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

Randomly packed particle beds (RPPB) are widely used in industry, and their internal flow and heat trans-fer 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 (Nu(ex)) 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 char-acteristics 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 veloc-ity (u(a,in)) and electromagnetic induction heating power (Q(e)) on the experimental results are investigated under different particle sizes (d(p) = 6 and 8 mm). The results show that when d(p) and Q(e) are constant, the overall Nu(ex) 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 Re-dh are 2650 and 5300 when d(p) = 6 and 8 mm, respectively. At fixed d(p) and u(a,i)n, Nu(ex),T decrease linearly with the in-crease of Re-dh. In the case of low Q(e), the exergy transfer between the particles and air is mainly based on pressure exergy transfer. As the Q(e) 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. (c) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study develops correlation equations to predict the exergy transfer characteristics within randomly packed particle beds (RPPB) and investigates the effects of different factors on the heat transfer and pressure loss. The results indicate that the exergy transfer coefficient decreases exponentially with increasing Reynolds number, even becoming negative, and the critical Reynolds number depends on the particle size and other factors.