Experimental investigation of flow and exergy transfer characteristics in the air-cooled randomly packed particle bed based on second law analysis

Abstract

Randomly packed particle beds (RPPB) are widely used in industry, and their internal flow and heat transfer characteristics have a significant impact on the energy efficiency of the overall system. Based on the second law of thermodynamics theory, the correlation equation to predict the exergy transfer coefficient (ETC), exergy transfer Nusselt number (Nuex) within the RPPB is developed, which can reflect temperature and pressure exergy transfer. It effectively reveals the relationship between the role of heat transfer and pressure loss. The experiments are then carried out using stainless-steel particles packed bed with an electric induction heating system to provide a uniform internal heat source. The ambient air is used as the test fluid to cool the heated particles. The pressure drops and forced convection heat transfer characteristics of air-cooled high-temperature particles are investigated. The experiments are performed for flows with modified Reynolds number Redh in the range of 811–6810. The effects of cooling air inlet velocity (ua,in) and electromagnetic induction heating power (Qe) on the experimental results are investigated under different particle sizes (dp = 6 and 8 mm). The results show that when dp and Qe are constant, the overall Nuex decreases exponentially with increasing Redh, and even becomes negative. It means that the heat gain of the air through the RPPB is lower than the pressure loss. The critical Redh are 2650 and 5300 when dp = 6 and 8 mm, respectively. At fixed dp and ua,in, Nuex,T decrease linearly with the increase of Redh. In the case of low Qe, the exergy transfer between the particles and air is mainly based on pressure exergy transfer. As the Qe increases, the temperature exergy transfer gradually increases and dominates, and Nuex shifts from negative to positive. This study delves deeper into the forced convective heat transfer process in RPPB, comprehensively considering the effects of heat transfer and pressure loss.