Experimental and mathematical modelling studies of a compact vapor compression refrigeration system

In this thesis, heat transfer phenomena in a cooling system with vapor compression refrigeration cycle is investigated through experimental and mathematical modeling studies. Experimental studies were carried out in a compact cooling system using R134a as refrigerant. The effect of air velocities on evaporator and condenser on the system performance is experimentally considered. A mathematical model is developed to calculate instantaneous temperature values of system components at three different air velocities on heat exchangers. The generated model is solved by using MathCAD with the evaporation and condensation pressures being taken from the experimental data. The obtained values are compared with the temperature outputs from experiments. On the other hand, the temperature changes depending on time of the air in the cooling volume and a material placed in the volume was obtained from experiments and mathematical models, and these curves of the temperature changes were compared. Vapor compression refrigeration system in which the experimental works are performed has an hermetic compressor, expansion valve, evaporator, evaporator fan, condenser and condenser fan as basic components. A capillary tube is used as an expansion valve for the refrigerator. As the evaporator and condenser, plate-fin heat exchangers with staggered tube arrangement that are frequently used in household refrigeration applications were used. This cooling system is a compact structure located at the bottom of the cooling volume. The cooling volume, which has a constant volume, is wanted to be cooled to 5°C from room temperature and maintained around this temperature. Cooling operation is provided by the cold air taken from the evaporator located under the cooling volume using the evaporator fan. The cold air is delivered to the cooled cabinet by the air blowing duct, and after receiving some heat from the cabinet, it is conveyed back to the evaporator by the air suction duct. In experimental studies, firstly temperature measurements are made for certain points in the refrigeration cycle components, the volume to be cooled and the air ducts by connecting thermoelements. Simultaneously conducted in the same experiment with temperature measurements, measurements of the power are taken by the system to determine the performance of the cooling system. These measurements are repeated for six different combinations by changing the supply voltage of the evaporator and condenser fans. The flow rate of the evaporator fan, a parameter that affects the performance of the cooling system, is measured for fan supply voltage values of 6V, 9V and 12V by using a setup designed for the blowing duct. In addition, a load is placed on the cooling system for a single fan supply voltage combination to examine the temperature change of the load and the cooling volume. In these experiments, a load package with similar thermal properties of a foodstuff (fillet steak) was used as the load. Apart from the temperature change experiments, two different mass loss experiments are carried out in which foodstuff (fillet steak) and water are placed in the system. These experiments are carried out by continuously measuring the mass using a precision balance placed in the cooling volume. In the section where the Mathematical Model is created, the cooling volume is first investigated. It is aimed to obtain the time-dependent cooling curves of the cooling volume and material placed in the volume. For this reason, two differential equations are established in accordance with the energy balance for the control volumes of the cooling volume and the cooled material. In these equations, the temperature of the air entering the cabinet (cooling volume) is considered constant. On the other hand, it is assumed that the air at the exit of the cabinet is the function of the time and equal to the temperature of the air inside the cabinet. The heat gain from the piping system of the refrigeration cycle is neglected. The natural and forced convection expressions were used to determine the heat gain from the cabinet. The inside temperature of the cabin and the temperature of the material are taken as 24.5 °C (room temperature), the initial conditions. The two equations are solved in the MathCAD program with these initial conditions and assumptions to obtain the cooling curves of the air and the material inside the cabinet. Two different solutions have been obtained for the cases in which the material placed in the system was a load package used in experiments and was real foodstuff. In the case of real food material, the heat transfer due to transferred moisture to the air from the food should also be considered. For this reason, the amount of heat transfer due to water evaporation is added to the energy balance equations established for the control volumes. The other mathematical model is created for the whole refrigeration cycle in order to obtain instantaneous tempreratures of the components of the cycle. The model created by making suitable assumptions for all components are solved using the MathCAD program. As the models are constructed in this section, the evaporation and condensation pressures are taken from the experimental datas as inputs. The number of revolutions, power and displacement volume of the compressor are taken from the catalog information in order to determine mass flow rate circulating in the cycle.In the compressor model, it is assumed that the compression process is adiabatic and reversible. The expansion process in the capillary tube is assumed to be adiabatic, and the enthalpies at the inlet and outlet of the component are considered to be equal. In the models of the condenser and the evaporator, similar expressions are used because both heat exchangers are plate-fin heat exchangers with staggered tube arrangement. The correlation used to find the heat transfer coefficient in external flow side for this type of heat exchanger is the same for the both components. On the internal flow side, a correlation that is valid for the cases in which condensation occurs in circular tubes is used in the part of the condenser where condensation occurs, In the phase-change part of the evaporator, a correlation is selected which is valid for the case of evaporation in straight-line pipelines. On the other hand, in the parts of both components where the phase of the refrigerant does not change, the same correlation obtained from the literature was used. In the mathematical model, all dimensions of the heat exchangers were taken as input. After calculating the fin efficiencies of the heat exchangers, the heat transfer quantities in these components were determined using the ε-NTU method. Thus exit enthalpies have been obtained. As the evaporation and condensation pressures are known, the temperatures at the exits of the components were obtained. In the last part of the thesis, the amount of energy consumed by the system, run-time and some performance values obtained from the experimental studies for the different flow rates of the evaporator fan of the cooling system are examined. According to the performance outputs, it can be said that as the air velocity on the evaporator increases, the runtime of the system decreases. In other words, the time the system runs in a certain period of time is decreasing. Therefore the duration of reaching the desired temperature of cooling volume is reduced so that the amount of energy consumed decreases during this period. In a similar way, it is seen that the run-time is also effective in the amount of energy that the systems consume after they have stabilized. Then, the cooling curves obtained from the experiment and the mathematical model are compared for the load package in the cooling volume. On the other hand, cooling curves of the load package and a real foodstuff (fillet steak) are compared using the results obtained from the model. It is observed that the temperature of the load pack placed at room temperature reaches about 6 °C after 7 hours, the temperature of the foodstuff placed at the same temperature reaches about 9°C because of the amount of moisture passing from the food to the cooling volume. Finally, the temperature values of the cycle components.obtained from the experiments are compared with the values obtained from the mathematical model for the three different evaporator fan voltage combinations. Although there are some differences between the data obtained for some points due to the assumptions made for the cycle components, very similar results have been achieved from both studies. It is observed that as the air velocity on the evaporator increases, the temperature range that the system operates also increases.