Transient thermal analysis of multilayer pipeline with phase change material

Based on thermal energy storage concept, multilayer composite pipeline with phase change material (PCM) has been proposed recently as an effective method to meet flow assurance challenges in deep water environment by releasing latent heat and extending cool-down time under shut-in condition. This work presents a mathematical model for the start-up and cool-down processes in a multilayer pipeline with an inner steel pipe, an intermediate PCM layer, and an external thermal insulation layer. With enthalpy formulation adopted for the PCM layer, the governing equations for heat conduction in the composite pipe walls and for energy transport in the produced fluid are solved numerically by explicit finite difference methods. The effects of the PCM thickness ratio, the phase change temperature, and the thermal conductivity ratio between the thermal insulation layer and phase change material are investigated. The effect of PCM hysteresis on the transient heat transfer process is also evaluated. It is found that there is an optimal PCM thickness ratio for a given combination of PCM and thermal insulation materials that maximizes the overall cool-down time. The cool-down time and the optimum PCM thickness ratio increase when the phase change temperature decreases. For a given PCM, the optimum PCM thickness ratio decreases for increasing thermal conductivity ratio. There is an interval of thermal conductivity ratio between 0.2 and 0.8 within which the PCM layer with optimum thickness ratio provides a significant increase in the cool-down time.