SUMMARY

Two-phase flow, evaporation, and boiling 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. In the proposed study, small hydraulic diameter (100 < Dh < 200 m) channels are fabricated on a copper substrate by electroforming copper on to a mask patterned by X-ray lithography and are sealed using diffusion bonding to ensures leak proof operation up to 10 MPa. Measurements of local condensation heat transfer coefficients in small quality increments has typically been found to be difficult due to the low heat transfer rates at the small flow rates in these microchannels. In the proposed study, a novel measurement technique will be used to address this issue. Subcooled refrigerant (R134a) will be supplied to a precisely controlled electric heater that pre-conditions the refrigerant to the desired quality, followed by condensation in the test section. Further downstream, another precisely controlled electric heater will be used to heat the refrigerant to a superheated state. Energy balances on the pre- and post-heaters will be used to establish the refrigerant inlet and outlet states at the test section. Cooling of the refrigerant in the test section will be accomplished using water at a high flow rate to ensure that the condensation side presents the governing thermal resistance. The water-side temperature will be controlled to obtain the desired incremental condensation rates. This method will be used to accurately determine heat transfer coefficients for refrigerant R134a for 200 < G < 800 kg/m2-s and 0 < x < 1 at four different saturation temperatures between 30 and 60oC. Condensation heat transfer and pressure drop models will be developed using flow regime maps for similar flows available in the literature. The proposed study will lead to a comprehensive understanding of condensation in microchannels for use in high-flux heat transfer applications.