ORCID Profile
0000-0003-3644-9400
Current Organisation
Zhejiang University
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Publisher: Cambridge University Press (CUP)
Date: 15-06-2021
DOI: 10.1017/JFM.2021.442
Publisher: Cambridge University Press (CUP)
Date: 22-02-2023
DOI: 10.1017/JFM.2023.78
Abstract: In the present work, direct numerical simulation of a laboratory-scale lean premixed reacting jet in cross-flow was performed to explore the flow–flame structures and turbulence–flame interactions. A jet of lean premixed ethylene–air mixtures (equivalence ratio $\\phi = 0.6$ ) was injected into a hot vitiated cross-flow. Both non-reacting and reacting cases were simulated. It was found that the reacting jet penetrates deeper in the cross-flow with a weaker shear layer compared with the non-reacting one. The wake of the non-reacting and reacting jet is characterized by vertical vortices and recirculation zones, respectively. As for the flame structure of the reacting case, the reaction intensity varies considerably in different flame zones. The heat release rate on the leeward side is higher than that on the windward side, but lower than that of the corresponding laminar flame. The analysis of the turbulence–flame interactions of the reacting case showed that the large local Damköhler number ( $Da$ ) related to reaction-induced dilatations results in an increased tendency of the scalar gradient to align with the most extensive strain rate, which is more evident in the regions with high heat release rate on the leeward side. Negative dilatation regions with positive tangential strain rate and negative normal strain rate are observed on the windward side. High positive dilatations appear on the flame front of the leeward side. The tangential strain rate is negatively correlated with the normal strain rate and curvature. Regions with a high local $Da$ on the windward side correspond with high positive curvature regions.
Publisher: Elsevier BV
Date: 02-2014
Publisher: American Chemical Society (ACS)
Date: 03-10-2012
DOI: 10.1021/EF300771J
Publisher: American Chemical Society (ACS)
Date: 13-12-2012
DOI: 10.1021/EF301699A
Publisher: Elsevier BV
Date: 04-2014
Publisher: Elsevier BV
Date: 07-2015
Publisher: Elsevier BV
Date: 11-2018
Publisher: SAGE Publications
Date: 09-2013
DOI: 10.1260/1756-8277.5.3.225
Abstract: A fully compressible 3D solver for reacting flows has been developed and applied to investigate a turbulent lifted jet flame in a vitiated coflow by means of direct numerical simulation (DNS) to validate the solver and analyze the flame characteristics. An eighth-order central differencing scheme is used for spatial discretization and a fourth-order Runge-Kutta method is employed for time integration. The DNS results agree well with the experimental measurements for the conditional means of reactive scalars. However, the lift-off height is under predicted. The mean axial velocity develops into a self-similar profile after x/D = 6. The normalized flame index is employed to characterize the combustion regime. It is found that at the flame base the gradients of the reactants are opposed and diffusion combustion is dominant. Further downstream, the contribution of premixed combustion increases and peaks at x/D = 8. Finally, the stabilization process is examined. The turbulent lifted flame is proved to stabilize in the lean mixtures and low scalar dissipation rate regions.
Publisher: Elsevier BV
Date: 2020
Publisher: Elsevier BV
Date: 12-2019
Publisher: AIP Publishing
Date: 10-2020
DOI: 10.1063/5.0023391
Abstract: In the present work, direct numerical simulations of non-reactive and reactive turbulent boundary layers laden with inert particles were performed to investigate the influences of combustion on particle motions in wall-bounded flows. It was found that the coherent vortical structures of turbulent boundary layers are significantly modified by combustion. The streamwise velocity of particles is enhanced, while the velocity fluctuation is reduced in the reactive case due to the relaminarization effects. The preferential concentration of particles in the reactive case was found to be attenuated based on both visualization and the Voronoi analysis. The particle concentration in the outer layer of the reactive case is reduced because of the thermal expansion effects. However, the concentration is increased in the near-wall regions. The quadrant analysis of the Reynolds stress component was applied to explain this behavior. It was revealed that the magnitude of the mean Reynolds stress component is attenuated as a result of relaminarization. The frequency of the sweep event is increased but that of the injection event is reduced in the near-wall regions, which is responsible for the high particle concentration near the wall of the reactive case. The present study improves fundamental understandings of particle behaviors in non-reactive and reactive turbulent boundary layers.
Publisher: Elsevier BV
Date: 2021
Publisher: Informa UK Limited
Date: 21-02-2018
Publisher: AIP Publishing
Date: 05-2021
DOI: 10.1063/5.0048680
Abstract: Accurate prediction of temporal evolution of turbulent flames represents one of the most challenging problems in the combustion community. In this work, predictive models for turbulent flame evolution were proposed based on machine learning with long short-term memory (LSTM) and convolutional neural network-long short-term memory (CNN-LSTM). Two configurations without and with mean shear are considered, i.e., turbulent freely propagating premixed combustion and turbulent boundary layer premixed combustion, respectively. The predictions of the LSTM and CNN-LSTM models were validated against the direct numerical simulation (DNS) data to assess the model performance. Particularly, the statistics of the fuel (CH4 for the freely propagating flames and H2 for the boundary layer flames) mass fraction and reaction rate were examined in detail. It was found that generally the performance of the CNN-LSTM model is better than that of the LSTM model. This is because that the CNN-LSTM model extracts both the spatial and temporal features of the flames while the LSTM model only extracts the temporal feature of the flames. The errors of the models mainly occur in regions with large scalar gradients. The correlation coefficient of the mass fraction from the DNS and that from the CNN-LSTM model is larger than 0.99 in various flames. The correlation coefficient of the reaction rate from the DNS and that from the CNN-LSTM model is larger than 0.93 in the freely propagating flames and 0.99 in the boundary layer flames. Finally, the profiles of the DNS values and predictions conditioned on axial distance were examined, and it was shown that the predictions of the CNN-LSTM model agree well with the DNS values. The LSTM model failed to accurately predict the evolution of boundary layer flames while the CNN-LSTM model could accurately predict the evolution of both freely propagating and boundary layer flames. Overall, this study shows the promising performance and the applicability of the proposed CNN-LSTM model, which will be applied to turbulent flames a posteriori in future work.
Publisher: AIP Publishing
Date: 05-2018
DOI: 10.1063/1.5026739
Abstract: In this work, three different flamelet tabulation methods for spray combustion are evaluated. Major differences among these methods lie in the treatment of the temperature boundary conditions of the flamelet equations. Particularly, in the first tabulation method (“M1”), both the fuel and oxidizer temperature boundary conditions are set to be fixed. In the second tabulation method (“M2”), the fuel temperature boundary condition is varied while the oxidizer temperature boundary condition is fixed. In the third tabulation method (“M3”), both the fuel and oxidizer temperature boundary conditions are varied and set to be equal. The focus of this work is to investigate whether the heat transfer between the droplet phase and gas phase can be represented by the studied tabulation methods through a priori analyses. To this end, spray flames stabilized in a three-dimensional counterflow are first simulated with detailed chemistry. Then, the trajectory variables are calculated from the detailed chemistry solutions. Finally, the tabulated thermo-chemical quantities are compared to the corresponding values from the detailed chemistry solutions. The comparisons show that the gas temperature cannot be predicted by “M1” with only a mixture fraction and reaction progress variable being the trajectory variables. The gas temperature can be correctly predicted by both “M2” and “M3,” in which the total enthalpy is introduced as an additional manifold. In “M2,” variations of the oxidizer temperature are considered with a temperature modification technique, which is not required in “M3.” Interestingly, it is found that the mass fractions of the reactants and major products are not sensitive to the representation of the interphase heat transfer in the flamelet chemtables, and they can be correctly predicted by all tabulation methods. By contrast, the intermediate species CO and H2 in the premixed flame reaction zone are over-predicted by all tabulation methods.
Publisher: AIP Publishing
Date: 2021
DOI: 10.1063/5.0035538
Abstract: Direct numerical simulations of non-premixed swirling fuel-rich/fuel-lean flames within a high-pressure model gas turbine combustor are conducted to investigate the flow and flame structures, as well as the transport mechanisms of both turbulent kinetic energy (TKE) and enstrophy. The effects of non-premixed flames upon these characteristics are also analyzed through comparison with the corresponding non-reacting swirling flows. We demonstrate that the turbulence state in the swirling flows behaves axisymmetrically overall in the current cylindrical laboratory-type combustor and is more likely to be cigar shaped in the presence of combustion. The analysis of TKE budgets within non-reacting swirling flows indicates that TKE is predominantly produced by mean shear in the shear layers and redistributed by transport effects from the inner shear layer (ISL) to the internal-recirculation zone however, these transport effects are suppressed by combustion in fuel-lean non-premixed flames. Although the total pressure effects consume TKE with a similar magnitude in all cases, the essential cause is different. The influence of combustion upon TKE budgets is more significant for fuel-lean flames than for fuel-rich flames as a result of the stronger burning intensity in the ISL of the former. Analysis of enstrophy dynamics shows that dilatation and baroclinic torque play relatively noticeable roles in swirling non-premixed flames, unlike their negligible effects in high-intensity homogeneous isotropic turbulence. The augmentation of baroclinic torque caused by non-premixed swirling combustion mainly arises from the remarkable decrease in density and enhancement of preferential alignment between the vorticity and baroclinic torque vectors.
Publisher: Elsevier BV
Date: 12-2017
Publisher: Elsevier BV
Date: 2021
Publisher: Elsevier BV
Date: 08-2014
Publisher: American Chemical Society (ACS)
Date: 27-07-2012
DOI: 10.1021/EF3007728
Publisher: Elsevier BV
Date: 09-2019
Publisher: American Chemical Society (ACS)
Date: 16-06-2020
Publisher: American Chemical Society (ACS)
Date: 27-08-2021
Publisher: Elsevier BV
Date: 10-2011
Publisher: Elsevier BV
Date: 02-2019
Publisher: Elsevier BV
Date: 12-2018
Publisher: Elsevier BV
Date: 10-2012
Publisher: American Chemical Society (ACS)
Date: 20-09-2023
Publisher: Elsevier BV
Date: 12-2018
Publisher: Elsevier BV
Date: 08-2020
Publisher: Elsevier BV
Date: 2015
Publisher: AIP Publishing
Date: 03-2021
DOI: 10.1063/5.0042732
Abstract: Accurate modeling of the unresolved flame surface area is critical for the closure of reaction source terms in the flame surface density (FSD) method. Some algebraic models have been proposed for the unresolved flame surface area for premixed flames in the flamelet or thin reaction zones (TRZ) regimes where the Karlovitz number (Ka) is less than 100. However, in many lean combustion applications, Ka is large (Ka & 100) due to the strong interactions of small-scale turbulence and flames. In the present work, a direct numerical simulation (DNS) database was used to evaluate the performance of algebraic FSD models in high Ka premixed flames in the context of large eddy simulations. Three DNS cases, i.e., case L, case M and case H, were performed, where case L is located in the TRZ regime with Ka & 100 and case M and case H are located in the broken reaction zones regime with Ka & 100. A convolutional neural network (CNN) model was also developed to predict the generalized FSD, which was trained with s les of case H and a small filter size, and was tested in various cases with different Ka and filter sizes. It was found that the fraction of resolved FSD increases with increasing filtered progress variable c̃ and decreasing subgrid turbulent velocity fluctuation u′Δ. The performance of CNN and algebraic models was assessed using the DNS database. Overall, the results of algebraic models are promising in case L and case M for a small filter size the CNN model performs generally better than the algebraic models in high Ka flames and the correlation coefficient between the modeled and actual generalized FSD is greater than 0.91 in all cases. The effects of c̃ and u′Δ on the performance of different models for various cases were explored. The algebraic models perform well with large values of c̃ and small values of u′Δ in high Ka cases, which indicates that they can be applied to high Ka flames in certain conditions. The performance of the CNN model is better than the algebraic models for a large filter size in high Ka cases.
Publisher: AIP Publishing
Date: 08-2017
DOI: 10.1063/1.4999335
Abstract: In the present work, the flamelet rogress variable (FPV) approach based on two mixture fractions is formulated for pulverized coal combustion and then evaluated in laminar counterflow coal flames under different operating conditions through both a priori and a posteriori analyses. Two mixture fractions, Zvol and Zchar, are defined to characterize the mixing between the oxidizer and the volatile matter/char reaction products. A coordinate transformation is conducted to map the flamelet solutions from a unit triangle space (Zvol, Zchar) to a unit square space (Z, X) so that a more stable solution can be achieved. To consider the heat transfers between the coal particle phase and the gas phase, the total enthalpy is introduced as an additional manifold. As a result, the thermo-chemical quantities are parameterized as a function of the mixture fraction Z, the mixing parameter X, the normalized total enthalpy Hnorm, and the reaction progress variable YPV. The validity of the flamelet chemtable and the selected trajectory variables is first evaluated in a priori tests by comparing the tabulated quantities with the results obtained from numerical simulations with detailed chemistry. The comparisons show that the major species mass fractions can be predicted by the FPV approach in all combustion regions for all operating conditions, while the CO and H2 mass fractions are over-predicted in the premixed flame reaction zone. The a posteriori study shows that overall good agreement between the FPV results and those obtained from detailed chemistry simulations can be achieved, although the coal particle ignition is predicted to be slightly earlier. Overall, the validity of the FPV approach for laminar pulverized coal combustion is confirmed and its performance in turbulent pulverized coal combustion will be tested in future work.
Publisher: Cambridge University Press (CUP)
Date: 09-07-2021
DOI: 10.1017/JFM.2021.535
Publisher: American Chemical Society (ACS)
Date: 25-02-2020
Publisher: American Chemical Society (ACS)
Date: 10-10-2022
Publisher: American Chemical Society (ACS)
Date: 10-10-2012
DOI: 10.1021/EF301253Y
Publisher: Elsevier BV
Date: 10-2018
Publisher: AIP Publishing
Date: 11-2020
DOI: 10.1063/5.0028877
Abstract: Flame stretch and its related quantities are three-dimensional (3D), while most planar imaging techniques, widely used in turbulent combustion, can only provide lower-dimensional information of these quantities. In the present work, based on a direct numerical simulation (DNS) database, artificial neural network (ANN) and random forest (RF) models were developed to predict the 3D flame stretch and its related quantities such as the tangential strain rate, displacement velocity, and curvature from lower-dimensional information that can be accessed experimentally. It was found that the performance of the RF model is better than that of the ANN model. In the RF model, the correlation coefficients between the modeled and actual values are more than 0.97, and the determination coefficients are over 0.95. The model performance deteriorates with increasing turbulent intensity. The probability density functions of various quantities predicted by the RF model are in good agreement with those of the DNS. Compromising the model performance and the computational cost, a simplified RF model was proposed by using a few optimal input features. It was found that the discrepancies between the modeled and actual values mainly occur in highly curved regions, which explains the observation that the prediction errors increase with increasing turbulent intensity. Overall, the predictions of the simplified RF model agree well with the actual values.
Publisher: Elsevier BV
Date: 2019
Publisher: AIP Publishing
Date: 2021
DOI: 10.1063/5.0029904
Abstract: Fully compressible Large Eddy Simulations (LES) are performed to investigate the effect of wall boundary conditions on the nonlinear response of turbulent premixed flames. For the unforced flames, the flame length and flame angle from the present LES results in the isothermal case are in better agreement with experimental results than those in the adiabatic case. For the forced flames, both litudes and phases of nonlinear flame response at moderate frequency are well reproduced in the isothermal case. While in the adiabatic case, the litudes are well reproduced, the prediction of phases is slightly underestimated. At high frequency, LES results from the isothermal case provide reasonable agreement with the experimental results in general, while the adiabatic case gives unreasonable prediction of nonlinear flame response. Results show that wall boundary conditions affect the spatial distributions of heat release fluctuations by changing the temperature field, which is affected by the evolutions of the flame structure and flow field in the nonlinear oscillation cycle. At moderate frequency, heat release fluctuations in different parts of the combustor are in phase, and wall boundary conditions have limited influence on the global flame response, while at high frequency, wall boundary conditions have significant influence on the global flame response since the heat release fluctuations in different parts of combustor are out of phase. Accurate prediction of flame response at high frequency needs accurate calculation of the wall temperature.
Publisher: Elsevier BV
Date: 2019
Publisher: Elsevier BV
Date: 03-2018
Publisher: Elsevier BV
Date: 03-2012
Publisher: Elsevier BV
Date: 06-2019
No related grants have been discovered for Kun Luo.