Research On Calculation And Application Of Unsteady Flow In Irrigation Network

The pipeline has become a key technology of water-saving agriculture in the worldbecause of high efficiency, simple construction, less land occupation and low maintenancecosts. The irrigation pipe network always uses PVC、PE and other plastic pipe. The pressureof this pipe is relatively low, with often led broken and made the whole system fall intoparalyzed state by water hammer caused by rapid opening and closing valve and pumpstopped. The water hammer damage generated in network is higher than the same excitationin pipeline. With the development of drip irrigation and sprinkler irrigation, the pipe materialdeveloped to the light direction and the structure of network will be more complex, and thewater hammer problems will be more and more outstanding. So research on the unsteady flowproblems in irrigation network has important significance for correct management and safeoperation.This paper studies the process of unsteady flow in irrigation network by experiment andnumerical simulation. As initial conditions of unsteady flow calculation, the constant flow isdiscussed in prior. The main research content includes: Study on the resistance characteristicsand flow characteristics of PVC tee pipes and PVC ball valve; Study on the water hammer ofsimple PVC pipe network by Tests and analysis; Discuss and build unsteady flow model ofdrip irrigation and sprinkler irrigation pipe network, and research on water hammer protection;Design the calculation program of unsteady flow. The main results are as follows:(1) The resistance of PVC tee pipes was studied and the data was used in the networkcalculation. Due to boundary effects in the bifurcation increased the flow mixing andturbulence, the water flow advanced into the similar pipe drag square zone friction coefficient.The Reynolds number which the local resistance coefficient tended to stable value wan about1.5×105。The resistance coefficient quantitative expression of7kinds of pipes was obtainedby experiment and numerical simulation, which the local resistance coefficient was correlatedwith the flow by second-order parabolic equations. Flow characteristics analysis showed thatthe main reason caused the main branch to side branch due to the loss of direction changesand velocity distribution cause by centrifugal force, and changes main branch to straightbranch was due to the vortex motion and velocity gradient in the straight caused by water shearing and transverse circulation in the large flow ratio.(2) The resistance of ball valve was the important parameter to calculate the steady flowand unsteady flow.5kinds of power function of PVC ball resistance coefficient with openingdegree was obtained by experiment and numerical simulation. The flow field analysis showedthat there has flow region and large vortex in the small opening degree which caused greaterhydraulic losses.(3) Through comparison of experiment and calculation showed that the MOC methodcan better predict the maximum water hammer pressure in pipe network which the relativeerror between experiment and calculation is less than13%. The pressure wave attenuation ofsimulation is slower than experiment because of the constant flow friction is used in thecalculation. The maximum water hammer pressure is reduced with the closing time extension.The maximum water pressure of difference closure rules was distinct, with the linear closingpattern was maximal, the two-stage straight closing pattern by the second, and the curveclosing pattern was lowest. In the curve closing pattern of power function, the pressuregenerated by power function pattern was less than the parabolic closed mode.(4) The unsteady flow mathematical model of drip irrigation pipe network and sprinklepipe network was established and the optimal step processing of pipe segments was studied. Amethod which used the equivalent capillary to replace primary capillary was presented. It wasaffirmed that the processed reflection and transmission of boundary water hammer wave wassimilar to original. The calculation of typical network showed that the double wave was easilyformatted in pipe network when several valve closed simultaneously, and the value ofmaximum water hammer pressure was greater than no overlapping wave. The maximumwater hammer pressure in pipe network was related to the distance of operation value, withthe distance was closer, the pressure was greater. The maximum head of calculation networkwas about2~5times of steady and the maximum water hammer occurred at a time when thevalve completely closed. From the point of view to the wave attenuation, attenuation of waterhammer wave in the pipe network was significantly larger than the long distance pipelineswith conveying fluid because of large nodes and branches in the network which caused muchresistance losses.(5) Arranged air tank in pipe network before operation valve and outlet location of pumpcould decrease the maximum water hammer pressure by about30%, and could control thenegative pressure effectively to reduce the probability of water hammer damage.(6) The network information expression, design method of coding and unsteady flowcalculation in view of the complex network was introduces based on Visual Basic6platformsystem. It showed that the designed procedure with simple input, calculation and convenient output, which can be used to calculate the actual pipe network.(7) The mathematical model established in this paper can be conveniently calculate thewater hammer variation of each part of the whole pipe network, pumping station and the ruleof maximum water hammer pressure, so that indicate the water hammer pressure endangerdegree, range and location to provide a basis for pipe network protection.