Analysis of the condensation process and air maldistribution in finned tube and minichannel condensers

• Autores: Alessandro Pisano
• Directores de la Tesis: Santiago Martínez Ballester (dir. tes.), José Miguel Corberán Salvador (dir. tes.)
• Lectura: En la Universitat Politècnica de València ( España ) en 2017
• Idioma: español
• Tribunal Calificador de la Tesis: Alberto Coronas Salcedo (presid.), José Gonzálvez Macia (secret.), Wiebke Brix Markussen (voc.)
• Enlaces
  • Tesis en acceso abierto en: RiuNet
• Resumen

  • This PhD work has been dedicated to the improvement of the modelling of air condensers of both round tube and fins (RTPFs) and Minichannel technologies. The calculation platform employed is IMST-ART. This is a dedicated software for the design of refrigeration, air-conditioning and heat pump equipment following the vapor compression cycle. The model implemented in IMST-ART for condensers and evaporators is the combination of a segment-by-segment approach with the numerical method SEWTLE (Semi Explicit method for Wall Temperature Linked Equations) for the solution of the resulting system of equations.

    The target of the first part of this thesis was the comparative analysis of the empirical correlations aimed at the evaluation of the heat transfer coefficients and pressure drop in both the air and refrigerant sides of a condenser. The Literature review pointed out the presence of many studies concerning the condensation modelling. Therefore, after selecting the most interesting to compare, the first objective of this first part of the PhD became the identification of a suitable methodology for defining the best combination of correlations for the estimation of the thermo-hydraulic performance of the condensers. After an in-depth analysis of different possibilities, a well-defined methodology was identified as the best for the purpose. In the thesis, it was successfully applied to the identification of the best set of correlations for the heat transfer coefficients and friction factors for both the round-tube and minichannel condensers.

    The second part of the PhD was targeted to the improvement of the condensation modelling. In particular, the attention was focused on the analysis of the phenomena taking place at the beginning of the condensation process, when the superheated vapor finds the wall of the condenser being at a temperature lower than the refrigerant saturation temperature, i.e. convective condensation superheated vapor zone (CSH zone). It is well known that, in this zone, the condensation starts with some kind of droplet/thin film condensation on the walls. Afterwards, the bulk of the refrigerant flow reaches the saturation temperature and the condensation occurs at saturated conditions. Hence, the PhD thesis has been dedicated to the implementation in the general model for condensers (in IMST-ART software) of this CSH zone, which it was found to have an important effect on the prediction of the wall temperatures distribution in the tested air condensers. Two different numerical solutions were implemented and compared, i.e. Temperature and Enthalpy approaches, and validated against experimental results. Prediction results are very similar, thus the Enthalpy approach was selected because it required lower computational time.

    The final part of thesis was oriented towards the study of the effect of airflow maldistribution on the performance of air condensers. An innovative experimental methodology for generating and measuring any uneven air velocity profile at the inlet of a heat exchanger was first developed in a dedicated wind tunnel and then applied for the analysis of the performance degradation of one sample of condenser of each RTPFs and Minichannel technologies. Three different velocity profiles were produced and tested along a wide set of operating conditions, including different refrigerant charges and hence subcoolings. The experimental results showed that, although differences in wall temperature distribution were significant, the effect of air maldistribution on the performance of the two tested condensers was small. The improved model was validated against the experimental results and also showed little effect on condenser performance. Finally, the agreement between the results of the simulation and the experimental results was very satisfactory.