Thermal-fluid simulation of air-to-CO2 finned coil evaporator
The increasing global pressure to phase out CFC‘s and HFC‘s has resulted in research focusing again on the use of natural refrigerants. CO2 is one alternative that has very good thermal-fluid characteristics. However, a low critical point of 31.1 and 73.2 summons some challenges in the use of CO2 in vapour compression heat pump cycles. This necessitates the implementation of a trans-critical cycle, generating much higher pressures than conventional heat pumps. The development of these trans-critical CO2 heat pumps requires research and technical improvements in their components. One such component is the finned coil evaporator used to transfer energy from air to the refrigerant. To aid in the design and development of this component, simulation models are required. However, the accuracy of these models depends strongly on the empirical correlations implemented, and therefore the use of accurate heat transfer and pressure drop correlations are important. Since CO2 has some unique thermal-fluid characteristics, heat transfer and pressure drop correlations are still a prime research specific. Thus, the present study aimed to develop a simulation model that incorporates current and accurate refrigerant-, and moist air-, heat transfer and pressure drop correlations. Using the NIST software package, EVAP-COND, verification of the simulation model were achieved, where the largest difference in the prediction of heat transfer rate was 1.7% with EVAP-COND as reference. The discrepancies are attributed to the updated correlations used in the present study. The developed model predicted 92.6% of the EVAP-COND predictions, to within ±20%. The installation of a fully instrumented finned coil heat exchanger was done to upgrade an existing test bench. This enabled the generation of experimental data for a number of operating conditions and the validation of the simulation model with a maximum difference in heat transfer rate of 8.7%. However, the amount of lubricant in the system had significant detrimental effects on the heat transfer coefficient. A first order attempt at the implementation of a degradation factor in a modified simulation model had some success. In addition, large measurement uncertainties resulted in the experimental latent heat transfer rate data being disregarded. The simulation of five extra RH conditions in EVAP-COND and the developed simulation served as an addition to the experimental conditions, which showed that the simulation agrees with the trend of an increase in inlet relative humidity as reported from literature. The simulation model was able to predict 82% of the experimental data to within ±20%. The developed simulation identified dehumi-dification to occur under the same conditions as EVAP-COND. Also, the developed simulation calculates the saturation point in the same vicinity as found in both EVAP-COND and the experimental data. Good agreements between the data sets lead to the conclusion that the correlation of Wang et al. (2002) is applicable to the geometry of the wavy finned coil and that the Cheng et al. (2008a&b) correlation is applicable for the experimental ranges of this study. It is recommended that the effects of lubricant be included in a further model development while a validation over a wider range of operating conditions would be of great interest.
- Engineering