59009 - Optimization of Fuel Nozzle Diameter in a Novel Cross Flow Lean Direct Injection Burner 

The gas turbine industry demands various performance criteria like efficient usage of fuel, uniform exit temperature distribution, a wide range of flame stability while also maintaining very less overall emissions. Lean Direct Injection (LDI) concept proves to be an ultra-low NOx combustion scheme for future gas turbine combustors because of its ability to operate at very lean conditions. For an LDI burner, the fuel nozzles play a vital role in deciding a balance between these various performance parameters. This work focuses on finding the optimum diameter of fuel nozzles in terms of lean blow out limit and efficient utilization of fuel in a novel multi-swirl LDI burner having a cross-flow mixing between fuel jets and swirling air. Seven sets of nozzle diameters have been chosen within a range of 0.4 mm to 1.7 mm for optimization. These nozzle diameters sweep fuel jet-to-swirl air momentum ratios that shift the balance from better lateral spread to greater penetration into the air flow.  At first, lean blow out limits were detected from experiments using two different fuels, methane and liquified petroleum gas (LPG), within a range of air flow rates. It was seen that with the decrease in diameter below 1 mm the flame extinguished at a higher equivalence ratio. On the other hand with the increase in diameter above 1 mm, there were no significant changes observed in lean blow out limits. Then the diameters were compared through CFD simulations using ANSYS CFX with two different combustion models, namely, Eddy Dissipation and PDF Flamelet. The air and fuel flow rates were kept fixed, with the fuel used being methane. Various key parameters like average temperature, maximum temperature, temperature pattern factor, and area average mass fractions of gases like O2, CO2, CO, CH4 and NO, were calculated at a plane 26 mm downstream from the burner plate. Apart from hot flow simulations, cold flow simulations were performed as well to visualize the distribution of methane concentration and also to quantify the jet penetration heights. The combined results from CFD showed that with the decrease in diameter the fuel jet was able to penetrate deeper into the air swirl by overcoming the air momentum, which not only results in better mixing and combustion of fuel but also gives more uniform temperature distribution and thus lower pattern factor. The optimum nozzle diameter obtained from this perspective was 0.6 mm. Thus any fuel nozzle diameter between 0.6 mm and 1 mm may be chosen for this LDI burner, according to preference between a leaner range of operation for lesser overall emissions or higher utilization of fuel for better fuel economy.