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Show in N 0 X production would be caused by the burner design. In each test case, the composition of 02, C02 , C O and N O across the diameter of the furnace were measured at various axial distances away from the burner using an intrusive rapid quench sampling probe. Similar radial profiles of gas temperature were measured using a suction pyrometer. At selected locations inside the furnace a laser based technique called Coherent Anti- Stokes Raman Spectroscopy (CARS, see Hughes et al., 1995) was used to obtain the mean and fluctuating components of gas temperature to generate temperature pdfs. The purpose for measuring temperature pdfs was to check if the presumed pdf method could adequately represent the influence of fluctuating temperature on N O x production. Other measurements included radiative heat transfer on furnace walls by 27r radiometer and the gas composition at the furnace exit by conventional continuous gas analyzers. SIMULATION PROCEDURE Based on the physical dimensions of the burners and the pilot-scale tunnel furnace, two computational grids were generated: one for each test case. To take advantage of geometric symmetry in the H N B test case, the computational grid was made to cover only a quarter of the burner and the furnace (about 77,000 grid nodes in total). For the U L N B case, there is no symmetry and its grid size amounted to 132,000 grid points. Using the measured air and natural gas flow rates at the inlet, a combustion simulation for each test case was performed. This involved solving for the overall fluid flow, combustion and heat transfer characteristics over the computational grid. It is important to perform the combustion simulation prior to the N O x simulation because the mechanisms of N O x formation are closely intertwined with the natural gas combustion chemistry. Based on the prediction of the overall combustion characteristics, the N O model was executed as a post-processor to obtain the spatial distribution of species N , H C N and N O within the calculation domain. This procedure assumes that the active species in the N O model (NO, N and H C N ) are present in quantities too small to affect the overall combustion, heat transfer and fluid flow characteristics in the furnace. R E S U L T S A N D DISCUSSION High NOx Burner Test Case The measured profiles of 02, CO and temperature at various axial distances away from the burner face are shown in Figures 4 and 5. Each profile describes the radial variation from the centre-line of the furnace (i.e. radial distance = 0.0) towards the wall. At axial distance x = ID (i.e. 1 burner diameter) corresponding to the exit of the burner tile, 0 2 is seen to drop sharply to zero towards the furnace centre while C O increases drastically, indicating that a narrow jet flame has formed and created a rather small high temperature fuel-rich zone at the furnace core. This diffusion flame propagates and expands along the axis of the furnace as natural gas and air continue to mix along the jet flow until all the fuel is consumed. Correspondingly, the measured 02, C O and temperature profiles gradually flatten with increasing axial distance, x. It is interesting to note that 6 |