Heat Transfer and Pressure Drop During Condensation of Refrigerants in Microchannels
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Two-phase flow, boiling, and condensation in microchannels have received considerable attention in the recent past due to the growing interest in the high heat fluxes made possible by these channels. This dissertation presents a study on the condensation of refrigerant R134a in small hydraulic diameter (100 < Dh < 160 mm) channels. A novel technique is used for the measurement of local condensation heat transfer coefficients in small quality increments, which has typically been found to be difficult due to the low heat transfer rates at the small flow rates in these microchannels. This method is used to accurately determine pressure drop and heat transfer coefficients for mass fluxes between 300 and 800 kg/m2-s and quality 0 < x < 1 at four different saturation temperatures between 30 and 60oC. The results obtained from this study capture the effect of variations in mass flux, quality, saturation temperature, hydraulic diameter, and channel aspect ratio on the observed pressure drop and heat transfer coefficients. Based on the available flow regime maps, it was assumed that either the intermittent or annular flow regimes prevail in these channels for the flow conditions under consideration. Internally consistent pressure drop and heat transfer models are proposed taking into account the effect of mass flux, quality, saturation temperature, hydraulic diameter, and channel aspect ratio. The proposed models predict 95% and 94% of the pressure drop and heat transfer data within ±25%, respectively. Both pressure drop and heat transfer coefficient increase with a decrease in hydraulic diameter, increase in channel aspect ratio and decrease in saturation temperature. A new non-dimensional parameter termed Annular Flow Factor is also introduced to quantify the predominance of intermittent or annular flow in the channels as the geometric parameters and operating conditions change. This study leads to a comprehensive understanding of condensation in microchannels for use in high-flux heat transfer applications.