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• A multi-zone model for unsteady heat transfer is proposed for an open volumetric air receiver. • The model includes all the modes of heat transfer in receiver internals. • Detailed investigations are performed to identify the important parameters. • Recommendations are made for the scale-up receiver design. A comprehensive, multi-zone, unsteady heat transfer model is developed for a straight absorber pore-based open volumetric air receiver. This includes heat exchange between the porous absorbers, absorbers and receiver casing, and absorbers and return air. The validation revealed its predictive capability within an uncertainty of ±7%. The model is used for the scale-up of receiver design with several intermediate absorber layers. The major recommendations for scale-up are a) multiple absorber layers is beneficial for mitigating thermal stress, b) higher flux concentration is required for non-volumetric heating, to achieve the desired air temperature, compared to volumetric heating, c) air return ratio should be 0.6, d) absorber porosity should be 0.6 for volumetric heating and higher for non-volumetric heating, e) absorber gap to length ratio should be 0.15 -0.25, f) the radiative heat loss is substantial for non-volumetric heating; therefore, the exposed surface area to ambient should be reduced, and g) absorber diameter to length ratio should be 1-2. The developed approach is generic and adaptable for the different open volumetric air receiver designs. [ABSTRACT FROM AUTHOR]

This paper aims at (a) comparative assessment of the different volume of fluid (VoF) based numerical schemes for a rising, single air bubble in isothermal liquid (water) layers and (b) investigation of bubble rise in thermally stratified liquid (therminol) layers. For numerical investigation of bubble rise in isothermal liquid layers, three available numerical schemes, Upwind, Quadratic Upstream Interpolation for Convective Kinematics (QUICK) and Monotonic Upwind Scheme for Conservation Laws (MUSCL) schemes, combined with the pressure-velocity coupling(p-v) approaches, such as, Pressure Implicit with Splitting of Operators (PISO) and Semi Implicit Method for Pressure-Linked Equations (SIMPLE), are considered. The wake analysis revealed that the region of influence grows beyond 10 times the diameter of the bubble. Moreover, based on the comparative assessment, MUSCL scheme with PISO is selected, for investigating the bubble rise in thermally stratified therminol layers. Based on these investigations, a 3D diagram, describing bubble shape as f (Ra, Eo, Ga), is proposed, and a new insight to the micro-convection, inside the rising bubbles is provided. Furthermore, a time-scale analysis is performed to describe the heat transfer mechanisms, (a) inside air bubble, and (b) between air bubble and the external surrounding liquid. Thus, the findings will be useful for the design of heat exchangers or cooling devices, which rely on the heat transfer augmentation with rising air bubble. [ABSTRACT FROM AUTHOR]