Conduction, heat transfer mode

The heat equation in the anode and cathode gas channels involves convection and conduction heat transfer modes and no heat generation. The equation is expressed as... 

The combined CFD—DEM approach was extended to investigate the effects of some important parameters closely related to the van der Waals force such as particle size and Hamaker constant (Hou et al., 2012a). The heat transfer characteristics of cohesive particles were demonstrated in three flow regimes in Fig. 19. It revealed that the convective heat transfer is dominant for large particles while the conductive heat transfer becomes important with the decrease of particle size. This is mainly attributed to the increase of surface area per unit volume. Two transitional points with the increase of Hamaker constant were found in the variation of heat fluxes by convective and conductive heat transfer modes as shown in Fig. 20. 

There are three heat-transfer modes, ie, conduction, convection, and radiation, each of which may play a role in the selection of a heat exchanger for a particular appHcation. The basic design principles of heat exchangers are also important, as are the analysis methods employed to determine the right size heat exchanger. 

Radiative heat transfer is perhaps the most difficult of the heat transfer mechanisms to understand because so many factors influence this heat transfer mode. Radiative heat transfer does not require a medium through which the heat is transferred, unlike both conduction and convection. The most apparent example of radiative heat transfer is the solar energy we receive from the Sun. The sunlight comes to Earth across 150,000,000 km (93,000,000 miles) through the vacuum of space. FIcat transfer by radiation is also not a linear function of temperature, as are both conduction and convection. Radiative energy emission is proportional to the fourth power of the absolute temperature of a body, and radiative heat transfer occurs in proportion to the difference between the fourth power of the absolute temperatures of the two surfaces. In equation form, q/A is defined as ... 

Conductive heat transfer is the dominant mode of intraparticle heat transfer. Under low Reynolds number flow situations, conductive heat transfer is also an important mode for fluid heat transfer. This section analyzes the conductive heat transfer characteristics of a... 

The governing heat transfer modes in gas-solid flow systems include gas-particle heat transfer, particle-particle heat transfer, and suspension-surface heat transfer by conduction, convection, and/or radiation. The basic heat and mass transfer modes of a single particle in a gas medium are introduced in Chapter 4. This chapter deals with the modeling approaches in describing the heat and mass transfer processes in gas-solid flows. In multiparticle systems, as in the fluidization systems with spherical or nearly spherical particles, the conductive heat transfer due to particle collisions is usually negligible. Hence, this chapter is mainly concerned with the heat and mass transfer from suspension to the wall, from suspension to an immersed surface, and from gas to solids for multiparticle systems. The heat and mass transfer mechanisms due to particle convection and gas convection are illustrated. In addition, heat transfer due to radiation is discussed. 

In VMD, the conductive heat transfer across the membrane is very low, mainly due to the low pressure on the permeate side of the membrane and could be neglected. Thus latent heat of vaporization is the only mode for heat transfer to be considered through the membrane [17,47,77]. Equation 19.3 can be used for calculating the rate of heat transfer across the membrane. 

In most steady-state heat transfer problems, more than one heat transfer mode may be involved. The various thermal resistances due to thermal convection or conduction may be combined and described by an overall heat transfer coefficient, U. Using U, the heat transfer rate, Q, can be calculated from the terminal and/or system temperatures. The analysis of this problem is simplified when the concepts of thermal circuit and thermal resistance are employed. 

Convection, conduction, radiation, electromagnetic fields, combination of heat transfer modes Intermittent or continuous ... 

It is possible, indeed desirable in some cases, to use combined heat transfer modes, e.g., convection and conduction, convection and radiation, convection and dielectric fields, etc., to reduce the need for increased gas flow that results in lower thermal efficiencies. Use of such combinations increases the capital costs, but these may be offset by reduced energy costs and enhanced product quality. No generalization can be made a priori without tests and economic evaluation. Finally, the heat input may be steady (continuous) or time-varying also, different heat transfer modes may be deployed simultaneously or consecutively depending on the individual application. In view of the significant increase in the number of design and operational parameters resulting from such complex operations, it is desirable to select the optimal conditions via a mathematical model. 

Use of superheated steam in direct dryers Increased use of indirect (conduction) heating Use of combined (or integrated) heat transfer modes Use of volumetric heating (microwave [MW]/radio-frequency [RF] fields) in specialized situations Use of two-stage (or multistage) dryers Use of intermittent heat transfer Use of novel combnstion technologies (e.g., pulse combustion for flash drying)... 

Heat transfer takes place by three mechanisms conduction, convection, and radiation. In conductive heat transfer, the heat flows from regions of high temperature to regions of low temperature. The transfer takes place due to motion at the molecular level. Matter must be present in order for conduction to occur. The material itself does not need to be in motion for conduction to take place in fact, many times the conducting medium will be stationary. In a solid material, the only mode of heat transfer is conduction [16]. In convection, heat transfer is due to the bulk motion of the fluid. Convective heat transfer only occurs in fluids. In radiation, heat or radiant energy is transferred in the form of electromagnetic waves. 

For heat transfer within the fluid the similarity parameter of importance is the Peclet number, which is the ratio of heat convection and heat conduction. We will revisit heat transfer modes later but for the purposes of flow characterization, the Peclet number is defined in Equation (3.26) below, where it is also given as the product of the Reynolds number and the Prandtl number, Pr (kinematic viscosity thermal diffusivity)... 

Different types of heat transfer processes are called modes. The main modes of heat transfer are convection, radiation, and conduction. For a temperature gradient that exists between a surface and a moving fluid, one should use the term convection. The radiation mode of heat transfer is driven by electromagnetic waves emitted from all surfaces of finite temperature, so there is a net heat transfer by radiation between two surfaces at different temperatures. When a temperature gradient exists in a stationary medium, heat fiows under the law of conduction heat transfer. In the case of solid materials, such as polymers, conduction is the dominant mechanism for heat transfer, involving mainly lattice vibrations and, in few cases, the transfer of kinetic thermal energy from one electron to another. 

Heat transfer through these various insulations can occur by several different mechanisms, but generally involves solid conduction, gas conduction and convection, and radiation. The purpose of any insulation is to minimize the summed transfer of heat by these various mechanisms. The apparent thermal conductivity of an insulation, measured experimentally to incorporate all of these heat transfer modes, offers the best means by which to compare the different types of insulation. Table 7.1 provides a listing of some accepted thermal conductivity values for the more popular insulations. 

Dry heat transfer due to conduction, convection and radiation depends upon a number of clothing and fabric parameters like fabric structure, thickness, porosity, fabric layers as well as environmental parameters. However, one mode may or may not be present under different circumstances e.g. in a still atmosphere where the fabric is kept between two static bodies without any kind of air flow between them, heat transfer due to natural and forced convection can be neglected and in this case heat transfer is only due to conduction and radiation. In an evacuated chamber/vacumn conductive heat transfer through still air can be neglected and heat transfer takes place mostly due to radiation and conduction through the solid material. 

Chem and Holmes [57] developed a model where the heat transfer between small blood vessels and tissue is separated into three modes a perfusion mode (equilibration of blood and tissue temperature), convective mode q (convective heat transfer from flowing blood against a tissue temperature gradient) and thermal conduction mode qp (conductive heat transfer due to small temperature fluctuations in the blood along the tissue temperature gradient) ... 

The heat transfer in fluidized beds of monodisperse particle has been extensively investigated in the past. Heat transfer in a packed/fluidized bed with an interstitial fluid may involve many mechanisms as shown in Fig. 1 (Yagi and Kunii, 1957). These mechanisms can be classified into three heat transfer modes in fluidized beds fluid—particle or fluid—wall convection particle-particle or particle—wall conduction and radiation. Different heat transfer models are developed for these mechanisms, as described in the following. 

Figure 8 Relative contributions of the heat transfer modes considered to the overall heat transfer as a function of ks/kf. line 1, heat conduction Qnsfs/ line 2, heat conduction Qcsfs, line 3, heat conduction line 4, the solid-solid radiation between particle surfaces dashed-line, the percentage of total conduction. Reprinted from Cheng and Yu (2013) with permission from ACS.

The heat transfer between an immersed tube and a fluidized bed depends on many factors, such as the contacts of particles with the tube, porosity, and gas flow around the tube. These factors are affected by many variables related to operational conditions. Gas velocity is one of the most important parameters in affecting the heat transfer, which can be seen in Fig. 23. With the increase of from 0.08 to 0.50 m/s, the overall FITC increases. However, when the exc is further increased from 0.50 to 0.80 m/s, the HTC decreases. The effect of particle thermal conductivity on the local HTC was also examined and shown in Fig. 25A (Hou et al., 2012b). The local HTC increases with the increase of from 1.10 to 100 W/(mK). However, such an increase is not significant for from 100 to 300 W/(m K). The variation of percentages of different heat transfer modes with ks is further shown in Fig. 25B. When is lower than 100 W/ (m K), the conductive heat transfer increases with the increase offej, while the convective heat transfer decreases. Further increase of fej has no significant effects. 

The effect of wall thermal conductivity is pronounced at high inlet velocities, where increased efficiency is observed in the case of high solid thermal conductivity, as seen in Fig. 5.5 pertaining to U-m = 3.7 m/s. In this graph, axial profiles of the energy heat balance terms in the solid are provided in terms of all modeled heat transfer modes heat generated via reactions on the surface, heat convected to the... 

The heat transfer characteristics of the model are simulated via heat structures. Heat structures are one-dimensional abstract components used to represent the solid mass of the reactor plant. The one dimension is in the radial direction, perpendicular to the direction of fluid flow. Heat transfer in the axial direction (parallel to fluid flow) is represented by using several fluid volumes in the axial direction and connecting a heat structure to each. TRACE heat structures also permit two-dimensional conduction in Cartesian, cylindrical or spherical geometry. Conduction, convection, and radiation heat transfer modes can all be represented with heat structures. Heat structures, with their associated boundary conditions and neutron kinetics calculations, provide the ability to link thermal conditions among the plant structural, fuel (power components), coolant, and ambient environment (radiation enclosures). 

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