NUSSELT NUMBER FOR THE NATURAL CONVECTION AND SURFACE THERMAL RADIATION IN SOLAR COLLECTORS
In this paper, a numerical investigation of the two modes of heat transfer, natural convection and surface thermal radiation, in a tilted slender cavity such a collector is presented. The 2-D conservation of mass, momentum and energy are coupled with a radiative model through the boundaries and solved by the finite volume method. The studied parameters are: aspect ratios (8≤A≤16), inclination angles (15°≤λ≤35°) and Rayleigh numbers (104≤Ra≤106). The results indicated that the radiative surface radiation coupled with the natural convection modifies the flow patterns and the average heat transfer in the slender cavity between the absorber plate and the glass in the collector. The convective heat transfer coefficient and the radiative heat transfer coefficient as a function of the aspect ratio and the inclination angles are shown. It was found that the radiative heat transfer contributes more than 40％ of the total heat transfer. A comparison between the present Nusselt numbers against the ones used for the design of solar collectors reported in the literature is presented. 1. INTRODUCTION For the design of the performance of solar collectors, the overall heat losses play an important roll in the collector efficiency. It is known that the mayor heat losses are from the top through the glass cover, thus the accurate calculation of the heat loss from the top of the collector to the surroundings is very important. Generally, heat transfer loss through the top per unit area is calculated by considering approximated heat transfer coefficients, such as the ones for two inclined parallel plates (Duffie and Beckmann, 1991). Because, one of the objectives in designing solar collectors is to reduce the heat loss through the covers, care must be taken, since improper design can lead to increase rather than decrease convection losses (Garg and Datta, 1984). To know the amount of energy that is lost by convection and radiation separately may guide to a better design in solar collectors and improve its efficiency. Theoretically, it is possible, to determine separately the contribution of the convective and radiative heat transfer coefficients by solving the combined heat transfer by convection and radiation in a differentially cavities (Riduane et al., 2004), these coefficients can be used in the models of global balances for the design of solar collectors and then to evaluate the overall heat losses. Recently, Alvarado et al., 2007 presented a summary of the literature about this problem and they concluded that the literature review shows that the effect of the inclination angle on natural convection in square and slender cavities has been studied, but the effect of the inclination angle on the interaction between surface radiation and natural convection is mostly related with square cavities. Then, as the conjugated heat transfer depends on parameters like the inclination angle, aspect ratio, etc.,
G.(A)lvarez J.Xam(a)n J.J.Flores R.Alvarado
Centro Nacional de Investigación y Desarrollo Tecnológico.CENIDET-DGEST-SEP Prol.Av.Palmira s/n.Col.Palmira.Cuernavaca, Morelos, C.P.62490, México.
国际会议
2007世界太阳能大会(Proceedings of ISES Solar World Congress 2007)
北京
英文
2007-09-18（万方平台首次上网日期，不代表论文的发表时间）