Silicon Microchannel Heat Sink Research Articles

Characteristics of Flow Boiling Oscillations in Silicon Microchannel Heat Sinks

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.

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.

Measurements and high-speed visualizations of flow boiling of a dielectric fluid in a silicon microchannel heat sink

Experiments were conducted to investigate flow boiling heat transfer to a dielectric fluid in a silicon chip-integrated microchannel heat sink. Twenty-four microchannels, each 389 μm × 389 μm in cross-section, were fabricated into the 12.7 mm × 12.7 mm silicon substrate. High-speed visualizations (at 12,500 frames per second) were performed simultaneously with heat transfer and pressure drop measurements to investigate the physics of flow boiling in parallel microchannel arrays. At low heat fluxes, bubbly flow is dominant, with the bubbles coalescing to form vapor slugs as the heat flux is increased. At high heat fluxes, the flow regimes in the downstream portion of the microchannels are characteristic of alternating wispy-annular flow and churn flow, while flow reversal is observed in the upstream region near the microchannel inlet. Local heat transfer measurements, obtained at three flow rates ranging from 35 to 60 ml/min, show that at lower heat fluxes, the heat transfer coefficient increases with increasing heat flux. The heat transfer coefficient in fully developed boiling is seen to be independent of flow rate in this range. At higher heat fluxes (exceeding 542, 673, 730 kW/m 2, respectively, for flow rates of 35, 47 and 60 ml/min), this trend is reversed, and the heat transfer coefficient decreases with further increases in heat flux due to partial dryout in some of the microchannels. Heat fluxes at which fully developed boiling is achieved depend on the flow rate. The pressure drop in fully developed boiling increases with increasing heat flux and is independent of flow rate for the test conditions considered in this work.

Experimental microchannel heat sink performance studies using nanofluids

In this study, microchannel heat sink (MCHS) performance using nanofluids as coolants is addressed. We first carried out a simple theoretical analysis that indicated more energy and lower MCHS wall temperature could be obtained under the assumption that heat transfer could be enhanced by the presence of nanoparticles. Experiments were then performed to verify the theoretical predictions. A silicon MCHS was made and CuO–H 2O mixtures without a dispersion agent were used as the coolants. The CuO particle volume fraction was in the range of 0.2 to 0.4%. It was found that nanofluid-cooled MCHS could absorb more energy than water-cooled MCHS when the flow rate was low. For high flow rates, the heat transfer was dominated by the volume flow rate and nanoparticles did not contribute to the extra heat absorption. The measured MCHS wall temperature variations agreed with the theoretical prediction for low flow rate. For high flow rate, the measured MCHS wall temperatures did not completely agree with the theoretical prediction due to the particle agglomeration and deposition. It was also found that raising the nanofluid bulk temperature could prevent the particles from being agglomerated into larger scale particle clusters. The experimental result also indicated that only slightly increase in pressure drop due to the presence of nanoparticles in MCHS operation.

Analysis of microchannel heat sink performance using nanofluids

In this study, silicon microchannel heat sink performance using nanofluids as coolants was analyzed. The nanofluid was a mixture of pure water and nanoscale Cu particles with various volume fractions. The heat transfer and friction coefficients required in the analysis were based on theoretical models and experimental correlations. In the theoretical model, nanofluid was treated as a single-phase fluid. In the experimental correlation, thermal dispersion due to particle random motion was included. The microchannel heat sink performances for two specific geometries, one with W ch = W fin = 100 μm and L ch = 300 μm, the other with W ch = W fin = 57 μm and L ch = 365 μm, were examined. Because of the increased thermal conductivity and thermal dispersion effects, it was found that the performances were greatly improved for these two specific geometries when nanofluids were used as the coolants. In addition to heat transfer enhancement, the existence of nanoparticles in the fluid did not produce extra pressure drop because of small particle size and low particle volume fraction.

Microscale heat transfer enhancement using thermal boundary layer redeveloping concept

We demonstrated a new silicon microchannel heat sink, composing of parallel longitudinal microchannels and several transverse microchannels, which separate the whole flow length into several independent zones, in which the thermal boundary layer is in developing. The redeveloping flow is repeated for all of the independent zones thus the overall heat transfer is greatly enhanced. Meanwhile, the pressure drops are decreased compared with the conventional microchannel heat sink. Both benefits of enhanced heat transfer and decreased pressure drop ensure the possibility to use “larger” hydraulic diameter of the microchannels so that less pumping power is needed, which are attractive for high heat flux chip cooling. The above idea fulfilled in microscale is verified by a set of experiments. The local chip temperature and Nusselt numbers are obtained using a high resolution Infrared Radiator Imaging system. Preliminary explanation is given on the decreased pressure drop while enhancing heat transfer. The dimensionless control parameter that guides the new heat sink design and the prospective of the new heat sink are discussed.

Improved heat sinking for laser-diode arrays using microchannels in CVD diamond

This work proposes a novel cooling system for high-power laser-diode arrays, for which the maximum optical output power density per unit surface area is limited by the temperature rise due to self-heating. The proposed system uses a microchannel heat sink made of chemical-vapor-deposited (CVD) diamond, whose high thermal conductivity increases the efficiency of the channel-wall fins and reduces the array-to-coolant thermal resistance using a simple model for the combined conduction and convection problem. The resistance is calculated to be 75% less than that for a conventional configuration using a silicon microchannel heat sink. The present analysis strongly motivates a future experimental study.

High-power continuous wave 690 nm AlGaInP laser-diode arrays

High-power diode laser arrays emitting at 690 nm have been developed for solid-state laser pumping. The laser diode bars (fill factor ≊0.7) have been fabricated from single quantum well AlGaInP-based heterostructures. Using silicon microchannel heatsinks, a record high 360 W/cm2 per emitting aperture is achieved under continuous wave operation.

Two-dimensional surface emitting distributed feedback laser arrays

The demonstration of two-dimensional arrays of surface-emitting distributed feedback lasers is reported. A 30-element array is shown to produce up to 6.8 W of pulsed output power. A 27-element array bonded on a silicon microchannel water-cooled heat sink is shown to produce as much as 3 W of continuous-wave output power. The high continuous wave power is possible due to the low thermal resistance in the p-side down design of the surface-emitting distributed feedback lasers and effective heat extraction by a silicon microchannel heat sink.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>