Combustors ignition

The kinetic simulations of the pulse combustor ignition can be carried out under conditions which closely approximate those in a continuously stirred tank reactor (cstr). In those calculations, hot product gases are steadily mixed with cold, unbumed reactants until the mixtures ignite. The reaction mechanisms used are valid for high temperatures, and the most important, sensitive reaction is reaction (3), and the combined influences of chemical kinetics, acoustics, and fluid dynamics can all be incorporated into a coherent practical design model [20]. 

Combustors All gas turbine combustors perform the same function They increase the temperature of the high-pressure gas at constant pressure. The gas turbine combustor uses veiy little of its air (10 percent) in the combustion process. The rest of the air is used for cooling and mixing. The air from the compressor must be diffused before it enters the combustor. The velocity leaving the compressor is about 400-500 ft/sec (130-164 m/sec), and the velocity in the combustor must be maintained at about 10-30 ft/sec (3-10 iTi/sec). Even at these low velocities, care must be taken to avoid the flame to be carried downstream. To ensure this, a baffle creates an eddy region that stabi-hzes the flame and produces continuous ignition. The loss of pressure in a combustor is a major problem, since it affecls both the fuel consumption and power output. Total pressure loss is in the range of 2-8 percent this loss is the same as the decrease in compressor efficiency. 

The startup speed and temperature acceleration curves as shown in Figure 19-2 are one such safety measure. If the temperature or speed are not reached in a certain time span from ignition, the turbine will be shutdown. In the early days when these acceleration and temperature curves were not used, the fuel, which was not ignited, was carried from the combustor and then deposited at the first or second turbine nozzle, where the fuel combusted which resulted in the burnout of the turbine nozzles. After an aborted start the turbine must be fully purged of any fuel before the next start is attempted. To achieve the purge of any fuel residual from the turbine, there must be about seven times the turbine volume of air that must be exhausted before combustion is once again attempted. 

Other properties of interest are carbon residue, sediment, and acidity or neutralization number. These measure respectively the tendency of a fuel to foul combustors with soot deposits, to foul filters with dirt and rust, and to corrode metal equipment. Cetane number measures the ability of a fuel to ignite spontaneously under high temperature and pressure, and it only applies to fuel used in Diesel engines. Typical properties ol fuels in the kerosene boiling range are given in Table 1. 

In a solid-propellant rocket motor, the propellant is contained within the wall of the combustion chamber, as shown in Fig. 1. This contrasts with liquid systems, where both the fuel and oxidizing components are stored in tanks external to the combustion chamber and are pumped or pressure-fed to the combustor. In hybrid systems, one component, usually the fuel, is contained in the combustion chamber, while the other component is fed to the chamber from a separate storage tank, as in liquid systems. The solid-propellant motor also has an ignition system located at one end to initiate operation of the rocket. The supersonic nozzle affects the conversion of... 

Ignition sequence of a helicopter engine. Hot gases (yellow) are injected through two burners and the flame propagates so that all 18 burners are eventually ignited. (From Boileau, M., Ignition of two-phase flow combustors. PhD, Institut National Polytechnique de Toulouse and CERFACS, 2007.)... 

The fuel bed was ignited on the top surface, thereby satisfying the conditions for a countercurrent batch bed combustion. After the ignition the process controlled itself and the temperature rose to a value around 1200 °C, which is a temperature level prevailing in true grate combustors. 

Results of an experimental program in which aluminum particles were burned with steam and mixtures of oxygen and argon in small-scale atmospheric dump combustor are presented. Measurements of combustion temperature, radiation intensity in the wavelength interval from 400 to 800 nm, and combustion products particle size distribution and composition were made. A combustion temperature of about 2900 K was measured for combustion of aluminum particles with a mixture of 20%(wt.) O2 and 80%(wt.) Ar, while a combustion temperature of about 2500 K was measured for combustion of aluminum particles with steam. Combustion efficiency for aluminum particles with a mean size of 17 yum burned in steam with O/F) / 0/F)st 1-10 and with residence time after ignition estimated at 22 ms was about 95%. A Monte Carlo numerical method was used to estimate the radiant heat loss rates from the combustion products, based on the measured radiation intensities and combustion temperatures. A peak heat loss rate of 9.5 W/cm was calculated for the 02/Ar oxidizer case, while a peak heat loss rate of 4.8 W/cm was calculated for the H2O oxidizer case. 

The combustor was ignited on propane. After a short period of operation on propane, aluminum flow was started and the propane flow was turned off. Once aluminum combustion was established, the oxygen flow was gradually decreased until only operation on H2O could be sustained. Typically, one to two minutes of aluminum firing on an O2/H2O mixture was required before the combustor was warmed up enough to make H2O only operation possible. Total duration of test runs was usually about 10 min. Preheating was not required for test runs using an 02/Ar oxidizer. 

Temperatures estimated from the measured intensity distributions at each port location during an 02/Ar oxidizer run in the 24-inch long combustor are plotted in Fig. 8.4, along with the measured hemispherical emissive power in the wavelength range from 425 to 800 nm. (The hemispherical emissive power, E, is related to the radiant intensity, /, by = ttI. Radiant intensity is also referred to as radiance.) The stoichiometry, 0/F)/ 0/F)st, for this run was about f.fO. The measured combustion temperature was about 2900 K, as compared to an adiabatic flame temperature of about 3650 K. The intensity measurements indicate that ignition occurs about 12 in. downstream from the injector. The intensity is near its peak at the most downstream port location, which indicates that combustion is still underway at that location. 

It was found that when using an 02/Ar oxidizer stable combustion could be maintained with cold combustor walls, while a warmup period of approximately two minutes assisted with O2 was required when using the H2O oxidizer before H2 0-only operation was possible. Apparently, with the lower heat release using the H2O oxidizer, the recirculated combustion products do not stay hot enough to ignite the incoming fuel when the combustor walls are cold. 

Based on the measured distribution of radiation intensity along the combustor length, the average radiant heat loss rate for the entire combustor volume for the 02/Ar case is estimated at about 4.0 W/cm, corresponding to a total radiant heat loss of about 20 kW or 160 cal/g of combustion products. Heat loss of 160 cal/g, together with an estimated combustion products temperature of 3050 K, implies that the reactedness of the mixture is on the order of 60% to 70%. The lower estimate of reactedness corresponds to a case in which the unburned aluminum never ignited. The higher estimate of reactedness corresponds to a case where the unburned aluminum is at the same temperature as the rest of the mixture, which implies that it is in vapor phase. The actual reactedness should fall between these two extremes. 

Figure 12.4 The comparison of predicted mean temperature fields in long and short combustors at t = 14.9 ms (a), 22.1 (6), and 58.1 ms (c) after ignition behind a bluff body. Boundary condition at outlets is ABC of Eq. (12.19). Mean velocity at the inlet Uin = 10 m/s. Other conditions are po = 0.1 MPa, To = 293 K, fco = 0.06 J/kg, lo = 4 mm. A set of graphs below compares mean absolute velocity distributions in the different cross-sections (/ to VII) of both combustors (from left to right x = 0, 80, 100, 112, 135, 235, and 330 mm). Solid line — short combustor, dashed line — long combustor, j/max is the height of the corresponding cross-section of the combustor...

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