Cold-gas experiments to study the flow separation characteristics of a dual-bell nozzle during its transition modes

An experimental investigation has been carried out to study the effect of test environment on transition characteristics and the flow unsteadiness associated with the transition modes of a dual-bell nozzle. Cold-gas tests using gaseous nitrogen were carried out in (i) a horizontal test-rig with nozzle exhausting into atmospheric conditions and, (ii) a high altitude simulation chamber with nozzle operation under self-evacuation mode. Transient tests indicate that increasing δP ₀/δt (the rate of stagnation chamber pressure change) reduces the amplitude of pressure fluctuations of the separation shock at the wall inflection point. This is preferable from the viewpoint of lowering the possible risk of any structural failure during the transition mode. Sea-level tests show 15–17% decrease in the transition nozzle pressure ratio (NPR) during subsequent tests in a single run primarily due to frost formation in the nozzle extension up to the wall inflection location. Frost reduces the wall inflection angle and hence, the transition NPR. However, tests inside the altitude chamber show nearly constant NPR value during subsequent runs primarily due to decrease in back temperature with decrease in back pressure that prevents any frost formation.     

Characteristics of Sonic Jets with Tabs

The results of an experimental investigation on the effect of a vortex generator in the form of a mechanical tab placed at the nozzle exit on the evolution of jet and its decay are reported in this paper. Jets from a sonic nozzle with and without tabs operated at nozzle pressure ratios from 2 to 7 were studied. Tabs with two combinations of length-to-width ratio were investigated by keeping the blockage area constant. The tabs offered a blockage of 10.18% of the nozzle exit area. The centerline pitot pressure decay shows that for the tabbed jet a maximum core reduction of about 75% can be achieved at a nozzle pressure ratio (NPR) 7 compared to an uncontrolled jet. The shadowgraph pictures show that the tabs drastically weaken the shock structure in the jet core and disperse the supersonic zone of the flow making them occupy a greater zone of the flow field compared to the plain nozzle. This causes the waves to become weaker and the jet to spread faster. The tabs are found to shed counter-rotating vortices all along the edges, resulting in enhanced mixing. Isobaric contours reveal that the streamwise vortices cause an inward indentation of the entrained mass into the jet core and an outward ejection of core flow. To understand the distortion introduced by tabs on the jet cross-section and its growth leading to bifurcation of the jet, a surface coating visualization method was developed and employed.     

LES and RANS modelling of under-expanded jets with application to gaseous fuel direct injection for advanced propulsion systems

A density-based solver with the classical fourth-order accurate Runge-Kutta temporal discretization scheme was developed and applied to study under-expanded jets issued through millimetre-size nozzles for applications in high-pressure direct-injection (DI) gaseous-fuelled propulsion systems. Both large eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) turbulence modelling techniques were used to evaluate the performance of the new code. The computational results were compared both quantitatively and qualitatively against available data from the literature. After initial evaluation of the code, the computational framework was used in conjunction with RANS modelling (k-ω SST) to investigate the effect of nozzle exit geometry on the characteristics of gaseous jets issued from millimetre-size nozzles. Cylindrical nozzles with various length to diameter ratios, namely 5, 10 and 20, in addition to a diverging conical nozzle, were studied. This study is believed to be the first to provide a direct comparison between RANS and LES within the context of nozzle exit profiling for advanced high-pressure injection systems with the formation of under-expanded jets. It was found that reducing the length of the straight section of the nozzle by 50% resulted in a slightly higher level of under-expansion (∼2.6% higher pressure at the nozzle exit) and ∼1% higher mass flow rate. It was also found that a nozzle with 50% shorter length resulted in ∼6% longer jet penetration length. At a constant nozzle pressure ratio (NPR), a lower nozzle length to diameter ratio resulted in a noticeably higher jet penetration. It was found that with a diverging conical nozzle, a fairly higher penetration length could be achieved if an under-expanded jet formed downstream of the nozzle exit compared to a jet issued from a straight nozzle with the same NPR. This was attributed to the radial restriction of the flow and consequently formation of a relatively smaller reflected shock angle. With the conical nozzle used in this study and a 30 bar injection pressure, an under-expanded hydrogen jet exhibited ∼60% higher penetration length compared to an under-expanded nitrogen jet at 100 μs after start of injection. Moreover, the former jet exhibited ∼22% higher penetration compared to a nitrogen jet issued through the conical profile with 150 bar injection pressure.     

Effect of cross-wire and tabs on sonic jet structure

An experimental study has been conducted to investigate the effectiveness of passive controls in the form of small tabs and a cross-wire projecting normally into the flow at the nozzle exit, on the characteristics of an axisymmetric sonic jet operated at three underexpansion levels. In this investigation, NPR 3, 5 and 7 were studied. The cross-wire and tabs were found to reduce the amplitude and axial extent of Pitot pressure oscillations and also the supersonic core length. Both cross-wire and tabs were found to be effective in modifying the jet structure significantly, resulting in faster characteristic decay of the jet at all the NPRs. Shadowgraph pictures captured the widening of the supersonic zone in the direction normal to the tabs/cross-wire.      

A fundamental study of a variable critical nozzle flow

Critical nozzle is defined as a device to measure the mass flow rate with only the nozzle supply conditions, making use of the flow choke phenomenon at the nozzle throat. Recently, such critical nozzles have been extensively utilized to measure the mass flow rate in a variety of industrial applications. For the measurement of mass flow rates in a wide range of operation conditions, the critical nozzle is required to be designed with different diameters. The objective of the present study is to investigate the effectiveness of a variable critical nozzle. A rod with a small diameter is inserted into the critical nozzle to change the effective cross-sectional area of the critical nozzle. Experimental work is performed to measure the mass flow rate of the critical nozzle with rod. Computational work is carried out using the two-dimensional, axisymmetric, compressible Navier–Stokes equations which are discretized using a fully implicit finite volume method. The diameter of the rod is varied to obtain different mass flow rates through the variable critical nozzle. Computational results predict well the measured mass flow rates. The boundary layer displacement and momentum thickness at the throat of the critical nozzle are given as a function of Reynolds number. The discharge coefficient of the critical nozzle is given as an empirical equation.     

Separated two-phase flow in a nozzle

This work was performed to extend and further test the method of handling separated two-phase flow by studying each phase separately and, particularly, by placing emphasis on the study of the gas phase with interface transport expressions showing the influence of the liquid phase on it. A one-dimensional flow model for accelerating flows was used in conjunction with experimental data to obtain the pressure distribution and velocity distribution in a converging nozzle for several values of flow quality and nozzle inlet stagnation pressure. The results tend to support the use of the model (which includes the assumption that the gas is in critical flow when the two-phase mixture is in critical flow) and give some insight regarding the nature of the liquid distribution near the nozzle throat.     

Simulation of the starting flow in a wedge–like nozzle

The starting process of the flow in a wedge-like expansion nozzle of a shock tunnel is simulated by an unsplit 2-D GRP scheme on an unstructured grid. The scheme is briefly outlined and results are presented and discussed in comparison to the experimental (shadowgraph) findings obtained by Amann. The simulated pattern of reflected and transmitted shock waves in the nozzle inlet region and inside the nozzle is found to agree well with the experimental data. Received 5 April 1996 / Accepted 16 June 1997     

The upward flow of air betweem two waterfalls: An unusual choked flow problem

An experimental and theoretical investigation of the effect of air rising between two opposed waterfalls is presented. It is found experimentally that as the air flow is increased, the waterfalls are drawn more closely together until a critical air flow rate is reached at which the waterfalls collapse together. A theoretical analysis of this phenomenon is presented and the collapse condition is shown to be analogous to the choked flow of air through a nozzle the cross-sectional area of which is strongly pressure dependent. This dependency results in a very low effective “sonic” velocity and “choked” flow and the theoretical predictions are in reasonable agreement with the experiment results. The relevance of this work to the PWR refill problem is discussed.     

Heterogeneous nucleation in CFD simulation of flashing flows in converging–diverging nozzles

Flashing flow is an important phenomenon in many industrial contexts; however simulation of these flows remains difficult. CFD simulations are able to describe the distribution and evolution of 3D structures in the flow but are dependent on good closure relations for interphase transfer. Nucleation during flashing flow is often neglected in CFD simulation where a minimum starting vapour fraction and a constant bubble number density are given. Models that include nucleation have used wall nucleation terms from 1D system code models, averaged over the domain. In this work, three models for wall nucleation are tested and compared with experimental data from a converging–diverging nozzle. Nucleation is applied at the walls of the domain, and various models are investigated. Good agreement with the critical flow rate and axial profiles are found, but agreement with the radial void fraction data is not satisfactory. Methods of addressing this are explored, and it is found that including a small bulk heterogeneous nucleation term gives the best agreement with the radial profiles, with negligible impact on the axial average properties.     

Experimental study on mixing enhancement in two dimensional supersonic flow

 Studies on mixing enhancement with two dimensional (2D) lobed nozzle have been conducted in a dual stream supersonic flow facility. The distributions of momentum flux, stagnation pressure and stagnation temperature across a plane at different axial distances from the nozzle exit were considered as a measure of mixing. The results indicated an enormous enhancement in mixing when 2D lobed nozzle was employed in comparison with conventional plain 2D nozzle. The enhanced mixing performance could be attributed to the large scale axial vortices observed in the flow-field of subsonic lobed nozzles by earlier investigators. Received: 27 December 1996 / Accepted: 21 August 1997     

Impaktströmung und die zusammengesetzte Schlitzdüse

The paper is an experimental study of a special kind of slot nozzle. The flow field with recirculation area results from a structure built in the nozzle (the so called combined slot nozzle). As the nozzle to wall distance changes, quantitative changes of the flow field occur. They can be seen in the development of the wall pressure as well as in the development of the mass transfer coefficient. Two different sorts of this development have been observed, corresponding to two different states of the flow field. The change of one into the other is very rapid and hysteretic in character.     

Bifurcations of the flow characteristics of an adjustable supersonic nozzle

A ground-based experimental study of the flow characteristic of an adjustable highaltitude nozzle was performed. It is shown that the flow characteristic of the adjustable nozzle can significantly depend on the design of its supersonic part and operation conditions. It is found that the operation such nozzles can involve different flow regimes of the working fluid depending on the position of the regulator; under certain conditions, there may be an abrupt change in the flow regime, which leads to an abrupt change (bifurcation) of the flow characteristic of the nozzle.     

采用NND方法计算三维喷管气流场

本文运用NND显式差分格式,计算了三维喷管气流场。气流场计算的基本方程为一般贴体坐标系下三维守恒型的欧拉方程。采用了时间分裂法和Steger-Warming矢通量分裂技术。在喷管内沿周向的每个由轴线和壁面构成的子午面上根据泊松方程生成贴体网格。本文运用三维程序计算了轴对称JPL喷管,同时与实验结果和前人采用轴对称二维程序所计算的结果做了对比。最后,本文还计算了三维矢量喷管,计算结果与现有的实验结果一致。通过轴对称JPL喷管和三维矢量喷管的计算考核,表明建立的算法和编写的计算程序是正确的。文中提出了采用子午面形式的贴体网格时奇性轴的处理方法。计算结果表明在喷管壁面处,马赫数与压强的计算结果与实验值吻合较好,而在喷管轴线处,只有当网格较密时,才能得出与实验结果接近的计算结果。     

Numerical simulation of flow in Hartmann resonance tube and flow in ultrasonic gas atomizer

The gas flow in the Hartmann resonance tube is numerically investigated by the finite volume method based on the Roe solver.The oscillation of the flow is studied with the presence of a needle actuator set along the nozzle axis.Numerical results agree well with the theoretical and experimental results available.Numerical results indicate that the resonance mode of the resonance tube will switch by means of removing or adding the actuator.The gas flow in the ultrasonic gas atomization (USGA) nozzle is also studied by the same numerical methods.Oscillation caused by the Hartmann resonance tube structure,coupled with a secondary resonator,in the USGA nozzle is investigated.Effects of the variation of parameters on the oscillation are studied.The mechanism of the transition of subsonic flow to supersonic flow in the USGA nozzle is also discussed based on numerical results.     

Liftoff of a Co-Flowing Non-Premixed Turbulent Methane Flame: Effect of the Fuel Nozzle Orifice Geometry

This paper reports an experimental study on the effect of the fuel nozzle orifice geometry on the stability of turbulent non-premixed methane flame. Different internal geometries (orifice equivalent diameter, length to diameter ratio and contraction angle) of a circular and a rectangular nozzle with an aspect ratio of 2 were examined. The strength of the co-airflow was also varied to assess its impact on the jet flame stability. The experimental data revealed that the level of turbulence in the jet near-field is, in general, higher for the rectangular nozzle in comparison with the circular nozzle. This high level of turbulence was found to accelerate the liftoff transition of the attached flame. The results revealed also that there is a clear interplay between the flame liftoff height and the jet flow characteristics. That is, a rectangular jet, which spreads faster along the minor axis and generates higher near-field turbulence, results in a flame base sitting closer to the nozzle exit in comparison with that of its circular nozzle counterpart. Finally, the presence of a moderate co-airflow resulted in a higher flame liftoff velocity and height. It also led to the appearance of a hysteresis phenomenon in the low jet velocity range regardless of the exit orifice shape of the fuel nozzle.     

Control of a submerged jet in a thin rectangular cavity

An innovative method is presented for control of an oscillatory turbulent jet in a thin rectangular cavity with a thickness to width ratio of 0.16. Jet flow control is achieved by mass injection of a secondary jet into the region above the submerged primary jet nozzle exit and perpendicular to the primary nozzle axis. An experimental model, a 2-D and a 3-D computational fluid dynamics (CFD) model are used to investigate the flow characteristics under various secondary injection mass flow rates and injection positions. Two-dimensional laser Doppler anemometry (LDA) measurements are compared with results from the CFD models, which incorporate a standard k–ε turbulence model or a 2-D and 3-D realisable k–ε model. Experimental results show deflection angles up to 23.3° for 24.6% of relative secondary mass flow are possible. The key to high jet control sensitivity is found to be lateral jet momentum with the optimum injection position at 12% of cavity width (31.6% of the primary nozzle length) above the primary nozzle exit. CFD results also show that a standard k–ε turbulence closure with nonequilibrium wall functions provides the best predictions of the flow.     

Tangential Slot Film Cooling in a Nozzle. Comparison of Calculation and Experiment

To test a method of calculating the boundary layer in liquid rocket engine (LRE) nozzles, developed by the authors and based on a differential three-parameter model of turbulence, nozzle flow with tangential-slot coolant injection in subsonic part of the nozzle is studied numerically. The effect of nozzle flow acceleration on the turbulence parameters and the dependence of the gas film effectiveness on the nozzle-inlet turbulence intensity are investigated. The numerical results are compared with available experimental data.     

Bubble formation process from a novel nozzle design in liquid cross-flow

An experimental study is reported that investigated the bubble formation from a novel nozzle design in a liquid cross-flow using high speed imaging. Different configurations and orientations of the novel nozzle design were considered over a range of gas-to-liquid flow rate ratios (GLR) from 0.00031 to 0.00204. The results show that for all cases, the novel nozzle generated smaller bubbles at higher detachment frequency compared to the standard nozzle. At low liquid velocities, the novel nozzle generated bubbles that were 30% smaller in size at a detachment frequency 2–3 times higher than that for the standard nozzle. It was also found that the bubble diameter and the detachment frequency are almost independent of the liquid velocity. The underlying physical process of bubble formation and detachment in the novel nozzle under liquid cross-flow was also investigated. It was observed that the process comprised of three phases: expansion, collapse and pinch off. It was also found that the rebound force of the bubble from a side-hole under the influence of liquid drag force and hydrostatic pressure plays a key role in the early bubble detachment. The results demonstrated that the novel nozzle design performs better than the standard nozzle in the liquid cross-flow, especially at high GLRs.     

基区无吹氦环柱型高超音速低温喷管流场分析

设计了小基区、基区不吹He的环柱型高能化学激光器环形HYLTE喷管 阵列,并使用数值模拟的方法对环形HYLTE喷管阵列流场的气动性能和光学性能进行了分 析. 结果表明,该设计有利于形成ν= 1 →ν = 0之间振转跃迁谱线,不利 于ν = 2→ν =1之间振转跃迁谱线的形成,由于光腔区绝大多数的HF(2)分 子碰撞去激活后成为HF(1)分子,从而使1P谱线的小信号增益倍增,无基 区吹He的设计有利于提高高能化学激光 器的效率. 采用小基区的设计可以增大喷管阵列的功率流密度,无基区吹He的设计不但可 以使复杂的环形高超音速低温喷管叶片的加工与装配得到简化,而且降低了激光器工作 气体的消耗. 因此该设计不但有利于实现高能激光器的紧凑化设计,还有利于节省高能 激光器建造和运行的成本.     

Flow structure at the initial section of a supersonic jet exhausting from a nozzle with chevrons

The spatial structure of the flow in a supersonic underexpanded jet exhausting from a convergent nozzle with vortex generators (chevrons) at the exit is experimentally studied. Exhaustion of a supersonic underexpanded jet from a nozzle with chevrons at the nozzle exit is numerically simulated with the use of the Fluent commercial software package. The experimental and numerical data are demonstrated to be in reasonable agreement. The influence of chevrons on the process of gas mixing is estimated.