Flow boiling oscillation characteristics in two silicon microchannel heat sink configurations are presented. One is a standard heat sink with 45 straight parallel channels, whereas the second is similar except with cross-linked paths at three locations. Data are presented over a flow range of 20–50ml∕min(91–228kg∕(m2s)) using distilled water as the working fluid. The heat sinks have a footprint area of 3.5cm2 and contain 269μm wide by 283μm deep reactive ion etching channels. Flow oscillations are found to be similar in characteristic trends between the two configurations, showing a decreasing frequency with increasing heat flux. The oscillation amplitudes are relatively large and identical in frequency for the inlet temperature, outlet temperature, inlet pressure, and pressure drop. Oscillation properties for the standard heat sink at two different inlet temperatures and various flow rates are correlated for different heat fluxes. This work additionally presents a first glimpse of the cross-linked heat sink performance under flow boiling instability conditions.

Microchannel heat transfer governs the performance of the microchannel heat sink, which is a recent technology aimed at managing the stringent thermal requirements of today’s high-end electronics. The microencapsulated form of liquid crystals has been well established for use in surface temperature mapping, while limited studies are available on the use of the un-encapsulated form. This latter form is advantageous since it offers the potential for high spatial resolution, which is necessary for microgeometries. A technique for using un-encapsulated thermochromic liquid crystals (TLCs) in order to measure the local heat transfer coefficient in microchannel geometries is shown in the present study. Measurements were made in a closed loop facility combined with a microscopic imaging system and automated data acquisition. A localized TLC calibration was used to account for a non-uniform coating and variation of lighting conditions. Three test section configurations were investigated with each subsequent configuration arising due to a shortfall in the previous. Two of these configurations are comprised of single wall heated rectangular channels, while the third is a circular tube channel. Validation results are also presented; overall, the methods developed and utilized in this study have been shown to provide the local heat transfer coefficient in microchannels.

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This paper investigates the heat transfer performance of a radial microchannel heat exchanger with varying crosssectional-area channels. The thermal performance of axially varying cross-sectional-area channels is compared with uniform cross-sectional-area channels. The first model is a one-dimensional thermal-resistance based model, and the second model is a three-dimensional conjugate computational fluid dynamics analysis using FLUENT software. The heat sink has a footprint area of 3:5 cm 2 and the fluid flows radially inward. The inlet aspect ratio is varied from 0.4 to 1.0, and the outlet aspect ratio is fixed at 0.5. Inclusion of axial conduction effects are found to be imperative for accurate modeling of a radial configuration using the one-dimensional thermal-resistance model. The analysis shows that when constrained by a fixed channel-outlet area, increasing the channel-inlet area will improve the thermal performance. At low pumping powers, the present scheme is found to have thermal performance that is equivalent to or better than the performances with other experimentally and numerically investigated microchannel heat sink designs. Nomenclature A 3=4 = three-or four-wall-heated area Cp = specific heat capacity, kJ=kg K D h = hydraulic diameter, m f = friction factor H = height, m q 00 = heat flux, W=m 2 k = thermal conductivity, W=m K L = length, m _ m = mass flow rate, kg=s N chn = number of channels Nu = Nusselt number P = pressure, Pa Pr = Prandtl number R = thermal resistance, K=W Re = Reynolds number based on hydraulic diameter r = radial coordinate, m r = r o r=r o r i T = temperature, K V = velocity, m=s W = width, m z = streamwise coordinate, m z = z=L chn z = z=RePrD h = channel aspect ratio (width/height) 1 = unit cell angle 2 = channel angle = nondimensional temperature = dynamic viscosity, N s=m 2 = kinematic viscosity, m 2 =s = density, kg=m 3 Subscripts b = fluid bulk parameter chn = channel parameter i = inner radius parameter in = inlet parameter j = counter o = outer radius parameter out = outlet parameter w = wall parameter