Date of Award

Spring 2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Hisham Hegab

Abstract

Theoretical models of counter flow microchannel heat exchangers subjected to scaling and secondary effects are developed in this dissertation. The scaling effects studied include axial heat conduction and viscous dissipation, while the secondary effects considered in this dissertation is that of external heat transfer via heat flux and temperature. The theoretical models developed are one-dimensional and consist primarily of ordinary governing equations that describe the axial variation of hot and cold fluid. For the case of axial heat conduction, the axial variation of wall temperature is also modeled. The models are dependent on various factors, such as Reynolds number, Prandtl number, microchannel hydraulic diameter, microchannel length, microchannel profile, substrate spacing, thermal conductivities of the fluids and wall, and fluid inlet temperature difference. The individual effect of all these parameters with respect to each of the scaling and secondary effects is studied using each model. The governing equations are solved using numerical method, specifically finite difference method.

Studies are done for Reynolds number between 1 and 1500 for thermal models on axial heat conduction, and external heat transfer. On the other hand, the Reynolds number for the model studying viscous dissipation is varied between 1 and 1000. The effect of Prandtl number on the models is analyzed using air, ethylene glycol and water. The influence of profile on the thermal performance of microchannel heat exchanger is studied using rectangular, trapezoidal and triangular microchannels. The aspect ratio of rectangular microchannels is varied between 1 and 0.125, while that of trapezoidal microchannels considered are 0.5, 0.25 and 0.125. Only one aspect ratio for triangular microchannels is considered in this study due to the manufacturing constraints on silicon based microchannel heat exchangers. It is 1.414. The effect of microchannel hydraulic diameter is studied for hydraulic diameter of 100µm, 200µm and 300µm for all models except that for viscous dissipation. For the model analyzing viscous dissipation, the influence of hydraulic diameter is studied using microchannels with hydraulic diameter of 200µm, 300µm and 400µm. The effect of substrate spacing for all models is analyzed by varying this parameter is between 100µm and 300µm in increments of 100µm. The length is varied between 2.54cm to 5.08cm to 7.62cm in order to study the effect of length on the thermal performance of microchannel heat exchangers of each model. The effect of difference between the inlet temperatures of the fluids on the model is also studied primarily for temperature differences of 25°C, 50°C, and 75°C.

All these effects except viscous dissipation are prominent at low Reynolds numbers; at high Reynolds numbers these effects have little influence on the effectiveness of the fluids. Viscous dissipation, is influential at high Reynolds numbers rather than at low Reynolds numbers. In addition it is observed from the solutions of the models that all these effects have the strongest influence on gases rather than on liquids. Based on the analysis of solutions, it can be generalized that square microchannels have the best performance between rectangular, trapezoidal and triangular microchannels under similar operating conditions.

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