Investigations of single and two phase heat transfer under natural and forced flow conditions using an instrumented glass MEMS device

Small, fast and densely populated computer chips have resulted in very high dissipation power that must be accommodated with advanced cooling techniques. Single phase and two phase heat transfer involving boiling phenomenon has emerged as prime candidates for removing high heat fluxes in microchannels. The two phase heat transfer is governed by a limiting condition termed as critical heat flux (CHF). To prevent device failures micro-channels cooling systems must be operated below the CHF.;The current state-of-art devices for investigations of convective heat transfer in microtubes and microchannels are fabricated in silicon, stainless steel, or copper; conduction losses through these high thermal conductivity materials result in conjugate effects and experimental uncertainties. For forced convective boiling analysis microchannels are fabricated in glass using the microfabrication technology. The use of low thermal conductivity glass improves the accuracy of measured CHF values and allow for visualization of the boiling phenomena, and identification of the flow configuration at CHF condition. The device is instrumented with heaters and voltage taps to measure the temperature distribution along the flow direction. Experimental results of the pressure drop and two phase heat transfer phenomena for pool and forced convective boiling are discussed with water as the medium.;In the present work an investigation of natural and forced convection heat transfer is done. Pool boiling investigation shows the device is able to detect the bubble dynamics, i.e. bubble initiation, growth and departure using local temperature measurements. Effect of surface roughness and nanofluids on CHF for pool boiling is studied. Nanofluids using single-wall and multi-wall carbon nanotubes, and nickel and bismuth-telluride nanoparticles were analyzed for pool boiling. Nanofluids were found to increase the CHF by 55.1% (SWCNTs), 221.2% (MWCNTs), 226.3% (42.9 mg/ml nickel nanoparticles), and 37.7% (Bi 2Te3 nanoparticles of diameter ∼13 nm). A small concentration of CNTs was found to enhance CHF by maximum amount because of the high thermal conductivity associated with them. CHF was also found to increase with an increase in the concentration of nickel nanoparticles.;For forced convective heat transfer single phase and boiling experiments were done for microchannels of different hydraulic diameters (130.59 mum, 186.9 mum, 206.3 mum, and 260.4 mum). Pressure drop analysis was done to characterize the flow behavior and the experimental values were compared with Hausen and Shah correlations. The correlations seem to over predict the Poiseuille number for microchannel of hydraulic diameter 130.59 mum and under predict for microchannels of hydraulic diameters 186.9 mum, 206.3 mum, and 260.4 mum respectively. Based on the hydrodynamic entrance length the flow was considered fully developed and in the laminar regime (Reynolds number varying from 20--300). The single phase heat transfer coefficient was compared with different correlations. The CHF condition was marked by a sudden rise in the wall temperature at which point the experiment was shut-off in order to prevent the burn-out of the device. Wall superheats of 20--40ºC were observed for CHF depending upon the flow conditions and microchannel geometry. CHF was also found to increase with mass flux and with the hydraulic diameters of different microchannels. High resolution images which define different stages during two-phase phenomena in microchannels were obtained. These stages range from bubble nucleation, increase in the number of active nucleation sites, bubble coalescence, slug flow, and eventually CHF condition. The void fraction was found to be of the order of 80--85% which indicates the transition of bubbly flow to the slug flow thereby causing CHF. No vapor backflow was observed in the microchannels during CHF experiments. The bubbles were found to increase in diameter with increasing heat flux eventually filling the microchannel. The wall temperature increased almost linearly during the single and two-phase condition and the boiling plateau was found to be suppressed.