Santiago Martínez-Ballester

A comparative study between a two-dimensional numerical minichannel evaporator model and a classical effectiveness–NTU approach under different dehumidifying conditions

In this article, a two-dimensional numerical model for a minichannel evaporator is implemented. This model takes into account the variation of wall temperature and moist air properties in both longitudinal and transverse directions. The verification of the current model is done with an analytical effectiveness–number of transfer units (ϵ–NTU) approach, and the results of the two approaches show very good agreement and consistency. Different refrigeration and air-conditioning applications have been chosen that represent various inlet conditions to the evaporator. A range of tube temperatures has also been selected to allow different dehumidifying scenarios for the tube and fin. A comparative study is conducted between the current model results and the results of the traditional ϵ–NTU method based on two different models of simultaneous heat and mass transfer. Significant deviations in results between the current model and the traditional ϵ–NTU approach are found, especially in latent heat transfer. These deviations are mainly due to the assumptions that are normally adopted by the ϵ–NTU method and fin theory, such as no variation in moist air temperature and humidity ratio along the direction between tubes, no accounting for partially wet fin conditions, and finally, the assumption of a constant average saturation line slope within a specific evaporator segment.

Error estimation of single phase effectiveness and LMTD methodologies when applied to heat exchangers with phase change

Single phase formulas based on logarithmic mean temperature difference (LMTD) or Effectiveness (ɛ-NTU) are widely and wrongly used for the thermal analysis of evaporators and condensers. Those formulas do not take into account that temperature variation during phase change is due to pressure variation and/or concentration changes when using nonazeotropic refrigerant mixtures. This paper first presents the correct evaluation of the mean temperature difference and effectiveness, for parallel and counter flow arrangements, under the hypothesis of linear temperature variation, for both evaporator and condenser cases. Then, the analytical solution is employed to evaluate the error of applying the single phase formulas of LMTD and Effectiveness to the phase change part of evaporators and condensers.

A discussion about the methodology for validating a model of a finned-tube condenser considering different correlations for the heat transfer coefficients and pressure drop

The selection of suitable correlations for calculating heat transfer coefficients and pressure drop plays a fundamental role in the use of semi-empirical models for the simulation of the performance of a heat exchanger. Therefore, a discussion about the best way for validating a condenser model and choosing the best set of correlations for both the heat transfer coefficients and pressure drop is presented. The studies were performed for both the air and refrigerant side in a round tube and plate fin condenser. A test campaign was specially designed to cover a wide range of key parameters, such as air velocity, condensation temperature, refrigerant mass flow rate, and condenser subcooling. The options for defining the boundary conditions in the model and the accuracy metrics are discussed in detail, allowing the identification of the most suitable correlations. By using this set of correlations, the prediction error is within an error band of ±0.4°C for the condensation temperature and ±0.6% in terms of capacity.