Victor Burger

Fuel Influence on Targeted Gas Turbine Combustion Properties: Part II — Detailed Results

The paper presents the results from a study that formed part of a bilateral project between DLR-VT and Sasol Technology Fuels Research aimed at investigating the potential influence of physical and chemical fuel properties on ignition and extinction limits within heterogeneous gas turbine combustion. The threshold of flame extinction and re-ignition behaviour of a range of alternative fuels was investigated in a representative aero-combustor sector to determine the relative influence of physical properties and chemical reaction timescales.A matrix of eight test fuels was selected for use during the study and included conventional crude-derived Jet A-1, synthetic paraffinic kerosene, linear paraffinic solvents, aromatic solvents and pure compounds. All test fuels were characterised through full specification analyses, distillation profiles and two-dimensional gas chromatography.The ignition and extinction behaviour of the test fuel matrix was evaluated under simulated altitude conditions at the Rolls-Royce Strategic Research Centre’s sub-atmospheric altitude ignition facility in Derby, UK. A twin sector segment of a Rich Quench Lean (RQL) combustor was employed with fuel supplied to a single burner. Combustor air inlet conditions were controlled to 41.4 kPa and 265 K. Fuel temperature was controlled to 288 K.In addition to the standard extinction and ignition detection systems, optical diagnostics were applied during the test programme. Simultaneous high-speed imaging of the OH* chemiluminescence, and broadband flame luminosity was employed to capture the main reaction zones, the global heat release and distribution of radiative soot particles respectively.Lean extinction points were determined using both a photodiode as well as from the OH* chemiluminescence data. The position of extinction and overall combustor ignition and extinction timescales were determined. The diagnostic methodology that was used to obtain the results reported in this paper is discussed in greater detail in a separate complementary paper.All eight fuels, including the fully synthetic Jet A-1 fuels that formed part of the test matrix, yielded performance that was comparable to that obtained with conventional crude-derived Jet A-1.© 2014 ASME

A Method for Determining the Laminar Flame Speed of Jet Fuels Using Combustion Bomb Pressure

This paper describes the use of a spherical combustion bomb to determine the laminar flame speed and Markstein length of a selection of hydrocarbon fuels. The fuels nominally represented Jet A-1 but some were doped with various component compounds which were chosen so as to vary particular jet fuel specification in relative isolation.Analyses of this kind are typically based on optical measurements and, to simplify the analysis, an approximation of constant pressure is usually achieved by limiting the useable data to the early stages of flame propagation only. The analysis methodology presented in this paper differs inasmuch that calculations were based solely on the recorded pressure data. Moreover, by deducing the response of the flame speed to pressure and temperature, it was possible to utilize the whole combustion pressure record which significantly increased the volume of useful data that could be obtained from each experiment. Other practical difficulties that are often encountered such as flame winkling at large diameters, especially with rich mixtures, were minimized by using a small bomb of only 100mm diameter. The method of analysis via the pressure trace rendered any flame winkling easily discernable wherefrom it could be easily eliminated.For each fuel, at least six repeat combustion pressure records (about 90 data points each) were obtained for each of six different air-fuel ratios spanning the range from lean to rich and the whole sequence was repeated at a higher initial temperature. This provided a database of over 6000 individual calculations of laminar flame speed from which the relevant parameter coefficients were obtained by means of a regression technique. It was found that the effects of changing the blend composition could be discerned in the various laminar flame speed results and that significant variation in laminar flame speed could possibly be “tailored” into a synthetic jet fuel formulation.Copyright

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

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.Copyright

Assessment of the Role of Fuel Autoignition Delay at the Limits of Gas Turbine Combustion and Ignition

The influence of fuel autoignition chemistry is known to be relevant when approaching the limits of lean blowout and lean ignition in a continuous combustion environment. This was investigated by employing four reference fuels having very different autoignition delay profiles but similar boiling points to interrogate various test environments and thereby to assess the relevance of the differences in autoignition chemistry. A combustion bomb apparatus was used to characterize the reference fuels together with a sample of commercial Jet A-1 for comparison. The measurements were cross-checked using a chemical kinetic simulation model. A continuous combustion rig was used to study the threshold ignition and blowout performance of the pre-vaporized reference fuels and a laminar flame speed bomb was used to study the influence of autoignition chemistry on normal, stoichiometric combustion and normal ignition conditions. In all the experiments, the results reflected the distinctive differences of the test fuels in terms of their autoignition delay timescales. The findings were interpreted against the background of the commercial jet fuel autoignition chemistry and the relevance of traditional autoignition delay metrics such as Octane or Cetane rating. Notwithstanding the influence of fuel evaporation and mixing timescales which can exert an overriding influence in a practical, gas turbine application, it was concluded that the fuel’s autoignition delay timescale also plays a very significant role in threshold operational situations.Copyright

Fuel Influence on Targeted Gas Turbine Combustion Properties: Part I — Detailed Diagnostics

The threshold combustion performance of different fuel formulations under simulated altitude relight conditions were investigated in the altitude relight test facility located at the Rolls-Royce plc. Strategic Research Centre in Derby, UK. The combustor employed was a twin-sector representation of an RQL gas turbine combustor. Eight fuels including conventional crude-derived Jet A-1 kerosene, synthetic paraffinic kerosenes (SPKs), linear paraffinic solvents, aromatic solvents and pure compounds were tested. The combustor was operated at sub-atmospheric air pressure of 41 kPa and air temperature of 265 K. The temperature of all fuels was regulated to 288 K. The combustor operating conditions corresponded to a low stratospheric flight altitude near 9 kilometres.The experimental work at the Rolls-Royce (RR) test-rig consisted of classical relight envelope ignition and extinction tests, and ancillary optical measurements: Simultaneous high-speed imaging of the OH* chemiluminescence and of the soot luminosity was used to visualize both the transient combustion phenomena and the combustion behaviour of the steady burning flames. Flame luminosity spectra were also simultaneously recorded with a spectrometer to obtain information about the different combustion intermediates and about the thermal soot radiation curve. This paper presents first results from the analysis of the weak extinction measurements. Further detailed test fuel results are the subject of a separate complementary paper [1].It was found in general that the determined weak extinction parameters were not strongly dependent on the fuels investigated, however at the leading edge of the OH* chemiluminescence intensity development in the pre-flame region fuel-related differences were observed.Copyright

Fuel Composition Influence on Gas Turbine Ignition and Combustion Performance

The influence of different jet fuel compositions on aviation gas turbine combustion performance was investigated. Eight fuels including conventional crude-derived Jet A-1 kerosene, fully synthetic Jet fuel, synthetic paraffinic kerosenes, linear paraffinic solvents, aromatic solvents and pure compounds were tested. The tests were performed in the altitude relight test facility located at the Rolls-Royce Strategic Research Centre in Derby (UK). The combustor employed was a twin-sector representation of an RQL gas turbine combustor. The combustor was operated at sub-atmospheric air pressure of 41 kPa and air temperature of 265 K. The temperature of the fuels was regulated to 288 K. The combustor operating conditions corresponded to a simulated low stratospheric flight altitude near 9,000 metres.The experimental work at the Rolls-Royce (RR) test-rig consisted of classical relight envelope ignition and extinction tests, and ancillary optical measurements: Simultaneous high-speed imaging of the OH* chemiluminescence and of the soot luminescence was applied to obtain spatial and temporal resolved insight into the ongoing processes. Optical emission spectroscopy was also applied simultaneously to obtain spectral and temporal resolved insight into the flame luminescence.First results from the analysis of the OH* chemiluminescence and detailed fuel analysis results were presented in previous papers [1, 2]. This article presents further results from the analysis of the soot luminescence imaging and flame spectra.It was found in general that the combustion performance of all test fuel formulations was comparable to regular Jet A-1 kerosene. Fuel related deviations, if existent, are found to be small.Copyright