Study On The Hydraulic Conveyance Theory Of Coarse Particles In Horizontal Sediment Laden Flow Pipeline

During the dredging of small and medium-sized reservoir pipelines,large pebble particles in the silts are inevitably sucked into the channel,causing pipeline blockage and bringing many troubles to engineering practices such as reservoir dredging.During the movement of coarse pebble particles driven by high sand-content water flow in the dredging pipeline,the coarse pebble particles will have a feedback effect on the flow field structure of the high sand-content water flow,which leads to an increase in energy consumption of the pipeline system and a decrease in overall conveying efficiency.The coarse particle pipeline hydraulic transport motion mechanism and its reciprocal feedback effect is a key technical challenge in pipeline sand discharge.Therefore,this paper adopts current situation analysis,model test,and theoretical derivation research methods.It combines the fundamental theories of particle media mechanics,fluid mechanics,and sediment dynamics to research the mechanism of coarse particle movement and its feed-forward effect in sand drainage pipelines,to clarify the solid-liquid two-phase flow transport mechanism under different coarse particle size,coarse particle sand transport rate,and pipeline flow velocity,to clarify the pressure the distribution law of pipe pressure under diverse pipeline conveying mechanism is defined.The inner connection between coarse particle size and coarse particle sand transport rate,along-range loss of pipe time-averaged pressure,the probability distribution of pulsation pressure amplitude,and power spectrum distribution is constructed to provide a theoretical reference for future pipeline sand dredging work.The results show that:(1)Sort out the current situation of sediment siltation in small and medium-sized reservoirs in Henan Province.The total volume of reservoirs in Henan Province is 716 million m~3,the sediment accumulation is mainly medium and coarse sand,most of the reservoirs are damaged due to sediment accumulation,and only a few cities have carried out reservoir dredging for a few reservoirs;three cities,Nanyang,Luoyang,and Zhumadian,account for the highest proportion of sediment accumulation in Henan Province,all of them are higher than 15%;meanwhile,the advantages and disadvantages of standard dredging methods for small and medium-sized reservoirs are clarified,and the necessity of pipeline sand dredging is emphasized.(2)The horizontal pipeline clear water coarse particle transport test was carried out,and a high-speed camera PTV(Particle Track Velocimetry)observation system was used in the observation section of the pipeline.Based on the movement state of coarse particles in the pipeline,three transport mechanisms were determined,namely,aggregate conveying,layer shift conveying,and mobile conveying,and the characteristics of the pressure distribution at the bottom of the pipeline for the three transport mechanisms were studied;the results of the pipeline clean water test were compared with those of the sediment laden water flow test under the same working conditions,and based on the characteristics of the overall distribution of the pressure in the sediment laden water flow pipeline,it was concluded that the movement state of coarse particles in the sediment laden pipeline flow could also be classified into aggregate conveying,layer shift conveying,and mobile conveying.(3)Study the influence mechanism of coarse particle size on pipeline hydraulic conveying.The changing trend of different pressure with particle size is the same between pipeline clear water and sediment laden water conveying;only part of the form of the fitting function has changed;sediment laden water flow carries a lot of suspended fine particles,and the internal turbulence of pipeline flow is enhanced;when sediment laden pipeline water conveying,the proper function of a normal distribution with coarse particle size is not monotonous,and the distribution form of the appropriate part of pulsating pressure is significantly affected by the sand carried by pipeline flow;the pulsating pressure frequency is also concentrated in the main frequency range of 2～20 Hz during the pipeline sediment laden water flow conveying,with the increasing coarse particle size,and the central frequency gradually shifts from high frequency to low frequency,especially the energy in the main frequency range of 5～15 Hz increases significantly.(4)Study the influence mechanism of coarse particle sand transport rate on pipeline hydraulic conveying.The variance of instantaneous pressure and pulsating pressure increases under the same coarse particle transport rate,the internal turbulence of pipeline flow increases,the energy loss of pipeline transport accelerates,and the energy consumption frequency range increases significantly(from 0～5 Hz to 0～24 Hz)when the sand is held by pipeline flow.With the increase of coarse particle transport rate,the pressure variation law at the bottom of the pipeline for clear water and sediment laden water flow transport tends to be the same when layer shift conveying is transformed into the aggregate conveying,but the pulsating peak pressure increases with the increase of coarse particle transport rate as an exponential function with natural number e as the base;the energy consumption frequency range increases significantly(from 0～13 Hz to 0～24 Hz)when the pipeline flow holds sand.