Numerical study of natural convection loss from open cavities

Abstract

In this paper, the natural convection occurring from open cavities is analysed. Three different cavity shapes are studied namely cubical, spherical and hemispherical geometries having equal heat transfer area. The numerical analysis is performed on three dimensional (3-D) cavity models using the Fluent CFD software. The studies are performed for cavities having isothermal wall temperatures of 100 °C, 200 °C and 300 °C. The effect of opening ratios ( d/ D or a/ H) of 1, 0.5 and 0.25 on the convective loss is studied for the cubical and hemispherical cavity shapes while the spherical cavity is analysed for opening ratios of 0.5 and 0.25. The effect of inclination of the cavity on the natural convection is studied for five inclinations; 0° (aperture facing sideways), 30°, 45°, 60° and 90° (aperture facing vertically downwards). The natural convection loss is found to increase with an increase in opening ratio. The increase in natural convection loss for different inclinations is found to vary between 30% and 80% when the opening ratio is increased from 0.25 to 0.5 for all cavity shapes. A similar increase in natural convection loss is observed when the opening ratio is increased from 0.5 to 1 for the cubical and hemispherical cavities. The natural convection loss increases with increase in cavity wall temperature. A decrease in loss is observed with increase in cavity inclination; the highest convective loss being at 0° inclination and the lowest at 90°. For all the cavity shapes analysed with opening ratios of 0.5 and 0.25, it is observed that the hemispherical open cavity has the highest natural convection loss when compared to cubical and spherical cavities. The hemispherical cavity has higher convective loss than the cubical cavity for an opening ratio of 1. A Nusselt number correlation involving the effect of cavity shape, Rayleigh number, inclination angle and the opening ratio has been developed from this study.