Critical Heat Flux During Flow Boiling in Microchannels: A Parametric Study

The application of flow boiling in microchannels in copper cooling elements for very high heat flux dissipation from microprocessor chips is one of the promising technologies to replace air cooling and water cooling of these units, particularly in mainframes and servers. Recently, the authors have proposed a new theoretical model to predict the critical heat flux (CHF) in microchannels, and it is used here to perform a parametric study to investigate the effects of fluid, saturation temperature, mass flux, inlet subcooling, microchannel diameter, and heated length on CHF for this application. The parametric study shows that CHF is increased by: (i) decreasing channel length, (ii) lowering saturation temperature, (iii) increasing mass flux, (iv) increasing inlet subcooling, and (v) increasing microchannel diameter. The best coolant is water, but water is not feasible for the present application because of its very low saturation pressure at 30-40C. Of the other four fluids simulated, their order of merit from best to worst is as follows: R-245fa, R-134a, R-236fa, and FC-72. FC-72, however, has a low saturation pressure (in fact, it would operate under vacuum at the saturation temperatures of 30-40C envisioned here) and is not a candidate fluid for the flow boiling coolant here. Furthermore, the authors have also recently proposed a diabatic flow map for microchannels based on their database for R-134a and R-245fa in 0.5- and 0.8-mm channels. The new CHF model has been incorporated into their map here to predict the transition from annular flow to dry-out, which is a critical design limitation for microprocessor coolers. Importantly, this map then provides the feasible operating range of such coolers with flow boiling as the cooling process, in terms of mass flux and maximum vapor quality at the outlet to avoid CHF

Characterization and prediction of two-phase flow regimes in miniature tubes

Visual observations of two-phase regimes for R134a and R245fa flowing in 0.509 mm and 0.790 mm horizontal tubes are documented and compared to the predictions of the analytical flow regime models available in the literature. Annular flow was found to dominate the behavior of these two miniature channels, with a significant Slug flow regime at intermediate qualities. Despite the horizontal orientation of these tubes, there were no observations of Stratified flow and a very limited region of Bubble flow. A comparison of the more than 2200 flow regime observations to the predictions of the Taitel-Dukler flow regime methodology revealed that 67% of the empirically observed flow pattern data were correctly identified. The Ullmann-Brauner model, based on an air-water database, correctly predicted the appropriate flow regime for 81% of the reported data. Proposed modifications in the Bubble-to-Slug and Slug-to-Annular transition criteria, respectively, were shown to provide a modest further improvement in the overall predictability to 90% of the observed data for the two refrigerants studied. (C) 2010 Elsevier Ltd. All rights reserved

Prediction of symmetry during intermittent and annular horizontal two-phase flows

An optical method is developed for a 2.95 mm inner diameter circular mini-channel to estimate liquid film thicknesses. The greyscale pictures obtained with a high-speed camera are processed to determine the liquid-vapour interface positions for annular and intermittent flows. The experiments are thus performed for a large range of flow conditions. The saturation temperatures tested ranged from 20 °C to 100 °C in steps of 10 °C and the mass velocities are 50, 100, 200, 300 and 400 kg m−2 s−1. A new parameter ranging from 0 to 1, the symmetry, is defined to account for the level of non uniformity of liquid distribution around the tube perimeter. New experimental data are presented (270 data points), that cover a range of symmetry parameter from 0.35 to 1.00. These data are merged with those available in the literature (406 data points with symmetry parameter from 0.71 to 1.00). A sensitivity analysis of the dimensionless numbers of major influence is run and a new correlation is proposed, that enables to predict over 90% of the data points in an error bandwidth of 10%. This correlation is proposed as criterion for asymmetry

Two-phase heat transfer measurements of R254fa at high saturation temperatures in horizontal mini-channels

Heat transfer measurements for R254fa were conducted. The heat transfer coefficient was determined for a smooth stainless steel tube with an inner tube diameter of 3 mm. The experiments were conducted for five mass fluxes (100, 300, 500, 700 and 1000 kg/(m².s)), three heat fluxes (10, 30 and 50W/m²) and at three saturation temperatures (40°C, 70°C and 125°C). The experiments were used to determine the influence of the saturation temperature, mass flux, heat flux and vapour quality on the heat transfer coefficient. At a low saturation temperature, the heat transfer coefficient increases with an increasing mass flux. However, at a high saturation temperature the heat transfer coefficient decreases with an increasing mass flux. Furthermore, the heat transfer coefficient increases with increasing vapour quality at a low saturation temperature. On the contrary, the heat transfer coefficient decreases at higher saturation temperatures

Experimental testing of a low-temperature organic Rankine cycle (ORC) engine coupled with concentrating PV/thermal collectors: Laboratory and field tests

International audienceA detailed experimental investigation of a small-scale low-temperature organic Rankine cycle (ORC) with R-404A is presented. The tests are first conducted at laboratory conditions for detailed evaluation of the main components at both design and off-design conditions, for variable heat input up to 48 kW th and hot water temperature in the range of 65e100 C. A scroll compressor in reverse operation is used as expansion machine and a dedicated helical coil heat exchanger is installed, suitable for high-pressure and temperature operation. The ORC pump is a diaphragm pump coupled with an induction motor. The rotational speeds of both the expander and pump are regulated with frequency inverters, in order to have the full control of the engine operation. The ORC has been then connected with concentrating PV/ thermal collectors, which produce electricity and heat and provide it to the ORC. These field tests are also presented with the overall focus on the performance of the whole ORC unit and its power contribution to the solar field. The tests have revealed that such low-temperature ORC unit can have adequate efficiency and that its coupling with a solar field is feasible, increasing the power production of the whole system