Experimental and numerical study of evaporating flow heat transfer in micro-channel

Hoki Lee

doi:10.7273/000005900

This dissertation presents the design of a MEMS-based micro-channel evaporator, fabricated and characterized to maximize the efficiency of evaporation. Efficiency of evaporator is defined to be the amount of energy used to evaporate fluid over the amount of energy input to the micro-channel evaporator. Experiment and numerical results are presented for steady and transient evaporation heat transfer from open top micro-channels. The radial channels were fabricated in two geometries: one was a rectangular SU8 wick structure 40[mu]m high and 5[mu]m wide with channel widths range from 10 to 70[mu]m. The second was a tapered 40[mu]m high and 80[mu]m wide at the outer radius and narrowing to 5[mu]m wide at inner radius. The radial channels were fabricated on two membrane materials one was two-micron thick silicon and the other was the three hundred-nanometer silicon nitride. Transient state evaporation tests were done at cycle frequencies of 10 Hz, 20Hz and 50Hz. An energy balance was experimentally determined on the radial micro-channel evaporators, including heat into the channels, conduction heat transfer radially along the channels and latent heat transfer through evaporation of the working fluid from the channels. A numerical analysis was used to simulate the experimental measurements. The numerical integration calculated conduction heat transfer using axisymmetric FDTD integration and mass transfer from evaporation using heat balance equation in the micro-channel. The experimental and the numerical results were compared to validate the numerical model. The 40[mu]m high and 5[mu]m wide SU8 features with 70[mu]m channels and the tapered channels were found to have the best overall performance of evaporators with silicon membrane. These wick dimensions yielded a mass evaporation rate of 6.5mg/min and 6.2mg/min, and wick efficiencies of 28% and 26% with an input energy of 34mW for steady state conditions. For transient state conditions of 10Hz 10% duty cycle, the tapered channel evaporator yielded a mass evaporation rate of 8.2mg/min and wick efficiency of 33.6% with an average input power of 34mw. Use of a low conductivity SiN membrane evaporator increased the mass evaporation rate to 8.2mg/min and the wick efficiency to 88% for an input energy of 13mW.

Experimental and numerical study of evaporating flow heat transfer in micro-channel

Files and links (1)

Abstract

Metrics

Details