An experimental investigation is carried out to study the heat transfer and pressure drop in the single-phase flow of water in a microchannel. The effect of dissolved gases on heat transfer and pressure drop is studied as the wall temperature approaches the saturation temperature of water, causing air and water vapor mixture to form bubbles on the heater surface. A set of six parallel microchannels, each approximately 200 micrometers square in cross section and fabricated in copper, with a hydraulic diameter of 207 micrometers, is used as the test section. Starting with air-saturated water at atmospheric pressure and temperature, the air content in the water is varied by vigorously boiling the water at elevated saturation pressures to provide different levels of dissolved air concentrations. The single-phase friction factor and heat transfer results are presented and compared with the available theoretical values. The friction factors for adiabatic cases match closely with the laminar single-phase friction factor predictions available for conventional-sized channels. The diabatic friction factor, after applying the correction for temperature dependent properties, also agrees well with the theoretical predictions. The Nusselt numbers, after applying the property corrections, are found to be below the theoretical values available in literature for constant temperature heating on all four sides. The disagreement is believed to be due to the three-sided heating employed in the current experiments. The effect of gas content on the heat transfer for the three gas concentrations is investigated. Nucleation was observed at a surface temperature of 90.5°C, for the reference case of 8.0 ppm. For the degassed cases (5.4 ppm and 1.8 ppm), nucleation is not observed until the surface temperature reached close to 100°C. An increase in heat transfer coefficient for surface temperatures above saturation is observed. However, a slight reduction in heat transfer is noted as the bubbles begin to nucleate. The presence of an attached bubble layer on the heating surface is believed to be responsible for this effect.
Single phase flow pressure drop and heat transfer in rectangular metallic microchannels
