| 1. |
- Prins, M. J., et al.
(författare)
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Biomass pyrolysis in a heated-grid reactor: Visualization of carbon monoxide and formaldehyde using Laser-Induced Fluorescence
- 2011
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Ingår i: Journal of Analytical and Applied Pyrolysis. - : Elsevier BV. - 1873-250X .- 0165-2370. ; 92:2, s. 280-286
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Tidskriftsartikel (refereegranskat)abstract
- The development of improved biomass pyrolysis models is vital for more accurate modelling and design of biomass conversion equipment. Such improved models must be based on reliable experimental data: biomass should be pyrolyzed at high heating rates and the reaction products should be measured using an on-line, non-intrusive method. Therefore, a heated grid reactor with heating rate of 300-600 K/s was used to study pyrolysis of biomass at temperatures in the range of 500-700 degrees C. The formation of formaldehyde and carbon monoxide from wood at high heating rates was successfully visualized using Laser-Induced Fluorescence (LIF). A thin vertical laser line or sheet was present directly above the biomass lying on the heated grid. Two-photon excitation at 230 nm was applied to induce fluorescence of carbon monoxide present in the volatiles, whereas excitation of formaldehyde was done at 355 nm. Visualization of these compounds shows that the release rises strongly with temperature; this typically happens on a timescale in the order of seconds. In principle, the method described allows for the determination of truly primary products. Future research is recommended, aimed at quantifying the concentrations measured by LIE. Care must be taken to calibrate for quenching of the fluorescence signal. Avoiding secondary reactions taking place in the gas phase is another experimental challenge. (C) 2011 Elsevier B.V. All rights reserved.
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| 2. |
- Yu, J. F., et al.
(författare)
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Heat transfer and flame stabilization of laminar premixed flames anchored to a heat-flux burner
- 2016
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 0360-3199. ; 41:3, s. 2037-2051
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Tidskriftsartikel (refereegranskat)abstract
- Measurement of the burning velocity of unstretched laminar hydrogen/air premixed flames suffers from large uncertainties owing to the highly diffusive nature of hydrogen that can give rise to flame instability. This paper reports on a numerical study of the structures and stability of laminar premixed CH4/O2/CO2 flames and H2/O2/N2 flames anchored to a heat-flux burner using a high-order numerical method with detailed chemical kinetic mechanisms and detailed transport properties. The aim is to elucidate the effect of the flow and temperature inhomogeneity generated by the burner plate holes on flame structures and burning velocity. Heat transfer flux between the burner plate and the surrounding gaseous mixture is investigated under various standoff distances and burner plate temperatures. The burning velocity and the detailed flow, temperature and species distributions in flames with a zero net heat flux between the flames and the burner plate are analyzed. It is found that for the methane flames, when the standoff distance is sufficiently small, the burner can essentially suppress the intrinsic flame instability, but the plate holes can give rise to flame wrinkles of the size of the holes. At high standoff distances, the non-uniformity of the flow from the burner plate holes has a minor effect on the flame surface wrinkling; however, large-scale cellular structures can appear on the flame surface due to intrinsic flame instability. For the studied methane flames the effect of non-uniformity of the flow from the burner plate holes on the burning velocity is fairly small. For the studied hydrogen flames the burner plate could not totally suppress the intrinsic flame instability. The intrinsic flame instability can give rise to a significant increase in the flame surface area and mean burning velocity, with more than 25% increase in the burning velocity.
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| 3. |
- Goswami, M., et al.
(författare)
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Experimental and modeling study of the effect of elevated pressure on lean high-hydrogen syngas flames
- 2015
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 35, s. 655-662
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Tidskriftsartikel (refereegranskat)abstract
- New laminar burning velocity measurements of 85: 15% (by volume) H-2-CO and H-2-N-2 mixtures with O-2-He oxidizer are reported at lean conditions and elevated pressures (1-10 atm). Experiments are conducted using the heat flux method at initial temperature of 298 K. In this technique a near adiabatic flame is stabilized by balancing the heat loss from the flame to the burner with heat gain to the unburnt gas mixture such that no net heat loss to the burner is observed. A new facility was designed for such high pressure burner stabilized flame experiments. The results obtained are compared with five chemical kinetic schemes from literature for syngas mixtures at elevated pressures. Large differences are observed between the kinetic schemes and the experiments which can be attributed to certain key chemical reactions. A study of the kinetics is performed through reaction rate and sensitivity analysis which indicate that a high uncertainty still remains in important reactions that drive the production and consumption of species such as H, HO2 and OH. For lean mixtures the reaction H + O-2(+M) = HO2(+M) contributes significantly to the deviation of models from the experiments. The present analysis in the lean mixture regime suggests the need for further studies in assessment and modification of rate constants for this reaction. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 4. |
- Goswami, Mayuri, et al.
(författare)
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The effect of elevated pressures on the laminar burning velocity of methane plus air mixtures
- 2013
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Ingår i: Combustion and Flame. - : Elsevier BV. - 0010-2180. ; 160:9, s. 1627-1635
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Tidskriftsartikel (refereegranskat)abstract
- In spite of the large amount of research spent on the evaluation of the high pressure dependence of laminar burning velocity of methane + air flame, there still exists a large uncertainty in the data for various reasons. In order to reduce the scatter to acceptable levels, the Heat Flux Method (HFM), known as a potential method with high accuracy, has been extended to higher pressures. New measurements of the laminar burning velocity of methane + air flames are presented. Non-stretched planar flames were stabilized on a perforated plate burner which was placed in a high pressure environment. The experimental results are reported for a pressure range between 1 and 5 atm. The equivalence ratio was varied from 0.8 to 1.4. Comparisons with several recent literature sources (experiments) show good agreement. An exhaustive literature survey was performed to study the numerous existing laminar burning velocity correlations for its pressure dependence. It is indicated from the literature that many of the deduced correlations use stretched laminar burning velocity results. Many used only few data points for the pressure behavior and correlations and therefore show wide discrepancies. As the heat flux method furnishes quality results with reduced errors, the results were further utilized in deducing a power-law pressure dependence. Numerical simulations were also performed using two widely used chemical reaction mechanisms, which were further involved in comparing correlations. The proposed power exponent beta(1) shows a non-monotonic behavior at equivalence ratio around 1.4 in experiments and simulations. Through species and reaction flux analysis it was observed that CH3 consumption through various reactions remain pressure dependent and show non-monotonic behavior at equivalence ratio around 1.4. (c) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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| 5. |
- Prins, A. J., et al.
(författare)
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Visualization of Biomass Pyrolysis and Temperature Imaging in a Heated-Grid Reactor
- 2009
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Ingår i: Energy & Fuels. - : American Chemical Society (ACS). - 0887-0624 .- 1520-5029. ; 23:1, s. 993-1006
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Tidskriftsartikel (refereegranskat)abstract
- The main advantage of a heated-grid reactor for studying pyrolysis kinetics of solid fuel samples is that high heating rates of up to 1000 K/s can be obtained. However, one of the concerns is whether the temperature distribution over the grid material is uniform and whether the presence of a thermocouple welded to the grid causes any measurement errors. Biomass samples were placed on the heated-grid reactor, and the volatiles, emitted in the biomass pyrolysis process as hot gas plumes, were imaged with an infrared camera with a high framing speed. The temporal resolved infrared images indicate that the pyrolysis process does not take place at the same rate everywhere on the grid. Two-dimensional temperature images of a heated grid made of stainless steel were recorded using the method of laser-induced thermometry with thermographic phosphors. As expected from a heat-transfer model, measured temperatures were found to be significantly higher than temperatures indicated by a thermocouple welded to the bottom of the grid. It was also observed that there is a large temperature gradient between the two electrodes on which the grid is connected. It is shown that replacing a wire mesh by a foil as a grid material may lead to more homogeneous temperature distribution. The paper recommends additional research to demonstrate the suitability of the heated-grid reactor for carrying out accurate measurements.
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| 6. |
- Akkerman, V., et al.
(författare)
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Flow-flame interaction in a closed chamber
- 2008
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 20:5, s. 21-
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Tidskriftsartikel (refereegranskat)abstract
- Numerous studies of flame interaction with a single vortex and recent simulations of burning in vortex arrays in open tubes demonstrated the same tendency for the turbulent burning rate proportional to U-rms lambda(2/3), where U-rms is the root-mean-square velocity and lambda is the vortex size. Here, it is demonstrated that this tendency is not universal for turbulent burning. Flame interaction with vortex arrays is investigated for the geometry of a closed burning chamber by using direct numerical simulations of the complete set of gas-dynamic combustion equations. Various initial conditions in the chamber are considered, including gas at rest and several systems of vortices of different intensities and sizes. It is found that the burning rate in a closed chamber (inverse burning time) depends strongly on the vortex intensity; at sufficiently high intensities it increases with U-rms approximately linearly in agreement with the above tendency. On the contrary, dependence of the burning rate on the vortex size is nonmonotonic and qualitatively different from the law lambda(2/3). It is shown that there is an optimal vortex size in a closed chamber, which provides the fastest total burning rate. In the present work, the optimal size is six times smaller than the chamber height.
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| 7. |
- Goswami, M., et al.
(författare)
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Experimental and modelling study of the effect of elevated pressure on ethane and propane flames
- 2016
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Ingår i: Fuel. - : Elsevier BV. - 1873-7153 .- 0016-2361. ; 166, s. 410-418
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Tidskriftsartikel (refereegranskat)abstract
- Laminar burning velocities, S-L, of ethane + air and propane + air flames within an equivalence ratio range between 0.8 and 1.3 were determined at atmospheric and elevated pressures up to 4 atm. Measurements were performed in non-stretched flames, stabilized on a perforated plate burner at adiabatic conditions, created using the heat flux method. Initial unburnt gas temperature was 298 K. These new experimental results were compared with available literature data and predictions using three kinetic schemes: USC Mech II, San Diego mechanism and Aramco Mech 1.3. The models behave differently in reproducing S-L of ethane and propane flames with closer agreement between Aramco Mech 1.3 and the present measurements. The pressure dependence of the laminar burning velocities was analysed using the expression S-L = S-L0(P/P-0)(beta). Large deviations of the derived power exponent, beta, were observed for different experimental datasets and between model predictions and the measurements. To elucidate these differences in the performance of the three mechanisms, sensitivity analyses of the burning velocity and of the power exponent beta were performed. It was demonstrated that the power exponent beta may serve as an independent target for model validation and improvement. When comparing beta coefficients derived from the present and previous measurements of S-L in methane, ethane, propane and n-pentane flames using the heat flux method, important similarities were found at lean conditions with large disparity in rich mixtures. Neither experiments nor modelling support the linear dependence of the power exponent beta with equivalence ratio for flames of alkanes. (C) 2015 Elsevier Ltd. All rights reserved.
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| 8. |
- Goswami, M., et al.
(författare)
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Laminar burning velocity of lean H-2-CO mixtures at elevated pressure using the heat flux method
- 2014
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Ingår i: International Journal of Hydrogen Energy. - : Elsevier BV. - 1879-3487 .- 0360-3199. ; 39:3, s. 1485-1498
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Tidskriftsartikel (refereegranskat)abstract
- Laminar burning velocity measurements of 50:50 and 85:15% (by volume) H-2-CO mixtures with O-2-N-2 and O-2-He oxidizers were performed at lean conditions (equivalence ratio from 0.5 to 1) and elevated pressures (1 atm-9 atm). The heat flux method (HFM) is employed for determining the laminar burning velocity of the fuel-oxidizer mixtures. HFM creates a one-dimensional adiabatic stretchless flame which is an important prerequisite in defining the laminar burning velocity. This technique is based on balancing the heat loss from the flame to the burner with heat gain to the unburnt gas mixture, in a very simple way, such that no net heat loss to the burner is obtained. Instabilities are observed in lean H-2-CO flames with nitrogen as the bath gas for pressures above 4 atm. Stable flames are obtained with helium as the bath gas for the entire pressure range. With the aim to cater stringent conditions for combustion systems such as gas turbines, an updated H-2-CO kinetic mechanism is proposed and validated against experimental results. The scheme was updated with recent rate constants proposed in literature to suit both atmospheric and elevated pressures. The proposed kinetic model agrees with new experimental results. At conditions of high pressure and lean combustion, reactions H + O-2 = OH + O and H + O-2 (+M) = H-2 (+M) compete the most when compared to other reactions. Reaction H + HO2 = OH + OH contributes to OH production, however, less at high-pressure conditions. At higher CO concentrations and leaner mixtures an important role of reaction CO + OH = CO2 + H is observed in the oxidation of CO. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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| 9. |
- Goswami, M., et al.
(författare)
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Numerical Simulations of Flat Laminar Premixed Methane-Air Flames at Elevated Pressure
- 2014
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Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 1563-521X .- 0010-2202. ; 186:10-11, s. 1447-1459
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Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional axisymmetric simulation of stoichiometric methane-air flames stabilized on flat burners at elevated pressure is reported in the present work. Such flames, in practice, are experimentally obtained using the heat flux method for measurement of laminar burning velocity of fuel-oxidizer mixtures (Bosschaart and de Goey, 2004; Goswami et al., 2013). The method makes use of a burner with a perforated brass burner plate. The dimensions of such a plate play an important role in creating flat flames. The present investigation is focused on studying laminar premixed flame structure numerically at elevated pressure up to 15 bar using a one-step and a detailed chemical reaction mechanism. Three burner plate models (of varying hole diameter and porosity) are used in the simulations for pressures up to 7 bar with a one-step mechanism. The surface area increase of the flame was evaluated based on an isotherm at 900 K and the net reaction rate of methane compared to a flat flame. The comparison of these models shows that the surface area increase can significantly be reduced by choosing a smaller hole diameter and larger porosity. The results of the detailed simulations using an appropriate chemical reaction mechanism up to 15 bar using a burner plate model, which is similar to the ones used in experiments (mentioned above), show a nonlinear increase of the flame curvature with elevating pressure. A hole diameter of 0.25 mm and a pitch of 0.29 mm is suggested for a burner plate in such experiments. Flame structure at elevated pressure is also analyzed further based on species profiles obtained.
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| 10. |
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| 11. |
- Valiev, Damir, 1981-
(författare)
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Flame Dynamics and Deflagration-to-Detonation Transition
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Various premixed flame phenomena are studied by means of direct numerical simulations of the complete system of hydrodynamic equations. Rigorous study of flame dynamics is essential for all premixed combustion problems where multidimensional effects cannot be disregarded.The present thesis consists of six parts. The first part deals with the fundamental problem of curved stationary flames propagation in free-slip tubes of different widths. It is shown that only simple "single-hump" slanted stationary flames are possible in tubes wider than some stability limit. The flame dynamics is shown to be governed by a large-scale stability mechanism resulting in a highly slanted flame front.The second part of the thesis is dedicated to studies of acceleration and fractal structure of outward freely propagating flames. It is shown that the development of Landau-Darrieus instability results in the formation of fractal-like flame front structure. Two-dimensional simulation of radially expanding flames displays a radial growth with 1.25 power law temporal behavior. It is shown that the fractal excess for 2D geometry obtained in thenumerical simulation is in good agreement with theoretical predictions.In third part the flame acceleration in tubes with non-slip at the walls is studied in the extremely wide range of flame front velocity. Flame accelerates from small initial velocity to supersonic speed in the laboratory reference frame. Flame acceleration undergoes three stages: 1) initial exponential acceleration in the quasi-isobaric regime, 2) almost linear increase of the flame speed to supersonic values, 3) saturation to a stationary high-speed deflagration velocity, which is correlated with the Chapman-Jouguet deflagration speed. The saturation velocity is in line with previous experimental results.In fourth part the role of viscous stress in heating of the fuel mixture in deflagration-to-detonation transition in tubes is studied both analytically and numerically. The developed analytical theory determines temperature distribution ahead of an accelerating flame. The heating effects of viscous stress and the compression wave become comparable at sufficiently high values of the Mach number. Viscous stress makes heating and explosion of the fuel mixture preferential at the walls.In fifth part we reveal the physical mechanism of ultra-fast flame acceleration in obstructed channels used in modern experiments on detonation triggering. It is demonstrated that delayed burning between the obstacles creates a powerful jet-flow, driving the acceleration. The flame front accelerates exponentially; theanalytical formula for the growth rate is obtained. The theory is validated by extensive direct numerical simulations and comparison to previous experiments.The last part of the thesis concerns the transition from deflagration to detonation. It is found that in sufficiently wide free-slip channels and for sufficiently fast flames Landau-Darrieus instability may invoke nucleation of hot spots within the wrinkled flame folds, triggering an abrupt transition from deflagrative to detonative combustion. Results on DDT in channels with non-slip at the walls are also presented.
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