(传热英文课件)chapter 7(Natural Convection Systems).ppt

Chapter 7 College of Nuclear Science and Technology * At very low Grashof numbers, there are very minute free-convection currents and the heat-transfer occurs mainly by conduction across the fluid layer. As the Grashof number is increased, different flow regimes are encountered, as shown, with a progressively increasing heat-transfer as expressed through Nusselt number Chapter 7 College of Nuclear Science and Technology * Although some open questions still remain, the experiments of Reference 18 may be used to predict the heat transfer to a number of liquids under constant-heat-flux conditions. The empirical correlations obtained were: Chapter 7 College of Nuclear Science and Technology * The heat-flux is calculated as The results are sometimes expressed in the alternate form of an effective or apparent thermal conductivity ke, defined by Chapter 7 College of Nuclear Science and Technology * By comparing Equation (7-56) and (7-57), we see that In the building industry the heat transfer across an air gap is sometimes expressed in terms of R values, so that Chapter 7 College of Nuclear Science and Technology * In terms of the above discussion, the R value would be Heat transfer in horizontal enclosed spaces involves two distinct situations. The upper plate is maintained at a higher temperature than the low plate, the lower density fluid is above the higher-density and no convections will be experienced. Lower plate has a higher temperature than the upper plate Chapter 7 College of Nuclear Science and Technology * Transient natural-convection heating or cooling in closed vertical or horizontal cylindrical enclosures may be calculated with For the range 0.75L/d2.0. The Grashof number is formed with the length of the cylinder L. Chapter 7 College of Nuclear Science and Technology * The analysis and experiments indicate that it is possible to represent the effective thermal conductivity for fluids between concentric spheres with the relation. Th