC, the heat flux of the evaporator increases when increasing air volume flow through evaporator The heat flux of micro tube evaporator is higher than the macro evaporator FNA 1.32 times, at the same experimental conditions Besides, the cooling capacity of the evaporator increase when increasing air volume flow The maximum cooling capacity of microtube evaporator is 840W The cooling capacity of micro tube evaporator is 1.4 times higher than the cooling capacity of macro evaporator FNA at the air volume flow is 47.5 l/s and the inlet air evaporator temperature is 30oC MỤC LỤC CHƯƠNG I: TỔNG QUAN 1.1 Tính cấp thiết đề tài 1.2 Tổng quan nghiên cứu liên quan 1.3 Mục đích đề tài 24 1.4 Phương pháp nghiên cứu 24 1.5 Đối tượng phạm vi nghiên cứu 25 1.6 Nội dung nghiên cứu 25 1.7 Giới hạn đề tài 25 CHƯƠNG II: CƠ SỞ LÝ THUYẾT 26 2.1 Lý thuyết truyền nhiệt 26 2.2 Lý thuyết đo lường 27 2.3 Tính tốn chu trình lạnh 28 CHƯƠNG III: THIẾT LẬP THỰC NGHIỆM 32 3.1 Thiết kế mơ hình hệ thống thí nghiệm 32 3.1.1 Thiết kế mơ hình 32 3.1.2 Hệ thống thí nghiệm 34 3.2 Mơ tả hệ thống thí nghiệm 36 CHƯƠNG IV: KẾT QUẢ VÀ THẢO LUẬN 43 4.1 Thực nghiệm thiết bị bay FNA 43 4.2 Thực nghiệm thiết bị bay ống micro 44 4.3 Thực nghiệm thay đổi lưu lượng khơng khí 45 CHƯƠNG V: KẾT LUẬN VÀ KIẾN NGHỊ 51 5.1 Kết luận 51 5.2 Kiến nghị 51 TÀI LIỆU THAM KHẢO 53 DANH MỤC CÁC KÝ HIỆU VÀ CHỮ VIẾT TẮT Qt : Nhiệt lượng truyền qua thiết bị bay hơi, W mt : Lưu lượng khối lượng, kg/s cp : Nhiệt dung riêng, kJ/kg dt : Độ chênh nhiệt độ đầu vào đầu thiết bị bay hơi, oC qt : Mật độ dòng nhiệt, W/m2 k : Hệ số truyền nhiệt tổng, W/m2K A : Diện tích truyền nhiệt, m2 ht : Năng suất lạnh tổng, W hs : Nhiệt q trình làm lạnh khơng khí, W hl : Nhiệt ẩn q trình làm lạnh khơng khí, W ρ : Tỷ trọng khơng khí, kg/m3 V : Lưu lượng thể tích, m3/s hwe : Nhiệt ẩn bay nước, kJ/kg dwkg : Độ chênh lệch độ ẩm, kg nước/kg khơng khí khơ F : Diện tích mặt cắt ngang ống gió, m2 v : Tốc độ khơng khí trung bình, m/s TBBH : Thiết bị bay ME : Thiết bị bay ống Micro NE : Thiết bị bay FNA-0.25/1.3 F : Diện tích mặt cắt ngang ống gió, m2 v : Tốc độ khơng khí trung bình, m/s Tin : Nhiệt độ khơng khí đầu vào thiết bị bay hơi, oC Tout_micro : Nhiệt độ khơng khí đầu thiết bị bay ống micro, oC Tout_FNA-0.25/1.3 : Nhiệt độ không khí đầu TBBH FNA-0.25/1.3, oC RHin : Độ ẩm tương đối đầu vào thiết bị bay hơi, % RHout_micro : Độ ẩm tương đối đầu thiết bị bay ống micro, % RHout_FNA-0.25/1.3 : Độ ẩm tương đối đầu TBBH FNA-0.25/1.3, % FNA : Thiết bị bay FNA-0.25/1.3, ZHONGLI, KEWELY Nel : Công suất điện tiêu thụ, kW Ndc : Công suất động cơ, kW S : Hệ số an tồn Ne : Cơng suất hữu ích, kW ηel : Hiệu suất động điện ηtđ : Hiệu suất truyền động Ni : Công nén thị, kW ηe : Tổn thất ma sát ηi : Hiệu suất thị Ns : Công nén đoạn nhiệt, kW m : Khối lượng môi chất qua máy nén, kg/s l : Công nén riêng, kJ/kg qo : Năng suất lạnh riêng, kJ/kg Qo : Năng suất lạnh, kW F : Diện tích trao đổi nhiệt thiết bị bay hơi, m2 tlm : Độ chênh nhiệt độ trung bình logarit, K αair : Hệ số trao đổi nhiệt khơng khí, W/m2K αR134a : Hệ số trao đổi nhiệt môi chất lạnh R134a, W/m2K δ : Độ dày bề mặt trao đổi nhiệt, m λ : Hệ số dẫn nhiệt, W/mK qk : Công suất nhiệt riêng, kJ/kg Qk : Công suất nhiệt, kW FNA-0.8/3.4 : Thiết bị ngưng tụ hãng ZHONGLI, KEWELY DANH MỤC BẢNG VÀ ĐỒ THỊ Bảng 1.1 Thể tóm tắt nghiên cứu thiết bị vi kênh sử dụng môi chất lạnh R134a .6 Bảng 1.2 Tóm tắt số nghiên cứu thiết bị bay vi kênh với thông số khác 13 Bảng 2.1 Các điểm nút chu trình làm lạnh mơi chất R134a 28 Bảng 3.1 Thơng số kích thước thiết bị bay 32 Bảng 4.1 Các điểm thực nghiệm chu trình thiết bị bay FNA 42 Bảng 4.2 Các điểm thực nghiệm chu trình thiết bị bay ống micro 28 Bảng 4.3 Bảng thông số kết thực nghiệm 32 DANH MỤC HÌNH ẢNH Hình 1.1 Sơ đồ ngun lý thực nghiệm [1] Hình 1.2 Sơ đồ nguyên lý thực nghiệm [3] Hình 1.3 Sơ đồ nguyên lý hệ thống thực nghiêm [15] Hình 1.4 Mơ hình thực nghiêm [15] .9 Hình 1.5 Sơ đồ nghiên lý thực nghiệm [20] 11 Hình 1.6 Sơ đồ nguyên lý thực nghiệm [24] 12 Hình 1.7 Bộ tản nhiệt micro với hình dáng khác [33] 15 Hình 1.8 Sơ đồ nguyên lý tản nhiệt với vị trí đầu vào/ra khác [34] 16 Hình 1.9 Sơ đồ nguyên lý tản nhiệt vi kênh với ống góp khác [34] 16 Hình 1.10 Sơ đồ nguyên lý với hình dạng vật lý vi kênh khác [34] 17 Hình 1.11 Bộ tản nhiệt vi kênh loại I [34] 17 Hình 1.12 Thơng số hình học vi kênh hình Ω lõm [36] 18 Hình 1.13 Sơ đồ nguyên lý thực nghiệm [36] 18 Hình 1.14 Bộ tản nhiệt vi kênh với hình dáng khác (hình thoi, hình trịn hình lục giác) [37] 19 Hình 1.15 Bộ tản nhiệt vi kênh với hình dáng khác (zigzag, cong bước kênh) [40] 21 Hình 1.16 Mơ hình thí nghiệm hệ thống thực nghiệm [41] 22 Hình 1.17 Hệ thống thực nghiệm [43] 23 Hình 1.18 Hệ thống thực nghiệm [44] 23 Hình 2.1 Vị trí đặt cảm biến ống gió tiết diện hình trịn hình chữ nhật 28 Hình 2.2 Đồ thị p-h trình làm lạnh R134a 29 Hình 2.3 Biến thiên nhiệt độ thiết bị bay 30 Hình 3.1 Mơ hình thiết bị bay FNA 32 Hình 3.2 Mơ hình thiết bị bay ống micro 33 Hình 3.3 Sơ đồ nguyên lý thực nghiệm 34 Hình 3.4 Hình ảnh hệ thống thí nghiệm thực tế 35 Hình 3.5 Hình ảnh máy nén EE80Y-E 36 Hình 3.6 Hình ảnh thiết bị ngưng tụ FNA-0.8/3.4 37 Hình 3.7 Hình ảnh ống mao sử dụng hệ thống 37 Hình 3.8 Hình ảnh điều khiển trung tâm 38 Hình 3.9 Hình ảnh thiết bị đo nhiệt độ hai kênh Extech 421509 39 Hình 3.10 Hình ảnh thiết bị đo độ ẩm Tenmars TM-181 40 Hình 3.11 Hình ảnh thiết bị đo tốc độ gió Prova AVM 03 40 Hình 3.12 Hình ảnh Ampe kiềm VC3266L+ 41 Hình 3.13 Hình ảnh camera nhiệt Fluke Ti9 41 Hình 4.1 Đồ thị p-h chu trình sử dụng thiết bị bay FNA 42 Hình 4.2 Đồ thị p-h chu trình sử dụng thiết bị bay ống micro 43 Hình 4.3 Mối quan hệ lưu lượng khơng khí nhiệt độ đầu TBBH 44 Hình 4.4 Mối quan hệ lưu lượng khơng khí độ ẩm tương đối đầu TBBH 45 Hình 4.5 Mối quan hệ lưu lượng khơng khí qua TBBH mật độ dịng nhiệt 46 Hình 4.6 Mối quan hệ lưu lượng khơng khí qua TBBH Năng suất lạnh 46 Hình 4.7 Biểu đồ độ ẩm TBBH ống micro lưu lượng khơng khí 47.5 l/s 48 Hình 4.8 Phân bố trường nhiệt độ 49 CHƯƠNG I: TỔNG QUAN 1.1 Tính cấp thiết đề tài Các ngành kỹ thuật đóng vai trị chủ chốt cơng cơng nghiệp hóa – đại hóa nước ta Cùng với phát triển khoa học công nghệ, ngành công nghệ kỹ thuật nhiệt bước cải thiện để đáp ứng nhu cầu công nghiệp đời sống Một vấn đề nhà nghiên cứu để tâm vấn đề kích thước thiết bị, hệ thống Các thiết bị giải nhiệt – làm mát thiết bị địi hỏi có kích thước nhỏ gọn để thuận tiện cho việc lắp đặt thẩm mỹ Cùng với vấn đề kích thước u cầu hiệu quả, cơng suất, giá thành địi hỏi để đáp ứng nhiều lĩnh vực Nắm bắt u cầu đó, nhà khoa học khơng ngừng nghiên cứu giải pháp nhằm nâng cao chất lượng sản phẩm theo nhiều chiều hướng khác Nhiều thí nghiệm thực để thu hẹp kích thước khả chiếm diện tích thiết bị mà có hiệu tốt, chi phí chế tạo, lắp đặt hợp lý Trong năm đầu kỷ 21, công nghệ micro nhiều nhà khoa học áp dụng nghiên cứu kết thu ngày tốt khả trao đổi nhiệt tăng lên, kích thước trao đổi nhiệt giảm xuống, hiệu suất truyền nhiệt ngày cải thiện Trong có nhiều ứng dụng nghiên cứu là: làm mát tuabin, làm mát lò phản ứng nhiệt hạch, làm mát vòi phun động tên lửa, làm mát điện tử công suất hệ thống điện tử hàng không loại xe hybrid, làm mát hồ chứa hydro, hệ thống gas lạnh, điều khiển nhiệt trọng lực vòng mao mạch bơm Để tiếp nối nghiên cứu trước đây, em định tìm hiểu nghiên cứu trình bay thiết bị bay kênh micro Từ đó, đưa quan điểm, phương pháp góp phần tối ưu hóa cho khả bay thiết bị trao đổi nhiệt kênh micro nghiên cứu trình lưu chất qua thiết bị; yếu tố ảnh hưởng đến trình truyền nhiệt, lưu chất thiết bị bay ống micro tiến hành nghiên cứu mơ để so sánh với kết thực nghiệm mà tác giả nghiên cứu Kiến nghị nghiên cứu tiếp bước nghiên cứu 52 TÀI LIỆU THAM KHẢO [1] Thiangtham P, Keepaiboon C, Kiatpachai P, Asirvatham LG, Mahian O, Dalkilic AS, et al., "An experimental study on two-phase flow patterns and heat transfer characteristics during boiling of R134a flowing through a 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microchannel heat sink", International Communications in Heat and Mass Transfer, 2011, 474-80 [41] Dang T, "AN EXPERIMENTAL STUDY ON SUBCOOLING PROCESS OF A TRANSCRITICAL CO2 AIR CONDITIONING CYCLE WORKING WITH MICROCHANNEL EVAPORATOR", 2017, 1505-14 [42] Ho D, Dang T, Le C, Nguyen H, "An experimental comparison between a microchannel cooler and conventional coolers of a CO2 air conditioning cycle", 2017, 656-61 [43] Doan M, Le T, Dang T, Teng J-t, "A Numerical Simulation on Phase Change of Steam in a Microchannel Condenser", 2017, [44] Doan M, Dang T, "The Effect of Cooling Water on Condensation of Microchannels", 2017, 51-6 57 [45] Nguyen H, Dang T, V Chau K, "Numerical Simulation on Heat Transfer Phenomena in Microchannel Evaporator of A CO2 Air Conditioning System", 2017, [46] PGS.TS Nguyễn Đức Lợi, Phạm Văn Tùy, Kỹ thuật lạnh sở, Nhà xuất giáo dục, 2009 [47] PGS.TS Nguyễn Đức Lợi, Phạm Văn Tùy, Máy nén thiết bị lạnh, Nhà xuất giáo dục, 2009 58 IJISET - International Journal of Innovative Science, Engineering & Technology, Vol Issue 3, March 2018 ISSN (Online) 2348 – 7968 www.ijiset.com An Experimental Study on Heat Transfer Characteristics of Refrigerant R134a in a Microtube Evaporator Giadat Nguyen, Hoangtuan Nguyen, and Thanhtrung Dang Department of Thermal Engineering, HCMC University of Technology and Education, Vietnam Abstract The heat transfer characteristics of a microtube evaporator use refrigerant R134a were studied experimentally This evaporator was made from copper tubes and aluminum foils with overall dimensions (L*W*H) of 259.3*43*200mm The evaporator has 10 passes, each pass including copper pipes (with hydraulic diameter of 0.82mm) and 83 aluminum foils (with the thickness of 0.3 mm); the distance between the two foils is 2.6 mm The experiment was conducted based on the same heat transfer area and overall dimensions a conventional evaporator (macro evaporator) In this study, the heat flux increases with increasing air flow rate of the evaporator at an inlet air temperature of 30oC Besides, the cooling capacity and the COP of the micro tube evaporator are also presented in this study Keywords: Experimental Study, Heat Transfer, R134a, MultiMicro Tubes, Evaporator Introduction In recent years, micro-technology is one of the most interesting fields in research Scientists and researchers have conducted many impressive and applicable studies in new trends of this technology Especially, for thermal engineering, thousands of good results have been published such as: improving the heat transfer performance, reducing the dimensions of heat exchangers or improving the heat transfer efficiency Moreover, many relating applications have been developed such as electronic cooling, cooling turbine blades, cooling fusion reactor blankets, cooling the nozzles of rocket engines, cooling power electronics in avionics and hybrid vehicles, cooling hydrogen storage reservoirs, refrigeration cooling, thermal control in microgravity and capillary–pumped loops In addition, comparing with the conventional heat exchangers, the heat exchangers using microchannels provide other benefits, such as decreasing the required size, the weight, the pumping power, and the amount of working fluid For topic of refrigerants in microchannels, Fayyadh et al [1] conducted experiments to investigate flow boiling heat transfer of R134a in a multi microchannel heat sink at 6.5 bar system pressure and covered a footprint area-based heat flux range 11.46–403.1 kW/m2 as well as mass flux range 50–300 kg/m2 s Three flow patterns were observed namely bubbly, slug and wavy-annular flow when the heat flux increased gradually The heat transfer coefficient increased with heat flux and there was no mass flux effect In addition, Mahmoud et al [2] studied the surface effects to flow boiling of R134a in microtubes and showed that the flow boiling characteristics in the welded tube were completely different from those in the seamless cold drawn tube In the seamless tube, the heat transfer process was dominated by the nucleate boiling mechanism while the welded tube did not show a clear dominant mechanism Besides, Thiangtham et al [3] and Keepaiboon et al [4] experimentally studied heat transfer and pressure drop characteristics of flow boiling of R134a in a multimicrochannel heat sink They gave results that: The heat flux and saturation temperature have significant effects on the variation of flow patterns For pressure drop, experimental results indicated that the total pressure was dominated by frictional pressure drop The increase of mass flux also increased the frictional pressure gradient, whereas the increase of saturation temperature reduced the frictional pressure gradient In addition, the heat flux also had an insignificant effect on the frictional the pressure gradient Micro device for liquid cooling by evaporation of R134a was studied by Wibel et al [5] This system is realized by the usage of a micro heat exchanger using microchannels with 100 µm in height and in width and a conventional cooling cycle In the experiments, the compact multilayer micro heat exchangers enabled the direct cooling of a non-continuous water flow from 55 ◦C to ≈ ◦C at a transferred cooling power of 650 W Researchers and scientists were also interested in microchannel heat exchangers A CFD study of the parameters influencing heat transfer in microchannel slug flow boiling was conducted by Magnini and Thome [6] The results indicated that the heat transfer coefficient 82 IJISET - International Journal of Innovative Science, Engineering & Technology, Vol Issue 3, March 2018 ISSN (Online) 2348 – 7968 www.ijiset.com slightly decreases with an increase of the heat flux when the bubble frequency is constant An increase of the mass flow rate reduces the heat transfer performance due to the thicker liquid film The heat transfer coefficient is improved by smaller channels and larger bubble frequency Besides, Huang and Thome [7] conducted experimental study on flow boiling pressure drop in multi-microchannel evaporators with different refrigerants With the present test conditions, the channel pressure drop increased with the inlet subcooling and inlet orifice width but slightly affected by the outlet saturation temperature Markal et al [8] experimentally investigated saturated flow boiling heat transfer and pressure drop in square microchannel The results showed that the local two phase heat transfer coefficient decreases with an increase of the heat flux or the local vapor quality for all the mass flux values considered; while, this heat transfer coefficient increases significantly with an increase in the mass flux In addition, experiments of pressure drop and heat transfer with the different orientations of gravity were conducted by Lee et al [9] In this paper, the influence of orientation on twophase heat transfer was significant for low mass velocities with G/ρ f < 0.22 m/s and negligible for G/ρ f > 0.22 m/s Moreover, adiabatic two-phase gas–liquid flow behaviors during upward flow in a vertical circular micro-channel were studied by Saisorn and Wongwises [10] The flow visualization results also indicated that the flow pattern map for vertical upward flow is not completely compatible with that for horizontal flow In addition, vertical upward flow can issue higher pressure drop when compared with the horizontal channel Furthermore, gas–liquid two-phase flow in microchannel was also studied by Triplett et al [11] Comparing with relevant flow regime transition models, the results in [11] are poor agreement Liu et al [12] conducted an experimental investigation of two-phase slug flow distribution in horizontal multi-parallel microchannels It was found that the phase distribution characteristics of two-phase flow in parallel channels highly depended on the inlet gas slug length and the inlet real velocity The channels in the front of the header can influence the phase distribution of the adjacent channel in the rear Thermal design and operational limits of twophase micro-channel heat sinks of Kim and Mudawar [13] showed that maximum heat flux is dominated by different limits for different flow rate ranges, and it may be increased significantly as decreasing bottom wall temperature Besides, Lee and Mudawar [14] studied about transient characteristics of flow boiling in large microchannel heat exchangers and stated that heat transfer mechanisms depending on quality range, with low qualities associated with slug flow and dominated by nucleate boiling, and high qualities by annular flow and convective boiling The influence of configuration and dimension is also interesting topic of microchannel Tran et al [15] conducted a study on five different channel shapes using a novel scheme for meshing and a structure of a multi-nozzle microchannel heat sink In channels of all shapes in this study, the best thermal performance was achieved by a circular channel shape In an experimental study on the effects of inlet/outlet locations conducted by Dang and Teng [16], for two types (I-type and S-type) of the microchannel heat exchangers, the heat flux and pressure drop obtained from the S-type are higher than those from the I-type, even though the performance indexes of both heat exchangers are essentially the same Alfaryjat et al [17], Gunnasegaran et al [18], and Mohammed et al [19] studied the effect of geometrical parameters and channel shape on heat transfer characteristics of microchannel heat sinks (MCHS) The results showed that better uniformities in heat transfer coefficient and temperature can be obtained in heat sinks having the smallest hydraulic diameter In addition, the temperature and the heat transfer coefficient of the zigzag MCHS is the least and greatest, respectively, among various channel shapes The circular cross-section has the least of the pressure drop Besides, Hasan et al [20] studied about the influence of channel geometry on the performance of a counter flow microchannel heat exchanger with different channel configuration such as: circular channel, rectangular channel, square channel, triangular channel, and trapezoidal channel They showed that circular channel brought the highest heat transfer and dynamic efficiency From the above literature reviews, it has been shown that many researchers have investigated the heat transfer characteristics of the microchannel heat exchangers However, most researchers have studied on the rectangular, triangular, trapezoidal or circular microchannel, but there are no more experiments on multimicro tubes In addition, research on a micro tube evaporator used in a refrigeration system has not been studied in more detail Therefore, in this study, the heat transfer characteristic of micro tube evaporator with the change of air volume flow through the evaporator will be discussed In particular, the heat transfer characteristics of micro tube evaporator will be investigated to compare with a macro evaporator 83 IJISET - International Journal of Innovative Science, Engineering & Technology, Vol Issue 3, March 2018 ISSN (Online) 2348 – 7968 www.ijiset.com Experimental Setup 2.1 Experimental System The microtube evaporator was designed and fabricated based on the heat transfer area and overall dimensions of a macro evaporator of ZHONGLI, KEWELY with the model FNA-0.25/1.3 Table Dimensions of evaporator Dimension (mm) Pipe diameter (mm) Number of tubes Number of aluminum foils Heat exchanger area (m2) Micro tube evaporator Macro evaporator FNA-0.25/1.3 225x43x200 225x45x210 0.18 0.8 80 16 90 83 1.48 1.48 dimensions of the microtube evaporator are slightly smaller than those of the macro evaporator The micro tube evaporator was made from copper tube and aluminum foil The microtube evaporator is made of copper tube and aluminum foil This evaporator has 10 passes, each pass with copper pipes (with hydraulic diameter of 0.82mm) and 80 aluminum foils (with the thickness of 0.3 mm); the distance between the two foils is 2.6 mm In addition, the overall dimensions of the evaporator are the length L = 259.3mm, the width W = 43 mm, and the height H = 200mm, as shown in Fig The experimental system is a refrigeration system using R134a refrigerant to compare with the heat transfer characteristics between the micro tube heat exchanger and the macro evaporator Fig The experimental test loop (V1: Stop valve 1; V2: Stop valve 2; T1, T2, T3, T4: Temperature sensors; ME: Micro tube evaporator; NE: Macro evaporator FNA-0.25/1.3) Fig A photo of the experimental system (1 Compressor; Condenser; Macro evaporator FNA-0.25/1.3; Micro tube evaporator; Capillary tube; Gate valve) Fig Dimensions of the micro tube evaporator All dimension parameters are shown in Table The two evaporators have the same heat transfer area and the Experimental system was divided into two cycles: the first cycle experiments on the micro tube evaporator, the second cycle experiments on the macro evaporator with the model FNA-0.25/1.3 (abbreviated by FNA), as shown in 84 IJISET - International Journal of Innovative Science, Engineering & Technology, Vol Issue 3, March 2018 ISSN (Online) 2348 – 7968 www.ijiset.com Fig and Fig With Cycle 1: the valve V1 opens and the valve V2 closes The vapor R134a refrigerant enters the compressor and is compressed to high pressure The high-pressure superheated vapor passing through condenser is cooled into the liquid high pressure The liquid refrigerant continues to move through the capillary, where the pressure and temperature of the liquid decrease after passage of the capillary due to the expansion process After passing through the capillary tube, the refrigerant continues through the micro tube evaporator to cool the air In here, the temperature and humidity parameters are taken by the temperature and humidity sensors, the data is sent to the central controller and is displayed on the computer The saturated vapor from the micro tube evaporator continues to the compressor suction, so the cycle continues With cycle 2: the process is similar to cycle 1; however, the valve V1 is closed and the valve V2 is opened The refrigerant will go through the macro evaporator FNA without through the micro tube evaporator The compressor used in the system is the piston compressor with the model EE80Y-E; it has a capacity of ¼ Hp The condenser model FNA-0.8/3.4 has a capacity of 800W COP = ht Pt Power input is calculated by: P t = U I cosφ where U is Voltage, I is current, φ is power factor (7) (8) Results and Discussion For experimental conditions at constant ambient temperature 30oC, the air volume flow going through the evaporator was varying from 19 l/s to 47.5 l/s, all other parameters were not changed and used on the same experimental system 2.2 Data Analysis With the experimental conditions in this study, the parameters the heat transfer characteristics of the fluid such as heat flux, heat transfer rate of the heat exchanger will be mentioned Dang and Teng [16] as follows: The heat transfer rate Q t is given as follows: (1) Qt = mc p dt Fig Outlet air temperature versus air volume flow of evaporator Heat flux is calculated by: qt = Qt A (2) Or: (3) qt = kt dt where m is mass flow rate, c p is specific heat, dt is temperature difference, q t is heat flux, A is heat transfer area, k t is overall heat transfer coefficient Total cooling capacity can be expressed as: (4) ht = hs + hl The sensible heat in a cooling process of air can be calculated as: (5) h s = c p ρ V dt where h s is sensible heat, c p is specific heat of air, ρ is density of air, V is air volume flow Latent heat due to the moisture in air can be calculated as: (6) h l = V ρ h we dw kg where h l is latent heat, h we is latent heat of vaporization of water, dw kg is humidity ratio difference The Coefficient of Performance (COP) of the cycle was quantified by: Fig Outlet air relative humidity versus air volume flow of evaporator The outlet air temperature of micro tube evaporator decreases as the air volume flow decreases, while the outlet relative humidity increases The outlet air temperature of macro evaporator FNA also decreases as the air volume flow decreases, but higher than the outlet air 85 IJISET - International Journal of Innovative Science, Engineering & Technology, Vol Issue 3, March 2018 ISSN (Online) 2348 – 7968 www.ijiset.com temperature of micro tube evaporator, as shown in Fig and Fig Fig COP versus air volume flow of evaporator Fig Heat flux versus air volume flow of evaporator The experimental relationships between the air volume flow through evaporator and heat flux are shown in Fig The results indicated that the heat flux increased as the air volume flow through evaporator increased at the ambient temperature 30oC and all other parameters are constant For micro tube evaporator, the heat flux can reach a maximum of 163.2 W/m2 and 1.32 times higher than the macro evaporator FNA The heat transfer characteristics of micro tube evaporator and macro evaporator FNA are also compared by thermal camera Fig shows the heat distribution of the micro tube evaporator is very uniform while the heat is concentrated only in the lower part of the macro evaporator FNA, at the same experimental conditions a) Temperature profile of the microtube evaporator Fig Cooling capacity versus air volume flow of evaporator Furthermore, Fig and Fig show the comparisons of the air volume flow difference and the cooling capacity and COP of the two evaporators: the micro tube evaporator and the macro evaporator FNA The cooling capacity and the COP of the evaporator increase when increasing air volume flow The maximum cooling capacity and COP number of micro tube evaporator are 840W and 4.58, respectively The COP of micro tube evaporator is 1.4 times higher than the COP of macro evaporator FNA at the air volume flow is 47.5 l/s and the inlet air temperature is 30oC b) Temperature profile of the macro evaporator FNA Fig A photo of the evaporators was taken by a thermal camera Conclusions An experiment study of the heat transfer characteristics of evaporation process in a microtube evaporator has been studied by changing of the air volume flow of the microtube evaporator In addition, the results of the study were compared on the same conditions with the macro evaporator FNA The results show that, at the same ambient temperature of 30oC, the heat flux of the evaporator increases when 86 IJISET - International Journal of Innovative Science, Engineering & Technology, Vol Issue 3, March 2018 ISSN (Online) 2348 – 7968 www.ijiset.com increasing air volume flow through evaporator The heat flux of micro tube evaporator is higher than the macro evaporator FNA times, at the same experimental conditions Besides, the cooling capacity and the COP of the evaporator increase when increasing air volume flow The maximum cooling capacity and the COP of microtube evaporator are 840W and 4.58, respectively The COP of micro tube evaporator is 1.4 times higher than the COP of macro evaporator FNA at the air volume flow is 47.5 l/s and the inlet air evaporator temperature is 30oC Moreover, the micro tube evaporator distributes heat more uniformly than the macro evaporator FNA, at the same experimental conditions Acknowledgments The supports of this work by the project No.B2015.22.01 (sponsored by the specific research fields of the Ministry of Education and Training, Vietnam) are deeply appreciated References [1] Fayyadh EM, Mahmoud MM, Karayiannis TG, "Flow Boiling Heat Transfer of R134a in Multi Micro Channels", International Journal of Heat and Mass Transfer, Vol 110, 2017, pp 422-436 [2] Mahmoud MM, Karayiannis TG, Kenning DBR, "Surface effects in flow boiling of R134a in microtubes", International Journal of Heat and Mass Transfer, 2011, pp 3334-3346 [3] Thiangtham P, Keepaiboon C, Kiatpachai P, Asirvatham LG, Mahian O, Dalkilic AS, et al., "An experimental study on twophase flow patterns and heat transfer characteristics during boiling of R134a flowing through a multi-microchannel heat sink", International Journal of Heat and Mass Transfer, 2016, pp 390-400 [4] Keepaiboon C, Thiangtham P, Mahian O, Dalklỗ AS, Wongwises S, "Pressure drop characteristics of R134a during flow boiling in a single rectangular micro-channel", International Communications in Heat and Mass Transfer, 2016, pp 245-253 [5] Wibel W, Schygulla U, Brandner JJ, "Micro device for liquid cooling by evaporation of R134a", Chemical Engineering Journal, 2011, pp 705-712 [6] Magnini M, Thome JR, "A CFD study of the parameters influencing heat transfer in microchannel slug flow boiling", International Journal of Thermal Sciences, 2016, pp 119-136 [7] Huang H, Thome JR, "An experimental study on flow boiling pressure drop in multi-microchannel evaporators with different refrigerants", Experimental Thermal and Fluid Science, 2017, pp 391-407 [8] Markal B, Aydin O, Avci M, "An experimental investigation of saturated flow boiling heat transfer and pressure drop in square microchannels", International Journal of Refrigeration, 2016, pp 1-11 [9] Lee H, Park I, Mudawar I, Hasan MM, "Micro-channel evaporator for space applications – Experimental pressure drop and heat transfer results for different orientations in earth gravity", International Journal of Heat and Mass Transfer, 2014, pp 12131230 [10] Saisorn S, Wongwises S, "Adiabatic two-phase gas–liquid flow behaviors during upward flow in a vertical circular microchannel", Experimental Thermal and Fluid Science, 2015, 158168 [11] Triplett KA, Ghiaasiaan SM, Abdel-Khalik SI, Sadowski DL, "Gas–liquid two-phase flow in microchannels Part I: twophase flow patterns", International Journal of Multiphase Flow, 1999, pp 377-394 [12] Liu Y, Sun W, Wang S, "Experimental investigation of twophase slug flow distribution in horizontal multi-parallel microchannels", Chemical Engineering Science, 2017, pp 267-276 [13] Kim S-M, Mudawar I, "Thermal design and operational limits of two-phase micro-channel heat sinks", International Journal of Heat and Mass Transfer, 2017, pp 861-876 [14] Lee S, Mudawar I, "Transient characteristics of flow boiling in large micro-channel heat exchangers", International Journal of Heat and Mass Transfer, 2016, pp 186-202 [15] Tran N, Chang Y-J, Teng J-t, Greif R, "A study on five different channel shapes using a novel scheme for meshing and a structure of a multi-nozzle microchannel heat sink", International Journal of Heat and Mass Transfer, 2017, pp 429-442 [16] Dang T, Teng J-t, "The effects of configurations on the performance of microchannel counter-flow heat exchangers–An experimental study", Applied Thermal Engineering, 2011, pp 3946-3955 [17] Alfaryjat AA, Mohammed HA, Adam NM, Ariffin MKA, Najafabadi MI, "Influence of geometrical parameters of hexagonal, circular, and rhombus microchannel heat sinks on the thermohydraulic characteristics", International Communications in Heat and Mass Transfer, 2014, pp 121-131 [18] Gunnasegaran P, Mohammed HA, Shuaib NH, Saidur R, "The effect of geometrical parameters on heat transfer characteristics of microchannels heat sink with different shapes", International Communications in Heat and Mass Transfer, 2010, pp 1078-1086 [19] Mohammed HA, Gunnasegaran P, Shuaib NH, "Influence of channel shape on the thermal and hydraulic performance of microchannel heat sink", International Communications in Heat and Mass Transfer, 2011, pp 474-480 [20] Hasan MI, Rageb AA, Yaghoubi M, Homayoni H, "Influence of channel geometry on the performance of a counter flow microchannel heat exchanger", International Journal of Thermal Sciences, 2009, pp 1607-1618 87 ... cứu thực nghiệm thiết bị dạng nhiều ống micro Ngồi ra, chưa có nghiên cứu thiết bị bay ống micro hệ thống lạnh Vì vậy, nghiên cứu này, đặc tính truyền nhiệt thiết bị bay ống micro với thay đổi lưu... qua thiết bị nghiên cứu Đặc biệt, đặc tính truyền nhiệt thiết bị bay ống micro nghiên cứu so sánh với thiết bị bay FNA-0.25/1.3 (gọi tắt FNA) Hai thiết bị có kích thước diện tích trao đổi nhiệt. .. nghiệm thiết bị bay ống micro thiết bị bay FNA 1.5 Đối tượng phạm vi nghiên cứu Hệ thống thực nghiệm hệ thống lạnh sử dụng môi chất R134a thí nghiệm hai thiết bị bay khác là: thiết bị bay ống micro