Papers by Iskender Gokalp
International Journal of Heat and Mass Transfer, 2002
This paper presents a new correlation for mass transfer from single liquid droplets into a turbul... more This paper presents a new correlation for mass transfer from single liquid droplets into a turbulent environment. Experiments were carried out under ambient room temperature and pressure. Homogeneous isotropic turbulence with zero-mean velocity was generated by eight identical electrical fans placed on the eight corners of a cubic chamber. The LDV technique was used to characterize the turbulence inside the chamber. The vaporization of fiber suspended droplets of five different n-alkanes and the bi-component droplet of n-heptane and n-decane mixtures subjected to varying turbulent kinetic energy is investigated by imaging techniques. For mono-component droplets the d2-law holds for all fuels and turbulent kinetic energies, and the vaporization rates increase with increasing the turbulent kinetic energy. Bi-component droplets exhibit a sequential vaporization behavior for all mixtures and turbulent kinetic energies. The instantaneous vaporization rates increase with increasing turbulence kinetic energy and increasing volume fraction of the highest volatility component. The proposed correlation predicts the vaporization rates of mono and bi-component n-alkane droplets subjected to isotropic turbulence with zero-mean velocity.
Energy & Fuels, 2008
This paper deals with the gas-phase thermodynamic properties of organic compounds formed during b... more This paper deals with the gas-phase thermodynamic properties of organic compounds formed during biomass thermochemical energetic conversion. The standard enthalpies of formation at 298.15 K are determined by means of quantum chemistry calculations along with a protocol developed for general organic compounds. The resultant data, currently not available in the literature for most of these compounds, are critical to the modeling of pyrolysis, gasification, and combustion chemistry of biomass.
Journal of Supercritical Fluids, 2010
Nowadays, numerous projects are focussed on finding non-polluting energy sources. Interest in the... more Nowadays, numerous projects are focussed on finding non-polluting energy sources. Interest in the use of biomass for energy purposes has been also revived in this context. Among others, agricultural biomass waste is one of the most promising resources. Its conversion into biofuel for further application via combustion increases its economic interest despite the initial low calorific value. In the present study, the conversion process is carried out in supercritical water (SCW), thus improving significantly the energy balance by eliminating the need for drying the biomass. The SCW dissolution, hydrolysis and oxidation of pure substances in a hydrothermal diamond anvil cell (HDAC) have been studied previously and unique results concerning the homogeneous dissolution of naphthalene, cellulose and glucose were acquired. The current investigation was performed for samples representing real biomass (winery residues) rather than pure substances. Visualizations of the SCW biomass gasification process were obtained and the products of the decomposition were also analyzed. It is expected that this work will contribute in valorizing winery waste by decreasing the cost required for its elimination and will also contribute significantly to green house gases mitigation.
Energy & Fuels, 2009
ABSTRACT Premixed turbulent methane/air flames with conical (or Bunsen) configuration are compute... more ABSTRACT Premixed turbulent methane/air flames with conical (or Bunsen) configuration are computed in two stages for selected equivalence ratios. First, the turbulent cold-flow field inside the combustion chamber is modeled, and turbulence characteristics are computed using the k−ε turbulence model and its variants. Second, the flame-front properties are investigated by two different turbulent premixed combustion models, namely, the Zimont model and the coherent flame model. All computations are performed with the Fluent software. The computations are confronted to the experimental data corresponding to turbulent flames in the corrugated flamelet regime. Experimental results concern turbulent premixed methane/air flames stabilized on a Bunsen-type burner; they are obtained mainly by LDA for the cold- and hot-flow velocity statistics and by laser-induced Mie and Rayleigh scattering techniques for flame-front statistics. The computations are in agreement with the experimental data; in particular, the decrease of both the flame height and the flame brush thickness with the increase of the equivalence ratio is well-reproduced by the computations.
The objective of the work reported herein is to demonstrate the ability of the PIV technique to p... more The objective of the work reported herein is to demonstrate the ability of the PIV technique to provide correct measurements of the velocity field in steady underexpanded hot free jets. A hot-gas generator capable to create jets with an initial diameter of 25 mm, total pressure up to 4 MPa, and total temperature up to 2200 K is presented together with the associated technique of stagnation conditions determination. The implementation of the PIV method is described paying attention to the choice of the PIV system components and to the seeding technique. Experimental results are presented on the observed jet structure and directly measured geometry of the Mach disk. The results on the velocity field are analyzed considering the effect of the kind of seeding particles as well as the jet stagnation conditions. Results from the experiment and numerical simulation are compared to validate the measurements of the velocity field.
The characterization of near nozzle dense sprays and axisymmetric gas jets using X-ray flash tech... more The characterization of near nozzle dense sprays and axisymmetric gas jets using X-ray flash techniques is presented. Flash radiography and X-ray induced fluorescence imaging (X.I.F. imaging), using a flash X-ray developed at GREMI, offer two complementary diagnostics particularly efficient in high pressure conditions. In this work, a compact flash X-ray device is used to freeze fluid motions. Single shot radiographs of argon jets and water sprays expanding in ambient air have been performed. Radial density profiles were measured and quantitative density measurements have been extracted for argon, nitrogen-argon mixture and water jets, using flash X-ray radiography. UV fluorescence emissions due to X-ray excitation of the jet species were imaged on a gated intensified CCD camera.
Combustion Science and Technology, 2006
The goal of this paper is to investigate the predictive capability of two turbulence models which... more The goal of this paper is to investigate the predictive capability of two turbulence models which are the k-ε model and the Reynolds Stress Model (RSM) within flamelet approach. A co-flow axisymetric turbulent non-premixed hydrogen flame investigated experimentally by and is used as a test case. The chemical mechanism of Yetter's and al. (1991) is adopted for the generation of the flamelet library. It consists of 10 chemical species and 21 reactions. The comparisons with experimental data demonstrate that predictions based on the Reynolds stress turbulence model are slightly superior to those obtained using the k-ε model. Overall, profile predictions of axial velocity, turbulent kinetic energy, mixture fraction, flame temperature and major species are in reasonable agreement with data and compare favourably with the results of earlier investigations.
There is a general consensus that hydrogen may become 'the fuel of the future'. One of the issues... more There is a general consensus that hydrogen may become 'the fuel of the future'. One of the issues concerning the hydrogen is its production well-controlled and economically feasible processes. The hydrogen production from biomass is one of the most promising routes because of its CO 2 neutral character. This investigation concerns the optimisation of biomass gasification in supercritical water. It is carried out for samples representing real biomass (winery residues). The effluent (one kind of winery residue) is placed in a Hydrothermal Diamond Anvil Cell (HDAC) to be decomposed into hydrogen and other gases. Promising visualisations of the sample gasification sequences have been observed under supercritical conditions.
Symposium (international) on Combustion, 1998
A comparison between the dynamic and scalar timescales in turbulent premixed flames is presented.... more A comparison between the dynamic and scalar timescales in turbulent premixed flames is presented. Methane-air turbulent Bunsen-type flames are used in this study. They are all lean flames and basically in the flamelet regime. Laser Doppler velocimetry (LDV), Mie scattering and Rayleigh scattering techniques allowed the determination of dynamic (both conditional and nonconditional) and scalar (density) timescales. For both dynamic and scalar temporal scales, we used the autocorrelation function of the LDV signal and the Rayleigh scattering signal, respectively. The main results of the paper indicate that there are significant differences between the conditional (in the reactants) dynamic timescales and those evaluated from nonconditional velocity measurements. The nonconditional velocity timescales are much closer to the scalar timescales, which indicates that they are strongly influenced by the flame front dynamics. Therefore, to evaluate the turbulent premixed flame regimes, only turbulence parameters based on conditional measurement are meaningful, as the nonconditional ones incorporate part of the flame response to the turbulence structure in the reactants. Comparisons between scalar and dynamic timescales, as well as between integral and dissipation timescales, are made. They can be useful in turbulent premixed combustion models for the mean reaction term. In general, the results presented here indicate a complex effect of the u′/SL, ratio, which seems to depend on the way this parameter is varied.
Proceedings of The Combustion Institute, 2011
Direct initiation and propagation of detonation through a cryogenic two-phase flow constituted by... more Direct initiation and propagation of detonation through a cryogenic two-phase flow constituted by liquid oxygen droplets in gaseous hydrogen at 100 K are experimentally investigated. The influence of droplet size distribution is characterized in a cryogenic gaseous helium and liquid oxygen two-phase flow. Droplet sizing and detonation experiments are conducted by varying different parameters: distance from the injector, helium and hydrogen mass flow rates, global equivalence ratio and addition of gaseous nitrogen. Droplet size distributions reveal quick vaporization of the smallest droplets of the cryogenic jet. Results in terms of wave velocity, pressure, and detonation cells show that a detonation wave can be directly initiated, with a propagation wave velocity of 20% higher than the Chapman–Jouguet value. Cell size measurements show that the mixture sensitivity is not affected by the presence of droplets. Addition of gaseous nitrogen reduces only slightly the peak pressure, but the detonation velocity is reduced by about 30%.
Studies in History and Philosophy of Science, 1990
Science Technology & Human Values, 1990
Combustion and Flame, 2006
The combustion of aluminum droplets in wet (3 mol% H2O) and dry CO2 is studied in order to identi... more The combustion of aluminum droplets in wet (3 mol% H2O) and dry CO2 is studied in order to identify the influence of the two atmospheres on the surface processes. Millimeter-sized samples are maintained contactless in an aerodynamic levitation system and are heated continuously during burning by a laser. Ignition and combustion of the aluminum droplet are observed with a high-speed camera, the Al surface temperature is measured by an optical pyrometer, and unburnt residues are analyzed by X-ray diffraction. The determination of the burning rates and of the droplet temperatures reveals no differences between wet and dry CO2 (β=1.28±0.05 mm2/sβ=1.28±0.05 mm2/s, T=2600±50 KT=2600±50 K), which shows that the gas-phase combustion regime is not affected by the presence of water vapor. However, the oxide cap, initially formed by the oxide coating breakdown at ignition, is progressively removed in wet CO2, whereas it is unvarying in dry CO2. Comparison between Al burning in CO2/H2 and in CO2/(Ar or He) demonstrates that the oxide cap regression in a wet atmosphere is related to a chemical effect of hydrogen produced in the flame, and then diffusing and reacting at the droplet surface. It is suggested that the adsorption mechanism of H2 on the Al surface may slow down the contribution of adsorbed oxygen-containing species (CO) to the oxide cap, which would consequently promote its decomposition (removal). Furthermore, the carbon dissolution process is observed in wet and dry CO2. When the carbon concentration reaches the saturation limit in the burning Al droplet (xC=0.23xC=0.23 at T=2600 KT=2600 K), the excess of carbon is ejected at the surface and forms a solid coating. In the absence of the oxide cap (wet CO2), the refractory carbon coating prevents strong surface oxidation, and the combustion definitely stops. In the presence of the oxide cap (dry CO2), the carbon coating reacts and produces an oxycarbide phase which is melted by the laser heating; a new burning regime occurs mainly controlled by direct surface reactions, and leading to the slow oxidation of the droplet and the expulsion of dissolved carbon into CO. Finally, a qualitative model of the combustion of aluminum in CO2 atmospheres is proposed.
Combustion Science and Technology, 2005
In this paper, the influence of carbon dioxide on the combustion of aluminum droplets is investig... more In this paper, the influence of carbon dioxide on the combustion of aluminum droplets is investigated. Millimeter-sized droplets were heated and ignited by a laser in an aerodynamic levitation system in several CO2 containing atmospheres (H2O/CO2, H2O/CO2/N2) with a large range of compositions (wet – xH2O < 3%, 80/20, 50/50, 12.5/87.5, 50/25/25, 20/40/40). The combustion processes were observed with a high-speed CCD
Proceedings of The Combustion Institute, 2007
Flame surface characteristics were measured in turbulent premixed flames of hydrogen-enriched met... more Flame surface characteristics were measured in turbulent premixed flames of hydrogen-enriched methane-air mixtures at elevated pressures. The equivalence ratio range was from 0.6 to 0.8 and the hydrogen mole fraction in the fuel was varied from 0 to 0.2. The pressure range covered from atmospheric to 0.9 MPa. Planar Mie scattering measurements were performed to obtain the flame front images, which were further analyzed for fractal parameters, flame front curvature statistics, and flame surface density. Non-dimensional turbulence intensity, u 0 /S L , changed in proportion to the pressure and was 1.53 at atmospheric pressure and 6.51 at 0.9 MPa. With increasing pressure the flame images displayed finer structures indicating that the flame surface area was increasing with pressure. This was also indicated by the probability density function of the flame front curvature as a function of the pressure. Fractal dimension showed a strong dependence on pressure and increased from about 2.1 at atmospheric pressure to about 2.25 at 0.9 MPa. Fractal parameters, including inner and outer cut-offs, agree to a certain extend with the previous experimental data obtained at atmospheric and elevated pressures. The sensitivity of the fractal dimension to u 0 /S L was found to depend on the way u 0 /S L was varied. A strong correlation between the inner cut-off scale and the average flame radius of curvature was demonstrated. The implications of these results for flame surface density estimations are discussed.
Physics of Fluids, 1998
In recent years several transport equation models for the scalar dissipation rate have been propo... more In recent years several transport equation models for the scalar dissipation rate have been proposed to replace the well known algebraic expression based on equality of mechanical and scalar turbulent time scales. In this study various transport equation models are compared with each other and the model equation of Yoshizawa [J. Fluid Mech. 195, 541 (1988)] is given special attention. The latter is shown to allow an algebraic solution that is different from the classical ``equal-scales'' algebraic model. The constants that appear in this equation are assigned values based on similarity behavior in turbulent jets and based on studies of homogeneous isotropic turbulence. Both algebraic models and the transport equation models are compared and applied to isothermal variable density jets and jet diffusion flames. It is found that general features, such as the behavior of scalar fluctuation intensities of variable density turbulent jets are relatively well predicted by all the models. Differences between the models exist regarding the predicted time scale ratios.
Combustion Theory and Modelling, 1997
Combustion and Flame, 2008
Density functional theory quantum chemical calculations corrected with empirical atomic increment... more Density functional theory quantum chemical calculations corrected with empirical atomic increments have been used to examine C
Proceedings of The Combustion Institute, 2005
The influence of nitrogen on the aluminum droplet combustion under forced convection conditions h... more The influence of nitrogen on the aluminum droplet combustion under forced convection conditions has been studied. An aerodynamic levitation technique of millimetric size liquid droplets heated with a CO2 laser has been adopted to characterize the combustion of aluminum droplets and, in particular, to observe the surface phenomena. The determination of the burning rate and of the droplet temperature in several atmospheres (H2O/O2, H2O/Ar, H2O/N2, and air) has shown that they depend only on the nature and concentration of the oxidizers (O2 and H2O); a comparison of experiments in nitrogen and in argon containing mixtures demonstrated that N2 did not influence the gas phase combustion. However, for nitrogen containing atmospheres we observed the formation of solid aluminum nitride (AlN) at the droplet surface after a latency time depending on the nitrogen pressure. AlN first interacts with the oxide cap producing an aluminum oxynitride, then completely covers the droplet, and finally prevents combustion. The existence of a latency time varying with the nitrogen pressure suggests that the AlN formation is controlled by heterogeneous kinetics. The phenomenon of oxide cap regression during combustion was also observed in all gases, and it is attributed to a chemical decomposition process of alumina by aluminum forming gaseous AlxOy species. Therefore, nitrogen effects are significant at the droplet surface rather than in the gas phase, and it is suggested that N2 is probably one of the main species causing the manifestation of unsteady processes during aluminum droplet burning.
Symposium (international) on Combustion, 1996
One-and two point laser-induced Rayleigh scattering techniques have been used to fully characteri... more One-and two point laser-induced Rayleigh scattering techniques have been used to fully characterize the instantaneous density field in turbulent open conical flames of methane-air in the flamelet regime. The instantaneous density information is used to deduce space and time statistics of instantaneous flame fronts along constant progress variable contours. They include the integral and dissipation time scales of the scalar field, flamelet crossing frequencies, transit times, and convection velocities. The mean flamelet wrinkling scale and the mean flamelet orientation angle are also deduced. These flamelet space, time, and velocity characteristics are used to implement some existing flame surface density models and compared with some DNS predictions.The results show that the integral and dissipation time scales of the scalar field (the progress variable c) vary through the flame brush and decrease with increasing (c). The characteristic flamelet wrinkling scale, determined directly from two-point measurements, also decreases with increasing (c). The flamelet convection velocity along iso-(c) contours, and the flamelet normal velocity have been, found independent of (c). The mean flamelet orientation angle is also independent of (c) and is equal to 0.7, as also found by DNS. The experimental results have been used to implement the BMCL closures for the mean rate of creation of products, both for their temporal and spatial versions.
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Papers by Iskender Gokalp