Carl Louis Viljoen

An Investigation Of Anomalies Identified Within The ASTM Research And Motor Octane Scales

This paper describes the preliminary findings arising from a project to develop a blend-property model for gasoline. An engine model was used in conjunction with various auto-ignition models, including the Shell model, to analyze engine test results and the ASTM guide curves that link octane number to compression ratio. Good correlation for both RON and MON was achieved for 40<ON<120 provided that a non-linear pressure dependency was included in the auto ignition model. This was regarded as a possible indication of molecular cage formation. The results confirmed that physical factors such as the rate of normal combustion and fuel evaporative cooling have a significant impact on the measured octane number and are thus critical for inclusion in any model that purports to predict the autoignition point.

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