Numerical investigation of the heat transfer enhancement using corrugated pipes in a PCM for grey water harnessing

Heat transfer from working fluids within plane pipes of heat exchangers (HEs) has an upper limit, which can easily be enhanced. Due to physical constraints the best option for enhancement is to boost the heat transfer coefficient (HTC). Amongst the passive techniques, the use of corrugated pipes is the most effective, especially suited for the application of harnessing waste heat from grey water (GW) to be stored in phase change materials (PCMs) and released back to cold water (CW). An experimental setup validates a numerical model of a corrugated pipe in a PCM to harness GW heat. Based on this validation discrete numerical models of varying rib heights and pitch lengths of corrugated pipes are analysed with changing GW/CW mass-flow rates in this application. The performance of each pipe along with the associated effects on the PCM thermal storage are evaluated, for both melting and freezing. Results show that both the flow and heat transfer characteristics are best improved on both sides of a corrugated pipe having a rib height of 4.5 mm and a pitch length of 30 mm. At a mass-flow rate of 0.1 kg/s the practical application-based Thermal Performance Factor (TPF) for this particular corrugated pipe increases by 3.1 for melting and 2.4 for freezing. Results also show that the use of corrugated pipes is 60-70% more effective during melting as compared to freezing of the PCM.