Designs for high flow natural convection solar air heaters

Abstract

Previous investigations have mainly focused on enhancing the thermal performance of a natural convection solar air heaters (SAH) at the expense of hydraulic performance by incorporating protruded surfaces and, thereby compromising on air mass flow rate significantly due to considerable pressure drop. Low mass flow rate of SAH makes it unsuitable for wide applications. In a first, design investigations using experimentally validated numerical model of SAH is reported that enhances flow rate by more than 100% in comparison to conventional flat plate SAH design. Integration of bell-shaped designs at the inlet of SAH adds to the ram-air effect that converts dynamic pressure into static pressure thereby manifesting into an excellent enhancement of air flow rate as well as in heat transfer associated with less hydraulic losses. The high-flow SAH was further investigated for building application using unsteady first-law of thermodynamic equation and it was observed to be 33% efficient over conventional design. An independent correlation for Nusselt number variation with Rayleigh numbers and bell mouth ratio of the form Nu(Racosθ)m(h/R)n has been developed and found to be in good agreement with the data. © 2019 International Solar Energy Society