Análisis numérico de los mapas de patrones de flujo del refrigerante R600a en un colector/evaporador de una bomba de calor asistida por energía solar

This research work presents a detailed description of the flow patterns maps generated in a horizontal pipe of the collector/evaporator of a direct-expansion solar-assisted heat pump, using R600a refrigerant as working fluid. The study was performed in a pipe with an internal diameter of 3.8 mm and a length of 1000 mm, mass velocities varying between 197.59 and 267.26 kg·m-2·s-1 and heat flux between 72.83 and 488.27 W·m-2; during the experimental tests, an incident solar radiation between 0 and 652.9 W·m-2 was present. The Wojtan, Ursenbacher and Thome correlation was considered for the analysis and the model used does not require iterative calculations; moreover, it carries out a detailed analysis of the different zones present along the pipe. The predominant zones in this study are intermittent, annular and dryout, found in the five tests, however, due to the working conditions in all tests with the exception of test A, mist and stratified-wavy flow were found.En la presente investigación se detallan los mapas de patrones de flujo que se generan en una tubería horizontal de un colector/evaporador componente de una bomba de calor de expansión directa asistida por energía solar, utilizando el refrigerante R600a como fluido de trabajo. El estudio se realizó en una tubería de 3,8 mm de diámetro interno y 1000 mm de longitud, las velocidades de masa variaron entre 197,59 y 267,26 kg·m-2·s-1, el flujo de calor entre 72,83 y 488,27 W·m-2, durante las pruebas experimentales se presentó una radiación solar incidente entre 0 y 652,9 W·m-2. Se consideró para el análisis la correlación de Wojtan, Ursenbacher y Thome, el modelo utilizado no requiere de cálculos iterativos, además, plantea un análisis detallado de las diferentes zonas presentes a lo largo de la tubería, evidenciando una mayor precisión en los resultados. Las zonas predominantes en los resultados de este estudio son intermitente, anular y secado, encontrados en las cinco pruebas, sin embargo, por las condiciones de trabajo en todas las pruebas a excepción de la prueba A, se encontró el flujo niebla y estratificado-ondulado

Análisis numérico de un sistema de calentamiento de agua utilizando un colector solar de placa plana

The objective of the present research work was to carry out a numerical analysis by means of CFD of a flat plate solar collector, in addition to a comparison with experimental results. The working fluid reached a maximum outlet temperature of 20.16 °C at 12:00, the value of solar radiation was determined for the geographical coordinates latitude -0.2252 and longitude -77.84; similarly, at this time it was possible to obtain a temperature of 27.12 °C on the collector surface, as peak value. The lowest performance of the heat transfer device was determined at 10:00 with an outlet water temperature and maximum temperature on the collector surface of 18.65 and 20.48 °C, respectively. The experimental results showed a maximum temperature of 20.93 °C and a minimum temperature of 19.4 °C, resulting in a 4.01 % error between the computational simulation and the experimental data.El objetivo de la presente investigación fue realizar un análisis numérico mediante CFD de un colector solar de placa plana; además, se realizó una comparación con resultados experimentales. El fluido de trabajo alcanzó una temperatura máxima de salida de 20.16 °C a las 12:00, el valor de la radiación solar se determinó para las coordenadas geográficas latitud –0.2252 y longitud –77.84; de forma similar, en este horario fue posible obtener una temperatura de la superficie del colector de 27.12 °C, como valor pico. Se determinó el menor rendimiento del dispositivo de transferencia de calor a las 10:00 con un valor de temperatura de salida del agua y temperatura máxima en la superficie del colector de 18.65 y 20.48 °C, respectivamente. Los resultados experimentales mostraron una temperatura máxima de 20.93 °C y una temperatura mínima de19.4 °C, derivando en un error de 4.01 % entre la simulación computacional y los datos experimentales

Evaluation of Correlations of Flow Boiling Heat Transfer of R600a in a Flat-Plate Solar Collector/Evaporator

In the current research work, the behavior of the boiling heat transfer coefficient in a flat-plate solar collector/evaporator, component of a direct-expansion solar-assisted heat pump (DX-SAHP) was studied using a hydrocarbon refrigerant with zero Ozone Depletion Potential (ODP) and low Global Warming Potential (GWP). The main dimensions of the collector/evaporator are 0.8, 3.8, and 1000 mm of the fin thickness, internal diameter, and length, respectively. Five experimental tests were realized at different times of the day for obtaining the results, with mass velocities varying between 197.59 and 267.26 kg·m-2·s-1, and the heat flux having values between 72.83 and 488.27 W·m-2. The operating values in tests, such as refrigerant pressure and temperature, were taken in a built prototype. The numerical analysis was carried out considering different correlations proposed by Chen, Wojtan, and Kattan. The Wojtan mathematical model offered the best projection of the heat transfer effect for the different transition zones of a two-phase flow along the pipeline. The boiling heat transfer coefficients had approximate maximum values of 8.2, 8.5, 7.8, 6.7, and 5.8 kW·m-2·K-1 for the A, B, C, D, and E tests prediction by Wojtan. Moreover, the boiling heat transfer coefficients increased as the mass velocity enhanced and the rise of vapor quality as the mass velocity was fixed. In this study, the effect of solar radiation, vapor quality on the measured heat transfer coefficient was analyzed

CFD Analysis of Heat Transfer Enhancement in a Flat-Plate Solar Collector with Different Geometric Variations in the Superficial Section

Nowadays, there is an increasing need for improving the inefficient ways for obtaining thermal energy from renewable sources to fulfil the industrial and typical needs in heat transfer processes that may be covered using solar assisted heat pumps due to their appropriate performance in the thermal energy transfer process. To improve the efficiency of the collector/evaporator by increasing the heat flux to the refrigerant, in this research, a numerical and computational fluid dynamics (CFD) analysis is conducted with geometrical variations in the surface section of a collector/evaporator. The performance was compared to the results of a base case, replicating its limit and environmental conditions such as the initial temperature of 5.5 °C, incident solar radiation of 464.1 W·m-2, the operating temperature of 17.6 °C and other parameters. The surface geometrical variations involved in this study show a surface area similar to the base case. However, different lengths of the fluid path were considered due to the new geometrical shapes represented with less thermal resistances and correct distribution of the fluid in the collector/evaporator, obtaining temperature variations of 3.78, 5.47, 5.56 °C and a maximum value of 5.63 °C, including the corresponding variation of the heat flux. Considering the geometric changes in the superficial section of a flat-plate solar collector, it is possible to implement these variations in different kinds of heat exchangers in order to analyze the efficiency in these devices and the impact in the global systems where the heat exchangers are used