Combustion reaction rate

Extinction can be ultimately defined as the reduction of the combustion reaction rates below a critical threshold that... 

The combustion reaction rate is controlled both by the availability of fuel and oxygen kinetic effects (temperature). In full-scale fire modeling, the resolvable length and time scales are usually much larger than those associated with the scales of the chemical combustion reaction, and it is common to assume that the reactions are infinitely fast. The local reaction rate depends on the rate at which oxygen and fuel are transported toward the surface of stoichiometric mixture fraction, shown in Figure 20.2 as a point where both oxygen and fuel mass fractions go to zero. For almost 20 years, the EBU or eddy dissipation models were the standard models used by the combustion CFD community. With the EBU, in its simplest form, the local rate of fuel consumption is calculated as [3] ... 

Figure 13. Typical polarization and relaxation transients of catalyst potential, (Fwr-// ), and ethylene combustion reaction rate, r, on Ir02/YSZ catalyst. Galvanostatic anodic polarization with / = 10 pA during 100 min. (a), (b), (c) and (d) designate the subsequent transient steps. Catalyst loading 77 pg IrCh. Feed composition at pm = 100 kPa PC2H4 = 12.5 Pa, - 1.25 kPa, balance helium. Flow rate 200 mL min" STP 7 =375°C. Reprinted from J. Electroanal. Chem., Q. F6ti, V. Stankovii, I. BolzoneUa, and Ch. Comninel-lis. Transient Behavior of Electrochemical Promotion of Gas-Phase Catalytic Reactions, (2002), in press, with permission from Elsevier Science,...

Generally, the rate of heat transfer is low near the burner wall because the temperature differences are very small. (Load movement is counterflow to flame movement thus, the flame reactants are coolest as they leave any one zone whereas the load pieces are hottest as they leave any one zone.) As the distance from the burner wall increases, the load surface is colder and the flame temperature is hotter because the combustion reaction rate accelerates. However, a control T-sensor 15 ft (4.6 m) from the burner wall limits the furnace temperature at that point. (This temperature is held to a setpoint determined by the operator or by a model.) With high-spin burners, as one follows the temperature profile away from its maximum and in the direction of flame reactant flow, the furnace temperature declines quickly to the setpoint, and thereafter drops rapidly to the exit. 

The variation of the combustion reaction rate with temperature will be governed by the Arrhenius equation. For a reaction which is first order in fuel concentration ... 

To make our combustion fundamentals considered above applicable to dust explosions we need only to add in the influence of particle size on reaction rate. Assuming the combustion reaction rate is now determined by the surface area of solid fuel particles (assumed spherical) exposed to the air, the heat release term (2) in Equation (15.4) becomes ... 

The development of combustion theory has led to the appearance of several specialized asymptotic concepts and mathematical methods. An extremely strong temperature dependence for the reaction rate is typical of the theory. This makes direct numerical solution of the equations difficult but at the same time accurate. The basic concept of combustion theory, the idea of a flame moving at a constant velocity independent of the ignition conditions and determined solely by the properties and state of the fuel mixture, is the product of the asymptotic approach (18,19). Theoretical understanding of turbulent combustion involves combining the theory of turbulence and the kinetics of chemical reactions (19—23). 

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. 

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... 

The majority of the NOx produced in the combustion chamber is called thermal NOx. It is produced by a series of chemical reactions between the nitrogen (N2) and the oxygen (O2) in the air that occur at the elevated temperatures and pressures in gas turbine combustors. The reaction rates are highly temperature dependent, and the NOx production rate becomes significant above flame temperatures of about 3300 °F (1815 °C). Figure 10-19 shows schematically, flame temperatures and therefore NOx production... 

The Chapman-Jongnet (CJ) theory is a one-dimensional model that treats the detonation shock wave as a discontinnity with infinite reaction rate. The conservation equations for mass, momentum, and energy across the one-dimensional wave gives a unique solution for the detonation velocity (CJ velocity) and the state of combustion products immediately behind the detonation wave. Based on the CJ theory it is possible to calculate detonation velocity, detonation pressure, etc. if the gas mixtnre composition is known. The CJ theory does not require any information about the chemical reaction rate (i.e., chemical kinetics). 

Combining volumes, law of, 26, 236 Combustion, heat of hydrogen, 40 Complex ions, 392 amphoteric, 396 bonding in, 395 formation, 413 geometry of. 393 in nature, 396 isomers, 394 linear, 395 octahedral, 393 significance of, 395 square planar, 395 tetrahedral, 394 weak acids, 396 Compound, 28 bonding in, 306 Concentration and equilibrium, 148 and E zero s, 213 and Le Chatelier s Principle, 149 effect on reaction rate, 126, 128 molar, 72... 

In the search for a better approach, investigators realized that the ignition of a combustible material requires the initiation of exothermic chemical reactions such that the rate of heat generation exceeds the rate of energy loss from the ignition reaction zone. Once this condition is achieved, the reaction rates will continue to accelerate because of the exponential dependence of reaction rate on temperature. The basic problem is then one of critical reaction rates which are determined by local reactant concentrations and local temperatures. This approach is essentially an outgrowth of the bulk thermal-explosion theory reported by Fra nk-Kamenetskii (F2). 

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