Jet ventilation is widely used in the ventilation design of highway and railway tunnels as an important air supply method during tunnel operation and disaster periods.This ventilation method has also been applied for ...Jet ventilation is widely used in the ventilation design of highway and railway tunnels as an important air supply method during tunnel operation and disaster periods.This ventilation method has also been applied for fire control in immersed tunnels.We conduct numerical simulations using computational fluid dynamics(CFD)to study positive ventilation in the upstream and reverse ventilation in the downstream(P-R)for an extra-wide immersed tunnel.The effects of fire source location and jet fan air velocity response strategy on the ceiling temperature decay,carbon monoxide(CO)distribution,and smoke exhaust efficiency were investigated for varying fire source locations.The results show that flames will be tilted to the side of the jet fan with a smaller air velocity.Additionally,the jet fan air velocity should be adjusted based on the relative distance between the fire source and the smoke vent.Among the studied scenarios,the most effective outcome was achieved when the air velocity was adjusted to 25 m/s on the side near the smoke vent.Also in this scenario,the phenomenon of smoke deposition was effectively mitigated and the average smoke exhaust efficiency reached 87%.Moreover,we found that the temperature decay of the tunnel follows an exponential decay law.The temperature decay rate is significantly higher on the side closest to the smoke vent compared to the farther side.This research provides a theoretical basis for smoke control strategies for fires that occur in immersed tunnels.展开更多
Hursts rescaled range (R/S) analysis and Wolfs attractor reconstruction technique have been adopted to estimate the local fractal dimensions and the local largest Lyapunov exponents in terms of the time series pressur...Hursts rescaled range (R/S) analysis and Wolfs attractor reconstruction technique have been adopted to estimate the local fractal dimensions and the local largest Lyapunov exponents in terms of the time series pressure fluctuations obtained from a gas liquid solid three phase self aspirated reversed flow jet loop reactor,respectively.The results indicate that the local fractal dimensions and the local largest Lyapunov exponents in both the jet region and the tubular region inside the draft tube increase with the increase in the jet liquid flowrates and the solid loadings,the local fractal dimension profiles are similar to those of the largest Lyapunov exponent,the local largest lyapunov exponents are positive for all cases,and the flow behavior of such a reactor is chaotic.The local nonlinear characteristic parameters such as the local fractal dimension and the local largest Lyapunov exponent could be applied to further study the flow properties such as the flow regime transitions and flow structures of this three phase jet loop reactor.展开更多
The local chaos characteristics of the time series pressure fluctuations of gas liquid two phase flow in a self aspirated reversed flow jet loop reactor are studied by the deterministic chaos analysis technique. It...The local chaos characteristics of the time series pressure fluctuations of gas liquid two phase flow in a self aspirated reversed flow jet loop reactor are studied by the deterministic chaos analysis technique. It is found that the estimated local largest Lyapunov exponent is positive in all cases and the profile is similar to that of the local fractal dimension in this reactor. The positive largest Lyapunov exponent shows that the reactor is a nonlinear chaotic system. The obvious distribution indicates that the local nonlinear characteristic parameters such as the Lyapunov exponent and the fractal dimension could be applied to further study the flow characteristics such as the flow regine transitions and flow structures of the multi phase reactors.展开更多
The local liquid--phase characteristics of the gas--liquid two-phase and gas--liquid--solid threephase self-aspirated reversed flow jet loop reactor with a concentric gas--liquid injection nozzle were studied experime...The local liquid--phase characteristics of the gas--liquid two-phase and gas--liquid--solid threephase self-aspirated reversed flow jet loop reactor with a concentric gas--liquid injection nozzle were studied experimentally. They facilitate the evaluation of local phenomena. The local instantaneous liquid velocities at different axial positions of the reactor were measured by using the modified pilot tube.The local liquid-phase turbulent structural parameters such as time-averaged velocity. turbulent nuctuating velocity and turbulent micro scale were calculated with the aid of the statistical theory of turbulence. In particular, effects of liquid jet flowrates and solid loadings on the profiles of the liquid--phase turbulent structural parameter both in the jet effective region and in the tubular region inside the draft tube were discussed.展开更多
According to the behaviors of a bubble in the ship wake flow, the numericai simulation is divided into two stages, quasi-spherical motion and non-spherical motion, based on whether the bubble is captured by the vortex...According to the behaviors of a bubble in the ship wake flow, the numericai simulation is divided into two stages, quasi-spherical motion and non-spherical motion, based on whether the bubble is captured by the vortex or not. The one-way coupled particle tracking method （PTM） and the boundary element method （BEM） are adopted to simulate these two stages, respectively. Meanwhile, the initial condition of the second stage is taken as the output of the first one, and the entire simulation is connected and completed. Based on the numerical results and the published experimental data, the cavitation inception is studied, and the wake bubble is tracked. Besides, the split of the bubble captured by the vortex and the following sub-bubbles are simulated, including motion, deformation, and collapse. The results provide some insights into the control on wake bubbles and optimization of the wake flow.展开更多
If cuttings carrying performance is poor and cuttings removal is not in time during the drilling of horizontal wells,drilling cuttings will accumulate in the lower sections,leading to backing pressure,BHA binding and ...If cuttings carrying performance is poor and cuttings removal is not in time during the drilling of horizontal wells,drilling cuttings will accumulate in the lower sections,leading to backing pressure,BHA binding and even drill pipe sticking.In this paper,a new type of Helmholtz pulse grinding bits suitable for horizontal wells was designed based on the theory of Helmholtz oscillation chamber to generate pulse,jet pump and high pressure jet after the formation of cuttings beds was analyzed.In this type of bit,a high-speed pulse jet is used to assist rock breaking,a reverse jet is used to remove the cuttings at the bottom of the bit under negative pressure,and its inner grinding structure is used to reduce the particle size of cuttings.By using this bit,efficient cuttings removal and rock breaking will be both realized,the chip hold-down effect will be reduced and the cuttings beds in a horizontal well will be also removed.Then,the hydraulic models were established for a pulse generation device,an efficient rock breaking device and a reverse swabbing device,respectively.It is shown from the simulation results that the optimal resonance flowrate increases with the increase of the diameters of an inlet chamber and a feedback chamber and with the decrease of the diameter of a resonance chamber,and it is approximately in linear relationship with each factor.The optimal flowrate ratio of the reverse swabbing device increases first and then decreases with the increase of dimensionless flowrate ratio,and decreases with the increase of dimensionless area ratio.It is indicated from example analysis that the inherent frequency of Helmholtz oscillation chamber is 24.00 Hz,the optimal oscillation flowrate is 23.92 L/s and the optimal flowrate ratio is 0.59.Based on case studies,the accuracy of hydraulic models is verified.It is concluded that this new type of bits provides a new solution to the accumulation of cuttings beds.展开更多
文摘Jet ventilation is widely used in the ventilation design of highway and railway tunnels as an important air supply method during tunnel operation and disaster periods.This ventilation method has also been applied for fire control in immersed tunnels.We conduct numerical simulations using computational fluid dynamics(CFD)to study positive ventilation in the upstream and reverse ventilation in the downstream(P-R)for an extra-wide immersed tunnel.The effects of fire source location and jet fan air velocity response strategy on the ceiling temperature decay,carbon monoxide(CO)distribution,and smoke exhaust efficiency were investigated for varying fire source locations.The results show that flames will be tilted to the side of the jet fan with a smaller air velocity.Additionally,the jet fan air velocity should be adjusted based on the relative distance between the fire source and the smoke vent.Among the studied scenarios,the most effective outcome was achieved when the air velocity was adjusted to 25 m/s on the side near the smoke vent.Also in this scenario,the phenomenon of smoke deposition was effectively mitigated and the average smoke exhaust efficiency reached 87%.Moreover,we found that the temperature decay of the tunnel follows an exponential decay law.The temperature decay rate is significantly higher on the side closest to the smoke vent compared to the farther side.This research provides a theoretical basis for smoke control strategies for fires that occur in immersed tunnels.
文摘Hursts rescaled range (R/S) analysis and Wolfs attractor reconstruction technique have been adopted to estimate the local fractal dimensions and the local largest Lyapunov exponents in terms of the time series pressure fluctuations obtained from a gas liquid solid three phase self aspirated reversed flow jet loop reactor,respectively.The results indicate that the local fractal dimensions and the local largest Lyapunov exponents in both the jet region and the tubular region inside the draft tube increase with the increase in the jet liquid flowrates and the solid loadings,the local fractal dimension profiles are similar to those of the largest Lyapunov exponent,the local largest lyapunov exponents are positive for all cases,and the flow behavior of such a reactor is chaotic.The local nonlinear characteristic parameters such as the local fractal dimension and the local largest Lyapunov exponent could be applied to further study the flow properties such as the flow regime transitions and flow structures of this three phase jet loop reactor.
文摘The local chaos characteristics of the time series pressure fluctuations of gas liquid two phase flow in a self aspirated reversed flow jet loop reactor are studied by the deterministic chaos analysis technique. It is found that the estimated local largest Lyapunov exponent is positive in all cases and the profile is similar to that of the local fractal dimension in this reactor. The positive largest Lyapunov exponent shows that the reactor is a nonlinear chaotic system. The obvious distribution indicates that the local nonlinear characteristic parameters such as the Lyapunov exponent and the fractal dimension could be applied to further study the flow characteristics such as the flow regine transitions and flow structures of the multi phase reactors.
文摘The local liquid--phase characteristics of the gas--liquid two-phase and gas--liquid--solid threephase self-aspirated reversed flow jet loop reactor with a concentric gas--liquid injection nozzle were studied experimentally. They facilitate the evaluation of local phenomena. The local instantaneous liquid velocities at different axial positions of the reactor were measured by using the modified pilot tube.The local liquid-phase turbulent structural parameters such as time-averaged velocity. turbulent nuctuating velocity and turbulent micro scale were calculated with the aid of the statistical theory of turbulence. In particular, effects of liquid jet flowrates and solid loadings on the profiles of the liquid--phase turbulent structural parameter both in the jet effective region and in the tubular region inside the draft tube were discussed.
基金Project supported by the Key Program of National Natural Science Foundation of China(No.50939002)the Joint Fund of the National Natural Science Foundation of China and the China Academy of Engineering Physics (No.10976008)the National Defense Basic Scientific Research Program of China (No.B2420110011)
文摘According to the behaviors of a bubble in the ship wake flow, the numericai simulation is divided into two stages, quasi-spherical motion and non-spherical motion, based on whether the bubble is captured by the vortex or not. The one-way coupled particle tracking method （PTM） and the boundary element method （BEM） are adopted to simulate these two stages, respectively. Meanwhile, the initial condition of the second stage is taken as the output of the first one, and the entire simulation is connected and completed. Based on the numerical results and the published experimental data, the cavitation inception is studied, and the wake bubble is tracked. Besides, the split of the bubble captured by the vortex and the following sub-bubbles are simulated, including motion, deformation, and collapse. The results provide some insights into the control on wake bubbles and optimization of the wake flow.
文摘If cuttings carrying performance is poor and cuttings removal is not in time during the drilling of horizontal wells,drilling cuttings will accumulate in the lower sections,leading to backing pressure,BHA binding and even drill pipe sticking.In this paper,a new type of Helmholtz pulse grinding bits suitable for horizontal wells was designed based on the theory of Helmholtz oscillation chamber to generate pulse,jet pump and high pressure jet after the formation of cuttings beds was analyzed.In this type of bit,a high-speed pulse jet is used to assist rock breaking,a reverse jet is used to remove the cuttings at the bottom of the bit under negative pressure,and its inner grinding structure is used to reduce the particle size of cuttings.By using this bit,efficient cuttings removal and rock breaking will be both realized,the chip hold-down effect will be reduced and the cuttings beds in a horizontal well will be also removed.Then,the hydraulic models were established for a pulse generation device,an efficient rock breaking device and a reverse swabbing device,respectively.It is shown from the simulation results that the optimal resonance flowrate increases with the increase of the diameters of an inlet chamber and a feedback chamber and with the decrease of the diameter of a resonance chamber,and it is approximately in linear relationship with each factor.The optimal flowrate ratio of the reverse swabbing device increases first and then decreases with the increase of dimensionless flowrate ratio,and decreases with the increase of dimensionless area ratio.It is indicated from example analysis that the inherent frequency of Helmholtz oscillation chamber is 24.00 Hz,the optimal oscillation flowrate is 23.92 L/s and the optimal flowrate ratio is 0.59.Based on case studies,the accuracy of hydraulic models is verified.It is concluded that this new type of bits provides a new solution to the accumulation of cuttings beds.
