A new equivalent Reynolds number model for vapor shear-controlled condensation inside smooth and micro-fin tubes

Vapor shear-controlled condensation inside smooth and micro-fin tubes is an important process for the refrigeration and air-conditioning industry. While many correlations have been developed to predict the condensing coefficient inside smooth tubes, most are not accurate over a wide range of diameters (1 to 20 mm) and reduced pressures. Currently, the Shah correlation is favored. Cavallini has recently developed a semi-empirical correlation for round microfin tubes. This correlation is based on existing, published data and is limited to the internal microfin dimensions of existing tubes. So far, no generalized theoretically based model has been developed for micro-fin tubes. Consequently, a new equivalent Reynolds number model to predict the condensation heat transfer coefficient in smooth and micro-fin tubes is proposed in this study. This was accomplished by theoretical and experimental approaches.; The equivalent Reynolds number model is based on the following concept: The two-phase flow of the vapor core and an annular liquid film can be replaced by an equivalent all liquid flow under the condition that the equivalent liquid flow produces the same wall shear stress as that of the two-phase flow. The resulting equivalent all-liquid Reynolds number can be used in a single-phase heat transfer correlation (for example, the Petukhov equation) to predict the condensation coefficient. A small correction factor is also applied to account for the different driving temperature differences used for condensation and single-phase flow.; In this study, R-134a, R-22 and R-404a condensation heat transfer and adiabatic two-phase pressure drop data were taken for three smooth tubes: two single copper tubes (3.25 and 6.20 mm ID) and an extruded aluminum tube (2.13 mm D{dollar}rmsb{lcub}h{rcub}).{dollar} These data plus the data collected from other sources, including smooth tube and micro-fin tube data (for R-32, R-125 and R-410a, etc.), were used to validate the equivalent Reynolds number model. The predictive ability of the Shah correlation for different tube diameters and different reduced pressures was checked, and a modified Shah correlation was proposed. A new correlation was developed to predict two-phase pressure gradients inside small diameter tubes and validated for 0.96 mm {dollar}leq{dollar} D {dollar}leq{dollar} 6.20 mm.