Two-phase Heat Transfer In Microchannels Research Articles

Single- and Two-Phase Thermal Transport in Microchannels With Embedded Staggered Herringbone Mixers

Improving mixing is an effective means to enhance single- and two-phase heat transfer in microchannels. However, it is challenging to induce since the flow in microchannels is laminar in the most working conditions. We report that heat transfer rate and critical heat flux (CHF) on 1-methoxyheptafluoropropane (HFE-7000) can be significantly enhanced by patterning embed- ded micromixers on the bottom walls in a parallel silicon microchannel array, which consists of five parallel channels (height, width, length: 250 µm × 220 µm × 10 mm). Compared with a plain-wall microchannel array at a mass flux range of 1018 to 2206 kg/m 2 · s and a heat flux range of 10 to 198 W/cm 2 ,s ingle- phase heat transfer rate, two-phase heat transfer rate, and CHF are enhanced up to 221%, 160%, and 61% using microscale staggered herringbone mixers in microchannels, respectively. These mixers consist of 7 or 3.5 Hz with 12 staggered herringbone grooves (50 µm in depth and width) with 90° between two asym- metric arms in each cycle. Its asymmetry is defined in accordance with the off center position of the apex of the herringbone groove. Finally, experimental results suggest that the locations and coverage of the micromixers have significant impacts on both single and two-phase heat transfer in microchannels.(2013-0372)

Effect of Flow Pulsation on the Heat Transfer Performance of a Minichannel Heat Sink

Heat sinks with liquid forced convection in microchannels are targeted for cooling electronic devices with a high dissipated power density. Given the inherent stability problems associated with two-phase microchannel heat transfer, this paper investigates experimentally the potential for enhancing single-phase convection cooling rates by applying pulsating flow. To this end, a pulsator device is developed which allows independent continuous control of pulsation amplitude and frequency. For a single minichannel geometry (1.9 mm hydraulic diameter) and a wide range of parameters (steady and pulsating Reynolds number, Womersley number), experimental results are presented for the overall heat transfer enhancement compared to the steady flow case. Enhancement factors up to 40% are observed for the investigated parameter range (Reynolds number between 100 and 650, ratio of pulsating to steady Reynolds number between 0.002 and 3, Womersley number between 6 and 17). Two regimes can be discerned: for low pulsation amplitude (corresponding to a ratio of pulsating to steady Reynolds number below 0.2), a small heat transfer reduction is observed similar to earlier analytical and numerical predictions. For higher amplitudes, a significant heat transfer enhancement is observed with a good correspondence to a power law correlation. This work establishes a reference case for future studies of the effect of flow unsteadiness in small scale heat sinks.

Assessment of the effectiveness of nanofluids for single-phase and two-phase heat transfer in micro-channels

Experiments were performed to explore the micro-channel cooling benefits of water-based nanofluids containing small concentrations of Al 2 O 3 . The high thermal conductivity of nanoparticles is shown to enhance the single-phase heat transfer coefficient, especially for laminar flow. Higher heat transfer coefficients were achieved mostly in the entrance region of micro-channels. However, the enhancement was weaker in the fully developed region, proving that nanoparticles have an appreciable effect on thermal boundary layer development. Higher concentrations also produced greater sensitivity to heat flux. Despite this enhancement, the overall cooling effectiveness of nanoparticles was quite miniscule because of the large axial temperature rise associated with the decreased specific heat for the nanofluid compared to the base fluid. For two-phase cooling, nanoparticles caused catastrophic failure by depositing into large clusters near the channel exit due to localized evaporation once boiling commenced. These and other practical disadvantages bring into question the overall merit of using nanofluids in micro-channel heat sinks.