External Pressure Pulse Research Articles

Spray characteristics and combustion mechanism influence on combustion stability of UDMH/NTO pre-combustor under external excitation

To study the combustion stability of liquid rocket engines with hypergolic propellant more comprehensively, external pressure pulse excitation was applied to the UDMH (unsymmetrical dimethylhydrazine)/NTO (methylhydrazine) pre-combustor. The effects of pressure pulse intensity, spray characteristics (droplet diameter and spray cone angle), and combustion mechanism on combustion stability characteristic frequencies and attenuation characteristics after excitation were investigated. The simulation results show that all operation conditions capture the first three longitudinal frequencies of the pre-combustor, and all factors have a greater impact on frequencies with higher orders. Increasing the pulse intensity and reducing the droplet diameter are not conducive to combustion stability. The influence of the spray cone angle on the combustion stability is not monotonous. The pre-combustor has good stability when the spray cone angle is 70°. Different combustion mechanisms have a significant effect on the results before and after pressure pulses, and considering a more complete combustion process may increase the attenuation time in the pre-combustor. The obtained rules can provide a reference for the design and simulation of hypergolic liquid rocket engines, which is beneficial for predicting and evaluating the combustion stability of liquid rocket engines.

Case study: Analysis and research on pipeline vibration of liquid oxygen kerosene of rocket engine and vibration reduction measures Measures

Abnormal vibration often occurs in the liquid oxygen kerosene transmission pipe- line of rocket engines, seriously threatening the safety of rocket engines. Improper handling will result in rocket launch failure and enormous economic losses. Therefore, the vibration of the transmission pipeline must be studied. In this paper, a three-dimensional high-pressure transmission pipeline model comprising a corrugated pipe, a multi-section bending pipe, and other auxiliary structures is established. Using the two-way fluid–solid coupling method, vibration analysis is performed on the pipeline under external pressure pulse excitation. The accuracy of the computation results is verified by a thermal test. Two vibration reduction strategies are presented and validated by simulation in accordance with the findings of the vibration study. The main conclusions are as follows: (1) At the same frequency, the amplitude distribution of vibration acceleration significantly correlates with the flow field pressure, indicating that fluid pressure fluctuation is the root cause of the abnormal vibration of the pipeline, and the vibration of the pipeline increases with the average pressure. (2) The time-average value and fluctuation amplitude of the larger stress and strain are mainly concentrated in the two supports, namely, the inside of the elbow and the bellows, which is different from the distribution of vibration acceleration. Such places are prone to structural failure and should be given attention. (3) The guide plate and the support enhancement method can reduce the vibration, but the support enhancement method has a better effect, reducing the vibration velocity and acceleration by 86.4% and 93%, respectively.

Нестационарные процессы в подкрепленном тонкостенной оболочкой вязкоупругом цилиндре при импульсном нагружении

Solid propellant rocket motor is considered as hollow viscoelastic cylinder inserted in multilayered elastic shell-like case. The material of propellant is considered to be compressible. An estimation of maximum unsteady stresses on cylinder-shell boundary and shell under growing pressure on interior or external cylindrical surface were calculated by FEM. Four corner isoparametric finite element is utilized. Numark method to integrate by time the dynamic equations is used. The problem of linear viscoelasticity have been employing of the Schapery method. `In the case of internal pressure, the possibility of tensile radial stresses on the contact surface of the propellant-shell during the transition process has been established. The dependence of the maximum contact stresses as well as circumferential stresses in the shell on the shell thickness is established. In the case of external pressure pulse, the presence of significant tensile radial stresses on the propellant-shell interface is shown. Insignificant tensile circumferential stresses in the transient wave process are possible in the shell.

Transient radiation by a submerged fluid-filled cylindrical shell

The radiation by a submerged fluid-filled cylindrical shell in response to a transient external pressure pulse is considered, and a semi-analytical model based on the Reissner–Mindlin shell theory is employed to simulate the interaction numerically. Two types of radiated waves that have been previously seen in experimental images for a submerged evacuated cylindrical shell are observed in both the external and internal fluids, the symmetric Lamb waves S0 and the antisymmetric Lamb (or pseudo-Rayleigh) waves A0. The third type of radiated waves is also observed that has not been explicitly imaged either experimentally or numerically for a submerged evacuated cylindrical shell, and it is demonstrated that these waves are the Scholte–Stoneley waves A. The effect that the complex structure of the radiated field has on the wave phenomena in the internal fluid is analyzed for shells of several different thicknesses, and the results of this analysis are summarized in the form of diagrams suitable for the use at the pre-design stage.

Molten metal and water direct contact interaction research – II. Numerical analysis

In the fuel–coolant interaction (FCI), thermal fragmentation of molten metal droplets could result in the propagation of pressure waves and subsequent fragmentation of other droplets and lead to steam explosion at last. It is recognized that the direct contact heat transfer and interface behavior are very important in the thermal fragmentation process. Two typical experimental phenomena are observed and introduced in the part I of the paper, named no interaction mode and explosive interaction mode. In this paper, a two-dimensional multi-phase thermal hydraulic code with the Volume of Fluid Method (VOF) is developed to investigate the direct liquid–liquid contact interaction behavior with large temperature difference. Two typical experiment phenomena are simulated by using the developed code, which are observed in the experiment and introduced in part I of this paper. Comparing with the experimental visual data, the simulation results are in a good agreement. At last, the thermal fragmentation process of melt drop triggered by external pressure pulse during FCI is simulated by using the developed code. The calculation results suggest that the growth and breaking up of filaments are the essential mechanism of molten metal droplet fragmentation.

Local facesheet pulse buckling in a curved, composite sandwich panel

Local facesheet buckling of a curved sandwich panel subjected to external pressure pulse loading was investigated in this paper. Equations of motion for the facesheet transient deformations in the sandwich shell were derived from Lagrange’s equations of motion, and solutions using this approach compared well with FEA results from ABAQUS Standard. Both facesheet fracture during stable deformation response and local dynamic pulse buckling of facesheets were considered as possible modes of failure in the curved sandwich panel. Parametric studies indicated that local facesheets buckling is more likely to occur than facesheet fracture in thin and deep curved sandwich panels. The facesheet laminate layup can also be adjusted to improve the local buckling resistance of the curve panel.

Numerical simulation of fragmentation of melt drop triggered by external pressure pulse in vapor explosions

The fragmentation of molten drops is the key process in the fuel–coolant interaction (FCIs) which may occur during the course of a severe accident in a light water reactor (LWR). However, the mechanisms of this complicated process cannot be clarified sufficiently by experimental studies due to the rapid reaction. In this paper, a multi-phase thermal hydraulic code with the volume of fluid method (VOF) is developed and the fragmentation process of melt drops triggered by external pressure pulse is numerically analyzed to investigate the mechanism of fragmentation in vapor explosions. The simulation results show that the fragmentation process can be divided into several stages, including vapor film collapse, melt drop-coolant direct contact, formation of high pressure spots, rapid growth of a filament around the molten metal drop, rapid fuel coolant interaction area expansion, breaking up of the filament, and mixing of fragments with water. The calculation results are similar to Ciccarelli and Frost’s (Nucl. Eng. Des., 146, 109–132) experiment data. The simulation results suggest that growth and breaking up of a filament are the essential mechanism of melt tin drop fragmentation. Penetration and evaporation of the water jets, which are assumed as fragmentation mechanism in Kim’s model (Nucl. Sci. Eng., 98, 16–28, 1988), are not observed. In the calculation case, when molten metal density is hypothetically smaller, the water penetration is observed. Besides, the effects of external pressure pulse and molten metal temperature on the growth of filament and the explosion bubble are discussed.

PUTTING CORONAL SEISMOLOGY ESTIMATES OF THE MAGNETIC FIELD STRENGTH TO THE TEST

The magnetic field strength inside a model coronal loop is estimated using coronal seismology, to examine the reliability of magnetic field strengths derived from observed, transverse coronal loop oscillations. Three-dimensional numerical simulations of the interaction of an external pressure pulse with a coronal loop (modeled as a three-dimensional density enhancement inside a two-dimensional magnetic arcade) are analyzed and the observed properties of the excited transverse loop oscillations are used to derive the value of the local magnetic field strength, following the method of Nakariakov & Ofman. Due to the (unexpected) change in periodicity, the magnetic field derived from our observed oscillation is substantially different from the actual (input) magnetic field value (approximately 50%). Coronal seismology can derive useful information about the local magnetic field, but the combined effect of the loop curvature, the density ratio, and aspect ratio of the loop appears to be more important than previously expected.

Vapor film collapse triggered by external pressure pulse and the fragmentation of melt droplet in FCIs

The fragmentation process of high-temperature molten drop is a key factor to determine the ratio heat transferred to power in FCIs, which estimates the possible damage degree during the hypothetical severe accident in the nuclear reactors. In this paper, the fragmentation process of melt droplet in FCIs is investigated by theoretic analysis. The fragmentation mechanism is studied when an external pressure pulse applied to a melt droplet, which is surrounded by vapor film. The vapor film collapse which induces fragmentation of melt droplet is analyzed and modeled. And then the generated pressure is calculated. The vapor film collapse model is introduced to fragmentation correlation, and the predicted fragment size is calculated and compared with experimental data. The result shows that the developed model can predict the diameter of fragments and can be used to calculate the fragmentation process appreciatively.

Extreme solar‐terrestrial events of October 2003: High‐latitude and Cluster observations of the large geomagnetic disturbances on 30 October

The extremely large solar eruption on 28 October 2003 caused an intense geomagnetic storm at Earth. A second solar eruption on 29 October resulted in a reintensification of the storm about a day later. Similarities and differences between these two events in terms of solar eruption, solar wind driver, and their resulting effect on the near‐Earth environment are investigated and put into context of previous works on storm geoeffectivness. Within the second storm some of the strongest substorms in the history of magnetic recordings occurred in northern Scandinavia. The aim of this study is to investigate the cause and resulting effects of these extreme geomagnetic disturbances on the ionosphere and upper atmosphere, focusing on the northern Scandinavian sector where these disturbances reached extremely high values. During this time period, well after the initial arrival of the Interplanetary Coronal Mass Ejection (ICME), the Cluster spacecraft were located at the flank of the magnetospheric tail. The satellites were passed several times by an inward and consecutively outward moving magnetopause in close relation to the substorm intensifications in northern Scandinavia. We propose that the evolution of these magnetospheric substorm intensifications are influenced by the changing dynamics of the solar wind in the form of increased pressure occurring after a prolonged period of southward Interplanetary Magnetic Field (IMF) and thus excessive energy loading into the magnetosphere prior to the onset of the intensifications. We present evidence of external pressure pulse triggering and possibly also quenching of these substorm onsets and recoveries. In addition, EISCAT data have been used to investigate the detailed local behavior of the ionospheric plasma, giving rise to such extreme disturbances. We found that in this case, extreme combinations of enhanced conductivity and intense electric field resulted in very high current intensities (westward electrojet ∼7.4 MA) and very fast onset of such currents. The associated geomagnetically induced currents caused power failures in southern Sweden.

On the propagation of magnetohydrodynamic perturbations in a Harris‐type current sheet: 2. Propagation on continuous modes and resonant absorption

We study the propagation of magnetohydrodynamic perturbations in a one‐dimensional (1‐D) model of the magnetotail current sheet. Starting from a general analysis of the MHD linear response of a Harris current sheet to external pressure pulse, we reconstruct the magnetic, velocity, and pressure perturbations for successive times and at different positions. The spatial‐temporal evolution of the signal propagating on the discrete modes has been studied by Fruit et al. [2002]. We here describe the propagation in the continuous spectrum domain that results from the existence of a spatial singularity in the differential equation defining the modes of propagation. At t = 0, the current sheet is supposed to be excited by an external pressure pulse. We estimate the energy that propagates in the continuum domain and will thus be absorbed by the resonant absorption process. Initial pulses of different shapes and characteristic spatial and temporal scales are used. Since we perform the complete inverse Laplace and Fourier transform of the continuous modes, we get the full description of the perturbations in real space, hence, the typical temporal and spatial scales of the absorption process. For example, starting with a pulse of total amplitude of 1 Pme (magnetic pressure in the lobe) with a spatial scale of 4 Earth radii and a temporal scale of 20 s, we show that 2% of the initial plasma thermal energy are transferred to the sheet over a distance of 20 Earth radii and in less than 10 min. Pulses of larger scales have no energetic consequences.

The Trigger Mechanism of Vapor Explosion

In the present study, trigger mechanisms of the vapor explosion are experimentally investigated. The interfacial behavior between high temperature molten liquid and low temperature water are experimentally investigated by using a molten material droplet and external pressure pulse. As the results, it is indicated that spontaneous vapor explosion hardly occur in high temperature water near saturation temperature since vapor film is stable. The vapor explosion can occur even in high temperature water near saturation temperature in case that the external pressure pulse is applied to high temperature molten material. Vapor explosion can not occur when the interfacial temperature between the molten material and water is lower than the material melting temperature, even if the vapor film around the molten material is collapsed by the external pressure pulse. It is clarified that the impossibility of the trigger process for the vapor explosion can be judged by comparing the interfacial temperature and the molten material temperature. The results obtained in the present experiments are applied to the results of the large-scale experiments using uranium dioxide. The results indicate that the possibility of the vapor explosion of the uranium dioxide and water under the present LWR operational condition is extremely unlikely. It should be noted that the present criteria should be applicable in case that the melting temperature does not decrease by containing the metal component.

Aeroelastic Response of Nonlinear Wing Sections Using a Functional Series Technique

This paper addresses the problem of the determination of the subcritical aeroelastic response and flutter instability of nonlinear two-dimensional lifting surfaces in an incompressible flow-field via indicial functions and Volterra series approach. The related aeroelastic governing equations are based upon the inclusion of structural and damping nonlinearities in plunging and pitching, of the linear unsteady aerodynamics and consideration of an arbitrary time-dependent external pressure pulse. Unsteady aeroelastic nonlinear kernels are determined, and based on these, frequency and time histories of the subcritical aeroelastic response are obtained, and in this context the influence of the considered nonlinearities is emphasized. Conclusions and results displaying the implications of the considered effects are supplied.

The Trigger Mechanism of Vapor Explosion.

In the present study, trigger mechanisms of the vapor explosion are experimentally investigated. The interfacial behavior between high temperature molten liquid and low temperature water are experimentally investigated by using a molten material droplet and external pressure pulse. As the results, it is indicated that spontaneous vapor explosion hardly occur in high temperature water near saturation temperature since vapor film is stable. The vapor explosion can occur even in high temperature water near saturation temperature in case that the external pressure pulse is applied to high temperature molten material. Vapor explosion can not occur when the interfacial temperature between the molten material and water is lower than the material melting temperature, even if the vapor film around the molten material is collapsed by the external pressure pulse. It is clarified that the impossibility of the trigger process for the vapor explosion can be judged by comparing the interfacial temperature and the molten material temperature. The results obtained in the present experiments are applied to the results of the large-scale experiments using uranium dioxide. The results indicate that the possibility of the vapor explosion of the uranium dioxide and water under the present LWR operational condition is extremely unlikely. It should be noted that the present criteria should be applicable in case that the melting temperature does not decrease by containing the metal component.

Monte Carlo Modeling and Analysis of Pressure Sensor Measurements During Suborbital Flight

The response of an ionization pressure sensor onboard a rotating suborbital spacecraft is investigated with data analysis and direct simulation Monte Carlo computations. The sensor housed in a chamber was connected to the spacecraftsurfacewith a tubeand recorded asymmetricram-wakepressurepulsesduring thenonthrusting period ofthemission.Three-dimensionalMonteCarlocomputationsoftheexternalandinternale owsareperformedusing a domain that includes the spacecraft and the pressure apparatus. Freestream parameters correspond to altitudes between 130 and 275 km. The e ux and composition at the sensor tube entrance is found to depend on the tube’ s orientation with the freestream during the spacecraft rotation. The predicted external pressure pulse differs from measurementsbecauseof internale ow effects. Thee owstructureisthree-dimensional at the tubeentry region and becomes axisymmetric a few tube diameters inside. The temperature, density, and surface pressure distributions inside the tube and chamber are found to depend on the tube orientation with the freestream and demonstrate the coupling between external and internal e ows. The predicted pressure pulse is in good qualitative agreement with measurements. Differences in magnitude are attributed to the uncertainty in the freestream parameters.

Dynamic short-wave buckling of thin-walled cylindrical shells upon local action of an external pressure pulse

We present the results of experimental investigations of dynamic instability (buckling) of thin-walled cylindrical shells upon local action of an external pressure impulse. In order to create the local short-term loading, we used pulsed radiation from a CO2 laser. The test pieces were smooth and reinforced shells made from aluminum alloys. The nature of the buckled wave in the treated region and the magnitude of the critical impulse correspond to the short-wave type of buckling in the shell and are determined by the amplitude of the pressure pulse. Axial static compression of the shell leads to a decrease in the critical impulse and an increase in the number of waves in the meridional direction within the treated region.

Axisymmetric dynamic buckling of submerged cylindrical shells

Dynamic buckling response of tubes immersed in water and subjected to an axisymmetric external throughwater pressure pulse is investigated using the finite element method. A loading routine describing the hydrodynamic net pressure, incorporating incident and reflected pressure pulses on the structure was developed. This is interfaced with the general-purpose finite element code ABAQUS to solve for the structural response which has non-linearity in both material properties and displacements. Idealized geometric imperfections in the radius are used in the finite element model to trigger the radial buckling mode. The same numerical procedure was employed to predict the dynamic buckling response of tubes subjected to impulsive loading in air. Comparisons are made with plastic flow buckling theory and experiments reported in literature.

Experimental study of the dynamic stability of spherical shells with axial reinforcement when subjected to an external pressure pulse

Experimental study of the dynamic stability of spherical shells with axial reinforcement when subjected to an external pressure pulse

Experimental investigation of cylindrical shells with an opening under an external pressure pulse

The results of experimental investigation of the deformation of a shell with a circular opening under a pulse of uniform external pressure are presented. A multifactor regression equation is derived on the basis of experimental data. The resultant equation is analyzed statistically. Relationships suitable for the practical computation of the residual deflection and critical pulse are given.

Analytic Predictions of Boiling Film Stability for Conditions Consistent with Light Water Reactor Degraded Core Conditions

An improved understanding of boiling film dynamics as it relates to energetic steam explosions during degraded core conditions in light water reactors is developed. Several models have been developed and used to predict the characteristics of film boiling when a molten fuel drop suddenly comes into contact with water. An incompressible model and an approximate compressible model, utilizing Gilmore's equation are developed consistent with past works and are determined to have several shortfalls. To improve the treatment of compressibility effects, a model employing Lagrangian equations is developed. This improved model predicts that applying an external pressure pulse can make a stable film go unstable and decreasing water subcooling stabilizes film oscillations; both predictions are consistent with experimental observations. However, the improved model predicts stable film boiling at low melt temperatures that cannot support such boiling. Modeling Taylor surface instability effects at the water/steam interface indicates that the surface area change due to this surface instability can stabilize the film oscillations.