tailieunhanh - Ebook Fundementals of heat and mass transfer kotandaraman (3rd edition): Part 2

(BQ) Part 2 book "Fundementals of heat and mass transfer kotandaraman" has contents: Convective heat transfer—practical correlations—flow over surfaces, forced convection, natural convection, phase change processes—boiling, condensation freezing and melting, heat exchangers, thermal radia tion, mass transfer. | 8 CONVECTIVE HEAT TRANSFER Practical Correlations - Flow Over Surfaces INTRODUCTION In chapter 7 the basics of convection was discussed and the methods of analysis were enumerated, correlations were obtained for laminar flow over flat plate at uniform temperature, starting from basic principles and using the concept of boundary layer. The application of these correlations are limited. However these equations provide a method of correlation of experimental results and extension of these equations to practical situations of more complex nature. Though the basic dimensionless numbers used remain the same, the constants and power indices are found to vary with ranges of these parameters and geometries. In this chapter it is proposed to list the various types of boundaries, ranges of parameters and the experimental correlations found suitable to deal with these situations, as far as flow over surfaces like flat plates, cylinders, spheres and tube banks are concerned. Equations for heat transfer in laminar flow over flat plate were derived from basics in Chapter 7. In this chapter additional practical correlations are introduced. Though several types of boundary conditions may exist, these can be approximated to three basic types. These are (i) constant wall temperature, (as may be obtained in evaporation, condensation etc., phase change at a specified pressure) (ii) constant heat flux, as may be obtained by electrical strip type of heating and (iii) flow with neither of these quantities remaining constant, as when two fluids may be flowing on either side of the plate. Distinct correlations are available for constant wall temperature and constant heat flux. But for the third case it may be necessary to approximate to one of the above two cases. . Laminar flow: The condition is that the Reynolds number should be less than 5 × 105 or as may be stated otherwise. For the condition that the plate temperature is constant the following equations are valid with fluid