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 ) 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 1 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 2 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/cm 2 was evaluated by numerical method. Besides, for the Influence of Gravity on the Performance Index of Microchannel Heat Exchangers-Experimental Investigations Thanhtrung Dang, Jyh-tong Teng, and Jiann-cherng Chu Proceedings of the World Congress on Engineering 2011 Vol III WCE 2011, July 6 - 8, 2011, London, U.K. ISBN: 978-988-19251-5-2 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online) WCE 2011