Propagating laminar flame characteristics for single and two phase alternative fuel mixtures

This thesis investigates enhanced methods for analysing non-linear effects in propagating laminar flames, enabling more accurate evaluation of laminar flame characteristics such as Markstein length and unstretched flame speed whilst proposing a new method for evaluating extinction stretch rate. Furthermore, a new cloud-combustor is developed and commissioned enabling laminar flame characteristics through droplet fuel mists to be explored again utilising advanced non-linear analysis. Re-analysis of previous low-ignition energy methane-water flames reveals the analytical non-linear characteristic. The analysis also demonstrates the need for a larger chamber to avoid pressurised effects during the latter stages of propagation, potentially reducing the accuracy of the adopted methodology. Non-linear analysis shows interesting trends concerning Markstein length at higher water loading in particular when it increase to 15% (by volume), and laminar burning rate decreased. The non-linear analysis technique is deployed to analyse four hydrocarbon fuels, two traditional paraffinic fuels in methane and propane, and two alternative alcohol fuels namely ethanol and methanol. It is shown that overdriven flame data can be used to predict flame extinction stretch rate, as long as a sufficient time period is disregarded to allow the effects of the early ignition-affected period to subside. The new technique proposed for evaluated critical extinction stretch rate shows good agreement with the traditional counter-flowing flame technique. Results for the four fuels reveal a common profile for extinction stretch-rate as a function of equivalence ratio, which was anticipated due to the similar fundamental combustion characteristics of the chosen fuels. Based on the non-linear analysis, it is shown analytically that this common profile may be represented by a combination of the iv unstretched laminar burning velocity, the Markstein length and the expansion ratio of the fuel. Ethanol in air is used to benchmark Cardiff University’s new, large 35Litre ‘Cloud Combustor’ for an investigation of flame propagation through fuel mists across a wide range of equivalence ratios. Non-intrusive, in-situ droplet sizing with concurrent flame propagation is achieved for the first time. The fuel mist flame data was subsequently compared to that for pure vapour mixtures at nominally identical ambient conditions in order to study the reported enhancement in flame speed exhibited in previous studies, and to compare qualitatively against conflicting published views reported in literature. It was found that with the onset of instabilities at certain droplet size an enhancement in flame speed could be shown for rich mist flames compared to those of analogous vapour flames. Based on mechanisms detailed elsewhere that provide a possible explanation for this enhancement full discussion and correlations that help to understand the nature of flame speed through droplet mists are presente

Flashback Avoidance in Swirling Flow Burners

AbstractLean premixed combustion using swirling flows is widely used in gas turbines and combustion. Although flashback is not generally a problem with natural gas combustion, there are some reports of flashback damage with existing gas turbines, whilst hydrogen enriched fuel blends cause concerns in this area. Thus, this paper describes a practical approach to study and avoid flashback in a pilot scale 100kW tangential swirl burner. The flashback phenomenon is studied experimentally via the derivation of flashback limits for a variety of different geometrical conditions. A high speed camera is used to visualize the process and distinguish new patterns of avoidance. The use of a central fuel injector is shown to give substantial benefits in terms of flashback resistance. Conclusions are drawn as to mitigation technologies

Characterization of critical stretch rate using outwardly propagating spherical flames

Outwardly propagating spherical flames within a constant volume combustion chamber (CVCC) are studied to analyse the non-linear relationship between flame stretch and flame speed, enabling a critical appraisal of a methodology proposed for characterizing the critical stretch rate. Four fuels, namely methane, propane, methanol and ethanol in air, are chosen to investigate the correlation between maximum critical stretch rate and the flame extinction across a range of equivalence ratios at various ambient conditions in under-driven flames, and to compare the hypothesis against data from the traditional counter-flowing flame technique. Flame propagation is recorded via high-speed Schlieren photography, and low ignition energies are achieved via a variable capacitive-discharge supply, enabling the critical early stages of flame propagation, critical stretch rate and the sensitivity of the non-linear methodology to ignition energy to be systematically analysed. The non-linear methodology shows partial agreement with extinction stretch rate from counter flowing flames, particularly in the case of gaseous fuels. Although the fuel vapour data lies between previous extinction stretch rate measurements using the counterflowing flame methodology, and predictions from chemical kinetic schemes, a 40% deviation is observed. A mathematical expression was produced to determine the critical stretch at the specific conditions of the present work. Key words: laminar burning velocity, liquid fuels, gaseous fuels, premixed flames, critical stretch rate, extinction stretch rat

Flashback avoidance in swirling flow burners

Lean premixed combustion using swirling flows is widely used in gas tur - bines and combustion. Although flashback is not generally a problem with natural gas combustion, there are some reports of flashback damage with existing gas turbines, whilst hydrogen enriched fuel blends cause concerns in this area. Thus, this paper describes a practical approach to study and avoid flashback in a pilot scale 100 kW tangential swirl burner. The flash - back phenomenon is studied experimentally via the derivation of flashback limits for a variety of different geometrical conditions. A high speed camera is used to visualize the process and distinguish new patterns of avoidance. The use of a central fuel injector is shown to give substantial benefits in terms of flashback resistance. Conclusions are drawn as to mitigation technologi

Estudio de la producción de gas de síntesis en un gasificador que opera con residuos sólidos

Estudio de la producción de gas de síntesis en un gasificador que opera con residuos sólido

Influence of ambient conditions on laminar burning velocity, ignition and flame extinction for ethanol air mixtures

Experimental studies of laminar ethanol - air gaseous flames have been undertaken in a large (34 l) cylindrical constant volume combustion bomb to investigate combustion fundamentals at varying ambient conditions. This vessel has been designed to minimise the influence of boundary walls, hence extending the quasi steady pressure region over which meaningful data may be obtained. Gaseous homogeneous mixtures are achieved by injecting liquid ethanol into the bomb which pre-vaporises prior to ignition. Initial pressure and equivalence ratio are predetermined using partial pressure methodology. Flame propagation is recorded utilising high-speed Schlieren photography, and low ignition energies were achieved via a variable discharge system enabling the sensitive early stages of flame propagation and extinction limits to be studied. Data is presented in terms of flame speed against stretch rate from which Markstein lengths and laminar burning velocities are derived for a variety of different initial conditions. The effect of ignition energy, initial pressure (from sub-atmospheric to elevated pressure) along with the effect of increasing initial temperature is studied. Results are discussed in terms of those of previous workers, and compared with predictions from detailed chemical kinetic schemes. Nonlinear trends witnessed during early stage flame propagation are further investigated as a suitable method for deriving extinction stretch rate