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Investigation On Gas-Liquid Two-Phase Flow-Induced Vibrations Of A Horizontal Elastic Pipe

Abstract This paper is concerned with experimental analyses on the vibration behaviors of a horizontal pipe containing gas-liquid two-phase flow with different flow patterns. The effects of flow patterns and superficial velocities on pressure fluctuations and structural responses are evaluated in detail. Numerical simulations on the fluid-structure interactions within the pipe are carried out using the volume of fluid method and the finite element method. A strongly partitioned coupling method is adopted to ensure the compatibility and equilibrium interface conditions between the fluid and the elastic pipe. The accuracy of the numerical solutions is confirmed by comparing with experimental results. It is found that the fluctuation frequency of the pressure fluctuations of the two-phase flow ranges from 0Hz to 5Hz. The standard deviation value of the pressure fluctuation of the two-phase flow increases with an increase in the superficial liquid velocity, and the maximum magnitude appears in slug flows. The vibration responses of the pipe exhibit strong dependence on the momentum flux of the two-phase flow, which mainly excites the fundamental flexural vibration mode of the pipe. The magnitude of vertical vibration response of the pipe is equal to that of the lateral vibration response, and the vibration response measured at the middle of the pipe does not contain the second-order operating mode. Moreover, the STD value of the structural responses of the pipe increases proportionally with an increase in the gas flow rate, while the predominant vibration frequency of the pipe slightly increases.

Thermally Assisted Rotational Autofrettage of Long Cylinders With Free Ends

Abstract Autofrettage is a widely employed process for strengthening cylindrical or spherical pressure vessels. The process involves applying a uniform load to the inner wall of a vessel to cause a controlled plastic deformation, where the vessel yields starting from the inner wall up to an intermediate radius. When the load is removed, elastic recovery takes place and compressive residual stresses are induced in the vicinity of the inner wall, which strengthen the vessel against high static and pulsating loads during service. Based on the load employed, autofrettage can be of five types—hydraulic, swage, explosive, thermal, and rotational. This work analyzes a rotational autofrettage augmented by a thermal load where the load is applied by rotating the cylinder about its axis while maintaining a temperature gradient across the wall. The combined centrifugal and thermally induced stresses cause plastic deformation in the cylinder. When the cylinder is unloaded by bringing it to rest and cooling down to room temperature, compressive hoop residual stresses are introduced in the vicinity of the inner wall. A finite element method model of the proposed thermally assisted rotational autofrettage is developed for a cylinder made of AH36 mild steel in a commercial package ABAQUS®. The results indicate that the thermal load reduces the rotational speed required for autofrettage, when compared to a conventional pure rotational autofrettage. The thermal load also mitigates the tensile axial residual stresses, which are typical in a purely rotational autofrettage. A conceptual design of the experimental setup is also presented.

Deformation and Dynamic Response of Steel Belt Staggered Multilayer Cylindrical Shell Under External Blast Loading

Abstract The deformation and dynamic response of a multilayer cylindrical shell composed of an inner shell and fourteen outer layers under external blast loads of different trinitro-toluene equivalency weights were studied. A numerical model using the thermo-viscoplastic constitutive model and considering fluid–structure coupling between explosion wave and structure was developed. The displacement in axial direction and cross section, as well as the effective strain responses, were analyzed to demonstrate the potential deformation of the shell structure. Results demonstrate that different materials cause inconsistent displacement and separation to develop in the inner and outer shells. In order to address the problem that the displacement of the inner shell is hard to measure due to the shielding and covering of the outer shell, a theoretical formula for calculating the maximum displacement of the inner shell was developed. The deflection process and stress triaxiality histories of the inner shell were investigated, and the results showed that compressive stress is the primary cause of plastic deformation. Additionally, the delamination that appeared in the outer shell was discussed, and it was revealed that there are two factors of delamination: (1) Stress waves spread across adjacent layers in the opposite direction because steel belts were wound in the opposite direction between the two adjacent layers; (2) Outer layers experienced uneven compressive loads. The results will be helpful to provide a reference for the intrinsic safety design of such multilayer cylindrical structures for hydrogen storage, etc.

Creep-Fatigue Life Property of P91 Welded Piping Subjected to Bending and Torsional Moments at High Temperature

Abstract In recent years, the role of thermal power plants has shifted from providing a baseload to providing supplemental supply to compensate for fluctuations in the output of renewable energy sources. Thus, the operation of these plants involves frequent startup and shutdown cycles, which lead to extensive damage caused by creep and fatigue interactions. In addition, the piping utilized in thermal plants is subjected to a combined stress state composed of bending and torsional moments. In this study, a high-temperature fatigue testing machine capable of generating such a bending-torsional loading was developed. Creep-fatigue tests were conducted on P91 steel piping with weldment. The results clarified that the creep-fatigue life was reduced by the superposition of the torsional and bending moments and that it was further reduced by a holding load. It was shown that the creep-fatigue life of piping with weldment can be estimated accurately using the equivalent bending moment, which is composed of the torsional and bending moments. It was also confirmed that crack occurred in the heat-affected zone (HAZ) of the welded part, which has been often observed in actual thermal power equipment. From the finite element analysis, it was identified that cracking was initiated in the HAZ due to the accumulation of creep strain and increase in the hydrostatic pressure component during a holding load.

Continuum Damage Mechanics Modeling of High-Temperature Flaw Propagation: Application to Creep Crack Growth in 316H Standardized Specimens and Nuclear Reactor Components

Abstract Predicting creep crack growth (CCG) of flaws found during operation in high-temperature alloy components is essential for assessing the remaining lifetime of those components. While defect assessment procedures are available for this purpose in design codes, these are limited in their range of applicability. This study assesses the application of a local damage-based finite element methodology as a more general technique for the prediction of CCG at high temperatures on a variety of structural configurations. Numerical results for stainless steel 316H, which are validated against experimental data, show the promise of this approach. This integration of continuum damage mechanics (CDM) based methodologies, together with adequate inelastic models; into assessment procedures can therefore inform the characterization of CCG under complex operating conditions while avoiding excessive conservatism. This article shows that such modeling frameworks can be calibrated to experimental data and used to demonstrate that the degree of triaxiality ahead of a growing creep crack affects its rate of growth. The framework is also successfully employed in characterizing CCG in realistic reactor pressure vessel geometry under an arbitrary loading condition. These results are particularly relevant to the nuclear power industry for defect assessment and inspections as part of codified practices of structural components with flaws in high-temperature reactors.