Heat transfer and pressure drop during condensation of refrigerants in microchannels

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. Condensation in such channels has been studied by few investigators, although heat removal and rejection applications can benefit from high heat flux condensation. Most of the studies on small diameter channels have used isothermal air-water mixtures to simulate two-phase flow. However, due to the adiabatic flow in these studies, the results are not directly applicable to phase-change situations. In the current study, small hydraulic diameter (100 < Dh < 160 mum) channels were fabricated on a copper substrate by electroforming copper on to a mask patterned by X-ray lithography. The channels were sealed using diffusion bonding, which ensures leak proof flow at saturation pressures as high as 10 MPa. Measurements of local condensation heat transfer coefficients in small quality increments have typically been found to be difficult due to the low heat transfer rates at the small flow rates in these microchannels. In the current study, a novel measurement technique was used to address this issue. Subcooled refrigerant (R134a) was supplied to a precisely controlled electric heater that preconditions the refrigerant to the desired quality, followed by condensation in the test section. Further downstream, another precisely controlled electric heater was used to heat the refrigerant to a superheated state. Energy balances on the pre- and post-heaters were used to establish the refrigerant inlet and outlet states at the test section. Cooling of the refrigerant in the test section was accomplished using water at a high flow rate to ensure that the condensation side presents the governing thermal resistance. The water-side temperature was controlled to obtain the desired incremental condensation rates. This method was 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 60°C.; The measured heat transfer coefficients and pressure drops are analyzed and compared with the limited heat transfer and pressure drop models available in the literature for similar flow conditions and explanations for agreements/disagreements with the proposed study are provided. Based on the available flow regime maps in the literature, it was concluded that either the intermittent, or the annular flow regime will predominate for the channels and the flow conditions under consideration. Based on these flow regimes, internally consistent condensation heat transfer and pressure drop models were developed. The proposed pressure drop and heat transfer models predict 95% and 94% of the data within +/-25%. The proposed models were then used to analyze the effect of various parameters like mass flux, saturation temperature, aspect ratio and diameter. As the mass flux increases, both the pressure drop and heat transfer increase due to an increase in flow velocities. As the saturation temperature decreases, the void fraction increases due to a decrease in the vapor to liquid density ratio. This increase in void fraction leads to an increase in flow velocities, which in turn leads to an increase in pressure drop and heat transfer. As the aspect ratio increases, both the pressure drop and heat transfer coefficients increase due to an increased occurrence of slugs. As the channel hydraulic diameter decreases, the pressure drop and heat transfer coefficient increase due to a decrease in film thickness and channel diameter.; The results from the current study thus make an important contribution to the understanding of pressure drop and heat transfer mechanisms during condensation in microchannels. The proposed model may be used by engineers for analyzing condensing two-phase flow in microchannels.