Thermal energy storage unit using phase change materials (PCMs) is a prominent method due to the higher storage capacity with a small change in temperature. In this study 3D numerical investigation of a vertical latent heat storage unit (LHSU) under simultaneous charging and discharging (SCD) condition is performed. The enthalpy-porosity method with a commercial CFD tool is used to simulate the melting behavior. Prior to the parametric investigation, the numerical methodology is validated with experiments. Lauric acid is used as a PCM due to its suitability for thermal storage. Water is used for heat transfer fluids (HTFs). The influence of heat transfer fluids' mass flow rates viz. 0.25, 0.5, 0.75, and 1 lit/min and inlet direction (parallel and counter flow) on the melting process are examined. Results depicted that the mass flow rate and direction of HTFs do not affect the melting process.