Huiying Wu

Pressure drop and heat transfer of Al2O3-H2O nanofluids through silicon microchannels

Experimental investigations were performed on the single-phase flow and heat transfer characteristics through the silicon-based trapezoidal microchannels with a hydraulic diameter of 194.5 µm using Al2O3-H2O nanofluids with particle volume fractions of 0, 0.15% and 0.26% as the working fluids. The effects of the Reynolds number, Prandtl number and nanoparticle concentration on the pressure drop and convective heat transfer were investigated. Experimental results show that the pressure drop and flow friction of the nanofluids increased slightly when compared with that of the pure water, while the Nusselt number increased considerably. At the same pumping power, using nanofluids instead of pure water caused a reduction in the thermal resistance. It was also found that the Nusselt number increased with the increase in the particle concentration, Reynolds number and Prandtl number. Based on the experimental data, the dimensionless correlations for the flow friction and heat transfer of Al2O3-H2O nanofluids through silicon microchannels were proposed for the first time. The agglomeration and deposition of nanoparticles in the silicon microchannels were also examined in this paper. It was found that the Al2O3 nanoparticles deposited on the inner wall of microchannels more easily with increasing wall temperature, and once boiling commenced, there is a severe deposition and adhesion of nanoparticles to the inner wall, which makes the boiling heat transfer of nanofluids in silicon microchannels questionable.

Pressure drop and flow boiling instabilities in silicon microchannel heat sinks

A simultaneous visualization and measurement experiment has been performed to study pressure drop and flow instabilities at various mass fluxes and heat fluxes during flow boiling of deionized water in a silicon microchannel heat sink. The silicon micro heat sink consisted of eight parallel microchannels 60 mm long, having a trapezoidal cross section with a hydraulic diameter of 72.7 µm. It is found that (1) the onset of nucleate boiling (ONB) depends on both the amount of heat flux and mass flux. The mass flux, at which the onset of nucleate boiling occurs, increases as the heat flux is increased; (2) the difference in mass fluxes between the ONB and the onset of flow instability (OFI) decreases as the heat flux is increased; (3) both oscillation amplitude and oscillation period of various measurements near the OFI are small; (4) as the mass flux is decreased further from the OFI while keeping the heat flux constant, the following boiling instability flow modes occurred sequentially: the liquid/two-phase alternating flow (LTAF) and the liquid/two-phase/vapor alternating flow (LTVAF); (5) the occurrence of LTAF and LTVAF, as well as their induced large-amplitude/long-period oscillations of various measurements, are owing to the reversed flow of vapor core because of the limited expansion space in the microchannels; (6) the appearance of the reversed flow of the vapor core in parallel microchannels is not in phase due to the flow or nucleation nonuniformity. The results presented in this paper help us to better understand the two-phase flow instabilities occurring in the silicon-based micro heat sinks.

Injection flow during steam condensation in silicon microchannels

An experimental investigation with the combined use of visualization and measurement techniques was performed on flow pattern transitions and wall temperature distributions in the condensation of steam in silicon microchannels. Three sets of trapezoidal silicon microchannels, having hydraulic diameters of 53.0 µm, 77.5 µm and 128.5 µm, respectively, were tested under different flow and cooling conditions. It was found that during the transitions from the annular flow to the slug/bubbly flow, a peculiar flow pattern injection flow appeared in silicon microchannels. The location at which the injection flow occurred was dependent on the Reynolds number, condensation number and hydraulic diameter. With increase in the Reynolds number, or decrease in the condensation number and hydraulic diameter, the injection flow moved towards the channel outlet. Based on the experimental results, a dimensionless correlation for the location of injection flow in functions of the Reynolds number, condensation number and hydraulic diameter was proposed for the first time. This correlation can be used to determine the annular flow zone and the slug/bubbly flow zone, and further to determine the dominating condensation flow pattern in silicon microchannels. Wall temperature distributions were also explored in this paper. It was found that near the injection flow, wall temperatures have a rapid decrease in the flow direction, while upstream and downstream far away from the injection flow, wall temperatures decreased mildly. Thus, the location of injection flow can also be determined based on the wall temperature distributions. The results presented in this paper help us to better understand the condensation flow and heat transfer in silicon microchannels.

Condensation heat transfer and flow friction in silicon microchannels

An experimental investigation was performed on heat transfer and flow friction characteristics during steam condensation flow in silicon microchannels. Three sets of trapezoidal silicon microchannels, with hydraulic diameters of 77.5 µm, 93.0 µm and 128.5 µm respectively, were tested under different flow and cooling conditions. It was found that both the condensation heat transfer Nusselt number (Nu) and the condensation two-phase frictional multiplier (2Lo) were dependent on the steam Reynolds number (Rev), condensation number (Co) and dimensionless hydraulic diameter (Dh/L). With the increase in the steam Reynolds number, condensation number and dimensionless hydraulic diameter, the condensation Nusselt number increased. However, different variations were observed for the condensation two-phase frictional multiplier. With the increase in the steam Reynolds number and dimensionless hydraulic diameter, the condensation two-phase frictional multiplier decreased, while with the increase in the condensation number, the condensation two-phase frictional multiplier increased. Based on the experimental results, dimensionless correlations for condensation heat transfer and flow friction in silicon microchannels were proposed for the first time. These correlations can be used to determine the condensation heat transfer coefficient and pressure drop in silicon microchannels if the steam mass flow rate, cooling rate and geometric parameters are fixed. It was also found that the condensation heat transfer and flow friction have relations to the injection flow (a transition flow pattern from the annular flow to the slug/bubbly flow), and with injection flow moving toward the outlet, both the condensation heat transfer coefficient and the condensation two-phase frictional multiplier increased.

Characterization on the performance of a fractal-shaped microchannel network for microelectronic cooling

Previous theoretical and analytical studies have shown that microchannel heat sinks with a fractal-shaped network have many advantages over traditional parallel microchannels with respect to thermal resistance, temperature uniformity and pressure drop. However, to the best knowledge of the authors, no experimental investigations on fractal-shaped microchannel network heat sinks have been conducted so far to verify their performance. In this paper, we designed and fabricated a silicon-based microchannel heat sink with a single-layered fractal-shaped microchannel network using MEMS technology, and experimentally studied its pressure drop and thermal resistance characteristics under different mass flow rate and heat flux conditions. Numerical simulations are performed to predict the heat sink performance under the same experimental conditions. It is found that the experimentally measured pressure drop in the heat sink has a nonlinear relationship with the mass flow rate, which agrees very well with the numerical simulation result. It is also found that the experimentally measured thermal resistance is also in reasonably good agreement with the numerical simulation, and therefore indirectly verifies the conclusion of previous numerical simulations that the performance of the fractal-shaped microchannel network is better than that of traditional parallel microchannels.

Experimental Study on Thermal Performance of a Silicon-Based Micro Pulsating Heat Pipe

In this paper, an experimental investigation was conducted on the thermal performance of a silicon-based micro-pulsating heat pipe (SMPHP) using FC-72 and R113 as working fluids. The SMPHP, covering an area of 46 × 19mm2 , consisted of fourteen meandering trapezoidal channels with a hydraulic diameter of 352μm. The effects of gravity, filling ratio, and working fluids on the overall thermal resistance of the SMPHP were discussed. Experimental results show that gravity had an impact on the thermal performance of the SMPHP, and self-sustained oscillation could not be achieved at the horizontal orientation. The SMPHP worked as a true pulsating device when the filling ratio varied from 30% to 65%. For FC-72 and R113, there was an optimal filling ratio of 55% and 41%, respectively, for the best thermal performance of the SMPHP. As compared to the SMPHP with 0% filling ratio (or charged with the air), the thermal resistances of the SMPHP charged with FC-72 (at 55% filling ratio) and R113 (at 41% filling ratio) were decreased maximally by 7.24°C/W (or 56.5%) and 7.51 °C/W (or 59.7%), respectively. It is also found that the R113 was favorable for the operation of the SMPHP at lower power inputs, while FC-72 was favorable at relatively higher power inputs.Copyright