Heat release rate flaming combustion

Samples are 150 x 150 mm and are cut from cabin components or tested in representative thickness. Samples are exposed to a radiemt heat flux of 35 kW/m in a vertical orientation and ignited by an impinging pilot flame. The rate of heat released by flaming combustion is deduced from the calibrated temperature rise of the air stream passing over the sample surface at a prescribed flow rate. The maximum rate of heat release cannot be greater than 65 kW/m over the entire... 

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. 

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. 

To examine the effect of turbulence on flames, and hence the mass consumption rate of the fuel mixture, it is best to first recall the tacit assumption that in laminar flames the flow conditions alter neither the chemical mechanism nor the associated chemical energy release rate. Now one must acknowledge that, in many flow configurations, there can be an interaction between the character of the flow and the reaction chemistry. When a flow becomes turbulent, there are fluctuating components of velocity, temperature, density, pressure, and concentration. The degree to which such components affect the chemical reactions, heat release rate, and flame structure in a combustion system depends upon the relative characteristic times associated with each of these individual parameters. In a general sense, if the characteristic time (r0) of the chemical reaction is much shorter than a characteristic time (rm) associated with the fluid-mechanical fluctuations, the chemistry is essentially unaffected by the flow field. But if the contra condition (rc > rm) is true, the fluid mechanics could influence the chemical reaction rate, energy release rates, and flame structure. 

There are many different aspects to the field of turbulent reacting flows. Consider, for example, the effect of turbulence on the rate of an exothermic reaction typical of those occurring in a turbulent flow reactor. Here, the fluctuating temperatures and concentrations could affect the chemical reaction and heat release rates. Then, there is the situation in which combustion products are rapidly mixed with reactants in a time much shorter than the chemical reaction time. (This latter example is the so-called stirred reactor, which will be discussed in more detail in the next section.) In both of these examples, no flame structure is considered to exist. 

The integral of the heat release rate with respect to temperature is taken over the entire region in which the reaction rate is non-zero. In fact, due to the rapid decrease in the reaction rate as the temperature falls, the basic contribution to the integral is made by a comparatively narrow temperature interval adjacent to the combustion temperature. Substituting the appropriate law for the reaction rate (mono- or bimolecular, etc.), we obtain the corresponding, rather cumbersome expressions for the flame velocity which we do not give here. The basic formula (1.3.5) allows us to analyze the influence of various parameters of the mixture on the velocity. 

At first glance it is surprising that the heat capacity of the mixture, Q, enters the denominator in the formula for the velocity since we know that as the heat capacity increases, so does the flame velocity. The fact is that for a constant initial temperature, growth of the heat capacity of the mixture induces an increase in the combustion temperature, but the increase of the integral of the heat release rate outweighs the increase in Q as the temperature is raised. 

From the technology of combustion we move to the molecular mechanism of flame propagation. We shall give a molecular-kinetic expression for the heat release rate by calculating the frequency v of collisions of fuel molecules with other molecules (v is proportional to the molecular velocity and inversely proportional to the mean free path), further taking into account that only a small (1/j/) part of all collisions are effective. The quantity 1/v—the probability of reaction taken with respect to a single collision— depends on the activation heat of an elementary reaction event, as well as on the fraction of all molecules comprised of those radicals or atoms by means of which the reaction occurs. The molecular-kinetic expression for the coefficient of thermal conductivity follows from formulas (1.2.4) and (1.2.3). 

If we are dealing with mutual diffusion of gases which are close in molecular weight (e.g., carbon monoxide and air), it may be shown that the temperature of the flame pellet will prove to be equal to the theoretical combustion temperature of the mixture. This equality depends on the existence in the kinetic theory of gases of a simple relation between the diffusion coefficient (on which the supply of reagents and heat release rate depend) and the thermal conductivity (on which the heat evacuation depends). 

Heat release rate is another relevant measure of the combustibility of a material along with ease of ignition and flame spread. Smith (55) points out that the release rate data, obtained under different test exposures, will be useful in predicting the performance in actual fires under different fuel loading. Release rate data can thus be used—along with other... 

FIGURE 26.6 Relationship between FIGRA and THR measured in MCC (i.e., PCFC). FIGRA = PHRR/ TTPHRR, FPI = TTI/PHRR, where PHRR is peak heat release rate, TTPHRR is time to peak heat release rate, and TTI is time to ignition. (Based on Lin, T.S. et al., Correlations between microscale combustion calorimetry and conventional flammability tests for flame retardant wire and cable compounds, in Proceedings of 56th International Wire and Cable Symposium, 2007, pp. 176-185.)... 

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