Heat removal and ventilation limitations of the solar chimney attached with a built enclosure: Correlations of thermal Rayleigh numbers, port arrangements and discrete heating elements

Abstract

For energy efficient ventilation and low carbon design, solar chimney has been extensively applied on buildings. In the present work, three representative solar chimneys were introduced, i.e., Model 1 with top-horizontal outlet, Model 2 with upper-vertical outlet and Model 3 with middle-vertical outlet. Updraft ventilation from the built enclosure to the outlet and heat transfer from the absorber to the fluid by heat convection were numerically and theoretically investigated, and their limitations and correlations with governing parameters were also discussed. Streamlines and heatlines were used to visualize transport paths of air and heat respectively. Reverse flow could not be observed in the solar chimney channels of Model 2 and Model 3; Therefore, total volume flow rate induced by the solar chimneys Model 2 and Model 3 and convection heat transfer rate of the absorber increases positively with thermal Rayleigh numbers (Ra). Model 2 chimney produces greater thermal buoyancy than that by Model 3, and its air flow rate is higher than that of Model 3. Generally, volume flow rate induced in Model 1 is higher than those of Model 2 and Model 3 as Ra is no more than 2.5 × 106, and simultaneously, convection heat transfer rate Nusselt number (Nu) is also greater than those of Model 2 and Model 3. When Ra exceeded 2.5 × 106, reverse air flow was intensified further, Nu of Model 1 started to decline. The three models with discrete heat sources could induce more air from indoor than that respective model without discrete heat sources. The maximum increment rate is obtained by the Model 1 solar chimney with discrete heat sources, then Model 2 and the least done by Model 3. Their power relationships between Nu and Ra, V*and Ra are regressed respectively. The dependence of volume flow rate on broad thermal Rayleigh numbers covering all the three flow regimes showed that there were linear correlations between discrete numerical volume flow rates and the scaling predictions, except in Model 1 for high Ra (exceeding critical value 9.0 × 105), where volume flow rate was remarkably reduced due to reverse air flow. This research could provide useful theoretical information for applying small sized vertical solar chimneys attached to the building sidewall to effectively reduce built energy consumption.