How to achieve full liquid conditions at the capillary tube inlet of a household refrigerator

[EN] The capillary tube with a liquid-to-suction heat exchanger (CT-LSHX) is a component that is widely used in household refrigerators. Recent works have indicated that even when measuring subcooled conditions at the condenser outlet, the condition at the capillary tube inlet is a two-phase flow. The present work was dedicated to analyzing the actual refrigerant conditions at the capillary tube inlet and to investigating how full liquid conditions could be achieved. The research was performed using a typical household refrigerator with corresponding fresh food and freezer compartments, replacing the original refrigerant-to-air condenser with a refrigerant-to-water condenser. This allowed, first, the condensation conditions to be controlled and, second, the estimation of the refrigerant conditions at the condenser outlet from the heat exchanger balance. The obtained results indicated the presence of a non-equilibrium two-phase flow, composed of subcooled vapor and subcooled liquid, at the capillary tube inlet, with both liquid and vapor entering the capillary tube as a vortex with small, fast fluctuations of the liquid level. This non-equilibrium indicated that the subcooling, evaluated from the pressure and temperature of the refrigerant at the condenser outlet, was only apparent and did not allow the evaluation of the actual enthalpy. Finally, by using a smaller capillary tube diameter and increasing the compressor speed, full liquid conditions at the capillary tube inlet were achieved. Furthermore, a performance comparison between the original and the new design revealed that the COP was higher with full liquid conditions.In this project, the work of Laetitia Bardoulet was partially supported by the Santiago Grisolia 2015 program, which is funded by the Generalitat Valenciana, with the reference number GRISOLIA/2015/021.Bardoulet, L.; Corberán, JM.; Santiago Martínez-Ballester (2019). How to achieve full liquid conditions at the capillary tube inlet of a household refrigerator. International Journal of Refrigeration. 100:265-273. https://doi.org/10.1016/j.ijrefrig.2019.02.006S26527310

Development of an experimental method to generate a non-uniform air flow distribution at the inlet of a heat exchanger

As already demonstrated by many authors, the non-uniformity of the air distribution at the inlet of both condensers and evaporators entails not negligible performance degradation. Its experimental quantification is mainly complicated by the difficulties connected to the air velocity measurement in a traditional air-conditioning installation. For this reason, many works are supported by the application of CFD models or others simulation tools. In order to analyze experimentally the effect of the nonuniform air distribution on the performance of a condenser, in this work a simple method for generating a specific air velocity profile at the inlet of a heat exchanger will be presented. It consist in allocating several filters along the heat exchanger height. Once characterized the filter, identifying the relation between the length of the filters, the pressure drop and the air velocity, any type of air velocity profile can be generated. The method presents high flexibility and allows being used with any typology of heat exchanger. In this work, the method will be used for generating a non-uniform air velocity profile at the inlet of two condenser: a round tube plate fins heat exchanger and a microchannel heat exchanger. The results show that, with a specific distribution of the filters, the nonuniform air velocity profile characterizing the A-shape condensation units has been reproduced with a very good agreement.The work of Alessandro Pisano on this project was partially supported by the Ministry for Economy and Finance of Spain, under the Training of Researcher program (FPI). Financial support from the Ministry for Economy and Finance of Spain, project numbers: DPI2011-26771-C02-01 is gratefully acknowledged

Two-dimensional numerical modeling for the air-side of minichannel evaporators accounting for partial dehumidification scenarios and tube-to-tube heat conduction

This paper presents a comparative study between the air-side heat transfer results of a two-dimensional numerical model for minichannel evaporators (Fin2D-W), and the results of the classical ε¿NTU approach. This study is carried out for different dehumidifying conditions, and for scenarios with different degree of temperature difference between tubes due to the refrigerant superheat. The Fin2D-W model solves the two-dimensional heat conduction in the fin wall considering also a 2D discretization for the air flow in contact with it. Thus, the presented model is able to capture the partial dehumidification scenarios and the effect of the heat conduction between tubes. The results analyze the differences, due to these phenomena, between the proposed model and the classical approach. Significant deviations between the two models are reported, especially in the cases of partially wet fin and high values of superheat, resulting in being up to ≈ 52% in total heat transfer.Abdelrahman H. Hassan is partially supported by the Santiago Grisolia 2012 program, which is funded by the Valencian Community under reference number: GRISOLIA/2012/005. Financial support from the Spanish Ministry of Economy and Finance, project number: DPI2011-26771-C02-01, is also gratefully acknowledged.Hassan, AHA.; Martínez Ballester, S.; Gonzálvez Maciá, J. (2016). Two-dimensional numerical modeling for the air-side of minichannel evaporators accounting for partial dehumidification scenarios and tube-to-tube heat conduction. International Journal of Refrigeration. 67:90-101. https://doi.org/10.1016/j.ijrefrig.2016.04.003S901016

A new moving boundary model for evaluating the performance of wet fins: Application to minichannel evaporators

[EN] In this paper, firstly, an innovative numerical model (Fin1D-MB) for wet fins of a minichannel evaporator was developed and verified. The proposed model is based on fin theory in conjunction with the moving boundary (MB) technique between wet and dry portions along the fin height. Secondly, a comparative study between the Fin1D-MB model and a comprehensive two-dimensional model (Fin2D-W) was implemented under different dehumidifying conditions. The results showed a good agreement between the two models in predicting the fin temperature profile and dehumidifying condition. However, the Fin1D-MB model saved computational cost by up to 95%, compared to the Fin2D-W model. Nevertheless, the relative deviations in the air-side heat transfer rates should be taken into consideration, especially in the partially wet fin region. The main source of these deviations is the assumption of uniform air temperature and humidity ratio along the fin height used by the Fin1D-MB model.The authors would like to gratefully acknowledge the financial support from the Spanish Ministry of Economy and Finance to project number DPI2011-26771-C02-01. The proofreading of this paper was funded by the Centro de Lenguas, Universitat Politecnica de Valencia.Hassan, A.; Martinez-Ballester, S.; Gonzálvez-Maciá, J. (2017). A new moving boundary model for evaluating the performance of wet fins: Application to minichannel evaporators. Applied Thermal Engineering. 127:566-579. https://doi.org/10.1016/j.applthermaleng.2017.08.055S56657912

Visualization of refrigerant flow at the capillary tube inlet of a high-efficiency household refrigerator

[EN] The subcooled condition at the condenser outlet ensures complete condensation, which is necessary in vapor compression systems to increase the cooling capacity and ensure the liquid conditions at the expansion device inlet. However, in household refrigerators, recent works indicate the presence of two-phase flow at the capillary tube inlet. These systems behave quite differently from other refrigeration systems due to the extremely low capacity In the present work, a test bench was built to visualize the refrigerant flow at the condenser outlet and at the capillary tube inlet of a commercial household refrigerator. A transparent tube replaced the end of the condenser and three transparent filters were installed with different orientations. Different positions of the capillary tube within the filters were also tested. Despite measuring a certain subcooling, all the reported visualizations showed that the capillary tube was steadily drawing in two-phase flow.Santiago Grisolia programme - Generalitat Valenciana GRISOLIA/2015/021Martínez-Ballester, S.; Bardoulet, L.; Pisano, A.; Corberán, JM. (2017). Visualization of refrigerant flow at the capillary tube inlet of a high-efficiency household refrigerator. International Journal of Refrigeration. 73:200-208. https://doi.org/10.1016/j.ijrefrig.2016.09.019S2002087

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

[EN] Single phase formulas based on logarithmic mean temperature difference (LMTD) or Effectiveness (E-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 condensersCorberán, JM.; Martínez-Ballester, S.; Gonzálvez-Maciá, J.; La-Barbera, C. (2016). Error estimation of single phase effectiveness and LMTD methodologies when applied to heat exchangers with phase change. Journal of Physics: Conference Series (Online). 745(3):1-8. https://doi.org/10.1088/1742-6596/745/3/032125S18745

Development and Validation of a Minichannel Evaporator Model under Different Dehumidifying Conditions

This paper presents a numerical model (Fin1D-MB) to predict the performance of minichannel evaporators under dehumidifying conditions. The presented model applies a segment-by-segment discretization to the evaporator, adding in each segment a novel bi-dimensional discretization to the fluids flow, fin, and tube wall. The Fin1D-MB model introduces a new approach to model the air-side heat transfer by using a composed function for the fin wall. This function is based on the fundamental one-dimensional fin theory and the technique of movable boundaries between wet and dry portions along the fin height. This modeling scheme allows capturing the heat conduction between tubes and different dehumidifying scenarios for the fin and tube. The proposed model was validated against experimental results for a minichannel evaporator operating with R134a at various test conditions. Generally, the numerical results were in good agreement with the measured data. The predicted inlet refrigerant and outlet air temperatures, refrigerant-side pressure drop, and cooling capacity were within ±0.5 oC, ±20%, and ±5% error bands, respectively

Evaluación del efecto del sobrecalentamiento en el modelado de la deshumidificación para un evaporador de minicanales

El trabajo actual presenta un modelo para el lado del aire de un evaporador de minicanales, empleando para las aletas un enfoque numérico más fundamental que otros modelos actuales. Esté enfoque numérico tiene en cuenta la conducción de calor 2D en cualquier elemento del tubo y aleta, por tanto no aplica la teoría de aleta, y permite una representación más detallada de las propiedades del aire. Después de la verificación con soluciones analíticas, se realizó un estudio comparativo entre el modelo propuesto Fin2D y el enfoque clásico de ε-NTU. La principal diferencia entre ambos enfoques es que el de ε-NTU asume la aleta como adiabática a mitad longitud. Ello se hace pese a la presencia de una diferencia de temperaturas entre los tubos de los extremos de la aleta, existente p.e. entre un tubo con sobrecalentamiento y otro evaporando. Este supuesto, que se utiliza extensamente, impide la conducción de calor entre los tubos a través de la aleta, resultando en una predicción incorrecta de la deshumidificación. Los resultados muestran desviaciones significativas en la transferencia de calor latente y total entre ambos enfoques, especialmente en el caso de la aleta parcialmente mojada y para valores altos del recalentamiento entre los tubos vecinos del evaporador.El trabajo de Abdelrahman Hussein Hassan en este proyecto ha sido parcialmente suportado por Generalitat Valenciana, bajo la programa de Santiago Grisolía 2012 con referencia GRISOLIA/2012/005. El apoyo económico del Ministerio de Economía y Competitividad de España, Numero de proyecto: DPI2011-26771-C02-01 es también ampliamente agradecido

Spatial Analysis of the Relationship between Mortality from Cardiovascular and Cerebrovascular Disease and Drinking Water Hardness

Previously published scientific papers have reported a negative correlation between drinking water hardness and cardiovascular mortality. Some ecologic and case–control studies suggest the protective effect of calcium and magnesium concentration in drinking water. In this article we present an analysis of this protective relationship in 538 municipalities of Comunidad Valenciana (Spain) from 1991–1998. We used the Spanish version of the Rapid Inquiry Facility (RIF) developed under the European Environment and Health Information System (EUROHEIS) research project. The strategy of analysis used in our study conforms to the exploratory nature of the RIF that is used as a tool to obtain quick and flexible insight into epidemiologic surveillance problems. This article describes the use of the RIF to explore possible associations between disease indicators and environmental factors. We used exposure analysis to assess the effect of both protective factors—calcium and magnesium—on mortality from cerebrovascular (ICD-9 430–438) and ischemic heart (ICD-9 410–414) diseases. This study provides statistical evidence of the relationship between mortality from cardiovascular diseases and hardness of drinking water. This relationship is stronger in cerebrovascular disease than in ischemic heart disease, is more pronounced for women than for men, and is more apparent with magnesium than with calcium concentration levels. Nevertheless, the protective nature of these two factors is not clearly established. Our results suggest the possibility of protectiveness but cannot be claimed as conclusive. The weak effects of these covariates make it difficult to separate them from the influence of socioeconomic and environmental factors. We have also performed disease mapping of standardized mortality ratios to detect clusters of municipalities with high risk. Further standardization by levels of calcium and magnesium in drinking water shows changes in the maps when we remove the effect of these covariates

El Mètode Ballester d'Aprenentatge

A la pràctica docent, no només convé tenir coneixement de la ciència específica, sinó també de l'evolució de la psicologia educativa, és a dir, de com aprenen els estudiants. La investigació en psicologia educativa més recent, i més rellevant des del punt de vista en què ens situem nosaltres, és la del constructivisme, iniciada a partir del psicòleg bielorús Lev Semionovitch Vigotski