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It is hypothesized that increases in pressure drop due to vapor generation
during boiling in microchannels can be reduced by extraction of vapor at its point of
inception. Ultimately, this local vapor extraction decreases the pressure drop required
to drive the flow through a fractal-like branching, microchannel flow network within
the heat sink. Indeed, by lowering the overall flow rate by vapor extraction the
pressure drop can, in principle, be lowered to that of single-phase flow. In the current
study, the feasibility of vapor extraction and its influence on the pressure drop across
the microchannels are investigated. The concept also has the potential to separate flow
independent of heat sink orientation or the influence of gravity.
The fractal-like flow network used here is one that has been previously shown
to reduce pressure drop and yield a more uniform surface temperature distribution for
single-phase flows than that observed in parallel microchannel flow networks. The
disk shaped heat sink was covered with two porous Nylon membranes with an average
pore size of 0.45 microns that were backed with a porous aluminum block. Deionized,
degassed water was used as the working fluid.
A theoretical model was developed to predict the pressure drop across the flow
network as a function of inlet mass flow rate, heat input, and pressure difference
driving extraction across the membranes. Results of the model predictions are
presented and discussed in both dimensional and non-dimensional format. Model
predictions were used to discuss the trends and physical implications of local vapor
extraction. It was observed that conditions existed where further increases in the
membrane pressure difference had no influence vapor extraction and network pressure
drop values. Experimental data was also collected and analyzed in order to assess the
validity of the theoretical model.
The predictive model and experimental data indicated that the two-phase
network pressure drop is reduced by locally extracting vapor from the heat sink. The
network pressure drop was reduced by as much as 80% theoretically and 50%
experimentally in the predictive model and experimental data, respectively, for similar
test conditions. A sensitivity analysis was performed on several parameters used in
the model in an attempt to help explain some of the discrepancies observed between
the experimental results and model predictions. Suggestions are made to improve the
theoretical model pressure predictions and to investigate the influence of vapor
extraction on the heat transfer coefficient values. |
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