Theoretical and Experimental Evaluation of Microchannel Condensers Applied to Household Refrigerators

The aim of this work is to study, theoretically and experimentally, the impact of using forced-draft microchannel condensers in a specific model of household refrigerator. To this end, a mathematical model was developed to predict the condenser heat released rate and the air-side pressure drop as a function of geometry and operating conditions. This model was then added to an existing simulation platform of household refrigerators, in an attempt to study the effect of the heat exchanger on the appliance performance. Experiments were carried out in an open-type wind tunnel and in a climate-controlled test chamber, to validate both the condenser and the system mathematical models. It has been found that the predictions of the condenser model for the heat transfer rate and air-side pressure drop were kept within ±10% and ±20% error bands, respectively. It has also been found that the appliance energy consumption was predicted with maximum errors of ±3.5%. An extensive sensitivity analysis was also carried out revealing a potential drop of 13% in energy consumption with the use of a 20 passes, 200mm high, 180mm wide, 72mm deep, 200 fins/meter and 46 rectangular ports microchannel condenser

An Experimental Study of HC-600a Flow Through Variable Expansion Devices for Household Refrigerators

This work focuses on the HC-600a flow through small capacity fixed and variable expansion devices. To this end three electronic expansion valves of the same innovative design but with distinct orifices, were firstly tested in a dry nitrogen flow testing apparatus by varying the opening ratio an the operating conditions. Secondly, equivalent capillary tubes were defined based on the measured nitrogen mass flow rates through an updated version of the well know Kipp Schmidt’s equation. Thirdly, simulations were carried out with an in-house validated capillary tube model to estimate the HC-600a mass flow rate, using the length and inner diameter of the equivalent capillary tubes as input data. A test rig was also designed, constructed and used to measure the valve HC-600a mass flow rates under different operation conditions and opening ratios. Comparative analyses between the valve and capillary tube mass flow rates were then carried out under different conditions. In-depth knowledge about the two-phase capillary tube and valve flows were then used to explain the observed differences

Uma Metodologia para a seleção do par tubo capilar: carga de refrigerante que maximiza o desempenho de refrigeradores domésticos

Dissertação (mestrado) - Universidade Federal de Santa Catarina, Centro Tecnológico. Programa de Pós-Graduação em Engenharia MecânicaNo Brasil, o consumo de energia no setor residencial é liderado pelo segmento de refrigeração e condicionamento de ar, com uma participação percentual da ordem de 50%, o que corresponde a aproximadamente 10% de toda eletricidade produzida no país. Tal valor se deve tanto ao grande número de produtos no mercado como ao seu baixo desempenho termodinâmico. Em um sistema de refrigeração por compressão mecânica de vapor, largamente utilizado na refrigeração doméstica, o ponto de operação do sistema depende da interação entre seus componentes. As pressões de condensação e evaporação afetam diretamente a capacidade do compressor e vazão mássica fornecida pelo tubo capilar e vice-versa. Já a carga de refrigerante afeta tanto a pressão de condensação como a de evaporação, através do excesso ou da falta de fluido nos trocadores de calor. Diferentes autores mostraram que o coeficiente de desempenho de um refrigerador doméstico pode ser melhorado pela simples escolha do tubo capilar e da carga de refrigerante. Porém, há escassez de trabalhos na literatura com foco no efeito simultâneo de tais parâmetros. Neste trabalho desenvolveu-se um procedimento experimental que permite a variação da carga de refrigerante e da restrição do dispositivo de expansão em refrigeradores domésticos. Para tanto, utilizam-se um dispositivo de carga de fluido refrigerante e uma válvula de expansão micrométrica instalada em série com um tubo capilar de diâmetro superior ao original. O dispositivo de carga é constituído por um cilindro suspenso em uma célula de carga e por duas válvulas solenoides de ação direta, conectadas às linhas de sucção e descarga do compressor. A combinação "válvula + tubo capilar" permite aumentar ou restringir a abertura do dispositivo de expansão em relação à situação original. Um modelo numérico foi também desenvolvido para simular o escoamento de fluido refrigerante através de tubos capilares diabáticos e adiabáticos. Esse programa permitiu a determinação do diâmetro interno do tubo capilar que origina a mesma vazão mássica obtida com o conjunto válvula mais capilar. O consumo de energia foi medido em regime permanente, o que diminui substancialmente o tempo de ensaio, sem comprometer as análises realizadas. O ponto ótimo de operação do refrigerador foi obtido através do mapeamento do consumo de energia em todas as condições de operação impostas ao sistema. Ao todo, foram realizados 95 ensaios experimentais. Verificou-se que há uma ampla região de operação onde o consumo de energia é mínimo, região esta que engloba diversas combinações de tubo capilar e carga de refrigerante. Por sua vez, uma combinação inadequada de carga de refrigerante e restrição do dispositivo de expansão pode provocar aumentos de consumo de energia de até 30%. Por fim, propôs-se uma correlação empírica para estimar o consumo de energia de um refrigerador doméstico específico em função da restrição do dispositivo de expansão e da carga de refrigerante. Um procedimento, baseado num número reduzido de testes experimentais, foi também desenvolvido e validado para derivar tais parâmetros. Um algoritmo de minimização foi utilizado para determinar o ponto ótimo de operação.According to the 2007 National Brazilian Energy Consumption Habits Research, the refrigeration and air conditioning equipment are responsible by 50% of the total energy consumption of the residential sector, which correspond to approximately 10% of the Brazilian overall energy production. This figure highlights the importance of undertaking research activities focused on the energy consumption of household refrigerators. There is plenty of evidence in the literature showing that each system requires an optimum refrigerant charge in order to reach a maximum efficiency level. In other words, the energy consumption is not only affected by the system components but also by the refrigerant charge. In this study an experimental apparatus was developed to simultaneously vary the refrigerant charge and the restriction of the expansion device. This apparatus is comprised of a charging device connected to a modified household refrigerator with a larger inner diameter capillary tube, and a step motor actuated needle valve installed in series and before the capillary tube. The charging device is comprised of a refrigerant cylinder, a load cell and two solenoid valves connected to the compressor suction and discharge lines. The combination of needle valve plus capillary tube allows the modulation of the restriction of the expansion device to values lower or higher than that of the original system. A mathematical model to simulate the refrigerant flow through diabatic capillary tubes was developed, validated and used to calculate the internal diameter of a capillary tube which is equivalent in terms of mass flow rate to the restriction imposed by the capillary tube and needle valve combination. Steady-state energy consumption tests were carried out, varying the refrigerant charge and the restriction of the expansion device. Such an approach was adopted to speed up the process without losing the experimental tendency. A total of 95 measurements were made. The energy consumption was plotted in a contour map with the restriction of the expansion device on the x-axis and the refrigerant charge on the y-axis. It was shown that there is a wide region where the energy consumption reaches a minimum, which stretches over a range of expansion device restriction and refrigerant charge. It was also shown that an inappropriate combination of charge and expansion device can increase the energy consumption by up to 30%. Ultimately, an empirical correlation was proposed to estimate the energy consumption of a household refrigerator based on the restriction of the expansion device and refrigerant charge. A new methodology, based on a small amount of experimental data was proposed and validated. A minimization algorithm was also developed and used to identify the point of minimum energy consumption

Non-Condensable Gases and their Effect on the Dynamic Behavior of Domestic Refrigerators

The aim of this work was to experimentally investigate the effect of non-condensable gases on the thermal acoustic behavior of domestic refrigerators under dynamic conditions. To this end, an acrylic made filter dryer was installed in the system for a proper visualization of the refrigerant flow at the capillary inlet. An accelerometer was also installed at the evaporator inlet to capture vibrations/noise signals, caused by the refrigerant pattern at the inlet of the expansion device. During the experiments the energy consumption was measured with the system systematically doped with precise amounts of nitrogen. It has been found that the energy consumption increases by 13%, when a mass fraction of 0.43% of N2 is added to the system. Moreover, it has been found that the energy consumption increases almost linearly with the N2 mass fraction. Pull-down tests were also carried out, showing a clear relationship between the N2 amount and the power and mass flow rate oscillations, soon after the compressor start-up, caused by the unbalance between the compressor and capillary tube mass flow rates. It was also observed that the noise level is higher when liquid refrigerant enters the capillary, thus increasing the corresponding mass flow rate

A Design Approach for Liquid Separators Applied to Household Refrigerators

The objective of this study was to develop a methodology for designing a T-junction liquid-vapor separator for household refrigerators. The influence of the following independent parameters on the separator performance was studied: i) refrigerant mass flow rate, ii) inlet vapor quality, iii) evaporation temperature, iv) critical droplet diameter, v) critical bubble diameter and vi) suction ratio. For modeling purposes, the T-junction was divided into three sub-models: inlet, vapor and liquid branches. The inlet branch sub-model revealed that a proper phase separation occurs with larger diameters as the refrigerant mass flow rate and vapor quality increase. The dimensions of the vapor branch are governed by the droplet diameter, vapor mass flow rate and evaporation temperature while the dimensions of the liquid branch are dictated by concepts such as time of liquid degassing and residence time. Additionally, a testing facility was designed and constructed to allow the flow visualization through acrylic made T-junctions of distinct geometries. For a typical HC-600a domestic refrigeration condition a T-junction with an inlet diameter of 7 mm, a vertical diameter of 28 mm and total height of 40 mm was selected as one of the suitable options. The separator geometry and flow phenomena were experimentally investigated and associated with the refrigerating capacity of the fresh food and freezer compartments during closed door conditions

Theoretical and Experimental Analysis of Expansion Devices for Meso-Scale Cooling Systems

Nowadays there is a growing interest for compact cooling systems, the so-called meso-refrigerating systems, and thus there is also a need for innovative components which could be applied in these systems. The expansion device of meso-refrigerating systems must be properly sized not only to improve the system performance but also to work properly under varying load conditions. The aim of this work is to study the expansion process of HFC-134a through two distinct expansion devices in order to get a better understanding of the fluid flow phenomena and to support the development of mathematical models. To this end an experimental apparatus was designed and constructed in order to reproduce and control typical operating conditions of meso-cooling systems. The experiments were designed according to the factorial design technique. The first part of this work focused on small diameter adiabatic capillary tubes, when it was found that the capillary tube I.D. has the greatest impact on the refrigerant mass flow rate. It was also found that the developed model predicted 90% of the experimental data points within a band of error of ± 10%. Further experiments were carried out to study the effect of the ingestion of vapor bubbles on the mass flow rate. The second part of this work focused on pulsating capillary tubes – series association of a PWM-controlled metering valve, an intermediate chamber and an adiabatic capillary tube – when it was found that the refrigerant mass flow rate is greatly affected by the valve duty cycle. The fluid flow phenomenon for this particular expansion device was not well captured by the model which provided mass flow rate deviations of ±30 % and overestimated the pressure in the intermediate chamber

A Capillary Tube-Refrigerant Charge Design Methodology for Household Refrigerators-Part II: Equivalent Diameter and Test Procedure

In the first part of this work an experimental apparatus was designed and constructed to map the energy consumption of a household refrigerator subjected to several combinations of refrigerant charge and expansion restriction. In the second part, the expansion restriction imposed by the pair metering valve-capillary tube was converted into an equivalent tube diameter applying two different procedures: dry nitrogen flow and mathematical modeling. An empirical correlation to estimate the energy consumption based on the capillary tube inner diameter and refrigerant charge was also developed and used during the minimization process. Different strategies were also explored in order to reduce the amount of experiments to a minimum. It was found that at least 14 data points, collected with three different refrigerant charges, are required to ensure the convergence of the energy consumption minimization process

Um estudo sobre a aplicação de evaporadores de microcanais em refrigeradores frost-free

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia Mecânica, Florianópolis, 2021.A refrigeração doméstica enfrenta regulamentações energéticas cada vez mais rigorosas, o que estimula a constante busca por aumento de eficiência e redução de custos. O objetivo deste trabalho foi propor e avaliar um conceito de evaporador no-frost com a tecnologia de microcanais, atendendo às restrições e especificações de um refrigerador frost-free moderno. O trabalho foi dividido em quatro etapas: (i) avaliação isolada do evaporador, (ii) avaliação in loco sob condições secas, (iii) avaliação in loco sob condições de formação de geada e (iv) avaliação in loco sob operação de degelo. Na primeira etapa, o desempenho termo-hidráulico de dezesseis protótipos de evaporadores de microcanais foi caracterizado em um calorímetro túnel de vento, indicando o potencial da tecnologia a aplicações domésticas. Em seguida, o desempenho de um refrigerador frost-free dotado de evaporadores de microcanais foi avaliado sob condições secas, indicando aumentos na capacidade de refrigeração e reduções no consumo de energia quando comparados à tecnologia convencional. Na terceira etapa, os evaporadores foram submetidos a condições severas de formação de geada in loco, e um modelo matemático foi proposto e validado para otimizar o desempenho dos evaporadores tanto sob condições secas como úmidas. Os resultados indicaram que diversas configurações de evaporadores de microcanais superam o desempenho energético do tubo-aleta em condições secas, porém exigem processos de degelo mais frequentes. Neste contexto, a quarta e última etapa propôs e avaliou um novo conceito de degelo para evaporadores de microcanais. Os resultados mostraram que os evaporadores de microcanais podem atingir reduções no consumo de energia de até 2,6 % em comparação à tecnologia convencional, mesmo com um aumento da frequência de degelo. Além disso, desempenhos similares ao evaporador tubo-aleta foram alcançados com evaporadores de microcanais até 47 % menores em volume.Abstract: The domestic refrigeration sector is currently facing stricter energy regulations, which leads the industry to seek more cost-efficient components. This study proposed and evaluated a concept of no-frost evaporator using the so-called microchannel technology, which meets the restrictions and specifications of a modern frost-free refrigerator. The work was divided into four stages: (i) evaluation of the evaporator at component-level, (ii) in loco assessment under dry conditions, (iii) in loco assessment under wet conditions and (iv) in loco assessment under defrost. In the first stage, the thermo-hydraulic performance of sixteen microchannel evaporator prototypes was characterized in a wind tunnel calorimeter, indicating the high potential of such technology for domestic application. Then, in the second stage, the performance of a frost-free refrigerator with microchannel evaporators in place was evaluated under dry conditions, resulting in significant increases in cooling capacity and energy consumption savings when compared to finned-tube technology. In the third stage, the evaporators in place were subjected to severe frost formation conditions, and a mathematical model was proposed and validated to optimize the performance of the evaporators under both dry and wet conditions. The results showed that, in general, several microchannel evaporators geometries outperform the energetic performance of the finned-tube evaporators in dry conditions, however, they require more periodic defrosting processes. In this context, the fourth and final stage proposed and evaluated a new defrost concept for microchannel evaporators, allowing the evaluation of the overall system performance. Reductions in global energy consumption of up to 2.6 % have been achieved with microchannel evaporators. Similar finned-tube performances were achieved with microchannel evaporators geometries up to 47 % smaller

Evaporator Frosting in Refrigerating Appliances: Fundamentals and Applications

Modern refrigerators are equipped with fan-supplied evaporators often tailor-made to mitigate the impacts of frost accretion, not only in terms of frost blocking, which depletes the cooling capacity and therefore the refrigerator coefficient of performance (COP), but also to allow optimal defrosting, thereby avoiding the undesired consequences of condensate retention and additional thermal loads. Evaporator design for frosting conditions can be done either empirically through trial-and-error approaches or using simulation models suitable to predict the distribution of the frost mass along the finned coil. Albeit the former is mandatory for robustness verification prior to product approval, it has been advocated that the latter speeds up the design process and reduces the costs of the engineering undertaking. Therefore, this article is aimed at summarizing the required foundations for the design of efficient evaporators and defrosting systems with minimized performance impacts due to frosting. The thermodynamics, and the heat and mass transfer principles involved in the frost nucleation, growth, and densification phenomena are presented. The thermophysical properties of frost, such as density and thermal conductivity, are discussed, and their relationship with refrigeration operating conditions are established. A first-principles model is presented to predict the growth of the frost layer on the evaporator surface as a function of geometric and operating conditions. The relation between the microscopic properties of frost and their macroscopic effects on the evaporator thermo-hydraulic performance is established and confirmed with experimental evidence. Furthermore, different defrost strategies are compared, and the concept of optimal defrost is formulated. Finally, the results are used to analyze the efficiency of the defrost operation based on the net cooling capacity of the refrigeration system for different duty cycles and evaporator geometries