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Show · This paper .describes the development and results of a simple, nonlinear model for premixed combUStion, referred to as PCOM (Premixed Combustion Oscillation Model). The model represents the relevant processes occurring in a fuel nozzle and combustor which are analogous to current LPM turbine combustors. As an alternative to either linear analysis or det~le.d modeling, this model is based on a time-dependent, nonlinear control volume analysis. A Similar approach was used by Richards et al. (1993) and Narayanaswami and Richards (1995) to successfully describe experimental oscillations observed in several different styles of pulse combustors. Daw et al. (1995) showed that this type of modeling can be particularly useful to understand laboratory observations of nonlinear, chaotic behavior in oscillating combustion. The technique used in the model may also be valuable to understand oscillations in low NOx industrial burners. Reardon (1995) developed a similar analysis to the one presented here, the primary difference being that Reardon modeled the combustion response with a specified time lag. In this paper, the combustion is modeled as a well-stirred reactor having finite kinetics. While a well stirred reactor is an obvious simplification to a premixed gas turbine combustor, it does represent a valuable limiting case. PC OM was developed to help explain specific experimental observations (Richards et al. 1995), and to provide guidance for the development of active control schemes based on nonlinear concepts. Comparison to experimental data shows that much of the dynamic behavior observed in the lab is also predicted by this model, including the effects of inlet air temperature and some open loop "active" control results. As discussed later, active control of oscillations can be accomplished by modulating the fuel flow at various frequencies and pulse widths. The model has also proven valuable to understanding the fundamental driving mechanisms of combustion instability and the effects of nozzle geometry. Model Description Flow properties and species concentrations in the nozzle, combustion, and tailpipe regions are determined using a control volume formulation of the conservation equations. The development is based upon the integral form of the conservation laws as presented in most texts (Moody, 1990), and also in Richards et al. (1993). PC OM assumptions include ideal gas behavior, variable specific heats, and uniform conditions within each region. The assumption of uniform conditions within each region reduces the volume and surface integrals to algebraic expressions, resulting in a set of first-order differential equations. The eight conservation equations utilized in the model are consolidated in Table 1. A thorough explanation of PCOM's development can be found in Janus and Richards (1996). The model geometry for the premixed combustor is shown in Figure 1. A mixture of fuel and air enter the nozzle at a specified equivalence ratio and temperature. The premixed flow passes through the swirl vanes and past a so-called. by~ass fu~l port w?ere additio~al f~el can be injected. Bypass fuel is a term denoting . fuel which IS not Included In the fuel/~r mixture entering the nozzle. Bypass fuel injection may I~corporate both a steady ~d a flu~tuatIng component if desired. The injected fuel mixe~ WIth the nozzle m~s flow. In a speCified vol~me, and continues to the end of the nozzle where It enters the combustIon region. The combustIon region is treated as a perfectly stirred reactor. The pressure and temperature are calculated in 2 |
