Bubble Temperature Effect on the Heat Transfer Performance of R449a During Flow Boiling Inside a Horizontal Smooth Tube

Since the Montreal Protocol (dated 1987), the reduction of the environmental impact has been one of the main goals in the HVAC sector, which has led to the replacement of widely used fluids with new environmentally friendly ones. Nevertheless, only new fluids with suitable heat transfer features can be used. The refrigerant mixture R449a, one of the fourth-generation refrigerants, was tested during flow boiling inside a horizontal smooth tube. The experiments were carried out at six different mass fluxes G ∈ [175;400] kg·m−2·s−1 and four different bubble temperatures Tb ∈ [2.5;10] °C, while the nominal values for inlet and outlet quality were selected as xTi = 0.1 and xTo = 0.9, respectively. The results highlighted that, as the bubble temperature increases, it has an opposite effect on the pressure drop per unit length and the heat transfer coefficient: the former decreases while the latter grows. The comparison between experimental results and the correlations showed that the Zhang and Webb formula provides the best prediction of pressure drop, while the models provided by Bertsch yield the most reliable predictions for the heat transfer coefficient. Nevertheless, for both quantities, other correlations with similar performances are available.