Thermal–hydraulic performance of small scale micro-channel and porous-media heat-exchangers

doi:10.1016/S0017-9310(00)00169-1

International Journal of Heat and Mass Transfer

Volume 44, Issue 5, March 2001, Pages 1039-1051

https://doi.org/10.1016/S0017-9310(00)00169-1 Get rights and content

Abstract

Fluid flow and forced convection heat transfer in micro-heat-exchangers with either micro-channels or porous media have been investigated experimentally. The influence of the dimensions of the micro-channels on the heat transfer performance was first analyzed numerically. Based on these computations, deep micro-channels were used for the experimental studies reported here. The measured performance of both micro-channel and porous-media micro-heat-exchangers are compared with those of similar heat-exchangers tested by other researchers. It is shown that the heat transfer performance of the micro-heat-exchanger using porous media is better than that of the micro-heat-exchanger using micro-channels, but the pressure drop of the former is much larger. Over the range of test conditions, the maximum volumetric heat transfer coefficient of the micro-heat-exchanger using porous media was 86.3 MW/(m³ K) for a water mass flow rate of 0.067 kg/s and a pressure drop of 4.66 bar. The maximum volumetric heat transfer coefficient of the micro-heat-exchanger using deep micro-channels was 38.4 MW/(m³ K) with a corresponding mass flow rate of 0.34 kg/s and a pressure drop of 0.7 bar. Considering both the heat transfer and pressure drop characteristics of these heat-exchangers, the deep micro-channel design offers a better overall performance than either the porous media or shallow micro-channel alternatives.

Introduction

In recent years, with the rapid progress in Micro-Electro-Mechanical Systems (MEMS), many micro-machining methods have been developed to build micro-devices such as micro-motors, micro-sensors, micro-mechanical gyroscopes, micro-pumps, micro-valves, micro-rockets, micro-gas-turbines, micro-heat-exchangers, etc. Micro-heat-exchangers and micro-channel heat sinks can be applied in many important fields: micro-electronics, aviation and aerospace, medical treatment, biological engineering, materials sciences, cooling of high temperature superconductors, thermal control of film deposition, cooling of powerful laser mirrors and other applications where lightweight, small heat-exchangers are required.

Compared with conventional heat-exchangers, the main advantage of the micro-heat-exchangers is their extremely high heat transfer area per unit volume. As a result, the overall heat transfer coefficient per unit volume can be as greater than 100 MW/(m³ K), which is much higher than conventional heat-exchangers (1–2 orders of magnitude). Micro-channel heat-exchangers which are a common type of micro-heat-exchanger generally consist of many channels fabricated from very thin foils of silicon or metallic materials. Such foils can be used to form micro-channel heat sinks or can be welded together to form cross-flow, parallel-flow or counter-flow micro-heat-exchangers. The concept of “micro-channel heat sinks” was first introduced in 1981 by Tuckerman and Pease [1] who demonstrated experimentally that a heat flux of 1300 W/cm² can be continuously dissipated while maintaining a temperature difference of less than 70°C. Since then, there have been many other related experimental works (e.g., [2], [3], [4]). The micro-heat-exchanger for exchanging heat between hot and cold fluids was first developed in 1985 by Swift et al. [5]. To facilitate fabrication and inlet and outlet flow conditions, most micro-channel heat-exchangers adopt a cross-flow arrangement. The dimensions and the heat transfer performance of different micro-heat-exchangers investigated previously are listed in Table 1.

Most micro-channel heat sinks use deep channels [2], [3] (the micro-channel depth to width ratio is more than 1), while micro-channel heat-exchangers generally have shallow channels [5], [6], [7], [8], [9], [10] (the depth to width ratio is less than 1). The reasons probably lie in the limits of fabrication, hardness and compactness of the heat transfer surfaces. Although decreasing the equivalent hydraulic diameter of the micro-channels can increase the heat transfer coefficient, the pressure drop is also greatly increased. Furthermore, the rate of increase of the pressure drop is far greater than that of the heat transfer coefficient. Both the heat transfer and the flow resistance should be considered in the design of micro-heat-exchangers. The equivalent hydraulic diameters of deeper channels for fixed channel width are relatively larger, so the flow resistance is relatively smaller while the heat transfer performance of deep channels is theoretically better than that of shallow channels according to Tuckerman [2], Keyes [11], Samalam [12], etc. Harms, et al. [13] experimentally showed that deeper channels provide better flow and heat transfer performance. However, there is little research related to the performance of two-fluid micro-channel heat-exchangers using deep channels, so more research work is needed.

Forced convection heat transfer in porous media has many important applications such as chemical particle beds, petroleum processing, transpiration cooling, solid matrix heat-exchangers, packed-bed regenerators, and heat transfer enhancement. Therefore, fluid flow and convection heat transfer in porous media have been received much attention for the past five decades, e.g., [14], [15], [16], [17], [18], [19], [20], [21]. Previous results have shown that flow through porous media can greatly enhance the convection heat transfer [18], [21]. There has been much research on tubular porous heat-exchangers [22], [23], but the performance of plate porous heat-exchangers for exchanging heat between two fluids has received little attention. Jiang, et al. [24], [25] theoretically analyzed the flow and heat transfer performance of small plate porous heat-exchangers. Since there is little relevant research work in the literature, this novel kind of micro-porous heat-exchanger needs further development and their flow and heat transfer performance need to be thoroughly investigated.

In this paper, the flow and heat transfer performances of a micro-channel heat-exchanger and a micro-porous heat-exchanger are theoretically and experimentally investigated and evaluated.

Section snippets

Experimental apparatus and data analysis

The experimental apparatus, shown schematically in Fig. 1, consisted of water tanks, pumps, a test section, regulator valves, accurate manometers, instrumentation to measure temperatures, an electric heater system and filters. The experimental system had both hot water and cold water loops. The hot water (distilled water) was circulated through a closed circuit. The cold water was supplied from a cold water tank. The test section contained either a micro-channel heat-exchanger (MCHE) or a

Influence of the dimensions of the micro-channels on the thermal–hydraulic performance

A three-dimensional combined thermal conduction and forced convection heat transfer numerical analysis of the micro-channels was used to analyze the influence of the micro-channel size on the thermal–hydraulic performance of the micro-channel heat-exchanger and to provide theoretical basis for the design and optimization of its structure.

Mala and Li [28] found that the difference between the measured Δp/Δl and correlation from conventional theory was small for micro-tube diameters more than

Micro-heat-exchanger performance evaluation

Generally, heat-exchanger performance includes many aspects, e.g., heat transfer, flow resistance, mechanical and economical performance, etc. The present paper evaluates the performance of different types of micro-heat-exchangers based on heat transfer and flow resistance aspects.

The flow and heat transfer performance of MHE1 and MHE2 are compared with another micro-channel heat-exchanger, MHE3, and a conventional plate heat-exchanger, HE4, in Fig. 8, Fig. 11 with water as the working fluid.

Conclusion

Fluid flow and forced convection heat transfer in MCHE and MPHE was investigated theoretically and experimentally. The conclusions can be summarized as:

1.
For the same flow rate, the pressure drop in the MCHE with deep channels is much lower than in the MCHE with shallow channels, but the difference between their heat transfer rates is not large. The maximum volumetric heat transfer coefficient in the MCHE with deep channels was 38.5 MW/(m³ K) for water flow rate of 0.34 kg/s and a pressure drop

Acknowledgements

The project was financially supported by the National Natural Science Foundation of China (No. 59506004) and the Natural Science Fundamental Research Foundation of Tsinghua University, Beijing, China.

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