SUMMARY

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 refrigerant (R134a) condensation study done on small hydraulic diameter (100e-6 < D < 160e-6 m) channels fabricated from Copper. A novel technique is used for the measurements 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 refrigerant R134a for 200 < G < 800 kg/m^2-s and 0 < x < 1 at four different saturation temperatures between 30 and 60 C. The results obtained from this study capture the effect of variation in hydraulic diameter, quality, saturation temperature and channel aspect ratio on the observed pressure drop and heat transfer coefficients. Existing pressure drop model show considerable scatter while the existing heat transfer models significantly under predict the heat transfer coefficients, observed in the current study. Based on the existing flow regime maps it was assumed that either the intermittent or annular flow regimes exist 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 diameter, saturation temperature, quality and channel aspect ratio. Both pressure drop and heat transfer coefficient increase with decrease in hydraulic diameter, increase in channel aspect ratio and decreases in saturation temperature. This study leads to a comprehensive understanding of condensation in microchannels for use in high-flux heat transfer applications.