Flow measurement at the short structures in hydraulic complex conditions: HE 'Djerdap 2' case study

Abstract

Hydraulic conditions at short structures in most cases prevents the existence of cross sections with parallel streamlines and fully developed turbulent flow profile. In such conditions the flow measurement is rather challenging. In spite of recent developments in flow measuring methods, there is no universal device that can be used in inappropriate hydraulic conditions with adequate accuracy. This paper gives the overview of the contemporary flow measuring methods with the focus on its usability in hydraulically difficult conditions. As a case study, the problem of flow measurements at the intake of the aggregates of Hydroelectric Power Plant (HE) 'Djerdap 2' is presented. Due to its position in Danube and leftovers of preconstruction works in river bed, there is a significant incoming angle of the water to the HE 'Djerdap' turbines. The streamlines angle is larger at the Serbian side then at the Rumanian side, resulting the worse efficiency of the Serbian turbines. To analyze the effect of the curved incoming stream to the turbine performance, the true flowrate has to be measured. Since the Kaplan's short turbines were used and no regular cross section where flow can be measured using standard methods exists, available measuring methods for velocity field measurement were analyzed in this paper. For selected method based on electromagnetic velocity measurement devices, the field check was performed and presented. The paper concludes with the assessment of accuracy and total costs for selected flow measurement method.

Merenje protoka na objektima gde ne postoje uslovi za formiranje pravolinijskog razvijenog turbulentnog strujanja predstavlja poseban izazov. I pored značajnog napretka u razvoju merne tehnike, ne postoji univerzalna metoda kojom je moguće izmeriti protok sa u napred poznatom tačnošću. U ovom radu je napravljen presek trenutno raspoloživih metoda sa osvrtom na njihovu upotrebljivost u složenim uslovima kao što je slučaj prostornog rasporeda brzina. Kao primer, razmatran je problem merenja protoka na zahvatima agregata hidroelektrane HE 'Đerdap 2'. Zbog asimetričnog položaja elektrane u odnosu na maticu toka reke Dunava, kao i zbog toga što radovi na čišćenju dna uzvodno od brane nisu sprovedeni do kraja, voda na turbine dotiče pod izvesnim uglom. Taj ugao je veći kod turbina bližih sredini brane (kod srpskih turbina) dok je kod turbina postavljenih uz levu obalu (rumunsku) zanemarljiv. Da bi se analizirao uticaj kosog dostrujavanja na rad turbina, neophodno je izmeriti protoke. Kako su kod primenjenih kratkih cevnih agregata loši hidraulički uslovi za kvalitetno merenje protoka standardnim metodama, u radu se analiziraju moguće varijante snimanja prostornog rasporeda brzina. Za slučaj korišćenja elektormagnetnih sondi, obavljena je i provera uticaj a smetnji od strane magnetnog polja generatora. Na kraju, procenjena je moguća tačnost merenja protoka kao i cena takvog sistema.