Characteristic features of the process of mixture formation upon fuel injection into a high-temperature air flow

Abstract

Physical models of the processes of interphase heat and mass transfer and computational techniques based on them are suggested. The problem of the interaction of fuel droplets with a high-temperature air flow is solved numerically. It is shown that injection of a fuel by a spray atomizer may cause a substantial change in the gas temperature. With pneumatic spraying of a fuel by a cold air jet the influence of interphase exchange on the characteristics of the main flow is insignificant. Upon injection of a hot spraying air the influence of fuel injection on the formation of temperature fields is more appreciable. However, here the gas is cooled less than in the case of spraying by a jet. Introduction. Combustion of a fuel in the combustion chambers of a gas-turbine engine and a gas-turbine plant is closely connected with the processes of mixing (1). Investigations of these processes carried out by both ex- perimental and computational methods (1-3) have recently become especially crucial because of the necessity of solv- ing ecological problems. One of the most pressing problems at present is account for the influence of droplets on an air flow. In some of the regimes of chamber operation this may lead to a substantial, almost twofold, change in the long range of a fuel spray and, consequently, to corresponding changes in the distributions of the concentrations of fuel phases. In this work we set out to calculate the fields of air velocity and temperature as well as of the distribution of a liquid fuel in a module combustion chamber with account for the processes of heating and evaporation of droplets in those regimes typical of combustion chambers in which there is a substantial interphase exchange. The influence of interphase heat exchange on mixing in the working volume of the module was considered earlier in (4). It was shown that the computational technique describes well the experimental data on the configuration of the fuel spray. In the present work, the technique is supplemented with account for mass exchange between liquid and gas phases. It is clear that when a "cold" fuel is supplied into a "hot" air flow, the droplets are heated and the air sur- rounding them is cooled. It is evident that at small flow rates of the fuel this cooling can be neglected. The aim of this work is to answer two questions: how much the air flow is cooled in the range of parameters typical of real com- bustion chambers, and how far the region of flow cooling extends. Moreover, the dependence of the flow charac- teristics on the means of fuel spraying (jetty or pneumatic) and also on the spraying air temperature is investigated. Statement of the Problem. The model selected for investigation is a straight channel of rectangular cross sec- tion 50 × 30 mm and 150 mm long into which air is supplied at a velocity U0 and temperature T0. Injection of a fuel with a temperature Tf into the channel at a velocity Vf is made through a hole in the upper wall of the channel with the aid of an injector installed along the normal to the longitudinal axis of the channel halfway between the side walls. In modeling the pneumatic injector it is considered that, coaxially with the fuel supply, the spraying air is fed at a velocity U1 and temperature T1 into the channel through a rectangular hole of size 4.5 × 3.75 mm. In modeling a jetty injector, we assume that the spraying air is absent. Calculations were carried out for the velocity and tempera- ture of the main air flow U0 = 20 m ⁄ sec and T0 = 900 K, Vf = 8 m/sec, Tf = 300 K. The gas pressure at the channel inlet was equal to 100 kPa. The variable parameters of the calculation were the velocity and temperature of the spray- ing air: U1 = 0-20 m ⁄ sec and T1 = 300-900 K, as well as the summed coefficient of air excess through the module α = 1.35-5.4. The values of the regime parameters are presented in Table 1. Regime 1 corresponds to jet spraying of