Flaming combustion heat release

Indeed, in normal (slow) combustion, which may propagate only due to heat conduction, the heat flux is a quantity of the same order as the combustion heat released in unit time. The width of the front should be of the same order as the product of the chemical reaction time and the flame propagation velocity. 

It is very important to understand the fuel properties when OEC is involved in a high-temperature process since the characteristics of OEC are very different in terms of the reaction intensity, the equilibrium temperature, and flame patterns, as well as pollutant emissions. Therefore, the properties of the fuels commonly fired in different industries and some basic concepts applied to combustion of each fuel will be presented in this chapter. Understanding them will enable combustion engineers and scientists to optimize a process with a desirable flame and heat release pattern, low NOx, CO, and other pollutant emissions, and safe and stable operation with minimum requirement of maintenance. 

Figures 12.4 and 12.5 show the distribution of OH and CH in swirl-stabilized propane-air flames using 25%/75% combustion air distribution in the inner and outer annulus of the burner, respectively. The results show that the reaction zone moves upstream towards the burner with high-shear, see Fig. 12.4. The distribution of CH shows a measure of the heat-release rate in flames. The heat-release rate is slow with the low-shear case to give a wider and longer flame (see Fig. 12.5). Figure 12.6 shows the distribution of OH in a kerosene flame with 55 /30 swirl distribution and 25%/75% combustion air (a) and 75%/25% combustion air distribution (b) in the two annular passages of the burner. Stronger...

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Figure 4 illustrates the trend in adiabatic flame temperatures with heat of combustion as described. Also indicated is the consequence of another statistical result, ie, flames extinguish at a roughly common low limit (1200°C). This corresponds to heat-release density of ca 1.9 MJ/m (50 Btu/ft ) of fuel—air mixtures, or half that for the stoichiometric ratio. It also corresponds to flame temperature, as indicated, of ca 1220°C. Because these are statistical quantities, the same numerical values of flame temperature, low limit excess air, and so forth, can be expected to apply to coal—air mixtures and to fuels derived from coal (see Fuels, synthetic). 

To analy2e premixed turbulent flames theoretically, two processes should be considered (/) the effects of combustion on the turbulence, and (2) the effects of turbulence on the average chemical reaction rates. In a turbulent flame, the peak time-averaged reaction rate can be orders of magnitude smaller than the corresponding rates in a laminar flame. The reason for this is the existence of turbulence-induced fluctuations in composition, temperature, density, and heat release rate within the flame, which are caused by large eddy stmctures and wrinkled laminar flame fronts. 

The physics and modeling of turbulent flows are affected by combustion through the production of density variations, buoyancy effects, dilation due to heat release, molecular transport, and instabiUty (1,2,3,5,8). Consequently, the conservation equations need to be modified to take these effects into account. This modification is achieved by the use of statistical quantities in the conservation equations. For example, because of the variations and fluctuations in the density that occur in turbulent combustion flows, density weighted mean values, or Favre mean values, are used for velocity components, mass fractions, enthalpy, and temperature. The turbulent diffusion flame can also be treated in terms of a probabiUty distribution function (pdf), the shape of which is assumed to be known a priori (1). 

When plastics are used, their behavior in fire must be considered. Ease of ignition, the rate of flame spread and of heat release, smoke release, toxicity of products of combustion, and other factors must be taken into account. Some plastics bum readily, others only with difficulty, and still others do not support their own combustion A plastic s behavior in fire depends upon the nature and scale of the fire as well as the surrounding conditions. Fire is a highly complex, variable phenomenon, and the behavior of plastics in a fire is equally complex and variable (Chapter 5, FIRE). 

The basic approach taken in the analytical studies of composite-propellant combustion represents a modification of the studies of double-base propellants. For composite propellants, it has been assumed that the solid fuel and solid oxidizer decompose at the solid surface to yield gaseous fuel and oxidizing species. These gaseous species then intermix and react in the gas phase to yield the final products of combustion and to establish the flame temperature. Part of the gas-phase heat release is then transferred back to the solid phase to sustain the decomposition processes. The temperature profile is assumed to be similar to the situation associated with double-base combustion, and, in this sense, combustion is identical in the two different types of propellants. 

Flames submitted to convective disturbances experience geometrical variations, which can in turn give rise to heat release unsteadiness. This process can be examined by considering different types of interactions between incident velocity or equivalence ratio modulations and combustion. The flame dynamics resulting from these interactions give rise to sound radiation and... 

Flame dynamics is intimately related to combustion instability and noise radiation. In this chapter, relationships between these different processes are described by making use of systematic experiments in which laminar flames respond to incident perturbations. The response to incoming disturbances is examined and expressions of the radiated pressure are compared with the measurements of heat release rate in the flame. The data indicate that flame dynamics determines the radiation of sound from flames. Links between combustion noise and combustion instabilities are drawn on this basis. These two aspects, usually treated separately, appear as manifestations of the same dynamical process. 

The analysis of combustion dynamics is then intimately linked to an understanding of perturbed flame dynamics, the subsequent generation of unsteady rates of heat release, and the associated radiation of sound and resulting acoustic feedback. In practical configurations, the resonance loop involves the flow, the combustion process, and the acoustic modes of the system as represented schematically in Figure 5.2.2. 

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