Large Eddy Simulation(LES)has been employed for the investigation of supersonic flow characteristics in five ducts with varying cross-sectional geometries.The numerical results reveal that flow channel configurations ...Large Eddy Simulation(LES)has been employed for the investigation of supersonic flow characteristics in five ducts with varying cross-sectional geometries.The numerical results reveal that flow channel configurations exert a considerable influence on the mainstream flow and the near-wall flow behavior.In contrast to straight ducts,square-to-circular and rectangular-to-circular ducts exhibit thicker boundary layers and a greater presence of vortex structures.Given the same inlet area,rectangular-to-circular ducts lead to higher flow drag force and total pressure loss than square-to-circular ducts.Characterized by the substantial flow separation and shock waves,the"S-shaped duct shows significant vertically-asymmetric characteristics.展开更多
To reduce the drag generated by the recirculation flow at the rocket base in a RocketBased Combined Cycle(RBCC)engine operating in the ramjet/scramjet mode,a novel annular rocket RBCC engine based on a central plug co...To reduce the drag generated by the recirculation flow at the rocket base in a RocketBased Combined Cycle(RBCC)engine operating in the ramjet/scramjet mode,a novel annular rocket RBCC engine based on a central plug cone was proposed.The performance loss mechanism caused by the recirculation flow at the rocket base and the influence of the plug cone configuration on the thrust performance were studied.Results indicated that the recirculation flow at the rocket base extended through the entire combustor,which creates an extensive range of the"low-kineticenergy zone"at the center and leads to an engine thrust loss.The plug cone serving as a surface structure had a restrictive effect on the internal flow of the engine,making it smoothly transit at the position of the large separation zone.The model RBCC engine could achieve a maximum thrust augmentation of 37.6%with a long plug cone that was twice diameter of the inner isolator.However,a shorter plug cone that was half diameter of the inner isolator proved less effective at reducing the recirculation flow for a supersonic flow and induced an undesirable flow fraction that diminished the thrust performance.Furthermore,the effectiveness of the plug cone increased with the flight Mach number,indicating that it could further broaden the operating speed range of the scramjet mode.展开更多
It has been found that the static pressure distribution along the axial direction of liquid kerosene is lower than that of the gaseous kerosene under the same flow condition and overall equivalent ratio from previous ...It has been found that the static pressure distribution along the axial direction of liquid kerosene is lower than that of the gaseous kerosene under the same flow condition and overall equivalent ratio from previous studies.To further investigate this phenomenon,a compressible two-phase parallel simulation method is utilized to analyze the mixing and combustion characteristics of gaseous and liquid kerosene jets in a cavity-based supersonic combustor.The numerical results are consistent with the experiments and demonstrate that gaseous injection leads to a cavity shear layer that dives deeper into the cavity,forming two recirculation zones in the front and rear of the cavity.In contrast,the cavity shear layer is closer to the mainstream during liquid injection,and only a large recirculation zone is formed in the rear of the cavity.As a result of the cavity shear layer and the recirculating flow,the fuel vapor of gaseous injection accumulates in the front of the cavity,while for the liquid injection,the fuel vapor disperses in the cavity,cavity shear layer,and the region above,and the rear of the cavity has a higher fuel vapor concentration than the front.This unique fuel distribution causes the combustion area to be concentrated in the cavity during the gaseous injection but dispersed inside and downstream of the cavity during the liquid injection.As a result,forming a thermal throat under the same conditions is more challenging during liquid injection,and the generated static pressure distribution is lower than that during the gaseous injection.展开更多
The asymmetric separation has a crucial effect on the performance of the scramjet.In this study,the asymmetric separation and combustion in both rectangular and circular scramjets are investigated numerically,and the ...The asymmetric separation has a crucial effect on the performance of the scramjet.In this study,the asymmetric separation and combustion in both rectangular and circular scramjets are investigated numerically,and the effect of injection scheme is analyzed.The characteristics of the flow field are analyzed based on sufficient code verification.In the rectangular scramjet,the separation tends to occur in the corner due to the corner boundary-layer effect.The separation is asym-metric and only two corners have serious separation.The fuel penetration depth in the separation zone increases and the combustion is intensified.When the injection scheme is uniform,both the combustion and separation become weak.In the circular scramjet,the separation and combustion are basically axisymmetric in the scramjet with one-row injection scheme.The asymmetric combustion becomes obvious in cases with multi-row injection scheme.When the injection orifices distribute intensively on the top and bottom sides,the strongest and weakest separations occur near these two sides respectively.When the distribution of orifices becomes uniform,the direction of separation cannot be predicted.For multi-row cases,the nonuniform injection scheme could result in violent combustion and asymmetric flow structures compared with the uniform injection scheme.展开更多
The atomization process of a liquid jet in a divergent cavity-based combustor was investigated experimentally using high-speed photography and schlieren techniques under a Mach number 2.0 supersonic crossflow.Gas-liqu...The atomization process of a liquid jet in a divergent cavity-based combustor was investigated experimentally using high-speed photography and schlieren techniques under a Mach number 2.0 supersonic crossflow.Gas-liquid flow field was studied at different divergent angles and injection schemes.It is found that complex wave structures exist in the divergent cavity-based combustor.The spray field can be divided into three distinct zones:surface wave-dominated breakup zone,rapid atomization zone and cavity mixing zone.A dimensionless spray factor is defined to describe the concentration of spray inside the cavity qualitatively.As a result,it is revealed that for the large divergent angle cavity,the injection scheme near the upstream inlet has a higher penetration depth but a lower spray distribution,where the injection scheme near the cavity has a more spray distribution.For the small divergent angle cavity,the injection scheme near the upstream inlet also has a higher penetration depth and the injection scheme near the start point of the divergent section has a more sufficient spray distribution.The small divergent angle cavity-based combustor with the upstream wall transverse injection is an optimized injection scheme to improve both penetration and spray distribution inside the cavity.Finally,a penetration depth formula is proposed to explain the spray and distribution behaviors in the divergent cavity-based combustor.展开更多
The injection and atomization process of a liquid fuel jet is critical for an ignition start of a scramjet engine.Airwall-mounted crossflow injection strategy is widely used in scramjet combustors,avoiding high total ...The injection and atomization process of a liquid fuel jet is critical for an ignition start of a scramjet engine.Airwall-mounted crossflow injection strategy is widely used in scramjet combustors,avoiding high total pressure loss and allowing the liquid fuel to rapidly undergo atomization,mixing,and evaporation.In this review,research progress on a liquid jet in supersonic crossflow was evaluated from aspects of atomization mechanism and spray characteristics.When a liquid jet is injected into a supersonic crossflow,primary and secondary breakups occur successively.The surface instability of liquid can significantly affect the breakup process.This review discusses the current understanding of the breakup process and spray characteristics of a liquid jet in supersonic crossflow including the mechanism of atomization and the characteristics of distribution and atomization.The development of windward Rayleigh-Taylor(R-T)unstable waves is the main factor in column breakup.The development of Kelvin-Helmholtz(K-H)unstable waves along the circumferential direction of the jet or droplets is the main factor of surface and droplet breakups.The liquid-gas momentum ratio is the most important factor affecting the penetration depth.The span width of the liquid jet is affected by the windward area.Breakup and coalescence lead to a transformation of the size distribution of droplets from S-or C-shaped to I-shaped,and the velocity distribution of the droplets on the central symmetry plane has a mirrored S-shape.The droplet distribution on the spanwise cross-section retains a structure similar to an“Ω”shape.At last,some promising recommendations have been proposed,namely a theoretical predictive model which can describe the breakup mechanism of a liquid jet,the distribution characteristics and droplets size distribution of a liquid jet under a cavity combustion chamber,especially for enthalpy flows with complex wave structures.展开更多
The ignition process and flame propagation with ethylene fuel in cavity-stabilized scramjet by a Multi-Channel Gliding Arc(MCGA)at Mach 2.0 were investigated.Effects of equivalence ratios on the MCGA-assisted ignition...The ignition process and flame propagation with ethylene fuel in cavity-stabilized scramjet by a Multi-Channel Gliding Arc(MCGA)at Mach 2.0 were investigated.Effects of equivalence ratios on the MCGA-assisted ignition process and flame propagation of the scramjet were recorded by two high-speed cameras from different view angles.The discharge characteristics of MCGA are also collected synchronously with the high-speed cameras.The distributions of temperature,velocity,and equivalence ratios in non-reactive flows of the cavity were simulated by Reynolds Averaged Navier-Stokes(RANS)model.The results show that MCGA can achieve reliable ignition with the Global Equivalence Ratios(GER)between 0.06 and 0.17.The ignition process is composed of flame kernel generation,flame development,and stable combustion.The time from flame kernel generation to the establishment of global flame decreases as GER decreases from 0.17 to 0.08.In the streamwise direction,the flame first develops to the Cavity Leading Edge(CLE)because of the influence of the cavity recirculation zone and then uplifts into the cavity shear layer,and finally develops to the Cavity Trailing Edge(CTE).In the spanwise direction,the flame width is less than 50%of the width of the cavity before developing to CLE and begins to develop towards the two sides of the combustor after reaching CLE,which is affected by the angular recirculation zone on both sides of CLE.The ignition processes by MCGA in the scramjet combustor are significantly affected by local distributions of equivalence ratios and velocity in the cavity.展开更多
The mixing and combustion characteristics in a cavity flameholding combustor under inlet Mach number 2.92 are numerically investigated with ethylene injection.Dimensionless distance is defined as the ratio of the actu...The mixing and combustion characteristics in a cavity flameholding combustor under inlet Mach number 2.92 are numerically investigated with ethylene injection.Dimensionless distance is defined as the ratio of the actual distance to the height of the combustor entrance.The cavity shear-layer mode,the lifted cavity shear-layer mode,and jet wake mode with upstream separation are observed respectively with dimensionless distance equals to 1.5,4.5,and 7.5.In both non-reacting and reacting flow fields,the numerical results are essentially in agreement with the schlieren photography,flame chemiluminescence images,and wall pressure,which verify the reliability of the numerical method.The results of non-reacting flow fields show that the BackwardFacing Step(BFS)can promote the flow separation downstream at a fixed distance.The more forward the separation position is,the larger the separation zone is in the non-reacting flow field.Furthermore,the larger the separation zone is,the higher the intensity of combustion in the reacting flow field is.A reasonable distance can reduce the total pressure loss generated by the shock waves in the combustor.The flame presents remarkable three-dimensional characteristics in the reacting flow fields.When dimensionless distance equals to 4.5,there are flames near the side wall above the cavity and it is difficult for the flame stabilization in the center of the combustor,while the combustion intensity in the center of the combustor is higher than that near the side wall when dimensionless distance equals to 7.5.In the cavity flameholding combustors with a backward-facing step,the higher combustion intensity may bring much total pressure loss to the combustor.Thus,it is a good choice to achieve better thrust performance when dimensionless distance equals to 4.5 compared to the other two combustors.展开更多
Developing supersonic combustion models with efficiency,accuracy and practicality is important foundation to deeply understand the complex combustion processes in scramjet engines.Characterized by efficiency and intui...Developing supersonic combustion models with efficiency,accuracy and practicality is important foundation to deeply understand the complex combustion processes in scramjet engines.Characterized by efficiency and intuition,the flamelet-like models are widely used models in computational combustion methods.However,the supersonic combustion flow field has the nature of strong compressibility,multiple modality,and multiple scales,which poses a great challenge to the traditional flamelet-like models with fixed boundary conditions,and then the complex chemical reaction mechanisms that may face will impose additional computational burden.In this context,this paper reviews the flamelet-like models used in scramjet engines,and summarizes prominent issues in the application practice,including modeling partially premixed combustion,defining progress variable,solving temperature efficiently,evaluating assumed Probability Density Function(PDF)models,and treating mixture fraction variance.Furthermore,possible prospects and directions of improvements are proposed and highlighted for the flamelet-like models.To fully describe the physicochemical scenario and address the raised challenges,these improvements are dedicated to dealing with the compressibility,temperature rise,time-scales,species of interest,multi-inlet combustion,the progress variable definition,and the higher Mach number flight condition.展开更多
The combustion modes in two different scramjet combustors with the mass flow rates of 1.8 kg/s and 3.6 kg/s are experimentally investigated to explore the scaling effects on supersonic combustion with a Mach number 2....The combustion modes in two different scramjet combustors with the mass flow rates of 1.8 kg/s and 3.6 kg/s are experimentally investigated to explore the scaling effects on supersonic combustion with a Mach number 2.0 inflow.It is found that the scramjet combustor with a larger scale can broaden the flame rich blowout limit.As the Equivalence Ratio(ER)increases,the combustion in the small-scale combustor maintains in the cavity-stabilized mode,and the flamebase moves downstream along the cavity shear layer;however,the combustion in the large-scale combustor gradually transfers from the cavity-stabilized mode to the jet-wake-stabilized mode.The differences in the cavity residence time,the ignition delay time and the Damkohler number caused by different scales of the scramjet combustor are likely to account for the scaling effects on the combustion modes.展开更多
Although millions of patients have clinically recovered from COVID-19,little is known about the immune status of lymphocytes in these individuals.In this study,the peripheral blood mononuclear cells of a clinically re...Although millions of patients have clinically recovered from COVID-19,little is known about the immune status of lymphocytes in these individuals.In this study,the peripheral blood mononuclear cells of a clinically recovered(CR)cohort were comparatively analyzed with those of an age-and sex-matched healthy donor cohort.We found that CD8^(+)T cells in the CR cohort had higher numbers of effector T cells and effector memory T cells but lower Tc1(IFN-γ^(+)),Tc2(IL-4^(+)),and Tc17(IL-17A^(+))cell frequencies.The CD4^(+)T cells of the CR cohort were decreased in frequency,especially the central memory T cell subset.Moreover,CD4^(+)T cells in the CR cohort showed lower programmed cell death protein 1(PD-1)expression and had lower frequencies of Th1(IFN-γ^(+)),Th2(IL-4^(+)),Th17(IL-17A^(+)),and circulating follicular helper T(CXCR5^(+)PD-1^(+))cells.Accordingly,the proportion of isotype-switched memory B cells(IgM−CD20^(hi))among B cells in the CR cohort showed a significantly lower proportion,although the level of the activation marker CD71 was elevated.For CD3−HLA-DR−lymphocytes in the CR cohort,in addition to lower levels of IFN-γ,granzyme B and T-bet,the correlation between T-bet and IFN-γ was not observed.Additionally,by taking into account the number of days after discharge,all the phenotypes associated with reduced function did not show a tendency toward recovery within 4-11 weeks.The remarkable phenotypic alterations in lymphocytes in the CR cohort suggest that severe acute respiratory syndrome coronavirus 2 infection profoundly affects lymphocytes and potentially results in dysfunction even after clinical recovery.展开更多
基金the National Natural Science Foundation of China(Grant Nos.92252206,12102471 and 11925207)。
文摘Large Eddy Simulation(LES)has been employed for the investigation of supersonic flow characteristics in five ducts with varying cross-sectional geometries.The numerical results reveal that flow channel configurations exert a considerable influence on the mainstream flow and the near-wall flow behavior.In contrast to straight ducts,square-to-circular and rectangular-to-circular ducts exhibit thicker boundary layers and a greater presence of vortex structures.Given the same inlet area,rectangular-to-circular ducts lead to higher flow drag force and total pressure loss than square-to-circular ducts.Characterized by the substantial flow separation and shock waves,the"S-shaped duct shows significant vertically-asymmetric characteristics.
基金supported by the National Natural Science Foundation of China(Nos.11925207 and 92252206)the Hunan Province Graduate Innovation Project,China(No.XJCX2023059)。
文摘To reduce the drag generated by the recirculation flow at the rocket base in a RocketBased Combined Cycle(RBCC)engine operating in the ramjet/scramjet mode,a novel annular rocket RBCC engine based on a central plug cone was proposed.The performance loss mechanism caused by the recirculation flow at the rocket base and the influence of the plug cone configuration on the thrust performance were studied.Results indicated that the recirculation flow at the rocket base extended through the entire combustor,which creates an extensive range of the"low-kineticenergy zone"at the center and leads to an engine thrust loss.The plug cone serving as a surface structure had a restrictive effect on the internal flow of the engine,making it smoothly transit at the position of the large separation zone.The model RBCC engine could achieve a maximum thrust augmentation of 37.6%with a long plug cone that was twice diameter of the inner isolator.However,a shorter plug cone that was half diameter of the inner isolator proved less effective at reducing the recirculation flow for a supersonic flow and induced an undesirable flow fraction that diminished the thrust performance.Furthermore,the effectiveness of the plug cone increased with the flight Mach number,indicating that it could further broaden the operating speed range of the scramjet mode.
基金supported by the National Natural Science Foundation of China (Nos.92252206,11925207,T2221002 and 12102472)。
文摘It has been found that the static pressure distribution along the axial direction of liquid kerosene is lower than that of the gaseous kerosene under the same flow condition and overall equivalent ratio from previous studies.To further investigate this phenomenon,a compressible two-phase parallel simulation method is utilized to analyze the mixing and combustion characteristics of gaseous and liquid kerosene jets in a cavity-based supersonic combustor.The numerical results are consistent with the experiments and demonstrate that gaseous injection leads to a cavity shear layer that dives deeper into the cavity,forming two recirculation zones in the front and rear of the cavity.In contrast,the cavity shear layer is closer to the mainstream during liquid injection,and only a large recirculation zone is formed in the rear of the cavity.As a result of the cavity shear layer and the recirculating flow,the fuel vapor of gaseous injection accumulates in the front of the cavity,while for the liquid injection,the fuel vapor disperses in the cavity,cavity shear layer,and the region above,and the rear of the cavity has a higher fuel vapor concentration than the front.This unique fuel distribution causes the combustion area to be concentrated in the cavity during the gaseous injection but dispersed inside and downstream of the cavity during the liquid injection.As a result,forming a thermal throat under the same conditions is more challenging during liquid injection,and the generated static pressure distribution is lower than that during the gaseous injection.
基金supported by the National Natural Science Foundation of China(Nos.11925207,12002381)the Scientific Research Plan of National University of Defense Technology in 2019,China(No.ZK19-02)+1 种基金the Postgraduate Scientific Research Innovation Project of Hunan Province,China(No.CX20200084)the Equipment Pre-research Foundation of Key Laboratory,China(No.6142703200311)。
文摘The asymmetric separation has a crucial effect on the performance of the scramjet.In this study,the asymmetric separation and combustion in both rectangular and circular scramjets are investigated numerically,and the effect of injection scheme is analyzed.The characteristics of the flow field are analyzed based on sufficient code verification.In the rectangular scramjet,the separation tends to occur in the corner due to the corner boundary-layer effect.The separation is asym-metric and only two corners have serious separation.The fuel penetration depth in the separation zone increases and the combustion is intensified.When the injection scheme is uniform,both the combustion and separation become weak.In the circular scramjet,the separation and combustion are basically axisymmetric in the scramjet with one-row injection scheme.The asymmetric combustion becomes obvious in cases with multi-row injection scheme.When the injection orifices distribute intensively on the top and bottom sides,the strongest and weakest separations occur near these two sides respectively.When the distribution of orifices becomes uniform,the direction of separation cannot be predicted.For multi-row cases,the nonuniform injection scheme could result in violent combustion and asymmetric flow structures compared with the uniform injection scheme.
基金supported by the National Natural Science Foundation of China(Grant Nos.12202488 and 12002377)the Natural Science Program of National University of Defense Technology(Grant No.ZK22-30)Independent Cultivation Project for Young Talents in College of Aerospace Science and Engineering.
基金the support from the National Natural Science Foundation of China(Nos.11902353,12272408,11925207,12102472,and T2221002)the Hunan Provincial Postgraduate Research Innovation Project of China(No.CX20210035)。
文摘The atomization process of a liquid jet in a divergent cavity-based combustor was investigated experimentally using high-speed photography and schlieren techniques under a Mach number 2.0 supersonic crossflow.Gas-liquid flow field was studied at different divergent angles and injection schemes.It is found that complex wave structures exist in the divergent cavity-based combustor.The spray field can be divided into three distinct zones:surface wave-dominated breakup zone,rapid atomization zone and cavity mixing zone.A dimensionless spray factor is defined to describe the concentration of spray inside the cavity qualitatively.As a result,it is revealed that for the large divergent angle cavity,the injection scheme near the upstream inlet has a higher penetration depth but a lower spray distribution,where the injection scheme near the cavity has a more spray distribution.For the small divergent angle cavity,the injection scheme near the upstream inlet also has a higher penetration depth and the injection scheme near the start point of the divergent section has a more sufficient spray distribution.The small divergent angle cavity-based combustor with the upstream wall transverse injection is an optimized injection scheme to improve both penetration and spray distribution inside the cavity.Finally,a penetration depth formula is proposed to explain the spray and distribution behaviors in the divergent cavity-based combustor.
基金supports from the National Natural Science Foundation of China(Nos.11902353,12272408,12102472,11902351,and 12102462)the National Science Fund for Distinguished Young Scholars,China(No.11925207)the Hunan Provincial Postgraduate Research Innovation Project,China(No.CX20210035).
文摘The injection and atomization process of a liquid fuel jet is critical for an ignition start of a scramjet engine.Airwall-mounted crossflow injection strategy is widely used in scramjet combustors,avoiding high total pressure loss and allowing the liquid fuel to rapidly undergo atomization,mixing,and evaporation.In this review,research progress on a liquid jet in supersonic crossflow was evaluated from aspects of atomization mechanism and spray characteristics.When a liquid jet is injected into a supersonic crossflow,primary and secondary breakups occur successively.The surface instability of liquid can significantly affect the breakup process.This review discusses the current understanding of the breakup process and spray characteristics of a liquid jet in supersonic crossflow including the mechanism of atomization and the characteristics of distribution and atomization.The development of windward Rayleigh-Taylor(R-T)unstable waves is the main factor in column breakup.The development of Kelvin-Helmholtz(K-H)unstable waves along the circumferential direction of the jet or droplets is the main factor of surface and droplet breakups.The liquid-gas momentum ratio is the most important factor affecting the penetration depth.The span width of the liquid jet is affected by the windward area.Breakup and coalescence lead to a transformation of the size distribution of droplets from S-or C-shaped to I-shaped,and the velocity distribution of the droplets on the central symmetry plane has a mirrored S-shape.The droplet distribution on the spanwise cross-section retains a structure similar to an“Ω”shape.At last,some promising recommendations have been proposed,namely a theoretical predictive model which can describe the breakup mechanism of a liquid jet,the distribution characteristics and droplets size distribution of a liquid jet under a cavity combustion chamber,especially for enthalpy flows with complex wave structures.
基金supported by the National Natural Science Foundation of China(Nos.12172379,11925207,and 91741205)the Foundation for Outstanding Young Scholars of National University of Defense Technology,China.
文摘The ignition process and flame propagation with ethylene fuel in cavity-stabilized scramjet by a Multi-Channel Gliding Arc(MCGA)at Mach 2.0 were investigated.Effects of equivalence ratios on the MCGA-assisted ignition process and flame propagation of the scramjet were recorded by two high-speed cameras from different view angles.The discharge characteristics of MCGA are also collected synchronously with the high-speed cameras.The distributions of temperature,velocity,and equivalence ratios in non-reactive flows of the cavity were simulated by Reynolds Averaged Navier-Stokes(RANS)model.The results show that MCGA can achieve reliable ignition with the Global Equivalence Ratios(GER)between 0.06 and 0.17.The ignition process is composed of flame kernel generation,flame development,and stable combustion.The time from flame kernel generation to the establishment of global flame decreases as GER decreases from 0.17 to 0.08.In the streamwise direction,the flame first develops to the Cavity Leading Edge(CLE)because of the influence of the cavity recirculation zone and then uplifts into the cavity shear layer,and finally develops to the Cavity Trailing Edge(CTE).In the spanwise direction,the flame width is less than 50%of the width of the cavity before developing to CLE and begins to develop towards the two sides of the combustor after reaching CLE,which is affected by the angular recirculation zone on both sides of CLE.The ignition processes by MCGA in the scramjet combustor are significantly affected by local distributions of equivalence ratios and velocity in the cavity.
基金supported by the National Natural Science Foundation of China(Nos.11925207 and 12002381)the Scientific Research Plan of the National University of Defense Technology in 2019,China(No.ZK19-02)the Science and Technology Foundation of State Key Laboratory,China(No.6142703200311).
文摘The mixing and combustion characteristics in a cavity flameholding combustor under inlet Mach number 2.92 are numerically investigated with ethylene injection.Dimensionless distance is defined as the ratio of the actual distance to the height of the combustor entrance.The cavity shear-layer mode,the lifted cavity shear-layer mode,and jet wake mode with upstream separation are observed respectively with dimensionless distance equals to 1.5,4.5,and 7.5.In both non-reacting and reacting flow fields,the numerical results are essentially in agreement with the schlieren photography,flame chemiluminescence images,and wall pressure,which verify the reliability of the numerical method.The results of non-reacting flow fields show that the BackwardFacing Step(BFS)can promote the flow separation downstream at a fixed distance.The more forward the separation position is,the larger the separation zone is in the non-reacting flow field.Furthermore,the larger the separation zone is,the higher the intensity of combustion in the reacting flow field is.A reasonable distance can reduce the total pressure loss generated by the shock waves in the combustor.The flame presents remarkable three-dimensional characteristics in the reacting flow fields.When dimensionless distance equals to 4.5,there are flames near the side wall above the cavity and it is difficult for the flame stabilization in the center of the combustor,while the combustion intensity in the center of the combustor is higher than that near the side wall when dimensionless distance equals to 7.5.In the cavity flameholding combustors with a backward-facing step,the higher combustion intensity may bring much total pressure loss to the combustor.Thus,it is a good choice to achieve better thrust performance when dimensionless distance equals to 4.5 compared to the other two combustors.
基金the National Natural Science Foundation of China(Nos.12002381 and 11925207)the Science and Technology Foundation of State Key Laboratory,China(No.6142703200311)the Scientific Research Plan of National University of Defense Technology in 2019,China(No.ZK19-02).
文摘Developing supersonic combustion models with efficiency,accuracy and practicality is important foundation to deeply understand the complex combustion processes in scramjet engines.Characterized by efficiency and intuition,the flamelet-like models are widely used models in computational combustion methods.However,the supersonic combustion flow field has the nature of strong compressibility,multiple modality,and multiple scales,which poses a great challenge to the traditional flamelet-like models with fixed boundary conditions,and then the complex chemical reaction mechanisms that may face will impose additional computational burden.In this context,this paper reviews the flamelet-like models used in scramjet engines,and summarizes prominent issues in the application practice,including modeling partially premixed combustion,defining progress variable,solving temperature efficiently,evaluating assumed Probability Density Function(PDF)models,and treating mixture fraction variance.Furthermore,possible prospects and directions of improvements are proposed and highlighted for the flamelet-like models.To fully describe the physicochemical scenario and address the raised challenges,these improvements are dedicated to dealing with the compressibility,temperature rise,time-scales,species of interest,multi-inlet combustion,the progress variable definition,and the higher Mach number flight condition.
基金supported by the National Natural Science Foundation of China(Nos.11925207,11902353 and 91741205)the Foundation of Innovation-oriented Province Construction of Hunan(No.2019RS2028)。
文摘The combustion modes in two different scramjet combustors with the mass flow rates of 1.8 kg/s and 3.6 kg/s are experimentally investigated to explore the scaling effects on supersonic combustion with a Mach number 2.0 inflow.It is found that the scramjet combustor with a larger scale can broaden the flame rich blowout limit.As the Equivalence Ratio(ER)increases,the combustion in the small-scale combustor maintains in the cavity-stabilized mode,and the flamebase moves downstream along the cavity shear layer;however,the combustion in the large-scale combustor gradually transfers from the cavity-stabilized mode to the jet-wake-stabilized mode.The differences in the cavity residence time,the ignition delay time and the Damkohler number caused by different scales of the scramjet combustor are likely to account for the scaling effects on the combustion modes.
基金supported by the Major Projects of Technological Innovation in Hubei Province(2019ABA089)the Kunming Science and Technology Department(2020-1-N-037).
文摘Although millions of patients have clinically recovered from COVID-19,little is known about the immune status of lymphocytes in these individuals.In this study,the peripheral blood mononuclear cells of a clinically recovered(CR)cohort were comparatively analyzed with those of an age-and sex-matched healthy donor cohort.We found that CD8^(+)T cells in the CR cohort had higher numbers of effector T cells and effector memory T cells but lower Tc1(IFN-γ^(+)),Tc2(IL-4^(+)),and Tc17(IL-17A^(+))cell frequencies.The CD4^(+)T cells of the CR cohort were decreased in frequency,especially the central memory T cell subset.Moreover,CD4^(+)T cells in the CR cohort showed lower programmed cell death protein 1(PD-1)expression and had lower frequencies of Th1(IFN-γ^(+)),Th2(IL-4^(+)),Th17(IL-17A^(+)),and circulating follicular helper T(CXCR5^(+)PD-1^(+))cells.Accordingly,the proportion of isotype-switched memory B cells(IgM−CD20^(hi))among B cells in the CR cohort showed a significantly lower proportion,although the level of the activation marker CD71 was elevated.For CD3−HLA-DR−lymphocytes in the CR cohort,in addition to lower levels of IFN-γ,granzyme B and T-bet,the correlation between T-bet and IFN-γ was not observed.Additionally,by taking into account the number of days after discharge,all the phenotypes associated with reduced function did not show a tendency toward recovery within 4-11 weeks.The remarkable phenotypic alterations in lymphocytes in the CR cohort suggest that severe acute respiratory syndrome coronavirus 2 infection profoundly affects lymphocytes and potentially results in dysfunction even after clinical recovery.