外文翻譯--關(guān)于二級液壓節(jié)流錐閥的低汽蝕研究

1、<p><b>　　中文4400字</b></p><p><b>　　外文翻譯</b></p><p>　　Research on low cavitation in water hydraulic two-stage throttle poppet valve </p><p>　　Abstract: Cav

2、itation has important effects on the performances and lifespan of water hydraulic control valve, such as degrading efficiency, intense noise, and severe vibration. Two-stage throttle valve is a practicable configuration

3、to mitigate cavitation, which is extensively used in water hydraulic pressure relief valves and throttle valves. The pressure distribution inside a medium chamber located between two throttles of a two-stage throttle val

4、ve is investigated through numerical simulation</p><p>　　Keywords: computational fluid dynamics simulation cavitation load rigidity passage area ratio two-stage throttle valve water hydraulics </p&

5、gt;<p>　　1 INTRODUCTION </p><p>　　Water hydraulic systems are operated with raw water (pure tap water) substituting for mineral oil. They have advantages in terms of durability, reliability, safety,

6、 and cleanness. Such systems are becoming more and more popular, especially in fields of steel and glass production, coal and gold mining, food and medicine processing, nuclear power generation, ocean exploration, and un

7、derwater robotics [1–5]. </p><p>　　Because the opening of a water hydraulic control valve is very small compared with that of oil valve, the water flow velocity through the water hydraulic control valve is l

8、arger under the same pressure condition; thus cavitation erosion may occur due to the high vapour pressure of water. Cavitation has an important effect on the performance and lifespan of water hydraulic control valve, su

9、ch as degrading efficiency, intense noise, and severe vibration. Previously, a number of studies on the rela</p><p>　　Aoyama et al. [7] studied experimentally the unsteady cavitation performance in an oil hy

10、draulic poppet valve. It was found that, as absolute values of the variation rates of inlet and outlet pressure increased, the incipient cavitation index exhibited a tendency to decrease, whereas the final cavitation ind

11、ex a tendency to increase under all geometrical parameters. As the absolute values of the variation rates of inlet and outlet pressure further increased, the hysteresis between the incipient </p><p>　　Ishiha

12、ra et al. [8] studied oil flow unsteadiness effect on cavitation phenomena at sharp-edged orifices. The rate of pressure drop across the orifice was kept constant, and cavitation incipience and finale were recorded by us

13、ing scattered laser beams showing that (a) there existed two types of cavitation, namely, gaseous cavitation and vapourous cavitation, and (b) cavitation incipience and finale varied with the initial condition, the tempe

14、rature of hydraulic oil, and the rate of pressure drop</p><p>　　Johnston et al. [9] carried out an experimental investigation of flow and force characteristics of hydraulic poppet and disc valves using water

15、 as the working fluid. The axisymmetric valve housing was constructed from clear perspex to facilitate flow visualization; tests were performed on a range of different poppet and disc valves operating under steady and no

16、n-cavitating conditions, for Reynolds numbers greater than 2500. Measured flow coefficients and force characteristics showed obvious diff</p><p>　　Vaughan et al. [10] conducted computational fluid dynamics (

17、CFD) analysis on flow through poppet valves. Simulations were compared with experimental measurements and visualized flow patterns. A qualitative agreement between simulated and visualized flow patterns was identified. H

18、owever, errors in the prediction of jet separation and reattachment resulted in quantitative inaccuracies. These errors were due to the limitations of the upwind differencing scheme employed and the representation of tur

19、</p><p>　　Ueno et al. [11] investigated experimentally and numerically the oil flow in a pressure control valve under an assumption of non-cavitating conditions for various configurations of the valves on th

20、e basics of a finite difference method. They concluded that the main noise of the testing valves was generated from cavitation, and the noise was affected by the valve configuration. Pressure measurements and flow visual

21、ization at two locations in a valve chamber were also performed on the basis of two</p><p>　　Martin et al. [12] investigated cavitation in spool valves in order to identify damage mechanisms of the related c

22、omponents. Tests were conducted in a representative metal spool valve as well as a model being three times larger. Data taken under non-cavitating conditions with both of these valves showed that the orientation of high-

23、velocity angular jets would be shifted due to variations in valve opening and Reynolds number. By means of high-frequency response pressure transducers strategically </p><p>　　Gao et al. [13] performed a sim

24、ulation of cavitating flows in hydraulic poppet valves by means of an renormalization group (RNG) k –1 turbulence model, which was derived from the nstantaneous Navier–Stokes equations based on the RNG theory. Experiment

25、s were conducted to catch cavitation images around the seat of a poppet valve from perpendicular directions, using a pair of industrial fibre scopes and a high-speed visualization system. The binary cavitating flowfield

26、distributions obtained throug</p><p>　　Oshima et al. [14] experimentally investigated the influences of (a) chamfer length in the valve seat, (b) the poppet angle, and (c) the oil temperature on the flow cha

27、racteristics and the cavitation phenomena, using water instead of oil as the working medium in water poppet throttles. The cavitation phenomena were directly observed and the pressure distribution between the valve seat

28、and poppet surface was measured in water poppet throttles. Comparison analyses on the condition of critical cavi</p><p>　　As an extension of Liu et al. [15, 16], this research will focus on the investigation

29、 of cavitation characteristics in the water hydraulic two-stage throttle poppet valve. This objective entails the following research tasks: (a) on the basis of the RNG k – 1 turbulent model, numerical simulations will be

30、 conducted to investigate the pressure distribution inside the medium chamber located between the two throttles, (b) the effect of the passage area ratio of the two throttles and pressure between</p><p><

31、b>　　2 </b></p><p>　　SIMULATION OF PRESSURE BETWEEN TWO THROTTLES </p><p><b>　　2.1 </b></p><p>　　Statement of problems </p><p>　　If water pressure d

32、ecreases below the saturation </p><p>　　vapour pressure pv (absolute pressure) of water under a given temperature, the vapour or gas will spill out of the water, and then cavitation will occur. For water, pv

33、 . 0.023 MPa when the temperature is 20 8C. Generally, the likelihood of the cavitation for the throttle valve can be measured by a cavitation index (K ), which can be expressed as follows </p><p>　　The crit

34、ical cavitation index (Kc) is the minimum cavitation index for the throttle under non-cavitating flow in throttle valve. For a single-stage throttle valve, Ksc . 0.4 [14, 17–20]. It denotes that, if the cavitation index

35、K is less than 0.4, the cavitation will occur. Therefore, the cavitation index (K ) should be larger than 0.4 so as to avoid cavitation in the throttle valve. </p><p>　　The larger the pressure drop across th

36、e throttle valve, the smaller the cavitation index (and thus greater probability of cavitation for the throttle valve). The two-stage throttle valve is a practicable configuration to mitigate cavitation, being extensivel

37、y used in water hydraulic pressure relief valves and throttle valves. Figure 1(a) is a scheme of water hydraulic two-stage throttle valve, which consists of a poppet and a seat with a step shape bore. There are two tande

38、m throttles to take th</p><p>　　For the two-stage throttle valve, the definition of cavitation index Kt can be analogously expressed as follows </p><p>　　Actually, for each stage throttle, the c

39、ritical conditions for avoiding cavitation in a single-stage throttle should be satisfied. Then </p><p>　　The pressure distribution inside the medium chamber located between the two throttles should be obtai

40、ned to calculate cavitation indices for both stages. CFD simulation will be undertaken to explore the pressure distribution of the medium chamber. </p><p><b>　　2.2 </b></p><p>　　Math

41、ematical model </p><p>　　In view of the axisymmetric structure of the two-stage throttle valve, the computational domain is simplified to a two-dimensional axisymmetric geometrical model (ABCDE), as shown in

42、 Fig. 1(a); its grids are generated (Fig. 1(b)) through an advanced grid preprocessor (Gambit of software package FLUENT) [21, 22]. In Fig. 1(a), AB and BC denote fixed wall of seat, ED is fixed wall of poppet, CD the in

43、let, and AE the outlet. The water is assumed to be incompressible viscous fluid with ρ= 998.2 kg/m</p><p>　　The RNG k –1 equation model (turbulence kinetic energy equation k and dissipation equation 1) and t

44、he multiphase model are used in this research. The Boussinesq hypothesis is employed to relate </p><p>　　Reynolds stresses to the mean velocity gradients The turbulence kinetic energy (k) and its rate of dis

45、sipation (1) are obtained from transport equations (6) and (7), respectively </p><p>　　In this research, the multiphase model involves liquid and gas phases that are considered as interpenetrating continua.

46、The law of conservation of mass and momentum is satisfied in each phase. The derivation of the conservation equations can be done by ensembling the local instantaneous balances for all of the phases. Volume fractions (ap

47、) represent the space occupied by gas phase. The continuity equation for gas phase can yield </p><p>　　In these equations, Gk represents the generation of turbulence kinetic energy due to the mean velocity g

48、radient; sk and s1 are the inverse effective turbulent Prandtl numbers for k and 1, respectively.</p><p><b>　　2.3 </b></p><p>　　Simulation results Sensitivity analyses of the variati

49、ons of pressures inside the medium chamber along with the changing passage area ratio of the two throttles and the inlet and outlet pressures were performed. As a large number of numerical simulations were conducted, it

50、is impossible to include all of the results here. Instead, representative outcomes are explicated to demonstrate important findings. The simulated contours of the volume fraction of water vapour are shown in Fig. 2. The

51、passag</p><p>　　The simulated contours of pressure inside the medium chambers are presented in Fig. 3 when the passage area ratio varied from 0.1 to 6.0 under conditions of pin . 10 MPa and pout . 1 MPa. It

52、can be seen from Fig. 3 that, when r was less than 0.2 or larger than 2.0, the pressure drop would be solely taken by the front or rear throttle of the two-stage throttle valve. This looked like a single-stage throttle v

53、alve, with its anti-cavitation capability being similar to that of single-stage throttle </p><p>　　In order to clarify the correlations among pressure levels inside the medium chamber and the related impact

54、factors (r, pin, and pout), a virtual line (l ) was employed to locate the pressure levels inside the medium chamber. They ranged from the midpoint of line AE to the midpoint of line CD. The virtual line was divided into

55、 eight parts, and the corresponding nodes were marked by ordinal numbers (from 0 to 8); the pressure levels on the virtual line were denoted as pl. </p><p>　　The effects of the passage area ratio (r) on the

56、pressure (pl) under condition of pin . 10 MPa and pout . 4 MPa are illustrated in Fig. 4. The pressure level would vary with the passage area ratio. The larger the passage area ratio r, the higher the pressure level on t

57、he virtual line. However, when r is less than 0.2, the pressures inside the medium chamber would become low. This was attributed to the small front throttle, which could be considered as a jet nozzle. The fluid would hav

58、e less dire</p><p>　　關(guān)于二級液壓節(jié)流錐閥的低汽蝕研究</p><p>　　摘要：汽蝕對于液壓控制閥的性能和壽命有重要影響,如引起效率的降低,產(chǎn)生強(qiáng)烈的噪音和振動。二級節(jié)流閥目前作為一種來減小汽蝕的裝置被廣泛用于液壓安全閥和節(jié)流閥?？梢酝ㄟ^數(shù)值模擬的方法來研究介質(zhì)在二級節(jié)流閥介質(zhì)腔的壓力分布。對兩個節(jié)流閥的截面比和介質(zhì)腔內(nèi)的進(jìn)出口壓力產(chǎn)生的影響進(jìn)行研究。仿真結(jié)果表明,

59、(1)介質(zhì)腔內(nèi)的壓力不是固定的,(b)介質(zhì)腔最大和最小壓力的位置都是固定的,不會隨著通道面積比率或進(jìn)出口壓力而變化,(c)流過前面節(jié)流閥和流過二級節(jié)流閥的總壓降的壓降比值幾乎不變。然后建立二級節(jié)流閥臨界汽蝕指標(biāo),。通過該二級液壓節(jié)流閥得到設(shè)計的經(jīng)驗準(zhǔn)則。對臨界汽蝕指標(biāo)和通道的兩個截面比例之間的關(guān)聯(lián)性進(jìn)行實驗研究。相關(guān)驗證實驗在一個定制的測試儀器中進(jìn)行。 實驗結(jié)果和模擬研究是一致的。進(jìn)一步的分析表明,(a)大的反壓力不僅可以提高抗汽蝕能力

60、還可以改善二級液壓節(jié)流閥的總負(fù)荷剛性、(b)一個適當(dāng)?shù)耐ǖ烂娣e比例有利于提高二級液壓節(jié)流閥的抗汽蝕能力(c)通道面積比例為0.6的二級液壓節(jié)流閥會有最好的抗汽蝕能力，而且產(chǎn)生汽蝕的風(fēng)險最低。</p><p>　　關(guān)鍵詞:流體動力學(xué)仿真計算、汽蝕、負(fù)載剛度、通道面積比率、二級節(jié)流閥。</p><p><b>　　1</b></p><p><

61、;b>　　介紹</b></p><p>　　水壓操作系統(tǒng)用原水(純自來水)替代礦物油。就他們優(yōu)勢而言，有耐久性、可靠性、安全性、以及清潔。這種系統(tǒng)特別是在鋼和玻璃生產(chǎn)、煤炭和金礦開采、食品和醫(yī)藥加工、核電、海洋勘查、水下機(jī)器人等領(lǐng)域正變得越來越流行。</p><p>　　和油閥相比水壓控制閥門開口是非常小的,所以在相同的壓力條件下通過水壓控制閥門的水流速會更大，但是由于具

62、有較高壓力的蒸汽水的存在，就可能出現(xiàn)汽蝕。汽蝕對水液壓控制閥的性能和壽命有重要影響,如降低效率、強(qiáng)烈的噪聲以及嚴(yán)重的振動。之前,有許多關(guān)于汽蝕現(xiàn)象、流量系數(shù)、驅(qū)動力和壓力在閥門分布、通過增加閥門出口壓力減小汽蝕、修改節(jié)流形狀、提高閥門零件材料的耐腐蝕級別、以及控制最大流體溫度、流速峰值的研究。[6]研究利用提升型保持閥的可視化流體來減少汽蝕。通過控制流量、上下游壓力和閥門開啟的集聚流動等一些措施來減小噪音和汽蝕。</p>

63、<p>　　Aoyama[7]實驗研究了在油壓提升閥中出現(xiàn)的不穩(wěn)定汽蝕現(xiàn)象，結(jié)果發(fā)現(xiàn),隨著入口和出口壓力變化速率絕對值的增加，初期的汽蝕指數(shù)表現(xiàn)出一種減小的傾向,而最后的汽蝕指數(shù)有增加到所有的幾何參數(shù)之下的趨勢,。隨著入口和出口壓力變化速率的絕對值進(jìn)一步增大,滯環(huán)之間初期和最終的汽蝕指數(shù)會比以往任何時候每個組合閥門和閥座的都大。</p><p>　　Ishihara[8]研究了不穩(wěn)定的油液流動對小孔通道

64、的汽蝕現(xiàn)象影響。使通過孔口的壓降速度保持不變,利用分散雷射光束記錄的最初和最終汽蝕表現(xiàn)出(a)存在著兩種類型的汽蝕,即氣態(tài)的汽蝕和類蒸汽汽蝕現(xiàn)象(b)初期和最終汽蝕隨著初始條件,液壓油的溫度,以及壓降的速率變得多樣化。</p><p>　　Johnston[9]進(jìn)行了一項關(guān)于在提升和圓盤液壓閥中流動和力學(xué)特性的實驗研究,并采用水作為工作介質(zhì)。制造出具有軸對稱性的有機(jī)玻璃閥門外殼,便于促進(jìn)可視化流動,實驗研究了在穩(wěn)

65、定且沒有汽蝕雷諾數(shù)大于2500的條件下一系列不同的對提升和圓盤閥的操作。測量顯示的流量系數(shù)和力學(xué)特性隨著閥門幾何形狀和開口有著明顯的不同。</p><p>　　Vaughan[10]對流過提升閥的油液進(jìn)行了流體動力學(xué)計算(CFD)分析。仿真被用于比較實驗測量和可視化流動模式。一種關(guān)于模擬和可視化的流動模式的可行性被認(rèn)同。然而,在預(yù)測射流分離和復(fù)位時的錯誤導(dǎo)致了定量的不精確性。這些錯誤是由于本身的局限性造成的,可以

66、采用迎風(fēng)差分方案表示k - 1湍流模型,但該模型用于循環(huán)流量時是不準(zhǔn)確的。</p><p>　　Ueno [11]用實驗和數(shù)值模擬的方法研究了用最基本的有限差分在假設(shè)沒有汽蝕在條件下對不同構(gòu)造壓力控制閥的油液流動,。他們總結(jié)說,測試閥的主要噪聲是由汽蝕造成的,同時噪聲也受閥門結(jié)構(gòu)的影響。對閥腔兩個位置的壓力測量和可視化流動在二維模型的基礎(chǔ)上進(jìn)行研究。通過測量和計算結(jié)果的比較,可以建立低噪聲閥門設(shè)計的幾個準(zhǔn)則。&l

67、t;/p><p>　　Martin[12]通過研究芯閥的汽蝕現(xiàn)象問題用來識別和損傷機(jī)制相關(guān)的組件。在典型的金屬芯閥和一個三倍大的模型中進(jìn)行測試。兩種液壓閥在沒有汽蝕條件下得到的數(shù)據(jù)表明,由于閥門開啟度和雷諾數(shù)的影響，閥的高噴流方向的角度會發(fā)生變化。通過放置在閥室的高頻壓力響應(yīng)傳感器,可以利用噪聲和汽蝕指數(shù)之間的相關(guān)聯(lián)系來檢測汽蝕。汽蝕可能通過對一個不變排放量的固定開啟度閥門進(jìn)行能量譜比較來檢測。在這次調(diào)查中所定義的初

68、期汽蝕指數(shù)是與兩個閥門雷諾數(shù)有關(guān)的。</p><p>　　Gao [13]通過基于RNG n - s方程理論中派生出來的重整化群(RNG紊流模型),對液壓提升閥中的汽蝕流動進(jìn)行了模擬。試驗研究用一對工業(yè)纖維和一個高速可視化系統(tǒng)從垂直方向捕捉提升閥座周圍的汽蝕圖像。通過數(shù)字處理原汽蝕圖像所獲得的汽蝕二進(jìn)制分布流場與數(shù)值結(jié)果達(dá)到令人滿意的一致性;由汽蝕流動引起的閥體和提升閥的振動可以利用被稱作激光測距應(yīng)變裝置的渦流位

69、移傳感器進(jìn)行測量。實驗表明錐閥的開口和錐度對汽蝕的強(qiáng)度有非常重要的影響。然而在此研究中,計算抗汽蝕能力時只分析了大量下游的初期汽蝕，出口壓力的影響則未被考慮。</p><p>　　Oshima[14]通過試驗研究了(a)閥座的槽長度和(b)提升閥角度,和(c)油溫對流動特性和汽蝕現(xiàn)象的影響,并在液壓提升閥用水代替油作為工作介質(zhì)。汽蝕現(xiàn)象可以被直接觀察到而且也對水壓提升閥中閥座和提升閥表面壓力分布進(jìn)行了檢測分析。在

70、臨界汽蝕的條件下對油和水進(jìn)行比較分析。實驗發(fā)現(xiàn)水壓提升閥的流量系數(shù)和臨界汽蝕數(shù)據(jù)不同于油壓的,因為水具有較高的密度和較低的粘度。最近,Liu[15、16]進(jìn)行了液壓系統(tǒng)中二級液壓閥的流動、汽蝕特性的試驗研究;他們得出的結(jié)論是二級液壓閥比單級的具有更強(qiáng)的抗汽蝕能力，而且座位的形狀對閥的抗汽蝕能力也有影響。汽蝕阻塞只是當(dāng)指數(shù)小于0.4的時候出現(xiàn)。他們還進(jìn)行了數(shù)項關(guān)于汽蝕和控制閥結(jié)構(gòu),汽蝕和兩個節(jié)流閥的通道截面比的研究。然而,兩個節(jié)流閥之間的

71、壓力被假設(shè)為常數(shù),這可能不適用于具有介質(zhì)腔的二級液壓閥。</p><p>　　作為Liu[15、16]的擴(kuò)展,本實驗將要集中于二級液壓提升閥的汽蝕特性的研究。為了達(dá)到這一目標(biāo)需要進(jìn)行以下研究工作:(a)在RNG紊流模型k - 1基礎(chǔ)上,用數(shù)值模擬的結(jié)果對位于兩個節(jié)流閥之間的介質(zhì)內(nèi)腔壓力分布進(jìn)行研究,(b)兩個節(jié)流閥之間的通道截面積比和壓力對抗汽蝕能力的影響將要被研究(c)根據(jù)低汽蝕和低噪聲的原則建立二級節(jié)流閥的設(shè)

72、計標(biāo)準(zhǔn),(d)并且將通過定制的測試儀器進(jìn)行實驗,用來證明的仿真方法的良好適用性和設(shè)計原則。</p><p><b>　　2</b></p><p>　　模擬兩個節(jié)流閥之間的壓力</p><p><b>　　2.1</b></p><p><b>　　狀態(tài)問題</b></p

73、><p>　　如果水的壓力低于一個特定溫度下的飽和蒸汽壓力pv(絕對壓力)，蒸汽或氣體將要從水中濺出來,然后將發(fā)生汽蝕。對于絕對壓力是0.023 MPa，溫度是20度的水，一般來說,節(jié)流閥汽蝕的可能性可以用一個汽蝕指數(shù)(K)進(jìn)行測量,它可以表示如下</p><p>　　臨界汽蝕指數(shù)(Kc)是在沒有汽蝕流動的節(jié)流閥中最低限度的汽蝕指標(biāo)。對于節(jié)流閥0.4[14]。它表示：如果汽蝕指數(shù)K值小于0.4

74、,將發(fā)生汽蝕。因此,應(yīng)該使汽蝕指數(shù)(K)應(yīng)大于0.4來避免在節(jié)流閥中發(fā)生汽蝕。</p><p>　　通過過節(jié)流閥的壓降越大，汽蝕指數(shù)就越小(節(jié)流閥發(fā)生汽蝕的可能性也越小)。二級節(jié)流閥是一種用來減小汽蝕的可行配置結(jié)構(gòu),而且正在被越來越廣泛地應(yīng)用于液壓安全閥和節(jié)流閥。圖1(a)是一種輸水方案的二級液壓節(jié)流閥,它是由一個提升閥和一個具有階梯孔的閥座組成的。由兩個串聯(lián)的節(jié)流閥分擔(dān)總壓降。因此,通過每一級節(jié)流閥的壓降小于閥

75、的總壓降,這和單級節(jié)流閥是不同的。因此，每一級節(jié)流閥的的汽蝕指應(yīng)該比單級節(jié)流閥大，這樣就降低了出現(xiàn)汽蝕和侵蝕的可能性。</p><p>　　對于二級節(jié)流閥，相比較之下其汽蝕指數(shù)Kt的定義可以表達(dá)如下</p><p>　　實際上,對于每一級節(jié)流閥,在一個單級節(jié)流閥中避免汽蝕的臨界條件都應(yīng)該滿足如下條件</p><p>　　然后利用兩個節(jié)流閥之間的介質(zhì)腔內(nèi)的壓力分布來計

76、算每級的汽蝕指數(shù)?？梢杂肅FD仿真來探討介質(zhì)腔內(nèi)的壓力分布。</p><p><b>　　2.2</b></p><p><b>　　數(shù)學(xué)模型</b></p><p>　　對于具有軸對稱結(jié)構(gòu)的二級節(jié)流閥，計算區(qū)域被簡化為一種二維軸對稱幾何模型(ABCDE),如圖1(a)。;它的網(wǎng)格(圖1)通過(b)一種先進(jìn)的網(wǎng)格預(yù)處理程序

77、(流體力學(xué)分析軟件)[21、22]生成。在圖1(a)、AB和BC表示固定的墻體,ED設(shè)置為提升閥、CD為進(jìn)口和AE為出口。假設(shè)水是ρ= 998.2Kg/m3,μ=1.007×10-6m2/s的不可壓縮且沒有粘性的液體,壓力和溫度對水密度和粘度的影響可以忽略不計;假設(shè)所有的墻都是隔熱的且以零速度滑動。入口和出口邊界可以指定為需要的壓力等級。</p><p>　　方程模型(凱西湍流動能方程和耗散方程）和多相

78、模型被用于此研究。假定雷諾應(yīng)力和湍流的平均速度梯度動能(k)，與通過傳導(dǎo)方程（6）和（7）獲得的耗散速率有一定的關(guān)系。在這個研究中，包括液相和氣相的多相模型都被認(rèn)為是互相關(guān)聯(lián)的。在各個相中都滿足質(zhì)量和動量守恒定律。守恒方程的推導(dǎo)可以由所有相的局部瞬時平衡來完成。體積分?jǐn)?shù)代表被氣相所占的空間?？梢缘玫綒庀嗟倪B續(xù)性方程。</p><p>　　在這些方程中,考慮到平均流速梯度 Gk代表了一系列的湍流動能;sk及s1分別

79、表示逆有效的無序拉丁字母k和l。</p><p><b>　　2.3</b></p><p>　　仿真結(jié)果對兩個節(jié)流閥的介質(zhì)腔壓力變化隨著通道面積比和進(jìn)出口壓力等發(fā)生的變化進(jìn)行了較深入有效的研究。即使有大量的數(shù)值模擬分析,在這里也不可能包括所有的結(jié)果。相反,具有代表性的結(jié)果被用來證明重要的發(fā)現(xiàn)。</p><p>　　如圖2展示了水蒸氣體積分?jǐn)?shù)的

80、模擬輪廓圖。通道截面積比(r)能顯著影響汽蝕的發(fā)生和汽蝕在入口為10兆帕出口為1兆帕條件下的強(qiáng)度。有一些關(guān)鍵的通道面積比率包括r為0.2,0.5和2.0。當(dāng)r小于0.2時,會在二級節(jié)流閥的介質(zhì)腔內(nèi)部形成水蒸氣，并且相應(yīng)的水蒸氣最大體積分?jǐn)?shù)會增加到85.3%,如圖2(a)所示。這是由于高流速和強(qiáng)烈的介質(zhì)內(nèi)腔再循環(huán)。再循環(huán)將會導(dǎo)致低壓區(qū)的形成和潛在汽蝕的出現(xiàn)。隨著r水平的增加，水蒸氣的體積分?jǐn)?shù)將會降低。當(dāng)r范圍在0.5 ~ 0.8時,水蒸氣

81、的最大體積分?jǐn)?shù)會下跌(到0.162%或以下),因此出現(xiàn)汽蝕的可能性也會減小。然而，隨著通道面積比率(例如當(dāng)r 2.0)的進(jìn)一步增加，水蒸氣最大體積分?jǐn)?shù)的會增加到5.15%-26.7%,如圖2(c)和(d)所示;因此會再次發(fā)生汽蝕。</p><p>　　圖3顯示了不同通道面積比率從0.1至6.0的時侯，在入口壓力為10兆帕出口為1兆帕的條件下介質(zhì)腔的壓力模擬輪廓圖。從圖3可以看出,當(dāng)r小于0.2或大于2.0時,壓降

82、會在二級節(jié)流閥的前方或后方單獨(dú)的產(chǎn)生。這看起來像一個單級節(jié)流閥,其抗汽蝕能力也類似于單級節(jié)流閥。當(dāng)r在0.2 ~ 2.0之間時,總壓降將被兩個串聯(lián)的節(jié)流閥共同分擔(dān)。很顯然,一個合理的r水平會幫助抑制二級液壓節(jié)流閥的初期汽蝕的產(chǎn)生。</p><p>　　為了闡明介質(zhì)腔壓力等級和相關(guān)影響因素(r,入口,出口壓力)之間的相關(guān)性,虛線(l)被用來設(shè)定介質(zhì)腔的壓力等級。他們是從AE線中點到對CD線中點。虛線被分為八個部分,

83、相應(yīng)的節(jié)點用有序數(shù)字進(jìn)行標(biāo)記(從0到8);虛線上的壓力等級用pl表示。</p><p>　　在圖4中顯示了通道面積比率(r)對在入口壓力為10兆帕出口為4兆帕的條件下壓力(pl)的影響。壓力等級會隨著通道截面積的比例變化。通道截面積比r越大,虛線上的壓力等級也越高。然而當(dāng)r小于0.2時,介質(zhì)腔內(nèi)的壓力會變低。這是由于小的前節(jié)流閥,也可被視為一種射流噴嘴。液體在通過前節(jié)流閥時會有更小的方向變化和收縮噴射。通過前節(jié)流