Abstract
The natural convection flow in the air gap between the absorber plate and glass cover of the flat plate solar collectors is predominantly evaluated based on the lumped capacitance method, which does not consider the spatial temperature gradients. With the recent advancements in the field of computational fluid dynamics, it became possible to study the natural convection heat transfer in the air gap of solar collectors with spatially resolved temperature gradients in the laminar regime. However, due to the relatively large temperature gradient in this air gap, the natural convection heat transfer lies in either the transitional regime or in the turbulent regime. This requires a very high grid density and a large convergence time for existing CFD methods. Higher order numerical methods are found to be effective for resolving turbulent flow phenomenon. Here we develop a non-dimensional transient numerical model for resolving the turbulent natural convection heat transfer in the air gap of a flat plate solar collector, which is fourth order accurate in both spatial and temporal domains. The developed model is validated against benchmark results available in the literature. An error of less than 5% is observed for the top heat loss coefficient parameter of the flat plate solar collector. Transient flow characteristics and various stages of natural convection flow development have been discussed. In addition, it was observed that the occurrence of flow mode transitions have a significant effect on the overall natural convection heat transfer.
Highlights
Solar collectors trap the solar radiation and convert it into thermal energy which has many applications, such as cooking, indoor space heating, hot water generation, distillation, steam production, solar air conditioning, etc
The natural convection phenomenon inside this air gap lies in the turbulent regime due to a large temperature difference between the absorber plate and glass cover of an FlatPlate Solar Collector (FPSC)
In the following figures we have shown only the iso-thermal contours to clearly depict the occurrence of flow mode transition
Summary
Solar collectors trap the solar radiation and convert it into thermal energy which has many applications, such as cooking, indoor space heating, hot water generation, distillation, steam production, solar air conditioning, etc. The natural convection phenomenon inside this air gap lies in the turbulent regime due to a large temperature difference between the absorber plate and glass cover of an FPSC. Multi-mode models were developed having multiple nodes in the absorber plate, heat transfer fluid, glass cover, etc. The FPSC performance is analysed on specific collector prototypes and compared with the experimental results of these prototypes only These models are 2nd order convergent requiring a very long convergence time and a very high grid density to model any practical solar collector, as the convection heat transfer in experimental solar collectors lies in the transitional or turbulent regime. 4th order accuracy to completely model the heat transfer behaviour of the solar collector and to analyse the transient fluid flow behaviour inside the air gap of the FPSC
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