دراسة توزيع الزعانف وترتيبها على أداء وحدة التخزين الحراري

Investigation of Fin Distribution and Arrangement on the Performance of Thermal Storage Unit

Abstract

Recently, there has been a significant trend to use renewable energies such as solar energy and others, but at the same time some of these energies are available for specific periods. For example, solar energy can only be obtained during the day and is unavailable at night and one of the most important uses of phase change materials (PCMs) is to store these energies and then use them at the suitable time. It is worth saying that most of these materials have low thermal conductivity and to improve this property, there are numerous and different techniques can be used.
In this study, four sinusoidal metal fins, with four different arrangements, are installed in the annular region of a latent heat storage (LTS) unit. The LTS unit consists of a copper tube with a 500 mm length, 23 mm internal diameter and 2 mm thickness surrounded by a transparent tube of a 74 mm internal diameter and 1 mm thickness. The fin dimensions are 20 mm long, 500 mm wide, and 2 mm thick. The annular space of the LTS unit is fully filled by the PCM (RT-42 paraffin wax).
The thermal properties (temperature, liquid mass fraction, gained thermal energy, and total melting time) of the PCM, with and without fins, are investigated numerically and experimentally. For this purpose, five main cases are conducted; case 1 represents the normal (without fins) case of the LTS unit and the other cases represent the arrangements of fins. These arrangements are a plus shape (case 2), cross shape (case 3), three fins in the bottom part and one in the upper part of the LTS unit (case 4) and four fins in the bottom part of the LTS unit (case 5). The effect of inlet temperature (three values of 60, 70 and 80 °C) of the heat transfer fluid (HTF), on the thermal performance of the PCM, is also investigated. Numerically, the investigation is done using ANSYS-Fluent software (2021 version).
The results indicate that the inlet temperature of the HTF has a clear effect on the melting process of the PCM. Where for 1 °C increase in the inlet temperature, there is a one-minute reduction in the total melting time. This is approximately for all cases of the LTS unit (with and without fins). Also, for 10 °C increase in the inlet temperature, there is an improvement of about 8% in the thermal energy and 9% in the temperature of the PCM. This is approximately for both the numerical and experimental results. The results also show that adding sinusoidal fins into the LTS unit, regardless of their arrangement, significantly improves the thermal performance of the PCM. Numerically, it is found that the optimum case of the fins arrangement for the effect on the thermal characteristics of the PCM is case 5 which involves four fins in the bottom part of the LTS unit. Where in this case, the total melting time of the PCM is reduced by 65%. Also in this case, the gained thermal energy of the PCM is improved by approximately 47%, 49% and 51% at 60, 70 and 80 °C inlet temperatures, respectively. This is by comparing to the normal case of the LTS unit. Experimentally, almost the same improvements are obtained.
By comparing the numerical results with the experimental ones, it is found that there is an excellent agreement between them especially at the beginning of the melting process of the PCM. The maximum difference between these results is about 4.5% for the liquid mass fraction at 60 min and 4% for the temperature of the PCM at 70 min of the melting process. For the thermal energy and the total melting time, the maximum differences between the numerical and experimental results are 4.5% at 160 min and 4.8%, respectively. This is for the modified case of the LTS unit.