Combustion rate

Emissions Control. From the combustion chemistry standpoint, lean mixtures produce the least amount of emissions. Hence, one pollution prevention alternative would be to use lean premixed flames. However, lean mixtures are difficult to ignite and form unstable flames. Furthermore, thek combustion rates are very low and can seldom be appHed dkectly without additional measures being taken. Consequently the use of lean mixtures is not practical. 

Ease of ignition, combustion rate and combustion temperature all increase when the oxygen content of an atmosphere >21 % (Chapter 6). 

A deflagration can best be described as a combustion mode in which the propagation rate is dominated by both molecular and turbulent transport processes. In the absence of turbulence (i.e., under laminar or near-laminar conditions), flame speeds for normal hydrocarbons are in the order of 5 to 30 meters per second. Such speeds are too low to produce any significant blast overpressure. Thus, under near-laminar-flow conditions, the vapor cloud will merely bum, and the event would simply be described as a large fiash fire. Therefore, turbulence is always present in vapor cloud explosions. Research tests have shown that turbulence will significantly enhance the combustion rate in defiagrations. 

These mechanisms may cause very high flame speeds and, as a result, strong blast pressures. The generation of high combustion rates is limited to the congested area, or the area affected by the turbulent release. As soon as the flame enters an area without turbulence, both the combustion rate and pressure will drop. 

A flash fire results from the ignition of a released flammable cloud in which there is essentially no increase in combustion rate. In fact, the combustion rate in a flash fire does increase slightly compared to the laminar phase. This increase is mainly due to the secondary influences of wind and surface roughness. 

Figure 2.1 identifies the conditions necessary for the occurrence of a flash fire. Only combustion rate differentiates flash fires from vapor cloud explosions. Combustion rate determines whether blast effects will be present (as in vapor cloud explosions) or not (as in flash fires). 

It is assumed that the target surface faces toward the radiation source so that it receives the maximum incident flux. The rate of combustion depends on the release. For a pool fire of a fuel with a boiling point above the ambient temperature (Tg), the combustion rate can be estimated by the empirical relation ... 

High-energy ignition of an unobstructed cloud by a jet flame emerging from a partially confined explosion produces a high combustion rate in the jet-flow region. 

Cylindrical geometry is obtained by placing two plates parallel to each other and introducing a gas mixture between them. The gas is usually ignited in the center. Obstacles are introduced to enhance the combustion rate (Figure 4.8). 

The channel experiments produced results similar to those from tubes. Introduction of venting (decrease of the degree of confinement) greatly reduces effectiveness of the positive-feedback mechanism. Obstacles appear to enhance the combustion rate considerably. 

In relatively low-reactive fuel-air mixtures, a detonation may only arise as a consequence of the presence of appropriate boundary conditions to the combustion process. These boundary conditions induce a turbulent structure in the flow ahead of the flame front. This turbulent structure is a basic element in the feedback coupling in the process by which combustion rate can grow more or less exponentially with time. This fundamental mechanism of a gas explosion has been described in Section 3.2. 

Experimental research has shown that a vapor cloud explosion can be described as a process of combustion-driven expansion flow with the turbulent structure of the flow acting as a positive feedback mechanism. Combustion, turbulence, and gas dynamics in this complicated process are closely interrelated. Computational research has explored the theoretical relations among burning speed, flame speed, combustion rates, geometry, and gas dynamics in gas explosions. 

The major mechanism of a vapor cloud explosion, the feedback in the interaction of combustion, flow, and turbulence, can be readily found in this mathematical model. The combustion rate, which is primarily determined by the turbulence properties, is a source term in the conservation equation for the fuel-mass fraction. The attendant energy release results in a distribution of internal energy which is described by the equation for conservation of energy. This internal energy distribution is translated into a pressure field which drives the flow field through momentum equations. The flow field acts as source term in the turbulence model, which results in a turbulent-flow structure. Finally, the turbulence properties, together with the composition, determine the rate of combustion. This completes the circle, the feedback in the process of turbulent, premixed combustion in gas explosions. The set of equations has been solved with various numerical methods e.g., SIMPLE (Patankar 1980) SOLA-ICE (Cloutman et al. 1976). 

This approach makes it possible to model a vapor cloud explosion blast by consideration of the two major characteristics of such a blast. These are, first, its scale, as determined by the amount of combustion energy involved and, second, its initial strength, as determined by combustion rate in the explosion process. 

In the overview of experimental research, it was shown that explosive, blastgenerating combustion in gas explosions is caused by intense turbulence which enhances combustion rate. On one hand, turbulence may be generated during a gas explosion by an uncontrolled feedback mechanism. A turbulence-generative environment, in the form of partially confining or obstructing structures, must be present for this mechanism to be triggered. 

The jet by which the propane is released. The jet s propane-air mixture is in intensely turbulent motion and will develop an explosive combustion rate and blast effects on ignition. 

In the rest of the cloud, which is unconfined and unobstructed, no explosive combustion rates can be maintained nor developed. 

S.K. Sinha W.D. Patwardhan, Explosiv-stoffe 16 (10), 223-25 (1968) CA 70,49144 (1969) The mechanism causing the plateau effect in the combustion of proplnts with ad-mixt of Pb compds (ie, the independence of pressure of the combustion rate in a certain range) is discussed. This effect is caused by the transport of free Pb alkyl radicals from the foam zone to the fizz zone, which decomn there, causing a more efficient combustion, and increase the temp of this zone by reaction1 with NO. An increase of pressure is assumed to displace the free radicals from this zone because of the increase of the collision rate . this leads... 

CA 78, 161665 (1973) A math analysis of the theory is presented on the basis of the combustion rate, the thermal conductivity, the heat capacity, the surface temp of the proplnt grains, and other factors. Expts were made to determine the relation of the combustion rate to acceleration for various proplnts. The rate of combustion at 70 atm was compared with the initial rate. The. relation of the critical pressure of transitional laminar combustion to acceleration, and the dependence of the combustion rate of nitroglycol to the pressure at various acceleration rates were determined. Exptl observations were compared with results of theoretical calcns... 

Laminar flame speed is one of the fundamental properties characterizing the global combustion rate of a fuel/ oxidizer mixture. Therefore, it frequently serves as the reference quantity in the study of the phenomena involving premixed flames, such as flammability limits, flame stabilization, blowoff, blowout, extinction, and turbulent combustion. Furthermore, it contains the information on the reaction mechanism in the high-temperature regime, in the presence of diffusive transport. Hence, at the global level, laminar flame-speed data have been widely used to validate a proposed chemical reaction mechanism. 

The difficult part of the problem is to identify and describe the mechanism by which the acoustic wave modulates the combustion rate. There are many possible mechanisms by which an acoustic wave can influence combustion, and the dominant mechanism varies with the design of the combustion device. Possible coupling mechanisms include... 

It is worth noticing that the "turbulent burning rates" reported in Figure 7.1.2 have been defined similarly but not exactly as the "turbulent flame speed" mentioned in Section 7.1.2. The mixture has been ignited at the center of the bomb and the dependence of the pressure on time has been recorded. This has enabled to determine the derivative of the burned mixture volume. This derivative is ascribed to a spherical surface whose volume is simply equal to the volume of fully burned products, thus leading to an estimate of the turbulent combustion rate. 

Fig. 6-1. The response to fossil fuel burning of atmospheric carbon dioxide. The fossil fuel combustion rate is shown at the bottom of the figure, and the calculated carbon dioxide level appears as a solid line at the top of the figure. Observed carbon dioxide values are plotted as triangles (Broecker and Peng, 1982). The observations have been normalized to preindustrial theoretical values.

These studies have found that increased confinement leads to flame acceleration and increased damage. The flame acceleration is caused by increased turbulence which stretches and tears the flame front, resulting in a larger flame front surface and an increased combustion rate. The turbulence is caused by two phenomena. First, the unburned gases are pushed and accelerated by the combustion products behind the reaction front. Second, turbulence is caused by the interaction of the gases with obstacles. The increased combustion rate results in additional turbulence and additional acceleration, providing a feedback mechanism for even more turbulence. 

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