Experimental investigation of heat transfer and pressure drop characteristics of ammonia-water in a mini-channel annulus

The ammonia-water mixture is commonly used in absorption and compression-resorption heat pumps. The heat transfer performance of a vertically oriented mini-channel annulus operated with an ammonia-water mixture under absorption conditions has been experimentally investigated. Heat exchangers comprised of annuli can be used in compression-resorption heat pumps. Measurements have been executed in a channel with a hydraulic diameter of 0.4 mm and a length of 0.8 m with an average eccentricity of 0.6. The experiments are used to determine the heat transfer coefficient and pressure drop during absorption for different operating conditions along the channel. The measured heat transfer coefficients vary from 1000 to 10,000 W m−2 K−1. Results are presented as function of heat flux, mass flux and vapor quality in order to investigate the dependency of heat transfer coefficients on the given variables. Mass flux is directly measured; vapor quality is obtained from equations of state with pressure and temperature at the inlet and outlet of each channel as input, assuming equilibrium conditions. The heat transfer coefficient increases with increasing mass flux, increasing inlet vapor quality and increasing heat flux. The heat transfer coefficient increases sharply between mass fluxes of 120 and 175 kg m−2 s−1 at low inlet vapor qualities and constant heat flux. The pressure drop shows an increasing trend with increasing mass flux and vapor inlet quality. The pressure drop measurements have been compared against empirical models from literature originally designed for tubes. One of these models is able to predict the measured pressure drop in the current channel within 25% deviation. The heat transfer performance was compared against empirical models from literature, which show very little agreement with the results from the experiments. The models are intended to predict condensation heat transfer in tubes, so they cannot fully take the annular geometry, eccentricity and mass transfer resistances into account, causing large discrepancies between predicted and experimental heat transfer coefficients.