OCR Text |
Show In a diffusion flame of gas or evaporated hydrocarbon fuel, a great deal of soot particles m ay be formed. These particles are formed at the first stage of the reaction and, then, they burn down if the sufficient amount of oxygen is available. The soot particles produce intense radiation, therefore the gas temperature in the reaction zone decreases by 300-400 K. The gas temperature in the zone of intensive reactions of the diffusion flame (a»l) is considerably lower than in the flame fronts of homogeneous fuel-air mixtures with a»l, since the flame of homogeneous mixtures does not produce radiation as intensely as the soot particles do. For example, if the rated thermodynamic temperature of the complete combustion of the "natural gas/air" stoichiometric mixture (a=l) is 2200 K [5] for the initial temperature of r0=293 K, then the highest gas temperature in the diffusion flame front should approximate 1800-1900 K. In the other regions of the flame, the temperature also decreases due to the radiation. This phenomenon facilitates decreasing the rate of both the "prompt" N O x and the "thermal" N O x formation. In order to reduce the residence time of gas at the highest temperatures, it is necessary to increase gas flow velocity. However, if the air flow velocity is higher than 10 m/s, the diffusion flame of the fuel jets becomes unsteady. Therefore, the special installations - flame holders - are required in order to maintain the flame in the vicinity of fuel nozzles. A s the flame holders, bluff bodies with the reverse flow regions behind them are usually used. In the reverse flow regions, fuel-air combustion with excess air coefficient (awl) is maintained. The combustion products in these regions have the highest temperature equal to 2200 K. The regions secure firing the fuel jets in the concurrent air flow. In this case, a stabilized diffusion flame with steady-state combustion at the flow velocities up to 30 m/s is maintained. O n the other hand the reverse flow regions are intense generators of both "prompt" N O x and "thermal" N O x . Therefore, the reverse flow zones in the proposed combustion chambers should possess spaces as small as possible. In this case, throughout the reverse flow zones, the amount of air and fuel necessary for establishing the steady-state combustion at required conditions with respect to velocity, temperature, and pressure will be minimal. DIFFUSION FLAMES: A CALCULATION In order to estimate a required combustion chamber length Lc, a calculation of combustion of natural gas jets in a concurrent air flow (Figure 1) was carried out. The initial contact between the gas jet and air flow was chosen as a point of the diffusion flame firing. In the calculation, a mathematical model proposed in [6] was used. It is based on the boundary layer model. Within this model, a combustion process is described by an irreversible single-stage reaction provided that the thermodynamic equilibrium exists at every flow point. A n assumption that the heat and admixture transfer processes were similar permitted to express all thermodynamic parameters through the one, namely, the passive admixture concentration. T o model the turbulence, a standard two-parameter k-e model was used. Also an equation for passive admixture concentration fluctuation intensity was involved in order to calculate completeness of the combustion. First and foremost, the calculation results proved an obvious fact that, for the rated excess air for the entire chamber, the diffusion flame length Lf (distance at which the coefficient of combustion completeness equals to r\=0.99) is directly proportional to the characteristic transversal size of the 3 |