Combustion and Turbulence

A more detailed description of the used models for combustion and turbulent flow is given in 13]. 

Simonin O (1996) Combustion and turbulence in two-phase flows, von Karman... 

R. BOTghi Background on droplets and sprays. In Combustion and Turbulence in Two-Phase Flows, Lecture series 1996-02, Von Karman Institute for Fluid Dyanamics. 

The balanced equation for turbulent kinetic energy in a reacting turbulent flow contains the terms that represent production as a result of mean flow shear, which can be influenced by combustion, and the terms that represent mean flow dilations, which can remove turbulent energy as a result of combustion. Some of the discrepancies between turbulent flame propagation speeds might be explained in terms of the balance between these competing effects. 

A unified statistical model for premixed turbulent combustion and its subsequent application to predict the speed of propagation and the stmcture of plane turbulent combustion waves is available (29—32). 

In general, comprehensive, multidimensional modeling of turbulent combustion is recognized as being difficult because of the problems associated with solving the differential equations and the complexities involved in describing the interactions between chemical reactions and turbulence. A number of computational models are available commercially that can do such work. These include FLUENT, FLOW-3D, and PCGC-2. 

To achieve complete combustion (i.e., the combination of the combustible elements and compounds of a fuel with all the oxygen that they can utilize), sufficient space, time, and turbulence and a temperature high enough to ignite the constituents must be provided. 

The three T s of combustion—time, temperature, and turbulence—govern the speed and completeness of the combustion reaction. For complete combustion, the oxygen must come into intimate contact with the combustible molecule at sufficient temperature and for a sufficient length of time for the reaction to be completed. Incomplete reactions may result in the generation of aldehydes, organic acids, carbon, and carbon monoxide. 

Large Fans These could be used to dilute a vapor cloud below its LFL with ambient air (see, for example, Whiting and Shaffer, Feasi-bihty Study of Hazardous Vapor Amelioration Techniques, Proc. 1978 Nat. Conf. on Control of Hazardous Material Spills, USEPA, Miami Beach, April 1978). But caution must be exercised because the turbulence produced by fans will likely promote rapid combustion and a resulting UVCE unless vapors are diluted below the LFL. Nevertheless, in new plants, strategic placement of air coolers may provide enough air flow to reduce the risk of a UVCE. 

When visualizing a combustion process, it is useful to think of it in terms of the three Ts time, temperature, and turbulence. Time for combushon to occur is necessary. A combustion process that is just initiated, and suddenly has its reactants discharged to a chilled environment, will not go to completion and will emit excessive pollutants. A high enough temperature must exist for the combustion reaction to be initiated. Combushon is an exothermic reachon (it gives off heat), but it also requires energy to be inihated. This is iUustrated in Fig. 6-5. 

Flare and Burners - Certainly the oldest and still widely used technology through some parts of the world is flaring. Flares are used in the petroleum, petrochemical, and other industries that require the disposal of waste gases of high concentration of both a continuous or intermittent basis. As other thermal oxidation technologies, the three T s of combustion of time, temperature, and turbulence are necessary to achieve adequate emission control. 

GASFLOW models geometrically complex containments, buildings, and ventilation systems with multiple compartments and internal structures. It calculates gas and aerosol behavior of low-speed buoyancy driven flows, diffusion-dominated flows, and turbulent flows dunng deflagrations. It models condensation in the bulk fluid regions heat transfer to wall and internal stmetures by convection, radiation, and condensation chemical kinetics of combustion of hydrogen or hydrocarbon.s fluid turbulence and the transport, deposition, and entrainment of discrete particles. 

Fundamental, laminar, and turbulent burning velocities describe three modes of flame propagation (see the Glossary for definitions). The fundamental burning velocity, S, is as its name implies, a fundamental property of a flammable mixture, and is a measure of how fast reactants are consumed and transformed into products of combustion. Fundamental burning velocity data for selected gases and vapors are listed in Appendix C of NFPA68 (1998). 

Khitrin, L. N. et al. 1965. Peculiarities of Laminar- and Turbulent-Flame Flashbacks. Proe. 10th Sympos. (Inti.) on Combustion, pp. 1285-1291. 

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. 

Turbulence may arise by two mechanisms. First, it may result either from a violent release of fuel from under high pressure in a jet or from explosive dispersion from a ruptured vessel. The maximum overpressures observed experimentally in jet combustion and explosively dispersed clouds have been relatively low (lower than 1(X) mbar). Second, turbulence can be generated by the gas flow caused by the combustion process itself an interacting with the boundary conditions. 

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). 

On the other hand, turbulence may also be generated by external sources. For example, fuels are often stored in vessels under pressure. In the event of a total vessel failure, the liquid will flash to vapor, expanding rapidly and producing fast, turbulent mixing. Should a small leak occur, fuel will be released as a high-velocity, turbulent jet in which the fuel is rapidly mixed with air. If such an intensely turbulent fuel-air mixture is ignited, explosive combustion and blast can result. 

The key to efficient destruction of liquid hazardous wastes lies in minimizing unevaporated droplets and unrcacted vapors. Just as for the rotary kiln, temperature, residence time, and turbulence may be optimized to increase destruction efficiencies. Typical combustion chamber residence time and temperature ranges arc 0.5-2 s and 1300-3000°F. Liquid injection incinerators vary in dimensions and have feed rates up to 1500 gal/h of organic wastes and 4000 gal/h of aqueous waste. 

W, R. Hawtlionie, D. S. Wenddell, aiid M. C. Ho((d, "Mixing and Combustion in Turbulent Gas Jets," in Third Symposium on Combustion, Williaiiis Wil- kins, Baltimore, 1949. 

By recirculating a part of the flue gas to the furnace, the combustion zone turbulence is increased, the temperature is lowered and the oxygen concentration is reduced. All of these factors lead to a reduction of NO, fonnation. 

Pulverized fuel coal burners (typically turbulent air burners, vertical burners, or nozzle burners) receive hot primary air containing the PF and introduce the mixture to secondary air in such a way that it provides a stable flame. The flow rates of both primary and secondary air are controlled by dampers. An ignitor is required to initiate combustion, and the flame front is maintained close to the burner, with the heat of combustion used to ignite incoming PF. A flame safety device electronically scans the flame and initiates corrective action if required. 

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. 

Tabaczynski, R. Trinker, F. H., and Shannon, B. A. S., Further refinement and validation of a turbulent flame propagation model for spark-ignition engines. Combustion and Flame, 39, 111-121, 1980. 

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