Influence of Fuel Physical Properties and Reaction Rate on Threshold Heterogeneous Gas Turbine Combustion

Abstract

The paper presents the findings from a study of the lean blowout (LBO) behaviour of sixteen fuel blends in a heterogeneous laboratory combustor. The LBO results were correlated with fuel blend properties that included the D86 distillation profile, density, viscosity, flash point and ignition delay as represented by derived cetane number (DCN). A spherical bomb was employed to measure laminar flame speed and Markstein length based on pressure measurements. The experiments were conducted with two different starting temperatures and over a range of air fuel ratios from rich to lean. The atomisation behaviour of the fuels was evaluated using a pressure atomised nozzle and a laser diffraction particle sizer. The data allowed the Sauter mean diameter (SMD) values at extinction to be estimated based on the fuel pressure. Each individual LBO test was conducted at constant air flow rate with the extinction point being attained by reducing the fuel flow rate. The test series for each fuel spanned a range of air flow rates based on combustor liner relative pressure drops from 1% to 6%. These results exhibited three distinct regions (A1, A2 and B) that were evident to varying degrees in the results obtained with all sixteen test fuels. The transition between A1 and A2 was ascribed to combustor flow and was shown to be independent of the fuel being tested. The transition between B and A2 was ascribed to the change from the LBO behaviour being dominated by atomization to it being a mixing / turbulence dominated regime. The individual transitions were found to be dependent on the test fuel blend. In order to accommodate the LBO results in a multivariate analysis the observed trends were represented by three parameters that were determined through curve fitting to the different regions. The three parameters were the SMD and air mass flow rate at the transition between region B and A2 and a projected LBO equivalence ratio at zero air mass flow. The data was cross correlated between all determined properties and it was shown that the extinction behaviour correlated with chemical reactivity, flame stretch, density and volatility to different degrees in the two regions of operation. It was concluded that there is potential for influencing threshold extinction limits through both chemical and physical jet fuel properties, and the need to take cognisance thereof in fuel formulation, was highlighted.