Flow visualization observations are described for natural convection flow in rectangular inclined enclosures. Observations are made in air-filled enclosures of small and moderate aspect ratio (0.25 ⩽ Ax ⩽ 7), angles of inclination from 0 to 90 deg and Rayleigh number between 5 × 103 and 2.5 × 105. For the range of parameters considered we determined the transition from stationary to nonstationary flow and the transition from two-dimensional to three-dimensional flow. Also, the different stationary flow structures are described. In addition measurements of the velocity profile of the stationary flow have been performed by means of a laser doppler anemometer. Special attention has been given to the side wall boundary condition of the enclosure.
This paper presents theoretical results on natural convection in vertical air-filled enclosures with isothermal hot and cold walls. The flow is considered to be two-dimensional, laminar, and stationary. The effect of stratification of the fluid in the core region on the heat transfer and the natural convection flow is discussed. Local heat transfer relations considering this stratification are given. The Rayleigh number varied from 104−106, the aspect ratio from 1–18, and the side walls were both perfectly conducting and adiabatic.
The boundary condition at the hot absorber plate in a solar collector will influence the natural convection in the enclosure. For the isoflux boundary condition and an isothermal cold wall an experimental and numerical study has been made for Ra numbers from 105 to 107 and inclinations from 20 to 90 deg with the horizontal. For vertical enclosures the heat transfer by natural convection was about 19 percent above that for an isothermal hot plate. This decreases with angle of inclination, to 9 percent at 20 deg. For solar collectors it means that for cases where the absorber plate is not isothermal the convective losses can be about 10 percent above the usually expected values.
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