Our beloved Mr. Banajith and Mr. Bishnu for keeping the lab room tidy at all times. I would like to thank Mr. Prakash and his workshop team, Mr. Satish and Mr. Vineel for helping me build test facility and conduct laser experiments.
Introduction
General Introduction
Gas based generators are of two types based on the drive viz. Gas Engine and Gas Turbine. In both cases, the combustion takes place at elevated/high pressure conditions due to the compression stage.
Combustion characteristics - fundamental concepts and definitions
- Combustion
- Stoichiometric mixture and Equivalence ratio
- Equilibrium temperature
- Laminar burning velocity
- Stretch
- Intrinsic instabilities
Due to non-uniform flow conditions, the balance/alignment between convection, diffusion and reaction is affected, eventually leading to flame disturbances. Uniform small cells appearing uniformly over the entire flame surface are a unique feature of hydrodynamic (or Darrieus-Landau) instability.
Organization of thesis
This instability affects all the spherical flames anchored in a combustion chamber, regardless of the type of fuel, oxidizer and initial conditions. Appendix-1: The latest literature review of 800 producer gas compositions is presented in this section.
Literature Review
General introduction
Flame measurement techniques
- Burner method
- Planar flame method
- Stagnation flame method
- Spherically expanding flame method
- Summary of Flame measurement techniques
Heat loss to the surround and rim of the burner is significant at the end of the flame. The main feature is the supply of thermal energy by preheating the unburned gases to compensate for heat loss from the flame to the burner.
Spherical flame combustion test facilities
The inner chamber is first filled with a combustible mixture to the desired initial pressure and temperature. The outer chamber will be filled with inert gases at the same thermodynamic conditions as the inner chamber.
Summary of Spherical flame combustion test facilities
Producer gas
- Composition variation
- Laminar burning velocity and intrinsic instabilities
- Summary of Producer gas combustion characteristics
CH4 and 65% (CO2:N2) burning in air using a divergent channel method at 1 bar and up to 650 K, and reported that the addition of CH4 increases the LBV of the mixture as methane was added only by replacing the diluents in the fuel. 84] determined the occurrence of thermo-diffusive and hydrodynamic instabilities in CO: H2 mixtures mixed with CO2/CH4.
Objectives
Design of experiments
- Effect of Non-combustible components
- Effect of Combustible components
- Effect of Initial pressure
Same as the intra-CC study, the intra-NCC compositions are calculated and reported in Figure 2-9. The CC is considered as 25CH4:25CO:25H2:25Y, where Y represents rich combustible species, which can be CH4 or CO or H2, and referred to as the intra-CC study.
Experimental setup and Methodology
Design of single-chamber test rig
Design Specifications of Developed Chamber
Experimental Procedure
The chamber was purged with dry (dehumidified) air supplied by a laboratory compressor, raising the core temperature to about 300 ± 5 K, and then evacuated. The chamber was then evacuated and later purged to remove the post-combustion products and finally reduce the temperature to normal.
Data Processing
- Confined flame method (Unsteady pressure-time data-driven)
- Unconfined Flame Method (Radius-time data-driven)
- Thermo-diffusive instability
- Hydrodynamic instability
- Numerical study
- Uncertainty analysis
- Summary
The radius of the spherical flame (smooth and free of instabilities) was obtained with image processing. Finally, laminar burning velocity (𝑆𝑢0) defined as the relative velocity of the reactants propagating normally to an adiabatic and stretch-free planar flame was estimated using Eq. This indicates the stability of the flame against an imbalance in thermal and mass diffusion effects.
Most models to estimate Leeff were formulated using a weighted average of the Le of the different species available in the mixture, and the procedure to arrive at these weights varied with the model chosen, such as the arithmetic/harmonic mean [109], vol. /mass fraction [110], and effective diffusion [110]. 111] used a weighting factor of the normalized heating values for each type of fuel, i.e., the ratio of the heating value of a species to the heating value of the mixture. The critical radius was the radius of the spherical flame where the flame started to develop uniform cells on the surface in a significant amount.
To calculate the critical radius, pixel intensity was monitored in a rectangular inspection area of 69920 pixels (two rectangles of size 190 × 184 to remove the electrodes) within the diameter of the optical access window, as shown in Figure 3-14.
Results and Discussion
Validation of the experimental rig and procedure
The uncertainty in all cases is not included in Figure 4-1 for readability of the data and is presented in the rest of the figures.
Chamber geometry, thermodynamic model, and processing data range selection
59] were lower than the present measurements due to the application of a linear strain extrapolation scheme in their work, respectively. The entire range of pressure-time data was not considered for the post-processing, as portions of data were affected by ignition effects, heat losses to the wall, hydrodynamic instability, etc. free of the above effects were selected for the accurate estimation of LBV.
The following results show that a purely spherical chamber without intrusion, together with Omari et al.
Effect of chamber volume and geometry on laminar burning velocity
- Flame stretch rate and preferential diffusion instability
- Burned Gas Markstein Length
- Flame stability - Effective and critical Lewis Numbers
- Unstretched Laminar Burning Velocity
Flame stretch rates decreased with an increase in chamber volume due to a decrease in the ratio of the flame thickness (𝛿𝑓) to the flame radius (𝑟𝑓) in the considered data series. Estimated flame stretch rates could not explain the sudden increase in the value of LBV at 20 L. This change was due to the increase in the significant presence of deficit species with high mass diffusivity.
Hydrogen-rich PG showed the lowest/highest Leeff for 25/45% CC blends due to the respective change in thermal diffusivity. Figures clearly showed that for a given CH4/CO/H2 rich PG and volume fraction of CC, the LBV varied non-linearly with change in the ratio of CO2 and N2. Observations from the figures showed that LBV increased linearly with an increase in volume fraction of CC at a given volume fraction of NCC for all cases studied due to the increase in heating value and flame temperature.
A linear variation in the slope values was observed for H2 and CO-rich cases with a decrease in XCO2 (and concomitant increase in XN2) in diluents.
- Unstretched Laminar Burning Velocity
In the H2-rich case, the size of the LBV increased with an increase in CO and a decrease in CH4 at all equivalence ratios. The peak LBV shifted to the richer side, i.e. and the magnitude of the peak values continuously increased with an increase in CO and subsequent decrease in CH4, which was typical behavior of H2-CO mixtures [58,122]. The LBV variation with an increase in equivalence ratio in each fuel composition became non-linear with a simultaneous increase/decrease in CH4/CO content respectively as the peak shifted to = 1.1.
In the CO-rich case, the extent of LBV increased with increasing H2 (and decreasing CH4) at any given equivalence ratio, and a similar trend was reported by Xie et al. In the CH4-rich case, the magnitude of the LBV increased with increasing H2 (and decreasing CO) at any given equivalence ratio. The maximum LBV value was reached at = 1.1 in all CH4-rich cases, while the magnitude decreased slightly with increasing H2 (and decreasing CO) due to decreasing flame temperature.
In the triple fuel variation considered, GRI Mech 3.0 performed better with CO and H2-rich mixtures, while FFCM performed better with CH4-rich mixtures.
Effect of Initial pressure on the LBV and flame stability of RPG-air mixtures
- Intrinsic instabilities
The mean Lb values for H2-rich mixtures varied differently from the trend observed in CH4- and CO-rich mixtures. Conversely, few of the lean ( = 0.8) mixtures of certain compositions were unstable to thermodiffusive effects due to the active presence of H2. The mole fraction of H2 in these compositions never exceeded 25%, which concludes the crucial role of H2 in inducing the thermodiffusive instability in the RPG air mixtures.
All mixtures gave thinner flames at elevated pressures due to an increase in reaction rate. A similar conflict was observed for the other parameters considered due to the diversity of the three types of fuel present in the fuel mixture. Compositions on the CH4-rich side (along with an increase in H2 and a decrease in CO) were more susceptible to thermo-diffusion instabilities.
Larger mole fractions of CH4 in the mixture reduced the susceptibility to hydrodynamic instability, while increasing CO suppressed the thermo-diffusive instability.
Conclusions
Effect of chamber volume on laminar burning velocity
The calculated flame extension rate could not alone explain the sudden increase in the LBV value at 20 L. A significant change in the slope of dp/dt indicated that the onset of flow instability resulted in flame acceleration. The stretching rate and stretched LBV predicted by the simulations did not change with an increase in chamber volume compared to the experiments, and the possible reason could be
It is recommended to use a spherical chamber with a volume in the region of 4.4 L to have less stretch effects, resulting in LBV with minimal stretch effect at elevated pressures and temperatures.
LBV obtained from simulations based on GRI Mech 3.0 deviated from experimental values with an increase in XCC and XN2. The values became negative when XCC dropped below 20% for hydrogen- and carbon monoxide-rich mixtures. This was due to a reduction in the heating value of fuel, flame temperature and participation of CO2.
At 25%CC+75%NCC, the LBV of CH4-rich mixtures was higher for all combinations of NCC than H2/CO-rich mixtures due to higher flame temperature. CO-rich mixtures had the lowest values of LBV at all mixture conditions due to low flame temperature and thermal diffusivity. From the intra-NCC study, the non-linearity of the trend line increased either with an increase in XCO2 (decrease in XN2) in all cases considered indicating the involvement of CO2 in the kinetics rather than being inert.
As a whole, a higher volume fraction of fuel/inert species in combustible/non-combustible components determines the combustion characteristics of PG.
The overall contribution of the present work
The application potential of the present work
Future scope of the present work
Computational studies of the laminar burning velocity of a generator gas-air mixture at typical engine conditions. Effects of fuel compositions on synthesis gas/product gas laminar diffusion flame heat generation and emission. Pang, Investigation of Hydrogen Sulfide Removal from Simulated Biomass Gasification Producer Gas with Titanomagnetite, Biomass and Bioenergy 109 (Nov.
Reyes, Characterization of the Combustion of Biomass Producer Gas in a Constant Volume Combustion Bomb, Fuel. Pianthong, Effect of pilot fuel quantity on the performance and emission of a dual gas producer - Diesel engine, Energy Procedia. Said, Effect of producer gas staged combustion on the performance and emissions of a single shaft microgas turbine running in a dual fuel mode, J.
Paul, Effects of fuel compositions on heat generation and emission of syngas/producer gas laminar diffusion flame, Int. Ramli, Investigation of the effects of operating factors on the resulting producer gas from gasification of oil palm leaves with a single neck downdraft gasifier, Forny. Kim, Gasification of dried sewage sludge in a newly developed three-stage gasifier: Effect of each reactor temperature on producer gas composition and impurity removal, Energ.
![Figure 2-6 Schematic diagram of a typical double/dual-chamber test facility [37]](https://thumb-ap.123doks.com/thumbv2/azpdfnet/10343248.0/43.892.180.753.112.600/figure-schematic-diagram-typical-double-dual-chamber-facility.webp)
