Low Ambient Pressure Effects On Flood Discharge Atomization

Over the past three decades,operation troubles and slope failures caused by flood discharge atomization have jeopardized hydropower station operations,and these issues are of widespread concern.Flood discharge atomization is a research subject of great importance for the development of basic safety theories applicable to major water conservancy projects,which is a complex two-phase flow problem involving air-water interactions in formation processes,and whose prediction and prevention are not only related to the economic cost and operational safety for the engineering project construction but also influence on the ecological safety of the surrounding environment.For high dam projects characterized by a high-water head,large discharge,and narrow canyon,the problem of flood discharge atomization is more prominent due to the uncertainty effects induced by the low-pressure environment at high altitudes.Consequently,it is urgent to examine the effects of low ambient pressure on flood discharge atomization.Since the major atomization source in ski-jump energy dissipation processes is random splashing by falling waterjets,in this thesis,random splash experiments under low ambient pressure were established to investigate the mechanism by which low ambient pressure influences flood discharge atomization,and random splash simulation models suited for low ambient pressure were presented to predict the influence scopes of atomized areas induced by high-altitude project flood discharge in environments with low ambient pressure.A series of studies and results（as follows）are intended to promote the risk perceptions and control methods of flood discharge atomization under complex discharge conditions from experience to scientific understanding to provide theoretical and technical support for the structural design and long-term safe operation of large-scale water resources and hydropower engineering.（1）To guide the protection of flood discharge atomization for high dam projects at high altitudes effectively,a large-sized experimental platform under the coupling conditions of low ambient pressure and high waterjet velocity was set up in the context of high-altitude hydropower station discharge.Then,a set of experimental devices based on high-speed waterjet atomization was developed.These devices generated atomized rain sources via collisions between a high-speed rectangular waterjet and a water cushion.In terms of high precision non-contact measurement and control in a local high humidity environment characterized by low ambient pressure,the technical points of testing instrument development regarding the atomized rain intensity and the atomized raindrop instrument installation maintenance were also described.This study can enable the establishment of an experimental foundation to measure the effects of low ambient pressure on flood discharge atomization.（2）In the random splash experiments involving atomized rain intensity under low ambient pressure,time-averaged characteristics of atomized point rainfall intensities in a horizontal measurement plane and time-history characteristics of atomized area rainfall intensities in an inclined measurement plane were compared under different low ambient pressure conditions.From above experiments the results are followed.When the ambient pressure decreases as other conditions remain fixed,the variation laws for atomized rain splashed by the release from waterjets show that the atomized rain intensity and the atomized source volume increase,and the characteristic rain intensity lines migrate linearly downstream;while the trend of atomized rain intensity reduction along the releasing centreline is similar to that observed under indoor atmospheric pressureⅠconditions（（P₁=0.99 P₀）and（P₀=101.325 k Pa））.Moreover,with ambient pressure levels decreasing from the standard atmospheric pressure condition（P₀）under constant inflow discharge,as the ambient pressure decrease by every 0.10 P₀,the average precipitation volume（L/h）of the atomized rainfall area in the inclined plane increases by 11.06%～20.48%,and the distance between the 10 mm/h atomized rain intensity line of the horizontal plane and the constricted nozzle outlet is extended to the downstream area by approximately 1%.This augmented knowledge can facilitate the construction of partition protection and numerical predictive systems for flood discharge atomization in high-altitude hydropower stations.（3）In the random splash experiments involving atomized raindrop size distributions under low ambient pressure,the effects of low ambient pressure on the atomized raindrop size distribution characteristics in different atomized rain intensity regions were analysed,and correlation analysis for the influence of ambient pressure,inflow discharge and measurement position on atomized raindrop size distribution was performed.The results of the above experiments are as follows.When the other parameters are constant,a decrease in ambient pressure can increase the total number concentration（number/m³）of atomized raindrops splashed by the release from waterjets.Meanwhile,an ambient pressure decrease can increase the total volume at diameter size grades of the mode and dominant diameters,and increase the peak of individual number concentration(number/（m³·mm）.However,the resulting decrease in ambient pressure has only a small effect on the spectrum trend and typical particle size ranges of the atomized raindrop size distributions,and on the number percentages of atomized raindrops in different particle size segments.Furthermore,with 5 mm as the critical particle size of an atomized raindrop in the atomized rainstorm zone（≥10mm/h）,the number of smaller particles is dominant and increases in all its size range,while that of larger particles increases at part of its size range when the ambient pressure levels decreasing from the indoor atmospheric pressureⅡconditions（P₁=1.02 P₀）under constant inflow discharge.However,the precipitation contribution of nonoptimal frequency band particles is more than 75%.Additionally,under the influence of various factors（the ambient pressure,inflow discharge and measurement position）,the scale spectra of atomized raindrop individual number concentration all remain a single peak spectral type skewed towards a small particle size,approximately matching the gamma distribution curve.The main peak particle size grades are mostly 0.562 mm and 0.687mm.This study is not only helpful for analysing the enhancement effect on atomized rain induced by low ambient pressure but also beneficial for numerically predicting flood discharge atomization in a high-altitude environment.（4）Random splash simulation models characterized by low pressure were developed in combination with the experimental results and applied for high-altitude high-engineering earth-rockfill dam projects.For the Quxue hydropower station,the influence scopes of atomized areas were analysed under numerous calculation conditions,including typical discharge conditions under different low ambient pressures and various discharge conditions under actual low ambient pressure.Regarding the Rumei hydropower station,for different discharge conditions under actual low ambient pressure,the influence scopes of atomized areas generated by different types of flip buckets were predicted,and their variation laws relative to normal ambient pressures were summarized.The predicted results show that with ambient pressure levels decreasing from the reference atmospheric pressure condition（₀P’=101.457 k Pa）with a certain gradient of 0.10₀P’,the maximum variation in the characteristic boundaries ranges within 10 m in numerical atomized areas enveloped by atomized rain lines of 10 mm/h at the Quxue hydropower station and of 40 and 10 mm/h at the Rumei hydropower station.The recommended relocation site for the village in the Quxue hydropower station under actual low ambient pressure（0.80₀P’）is an area downstream the x coordinate of 826.87 m in the calculation domain.This location is determined by referring to the critical value of natural light rain intensity of 0.42 mm/h.In the atomized rainstorm zone（≥10 mm/h）of the Rumei hydropower station with actual low ambient pressure of 0.75₀P’,the influence heights on the left bank are maximally equal to 0.83～0.85 times the maximum dam height of 315 m,and the distances between the downstream boundary of the 10 mm/h rain intensity line and the3#tunnel spillway outlet located downstream are within（666.80～692.40）m.As mentioned above,these applied studies can offer effective methods that more fully exploit numerical random splash models for partition protections of atomized areas for high-altitude hydropower stations.