Xin Wang,Ya-an Hu,Jian-min Zhang

Hydraulic Engineering Department, Nanjing Hydraulic Research Institute, Nanjing 210029,China

State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Nanjing 210029,China

Abstract:To study the serious valve top gap cavitation of Datengxia high head single-lift ship lock,the 1:1 scale slice experiment is carried out,to truly reveal the gap flow characteristics,as well as the valve top gap cavitation characteristics and the anti-cavitation mechanism of natural aeration.Threekinds of cavitation,namely, the throat cavitation,the mainstream inner cavitation,and thevalve plate cavitation,are determined and they are found to occur step-by-step in the gap section in the development of the cavitation.According to the anti-cavitation mechanism of the natural aeration, the pressure of thegap flow through the ventilation isincreased,to avoid the mainstream inner cavitation and the valve plate cavitation,and to weaken the throat cavitation. The negative pressure zonein the gap section gradually extends with the development of thecavitation until the gap section is filled with a stable -10 m water head.When the natural aeration measure is employed,the pressure in the gap section approximately reaches the -2 m water head,and the cavitation disappears.The systematic tests reveal the quadratic polynomial relationship between the pressure in the gap and the ventilation per meter width.The pressure,which corresponds to the maximum value of the ventilation at the beginning of the gap,approximately reaches the-2 m water head, and the pressure and the ventilation reach the equilibrium statelimit. When the pressure in the gap increases when the valve is opened,the ventilation gradually decreases until the natural aeration stops.The gap section length for the high head valves hasa great effect on the natural aeration and should be long enough to maintain the stable negative pressure in the gap,whereas the throat width has a minimal effect.Resultscan be used for the anti-cavitation design of the high head lock valve.

Key words:High head lock,valve top gap cavitation,natural aeration,slice experiment

Introduction

The valve plays an important role as the throat of the ship lock?s water delivery system.Moreover,the operating condition of the valve is directly related with the safety and the efficiency of the ship lock.For a ship lock with a high water head of more than 20 m,the cavitation in the valve section is a critical factor for the vibration,the structural safety,and the operation stability of the valve and the hydraulic hoist[1].For example,the Gezhouba ship lock with 27 m of running water head is the first large-scale high-head ship lock in China.The valve cavitation of this ship lock causes a series of adverse consequences.The M20 seal bolts of reverse tainter valves are cut off, and the vibration acceleration of the valve is up to 4 g[2-3].The steel plates of the valve and the 24 mm thick lintel are eroded through the thickness.The valve cavitation was widely studied[4],which has several types,including the top gapcavitation,the bottom edgecavitation,the step-down floorcavitation,and the valve slot cavitation.The top gap cavitation,with serious water jet and high frequency cavitation,is the strongest among these cavitation types[5-6](Fig.1).

The development of the ship lock projects in China leads to the construction of several high head locks,such as the Shuikou three-step ship lock (with the water head of the middle step of 41.7 m),the Wuqiangxi three-step ship lock (with the water head of the middle step of 42.5 m),and the three-gorge five-step ship lock (with the water head of the middle step of 45.2 m).A new cavitation resistance technology for high head valves is developed to address the urgent need problem of the high head valve cavitation[7-8].This technology involves a combined measure,including a new valve culvert shape model,a reasonable initial submergence depth,and the natural aeration[9-10].The cavitation problem of the high head valves is properly resolved.The natural aeration measure can suppress the top gap cavitation with a good inhibitory effect on the bottom edge cavitation[11-12].Moreover,this measure enables the aerated flow to reach the bottom edge of the valve[13-14].Consequently,the natural aeration has become a necessary measure for the protection against cavitation of high head valves.The natural aeration involves a negative pressure at the top gap section by setting a reasonable lintel aerator[15-16].The effect of ventilation is a closely related factor.Thus,the design of the lintel body isimportant[17-18].

Fig.1 Sketch of the top gap cavitation of valve

The Datengxia ship lock with a working water head of 40.25 m represents the top level of the single step lock at home and abroad.The valve hydraulic problems are very complex.Further research must be carried out to demonstrate whether the present lintel body could adapt to a higher head valve. In view of the large length and width ratio of the narrow top gap,the gap flow can be regarded as two-dimensional.Therefore,the 1:1 slice experimental method,which could truly reflect the characteristics of the gap flow,was proposed for the investigation of the lintel body and the aeration effect[19-20].In this work,a Datengxia high head valve 1:1 slice experiment is carried out by using a high-speed and high-pressure gap flow experimental setup.The lintel top gap cavitation characteristics and the anti-cavitation mechanism of the natural aeration are explored,and a reasonable lintel body is proposed for thehigh head lock.

1.Design of 1:1 slice experiment

Figure 2 shows the general layout of the gap flow experimental setup for the high-speed and highpressure gap flow.Figure 3 shows the gap flow experimental setup.As the gap flow can be treated as a planar problem (the valve width of the Datengxia ship lock is5000 mm,whereasthe throat width isonly approximately 20 mm),the slice width has a minimal influence on the experiment.The slice width is determined to be 120 mm according to the experimental condition and the layout of the small vent pipes along the valve width.The working water head of the setup can reach 100 m.The dimension of the scope observation window located in the experimental section is 600 mm×800 mm (width×height).Thus,the setup can meet the demand of the top gap cavitation experiment for thehigh head valve.

Fig.2 General layout of the gap flow experimental setup

The Datengxia ship lock uses a reverse tainter valve,and the designed top gap body and the physical model are shown in Fig.4.The left border represents the outer plate of the reverse tainter valve,whereas the right border representsthelintel.A narrow gap section is formed between these borders.The throat is the narrowest section in the entire top gap,and a negative pressure is formed behind it along the flow direction.The lintel aerator is set behind the throat.Several small vent pipes with a diameter of 20 mm are arranged every 60 mm width between the aerator and the horizontal main ventilation pipe.The natural aeration is achieved under the negative pressure formed in the gap to inhibit the top gap cavitation.

Fig.3 (Color online)Sketch of the valve?s top gap cavitation

Fig.4 (Color online)Diagram of the top gap body and physical model

The 1:1 slice model of the lintel gap section is made of a 120 mm wide plexiglass.Two small vent pipes are symmetrically arranged in the aerator.The gap flow model,the water head,and the flow speed are all exactly the same as those of the real lock.To investigate the cavitation characteristics and the natural aeration effect of the lintel,various physical measurements are made,including the cavitation and aerated flow pattern,the lintel ventilation capacity,the hydrodynamic pressure in the gap section,the cavitation noise,and the vibration.The measurement equipment correspondingly comprises a high-speed camera,vortex air flow meters,pressure sensors,hydrophones,and vibration acceleration sensors.Five pressure points(e.g.,P1-P5)are set at the side of the valve outer plate(Fig.3).The first point (P1)is set in the throat section,the second point (P2)is set in the beginning of the gap section,and P5 is set at the end.The upstream pressure in the top gap reaches up to 61 m water head,and that of the downstream is 20.75 m water head.The flow speed at the throat exceeds30 m/s.

2.Anti-cavitation mechanism of lintel natural aera- tion

2.1 Cavitation and aerated flow pattern

For the Datengxia ship lock with a high working head,the flow speed at the throat of the top gap in the opening process of the valve reaches up to 37 m/s.The cavitation pattern of the lintel gap section is closely related to the pressure condition in the upstream and the downstream of the valve.Figure 5 shows several typical cavitation and aerated patterns under different conditions. The development process of the gap cavitation can be divided into five stages.At the first stage,one sees the initial cavitationat the throat sill.Unstable cavitation bubbles quickly flash and disappear behind the throat (Fig.5(a)).The throat sill cavitation is caused by the flow shedding.The throat sill is the most vulnerable part for cavitation in the entire gap.At the second stage,the weak cavitation develops from the initial cavitation.Many tiny bubbles are constantly generated and rolled in the aerator (Fig.5(b)). At the third stage,the cavitation develops,and the gap reaches the critical ventilation condition.The cavitation cloud extends to the gap section behind the aerator.At this stage,the downstream water level is still relatively high,and the cavitation occurs in the throat.The cavitation flow rolls in the aerator with a wavy diffusion orientation downstream the beginning of the gap section along the lintel wall.Then,a stable fine cloud cavitation flow is formed (Fig.5(c)).At the fourth stage,one sees the critical blocking cavitation.When the downstream outlet pressure continues to decrease until the upstream pressure does not change with the the decrease of the downstream pressure,the gap cavitation reaches the critical blocking state.The gap flow rate,the flow velocity,and the upstream pressure are all fixed. The scope of the lintel covered by the cavitation flow generated from the throat drop sill is extended. Meanwhile,a new kind of cavitation occurs in the main flow under a negative pressure when it passes the throat.The mainstream inner cavitation is a bubble cavitation,mainly caused by bubbles in the high air content flow.A vertical cavitation airflow appears in the mainstream center and is mixed with the throat cavitation flow in the gap section (Fig.5(d)).At the fifth stage,one sees thestrong cavitation.When the downstream outlet pressure is further reduced,the cavitation generated at the throat drop sill develops further and continues to extend toward the outlet of

Under the strong cavitation condition,the natural aeration occurs when the vent valve is turned on.When the air is automatically entrained into the gap flow,the cavitation noise decreases significantly,and the current cavitation flow pattern changes.The cavitation occurs at the left valve plate,and that of the middle mainstream disappears.The aerated flow mixes with the throat sill cavitation,and the air in the aerator flows and diffuses along the lintel wall toward the downstream.A clear interface is formed between the mainstream and the aerated flow (Fig.5(f)).The air is gradually entrained into the mainstream and mixed evenly at the outlet of the gap section.Therefore,the variation of the flow patterns(aerated or not)indicates that the anti-cavitation measure of the natural aeration of the lintel includes two aspects.The valve plate cavitation and the mainstream inner cavitation are eliminated first,and the throat sill cavitation is suppressed through the natural aeration simultaneously.

2.2 Pressure characteristicsof gap section

The flow pattern is closely related with the pressure.Therefore,the cavitation growth process and the anti-cavitation mechanism of the natural aeration measure in the top gap can be investigated further through the pressure analysis at the five points(Fig.4).The parameter,that is,the downstream cavitation numberis introduced to characterize the cavitation condition in thegap section

Fig.5 (Color online) Flow pattern of cavitation developing processand comparison of aerated and not aerated cases

For the body of 20 mm in throat width and 320 mm in gap section length,the downstream cavitation number is equal to 0.54,when the critical blocking cavitation occurs in the gap section.Then,the cavitation number gradually decreases and the cavitation continuously develops through the reduction of the downstream pressure.Figure 6 shows the pressure distributions in the gap section under several different cavitation conditions of the process.In the critical blocking cavitation,only the pressure at the P2 point at the beginning of the gap is negative.The cavitation is strengthened,and the cavitation flow spreads into the gap section with a decrease of the downstream cavitation number.Meanwhile,the pressure in the gap becomes negative and reaches the maximum value of approximately -10 m water head.Then,the negative pressure zone continuously extends with the decrease of the cavitation number.When the downstream cavitation number is 0.28,a strong cavitation is observed,and the whole gap section reaches the stable negative condition of -10 m water head.

Fig.6 (Color online)Pressure variation with the growth of the cavitation in the gap section (9.81kPa)

The two conditions of aerated and not aerated in the strong cavitation as before,are still taken as an example.Figure 7 shows the comparison of the pressure distributions of the gap section under the two conditions.In the strong cavitation without aeration,a very stable negative pressure of approximately -10 m water head exists in most of the gap section,which causes the strong cavitation,to flood the gap section.The pressures at the throat (P1)and the outlet of the gap(P5)are higher than the inside ones,affected by the upstream and downstream pressures.Under the same condition as the lintel aeration,the pressure of the gap flow is increased to approximately -2 m water head from -10 m water head,thereby reaching the equilibrium state of the pressure and the ventilation.The negative pressure in the gap section still needs to be filled for the stable lintel aeration.

Fig.7 (Color online)Pressure distribution in the gap section (9.81kPa)

2.3 Cavitation noise and vibration characteristics

Figure 8 shows the comparison of the cavitation noise intensity curves when the lintel is aerated or not aerated under the same working conditions.The cavitation noise intensity without lintel aeration is evidently stronger than that with aeration,approximately 5 times as stronger.Thus,the natural aeration measure has a perfect inhibitory effect on the cavitation and is the key factor in avoiding the “sound vibration”in the project.

Fig.8 Comparison of cavitation noise with or without aeration

One three-direction vibration acceleration sensor is set in the side wall of the lintel gap section to measure the vibration responses in the two cases of strong cavitation with and without aeration.The vibration responses under different conditions,including various throat widths,are listed in Table 1.The vibration acceleration decreases by 70%-80% after the ventilation.Thus,the cavitation is the main cause of the vibration,and a close relationship exists between them.The vibration weakens when the cavi-tation is inhibited by thenatural aeration.

3.Optimization experiment of lintel body

3.1 Recommended lintel body

The throat width and the gap section length are the two important parameters of the Datengxia shiplock?s lintel body.To study their effects,three values of width (e.g.,15 mm,20 mm and 25 mm throat widths)and length (320 mm,270 mm and 220 mm gap section lengths)are adopted for every parameter.

Table 1 Comparison of vibration with or without aeration

The critical natural aeration conditions of different width and length schemes are basically consistent throughout the systematic experiments.The critical natural aeration condition presents a linear relationship when it is described by the upstream pressure ( pu)and the downstream pressure( pd)

The experimental result shows that the lintel natural aeration flows in three different throat width schemes are consistent in the opening process of the valve.Therefore,the throat width hasa minimal effect on the natural lintel aeration.

The relative pressure ( )R and the average aeration concentration ( )C aredefined as:

whereaQ is the ventilation flow,wQ is the flow passed through thegap.

Fig.9 (Color online)Critical natural aeration condition

The relationships between the relative pressure and the averageaeration concentration for the three gap section lengths(with the throat width of 20 mm)are shown in Fig.10.When the gap,such asof 320 mm or 270 mm in length,is relatively long,the aeration concentration is basically consistent.However,when the gap length is 220 mm,the aeration concentration decreases evidently because the gap section cannot form enough negative pressure for the short gap.Thus,the gap section length should not be too short for the Datengxia ship lock,and a 20 mm throat width and a 270 mm gap length are recommended.

3.2 Relationship between ventilation and pressure in the gap section

Fig.10 Relationship between relative pressure and aeration concentration

Fig.11 Ventilation against the valve relative opening

For the Datengxia ship lock,6 min is needed to open the valve continuously.Figure 11 shows the ventilation of the top gap of the valve in the opening process.The natural aeration is smooth in the range of relative opening of 0.6.Figure 12 shows the pressure distributions in the gap section of relative opening in the range of 0.1-0.6.A close relationship exists between the natural aeration ability and the pressure in the gap.In the small relative openness of 0.1-0.3,the pressure distribution in the gap and the lintel ventilation are stable.The downstream pressure gradually increases with the increase of the relative openness of the valve and affects the pressure in the gap.After the relative openness reaches 0.4,the value and the scope of the negative pressure in the gap section,as well as the natural ventilation,are gradually decreased because of the downstream pressure.When the relative openness reaches 0.6,only the pressure at P2 at the beginning of the gap remains negative among the five points,and the lintel ventilation evidently decreases.When the relative opening reaches 0.7,the lintel cannot aerate naturally,and a weak cavitation occurs at the throat.Therefore,the natural aeration capacity and the cavitation condition are closely related to the pressure in the gap.A strong cavitation will not occur when the natural aeration measureisadopted.

The typical measuring point (P2)at the beginning of thegap istaken asan example.Figure 13 shows the relationship between the pressure at P2 and the lintel ventilation per meter width obtained from the systematic experiments.A clear quadratic polynomial relation exists between the aforementioned factors

whereaq is the ventilation per meter width,p is the pressure at the beginning of thegap.

The pressure,which corresponds to the maximum value of ventilation at the beginning of the gap,is approximately -2 m water head.The pressure and theventilation reach theequilibrium state limit.

4.Conclusions

A gap flow experimental setup with high speed and pressure is adopted for the 1:1 lintel slice experiment for the high head valve of the Datengxia ship lock.The lintel cavitation characteristics,the anticavitation mechanism of the natural aeration,and the lintel body are investigated,and some conclusions are obtained asfollows:

(1)Three kinds of cavitation,namely,the throat sillcavitation,the mainstream inner cavitation,and the valve outer platecavitation,are found to occur step-by-step with the development of cavitation.Under a strong cavitation,the three kinds of cavitation simultaneously exist and fill the entire gap section.The flow near the gap outlet is disordered with a back jet.

(2)The anti-cavitation mechanism of the lintel?s natural aeration increases the pressure in the gap section through ventilation. The valve plate and the mainstream inner cavitation are eliminated and have disappeared,and the throat sill cavitation is also suppressed.

Fig.12 (Color online)Pressure distribution on the valve plate for different values of openness(9.81 kPa)

Fig.13 Relationship between the ventilation and the pressure at the beginning of thegap

(3)For a high head valve,the gap section length hasa great effect on the natural aeration and should be long enough to maintain the stable negative pressure in the gap,whereas the throat width has a minimal effect.

(4)The value and the scope of the negative pressure in the gap increases with the development of cavitation without the lintel natural aeration.Under a strong cavitation,a very stable negative pressure of approximately -10 m water head exists in most of the gap section. Meanwhile, the pressure of the gap flow increases to approximately -2 m water head under the same condition with thelintel aeration.

(5)A quadratic polynomial relation,that is,a close relationship between the ventilation and the gap pressure,is observed on the basis of systematic experiments.The pressure,which corresponds to the maximum value of ventilation at the beginning of the gap,is approximately -2 m water head,and the pressureand the ventilation reach the equilibrium state limit.