Molecular Mixing and Flowfield Measurements in a Recirculating Shear Flow. Part II: Supersonic Flow

Fundamental aspects of mixing between two gaseous streams in a complex geometry are studied and discussed. In the present paper, a supersonic top-stream is expanded over a 30° ramp, through which a secondary lower-stream is injected. The mass flux through the secondary stream is purposely insufficient to provide the entrainment requirements of the resulting shear layer, causing it to attach to the lower guidewall. Part of the shear layer fluid is directed upstream forming a recirculation zone, with enhanced mixing characteristics. The pressure coefficient of the device is quantified as a function of velocity ratio. The effect of heat release on the pressure coefficient is also reported. Molecular mixing was measured employing “flip” experiments based on the hypergolic hydrogen-fluorine chemical reaction. The amount of mixing for the expansion-ramp geometry is found to be higher than in classical free shear layers. However, as in free shear layers, the level of mixing decreases with increasing top-stream velocity. Results for a similar configuration with subsonic/transonic flow in the top stream are reported in Part I of this two-part series.     

Mixing Measurements in a Supersonic Expansion-Ramp Combustor

This paper reports results on molecular mixing for injection via an expansion-ramp into a supersonic freestream with M ₁ = 1.5. This geometry produces a compressible turbulent shear layer between an upper, high-speed “air” stream and a lower, low-speed “fuel” stream, injected through an expansion-ramp at α = 30° to the high-speed freestream. Mass injection is chosen to force the shear layer to attach to the lower guide wall. This results in part of the flow being directed upstream, forming a recirculation zone. Employing the hypergolic hydrogen-fluorine chemical reaction and pairs of “flip” experiments, molecular mixing is quantified by measuring the resulting temperature rise. Initial experiments established the fast-chemistry limit for this flow in terms of a Damköhler number (Da). For Da ≥ 1.4, molecularly mixed fluid effectively reacts to completion. Parameters varied in these experiments were the measurement station location, the injection velocity of the (lower) “fuel” stream, the stoichiometry for the flip experiments, and the density ratio of the fuel and air streams. As expected, mixing increases with increasing distance from the injection surface. The mixed fluid fraction increases by 12% when changing the fuel-to-air stream density ratio from 1 to 0.2. Comparisons with measurements at subsonic (high-speed) “air” stream velocities show that the trend of decreasing mixing with increasing speed documented in free-shear layer flows is also encountered in these flows. The current geometry produces higher mixing levels than do free shear layers.     

Flow patterns and mixing characteristics of gaseous fuel multiple injections in a non-reacting supersonic combustor

Using the particle-based laser scattering imaging technique, schlieren system and surface oil-flow visualization technique, the flow patterns and mixing characteristics of multiple injections with tandem multi-orifices and parallel multi-orifices in a supersonic vitiated air flow were investigated in this paper. All injectors have a declined angle of 30 degree to the freestream direction. The distance between the tandem orifices and that between the parallel orifices was varied. The experimental results showed that decreasing the distance between the tandem orifices will reduce the pressure and velocity of the stream upstream of the second jet, which results in the increase of the penetration height of the second injection and quick mixing of the whole field. For the small distance between the parallel multi-orifices, the bow shock waves upstream of the injected jets connect with each other and the air stream entered into the gap between the jets is not enough, resulting in the decrease of the mixing effect. Large distance between the parallel multi-orifices decreases the interaction between the injection jets. For the mixing enhancement, there should be a proper optimized distance between the parallel injection orifices.     

非平衡流模拟参数ρ∞L有效性的数值分析

在非平衡流相似律现有研究成果的基础上,通过对化学反应特征时间、具体流场区 域热化学状态、辐射输运方程的分析,并结合若干典型算例,讨论双体碰撞反应的非 平衡尺度效应模拟参数ρ∞L对高空高超声速飞行器全流场适用的条件.结果表明,当模拟参数ρ∞L值低于一定范围时, 在高超声速流场非平衡区中离解非平衡起主导作用,此时模拟参数 ρ∞L不仅对全流场的无量纲参数分布有效,而且对流场电离特性、模型气动热特性、分子光谱范围的驻 点辐射谱强度都是有效的.模拟参数ρ∞L的有效性与自由流速度、不同类物理量和流场不同区域均相关: 当自由流速度增加时, 对全场有效的ρ∞L要求ρ∞L 值减小;与无量纲压力、密度和温度相比, 对化学组元特别是离子组元分布有效的ρ∞L所要求的ρ∞L 值更小; 与模型头部区相比, 在身部区有效的ρ∞L所要求的ρ∞L值更小.     

Numerical Investigation of the Robustness of an Axisymmetric Separating/Reattaching Flow to an External Perturbation Using ZDES

The present article focuses on the influence of an external perturbation on the dynamics and the topology of an axisymmetric separating/reattaching flow for a Mach number equal to 0.7. Zonal Detached Eddy Simulation (ZDES) is used to reproduce the flow dynamics downstream an axisymmetric backward facing step with and without a surrounding mixing layer. The instantaneous flowfield shows the influence of the outer shear layer whose structures interact with the separated flow of interest. The instantaneous flowfield depicts the scales of the two axisymmetric mixing layers. Then, the statistical properties of the flow illustrate an excellent agreement between the experiments (PIV and LDV) and the numerical simulation (ZDES). The second-order moment of the pressure coefficient unveils the very high fluctuation levels of the perturbed case compared to the unperturbed one. The analysis of the spectral content reveals a remarkable alteration of the spatial organization for the fluctuating pressure field at the wall and the normal velocity component fluctuations in the afterbody shear layer. Finally, the impact of the perturbation on the energy distribution of the afterbody unsteady side-loads is evidenced by single-point power spectral densities.     

Mixing Characteristics in a Supersonic Combustor with Gaseous Fuel Injection Upstream of a Cavity Flameholder

Non-reacting experiments and numerical simulations have been performed to investigate the mixing characteristics in a supersonic combustor with gaseous fuel injection upstream of a flameholding cavity in a supersonic vitiated air flow with stream Mach number 1.7. Using helium as simulated fuel, the acetone vapor is adulterated into the fuel jet. The fuel distribution in spanwise and streamwise direction is imaged by the planar laser-induced fluorescence (PLIF) measurement. According to the similarity of experimental observations with different cavities, the typical L/D = 7 cavity with aft wall angle 45° is chosen and the flowfield with the injection is calculated by Large Eddy Simulation. Experimental and numerical results have shown that most of the fuel flow away upon the open cavity with the lifting counter-rotating vortex structures induced by the transverse jet. Only a small portion of the fuel is convected into the cavity shear layer by the vortex interaction of the jet with cavity shear layer, and then transported into the cavity due to the cavity shear layer motion and the interaction of the shear layer with the cavity trailing edge.     

The influence of fuel injection and heat release on bulk flow structures in a direct-injection, swirl-supported diesel engine

Particle image velocimetry is applied to measure the vertical (r–z) plane flow structures in a light-duty direct-injection diesel engine with a realistic piston geometry. The measurements are corrected for optical distortions due to the curved piston bowl walls and the cylindrical liner. Mean flow fields are presented and contrasted for operation both with and without fuel injection and combustion. For operation with combustion, the two-dimensional divergence of the measured mean velocity fields is employed as a qualitative indicator of the locations of mean heat release. In agreement with numerical simulations, dual-vortex, vertical plane mean flow structures that may enhance mixing rates are formed approximately mid-way through the combustion event. Late in the cycle a toroidal vortex forms outside the bowl mouth. Imaging studies suggest that soot and partially oxidized fuel trapped within this vortex are slow to mix with surrounding fluid; moreover, the vortex impedes mixing of fluid exiting the bowl with air within the squish volume.     

Developing and fully developed turbulent flow in ribbed channels

Wall-mounted roughness features, such as ribs, are often placed along the walls of a channel to increase the convective surface area and to augment heat transfer and mixing by increasing turbulence. Depending on the relative roughness size and orientation, the ribs also have varying degrees of increased pressure losses. Designs that use ribs to promote heat transfer encompass the full range of having only a few streamwise ribs, which do not allow fully developed flow conditions, to multiple streamwise ribs, which do allow the flow to become fully developed. The majority of previous studies have focused on perturbing the geometry of the rib with little attention to the spatially and temporally varying flow characteristics and their dependence on the Reynolds number. A staggered rib-roughened channel study was performed using time-resolved digital particle image velocimetry (TRDPIV). Both the developing (entry region) and a fully developed region were interrogated for three Reynolds numbers of 2,500, 10,000, and 20,000. The results indicate that the flow was more sensitive to Reynolds number at the inlet than within the fully developed region. Despite having a similar mean-averaged flowfield structure over the full Reynolds number range investigated, the population and distribution of coherent structures and turbulent dissipation within the fully developed region were also found to be Reynolds number dependent. Exploring the time-accurate flow characteristics revealed that in addition to vortices shed from the rib shear layer, the region of the rib wake was governed by a periodic process of bursting of the wake vortices resulting in the intermittent ejection of the inter-rib recirculation region into the core flow. This periodic process was the driving mechanism resulting in mixing and heat transfer augmentation. A quadrant-splitting burst analysis was also performed to determine the characteristic frequency and duration of inter-rib bursting as well as the wake shedding frequency, both of which were determined to be Reynolds number dependent.     

Transient Flow Patterns of Multiple Plasma Synthetic Jets Under Different Ambient Pressures

The plasma synthetic jet is a new active flow control technique, which has great potentials for supersonic flow control A plasma synthetic jet actuator (PSJA) for supersonic flow control which operates under low ambient pressure is designed In order to explore the transient jet flowfield, high-speed Schlieren and electronic measurement systems are utilized to test the single-shot operating characteristics of the actuator under two ambient pressure conditions. The evolution of the jet boundary, which depicts the transient jet flowfield and can be used to estimate the flow control capability, is captured. It is found that the ambient pressure is the primary reason to affect the arc energy deposition, which directly determines the velocity of blast wave and jet front. In addition, the flow patterns of PSJA under various ambient pressures show some similarities. The jet evolution can be divided into three stages, i.e. pressure dominant stage, the inertia dominant stage and vortex ring dominant stage. During the pressure dominant stage, the PSJA could be treated as jet. For the inertia dominant stage, the Froude number decreases from 2263.82 to 21.73, indicating the inertia effect gets weakened with an intensified buoyancy effect, but the inertia effect still holds the dominant position, and this stage may be considered as the end mark of the injection process. As for the vortex ring dominant stage, large scale vortex ring becomes significant and the jet front velocity is very low but with apparent fluctuation.     

Flow analysis of an innovative compact heat exchanger channel geometry

In the framework of CEA R&D program to develop an industrial prototype of sodium-cooled fast reactor named ASTRID, the present work aims to propose an innovative compact heat exchanger technology to provide solid technological basis for the utilization of a Brayton gas-power conversion system, in order to avoid the energetic sodium–water interaction if a traditional Rankine cycle was used. The aim of the present work is to propose an innovative compact heat exchanger channel geometry to potentially enhance heat transfer in such components. Hence, before studying the innovative channel performance, a solid experimental and numerical database is necessary to perform a preliminary thermal–hydraulic analysis. To do that, two experimental test sections are used: a Laser Doppler Velocimetry (LDV) test section and a Particle Image Velocimetry (PIV) test section. The acquired experimental database is used to validate the Anisotropic Shear Stress Transport (ASST) turbulence model. Results show a good agreement between LDV, PIV and ASST data for the pure aerodynamic flow. Once validated the numerical model, the innovative channel flow analysis is performed. Principal and secondary flow has been analyzed, showing a high swirling flow in the bend region and demonstrating that mixing actually occurs in the mixing zone. This work has to be considered as a step forward the preposition of a reliable high-performance component for application to ASTRID reactor as well as to any other industrial power plant dealing needing compact heat exchangers.     

Numerical Study of Ignition Process in Turbulent Shear-less Methane-air Mixing Layer

In this work, ignition process in a turbulent shear-less methane-air mixing layer is numerically investigated. A compressible large eddy simulation method with Smagorinsky sub-grid scale model is used to solve the flow field. Also, a thickened flame combustion model and DRM-19 reduced mechanism are used to compute species distribution and the heat release. Non-reacting mean and RMS axial velocity profiles and mean mixture fraction are validated against experimental data. Instantaneous mixture fraction contours show that the large bursts penetrate from the fuel stream into that of the oxidizer and vice versa and a random behaviour in the cross-stream direction. Flame kernel initiation, growth and propagation are analysed and compared with the experimental data. The ignition results show that the flame is not stable and blow-off occurs, but a more detailed investigation shows that local and short time flame stabilization exist during blow-off. During these local stabilization, heat release increased at the upstream edge of the flame. Most_upstream flame edge scalar analysis shows that the methane mass fraction has a dominant role in the local flame stabilization. OH, HO₂, CH₂O and heat release contours demonstration reveal that HO₂ and CH₂O mass fraction as well as the heat release reach a maximum on the border of the flame, but the maximum OH concentration is located in the middle of flame kernel.     

Influence of lateral mass flux on mixed convection heat and mass transfer over inclined surfaces in porous media

 The effect of lateral mass flux on mixed convection heat and mass transfer in a saturated porous medium adjacent to an inclined permeable surface is analyzed. A similarity solution is obtained when surface temperature and concentration, free stream velocity and injection/suction velocity of fluid are prescribed as power functions of distance from the leading edge. The cases when the flow and buoyancy forces are in the same and opposite directions are discussed both for aiding and opposing buoyancy effects. The governing parameters are the mixed convection parameter Gr, the Lewis number Le, the buoyancy ratio N, the lateral mass flux parameter f _w, representing the effects of injection or withdrawal of fluid at the wall, and λ which specifies three cases of the inclined plate. The interactive effect of these parameters on heat and mass transfer rates are presented. It is observed that the diffusion ratio (Le) has a more pronounced effect on concentration field than on flow and temperature fields. It is found that the rates of heat and mass transfer increase with suction and decrease with injection of the fluid. Received on 31 August 2000 / Published online: 29 November 2001     

Direct Numerical Simulation of a Square Jet Ejected Transversely into an Accelerating, Laminar Main Flow

A numerical simulation of a square jet ejected transversely into a laminar boundary-layer flow was performed at a jet-to-main-flow velocity ratio of 9.78 and jet Reynolds number of 6330. The jet consisted of a single pulse with a duration equal to the time required for the jet fluid to travel 173 jet widths. A strongly-favourable streamwise pressure gradient was applied to the boundary layer and produced a freestream acceleration that is above the typical threshold required for relaminarization. The results of the simulation illustrate the effect of the favourable streamwise pressure gradient on the flowfield created by the transverse jet. Notably, the horseshoe vortex system created upwind of the jet remains steady in time and does not induce noticeable fluctuations in the jet flow. The upwind and downwind shear layers of the jet roll-up through a Kelvin–Helmholtz-like instability into discrete shear-layer vortices. Jet vorticity in the upwind and downwind shear layers accumulates near the corners of the jet and produces two sets of vortex pairs, the former of which couple with the shear-layer vortices to produce large, counter-rotating vortices in the freestream, while the latter are unstable and periodically produce hairpin vortices in the main-flow boundary layer and elongated vortices in the freestream behind the jet. The departure of the jet flowfield from the vortical structures typically observed in transverse jets illustrates the substantive effect of the favourable streamwise pressure gradient on the flowfield created by the jet.     

Oscillation of shallow flow past a cavity: Resonant coupling with a gravity wave

Self-excited oscillations of flow past a cavity are generated in a shallow free-surface system. The shear layer past the cavity opening has two basic forms: a separated free-shear flow; and a shear flow along a slotted plate. Instabilities of these classes of shear flows can couple with the fundamental gravity-wave mode of the adjacent cavity. The dimensionless frequencies of both types of oscillations scale on the length of the cavity opening, rather than the gap distance between the slats, i.e., a large-scale instability is always prevalent. A technique of high-image-density particle image velocimetry allows acquisition and interpretation of global, instantaneous images of the flow pattern, including patterns of vorticity and Reynolds stress correlation. Use of a cinema approach provides representations of the timewise evolution of the global, instantaneous flow structure, and thereby definition of the amplitude peaks and phase angles of the coupled fluctuations via auto- and cross-spectral techniques. These methods, along with global, averaged representations of the fluctuating flow field, provide insight into the onset of fully coupled (phase-locked) oscillations of the shear flow past the resonator cavity. The common, as well as the distinctive, features of the resonant-coupled instability of the shear flow past the slotted plate are characterized, relative to the corresponding coupled instability of the free-shear layer. Varying degrees of resonant coupling between the unstable shear layer and the adjacent resonator are attained by variations of the inflow velocity, which yield changes of the predominant oscillation frequency, relative to the resonant frequency of the adjacent cavity. Well-defined, coherent oscillations are indeed attainable for the case of the shear flow along the slotted plate, though their amplitude is significantly mitigated relative to the case of a free-shear layer. The degree of organization of the self-excited, resonant-coupled oscillation and the manner in which it varies with open area ratio and geometry of the plate are interpreted in terms of the flow structure on either side of, and within, the slotted plate; these features are compared with the corresponding structure of the free-shear layer oscillations.     

Heat Transfer and Gas Flow through Feed Stream within Horizontal Pipe

In the feeding process, the feed stream forms a moving packed bed of particle from the feedstock in the feed channel. When the feeding is at emergency interruption especially in the case of flooding and uncontrollable discharge, the hot gases from reactor would infiltrate into the feed stream. The high heat penetration into feed stream would affect the feeder performance. In this paper, transient thermal response of feed stream within horizontal pipe is described mathematically with a gas flow and heat transfer model. Influences of varied factors on the thermal penetration into feed stream are examined for different conditions. The temperature of the packed-bed particles and the gas velocity distribution curves are obtained for the feeding service at interruption and at normal operating conditions. The numerical results show that the thermal penetration to the packed-bed particles by the seepage flow fluid is high only in the position near the gas entrance. The thermal penetration depth tends to increase with the seepage flow velocity and decrease with feeding rate. There is no appreciable thermal penetration in the feed stream when the feeding service is at normal running. The operating conditions and the porosity of solid bed have importance effects on the gas velocity and temperature field in the thermal penetration zone. A test system is set up to determine the transient thermal response experimentally for the packed bed of particles within a horizontal pipe. The model results are found to compare favorably with the experimental data.     

Flow structure and wall layer control in a lower half heated upper half cooled enclosure with superimposed temperature deviations

This paper presents an investigation on the effects of superimposed temperature deviations as a control technique for the flows and mixing in lower half heated upper half cooled enclosures. Results show that the strength of the wall layer depends on the difference between the wall surface temperature and the fluid core temperature. The location of the head-on collision between a pair of upward/downward wall layers, which controls the mixing and fluid exchange between the two halves, is determined by the wall layer flow momentum strengths. Elevating/reducing the wall temperature by a superimposed temperature deviation is an effective control for the flow and mixing in such enclosures. Heat transfer analysis shows that the superimposed temperature deviations have minor effects on the total heat flow rate from the lower walls. Thus, this technique can be applied onto reactor vessels without modifying the reactor vessel configuration.     

Role of inlet reactant mixedness on the thermodynamic performance of a rotating detonation engine

Rotating detonation engines have the potential to achieve the high propulsive efficiencies of detonation cycles in a simple and effective annular geometry. A two-dimensional Euler simulation is modified to include mixing factors to simulate the imperfect mixing of injected reactant streams. Contrary to expectations, mixing is shown to have a minimal impact on performance. Oblique detonation waves are shown to increase local stream thermal efficiency, which compensates for other losses in the flow stream. The degree of reactant mixing is, however, a factor in controlling the stability and existence of rotating detonations.     

Entry region of louvered fin heat exchangers

The dominant thermal resistance for most compact heat exchangers occurs on the gas side and as such an understanding of the gas side flowfield is needed before improving current designs. Louvered fins are commonly used in many compact heat exchangers to increase the surface area and initiate new boundary layer growth. For this study, detailed flowfield measurements were made in the entry region of several louvered fin geometries whereby the louver angle, ratio of fin pitch to louver pitch, and Reynolds number were all varied. In addition to mean velocity measurements, time-resolved velocity measurements were made to quantify unsteady effects.

The results indicated larger fin pitches resulted in lower average flow angles in the louver passages and longer development lengths. Larger louver angles with a constant ratio of fin pitch to louver pitch resulted in higher average flow angles and shorter development lengths. As the Reynolds number increased, longer development lengths were required and higher average flow angles occurred as compared with a lower Reynolds number case. Time-resolved velocity measurements indicated some flow periodicity behind the fully developed louver for a range of Reynolds numbers. The Strouhal number of these fluctuations was constant for a given louver geometry, but the value increased with increasing fin pitch.     

Numerical and experimental evidence of the Fabri-choking in a supersonic ejector

The purpose of a supersonic ejector consists in the mixing of two fluids with different stagnation pressures in order to obtain a fluid at an intermediate stagnation pressure at the discharge. Depending on the geometry of the ejector and on the operating conditions, the entrained secondary stream may reach sonic/supersonic velocities within the ejector, leading to the capping of the entrained mass flow rate for fixed reservoir conditions. Although the associated limitation of the entrainment ratio (due to choking) is a well known phenomenon, there is still a lack of understanding of the complex flow phenomena at play within supersonic ejectors, and further detailed knowledge and modeling of the choking process is necessary. This paper presents a detailed analysis of the choking phenomenon through advanced post-processing of CFD calculations which are validated with experimental results both at the global and the local scales. This in-depth investigation of the choking phenomenon within the ejector is proposed both qualitatively and quantitatively for given reservoir conditions. The complex flow signature highlighted by means of the numerical results is then investigated and corroborated through experimental shadowgraphy. Studies combining experimental results (including visualizations) with numerical simulations are rather scarce in the open literature and to the knowledge of the authors, this study is the first one that proposes such a detailed analysis. For the present ejector geometry and operating conditions, the choking phenomenology of the secondary stream is found to closely correspond to the model of the Fabri-choking early postulated in Fabri and Siestrunck (1958).     

Boundary layer flow over a moving surface in a nanofluid with suction or injection

An analysis is performed to study the heat transfer characteristics of steady two-dimensional boundary layer flow past a moving permeable flat plate in a nanofluid. The effects of uniform suction and injection on the flow field and heat transfer characteristics are numerically studied by using an implicit finite difference method. It is found that dual solutions exist when the plate and the free stream move in the opposite directions. The results indicate that suction delays the boundary layer separation, while injection accelerates it.