Reinhard Seiser

Extinction and Autoignition of n-Heptane in Counterflow Configuration

A study is performed to elucidate the mechanisms of extinction and autoignition of n-heptane in strained laminar flows under nonpremixed conditions. A previously developed detailed mechanism made UP of 2540 reversible elementary reactions among 557 species is the starting point for the study. The detailed mechanism was previously used to calculate ignition delay times in homogeneous reactors, and concentration histories of a number of species in plug-flow and jet-stirred reactors. An intermediate mechanism made up of 1282 reversible elementary reactions among 282 species and a short mechanism made up of 770 reversible elementary reactions among 160 species are assembled from this detailed mechanism. Ignition delay times in an isochoric homogeneous reactor calculated using the intermediate and the short mechanism are found to agree well with those calculated using the detailed mechanism. The intermediate and the short mechanism are used to calculate extinction and autoignition of n-heptane in strained laminar flows. Steady laminar flow of two counter flowing Streams toward a stagnation plane is considered. One stream made up of prevaporized n-heptane and nitrogen is injected from the fuel boundary and the other stream made up of air and nitrogen is injected from the oxidizer boundary. Critical conditions of extinction and autoignition given by the strain rate, temperature and concentrations of the reactants at the boundaries, are calculated. The results are found to agree well with experiments. Sensitivity analysis is carried out to evaluate the influence of various elementary reactions on autoignition. At all values of the strain rate investigated here, high temperature chemical processes are found to control autoignition. In general, the influence of low temperature chemistry is found to increase with decreasing strain. A key finding of the present study is that strain has more influence on low temperature chemistry than the temperature of the reactants.

Temperature cross-over and non-thermal runaway at two-stage ignition of n-heptane

Abstract To calculate ignition delay times a skeletal 56-step mechanism for n-heptane is further reduced to a short 30-step mechanism containing two isomers of the n-heptyl-redical and reactions describing both the high temperature and the low temperature chemistry. This mechanism reproduces ignition delay times at various pressures and temperatures reasonably well. Steady state assumptions for many of the intermediate species are introduced to derive separately two global mechanisms for the low temperature regime as well as for the intermediate and high temperature regime. In those formulations the OH radical is depleted by fast reactions with the fuel, as long as fuel is present. Its steady state relation shows that the OH concentration would blow up as soon as the fuel is depleted. Therefore the depletion of the fuel is used as a suitable criterion for ignition. In the intermediate temperature regime the first stage ignition is related to a change from chain-branching to chain-breaking as the temperature crosses a certain threshold. The chain branching reactions result in a build-up of ketohydroperoxides which dissociate to produce OH radicals. This is associated with a slight temperature rise which leads to a crossing of the threshold temperature with the consequence that the production of OH radicals by ketohydroperoxides suddenly ceases. The subsequent second stage is driven by the much slower production of OH radicals owing to the dissociation of hydrogen peroxide. The OH radicals react with the fuel at nearly constant temperature until the latter is fully depleted. In all three regimes analytical solutions for the ignition delay time are presented. The reduced 4-step mechanism of the low temperature regime leads with the assumption of constant temperature to linear differential equations, which are solved. The calculated ignition delay times at fuel depletion compares well with those of the 30-step mechanism. The analysis for the intermediate temperature regime starts from a 4-step subset of a 9-step reduced mechanism. It contains the cross-over dynamics in form of a temperature dependent stoichiometric coefficient which is analysed mathematically. The resulting closed form solutions describe the first stage ignition, the temperature cross-over and the second stage ignition. They also identify the rate determining reactions and quantify the influence of their rates on the first and the second ignition times. The high temperature regime is governed by a three-step mechanism leading to a nonlinear problem which is solved by asymptotic analysis. While the dissociation reaction of the ketohydroperoxide dominates the low temperature regime and the first stage ignition of the intermediate temperature regime, the hydrogen peroxide dissociation takes this role for the second stage of the intermediate and in the high temperature regime. The overall activation energy of the ignition delay time in the low temperature regime is the mean of the activation energies of two reactions only. The overall activation energy of the ignition delay time in the high temperature regime is shown to be related to the activation energies of only three but different rate determining reactions.

Ignition in the viscous layer between counterflowing streams: asymptotic theory with comparison to experiments

A formulation is given for describing ignition in nonpremixed systems. Steady laminar flow of two counterflowing streams toward a stagnation plane is considered. One stream comprises fuel and the other oxygen. The characteristic Reynolds numbers of the counterflowing streams are presumed to be large so that the thickness of the viscous layer formed in the vicinity of the stagnation plane is small. The chemical reaction between fuel and oxygen that takes place in the viscous layer is described by a one-step overall process. The activation energy of the reaction is presumed to be large in comparison to the thermal energy. The asymptotic theory developed here makes available explicit formulae for predicting ignition in the viscous layer. From these results a simple but reasonably accurate method is developed for deducing the activation energy, E, and frequency factor, B, of the rate of the one-step reaction between the fuel and oxygen. To illustrate the application of this method, experiments are carried out in the counterflow configuration. The fuels tested are n-heptane, n-decane, JP-10, and toluene. Experimental data obtained are the velocities and temperatures of counterflowing streams at ignition. Values of E and B are obtained by using the experimental data in the formulae given by the asymptotic theory. These values of E and B are found to agree well with those obtained from numerical calculations.

Non-premixed and Premixed Extinction and Autoignition of C2H4, C2H6, C3H6, and C3H8

Experimental studies are conducted on laminar non-premixed and premixed flames stabilized in the counterflow configuration. The fuels tested are ethene (C 2 H 4 ), ethane (C 2 H 6 ), propene (C 3 H 6 ), and propane (C 3 H 8 ). Studies on non-premixed systems are carried out by injecting a fuel stream made up of fuel and nitrogen (N 2 ) from one duct and an oxidizer stream made up of air and N 2 from the other duct. Studies on premixed systems are carried out by injecting a premixed reactant stream made up of fuel, oxygen, and nitrogen from one duct and an inert-gas stream of N 2 from the other duct. Critical conditions of extinction are measured by increasing the flow rates of the counterflowing streams until the flame extinguishes. Critical conditions of autoignition are measured by preheating the oxidizer stream of the non-premixed system and the inert-gas stream of the premixed system. Experimental data for autoignition are obtained over a wide range of temperatures of the heated streams. In addition, for premixed systems, experimental data are obtained for a wide range of values of the equivalence ratio including fuel-lean and fuel-rich conditions. Numerical calculations are performed using a detailed chemical-kinetic mechanism and compared with measurements. The present study highlights the influences of non-uniform flow-feld on autoignition in premixed systems that were not available from previous studies using shock tubes. For the premixed system considered here, the changes in the strain rates at extinction with equivalence ratio are found to be similar to previous observations of changes in laminar burning velocities with equivalence ratio. The studies on autoignition in the premixed system show that the temperature of the inert-gas stream at autoignition reaches a minimum for a certain equivalence ratio. For premixed systems, abrupt extinction and autoignition are not observed if the value of the equivalence ratio is less than some critical value.

Experimental Investigation of Combustion of Jet Fuels and Surrogates in Nonpremixed Flows

Experimental studies are carried out to characterize nonpremixed combustion of jet fuels and a number of their surrogatesinlaminarnonuniform flows.Thecounterflowconfigurationisemployed.Criticalconditionsofextinction and autoignition are measured for jet propellant 8, Jet-A, and Fisher–Tropsch jet propellant 8. Thirteen surrogates of jetpropellant 8andone surrogate of Fisher–Tropsch jet propellant 8are tested. Itis foundthat criticalconditions of extinction and autoignition of jet propellant 8 and Jet-A are similar, while Fisher–Tropsch jet propellant 8 is morereactivethanjetpropellant8andJet-A.Amongthesurrogatestested,surrogateH[madeupofn-decane(80%) and 1, 3, 5-trimethylbenzene (20%) by liquid volume] and surrogate C [made up of n-dodecane (60%), methylcyclohexane (20%), and o-xylene (20%) by liquid volume] best reproduce extinction and autoignition characteristics of jet propellant 8. Surrogate G [made up ofn-decane (60%) and iso-octane (40%) by liquid volume] best reproduces the combustion characteristics of Fisher–Tropsch jet propellant 8.

Ignition and extinction of low molecular weight esters in nonpremixed flows

An experimental and kinetic modeling study is carried out to characterize combustion of low molecular weight esters in nonpremixed, nonuniform flows. An improved understanding of the combustion characteristics of low molecular weight esters will provide insights on combustion of high molecular weight esters and biodiesel. The fuels tested are methyl butanoate, methyl crotonate, ethyl propionate, biodiesel, and diesel. Two types of configuration – the condensed fuel configuration and the prevaporized fuel configuration – are employed. The condensed fuel configuration is particularly useful for studies on those liquid fuels that have high boiling points, for example biodiesel and diesel, where prevaporization, without thermal breakdown of the fuel, is difficult to achieve. In the condensed fuel configuration, an oxidizer, made up of a mixture of oxygen and nitrogen, flows over the vaporizing surface of a pool of liquid fuel. A stagnation-point boundary layer flow is established over the surface of the liquid pool. The flame is stabilized in the boundary layer. In the prevaporized fuel configuration, the flame is established in the mixing layer formed between two streams. One stream is a mixture of oxygen and nitrogen and the other is a mixture of prevaporized fuel and nitrogen. Critical conditions of extinction and ignition are measured. The results show that the critical conditions of extinction of diesel and biodiesel are nearly the same. Experimental data show that in general flames burning the esters are more difficult to extinguish in comparison to those for biodiesel. At the same value of a characteristic flow time, the ignition temperature for biodiesel is lower than that for diesel. The ignition temperatures for biodiesel are lower than those for the methyl esters tested here. Critical conditions of extinction and ignition for methyl butanoate were calculated using a detailed chemical kinetic mechanism. The results agreed well with the experimental data. The asymptotic structure of a methyl butanoate flame is found to be similar to that for many hydrocarbon flames. This will facilitate analytical modeling, of structures of ester flames, using rate-ratio asymptotic techniques, developed previously for hydrocarbon flames.

Experimental study of methane fuel oxycombustion in a spark-ignited engine

An experimental investigation of methane fuel oxycombustion in a variable compression ratio, spark-ignited piston engine has been carried out. Compression ratio, spark-timing, and oxygen concentration sweeps were performed to determine peak performance conditions for operation with both wet and dry exhaust gas recirculation (EGR). Results illustrate that when operating under oxycombustion conditions an optimum oxygen concentration exists at which fuel-conversion efficiency is maximized. Maximum conversion efficiency was achieved with approximately 29% oxygen by volume in the intake for wet EGR, and approximately 32.5% oxygen by volume in the intake for dry EGR. All test conditions, including air, were able to operate at the engines maximum compression ratio of 17 to 1 without significant knock limitations. Peak fuel-conversion efficiency under oxycombustion conditions was significantly reduced relative to methane-in-air operation, with wet EGR achieving 23.6%, dry EGR achieving 24.2% and methane-in-air achieving 31.4%. The reduced fuel-conversion efficiency of oxycombustion conditions relative to air was primarily due to the reduced ratio of specific heats of the EGR working fluids relative to nitrogen (air) working fluid.

Experimental study of methane fuel oxycombustion in an SI engine

An experimental investigation of the thermal efficiency, combustion efficiency, and CoV IMEP, of methane fuel oxycombustion in an SI engine has been carried out. Compression ratio, spark-timing, and oxygen concentration were all varied. A variable compression ratio SI engine was operated on both wet and dry EGR working fluids, with results illustrating that the efficiency of the engine operating with a large amount of EGR was significantly reduced relative to methane-in-air operation over all oxygen concentrations and compression ratios. The maximum thermal efficiency of wet EGR, dry EGR, and air was found to be 23.6%, 24.2%, and 31.4%, respectively, corresponding to oxygen volume fractions of 29.3%, 32.7% and 21%. Combustion efficiency was above 98% for wet EGR and approximately 96% for dry EGR. CoV IMEP was low for both cases. The much lower efficiency of both EGR cases relative to air is primarily a result of the reduced specific-heat ratio of the EGR working fluids relative to air working fluid.Copyright

Ambient-temperature co-digestion of low-solids municipal and industrial waste mixtures: Insights from molecular analyses

ABSTRACT The performance of ambient temperature anaerobic co-digestion was investigated for mixtures of six substrates: canned tomato and salsa waste, portable toilet waste, septic tank waste, winery waste, beer and cider waste, and fats, oils, and grease (FOG). Laboratory semi-continuous reactor studies and molecular biological analyses revealed that beer/winery, and tomato/FOG/winery/beer mixtures resulted in the best performance in terms of biogas production (515 and 371 mL CH4/g VS, respectively) and methanogenic populations. A portable toilet/septage mixture resulted in the overall poorest performance and inhibition of microbial activity was evident. Average methane content was ~70% for all mixtures tested. The findings of this study reveal that healthy methanogen populations were present, further supporting the feasibility of biogas production via the novel feedstock mixtures in ambient temperature lagoons. Implications: Disposal of septic tank waste and other high chemical oxygen demand (COD) 10 industrial food processing waste at a small wastewater treatment plant is uncommon, because it can upset the treatment process and requires additional power for treatment. Ambient-temperature covered lagoon digesters can be an alternative low-cost technology for co-digestion of these recalcitrant waste streams while generating bioenergy. The results of this study demonstrated that there is potential for implementation of unheated covered lagoon digester systems 15 for conversion of liquid wastes for production of renewable biomethane while eliminating the need to treat these wastes at a wastewater treatment plant.

Nonpremixed and Premixed Combustion of Mixtures of Producer Gas and Methane

Abstract Experimental and kinetic modeling studies are carried out to characterize nonpremixed and premixed combustion of producer gas and mixtures of producer gas and methane. The producer gas, employed in the study, is made up of 55.00% carbon monoxide, 2.34% hydrogen, 12.72% methane, 5.24% ethene, and 24.7% carbon dioxide by mass. The primary focus is on characterizing the chemical influence of addition of producer gas on combustion of methane. The kinetic modeling studies are carried out employing a detailed chemical-kinetic mechanism, called the San Diego Mechanism, a skeletal mechanism, and a reduced mechanism made up of five global steps. Experiments on nonpremixed combustion are carried out employing the counterflow configuration. Critical conditions of extinction are measured for producer gas, methane, and mixtures of producer gas and methane. They are compared with predictions obtained using the detailed, skeletal, and reduced mechanism. Critical conditions of autoignition are measured for producer gas and compared with the predictions obtained employing the detailed mechanism. Experimental data and predictions show that with increasing amounts of producer gas in the mixture, the flame is more difficult to extinguish. Flame structures show that at a fixed value of the strain rate, leakage of oxygen from the reaction zone decreases with increasing amounts of producer gas in the combustible mixture. This is attributed to enhanced consumption of oxygen in an overall chain branching step that consumes hydrogen. Thus, the increase in the overall reactivity of the combustible mixture is attributed to presence of hydrogen in producer gas. Computations are performed, using the detailed mechanism and the skeletal mechanism, to investigate aspects of premixed combustion of stoichiometric mixtures of producer gas, methane, oxygen, and nitrogen at fixed values of adiabatic temperature. Burning velocities are calculated. They are found to be less sensitive to the amount of producer gas in the mixture. This is qualitatively different from that observed for nonpremixed combustion. It is attributed to complete consumption of all reactants including oxygen and combined influences of H2 and CO in producer gas on overall combustion of methane. A third set of computations was performed on strained premixed flames stabilized in counterflow between a stream of a stoichiometric mixture of producer gas, methane, oxygen, and nitrogen, and a stream of nitrogen at fixed adiabatic temperature. Critical conditions of extinction were obtained. The strain rate at extinction increased with increasing amounts of producer gas. The increase was comparable to that observed for nonpremixed combustion of these fuels.