外文文獻(xiàn)翻譯-液壓沖擊原理

1、<p><b>　　外文文獻(xiàn)及譯文</b></p><p><b>　　液壓沖擊原理</b></p><p>　　我們?cè)诜治鲆簤簺_擊現(xiàn)象和合理的流體方程之前，首先先來(lái)描繪一般的關(guān)于壓力傳遞的機(jī)械理論。通過(guò)參與這個(gè)關(guān)于閥門定位在一個(gè)較長(zhǎng)點(diǎn)幾乎沒(méi)有摩擦的管道傳輸液體于兩個(gè)蓄能源之間的結(jié)果之后是必要的。這個(gè)閥門連接的順流管道截面和逆流管道截面

2、考慮是一樣的。壓力沖擊流將通過(guò)閥門操作傳遞在兩個(gè)管道之間，并且假設(shè)閥門的關(guān)閉速度不應(yīng)用于堅(jiān)固圓管理論。</p><p>　　如果閥門是關(guān)閉的，而液體的流向是逆方向的，緩慢前進(jìn)，結(jié)果導(dǎo)致液體被壓縮和管道的橫截面膨脹。閥門的壓力增加導(dǎo)致高壓液體逆向流動(dòng)，延長(zhǎng)了液體流過(guò)圓管通向閥這段管道的時(shí)間。這種高壓液體的流動(dòng)類似聲音的傳播，是依靠液體和管道材料作為介質(zhì)的。</p><p>　　同樣，閥的順流

3、面流動(dòng)的延遲，將導(dǎo)致減小壓力在閥門處。這個(gè)結(jié)果否定了高壓液體的流動(dòng)是沿著順流管道的，阻止液體流動(dòng)，假設(shè)流體壓力在順流管道是不能減小液體壓力的或者蒸汽壓力或者溶解氣體釋放的壓力，各種愿意的考慮是不同的。</p><p>　　這樣，關(guān)閉著的閥門導(dǎo)致高壓液體的流動(dòng)是沿著管道的，盡管那些流動(dòng)有著各種不同的征兆。相對(duì)于穩(wěn)定的壓力流經(jīng)閥門開啟的管道。這種影響是關(guān)于液體流動(dòng)的延遲在兩種管道截面之間，管道自身受到影響由于液體逆向

4、產(chǎn)生高壓，管壁膨脹。同時(shí)，順流管道縮短，由于流經(jīng)液體的壓力降低，這種管道橫截面的巨大變形是由于管道材料的，并且能夠被證明。例如，使用薄壁型橡膠管材。高壓液體沿著液流前進(jìn)。實(shí)踐證明，由于液體的張力流向沿著管壁，它的速度接近于聲速。在這種管道材料中，然而，這是一種次要作用，當(dāng)認(rèn)識(shí)到它的存在，能夠解釋一部分壓力的傳遞時(shí)間隨著閥門關(guān)閉特點(diǎn)，它幾乎沒(méi)有影響到壓力標(biāo)準(zhǔn)應(yīng)用在壓力沖擊現(xiàn)象。</p><p>　　在閥門關(guān)閉之后，

5、這時(shí)是受壓時(shí)間將主要依靠系統(tǒng)的邊界條件，為了描繪閥門關(guān)閉的結(jié)果在同一個(gè)系統(tǒng)上，它將很容易說(shuō)明在大量的圖表上面，管道在每個(gè)時(shí)間段的情形。</p><p>　　由于閥門的關(guān)閉是瞬時(shí)的，液體接近每一段管道的閥門會(huì)帶來(lái)停止，并且高壓液體流動(dòng)情況可能已經(jīng)流過(guò)每一段管道。在適應(yīng)的流速c和一段時(shí)間t，這時(shí)液體已經(jīng)流過(guò)了一段距離1=ct，在每一段管道內(nèi)，這時(shí)管道的橫截面是變形也有一段距離1。</p><p&g

6、t;　　高壓液體到達(dá)蓄能站通過(guò)管道的時(shí)間為t=1/c，在這段距離中出現(xiàn)了一個(gè)不穩(wěn)定的位置，是在管道與蓄能站連接處。由于是不可能出現(xiàn)層流在蓄能站連接處，而保持壓力不同及其它的值在閥門關(guān)閉之前，流過(guò)每一個(gè)蓄能站的時(shí)間為1/c，在逆向管道這邊是高壓液體的流動(dòng)朝向閥門的關(guān)閉。減小管壁的壓力到其原值，并且恢復(fù)管壁的橫截面積。這時(shí)液體的流動(dòng)需要產(chǎn)生差值。從管道流向蓄能站，在管道的前段的液體流動(dòng)有比較高的壓力比蓄能站。現(xiàn)在，由于系統(tǒng)假設(shè)沒(méi)有摩擦，這種

7、巨大的逆向流動(dòng)會(huì)有精確的對(duì)比和最初的流動(dòng)速度。</p><p>　　在順流蓄能站，存在相反的情況，導(dǎo)致液體壓力上升流向和確定的順流流向從蓄能站到閥門。</p><p>　　由于這里考慮的是簡(jiǎn)單的管道，恢復(fù)高壓液體在管道和閥門之間的時(shí)間為21/c。整個(gè)逆流管道也是同樣，在返回最初的壓力和流向在管道外也被確定時(shí)間為21/c，由于液體已經(jīng)到達(dá)閥門，意味著沒(méi)有液體提前在提供的逆向一個(gè)低的壓力區(qū)域形

8、成在閥門外，破壞了流向和給上升的壓力減小流動(dòng)流向逆方向的閥門。再一次，帶來(lái)流動(dòng)的停止沿著管道且減小壓力在管道中。它已經(jīng)被假設(shè)在閥門處壓力下降，減小蒸發(fā)壓力。由于系統(tǒng)已經(jīng)假設(shè)沒(méi)有摩擦，所有的液面會(huì)有相同，絕對(duì)的，巨大的壓力增加。在穩(wěn)定的運(yùn)動(dòng)壓力下，會(huì)通過(guò)閥門的關(guān)閉產(chǎn)生。如果壓力增長(zhǎng)是h，這時(shí)所有的液面是h，因此，液體逆流經(jīng)過(guò)閥門的時(shí)間為21/c，存在一個(gè)值-h,同時(shí)，減少所有沿著管道的點(diǎn)從h降到最初的壓力時(shí)間逆向流動(dòng)到蓄能站的時(shí)間為31/

9、c。</p><p>　　類似的，恢復(fù)液體最初的順流到閥門的時(shí)間為21/c，并且流向從順流管道流向閥門關(guān)閉，這會(huì)在閥門處帶來(lái)流動(dòng)停止，導(dǎo)致壓力上升。在整個(gè)順流管道的每一段時(shí)間內(nèi)壓力h上升到最初的壓力在流動(dòng)停止時(shí)。</p><p>　　因此，在31/c時(shí)是一種不穩(wěn)定的情形類似于在t=1/c的情形，出現(xiàn)在蓄能站和管道的連接處存在著不同。即是逆流管道壓力下降到最初壓力和順流管道上升到最初壓力，然

10、而，這種液體流動(dòng)恢復(fù)機(jī)構(gòu)所用時(shí)間是相同的t=1/c。結(jié)果是逆流流向蓄能站，它有效地恢復(fù)環(huán)境沿著管道到它的最初值。當(dāng)液體到達(dá)關(guān)閉的閥門時(shí)，沿著每一段管壁都是相同的時(shí)間t=0，然而，由于閥門一直是關(guān)閉的，這種情形不能保持循環(huán)流動(dòng)周期。</p><p>　　管道系統(tǒng)采用循環(huán)流動(dòng)周期，瞬時(shí)選擇一種專門的機(jī)械情形，管道的順流和逆流對(duì)于閥是一樣的。實(shí)際 ，這是不同的。因而，所描繪的周期將一直被使用，除了壓力變化在兩管道之間不

11、再表示相同相位關(guān)系，每一個(gè)壓力周期的變化將是41/c，那里1和c代表著每一段管道適應(yīng)的時(shí)期，這是重要的標(biāo)記，一旦閥門是關(guān)閉的，這兩個(gè)管道將做出相應(yīng)的流動(dòng)到任何一段距離。</p><p>　　通過(guò)上述沖擊周期的描繪，可以劃分壓力-時(shí)間關(guān)系，在某一點(diǎn)沿管道上，這些變化的出現(xiàn)是類似的。通過(guò)時(shí)間在任何一點(diǎn)h，液體到達(dá)某一點(diǎn)，系統(tǒng)假設(shè)流動(dòng)速度為一個(gè)常數(shù)c，這主要集中在壓力沖擊依靠的方法是限制壓力的升高和減小閥的啟閉速度。然

12、而，存在著很重要的一點(diǎn)，沒(méi)有減小開啟壓力，將發(fā)生直到閥的關(guān)閉時(shí)間先于另一個(gè)管道。減小壓力達(dá)到出現(xiàn)閥門慢速關(guān)閉的結(jié)果先于忽略液體逆流到閥門關(guān)閉。由于沒(méi)有影響，返回到閥門時(shí)間21/c前，從閥門開始運(yùn)動(dòng)沒(méi)有壓力減小能夠到達(dá)如果閥門沒(méi)有打開超過(guò)了時(shí)間。一般來(lái)說(shuō)，閥門的關(guān)閉小于管道涉及的速度并且它將比21/c短。</p><p>　　在沒(méi)有摩擦的情況下，周期的繼續(xù)是不確定的。然而，實(shí)際中，摩擦力是壓力損失在很短的時(shí)間內(nèi)，系

13、統(tǒng)的摩擦損失越高，忽略摩擦力的影響導(dǎo)致結(jié)果越嚴(yán)重。事實(shí)上，閥門的頂點(diǎn)低相對(duì)于蓄能站頂點(diǎn)。然而，由于緩慢的流動(dòng)，摩擦點(diǎn)的損失減少。沿著管壁并且這個(gè)點(diǎn)向著蓄能站的方向增長(zhǎng)。由于液體的每一層，在閥門和蓄能站中會(huì)帶來(lái)停止，通過(guò)流動(dòng)最初的液面，所以大多在第二個(gè)液面位置相應(yīng)的摩擦點(diǎn)恢復(fù)流向。閥門導(dǎo)致影響整個(gè)時(shí)間21/c。由于流動(dòng)是相反的在管道中時(shí)間為21/c和41/c。這個(gè)位置影響主要在閥門，由于重新建立一個(gè)新的摩擦損失，在確切的事例中，例如，長(zhǎng)距

14、離油管，在閥門關(guān)閉之前，它將上升一部分壓力。</p><p>　　隨著假設(shè)條件對(duì)摩擦周期的描繪，提及到使壓力下降的條件，如果這些情況發(fā)生，這時(shí)流向圓管已經(jīng)分離出類似的周期描繪，可能中斷通過(guò)形成蒸氣壓力減小的位置有蒸氣生成。因此，系統(tǒng)描述可能發(fā)生在閥門的順流時(shí)間0或者逆流時(shí)間21/c形成一個(gè)腔。由于一段時(shí)間液體沿管壁流動(dòng)在一個(gè)壓力梯度下，在這個(gè)腔和系統(tǒng)邊界之間。這種方法是通常由于產(chǎn)生額外壓力在最后的腔中。這種現(xiàn)象一

15、般涉及到像圓管的分離和通常的制作更多的錯(cuò)綜復(fù)雜的由于釋放溶解的氣體在附近的腔中。</p><p>　　沖擊壓力也許被定義為在一些封閉的管道中應(yīng)用，通過(guò)兩個(gè)基本的方程，分別是運(yùn)動(dòng)平衡方程和連續(xù)應(yīng)用在一個(gè)短的流體圓管。它依靠可變的流體平均壓力和速度在任何一段管道的橫截面，且不依靠可變的時(shí)間和距離。通?？紤]實(shí)際的穩(wěn)流方向。摩擦力將被假設(shè)與速度平方成比例，并且穩(wěn)流摩擦關(guān)系將被假設(shè)應(yīng)用在非穩(wěn)定事例中。</p>

16、<p>　　Hydraulic transient theory</p><p>　　Before we embarking on the analysis of pressure transient phenomena and the derivation of the appropriate wave equations,it will be usefull to describe the gen

17、eral mechanism of pressure propagation by reference to the events fllowing the instantaneous closure of a value postioned at the med-length point of a frictionless pipeline carrying fluid between two reservoirs.The two p

18、ipeline sections upstream and downstream of the value are identical in all respects.Transient pressure waves will be prop</p><p>　　As the valve is closed,so the fluide approaching its upstream face is retard

19、ed with a consequent compression of the flude and an expansion od the pipe cross-section.The increase in pressure at the valve results in a pressure wave being propagated upstream which conveys the retardation of flow to

20、 the column of fluid approaching the valve along the upstream pipeline.This pressure wave travels through the fluid at the appropriate sonic velocity,which will be shown to depend on the properties of the</p><

21、p>　　Similarly,on the downstream side of the valve the retardation of flow results in a reduction in pressure at the valve,with the result that a negative pressure waves is propagated along the downstream pipe which,in

22、 turn,retards the fluid flow.It will be assumed that this pressure drop in the downstream pipe is insufficient to reduce the fluid pressure to either its vapour pressure or its dissolved gas release pressure,which may be

23、 considerable different.</p><p>　　Thus,closure of the valve results in propagation of pressure waves along both pipes and,although these waves are of different sign relative to the steady pressure in the pip

24、e prior to valve operation,the effect is to retard the flow in both pipe sections.The pipe itself is affected by the wave propagation as the upstream pipe swells as the pressure rise wave passes along it,while the downst

25、ream pipe contracts due to the passage of the pressure reducting wave.The magnitude of the deformation of t</p><p>　　Following valve closure,the subsequent pressure-time history will depend on the conditions

26、 prevailing at the boundaries of the system.In order to describe the events following valve closure in the simple pipe system outlined above,it will be easier to refer to a series of diagrams illustrating conditions in t

27、he pipe at a number of time steps.</p><p>　　Assuming that valve closure was instantaneous,the fluid adjacent to the valve in each pipe would have been brought to rest and pressure waves conveying this inform

28、ation would have been propagated at each pipe at the appropriate sonic velocity c.At a later time t,the situation is as shown in fig.The wavefronts having moved a distance 1=ct,in each pipe,the deformation of the pipe cr

29、oss-section will also have traveled a distancel as shown.</p><p>　　The pressure waves reach the reservoirs terminating the pipes at a time t=1/c.at this instant,an unbalanced situation arises at the pipe-res

30、ervior junction,as it is clearly impossible for the layer of fluid adjacent to the reservoir inlet to maintain a pressure different to that prevailing at that depth in the reservoir.Hence,a restoring pressure wave having

31、 a magnitude suffcient to bring the pipeline pressure back to its value prior to valve closure is transmitted from each reservoit at a time </p><p>　　At the downstream reservoir,the converse occurs,resulting

32、 in the propagation of a pressure rise wave towards the valve and the establishment of a flow from the downstream reservoir towards the valve.</p><p>　　For the simple pipe considered here,the restoring press

33、ure waves in both pipes reach the valve at a time 21/c.The whole of the upstream pipe has,thus,been returned to its original pressure and a flow has been established out of the pipe.At time 21/c,as the wave has reached t

34、he valve,there remains no fluid ahead of the wave to support the reversed flow.A low pressure region,therefore,forms at the valve,destroying the flow and giving rise to a pressure reducing wave which is transmitted upstr

35、eam f</p><p>　　Similarly,the restoring wave from the downstream reservoir that reached the valve at time 21/c had established a reversed flow along the downstream pipe towards the closed valve .This is broug

36、ht to rest at the valve,with a consequent rise in pressure which is transmitted.downstream as a +h wave arriving at the downstream reservoir at 31/c,at which time the whole of the downstream pipe is at pressure +h above

37、the initial pressure whth the fuid at rest.</p><p>　　Thus,at time 31/c an unbalanced situation similar to the situation at t=1/c again arises at the reservoir –pipe junctions with the difference that it is t

38、he upstream pipe which is at a pressure below the reservoir pressure and the downstream pipe that is above reservoir pressure .However,the mechanism of restoring wave propagation is identical with that at t=1/c,resulting

39、 in a-h wave being transmitted from the upstream reservior,which effectively restores conditions along the pipe to their initi</p><p>　　The pipe system chosen to illustrate the cycle of transient propagation

40、 was a special case as,for convenience,the pipes upstream and downstream of the valve were identical.In practice,this would be unusual.However,the cycle described would still apply,except that the pressure variations in

41、the two pipes would no longer show the same phase relationship.The period of each individual pressure cycle would be 41/c,where I and c took the appropriate values for each pipe.It is important to note that on</p>

42、<p>　　The period of the pressure cycle described is 41/c.However,a term ofen met in transient analysis is pipe period,this is defined as the time taken for a restoring reflection to arrive at the source of the init

43、ial transient propagation and,thus,has a value 21/c.In the case described,the pipe period for both pipes was the same and was the time taken for the reflection of the transient wave propagated by valve from the reservoir

44、s.</p><p>　　From the description of the transient cycle above,it is possible to draw the pressure-time records at points along the pipeline.These variations are arrived at simply by calculating the time at w

45、hich any one of the±h waves reaches a point in the system assuming a constant propagation velocity c.The major interest in pressure transients lies in methods of limiting excessive pressure rises and one obcious met

46、hod is to reduce valve speeds.However,reference to fig.illustrates an important point no r</p><p>　　In the absence of friction , the cycle would continue indefinitely .However ,in practice, friction damps th

47、e pressure oscillations within a short period of time .In system where the frictional losses are high,the neglect of frictional effects can result in a serious underestimate of the pressure rise following valve closure.I

48、n these case,the head at the valve is considerably lower than the reservoir head.However,as the flow is retarded,so the frictional head loss is reduced along the pipe and th</p><p>　　In addition to the assum

49、ptions made with regard to friction in the cycle description,mention was also made of the condition that the pressure drop waves at no time reduced the pressure in the system to the fluid vapour pressure.If this had occu

50、rred,then the fluid column would have separated and the simple cycle described would have been disrupted by the formation of a vapour cavity at the position where the pressure was reduced to vapour level.In the system de