Combustion correlation

The use of flame retardants came about because of concern over the flammabiUty of synthetic polymers (plastics). A simple method of assessing the potential contribution of polymers to a fire is to examine the heats of combustion, which for common polymers vary by only about a factor of two (1). Heats of combustion correlate with the chemical nature of a polymer whether the polymer is synthetic or natural. Concern over flammabiUty should arise via a proper risk assessment which takes into account not only the flammabiUty of the material, but also the environment in which it is used. 

At present, a petrographic combustion correlation is being worked out which should provide ultimately a general classification of coals for use in several combustion processes. The results of this study will be reported at a later date. 

Preliminary experimental investigation of biomass combustion in a stoker combustor has shown that the useful heat output or heat efficiency of a biomass fuel combustion in lab-scale trials can be adequately represented by using the stoichiometric relationship and measured incomplete combustion correlations. 

The rise in radiant heat flux levels incident at the crown due to combustion during firing cycles is quantified, with a nominal increases of 105 kW/m at the two locations closest to the batch feeder and increases of approximately 155 kW/m at the two locations in the center of the combustion space. The contribution of radiant heat flux to the crown from combustion is not as appreciable near the working end with increases due to firing of approximately 85 kW/m, and 12 kW/m at the final two measured locations, respectively. The contribution of radiant incident flux on the crown from combustion correlates well with firing conditions in each port location. 

The Octane Requirement Increase, ORI, is a phenomenon manifested by the appearance of knocking and is due to the increase in engine octane demand with time. This phenomenon is correlated with the increase of solid deposits in the combustion chamber. Although the causes have not been determined with certainty, some companies have patented additives which modify the deposits. The effect is to limit the increase in octane demand (Bert et al., 1983 Chevron, 1988 Nelson et al., 1989). 

Limiting Oxygen Index. The minimum concentration of oxygen in an O2/N2 mixture that supports combustion of a vertically mounted test specimen is called the limiting oxygen index (3,4). Test specimens are 0.65 x 0.3 cm x 12.5 cm. The principal advantage of this test is its reproducibiUty which makes it useful for quaUty control. The main disadvantage is that the results rarely correlate with the results of other fire tests. 

Some additional methods of classification are under development that center on the use of lignite for combustion in utihty boilers or electric power generation. Correlations based on the sodium concentration in the lignitic ash (10), or soluble A1 concentration (11) are used. The classifications are often given in terms of the severity of boiler fouling. 

Burning times for coal particles are obtained from integrated reaction rates. For larger particles (>100 fim) and at practical combustion temperatures, there is a good correlation between theory and experiment for char burnout. Experimental data are found to obey the Nusselt "square law" which states that the burning time varies with the square of the initial particle diameter (t ). However, for particle sizes smaller than 100 p.m, the Nusselt... 

E. J. Badin, Coal Combustion Chemistry—Correlation Fispects Elsevier, New York, 1984, Chapt. 6, p. 68. 

The hydrogen content, heat of combustion, specific heat, and thermal conductivity data herein were abstracted from Bureau of Standards MisceUaneous Pubhcation 97, Thermal Propei tie.s of Petroleum Products. These data are widely used, although other correlations have appeared, notably that by Linden and Othmer Chem. Eng. 54[4, 5], April and May, 1947). 

In fact, this correlation is almost exact for many compounds. For some combustible gases and vapors, P sometimes has an exponent somewhat less than 1. 

A comprehensive collection of estimates of TNT equivalencies was deduced from damage patterns observed in major accidental vapor cloud explosions (Gugan 1978). From these estimates, it can be concluded that there is little, if any, correlation between the quantity of combustion energy involved in a vapor cloud explosion... 

A fireball is assumed to bum with a constant temperature in the isothermal fireball model of Lihou and Maund (1982). Combustion is controlled by the supply of air and ceases after a time which is correlated empirically with the mass of flammable gas in the initial vapor sphere. It is assumed that a fraction (1 — /c) of the fuel is used to form soot, and the remaining fractionbums stoichiometrically, producing an increase of /ij moles per mole of flammable gas. The stoichiometric molar ratio of air to flammable gas is p, and dVIdt is the volumetric rate of air entrainment. The rate of increase of volume can now be written as ... 

Hymes (1983) presents a fireball-specific formulation of the point-source model developed from the generalized formulation (presented in Section 3.5.1) and Roberts s (1982) correlation of the duration of the combustion phase of a fireball. According to this approach the peak thermal input at distance L is given by... 

Glowiak (Ref 5) claims that detonation velocity, heat of combustion and Trauzl Block test all correlate with a true OB for 25 nitro compounds (some containing diazo or azido groups). This writer has not seen his original publication CA does not specify intelligibly what is meant by true OB... 

Generally speaking, these studies of erosive burning have been able to correlate the observed effects. Until the structure of the combustion zone is defined and quantitatively characterized in detail, it would appear that the currently available bases for correlating erosive-burning data are adequate. 

During the operation of some solid-propellant motors, several investigators have observed oscillations occurring at low frequences (0-500 cps), as shown in Fig. 23. These oscillations cannot be associated with any of the acoustic modes of the combustion chamber. Angelus (All) was one of the first to investigate these low-frequency oscillations later, Yount and Angelus (Yl) observed that the amplitude of the oscillations decreased and the frequency increased with increasing mean chamber pressure. They correlated... 

In an effort to determine the processes responsible for this type of behavior, Akiba and Tanno (A3), Sehgal and Strand (S2), and Beckstead (B6) have studied the coupling between the dynamics of the combustion process and the dynamic ballistics of the combustion chamber as described by Eq. (7). Each of these investigators has postulated admittedly simplified but slightly different combustion models to couple with the transient ballistic equations. Each has examined the combined equations for regions of instability. The results of these studies suggest a correlation between the L of the motor (the ratio of combustion-chamber volume to nozzle throat area) and the frequency of the oscillations. 

This expression seemed to correlate the data of Anderson (A5) for non-aluminized propellants, but did not work for aluminized propellants. In later work, Sehgal (S2) has studied the aluminum effect in greater detail. He reports that the effect of aluminum appears to cause incomplete combustion. Price (P10) has reported essentially the same observation. Beckstead derived an expression between the frequency of the oscillations and the L of the combustion chamber. The resulting equations were then shown to correlate experimental data. 

Another contributing mechanism is the direct cooling of hot propellant surface by contact with the injected fluid. The fluid should cause the decomposing surface to reduce its pyrolysis rate to a point where combustion cannot be sustained. In addition, the presence of water on the surface would obstruct heat transfer from the gas-phase reaction zones to the solid surface, thus augmenting the cooling of the surface. Proponents of these two approaches have correlated the injection data on the basis of mass of fluid required per unit area of surface, but theoretical justifications for the use of this particular correlating parameter have not been presented. 

A third approach has been suggested by Jaroudi (Jl), who points out that one necessary condition to prevent reignition of the propellant is to ensure that the gas temperature resulting from thermal equilibrium between the injected fluid and the combustion products is below the propellant autoignition temperature. This approach leads to the conclusion that the ratio of coolant mass flow to propellant mass flow is the critical correlating parameter. 

Correlation between liftoff height and jet velocity with partial premixing of air to fuel stream. (From Lee, B.J., Cha, M.S., and Chung, S.H., Combust. Sci. Technoh, 127, 55,1997.)... 

Shy, S.S., Lin, W.J., and Peng, K.Z., High-intensity turbulent premixed combustion General correlations of turbulent burning velocities in a new cruciform burner, Proc. Combust. Inst., 28, 561, 2000. 

Stone, R., Clarke, A., and Beckwith, P, Correlations for the laminar-burning velocity of methane/diluent/air mixtures obtained in free-fall experiments. Combust. Flame, 114, 546, 1998. 

RoCek et al. have also measured rate coefficients for a series of cyclo-alkanes, (CH2) (/i = 4 to 14), and find the analogue of kcHj in equation (25) to fluctuate with ring size in a manner corresponding exactly to the enthalpy of combustion of the cycloalkane concerned per methylene group, provided n is greater than five, i.e. there exists a direct correlation between reactivity and thermochemical strain. 

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