A numerical study on the natural convective heat loss of an isothermal upward-facing cylindrical cavity

Abstract

• Uncovered the influences of surface temperature, tilt angle and aperture area. • Developed an empirical correlation with high prediction accuracy. • Proposed a strategy that reduces the air velocity to control convective heat loss. The development of second-stage concentrators and lens systems facilitates the application of upward-facing cavities in solar energy utilization. To design a cavity with high efficiency and low cost, convective heat loss must be determined then controlled. Taking the commonly used isothermal cylindrical cavities as the object, natural convective heat loss characteristics were analysed numerically. The influences of surface temperature, tilt angle and aperture ratio were elaborated. Results show that the natural convective heat loss keeps increasing with increasing aperture ratio. As the tilt angle reduces, the natural convective heat loss increases before attaining a maximum at about −30° after which it drops. In short, the natural convective heat loss is proportional to the air velocity, the higher the air velocity, the larger the heat loss. A strategy controlling the air velocity is thus proposed to control the convective heat loss. Unlike isoflux conditions, a clockwise vortex forms inside the cavity for the tilt angle less than −30°. Large prediction errors are presented for using previous correlations to predict the natural convection heat loss of upward-facing cylindrical cavities. Hence, a novel empirical correlation based upon the influence characteristics of studied factors and with high accuracy is developed.