Influence of lubricant oil on heat transfer performance of refrigerant flow boiling inside small diameter tubes. Part II: Correlations

The predictive ability of the available state-of-the-art heat transfer correlations of refrigerant–oil mixture is evaluated with the present experiment data of small tubes with inside diameter of 6.34 mm and 2.50 mm. Most of these correlations can be used to predict the heat transfer coefficient of 6.34 mm tube, but none of them can predict heat transfer coefficient of 2.50 mm tube satisfactorily. A new correlation of two-phase heat transfer multiplier with local properties of refrigerant–oil mixture is developed. This correlation approaches the actual physical mechanism of flow boiling heat transfer of refrigerant–oil mixture and can reflect the actual co-existing conditions of refrigerant and lubricant oil. More than 90% of the experiment data of both test tubes have less than ±20% deviation from the prediction values of the new correlations.