Two-dimensional numerical calculation of the flow field about a scoop inlet by the piecewise linear method

The piecewise linear method (PLM) based on time operator splitting is used to solve the unsteady compressible Euler equations describing the two-dimensional flow around and through a straight wall inlet placed stationary in a rapidly rotating supersonic flow. The PLM scheme is formulated as a Lagrangian step followed by an Eulerian remap. The inhomogeneous terms in the Euler equations written in cylindrical coordinates are first removed by Sod's method and the resulting set of equations is further reduced to two sets of one-dimensional Lagrangian equations, using time operator splitting. The numerically generated flow fields are presented for different values of the back pressure imposed at the downstream exit of the inlet nozzle. An oblique shock wave is formed in front of the almost whole portion of the inlet entrance, the incoming streamlines being deflected towards the higher pressure side after passing through the oblique shock wave and then bending down to the lower pressure side. A reverse flow appears inside the inlet nozzle owing to the recovery pressure of the incoming streams being lower than the back pressure of the inlet nozzle.     

Experimental investigation of parameters effect on heat transfer of spray cooling

The effects of spray height, nozzle spray angle, inlet pressure and spray incident angle on heat transfer of spray cooling were studied by an experimental method. Multi-points thermocouples and infrared imaging device were used to measure temperature distribution on heating surface. A Doppler anemometry and a camera were applied to study the spray flow field. The mechanism of heat transfer of spray cooling was concluded on the basis of experimental data and spray characteristics. It is showed that parameters affect heat transfer by way of changing the flow field on the heating surface. Heat transfer performance can be optimized by a smaller spray angle nozzle, which sprays at a smaller spray height and a higher inlet pressure. The effect of incident angle on heat transfer depends on nozzle spray angle and the definition of distance of nozzle to surface.     

Oscillations of the fluid flow and the free surface in a cavity with a submerged bifurcated nozzle

The free surface dynamics and sub-surface flow behavior in a thin (height and width much larger than thickness), liquid filled, rectangular cavity with a submerged bifurcated nozzle were investigated using free surface visualization and particle image velocimetry (PIV). Three regimes in the free surface behavior were identified, depending on nozzle depth and inlet velocity. For small nozzle depths, an irregular free surface is observed without clear periodicities. For intermediate nozzle depths and sufficiently high inlet velocities, natural mode oscillations consistent with gravity waves are present, while at large nozzle depths long term self-sustained asymmetric oscillations occur.For the latter case, time-resolved PIV measurements of the flow below the free surface indicated a strong oscillation of the direction with which each of the two jets issue from the nozzle. The frequency of the jet oscillation is identical to the free surface oscillation frequency. The two jets oscillate in anti-phase, causing the asymmetric free surface oscillation. The jets interact through a cross-flow in the gaps between the inlet channel and the front and back walls of the cavity.     

Study on micronozzle flow and propulsion performance using DSMC and continuum methods

In this paper, both DSMC and Navier–Stokes computational approaches were applied to study micronozzle flow. The effects of inlet condition, wall boundary condition, Reynolds number, micronozzle geometry and Knudsen number on the micronozzle flow field and propulsion performance were studied in detail. It is found that within the Knudsen number range under consideration, both the methods work to predict flow characteristics inside micronozzles. The continuum method with slip boundary conditions has shown good performance in simulating the formation of a boundary layer inside the nozzle. However, in the nozzle exit lip region, the DSMC method is better due to gas rapid expansion. It is found that with decreasing the inlet pressure, the difference between the continuum model and DSMC results increases due to the enhanced rarefaction effect. The coefficient of discharge and the thrust efficiency increase with increasing the Reynolds number. Thrust is almost proportional to the nozzle width. With dimension enlarged, the nozzle performance becomes better while the rarefaction effects would be somewhat weakened.The project supported by the National Natural Science Foundation of China (10372099). The English text was polished by Boyi Wang     

Slowly mixing cylinder in a cone-shaped nozzle

We report on a surprising phenomenon in a turbulent jet setup which uses a cone-shaped nozzle with an excentric inlet. Inside the nozzle a slowly mixing, rotating cylinder surface was observed. The speed of mixing on this cylinder surface is reduced by approximately a factor of 8 compared to the remaining flow field of the nozzle. The phenomenon seems to be independent of the Reynolds number and the pressure distribution inside the nozzle.     

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     

Nozzle startup in an oncoming flow

Three variants of the startup of an axisymmetric convergent-divergent nozzle are considered with the static pressures at the entry and exit of the nozzle being the same at the beginning of the process. The subsonic startup corresponds to open nozzle acceleration in air. The supersonic startup simulates the sudden opening of a cover at the nozzle inlet under supersonic flight conditions. A successful nozzle startup with the formation of steady supersonic flow along the whole channel is realized in the third variant of supersonic startup with gas injection through a small region of the wall of the divergent nozzle section. The investigation is performed numerically, on the basis of the Euler equations for axisymmetric gas flows.     

自激振荡脉冲射流喷嘴装置系统频率特性理论研究

根据相似系统原理和流体网络理论建立了自激振荡脉冲射流喷嘴装置的等效网络模型,利用系统传递函数推导了系统频率特性方程并进行了数值计算。结果表明:喷嘴装置的固有频率主要由喷嘴形状、结构参数、入口流速、射流中压力扰动波波速决定;自激振荡腔腔径、自激振荡腔腔长、上喷嘴直径、下喷嘴直径都对系统频率特性影响很大。提出了相应的自激振荡脉冲射流喷嘴设计准则,即喷嘴装置在最佳阻尼比下产生谐波共振。     

Numerical study of the spontaneous nucleation of self-rotational moist gas in a converging–diverging nozzle

Spontaneous nucleation is the primary way of droplet formation in the supersonic gas separation technology, and the converging–diverging nozzle is the condensation and separation unit of supersonic gas separation devices. A three-dimensional geometrical model for the generation of self-rotational transonic gas flow is set up, based on which, the spontaneous nucleation of self-rotational transonic moist gas in the converging–diverging nozzle is carried out using an Eulerian multi-fluid model. The simulated results of the main flow and nucleation parameters indicate that the spontaneous nucleation can occur in the diverging part of the nozzle. However, different from the nucleation flow without self-rotation, the distributions of these parameters are unsymmetrical about the nozzle axis due to the irregular flow form caused by the self-rotation of gas flow. The nucleation region is located on the position where gas flows with intense rotation and the self-rotation impacts much on the nucleation process. Stronger rotation delays the onset of spontaneous nucleation and yields lower nucleation rate and narrow nucleation region. In addition, influences of other factors such as inlet total pressure p ₀, inlet total temperature T ₀, the nozzle-expanding ratio ? and the inlet relative humidity ф ₀ on the nucleation of self-rotational moist gas flow in the nozzle are also discussed.     

LES of supersonic turbulent flow in nozzles and diffusers

Large-eddy simulations of supersonic nozzle and diffuser flows with circular cross-sections using high-order compact schemes and an explicit filtering version of the approximate deconvolution method are presented in this paper. Two flow cases each for nozzle and diffuser having different outlet to inlet area ratios are presented. The effect of the geometry variations on the Reynolds stresses as well as on the production and pressure-strain terms in their transport equations is demonstrated. A Green’s function analysis of the Poisson equation for pressure fluctuations using LES data is presented and the results show similar trends as found in previous analyses using DNS data. The effects of geometry changes on the rapid and slow parts of pressure-strain correlations is also demonstrated.     

超音速气流粉碎喷嘴数值模拟

对气流粉碎装置的喷嘴结构和参数进行设计与优化,采用流体动力学软件对所设计喷嘴进行流场模拟,对所喷嘴效果进行检验。分别讨论了锥顶角和内腔造型对超音速喷嘴性能的影响,通过结果比较得出,入口压力3.5MPa、入口直径为6mm的喷嘴为设计的最佳喷嘴。超音速喷嘴锥顶角在8度到12度之间变化时,对喷嘴的性能影响不大,内腔造型为光滑曲面的超音速喷嘴性能最佳。     

Shock and wave dynamics in cavitating compressible liquid flows in injection nozzles

Due to the exceptional high inlet pressures up to 2,000 bar flow dynamics and efficiency of modern injection systems are controlled by high frequency wave dynamics of the compressible liquid flow. Corresponding to alternating shock and expansion waves the liquid fluid evaporates and recondenses instantaneously. Here we present CFD simulations of the time accurate evolution of cavitating flows in 2-D plane and in six-hole injection nozzles with focus on the wave dynamics just after initialisation of the flow and within the time scale Δt ≤ 10^?4 s of pilot and multi-point injection. Due to shock reflections at the bottom of the sack hole the instantaneous maximum pressure increases more than three times higher as compared with the prescribed pressure at the nozzle inlet. For instance, in case of an inlet pressure of 600 bar the maximum pressure in the sack and therefore ahead of the nozzle bore holes reaches about 2,100 bar. It is quite reasonable that this amplification of the pressure affects the evolution of the convective flow and therefore the mass flow through the nozzle bore holes.     

On the effect of the nozzle design on the performances of gas–liquid cylindrical cyclone separators

This paper constitutes an experimental study of the separation performances of a gas–liquid cylindrical cyclone (GLCC) separator that interests the oil industry. The global hydrodynamics behavior in the GLCC is characterized by flow visualization under various inflow operating conditions. The effect of the inlet nozzle design on the performances of the separator is studied by using three different nozzles, and it proves to be a key parameter. With an insufficient nozzle restriction, low swirl intensity is imparted to the flow. Due to inadequate centrifugal effects, liquid is prematurely carried over by the gas as flooding occurs in the separator upper part. High amounts of gas are also carried under by the liquid stream. On the other hand, with a too severe nozzle convergence, the important drag applied by the gas leads to liquid “short circuiting” the cyclone toward the gas outlet. In addition to the nozzle design, the separator performances are influenced by phenomena such as liquid bridging or the occurrence of the slug flow regime at the cyclone inlet. This paper leads to a better understanding of the links between the hydrodynamics in the GLCC and its operational limits, which is necessary to enable reliable scaling up tools.     

On a Degenerate Free Boundary Problem and Continuous Subsonic–Sonic Flows in a Convergent Nozzle

This paper concerns the well-posedness of a boundary value problem for a quasilinear second order elliptic equation which is degenerate on a free boundary. Such problems arise when studying continuous subsonic–sonic flows in a convergent nozzle with straight solid walls. It is shown that for a given inlet being a perturbation of an arc centered at the vertex of the nozzle and a given incoming mass flux belonging to an open interval depending only on the adiabatic exponent and the length of the arc, there is a unique continuous subsonic–sonic flow from the given inlet with the angle of the velocity orthogonal to the inlet and the given incoming mass flux. Furthermore, the sonic curve of this continuous subsonic–sonic flow is a free boundary, where the flow is singular in the sense that while the speed is C ^1/2 Hölder continuous at the sonic state, the acceleration blows up at the sonic state.     

Liquid flow in a simplex swirl nozzle

Pressure-swirl nozzles are widely used in applications such as combustion, painting, air-conditioning, and fire suppression. Understanding the effects of nozzle geometry and inlet flow conditions on liquid film thickness, discharge coefficient and spray angle is very important in nozzle design. The nozzle-internal flow is two-phase with a secondary flow which makes its detailed analysis rather complex. In the current work, the flow field inside a pressure-swirl nozzle is studied theoretically. Using the integral momentum method, the growth of the boundary layer from the nozzle entry to the orifice exit is investigated and the velocity through the boundary layer and the main body of the swirling liquid is calculated. A numerical modeling and a series of experiments have also been performed to validate the theoretical results. The effect of various geometrical parameters is studied and results are compared for viscous and inviscid cases. In addition, the condition in which the centrifugal force of the swirling flow overcomes the viscous force and induces an air core is predicted. The theoretical analysis discussed in this paper provides better criteria for the design and the performance analysis of nozzles.     

Stabilization of detonation combustion in a high-velocity flow of a hydrogen-oxygen mixture

The flow of a hydrogen-oxygen mixture diluted with argon in a supersonic axisymmetric nozzle consisting of an inlet cylinder, a convergent region, a cylindrical throat, and a divergent region is considered. The supersonic flow enters the channel along the axis of symmetry. The flow structure is calculated with allowance for hydrogen ignition. A possibility of stabilizing the combustion zone is studied and the forces acting on the nozzle from the flow are determined. The problem is solved in the two-dimensional approximation with account for detailed combustion kinetics.     

Numerical Analysis of the Impact of the Interior Nozzle Geometry on Low Mach Number Jet Acoustics

The flow and acoustic fields of subsonic turbulent hot jets exhausting from three divergent nozzles at a Mach number M=0.12 based on the nozzle exit velocity are conducted using a hybrid CFD-CAA method. The flow field is computed by highly resolved large-eddy simulations (LES) and the acoustic field is computed by solving the acoustic perturbation equations (APE) whose acoustic source terms are determined by the LES. The LES of the computational domain includes the interior of the nozzle geometry. Synthetic turbulence is prescribed at the inlet of the nozzle to mimic the exit conditions downstream of the last turbine stage. The LES is based on hierarchically refined Cartesian meshes, where the nozzle wall boundaries are resolved by a conservative cut-cell method. The APE solution is determined on a block structured mesh. Three nozzle geometries of increasing complexity are considered, i.e., the flow and acoustic fields of a clean geometry without any built-in components, a nozzle with a centerbody, and a nozzle with a centerbody plus struts are computed. Spectral distributions of the LES based turbulent fluctuated quantities inside the nozzle and further downstream are analyzed in detail. The noise sources in the near field are noticeably influenced by the nozzle built-in components. The centerbody nozzle increases the overall sound pressure level (OASPL) in the near field with respect to the clean nozzle and the centerbody-plus-strut nozzle reduces it compared to the centerbody nozzle due to the increased turbulent mixing. The centerbody perturbed nozzle configurations generate a remarkable spectral peak at S t=0.56 which also occurs in the APE findings in the near field region. This tone is generated by large scale vortical structures shed from the centerbody. The analysis of the individual noise sources shows that the entropy term possesses the highest acoustic contribution in the sideline direction whereas the vortex sound source dominates the downstream acoustics.     

以压力为基本求解变量数值模拟粘性超、跨音速流动

应用以压力为基本求解变量的SIMPLE方法 ,对一双喉喷管中的层流超音速流动和一扩压器中的紊流跨音速流动进行了数值计算。计算结果显示 ,本文的计算结果与文献数据及实验结果相符很好。表明本文方法对可压缩流动有很高的模拟精度。进而表明经过可压缩推广的SIMPLE方法适用于任何马赫数的流动计算     

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.     

Parametric Study of Alternating Flow Patterns in Non-Reacting Multiple-Swirl Flows

Multiple nozzle combustors, under certain conditions, may result in flowfields that differ between nozzles in an alternating pattern. Previous work has provided some clues on the parameters which govern the appearance of this behavior, but there is a lack of systematic studies. A series of non-reacting simulations of adjacent swirling flows is used to investigate the effect of nozzle exit flare angle and swirl number on the presence of the alternating flow pattern. Two-nozzle simulations are shown to accurately predict if an asymmetric flow characteristic appears and are therefore used in the parametric investigation. Alternating flow patterns are predicted at nozzle exit flare angles of 105 degrees (for a swirl number of 0.79) and 120 degrees (for a swirl number of 0.69 and 0.79). Under conditions close to the stability boundary between symmetric and asymmetric flows, the nozzle exit flare and increased swirl number push the shear layers against the dome wall so that the flows between each nozzle are largely opposite in direction. An increase in nozzle exit flare above 120^° results in separated flows exiting from the inlet and a return to a symmetric flow state. This is consistent with a proposed physical mechanism based on hydrodynamic stability in turbulent opposed jets.