Aerator Performance in Reducing Phenomenon of Cavitation in Supercritical Flow in Steep Channel Bed

Transcription

1 Engneerng Internatonal Conference UNNES Conservaton 13 Proceedng Aerator Performance n Reducng Phenomenon of Cavtaton n Supercrtcal Flow n Steep Channel Bed Yer Sutopo 1, Bud Wgnyosukarto, Istarto, dan Bambang Yulstyanto ( 1 Mahasswa Program Doktor Teknk Spl dan Lngkungan,Fakultas Teknk Unverstas Gadjah Mada Yogyakarta, Dosen Jurusan Teknk Spl FT UNNES, Emal: yerg3g@yahoo.com) ( Staf Pengajar Teknk Spl dan Lngkungan, Fakultas Teknk Unverstas Gadjah Mada Yogyakarta.) Abstract Ths research ams to (1) descrbe an ndex of cavtaton on a steep slope that has a channel wth a horzontal plane by and 5 ; and () fnd a soluton n the form of hydraulc structure n a low ts ndex cavtaton value. The hardware n the nstrument conssts of a seres of reflectve lght sensor wth flexble rubber membrane. One unt ADC 89 used to transform analog electrcal quantty nto bnary data. Mcrocontroller software n ths study made by usng assembly language and computer software usng the programmng language Delph 6.. These nstruments are mplemented on the long acrylc flume 11 m, wdth. m and a heght of.4 m and tlts and 5. At the end of the upper acrylc flume assocated wth ponds that are located at 5. m above the floor of the laboratory. Dscharge nflow was.9 l/s. cavtaton ndex measured at the locaton of the 4,53 m, 3,53 m, 6,3m, and 7,3 m on the lower. Based on research on the slope of flume wth of the horzontal plane as well as dscharge.9 l/s are: (1) on the locaton of 3,53 m from ; the sze of σ (ndex of cavtaton)=,41, p or pressure at the channel bed = 33,58 N/m, Froude number = 5,49; () on the locaton of the flume nlet 4,53 m, the sze of σ (ndex of cavtaton),41, p or pressure at the base of the channel = 7,48 N/m, Froude number = 5,61; (3) on the locaton of 6,3 m from, the sze σ (ndex of cavtaton)=.4, p or pressure at the base of the channel = 1,37 N/m, Froude number = 5,74; and (4) on the locaton of 7,3 yards from the, the sze σ (ndex of cavtaton)=.39, p or pressure at the base of the channel = 18.3 N/m, Froude number = Based on research on the slope of flume wth 5 of the feld horzontally debts as well as 13 l/s are: (1) on the locaton of 3,53 m, of the, the sze σ (ndex of cavtaton) =,38, p or pressure at the base of the channel = 3,53 N/m, Froude number = 6.5; () on the locaton from the 4,53 m, the sze σ (ndex of cavtaton)=,36, p or pressure at the base of the channel = 7,48 N/m, Froude number = 6.6; (3) on the locaton of 6,3 m from, the sze σ (ndex of cavtaton)=.333; p or pressure at the channel bed = 18.3 N/m, Froude number = 6.7; and (4) on the locaton of 7,3 m from the, σ (ndex of cavtaton),335, p or pressure at the channel bed = 1 N/m, Froude number = 6.8. Based on the results of the study, the aerator s mounted n the locaton from of 7,3 m, on.9 l/s dscharge, and the slope of flume by 5. The dmensons of the aerator based on research results and calculatons are (1) hgh deflector 6 mm, length 3 mm, wdth mm; () duct sze s mm, length 8 mm, and (3) ar vents dameter 1 mm. Installaton of aerator desgn results n the 7,3 m locaton from the lower beneft prevent the occurrence of cavtaton, due to the concentraton of ar bubbles at the channel bed reachng 57,98%. Keywords Index cavtaton ; concentraton an ar bubble C; aerator. I. INTRODUCTION Accordng to Borman as quoted by Falvey [1], the self ar entranment regme n the chute spllways nvolvng three regons are (1) no ar entranment; () developng; and (3) fully developed. Supercrtcal flow that occurs n the chute and the regon of no ar entranment as well as developng regon can cause cavtaton. Cavtaton occurs n an area where the pressure s lower as well as n regons where ar bubbles have yet to touch the channel bed or n developng regon. The hgher the flow velocty n steep channels, the greater the chance of the occurrence of cavtaton. Hgh velocty flow cause the occurrence of negatve or low pressure n the flow, especally at the bed of the steep channels. The force caused by negatve pressure wll draw the elements at the bed of the steep channels that would lead to exfolate the surface of the bed. Gradually a bed peel steep channels wll form small holes whch subsequently became a huge hole that s endangerng the structure. The symptoms descrbed above are often referred to as cavtaton. The bed eroson cause of the steep channel damage can be prevented by mprovng hgh pressure or by way of buldng the wall bed and sde channels on a very fne condton. However, buldng the bed and walls of very fne channels are usually not easly put nto practce or are very dffcult to acheve. For example, damage to ec.ft.unnes.ac.d - II-81 -

2 Engneerng Internatonal Conference UNNES Conservaton 13 Proceedng concrete structures due to eroson caused by the velocty of the flow of 9,4 m/s for 3 hours, ths has resulted n a hole as deep as 1.5 cm. On the condtons the same velocty f the polymer concrete s used, the sze of the hole of 1.5 cm was acheved after 6 hours of occurrence of eroson. However, the use of the materal as above also have physcal lmtatons, and can only wthstand eroson for a certan tme. The method s relatvely new and hghly effectve way to prevent damage due to eroson bed how to elmnate ar pockets along the boundary flow. Ths s done by nstallng ar entrament slots. It s desgned specfcally so that ncomng ar along the layer boundary. Ths method has been carred out to prevent damage due to eroson bed on a varety of hgh velocty flow ncludng on the chute spllways. Test of aeraton devce shows that n the boundary layer, ths tool affect an ncrease n pressure. Based on the smulatons ndcated that aeraton s able to ncrease the pressure on average up to 5% on top of the condtons before the tool s plugged. Geometry and locaton of the placement of the aeraton of slot untl now contnue to be studed, because there hasn't been any recommendatons that have been approved by the experts of hydraulcs. A combnaton of ramps and groove can form a space greater pressure behnd the ts ramps, compared to the ramp wthout the groove. Ths s possble due to a combnaton of ramps and the groove was able to ncrease the sze of the ncomng ar. The man desgn for varous aerator at Gouptan Project adopted ths type of combnaton,.e. wth ramps and groove. Ths research ams to (1) descrbe an ndex of cavtaton n flow more n steep channels that have a slope wth the horzontal plane by and 5 ; () fnd a soluton n the form of hydraulc structures/aerator n a low ts ndex cavtaton value; and (3) descrbe the effectveness of aerator n preventng the phenomenon of cavtaton n at the supercrtcal flow n the steep channels bed. II. II.1 The Steps to Determne Cavtaton Index RESEARCH METHOD ADC (Analog- Dgtal Converter) of 8 bts, the nterface wth the Assembly language Computers that process data nto graphs or curves usng the Delph language. Relatonshp between computer and data logger usng the USB Port Amplfers, amplfes the output sgnal from the sensor to be able to read ADC Pressure Sensor usng reflectve lght sensor Fg.1. Data acquston system of flow chart the pressure at steep channel bed ec.ft.unnes.ac.d - II-8 -

3 Engneerng Internatonal Conference UNNES Conservaton 13 Proceedng The pressure at the bed of a flow s read by the transducer that s manfested n the form of the vertcal scale of the ADC n the computer screen. The vertcal scale s not yet a pressure scale, therefore ths scale must be calbrated, so that a scale expressed n real terms as the pressure scale n mm HO. Calbraton steps follow the stages such as n the followng. (1) n the form of a rubber membrane daphragm mounted n cap the dameter s.5 nches; () further cap s connected to the ppe whch dameter.5 nches that vacuum condtons; (3) rubber membrane daphragm s connected to the pre amplfer and amplfer DC and DC output transducer; (4) was next wth software n the computer; (5) cap ppe n step 1 s connected to the vacuum pump through rubber hose; (6) a vacuum pump connected to a rubber hose through the manometer, manometer s nstrument calbraton of vertcal scale as a comparson between the results of transducer to the scale of the real pressure; (7) vacuum pump s pressed, ths s a manfestaton of postve pressure, then the computer wll show the scale of the response s postve, so that ths form of postve scale s the scale of the real postve pressure, then the scale of the manometer must read also, whch s the content of the lqud n the manometer s water, and then ts pressure p=ρ ar gh ar. II. Steps Determnng The Dstrbuton Of Ar Bubbles Concentraton Fg.. Flow chart to determne concentraton of ar bubbles The steps determne the concentraton dstrbuton of ar bubbles C: (1) take a pcture of ar bubbles usng a CCTV camera ftted wth a CCD sensor; () pcture of the ar bubbles are read usng software vdeo capture; (3) the vdeo mage s stored nsde the fle software vdeo capture; (4) the vdeo mage was made nto a stll mage (stll mage) usng a specal Batch Convert; (5) descrbe the dmensons and levels of greysh ar bubbles wth the Image software; (6) arrangng the table preparaton concentraton dstrbuton of ar bubbles C usng a spread sheet software; (7) make the curve s the relatonshp between the depth z and concentraton C ar bubbles usng a spread sheet; and (8) to calculate the equaton of dstrbuton ar bubbles usng a spread sheet. ec.ft.unnes.ac.d - II-83 -

4 Engneerng Internatonal Conference UNNES Conservaton 13 Proceedng III. THE RESULTS OF RESEARCH AND DISCUSSIONS III.1 Froude Number And Cavtaton Index At The Bottom Of Steep Channel Table 1. Index of cavtaton n a four pont of dscharge flume nlet,9 l/s slope of flume Poston ρ ar (kg/m 3 ) g (m /s) h pressure head (mm) 7,3 m on 6,3 m on 4,53 m on 3,53 m on Source: Draft report of the dssertaton, Yer Sutopo[3] p pressure at the channel bed N/m U (flow rate) m/s and Froude Number σ (cavtaton ndex) 998,8 9,81 18,414 18,31 3,91536/5,88, ,8 9,81 1,483 1,3748 3,4741/5,74, ,8 9,81 7,61 7,4819 3,191489/5,61, ,8 9,81 33,759 33,5889 3,143713/5,49,41 Based on Tables 1 and. below, t appears that the aerator can be mounted on the nlet 7,3 m flume, because the value of the cavtaton ndex σ ; (Thoma Number) between the value of.17 to.5 and Froude Numbers > 6,Arora [4]; whle n the poston of the other basc channels requred refnement. Accordng to Henry t. Falvey [1] σ = 3 cavtaton does not occur, σ = 1.8 was the early onset of cavtaton, and, 3 < σ< 1.8 cavtaton has been growng, whle σ<.3 ncluded n category supercavtaton. Poston Table. Index of cavtaton n a four pont of dscharge flume nlet,9 l/s slope of flume 5 ρ ar (kg/m 3 ) g (m /s) h pressure head (mm) 7,3 m on 6,3 m on 4,53 m m on 3,53 m m on Source: Draft report of the dssertaton, Yer Sutopo[3] p pressure at the channel bed N/m U (flow rate) m/s and Froude Number σ (cavtaton ndex) 998,8 9,81 1,76 1,14 3,63318/6,8,99 998,8 9,81 18,414 18,31 3,59589/6,7,36 998,8 9,81 7,61 7,4819 3,5593/6,6, ,8 9,81 3,69 3,5354 3,5349/6,5,319 III. The Draft Aerator Research a. Specfy the ndex of the cavtaton flow n the entre length of flum's research, partcularly on the lower ar ntake pont for the frst tme ths has happened or n regon of no ar entranment; ec.ft.unnes.ac.d - II-84 -

5 Engneerng Internatonal Conference UNNES Conservaton 13 Proceedng p pv where : Index of cavtaton; U / the aerator; p : Vapor pressure of water at a temperature of C; v wdth flum =, m; : Densty of water at C=998,8; p : Pressure pont revew the potental locaton of U : Average flow rate; b : Base b. Determne the locaton aerator based on the value of the ndex cavtaton on at a step a; c. Estmate the angle of nclne of the horzontal (ramp), gx Cos (tan tan ) n whch U, use the followng Equaton X : the dstance from the tp of a horzontal nclne to the pont that nfluenced the trajectory of water (jet); : the angle between the base of the spllway wth the horzontal; : angle of the ramp (ramp) towards horzontal; : the angle e between the tangent to the trajectory of the jet at ramp to the horzontal s calculated usng the equaton ( 1 A ) A where e r r A s a (jet trajectory coeffcent) or (the relatve ramp heght r factor); : angle of the ramp (ramp) towards basc flum; d. Usually the structure not only of aerator ramp alone, but requred offset duct or all at once. The equaton s used to determne the need for offsets or duct s h p z F z z {1 ( ) } cos {( ) z / z z z 1}. The decson requred offset or not f the value s h Y Y ) where the depth of the Groove (duct) D s based on the equaton p ( D d,15 e. Specfy concentraton an ar bubble n hlr aerator through research; f. Calculate the concentraton of the ar n the bottom of the channel launch aerator based on research results. If the concentraton of the ar base of the channels launch greater than.1 or 1%, then the other aerator s not requred. If the ar concentraton of less than.8 or equal to.1, then at that locaton should be nstalled a new aerator; ec.ft.unnes.ac.d - II-85 -

6 z Normal depth from channel bed Engneerng Internatonal Conference UNNES Conservaton 13 Proceedng g. In the constructon of the aerator the next. Based on the step (a) to step (g) above, then found the dmensons of the aerator as Fgure 3 below. Fgure 3. The dmensons of the desgn of the research results of aerator III.3 Concentraton Dstrbuton Of Ar Bubbles In Steep Channels Downstream Aerator Ar bubbles concentraton C (%) Fg. 4. Concentraton dstrbuton of ar bubbles that are 1.5 m aerator Accordng to Fgure 4 above, t appears that the concentraton of ar bubbles at the bed of the flow that s 1.5 m aerator s 57,98%. Accordng to Falvey [1], f the concentraton of ar bubbles at the bed of the channel s greater than.1 or 1%, then the other aerator s not requred. If the concentraton of ar bubbles s less than or equal to.1.8, then at that locaton should be nstalled a new aerator. Thus, the nstallaton of aerator n 7,3 m flume nlet on the lower beneft prevent the occurrence of cavtaton, due to the concentraton of ar bubbles at the bottom of the flow reachng 57,98%. Accordng to Pettersson [], ths s n accordance wth the opnon of Peterka that ar bubbles that have reached the bed of the channel can prevent damage due to cavtaton, as well as accordng to the Chanson that ar bubbles by 5-7% who have reached the steep channels bed can prevent damage due to cavtaton. ec.ft.unnes.ac.d - II-86 -

7 Engneerng Internatonal Conference UNNES Conservaton 13 Proceedng Table 3. The Mean Concentratons of Ar Bubbles n Fve Successve Frames C (%) No. z The normal depth from channel bed (mm) The mean concentratons of ar bubbles n fve successve frames C (%) C mean (%) Frame to ,14 57,81 56,14 6,81 5, 57,98,1 76,66 73,33 71,66 78,33 64,83 7,96 3 4, 9,8 88,95 87,8 93,95 79, 88,3 4 6,3 95,58 9,4 9,58 97,4 81,41 91,41 5 8,4 84,95 81,6 79,95 86,6 79, 8,43 6 1,5 91,96 88,63 86,96 93,63 81,73 88,58 7 1,6 81,8 77,95 76,8 8,95 8,63 8, 8 14,7 91,43 88,1 86,43 93,1 9,55 89,9 9 16,8 84,95 81,6 79,95 86,6 85,39 83,7 1 18,9 87,61 84,8 8,61 89,8 87,74 86, ,6 85,8 83,6 9,8 88,48 87,5 1 3,1 9,16 86,83 85,16 91,83 88,16 88, , 84,95 81,6 79,95 86,6 9,8 84, ,3 9,9 89,59 87,9 94,59 91,3 91,1 15 9,4 9,16 86,83 85,16 91,83 89,1 88, ,6 9,71 89,37 87,71 94,37 93,31 91, ,7 9,16 86,83 85,16 91,83 91,35 89, ,8 91,91 88,58 86,91 93,58 95,38 91,7 39,9 91,86 88,53 86,86 93,53 96,8 91, ,41 91,8 89,41 96,8 93,5 9,8 44,1 91,43 88,1 86,43 93,1 94,53 9,7 3 46, 9,16 86,83 85,16 91,83 93,95 89, ,3 9,9 89,59 87,9 94,59 96,45 9,3 5 5,4 89,31 85,98 84,31 9,98 97,94 89,7 6 5,5 94,41 91,8 89,41 96,8 96,8 93, ,6 94,41 91,8 89,41 96,8 98,6 93, ,7 9,9 89,59 87,9 94,59 91,83 91, ,8 94,41 91,8 89,41 96,8 96,8 93,41 3 6,9 93,61 9,8 88,61 95,8 94,54 9, ,65 88,31 86,65 93,31 93,95 9, ,1 91,1 87,78 86,1 9,78 96,8 9, , 93,45 9,1 88,45 95,1 98,15 93, ,3 94,94 91,61 89,94 96,61 96,98 94, 35 71,4 96,11 9,78 91,11 97,78 98,31 95, Source: draft report dssertaton, Yer Sutopo[3] ec.ft.unnes.ac.d - II-87 -

8 Engneerng Internatonal Conference UNNES Conservaton 13 Proceedng IV. CONCLUSION AND RECOMMENDATIONS IV.1 Cloncuson Frst, the slope of flume wth of the horzontal plane as well as dscharge.9 l/s (1) at the locaton of 3,53 m from ; the sze of σ (ndex of cavtaton)=,41; () at the locaton of 4,53 m from nlet flume, the sze σ (ndex of cavtaton),41; (3) at the locaton of 6,3 m from, the sze of σ (ndex of cavtaton)=.4; and (4) at the locaton of 7,3 m from, the sze of σ (ndex of cavtaton)=.39. Slope of Flume wth 5 of the feld horzontally dscharge as well as 11,4 l/s are: (1) at the locaton of 3,53 m from, the sze of σ (ndex of cavtaton)=,38; () at the locaton of 4,53 m from, the sze of σ (ndex of cavtaton)=,36; (3) at the locaton of 6,3 m from, the sze of σ (ndex of cavtaton)=.333; and (4) at the locaton of 7,3 m from,the sze of σ (ndex cavtaton)=,335; Secondly, the dmensons of the aerator based on research results and calculatons are (1) hgh deflector 6 mm, length 3 mm, wdth mm; () duct sze n mm, length 8 mm, and (3) ar vents dameter 1 mm; Thrd, the nstallaton aerator desgn results over at the locaton of 7,3 m flume nlet on the lower beneft prevent the occurrence of cavtaton, due to the concentraton of ar bubbles at the bottom of the flow reachng 57,98%. IV. Recommendaton The number of the aerator s mounted on the chute spllway does not need too much. On the length of chute spllway wth up to 4 m and a wdth of 1 m, t only takes one aerator unt combnaton of ramp or deflector, offsets, and the duct. ACKNOWLEDGEMENTS Researchers Say extended grattude to Prof. Ir. Djoko Legono, Ph.D. head of Hydraulcs Laboratory, Department of Cvl Engneerng and Envronment, Faculty of Engneerng, Gadjahmada Unversty (GMU) Yogyakarta, because allowed to carry out research n ths laboratory. REFERENCES Journal papers [1] Falvey, T. H., Cavtaton on chutes and spllways, (Engneerng Monograph, Denver Colorado, pp. 59-7, 199) [] Pettersson, K., Desgn of aerator for preventon of Cavtaton the Holjes Dam, Degree Project for the Master s Program n Cvl Engneerng, Royal Insttute of Technology (KTH) SE-1 44 STOCKHOLM, Sweden, 1. [3] Yer Sutopo, 1, Pemasukan udara alamah dan buatan (self and artfcal ar entranment) d saluran curam, Draft laporan dsertas, Teknk Spl FT UGM Yogyakarta, 1. [4] Arora, J..C., Gudelnes for prelmnary desgn of spllway aerators, (Water Resources Dvson Councl USA, 1) ec.ft.unnes.ac.d - II-88 -