Heat transfer effect

Relations for transport properties such as viscosity and thermal conductivity are also required if wall friction and heat-transfer effects are considered. 

In rotary devices, reradiation from the exposed shelf surface to the solids bed is a major design consideration. A treatise on furnaces, including radiative heat-transfer effects, is given by Ellwood and Danatos [Chem. Eng., 73(8), 174 (1966)]. For discussion of radiation heat-transfer computational methods, heat fliixes obtainable, and emissivity values, see Schornshort and Viskanta (ASME Paper 68-H 7-32), Sherman (ASME Paper 56-A-III), and the fohowing subsection. 

E = superficial liquid entrainment, Ib/hr/fL or, heat transfer effectiveness. 

In addition to impurities, other factors such as fluid flow and heat transfer often exert an important influence in practice. Fluid flow accentuates the effects of impurities by increasing their rate of transport to the corroding surface and may in some cases hinder the formation of (or even remove) protective films, e.g. nickel in HF. In conditions of heat transfer the rate of corrosion is more likely to be governed by the effective temperature of the metal surface than by that of the solution. When the metal is hotter than the acidic solution corrosion is likely to be greater than that experienced by a similar combination under isothermal conditions. The increase in corrosion that may arise through the heat transfer effect can be particularly serious with any metal or alloy that owes its corrosion resistance to passivity, since it appears that passivity breaks down rather suddenly above a critical temperature, which, however, in turn depends on the composition and concentration of the acid. If the breakdown of passivity is only partial, pitting may develop or corrosion may become localised at hot spots if, however, passivity fails completely, more or less uniform corrosion is likely to occur. 

For the lower heat transfer surfaces in Fig. 2.60 to contribute to the energy transport, the solid should be an effective conductor of heat through its thickness. In other words, conjugate heat transfer effects should not create a more significant resistance to heat flow than that of the fluid in the channel. Since the heat transfer coefficient is generally a maximum at CHF, this leads to... 

Most of heat transfer correlations are based on data obtained in flow boiling from relatively large diameter conduits and the predictions from these correlations show considerable variability. Effects of superficial liquid and gas velocity on heat transfer in gas-liquid flow and its connection to flow characteristics were studied by Hetsroni et al. (1998a,b, 2003b), Zimmerman et al. (2006), Kim et al. (1999), and Ghajaret al. (2004). However these investigation were carried out for tubes of D = 25—42 mm. These data, as well as results presented by Bao et al. (2000) in tubes of L> = 1.95 mm and results obtained by Hetsroni et al. (2001), Mosyak and Hetsroni (1999) are discussed in the next sections to clarify how gas and liquid velocities affect heat transfer. Effects of the channel size and inclination are considered. 

Fig. 1 shows the thermal decomposition curves of HDPE mixed with Al-MCM-41, with respect to time, at isothermal operating temperatures. Lag periods were formed at the initial stage of decomposition, possibly due to the heat transfer effect, which could delay the decomposition of a sample until the latter reaches the operating temperatures. As the reaction ten erature increased, the reaction time became noticeably shorter. The shortening of the reaction time was clearly observed when the reaction occurred at the reaction teirperatures between 420 and 460 °C. The HDPE on Al-MCM-41-P decomposed faster than that on blank and that on A1-MCM-41-D, as shown in Fig. 1(b). 

The flow velocity, pressure and dynamic viscosity are denoted u, p and fj and the symbol (...) represents an average over the fluid phase. Kim et al. used an extended Darcy equation to model the flow distribution in a micro channel cooling device [118]. In general, the permeability K has to be regarded as a tensor quantity accounting for the anisotropy of the medium. Furthermore, the description can be generalized to include heat transfer effects in porous media. More details on transport processes in porous media will be presented in Section 2.9. 

The following phenomena pertaining to bubble departure from a heated surface are discussed in this section bubble size at departure, departure frequency, boiling sound, and heat transfer effects by departing bubbles. 

Schraub, F. A., 1969, Spray Cooling Heat Transfer Effectiveness during Simulated Loss-of-Coolant Transients, ASME Paper 69WA/NE-8, ASME, New York. (4)... 

To predict the heat transfer effects, the engineer must have an adequate quantitative description of heat transfer between the tube wall and the fluid phases, heat transfer between the tube wall and the fluid phases, heat transfer between the two phases, the rate of phase change within the system, and the rate of heat transfer resulting from phase change. Unfortunately, present design procedures only provide estimates of the system performance. Many procedures have not been formulated in a systematic manner, and therefore it is difficult to pinpoint areas where the present understanding of the design process is weakest. 

The inclusion of radiative heat transfer effects can be accommodated by the stagnant layer model. However, this can only be done if a priori we can prescribe or calculate these effects. The complications of radiative heat transfer in flames is illustrated in Figure 9.12. This illustration is only schematic and does not represent the spectral and continuum effects fully. A more complete overview on radiative heat transfer in flame can be found in Tien, Lee and Stretton [12]. In Figure 9.12, the heat fluxes are presented as incident (to a sensor at T,, ) and absorbed (at TV) at the surface. Any attempt to discriminate further for the radiant heating would prove tedious and pedantic. It should be clear from heat transfer principles that we have effects of surface and gas phase radiative emittance, reflectance, absorptance and transmittance. These are complicated by the spectral character of the radiation, the soot and combustion product temperature and concentration distributions, and the decomposition of the surface. Reasonable approximations that serve to simplify are ... 

Table 3.5 shows that the study of chemical kinetics is critical in successful scale-up of catalytic systems, of gas-phase controlled systems, and of continuous tank stirred reactors (CSTR). For scale-up of batch systems consisting of gas or liquid compounds, chemical kinetics and heat transfer effects must be studied because the combination of these phenomenon determine the conditions for a runaway and thus involve the safety of the operation. 

The best data to use for determining whether an incompatibility exists will obviously be from testing the actual scenarios and conditions that are identified. However, this is often not practical or possible. Small-scale tests can be performed in a laboratory that can give an indication whether a reaction is expected. However, be wary of concluding that since no reaction is seen on a small scale, no effects will be realized in an industrial facility. Heat transfer effects and scale-up issues are especially important to be careful of when extrapolating small-scale results. Differences in heat transfer, mixing and other scale-up effects can cause a significant and potentially... 

Chen, L. and Wu, C., Heat transfer effect on the net work and/or power versus efficiency characteristics for the air standard Diesel cycles. Energy The International Journal, 21(12), 1201-1205, 1996. 

When reaction conditions within the particle are non-isothermal. Weisz and Hicks [8] showed that a suitable criterion defining conditions under which a reaction is not controlled by diffusion and heat transfer effects in the solid is... 

Selectivity in Catalytic Reactions Influenced by Mass and Heat Transfer Effects... 

Where both multicomponent and heat transfer effects appeared important, it was necessary to compare the results of selected runs via the MASC program under adiabatic conditions to the value obtained by multiplying the MTZ length prediction via SSMTZ (isothermal, multicomponent) by the ratio of MTZ lengths predicted by SSHTZ under adiabatic and isothermal conditions. If the values are essentially equivalent, the SSMTZ and SSHTZ programs could be used to analyze the MTZ data with a high degree of confidence. 

It should be emphasized that in all these cases, combined or superimposed phenomena must be dealt with, viz. for stage IV, fluiddynamics, kinetics of polymerization, and rheokinetic changes caused by chemical reactions for stage V, polymerization kinetics, crystallization kinetics and heat transfer effects a thermomechanical problem in combination with crystallization kinetics. Construction of a mathematical model requires simultaneous solution of a set of equations in order to describe these related phenomena. 

Greenberg. J. B. and Goldman, Y. (1989). Volatilization and burning of Pulverized coal with radiation heat transfer effects on a counter flow combustor. Combustion Sci. Tech., 64 1-17. 

Wray, W. O., Aida, T., and Dyer, R. B. (2002). Photoacoustic cavitation and heat transfer effects in the laser-induced temperature jump in water. Appl. Phys. B 74, 57—66. 

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