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MODEL STUDY ON FLOOD DISCHARGE ATOMIZATION OF HYDROPOWER STATIONS ADOPTING ENERGY DISSIPATION BY HYDRAULIC JUMP

Shiqiang Wu , Xiufeng Wu , Hui Zhou , Huiling Chen

Professor, Nanjing Hydraulic Research Institute, State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Nanjing, China, e-mail: sqwu@nhri.cn Senior engineer, Nanjing Hydraulic Research Institute, State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Nanjing, China, e-mail: xfwu@nhri.cn Senior engineer, Nanjing Hydraulic Research Institute, Nanjing, China, e-mail: hzhou@nhri.cn Professor, Nanjing Hydraulic Research Institute, Nanjing, China, e-mail: hlchen@nhri.cn

Abstract

The phenomenon of flood discharge atomization is a negative effect brought by the

flood discharge of high dams, and it has some certain influences on the dam safety and surrounding environments. The method of large-scale model test is adopted to research the flood discharge atomization of Xiangjiaba Hydropower Station. By use of the prototype observation results of flood discharge atomization of Wantang Hydropower Station which is similar to an energy dissipater. The feedback analysis on the scale effect of atomization models is conducted. The similarity law of atomization quantity is obtained which could be used to predict and evaluate the influences of flood discharge atomization of Xiangjiaba Hydropower Station. Key words flood discharge atomization; energy dissipation by hydraulic jump; model test; feedback analysis; scale effect The problem of flood discharge atomization is a key topic of research on high velocity flow, which has occurred along with the construction of high dams. So far, the methods for research on flood discharge atomization mainly include physical model, numerical model, and feedback analysis on prototype observation data. Due to complex phenomena of flood discharge atomization, and plentiful influential factors, the formation mechanism of atomization is not clear yet. The method of mathematical model (Li Dingfang, 2001,Chen Huiling 2002,Zhang Hua 2005,Zhou Hui 1994) could not completely simulate this phenomenon, and its accuracy and reliability could not meet the requirements of actual engineering application. The method of prototype observation (Wu Fusheng 1997,Wu Xiufeng 2001) is an

This work is supported by National Natural Science Foundation of China (50579084).

important means for researching the problem of flood discharge atomization, but it involves with plentiful observation work, and it is difficult to obtain relatively comprehensive and systematic observation data or to correctly evaluate the influences of flood discharge atomization. Due to the influences of scale effect, it is difficult for traditional hydraulic model test to completely simulate the phenomenon of flood discharge atomization (especially the mist of prototype). At present, the method of large-scale hydraulic model test (Chen Huiling 2000, 1995) is mainly adopted for assessing the influences of flood discharge atomization of hydropower stations, and for bringing forward protective countermeasures from thereof. This method has been verified through feedback analysis on the prototype observation results of flood discharge atomization of some hydropower stations, and has been proved its

Proceedings of 16th IAHR-APD Congress and 3rd Symposium of IAHR-ISHS

reliability for prediction and evaluation on these influences. As the hydraulic model test, the model of flood discharge atomization is designed based on the gravity similarity criterion, which ignores secondary factors, and ensures the similarity of main physical quantity of flow. However, scale effect exists in atomization quantity, and the conversion doesn’t follow the gravity similarity criterion, so it is necessary to research and analyze its conversion formula. In addition, the incidence of flood discharge atomization of hydropower stations adopting energy dissipation by hydraulic jump is extremely limited, and there is no detailed research on this dissipater previously (Wang Shanda 2003). According to the feedback analysis on the prototype observation results and model test results of Ertan, Ankang, Yantan and Dongfeng Hydropower Stations, etc., it is proved that the conversion relation of Lr0.5~1.5 exists between the prototype and model of rainfall intensity of flood discharge atomization (Chen Huiling 1995). However, Xiangjiaba Hydropower Station adopts energy dissipation by hydraulic jump, and the incidence of its atomization is extremely limited; also, there is lack of feedback analysis on the prototype and model test results, and it is to be demonstrated that whether the relation equation obtained by means of overflow dam and ski jump, etc. is applicable or not. Therefore, the comparative analysis is conducted on the prototype observation data of flood discharge atomization of Wantang Hydropower Station adopting similar energy dissipation method, and thus the rationality of the similarity law of atomization models is proved. Then, with Xiangjiaba Project as the research object, the method of large-scale model test is adopted to study the distribution scope of rain area and nepheloid zone caused by flood discharge atomization of this project, the influence degree of atomization is predicted, and some suggestions and preventing measures are given in this paper. Xiangjiaba Hydropower Station is the last step g1744g

in the cascade development of Jinshan River. As an incomplete seasonal regulation reservoir, its normal water level, dead water level, and installed gross capacity are 380m, 370m and 6,000MW respectively. The project is composed of dam, flood discharge release facilities, power house behind the dam, vertical ship lift, and water intakes for irrigation on both banks. Flood discharge release facilities consist of 10 middle tunnels and 12 crest outlets arranged at some interval, which are at the main river way and totally have a maximum flow discharge of 48680m3/s. At about 1.7km in the lower reach of the dam is Yuntainhua Enterprise mainly manufacturing nitrogen fertilizers, and its products are extremely sensitive to the air humidity, which is especially important for the prediction and evaluation on the influences of flood discharge atomization. Therefore, the method of energy dissipation by hydraulic jump for flood discharge is considered, so as to relieve the influences of flood discharge atomization.

1

TEST AND RESEARCH METHODS

The feedback analysis on many results of

model test and prototype observation on flood discharge atomization (Li Dingfang, 2001, Chen Huiling 2002,Zhang Hua 2005,Zhou Hui 1994, Zeng Xiang 1997, Su Jianming 2002) shows that, only when the Weber number (We) of the surface flow of model flood discharge is over 500, could the atomization parameter obtained from model measurement be converted to the prototype as some certain model law. The formula for calculation of Weber number We:

We = ( ρ RV 2 / σ )

0.5

(1)

Where, ρ indicates the density of water flow, which is 1000kg/m3 at 20 ; V indicates the velocity of nape; σ indicates the coefficient of surface tension, with 0.0725 N/m taken here; R is the radius of

October 20-23 2008, Hohai University, Nanjing, China

curvature, and generally, the surface radius of curvature of the water flow on the reverse arc surface of over flow dam is taken. Substitute the above numerical value into formula (1), we obtain: We=(270620.7ΔH·R)0.5 (2) Where, ΔH is the height difference from the water surface at the calculation point to the lower reach, m. As for the two situations in which the water level of reservoir is 370m and 380m respectively, the model scale Lr is determined to be 1:60 on the basis that the We number of the over flow dam close to the nose ridge meets the model atomization similarity requirement. Also, according to the release facility operation patterns, the test conditions are determined to be 5 hydrological conditions in flood season, namely flood occurring frequency of 50%, 20% 5% and 1% respectively, and flood occurring frequently in non-flood season.

Hydropower Station with a scale of 1:15, and could adopt the prototype observation effects of Wantang Hydropower Station for verifying the conversion relations of atomization quantity. Carry out feedback analysis by adopting the atomization model test results close to the flood discharge condition (discharge about 1218.0m3/s) of Wantang Hydropower Station. When the water level of the upper and lower reaches is 370.00m and 290.65m respectively, all crest outlets and middle tunnels are opened to release flood discharge, the discharge of prototype is 34365 m3/s, and the scale of flow discharge is close to Lr2.5, which basically meets the gravity similarity criterion.

2.1

FEEDBACK ANALYSIS ON CHARACTERISTIC VALUE OF RAIN INTENSITY

2

DISCUSSION ON THE SIMILARITY LAW OF FLOOD DISCHARGE ATOMIZA -TION RESULTING FROM ENERGY DISSIPATION BY HYDRAULIC JUMP

In the flood discharge atomization observation of Wantang Hydropower Station, the maximum rain intensity at 2.5m and 3.0m above the water surface of the stilling basin below the dam is 0.83mm/h and 0.11mm/h respectively, and the rain intensity at the area (0.15 0.2m above the water surface of the stilling basin) corresponding to the observation point in model is about 0.04 0.06mm/h. From the magnitude of rain intensity, the conversion relation ? between the two shall be Lr0.97 1.12, close to Lr, which means that the conversion relation of atomization rain intensity doesn’t follow the gravity similarity criterion. By combining the results of feedback analysis on similar physical model test results and prototype observation results in the past years, and by comprehensively considering the measurement errors and safety factors, the conversion of rain intensity of flood discharge atomization of Xiangjiaba Hydropower Station is determined to be Lr1.5.

Wantang Hydropower Station, having similar flood discharge configuration to that of Xiangjiaba Hydropower Station, also adopts the method of energy dissipation by hydraulic jump, and execute the prototype observation test of flood discharge atomization in September 2000. The water head of Wantang Hydropower Station is about 28m, while that of Xiangjiaba Hydropower Station is about 109m. It could be found the relation of 1:15:60 between Xiajiaba project model, the dam of Wantang project, and the dam of Xiangjiaba. Therefore, it could take the flood discharge atomization model of Xiangjiaba Hydropower Station approximately as that of Wantang

2.2

CONTRASTIVE ANALYSIS ON THE CHARACTERISTICS OF DROP SPECTRUM

The comparison of model and the prototype

drop spectrum distribution of Wantang Hydropower g1745g

Proceedings of 16th IAHR-APD Congress and 3rd Symposium of IAHR-ISHS

Station is as shown in Figure 1. It is found that the characteristics of raindrop size distribution of the two are relatively accordant, and that the raindrop size distribution obtained from model test is compliant with that of the prototype.

Fig. 1 Comparison of the drop diameter distribution between the field result for Wantang project and the model test values for Xiangjiaba project

2.3

ANALYSIS ON DISTRIBUTION OF RAIN AREAS

The atomization rain areas measured from

flood discharge atomization model concentrate nearby the distilling basin, and the farthest incidence is about 0.7~1.2m behind the distilling basin, while the farthest incidence of Wantang Hydropower Station is 10-20m behind the distilling basin, as shown in Figure 2. The conversion relation between the two is 14.3 16.7, namely the conversion relation of the incidence basically meets the gravity similarity criterion. From the comparative analysis on the characteristics of atomization rain areas of prototype observation and the model test results of Wantang Hydropower Station, it could be found that the scale 1:60 for flood discharge atomization models is feasible, which could be applied to prediction and evaluation on the influences of flood discharge atomization of Xiangjiaba Hydropower Station. Whereas the above feedback analysis, and considering the extremely limited incidence of the flood discharge atomization of the energy dissipation by hydraulic jump, conversion relation Lr1.5 is selected as rain intensity and Lr as the incidence scale of rain areas. g1746g

Fig. 2 Comparison of the atomized rainfall between the results of field measure of Wantang Project and model test for Xiangjiaba Project (unit: mm/h)

3

PREDICTIVE ANALYSIS ON THE IMPACTS OF FLOOD DISCHARGE ATOMIZATION

ATOMIZATION PHENOMENON FROM ENERGY DISSIPATION BY HYDRAULIC JUMP

The pivot of hydropower station is linked up

3.1

with the water flow of river way in the lower reach through energy dissipation by hydraulic jump. In flood discharging, the water flows out of middle tunnels and into the distilling basin to link up with the water flow inside the distilling basin and form vortex inside the basin. In the process of flood discharge, the water flow on the surface of dam has not much aeration, serious crushing or dripping of

October 20-23 2008, Hohai University, Nanjing, China

water drop. But inside the distilling basin, there is vortex, the water flow is in turbulent motion and aeration, some water drops and blocks are separated from the basin surface, and thrown into the sky and out of the basin. However, compared with ski jump energy dissipation, there is no bounce and splashing of water body inside the distilling basin, and by far less water drops and blocks are thrown out of the basin, therefore, its rain areas and rain intensity of flood discharge atomization are much smaller than those of ski jump energy dissipation engineering.

rain intensity inside the distilling basin could be found from the distribution of rain intensity, and a relatively small peak value of rain intensity behind the afterbay dam could also be found. No matter

3.2

DISTRIBUTION OF RAIN INTENSITY AND RAIN AREAS OF FLOOD DISCHARGE ATOMIZATION

The distributions of rain area and rain intensity

are as shown in Figure 3. Where, the impacted rain area scope is between S0+100m S0+460m, and the horizontal scope may reach 10 20m outside the guide wall of distilling basin. In case of overflowing of the left over flow dam, the rain area will mainly concentrate at the left distilling basin and within 10 20m outside the guide wall; in case of overflowing of the right over flow dam, the rain area will mainly concentrate at the right distilling basin and within 10 20m outside the right guide wall; in case of simultaneous overflowing of the left and right over flow dams, the rain areas will be distributed at the two distilling basins and within 10 20m outside the left and right guide walls. The highest impacted point in rain area is not over 310m elevation. The maximum rain intensity is about 1194mm/h within the distilling basin. An afterbay dam is built up at the tail end of distilling basin, so based on different relations between the water surface elevation of distilling basin and the water level of the lower reach, some secondary linkup flow patterns will occur. When flooded surface flow or mixed flow occurs behind the afterbay dam, the rain intensity below the afterbay dam will be bigger than that of other pure surface flow and wave current. Therefore, a relatively big peak value of

Fig. 3 Atomized rainfall isoline caused by released flood discharge for Xiangjiaba Project (300m elevation) (unit: mm/h)

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Proceedings of 16th IAHR-APD Congress and 3rd Symposium of IAHR-ISHS

jump turbulent motion areas in the distilling base, the bigger the rainfall will be, so will the raindrop diameter. Similarly, at positions far from the hydraulic jump areas, the rain intensity is reduced, and rain drops of small diameter increase. Rain areas are the sources of diffusion of atomized flow. Therefore, within the rain areas, the bigger the rainfall is, the more the rain drops will be, and the bigger the concentration of atomization sources diffused at wind field will be.

Fig. 3 (Continued)

how the peak value of rain intensity changes, the rain areas will be limited to the banks, so it is no influence to Yuntianhua Group located at the downstream of the dam. The distribution of rain areas keep basically constant under different water level due to the restrictions of the side walls of the distilling basins. However, the rain intensity is relatively great when the crest outlets release flood discharge. It is found from the actually measured rain intensity that, under the same conditions, if the flooded state of hydraulic jump is different, and the turbulent motion intensity of aeration is different, the rain intensity will be somewhat different. In case the water level of distilling basin rises, the rain intensity of flood discharge atomization will trail off.

Fig. 4

The drop diameter distribution caused by released flood discharge

3.4

ASSESSMENT ON THE IMPACT OF FLOOD DISCHARGE ATOMIZATION ON YUNTIANHUA GROUP

To analyze from the test results, the incidence

of rain areas caused by flood discharge atomization basically concentrates within an extremely limited scope nearby the distilling basin, so it will not directly affect the factory area and living area of Yuntianhua Group. As for the nape wind caused by flood discharge, the discharge flow of the sluice structure is relatively short and closely hangs to the overflow surface, forms a small scope of source areas of wind speed field only, and its atomized diffusion function on the flood discharge atomization is also extremely limited. In addition, the wind vortex is behind the place of atomization sources, and the atomized flow could not be diffused easily. Therefore, the humidity in the air in Yuntianhua living area will be similar to the natural one, and the diffusion function of nape

3.3

ANALYSIS ON CHARACTERISTICS OF RAINDROP SIZE DISTRIBUTION

In condition of flood occurring frequency of

1%, the characteristics of raindrop size distribution in rainfall areas will be as shown in Figure 4. Within the rain areas, the closer to the hydraulic g1748g

October 20-23 2008, Hohai University, Nanjing, China

wind on atomized flow won’t affect the factory area and living area of Yuntianhua.

REFERENCES

Li Dingfang. Report on the Numerical Simulation and Computation of Flood Discharge Atomization of Xiangjiaba Hydropower Station [R].Nanjing: Nanjing Hydraulic Research Institute, 2000, 2. Chen Huiling and Li Dingfang. Research on the Diffusion of Flood Discharge Atomization of Xiluodu Hydropower Station [R].Nanjing: Nanjing Hydraulic Research Institute, 2002, 5. Zhang Hua, Lian Jijian. Prediction of Influences of Underflow Flood Discharge Atomization on the Environment in the Lower Reach [J]. Journal of North China Electric Power University, 2005, l32 (1): 107-112. Zhou Hui, Chen Huiling. Fuzzy Comprehensive Assessment Method for Atomized-rain of Jet Overflow [J]. water transportation science research, 1994, (1) : 165-170. Wu Fusheng, Cheng Hesen, Li Zhangsu, and others. Prototype Observation Research on Environmental Pollution of Flood Discharge Atomization of High Dams [J]. Advances in Water Science, 1997(2): 189-196. Wu Xiufeng, Wu Shiqiang, Zhou Hui, and others. Prototype Observation of Flood Discharge Atomization of Wantang Hydropower Station [J]. Hydro-Science and Engineering, 2001, (4): 71-74. Chen Huiling and Huang Guoqing. Research on the Physical Model Test of Flood Discharge Atomization of Pivot of Xiluodu Hydropower Station [R]. Nanjing Hydraulic Research Institute, 2000, 2. Chen Huiling. Research on the Flood Discharge Atomization of Xiaowan Hydropower Station [R].Nanjing: Nanjing Hydraulic Research Institute, 1995. Wang Shanda. Research on the Influences of Flood Discharge Atomization Resulting from Energy Dissipation by Hydraulic Jump [A]. Symposia of the 17th National Hydrodynamics Forum and the 6th National Hydrodynamics Academic Conference [C]. Shanghai: 2003: 526-531. Zeng Xiang, Xiao Xingbin. Introduction to the Research on Flood Discharge Atomization of High Dams [J].Pearl River, 1997, (2): 22-25. Su Jianming and Li Haoran. The Influences of Flood Discharge Atomization of Ertan Hydropower Station on the Slope in the Lower Reach [J]. Hydrogeology and Engineering Geology, 2002, (2) : 22-24.

4

CONCLUSIONS

The feedback analysis on the model test results

and the prototype observation results of Wantang shows that, it is reasonable to determine the model scale according to the condition that the Weber number (We) is over 500 for the atomization model of hydropower stations adopting energy dissipation by hydraulic pump,the atomization incidence follows the gravity similarity criterion, but the intensity of atomized rainfall is based on the relation of Lr1.5. Also obvious scale effect of the atomization model is discovered and further researches with the prototype observation data or serial model tests are required. The results show that the impact of flood discharge atomization for Xiangjiaba Hydropower Station is only limited to a verysmall area nearby the distilling basin, and the rain intensity in the basin is very weak. Therefrom, the intensity of atomization source is determined to calculate the influences of the diffusion of atomized flow on the factory area and living area of Yuntianhua. The results also show that the flood discharge atomization of Xiangjiaba Hydropower Station won’t affect the factory area and living area of Yuntianhua, and that the energy dissipation by hydraulic jump effectively decreases the influences of flood discharge atomization.

ACKNOWLEDGEMENTS

Financial support was provided by the National Natural Science Foundation of China (50579084).

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