Critical Jet Velocity Research Articles

Mechanism and risk assessment of baffle-jetting-induced vibrations in fuel rods

As nuclear reactor cores approach the end of their lifespans, it is a crucial aspect to ensure that the structural integrity of the reactor internals is not affected. The baffle-former bolts are subjected to significant mechanical stress and neutron irradiation from the reactor core during the plant operation. These circumstances can cause the bolts to degrade and lose some of their load-bearing capacity over the reactor lifespan. The degraded baffle-former bolts can lead to separation/openings between the baffle plates which cause water jet cross-flow injecting through these openings. This phenomenon is called baffle jetting. This jet flow can induce the vibration of fuel rods located near the baffle gaps, resulting in grid-to-rod fretting and, in extreme cases, fuel rod failures.This paper investigates the risk of baffle jetting-induced fluidelastic instability in fuel assemblies. A 6 × 5 axisymmetric flexible rod bundle having a prototypical nuclear fuel assembly is designed and fabricated to simulate a part of a fuel assembly under combined axial flow and baffle jetting. A special displacement mechanism is used to vary the position of jet flow relative to the array (i.e. jet eccentricity). Two cases, the jet-gap aligned case, and the jet-rod aligned case, are investigated to determine the variation of critical jet velocity with the jet eccentricity. In addition, three axial flow velocity cases, VAxial = 1.0 m/s, 1.5 m/s, and 2.0 m/s are tested in order to determine the stability effect of axial flow velocity on jet-induced instability. The experiments show that the mock-up array response resembles fluidelastic instability behavior. The stability threshold of the mock-up array depends strongly on the relative position of the jet flow with respect to the array centerline. In the jet-rod aligned case, the axial flow shows a strong effect on the combined flow-induced instability.

Fluidelastic instability of a rod bundle subjected to jet cross-flow

Jet flow emanating from loss of coolant accident (LOCA) holes in the baffle plates of some pressurized water reactors (PWRs) was found responsible for flow-induced vibration in nuclear fuel bundles. This could cause fretting wear of the fuel rods at the spacer grid location. Experimental tests on prototypical rod bundles subjected to jet cross-flow showed a strong vibration response akin to fluidelastic instability of tube arrays in uniform cross-flow. Vibrations similar to flow-periodicity resonance response have also been reported.Fluidelastic instability (FEI) is the most critical flow-induced vibration mechanism. For fuel rods subject to jet-cross flow, it is important to identify the specific mechanism underlying the rod vibrations and in particular, confirm possible fluidelastic instability. In previous work, a few stability boundary correlations based on the Connors equation were developed for the baffle jetting (i.e. 2D jet) problem in PWRs. With the exception of these correlations, no theoretical model accounting for fluid–structure interaction under jet cross-flow has been reported.To investigate possible fluidelastic instability, the quasi-steady model, initially developed for tube arrays in uniform cross-flow, is extended to the problem of a rod bundle subjected to a jet cross-flow. In the extended quasi-steady model, a new formulation of the fluidelastic forces as functions of the projected rod area derivative is proposed to account for the variation of the projected area through the LOCA hole (i.e. circular jet).A significant limitation of the ‘classical’ quasi-steady model is the poor representation of the time delay effect, known to be critical for correct fluidelastic instability prediction. This shortcoming is addressed in the present work. Furthermore, unsteady flow effects are partly taken into account experimentally measuring the time delay parameter for this jet-cross-flow configuration.For the present highly compact rod array, a new test apparatus is designed and built to directly measure the time delay parameter. The new results show that the time delay to be non-constant, strongly dependent on Reynolds number while relative independent of reduced velocity for Low Re. For High Re, the time delay depends on both Re and reduced flow velocity. Based on the new measurements, a formulation for the time delay parameter, dependent on Reynolds number and reduced velocity is proposed for the highly packed fuel rod bundle.The new extended quasi-steady model conclusively confirms that the vibration phenomenon observed in experimental tests is clear fluidelastic instability induced by the jet cross-flow. Moreover, the model predicts the critical jet velocity for fluidelastic instability with an error of 9% when the new Reynolds number dependent time delay formulation is employed.

Study on Dispersion and Mixing Mechanism of Coal Slime Particles in Jet Mixing Flow Field

The jet flow field is characterized by the dispersion and mixing of multiphase flow, which is widely used in the field of coal slime flotation. In this paper, the behavior of coal slime surface modification, the behavior of material suspension and the effect of different jet fluids on coal slurry blending were studied. Based on the action mechanism of the jet flow field, a jet device suitable for coal slime graded mixing was proposed, and the mixing effect of the jet device was tested. The results show that the jet flow field has strong effects on material dissociation and dispersion, and the mixing effect of a single jet is equivalent to that of the laser particle size analyzer’s own agitation device after stirring for 2.5 min at 500 r/min speed. The SEM test of material surface morphology and the changes of Al and Si elements measured by EDS show that the jet flow field can effectively remove the fine mud wrapped on the surface of coal particles. The precondition of material suspension is to have the just-suspended capacity. The critical jet velocity of coal slime suspension is in the range of 6 m/s~9 m/s. The mixing ability of the jet stream has certain limitations. The increase or decrease of the jet height will cause the decrease of the suspension percentage of sampling points in the tank. The gas jet mode can promote the reagent acting on the surface of the bubble liquid film to form oil bubbles, which is more suitable for hydrophobic mineral flotation. The optimal speed of the gas jet is 0.86 m/s, and the shortest cycle period is 1.0 T (T is one material cycle period). The flotation perfection index of 0.5–0.25 mm and less than 0.075 mm coal slime increased by 2.67% and 26.78%, respectively, indicating that the overall idea of the jet mixing device proposed based on the experimental conclusion is feasible.

Blowout limits of inclined nonpremixed turbulent jet flames

The blowout behavior of inclined nonpremixed turbulent jet flames is investigated by varying the jet inclined angle in the range of -90° to 90° The critical jet velocity at blow-out limit is quantified experimentally for various nozzle diameters, different fuels and inclined angles. Numerical simulations are performed to emphasize the flow field difference for the positive and negative inclined angles. Physical modeling is conducted to incorporate the effect of the inclined angle on blow-out behavior. Major findings include: (1) The negatively inclined jet flames show more intense yellow luminosity with larger sooting zones than the positively inclined jet flames; (2) The blowout limit decreases appreciably with the jet inclined angle for the negatively inclined flames, while for the positively inclined jet flames, this decrease is relatively small; (3) Physical analysis of the flow development of inclined jets is conducted, indicating the centerline velocity along the jet trajectory decreases faster for the flame with smaller inclined angle. And the decrease rate is relatively larger for the negatively inclined jet flames; (4) Based on the analysis of the flow development as well as the characteristic velocity with the inclined angle variation, a model based on the Damköhler number (Da) accounting for the effect of jet inclined angle is developed to characterize the blowout limits of inclined jet flames. The proposed model successfully correlates the experimental data. The present findings provide new data and a basic scaling law for the blowout limit of nonpremixed inclined turbulent jet flames, revealing the effect of the relative angle between the jet momentum and buoyancy.

Initiation of a Confined Finite‐Thickness Explosive under the Impact of Shaped Charge Jet

AbstractThis work aims to study the critical initiation conditions of a confined finite‐thickness explosive under jet impact and the influence of shell constraints on the explosive initiation threshold. A theoretical calculation model for jet impact initiation of a confined explosive is established based on the Pop plot of the explosive. A jet impact test is conducted on a confined TNT explosive covered by a 50SiMnVB cover plate by using Φ40 mm shaped charge. A high‐speed camera is utilized to record the reaction process of shelled explosives. Numerical simulation software is adopted to calculate the reaction process of the confined explosive under the jet impact, and the critical initiation threshold of the confined explosive is obtained. Experimental results are verified, and the propagation and development of bow shock in the confined explosives under different cover thicknesses are analyzed. Then, the calculation results of the theoretical model are compared with the experimental simulation results. The findings show that the critical cover thickness of the confined TNT with a thickness of 43 mm is 20 mm, and the critical initiation threshold is 31 mm3 μs−2. The theoretical model can accurately calculate the critical initiation threshold and change the law of explosives under unconfined and confined conditions. When the thickness of the explosive and the acoustic impedance of the shell material increase, the critical initiation threshold of the explosive decreases. The shell reduces the critical jet velocity for explosive initiation, and the influence of the shell on the critical jet velocity gradually decreases as the thickness of the explosive increases.

Droplet breakup in a stagnation-point flow

Abstract

Critical velocity of active air jet required to enhance free opening rectangular exhaust hood

Using a radial active air jet ejected from the slot surrounding a circular exhaust hood opening can increase the air speed generated by the hood as well as achieve a larger capacity to exhaust pollutants. Critical parameters of the Aaberg hood have been identified to ensure the positive effect of the active jet. This paper proposed a novel type of enhanced exhaust hood using the rectangular opening in place of the circular one in Aaberg hood to meet the needs of different shape and size of exhaust hood. To understand the performance of this kind of exhaust hood and to determine the critical velocity of the active air jet, the flow characteristics and the centerline velocity are investigated by changing the exhaust rate, hood size, jet angle, jet velocity and jet slot width via experimental and numerical methods. Results show that the critical jet velocity exists between two working modes of the novel exhaust hood, one is the ineffective mode with the short-circuit jet, and the other is jet-enhanced mode with the developed radial jet. The near-wall circulation along the hood opening perimeter is not attributed to the existence of the jet. The suggested equation which error is proven to be within 12.5% for the critical velocity presented in this paper is simple in structure, and will help to facilitate the choice of the energy-efficient exhaust hood design in residential and industrial buildings to save energy that would be wasted on increasing the exhaust rate.

Experimental and numerical study of the effect of perimeter jet enhancement on the capture velocity of a rectangular exhaust hood

Using a radial jet at the outer edge of a circular exhaust hood can effectively reduce the velocity attenuation in front of the exhaust hood. The velocity distribution, especially the centerline velocity, is a key parameter in the effective design and use of an exhaust hood. In this paper, based on the derivation of the prediction formula of the suction velocity, the study of the suction velocity of jet-enhanced rectangular exhaust hood that considers the jet enhanced, viscous and geometrical effects is presented via the experimental and numerical methods. To understand the suction velocity distribution, the centerline velocity is investigated by varying the jet velocity, jet angle, jet slot width, exhaust rate and aspect ratio of the hood. It is found that the critical jet velocity condition is a most effective and energy-saving working condition for this kind of hood. Based on the critical jet velocity working condition, reducing the jet angle instead of increasing the exhaust rate is an airflow-rate-saving way to further increase the centerline velocity at a relatively large distance away from the hood opening. Calculation equations for the prediction of the centerline velocity of rectangular jet-enhanced exhaust hood are proposed. The error of the formula is proven to be within 10%, and its application range is proposed. The outcome of this work can help to efficiently design and use this type of energy-saving exhaust hood in residential and industrial local ventilation applications.

CFD Modeling of Active Volume Creation in a Non‐Newtonian Fluid Agitated by Submerged Recirculating Jets

AbstractComputational fluid dynamics (CFD) models were employed to investigate flow conditions inside a model reactor in which yield stress non‐Newtonian liquid is mobilized using submerged recirculating jets. The simulation results agree well with the experimental results of active volume in the reactor obtained using flow visualization by the authors in a previous study. The models developed are capable of predicting a critical jet velocity (vc) that determines the extent of active volume obtained due to jet mixing. The vc values are influenced both by the rheological properties of the liquid and the nozzle orientation. The liquid with higher effective viscosity leads to higher vc for a downward facing injection nozzle. However, an upward facing injection nozzle along with a downward facing suction nozzle generates enhanced complementary flow fields which overcome the rheological constraints of the liquid and lead to lower vc.

Kelvin–Helmholtz instability in an active region jet observed with Hinode

Over past ten years a variety of jet-like phenomena were detected in the solar atmosphere, including plasma ejections over a range of coronal temperatures being observed as extreme ultraviolet (EUV) and X-ray jets. We study the possibility for the development of Kelvin--Helmholtz (KH) instability of transverse magnetohydrodynamic (MHD) waves traveling along an EUV jet situated on the west side of NOAA AR 10938 and observed by three instruments on board Hinode on 2007 January 15/16 (Chifor et al., Astron. Astrophys.481, L57 (2008)). The jet was observed around LogT_e = 6.2 with up-flow velocities exceeded 150 km/s. Using Fe XII lambda186 and lambda195 line ratios, the measured densities were found to be above LogN_e = 11. We have modeled that EUV jet as a vertically moving magnetic flux tube (untwisted and weakly twisted) and have studied the propagation characteristics of the kink (m = 1) mode and the higher m modes with azimuthal mode numbers m = 2, 3, 4. It turns out that all these MHD waves can become unstable at flow velocities in the range of 112--114.8 km/s. The lowest critical jet velocity of 112 km/s is obtained when modeling the jet as compressible plasma contained in an untwisted magnetic flux tube. We have compared two analytically found criteria for predicting the threshold Alfven Mach number for the onset of KH instability and have concluded that one of them yields reliable values for the critical Alfven Mach number. Our study of the nature of stable and unstable MHD modes propagating on the jet shows that in a stable regime all the modes are pure surface waves, while the unstable kink (m = 1) mode in untwisted compressible plasma flux tube becomes a leaky wave. In the limit of incompressible media (for the jet and its environment) all unstable modes are non-leaky surface waves.

Spatial modes of capillary jets, with application to surface stimulation

AbstractSurface stimulation of any physical origin (electrohydrodynamic, thermocapillary, etc.) has the goal of generating localized perturbations on the free surface or the velocity field of a capillary jet. Among these perturbations, only the axisymmetric ones are determinant for the jet breakup. Often, the stimulation is weak enough for a linear model to be applicable. Then, the stimulation can be described by means of the Green functions for stresses, both normal and tangential to the interface, the calculations of which are, in addition, uncoupled from the hydrodynamic variables. If a harmonic forcing is applied, these Green functions are combinations of the spatial modes whose associated poles lie inside the appropriate integration contour of the complex wavenumber plane. This is the motivation for a comprehensive enumeration and description of the spatial modes, which has not been done up to now. Modes familiar from a temporal analysis, the dominant and subdominant capillary modes and the hydrodynamic modes, are present, along with modes specific to a spatial analysis. Most of the latter have already been mentioned in the literature for inviscid jets, but not analysed. A mode not previously found is reported. In addition, a description of the velocity field associated with each mode is provided, as a tool to understand their physical origin and behaviour. The relative importance of each mode in both normal- and tangential-stress stimulations is discussed. Finally, the well-known merging of poles below a critical jet velocity, leading to absolute instability, is analysed in the light of the modal description.

Non-coalescence of jets

The phenomenon of non-coalescence between fluid jets was first reported by Lord Rayleigh, more than a century ago. Rayleigh described the observation in words without any experimental data or pictures. To the best of our knowledge, this curious phenomenon received no attention from the scientific community since then. We present the first experimental demonstration of the non-coalescence of two and three jets of the same fluid, and of non-coalescence between a jet and droplets of the same fluid. Figure 1 shows two jets of silicone oil (viscosity 10 cSt at 25 !C) with diameter 500 lm, impinging obliquely onto a vertical jet of the same fluid and diameter. Instead of coalescing, the jets from the sides rebound off the middle jet due to lubrication by a thin film of air separating the jets. The layer of air is continuously replenished by the motion of the jets, resulting in indefinitely sustained noncoalescence between the jets. As the jet velocity (v) increases, the air film is drained and the two jets coalesce. Figure 2 shows the transition from coalescence to non-coalescence between two jets when the experiment is carried out below a critical jet velocity. We also observed non-coalescence between a jet and drops of the same fluid, an example of which is shown in Figure 3. The drops plunged into the jet from two sides without coalescing into it, bending it at two locations.

Bubble Formation from an Air Jet Injected into a Turbulent Boundary Layer

A theoretical and experimental study is conducted to investigate the bubble separation from an air jet injected into a liquid turbulent boundary layer. The following three patterns of bubble separation are dependent on the jet velocity: (a) a single bubble, (b) a coalescent bubble, and (c) a continuous jet. The critical jet velocity from (a) to (b) is theoretically estimated from the condition that the jet from the nozzle overtakes the rear end of the separated bubble, which shrinks due to surface tension. The calculated results generally reproduce the experimental results observed by a high-speed video camera. In addition, the process of bubble separation from the continuous jet (c) is classified into two patterns: (i) the bubble separates from the swell at the front end of the jet, or (ii) the jet breaks due to the instability at the liquid-gas interface. The separated bubble diameter of pattern (i) is theoretically determined by considering the force balance at the front end of the jet between the surface tension, the drag from the free stream, and the virtual mass force. The bubble diameter of pattern (ii) is calculated from the most unstable wavelength of Rayleigh instability. Both of these theoretical results also agree well with those obtained experimentally.

乱流境界層中に噴出した気流からの気泡分離機構に関する研究 : 連続気柱からの気泡分離(流体工学,流体機械)

A theoretical and experimental consideration is conducted to investigate the bubble separation from an air injected into a liquid turbulent boundary layer. There exist three patterns of bubble separation dependent on the jet velocity, i.e., (a) single bubble, (b) ccalescent bubble, and (c) continuous jet. First, the critical jet velocity from (a) to (b) is theoretically estimated from the condition that the jet from the nozzle overtakes the rear end of the separated bubble which shrinks due to the action of surface tension. The calculated results roughly reproduce the experimental results observed by the high-speed video camera. The process of bubble separation from the continuous jet (c) can be classified into two patterns, i.e., (i) the bubble separates from the swell at the front end of jet, (ii) the jet breaks due to the instability on the liquid-gas interface. The separated bubble diameter of pattern (i) is theoretically determined by considering the force balance at the front end of jet between surface tension, drag from the frees stream and virtual mass force. Also the bubble diameter of pattern (ii) is calculated from the most unstable wavelength of Rayleigh instability. Both theoretical results agree well with those obtained experimentally.

Blade Tip Clearance Flow and Compressor Nonsynchronous Vibrations: The Jet Core Feedback Theory as the Coupling Mechanism

This paper investigates the role of tip clearance flow in the occurrence of nonsynchronous vibrations (NSVs) observed in the first axial rotor of a high-speed high-pressure compressor in an aeroengine. NSV is an aeroelastic phenomenon where the rotor blades vibrate at nonintegral multiples of the shaft rotational frequencies in operating regimes where classical flutter is not known to occur. A physical mechanism to explain the NSV phenomenon is proposed based on the blade tip trailing edge impinging jetlike flow, and a novel theory based on the acoustic feedback in the jet potential core. The theory suggests that the critical jet velocity, which brings a jet impinging on a rigid structure to resonance, is reduced to the velocities observed in the blade tip secondary flow when the jet impinges on a flexible structure. The feedback mechanism is then an acoustic wave traveling backward in the jet potential core, and this is experimentally demonstrated. A model is proposed to predict the critical tip speed at which NSV can occur. The model also addresses several unexplained phenomena, or missing links, which are essential to connect tip clearance flow unsteadiness to NSV. These are the pressure level, the pitch-based reduced frequency, and the observed step changes in blade vibration and mode shape. The model is verified using two different rotors that exhibited NSV.

Numerical analysis of critical jet penetration

In this paper, a computational program for two dimensional unsteady compressible, elastic-plastic flow is used to study low speed jet penetration into a steel target. Based on the computational results, the dynamic process of penetration can be distinguished into two stages, cratering and steady penetrating. This is in agreement with the jet penetration mechanism developed by Cheng Che Min in 1972[1,2]. The consumption of the jet energy is discussed and the value of the critical jet velocity, which represents the strength resistance of the target, is estimated.