Characterisation of Laser Induced Spark-Ignited Flame Kernels in Premixed Fuel/Air Mixtures

Gaseous alternative fuels are promising solution for today’s increasing demand for clean and reliable power. The wide number of fuel types and sources implies that engine designers need to develop fuel flexible combustors. Also, to meet tightening emission laws, these combustors would be required to operate under ultra-lean, high pressure and high temperature environments. Such extreme conditions make ignition difficult to achieve especially with current spark plugs which has been the primary ignition source during the last one hundred years. Laser ignition has been proposed as an alternative ignition system capable of providing stable combustion under these conditions. The advantages provided by laser ignition over electric spark system include: the absence of flame quenching effects of electrodes which enhances the ignition of lean mixtures, less energy requirement for ignition at higher pressures, precise timing, and choice of suitable ignition location. To explore the benefits offered by the Laser ignition in practical combustors, there is a need to characterise the propagation behaviour of the laser flame kernel since successful ignition requires the transition from an ignited spark kernel to a self-sustained flame. The present thesis contributes to existing knowledge on laser ignition through investigation of different development characteristics of the ignited flame kernel. The first investigation involves high-speed imaging of the flow field characteristics of the flame kernel based on combined 2D Laser tomography and PIV techniques. The ignition was achieved by focussing a laser beam of 1064 nm wavelength on an atmospheric co-axial straight tube burner through which stoichiometric CH4/Air was flowed. The resulting flame kernel and its flow field were visualized through laser-sheet illumination and then captured using a high-speed camera. The observed flame kernel features from the tomographic images were consistent with previous research observation and provided insight to other phenomena such as induced vortex motion in the developing kernel. Additionally, the PIV data provided insight on how the local flow field velocities were changing during development of the flame kernel. The second investigation involves direct imaging of the flame kernel chemiluminescence to understand both the fluid dynamics and chemical reactivity of the laser flame kernel. The atmospheric burner used in this setup is made of co-axial contracting nozzles in which flowing fuel/air mixtures were repeatedly ignited by a focused laser beam of 1064 nm wavelength and 2 Hz frequency. To characterise the resulting flame kernel, 2D projection images of the kernel OH* chemiluminescence was captured using intensified CCD camera. The observed geometric features of the kernel were similar to the earlier observation. Additional data on the OH* luminosity provided insight on the reactivity of the kernel at various transition points during its development and the reason for the variation in growth rate of the flame kernel at different stages. The investigation was extended to include the effects of varying physical parameters such as laser pulse energy and flow velocity. The observation showed that the effect of increasing the pulse energy within a certain threshold limit is an enhanced early kernel growth, but the ultimate effect was insignificant. Although, a higher flow velocity had no remarkable effect on the size of the kernel, it resulted in faster propagation of the flame front downstream due to the combined effect of convection and increased turbulence. In the final study, the sensitivity of the kernel characteristics to changes in the fuel thermochemical properties was investigated based on direct imaging of the OH* chemiluminescence. The investigation comprises the effect of changing equivalence ratio, variation in fuels at constant Adiabatic Flame Temperature and variation in fuels at constant Laminar Flame Velocity. The results of the analysis showed linear dependence of most characteristics with equivalence in laminar flow regime but not in turbulent flows. For both constant temperature and constant laminar velocity mixtures, the results showed differences in the flame kernel characteristics depending on the fuel. This shows that no single thermochemical property is enough to uniquely define different fuel/air mixtures. Hence, further study on the inter-dependencies of the different thermochemical properties would be necessary for development of more robust model that would characterise flame kernel propagation in flexible combustion systems

Fifth International Microgravity Combustion Workshop

This conference proceedings document is a compilation of 120 papers presented orally or as poster displays to the Fifth International Microgravity Combustion Workshop held in Cleveland, Ohio on May 18-20, 1999. The purpose of the workshop is to present and exchange research results from theoretical and experimental work in combustion science using the reduced-gravity environment as a research tool. The results are contributed by researchers funded by NASA throughout the United States at universities, industry and government research agencies, and by researchers from at least eight international partner countries that are also participating in the microgravity combustion science research discipline. These research results are intended for use by public and private sector organizations for academic purposes, for the development of technologies needed for the Human Exploration and Development of Space, and to improve Earth-bound combustion and fire-safety related technologies

Experimental Study of Lean Blowout with Hydrogen Addition in a Swirl-stabilized Premixed Combustor

Lean premixed combustion is widely used to achieve a better compromise between nitric oxides emissions and combustion efficiency. However, combustor operation near the lean blowout limit can render the flame unstable and lead to oscillations, flashback, or extinction, thereby limiting the potential range of lean combustion. Recent interest in integrated gasification combined cycle plants and syngas combustion requires an improved understanding of the role of hydrogen on the combustion process. Therefore, in present study, combustion of pure methane and blended methane-hydrogen has been conducted in a swirl stabilized premixed combustor. The measurement techniques implemented mainly include particle image velocimetry, CH*/OH* chemiluminescence imaging, planar laser-induced fluorescence imaging of OH radical. By investigating the flow field, heat release, flow-flame interaction, and flame structure properties, the fundamental controlling processes that limit lean and hydrogen-enriched premixed combustion with and without confinement have been analyzed and discussed. As equivalence ratio decreases, for unconfined flames, the reduced flame speed leads flame shrinking toward internal recirculation zone (IRZ) and getting more interacted with inner shear layer, where turbulence level and vorticity are higher. The flame fronts therefore experience higher hydrodynamic stretch rate, resulting in local extinction, and breaks along the flame fronts. Those breaks, in turn, entrain the unburnt fuel air mixture into IRZ passing through the shear layer with the local vortex effect, further leading to reaction within IRZ. In methane-only flames, the width of IRZ decreases, causing flames to straddle the boundary of the IRZ and to be unstable. High speed imaging shows that periodic flame rotating with local extinction and re-light events are evident, resulting in high RMS of heat release rate, and therefore a shorter extinction time scale. With hydrogen addition, flames remain in relatively axisymmetric burning structure and stable with the aid of low minimum ignition energy and high molecular diffusivity associated with hydrogen, leading to lower heat release fluctuation and a longer extinction time scale. For confined flames, however, the hydrogen effect on the extinction transient is completely opposite due to spiraling columnar burning structure, in comparison of a relatively stable conical shape in methane flames

DEVELOPMENT OF A GAS-BASED APPLICATION FOR FIRE MODELING IN THE PROCESS INDUSTRIES

The major hazards with which the chemical industry is concerned are fire, explosion and toxic release. Of these three, fire is the most common. In assessing the damage potential and causes or errors which have led to these disasters, an analysis has to be done. The impacts of fires in the process industries may be predicted by the application of mathematical models. However, the applications of these models require competency in mathematics and computer programming. Therefore, the objective of this project is to develop an application called the Fire Simulation Tool (FiST), which is able to study the impact of fire in the process industry. The scope of work for this project is confmed to fire cases only, which are: flash fire, jet fire, pool fire and fireball. The FiST application is developed using Visual Basic (VB) programming language with integration of GIS tools. The mathematical models of the four types of fire are simulated and the results are integrated to GIS for better visualization. The development is done by customizing MapObjects using VB. With MapObjects user can incorporate mapping capabilities in their application. The methodology of the project includes utilizing established models in order to calculate the impact of fire. The development of this software has been divided into five different stages, which are planning the application, building the graphical user interface (GUI), writing the computer programme, software validation and verification and lastly, integrating the results from the tool with GIS application to present the simulation outcome as buffer zones around the centre of the accident. The results from FiST software is verified and validated with other risk assessment softwares such as: FRED (developed by Shell Global company, 2004), BIS (developed by ThermDyne Technologies Ltd, 2003) and SCIA (developed by EI-Harbawi, 2006) and with established data. The software is capable to estimate the thermal radiation and the impacts from the fire scenarios which include the probability of frrst, second and third degree of bums for the hmnan skin. The FiST application is useful and feasible because it is user-friendly, able to function as a stand-alone application and it is compatible with all windows operating system. Furthermore, the cost of developing the software is cheap and the application incorporates the risk tolerability limit for Malaysia

A study of lean premixed swirl-stabilized combustion of gaseous alternative fuels.

The burner was constructed of 1.5&inches; (3.8 cm) schedule 40 steel pipe. Fuel injectors were placed 40 cm upstream of the burner to ensure that the fuel and air were fully premixed prior to combustion. The fuel air mixture entered the combustion chamber in the annulus around a centerbody which contained 6 swirl vanes to impart an out of plane motion to the flow. The flow expanded into the combustion chamber which had an 8.1 cm inside diameter, and was exhausted into the ambient at the end of the combustion chamber. Pollutant emissions were measured using an electro-chemical gas analyzer and water-cooled stainless steel and expansion-cooled quartz probes. Flame extinction was studied by visual observation of the flame. Combustion related noise was recorded using a condenser microphone and digitized by a high speed data acquisition card. (Abstract shortened by UMI.)The effects of utilizing gaseous fuels with different compositions was studied for a lean premixed swirl stabilized burner typical of those used in land-based gas turbine engines. The experiments were performed at atmospheric temperature and pressure in a quartz glass combustor. The fuels utilized were binary mixtures containing either methane or propane as the primary component and hydrogen, oxygen, nitrogen or carbon dioxide as the secondary component. The combinations chosen represent constituents of various gaseous alternative fuels. In particular, focus was placed on hydrogen enriched hydrocarbon fuels proposed as a cross-over strategy to the hydrogen energy infrastructure. The operating parameters included fuel composition, total reactant flow rate, and the calculated adiabatic flame temperature. Global flame characteristics such as emissions of oxides of nitrogen (NOx) and carbon monoxide (CO), flame extinction, and combustion noise were studied. The internal characteristics of flames were also studied, including the velocity field and related flow properties, as well as the structures of the reaction zones

The effect of elevated water content on ethanol combustion

Ethanol is currently being considered as a potential alternative to traditional fuels. However, the fuel offers a low return in terms of energy output per dollar invested when compared to fossil fuels. More than 1/3 of the cost associated with bio-ethanol production is devoted to distillation and water removal. This study seeks to validate the use of hydrous ethanol as a practical fuel to be used in lieu of fossil fuels or anhydrous ethanol. Success would reduce the production cost associated with ethanol fuel. Hydrous ethanol was burned in a swirl-stabilized combustor, air is introduced at a constant flow rate through a dump diffuser, and fuels ranging from 0%-40% water by volume was tested for practicality. A stable flame was achieved with up to 35% water and the Lean Blow Out limit was determined for these fuels. Fuels ranging from 0% to 20% water were tested in greater detail. This included thermal mapping of the flame, exhaust temperature measurements, exhaust NOx, CO2, and O2 measurement, as well as CH* and OH* imaging of the low-flame region. Equivalence ratio was varied to include test points at 0.6, 0.8, 1.0 and 1.1. This range provides insight into flame behavior at extremely lean, lean, stoichiometric, and rich test conditions. Results revealed that exhaust heat rate, combustion efficiency, and combustor thermal efficiency were not affected negatively by elevated water content. However, flame temperature decreased as a result of water addition, particularly in the low flame region. CH*/OH* emissions in the low-flame region were also reduced due to the parasitic heat load of water vaporization and local quenching. The practical consequence of burning hydrous fuel was reduced exhaust temperature. This negative consequence, coupled with the desirable consequence of increased mass flow rate, did not appreciably affect the net exhaust heat rate. Reduced peak temperatures lead to exhaust NOx reductions. In conclusion, this study reveals that ethanol with proof as low as 140 behaves as a practical fuel and is recommended as a means of increasing the economic return when using ethanol as fuel in situations where increased volumetric consumption of the fuel is acceptable

Fire performance of residential shipping containers designed with a shaft wall system

seven story building made of shipping containers is planned to be built in Barcelona, Spain. This study mainly aimed to evaluate the fire performance of one of these residential shipping containers whose walls and ceiling will have a shaft wall system installed. The default assembly consisted of three fire resistant gypsum boards for vertical panels and a mineral wool layer within the framing system. This work aimed to assess if system variants (e.g. less gypsum boards, no mineral wool layer) could still be adequate considering fire resistance purposes. To determine if steel temperatures would attain a predetermined temperature of 300-350ºC (a temperature value above which mechanical properties of steel start to change significantly) the temperature evolution within the shaft wall system and the corrugated steel profile of the container was analysed under different fire conditions. Diamonds simulator (v. 2020; Buildsoft) was used to perform the heat transfer analysis from the inside surface of the container (where the fire source was present) and within the shaft wall and the corrugated profile. To do so gas temperatures near the walls and the ceiling were required, so these temperatures were obtained from two sources: (1) The standard fire curve ISO834; (2) CFD simulations performed using the Fire Dynamics Simulator (FDS). Post-flashover fire scenarios were modelled in FDS taking into account the type of fuel present in residential buildings according to international standards. The results obtained indicate that temperatures lower than 350ºC were attained on the ribbed steel sheet under all the tested heat exposure conditions. When changing the assembly by removing the mineral wool layer, fire resistance was found to still be adequate. Therefore, under the tested conditions, the structural response of the containers would comply with fire protection standards, even in the case where insulation was reduced.Postprint (published version

Numerical study of the characteristics of CNG, LPG and Hydrogen turbulent premixed flames

Numerical simulations have proven itself as a significant and powerful tool for accurate prediction of turbulent premixed flames in practical engineering devices. The work presented in this thesis concerns the development of simulation techniques for premixed turbulent combustion of three different fuels, namely, CNG, LPG and Hydrogen air mixtures. The numerical results are validated against published experimental data from the newly built Sydney combustion chamber. In this work a newly developed Large Eddy Simulation (LES) CFD model is applied to the new Sydney combustion chamber of size 50 x 50 x 250 mm (0.625 litre volume). Turbulence is generated in the chamber by introducing series of baffle plates and a solid square obstacle at various axial locations. These baffles can be added or removed from the chamber to adapt various experimental configurations for studies. This is essential to understand the flame behaviour and the structure. The LES numerical simulations are conducted using the Smagorinsky eddy viscosity model with standard dynamic procedures for sub-grid scale turbulence. Combustion is modelled by using a newly developed dynamic flame surface density (DFSD) model based on the flamelet assumption. Various numerical tests are carried out to establish the confidence in the LES based combustion modelling technique. A detailed analysis has been carried out to determine the regimes of combustion at different stages of flame propagation inside the chamber. The predictions using the DFSD combustion model are evaluated and validated against experimental measurements for various flow configurations. In addition, the in-house code capability is extended by implementing the Lewis number effects. The LES predictions are identified to be in a very good agreement with the experimental measurements for cases with high turbulence levels. However, some disagreement were observed with the quasi-laminar case. In addition a data analysis for experimental data, regarding the overpressure, flame position and the flame speed is carried out for the high and low turbulence cases. Moreover, an image processing procedure is used to extract the flame rate of stretch from both the experimental and numerical flame images that are used as a further method to validate the numerical results. For the grids under investigation, it is concluded that the employed grid is independent of the filter width and grid resolution. The applicability of the DFSD model using grid-independent results for turbulent premixed propagating flames was examined by validating the generated pressure and other flame characteristics, such as flame position and speed against experimental data. This study concludes that the predictions using DFSD model provide reasonably good results. It is found that LES predictions were slightly improved in predicting overpressure, flame position and speed by incorporating the Lewis number effect in the model. Also, the investigation demonstrates the effects of placing multiple obstacles at various locations in the path of the turbulent propagating premixed flames. It is concluded that the pressure generated in any individual configuration is directly proportional to the number of baffles plates. The flame position and speed are clearly dependent on the number of obstacles used and their blockage ratio. The flame stretch extracted from both the experimental and numerical images shows that hydrogen has the highest stretch values over CNG and LPG. Finally, the regime of combustion identified for the three fuels in the present combustion chamber is found to lie within the thin reaction zone. This finding supports the use of the laminar flamelet modelling concept that has been in use for the modelling of turbulent premixed flames in practical applications

MILD Combustion of Prevaporised Liquid Fuels

Combustion of liquid fuels is the dominant source in the global energy supply, and its crucial importance is expected to remain well into the foreseeable future. Whilst providing human beings with energy, combustion of liquid fuels also produces undesired byproducts, including pollutants and greenhouse gases. In response to the mounting concern for energy sustainability and the environment, concerted efforts have been invested in the development of advanced combustion technologies. Moderate or Intense Low-oxygen Dilution (MILD) combustion technology has a great potential to abate pollutant and greenhouse emissions while maintaining a high thermal efficiency. In practical applications of MILD combustion, hot exhaust gases are recirculated inside the combustion chamber, simultaneously preheating and diluting reactants. A combination of hot reactants' temperature and low local oxygen concentration across the entire combustion chamber lead to volumetric reactions, resulting in a more uniform temperature and heat distribution. As a consequence, the peak flame temperature is reduced, thereby suppressing the formation of pollutants, such as nitrogen oxides. Most of the previous studies on MILD combustion have been focused on simple gaseous fuels. There is a paucity of information concerning liquid fuels burning under MILD combustion conditions, despite their critical role in the world energy supply. This thesis aims to advance the understanding of MILD combustion of liquid fuels through a combined experimental and computational investigation. In this investigation, liquid fuels are prevaporised in order to avoid the complexity of spray dynamics. Thus the focus is on the fundamental aspects of chemical kinetics of these fuels under MILD combustion. This thesis consists of a compilation of four journal articles, presenting results and findings from a combination of experimental and numerical studies. The first part of the experimental studies were conducted in a pressurised reverse- flow MILD combustor burning prevaporised ethanol, acetone, and n-heptane. These fuels are chosen to represent different classes of hydrocarbons, namely, an alcohol, a ketone, and a long-chain alkane. The pollutant emissions and the combustion stability under a wide range of operating conditions are examined. This investigation identifies several key operating parameters, namely, fuel type, equivalence ratio, carrier gas, air jet velocity, and operating pressure inside the combustion chamber. In order to further investigate the stabilisation of MILD flames and assess the impact of important parameters independently, parametric studies of prevaporised ethanol, acetone, and n-heptane are performed in a well-controlled environment, namely in a Jet in Hot Coflow (JHC) burner. Turbulent jet flames of dimethyl ether (an isomer of ethanol) are also investigated and compared to ethanol flames. Simultaneous imaging of OH, CH2O, and temperature, together with digital photography and imaging of OH* chemiluminescence, are performed to reveal the flame structure. Reaction flux analyses of various fuels are conducted to complement the experimental results. These results reveal that the local oxygen concentration plays a significant role in the flame structure. A transitional flame structure (a strong OH layer connected with a weaker "tail") is observed in the ethanol and the DME flames in a 9% O2 coflow instead of a 3% O2 coflow. This occurrence of the transitional flame structure is considered as an indicator of flames deviating from the MILD combustion regime. Simulations of ethanol and DME flames reveal that the importance of H2/O2 pathways in their oxidation processes decreases and intermediate species pool changes as the oxygen level increases from 3% to 9%. This suggests that a three-fold increase in the oxygen concentration leads to fundamental changes in the chemical kinetics of ethanol and DME. It is also found that n-heptane flames do not have the characteristics of a typical MILD combustion flame as observed in the ethanol and the DME flames. A transitional flame structure is seen in the n-heptane flames even at the 3% O2 coflow. In the reverse-flow combustor, stable combustion of ethanol is established under all tested conditions. However, n-heptane flames become more unstable than ethanol and acetone flames at high equivalence ratios and pressures. Calculations suggest that n-heptane flames burn faster than acetone and ethanol flames under elevated pressures. This indicates that n-heptane flames may ignite prior to a thorough mixing with hot combustion products. Furthermore, the jet velocity also decreases linearly with the increasing operating pressure inside the combustor. This is suspected to weaken the mixing of fresh reactants and exhaust gases, thus contributing to the unsuccessful establishment of MILD combustion. One criteria of MILD combustion, based on heat release profiles, is adopted to investigate the distinctive behaviour of n-heptane. This numerical investigation is focused on two unique features identified in flames in the MILD combustion regime: the mismatch between the location of the peak net heat release rate (Zhmax) and the location of stoichiometric mixture fraction (Zst); the absence of a net negative heat release region. For ethanol flames, Zhmax and Zst are uncorrelated under all the oxygen levels and strain rates investigated, while the absence of a net negative heat release region is dependent on the strain rate. These results indicate that the transition boundary between the conventional combustion regime and the MILD combustion regime cannot be determined by the oxygen level alone. For n-heptane flames, a net negative heat release region exists despite a low O2 level and a high strain rate. This is attributed to changes between alternative pyrolytic channels of n-heptane under different conditions due to its complex chemistry. The fundamental aspects revealed by this study shed more light on the MILD combustion of more complex fuels. An improved understanding on the role of fuel structure in the establishment of MILD combustion is achieved by this work. The findings of this study are relevant to the implementation of MILD combustion technology in a variety of combustion devices.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 201