Vapor Extraction From Flow Boiling in a Fractal-Like Branching Heat Sink

Abstract

Increases in pressure 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 the microchannel network within the heat sink. Indeed, by lowering the overall flow rate by vapor extraction, the pressure drop can, in principle, be lowered below that of single-phase flow. In this present study the relative driving forces necessary for vapor extraction and for flow through the microchannels are investigated. The concept also has the potential to separate flow independent of orientation or 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. The disk shaped heat sink was covered with a porous Nylon membrane with an average pore size of 0.45 microns. Water was used as the working fluid with inlet subcooling of approximately 2.5 K and flow rates ranging between 8 and 12 g/min. The vapor extraction pressure was varied and maintained between 0 and 56 kPa below the average pressure between the inlet and exit of the heat sink. Heating varied from 18 to 30 Watts. Actual vapor extraction data are correlated with the exit quality predicted with no vapor extraction, the value of which is dependent upon mass flow rate, heat input and degree of subcooling. Network pressure drop data are correlated with the membrane pressure difference data.

Modeling In-Situ Vapor Extraction During Convective Boiling

“…1 Flow enters from the center of the disk and flows radially through the branching channels. In the experimental investigation, (Apreotesi et al [14,15]) a hydrophobic porous membrane with a porous aluminum backing for support serves as the porous top wall of the channels. The porous aluminum block had an embedded resistance heater to assure no vapor condensation within the block.…”

“…Studies suggest that the pressure drop across microscale heat sinks can also be improved by locally extracting vapor from twophase flow through a hydrophobic, porous membrane forming one wall of the channel [14][15][16]. Apreotesi et al [14,15] provided experimental results of diabatic boiling water flowing through a fractal-like microchannel heat sink with local vapor extraction that show a decrease in overall channel pressure drop as the extraction pressure difference increases. A study by David et al [16] with flow boiling in a microchannel heat sink used one wall fabricated from a hydrophobic porous membrane to allow venting of the vapor.…”

“…Pressure drop, temperature distribution and extracted vapor mass flow rate are presented. The results are compared with the experimental data of Apreotesi et al [14,15] for relatively low flow rates and low heat flux conditions; experimental high flow rates and high heat flux data are not available in the literature.…”

Altering Bubble Dynamics via In Situ Vapor Extraction

“…The application of vapor extraction to heat sinks was first introduced by Apreotesi et al [3] using a fractal-like branching microchannel flow network and by David et al [4] for several microscale configurations. The latter group extended these studies by employing a computational fluid dynamic analysis (Fang,et al [5]).…”

“…equation B is the slope of the departure diameter correlation in Eq (3),. and , K,  g , and P are characteristics of the extraction conditions determined by the application.Conclusions and RecommendationsHigh speed movies of confined extraction of individual vapor and gas bubbles supplied at a constant volumetric flow rate were studied for a range of confinement, or gap, heights.…”