Radiative heat losses

The rate of combustion is set equal to the rate of heat applied to warm the entrained air plus the radiative heat losses ... 

If the strain rate exerted on an edge flame becomes very small, especially for weak intensity flames, the radiative heat loss could influence the edge behavior significantly. Consequently, the propagation speed could decrease appreciably in low-strained flames [29], as marked in fhe dash-dot line. 

Yang, S.l. and Shy, S.S., Global quenching of premixed CH4/air flames Effects of turbulent straining, equivalence ratio, and radiative heat loss, Proc. Combust. Inst., 29,1841,2002. 

It is also well known that there exist different extinction modes in the presence of radiative heat loss (RHL) from the stretched premixed flame (e.g.. Refs. [8-13]). When RHL is included, the radiative flames can behave differently from the adiabatic ones, both qualitatively and quantitatively. Figure 6.3.1 shows the computed maximum flame temperature as a function of the stretch rate xfor lean counterflow methane/air flames of equivalence ratio (j) = 0.455, with and without RHL. The stretch rate in this case is defined as the negative maximum of the local axial-velocity gradient ahead of the thermal mixing layer. For the lean methane/air flames,... 

Future combustion devices may burn alternative fuels with higher carbon-to-hydrogen ratios and operate at higher pressures. The combustion of such fuels under these conditions will result in more intense turbulence, higher levels of soot formation, and the associated increase in radiative heat loss compared to more traditional fuels burned at lower pressures. Depending upon the design objectives, it may be desirable to control soot levels using predictive capabilities. 

The decrease in temperature predicted by the analysis is relatively small and has not been observed experimentally. Experiments with higher precision and accuracy are warranted for checking if this is an artifact of the present chemistry that does not include the effects of higher hydrocarbon formation and radiative heat loss. The peak temperature was found to decrease because of a decrease in the peak volumetric heat release rate caused by a broadening of the reaction zone. 

Thermolysis on insulating wool. Kaowool or other refractory wools are valuable for reduced radiative heat losses from a hot crucible. However, their use causes some increase in the extent of pyrolysis of substrate vapor by the crucible assembly and this, in rare instances, may spoil a metal atom synthesis. Only one example is known at present. The reaction of palladium atoms with benzyl chloride gives very low yields of t73-benzylpalladium chloride when the palladium is evaporated from an alumina crucible insulated with Kaowool, but a 30-50% yield with an uninsulated crucible. It has been established that this is due to enhanced formation of product-destroying radicals on the hot Kaowool. 

If we consider Ts constant and radiative heat loss negligible, Equation 14 reduces to the earlier law ... 

The low pressure behavior predicted by the collapsed model is very sensitive to the choice of Es (see Figure 12) when Es is large and when there is radiative heat loss, extinction will occur at some low pressure because the surface reaction for large Es is a more sensitive function of surface temperature than is radiative heat loss. Thus, at some low pressure, where the O/F flame is weak, the surface reaction, which is almost the entire source of heat, cannot overcome the heat loss. This is the... 

The pressure at which extinction occurs depends on the size of the strand. Both theoretical predictions and our observations of the flame indicate that the A/PA and O/F flame zones are thick enough (order of millimeters) at the extinction pressure for the flame to be susceptible to convective cooling by the entrained ambient gases as well as to a significant loss of available heat owing to escape of unreacted AP from the A/PA flame zone. No evidence has been found to show that radiative heat loss from the propellant surface is a major factor. However, Fein-auer s (27) results indicate that it is a contributory factor when carbon black has been added to the propellant. 

The final term in Eqn. (11.6A) is conductive, convective plus radiative heat loss from the gas. As discussed in section 11.3, it is assumed here to be zero, i.e. ... 

The assumption of no conductive, convective plus radiative heat loss from catalytic S02 oxidation converters is not perfectly correct. There is always some heat loss. Nevertheless ... 

Table 21.1. Bottom half of Table O.l s 3rd catalyst bed heatup path-equilibrium curve intercept worksheet. Input and output gas enthalpies are shown in rows 43 and 44. Note that they are the same. This is because our heatup path calculations assume no convective, conductive or radiative heat loss during catalytic SO2+V2O2 —> SO3 oxidation, Section 11.9. 1st and 2nd catalyst bed enthalpies are calculated similarly - using Tables J.2 and M.2. 

Table 21.2. Summary of Fig. 21.1 s temperatures, enthalpies and heat transfers. Note the continuing decrease in the gas s enthalpy as heat is transferred from gas to water and steam in Fig. 21.1 s boiler, superheater and economizer. All temperatures but the last are from Tables J.2, M.2 and 21.1. Note that a catalyst bed s input enthalpy is always the same as its output enthalpy. This is due to our assumption that there is no conductive, convective or radiative heat loss from the gas.

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