The core temperature of a healthy human body is around 370C. This can vary within a band of ±10C during the day depending upon physical activity level, emotional state, and climatic conditions. The extreme cold and hot climate can change core body temperatures beyond this band. The variation in body temperature not only reduces work efficiency but can also result in life-threatening heat-related injuries. The PCM based cool & warm vests can provide thermal comfort to persons working in these harsh conditions.A thermal manikin has been developed at IITJ for testing of PCM cool & warm vests under controlled thermal conditions in the climatic chamber. The Basal Metabolic Rate (BMR) can be expressed in the form of equivalent body surface heat flux or Metabolic Equivalent of Task (MET) for different activities. The present design of thermal manikin employs a copper torso with an IR heater as a heating element at the core. The heat generated on the heater is transferred to the torso surface through radiative and convective heat transfer. The surface heat flux will be measured using a handheld heat flux sensor and any deviation from the required heat flux will be controlled by controlling power dissipated to the heater. Uniform distribution of surface heat flux and temperature is one of the key requirements to be met by the manikin. Therefore, heat transfer analysis has been carried out using ANSYS Fluent to get a distribution of the same for different activities.The results of the analysis agree with the standard body parameters.The thermal manikin has been realized and the results are validated by experimentingthe manikin. Further, the thermal performance analysis has been carried out for PCM cool & warm vests using AnsysFluent. The analysis on cool vest has been carried out using AnsysFluent default energy equation with solidification/melting model and also using a separate UDS (User Defined Scalar) written in C++ language for solving “transport equation”. The analysis results for both cases are compared. The analysis was also carried out for four different PCMs namely ice,paraffin C-16, 1-dodecanol, and savE OM 21. The effect of activity level on the thermal performance of cool vest has also been studied. The analysis of warm vest has also been carried outusing Ansys Fluent for sub-zero ambient conditions.Further, the modeling of dendrite solidification in an under-cooled aqueous solution of sodium acetate trihydrate has been carried out. The enthalpy method has been used to compute the solid-liquid interface growing in undercooled melt. The interface temperature and grain growth have been modeled considering the curvature effect and solute undercooling. A 2-D computational grid of square control volumes has been used, and discretized governing equations were solved explicitly. The crystal anisotropy was imposed explicitly. Key Words: PCM vest, thermal manikin, solidification modeling, sodium acetate trihydrate, vest thermal analysis