Convective heat transfer applications

The present section reviews the concepts behind the Generalized Integral Transform Technique (GITT) [35-40] as an example of a hybrid method in convective heat transfer applications. The GITT adds to tiie available simulation tools, either as a companion in co-validation tasks, or as an alternative approach for analytically oriented users. We first illustrate the application of this method in the full transformation of a typical convection-diffusion problem, until an ordinary differential system is obtained for the transformed potentials. Then, the more recently introduced strategy of... 

Fig. 2 Convective heat transfer applications in pharmaceutical dr5dng (A) tray-drying of a static solids bed and (B) fluid bed-drying of a spherical particle.

The lower Emit of applicability of the nucleate-boiling equations is from 0.1 to 0.2 of the maximum limit and depends upon the magnitude of natural-convection heat transfer for the liquid. The best method of determining the lower limit is to plot two curves one of h versus At for natural convection, the other ofh versus At for nucleate boiling. The intersection of these two cui ves may be considered the lower limit of apphcability of the equations. 

Da.skalaki E. Natural convection heat transfer coefficients from vertical and horizontal surfaces for building applications. Energy and Buddings, vol. 20, no.. T, 1994. 

Used for convective flow in heat-transfer applications. 

A variety of studies can be found in the literature for the solution of the convection heat transfer problem in micro-channels. Some of the analytical methods are very powerful, computationally very fast, and provide highly accurate results. Usually, their application is shown only for those channels and thermal boundary conditions for which solutions already exist, such as circular tube and parallel plates for constant heat flux or constant temperature thermal boundary conditions. The majority of experimental investigations are carried out under other thermal boundary conditions (e.g., experiments in rectangular and trapezoidal channels were conducted with heating only the bottom and/or the top of the channel). These experiments should be compared to solutions obtained for a given channel geometry at the same thermal boundary conditions. Results obtained in devices that are built up from a number of parallel micro-channels should account for heat flux and temperature distribution not only due to heat conduction in the streamwise direction but also conduction across the experimental set-up, and new computational models should be elaborated to compare the measurements with theory. 

Rouson, D., S.R. Tieszen, and G. Evans, Modeling convection heat transfer and turbulence with fire applications A high temperature vertical plate and a methane fire, in Proceedings of the Summer Program. 2002, Center for Turbulence Research, Stanford University, pp. 53-70. 

This chapter describes the fundamental principles of heat and mass transfer in gas-solid flows. For most gas-solid flow situations, the temperature inside the solid particle can be approximated to be uniform. The theoretical basis and relevant restrictions of this approximation are briefly presented. The conductive heat transfer due to an elastic collision is introduced. A simple convective heat transfer model, based on the pseudocontinuum assumption for the gas-solid mixture, as well as the limitations of the model applications are discussed. The chapter also describes heat transfer due to radiation of the particulate phase. Specifically, thermal radiation from a single particle, radiation from a particle cloud with multiple scattering effects, and the basic governing equation for general multiparticle radiations are discussed. The discussion of gas phase radiation is, however, excluded because of its complexity, as it is affected by the type of gas components, concentrations, and gas temperatures. Interested readers may refer to Ozisik (1973) for the absorption (or emission) of radiation by gases. The last part of this chapter presents the fundamental principles of mass transfer in gas-solid flows. 

The applicability of the preceding pseudocontinuum approach to convective heat transfer of gas-solid systems without heat sources depends not only on the validity of the phase continuum approximation but also on the appropriateness of the local thermal equilibrium assumption. The local thermal equilibrium may be assumed only if the particle-heating... 

Development of a mechanistic model is essential to quantification of the heat transfer phenomena in a fluidized system. Most models that are originally developed for dense-phase fluidized systems are also applicable to other fluidization systems. Figure 12.2 provides basic heat transfer characteristics in dense-phase fluidization systems that must be taken into account by a mechanistic model. The figure shows the variation of heat transfer coefficient with the gas velocity. It is seen that at a low gas velocity where the bed is in a fixed bed state, the heat transfer coefficient is low with increasing gas velocity, it increases sharply to a maximum value and then decreases. This increasing and decreasing behavior is a result of interplay between the particle convective and gas convective heat transfer which can be explained by mechanistic models given in 12.2.2, 12.2.3, and 12.2.4. 

Fortunately, most cryogens, with the exception of helium II, behave as classical fluids. As a result, it has been possible to predict their behavior by using well-established principles of mechanics and thermodynamics applicable to many room-temperature fluids. In addition, this has permitted the formulation of convective heat transfer correlations for low-temperature designs of simple heat exchangers that are similar to those used at ambient conditions and utilize such well-known dimensionless quantities as the Nusselt, Reynolds, Prandtl, and Grashof numbers. 

The continuity equation (8.9) and the energy equation (8.12) are identical to those for forced convective flow. The x- and y-momentum equations, i.e., Eqs. (8.10) and (8.11), differ, however, from those for forced convective flow due to the presence of the buoyancy terms. The way in which these terms are derived was discussed in Chapter 1 when considering the application of dimensional analysis to convective heat transfer. In these buoyancy terms, is the angle that the x-axis makes to the vertical as shown in Fig. 8.3. 

Our preceding discussions of convection heat transfer have considered homogeneous single-phase systems. Of equal importance are the convection processes associated with a change of phase of a fluid. The two most important examples are condensation and boiling phenomena, although heat transfer with solid-gas changes has become important because of a number of applications. 

There are upper and lower limits of applicability of the equation above. The lower limit results because natural-convection heat transfer governs at low temperature differences between the surface and the fluid. The upper limit results because a transition to film boiling occurs at high temperature differences. In film boiling, a layer of vapor blankets the heat-transfer surface and no liquid reaches the surface. Heat transfer occurs as a result of conduction across the vapor film as well as by radiation. Film-boiling heat-transfer coefficients are much less than those for nucleate boiling. For further discussion of boiling heat transfer, see Refs. 5 and 6. 

Researchers and engineers extensively utilized second law analysis in the field of heat and fluid flow. Bejan developed the basic approach, methodology, and applications. In two-dimensional Cartesian coordinates, the local rate of entropy production per unit volume in a convective heat transfer is... 

Rao and Anantheswaran (1988) reviewed studies on convective heat transfer to canned fluids in detail. Here, the dimensionless groups and relationships applicable to both Newtonian and non-Nervtonian fluids are reviewed in brief The rotational Reynolds Reg number is defined as... 

In some ca.se.s, such as those encountered in space and cryogenic applications, a heat transfer surface is surrounded by an evacuated space and thus there is no convection heat transfer between a surface and the surrounding medium. In such cases, radiation becomes the only mechanism of heat transfer between the surface under consideration and llie surroundings. Using an energy balance, the radiation boundary condition on a surface can be expressed as... 

The use of fins is most effective in applications involving a low convection heat transfer coefficient. Thus, the use of fins is more easily justified when the medium is a gas instead of a liquid and the heat transfer is by natural convection instead of by forced convection. Therefore, it is no coincidence that in liquid-to-gas heat exchangers such as the car radiator, fins are placed on the gas side. 

Experimental investigations on convective heat transfer in liquid flows in microchannels have been in the continuum regime. Hence, the conventional Navier-Stokes equations are applicable. 

The modem microstmcture applications led to increased interest in convection heat transfer in micro conduits. Huid transport in micro channels has found applications in a number of technologies such as biomedical diagnostic techniques, thermal control of electronic devices, chemical separation processes, etc. 

Bayazitoglu, Y., Tunc, G., Wilson, K., and Tjahjono, I., (2005) Convective Heat Transfer for Single-Phase Gases in MicroChannel Slip Flow Analytical Solutions, presented at NATO Advanced Study Institute, Microscale Heat Transfer - Fundamentals and Applications in Biological and Microelectromechanical Systems, July 18-30, Altin Yunus - Qe me, Izmir, Turkey. 

As far as convective heat transfer is concerned, liquid and gaseous flows musf be considered separately. Liquid flow has been investigated experimentally, whereas analytical, numerical and molecular simulation techniques have been applied to understand the characteristics of gaseous flow and heat transfer. While the Navier-Stokes equations can still be applied, due to the small size of microchannels, some deviations from the conventionally sized applications have been observed. Flow regime boundaries are significantly different, as well as flow and heat transfer characteristics. 

The two types of processes currently used in dry-heat sterilization include 1) dry-heat batch sterilization/ oven sterilization and 2) dry-heat tunnel sterilization. Process 1 is the type of dry heat unit widely used in the pharmaceutical industry it uses the principle of convective heat transfer to heat the load. Process 2 is only found in large-scale processes, and the main application of this process is in the sterilization and depyrogenation of glass.f ... 

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