Numerical Study On A Novel Direct Contact Latent Heat Storage Unit For Discharging Performance By Introducing Jet Breakup Of Phase Change Material

The low thermal conductivity of the phase change material prolongs the charging and discharging process,thereby limiting the application of the latent heat thermal energy storage system.For enhancing the heat transfer performance in the discharging process,a novel heat storage unit was proposed in this paper.This novel unit was combined with the jet breakup of molten phase change material to formulate the counterflow between phase change material and heat transfer fluid.In this study,sodium acetate trihydrate was chosen as the phase change material,and heat transfer oil was treated as the heat transfer fluid.A 2D numerical model was used to compare the discharging performance of the novel heat storage unit with the conventional type and analysis the effect of changing the operational parameters and unit size on the discharging performance.In this model,VOF model was used to simulate the immiscible flow,the enthalpy-porosity model was introduced to describe the phase transition(liquid-solid)of SAT,the Wall-Adapting Local Eddy-Viscosity model was used to simulate the turbulent flow in the unit.The mesh independence was carried out by comparing four meshes with different resolutions of 27 923,122 130,148 317 and 172 457 cells.This study chosen the mesh with 148 317 elements to conduct the following investigation.The comparison at different SAT inlet pressures implies that the aforementioned model can satisfy the accuracy requirements for further study.The result showed that this novel unit exhibits a larger heat transfer surface and heat transfer region than conventional type.In addition,the strong eddies flow driven by the counterflow in the novel unit implies an enhanced convective heat transfer.However,the strong convection causes phase change material droplets to flow out of the novel unit.Hence,this paper studied the effect of different operational parameters and different unit sizes on the heat transfer performance and the mass of leaked sodium acetate trihydrate.Increasing the mass flow rate of heat transfer oil from 1.29 kg/s to 2.58 kg/s can enhance the heat output by 64.9 % and increase the mass flow rate of leaked sodium acetate trihydrate by 5.44 times within 10 s.Decreasing the inlet temperature of heat transfer oil from 38 ? to 18 ? can enhance the heat output by one time within 10 s.The mass of the leaked sodium acetate trihydrate can be reduced 44 %.Decreasing the mass flow rate of sodium acetate trihydrate from 3.12 kg/s to 2.34 kg/s can reduce the heat output by 26.78 % and decrease the mass of leaked sodium acetate trihydrate by86 %.Extending the horizontal dimension of the novel unit by 50 mm while keeping the unit volume constant can reduce the mass of leaked sodium acetate trihydrate by 56 %.The output heat can be enhanced by 7.8 % by this approach.