Abstract:
Recent studies of flow boiling within microchannels have shown high heat fluxes,
leading to successful design and testing of compact heat exchangers, cooling systems
for electronics and other high efficiency, miniature components. An experimental
study of flow boiling of a concentrated salt solution in microchannels is presented.
Experiments examined flow in single, circular microchannels, with a length of 25 mm
and diameters of 0.127 mm and 0.25 mm, and in arrays of five rectangular
microchannels, each with a length of 8.5 mm and a hydraulic diameter of 0.133 mm.
Inlet concentrations of lithium bromide were varied from 47% to 57% by mass, heat
inputs from 2.2 to 7.0 W per channel, and flow rates from 0.60 to 0.80 g/min per
channel. Single channel tests yielded as much as 0.05 g/min of water vapor for the
conditions examined. If an array of channels could produce vapor at a similar rate, it
may be possible to apply microchannel flow boiling in a compact desorber or
evaporator. Unequal flow distribution in the microchannel arrays limited testing to
heat input rates too low to verify array desorption.
Infrared visualization of flow at the exit of the microchannel was performed to
document solution exit condition based on flow rate, heat input rate, and lithium
bromide concentration. Visualization identified four basic exit conditions: steady
dripping at low heat input rates, unsteady dripping with occasional vapor at moderate
heat input rates, continuous droplet spraying at high heat input rates and flow rates,
and long period oscillation between unsteady dripping and droplet spraying at high
heat input rates and low flow rates.
