Analysis of one-dimensional pressure pulse propagation in a closed fluid system

This paper describes a unique numerical method for linear inviscid fluid hammer analysis based on the method of characteristics. The uniqueness lies in that it uses the analytical solutions of the wave equation in place of the compatibility relatins of the more conventional method of characteristics. The numerical solution is obtained by a simple superposition technique for tracing the waves traveling along each characteristic and extending the solution from one constant time line to the next. Using a predetermined finite difference net of grids with equal spacings, an elimination is made of the spatial interpolation, thereby maintaining the wave amplitudes in their full strength in the numerical procedure. This is in contrast to the case of a nonlinear problem in which the pressure peaks are always flattened to some degree in the interpolation procedure.The computer program NAHAMMER is a system analysis code adequate for short-term pressure transients of most engineering problems of significance involving a moderate pressure source. It considers the simplified one-dimensional, linear, inviscid set of governing equations with an isentropic flow assumption. A closed fluid-network system is considered to be composed of a multiple of one-dimensional pipe sections and components that are connected by various joints. An analytical solution is obtained under an acoustic approximation for a simple system and the result shows good agreement with the numerical solution. As examples of the application of the method, complex problems of engineering importance are calculated and the results are presented.     

Numerical study on pressure wave propagation in a mercury loop

On-beam tests were carried out at the Los Alamos Neutron Science Center-Weapons Neutron Research (LANSCE-WNR) facility in June 2005 to investigate pressure wave mitigation in mercury targets for the MW-class spallation neutron sources under international collaboration between US Spallation Neutron Source (SNS) and Japanese Spallation Neutron Source (JSNS). A mercury loop was used for the target, a so-called In-Beam Bubbling Test Loop (IBBTL). The loop consists of the rectangular pipe of 25 mm × 50 mm² in cross section, 1.5 mm in wall thickness and 2 m in total length approximately. The SNS team set 8 strain sensors on the pipe wall to measure the strain propagation caused by the pressure wave. The maximum strain appeared at 350 mm apart from the proton-bombarded point at 5.5 ms after the proton bombardment. It is known that the propagation velocity of the pressure wave in mercury is ca. 1500 m/s and that of the stress wave in stainless steel is ca. 5000 m/s. However, the apparent wave propagation velocity in the IBBTL was lower than those velocities and was observed to be 65 m/s. Numerical analysis was carried out to understand the strain propagation in the pipe wall of the IBBTL. Numerical results showed that the maximum strain at 350 mm apart from the beam spot appeared at 5.5 ms after proton bombardment in good agreement with experimental results.     

电离层对核电磁脉冲传播特性影响的分析

引入傅立叶变换,对高空核爆激励的瞬态电磁脉冲信号通过电离层向同步卫星轨道传播后的波形进行了分析。由于电离层的色散特性,使原来的瞬态电磁场不同频率的成分相互分离,频率高的相速度快,频率低的相速度慢,使到达卫星轨道的波形成为一衰减振荡,且波形持续时间拉大。考虑电离层的衰减,对20～lOOkm爆高的核电磁脉冲传播至卫星轨道后的电场仍是可以监测到的。     

Fracture mechanics analysis on the initiation and propagation of circumferential and longitudinal cracks in straight pipes and pipe bends

The safety potential against pipe fracture of a nuclear piping system is quantified using as an example the fuel circulating system of the THTR 300 MWe. A comparison of the size of cracks expected to occur during operation in the most unfavourable case with the critical crack sizes from the fracture mechanics aspect is used as a criterium for evaluation.A comprehensive test program was carried out to investigate the dependence of longitudinal and transversal crack sizes upon load (bending and internal pressure), pressure medium and temperature. The theoretical methods for predicting the critical crack sizes were checked by the test results.The conclusion is that pipe rupture does not need to be considered under the conditions investigated.     

Influence of high-voltage pulse parameters on the propagation of a plasma synthetic jet

In this work, a typical pin-to-pin plasma synthetic jet in static air is excited by a pulsed DC power supply. The influences of the pulse rising time, the amplitude and the repetition frequency of the pulse voltage on the jet flow have been investigated. First, using a high-speed Schlieren imaging technique, the induced shock waves and the fast jet flow generated by the plasma synthetic jet are characterized. With a deposited energy of 44 mJ per pulse, the velocity of the shock wave and the maximum velocity of the jet flow reach 320 m s⁻¹ and 100 m s⁻¹, respectively. Second, when the applied voltage increases from 12.8 kV to 16 kV, the maximum jet velocity increases from 66 m s⁻¹ to 93 m s⁻¹. On the other hand, as the pulse rising time varies from 50 ns to 500 ns, or the pulse repetition frequency increases from 5 Hz to 40 Hz, the jet velocity induced by the plasma synthetic jet is weakly dependent. In addition, a comparative study of the plasma synthetic jets using three commercial pulsed power supplies (XJ-15, NPG- 18, and PG-30) is implemented. It reveals that the maximum jet velocity of 120 m s⁻¹ is obtained in the case of PG-30, with the longest pulse rising time and the lowest breakdown voltage, while the maximum velocity of 33 m s⁻¹ is detected in the case of NPG-18, even though it has the shortest pulse rising time and the highest breakdown voltage.     

Creep of a straight pipe under combined bending and internal pressure

Several analyses are presented for the isothermal creep behaviour of a thin, constant thickness, circular crosssection cylindrical shell under the action of conbined bending and internal pressure loading. Results are presented in a way that should be helpful for general pipework design. Previous analyses are summarised and discussed.     

Simplified analysis of shrinkage in pipe to pipe butt welds

Generally some shrinkage is typical of butt welding of pipes. Shrinkage due to butt welding could be more pronounced and significant in thin wall stainless steel pipes because the thermal expansion coefficient is roughly one and half times that of carbon steel. An axisymmetric finite element evaluation of hoop shrinkage associated with circumferential butt welds in thin wall stainless steel pipes was performed. Actual shrinkage data for a larger (24 in. diameter, 0.375 in. wall thickness) pipe and a smaller (4 in. diameter, 0.237 in. wall thickness) pipe were utilized. The results indicate that very localized residual stresses in excess of yield strength produced during cooldown of metal in the weld and heat affected zones cause redistribution of the stresses. A simplified elastic–plastic analysis approach was developed with adjustments for section modulus and Poisson’s ratio, and the strains due to radial shrinkage were calculated for inside and outside surfaces of the pipe at the weld center line. From the strain point of view, the strain values in the circumferential direction were about 1.4% for the larger size pipe and 3.4% for the smaller size pipe. The strain values in the axial direction were 2.5% for the larger pipe and 5.9% for the smaller pipe. It is concluded that these levels of strains are not detrimental in nature. However, for the smaller pipe they are on the high side and it is recommended not to use the pipe for elevated temperature service. Residual stresses were also calculated for inside and outside surfaces of the pipe at weld center line using a simplified elastic–plastic approach and a bilinear stress–strain curve and compared with published data indicating a general agreement.     

The effect of internal pressure on the creep relaxation of a thin, straight pipe

The effect of constant internal pressure on the relaxation of bending moment on a long, thin, circular cross section cylindrical shell is determined. The inadequacy of several simple approximate analyses, previously proven useful in other creep applications, is established.     

Noise analysis of a pulse processor using SPICE

We describe a technique, simpler than earlier ones, for using SPICE in the computation of the 1/f, step, and delta noise indices of linear and time-valiant filters. The method is based on obtaining a weighting function from the results of simulation and is demonstrated for triangular and trapezoidal shapers. The results are compared with those obtained by direct calculation. The greatest discrepancies occur when the method is applied to gated integrators, but the errors are probably due to artifacts not related to the basic method described here     

Test results and calculations concerning crack propagation under cyclic thermal chock loading on a pressure vessel nozzle

The mechanical-technological and nondestructive tests to determine the state of the HDR pressure vessel revealed that no significant defects were present. For this reason it was proposed to weaken the pressure vessel purposefully by cyclic thermal shock applied locally in the A2 nozzle region as to bring about defined natural defects for the purpose of NDE measurements and fracture mechanics analysis.Throughout the duration of the test, operating variables and changes under thermal shock loading, such as strain, temperature and crack openings were recorded. After a total of 1751 thermal shock cycles had been attained a limited destructive test was made on nozzle A2 for the purpose of determining the actual geometry of individual cracks.For precalculation of crack propagation cyclic, stress intensity factors were determined on the basis of 2D and 3D linear elastic FE stress analysis. The crack growth was determined by integrating the Paris correlation.The results of the thermal shock test series V66 on HDR nozzle A2 show that further investigations into the behaviour of components which are cracked and exposed to pressurized water under superimposed static and cyclic load components are necessary in order to obtain crack propagation assessment criteria valid for the crack front.     

垂直加热通道内超临界水流动稳定性参数敏感性分析

给出了无量纲分析法与频域法相结合的稳定性分析方法的详细描述,并定义了影响稳定性的关键无量纲数。针对垂直加热通道内超临界水进行了密度波稳定性分析,并建立了稳定性边界。对系统入口阻力因数、出口阻力因数、摩擦因数、进出口压降和流动方向等进行了参数敏感性分析,结果表明高的入口阻力因数有利于系统的稳定,但高的出口阻力因数和高的摩擦因数不利于系统的稳定,系统进出口压差对系统的稳定性影响较小,向上流动比向下流动更有利于系统的稳定。计算结果对超临界水堆的堆芯和系统设计具有指导性作用。     

Effect of pulse voltage rising time on discharge characteristics of a helium-air plasma at atmospheric pressure

In this paper,the influence of voltage rising time on a pulsed-dc helium-air plasma at atmospheric pressure is numerically simulated.Simulation results show that as the voltage rising time increases from 10 ns to 30 ns,there is a decrease in the discharge current,namely 0.052 A when the voltage rising time is 10 ns and 0.038 A when the voltage rising time is 30 ns.Additionally,a shorter voltage rising time results in a faster breakdown,a more rapidly rising current waveform,and a higher breakdown voltage.Furthermore,the basic paraneters of the streamer discharge also increase with voltage rise rate,which is ascribed to the fact that more energetic electrons are produced in a shorter voltage rising time.Therefore,a pulsed-dc voltage with a short rising time is desirable for efficient production of nonequilibrium atmospheric pressure plasma discharge.     

Prediction of semi-elliptical defect form, case of a pipe subjected to internal pressure

The fatigue propagation of a semi-elliptical surface crack in a pipe subjected to internal pressure is always an open question in spite of a number of numerical works. The crack front development is generally predicted by empirical relationships or parameters c and m of the crack propagation law. This fatigue propagation in the case of a pipe subjected to internal pressure have been obtained using a special device allowed to produce an internal pressure up to 700 bars. The crack aspect ratio development is similar to that of a crack in a plate submitted to tension.     

数字化多道脉冲幅度分析技术研究

多道脉冲幅度分析技术正在朝数字化方向发展,基于数字信号处理技术的数字化多道具有脉冲处理能力强、速度快、稳定性高和灵活性强等特点。文章分析了数字核脉冲处理关键技术,利用MATLAB完成了梯形成形、数字基线估计、数字极零零极补偿、极零点识别等核心算法的模拟。给出了数字化多道初步总体设计方案,探讨了工程没计中的关键技术问题,为研制国产数字化核谱仪打下了基础。     

Optimized analysis of sharpening characteristics of a compact RF pulse source based on a gyro-magnetic nonlinear transmission line for ultrawideband electromagnetic pulse application

We constructed a compact high-power RF pulse generator based on a gyro-magnetic nonlinear transmission line (GNLTL) to produce a high-voltage pulse with a sub-nanosecond rise time and a relatively high repetition rate, which shows great potential for application in the high-power ultrawideband electromagnetic effect, etc. The influence of incident pulse parameters (rise time and voltage amplitude) and line length on the sharpening characteristics of the GNLTL were investigated experimentally to optimize the rising rate of the modulated pulse front. Based on the GNLTL equivalent circuit model consisting of an LC ladder network, the rise time, the voltage conversion coefficient and the rising rate properties of a modulated pulse were also numerically analyzed in a wider range. The results show that a>90 kV RF pulse with a rise time of 350 ps and a repetition rate of 1 kHz in burst mode is produced by the GNLTL at an axial biasing magnetic field of 22 kA m⁻¹ and a line length of 30 cm under the condition of a 70 kV incident pulse. Applying a faster and higher incident pulse is conducive to improving the sharpening effect of the GNLTL. Furthermore, within a certain range, increasing the line length of the GNLTL not only reduces the rise time, but increases the voltage conversion coefficient and the rising rate of a modulated pulse. Furthermore, considering the energy loss of ferrite rings, there is an optimal line length to obtain the fastest rising rate of a modulated pulse front edge.     

Investigation on the streamer propagation in atmospheric pressure helium plasma jet by the capacitive probe

In this letter, the streamer propagation in the atmospheric pressure helium plasma jet with afloating electrode nozzle driven by the kHz AC power supply is investigated. The current signal induced by the space charges and the mean propagation velocity of the guided ionization waves are measured by the capacitive probe method in the discharge region. The space charges in the guided ionization waves are found to increase with the applied voltage, which enhances both the electric field near the streamer head and the propagation velocity. The applicability of the streamer mechanism to the propagation of the guided ionization waves is validated by this electrical diagnostic method.     

Normalizing the influence of flaw length on failure pressure of straight pipe with wall-thinning

Burst tests using wall-thinned pipe of carbon steel for high-temperature use were conducted in order to examine the influence of length of wall-thinning on burst pressure. Then, three-dimensional elastic-plastic large deformation finite element analyses (EP-FEA) were performed to accurately predict the burst pressure obtained by the tests. The failure pressure corresponding to the burst pressure in tests was defined as the maximum pressure during the analysis including the instability condition after the peak of pressure. The results showed that the failure pressure obtained by EP-FEA agreed well with the experimental results. Finally, failure pressures of wall-thinned pipes with various sizes, thicknesses, flaw lengths and depths were examined by EP-FEA with the same procedure of analysis as validated in this paper. The results showed that, from the standpoint of influence of flaw length on failure pressure, it is preferable to normalize flaw length by pipe mean radius of the unflawed section R rather than by shell parameter (Rt)^0.5, where t is the thickness of the unflawed section.     

Probability of pipe breakage regarding transient flow in a small pipe network

A reliability model was developed for the calculations of the probability of pipe breakage associated with transient effect. The statistical distribution for the maximum pressure wave heights was determined from the results of 168 transient analyses. It was found that the statistical distribution for internal pressure among the random variables of reliability function is well matched with the Gumbel distribution. The probability of pipe breakage in a small pipe network was calculated according to pipe diameter, thickness, allowable stress, and internal pressure. From the results, it was found that transient effect significantly increases the probability of pipe breakage. Using the present reliability model, it could be possible to find a pipe that contains the high probability of pipe breakage in a water distribution system. If the reliability model developed in the present study is applied for the design, a safe design will be accomplished. Furthermore, it can be effectively used for the management and maintenance of a water distribution system.     

Comparisons between finite-element analysis predictions and pipe fracture experiments

This paper presents the results of ten finite-element analyses of cracked pipe subjected to bending loads compared to the corresponding experimental results produced from full-scale tests. As part of the presentation, detailed results from two international round-robin problems are presented. In all, nine through-wall cracked pipe and one surface cracked pipe is considered. The cracked pipe includes stainless, carbon, and welded pipe.