Condensation heat transfer in minichannels: A review of available correlations

Heat exchangers with enhanced performance are demanded in various engineering applications. Very often heat transfer devices are requested to guarantee not only high heat transfer coefficients but also small size and weight, thus limiting the charge of the operative fluid inside the heat exchanger. In order to increase the performance of condensers and to properly design new heat exchangers, it is essential to have predictive tools that are validated with experimental data. Sometimes, even well established semi-empirical correlations can be inaccurate in some microscale flow conditions or with new refrigerants. The present paper starts from the experimental database measured during condensation with different fluids (pure fluids and refrigerant blends) inside small channels (hydraulic diameter around 1 mm) at the Department of Industrial Engineering of the University of Padova. A critical review of available correlations for heat transfer during condensation in minichannels is presented. Predictions of heat transfer coefficients obtained applying selected models are compared with the experimental database that covers various refrigerants: hydrofluorocarbons (HFC, i.e. R32 and R134a), new hydrofluoroolefins (HFOs) with low global warming potential (R1234ze(E)), natural refrigerants (hydrocarbons such as propane) and zeotropic refrigerant blends of HFCs and HFOs (R32/R1234ze(E)). Refrigerant mixtures are studied because for some applications they may be a proper solution. For instance in the air-conditioning industry there are not dropin pure fluids to replace the high global warming potential (GWP) fluids currently employed (e.g. R410A). Unfortunately, the design of condensers working with zeotropic mixtures poses the additional problem to account for the mass transfer resistance that leads to a penalization of the heat transfer coefficient. Experimental data are necessary for the assessment of predicting correlations that can be used with these new refrigerants blends.