Experimental and numerical investigations of nano-additives enhanced paraffin in a shell-and-tube heat exchanger: a comparative study

The impact of metal oxides, metal nitrides and carbon allotropes based nano-additives on thermal conductivity and thermal storage performance of paraffin based latent heat storage (LHS) system is experimentally and numerically investigated. Aluminium oxide (Al2O3), aluminium nitride (AlN) and graphene nano-platelets (GnP) based nano-PCM samples are prepared with ultrasonic emulsification technique. Thermal performance enhancements of nano-PCM samples are investigated by conducting a series of charging and discharging experiments in shell-and-tube heat exchanger at various operating conditions. Moreover, a numerical model is developed to account for an impact of varying operating temperature, nano-additives particle size and volume fraction on the effective thermal conductivity and dynamic viscosity of nano-PCM. The numerical model is simulated to investigate the influence of effective thermal conductivity and dynamic viscosity on heat transfer and temperature distribution, phase transition rate and total enthalpy of the system. It is noticed that the charging rates for Al2O3, AlN and GnP based nano-PCM samples are significantly enhanced by 28.01%, 36.47% and 44.57% as compared to pure paraffin, respectively. Likewise, the discharging rates are augmented by 14.63%, 34.95% and 41.46%, respectively. However, the addition of nano-additives compromises the overall thermal storage capacity and augments the effective dynamic viscosity which has adverse impact on natural convection. Therefore, an optimum volume fraction of nano-additives is determined by conducting experimental examinations on Al2O3 based nano-PCM samples with volume fraction of 1%, 3% and 5%, at varied operating conditions. It is observed that by increasing volume fraction from 1% to 3%, the charging and discharging rates are significantly enhanced. However, an insignificant enhancement is noticed with further increase in volume fraction from 3% to 5%. Therefore, the optimum volume fraction of 3% is established. Furthermore, GnP based nano-PCM samples have demonstrated higher potential for thermal performance enhancement of LHS system and respective utilisation in practical applications