Experimental study on local heat transfer coefficients and the effect of aspect ratio on flow boiling in a microchannel
Flow boiling in integrated microchannel systems is a cooling technology that has received significant attention in recent years as an effective option for high heat flux microelectronic devices as it provides high heat transfer and small variations in surface temperature. However, there are still a number of issues to be addressed before this technology is used for commercial applications. Amongst the issues that require further investigation are the two-phase heat transfer enhancement mechanisms, the effect of channel geometry on heat transfer characteristics, two-phase flow instabilities, critical heat flux and interfacial liquid-vapour heat transfer in the vicinity of the wall. This work is an experimental study on two-phase flow boiling in multi- and single-rectangular microchannels. Experimental research was performed on the effect of the channel aspect ratio and hydraulic diameter, particularly for parallel multi-microchannel systems in order to provide design guidelines. Flow boiling experiments were performed using deionised water in silicon microchannel heat sinks with width-to-depth aspect ratios (𝑎�) from 0.33 to 3 and hydraulic diameters from 50 μm to 150 μm. The effect of aspect ratio on two-phase flow boiling local heat transfer coefficient and two-phase pressure drop was investigated as well as the two-phase heat transfer coefficients trends with mass flux for the constant heat fluxes of 151 kW m-2, 183 kW m- 2, 271 kW m-2 and 363 kW m-2. Wall temperature measurements were obtained from five integrated thin nickel film temperature sensors. An integrated thin aluminium heater enabled uniform heating with a small thermal resistance between the heater and the channels. The microfabricated temperature sensors were used with simultaneous high-speed imaging and pressure measurements in order to obtain a better insight related to temperature and pressure fluctuations caused by two-phase flow instabilities under uniform heating in parallel microchannels. The results demonstrated that the aspect ratio of the microchannels affects flow boiling heat transfer coefficients. However, there is not clear trend of the aspect ratio on the heat transfer coefficient. Pressure drop was found to increase with increasing aspect ratio. Wide microchannels but not very shallow, with 𝑎� = 1.5 and 𝐷�ℎ = 120 μm, have shown good heat transfer performance, by producing modest two-phase pressure drop of maximum 200 mbar for the highest heat flux and heat transfer coefficients of 200 kW m-2 during two-phase flow boiling conditions. For the high aspect ratio, values of 2 and 3 two-phase flow boiling heat transfer coefficients were measured to be lower compared to aspect ratio of 1.5. Microchannels with aspect ratios higher than 1.5 produced severe wall temperature fluctuations for high heat fluxes that periodically reached extreme wall temperature values in excess of 250 ˚C. The consequences of these severe wall temperature and pressure fluctuations at high aspect ratios of 2 and 3 resulted in non-uniform flow distribution and temporal dryout. Abrupt increase in two-phase pressure drop occurred for 𝑎� > 1.5. The effect of the inlet subcooling was found to be significant on both heat transfer coefficient and pressure drop. Furthermore, the effects of bubble growth on flow instabilities and heat transfer coefficients have been investigated. Although the thin film nickel sensors provide the advantage of much faster response time and smaller thermal resistance compared to classic thermocouples, they do not allow for full two-dimensional wall temperature mapping of the heated surface. An advanced experimental method was devised in order to produce accurate two-dimensional heat transfer coefficient data as a function of time. Infrared (IR) thermography was synchronised with simultaneous high-speed imaging and pressure measurements from integrated miniature pressure sensors inside the microchannel, in order to produce two-dimensional (2D) high spatial and temporal resolution two-phase heat transfer coefficient maps across the full domain of a polydimethylsiloxane (PDMS) microchannel. The microchannel was characterised by a high aspect ratio (𝑎� = 22) and a hydraulic diameter of 192 μm. The PDMS microchannel was bonded on a transparent indium tin oxide (ITO) thin film coated glass. The transparent thin film ITO heater allowed the recording of high quality synchronised high - speed images of the liquid-vapour distribution. This work presents a better insight into the two-phase heat transfer coefficient spatial variation during flow instabilities with two-dimensional heat transfer coefficient plots as a function of time during the cycles of liquid-vapour alternations for different mass flux and heat flux conditions. High spatial and temporal resolution wall temperature measurements and pressure data were obtained for a range of mass fluxes from 7.37 to 298 kg m-2 s-1 and heat fluxes from 13.64 to 179.2 kW m-2 using FC-72 as a dielectric liquid. 3D plots of spatially averaged two-phase heat transfer coefficients at the inlet, middle and outlet of the microchannel are presented with time. The optical images were correlated, with simultaneous thermal images. The results demonstrate that bubble growth in microchannels differs from macroscale channels and the confinement effects influence the local two-phase heat transfer coefficient distribution. Bubble nucleation and axial growth as well as wetting and rewetting in the channel were found to significantly affect the local heat transfer physical mechanisms. Bubble level heat transfer coefficient measurements are important as previous researchers have experimentally investigated local temperature and high speed visualisation in bubbles during pool boiling conditions and not flow boiling. The effect of the confined bubble axial growth to the two-phase heat transfer coefficient distribution at the channel entrance was investigated at low mass fluxes and low heat fluxes. The 3D plots of the 2D two-phase heat transfer coefficient with time across the microchannel domain were correlated with liquid-vapour dynamics and liquid film thinning from the contrast of the optical images, which caused suspected dryout. The 3D plots of heat transfer coefficients with time provided fine details of local variations during bubble nucleation, confinement, elongated bubble, slug flow and annular flow patterns. The correlation between the synchronised high-resolution thermal and optical images assisted in a better understanding of the heat transfer mechanisms and critical heat flux during two-phase flow boiling in microchannels.