Radiative heat loss , effect

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. 

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. 

For high-temperature systems, radiative heat loss can be very significant. In conventional systems, reflective shields or radiation mirrors and low-emissivity material are commonly used to mitigate radiative heat loss. It is expected that similar approaches would be effective in microreactors as well. ... 

Case and Jackson were able to show that anomalously high values were always associated with a fine grained ash structure and that continuous removal of this ash residue gave values in keeping with prediction. These observations suggest that the main effect may be on and R rather than on Tg. Indeed the present author s own observations are that the flame actually enters this fine ash structure, and raises it to incandescence. Thus may approach 900 °C, increasing the contribution from the heat capacity of the residue and possibly also increasing radiative heat losses. 

Figure 18.10 Effect of conductive, convective plus radiative heat loss and nonattainment of equilibrium on a first catalyst bed s final % SO2 oxidized. The two effects are seen to offset each other. (The heat loss heatup path is steeper because less heat is available to warm the gas.)...

The net heat flux is taken here to represent radiative heating in an environment at Tcx, with an initial temperature T,yj as well. From Equation (7.20) a more general form can apply if the flame heat flux is taken as constant. This nonlinear problem cannot yield an analytical solution. To circumvent this difficulty, the radiative loss term is approximated by a linearized relationship using an effective coefficient, hr ... 

The actual flame temperature is lower than the adiabatic equilibrium flame temperature because of heat loss from the flame. The actual flame temperature is determined by how well the flame radiates its heat and how well the combustion system, including the load and the refractory walls, absorbs that radiation. A highly luminous flame generally has a lower flame temperature than a highly nonluminous flame. The actual flame temperature will also be lower when the load and the walls are more radiatively absorptive. This occurs when the load and walls are at lower temperatures and have high radiant absorptivities. These effects are discussed in more detail in Chapter 4. As the gaseous combustion products exit the flame, they... 

Hot gases at Ts = 47 °C flow in a pipe of 5 cm diameter (Fig. 9P-14). For safety reasons, a radiative insulation is being considered by covering the pipe with a thin concentric metal shell. The enussivities of the pipe and shell are ep and et, respectively. The ambient temperature is Too = 27 °C (a) Neglecting the effect of convection, find the temperature of the shell and the reduction in heat loss to the ambient for ep = es = 0.1, and for ep = 1 and e, = 0.1. (b) For the same reduction of part (a), determine the required thickness of an asbestos insulation. 

Humidified air is less dense than dry air and the difference in densities can be used effectively to remove heat and water from the cathodes (i.e., the oxidant and coolant streams can be the same). Small fans and blowers can be used to provide more compact cell designs. The associated parasitic losses are small (e.g., <1%) even with large excess flows. For low power levels the cell cooling can be provided by convective and radiative heat transfer. The cell designs based on this approach must balance relatively open structures with the penalty of lower volumetric power densities. Applications requiring more electrical power (>1 kWJ must consider active cooling schemes almost without exception. 

One of the most effective ways to expand the limits of stable combustion is to reduce heat loss and at any rate partly recover heat of combustion products. A burner unit designed in the form of a geometrically closed cavity (3D matrix) made of a gas-permeable material (Fig. 12.11) is heated to high temperature due to convectional and radiative heat transfer from the flame front stabilized near the sm-face and from the reaction products to the irmer sm-face of the matrix. Due to an effective heat transfer from the reaction products to the... 

Many researchers have carried out investigations in interaction of radiation and conduction heat loss in determining the total thermal insulation of the porous medium. Lee [18] considered the fibers as infinitely long cylinders and obtained the effect of fiber orientation in the radiative properties of the fibrous media and found that the highest insulation is obtained when the fibers are oriented parallel to the direction of heat flow and the highest radiative heat transfer takes place when the fibers are perpendicular to the heat flow direction. 

Heat of combustion = 12.0 kJ/g Heat of gasification = 6.0 kJ/g Effective vaporization temperature = 380 °C Radiative loss fraction = 0.35 Conversion constants ... 

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