Performance enhancement of a shell-and-tube evaporator using Al2O3/R600a nanorefrigerant

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To meet the growing energy demand of modern refrigeration and air conditioning systems, thermal properties of conventional refrigerants must be improved. Suspension of nanoparticles into refrigerants, thereby forming a nanofluid, also known as nanorefrigerant is an observed method to enhance the thermophysical and heat transfer characteristics of the base refrigerant. In this paper, an attempt has been made to evaluate numerically overall performances of a shell and tube evaporator using Al2O3/R600a nanorefrigerant as the working substance. A model is developed using the available correlations of nanofluids to analyse the flow boiling heat transfer characteristics of the nanorefrigerant inside the tubes and to compare its performances with those of the pure refrigerant. Boiling and pressure drop models of nanorefrigerant in evaporator is solved numerically using EES software and the results show that along with overall heat transfer coefficient and cooling capacity of the evaporator, pumping power also increases with increase in nanoparticle concentration. Regression models of evaporator cooling capacity and total pumping power are developed using analysis of variance (ANOVA) tool in response surface methodology (RSM). A multi-object optimization is carried out to choose the right concentration of nanoparticles and tube diameter, which will maximize the amount of heat transfer and minimize the total pumping power of the fluids in the evaporator. Optimization results indicate that the best possible values of evaporator cooling capacity and total pumping power are 9.23 kW and 1.39 kW respectively for 13.7 mm tube diameter and nanoparticle weight fraction of 0.0268 within the investigated range of tube diameter of 10-18 mm and weight fraction of 0-5%. These optimization results contribute to the understanding of heat transfer and pressure drop balance on using nanorefrigerants and also define the limit of nanoparticle concentration for such evaporator configurations.