Iskender Gökalp

Characterization of flame front surfaces in turbulent premixed methane/air combustion

Abstract A detailed experimental investigation of the application of fractal geometry concepts in determining the turbulent burning velocity in the wrinkled flame regime of turbulent premixed combustion was conducted. The fractal dimension and cutoff scales were determined for six different turbulent flames in the wrinkled flame regime, where the turbulence intensity, turbulent length scale, and equivalence ratio were varied. Unlike previous reports, it has proved possible to obtain the fractal dimension and inner and outer cutoffs from individual flame images. From this individual data, the pdf distributions of all three fractal parameters, along with the distribution of th predicted increase in surface area, may be determined. The analysis of over 300 flame images for each flame condition provided a sufficient sample size to accurately define the pdf distributions and their means. However, the predicted S T S L , calculated using fractal parameters, was significantly below the measured values. For conical flames, a geometrical modification factor was employed to predict S T S L , however, this did little to improve the predictions. There appeared to be no dependence of the predicted S T S L on the approach flow turbulence. The cutoffs did not seem to vary significantly with any of the length scales in the approach flow turbulence, although the fractal dimension did appear to have a weak dependence on u′ S L and ReΛ. The probable reasons that fractal geometry does not correctly predict S T S L are that S T S L = A w A 0 does not hold in wrinkled turbulent premixed flames, that the flame front surface cannot be described by a single scaling exponent, or that these are not wrinkled flames even though they are within conventional definitions of the wrinkled flame regime.

Studies on the Ignition and Burning of Levitated Aluminum Particles

Abstract An experimental set-up is developed to investigate the burning of levitated aluminum particles ignited by a C02laser in air under high pressure conditions. Residence times of aluminum particles burning in the electrodynamic levitator are long enough to observe the total burning process, under normal and high pressures. Experiments allow to measure the ignition delay and the burning times of aluminum particles. These measurements and the accompanying high speed images give useful information on the burning processes of aluminum particles under various regimes. A numerical model is also developed to predict the burning rates of aluminum particles and the sizes of the alumina residues.

Numerical Study of the Continuous Detonation Wave Rocket Engine

This paper presents numerical results on the operation of the combustion chamber of a Continuous Detonation Wave Rocket Engine (CDWRE). The propagation of a transversal detonation wave in a layer of continuously injected gaseous mixture H2-O2 has been simulated by a reactive high-resolution Euler code in a 2D approach with periodicity boundary conditions. A parametric study has been conducted to investigate the effect of the injection parameters (injector relative area and injection pressure), combustion chamber length, and spacing between successive detonation fronts on the integral characteristics of the combustion chamber operation. It is found that the detonation wave stably propagates at the Chapman-Jouguet velocity. The spatial period between successive detonations is a geometric scaling factor whereas the injection pressure is a scaling factor for the injection mass flux and the wall pressure. The injection mass flux is the main quantity characterizing the detonation propagation conditions.

Droplet vaporisation characteristics of vegetable oil derived biofuels at high temperatures

To characterise the mechanisms occurring in the deposit formation during the combustion of vegetable oils used as biofuels in Diesel engines, it is necessary to investigate the vaporisation of vegetable oil droplets under various flow, pressure and temperature conditions. In the current work, experimental results about the vaporisation of rapeseed and sunflower oil methyl ester droplets are presented. The fibre-suspended droplet technique is used and the time evolution of droplet diameter during vaporisation is observed by imaging technique. Average and instantaneous vaporisation rates have been estimated from the d 2 (t) curves at temperatures between 473-1020 K and at atmospheric pressure. The droplets of vegetable oil methyl esters evaporate like mono component droplets with a very significant heating phase. A comparison with experimental results obtained with n-alkanes droplets (from n-pentane to n-decane) and prediction of the quasi-steady theory is also presented and discussed.

Variable density effects in axisymmetric isothermal turbulent jets: a comparison between a first- and a second-order turbulence model

Abstract The standard κ-e model and a second-order Reynolds stress model (RSM) are used to investigate variable density effects in axisymmetric turbulent jets. Without buoyancy, both models predict no effect of the varying density on the far field turbulence parameters and the classical effective diameter concept works for the decay rates. No conclusive disagreement with experimental data is observed. Effects of turbulence production due to buoyancy are found to be small compared to the effect of the mean buoyancy term in the momentum equation. However, this turbulence production has a large influence on the axial scalar flux, for which the experimental trend is predicted with the second-order model but not at all with the k-e model.

Ignition and combustion of levitated magnesium and aluminum particles in carbon dioxide

Abstract This article considers ignition and combustion of single particles of magnesium and aluminum in carbon dioxide at pressures 0.1-2 MPa. An experimental setup with an electrodynamic levitator inside a high-pressure chamber was employed. The CO2-laser was used for heating to ignition of the particles. The results show that ignition mechanisms of Mg and Al in CO2 are different. Experiments with Mg indicate the existence of the critical partial pressure of CO2, whereas the ignition probability of Al particles in CO2 is low but independent on pressure. Analysis of flame images and combustion parameters shows that the mechanism of Mg particle burning in CO2 corresponds to conventional models of vapor-phase diffusion-controlled combustion, whereas in the case of Al exothermic processes on the particle surface or close to it play a leading part in the burning process.

Mass transfer from liquid fuel droplets in turbulent flow

Abstract An experimental apparatus and strategy have been developed to investigate the influence of turbulence on the global mass transfer rates from fuel droplets. Heptane and decane droplets suspended in grid-induced turbulent flows have been investigated in the regime where the integral length scales of turbulence are, on average, 5 times larger than the initial droplet diameter. The turbulence intensity has been increased up to 44%. A new mass transfer parameter that distinguishes between the influences of the mean relative velocity and those of turbulence structure has been introduced. Mass transfer from heptane droplets has been found to be insensitive to turbulence. The same turbulence conditions, however, exert a significant influence on the mass transfer from decane droplets. It is shown that the influence of turbulence on the mass transfer from decane droplets can be correlated by a turbulence Reynolds number. The Frossling coefficient is found to increase with the turbulence intensity. The differences in the sensitivity of heptane and decane droplets to the turbulence influence on mass transfer are tentatively explained by introducing a “vaporization Damkohler number.” The present experimental results suggest that turbulence enhances the mass transfer from liquid droplets only for low values of this number.

Turbulence Effects on the Vaporization of Monocomponent Single Droplets

Abstract An experimental facility has been developed to study the effects of turbulence on droplet vaporization. The facility allows lo generate a zero mean velocity, isotropic and homogeneous turbulence and to vary systematically the turbulence kinetic energy. The influence of turbulence on suspended single droplets of five n-alkane hydrocarbons is investigated, by determining the average vaporization rates by image analysis techniques. The experiments have been conducted under normal pressure and temperature conditions; for all cases, the length scales of energetic turbulence eddies are larger than the initial droplet diameter. For all the investigated cases, it is found that the presence of turbulent velocity fluctuations increases the average vaporization rates compared to the stagnant case. The linear regression rate of the projected droplet surface area versus time is observed under all turbulence conditions. It is observed that droplets of the five investigated fuels respond differently to the same...

High-pressure droplet burning experiments in microgravity

High-pressure droplet burning characteristics of five fuels are investigated under normal and reduced gravity conditions. The reduced gravity experiments have been conducted by using the parabolic flights of the CNES Caravelle and the NASA KC-135 aircrafts. A fully automatic high-pressure droplet gasification facility has been developed for these experiments. Rapid videography is used to determine the time histories of burning droplets, from which average droplet burning rates are determined. For all experiments, the suspended droplet technique is used. Initial droplet diameters are about 1.5 mm. Subcritical and supercritical droplet burning regimes are explored. Droplet time histories are only determined for weakly sooting fuels, such as methanol. These results show that the D 2 law holds even under very high pressures and allows the estimation of an average droplet burning rate. For the heavily sooting n -alkane droplets, this result is used to deduce the average burning rate by measuring the droplet burning life-time. The experimental results for all fuels show that the droplet burning lifetime decrease strongly with increasing pressure in the subcritical regime. When the pressure is increased above the critical pressure of the pure liquid, the droplet burning life-time remain constant on the average. Therefore, the minimum burning time observed in the literature is not confirmed here. To take into account the residual gravity-enhanced natural convection effects, a correction based on the Grashof number is applied. This correction allows a comparison of various experiments under different gravity and pressure conditions and an estimate of the burning time without any convection effect. In the subcritical regime, this burning time varies as p −0.37 .

Flamelet-based modeling of no formation in turbulent hydrogen jet diffusion flames

Abstract The potential of using the laminar flamelet model for predictions of NOx emissions from turbulent hydrogen jet diffusion flames with various amounts of helium dilution is explored in this study. The flamelet approach treats turbulent flames as an ensemble of laminar flames subject to local fluid dynamic stretch which causes chemical kinetics to deviate from equilibrium. This flame stretch rate can be described as the scalar dissipation rate or as the strain rate. Numerical modeling is performed to assess the merits of these two choices by comparing predicted scaling behavior of NOx emission index with the experimental data of Driscoll and co-workers [1, 2]. The present study reveals that only with the scalar dissipation rate as the nonequilibrium parameter will the predicted NOx emission indices exhibit a clear scaling relation vs the Damkohler number. The flamelet predictions are found to improve when the Damkohler number increases. However, the absolute NOx levels are overpredicted, which is attributed to differential diffusion effects.