Study of cavitation phenomena based on a technique for visualizing bubbles in a liquid pressurized chamber

In this paper, the influence of nozzle geometry on cavitation and near-nozzle spray behavior under liquid pressurized ambient is studied. For this purpose, eight steel drilled plates, with different diameters and degrees of conicity of their holes, are analyzed. A special near-nozzle field visualization technique, using a test rig pressurized with fuel, is used. Due to the difference in refractive index between liquid and vapor phase, bubbles present at the outlet of the orifice are visualized. The pressure conditions at which bubbles start appearing at the orifice outlet are compared with those at which choked flow appears. The results showed that pressure conditions for inception of cavitation obtained in the visualization tests differs from those seen for choked flow (5–8% in terms of cavitation number). In addition to this, the images taken are analyzed to get the angle of the jet formed by fuel bubbles, showing that it increases significantly for those conditions more prone to cavitate. Furthermore, comparison of bubbles generation when increasing or decreasing backpressure indicates the presence of hysteresis in cavitation inception phenomena.     

Evaluating cavitation regimes in an internal orifice at different temperatures using frequency analysis and visualization

Experiments on a water cavitating orifice were conducted to investigate the influence of pressure and temperature on flow regime transition due to cavitation. The thermal effects could be important in cases with cryogenic cavitation or hot fluid injection. The investigations were based on CCD observations and a pressure fluctuations frequency analysis.The high-speed photographic recordings were used to analyze the cavitation evolution and individuate the frequency content of the two-phase flow by processing the pixel-intensity time-series data.The cavitating structures showed different behaviors and characteristics with variations in operating conditions, as the pressure inside the orifice and the flow temperature .The flow regime map for the cavitating flow was obtained using experimental observations to analyze the occurrence of the different two-phase flow regime transitions at various operating conditions.As the pressure at the orifice inlet increased, at the same downstream pressure, cavitation inception occurred. The decrease of the cavitation number brought a significant increase in cavitation zone extension. As the pressure drop inside the orifice increased, the cavitation was characterized by an evident increase in cavitation zone length to the outlet of the orifice. With a further cavitation number decrease, the transition to jet cavitation was evident.The temperature influenced both the cavitation intensity and the cavitation number at which different two-phase flow regime transitions occurred, which tended to increase with temperature.The vapor fraction was estimated using an image processing algorithm.The frequency content given by the pressure fluctuations was analyzed and compared with the frequency spectra obtained from the visual observations. The behavior of the different cavitating flows could be correlated to the frequency spectrum of the pressure fluctuations measured upstream and downstream of the orifice. The cavitation number reduction and consequent increase in cavitating area width were related to a corresponding significant increase in the amplitude of typical frequency components. The transition to jet cavitation was characterized by a significant increase in the first peak in the frequency spectrum; weaker spectral peaks were also present at high cavitation numbers.     

A semi-empirical model of two-phase flow of refrigerant-134a through short tube orifices

Measurements were conducted on Refrigerant-134a flowing through short tube orifices with length-to-diameter (L/D) ratios ranging from 5 to 20. Both two-phase and subcooled liquid flow conditions entering the short tube were examined for upstream pressures ranging from 896 to 1448 kPa and for qualities as high as 10% and subcoolings as high as 13.9°C. Data were analyzed as a function of the main operating variables and tube geometry. Semi-empirical models for both single- and two-phase flow at the inlet of the short tubes were developed to predict the mass flow of Refrigerant-134a through short tube orifices.

Choked flow conditions for Refrigerant-134a were typically established when downstream pressures were reduced below the saturation pressure corresponding to the inlet temperature. The flow rate strongly depended on the upstream pressure and upstream subcooling/quality. The mass flow also depended on cross-sectional area and short tube length. The mass flow model utilized a modified orifice equation that formulated the mass flow as a function of normalized operating variables and short tube geometry. For a two-phase flow entering the short tube, the modified orifice equation was corrected using a theoretically derived expression that related the liquid portion of the mass flow under two-phase conditions to a flow that would occur if the flow were a single-phase liquid. It was found that for sharp-edged short tubes with single- and two-phase flow, approximately 95% of the measured data and model's prediction were within ±15% of each other.     

An experimental study on developing air-water two-phase flow along a large vertical pipe: effect of air injection method

The flow structure in a developing air-water two-phase flow was investigated experimentally along a large vertical pipe (inner diameter, D_h: 0.48 m, ratio of length of flow path L to D_h: about 4.2). Two air injection methods (porous sinter injection and nozzle injection) were adopted to realize an extremely different flow structure in the developing region. The flow rate condition in the test section was as follows: superficial air velocity: 0.02–0.87 m/s (at atmospheric pressure) and superficial water velocity: 0.01–0.2 0.01–0.2 m/s, which covers the range of bubbly to slug flow in a small-scale pipe (D_h about 0.05 m).

No air slugs occupying the flow path were recognized in this experiment regardless of the air injection methods even under the condition where slug flow is realized in the small-scale pipe. In the lower half of the test section, the axial distribution of sectional differential pressure and the radial distribution of local void fraction showed peculiar distributions depending on the air injection methods. However, in the upper half of the test section, the effects of the air injection methods are small in respect of the shapes of the differential pressure distribution and the phase distribution. The comparison of sectional void fraction near the top of the test section with Kataoka's correlation indicated that the distribution parameter of the drift-flux model should be modeled including the effect of D_h and the bubble size distribution is affected by the air injection methods. The bubble size distribution is considered to be affected also by L/D_h based on comparison of results with Hills' correlation.     

A Brief Summary of L. van Wijngaarden's Work Up Till His Retirement

This paper attempts to provide an overview of Professor Leen van Wijngaarden's scientific work by briefly summarizing a number of his papers. The review is organized by topic and covers his work on pressure waves in bubbly liquids, bubble dynamics, two-phase flow, standing waves in resonant systems, and flow cavitation noise. A list of publications up till his retirement in March 1997 is provided in the Appendix.     

Numerical and experimental study of turbulent impinging twin-jet flow

The two dimensional impinging circular twin-jet flow with no-cross flow is studied numerically and experimentally. The theoretical predications are carried out through numerical procedure based on finite volume method to solve the governing mass, momentum, turbulent kinetic energy and turbulent kinetic energy dissipation rate. The parameters studied were jet Reynolds number (9.5 × 10⁴  Re  22.4 × 10⁴), nozzle to plate spacing (3  h/d  12), nozzle to nozzle centerline spacing (l/d = 3, 5 and 8) and jet angle (0°  θ  20°). It is concluded that the stagnation primary point moves away in the radial main flow direction by increasing the jet angle. This shift becomes stronger by increasing the nozzle to nozzle centerline spacing (l/d). A secondary stagnation point is set up between two jets. The value of pressure at this point decreases by decreasing Reynolds number and/or increasing the jet angle.

The sub atmospheric region occurs on the impingement plate. It increases strongly by increasing Reynolds number and decreases as the jet angle and/or a nozzle to plate spacing increases. The spreading of jet decreases by increasing nozzle to plate spacing. The intensity of re-circulation zone between two jets decreases by increasing of h/d and jet angle. The increase of turbulence kinetic energy occurs within high gradient velocity.     

Numerical study of choked cavitation in high temperature hydrocarbon liquid jets

A numerical study was conducted on a practical plain orifice injector issuing pressurized high-temperature aviation fuel, in order to simulate injection of fuel after use as a coolant in the active cooling system of a hypersonic vehicle. A three-dimensional unstructured mesh inside the orifice was created using ICEMCFD^TM S/W, and the CFD analysis was performed using FLUENT^TM S/W. A multiphase mixture model was used to simulate cavitating two-phase flow, and the full cavitation model was activated to predict the mechanism and effects of cavitation induced by the high fuel vapor pressures at elevated temperature conditions. The simulation was performed for fuel heated up to 553 K (280 °C) at an upstream pressure (P_inj) of up to 1.0 MPa, and various ambient pressures (P_∞). The results were compared with experimental data, and the simulation was found to predict the discharge coefficient (C_d) with respect to the fuel injection temperature (T_inj) quite well at the given conditions. The CFD analyses for high fuel temperature conditions revealed that the mainstream flow inside the injector separates from the orifice wall at the vena contracta due to the generated fuel vapor cavity, and the attached flow at the end of the cavity separates again to produce a very small recirculation zone. In addition, for a given pressure drop, the sharply decreasing trend of the mass flow rate (or C_d) with increasing T_inj varies depending on P_∞, because the mass flow choking is determined by the relationship between P_∞ and the vapor pressure (P_sat) at T_inj. Finally, C_d with respect to cavitation number was found to follow an almost identical line, even at different P_∞. This confirms that choked cavitation at high fuel temperature conditions depends on the downstream pressure of the orifice, and the effect of cavitation on C_d at high T_inj is well represented by the cavitation numbers, regardless of P_inj, P_∞, and T_inj.     

Choking and shock phenomena in a single component two-phase flow including vibrational effects

An analytical and experimental investigation including vibratory effects of flashing flow in a tube with a sharp edged entrance is presented. A free streamline flow model is applied to predict choking in single-component two-phase flow. By identifying three separate regimes (i.e. jet flow, two-phase homogeneous flow, and single-phase liquid flow) in the flashing flow system, an expression is obtained for the prediction of the minimum stagnation pressure loss under choked flow conditions. A normal shock located between the flashing two-phase mixture and the single-phase liquid was experimentally observed. The location of the shock is predicted as a function of the stagnation pressure drop across the tube. The analytical predictions are verified by experimental data.     

射流对绕水翼云空化流动抑制机理研究

为理解绕水翼云空化流动的发展机理和探究水翼吸力面开孔射流的影响,采用密度 修正的RNG $k$-$\varepsilon $湍流模型和Schnerr-Sauer空化模型对原始NACA66(mod) 水翼和采用射流后的 水翼的云空化非定常过程进行模拟和对比分析;采用在水翼吸力面近壁区设立监测线的方法对近壁区的流场进行监测,得到 近壁区汽相体积分数、回射流速度、压力及压力梯度的时空分布云图;开展了云空化流场特性的涡动力学分析,进而分析水 翼云空化的发生机理和射流抑制空化的抑制机理. 结果表明:游离型空泡在下游溃灭时产生强烈的局部高压,其向上游传播 导致前缘空穴的一次回缩,而空穴的二次回缩受回射流的影响. 回射流的发展区域受限于较高的压力梯度,高的压力梯度一 直存在,但回射流在一个周期内的首次出现需要时间的积累. 在水翼吸力面射流使得射流孔附近压力升高,弥补了由于空化 和绕流造成的压降,压力梯度增大,抗逆压能力增强,对回射流起到阻挡作用;另一方面,射流使得回射流区域面积和回射 流的强度也有所减小,从而对云空化的发展起到抑制的效果. $Q$准则的涡结构云图相比于汽相体积分数云图能显示复杂的 流动结构,前缘附着型空穴和尾缘游离型空穴内存在旋涡,回射流对空穴存在剪切作用造成空穴脱落. 而射流对空穴和回射 流的剪切和阻挡使云空化发展得到抑制.      

Correlations predicting frictional pressure drop and liquid holdup during horizontal gas-liquid pipe flow with a small liquid holdup

Experimental data and correlations available in the literature for the liquid holdup ε_L and the pressure gradient ΔP_TP/L for gas-liquid pipe flow, generally, do not cover the domain 0 < ε_L < 0.06. Reliable pressure-drop correlations for this holdup range are important for calculating flow rates of natural gas, containing traces of condensate. In the present paper attention is focused on reliable measurements of ε_L and ΔP_TPIL values and on the development of a phenomenological model for the liquid-holdup range 0 < ε_L < 0.06. This model is called the “apparent rough surface” model and is referred to as the ARS model. The experimental results presented in this paper refer to air-water and air-water + ethyleneglycol systems with varying transport properties in horizontal straight smooth glass tubes under steady-state conditions. The holdup and pressure gradient values predicted with the ARS model agree satisfactorily with both our experimental results and data obtained from the literature referring to small liquid-holdup values 0 < ε_L < 0.06. Further, it has been shown that in the domain 38 < < 72 mPa m the interfacial tension of the gas-liquid system has no significant effect on the liquid holdup. The pressure gradient, however, increases slightly with decreasing surface tension values.     

Inertial instability of the Stewartson -layer

Inertial stability of a vertical shear layer (Stewartson E^1/4-layer) on the sidewall of a cylindrical tank with respect to stationary axisymmetric perturbations is inverstigated by means of a linear theory. The stability is determined by two non-dimensional parameters, the Rossby number Ro = U/2ΩL and Ekman number E = v/ΩH², where U and L = (E/4)1/4H are the characteristic velocity and width of the shear layer, respectively, Ω the angular velocity of the basic rotation, v the kinematic viscosity and H the depth of the tank.

For a given Ekman number, the flow is more unstable for larger values of the Rossby number. For E = 10⁻⁴, which is a typical value of the Ekman number realized in rotating tank experiments, the critical Rossby number Ro_c for instability and the critical axial wavenumber m_c non-dimensionalized by L⁻¹ are found to be 1.3670 and 8.97, respectively. The value of Ro_c increases and that of m_c decreases with increasing E.     

主动射流控制水翼空化的数值模拟与分析

为了改善高速流动工况下水翼吸力面上流场的空化特性,提出了水翼表面主动射流对绕水翼周围流动加以控制的方法.基于密度分域滤波的FBDCM混合湍流模型联合Zwart-Gerber-Belamri空化模型,分析了来流空化数为0.83,来流攻角为8°,射流位置距水翼前缘为x=0.19c时,主动射流对于水翼吸力面上流动的空化特性和水动力特性影响.对回射流的强度进行了量化分析,以探究回射流与流场空化特性的关系.数值分析结果表明,在射流水翼吸力面上的时均空泡体积为原始水翼的1/15,使得流场内空化流动由云空化状态转变为较为稳定的片空化状态,显著地削弱了云空化的发展.此外,射流极大地改善了水翼的水动力性能,使得水翼的升阻比较原始水翼提高了22.9%,空泡的脱落频率减少了26.2%,空泡脱落所引起的振幅减小了9.1%.射流大幅降低了水翼吸力面上低压区面积,水翼吸力面上流体的逆向压力减小,回射流强度降低;同时,射流使水翼吸力面上的边界层减薄,增强了流动的抗逆压梯度能力,一定程度上阻挡了回射流向水翼前缘的流动,这也从机理上分析了主动射流抑制空化的原因.      

Mass flow rate measurements in gas–liquid flows by means of a venturi or orifice plate coupled to a void fraction sensor

Two-phase flow measurements were carried out using a resistive void fraction meter coupled to a venturi or orifice plate. The measurement system used to estimate the liquid and gas mass flow rates was evaluated using an air–water experimental facility. Experiments included upward vertical and horizontal flow, annular, bubbly, churn and slug patterns, void fraction ranging from 2% to 85%, water flow rate up to 4000 kg/h, air flow rate up to 50 kg/h, and quality up to almost 10%. The fractional root mean square (RMS) deviation of the two-phase mass flow rate in upward vertical flow through a venturi plate is 6.8% using the correlation of Chisholm (D. Chisholm, Pressure gradients during the flow of incompressible two-phase mixtures through pipes, venturis and orifice plates, British Chemical Engineering 12 (9) (1967) 454–457). For the orifice plate, the RMS deviation of the vertical flow is 5.5% using the correlation of Zhang et al. (H.J. Zhang, W.T. Yue, Z.Y. Huang, Investigation of oil–air two-phase mass flow rate measurement using venturi and void fraction sensor, Journal of Zhejiang University Science 6A (6) (2005) 601–606). The results show that the flow direction has no significant influence on the meters in relation to the pressure drop in the experimental operation range. Quality and slip ratio analyses were also performed. The results show a mean slip ratio lower than 1.1, when bubbly and slug flow patterns are encountered for mean void fractions lower than 70%.     

Vortex shedding behind rings and discs

The wake structure of discs and bluff rings has been investigated experimentally in a wind tunnel. The rings have an inner diameter d_i, and an outer diameter d_o and are classified according to the parameter (d_o + d_i)/(d_o − d_i) = d/w. the ratio of mean diameter to ring width. As d/w → ∞ the flow approaches that around a two dimensional bluff body whereas as d/w tends to unity the body approaches a solid disc. A distinct change in the vortex shedding pattern is found around d/w = 5. Below this critical value velocity fluctuations in the wake have a weak periodic component which is 180° out of phase across a diameter of the body. Above d/w = 5. regular and coherent axisymmetric vortex ring shedding is observed with shedding occurring alternately from the inner and outer circumferences of the bluff body. Flow visualization and conditional averaging of hot-wire data are used to investigate the vortex structure.     

CFG桩复合地基沉降影响因素分析

对绕2D水翼无分离流边界层内的初生空化形态进行了实验研究. 采用高速摄像机观测 了空化初生结构的形态，应用2D-LDV测量了空化初生时翼型周围的流动速度分布，并对实 验结果进行了分析. 结果表明：绕水翼无分离边界层内，初生空化结构中空泡伴随 着近壁湍流拟序结构的发生而出现，在初生空化条件下，形成空化涡结构，大量的微空泡产生于发夹 涡结构中，并在涡结构的猝发过程中出现生成---长大---溃灭---反弹---再溃灭的过程. 初生空 化涡结构具有空泡和近壁拟序结构双重特性.     

Experimental study of ventilated cavity flow over a 3-D wall-mounted fence

Ventilated cavity flow over a fixed height 3-D wall-mounted fence is experimentally investigated in a cavitation tunnel for a range of free-stream conditions. The impact of 3-D effects on cavity topology is examined, along with the dependence of the cavitation number and drag on the volumetric flow rate coefficient, fence height based Froude number and vapour pressure based cavitation number. Three different flow regimes are identified throughout the range of cavitation numbers for a particular free-stream condition. Generally, the cavity has a typical re-entrant jet closure the intensity of which is found to increase linearly with increasing Froude number. This increase in re-entrant jet intensity causes an increase in drag with Froude number for constant volumetric flow rate coefficient. At low Froude numbers the closure mechanism transitions from a single to a split re-entrant jet. The parameters used to characterize the cavity topology show a linear dependence on Froude number irrespective of the closure mode. The cavity topology and drag are found to be independent of vapour pressure based cavitation number.     

Liquid single- and gas—liquid two-phase flows of magnetic fluid in the presence of a high non-uniform parallel magnetic field

The effect of a non-uniform parallel high magnetic field on flow control characteristics is investigated experimentally for a magnetic fluid single-phase flow and an air—magnetic fluid two-phase flow in a vertical channel. It is found that as the magnetic field strength is increased, the friction factor of the single-phase flow increases significantly. For the two-phase flow, the friction pressure loss and the head pressure loss, which is smaller than the friction loss, are negligibly small compared with the magnetic pressure loss. In the case where air is injected 27.9d upstream from the maximum magnetic field, the air flow is blocked by the magnetic force in the entrance of the magnetic field, which leads to increases in both local void fraction and pressure drop there. In the case where air is injected 1.43d downstream from the maximum magnetic field, the air flow is accelerated, resulting in a decrease in void fraction and an increase in pressure rise. In the latter case and under the present range of experimental conditions, the magnetic pumping head reaches 0.02 MPa at the highest, and the maximum circulation flow rate reaches twice as high as non-magnetically driven flow rate.     

Turbulent flow structure and bubble distribution in an axisymmetric nonisothermal impinging gas-liquid jet

The flow structure of a bubbly impinging jet in the presence of heat transfer between the two-phase flow and the surface is numerically investigated on the basis of the Eulerian approach. The model uses the system of Reynolds-averaged Navier–Stokes equations in the axisymmetric approximation written with account for the inverse effect of the bubbles on the average and fluctuating flow parameters. The influence of the gas volumetric flow rate ratio and the dimensions of the bubbles on the flow structure in a gas-liquid impinging jet is studied, In the presence of gas bubbles the liquid velocity is higher than the corresponding value in the single-phase flow. A considerable, more than twofold, anisotropy between the axial and radial turbulent fluctuations in the gas-liquid impinging jet is shown to exist. An addition of air bubbles leads to a considerable growth in the liquid velocity fluctuations in the two-phase flow (up to 50% compared with the single-fluid liquid impinging jet). An increase in the disperse phase dimensions leads to intensification of turbulence of the liquid.     

Multiphase fluid dynamics and transport processes of low capillary number cavitating flows

To better understand the multiphase fluid dynamics and associated transport processes of cavitating flows at the capillary number of 0.74 and 0.54, and to validate the numerical results, a combined computational and experimental investigation of flows around a hydrofoil is studied based on flow visualizations and time-resolved interface movement. The computational model is based on a modified RNG k-ε model as turbulence closure, along with a vapor-liquid mass transfer model for treating the cavitation process. Overall, favorable agreement between the numerical and experimental results is observed. It is shown that the cavi- tation structure depends on the interaction of the water-vapor mixture and the vapor among the whole cavitation stage, the interface between the vapor and the two-phase mixture exhibits substantial unsteadiness. And, the adverse motion of the interface relates to pressure and velocity fluctuations inside the cavity. In particular, the velocity in the vapor region is lower than that in the two-phase region.     

Numerical simulation of multiphase cavitating flows around an underwater projectile

The present simulation investigates the multiphase cavitating flow around an underwater projectile. Based on the Homogeneous Equilibrium Flow assumption, a mixture model is applied to simulate the multiphase cavitating flow including ventilated cavitation caused by air injection as well as natural cavitation that forms in a region where the pressure of liquid falls below its vapor pressure. The transport equation cavitating model is applied. The calculations are executed based on a suite of CFD code. The hydrodynamics characteristics of flow field under the interaction of natural cavitation and ventilated cavitation is analyzed. The results indicate that the ventilated cavitation number is under a combined effect of the natural cavitation number and gas flow rate in the multiphase cavitating flows.