Improved flow pattern map for accurate prediction of the heat transfer coefficients during condensation of R-134a in smooth horizontal tubes and within the low-mass flux range

This paper presents an improved flow pattern map for predicting the heat transfer coefficients during condensation of R-134a inside a smooth horizontal tube. Experimental tests were conducted over the low-mass flux range of 75-300 kg/m² s, at a nominal saturation temperature of 40 °C, and with the test section vapour qualities ranging from 0.76 down to 0.03. This represents points within the annular, intermittent and stratified flow regimes. The results were used to modify the Thome-El Hajal flow pattern map to include a transition region between the stratified-wavy and annular or intermittent flow regimes. The revised flow pattern-based heat transfer correlation predicted the experimental data to a mean deviation of less than 6%.