Andy Yates

Correlating Auto-Ignition Delays And Knock-Limited Spark-Advance Data For Different Types Of Fuel

The knock-limited spark advance (KLSA) data for various engines and fuels were analysed using a comprehensive engine model to simulate the pressure-temperature history of the end-gas. Regression techniques were used to match the engine data with a three-stage Arrhenius model of the fuel ignition delay and to deduce parametric information regarding the behavioural characteristics of the system. The validity of the analysis results was cross-checked by classifying the fuels in terms of linear paraffins, iso-paraffins, olefins, aromatics or alcohols and subjecting specific examples of these classes of fuels to a detailed chemical-kinetic analysis to determine the essential characteristics of their associated auto-ignition delays. A further boundary condition for the analysis was provided by the octane numbers (RON and MON) of the fuel. These data required cautious treatment because the knocking criterion which is specified for the ASTM octane test method differs fundamentally from that used in a typical KLSA test. Issues relating to high-speed knock and fuel composition are addressed in this paper and some very relevant anomalies regarding the octane rating of aromatic and olefinic fuels are explained.

Understanding the Relation Between Cetane Number and Combustion Bomb Ignition Delay Measurements

A recently approved method for cetane determination using the Ignition-Quality Tester (IQT) is based on an ignition delay measurement in a combustion bomb apparatus, which is empirically correlated to cetane number. The correlation assumes that all fuels will respond to the different pressure and temperature domains of the IQT and the cetane test engine in the same way. This assumption was investigated at a more fundamental level by conducting IQT measurements at different pressure and temperature points and characterising the ignition delay of the fuel in terms of an Arrhenius autoignition model. The fuel model was combined with a mathematical model of the cetane engine and the concept was evaluated using a variety of test fuels, including the diesel cetane rating reference fuels. The analysis technique was able to accurately predict the cetane number in all cases.

An Investigation of the Ignition Delay Character of Different Fuel Components and an Assessment of Various Autoignition Modelling Approaches

An understanding of the ignition delay behaviour of spark ignition fuels, over a wide range of temperatures and pressures, was an essential prerequisite for an ongoing pursuit to develop a fundamentally-based predictive octane model for gasoline blends. The ignition delay characteristics of certain model fuel compounds such as linear and iso-paraffins, olefins, aromatics and alcohols were investigated by means of chemical kinetic modelling, employing CHEMKIN 3.7 using detailed molecular oxidation mechanisms obtained from the literature. The complexity of these mechanisms necessitated the parallel investigation of reduced kinetic models in some of the applications. Reduced kinetic models were also used to describe the blending behaviour of selected binary combinations of the model fuels. The complex ignition delay response in the temperature/pressure domain that was predicted by the detailed kinetic analyses was reduced to a simple system of three, coupled Arrhenius equations. This simplified expression was used to emulate experimental data that were obtained for the model fuels in a combustion bomb apparatus, the IQT™, as well as data from a single cylinder CFR engine under knocking conditions. A combination of the various approaches has led to new insights regarding the blending behaviour of various classes of fuel molecules in regard to their collective resistance towards autoignition. This is a critical requirement for understanding and modelling the chemical ignition delay as reflected by octane numbers.

A Further Study of Inconsistencies between Autoignition and Knock Intensity in the CFR Octane Rating Engine

Careful consideration of the development and operation of the ASTM knock detection system on the Cooperative Fuels Research (CFR) octane rating engine has shown that the pressure fluctuations, brought about by autoignition of the end-gas, do not contribute to measurement of knock intensity. The analyses of a variety of fuels at standard knock intensity revealed that knock intensity measured on the CFR engine is related to the rate of change of pressure prior to knocking and is consistent with the description and operation of, not only the original bouncing pin, but also the modern day electronic CFR knock measurement system. It was concluded that the use of octane number data to directly infer information about the autoignition behaviour of fuels should be done with caution.

Assessment of the Operational Performance of Fischer-Tropsch Synthetic-Paraffinic Kerosene in a T63 Gas Turbine Compared to Conventional Jet A-1 Fuel

The performance implications of operating on Synthetic-Paraffinic Kerosene (SPK) were investigated using a RR-Allison T63-A-700 Model 250-C18 B gas turbine and compared to conventional Jet A-1. The SPK was aromatic–free and possessed a greater hydrogen/carbon ratio than petroleum derived Jet A-1. The variation in aromatic content had several implications with respect to soot and NOx emissions. Reduced aromatics also implied a reduction in the radiative heat transfer to the combustor liner. A simple model was used to explore the effect of H/C ratio on the adiabatic flame temperature, the combustor exit temperature and the engine efficiency via the impact on the gas properties and these were compared to the experimental data. It was found that operation with SPK changed directionally toward improving energy extraction via a turbine and an overall efficiency gain of about 1.2% was attained with operation on SPK through increased combustion efficiency, a reduction in liner pressure loss and an improvement in the combustion products properties. A modified combustion liner was fitted to enable the thermal loading on the combustor liner to be investigated and the expected trend with the SPK fuel was confirmed and quantified.Copyright

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