Les Model Vs Experiment

Velocity measurements were performed and compared with the LES model results. One set of velocity profiles are presented in Fig. 4. The velocity profile predicted by the model agrees approximately with the measured velocity profiles. Fig. 5 presents the results of the LES prediction vs. experiment for several axial locations (x/d, where x is axial distance and d is the diameter of the inner pipe) with methane as the fuel. From this figure, one can see that the LES model results agree with experiment reasonably well. Comparisons were also done with Ethane as the fuel at one mixing length (x/d=24) in order to assure that the effects of buoyancy on the system are negligible. As the Froude number is much greater than one for both the methane and ethane experiments (15,800 for the methane jet vs. 11,500 for ethane), momentum rather than bouyancy dominates the flow [22], so that little difference in the mixing would be expected upon changing the fuel. The results (not shown), both experimentally and from the LES model, showed insignificant effects of buoyancy on the concentration profile.

Figure 6 shows a comparison of concentration RMS vs. radial position for the GA program based on experimental line-of-sight intensity ratios, and for the LES model. As can be seen from the figure, the RMS values obtained by the two methods are in reasonable agreement for the most part (given the error in finding the RMS concentration profile with the GA program, seen earlier in Figs. 2 and 3). It can be seen that the reconstructed concentration profile tends to underpredict the RMS concentration. The finite diameter of the laser beam will contribute to this. The Rayleigh range for the given laser is approximately 500 mm, and with the optical setup used, the beam diameter is 2.2 mm - defined as the diameter of the beam containing 86% of the laser power [23]. Thus, some smearing of the actual fluctuations of fuel concentration would occur, causing the measured signal RMS to be lower than what is really there (thus while temporal resolution was adequate, spatial resolution was marginal). By using an optical system that focuses the beam at the measurement point [23], the resolution of the measurements for finding the RMS of the concentration would be improved.

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