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.
Heat activated cooling provides an opportunity to recover and utilize wasted heat. In terms of thermal management of electronics, a heat-activated cooling cycle could be used to thermally manage a space such as a central computing facility. A microscale, fractal-like branching flow heat exchanger was designed and used to desorb ammonia from an aqueous ammonia solution. The fractal-like pattern employed in the present study was previously studied for high heat flux single-phase and two-phase boiling flow heat sink applications. For compatibility, the desorber was fabricated in 316 stainless steel. The desorber is compact, approximately 38 mm in diameter and 6.4 mm thick, and lightweight, weighing 20 grams. Heating was accomplished using Paratherm NF oil between 350 and 400 K. The mass fraction of ammonia in the strong solution inlet stream was 0.30 and the temperature was 300 K. Given a range of inlet solution mass flow rates between 0.42 and 0.92 g/s and oil flow rates between 1.67 and 10 g/s, the mass flow rate of vapor generated varied from 0.02 to 0.13 g/s. The mass fraction of ammonia in the exiting vapor stream varied between 0.8 and 0.96 while circulation ratios varied between 3.5 and 20. Heat exchanger performance is presented using LMTD and ε-NTU analyses. Overall heat transfer coefficients ranged from 7500 to 15,000 for the flow rates and driving temperature differences investigated. The configuration of the desorbers is such that the oil stream can be introduced to flow parallel or counter to the ammonia solution stream. The nature of the microchannels is such that desorption occurs in a co-flowing manner, limiting the vapor mass fraction. However, the advantages of the present design are lightweight, compact, modularity and orientation independence.
Desorption in micro-scale plate heat exchangers having a branching flow network is investigated as a function of oil flow rate, solution flow rate, manifold pressure and channel depth. The solution is an aqueous-ammonia solution with an inlet concentration held fixed at 30%. Mass flow rate and ammonia mass fraction of the generated vapor stream are characterized as is the heat exchange effectiveness of the various heat exchange desorbers. The effects of operating or exit plenum pressure and channel height on desorption and heat transfer characteristics are considered. Microscale channels are employed for enhanced heat and mass transport. The branching nature of the flow network is employed for flow symmetry and low pressure drop penalties. An operational model is generated to correctly size and efficiently integrate the desorber into an absorption cycle.
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