Numerical simulation of an advanced airblast fuel nozzle under rich-burn condition

Numerical study was performed to investigate the chemically reactive flows with sprays inside a staged turbine combustor (STC) rich-burn O B ) zone using a modified version of the KIVA-I1 code. The STC is cquipped with an advanced airblast duel-fuel nozzle. The duel-fuel nozzle has two fuel injection passages and four air flow passages. Parametric studies of the effect of the fuel-split ratio and the RB zone length on the fluid flow, temperature. and the NOx formation inside the S T C RB zone are given. From the fuel-split parametric study, it was found that this parameter has major impact on the NOx formation inside the RB zone. Study of the effect of RB zone length indicated that, as long as the flow and temperature fields inside the RB zone are not affected by the reduction of the RB zone length, the size of the RB zone can be optimiizd. Through this study, a better understanding of the physics of flow and temperature fields inside the RB zone was obtained. which enhances the STC design process. / Assistant Professor, Depmment of Mechanical Engineering and Engineering Mechanics. Member AIAA. Graduate Student, Department of Mechanical Engineering and Engineering Mechanics. Aerospace Engineer, Combustion Technology Branch. Member AIAA. This paper is a work of the US. Government and is not subject to copy right protection in the United States. , INTRODUCTION A concept of staged turbine combustor (STC) for reducing NOx emission is currently under study at NASA Lewis Research Center. The STC concept uses a rich-bum (RB) zone and a lean-combustion (LC) zone to prevent the formation of NOx. In between these two zones, a quickquench (QQ) section is used to bridge them. To block any possible back-flow from the QQ zone to the RB zone, a convergent section with 300 angle is used. Figure 1 shows the schematic diagram of one STC. The STC concept offers a broad operational range because of the good flame stability of the RB zone. An earlier numerical studies of a complete STC using KIVA-11’ showed some promising results about the performance of STC with a duel-fuel nozzle”. It was found that the temperature of the fluid leaving the LC zone was very uniform. Prediction of NOx emission indicated that there were no excessive thermal NOx produced in the Q W C zones for the cases studied. It is the purpose of this paper to funher investigate the effect of (1) the fuel-split ratio for an advanced airblast duel-fuel nozzle, and (2) the RE zone length on the fluid flow, temperature, and the NOx formation inside the STC RB zone The advanced airblast duel-fuel nozzle under study has two fuel injection passages and four air flow passages. The inner air flow passage has a counter-rotating swirlier with a 63O van angle. The van angle of the middle, outer, and dome air flow passages are 61.3O. 60.2’, and 60.Z0, respectively, and are all co-rotating. Figure 2 shows the schematic diagram of this fuel nozzle. To see the effect of fuel-split ratio on the flow, numerical studies were camed out for three fuel-split ratios using KIVA-I1 code. They were 50/50%, 60/40%, and 70/30% (outerlnner). For all these cases, a fixed RB zone length (L,,,,) was used. Numerical results indicated that the 70/30% fuel-split ratio gives better performance, in terms of the NOx production. In addition to the aforementioned parametric study, the effect of RB zone length on the flow was also investigated. It is argued that, to further reduce the NOx formation inside the RB zone, the residence time should also be reduced, which can be achieved by reducing the RB zone length. In addition, with a shorter RB zone length, the overall size and weight of STC is also reduced. It was for these reasons motivated this parametric study. Using the 60/40% fuel-split ratio with L, length as the baseline case for this study, two additional cases were chosen, one with 0.67 L, RB zone length and the other 1.2 Lrich. It was found that the flow and temperature fields inside the RB zone might become unstable if the RB zone length is t ~ ) shon. Through this parametric study, it is shown that an optimum design of the size of the RB zone can be derived. In the following sections, the problem description is given first. Numerical method of solution using KIVA-I1 is then briefly described along with the modification of the code. Afterwards, results are presented and discussed. DESCRWION OF PROBLEM The baseline geometry under investigation is shown in Figure 3. In this figure, L, and R are the length and the radius of the RB zone, respectively. It is noted that, to minimize the effect of the outflow boundary conditions (to be described latter) on the solution inside the RB zone, the outlet boundary of the RB zone was extended to the end of the convergence section, which is also the beginning of the QQ zone. The duel-fuel nozzle is located at the center of the RB zone inlet. The following inlet conditions, which are similar to those encountered in advanced combustion systems, were used: Temperature=lOC€fF(811 K) Pressure = 90 psia (6.205 x 106 dynes/cm') * Air mass flow rate = 1.09 Ibwsec (494.42 g/s) Air flow split = 7.8/19.1/25.5/47.6% (from inner to dome) * Air flow passage area = .007/.0117/.0156/.027 ft' (6.50/10.87/14.49/25.08 cm') Equivalence ratio = 2.0 Fuel split = 50/50%, 60/40%, and 70/30% Turbulent length scale = 0.25 of the respective flow passage width Turbulent kinetic energy = 1% of the respective 0.5 W2 where W is the mean axial velocity at the inlet The inlet boundary conditions were the spccification of the density (calculated from the temperature and pressure given above) and W (calculated from the mass flow rates and flow areas given above). The radial velocity component was set so that the inlet flow was tangent to the flow passages. The azimuthal or swirl velocity component was specified assuming wheel flow. The turbulent kinetic energy and length scale were specified at the values given above. The exit boundary condition was to specify the pressure while all other flow variables were extrapolated. The combustor walls were assumed to be adiabatic with a turbulent boundary layer. These conditions were enforced using wall functions'. NUMERICAL METHOD OF SOLUTION The problem was modeled as a compressible turbulent reacting flow and was closed by a K E turbulence model with wall function. Due to the geometrical and physical symmetry, the problcm was treated as axisymmetic with swirl. The numerical solutions were obtained by a modified version of KIVA-II, implemented to study the aforementioned objective. The KIVA-II code' developed at the Los Alamos National Laboratory is an advanced computer program for the numerical calculation of transient, twoand three-dimensional. chemically reactive fluid flows with sprays. It solves the unsteady equations of motion of a turbulent, chemically reactive mixture of ideal gases, coupled to the equations for a single-component vaporizing fuel spray. The numerical scheme is based on the arbitrary Lagrangian-Eulerian (ALE) method4' with implicit continuous Eulerian modification for low Mach number flows. A stochastic particle method is used to calculate the evaporation rate of liquid sprays. The effects of droplet oscillation, distortion, breakup, collision, and coalescence are considered in the computations. Several upwind convection schemes, such as the panial donor ccll and quasi-second-order upwind, are included. Two turbulence models, modified K E and subgrid scale models, are also available. For additional details of the KIVA-11, see Ref. 1. L The RB zone grid system was generated by an algebraic/elliptic method. Figure 4 shows the baseline case grid system (68x51 grid points). In this figure, the crosshatched area is the location of the ignition window, which was used to simulate a torch ignitor. Certain planes