Dimitrios C. Kyritsis

Comparative second-law analysis of internal combustion engine operation for methane, methanol, and dodecane fuels

A method for both combustion irreversibility and working medium availability computations in a high-speed, naturally-aspirated, four-stroke, internal combustion engine cylinder is presented. The results of the second-law analysis of engine operation with n-dodecane (n-C12H26) fuel are compared with the results of a similar analysis for cases where a light, gaseous (CH4) and an oxygenated (CH3OH) fuel is used. The rate of entropy production during combustion is analytically calculated as a function of the fuel reaction rate with the combined use of first- and second-law arguments and a chemical equilibrium hypothesis. It is shown theoretically that the decomposition of lighter molecules leads to less entropy generation compared to heavier fuels. This is verified computationally for the particular fuels and the corresponding decrease in combustion irreversibility is calculated. Special reference is made to the effect of the lower mixing entropy of the exhaust gas of an oxygenated fuel (CH3OH) as a contribution to the discussion of the advantages and disadvantages of the use of such fuels.

Mesoscale power generation by a catalytic combustor using electrosprayed liquid hydrocarbons

The development of a mesoscale catalytic combustor to be coupled with direct energy conversion modules for power production is presented. The combustor has a volume on the order of a few cubic centimeters and operates on JP8 jet fuel, which is electrosprayed at a flow rate on the order of 10 g/hr and equivalence ratios varying from 0.35 to 0.70. Temperatures in the range 900–1300 K are achieved with a � 5% uniformity over the top circular surface of the burner. Using gas chromatographic analysis of the exhaust gases, a combustion efficiency on the order of 97% is estimated. Remarkably, no fouling, nor soot, nor NOx were detected in the exhaust gases. The system resulted in clean and efficient combustion of even environmentally problematic liquid hydrocarbons.

The combustion of n-butanol/diesel fuel blends and its cyclic variability in a direct injection diesel engine

An experimental study is conducted to evaluate the effects of using blends of diesel fuel with n-butanol (normal butanol) up to 24 per cent (by volume), which is a promising fuel that can be produced from biomass (bio-butanol), on the combustion behaviour of a standard, high-speed, direct injection (DI), ‘Hydra’ diesel engine located at the authors’ laboratory. Combustion chamber and fuel injection pressure diagrams are obtained at four different loads using a developed, high-speed, data acquisition, and processing system. A heat release analysis of the experimentally obtained cylinder pressure diagrams is developed and used. Plots of histories in the combustion chamber of the gross heat release rate and other related parameters reveal some interesting features, which shed light on the combustion mechanism when using these blends. These results, combined with the differing physical and chemical properties of the n-butanol against those for the diesel fuel, aid the correct interpretation of the observed engine behaviour performance based on and emissions. Moreover, given the concern for the rather low cetane number of the n-butanol that may promote cyclic (combustion) variability, its strength is also examined as reflected in the pressure indicator diagrams, by analysing for the maximum pressure and its rate, dynamic injection timing and ignition delay, by using stochastic analysis for averages, standard deviations, probability density functions, autocorrelation, power spectra, and cross-correlation coefficients.

METHANE FLAME PROPAGATION IN COMPOSITIONALLY STRATIFIED GASES

ABSTRACT Flame propagation in compositionally stratified methane-air mixtures was studied experimentally as a function of the equivalence ratio distribution in the unburnt mixture. Stratification was established in a controlled manner using a convective-diffusive balance in a very slow fuel-air mixture flow in an optically accessible test chamber. The flame speed was shown to be significantly higher than the one corresponding to a homogeneous mixture of the local equivalence ratio for mixture compositions close to the lean flammability limit. Also a significant extension of the lean flammability limit was observed. It was established that the local spatial gradient of the equivalence ratio was not sufficient to describe the departure of stratified combustion from quasi-homogeneity. Instead, an appropriately defined integral parameter that depended on the history of flame propagation was shown to determine when the flame could not be treated as a series of premixed flamelets propagating at the local adiabatic flame speed.

VORTEX-INDUCED EXTINCTION BEHAVIOR IN METHANOL GASEOUS FLAMES: A COMPARISON WITH QUASI-STEADY EXTINCTION

Using a combination of HCHO planar laser-induced fluorescence and laser Doppler velocimetry measurements, the extinction behavior of methanol counterflow diffusion flames was examined experimentally under conditions in which the extinction was brought about by a vortex generated on the oxidizer side. Comparisons were made with quasi-steady extinction results for the same flames. It was found that the flames can withstand instantaneous strain rates as much as two-and-a-half times larger than the quasisteady ones. The finding was rationalized phenomenologically by comparing the characteristic times of the problem, that is, the mechanical time, the chemical time, and the vortex turnover time. Specifically, estimates of these times yielded the following ordering: sch svort sm. As a result, the vortex introduced an unsteady effect in the outer diffusive-convective layer of the flame, while the inner reactive-diffusive layer behaved in a quasi-steady manner. Consequently, the flame was subject to a damped strain rate through the outer layer. Results from a simple analytical model showed that the difference between vortexinduced extinction and quasi-steady extinction was much more modest in terms of instantaneous scalar

An experimental study of vortex-flame interaction in counterflow spray diffusion flames

The extinction behavior of methanol counterflow spray diffusion flames was investigated using a combination of formaldehyde planer laser-induced fluorescence (PLIF) and phase Doppler measurements. Extinction was brought about quasi-steadily, by progressively increasing the flow rates of both oxidizer and fuel side, and unsteadily, by generating a vortex on the oxidizer side. The unsteady experiments yielded values of extinction strain rates a factor of 2 larger than the quasi-steady values. The greater robustness of the spray flame under unsteady perturbation, was explained phenomenologically by estimating the timescales involved in the process. It was found that the vortex introduces unsteady effects in the outer diffusiveconvective layer of the flame. The inner reactive-diffusive layer, on the other hand, behaves in a quasisteady manner, since the characteristic chemical time is much smaller than the characteristic unsteady time. As a result, even though the instantaneous strain rate is much larger than the quasi-steady extinction strain rate, the flame is subject to a damped strain rate through the outer layer. An estimate of the thickness of the mixing layer, based on formaldehyde PLIF, provided a convenient means to compare the scalar dissipation rate and the Damkohler number between the two extinction modes, bypassing the need for detailed species measurements for the assessment of the mixture fraction and its gradient. Such a comparison showed that the difference between the two extinction modes was reduced to 25% on the average, consistent with expectations based on flame structure models from asymptotic theory. Spray flames exhibited longer time delays between the onset of extinction and reignition, as compared to gaseous flames. Estimates of the relevant Stokes number suggested that the difference may be attributed to droplet inertia effects.

Comparative Evaluation of Extinction through Strain among Three Alcoholic Butanol Isomers in Non-Premixed Counterflow Flames

AbstractButanol isomer diffusion flames were studied experimentally in a counterflow burner configuration with an emphasis on establishing a difference in extinction strain rate among three fuels with practically identical thermochemistry. An effect of molecular structure on the extinction strain rates of butanol isomers was observed and analyzed in terms of bond dissociation energies. The results indicate that although all isomers share essentially the same adiabatic flame temperature, n-butanol flames can sustain a consistently higher extinction strain rate than the flames of other isomers (isobutanol and sec-butanol). Extinction strain rates of isobutanol and sec-butanol were equal, within experimental error. Numerical simulation of n-butanol diffusion flames produced results consistent with those measured experimentally and provided insight into the distribution of major species and combustion intermediates. Stable annular flames were observed for all three isomers as a temporary step before extinction.

A methodology for velocity field measurement in multiphase high‐pressure flow of CO2 and water in micromodels

This paper presents a novel methodology for capturing instantaneous, temporally and spatially resolved velocity fields in an immiscible multiphase flow of liquid/supercritical CO2 and water through a porous micromodel. Of interest is quantifying pore-scale flow processes relevant to geological CO2 sequestration and enhanced oil recovery, and in particular, at thermodynamic conditions relevant to geological reservoirs. A previously developed two-color microscopic particle image velocimetry approach is combined with a high-pressure apparatus, facilitating flow quantification of water interacting with supercritical CO2. This technique simultaneously resolves (in space and time) the aqueous phase velocity field as well as the dynamics of the menisci. The method and the experimental apparatus are detailed, and the results are presented to demonstrate its unique capabilities for studying pore-scale dynamics of CO2-water interactions. Simultaneous identification of the boundary between the two fluid phases and quantification of the instantaneous velocity field in the aqueous phase provides a step change in capability for investigating multiphase flow physics at the pore scale at reservoir-relevant conditions.

Quantitative scalar dissipation rate measurements in vortex-perturbed counterflow diffusion flames

Even though the scalar dissipation rate at the stoichiometric surface, χ stoich , is recognized to be the most fundamental fluid time scale in laminar diffusion flames, their structure and extinction behavior are often characterized simply in terms of strain rate, a much more easily measurable observable. Yet, the two variables are different, especially in unsteady flamelets. An experimental technique based on line Raman imaging of major species is presented for the quantitative measurement of χ stoich in vortex-perturbed counterflow diffusion flames. Three formulations are evaluated, and it is shown that a formulation based on N 2 -mass fraction is the most appropriate, provided that N 2 is experimentally accessible and that there is no significant preferential diffusion. The technique is used to compare vortex-perturbed and quasi-steady extinction. The thesis that for a given composition of the counterflowing streams, extinction occurs at a given value of χ stoich , irrespective of the mode of perturbation, steady or unsteady, is verified experimentally and is contrasted with the observation that vortex-perturbed flames can sustain an almost double strain rate at extinction compared to steadily strained ones. The effect of two-dimensional phenomena on the results is discussed. Finally, a promising approximation of χ stoich using estimates of the thickness of mixing layer from temperature profiles, with significant simplifications in the required measurements, is investigated.

NITRIC OXIDE FORMATION DURING FLAME/VORTEX INTERACTION

The formation of nitric oxide was investigated in methane diffusion flames under the perturbation of laminar vortices. Laser-induced fluorescence and Raman spectroscopy were used to measure nitric oxide, major species, and temperature, and to obtain the time evolution of the scalar dissipation rate at the stoichiometric surface during the flame/vortex interaction. Vortices with two characteristic times were thrust into the flame and their effects were compared with steady flames at the same scalar dissipation rate. The experimental measurements showed that the vortex did not affect significantly the peak temperature, CO, and CO2 mass fraction during the interaction. NO, instead, was strongly affected. The peak mass fraction of NO in flames under vortex perturbation differed by almost a factor of 2 from steady flames with similar scalar dissipation rates. One-dimensional numerical simulations, used under the well-justified assumption that the vortices induced negligible curvature, yielded results in good agreement with the experimental measurements. The numerical computations were also used to investigate the effect of the characteristic time of the vortex on the different NO formation paths, that is, thermal, and prompt. The peak mass fraction of thermal NO was shown to be strongly dependent on the timescale of the unsteady perturbation, while prompt NO was almost independent. The emission index, that is, the production of NO normalized by the consumption of the fuel, behaved quasi-steadily for the two formation paths. The results were rationalized by considering the flame structure and the activation energy of the different formation paths.