Finned-tube heat exchanger, Validation, Correlation, Condensation.
As already demonstrated by others authors, when the performance of a heat exchanger is analyzed, a semi-empirical model allows getting good prediction of the experimental results provided that it is accompanied by the application of the suitable correlations for calculating heat transfer coefficients (HTC) and pressure drop (PD) in both refrigerant and air side. Many correlations for calculating these coefficients are available in literature, therefore choose the more suitable of them turns out to be not an easy task. This paper presents a discussion about the methodology to perform a comparison aimed to define the criterion for choosing the most suitable correlation to be used in a simulation model for prediction of heat exchangers performance in refrigeration systems. Differently from others works, where the results of each correlation are compared with those obtained by means specific experiments focused on the local phenomenon visualization, in this work the correlations are compared against the measurements of a complete finned-tube condenser integrated in a traditional air-to-water heat pump. The comparison has been carried out using a model able to discretize the heat exchanger through a finite volume method (FVM). The discussion is supported with a case study which was experimentally tested and numerically modeled. The condenser is a finned tube coil that characterized by two asymmetric circuits. The tube diameter is 7 mm and the fin pitch is 2.6 mm. The experimental test bench includes a single-speed 34.3 cm3 reciprocating-hermetic compressor, a plate evaporator and an electronic valve as expansion device. An accumulator allow adjusting the condenser sub-cooling. A specific test campaign has been designed in order to cover a wide range operating points of the condenser: different air velocities (from 1.5 m/s to 4 m/s), inlet temperatures (20-46°C) and sub-cooling (0-5-10°C) have been taken into account. All the correlations considered were programmed and simulated with the software IMST-ART. In order to avoid the overlapping of the effects of the different correlations, during the analysis of the two-phase flow, the heat transferred in the one-phase section of the exchanger and the air-side heat transfer coefficient were maintained constant. The analysis of air-side heat transfer coefficient and pressure drop was carried out in the same way but keeping constant the coefficients of the refrigerant side. Different accuracy parameters were defined in order to assist the final decision about which correlation is able to provide the best agreement with experimental data. The paper aims to present a discussion about the validation methodology, the analysis of the correlations impact on the model results rather than define the parameters to choose the best correlation.