BỘ GIÁO DỤC VÀ ĐÀO TẠO TRƯỜNG ĐẠI HỌC SƯ PHẠM KỸ THUẬT THÀNH PHỐ HỒ CHÍ MINH BÁO CÁO TỔNG KẾT ĐỀ TÀI KH&CN CẤP TRƯỜNG NGHIÊN CỨU ẢNH HƯỞNG LỰC TRỌNG TRƯỜNG ĐẾN CÁC ĐẶC TÍNH TRUYỀN NHIỆT VÀ LƯU CHẤT TRONG BỘ TRAO ĐỔI NHIỆT MICROCHANNEL S K C 0 9 MÃ SỐ:T2011-07TĐ/KHCN-GV S KC 0 3 Tp Hồ Chí Minh, 2011 BỘ GIÁO DỤC VÀ ĐÀO TẠO TRƯỜNG ĐẠI HỌC SƯ PHẠM KỸ THUẬT THÀNH PHỐ HỒ CHÍ MINH BÁO CÁO TỔNG KẾT ĐỀ TÀI KH&CN CẤP TRƯỜNG TRỌNG ĐIỂM NGHIÊN CỨU ẢNH HƯỞNG LỰC TRỌNG TRƯỜNG ĐẾN CÁC ĐẶC TÍNH TRUYỀN NHIỆT VÀ LƯU CHẤT TRONG BỘ TRAO ĐỔI NHIỆT MICROCHANNEL Mã số: T2011-07TĐ/KHCN-GV Chủ nhiệm đề tài: TS Đặng Thành Trung TP HCM, 12/2011 TRƯỜNG ĐẠI HỌC SƯ PHẠM KỸ THUẬT THÀNH PHỐ HỒ CHÍ MINH KHOA CƠ KHÍ ĐỘNG LỰC BÁO CÁO TỔNG KẾT ĐỀ TÀI KH&CN CẤP TRƯỜNG TRỌNG ĐIỂM NGHIÊN CỨU ẢNH HƯỞNG LỰC TRỌNG TRƯỜNG ĐẾN CÁC ĐẶC TÍNH TRUYỀN NHIỆT VÀ LƯU CHẤT TRONG BỘ TRAO ĐỔI NHIỆT MICROCHANNEL Mã số: T2011-07TĐ/KHCN-GV Chủ nhiệm đề tài: TS Đặng Thành Trung Thành viên đề tài: KS Đoàn Minh Hùng TP HCM, 12/2011 Ảnh hưởng lực trọng trường đến đặc tính truyền nhiệt lưu chất trao đổi nhiệt microchannel MỤC LỤC Thông tin kết nghiên cứu Information on research results Danh mục ký hiệu chữ viết tắt Phần Giới thiệu Phần Phương pháp thực nghiệm 13 Phần Các kết thảo luận 22 Phần Kết luận kiến nghị 30 Lời cảm ơn 31 Tài liệu tham khảo 36 Phụ lục 35 Ảnh hưởng lực trọng trường đến đặc tính truyền nhiệt lưu chất trao đổi nhiệt microchannel THÔNG TIN KẾT QUẢ NGHIÊN CỨU Thông tin chung: - Tên đề tài: Nghiên cứu ảnh hưởng lực trọng trường đến đặc tính truyền nhiệt lưu chất trao đổi nhiệt microchannel - Mã số: T2011-07TĐ/KHCN-GV - Chủ nhiệm: TS Đặng Thành Trung - Cơ quan chủ trì: Đại học Sư phạm Kỹ thuật Tp HCM - Thời gian thực hiện: 01/01/2011 đến 30/12/2011 Mục tiêu: - Đặt tảng cho hướng nghiên cứu lĩnh vực truyền nhiệt Micro/nano Bộ môn công nghệ Nhiệt-Điện lạnh, trường Đại học Sư phạm Kỹ thuật nói riêng trường đại học khác nước nói chung - Cố gắng bắt kịp nước tiên tiến hướng nghiên cứu tương lai lĩnh vực khí nhiệt lưu chất Tính sáng tạo: Nghiên cứu nghiên cứu nước nghiên cứu giới Kết nghiên cứu: Đạt yêu cầu đặt Sản phẩm: Hai BTĐN microchannel số cảm biến nhiệt độ 01 báo khoa học đăng tạp chí SCI, 01 báo đăng tạp chí EI 01 báo đăng hội nghị quốc tế Hiệu quả, phƣơng thức chuyển giao kết nghiên cứu khả áp dụng: Tính đến ngày 24/11/2011, kết nghiên cứu trích lục lần Scopus Trƣởng Đơn vị Chủ nhiệm đề tài Ảnh hưởng lực trọng trường đến đặc tính truyền nhiệt lưu chất trao đổi nhiệt microchannel (ký, họ tên) (ký, họ tên) INFORMATION ON RESEARCH RESULTS General information: Project title: Influence of gravity on the heat transfer and fluid flow behaviors of the microchannel heat exchangers Code number: T2011-07TĐ/KHCN-GV Coordinator: Thanhtrung Dang, Ph.D Implementing institution: Hochiminh city University of Technical Education Duration: from January 01, 2011 to December 30, 2011 Objective(s): Build the research on Micro/Nano heat transfer areas at the Department of Heat and Refrigeration Technology, Hochiminh city University of Technical Education in specially and other universities of Vietnam in generally Try to follow several developed countries about one of present and future researches regarding themo-fluidics Creativeness and innovativeness: The study is the first research in Vietnam and is also one of the new researches on the world Research results: The proposed objectives have been achied Products: Two microchannel heat exchangers A SCI journal paper, a EI journal paper, and a EI international conference paper Ảnh hưởng lực trọng trường đến đặc tính truyền nhiệt lưu chất trao đổi nhiệt microchannel Effects, transfer alternatives of reserach results and applicability: Up to November 24, 2011, the results have been cited more than six times by Scopus DANH MỤC CÁC KÝ HIỆU VÀ CHỮ VIẾT TẮT Ac diện tích mặt cắt, m2 BTĐN trao đổi nhiệt Dh đường kính quy ước, m f hệ số ma sát Fanning h hệ số tỏa nhiệt đối lưu, W/m2K k hệ số truyền nhiệt tổng, W/m2K L chiều dài kênh micro, m m lưu lượng khối lượng, kg/s NTU số truyền nhiệt đơn vị (Number of Transfer Unit) Nu số Nusselt p áp suất, Pa P đường kính ướt, m Q lượng nhiệt truyền qua thiết bị, W q mật độ dòng nhiệt, W/m2 R nhiệt trở, m2K/W Re số Reynolds T nhiệt độ, K Greek symbols  độ nhớt động lực học, Ns/m2  khối lượng riêng, kg/m3 Ảnh hưởng lực trọng trường đến đặc tính truyền nhiệt lưu chất trao đổi nhiệt microchannel  hệ số dẫn nhiệt, W/m K  vận tốc, m/s  hiệu suất  số hoàn thiện, W/kPa T nhiệt độ chênh lệch, K p tổn thất áp suất, Pa Ảnh hưởng lực trọng trường đến đặc tính truyền nhiệt lưu chất trao đổi nhiệt microchannel PHẦN GIỚI THIỆU Trong năm gần đây, công nghệ Mico/Nano ứng dụng nhiều lĩnh vực khoa học kỹ thuật Trong đó, thiết bị truyền nhiệt microchannel ứng dụng công nghệ này, ứng dụng lĩnh vực giải nhiệt linh kiện điện tử, kỹ thuật hóa học, nhà máy điện nguyên tử micro, Bộ trao đổi nhiệt (BTĐN) microchannel có ý nghĩa quan trọng phạm vi sử dụng cần không gian hẹp, mật độ dòng nhiệt cao hay thiết bị truyền nhiệt nhỏ gọn Có nhiều nghiên cứu BTĐN micro cho dòng chảy pha, dòng hai pha, số BTĐN micro dùng hệ thống điều hòa không khí với môi chất lạnh C02, tối ưu hóa cho BTĐN micro, ứng dụng BTĐN micro, Trong nghiên cứu đó, nghiên cứu liên quan đến BTĐN micro cho dòng chảy pha đề cập nhiều Một nghiên cứu tổng quan BTĐN microchannel liên quan đến vật lý dòng chảy, phương pháp gia công ứng dụng thực Bowman and Maynes [1] Trước tiên, nghiên cứu giới thiệu kết thực nghiệm mô số học dòng chảy lưu chất kênh micro Xa nữa, số phương pháp gia công cho thiết bị micro gia công micro, khắc hóa chất, gia công laxe, gia công máy xác, đề cập Tổng quan đặc tính truyền nhiệt dòng chảy lưu chất BTĐN micro thực Dang cộng [2] Tổng quan kết thực nghiệm liên quan đến truyền nhiệt đối lưu dòng chảy pha kênh micro thực Morini [3], với kết thu cho hệ số ma sát, trạng thái độ từ chảy tầng đến chảy rối hệ số Nusselt kênh có đường kính quy ước nhỏ mm Dang [4] nghiên cứu đặc tính truyền nhiệt dòng chảy lưu chất cho BTĐN micro có kênh hình chữ nhật cho mô số học lẫn Ảnh hưởng lực trọng trường đến đặc tính truyền nhiệt lưu chất trao đổi nhiệt microchannel thực nghiệm Brandner cộng [5] mô tả BTĐN micro ứng dụng phòng thí nghiệm công nghiệp Trong nghiên cứu họ, số trao đổi nhiệt micro giới thiệu BTĐN micro dùng vật liệu polymer, nhôm, gốm ceramic, Một phân tích hiệu suất tổn thất áp suất BTĐN micro có dòng chảy cắt thực Kang Tseng [6] Henning cộng [7] chế tạo BTĐN micro với dạng kênh chuẩn thẳng, dạng kênh ngắn thẳng dạng kênh hình sóng Kết thực nghiệm thể rẳng kênh chuẩn thẳng lựa chọn tốt cho dòng chảy có lưu lượng cao Bộ trao đổi nhiệt micro làm từ thép không gỉ W316L nghiên cứu Brandner [8] Trong nghiên cứu này, truyền nhiệt BTĐN micro nâng lên sử dụng dãy cột micro so le để đạt trạng thái chảy độ hay chảy rối Các BTĐN loại ngược chiều cắt dùng vật liệu ceramic gia công Alm cộng [9] Những thiết bị dùng lĩnh vực kỹ thuật nhiệt hóa học Hệ số truyền nhiệt BTĐN loại cắt đạt 22 kW/(m2K) Hallmark cộng [10] nghiên cứu thực nghiệm BTĐN dạng màng ống mao micro làm từ plastic Năng lượng lấy BTĐN hàm lượng điện cấp vào tăng lưu lượng dòng chảy Jiang cộng [11] nghiên cứu dòng chảy lưu chất truyền nhiệt đối lưu cưỡng BTĐN microchannel Trạng thái độ từ chảy tầng sang chảy rối tìm thấy khoảng hệ số Reynolds ≈ 600 Một phương pháp gia công cho bề mặt truyền nhiệt BTĐN micro thực Schulz cộng [12] Bởi phương pháp khắc axít kết hợp với mạ điện kim loại, dãy Đồng tạo nên bề mặt truyền nhiệt ống, nhiệt độ, mật độ dòng nhiệt hệ số truyền nhiệt thu từ ống micro cao giá trị thu từ ống trơn Lee cộng [13] nghiên Proceedings of the World Congress on Engineering 2011 Vol III WCE 2011, July - 8, 2011, London, U.K Influence of Gravity on the Performance Index of Microchannel Heat Exchangers-Experimental Investigations Thanhtrung Dang, Jyh-tong Teng, and Jiann-cherng Chu  Abstract—Influence of gravity on the heat transfer and fluid flow phenomenon of microchannel heat exchangers was presented experimentally The effect was determined by two cases: one with horizontal channels, the other with vertical channels For vertical channels, the hot water is flowing upward which is against the gravitational field, while the cold water is flowing downward which is in the same direction as the gravitational field In this study, the difference between the results obtained from horizontal channels and those from vertical ones is negligibly small; the impact of gravity on the fluid flowing through the microchannel heat exchangers was found to be small, with the maximum difference between the two cases being less than 8% Good agreements were achieved between the results obtained in the present study and the results obtained in literatures Index Terms—micro heat exchanger, gravity, heat transfer rate, pressure drop, performance index I INTRODUCTION The need for the development of effective cooling devices has raised much interest in microchannel heat transfer in recent years A review on micro heat exchanger related issues such as flow physics, fabrication methods, and applications was done by Bowman and Maynes [1] This review firstly introduced the experimental and numerical investigations of microchannel flow Friction and heat transfer measurements of gas flow and liquid flow were discussed in the paper The paper indicated that the transition Reynolds number is a function of surface roughness and channel geometry Moreover, in the paper, the heat exchanger designs – including their comparison and optimization – were also reviewed Furthermore, several fabrication methods including micromachining, chemical etching, laser machining, electroplating, and lamination, were discussed Manuscript received March 18, 2011 Thanhtrung Dang is with Department of Heat and Refrigeration Technology, Ho Chi Minh City University of Technical Education, Ho Chi Minh City, Vietnam (e-mail: trungdang@hcmute.edu.vn) Jyh-tong Teng, the corresponding author, is with Department of Mechanical Engineering, Chung Yuan Christian University, Taiwan (e-mail: jyhtong@cycu.edu.tw) Jiann-cherng Chu is with the Department of Mechanical Engineering, Chung Yuan Christian University, Taiwan (e-mail: mr.jcchu@gmail.com) ISBN: 978-988-19251-5-2 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online) Brandner et al [2] described microstructure heat exchangers and their applications in laboratory and industry Several micro heat exchangers were introduced: polymer microchannel heat exchanger with aluminum separation foil, electrically powered lab-scale microchannel evaporator, ceramic counter-flow microstructure heat exchanger, etc Ameel et al [3] presented an overview of the miniaturization technologies and their applications to energy systems Based on the MEMS technologies (silicon-based micromachining, deep X-ray lithography, and the micro mechanical machining), processes were discussed in the context of applications to fluid flow, heat transfer, and energy systems Review on experimental results concerning single-phase convective heat transfer in microchannels was presented by Morini [4], with additional review results obtained for the friction factor, the laminar-to-turbulent transition, and the Nusselt number in channels having a hydraulic diameter less than mm Mathew and Hegab [5] studied on the application of effectiveness-NTU relationship to parallel-flow microchannel heat exchangers Besides, development of nondimensional parameters (such as axial distance, temperature, and heat transfer rate) was carrier out However, the results were analyzed theoretically only Studies of effectiveness and pressure drop for micro cross-flow heat exchanger were presented by Kang and Tseng [6] At the same effectiveness, heat transfer rate and pressure drop were expressed as a function of average temperature However, in their study, they did not study for the cases with varying mass flow rates at each side Chein and Chen [7] presented a numerical study of the effect of inlet/outlet arrangement on the performance of microchannel heat sink Six types of heat sink were studied with the best performance being the V-type Because that if the microchanels have the same cross-section area and width of microchannel, the depth of microchannel obtained from V-shaped microchannel is deeper than that obtained from rectangular-shaped one So it is not easy to design a heat exchanger with the subtrate thickness from 1.2 to mm using V-type microchannels Foli et al [8] studied numerically on the heat flux, heat transfer rate, and pressure drop in channels with numerous aspect ratios However, the results in Ref [8] were presented without experiments A study on the simulations of a trapezoidal shaped micro heat exchanger was presented by Dang et al [9] Using the geometric dimensions and the flow condition associated with the micro heat exchanger, a heat flux of 13.6 W/cm2 was evaluated by numerical method Besides, for the WCE 2011 Proceedings of the World Congress on Engineering 2011 Vol III WCE 2011, July - 8, 2011, London, U.K microchannel heat exchanger, behaviors of the temperature and velocity profiles were determined Effect of flow arrangement on the heat transfer related behaviors of a microchannel heat exchanger was presented by Dang et al [10, 11] For all cases done in the study, the heat flux and performance index obtained from the counter-flow arrangement are always higher than those obtained from the parallel-flow one: the values obtained from the counter-flow are 1.1 to 1.2 times of those obtained from the parallel-flow Dang and Teng [12] studied effect of the substrate thickness of counter-flow microchannel heat exchangers on the heat transfer behaviors It was found that the actual heat transfer rate varies insignificantly with the substrate thicknesses varying from 1.2 to mm However, the results obtained in [12] only mentioned the heat transfer behaviors of the heat exchangers, while the fluid flow behaviors of the heat exchangers were not discussed Dang et al [13] presented an experimental study of the effects of gravity on heat transfer and pressure drop behaviors of a microchannel heat exchanger However, the results in [13] were presented only for a microchannel heat exchanger evaluated under the condition of rising the inlet temperature for the hot side To summarize, it is goal of this paper to study experimentally for the effects of gravity on the heat transfer and fluid flow behaviors of microchannel heat exchangers In the following section, two microchannel heat exchangers will be discussed under the condition of rising mass flow rate for the cold side II METHODOLOGY A Experimental set-up Three major parts are used in the experimental system: the test section (the microchannel heat exchanger), syringe system, and overall testing loop, as shown in Fig In this study, two microchannel heat exchangers were tested The heat transfer process of these devices is carried out between two liquids which are hot water and cold water; the hot and cold fluids are flowing in the opposite directions Fig shows the dimensions of the test sections The substrate material used for the heat exchangers is aluminum, with the thermal conductivity of 237 W/(mK), density of 2,700 kg/m3, and specific heat at constant pressure of 904 J/(kgK) For each microchannel heat exchanger, the top side for the hot water has 10 microchannels and the bottom side for the cold water also has 10 microchannels The length of each microchannel is 32 mm Microchannels have rectangular cross-section with the width and the depth being Wc and Dc, respectively In a microchannel heat exchanger, all channels are connected by manifolds for the inlet and outlet of hot water and for those of cold water, respectively The manifolds of the heat exchangers are of the same cross-sections: having a rectangular shape with a width of mm and a depth of 300 m ISBN: 978-988-19251-5-2 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online) P Exhaust air valve T Pump Buffer tank Heat exchanger T Pre-heater Heater P Balance Pump Water tank Balance Buffer tank Fig Schematic of the test loop for microchannel heat exchangers Table Geometrical parameters of microchannel heat exchangers Dimensions of the substrate (mm) L W T Dimensions of the channel (m) Wc Dc 46 46 26.5 26.5 1.2 1.2 500 500 No T1 T2 300 180 Fig shows the dimensions of the test section In this study, two microchannel heat exchangers were designed and manufactured, with their dimensions listed in Table Fig shows a photo of the microchannel heat exchanger These test sections were manufactured by precision micromachining [3] Each inlet hole or outlet hole of the heat exchangers has a cross-sectional area of mm2 The four sides of the heat exchanger were thermally insulated by the glass wool with a thickness of mm To seal the microchannels, two layers of PMMA (polymethyl methacrylate) are bonded on the top and bottom sides of the substrate by UV (ultraviolet) light process, as indicated in Fig The physical properties of the PMMA and the glass wool are listed in Table [14] WCE 2011 Proceedings of the World Congress on Engineering 2011 Vol III WCE 2011, July - 8, 2011, London, U.K Thermocouples, T-type Pump, Model PU-2087, made by Jasco Pump, VSP-1200, made by Tokyo Rikakikai Heater, Model AXW-8, made by Medilab Differential pressure transducer, Model PMP4110, made by Duck Micro electronic balance, Model TE-214S, made by Sartorious Table Uncertainty data for measured parameters Parameter Temperature Pressure Mass flow rate Channel height Channel width Channel length Fig Dimensions of the test section PMMA Test sample Fig A photo of the microchannel heat exchanger Table The physical properties of the PMMA and the glass wool Material PMMA Glass wool Density kg/m3 1420 154 Thermal conductivity W/(mK) 0.19 0.051 Experimental data for the microchannel heat exchanger were obtained under the constant room temperature of 25 ºC For this study, DI water (deionized water) was used as the working fluid Each inlet or outlet of the heat exchanger has a set of two thermocouples to record temperature values So, there are eight thermocouples in total At each side, a differential pressure transducer was used to measure the pressure drop To assess the accuracy of measurements presented in this work, the uncertainty values for measured parameters are listed in Table In addition, the uncertainties on the dimensions of microchannel evaluated by using a scanning laser made by Mitaka/Ryokosha model NH-3 The uncertainties of the scanning laser were estimated to be ± 0.03 µm Equipments used for the experiments are listed as follows [13]: ISBN: 978-988-19251-5-2 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online) Uncertainties  0.1 C  0.025% FS  0.0015 g  m  10 m  70 m In order to study the effects of gravity on heat transfer and fluid flow behaviors of the heat exchangers, all experimental conditions for the two microchannel heat exchangers were kept the same Throughout the paper, the experimental conditions of testing were discussed: the case is studied under condition of increasing the mass flow rate of the cold side Further details of the case are as follows: The inlet temperature and the mass flow rate of the hot side were fixed at 70 ºC and 0.2308 g/s, respectively; at the cold side, the inlet temperature was fixed at 22.5 ºC and the mass flow rates were varying from 0.2135 to 0.401 g/s B Data analysis In the following analyses, the major assumptions were made: - The fluid is a laminar flow - The fluid flow is incompressible and continuum - Heat transfer is steady - Negligible radiation heat transfer For the experiments carried out in this study, the effects on the heat transfer and fluid flow – such as heat flux, effectiveness, pressure drop, and performance index – of the heat exchangers will be discussed as follows The maximum heat transfer rate, Qmax is evaluated by (1) Qmax = (mc)min(Th,i – Tc,i) The heat transfer rate of the heat exchanger, Q is calculated by (2) Qc = mccc (Tc,o – Tc,i) The effectiveness (NTU method) is determined by  Qc Qmax (3) Heat flux is calculated by q  Or q = k Tlm = Tlm R Qc m c c c (Tc,o - Tc,i )  A nL cWc (4) (5) WCE 2011 Proceedings of the World Congress on Engineering 2011 Vol III WCE 2011, July - 8, 2011, London, U.K The overall thermal resistance R is determined by R = Rcond + Rconv (6) The log mean temperature difference is calculated by T  Tmin (7) Tlm  max Tmax ln Tmin where m is mass flow rate (subscripts h and c stand for the hot side and cold side, respectively), n is number of microchannels, c is specific heat, Th,i, Th,o, Tc,i and Tc,o are inlet and outlet temperatures of the hot and cold sides, respectively, q is heat flux, A is heat transfer area, k is overall heat transfer coefficient, Rcond  resistance, Rconv   is conductive thermal  1 is convective thermal resistance,  hh hc hh and hc are the convective heat transfer coefficients of the hot side and the cold sides, respectively,  is thickness of heat transfer,  is thermal conductivity, and Tlm is the log mean temperature difference The Reynolds number is calculated by: Re  wDh 2m    Wc  Dc  (8) Ac is the hydraulic diameter, w is velocity in P the z-direction,  is dynamic viscosity,  is density, Ac is where Dh  By using the estimated errors of parameters listed in Table 3, the maximum experimental uncertainties in determining Qc, Re, and  were 2,1%, 3.1%, and 3.3%, respectively, for all cases being studied III RESULTS AND DISCUSSION For the experimental system, the inlet temperature and the mass flow rate of the hot side were fixed at 70 ºC and 0.2308 g/s, respectively; at the cold side, the inlet temperature was fixed at 22.5 ºC and the mass flow rates were varying from 0.2135 to 0.401 g/s In this study, influence of gravity was determined by two cases: one with horizontal channels, the other with vertical channels For vertical channels, the hot water is flowing upward which is against the gravitational field, while the cold water is flowing downward which is in the same direction as the gravitational field Two microchannel heat exchangers T1 and T2 were tested: these two microchannel heat exchangers have the same physical configurations for their substrates, manifolds, and lengths of channels; only the cross-sectional areas of microchannels are different The microchannels of T1 have a rectangular cross-section with width of 500 m and depth of 300 m; the microchannel of T3, width of 500 m and depth of 180 m Parameters of the heat exchangers (T1 and T2) are listed in more detail in Table 46 T 1-horizontal Outlet temperature of hot side, C cross-sectional area, and P is wetted perimeter The total pressure drop of the heat exchanger is given by (9) pt  p h  p c where ph and pc are pressure drops of hot and cold sides, respectively The performance index,  , is determined by m c c c (Tc,o - Tc,i ) Qc (10)  pt p h  p c The experimental uncertainties were estimated, following the method described by Holman [15]; the final expressions for uncertainties were given as follows:   m   c Qc  mc  U Qc   cc   Tc ,o  Tc ,i          cc   Tc ,o  Tc ,i 2 2 U Re  m         Wc           Re  m         Wc   m   c   T  T c ,i  c    c    c ,o U   mc   cc   Tc ,o  Tc ,i  2    p h   p c               p h   p c          1/ 2 T 2-vertical 42 40 38 36 0.1500 0.2000 0.2500 0.3000 0.3500 0.4000 0.4500 Mass flow rate of cold side, g/s Fig Comparison of the outlet temperatures of hot side 1/ ISBN: 978-988-19251-5-2 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online) T 2-horizontal (11)   Dc        Dc  (12)            T 1-vertical 44 (13)    1/ Fig shows a comparison of the outlet temperature of hot side of two microchannel heat exchangers under the effect of gravity It is observed that the outlet temperatures of hot side obtained from horizontal channels and those from the vertical ones are negligibly small A comparison of the outlet temperatures of cold side of two microchannel heat exchangers is shown in Fig The outlet temperatures (for both the hot and the cold sides) are functions of the mass flow rate of cold side; the outlet temperatures decrease as the mass flow rate of the cold side increases WCE 2011 50 6000 48 5000 T 1-horizontal Outlet temperature of cold side, C Proceedings of the World Congress on Engineering 2011 Vol III WCE 2011, July - 8, 2011, London, U.K Total pressure drop, Pa T 1-vertical 46 44 42 T 1-horizontal T 1-vertical 40 T 2-horizontal T 2-horizontal T 2-vertical 4000 3000 2000 1000 T 2-verticcal 38 0.1500 0.2000 0.2500 0.3000 0.3500 0.4000 0.4500 100 140 220 260 300 Re number of cold side Mass flow rate of cold side, g/s Fig Comparison of the outlet temperatures of cold side Fig Comparison of the total pressure drops 22 Performance index, W/kPa 30 Heat transfer rate, W 180 28 26 T1-horizontal 24 T1-vertical T2-horizontal 22 T2-vertical 20 18 T 1-horizontal T 1-vertical 14 T 2-horizontal T 2-vertical 10 0.1500 0.2000 0.2500 0.3000 0.3500 0.4000 0.4500 Mass flow rate of cold side, g/s Fig Comparison of the heat transfer rates The outlet temperatures of hot side obtained from T1 is higher than those obtained from T2; however, the outlet temperatures of cold side obtained from T1 is lower than those obtained from T2 As a result, the heat transfer rate obtained from T2 is higher than that obtained from T1, as shown in Fig The results obtained from the present study are in good agreement with those obtained from [8] Foli et al [8] indicated that under the constant mass flow rate condition, the higher the heat flux, the lower the aspect ratio (defined as the ratio of the microchannel height to its width) It is shown from Fig that the heat transfer rates obtained from horizontal channels and those from the vertical ones are negligibly small The heat transfer rate of the heat exchangers is a function of the mass flow rate of cold side: it increases from 24.8 to 29.92 W with the mass flow rate of cold side rising from 0.2043 to 0.401 g/s (for the heat exchanger T2) ISBN: 978-988-19251-5-2 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online) 100 140 180 220 260 300 Re number of cold side Fig Comparison of the performance indices Because that the hydraulic diameter of channel in T2 is smaller than that of channel in T1, this results in the velocity in the channel of T2 to be higher than that of T1, leading to a higher total pressure drop in T2 than that in T1, as shown in Fig Besides, the Figure shows that the total pressure drop is a function of Reynolds number of cold side; the total pressure drop increases as rising the Re number of cold side Experimental results for effects of gravity on the behavior of pressure drop for the microchannel heat exchanger are also shown in Fig It is observed that the change of pressure drop between the two cases (horizontal channels and vertical channels) is negligibly small; the maximum change in pressure is 7.2% for a pressure drop from 1060 to 2044 Pa It was found that the pressure drop of T2 is times higher than that of T1, while the heat transfer rate of T2 is 1.06 times higher than that of T1 As a result, the performance index (defined as the ratio of the heat transfer rate to the pressure drop in the heat exchanger) obtained from T1 is higher than that obtained from T2, as shown in Fig For WCE 2011 Proceedings of the World Congress on Engineering 2011 Vol III WCE 2011, July - 8, 2011, London, U.K heat exchanger T1, a performance index of 21.68 W/kPa was achieved for water from the hot side having an inlet temperature of 70 C and a mass flow rate of 0.2308 g/s and for water from the cold side having an inlet temperature of 22.5 C and mass flow rate of 0.2135 g/s It is also observed that the change of performance between the two cases (horizontal channels and vertical channels) is negligibly small; the maximum change in performance is 5.5%, out of a performance index from 13.69 to 21.68 W/kPa In summary, it is concluded that the impact of gravity on the fluid flowing through the microchannel heat exchanger can be ignored as indicated in [8, 9, 13, 16] IV CONCLUSION An experimental work was done on two microchannel heat exchangers to carry out the evaluation of their performance for the varying the mass flow rates of the cold side These two microchannel heat exchangers have the same physical configurations for their substrates, manifolds, and lengths of channels; only the cross-sectional areas of microchannels are different For heat exchanger T1, a performance index of 21.68 W/kPa was achieved for water from the hot side having an inlet temperature of 70 C and a mass flow rate of 0.2308 g/s and for water from the cold side having an inlet temperature of 22.5 C and mass flow rate of 0.2135 g/s The impact of gravity on the fluid flowing through the microchannel heat exchanger was found to be small, with the maximum difference between the results of horizontal and vertical channels being less than 8% In addition, in this study, good agreements were achieved between the results obtained from the present study and the results obtained from the literatures external heat transfer, International Journal of Thermal Sciences, Volume 49, Issue 1, 2010, pp 76-85 [6] S.W Kang and S.C Tseng, Analysis of effectiveness and pressure drop in micro cross-flow heat exchanger, Applied Thermal Engineering, Volume 27, Isuue 5-6, 2007, pp 877-885 [7] R Chein and J Chen, Numerical study of the inlet/outlet arrangement effect on microchannel heat sink performance, International Journal of Thermal Sciences, Volume 48, Issue 8, 2009, pp 1627-1638 [8] K Foli, T Okabe, M Olhofer, Y Jin, and B Sendhoff, Optimization of micro heat exchanger: CFD, analytical approach and multi-objective evolutionary algorithms, International Journal of Heat and Mass Transfer, Volume 49, Issue 5-6, 2006, pp 1090-1099 [9] T.T Dang, Y.J Chang, and J.T Teng, A study on the simulations of a trapezoidal shaped micro heat exchanger, Journal of Advanced Engineering, Volume 4, Issue 4, 2009, pp 397-402 [10] T.T Dang, J.T Teng, and J.C Chu, Effect of flow arrangement on the heat transfer behaviors of a microchannel heat exchanger, Proceedings of the International MultiConference of Engineers and Computer Scientists 2010, Hongkong, 2010, pp 2209-2214 [11] T.T Dang and J.T Teng, Influence of flow arrangement on the performance index for an aluminium microchannel heat exchanger, IAENG Transactions on Engineering Technologies Volume 5, the American Institute of Physics (AIP), Vol 1285, 2010, pp 576-590 [12] T.T Dang and J.T Teng, Effect of the substrate thickness of counter-flow microchannel heat exchanger on the heat transfer behaviors, Proceedings of the international symposium on computer, communication, control and automation 2010, Taiwan, 2010, pp 17-20 [13] T.T Dang, J.T Teng, and J.C Chu, A study on the simulation and experiment of a microchannel counter-flow heat exchanger Applied Thermal Engineering, Volume 30, Issue 14-15, 2010, pp 2163-2172 [14] COMSOL Multiphysics version 3.5 – Documentation, Sept 2008 [15] J.P Holman, Experimental methods for engineers, McGraw-Hill, New York, 1984 [16] S.G Kandlikar, S Garimella, D.Q Li, S Colin, and M.R King, Heat transfer and fluid flow in minichannels and microchannels Elsevier, 2006 ACKNOWLEDGMENT The supports of this work by (1) the project (Project No NSC 99-2221-E-033-025) sponsored by National Science Council of the Republic of China in Taiwan and (2) the project (under Grant No CYCU-98-CR-ME) sponsored by the specific research fields at Chung Yuan Christian University, Taiwan, are deeply appreciated REFERENCES [1] [2] [3] [4] [5] W.J Bowman and D Maynes, A review of micro-heat exchanger flow physics, fabrication methods and application, Proceedings of ASME IMECE 2001, New York, USA, Nov 11-16, 2001, HTD-24280, pp 385-407 J.J Brandner, L Bohn, T Henning, U Schygulla, and K Schubert, Microstructure heat exchanger applications in laboratory and industry, Proceedings of ICNMM2006, ICNMM2006-96017, Limerick, Ireland, 2006, pp 1233-1243 T.A Ameel, R.O Warrington, R.S Wegeng, and M.K Drost, Miniaturization technologies applied to energy systems, Energy Conversion and Management, Volume 38, 1997, pp 969–982 G.L Morini, Single-phase convective heat transfer in microchannels: a review of experimental results, International Journal of Thermal Sciences, Volume 43, Issue 7, 2004, pp 631-651 B Mathew and H Hegab, Application of effectiveness-NTU relationship to parallel flow microchannel heat exchangers subjected to ISBN: 978-988-19251-5-2 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online) WCE 2011