Jian-rong Xu,Jian-hua Wu,Yu Peng,Fei Ma

1. Powerchina Huadong Engineering Corporation Limited, Hangzhou 311122,China

2.College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing 210098,China

Abstract:In the present work,a 3-Daerator device with backward lateral deflectors,called BLD-3-D aerator device,isdeveloped, and the lateral cavity and fin performance of the BLD-3-D aerator device are experimentally investigated.The findings show that,the relativelateral cavity length with backward lateral deflectorsisshorter than that with current lateral deflectors under thesame approach flow conditions,and on the basisof theresultsof therelative cavity length ratio between the lateral and bottom aeratorsthe BLD-3-D aerator deviceisof remarkable performancefor the water fin control thanks to the decrease of therelativelateral cavity length.

Key words:Aerator, backward lateral deflector,BLD-3-D aerator device,cavity,water fin

It is well known that the air entrainment into the flow could decrease or avoid the occurrence of the cavitation damage is informed about 60 years since the applications of the air entrainment to the Grand Goulee Dam in 1960[1-4].The combining structure of the bottom aerator and the lateral deflector is designed and used due to the cavitation damage of the side walls of the release works,which consists of a bottom aerator and two lateral deflectors placed after the working gate or in the discharge tunnel[5-7].However,the intense water fins occur easily and the flows are deteriorated due to the existence of the lateral deflectors in an aerator device.

Some investigations have been conducted about the water fins produced by the lateral deflectors.Those water fins take place when the lateral cavity length is larger than the bottom cavity length.In this occasion,the bottom cavity in length is fully exposed in the air from the lateral cavities in the two side walls.An index,the ratio of the lateral and bottom cavity lengths,βLB,is proposed in order to evaluate the characteristics of the water fins.The water fin intensity,either the length or the height,decreases with the decrease of βLB.Especially,the water fins disappear if βLB≤1[8].

The expression βLB≤ 1 means that it is necessary that the lateral cavity length is shorter than the bottom cavity length.Thus,this condition results in the relative small thickness of the lateral deflector,and then the relative small space of the air entrainment,which is not good enough for the air entrainment to decrease or avoid the cavitation damage.

On the basis of the knowledge of the exposure of the bottom cavity end in the air of the lateral cavity,and then the occurrence of the water fins,the bottom cavity end could be covered if moving the lateral deflector in the reverse flow direction.Thus,the water fins could be eliminated as long as the end position of thelateral cavity is before that of the bottom cavity.

Therefore, the present work developed a kind of the three dimension aerator device with the backward lateral deflectors,called BLD-3-D aerator device,placing the end position of the lateral deflector before that of the bottom aerator.

The objectives of this work are to observe the flow regimes through the BLD-3-D aerator device and to investigate the characteristics of the lateral cavity and fin performance for the BLD-3-D aerator device.

Figure 1 is the definition sketch of the BLD-3-D aerator device geometry and the flow regime.This BLD-3D aerator device is placed at the inclined section of a discharge tunnel with the free flow,and W = 0.35 m and i = 1:4 arethe width and bottom slope of the inclined section for the discharge tunnel model,respectively.

Fig.1 Definition sketch

The parameters of the BLD-3-D aerator device include:tr= 0.010 m and lr= 0.200 m,are the height and length of the ramp,and ts= 0.038 m,is the step height for the bottom aerator,tl= 0.010 m and ll= 0.020 m,are the thickness and length of the lateral deflector,respectively,b is the backward distance relative to the end position of the bottom aerator.Table 1 is the cases and parameters of the different lateral deflectors for the BLD-3-D aerator device,as well as case M1,its backward distance B =0 in order to compare the effects of the different lateral deflectors on the fin performance for the BLD-3-D aerator device,where B =b/tl.

Table 1 Cases and parameters for lateral deflector models

The experiments were conducted in High-speed Flow Laboratory,Hohai University. The experimental setup consists of a pump,an approach conduit,a large feeding basin,a test model,and a flow return system.The feeding basin issues an approach (subscript o)flow of depth hoand average velocity vo(Fig.1(a)),resulting in the approach flow Froude number Fro= vo/(gho)0.5.The model discharges are measured with discharge measurement weir instruments.The maximum pump capacity QM=400 L·s–1.

It is observed that,the water fins start from the end position (see point A in Fig.1)of the lateral cavity if occurrence.The water bodies exceed the free surface of the flow are called as the water fin in the present work.Hfand Lfare defined as the height and length of the water fins,respectively (Fig.1(a)).

Figure 2 shows the flow regimes through the BLD-3-D aerator device at a same approach flow Froude number,i.e.,Fro=5.23,for the different backward distancesof the lateral deflectors.

Where the cases listed in Table 1 was concerned,case M1 without backward distance produces the intense water fins (Fig.2(a)),while those water fins almost disappear when B = 20 for M5(Fig.2(e)).It is clearly observed that,either the water fin length or the water fin height decreases significantly with the increase of the backward distance(Figs.2(b)-2(e)).

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Fig.2 Flow regimes at Fr o= 5.23

Figure 3 demonstrates thevariations of the lateral cavity length (Llo/tland Ll/tl)with the approach flow Froude number (Fro).In Fig.3,Lloand Llare the the cavity length from the end sections of the lateral deflector and of the bottom aerator, respectively,and Ll= Llo–b,where b is the backward distance of the lateral deflector relative to the bottom aerator.

Some propertiesof those two lateral cavity lengths areasfollows:

For all the cases of lateral deflectors in Table 1,its lateral cavity length,either Llo/tlor Ll/tl,increases with the increase of the approach flow Froude number (Fro).Secondly,since the lateral deflector has some same geometries,i.e.,the same thickness and length of the lateral deflector,Llo/tlincreases with the increase of the backward distance B at the same Fro,such as Llo/tl(M4,B= 15)>Llo/tl(M3,B =10)>Llo/tl(M2,B =5)>Llo/tl(M1,B = 0).Those results should be related to the relative positions between the lateral and bottom cavities.Thirdly,the lateral cavity becomes small if large backward distance of the lateral deflector,like M5(B = 20),and in this occasion,it may be little affected by the bottom cavity (Fig.3(a)).

Fig.3 Variations of (a) L lo/t l and (b) L l/t l with Fr o

For the backward lateral deflectors,Ll/tlare all smaller than Llo/tlwith the exception of M1(B = 0),and Ll/tldecreases with the increase of the backward distance B at the same Fro,such as Ll/tl(M5,B = 20)

Figure 4 shows the variations of Ll/Lbwith Froand (1+B)-0.08Frofor the different cases of the backward lateral deflectors.Clearly,Ll/Lb(Fro)increases,and Ll/Lb(B)decreases linearly,respectively.On the basis of the experimental observations of the water fins(such as Fig.2),the changesof the intensity for the water fins are related closely to the index Ll/Lb,especially,the water fins disappear when B is large enough,like B=20,(see M5 in Fig.4(a)).Further,Ll/Lbcould be expressed by the variables of Froand B (see Fig.4(b))(R2=0.927), that is

Fig.4 Variationsof L l/L b with (a) Fr o and (b)(1+B)-0.08Fr o

Fig.5 Variations of (a) L f/h o with (1+B)-0.12Fr o2.51 and (b) H f/h o with (1+B)-0.17Fr o1.66 with respect to various B

Figure 5 shows the variations of Lf/howith (1+B)-0.12Fro2.51(R2= 0.892)and Hf/howith (1+B)-0.17Fro1.66(R2=0.858)at various B,respectively,including B=0,i.e.,without the backward distance of the lateral deflector,where the data of case M5 are not obtained because of no appearances of the water fins.The best fitsare

respectively.Equations(2)and (3)are valid for B =0-15, Fro= 4.66-5.85.Clearly,the backward distance of the lateral deflector can efficiently decrease the intensity of the water fins,either Lf/hoor Hf/ho, at the same approach flow Froude number.Figure 6 shows the variations of Lf/hoand Hf/howith Ll/Lbat various B,respectively.The best fits are on the basisof Fig.4(b)

Their determination coefficients are R2= 0.941 and R2= 0.947, respectively.The application conditions of Eqs.(4) and (5)are the same with Eqs.(2)and (3).In other words,both Lf/hoand Hf/hoare also the functions of the variable Ll/Lbon the basis of Eq.(1)and Fig.4.

Fig.6 Variations of (a) L f/h o with L l/L b and (b) H f/h o with L l/L b with respect to various B

In short,for a BLD-3-D aerator device,here,given Ll= Llo– b,the lateral cavity length Llwill be largely shortened when the end position of the lateral deflector moves upstream before that of bottom aerator.The more the lateral deflector moves(i.e.,backward distance b),the larger the lateral cavity length is shortened under the present experimental conditions.On the basis of the knowledge of the effect of the cavity length ratio on the water fins,either the length or the height[8]and of the effect of the backward distance of the lateral deflector on the lateral cavity length,the design of the backward lateral deflector is of significance in order to decrease or avoid the water finsfor a 3-D aerator device.In additions,the expressions of estimating the water fin intensities are presented with the ratio of the lateral and bottom cavity lengths,or the backward distance and the approach flow Froude number.