3 結(jié)果與分析
在渠道比降(i)分別為1/1 000,1/2 000,1/3 000 和1/5 000,圓頭量水柱的喉口收縮比(ε)分別為0.75,0.70,0.63,0.56,0.50和0.44,流量(Q)分別為0.015,0.025,0.035,0.045和0.055 m3/s的條件下,進(jìn)行了120組試驗(yàn),獲得了各試驗(yàn)條件下渠道及圓頭量水柱8個(gè)關(guān)鍵點(diǎn)處的水深,并分析了不同渠道比降條件下渠道流量與圓頭量水柱駐點(diǎn)水深、喉口收縮比的關(guān)系。
3.1 圓頭量水柱駐點(diǎn)水深與流量的關(guān)系
不同渠道比降和喉口收縮比時(shí)流量與圓頭量水柱駐點(diǎn)水深的關(guān)系見(jiàn)圖5。從圖5可以看出,駐點(diǎn)水深與流量之間呈現(xiàn)良好的指數(shù)相關(guān)關(guān)系,相關(guān)系數(shù)達(dá)0.997。同一渠道比降下,隨喉口收縮比的增大,流量與駐點(diǎn)水深關(guān)系曲線上移。由于臨界水深的不穩(wěn)定性,因此用駐點(diǎn)水深代替臨界水深計(jì)算理論流量是可行的。通過(guò)對(duì)流量與喉口收縮比、駐點(diǎn)水深之間相關(guān)關(guān)系的分析,建立了不同渠道比降時(shí)具有量綱和諧性的流量計(jì)算公式為:
(8)
(9)
(10)
(11)
式中:ε為喉口收縮比;Hs為駐點(diǎn)水深,m;b為喉口斷面水面寬度,m。
式(8)~(11)是在n=0.011的有機(jī)玻璃渠道試驗(yàn)基礎(chǔ)上推導(dǎo)得出的,灌區(qū)實(shí)際渠道的糙率普遍較0.011大,故本試驗(yàn)即將在糙率較大的末級(jí)渠道上進(jìn)行研究,以對(duì)其進(jìn)行進(jìn)一步修正。
3.2 圓頭量水柱喉口收縮比的選擇
圓頭量水柱喉口收縮比定義為喉口斷面面積(Ac,m2)與渠道襯砌斷面面積(A0,m2)之比[13],即ε=Ac/A0。喉口收縮比是影響圓頭量水柱測(cè)流精度的重要參數(shù),如果收縮比過(guò)大,喉口處難以形成臨界流;如果收縮比過(guò)小,則造成上游壅水高度增大,水頭損失增大。因此,只要將喉口收縮比限定在一定范圍內(nèi),那么因收縮比而導(dǎo)致的測(cè)流誤差將會(huì)限定在某一較小數(shù)值范圍內(nèi)。不同試驗(yàn)條件下圓頭量水柱上游的壅水高度見(jiàn)表3。
當(dāng)喉口收縮比為0.50~0.70時(shí),測(cè)流誤差較小,且上游壅水高度較小,基本控制在5 cm內(nèi),滿足灌區(qū)對(duì)于上下游水頭差不高于5 cm的要求,因此較適的喉口收縮比為0.50~0.70。鑒于渠道比降為1/5 000時(shí)上游壅水高度的變化趨勢(shì)與渠道比降為1/1 000,1/2 000和1/3 000時(shí)一致,故表3只列出了不同流量和適宜喉口收縮比(0.50~0.70)條件下渠道比降為1/1 000,1/2 000 和 1/3 000 時(shí)圓頭量水柱的上游壅水高度。表3表明,同一流量下,渠道上游壅水高度隨喉口收縮比的增大而減小。
3.3 圓頭量水柱的臨界淹沒(méi)度
圓頭量水柱的臨界淹沒(méi)度定義為不影響上游水位的最大下游水深與上游水深之比[14]。其測(cè)定方法為:在相同流量下,通過(guò)調(diào)節(jié)渠道尾門(mén)來(lái)改變下游水位,下游水位在一定范圍內(nèi)不影響上游水位,隨著下游水位的繼續(xù)升高,當(dāng)下游水位開(kāi)始影響上游水位時(shí),此時(shí)下游水深(H2)與上游水深(H1)的比值為臨界淹沒(méi)度,即σ=H2/H1,較高的臨界淹沒(méi)度可以保證圓頭量水柱較大范圍的自由出流。對(duì)同一喉口收縮比的圓頭量水柱而言,流量不同時(shí)臨界淹沒(méi)度也不同,一般當(dāng)圓頭量水柱滿足最大流量下的臨界淹沒(méi)度要求時(shí),其他流量下的臨界淹沒(méi)度也隨之滿足。本試驗(yàn)通過(guò)對(duì)不同渠道比降、喉口收縮比、流量下圓頭量水柱臨界淹沒(méi)度的測(cè)試,得出其臨界淹沒(méi)度可達(dá)0.90,說(shuō)明該量水柱具有較大的自由出流范圍。
3.4 圓頭量水柱的水頭損失
水流在通過(guò)裝有圓頭量水柱的U形渠道時(shí),由于側(cè)收縮的存在,速度沿水流方向增加,由于邊界層及水流混摻碰撞的影響產(chǎn)生了局部水頭損失,由摩阻引起的沿程水頭損失比由于喉口形狀變化所引起的局部水頭損失小得多,因此沿程水頭損失可以忽略不計(jì)。通過(guò)增加尾翼對(duì)圓柱形量水槽進(jìn)行體型優(yōu)化,得到的圓頭量水柱可以有效地減小水頭損失。試驗(yàn)結(jié)果表明,在渠道比降為1/2 000,喉口收縮比為0.63,流量為0.035 m3/s情況下,無(wú)尾翼(L/D=1)的圓柱形量水槽水頭損失為7.94%,而L/D= 3/2 和L/D=2的圓頭量水柱的水頭損失分別為6.82%和6.11%,可知相同喉口收縮比下有尾翼的圓頭量水柱比無(wú)尾翼圓柱形量水槽的水頭損失要小。圖6反映了圓頭量水柱L/D=3/2時(shí),不同渠道比降及喉口收縮比(ε=0.56,0.63,0.70,0.75)下圓頭量水柱的水頭損失占上游總水頭的比例,由于喉口收縮比過(guò)小(ε=0.50,0.44)時(shí)下游水頭損失較大,故本研究不考慮,相應(yīng)數(shù)據(jù)在圖6中未列出。
從圖6可以看出,當(dāng)喉口收縮比為0.56~0.75時(shí),圓頭量水柱的水頭損失hf最小為上游總水頭H1的1.5%左右,水頭損失最大為上游總水頭的 9.5% 左右。文獻(xiàn)[15]中長(zhǎng)喉道量水槽水頭損失約為13%,與之相比圓頭量水柱的水頭損失較小。
3.5 圓頭量水柱的測(cè)流精度
測(cè)流精度一般用測(cè)流相對(duì)誤差表示,相對(duì)誤差為計(jì)算流量值與實(shí)際流量值之差與實(shí)際流量值的百分比[16]。根據(jù)3.1節(jié)中擬合的公式(8)、(9)、(10)、(11)計(jì)算過(guò)槽流量,結(jié)果表明,計(jì)算流量與實(shí)際流量之間的最大誤差為6.79%,平均誤差僅為0.32%。在一定喉口收縮比范圍內(nèi),測(cè)流誤差均小于5%。如當(dāng)渠道比降為1/1 000、收縮比為0.50~0.70時(shí),測(cè)流誤差最大為4.70%,最小為0.29%。
4 結(jié)論與討論
本研究通過(guò)對(duì)U形渠道4種渠道比降、6種喉口收縮比的圓頭量水柱進(jìn)行試驗(yàn),分析了喉口收縮比、渠道比降等渠道結(jié)構(gòu)參數(shù)與圓頭量水柱水力性能的關(guān)系。在試驗(yàn)測(cè)流流量為0.015~0.055 m3/s條件下,建立了基于駐點(diǎn)水深的流量公式,該公式簡(jiǎn)單實(shí)用,測(cè)流誤差最大為6.79%;選擇的適宜喉口收縮比為0.50~0.70,具體選擇時(shí)應(yīng)視渠道比降大小而定,比降較小的渠道應(yīng)選取較大的喉口收縮比;比降較大的渠道應(yīng)選取較小的喉口收縮比。這與張魯婧等[17]關(guān)于矩形渠道半圓柱形量水槽的研究成果一致。
渠道過(guò)流能力取決于過(guò)水?dāng)嗝娴男螤?、尺寸、比降及糙率,量水設(shè)施的結(jié)構(gòu)應(yīng)根據(jù)實(shí)際情況加以變化,并與渠道條件相配合,對(duì)于不同比降、渠道尺寸適宜的圓頭量水柱體型及流量公式尚需進(jìn)一步試驗(yàn),以便于該量水設(shè)施的推廣應(yīng)用。
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Factors affecting hydraulic performance of water-measuring pillar with round head in U-shaped channel
LIU Ying1,WANG Wen-e1,HU Xiao-tao1,TAN Xiao-fan2
(1CollegeofWaterConservancyandArchitectureEngineering,NorthwestA&FUniversity,Yangling,Shaanxi712100,China;2PumingStreetOffice,Mianyang,Sichuan621000,China)
【Objective】 This paper studied the factors affecting hydraulic performance of water-measuring pillar with round head based on the principle of critical flow to improve its application in irrigation areas of North China.【Method】 The hydraulic performance on flow measurement formula,proper throat contraction ratio,head loss,critical submergence degree, and flow measurement precision of water-measuring pillar with round head with six contraction ratios (ε=0.75,0.70,0.63,0.56,0.50,and 0.44) in U-shaped channel was tested based on five discharge rates (Q=0.015,0.025,0.035,0.045,and 0.055 m3/s) and four channel slopes (i=1/1 000,1/2 000,1/3 000,and 1/5 000).【Result】 There existed a good exponential correlation between the stagnation-point depth of the water-measuring pillar with round head and the discharge with coefficient of correlation of 0.997.Discharge formulas of four channel slopes were obtained through the regression analysis of experiment data,the fitted discharge formula which was in line with the dimension concordant principle met the flow measurement accuracy,and the maximum error between the measured discharge and the calculated discharge was 6.79%.When the proper throat contraction ratio was within 0.50 to 0.70 and the upstream backwater as well as the water head loss was small,it had a high critical submergence degree of up to 0.90.【Conclusion】 The water-measuring pillar with round head has a simple structure and can resist sludge plugging, which suits the channels with sediment-carried water in North China.
U-shaped channel;water-measuring pillar with round head;hydraulic performance;throat contraction ratios
2013-10-22
國(guó)家自然科學(xué)基金項(xiàng)目(51179163,50909083);國(guó)家“十二五”科技支撐計(jì)劃項(xiàng)目(2011BAD29B01);中央高?;究蒲袠I(yè)務(wù)費(fèi)專項(xiàng)(QN2011127);西北農(nóng)林科技大學(xué)青年學(xué)術(shù)骨干支持計(jì)劃項(xiàng)目
劉 英(1989-),女,河北衡水人,在讀碩士,主要從事工程水力學(xué)研究。E-mail:lykl2008@126.com
王文娥(1975-),女,河南孟縣人,副教授,碩士生導(dǎo)師,主要從事節(jié)水灌溉技術(shù)、流體機(jī)械及排灌設(shè)備等研究。 E-mail:wangwene@nwsuaf.edu.cn
時(shí)間:2015-01-05 08:59
10.13207/j.cnki.jnwafu.2015.02.032
S274.4
A
1671-9387(2015)02-0228-07
網(wǎng)絡(luò)出版地址:http://www.cnki.net/kcms/detail/61.1390.S.20150105.0859.032.html