A study of the limit pressures of thick-walled pipes with part-through slots on the outside surface

An experimental study of the limit pressures of thick-walled pipes with part-through slots on the outside surface has been carried out. Three types of slots were tested on two groups of thick-walled pipes made of carbon steel. The postulated yield pressure is based on the inflexion point of the tangential strain vs pressure curve. Two geometric parameters combining the slot geometry and the pipe dimensions are proposed. The relative reductions in yield pressure caused by the slots are correlated with those geometric parameters. The correlations are reasonably good and empirical equations have been obtained. These equations can be used to estimate the reduction of yield pressure caused by slots on the outside surface of thick-walled pipes.     

Lower-bound shakedown analysis of spherical shells containing defects

In this paper, a lower-bound shakedown analysis of spherical shells containing defects is given based on the static shakedown theorem. The pseudo-residual stress field is simulated by the temperature parameter method. In order to get over the difficulty of large computative quantities, the yield condition is linearized, so that shakedown analysis is transformed into a linear programming problem. The shakedown and the limit loads of spherical shells containing part-through slots and gas holes are computed by this method, and the relationships between the carrying capacities of vessels and the geometric parameters of shells and defects are obtained. The effects of pits and gas holes on carrying capacities of vessels are then compard with each other.     

Equivalent torispherical pressure vessel heads

Lower bounds to limit pressures for ellipsoidal and torispherical heads of cylindrical shells have been calculated using a numerical optimisation technique. These have been used to explore the ideas of equivalence between torispherical and ellipsoidal heads. The geometrical proportions of torispherical heads which have optimum limit pressures have been found and these are compared with the results of three geometrical criteria of ‘equivalence’.     

Numerical analysis of limit load and reference stress of defective pipelines under multi-loading systems

The concepts of limit load and reference stress have been widely used in structural engineering design and component integrity assessment, especially in Nuclear Electric's (formerly CEGB) R5 and R6 procedures. The reference stress method has been proven to be successful in problems pertaining to creep growth, rupture damage, creep buckling, and more recently, elastic–plastic fracture toughness. An approximate method of reference stress determination relies on prior knowledge of limit loads for various configurations and loadings. However, determination of the limit loads for the problems with complicated geometric forms and loading conditions is not a simple task. In the present paper, a numerical solution method for radial loading is presented, the mathematical programming formulation is derived for the kinematic limit analysis of 3D structures under multi-loading systems, and moreover, a direct iterative algorithm used to determine the reference stress is proposed which depends on the evaluation of limit load. The numerical procedure is applied to determine the limit load and reference stress of defective pipelines under multi-loading systems. The effects of four kinds of typical part-through slots on the collapse loads of pipelines are investigated and evaluated in detail. Some typical failure modes corresponding to different configurations of slots and loading forms are studied.     

Characteristics of some biomass briquettes prepared under modest die pressures

Biomass material, including sawdust, rice husks, peanut shells, coconut fibres and palm fruit fibres, was densified into briquettes at modest pressures of 5–7 MPa using a piston and die type of press. The briquettes were tested to evaluate their relaxation behaviour, mechanical strength and burning characteristics. The sawdust briquettes were found to have better overall handling characteristics. But briquettes of different biomass materials required different optimum conditions of fabrication and generally showed a promising potential for further development.     

Properties of charcoal derived from hazelnut shell and the production of briquettes using pyrolytic oil

Hazelnut shells were converted to charcoal and to liquid, and gaseous products using pyrolysis at different temperatures. The chemical compositions and yields of the charcoals were determined as functions of the carbonization temperature. Higher heating values (HHVs) were estimated using both ultimate and proximate analyses. Hazelnut shells and the derived charcoal were densified to briquettes using pyrolytic oil or tar as binder. Briquette properties improved with an increase in briquetting pressures and percentages of binder materials. The best charcoal briquettes were obtained at 800 MPa pressure at 400 K.     

The vibration and buckling of freely supported non-homogeneous orthotropic conical shells subjected to different uniform pressures

The paper examines the vibration and stability of freely supported orthotropic truncated and complete conical shells with non-homogeneous material properties under uniform lateral and hydrostatic pressures. First, the basic relations have been obtained for orthotropic truncated conical shells, Young's moduli and density which vary continuously in the thickness direction. By applying the Galerkin method, the buckling pressures and lowest cyclic frequencies of truncated and complete conical shells are obtained. To verify the accuracy of the results, comparisons are made with results available in the open literature. This study also examines in detail the effects of the variations of conical shell characteristics, the effects of the non-homogeneity and the orthotropy on the critical pressures and lowest cyclic frequencies of truncated and complete conical shells.     

Buckling of externally pressurised barrelled shells: a comparison of experiment and theory

Details of collapse tests of two cylindrical and four bowed out mild steel shells are provided. The diameter of the tested shells was about 200 mm, their length varied from 75 to 100 mm whilst the wall thickness was about 3 mm. Experimental buckling/collapse pressures varied from 8 to 22 MPa. Zero radial displacements and zero rotations have been implemented at both ends of all tested shells. The sensitivity of buckling/collapse pressures to the initial, eigenmode type geometric imperfections has been assessed for both cylindrical and bowed out geometries. It appears that barrelling does not necessarily increase the sensitivity of buckling pressure to shape deviations from perfect geometry. Good agreement has been obtained between experimental results on these computer numerically controlled (CNC)-machined models and numerical predictions. Results show that, on a like-for-like basis, barrels were able to support a pressure 85% higher than mass equivalent cylinders.     

Buckling analysis of an orthotropic thin shell of revolution using differential quadrature

A method is developed to predict the buckling characteristics of an orthotropic shell of revolution of arbitrary meridian subjected to a normal pressure. The solution is given within the context of the linearized Sanders–Budiansky shell buckling theory and makes use of the differential quadrature method. Numerical results for buckling pressures and mode shapes are given for complete toroidal shells. Both completely free shells and shells with circumferential line restraints are covered. The loadings considered consist either of uniform pressure or circumferential bands of constant pressure. It is demonstrated that the differential quadrature method is numerically stable and converges. For isotropic toroidal shells, good agreement is observed with previously published analytical and finite element results. New results for buckling pressures and mode numbers are given for orthotropic shells and for band loaded shells.     

开槽气膜孔结构对气膜冷却和流动特性的影响

运用数值模拟的手段,从流动特性和冷却特性两方面评价了各种开槽气膜冷却孔结构的优劣。从流动的机理揭示了在相同的槽深下,不同的横槽结构对改善气膜冷却效率和流量系数的影响,并比较了在气膜孔出口和入口均开有横槽后对流动和冷却特性的影响。结果表明：开横槽后,气膜孔出口下游的冷却效率得到不同程度的改善,吹风比越大,改善的程度越明显。在横槽下游5D-10D的范围内,冷却效率的改善程度最大;在气膜孔出入口处均开有斜横槽的结构和用圆角过渡气膜孔入口处的横槽均是提高气膜冷却效率和减小气膜孔流动阻力的有效措施,而在气膜孔出口处的横槽用圆角过渡则不利于改善气膜冷却效果。     

碳酸氢钠分解的热重分析研究

进行了不同压力、气氛及升温速率下的热重试验,研究干法Na2CO3/NaHCO3循环脱除CO2技术吸收剂的预处理和再生过程中NaHCO3的分解特性.通过NaHCO3热解失重率(TG)和失重速率(DTG)曲线,获得相关热解特性参数,研究了CO2:含量、升温速率和压力对NaHCO3分解的影响.结果表明:改变反应气氛的试验以Na2CO3晶体的形成与长大为控制步骤,遵循随机成核随后生长机理;改变升温速率的试验以化学反应为控制步骤,遵循一级反应模型;加压试验以扩散过程为控制步骤,遵循三维扩散机理.给出了相应的反应机理函数.     

Computational limit analysis of anisotropic axisymmetric shells

In this paper a numerical method is suggested for the limit analysis of axisymmetric shells using Hill's 1948 yield criterion. Since von Mises' condition is a special case of Hill's 1948 criterion, the new method can be applied to both the isotropic von Mises' material and the anisotropic Hill's material. By means of finite element techniques and the kinematic theorem for limit analysis, we introduce an efficient iteration algorithm to compute the upper bound of the collapse load. Numerical results of the limit analysis of cylindrical-conical-cylindrical combined shells subjected to internal pressure are presented by means of Hill's as well as von Mises' conditions. From the present investigation, some comments are made on the design of anisotropic pressure vessels of cylindrical-conical-cylindrical combined shells.     

Force density limits in low-speed PM machines due to temperature and reactance

This paper discusses two of the mechanisms that limit the attainable force density in slotted low-speed permanent-magnet (PM) electric machines. Most of the interest is focused on the force density limits imposed by heating of the windings and by stator reactance. The study is based on analytical models for the force and reactance calculations and a lumped parameter thermal model. It is found that in a machine with an indirectly cooled stator, it is difficult to achieve a force density greater than 100 kN/m/sup 2/ due to temperature limits. A high force density is achieved by using deep slots, which lead to high reactance. The high reactance severely increases the converter kilovolt-ampere requirement and total system cost. It is also shown that the cost caused by the high reactance will also limit the force density reached. In machines with one slot per pole per phase, the reactance limited the useful slot depth to approximately 200 mm. However, in machines having a greater number of slots per pole per phase the reactance becomes no longer an important limiting factor for the slot depth and force density.     

Elastic buckling of,and first yielding in,thin torispherical shells subjected to internal pressure

With the aid of the non-linear shell buckling computer program BOSOR 4, the internal pressures at which elastic circumferential buckling (or wrinkling) take place in thin torispherical shells have been calculated. The maximum equivalent (or effective) stresses in the shells in the axisymmetric pre-buckled state were also obtained; from these, the pressures at which first yielding in the shells commences were determined for $\text{1 <}\text{σ}{}_{\mathrm{<mtext>yp</mtext>}}\text{E}\text{× 10}{}^3\text{< 4}$The calculations were performed for shells with $\text{diameter}\text{thickness}$ ratios of 500, 1000 and 2000; other geometric ratios, as detailed in the paper, were also varied. The computations were carried out for steel shells but the results have been presented in dimensionless form.Utilising the above results it is possible to determine whether a given torispherical end closure will buckle elastically or whether an elastic-plastic analysis of the shell is desirable. Factors which are conducive to elastic buckling are a high yield point, a low modulus of elasticity or a large value of the shell diameter-thickness ($\text{D}\text{t}$) ratio. For steel shells, elastic internal pressure buckling will occur (for some combinations of $\text{r}\text{D}$ and $\text{R}{}_{\mathrm{<mtext>S</mtext>}}\text{D}$) for $\text{D}\text{t}\text{= 2000}$ and $\text{σ}{}_{\mathrm{<mtext>yp</mtext>}}\text{E}\text{= 3 × 10}{}^{\mathrm{?3}}$. For $\text{D}\text{t}\text{= 1000}\text{and}\text{500}$, first yield always precedes elastic buckling for the parameters investigated. The failure mode for these cases is either elastic-plastic buckling or plastic collapse (an axisymmetric mode with large deformations).A comparison of the results of linear and non-linear elastic axisymmetric stress analyses of the shells shows that the linear theory sometimes underestimates the first yield pressure by considerable amounts. Limit pressures obtained from small-deflection shell theories can be too low in such cases.Also given in the paper are approximate simple expressions whereby the elastic internal buckling pressures of torispherical shells may be calculated. These expressions should be useful to designers.     

Effects of turbulence on nonpremixed ignition of hydrogen in heated counterflow

Experiments were conducted to determine the effects of turbulence on the temperature of a heated air jet required to ignite a counterflowing cold hydrogen/nitrogen jet. In contrast to pseudo-turbulent flows, where turbulence was generated by only a perforated plate on the fuel side, resulting in little effect on ignition in a hydrogen system, fully turbulent flows with perforated plates on both sides of the flow were found to produce noticeable effects. The difference was attributed to the fact that in fully turbulent flows, a significantly larger range of turbulent eddies extend to smaller scales than in pseudo-turbulent flows. At atmospheric pressure, the lowest turbulence intensity studied had ignition temperatures notably lower than laminar ones, while further increases in turbulence intensity resulted in rising ignition temperatures. As a result, optimal conditions for nonpremixed hydrogen ignition exist in weakly turbulent flows where the ignition temperature is lower than can be obtained in other laminar or turbulent flows at the same pressure. Similar trends were seen for all fuel concentrations and at all pressures in the second ignition limit (below 3-4 atm). At higher pressures, turbulent flows caused the ignition temperatures to continue to follow the second limit resulting in ignition temperatures higher than the laminar values. The extension of the second limit ends at the highest pressures (7 to 8 atm) where evidence of third limit behavior appears. Three mechanisms were noted to explain the experimental results. First, turbulent eddies similar in size to the ignition kernel can promote discrete mixing of otherwise isolated pockets of gas. Second, this mixing can promote HO₂ chain branching pathways, which can account for the enhanced ignition noted in the second limit where reaction is governed by crossover temperature chemistry. Third, turbulence limits the excursion times available for reaction, inordinately affecting the slower HO₂ reactions. This is responsible for the increasing ignition temperature with turbulence intensity and pressure.     

末级中空静叶除湿方法的数值研究

采用数值计算方法对某末级中空静叶片开双抽缝除湿方法进行了研究。通过研究抽吸缝除湿机理设计了两种方案的双抽吸缝结构,计算了一级采用双抽吸缝结构的叶栅流场。比较了两种双抽吸缝方案条件下不同叶高上的流场、湿度场以及叶片表面的压力情况。通过分析得到两种抽吸缝方案的计算结果,得到了两种双抽吸缝中占优的方案。     

Experimental study of a hydrogen-air rotating detonation engine with variable air-inlet slot

Rotating detonation engines have attracted considerable attentions in recent years. In this study, the experiments of initiating rotating detonation waves were performed on a H₂/air rotating detonation wave with the variable air-inlet slot. The results showed that the stability of detonation-wave pressure and velocity both initially increased and then decreased with the increase of slot width, and it could improve the stability of detonation-wave velocity via increasing the equivalence ratio. The intensity of reflected wave was strong for the tests of d = 0.5 mm, which leaded to the advance ignition of fresh mixture and a velocity deficit reaching up to 20%. The strong interaction between air plenum and combustor and bad mixing effect may be the reasons of forming unstable detonation wave for the tests of large-scale slots. The air-inlet slot of d = 1 mm, which got a best experiment results relative to other tests, had a wide equivalence-ratio scope to produce stable detonation wave.     

Transient helium heat transfer phase I—Static coolant

Transient heat-transfer data have been obtained for flat heating surfaces in static liquid and supercritical helium. Measurements start 2(10)^?5 s after step power inputs, and cover a heat flux range of 0.05–20 W/cm², pressures from 0.09–0.3 MPa, and four different heater orientations. Initial heat-transfer coefficients, being limited primarily by the Kapitza resistance, are 10–100 times greater than steady state, and the time to reach steady state varies from 10^?5 to 1s. For heat flux below the steady state peak nucleate boiling limit the temperature follows calculations based on pure conduction to the steady state nucleate boiling level. Above that limit the transient conduction period leads to an apparent metastable nucleation period followed by a transition to film boiling.     

Compressive behaviour of thin catalyst layers. Part I - Experimental study

In this study, the effect of compression is investigated experimentally on deformation and porosity of catalyst layers (CLs). Compression tests are performed on five CL samples with various microstructures using a thermomechanical analyzer and a custom-made machine Tuc-Ruc (Thickness under compression-Resistivity under compression). The results indicate that CLs have a linear behaviour with no plastic deformation at pressures less than 2 MPa even after 12 cycles. However, CLs showed plastic deformation, work hardening, and elastic shakedown under cyclic compression up to 5 MPa. In this pressure range, the material becomes stiffer and Young's modulus has increased by 50–113% after 8 loading cycles. Moreover, the material “settles down” after 6 cycles showing no further significant plastic deformation at higher pressures (up to 5 MPa). This behaviour suggests that CLs enter elastic shakedown region since after several cycles, plastic strain diminished, and they behave elastically afterwards. The compression tests on five samples yield Young's modulus of 30–45 MPa for pressures up to 2 MPa and Young's modulus of 37–70 MPa for pressures up to 5 MPa. The reason for slight change in Young's modulus is that the microstructure of CL changed, and the porosity decreased at higher pressures.