Combustion mode

Heat is produced by chemical reaction in a reaction zone. The heat is transported, mainly by conduction and molecular diffusion, ahead of the reaction zone into a preheating zone in which the mixture is heated, that is, preconditioned for reaction. Since molecular diffusion is a relatively slow process, laminar flame propagation is slow. Table 3.1 gives an overview of laminar burning velocities of some of the most common hydrocarbons and hydrogen. 

What are the mechanisms by which slow, laminar combustion can be transformed into an intense, blast-generating process This transformation is most strongly influenced by turbulence, and secondarily by combustion instabilities. A laminar-flame front propagating into a turbulent mixture is strongly affected by the turbulence. Low-intensity turbulence will only wrinkle the flame front and enlarge its surface area. With increasing turbulence intensity, the flame front loses its more-or-less smooth, laminar character and breaks up into a combustion zone. In an intensely turbulent mixture, combustion takes place in an extended zone in which 

Rgura 3.1. Temperature distribution across a iaminar flame. 

If such a process continues to accelerate, the combustion mode may suddenly change drastically. The reactive mixture just in front of the turbulent combustion zone is preconditioned for reaction by a combination of compression and of heating by turbulent mixing with combustion products. If turbulent mixing becomes too intense, the combustion reaction may quench locally. A very local, nonreacting but highly reactive mixture of reactants and hot products is the result. 

The pressure behind the nonreactive shock is much higher than the CJ detonation pressure, which is not attained until the reaction is complete. The duration of the 

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Multiftmctional SO removal catalyst systems have been ia commercial use siace 1985 ia the United States. Such systems have successfully reduced SO emissions in the FCCU regenerator by 50% when the regenerator is operating in the complete CO combustion mode (45). Modern-day additives can achieve a 50% SO reduction with only IS—2% of the additive in the circulating inventory, an amount small enough not to interfere with the cracking characteristics of the bulk FCC catalyst (45). 

Because O2 is necessary to convert SO2 to SO, decreasing O2 in the regenerator has been found to reduce the effectiveness of the SO removal additive. The SO additives used in regenerators operating in a partial CO combustion mode, where excess O2 is frequentiy limited to <0.2 vol % in the flue gas, are less successful in reducing SO. In such cases, SO removal is typically 20—30% less than for a full CO combustion (1 + % excess O2) case (45). 

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. 

Flow fields resulting from these combustion modes were computed by means of similarity methods (Section 4.2.1) and used to provide initial conditions for numerical computations. The main conclusion was that blast waves at some distance from the charge were very similar, regardless of whether the combustion mode was detonation or strong deflagration. 

Operating Region Regenerator Combustion Partial Combustion Mode Full Combustion Mode... 

Since most of the regenerators operating in full combustion mode usually operate with 1 % to 3% excess oxygen, the capturing efficiency of SOj - additive is often greater in full combustion than in partial combustion units. 

The conventional Cl combustion mode with highly stratified in-cylinder conditions creates a perennial soot-NO , trade-off that severely limits the... 

Currently, the ID model describes the essential features of the propagation of a combustion front in the reverse combustion mode. With an adapted version of the model, the combustion of biomass could be modeled accurately. To obtain... 

Not all metals bum heterogeneously. The determination of which metals will bum in a heterogeneous combustion mode can be made from a knowledge of the thermodynamic and physical properties of the metal and its oxide product [1],... 

The underfired combustion mode corresponded to the operation of an updraft gasifier. The tests carried out on wet wood chips and peat lumps (35-60%) showed that the combustion rates are many times higher than the same conditions for the overfired mode. Koistinen et al were able to gasify wood chips with a moisture content of 58% d.b. However, the off-gas was so humid that it was not ignitable until the end of each run when the drying had ceased. Koistinen et al concluded that the combustion rate increased linearly with an increased primary air flow rate. 

Detonation, Pseudo. Phenomenon of pseudo detonation was observed by Pangburn et al (Ref 1, p 7) during comparison of combustion modes in intermittent jet engines. The same phenomenon is also known as unstable double discontinuity or latent combustion phase Dunkle (Ref 2, p lie), under the heading "Coalescence of Shock and Combustion Waves , stated that if both shock wave front and combustion wave front move at the same velocity, the rapid photography camera frame in which the shock wave front is stationary and the frame in which the flame front is stationary are the same. This, however, is not always rhe case. 

Refs 1) D.F. Pangburn, "A Comparison of Combustion Modes in Intermittent Jet Engines , RPI (Rensselaer Polytechnic Institute) TechRept AE 5404(1955)... 

Though the physical structures of CMDB propellants are heterogeneous, similar to those of composite propellants, the base matrix used as a binder burns by itself and the combustion mode of CMDB propellants appears to be different from that of composite propellants and double-base propellants. The burning rate of a CMDB propellant is dependent on the type of crystalline particles incorporated. 

The burning surface of an HMX propellant only becomes covered with carbonaceous materials when the propellant is catalyzed with both LiF and C. This surface structure is similar to the burning surface of an H MX propellant catalyzed with a lead compound and C. The results indicate that the combustion mode and the action of LiF are the same as those resulting from the use of lead compounds to produce super-rate and plateau burning of nitramine propellants. 

The last ten years have witnessed a number of extensive field tests of underground coal gasification (UCG) in the United States and Europe. Model development is essential to the proper understanding of these test results and to the planning of future experiments. This report will focus upon the steady-state "permeation" or "packed bed" model of in situ gasification (forward combustion mode). In this useful but idealistic model the coal bed is assumed to be uniformly permeable to reactant and product gases. 

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