Analysis of the Drop-in Operation of a Refrigeration System by the Response Surface Methodology

The objective of this study is to evaluate the gains in a drop-in process of refrigerants in a basic refrigeration system using the Response Surface Methodology to optimization. A central composite design and analysis of variance are also used. In addition, the model will be analyzed and compared to experimental results, consolidating various parameters to get the best regions in a broad universe of options, with a reduced number of experiments. The response surface methodology is a set of statistical tool, which describes the entire region of interest through experimental specific points, by appropriate polynomial equation. The experimental results were extracted from a cooling test bench compound for a reciprocating compressor, evaporator, condenser and expansion valve. R22, R290, R1270, R438A, R404A, R134a, R410A and R32 were tested at the same experimental apparatus, the lubrificating oil was replaced in some cases. The higher value of coefficient of performance and cooling capacity were investigated. The results obtained with the MSR technique showed good agreement with the experimental work, (R2 exceeding 0.8) which indicates, good representation of the output variables of the cooling system. The tested optimization tools were able to identify areas of maximum, allowing for better comparison of results obtained by different refrigerants

Experimental comparison of a cascade refrigeration system operating with R744/R134a and R744/R404a.

This study evaluates the performance of a cascade system in subcritical operation using the pair R744 / R134, as an option to conventional systems in supermarkets, which usually uses R404A, or R22. The experimental apparatus consists of a variable speed reciprocating compressor for R744 and an electronic expansion valve that promotes direct evaporation of the CO2 inside a cold room (2.3m x 2.6m x 2.5m) to maintain the internal air temperature stable. The high-temperature cycle consists of a reciprocating compressor for R134a, a thermostatic expansion valve, and an air-cooled condenser. A plate heat exchanger, which is at the same time, the condenser for the R744 and evaporator to R134a completes the setup. Two parameters were manipulated: The superheating degree of the R744, 5-20 K, and the R744 compressor operation frequency, 40-65 Hz. In order to contribute to the improvement of the cooling processes, mainly about the sustainability and energy efficiency, a drop-in has been made at the high-temperature cycle, whose R134a load has been replaced by R404A. The alternative cooling system (R744 / R404A) was subjected to two stages of tests which allowed the energy comparison between the two refrigerant sets. Through the obtained results, it was estimated maximum COP equivalent to 1.36 and a minimum value of 1.06 for the R744 / R134a pair, demonstrating the applicability of this cascade system of variable thermal load conditions, the values ​​of capacity cooling, settled between 4.09 and 5.13 kW. The minimum value of the air temperature within the cold room was - 28 °C and -5 °C the maximum. Finally, it was found that the results obtained for R744 / R404A pair attended the air temperature condition inside the cold room with similar COP values. However, R744 / R404A pair operated at lower refrigeration capacities, these settled between 2.16 and 3.00 kW. This reduction is due to the large difference between the volumetric cooling capacity values of these HFCs in question, resulting in adaptation problems of the R404A to the compressor and the expansion mechanism

Photothermal conversion efficiency of nanofluids: An experimental and numerical study

This work investigated experimentally the photothermal conversion efficiency (PTE) of gold nanofluids in a cylindrical tube under natural solar irradiation conditions, which was also compared with a developed 3-D numerical model. The PTE of gold nanofluids was found to be much higher than that of pure water, and increased non-linearly with the nanoparticle concentration, reaching 76.0% at a concentration of 5.8 ppm. Significant non-uniform temperature distribution was identified both experimentally and numerically, and a large uncertainty can be produced in the PTE calculation by using only one-point temperature measurement. A mathematical model was also established to calculate the solar absorption efficiency without knowing the temperature field within the nanofluids, which can be used to predict the theoretical PTE for nanofluids based on their optical properties only

Thermal-Chemical Characteristics of Al-Cu Alloy Nanoparticles

This work investigated the oxidation, ignition, and thermal reactivity of alloy nanoparticles of aluminum and copper (nAlCu) using simultaneous thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC) method. The microstructure of the particles was characterized with a scanning electron microscope (SEM) and transmission electron microscope (TEM), and the elemental composition of the particles before and after the oxidation was investigated with energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The particles were heated from room temperature to 1200 °C under different heating rates from 2 to 30 K/min in the presence of air. The complete oxidation process of the nAlCu was characterized by two exothermic and two endothermic reactions, and the reaction paths up to 1200 °C were proposed. An early ignition of nAlCu, in the temperature around 565 °C, was found at heating rates ≥ 8 K/min. The eutectic melting temperature of nAlCu was identified at ∼546 °C, which played a critical role in the early ignition. The comparison of the reactivity with that of pure Al nanoparticles showed that the nAlCu was more reactive through alloying

An experimental study of convective flow boiling of refrigerants inside smooth and microfin tubes

A presente pesquisa trata de um estudo experimental da transferência de calor e da perda de carga de fluidos refrigerantes puros e suas misturas em mudança de fase convectiva no interior de tubos lisos e aqueles dotados de ranhuras internas. Para tanto, foi desenvolvido um equipamento experimental cujo componente básico é composto por um tubo horizontal, aquecido por intermédio de uma resistência elétrica do tipo fita, aderida à superfície externa do tubo. As condições de ensaio variaram numa ampla faixa, permitindo cobrir as condições verificadas na maioria das instalações frigoríficas. Os resultados experimentais foram agrupados em duas faixas de velocidades mássicas: elevadas (G > ou = 200 kg/s.m2), onde prepondera o padrão anular de escoamento, e reduzidas (G < 200 kg/s.m2), predominando o padrão estratificado. Os principais parâmetros que afetam o coeficiente de transferência de calor, tais como, velocidade mássica, fluxo de calor, tipo de refrigerante, temperatura de evaporação e diâmetro do tubo foram analisados. O desempenho termo-hidráulico, relativo ao efeito combinado da transferência de calor e da perda de carga dos tubos ranhurados, foi sensivelmente superior quando comparados aos tubos lisos. A análise dos resultados experimentais permitiu a proposição de correlações para a perda da carga, avaliada através do multiplicador bifásico, &#966L, e para coeficiente de transferência de calor, em tubos lisos e ranhurados. As correlações propostas se mostraram adequadas para aplicações práticas, proporcionando desvios reduzidos em relação aos resultados experimentais. Destacam-se as correlações obtidas para o multiplicador bifásico para tubos microaletados e para o coeficiente de transferência de calor para vazões reduzidas em tubos lisos. Diversos registros fotográficos dos principais padrões de escoamento foram levantados, tendo sido importante na análise e entendimento da mudança de fase.Present research deals with an experimental study of the heat transfer and pressure drop of pure and mixtures of refrigerants undergoing convective boiling inside horizontal smooth and microfin tubes. An experimental apparatus has been developed and constructed whose main component is a horizontal tube electrically heated. Experimental results have been grouped into two mass velocity ranges: the one corresponding to mass velocities lower than 200 kg/s.m2, where the stratified flow pattern is dominant, and that for mass velocities higher than 200 kg/s.m2, where typically the annular flow pattern can be found. Effects over the heat transfer coefficient of physical parameters such as mass velocity, heat flux, diameter, saturation temperature, and refrigerant have been investigated and analyzed. It has been found out that the thermo-hydraulic performance of microfin tubes is better than that of the smooth ones. Empirical correlations have been proposed for both the two-phase flow multiplier and the heat transfer coefficient for different ranges of operating conditions as well as for smooth and microfin tubes. Results from the proposed correlations can be deemed adequate for practical applications given the limited dispersion obtained with respect to their experimental counterpart. Noteworthy are the results obtained from correlations for both the two phase flow multiplier for microfin tubes and the heat transfer coefficient for the lower range of mass velocities in smooth tubes. Finally, worth mentioning is the photographic essay developed in present research involving the flow patterns that occur under convective boiling of refrigerants in horizontal tubes

Design and evaluation of an experimental refrigeration system working by solar adsorption

Este trabajo presenta una caracterización de un sistema de refrigeración empleando la radiación solar como única fuente de energía para generar el fenómeno de la adsorción asistido en un colector estacionario tipo placa plana. Este colector presenta un área de captación de 1.1m2 trabajando con carbón activado como adsorbato y empleando metanol como adsorbente. Los resultados experimentales permitieron compararse en dos condiciones de trabajo cuya finalidad fue enfriar una pequeña cantidad de agua reservada de 2 litros. Se registraron temperaturas de generación en el colector solar por encima de los 50°C para intensidades solares por encima de los 800 [watt/m2]. El COP del sistema se presentó por encima de los 0,4 según etapas de enfriamiento y adsorción isobárica. Se realizaron mediciones de radiación solar, se tomaron mediciones de temperatura del sistema en funcionamiento, dando como resultados diferencias de temperatura de hasta 8,6 °C entre la temperatura ambiente y la temperatura de la recámara de frio. Se tomaron mediciones de radiación local de hasta 1155 [watt/m2], dando resultados de eficiencia de nuestro colector de hasta 75% en horas específicas y de 35% en eficiencia total diaria. Aprovechando herramientas a nivel de diseño de experimentos (DOE), por medio de superficies de respuestas se estimó un modelo del comportamiento en equilibrio termodinámico que se utilizará como referencia en futuros sistema de control.This work presents a characterization of a cooling system using solar radiation as the only source of energy to generate the adsorption phenomenon assisted in a stationary flat plate collector. This collector has an area of 1.1m2 working with activated charcoal as adsorbate and using methanol as an adsorbent. The experimental results were compared in two conditions of tests with the objective to cool two liters of water. Temperatures of generation were recorded in the solar collector above 50 °C for solar radiation above 800 [W/m2]. The COP of the system presented values above 0.4, according to isobaric cooling and adsorption stages. Measurements of solar radiation, temperatures of the system were performed, resulting in temperature differences of up to 8.6 °C between the ambient temperature and the cold room. Local radiation samples of up to 1155 [W/m2] were obtained, showing efficiency of the collector up to 75%, at specific periods and 35% at total daily efficiency. Using the Design of Experiments (DOE), through response surfaces, a model of behavior in thermodynamic equilibrium to be used as a reference in futures control systems