No description available.

In this work, both experimental and computational studies have been performed to investigate the flame dynamics and combustion instability of a laboratory buoyant jet diffusion flame from different prospects. The motivation behind this study was to obtain a better understanding of the dynamics of jet diffusion flames, as part of a long-term effort in achieving flexible fuel utilisation such as interchangeable fuels and achieving more effective combustion control such as better combustion efficiency.In the experimental study of jet diffusion flames, the influences of parameters such as nozzle exit diameter, fuel flow rate, fuel types and burner geometries have been investigated, where the focus was on the effects of fuel mixture on the flame dynamics. The frequency spectra, flame vortex development and flickering frequencies were measured using flow visualisation techniques and data acquisition systems. It was observed that the fuel jet velocity and the type of burner had a weak influence on the pulsation frequency for all the tested diameters. In contrast it has been found that both the ambient condition and fuel variability do have significant effects on the flame flickering frequency. Flame structure and dynamics are very different for the methane, propane and mixed fuel jet flames. Since the measurements of variables such as entrainment properties are difficult to obtain under experimental conditions, it is more effective to deal with such problems numerically. In the second part of this study, the dynamics of the buoyant jet diffusion flame has been investigated by idealised axisymmetric direct numerical simulations (DNS). The physical problem is a fuel jet issuing vertically into an oxidant ambient. Taking the advantages of idealised computational conditions, the effects of nozzle velocity profile, initial momentum thickness, Froude number, Reynolds number and co-flow on the near-field dynamics of a jet diffusion flame have been investigated. The computational cases have shown the development of different vortical structures, which suggest that vortical structures depend on both buoyancy and jet nozzle velocity profile. The flickering frequency and flickering energy results provide supportive evidence of the above finding. The results of the co-flow case indicate no significant flame-vortex interaction, and the flame oscillation is being suppressed. In general, the study suggested that the velocity shear plays a significant role in the near-field flame dynamics, apart from the buoyancy effects.

An experimental study was conducted using axisymmetric co-flow laminar diffusion flames of methane-air, methane-oxygen and ethylene-air to examine the effect of pressure on soot formation and the structure of the temperature field. A liquid fuel burner was designed and built to observe the sooting behavior of methanol-air and n-heptane-air laminar diffusion flames at elevated pressures up to 50 atm. A non-intrusive, line-of-sight spectral soot emission (SSE) diagnostic technique was used to determine the temperature and the soot volume fraction of methane-air flames up to 60 atm, methane-oxygen flames up to 90 atm and ethylene-air flames up to 35 atm. The physical flame structure of the methane-air and methane-oxygen diffusion flames were characterized over the pressure range of 10 to 100 atm and up to 35 atm for ethylene-air flames. The flame height, marked by the visible soot radiation emission, remained relatively constant for methane-air and ethylene-air flames over their respected pressure ranges, while the visible flame height for the methane-oxygen flames was reduced by over 50 % between 10 and 100 atm. During methane-air experiments, observations of anomalous occurrence of liquid material formation at 60 atm and above were recorded. The maximum conversion of the carbon in the fuel to soot exhibited a strong power-law dependence on pressure. At pressures 10 to 30 atm, the pressure exponent is approximately 0.73 for methane-air flames. At higher pressures, between 30 and 60 atm, the pressure exponent is approximately 0.33. The maximum fuel carbon conversion to soot is 12.6 % at 60 atm. For methane-oxygen flames, the pressure exponent is approximately 1.2 for pressures between 10 and 40 atm. At pressures between 50 and 70 atm, the pressure exponent is about -3.8 and approximately -12 for 70 to 90 atm. The maximum fuel carbon conversion to soot is 2 % at 40 atm. For ethylene-air flames, the pressure exponent is approximately 1.4 between 10 and 30 atm. The maximum carbon conversion to soot is approximately 6.5 % at 30 atm and remained constant at higher pressures.

A pulsed fuel injector system was used to study flame structure, flame length, and emissions of ethylene jet diffusion flames over a range of injection times and duty-cycles with a variable air co-flow. In all cases the jet was completely shut off between pulses (fully-modulated) for varying intervals, giving both widely-spaced, non-interacting puffs and interacting puffs. Imaging of the luminosity from the flame revealed distinct types of flame structure and length, depending on the duration of the fuel injection interval. Flame lengths for isolated puffs (small injection times) were up to 83% less than steady state flames with the same injection velocities. With the addition of co-flow flame lengths grew to a maximum of 30% longer than flames without any co-flow. A scaling argument is also developed to predict the amount of co-flow that gives a 15% increase in mean flame length. Interacting flames with a small co-flow and small injection times (injection time = 5.475 ms) experienced flame length increases of up to 212% for a change in injection duty-cycle from 0.1 to 0.5. For interacting flames with long injection times (on time = 119 ms), essentially no change in flame length was noticeable over the same range of duty-cycles. Emission measurements suggest partial quenching of the reaction in isolated puffs with low duty-cycles and injection times (injection times less than 5.475 ms) resulting in high CO and UHC concentrations and low NO and NOx concentrations. With an increase in duty-cycle, the puffs began to interact and CO and UHC concentrations decreased while NO and NOx concentrations increased. For flames with injection times greater than 5.475 ms emission concentrations seem to be reasonably constant, with a slight increase in NO and NOx concentrations as the duty-cycle increased. Also the duty-cycle experienced in the vicinity of the probe is estimated and used as a scaling factor for the emission measurements.

Methane-air, ethane-air, and n-heptane-air over-ventilated co-flow laminar diffusion flames were studied up to pressures of 2.03, 1.52, and 0.51 MPa, respectively, to determine the effect of pressure on flame shape, soot concentration, and temperature. A spectral soot emission optical diagnostic method was used to obtain the spatially resolved soot formation and temperature data. In all cases, soot formation was enhanced by pressure, but the pressure sensitivity decreased as pressure was increased. The maximum fuel carbon conversion to soot, ηmax, was approximated by a power law dependence with the pressure exponent of 0.92 between 0.51 and 1.01 MPa, and 0.68 between 1.01 and 2.03 MPa with ηmax=9.5% at 2.03 MPa for methane-air flames. For ethane-air flames, the pressure exponent was 1.57 between 0.20 and 0.51 MPa, 1.08 between 0.51 and 1.01 MPa, and 0.58 between 1.01 and 1.52 MPa where ηmax=23% at 1.52 MPa. For nitrogen-diluted n-heptane-air flames, ηmax=6.5% at 0.51 MPa.

Methane-air, ethane-air, and n-heptane-air over-ventilated co-flow laminar diffusion flames were studied up to pressures of 2.03, 1.52, and 0.51 MPa, respectively, to determine the effect of pressure on flame shape, soot concentration, and temperature. A spectral soot emission optical diagnostic method was used to obtain the spatially resolved soot formation and temperature data. In all cases, soot formation was enhanced by pressure, but the pressure sensitivity decreased as pressure was increased. The maximum fuel carbon conversion to soot, ηmax, was approximated by a power law dependence with the pressure exponent of 0.92 between 0.51 and 1.01 MPa, and 0.68 between 1.01 and 2.03 MPa with ηmax=9.5% at 2.03 MPa for methane-air flames. For ethane-air flames, the pressure exponent was 1.57 between 0.20 and 0.51 MPa, 1.08 between 0.51 and 1.01 MPa, and 0.58 between 1.01 and 1.52 MPa where ηmax=23% at 1.52 MPa. For nitrogen-diluted n-heptane-air flames, ηmax=6.5% at 0.51 MPa.

The measurement of Polycyclic Aromatic Hydrocarbons (PAHs) in flames is essential for the understanding of soot formation. In comparison to conventional aromatics-sampling techniques, a new technique was proposed that involves fewer manual operations and no hazardous extraction solvents. Apparatus and experimental procedures of the newly proposed adsorptive-sampling and desorptive-extraction technique for aromatic-hydrocarbon measurements were established in this study. The capabilities and limitations of this new technique were assessed in terms of limits of detection, sampling locations and data repeatability. The accuracy of this technique was also evaluated. Aromatic-hydrocarbon species concentrations were measured in laminar co-flow diffusion flames of ethylene (C2H4) and synthetic paraffinic kerosene (SPK). The results obtained from the ethylene flame were compared to its numerical simulation, with the goal of achieving agreement within an order of magnitude. The differences between simulated values and experimental measurements, along with the limitations of the technique, were used as an indication of the accuracy of the technique.

The measurement of Polycyclic Aromatic Hydrocarbons (PAHs) in flames is essential for the understanding of soot formation. In comparison to conventional aromatics-sampling techniques, a new technique was proposed that involves fewer manual operations and no hazardous extraction solvents. Apparatus and experimental procedures of the newly proposed adsorptive-sampling and desorptive-extraction technique for aromatic-hydrocarbon measurements were established in this study. The capabilities and limitations of this new technique were assessed in terms of limits of detection, sampling locations and data repeatability. The accuracy of this technique was also evaluated. Aromatic-hydrocarbon species concentrations were measured in laminar co-flow diffusion flames of ethylene (C2H4) and synthetic paraffinic kerosene (SPK). The results obtained from the ethylene flame were compared to its numerical simulation, with the goal of achieving agreement within an order of magnitude. The differences between simulated values and experimental measurements, along with the limitations of the technique, were used as an indication of the accuracy of the technique.

An experimental study was conducted using axisymmetric co-flow laminar diffusion flames of methane-air, methane-oxygen and ethylene-air to examine the effect of pressure on soot formation and the structure of the temperature field. A liquid fuel burner was designed and built to observe the sooting behavior of methanol-air and n-heptane-air laminar diffusion flames at elevated pressures up to 50 atm. A non-intrusive, line-of-sight spectral soot emission (SSE) diagnostic technique was used to determine the temperature and the soot volume fraction of methane-air flames up to 60 atm, methane-oxygen flames up to 90 atm and ethylene-air flames up to 35 atm. The physical flame structure of the methane-air and methane-oxygen diffusion flames were characterized over the pressure range of 10 to 100 atm and up to 35 atm for ethylene-air flames. The flame height, marked by the visible soot radiation emission, remained relatively constant for methane-air and ethylene-air flames over their respected pressure ranges, while the visible flame height for the methane-oxygen flames was reduced by over 50 % between 10 and 100 atm. During methane-air experiments, observations of anomalous occurrence of liquid material formation at 60 atm and above were recorded. The maximum conversion of the carbon in the fuel to soot exhibited a strong power-law dependence on pressure. At pressures 10 to 30 atm, the pressure exponent is approximately 0.73 for methane-air flames. At higher pressures, between 30 and 60 atm, the pressure exponent is approximately 0.33. The maximum fuel carbon conversion to soot is 12.6 % at 60 atm. For methane-oxygen flames, the pressure exponent is approximately 1.2 for pressures between 10 and 40 atm. At pressures between 50 and 70 atm, the pressure exponent is about -3.8 and approximately -12 for 70 to 90 atm. The maximum fuel carbon conversion to soot is 2 % at 40 atm. For ethylene-air flames, the pressure exponent is approximately 1.4 between 10 and 30 atm. The maximum carbon conversion to soot is approximately 6.5 % at 30 atm and remained constant at higher pressures.

Soot formation is one of the most complex phenomena in combustion science and an understanding of the underlying physico-chemical mechanisms is important. This work adopted both experimental and numerical approaches to study soot formation in laminar counterfl ow diffusion flames. As polycyclic aromatic hydrocarbons (PAHs) are the precursors of soot particles, a detailed gas-phase chemical mechanism describing PAH growth upto coronene for fuels with 1 to 4 carbon atoms was validated against laminar premixed and counter- flow diffusion fl ames. Built upon this gas-phase mechanism, a soot model was then developed to describe soot inception and surface growth. This soot model was sub- sequently used to study fuel mixing effect on soot formation in counterfl ow diffusion flames. Simulation results showed that compared to the baseline case of the ethylene flame, the doping of 5% (by volume) propane or ethane in ethylene tends to increase the soot volume fraction and number density while keeping the average soot size almost unchanged. These results are in agreement with experimental observations. Laser light extinction/scattering as well as laser induced fluorescence techniques were used to study the effect of strain rate on soot and PAH formation in counterfl ow diffusion ames. The results showed that as strain rate increased both soot volume fraction and PAH concentrations decreased. The concentrations of larger PAH were more sensitive to strain rate compared to smaller ones. The effect of CO2 addition on soot formation was also studied using similar experimental techniques. Soot loading was reduced with CO2 dilution. Subsequent numerical modeling studies were able to reproduce the experimental trend. In addition, the chemical effect of CO2 addition was analyzed using numerical data. Critical conditions for the onset of soot were systematically studied in counterfl ow diffusion ames for various gaseous hydrocarbon fuels and at different strain rates. A sooting temperature index (STI) and a sooting sensitivity index (SSI) were proposed to present the sooting tendencies of different fuels and their sensitivities to strain rates.