Adiabatic combustion temperature

Direct reaction of transition metals (Ti, Ta, Nb, Zr) or A1 or Si results in adiabatic combustion temperatures of 2500-A500 C. 

The liquid propellant rocket combination nitrogen tetroxide (N204) and IJDMII (unsymmetrical dimethyl hydrazine) has optimum performance at an oxidizer-to-fuel weight ratio of 2 at a chamber pressure of 67 atm. Assume that the products of combustion of this mixture are N2, C02, H20, CO, H2, O, H, OH, and NO. Set down the equations necessary to calculate the adiabatic combustion temperature and the actual product composition under these conditions. These equations should contain all the numerical... 

Equation (4.4), which connects the known variables, unbumed gas pressure, temperature, and density, is not an independent equation. In the coordinate system chosen, //, is (lie velocity fed into the wave and u2 is the velocity coming out of the wave. In the laboratory coordinate system, the velocity ahead of the wave is zero, the wave velocity is uh and (u — u2) is the velocity of the burned gases with respect to the tube. The unknowns in the system are U, u2, P2, T2, and p2. The chemical energy release is q, and the stagnation adiabatic combustion temperature is T, for n-> = 0. The symbols follow the normal convention. 

Note Twl, Volatilization temperature [or stoichiometric adiabatic combustion temperature creating compound under ambient conditions T = 298 K, P = 1 atm) Tg, Meta/ boiling point at 1 atm Te, Decomposition temperature (see... 

FIGURE 9.1 Adiabatic combustion temperature of various metal-oxygen/nitrogen systems as a function of equivalence ratio . Initial conditions 298 K and 1 atm. 

FIGURE 9.2 Equilibrium product composition and adiabatic combustion temperature for stoichiometric A1 and 02 at 298 K and 1 atm as a function of assigned enthalpy. An assigned enthalpy of zero corresponds to the true ambient condition. (1) specifies a liquid product, (s) specifies a solid product and all other products are gas. 

FIGURE 9.10 Comparison of the plot of adiabatic combustion temperature and product composition of the stoichiometric Al-Q2 system at total pressure of 1 atm (Fig. 9.2) and that at 10 atm. 

FIGURE 9.12 Adiabatic combustion temperature of a stoichiometric Al—02 system containing various amounts of die inert diluent argon as a function of assigned enthalpy, other conditions being the same as those for Fig. 9.2. 

Flame stability requires adiabatic combustion temperatures as high as 1600-1800 °C, which must be reduced to 1100-1450 °C by means of cooling bypass air before delivering the hot compressed gas to the turbine to avoid damaging the inlet blades. At such temperatures, within the tens milliseconds residence time required for complete burnout of fuel and CO, significant amounts of NO are produced, mostly by the Zeldovich thermal mechanism [2]. 

Plenty of radiant heat is liberated, but only to the refractory brick walls. The bricks then reradiate the heat back into the gaseous products of reaction. This is called adiabatic combustion because no heat is lost from the combustion reaction to radiation. The adiabatic-combustion temperature for the preceding reaction [Eq. (21.1)1 is about 2300°F. The refractory used to contain this high temperature is manufactured from 90 percent alumina. Such refractory may be exposed to temperatures of up to 2900°F, without damage. 

The enthalpies are written in terms of H° - H0° because most tables of enthalpy are tabulated in this form. When all the heat evolved is used to heat up the product gases, the product temperature T is called the flame or adiabatic combustion temperature and ... 

Note that Eq. (24) is not valid for 2, since it leads to zero (n=2) and imaginary (n>2) values of velocity. The origin of this discrepancy is that when the dependence of reaction rate on conversion is sufficiently strong (e.g., for higher-order reactions), the reaction rate away from the adiabatic combustion temperature may also be significant (Merzhanov and Khaikin, 1988). Utilizing this idea, while maintaining the thin-zone approximation, Khaikin and Merzhanov (1966) derived an expression for the case of n th order kinetics ... 

Combustion with a thin reaction zone (discussed earlier), where T =7 c=7 o+0/Cp, and for which the combustion velocity is determined by the adiabatic combustion temperature... 

In an early work by Kottke and Niiler (1988), a cellular model was used to simulate the combustion wave initiation and propagation for the TH-C model system. The interactions between neighboring cells were described by the electrical circuit analogy to heat conduction. At the reaction initiation temperature (i.e., melting point of titanium), the cell is instantly converted to the product, TiC, at the adiabatic combustion temperature. The cell size was chosen to be twice as large as the Ti particles (44 /xm). Experimentally determined values for the green mixture thermal conductivity as a function of density were used in the simulations. As a result, the effects of thermal conductivity of the reactant mixture on combustion wave velocity were determined (see Fig. 21). Advani et al. (1991) used the same model, and also computed the effects of adding TiC as a diluent on the combustion velocity. 

Thermodynamic calculations can identify the adiabatic combustion temperature, as well as the equilibrium phases and compounds present at that temperature. The composition of the equilibrium final products is determined by minimizing the thermodynamic potential. For a system with gas and A solid number of components, at constant pressure, this may be expressed as... 

Thermodynamic calculations of adiabatic combustion temperatures and their comparison with experimentally measured values have been made for a variety of systems (cf Holt and Munir, 1986 Calcote et al, 1990 Glassman et al, 1992). Under conditions that lead to full conversion, a good agreement between theoretical and experimental values has generally been obtained (Table XX). 

System Adiabatic Combustion Temperature, Tt," (K) Measured Combustion Temperature, Tc (K) Lowest Melting Point On the Phase Diagram (K)... 

The thermodynamic calculations for the equilibrium combustion temperature and product compositions for a three-component (Ni-Al-NiO) reduction-type system are shown in Figs. 33a and b, respectively (Filatov et al, 1988). In Fig. 33a, the curves correspond to constant adiabatic combustion temperature at P= 1 atm. The maximum was calculated to be 3140 K for the 2Al+3NiO green mixture... 

Consider the combustion reaction between a solid reactant and a gas oxidizer present initially in the constant volume of a porous medium (see Section IV,D,1). In this case, thermodynamic calculations for the silicon-nitrogen system have been made for constant volumes (Skibska et al, 1993b). The calculations yield the adiabatic combustion temperature, as well as pressures and concentration, as functions of the silicon conversion. As shown in Fig. 34a, the reactant gas pressure (curve 3) increases even though conversion increases. This occurs because... 

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