Heat transfer mechanisms, influence

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 ... 

The other studies, which included the effect of vibration on steam condensation, nucleate boiling heat transfer, and scale deposition, investigated the relation of frequency and amplitude of vibration of the heat transfer surface to increases in these heat transfer mechanisms. The study of the effect of acoustic vibrations in water on forced convection heat transfer investigated the influence of frequency and amplitude of the standing waves on increasing heat transfer rates and the flow Reynolds numbers at which increases could be obtained. 

Stages (a), (b) and (e) involve direct heat transfer between a gaseous medium and the particles. As a unit process, the heat transfer mechanism is well understood and not specific to the calcination of limestone. Stages (c) and (d), however, are specific to limestone, are influenced by the design of lime kiln, and influence the properties of the quicklime. They are described in detail below. 

These researches laid the foundation for the heat transfer of deep underground, and have positive effect on the deep roadway heat transfer mechanism recognization and heat load calculation. These works gained various degrees of success. However, most of the researches considered the heat transfer surface as rigid boundary, ignored the influence from the permeability of the porous media, led to inaccurate understand of surrounding rock heat transfer and calculation of heat load. 

This work has described the influences of external applied radiation on the horizontal flame spread rates and heat transfer mechanisms of two typical insulation materials (rigid polyurethane and molded polystyrene foams) in bench-scale both in Hefei plain and on Tibetan plateau. The different flame spread rates of two kinds of insulation foams with increasing radiant flux were analyzed qualitatively. The mechanisms of heat transfer in flame spread process of PUR and EPS foams were discussed and an expression of flame spread rate under external radiations was obtained. The results are summarized as follows ... 

Soliman HM 1986 The mist-annular transition during condensation and its influence on the heat transfer mechanism. Int. J. Multiphase Flow, 12, pp. 277-288. 

Heat transfer and mass transfer occur simultaneously whenever a transfer operation involves a change in phase or a chemical reaction. Of these two situations, only the first is considered herein because in reacting systems the complications of chemical reaction mechanisms and pathways are usually primary (see HeaT-EXCHANGETECHNOLOGy). Even in processes involving phase changes, design is frequendy based on the heat-transfer process alone mass transfer is presumed to add no compHcations. But in fact mass transfer effects do influence and can even limit the process rate. 

Convection is influenced by the fluid flow adjacent to the solid surface. To appreciate the mechanics of this mode of heat transfer, the nature of the fluid flow in relation to the particular flow process must be known. Consideration of the flow structure created by the passage of a turbulent fluid over a smooth solid surface shows (see Fig. 4.24)... 

Deposit control is important because porous deposits, under the influence of heat flux, can induce the development of high concentrations of boiler water solutes far above their normally beneficial bulk values with correspondingly increased corrosion rates. This becomes an increasingly important feature with increase in boiler saturation temperature. In addition, deposits can cause overheating owing to loss of heat transfer. Finally, carryover of boiler water solutes, which can be either mechanical or chemical, can lead to consequential corrosion in the circuit, either on-load or off-load. Material so transported can result in corrosion reactions far from its point of origin, with costly penalties. It is therefore preferably dealt with by a policy of prevention rather than cure. 

The present model takes into account how capillary, friction and gravity forces affect the flow development. The parameters which influence the flow mechanism are evaluated. In the frame of the quasi-one-dimensional model the theoretical description of the phenomena is based on the assumption of uniform parameter distribution over the cross-section of the liquid and vapor flows. With this approximation, the mass, thermal and momentum equations for the average parameters are used. These equations allow one to determine the velocity, pressure and temperature distributions along the capillary axis, the shape of the interface surface for various geometrical and regime parameters, as well as the influence of physical properties of the liquid and vapor, micro-channel size, initial temperature of the cooling liquid, wall heat flux and gravity on the flow and heat transfer characteristics. 

Ackeskog et al. (1993) made the first heat transfer measurements in a scale model of a pressurized bubbling bed combustor. These results shed light on the influence of particle size, density and pressure levels on the fundamental mechanism of heat transfer, e.g., the increased importance of the gas convective component with increased pressure. 

The overall heat-transfer coefficient U depends upon the properties of the dry product and the method of heal transfer. The heat-transfer rate A is influenced by the mechanical design of the heating elements and the conditioning of the frozen mass. The temperature gradient AT is limited by the maximum allowable temperatures al the sublimation interface and dry-layer surface. In the constant-rate period, the lirst one-half to two-thirds of the drying cycle, about 8fl + of the water is removed. 

The differences in the various models reside in the choices regarding the mechanisms through which heat is transferred and generated. Since heat transfer has great influence on the overall performance of the cell, as already mentioned, the correct modeling of the temperature field is crucial. 

The performance of a fluidized bed combustor is strongly influenced by the fluid mechanics and heat transfer in the bed, consideration of which must be part of any attempt to realistically model bed performance. The fluid mechanics and heat transfer in an AFBC must, however, be distinguished from those in fluidized catalytic reactors such as fluidized catalytic crackers (FCCs) because the particle size in an AFBC, typically about 1 mm in diameter, is more than an order of magnitude larger than that utilized in FCC s, typically about 50 ym. The consequences of this difference in particle size is illustrated in Table 1. Particle Reynolds number in an FCC is much smaller than unity so that viscous forces dominate whereas for an AFBC the particle Reynolds number is of order unity and the effect of inertial forces become noticeable. Minimum velocity of fluidization (u ) in an FCC is so low that the bubble-rise velocity exceeds the gas velocity in the dense phase (umf/cmf) over a bed s depth the FCC s operate in the so-called fast bubble regime to be elaborated on later. By contrast- the bubble-rise velocity in an AFBC may be slower or faster than the gas-phase velocity in the emulsion... 

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