Natural Convective Heat Transfer from Horizontal and Near Horizontal Surfaces

A discussion of the flow situations considered in this book is provided. A relatively brief overview of past studies concerned with the types of flow being considered is given. The numerical solution procedure adopted in obtaining the results discussed in the book is then outlined. A discussion of the use of a characteristic length is then given. It is explained that if this characteristic length is used in defining the Nusselt and Rayleigh numbers, then the same relationship between the Nusselt number and the Rayleigh number will be obtained for all the surface shapes in a given type of flow situation.

In this chapter, consideration is given to natural convective heat transfer from horizontal heated plane surfaces of relatively simple shape that are imbedded in a surrounding adiabatic surface, the heated and adiabatic surfaces being in the same horizontal plane. Attention is limited to surfaces that have a uniform surface temperature, i.e., which are isothermal, and to cases where, if there is a heated surface, it is facing upward and if there is a cooled surface, it is facing downward. Natural convective heat transfer from heated surfaces having circular, square, and rectangular shapes is considered. In addition, consideration is also given to the natural convective heat transfer from a plane two-dimensional surface. The results for the rectangular surface for the various aspect ratio values considered have been expressed in terms of Nusselt and Rayleigh numbers based on the characteristic length and the variations obtained for all aspect ratios considered are shown to be effectively the same in the laminar and fully turbulent flow regions.

Natural convective heat transfer rates from horizontal heated isothermal surfaces of more complex shape are considered in this chapter. The surfaces considered are imbedded in a surrounding plane horizontal adiabatic surface, the heated and adiabatic surfaces being in the same horizontal plane. The surface shapes considered are (a) a circular heated surface with an adiabatic inner circular section, (b) a square-heated surface with an adiabatic inner square section, (c) a square-heated surface with an adiabatic inner rectangular section, (d) an I-shaped heated surface, and (e) a +-shaped (plus-shaped) heated surface. The results for these surface shapes show that if the variations of Nusselt number with Rayleigh number for the various shapes are expressed in terms of Nusselt numbers and Rayleigh numbers based on the characteristic length, then the variations of Nusselt number and Rayleigh number for all the surface shapes are essentially the same in the laminar and the fully turbulent flow regions.

Attention here has been given to natural convective heat transfer from a heated plane surface that is either recessed into an adiabatic surrounding surface or protrudes from a surrounding adiabatic surface. The case of a heated surface having a circular shape and the case of a heated surface that is two-dimensional are considered. The results show that for both the heated surface shapes considered, when the heated surface protrudes from the surrounding adiabatic surface the height to which the heated surface protrudes has only a very small effect on the heat transfer rate. However, the results also show that when the heated surface is recessed into the surrounding adiabatic surface the depth to which the surface is recessed has quite a significant effect on the heat transfer rate.

In this chapter, the effects of surface inclination and of flow interaction between two adjacent horizontal surfaces on the natural convective heat transfer rate are considered. The results indicate that the form of the variation of the Nusselt number for an inclined isothermal square surface with the angle of inclination is different for inclination angles of less than roughly 3o than it is for larger angles of inclination. The results also indicate that for a pair of adjacent horizontal square-heated surfaces, the magnitude of the dimensionless size of the gap between the horizontal surfaces has a relatively small effect on the heat transfer rates from the two heated surfaces. The results also indicate that the form of the variation of the mean heated surface Nusselt number with the dimensionless gap between the horizontal surfaces is strongly dependent on the Rayleigh number value.

In this chapter, attention is given to natural convective heat transfer from isothermal bodies in which there is heat transfer from both the top and the bottom surfaces of the body and where there is no surrounding adiabatic section. Two situations of this type are considered in this chapter. The first of these involves heat transfer from a two-sided circular plate with an adiabatic circular center section and the second involves heat transfer from a pair of vertically spaced two-sided circular plates. In both of these cases, the plates involved are assumed to be very thin and the heat transfer from the vertical side surfaces of the plates is, therefore, neglected.

In this chapter, attention is given to natural convective heat transfer from a horizontal upward-facing-heated surface that is imbedded in a surrounding adiabatic surface, the heated and adiabatic surfaces being in the same horizontal plane for the case where there is an adiabatic parallel plane covering the surface (a shroud) above and relatively near to the heated surface. In such an arrangement, the presence of the covering surface can have a significant influence on the rate of heat transfer from the heated surface. Attention in this chapter has been given to heated surfaces having a circular, a square, and a rectangular shape. For the square-and circular-shaped surfaces, the effect of the presence of the covering surface on the Nusselt number variation with Rayleigh number for the various dimensionless gap values is approximately the same. For the rectangular surface case, the aspect ratio of the surface has a very significant influence on the form of the variation of Nusselt number with Rayleigh number and on the effect the dimensionless-heated surface gap value on the Nusselt number variation.