The pressure effects of expanding the cross section of microchannels along the direction of flow are investigated across four rates of channel expansion in the flow boiling of R-134a. Prior investigation by the authors detailed the fabrication of four copper microchannel arrays and the pumped-loop apparatus developed to facilitate interchange of the microchannel specimens, allowing consistency across experiments. Significant beneficial pressure effects are observed to result from the expansion, including reduction by half of the pumping cost per flow rate at critical heat flux. The improvements are seen with small expansions, and greater expansion yields diminishing returns. The high pressure drops associated with microchannel evaporators are effectively reduced by expanding channel geometry, and the low-frequency system spectral response indicates that expanding channel arrays typically carry less energy in oscillations up to 2.5 Hz, suggesting amelioration of oscillatory instabilities. Results are discussed in light of a comparative force analysis, with the balance of these forces linked to the observed behavior of the pressure drop and heat flux relationship.

This work describes the experimental setup, method, and results of utilizing a micrometer to move an adjustable orifice immediately in front of an array of microchannels. Research by others indicates potential for significant improvement in delaying critical heat flux and increasing heat transfer coefficients when placing an orifice in front of each individual channel of a microchannel array. The experimental setup in this work allows incremental orifice size changes. This ability allows the experimentalist to find which orifice size provides enough pressure drop immediately in front of the channels to reduce oscillations. The design also allows for rapid change of orifice size without having to remove and replace any components of the test setup. Signal analysis methods were used to identify frequency and amplitude of pressure and temperature oscillations. Low mass flux experiments (300 kg m−2 s−1 and 600 kg m−2 s−1 of R134a in a pumped loop) showed reduced channel wall temperatures with smaller orifice sizes. The orifice concept was found to be more effective at reducing oscillations for the higher 600 kg m−2 s−1 flow rate.

This study discusses the simulation of flow boiling in a microchannel and numerically predicts the effects of channel geometry variation along the flow direction. Experimental studies by Pan and collaborators and suggestions from Mukherjee and Kandlikar have generated interest in expanding the cross section of a microchannel to improve boiling heat transfer. The motivation for this geometry change is discussed, constraints and model selection are reviewed, and Revellín and Thome's critical heal flux criterion is used to bound the simulation, via MATLAB, of separated flow in a heated channel. The multiphase convective heat-transfer coefficient is extracted from these results using Qu and Mudawar' s relationship and is compared to reported experimental values. Expanding channel geometry permits higher heat rates before reaching critical heat flux.

The effect of an expanding microchannel cross-section onflow boiling critical heat flux (CHF) is experimentally investigated across four rates of expansion. A pumped-loop apparatus is developed to boil R-134a in an array of microchanneis cut into copper; a test section is designed to facilitate interchange of the microchannel specimens, allowing consistency across experiments. An optimum expansion angle allowing maximum heat flux is observed, the location of which increases with the mass flow rate. The boiling number does not indicate any optimum in the range observed, showing a nearly monotonie increase with expansion angle. The familiar increase in critical heat flux with mass flux is observed, though expansion shifts the CHF-massflux curves in a favorable direction. The existence of an optimum expansion angle confirms an earlier qualitative hypothesis by the authors and suggests that microchannel heat sinks offer opportunities for methodical improvement of flow boiling stability and performance.

This work describes the experimental setup, method, and results of utilizing a micrometer to move an adjustable orifice immediately in front of an array of microchannels. Research by others indicates potential for significant improvement in delaying critical heat flux and increasing heat transfer coefficients when placing an orifice in front of each individual channel of a microchannel array. The experimental setup in this work allows incremental orifice size changes. This ability allows the experimentalist to find which orifice size provides enough pressure drop immediately in front of the channels to reduce oscillations. The design also allows for rapid change of orifice size without having to remove and replace any components of the test setup. Signal analysis methods were used to identify frequency and amplitude of pressure and temperature oscillations. Low mass flux experiments (300 kg m−2 s−1 and 600 kg m−2 s−1 of R134a in a pumped loop) showed reduced channel wall temperatures with smaller orifice sizes. The orifice concept was found to be effective at reducing oscillations for the higher 600 kg m−2 s−1 flow rate, but the data indicate that wall temperature reduction with inlet orifice use is not solely due to elimination of oscillations. Signal analysis was an effective method of identifying oscillations without the availability of pictorial representation of flow patterns in the channels.