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Epaminondas Mastorakos - One of the best experts on this subject based on the ideXlab platform.
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Non-linear response of turbulent premixed flames to imposed Inlet Velocity oscillations of two frequencies
FLOW TURBUL COMBUST, 2008Co-Authors: Epaminondas MastorakosAbstract:This paper describes an experimental study investigating the non-linear response of lean premixed air/ethylene flames to strong Inlet Velocity perturbations of two frequencies. The combustor has a centrally-placed bluff body and a short quartz section. The annulus between the bluff body and the flow tube, which also housed the acoustic pressure transducers, allowed the reactants into the combustor. The Inlet flow was perturbed using loudspeakers. High speed laser tomography, OH* chemiluminescence and OH Planar Laser Induced Fluorescence (PLIF) have been used for flow visualization, heat release and flame surface density (FSD) measurements respectively. The heat release fluctuations increased initially linearly with Inlet Velocity amplitude for a single frequency forcing, with saturation occurring after forcing amplitudes of around 15% of the bulk Velocity, which was found to occur due to vortex roll up and subsequent flame annihilation. The introduction of energy at the second frequency (i.e, the harmonic) was found to change the vortex formation and shedding frequency, depending on the level of forcing. This resulted in a non-linear flame response transfer function (defined as the amplitude of unsteady heat release divided by the amplitude of Velocity perturbation at the fundamental) whose amplitude depended greatly on the amount of harmonic content present in the perturbations. The introduction of higher harmonics reduced the flame annihilation events, which are responsible for saturation, thus reducing non-linearity in the amplitude dependence of the flame response. These results were further verified using sequential time-resolved OH PLIF measurements. The findings from this study suggest that the acoustic response of the flame was mostly due to flame area variation effected by modulation of the annular jet and evolution of the shear layers.
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investigation of the nonlinear response of turbulent premixed flames to imposed Inlet Velocity oscillations
Combustion and Flame, 2006Co-Authors: C. A. Armitage, Epaminondas Mastorakos, R Balachandran, R. S. CantAbstract:Acoustically forced lean premixed turbulent bluff-body stabilized flames are investigated using turbulent combustion CFD. The calculations simulate aspects of the experimental investigation by Balachandran et al. [R. Balachandran, B. Ayoola, C. Kaminski, A. Dowling, E. Mastorakos, Combust. Flame 143 (2005) 37–55] and focus on the amplitude dependence of the flame response. For the frequencies of interest in this investigation an unsteady Reynolds-averaged Navier–Stokes (URANS) approach is appropriate. The combustion is represented using a modified laminar flamelet approach with an algebraic representation of the flame surface density. The predictions are compared with flame surface density (FSD) and OH∗ chemiluminescence measurements. In the experiments the response of the flame has been quantified by means of a number of single-frequency, amplitude-dependent transfer functions. The predicted flame shape and position are in good agreement with the experiment. The dynamic response of the flame to Inlet Velocity forcing is also well captured by the calculations. At moderate frequencies nonlinear behavior of the transfer functions is observed as the forcing amplitude is increased. In the experiments this nonlinearity was attributed in part to the rollup of the reacting shear layer into vortices and in part to the collision of the inner and outer flame sheets. This transition to nonlinearity is also observed in the transfer functions obtained from the predictions. Furthermore, the vortex shedding and flame-sheet collapse may be seen in snapshots of the predicted flow field taken throughout the forcing cycle. The URANS methodology successfully predicts the behavior of the forced premixed turbulent flames and captures the effects of saturation in the transfer function of the response of the heat release to Velocity fluctuations.
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experimental investigation of the nonlinear response of turbulent premixed flames to imposed Inlet Velocity oscillations
Combustion and Flame, 2005Co-Authors: R Balachandran, B O Ayoola, Clemens F Kaminski, A P Dowling, Epaminondas MastorakosAbstract:This paper describes an experimental investigation of acoustically forced lean premixed turbulent bluff-body-stabilised flames in an enclosure short enough so that no coupling of the combustor downstream acoustics occurred for the frequencies studied here, which allows an unambiguous examination of the flame response to Inlet Velocity fluctuations. Special emphasis was placed on the amplitude dependence of this response. Measurements of the heat release rate were performed with OH∗ and CH∗ chemiluminescence, planar laser-induced fluorescence (PLIF) of OH from which the flame surface density (FSD) was computed, and simultaneous CH2O and OH PLIF imaging from which the local heat release rate (RX) was estimated. The global heat release measured with chemiluminescence and that integrated from the local FSD measurements were in close agreement, while a comparison between FSD and high-resolution RX imaging also showed good agreement. This suggests that estimates of the flame area are sufficient to determine heat release rate for this flow. The heat release response became nonlinear after Inlet Velocity amplitudes of around 15% of the bulk Velocity. This value depended on the forcing frequency and the equivalence ratio. The nonlinearity was found to occur when the shear layers rolled up into vortices. The vortices induced by the Inlet Velocity fluctuations not only generated flame area when the flame wrapped around them, but also caused cusps and even large-scale flame annihilation events, as observed in time-resolved OH PLIF images. Such events occurred when parts of the flame stabilised on the inner shear layer close to the recirculation zone collapsed on parts of the flame stabilised on the outer recirculation zone, a phenomenon that was made more prominent with increasing forcing amplitude. A further nonlinearity occurred at high amplitudes and at some equivalence ratios, where a significant leakage of energy to higher harmonics was observed, but the origin of this is not yet clarified. The present results suggest that the flame sheet kinematics play a major role in the saturation mechanism of lean premixed flame response, hence extending previous experimental and analytical results from laminar to turbulent flames. Heat release fluctuations due to local fluctuations of strain rate and curvature were less significant, while no localised extinction has been observed even at large forcing amplitudes.
Jingxuan Yang - One of the best experts on this subject based on the ideXlab platform.
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a model for prediction of maximum efficiency Inlet Velocity in a gas solid cyclone separator
Chemical Engineering Science, 2019Co-Authors: Qing Wei, Guogang Sun, Jingxuan YangAbstract:Abstract A new model for determining maximum-efficiency Inlet Velocity in the gas-solid cyclone separator was developed analytically in this study. Previous maximum-efficiency Inlet Velocity (VMEIV) models are derived at normal temperature and do not function properly under high-temperature conditions. The new model not only accounts for gas physical properties and cyclone structure dimensions, but also various particle characteristics as well as temperature; it can be applied to predict VMEIV at both normal and high temperatures. Based on Yang’s model, a hard sphere analogy of the soft sphere model was used to directly manage particle-wall collision as per the flow field of the cyclone. This model yields more accurate downward flow region width information than previously proposed models. If the radial displacement of a rebounded particle after collision is equal to the width of the downward flow region, then the corresponding Inlet Velocity is defined as the maximum-efficiency Inlet Velocity. Comparisons among different experimental results and other models’ results indicate that the proposed model more accurately predicts maximum-efficiency Inlet Velocity.
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prediction of the maximum efficiency Inlet Velocity in cyclones
Powder Technology, 2015Co-Authors: Jingxuan Yang, Guogang Sun, Minshu ZhanAbstract:Abstract The separation efficiency of cyclones is closely related to the Inlet Velocity, and the maximum-efficiency Inlet Velocity (MEIV) maximizes the separation efficiency. In current separation models, particles centrifuged on the wall are considered captured, and their further motions are no longer considered. We propose that particles centrifuged on the wall impact the wall and then rebound. If the energy in these particles is sufficient, they will rebound into the upward gas flow. Within the fast upward gas flow, particles quickly move into the vortex finder and escape from the cyclone. A faster Inlet Velocity imparts more energy to the particles. Therefore, an excessive Inlet Velocity causes rebounded particles to escape, decreasing efficiency. The particle motion discussed above is the reason for the MEIV phenomenon, which is different from previous explanations. Newton's law and the hard sphere model were used to describe the particles' motion. Taken together, a new approach to forecast MEIV was established. The effects of various particle characteristics on the MEIV of cyclones, which have not been considered by previous models to forecast the MEIV, are taken into account in this new approach. We observed good consistency between the prediction of the new model and experimental data.
Aiwu Fan - One of the best experts on this subject based on the ideXlab platform.
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a numerical investigation on non premixed catalytic combustion of ch4 o2 n2 in a planar micro combustor
Fuel, 2019Co-Authors: Shixuan Wang, Liang Zhao, Aiwu FanAbstract:Abstract Non-premixed catalytic combustion in micro-combustors has not been reported so far. The present study demonstrates the feasibility of non-premixed CH 4 /(O 2 + N 2 ) catalytic combustion in a planar micro-combustor with a height less than the quenching distance. The impacts of Inlet Velocity, combustor height, nominal equivalence ratio and O 2 concentration on the combustion characteristics were numerically investigated. First, it is found that with the increase of Inlet Velocity, the catalytic reaction zone moves downstream and the reaction intensity increases first and then decreases. Second, as the combustor height is increased, the catalytic reaction zone broadens and the largest heat release rate decreases, while the maximum temperature and combustion efficiency show non-monotonic variations. Thirdly, a slightly fuel-rich condition can increase Pt(s) coverage, enhance catalytic reaction on the oxidant side, and improve combustion efficiency. Finally, oxygen enrichment can significantly enhance both heterogeneous and homogeneous reactions, leading to a dramatic improvement in combustion efficiency.
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effect of the cavity depth on the combustion efficiency of lean h 2 air flames in a micro combustor with dual cavities
International Journal of Hydrogen Energy, 2017Co-Authors: Wei Yang, Ying Xiang, Aiwu Fan, Hong YaoAbstract:Abstract The impact of the cavity depth on the combustion efficiency of lean hydrogen/air flames in a micro-combustor with dual cavities was numerically investigated. The results show that under lower and higher Inlet velocities, the combustion efficiency varies in opposite tendencies with an increasing cavity depth. Namely, under lower Inlet Velocity, the combustion efficiency decreases as the cavity depth is increased, while it increases with an increasing cavity depth under higher Inlet Velocity. To reveal the underlying mechanisms responsible for these variation tendencies, we performed comprehensive analysis in terms of heat transfer from the flame to cavity walls, heat regeneration ratio via upstream wall, residence time in the cavity, and stretch rate at the flame tip. It can be summarized that the total heat flux at the cavity walls and the heat regeneration ratio via upstream wall are the two dominant factors under lower Inlet Velocity, while the stretch rate at the flame tip and the residence time in the cavity overwhelm the other two factors under higher Inlet Velocity.
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experimental investigation on non premixed methane air combustion in y shaped meso scale combustors with without fibrous porous media
Energy Conversion and Management, 2017Co-Authors: Daoguan Ning, Ying Xiang, Yi Liu, Aiwu FanAbstract:Abstract Diffusion methane-air flames in Y-shaped meso-scale combustors of three different diameters (i.e., d = 4, 5 and 6 mm) were experimentally studied. The impact of adding fibrous porous media on mixture ignitability, flame stability and flammable range were systematically investigated. Without porous media, the mixture can only be ignited at positions farther than a certain distance, which was defined as ignition distance in this paper. The ignition distance increases with an increasing Inlet Velocity and decreases with a decreasing channel diameter. The flame cannot be stabilized and propagates downstream with an inclination angle. The propagation Velocity increases with the increase of Inlet Velocity or with the decrease of channel diameter. With the addition of porous media, the mixture can be ignited near the splitter due to improved methane/air mixing. Moreover, the flames become more stable and stationary flames were observed under lower velocities in the combustors of d = 5 mm and 6 mm with fibrous porous media. Furthermore, the application of porous media can significantly expand the flammable range. In summary, the combustion performances can be notably improved by the application of fibrous porous media.
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experimental investigation and numerical analysis on flame stabilization of ch4 air mixture in a mesoscale channel with wall cavities
Combustion and Flame, 2015Co-Authors: Jianlong Wan, Aiwu Fan, Hong Yao, Yi Liu, Wei Liu, Xiaolong Gou, Daiqing ZhaoAbstract:Behaviors of premixed CH4/air flame in mesoscale channels with and without cavities were experimentally investigated. No stable symmetric flame was observed in the channel without cavities and flame is prone to inclining and pulsating. In contrast, flame can be effectively anchored in the presence of cavities. When the Inlet Velocity is increased sufficiently high, curved fluctuating flame front appears. Blow-off limits of the channel with cavities are several times larger than the corresponding burning Velocity of incoming CH4/air mixture, while the flashback limits are almost the same as the straight channel counterparts. These indicate that the cavities have a strong ability to extend the operational range of Inlet Velocity. Numerical simulation demonstrates that combined effects, i.e., the formation of recirculation zone and low Velocity zone in the cavities, preferential diffusion effect, as well as the preheating effect of upstream inner walls, are major mechanisms responsible for flame stabilization. Furthermore, numerical result reveals that large strain rate and heat loss rate exist at the transition point between the ramped cavity wall and the downstream inner wall, which results in flame splitting at high Inlet Velocity due to local extinction, and eventually leads to flame blow-off. In summary, the combustion behaviors in the mesoscale channel with cavities strongly depend on the interactions between the reaction zone, conjugate heat exchange and flow field. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Wenbo Bi - One of the best experts on this subject based on the ideXlab platform.
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numerical investigation on heat transfer performance of planar elastic tube bundle by flow induced vibration in heat exchanger
International Journal of Heat and Mass Transfer, 2016Co-Authors: Derong Duan, Peiqi Ge, Wenbo BiAbstract:Abstract This study numerically investigated the heat transfer enhancement of planar elastic tube bundle by flow-induced vibration based on a two-way fluid structure interaction (FSI) model in the range of Inlet Velocity 0.2–0.5 m/s. This two-way FSI calculation involved the unsteady, three-dimensional incompressible Navier–Stokes equation solved with finite volume approach and the dynamic equilibrium equation of tube bundle solved with finite element method combined with dynamic mesh scheme, which was verified by comparing with the published experimental results. Then the Nusselt number of the circumference of tube, local position, single tube and the overall tube bundle was presented. The performance evaluation criterion (PEC) was selected to study the heat transfer performance of planar elastic tube bundle. Results show that vibration frequency dominates the heat transfer enhancement at the local position near the mass-block. In the middle of tube bundle, vibration amplitude plays a significant role on heat transfer enhancement. Therefore, the average heat transfer enhancement of 12.97% and 4.58% at the middle two tubes is higher than that of 5.37% and 2.4% at the innermost and outermost tubes when the Inlet Velocity is 0.2 m/s and 0.5 m/s, respectively. In the range of Inlet Velocity 0.2–0.5 m/s, flow-induced vibration contributes to enhancing the heat transfer of planar elastic tube bundle at low Inlet Velocity, resulting in the heat transfer enhancement of 8.26%, 6.07%, 5.67% and 3.91% respectively. Compared to the mechanical vibration strengthening heat transfer technology, flow-induced vibration plays an advantage on energy conversion to improve heat transfer. PEC indicates that the higher Inlet Velocity is sometimes more acceptable to obtain a higher heat transfer coefficient in the industry production.
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numerical analysis on shell side flow induced vibration and heat transfer characteristics of elastic tube bundle in heat exchanger
Applied Thermal Engineering, 2016Co-Authors: Jiadong Ji, Peiqi Ge, Wenbo BiAbstract:Abstract Based on the sequential solution of bi-direction fluid-structure coupling method, the shell-side flow-induced vibration responses and the heat transfer characteristics of the elastic tube bundles in heat exchanger have been investigated in this paper. Taking into account the meshing convenience and computing efficiency, appropriate meshing strategy and step calculation method have been proposed. Effects of shell-side water Inlet Velocity on the flow-induced vibration responses and heat transfer characteristics of the elastic tube bundle have been discussed. The results indicate that the mainly vibration of each tube bundle induced by the shell-side fluid is the out-plane vibration. The vibration amplitudes of the two stainless steel blocks are basically consistent when the shell-side fluid Inlet Velocity is lower, and the vibration amplitude of the stainless steel block III is more intense when the shell-side fluid Inlet Velocity is higher. In addition, the heat transfer coefficient of each elastic tube bundle has increased significantly at low flow rate (or low Reynolds number) when the shell-side flow-induced the elastic tube bundle vibration.
R. S. Cant - One of the best experts on this subject based on the ideXlab platform.
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Temperature response of turbulent premixed flames to Inlet Velocity oscillations
Experiments in Fluids, 2008Co-Authors: B. Ayoola, G. Hartung, C. A. Armitage, J. Hult, R. S. Cant, Clemens F KaminskiAbstract:Flame–turbulence interactions are at the heart of modern combustion research as they have a major influence on efficiency, stability of operation and pollutant emissions. The problem remains a formidable challenge, and predictive modelling and the implementation of active control measures both rely on further fundamental measurements. Model burners with simple geometry offer an opportunity for the isolation and detailed study of phenomena that take place in real-world combustors, in an environment conducive to the application of advanced laser diagnostic tools. Lean premixed combustion conditions are currently of greatest interest since these are able to provide low NO_ x and improved increased fuel economy, which in turn leads to lower CO_2 emissions. This paper presents an experimental investigation of the response of a bluff-body-stabilised flame to periodic Inlet fluctuations under lean premixed turbulent conditions. Inlet Velocity fluctuations were imposed acoustically using loudspeakers. Spatially resolved heat release rate imaging measurements, using simultaneous planar laser-induced fluorescence (PLIF) of OH and CH_2O, have been performed to explore the periodic heat release rate response to various acoustic forcing amplitudes and frequencies. For the first time we use this method to evaluate flame transfer functions and we compare these results with chemiluminescence measurements. Qualitative thermometry based on two-line OH PLIF was also used to compare the periodic temperature distribution around the flame with the periodic fluctuation of local heat release rate during acoustic forcing cycles.
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investigation of the nonlinear response of turbulent premixed flames to imposed Inlet Velocity oscillations
Combustion and Flame, 2006Co-Authors: C. A. Armitage, Epaminondas Mastorakos, R Balachandran, R. S. CantAbstract:Acoustically forced lean premixed turbulent bluff-body stabilized flames are investigated using turbulent combustion CFD. The calculations simulate aspects of the experimental investigation by Balachandran et al. [R. Balachandran, B. Ayoola, C. Kaminski, A. Dowling, E. Mastorakos, Combust. Flame 143 (2005) 37–55] and focus on the amplitude dependence of the flame response. For the frequencies of interest in this investigation an unsteady Reynolds-averaged Navier–Stokes (URANS) approach is appropriate. The combustion is represented using a modified laminar flamelet approach with an algebraic representation of the flame surface density. The predictions are compared with flame surface density (FSD) and OH∗ chemiluminescence measurements. In the experiments the response of the flame has been quantified by means of a number of single-frequency, amplitude-dependent transfer functions. The predicted flame shape and position are in good agreement with the experiment. The dynamic response of the flame to Inlet Velocity forcing is also well captured by the calculations. At moderate frequencies nonlinear behavior of the transfer functions is observed as the forcing amplitude is increased. In the experiments this nonlinearity was attributed in part to the rollup of the reacting shear layer into vortices and in part to the collision of the inner and outer flame sheets. This transition to nonlinearity is also observed in the transfer functions obtained from the predictions. Furthermore, the vortex shedding and flame-sheet collapse may be seen in snapshots of the predicted flow field taken throughout the forcing cycle. The URANS methodology successfully predicts the behavior of the forced premixed turbulent flames and captures the effects of saturation in the transfer function of the response of the heat release to Velocity fluctuations.