| 1. |
- Chomiak, Jerzy, 1934, et al.
(författare)
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Flame kernel growth in a rotating gas
- 2008
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Ingår i: Combustion Science and Technology. - : Informa UK Limited. - 0010-2202 .- 1563-521X. ; 180:2, s. 391-399
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Tidskriftsartikel (refereegranskat)abstract
- The communication deals with an ignition kernel development in uniformly rotating mixture. A simple model is presented in order to predict the time evolution of the kernel length and its diameter under the assumption of ignition on the axis of rotation, which is preferred mode for rapid flame development. The analytic expressions for flame radius and length are compared with experimental results. The predicted radius growth rate is in good agreement with experimental data, whereas the length evolution rate predictions deviate substantially from measurements due to flame propagation effects involving quenching and perturbation of the surrounding flow by the growing kernel. An interesting general result supported by the theory and experiment is that the diameter growth rate of the cylindrical part of the kernel is about half the growth rate of the spherical kernel in a quiescent mixture and is independent of the rotation rate. Wall effects start to reduce the kernel development rate when the distance from the wall is less than double the kernel diameter. The effects are quite strong.
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| 2. |
- Chomiak, Jerzy, 1934, et al.
(författare)
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Simple criterion of importance of laminar flame instabilities in premixed turbulent combustion of mixtures characterized by low Lewis numbers
- 2023
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Ingår i: Physical Review E. - 2470-0045 .- 2470-0053. ; 107:1
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Tidskriftsartikel (refereegranskat)abstract
- By (i) highlighting the mitigation effect of strain rates on laminar flame instabilities and (ii) comparing peak growth rates of laminar flame instabilities with strain rates generated by small-scale turbulent eddies, a simple criterion of importance of the influence of the instabilities on an increase in premixed flame surface area in turbulent flows is suggested. The criterion implies that, even in lean hydrogen-air mixtures, laminar flame instabilities can significantly affect the flame area only in weak or moderate turbulence (the Karlovitz number defined using laminar flame speed, thermal flame thickness, and Kolmogorov time scale is on the order of 10 or less under room conditions).
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| 3. |
- Ganippa, Lionel Christopher, 1970, et al.
(författare)
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Cavitation: a Contributary Factor in the Transition from Symmetric to Asymmetric Jets in Cross-flow Nozzles
- 2004
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Ingår i: Experiments in Fluids. ; Vol. 36, s. pp. 627 - 634
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Tidskriftsartikel (refereegranskat)abstract
- The structure and evolution of cavitation and its influence on jet patterns from two transparent cross-flow nozzles with holes inclined at 90 degrees (nozzle A) and 80 degrees (nozzle B) to the nozzle axis have been investigated using high-speed motion pictures, flash photography and stroboscopic visualization. At the onset, cavitation inception was in the form of travelling bubbles, which were transported along the flow and clearly detached from the wall. As the flow was increased the bubbles grew and merged into a dense group of bubbles (cloud cavitation), partly unsteady and shedding. Further increasing the flow caused the cavitation at the entrance to transform mainly into a glassy appearance and at this stage the cavitation was well inside the hole and the spray appeared symmetric. When the flow was increased beyond this stage, cavitation extended to the exit of the hole, occupying a significant part of the hole on one side, resulting in a jet that atomized on the side where cavitation was most extensive and a non-atomizing jet on the side with less cavitation. The distribution of cavitation in the hole is very sensitive to the nozzle geometry and it substantially influences the spray dispersion.
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| 4. |
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| 5. |
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| 6. |
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| 7. |
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| 8. |
- Gjirja, Savo, 1945, et al.
(författare)
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Evaluation of the Results of Initial Engine Tests with STID Equipment
- 2001
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The report provides an evaluation of selected cylinder pressure records of a Wärtsilä 320 engine operating according to the STID principle with early, low pressure steam injection, and a discussion of the effects of the steam injection on emissions. It also contains an evaluation of the application potential of low pressure steam injection on engines.Due to pressure limitations of the available boiler, only tests with steam pressures up to 50 bar and steam temperatures up to 320°C could be performed. The steam pressure limitation caused that only injection during the intake stroke and early during the compression stroke was feasible which strongly restricted the scope of the study. However, the research has contributed considerably to the understanding of the STID process and in particular to evaluation of the steam effects on emissions and combustion. The low pressure steam injection is also one of the possible STID application versions to the normal, not thermally upgraded (hot), engines which requires only minor modifications of the steam generating facilities used in current ship installations.It is shown that the ignition and combustion processes in the engine are practically unaffected by injection of the required mass of steam, which typically shall not be larger than 3.5 times the mass of fuel, at which soot, unburned hydrocarbon and CO emissions start to grow. Within this range the NOx emissions can be described by a simple formula NOx =NOx,o(1-0.2m), where m is the steam to fuel mass ratio and NOx,o the emissions of the engine with out steam injection.The optimal way of using low pressure steam is to inject the steam into the inlet manifold of the engine which eliminates the complications of direct steam injection apparatus and eliminates the growth of maximum pressure. The steam temperature shall be as low as possible to minimize the reduction of volumetric efficiency of the engine and to reduce heat losses. The surplus steam enthalpy shall be used to additionally supercharge the engine using an ejector or tangential injection into the impeller of the compressor. It is recommended to utilize the low pressure steam injection on efficiency optimized engines by which both fuel consumption, NOx and soot emission reduction can be achieved
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| 9. |
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| 10. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
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A DNS study of the physical mechanisms associated with density ratio influence on turbulent burning velocity in premixed flames
- 2018
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Ingår i: Combustion Theory and Modelling. - : Informa UK Limited. - 1364-7830 .- 1741-3559. ; 22:1, s. 131-155
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Tidskriftsartikel (refereegranskat)abstract
- Data obtained in 3D direct numerical simulations of statistically planar, 1D weakly turbulent flames characterised by different density ratios σ are analysed to study the influence of thermal expansion on flame surface area and burning rate. Results show that, on the one hand, the pressure gradient induced within a flame brush owing to heat release in flamelets significantly accelerates the unburned gas that deeply intrudes into the combustion products in the form of an unburned mixture finger, thus causing largescale oscillations of the burning rate and flame brush thickness. Under the conditions of the present simulations, the contribution of this mechanism to the creation of the flame surface area is substantial and is increased by σ, thus implying an increase in the burning rate by σ. On the other hand, the total flame surface areas simulated at σ = 7.53 and 2.5 are approximately equal. The apparent inconsistency between these results implies the existence of another thermal expansion effect that reduces the influence of σ on the flame surface area and burning rate. Investigation of the issue shows that the flow acceleration by the combustion-induced pressure gradient not only creates the flame surface area by pushing the finger tip into the products, but also mitigates wrinkling of the flame surface (the side surface of the finger) by turbulent eddies. The latter effect is attributed to the high-speed (at σ = 7.53) axial flow of the unburned gas, which is induced by the axial pressure gradient within the flame brush (and the finger). This axial flow acceleration reduces the residence time of a turbulent eddy in an unburned zone of the flame brush (e.g. within the finger). Therefore, the capability of the eddy for wrinkling the flamelet surface (e.g. the side finger surface) is weakened owing to a shorter residence time.
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| 11. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
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A study of the effects of pressure-driven transport on developing turbulent flame structure and propagation
- 2004
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Ingår i: Combustion Theory and Modelling. ; 8(2), s. 211-225
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Tidskriftsartikel (refereegranskat)abstract
- Properties of solutions of a generalized normalized balance equation for the Favre-averaged combustion progress variable are numerically studied in the simplest case of a statistically planar, one-dimensional, stationary and uniform flow of unburned mixture. The focus is placed on the dependence of the solutions on pressure-driven transport for several closures of the mean rate of product creation. The results show that:(1) The flame structure is self-similar if the pressure-driven transport is sufficiently strong, but the self-similarity can be also obtained even for zero pressure-driven transport by using a particular closure of the mean rate of product creation;(2) Both burning velocity and flame thickness decrease if the pressure-driven transport increases and this effect can be reduced to a decrease in the asymptotically fully-developed quantities. An analysis of a more general progress variable balance equation, performed by invoking the sole assumption of the self-similarity of the flame structure, quantitatively confirms many numerical results, in particular,(1) the profile of the progress variable,(2) the scaling of the asymptotically fully-developed flame brush thickness and burning velocity, and(3) the development of the flame brush thickness and burning velocity in the cases of weak and strong pressure-driven transport. The analysis straightforwardly shows that the above general balance equation may be reduced to the Zimont equation with modified diffusivity provided that the flame structure is self-similar.
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| 12. |
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| 13. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
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A theoretical study of premixed turbulent flame development
- 2004
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Ingår i: Proceedings of the Combustion Institute, 30.
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Tidskriftsartikel (refereegranskat)abstract
- Flame development in a statistically stationary and uniform, planar, one-dimensional turbulent flow is theoretically studied. A generalized balance equation for the mean combustion progress variable, which includes turbulent diffusion and pressure-driven transport terms, as well as the mean rate of product creation, is introduced and analyzed by invoking the sole assumption of a self-similar flame structure, well-supported by numerous experiments. The assumption offers the opportunity to simplify the problem by splitting the aforementioned partial differential equation into two ordinary differential equations, which separately model spatial variations of the progress variable and time variations of flame speed and thickness. The self-similar profile of the progress variable, obtained in numerous experiments, is theoretically predicted. Closures of the normalized pressure-driven transport term and mean rate of product creation are obtained. The closed balance equation shows that turbulent diffusion dominates during the initial stage of flame development, followed by the transition to counter-gradient transport in a sufficiently developed flame. A criterion of the transition is derived. The transition is promoted by the heat release and pressure-driven transport. Fully-developed mean flame brush thickness and speed are shown to decrease when either density ratio or pressure-driven transport increases. Solutions for the development of the thickness are obtained. The development is accelerated by the pressure-driven transport and heat release.
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| 14. |
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| 15. |
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| 16. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
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Effects of Large-Scale Stretching on Turbulent Flame Speed
- 2006
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Ingår i: Turbulence, Heat and Mass Transfer 5. Proceedings of the Fifth International Symposium on Turbulence, Heat and Mass Transfer, Dubrovnik, Croatia, September 25-29, 2006. - 1567002293 ; , s. 573-576
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Konferensbidrag (refereegranskat)
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| 17. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
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Effects of premixed flames on turbulence and turbulent scalar transport
- 2010
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Ingår i: Progress in Energy and Combustion Science. - : Elsevier BV. - 0360-1285. ; 36:1, s. 1-102
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Tidskriftsartikel (refereegranskat)abstract
- Experimental data and results of direct numerical simulations are reviewed in order to show that premixed combustion can change the basic characteristics of a fluctuating velocity field (the so-called flame-generated turbulence) and the direction of scalar fluxes (the so-called countergradient or pressure-driven transport) in a turbulent flow. Various approaches to modeling these phenomena are discussed and the lack of a well-elaborated and widely validated predictive approach is emphasized. Relevant basic issues (the transition from gradient to countergradient scalar transport, the role played by flame-generated turbulence in the combustion rate, the characterization of turbulence in premixed flames, etc.) are critically considered and certain widely accepted concepts are disputed. Despite the Substantial progress made in understanding the discussed effects over the past decades, these basic issues strongly need further research.
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| 18. |
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| 20. |
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| 21. |
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| 22. |
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| 23. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
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Molecular transport effects on turbulent flame propagation and structure
- 2004
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Ingår i: Progress Energy and Combustion Science.
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Tidskriftsartikel (refereegranskat)abstract
- Abstract:Various experimental and DNS data show that premixed combustion is affected by the differences between the coefficients of molecular transport of fuel, oxidant, and heat not only at weak but also at moderate and high turbulence. In particular, turbulent flame speed increases with decreasing the Lewis number of the deficient reactant, the effect being very strong for lean hydrogen mixtures. Various concepts; flame instability, flame stretch, local extinction, leading point, that aim at describing the effects of molecular transport on turbulent flame propagation and structure are critically discussed and the results of relevant studies of perturbed laminar flames (unstable flames, flame balls, flames in vortex tubes) are reviewed. The crucial role played by extremely curved laminar flamelets in the propagation of moderately and highly turbulent flames is highlighted and the relevant physical mechanisms are discussed.
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| 24. |
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| 25. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
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On the Applicability of Coherent Flame Models to Simulations of Developing Turbulent Flames
- 2006
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Ingår i: Turbulence, Heat and Mass Transfer 5. Proceedings of the Fifth International Symposium on Turbulence, Heat and Mass Transfer, Dubrovnik, Croatia, September 25-29, 2006. - 1567002293 ; , s. 577-580
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Konferensbidrag (refereegranskat)
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| 26. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
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Scalar transport in self-similar, developing, premixed, turbulent flames
- 2005
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Ingår i: Proceedings of the Fourth Mediterranean Combustion Symposium, 6-10 October, 2005, Lisbon, Portugal.
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Konferensbidrag (refereegranskat)
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| 27. |
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| 28. |
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| 29. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
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Unburned mixture fingers in premixed turbulent flames
- 2015
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Ingår i: Proceedings of the Combustion Institute. - : Elsevier BV. - 1540-7489. ; 35:2, s. 1401-1408
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Tidskriftsartikel (refereegranskat)abstract
- Data obtained in 3D direct numerical simulations of statistically planar, 1D premixed turbulent flames indicate that the global burning velocity, flame surface area, and the mean flame brush thickness exhibit significant large-scale oscillations with time. Analysis of the data shows that the oscillations are caused by origin, growth, and subsequent disappearance of elongated channels filled by unburned gas. The growth of such an unburned mixture finger (UMF), which deeply intrudes into combustion products, is controlled by a physical mechanism of flame-flow interaction that has not yet been highlighted in the turbulent combustion literature, to the best of the present authors knowledge. More specifically, the fingers grow due to strong axial acceleration of unburned gas by local pressure gradient induced by heat release in surrounding flamelets. Under conditions of the present DNS, this physical mechanism plays an important role by producing at least as much flame surface area as turbulence does when the density ratio is equal to 7.5. Although, similarly to the Darrieus-Landau (DL) instability, the highlighted physical mechanism results from the interaction between a premixed flame and pressure field, it is argued that the UMF and the DL instability are different manifestations of the aforementioned interaction. Disappearance of an UMF is mainly controlled by the high-speed self-propagation of strongly inclined flame fronts (cusps) to the leading edge of the flame brush, but significant local increase in displacement speed due to large negative curvature of the front plays an important role also.
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| 30. |
- Sathiah, Pratap, 1978, et al.
(författare)
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Effects of Flame Development and Structure on Thermo-Acoustic Oscillations of Premixed Turbulent Flames
- 2004
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Ingår i: 2004 ASME Heat Transfer/Fluids Engineering Summer Conference, Charlotte, North Carolina, USA, July 11-15. - : ASMEDC. ; 4, s. 107-114
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Konferensbidrag (refereegranskat)abstract
- Abstract:Non-stationary confined premixed turbulent flames stabilized behind a bluff body are studied. A simple kinematic model of such flames was developed by Dowling who reduced the combustion process to the propagation of an infinitely thin flame with a constant speed. The goal of this work is to extend the model by taking into account the real structure of premixed turbulent flames and the development of turbulent flame speed and thickness. For these purposes, so-called Flame Speed Closure model for multi-dimensional simulations of premixed turbulent flames is adapted and combined with the aforementioned Dowling model. Simulations of the heat release rate dynamics for ducted flames due to oncoming flow oscillations have been performed. Typical results show that the oscillations in the integrated heat release rate follow the oncoming flow velocity oscillations with certain time delay, which controls the sign of the well-known Rayleigh integral and, hence, the global instability or stability of the combustor. The delays computed using the Dowling and the above approach are different, thus indicating the importance of resolving the flame structure when modeling ducted flame oscillations.
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| 31. |
- Sathiah, Pratap, 1978, et al.
(författare)
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Effects of Turbulent Flame Speed Development on Ducted Flame Oscillations
- 2004
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Ingår i: The Eleventh International Congress on Sound and Vibration. St. Petersburg.
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Konferensbidrag (refereegranskat)abstract
- Abstract:A confined premixed turbulent flame that oscillates behind a bluff body under the influence of imposed velocity perturbations is studied as a model of thermo-acoustic oscillations in the afterburner of an aeroengine, caused by the interaction between acoustic waves and unsteady heat release. A kinematic model of the flame dynamics, developed by Dowling, is extended to allow for the effects of turbulent flame speed development on the heat release in the system. For this purpose, an expression for developing turbulent flame speed, obtained by two of us [13], is incorporated into the model of Dowling. A theoretical solution for the transfer function between the oncoming flow velocity and total heat release rate is found by considering weak velocity oscillations and using a simplified expression for the developing flame speed. Numerical solutions are obtained for various magnitudes of the velocity oscillations and using the full model for the flame speed. Both solutions indicate that the development of turbulent flame speed notably changes the phase difference between the oscillations of the oncoming flow velocity and total heat release rate in the combustor. Consequently, according to the Rayleigh criterion, the flame speed development should be properly addressed when numerically investigating the stability of a combustor with respect to acoustic waves.
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| 32. |
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