Flow choking characteristics of slit-type energy dissipaters

In this paper, the behavior of the flow choking, including the critical and developing states, was experimentally investiga- ted by means of five slit-type outlets, characterized by the outlet width, the contraction angle and the opening of the working gate. The results showed that the approach flow Froude number of the critical choking decreases if the outlet width increases, or the con- traction angle decreases, or the opening increases. There is the hysteresis when the flow choking develops, i.e., the Froude numbers of the appearance and disappearance of the flow choking at the increasing discharge regime are all larger than those at the decreasing discharge regime. For various widths of the outlets, the differences between the critical Froude numbers at small opening are much larger than thoset at large opening. The change range from appearance to disappearance of the flow choking at small opening is smaller than that at large opening.

Flow choking over weir flow slit-type flip buckets

The flow choking may occur for weir flow slit-type flip buckets trader common operation conditions. An estimation method is developed through introducing a comprehensive coefficient to determine the approach flow Froude number for the flow choking to occur in those flip buckets. The error of the present method relative to the experimental data is less than 5%. The results show that, the Froude number for the flow choking to occur is related to the contraction ratio and the contraction angle of the flip buckets. When the flow choking occurs, the upper jet trajectory decreases and the lower one is almost not affected, and the dynamic pressures on the bottom and the sidewalls increase due to the flow profile rising on the flip buckets.

Analysis of choked two-phase flows of gas and particle in a C-D nozzle

Particle-gas two-phase flows show significantly different behaviors compared to single gas flow through a convergent-divergent nozzle. Non-equilibrium effects, thermal and velocity lag results to the inefficiency of nozzle performance. In the present studies, theoretical analysis and numerical simulations were carried out to investigate particle-gas flows in a C-D nozzle. Homogeneous equilibrium model that no lag in velocity and temperature occurs between particles and gas phase was used to derive mass flow rate and sound speed of multiphase flows. Two-phase flows are regarded as isentropic flows that isentropic relations can be used for homogeneous equilibrium model. Discrete phase model （DPM） where interaction with continuous phase and discrete random walk model were considered was used to calculate particle- gas flows. Particle mass loadings were varied to investigate their effects on choking phenomena of particle-gas flows. Mass flow rate and sound speed of mixture flows were theoretically calculated by homogeneous equilibrium model and compared with numerical results. Shock wave structure and particle number density were also obtained to be different at different particle mass loading and operating pressure conditions.

An experimental study on the choked flow characteristics of CO_(2) pipelines in various phases

High pressure pipeline transportation has been an established technology for economically transporting large amounts of CO_(2).However,there are still issues and associated risks that have to be effectively addressed and adequately understood.It is well known that a strong JouleThomson Cooling effect can occur when pressurized CO_(2) flows through a choke valve.Thus,to investigate the choking characteristics especially the temperature drop of high pressure CO_(2),a new laboratory scale experimental setup(total length of 14.85 m and the inner diameter of 15 mm)was constructed.Steady choked flow and transient choked flow tests were carried out respectively for pressurized CO_(2) in various initial phases.The phase transitions and temperature drop characteristics were then studied following the choked flow and the results show that the phase transitions in steady choked flow differs significantly from that in transient choked flow.For transient choked flow of various initial phases,all the flows downstream would transfer from single phase to gas-liquid twophase flow.Furthermore,the effect of water on transient choked flow of supercritical CO_(2) pipeline was investigated,and the phenomena of solid particles deposition was captured which was paramount importance of ensuring the safety operation of CO_(2)pipelines when throttling by the choke valves.

Delivery of inert gas through a vertical borehole using inert gas generator: A theoretical study

The delivery of the inert gas through a vertical borehole using inert gas generator or IGG is investigated.Potential limitations and/or transient effects are highlighted.During the analysis,the borehole diameter,borehole length,type of borehole and partial condensation prior to entering the borehole were varied.A choked flow will occur for a contraction exit or borehole of 0.3 m in diameter if no condensation prior to the contraction occurs.If partial condensation takes place,a borehole diameter of 0.3 m will be possible if almost 50%of the water vapour condensates.However,pressure losses along boreholes with a diameter of 0.3 or 0.4 m are significant and could pose a challenge if trying to mitigate the pressure losses.Adding a booster fan prior to the inlet of the 0.4 m lined borehole would still be a challenge.The corresponding case with a 0.5 m borehole presents much more favourable pressure losses.The 0.5 m diameter lined borehole should be regarded as the lower threshold.The rapid heating of the unlined borehole surface will increase the risk of thermal spallation and possibly imposing restrictions.Understanding the mechanisms during gas delivery will increase the likelihood of a successful inertisation.

Hydraulic jump and choking of flow in pipe with a change of slope

This paper investigates hydraulic jumps in sloping pipes by means of wall-resolved large eddy simulation(LES).The purpose is to achieve an improved understanding of jump behaviours driven by pipe discharge and slope.The LES model predicts the hydraulic jump as a 3-D two-phase flow,with air as the gas phase and water as the liquid phase.The predictions yield instantaneous velocity and pressure fields as well as fluid volume fraction.The instantaneous flow variables allow ensemble averages,which quantify the internal structures and integral properties of the hydraulic jump.The predicted instantaneous velocity shows spectra in consistency with the well-known Kolmogorov−5/3 law.The ensemble averages of air and water velocities,free-surface profile,roller length and aeration length,compare well with available experimental data.The jump behaviours are complex.Some aspects such as free-surface fluctuation and jump-toe oscillation resemble the classical hydraulic jump on horizontal floors.Others like the 3-D distributions of core jet,vorticity and aeration are much more complicated.Depending on the pipe discharge and slope,the resulting jump can be a complete or an incomplete jump.The incomplete hydraulic jump causes choked flow downstream.This has severe consequences on drainage conditions in sewer pipes laid on sloping terrain.This paper proposes using the Okubo-Weiss parameter as a new way to subtly delineate the region of hydraulic jump.It is much more efficient and less ambiguous,compared with traditional visual inspections.

The Effect of Secondary Flows on the Starting Pressure for a Second-throat Supersonic Ejector

The effect of the secondary flow on the starting pressure of a second-throat supersonic ejector has been investigated by adapting the height of the secondary flow inlet.The obtained results show that an optimum value of the secondary inlet height exists,and the starting pressure of the ejector becomes a minimum at that condition.Based on the results of the pressure measurements,a qualitative analysis has been made to clarify the flow behavior and the physical meaning of the performance diagram.It appears that the choking phenomenon of the secondary flow plays an important role in the starting process of the ejector.When the secondary inlet height is relatively small,the choked secondary flow and the supersonic primary flow could be employed to protect the static pressure in the suction chamber from being disturbed by the back pressure effect at a certain primary stagnation pressure,which is lower than the starting pressure for the case of the zero-secondary flow.However,as the secondary inlet height increases and exceeds a critical value,the static pressure in the suction chamber rapidly increases,and the starting pressure of the ejector increases accordingly.

A Study of the Gas Flow through a LNG Safety Valve

A safety valve functions to control an upper limit of pressure inside the LNG line of transportation. If the pressure inside the safety valve nozzle exceeds a predetermined value on the valve sheet which plugs the nozzle, an excess of LNG discharges through the gap between the nozzle exit and valve sheet. In this situation, the forces acting on the valve sheet are gasdynamic forces generated by the discharge of LNG and mechanical forces supported by the spring behind the valve sheet. The flow through the gap is very complicated, involving vortices, flow separation, and shock waves. These affect adversely on the system accompanying with noise and vibration. The present study aims at understanding the flow physics of safety valve. A computational work using the twodimensional, axisymmetric, compressible Navier-Stokes equations is carried out to simulate the gas flow between the nozzle exit and valve sheet, and compared with the theoretical results. It has been found that there exists a distance between nozzle exit and valve sheet in which the thrust coefficient at the valve sheet increases abruptly.

Study of Choked Flows through a Convergent Nozzle

When sonic nozzles of significantly smaller diameter are used as standard flow meters,the critical back pressure ratio is affected by the boundary layer at the nozzle throat.However,the effect of the boundary layer on choking criteria is still controversial.Then,the choking phenomenon of a convergent nozzle flow has been experimentally investigated using four convergent nozzles with the same diameter followed by a straight pipe of a variable length.As a result,it is shown that the critical back pressure ratio is smaller than that for the steady one-dimensional is-entropic flow and decreases as the boundary layer thickness increases.Moreover,the main flow Mach number at the nozzle exit is supersonic when the back pressure ratio is equivalent to the choking condition,and the Mach number increases as the boundary layer thickness increases.

Flow Characteristics and Aerodynamic Heating of Tube Trains in Choked/Unchoked Flow:A Numerical Study

Hyperloop has become one of the key reserve technologies for future high-speed rail transit.The gas in the tube is compressed and rubbed,leading to a strong aerodynamic heating effect.The research on the flow field characteristics and aerodynamic heating effect of hyperloop is in its infancy,and that on the flow field structure is lacking.In this study,the nozzle theory was used to make a preliminary judgment on the choked flow phenomenon in the hyperloop.Based on the flow results obtained under different working conditions,the identification basis of the choked flow phenomenon in the hyperloop was obtained.Furthermore,the effect of the choked/unchoked flow on the flow structure,temperature,and pressure distribution of the annular space in the tube was analyzed.Based on traditional high-speed railway aerodynamics,according to relevant theories and calculation in aerospace field,and combined with the model test data,the reliability verification analysis on the characteristics of the flow field are carried out.The structure of the flow filed in the tube can be divided into choked and unchoked.The judgment is dependent on whether the throat reaches the speed of sound.Under the choked flow,a normal shock wave is formed in front of the tube train.The temperature rise of the local flow field exceeds 50 K;the temperature rise of the stagnation region exceeds 88 K,and the pressure is approximately 1.7 times that of the initial pressure in the tube.When the flow is unchoked,differences arise in the distribution of the flow field corresponding to different incoming Mach numbers.When the incoming flow is supersonic,the flow field maintains a supersonic speed,and a bow-shaped shock wave is formed at the front of the tube train.Owing to the shock wave or expansion wave,the local flow field exhibits significant fluctuations in temperature and pressure.Conversely,when the incoming flow is subsonic,the flow field in the tube maintains a subsonic speed,and no shock wave structure is observed.

Hydraulics of crest spillway with large unit discharge and low Froude number

With respect to the crest spillway with large unit discharge and low Froude number, the hydraulics of the slit-type energy dissipater at the outlet should be noticed due to the complicated flow regimes. In the present paper, some issues about hydraulic characteristics were experimentally investigated by means of five slit-type outlets and four tetrahedrons, including the flow choking, impact to river banks and jet trajectory. The main findings are as follows. The critical Froude number for the flow choking decreases with increasing outlet width of the slit-type energy dissipater. If the flow Froude number is expressed by the parameters just before this energy dissipater, the tetrahedron placed inside the side wall of the outlet could efficiently avoid the flow impact to the river bank of same side, and compared with the jet trajectory of the slit-type energy dissipater, the outlet with tetrahedron has different trajectory trend, i.e., the distance of the jet trajectory decreases with the increase of the water head due to special form of the outlet tetrahedron.

Investigating the effect of blade sweep and lean in one stage of an industrial gas turbine's transonic compressor

In this paper,the simultaneous effects of the sweep and lean of the blades in one stage of a transonic compressor on its performance have been investigated.Then,with the help of numerical solution,fluid flows over these two modified geometries generated from the original sample were analyzed.Considering the applied constraints,the two generated rotor geometries have different geometrical characteristics;so that in rotor No.1,the blade has a backward sweep and it is less affected by lean,while in the modified rotor No.2,the blade has a forward sweep and it is more affected by lean.In the first sample,it is observed that the stage efficiency increases by 0.5%for operating design,while the stall margin reduces,and the chocking mass flow rate diminishes by 1.5%.Also regarding the second modified blade,the results indicate that the stall margin increases,the choking flow rate at the nominal rotational speed of the stage increases by 0.18%and the stage efficiency increases by 1%.The comparison of numerical results also shows that,in the first modified rotor,the pressure ratio of the stage diminishes by 0.01%;while in the second sample,the pressure ratio of the stage increases by the same amount.These results were then compared with the experimental results,showing a good agreement.