Flow boiling Heat Transfer Enhancement in Porous Microchannel Evaporator
Date Issued
2012
Date
2012
Author(s)
Chou, Chi-Sheng
Abstract
The microchannel evaporator,which possesses the advantage of high heat transfer coefficient,good temperature uniformity,and small requirement for coolant flow rates,is considered as a potential cooling technology.The porous structure with a large number of nucleation site density as well as the reentrant grooves is to enhance the heat transfer performance in the microchannels evaporator.
In present study,the flow boiling experiments were conducted with a plane and porous microchannels evaporator on one square inch copper substrates. Using water as working fluid,the mass flux from103~207 kg/m^2 s and the saturated pressure of 140kpa. Both microchannels have 62 channels(225μm in width；and 660μm in depth).The effects of powder size,thickness of structure upon heat transfer performance are investigated.The comparsions of heat transfer characteristics,pressure drop, pressure instability,and heat transfer enhanced effects between the plane and the porous microchannels evaporator are made.Finally,the comparisons of heat transfer performance,pressure drop,pressure instability between two different working fluid water and R-134a in microchannels.
The experiment results were substituted into the heat transfer correlations in which the surface tension force was taken into consideration.The mean average error was16.5%. Pressure drop raised by increasing heat fluxes,but did not vary with increasing mass flux.The experiment results were substituted into the separation model incorporating surface tension force. The mean average error was 21.3%. The pressure drop oscillation suggested that the presence of instability inside plane microchannels as well as the maximum amplitude of oscillation were found near the onset of nucleation.
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 225~375μm, and δ/dp ranged from 3~20, 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 3~4 in our work,withδ 225μm and dp 53μm.The average heat transfer coefficient enhanced about 3 times larger than the plane microchannels.
For the porous microchannels evaporator,the heat transfer results different from the plane microchannels evaporator,heat transfer coefficient varied with varing mass flux.Pressure drop in porous microchannel evaporator was raised by increasing heat fluxes.The pressure drop was higher than plane microchannels；however,the maximum pressure drop was not over 50%. The maximum amplitude of oscillation was 66% lower than plane microchannels.This result presented that the porous microchannels evaporator provided a stable boiling behavior when nucleation began.
For the porous microchannels： Working fluid water,the better ratio ofδ/dp was between 3~4；however, the better ratio ofδ/dp was between 8~12 when R-134a as working fluid.Surface tension force was probably the different choose between the better ratio ofδ/dp .The comparisons between two different working fluid water and R-134a in microchannels： The pressure results showed that water in the plane microchannels,its maximum amplitude of oscillation was larger than R-134a.The maximum amplitude of oscillation was obviously lower than the plane microchannels in two different working fluids.
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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