Shock Train Length Research Articles

Numerical investigation of spindle position effects on steam ejector performance with a nonequilibrium condensation model

AbstractThis research studied the performance of a spindle‐controlled steam ejector using models such as ideal gas and wet steam under various operating conditions based on computational fluid dynamics (CFD). A wet steam model incorporating nonequilibrium condensation was employed to simulate the complex flow phenomena within the ejector. The structure of the flow, entrainment ratio (Er), and shock wave characteristics of the steam ejector were examined in two different models. Results indicate that the spindle position has a substantial impact on steam ejector performance. For the ideal gas and wet steam models, the optimal spindle position (SP‐5) at a .1 MPa motive pressure achieves the highest entrainment ratios (Er) of 1.01 and 1.042, respectively. However, an ejector with a fixed geometry achieves Er values of only .517 and .549 for the ideal gas and wet steam models, under identical working conditions. This represents a substantial improvement of 89.8% over the fixed‐geometry ejector. The wet steam model consistently predicts 2%–4% higher Er values compared with the ideal gas model across all spindle positions. The study also reveals that increasing the motive pressure from .1 to .3 MPa reduces Er by up to 45.8% at the optimal spindle position, with the shock train length extending to 35% of the mixing chamber at .3 MPa. These findings offer insights for improving the design and optimization of variable‐geometry steam ejectors, potentially increasing efficiency in industrial applications.

On the Analysis of Scale Effects of Flame Stabilization in Scramjet Combustors

Scale effects of flame stabilization in scramjet combustors at Mach 2.52 are investigated through experiments and numerical simulations based on two geometrically similar combustors. As the equivalence ratio increases, the combustion mode will change from Scram to Dual to Ram. However, the larger combustor has a delayed mode transition. The scale effects of flame stabilization are also reflected in the changes of heat release, precombustion shock train length, jet penetration depth, and reaction zone location. The combustion flowfields of different-scale combustors exhibit numerous differences under different combustion modes. However, after shortening the isolator of the proportionally scaled small-scale combustor, the combustion flowfield is closer to that of the large-scale combustor. Further analysis reveals the reason for the inconsistent scale effects of flame stabilization under different combustion modes. The probable reason is that the boundary layer, which does not change proportionally with the combustor scale, has different effects on the combustion under the three modes. Compared with the Scram and Ram modes, the scale effect in the Dual mode is the most significant because the strong interaction between heat release and the boundary layer amplifies the slight difference in the relative thickness of the boundary layer.

Characteristics of self-excited shock wave oscillations in a supersonic isolator with exit duct configurations

The structure and dynamics of multiple shock waves inside a constant area isolator of a model scramjet engine are studied numerically using a finite volume solver. The flow is accelerated through a nozzle with an exit Mach number of 1.75 and continues in the constant area isolator where multiple shock waves are formed. To analyze the effect of duct acoustics on the dynamics of shock train oscillations, three ducts with a duct length of 0D, 50D, and 150D are attached to the exit of the isolator, where D is the isolator exit height. The wall static pressure variation along the bottom wall and four specific locations are monitored. The wall shear stress variation along the wall provides the location of separation and reattachment points in the flow field. The statistical analysis provided more insights into the oscillation mechanisms. The lower frequency components from the duct resonance are enhanced as the duct length is increased and it altered the dominant frequency of shock oscillation. The dependence of shock train length on the exit duct confirmation is analyzed by the pressure ratio method. The different frequency components in the flow field are analyzed using the power spectral density of wall static pressure fluctuations and are distinguished based on the frequency range. The cross-spectral analysis of the wall pressure fluctuations provided the correlation between pressure signals and their narrowband time delay. Thus, the origin and directions of the perturbations are obtained and discussed. The increase in exit duct length decreases the downstream disturbance velocity, indicating the duct resonance effect on shock train oscillations. • Simulation of the supersonic isolator flow field to understand the duct resonance effect on self-excited oscillations of shock train. • The duct resonance is varied by attaching ducts of different lengths to the exit of the isolator. • As the duct length is increased, the amplitude of shock oscillation is increased and the dominant frequency of shock oscillation is decreased. • The length of the shock train is increased with an increase in duct length. • The cross-spectral analysis of the wall pressure fluctuations provided the correlation between pressure signals and their narrowband time delay.

Shock Train Response to High-Frequency Backpressure Forcing

Numerical simulations are performed to study the effect of backpressure disturbances on a shock train. The basic problem consists of a shock train within a constant-area channel with an inflow and backpressure applied to the outlet. By subjecting the shock train to different fixed backpressures, it is shown that the shock train length varies linearly with backpressure, while the shock spacing is conserved. Through backpressure step forcing, it is shown that the response of the shocks, after an initial lag, is largely determined by the magnitude of the step change. Step increases in backpressure cause upstream shock movements with higher shock pressures. The opposite response occurs with backpressure decreases. With sinusoidal backpressure forcing, the shock train oscillates at the applied forcing frequency. The forcing propagates disturbances upstream, and therefore there is a noticeable forcing-response lag. Higher frequency disturbances are filtered as they travel upstream through the shock train while lower frequencies induce larger shock oscillations and produce upstream offsets in the shock positions. Further examination of the flow reveals a region of low-frequency motion below the leading shock, which becomes strongly amplified by the lowest backpressure forcing frequency.

Impacts of panel vibration on shock train structures and performance of two-dimensional isolators

As one of the most critical components in scramjet engines, isolators are designed to prevent the hypersonic inlets from pressure disturbances raised from combustors. Shock trains dominate the internal flow field of isolators, the characteristics of which is crucial to the performance of isolator. However, recent studies have primarily focused on rigid isolators without structural deformation. The impact of structural vibration on the flow structures and performance of isolators receives little attention. The current study focuses on this problem by investigating the effect of vibration panel at the foot of shock train on the flow and performance of isolator which is quantified by 11 parameters regarding separation zone, shock structures, flow asymmetry, and isolator performance. Results examine the difference due to vibration amplitude, frequency and wavelength. Analyses indicate that the panel vibration will cause the upstream movement and length increase of shock train and separation zones, the increase of flow asymmetry and decrease of isolator performance. With increasing vibration amplitude, the shock train and separation zone travel upstream with the increase of shock train length, the flow asymmetry increases with large transient side loads, and the influence on the performance of isolator isminor. With increasing vibration frequency, the shock train and separation slightly travel downstream with the decrease in length, the change of flow asymmetry is minor with large transient side loads and the change of performance is minor. With increasing mode number (decreasing wavelength), the shock train and separation zones move downstream with the increase of shock train length, the flow asymmetry increases with significant transient side load at 2nd structural mode, and the change of performance is minor.

A Steam Ejector Refrigeration System Powered by Engine Combustion Waste Heat: Part 1. Characterization of the Internal Flow Structure

With the escalating production of automobiles, energy efficiency and environmental friendliness have always been a major concern in the automotive industry. In order to effectively lower the energy consumption of a vehicle, it is essential to develop air-conditioning systems that can make good use of combustion waste heat. Ejector refrigeration systems have become increasingly popular for this purpose due to their energy efficiency and ability to recycle waste heat. In this article, the elements affecting the performance of a typical ejector refrigeration system have been explored using both experimental and numerical approaches. For the first time, the internal flow structure was characterized by means of comprehensive numerical simulations. In essence, three major sections of the steam ejector were investigated. Two energy processes and the shock-mixing layer were defined and analyzed. The results indicated that the length of the choking zone directly affects the entertainment ratio under different primary fluid temperature. The optimum enterainment ratio was achieved with 138 °C primary fluid temperature. The shock-mixing layer was greatly affected by secondary fluid temperature. With increasing of back pressure, the normal shock gradually shifted from the diffuser towards the throat, while the shock train length remains lunchanged.

Performance of Circular to Non Circular Shape Scramjet Isolators

This paper details the influence of geometry variation such as shape transition of the isolator from its entry to exit on overall performance of scramjet isolators. The experiments are conducted to assess the performance of two isolator geometries of circle to pentagon and circle to hexagon. The pseudo shock length is obtained for the two cases by observing the sudden pressure rise in pressure plots taken along the wall. It is found that the shock train length is shorter for circular to pentagon duct compared with the other duct. Hence, this duct is preferable in order to prevent inlet non-start induced by the pressure rise in the combustion chamber.

Effect of Swirling Secondary Flow on the Under-expanded Non-circular Supersonic Jets

AbstractEffects of swirling secondary flow on the decay characteristics of non-circular under-expanded supersonic primary jets were experimentally studied. Primary jet is issuing from a convergent-divergent nozzle with different exit geometry such as circle, cruciform, triangle and rectangle. In this study, two different methods were used to impart swirl on the secondary flow as follows, (i) introduction of- angular vanes inside the secondary flow duct and (ii) using tangential inlets at the secondary flow duct. This swirling secondary flow influences the boundary condition of the primary jet and results in reduction of jet core length and shock train length, it also enhances the jet decay rate. Maximum shock train length reduction of 70 % is achieved when swirling secondary flow (tangential injection method) is introduced over the circular primary jet.

Shock Train Unsteadiness Characteristics, Oblique-to-Normal Transition, and Three-Dimensional Leading Shock Structure

The structure and unsteadiness characteristics of a shock train in a constant-area ducted flow are studied experimentally as a function of duct back pressure, which is mechanically controlled by a valve. Increasing the back pressure pushes the shock train upstream where the approach conditions (including Mach number and boundary-layer momentum thickness) are different, as these quantities vary along the duct length. Despite nominally constant boundary conditions during a run, the shock train fluctuates about its time-averaged position. Schlieren imaging reveals that the unsteadiness characteristics, including the fluctuation amplitude and frequency content, are independent of back pressure. However, changes in the Mach stem length and shock angles demonstrate that the leading shock structure transitions from oblique to normal as back pressure increases. Time-averaged pressure profiles show that the shock train length decreases and the pressure rise across the shock train increases during this process, but these quantities are nominally constant when normalized by the approach conditions. Finally, stereo particle image velocimetry is used to study the three-dimensional structure of a normal leading shock. The velocity fields show a large separation region on the side-wall of the duct and a degree of axisymmetry that indicates a nominally conical shock structure.

Experimental and Numerical Analysis on the Internal Flow of Supersonic Ejector Under Different Working Modes

ABSTRACTSupersonic ejectors involve very complex phenomena such as interaction between supersonic and subsonic flows, shock trains, instabilities, which strongly influences the performance of supersonic ejector. In this study, the static pressure distribution along the ejector wall and Mach number distribution along the axis are used to investigate the internal flow field of supersonic ejector. Results indicate that when the back pressure is much less than the critical back pressure, there are two series of shock trains, and the change of the back pressure will not affect the flow field before the effective area section, so the entrainment ratio would remain constant. The second shock train moves further upstream and is combined with the first shock train to form a single shock train as the back pressure rises. When the back pressure is greater than the critical back pressure, the position of the shock train, the static pressure at its upstream and the entrainment ratio, will be affected. The “effective area section” in the mixing tube is obtained. The effective area section position moves downstream with the increase of the primary flow pressure, while it moves upstream with the increase of the secondary flow pressure. The entrainment ratio shows inversely proportional relationship with the effective section position. Besides, the first shock train length increases with the increase of primary flow pressure or secondary flow pressure. The critical back pressure represents direct proportional relationship to the first shock train length.

Numerical Investigation of Shock-Train Response to Inflow Boundary-Layer Variations

A dataset of normal shock trains in a rectangular cross-section channel has been created from direct numerical simulations in an effort to quantify the impact of inflow confinement ratio on the shock-train structure. To this end, the inlet boundary-layer momentum thickness was varied while the bulk inflow and outflow conditions remained constant. The simulations show that the shock train is displaced upstream as the inflow confinement ratio increases. Also, an increase in boundary-layer momentum thickness results in a reduction of the normal-like portion of the lambda-shock structures in the channel core. This leads to more numerous but weaker bifurcating shocks as well as an increase of the shock-train length. When the inflow boundary-layer thickness is varied temporally, the complex shock-train response depends on the excitation frequency. A resonant frequency is observed when different components of the shock train exhibit the highest amplification in terms of pressure jumps. Further, the domain upstream of the leading shock acts as a low-pass filter, which has the end result of limiting the axial shock-train motion. Nevertheless, even a small oscillation in boundary-layer momentum thickness of 0.45 mm (0.6% of the channel height) is seen to increase the shock-train length by two orders of magnitude (4 cm).

Investigation of corner separation and suction control in constant area duct

To control shock-wave/boundary-layer interaction with suction, numerical simulations in a constant-area rectangular isolator with asymmetry nozzle are conducted to understand the complex flow phenomena. Results show that three-dimensional effect significantly weakens isolator resistance to backpressure, which indirectly emphasizes enormous role of viscous effect and geometry to the dynamic behavior of shock train or pseudo-shock wave. The large-scale separated flow only locates at the corner of the upper wall. The mechanism analysis indicates that the flow is accelerated more rapidly along the bottom wall, thereby providing more momentum at the nozzle exit. The displacement and momentum thicknesses and their increasing rate on the upper wall are all greater than that of the bottom wall, which means that upper corner flow is subjected easily to separation. The vortex structure of different cross sections in the pseudo-shock region is formed in different ways and the large-scale vortex still exists after the airflow mixing, which contributes to the deceleration and pressurization. Through the control effect analysis of suction slot, the suction slot in spanwise direction on the side wall can shorten effectively the length of shock train, which indicates that removing the low-momentum fluid near the corners can control significantly the shock-wave/boundary-layer interactions.

Shock Train/Boundary-Layer Interaction in Rectangular Isolators

Shock trains in rectangular isolators with inflow Mach number of 2.5 and ARs of 3.0 and 6.0 are investigated. Knowledge gained is used to propose a rectangular isolator modification to existing empirical shock train length relations based on circular ducts. A novel, multiplane shadowgraph concept allows simultaneous flow visualization from multiple perspectives. Boundary-layer separation in the low-momentum corner flow regions is observed to occur upstream of the center-flow field by approximately one duct height. The leading edge shock train structure is composed of a hybrid oblique–normal shock front, with oblique shocks spawning from the corner flow separation transforming into a normal shock front near the centerline. Dynamic wall pressure measurements made along the duct major axis allow for direct comparison between the outboard and centerline sensor’s ability to detect incipient unstart, with the outboard sensor detecting the separation-induced pressure rise well ahead of centerline measurements. A modified shock train length relation for rectangular isolators accounts for the effects of potential boundary-layer momentum thickness asymmetry, upstream corner flow separation length scale, and AR magnitude. Improvements in correlation strength are discussed for both ARs, with coefficient of determination and root mean square error improvements on the order of 60% observed for the highest AR tested.

Comparison of a Direct-Connect and Freejet Dual-Mode Scramjet

Testing of identical dual-mode scramjet flowpath geometries in the freejet and direct-connect configurations was conducted in two separate facilities. These tests enabled a comparison study between the two configurations, which was directed toward the determination of the effects of inlet distortion and backpressure on the performance and operability of a dual-mode scramjet. Bulk flow conditions were matched between the two facilities at the isolator entrance plane to simulate Mach 4.8 flight, and a series of metrics were established to quantify similarities and differences. The effects of flowpath backpressure in the direct-connect case were seen to be isolated to regions close to the exhaust. An approximate 10% decrease in combustor pressure rise and consequently integrated pressure force were observed in the freejet configuration; shock train length, however, remained the same. Mode transition was delayed from an equivalence ratio of in the direct-connect case to in the freejet configuration. Further, ignition difficulties were experienced in the freejet tests which were not encountered in those of direct-connect tests, limiting the scope of comparable test points. This work represents the first attempt at a quantitative comparison in the literature of an identical direct-connect and freejet dual-mode scramjet.

Experimental Study of Combustor–Isolator Interactions in a Dual-Combustion Ramjet

Combustor–isolator interactions are investigated in a direct-connect dual-combustion ramjet experiment. The test apparatus contained an annular isolator, a constant cross-sectional area cylindrical supersonic combustor, and a subsonic-burning gas generator. The gas generator, in which a liquid hydrocarbon fuel was injected, produced a fuel-rich hot-gas mixture that is mixed with the annular supersonic airflow in the supersonic combustor. A precombustion compression field was generated, both in the isolator and the combustor, by the heat release. The annular isolator entrance Mach number was varied as 1.78, 1.98, and 2.23. The wall static pressure distributions measured in the isolator and the combustor are presented and analyzed to determine the precombustion shock-train length. The new correlating equations and the corresponding theoretical model that can predict the precombustion shock-train length and the wall pressure distribution are derived therefrom.

Numerical analysis of pseudo-shock flow diffusion phenomenon in variable cross-section ducts

The pseudo-shock wave (PSW) region of flow diffusion, which appears when a supersonic flow in a duct decelerates to a subsonic flow, is a complicated process involving a multi-faceted interaction between the duct's peripheral boundary layer and the central shock-wave field. It is made up of an upstream shock train region, comprised of lambda and oblique shock waves, and a downstream mixing region, both of which significantly influence the effective performance of the desired flow diffusion process. In this article, the PSW in two variable-cross-sectional ducts, edge- and corner-varied in transforming from a rectangular to a circular duct moving downstream, was researched through computational fluid dynamics (CFD) numerical simulation. The PSW flowfields in circular and rectangular ducts were also computed to be counterparts. The identical inflow parameters ( Ma = 2.0, T0 = 298 K and P0 = 100 kPa) in combination with different pressure ratios ( Pb/ Pi = 2.8—3.8) were considered. The characteristics of the PSW phenomenon, such as the leading edge of the PSW, length of the shock train, mass-weighted average parameters at the exit, and so on, were analysed. For these cases, with the same pressure ratio Pb/ Pi = 3.8, the shock train length in the corner-varied duct is the shortest, and its capability of supporting backpressure is the weakest of the four ducts. For these cases, with the same leading edge of the PSW at X = 0.167 L, a shorter and wider corner flow separation region appears in the variable-cross-sectional edge-varied duct in comparison with the rectangular duct. With regard to the length of shock train, level of internal drag, and capability to support backpressure, the edge-varied duct is the superior design choice. An advantage of the edge-varied duct is the improved internal drag: an additive thrust is produced to partly offset the drag due to the duct's axial divergence-angle setting. Based on an analysis of the PSW flow pattern in these ducts, it has been established that the wall-pressure distributions of the shock train can be well predicted by the modified Waltrup formula after the introduction of an equivalent diameter and an exponent α for Reθ.