Swirl Initial Conditions

Swirl conditions of various degrees of complexity were considered to initialize the simulations at the inlet. This included  

The combustor flows investigated here were characterized by peak inlet free-stream Mach numbers between 0.05 and 0.3 and by standard temperature and pressure (STP) conditions. Swirl (S) and radial (f ) numbers, defined in terms of circumferentially-averaged velocity data by 

75 and between 0 and 0.5, respectively. Other than passive excitation due to the swirl, the flow was unforced and was allowed to naturally develop its unsteadiness. Typical Reynolds numbers involved, based on the peak mean inlet axial velocity and diameter, were Re 70,000. 

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The sensitivity of the axisymmetric combustor flow dynamics to the actual choice of inlet velocity conditions was also examined. Figure 11.3 compares the results of initializing the simulations with the turbulent-pipe or LM-6000 swirling conditions and otherwise identical initial conditions S = 0.56, Uo — 100 m/s, STP). The flow visualizations depict the significant effects on the combustor vortex dynamics of changing the specifics of the velocity profiles used to initialize the LES, with noticeably more-axisymmetric features observed in the flow features for the LM-6000 case. The LM-6000 initial velocity conditions (Fig. 11.2a) involve a peak tangential velocity component located farther away from the axis and a more moderate radial gradient of the axial velocity. A clear consequence of these initial condition specifics, apparent in Fig. 11.3a, is that the LM-6000... 

The hybrid simulation approach used here for the swirl combustor configuration in Fig. 11.1 involves effective boundary conditions emulating the fuel nozzle and LES to study the flow within the combustor. Case studies ranging from single-swirler to more complex triple-swirler nozzles were investigated. The inlet boundary conditions used to initialize the combustor flow involve velocity... 

The model was initially run at MCR for a baseline condition. The velocity profiles were set to the design conditions at the model entrance. The mass flow readings, peripheral distribution, and swirl were then recorded. Baffle plates were then installed strategically within the model and the mass flow, peripheral distribution, and swirl were then remeasured. 

A series of experiments has been carried out in the group of G. Brenn, TU Graz, Austria, for water sprays and PVP/water (20% PVP and 80% water by mass) sprays in air with different liquid mass inflow rates [42]. Various atomizers with different dimensions of swirl chambers and exit diameters were used. At various cross-sections, the droplet sizes and velocities are recorded for different liquid inflow rates using phase Doppler anemometry (PDA) [52]. The present simulations concern the experimental data using the Delavan nozzle SDX-SD-90 with an internal diameter of 0.002 m and an outer diameter of 0.012 m at the nozzle throat and 0.016 m at the top [42]. The liquid inflow rates for water/air spray are 80kg/h and 120 kg/h, and a 112 kg/h flow rate is used for the spray of a PVP/water solution in air. The spray is injected into a cylindrical spray chamber with a diameter of 1 m. The carrier gas is at standard conditions. Measurements are taken at cross-sections of 0.08 m, 0.12 m, and 0.16 m away from nozzle exit. The experimental data at the closest position to the nozzle is used to generate initial data for the numerical computations [53]. 

The critical bath depth //l,i,s for the initiation of the first kind of swirl motion in an air-water system was measured five times under a range of experimental conditions. The arithmetic mean of the data is plotted in Figs. 5.7 and 5.8. The scatter in the experimental data was within 25%. The results indicate that l,i,s is about 25 mm irrespective of the bath diameter D, the inner diameter of nozzle, dau and air flow rate Qg. This trend suggests that the inertial force of the injected gas has no effect on the initiation of the first kind of swirl motion. 

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