CFD SIMULATION OF SHELL AND TUBE HEAT EXCHANGER FILLED WITH PHASE CHANGE MATERIAL FOR SOLAR WATER HEATING SYSTEM

Abstract

Solar energy, the most promising source of energy, requires thermal energy storage (TES) due to its intermittent nature. Storage of thermal energy can take the form of sensible heat storage (SHS), latent heat storage (LHS), and thermo-chemical storage (TCS). The amount of energy that is stored in SHS depends on the specific heat of the substance, the change in temperature, and the mass of the storage material since heat energy is retained by raising the temperature of the storage material without going through a phase transition. However, LHS involves a phase transition, when heated to the constant temperature, between solid-liquid, solid-solid, and liquid gas states and vice versa. Solid-solid phase transitions require a lower energy storage capacity than liquid-gas phase transitions which require a large increase in volume. As a result, the solid liquid transformation is most commonly used in LHS applications because it is more efficient than other transformations. In this study, the methods of enhancing the heat transmission in a latent heat thermal energy storage (LHTES) system with a heat exchanger of shell & tube type having a phase change material (PCM) in shell region, known as PCM heat exchanger (PCMHE), and carrying a number of tubes of circular, elliptical and square shape, to flow of a heat transfer fluid (HTF) using a two-dimension (2D) computational fluid dynamics (CFD) model has been presented. Enhancement of heat transfer in the LHTES system may be obtained either by using a suitable geometric configuration of PCMHE and/or by increasing the thermal conductivity of PCM. Heat transfer enhancements in LHTES systems can be achieved by using extended surfaces like fins and heat pipes. To enhance the heat transmission between tubes and PCM, a number of rectangular fins are attached to the outer surface of the circular, elliptical, and square tubes of the PCMHE. Geometrical modeling of PCMHE of shell and tube type, in which a solid Gallium as a PCM is filled in the outer shell and an HTF is flowing inside tubes, has been done in Solid Works. In this vi work, the outer shell of PCMHE is selected in two different shapes: square and circular. The tubes inside the outer shell of PCMHE are also selected in different shapes i.e. circular, square, and elliptical as well as in different configurations i.e. with and without fins. All the geometries of PCMHE modeled in Solid Works saves in IGES format and transported in the Ansys Fluent to convert them into finite element models by mesh generation of the geometries. Unstructured triangular elements are used for the mesh generation of all the geometries of PCMHE. After the mesh generation is completed, the meshed model is transferred to the Ansys Fluent setup and solution mode. In the setup mode of Ansys Fluent, further operations like solver type, time dependency, gravitational acceleration, material properties, boundary conditions, etc. are selected and then the solution is executed. 2D CFD numerical investigation of heat transfer & melting of PCM in different geometries of PCMHE has been performed. After the execution of the solution of all geometries of PCMHE has been completed, viewing and postprocessing of the results are performed. CFD investigations of heat exchanger geometries have been performed at different temperatures for liquid fraction and mean temperature of PCM as well as melting time and time for attainment of applied temperature. Results show that enhancement of heat transfer and reduction in melting time take place by employing a number of fins on tubes of PCMHE. It has also been concluded by comparing the different geometries of PCMHE that the configuration having the similar shape of shell and tubes (both shell and tubes are either square or circular) has the maximum heat transfer rate and lowest melting time compared to other geometries of PCMHE. Based on the available simulation data, the findings are validated and show good agreement, which suggests a deviation of 3.8 & 4.1 percent.