The three-dimensional Navier Stokes equation and the k-ε viscous model are used to simulate the attack angle characteristics of a hemisphere nose-tip with an opposing jet thermal protection system in supersonic flow ...The three-dimensional Navier Stokes equation and the k-ε viscous model are used to simulate the attack angle characteristics of a hemisphere nose-tip with an opposing jet thermal protection system in supersonic flow conditions. The numerical method is validated by the relevant experiment. The flow field parameters, aerodynamic forces, and surface heat flux distributions for attack angles of 0°, 2°, 5°, 7°, and 10° are obtained. The detailed numerical results show that the cruise attack angle has a great influence on the flow field parameters, aerodynamic force, and surface heat flux distribution of the supersonic vehicle nose-tip with an opposing jet thermal protection system. When the attack angle reaches 10°, the heat flux on the windward generatrix is close to the maximal heat flux on the wall surface of the nose-tip without thermal protection system, thus the thermal protection has failed.展开更多
This paper experimentally studied the effect of CO_(2) opposing multiple jets on the thermoacoustic instability and NO_(x) emissions in a lean-premixed model combustor.The feasibility was verified from three variables...This paper experimentally studied the effect of CO_(2) opposing multiple jets on the thermoacoustic instability and NO_(x) emissions in a lean-premixed model combustor.The feasibility was verified from three variables:the CO_(2) jet flow rate,hole numbers,and hole diameters of the nozzles.Results indicate that the control effect of thermoacoustic instability and NO_x emissions show a reverse trend with the increase of open area ratio on the whole,and the optimal jet flow rate range is 1-4 L/min with CO_(2) opposing multiple jets.In this flow rate range,the amplitude and frequency of the dynamic pressure and heat release signals CH~* basically decrease as the CO_(2) flow rate increases,which avoids high-frequency and high-amplitude thermoacoustic instability.The amplitude-damped ratio of dynamic pressure and CH*can reach as high as 98.75% and 93.64% with an optimal open area ratio of 3.72%.NO_(x) emissions also decrease as the jet flow rate increases,and the maximum suppression ratio can reach 68.14%.Besides,the flame shape changes from a steep inverted "V" to a more flat "M",and the flame length will become shorter with CO_(2) opposing multiple jets.This research achieved the synchronous control of thermoacoustic instability and NO_(x) emissions,which could be a design reference for constructing a safer and cleaner combustor.展开更多
A preliminary study on the criterion for opposed jet flame stabilization is reported. The experimental investigation consists of the measurement of flame blowoff by means of premixed lean liquefied petroleum gas and t...A preliminary study on the criterion for opposed jet flame stabilization is reported. The experimental investigation consists of the measurement of flame blowoff by means of premixed lean liquefied petroleum gas and the temperature distribution. The criterion for flame stabilization has been obtained. It is correlated with main stream velocity VPm, mass flux ratio of jet to main streamF, internal diameter of jet nozzled; and equivalent ratio of fuel-air ?. The stabilization character of opposed jet flame is described as a well-stirred reactor, and the critical zone acts as a strong ignition source to maintain flame stabilization. A criterion for flame stabilization by theoretical analyses is in good accordance with experimental results.展开更多
The technique of the use of multi-annular opposed jets as different from using swirl and bluff body creates an excellent recirculation zone with desired size in a large space.The size of recirculation,the magnitude of...The technique of the use of multi-annular opposed jets as different from using swirl and bluff body creates an excellent recirculation zone with desired size in a large space.The size of recirculation,the magnitude of reverse velocity and turbulence intensity are much greater than those formed by bluff body.Factors affecting the flowfield include the velocity ratio of the opposed jets to the primary air J,the diameter and construction of the opposed jet ring,secondary air velocity and configuration,and confined or unconfined flow condition and so on.This method is a promising way for flame stabilization in combustion technology.展开更多
Fluidized bed opposed jet mills are capable of meeting the continuously growing dema nd for contamination-free fine particles.In this type of jet mill,the solid material is entrained and accelerated by expanding gas j...Fluidized bed opposed jet mills are capable of meeting the continuously growing dema nd for contamination-free fine particles.In this type of jet mill,the solid material is entrained and accelerated by expanding gas jets that are focused onto a focal point in side a fluidized bed.The resulting particle collisions induce breakage.The process is affected by the relative particle velocities and the number of particle-particle collisions.Clearly,both quantities are distributed.However,to date,neither relative particle velocities nor collision frequencies in such units have been determined.The present work introduces an innovative method to assess the stressing conditions in jet mills experimentally.To this end,mixtures of glass and ductile metal microspheres were used,with the latter employed in small amounts.Inter-particle collisions between the aluminum and glass spheres lead to the formation of dents on the microparticles.The size and number of these dents are associated with the individual collision velocities and overall collision frequencies.The correlation between dent size and collision velocity was obtained from finite element calculations based on empirical data.The proposed approach was validated using particle image velocimetry during secondary gas injection into a fluidized bed reactor.In this case the effect of the distance between two opposed nozzles was examined.For a lab-scaled fluidized bed opposed jet mill the effects of gas pressure and hold-up were investigated.Relative particle velocities were found to be sign ificantly lower tha n the gas velocities,while the nu mber of contacts per particle was determined to be extremely high.展开更多
Experimental study and numerical simulation were performed for the averaged flow field of two closely spaced opposed jets with hot wire anemometer(HWA)and CFD software and the simulation was compared with the measurem...Experimental study and numerical simulation were performed for the averaged flow field of two closely spaced opposed jets with hot wire anemometer(HWA)and CFD software and the simulation was compared with the measurement and approximate analytic solution in the literature.The results showed that the exit velocity profile of the single jet was top-hat distribution due to the presence of boundary layers.At L<2D(where L is nozzle separation and D is nozzle diameter),bimodal distribution of exit velocity profile,low in the middle and high on both sides,was present,while such bimodal distribution of exit velocity profile was absent at L=2D.With increasing nozzle separation,the stagnation point offset of the impinging plane increased.With the same velocity ratio,stagnation point offset of the nozzles with uniform profiles was larger than that with top-hat profiles.The approximate analytic solution of the flow field of two opposed jets in the literature was only valid for the nozzles with uniform exit velocity profiles and the prediction accuracy became worse for the nozzles with top-hat exit velocity profiles.展开更多
The oscillation characteristics of two planar opposed jets were studied by using high-speed camera,smoke wire visualization and hot-wire anemometry(HWA).The oscillation periods of two planar opposed jets at different ...The oscillation characteristics of two planar opposed jets were studied by using high-speed camera,smoke wire visualization and hot-wire anemometry(HWA).The oscillation periods of two planar opposed jets at different nozzle separations and exit nozzle velocities were studied.The results showed two models of instabilities:streamwise quasi-periodic oscillation model and deflection oscillation model.The oscillation model was mainly streamwise quasi-periodic oscillation model at L<5H(where L was the nozzle separation and H was the nozzle slot height),and it was mainly deflection oscillation model at L>5H.The deflection oscillation model had a stable period which decreased with increasing normalized nozzle separation.展开更多
基金Project supported by the National Natural Science Foundation of China (Grant No. 90916018)the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 200899980006)the Natural Science Foundation of Hunan Province,China(Grant No. 09JJ3109)
文摘The three-dimensional Navier Stokes equation and the k-ε viscous model are used to simulate the attack angle characteristics of a hemisphere nose-tip with an opposing jet thermal protection system in supersonic flow conditions. The numerical method is validated by the relevant experiment. The flow field parameters, aerodynamic forces, and surface heat flux distributions for attack angles of 0°, 2°, 5°, 7°, and 10° are obtained. The detailed numerical results show that the cruise attack angle has a great influence on the flow field parameters, aerodynamic force, and surface heat flux distribution of the supersonic vehicle nose-tip with an opposing jet thermal protection system. When the attack angle reaches 10°, the heat flux on the windward generatrix is close to the maximal heat flux on the wall surface of the nose-tip without thermal protection system, thus the thermal protection has failed.
基金supported by the National Science Fund for Distinguished Young Scholars (Grant No. 51825605)。
文摘This paper experimentally studied the effect of CO_(2) opposing multiple jets on the thermoacoustic instability and NO_(x) emissions in a lean-premixed model combustor.The feasibility was verified from three variables:the CO_(2) jet flow rate,hole numbers,and hole diameters of the nozzles.Results indicate that the control effect of thermoacoustic instability and NO_x emissions show a reverse trend with the increase of open area ratio on the whole,and the optimal jet flow rate range is 1-4 L/min with CO_(2) opposing multiple jets.In this flow rate range,the amplitude and frequency of the dynamic pressure and heat release signals CH~* basically decrease as the CO_(2) flow rate increases,which avoids high-frequency and high-amplitude thermoacoustic instability.The amplitude-damped ratio of dynamic pressure and CH*can reach as high as 98.75% and 93.64% with an optimal open area ratio of 3.72%.NO_(x) emissions also decrease as the jet flow rate increases,and the maximum suppression ratio can reach 68.14%.Besides,the flame shape changes from a steep inverted "V" to a more flat "M",and the flame length will become shorter with CO_(2) opposing multiple jets.This research achieved the synchronous control of thermoacoustic instability and NO_(x) emissions,which could be a design reference for constructing a safer and cleaner combustor.
文摘A preliminary study on the criterion for opposed jet flame stabilization is reported. The experimental investigation consists of the measurement of flame blowoff by means of premixed lean liquefied petroleum gas and the temperature distribution. The criterion for flame stabilization has been obtained. It is correlated with main stream velocity VPm, mass flux ratio of jet to main streamF, internal diameter of jet nozzled; and equivalent ratio of fuel-air ?. The stabilization character of opposed jet flame is described as a well-stirred reactor, and the critical zone acts as a strong ignition source to maintain flame stabilization. A criterion for flame stabilization by theoretical analyses is in good accordance with experimental results.
文摘The technique of the use of multi-annular opposed jets as different from using swirl and bluff body creates an excellent recirculation zone with desired size in a large space.The size of recirculation,the magnitude of reverse velocity and turbulence intensity are much greater than those formed by bluff body.Factors affecting the flowfield include the velocity ratio of the opposed jets to the primary air J,the diameter and construction of the opposed jet ring,secondary air velocity and configuration,and confined or unconfined flow condition and so on.This method is a promising way for flame stabilization in combustion technology.
基金the German Research Foundation through the DFG priority program 1679"Dynamic Simulation of Interconnected Solids Processes".
文摘Fluidized bed opposed jet mills are capable of meeting the continuously growing dema nd for contamination-free fine particles.In this type of jet mill,the solid material is entrained and accelerated by expanding gas jets that are focused onto a focal point in side a fluidized bed.The resulting particle collisions induce breakage.The process is affected by the relative particle velocities and the number of particle-particle collisions.Clearly,both quantities are distributed.However,to date,neither relative particle velocities nor collision frequencies in such units have been determined.The present work introduces an innovative method to assess the stressing conditions in jet mills experimentally.To this end,mixtures of glass and ductile metal microspheres were used,with the latter employed in small amounts.Inter-particle collisions between the aluminum and glass spheres lead to the formation of dents on the microparticles.The size and number of these dents are associated with the individual collision velocities and overall collision frequencies.The correlation between dent size and collision velocity was obtained from finite element calculations based on empirical data.The proposed approach was validated using particle image velocimetry during secondary gas injection into a fluidized bed reactor.In this case the effect of the distance between two opposed nozzles was examined.For a lab-scaled fluidized bed opposed jet mill the effects of gas pressure and hold-up were investigated.Relative particle velocities were found to be sign ificantly lower tha n the gas velocities,while the nu mber of contacts per particle was determined to be extremely high.
文摘Experimental study and numerical simulation were performed for the averaged flow field of two closely spaced opposed jets with hot wire anemometer(HWA)and CFD software and the simulation was compared with the measurement and approximate analytic solution in the literature.The results showed that the exit velocity profile of the single jet was top-hat distribution due to the presence of boundary layers.At L<2D(where L is nozzle separation and D is nozzle diameter),bimodal distribution of exit velocity profile,low in the middle and high on both sides,was present,while such bimodal distribution of exit velocity profile was absent at L=2D.With increasing nozzle separation,the stagnation point offset of the impinging plane increased.With the same velocity ratio,stagnation point offset of the nozzles with uniform profiles was larger than that with top-hat profiles.The approximate analytic solution of the flow field of two opposed jets in the literature was only valid for the nozzles with uniform exit velocity profiles and the prediction accuracy became worse for the nozzles with top-hat exit velocity profiles.
文摘The oscillation characteristics of two planar opposed jets were studied by using high-speed camera,smoke wire visualization and hot-wire anemometry(HWA).The oscillation periods of two planar opposed jets at different nozzle separations and exit nozzle velocities were studied.The results showed two models of instabilities:streamwise quasi-periodic oscillation model and deflection oscillation model.The oscillation model was mainly streamwise quasi-periodic oscillation model at L<5H(where L was the nozzle separation and H was the nozzle slot height),and it was mainly deflection oscillation model at L>5H.The deflection oscillation model had a stable period which decreased with increasing normalized nozzle separation.