The interaction between radiation and turbulent natural convection in square and rectangular enclosures

• Main Author: Shati, Abdulmaged
• Other Authors: Beck, Stephen ; Blakey, Simon
• Published: University of Sheffield 2013
• Subjects: 621.4022
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.568146

Heat transfer by natural convection inside enclosed spaces with radiation interaction is of practical interest in many engineering applications, such as design of buildings for thermal comfort, nuclear reactors, solar collectors, and the cooling of electronic equipment. In the natural convection studies inside enclosed spaces, mostly two simple enclosed spaces are considered; first the square enclosure and secondly the rectangular cavity both with heated and cooled walls. In this study the effects of natural convection with and without the interaction of surface radiation in square and rectangular enclosures has been studied, numerically, theoretically and experimentally. The analyses were carried out over a wide range of enclosure aspect ratios ranging from 0.0625 to 16, including square enclosures in sizes from 40cm to 240cm, with cold wall temperatures ranging from 283 to 373 K, and hot to cold temperature ratios ranging from 1.02 to 2.61. The work was carried out using four fluids (Argon, Air, Helium and Hydrogen; whose properties vary with temperature). The numerical model used is explained in detail and validated using previous experimental and numerical results and also the author’s experiments. FLUENT software was used to carry out the numerical study. In the numerical study, turbulence was modelled using the RNG k-ϵ model with a non-uniform grid and using the Discrete Transfer Radiation Model (DTRM) for radiation. 2D and 3D numerical calculations were performed for square and rectangular enclosures (and for the range of parameters mentioned above) to produce the constants of the derived correlation equation and to compare them with experimental results. A dimensional analysis was established to produce correlation equations controlling the flow inside square and rectangular enclosures for pure natural convection and natural convection with radiation. This study starts from the partial differential equation which explaining the flow inside the cavities. The correlation equations for the new dimensionless group, which is the ratio between natural convection to radiation heat transfer, were provided for both square and rectangular enclosures. Also the correlation equations for the average Nusselt number with and without radiation were provided for square and rectangular enclosures. The constants, as a function of temperature ratio, of all the derived correlation equations are given along with these equations. These constants are correlated and a polynomial equations for each constant as a function of absolute temperature ratio is given. All these equations are valid for a different temperature ratio, different enclosure size, different aspect ratio and different fluid properties. This provides a generalised equation for heat transfer in square and rectangular enclosures both with and without radiation. These can be used to calculate Nusselt numbers for pure natural convection and natural convection with radiation interaction and also to calculate the ratio between convection to radiation heat transfer for both square and rectangular enclosures. To validate the numerical results, an experimental study was performed for square and rectangular enclosures. This was for three aspect ratios 2.0, 1.0 and 0.5. Tests were carried out for hot wall temperatures ranging from 50°C to 75°C and for a Rayleigh number ranging from 9.6×〖10〗^7≤Ra≤8×〖10〗^9. This allows the calculation of total heat transfer from the hot and cold walls using six thermocouples in each side. Experimental measurements of the velocity and turbulence intensity profiles were performed using the laser Doppler velocimtrey for three aspect ratios 2.0, 1.0 and 0.5 and hot wall temperature ranging from 50°C to 75°C. The results of this study are expected to contribute to the literature in this field and enhance the understanding of the natural convection with radiation interaction in rectangular enclosures. Also this will be a useful technique, as, by using the results it is possible to generalise the heat transfer in square and rectangular cavities filled with ideal gases.