OCR Text |
Show Combustion is model~ed ~ a bi-molecular reaction. Fuel and oxygen react at a rate determined by a one-step Arrhenius kinetic mechanism. Fuel properties used in the model are those of methane. Numeric Solution of the Governing Equations Solution of these equations is obtained by applying the Euler predictor-corrector algorithm. The solution method is straightforward and involves direct marching from one time to the next. The simulation is started by simply specifying a high initial temperature. Depending on the geometry and operating conditions, combustion will continue as a steady flame, blowout, or oscillate. It is emphasized that this model will predict any of these responses depending on the conditions specified. Periodic heat release is not imposed. The solution speed of PCOM is relatively fast. Given a simple combustor geometry and a specified time step, a CFD model running on an SGI Indig02 machine with a R400 MIPS processor could take over 30 days to simulate a single second of combustion. In comparison, PCOM running on a personal computer with a 100 MHz Pentium processor can simulate a second of combustion in less than 15 minutes. Experimental Description PC OM simulations are based upon the experimental rig shown in Figure 2. This atmospheric pressure combustion rig is designed to handle multiple premix fuel nozzles. The 9 cm 0.0. combustor body is constructed of quartz and is mounted inside a 35 .56 cm 0 .0 . steel containment pipe. The multiple viewing ports of the containment pipe allow complete optical access to the flame zone. Oscillations are typically excited around 300 Hz, which is the approximate quarter wavelength of the 60 cm combustor body. A detailed sketch of the premixed, swirl stabilized fuel nozzle is shown in Figure 3. A central 1.27 cm stainless steel tube supplies premixed fuel and air to the pilot flame on the nozzle axis. The pilot flame is lit with an internal spark electrode approximately 5.7 cm upstream of the nozzle exit. The pilot tube is surrounded by the lean premix fuel and air. The lean premixture enters the nozzle through two diametrically opposed tubes 21 cm upstream of the nozzle exit. The premixed fuel and air pass through a wire mesh flow straightener and then through straight finned swirl vanes angled 45 degrees from the nozzle axis. The fuel and air are mixed approximately 100 cm upstream of the nozzle to ensure thorough mixing. Multiple injection ports for premix fuel are provided along the nozzle length. Fuel flow through each injection port is individually metered. The bypass fuel entering through the injection ports may be choked at the nozzle interface or further upstream, depending upon the goal of the experiment. The length of the tubing between the choke point and the fuel nozzle is adjustable. This length is critical because acoustic resonance ~n the tube can contribu~e to combustion oscillations (Richards and Yip, 1995). The port IS choked at the nozzle Interface when testing for instabilities due to air supply variation. 4 |