Discrete particle modelling of buoyant convective particle-laden air flow in solar cavity free-falling particle receivers

Abstract

A coupled computational fluid dynamics and discrete particle method numerical model is developed to provide a systematic analysis of buoyant convective particle-laden air flow and heat losses from a solar cavity receiver with free-falling particles. The fundamental mechanisms of heat loss from a solar cavity particle receiver are poorly understood and difficult, if not impossible, to obtain experimentally. Studies on particle-laden air flow and heat transfer mechanisms in a solar cavity free-falling particle receiver are extremely limited. As such, this study is the first attempt to unravel the convective heat transfer mechanisms of a solar cavity particle receiver while considering the influence of the entrained air, air recirculation patterns, and solid–gas thermal dynamics. Furthermore, the receiver efficiency due to air recirculation and particle transport in addition to the relative losses due to heat exiting the receiver aperture and outlet ports is investigated. It is found that the particle mass flow rates have profound implications on the air recirculation patterns, entrained air, and heat fluxes of the solar receiver. The heat losses at the particle outlet greatly increase with increasing particle mass flow rate due to the higher thermal mass of the entrained air. An increase in the particle injection rate from 100,000 pps to 3,000,000 pps leads to an increase in the net heat by 48.6% although the aperture heat loss decreases by 24.4%. This is linked to the high entrained air velocities which recirculates much of the heated air in the cavity thereby reducing the aperture heat loss. This also infers a linear increase in the thermal efficiency and exponential decrease in exit loss ratios of the solar cavity receiver. It is also found that a competition exists between the various flow intensities of the particle curtain, the ambient air, and the entrained air velocities, which is quantified by the dimensionless thermal energy. Scientists and engineers can further optimize the geometric morphologies of solar cavity receivers after gaining a solid comprehension of the flow structures and thermal efficiencies of the solar receiver. To this end, a modified receiver geometry is proposed to mitigate convective heat losses. The modified receiver is about 26% efficient than the original receiver, which is linked to the 45° plate which impedes the velocity of the particle curtain at the outlet and reduces the entrained air velocity and heat losses from the cavity. Furthermore, a high particle initialization velocity greatly amplifies the heat losses at the particle outlet port. This high thermal mass at the outlet port amplifies the entrained air velocity regardless of the geometric morphology of the solar receiver. The results from the validated numerical model can be used by engineers and scientists to better optimize geometric morphologies of solar particle receivers.