Tannaz Harirchian

An investigation of flow boiling regimes in microchannels of different sizes by means of high-speed visualization

Boiling in microchannel heat sinks is attractive for high-performance electronics cooling due to the high heat transfer rates that can be achieved. However, the physics of flow boiling in microchannels, the flow patterns present, and the effect of microchannel size on the boiling regimes have not been investigated extensively, particularly with dielectric fluids. In the present work, experiments are conducted with a perfluorinated dielectric fluid, Fluorinert FC-77, to investigate the effect of channel size and mass flux (250 to 1600 kg/m2s) on microchannel flow boiling regimes by means of high-speed photography. Seven different silicon test pieces with parallel microchannels of widths ranging from 100 to 5850 mum, all with a depth of 400 mum, are considered. Flow visualizations are performed with a high-speed digital video camera while local measurements of the heat transfer coefficient and pressure drop are simultaneously obtained. The visualizations show that flow regimes in microchannels of width 400 mum and larger are similar, while those in the 100 mum wide microchannels are distinctly different. Also, unlike the 100 mum wide microchannels, in which bubble nucleation at the walls is suppressed at a relatively low heat flux, nucleate boiling is dominant over a wide range of heat flux for microchannels of width 400 mum and larger.

Microchannel Size Effects on Two-Phase Local Heat Transfer and Pressure Drop in Silicon Microchannel Heat Sinks With a Dielectric Fluid

Two-phase heat transfer in microchannels can support very high heat fluxes for use in high-performance electronics-cooling applications. However, the effects of microchannel cross-sectional dimensions on the heat transfer coefficient and pressure drop have not been investigated extensively. In the present work, experiments are conducted to investigate the local flow boiling heat transfer in microchannel heat sinks. The effect of channel size on the heat transfer coefficient and pressure drop is studied for mass fluxes ranging from 250 to 1600 kg/m2 s. The test sections consist of parallel microchannels with nominal widths of 100, 250, 400, 700, and 1000 μm, all with a depth of 400 μm, cut into 12.7 mm × 12.7 mm silicon substrates. Twenty-five microheaters embedded in the substrate allow local control of the imposed heat flux, while twenty-five temperature microsensors integrated into the back of the substrates enable local measurements of temperature. The dielectric fluid Fluorinert FC-77 is used as the working fluid. The results of this study serve to quantify the effectiveness of microchannel heat transport while simultaneously assessing the pressure drop trade-offs.© 2007 ASME

AN EXPERIMENTAL INVESTIGATION OF MICROCHANNEL SIZE EFFECTS ON FLOW BOILING WITH DE-IONIZED WATER

The heat transfer characteristics during flow boiling of deionized water in parallel microchannels are investigated. The silicon heat sinks contain an array of integrated heaters and diodes for localized heat-flux control and temperature measurement. The channel widths for the three different test pieces range from 250 μm to 2200 μm, with a nominal depth for all channels of 400 μm. The present study investigates the effects of the channel width and mass flux on the boiling performance. This study follows a previous study using a wetting dielectric liquid, and aims to understand the role of wetting since water is relatively non-wetting. From the results of the present study, a weak dependence of the boiling curve and heat transfer coefficient on mass flux was observed. Varying the channel width also does not have a strong effect on either the boiling curve or the heat transfer coefficient. The experimental results are compared to those obtained previously for a dielectric liquid. They are also compared with predictions from several correlations from the literature.Copyright

Dependence of Flow Boiling Heat Transfer Coefficient on Location and Vapor Quality in a Microchannel Heat Sink

Experiments were conducted to determine the influence of local vapor quality on local heat transfer coefficient in flow boiling in an array of microchannels. Additionally, the variation of local heat transfer coefficient along the length and width of the microchannel heat sink for given operating conditions was investigated over a range of flow parameters. Each test piece includes a silicon parallel microchannel heat sink with 25 integrated heaters and 25 temperature sensors arranged in a 5×5 grid, allowing for uniform heat dissipation and local temperature measurements. Channel dimensions ranged from 100 μm to 400 μm in depth and 100 μm to 5850 μm in width; the working fluid for all cases was the perfluorinated dielectric liquid, FC-77. The heat transfer coefficient is found to increase with increasing vapor quality, reach a peak, and then decrease rapidly due to partial dryout on the channel walls. The vapor quality at which the peak is observed shows a strong dependence on mass flux, occurring at lower vapor qualities with increasing mass flux for fixed channel dimensions. Variations in local heat transfer coefficient across the test piece were examined both along the flow direction and in a direction transverse to it; observed trends included variations due to entrance region effects, two-phase transition, non-uniform flow distribution, and channel wall dryout.Copyright

Flow Boiling in Silicon Microchannel Heat Sinks

The local flow boiling heat transfer and pressure drop in microchannel heat sinks are investigated with a dielectric fluid, Fluorinert FC-77. The effect of channel size on flow boiling is studied for mass fluxes ranging from 250 to 1600 kg/m2s for seven different test pieces consisting of parallel microchannels with nominal widths ranging from 100 to 5850 mum, all with a depth of 400 mum. High-speed visualizations are performed simultaneously with the local measurements of the temperature and pressure drop to investigate the flow boiling patterns and the conditions for transition between different regimes. The results of this study show that for microchannels of width 400 mum and greater, the heat transfer coefficients corresponding to a fixed wall heat flux as well as the boiling curves are independent of channel size, and have a weak dependence on channel width for smaller microchannels. This is consistent with the visualizations which show that flow regimes in microchannels of width 400 mum and larger are similar, while those in the 100 mum wide microchannels are distinctly different. Also, unlike the 100 mum wide microchannels, in which bubble nucleation at the walls is suppressed at a relatively low heat flux, nucleate boiling is dominant over a wide range of heat fluxes for microchannels of width 400 mum and larger.

A Systematic Investigation of the Effects of Microchannel Width, Depth, and Aspect Ratio on Convective Boiling Heat Transfer and Flow Regimes in Parallel Microchannels

Experiments are conducted with a perfluorinated dielectric fluid, Fluorinert FC-77, to investigate the effects of channel width, depth, and aspect ratio on flow boiling heat transfer and flow patterns in microchannels. Experiments are performed for a fixed mass flux of 630 kg/m2 s with eleven different silicon test pieces containing parallel microchannels of widths ranging from 100 μm to 5850 μm and depths ranging from 100 μm to 400 μm. Flow visualizations are performed using a high-speed digital video camera to determine the flow regimes, with simultaneous local measurements of the heat transfer coefficient and pressure drop. In recent work by the authors [1], it was shown that for a fixed channel depth, the heat transfer coefficient was independent of channel width for microchannels of width 400 μm and larger, with the flow regimes in these microchannels being similar; nucleate boiling was also found to be dominant over a wide range of heat fluxes. The aim of the present study is to expand the range of the microchannel dimensions considered, and specifically to investigate as independent parameters the effects of channel width and depth as well as the aspect ratio and cross-sectional area on boiling heat transfer in microchannels. Flow visualizations and heat transfer results show that the channel cross-sectional area is the important governing parameter determining boiling mechanisms and heat transfer in microchannels.Copyright