Heat Transfer Enhancement in Porous Microchannel Evaporator
Date Issued
2011
Date
2011
Author(s)
Liu, Bing-Han
Abstract
The microchannels evaporator, which possesses the advantages of high heat transfer coefficient, good temperature uniformity, and small requirement for coolant flow rates, is considered as a potential cooling technology. In recent years, the raising of heat dissipation rate in electrical products becomes an important issue. The heat-transfer enhanced microchannels are suitable for the applications. The porous structure with a large number of nuclear sites as well as the re-entrance cavities is expected to enhance the heat transfer performance in a microstructure. In the present study, porous microchannel evaporators are designed and manufactured. The effects of powder size, thickness of structure, and pore size distribution upon the heat transfer performance are investigated. The comparisons of heat transfer characteristics, pressure drop, pressure instability, and heat transfer enhanced effects between the plane and the porous microchannel evaporator are made.
The flow boiling experiments were conducted with a plane and a porous microchannel evaporator using R-134a as coolant. Both microchannels had 62 channels (225μm in width; and 660μm in depth) on copper substrates with one square inch in area.
For the plane microchannel evaporators, the results showed that the nucleation boiling and the force convection boiling mechanisms both appeared in microchannels. When the quality in the microchannels was smaller than 0.4, the heat transfer coefficient mainly increased with increasing heat flux and did not vary with the mass flow rate or the quality. This region (quality was under 0.4) was dominated by the nucleation boiling mechanism. On the other hand, when the quality was larger than 0.4, the heat transfer coefficient increased with a increasing mass flux. This region (quality was over 0.4) was dominated by the force convection boiling. The experiment results were substituted into the correlations in which the surface tension force was taken into consideration. The predictions showed a good agreement with experimental data. The critical heat flux (CHF) increased with increasing flow rates. A CHF correlation that incorporates the surface tension force showed an excellent accuracy for the experimental data. Pressure drop were raised by increasing flow rates and heat fluxes. The separation model incorporating surface tension force had a good agreement. The pressure drop oscillation suggested that the presence of instability inside the plane microchannels as well as the maximum amplitude of oscillation were found near the onset of nucleation (ONB).
For the porous microchannels evaporator, the experimental results depicted that the heat transfer coefficient reached a peak value at low quality and decreased with a increasing quality. However, the heat transfer coefficient did not vary with the mass flow rate. This was apparently different from the plane microchannels. The heat-transfer behavior dominated by the mass fluxes belongs to the force convection boiling mechanism. In contrast of the plane microchannel evaporator, the heat transfer coefficient in the porous microchannels evaporator had an enhancement of 5 times in average. The CHF in porous microchannel evaporator increased with increasing mass fluxes and did not enhanced significantly. Furthermore, the trend of pressure drop in porous microchannel was similar in the plane microchannels. The pressure drop was higher than plane microchannels; however, the maximum pressure drop was not over 50%. The amplitude of average pressure drop oscillation near the high heat flux as well as ONB was 1/6 and 1/2 smaller than in the plane microchannels. This result presented that the porous microchannels evaporators provided a stable boiling behavior when the nucleation began.
The porous microchannel evaporators were sintered under the following parameters: the powder diameter dp ranged from 1~100μm, thickness of porous structure δ ranged from 150~375μm, and δ/dp ranged from 2~20, respectively. The investigation on the effect of particle size dp as well as thickness δ indicated that the ratio of the thickness to the particle size δ/dp had a significance in the heat transfer performance. This ratio must be properly chosen in order to reach a better heat transfer performance. The better ratio of δ/dp was between 8~12 in our work. Moreever, the pore size distribution dominated the heat transfer behavior. Smaller pore size with a higher heat transfer capacity. The bi-porous structure was better than the mono-porous structure in about 2 times. To conclude the present study, the porous microchannel evaporator is highly potential for the industrial applications.
Subjects
microchannel
boiling heat transfer enhancement
porous structure
Type
thesis
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