Measurement of Condensation Heat Transfer Coefficients in Microchannel Tubes

Abstract

Abstract A technique for the measurement of condensation heat transfer coefficients in microchannels is reported. The high heat transfer coefficients and low mass flow rates in microchannels make it difficult to accurately measure these coefficients. The requirements for accurate heat duty measurement are in direct conflict with the requirements for deducing the heat transfer coefficients from measured temperatures and flow rates. In addition, measurement of local condensation heat transfer coefficients in small increments of quality is difficult to accomplish due to the low heat transfer rates for such quality changes. The present work reports a technique that addresses these requirements. The inlet and outlet qualities to a microchannel test section are measured through energy balances on a pre- and post-condenser. The test section is cooled using water at a high flow rate to ensure that the condensation side presents the governing thermal resistance. Heat exchange with a secondary cooling water stream at a much lower flow rate is used to obtain a large temperature difference, which is in turn used to measure the condensation duty. Local heat transfer coefficients are therefore measured in small increments for the entire saturated vapor-liquid region. This technique is demonstrated using a square microchannel geometry with a hydraulic diameter of 0.76 mm. Heat transfer coefficients for the condensation of refrigerant R134a in this geometry range from 2,110–10,640 W/m2–K over the mass flux range 150 < G < 750 kg/m2–s.