Limit and burst pressures for cylindrical shells with part-through slots

Pressure tests on aluminium alloy cylindrical shells having rectangular slots part through the wall thickness at mid-length were carried out to estimate plastic limit pressures for comparison with calculated lower bound limit pressures. The tests were continued to failure so that the results could be used in consideration of the ‘two criteria’ method of assessing the pressure carrying capacity of shells with defects. Twelve shells, nominally 5·5 in diameter and of different thicknesses, were tested. One had no slot, others had slots of different depths and side ratios.     

A simplified ratcheting limit method based on limit analysis using modified yield surface

Two effective approaches for obtaining ratchet boundaries of a structure undergoing cyclic loads are presented. The approaches use limit analysis of a structure whose yield surface is modified according to the cyclic load. In the first approach, Uniform Modified Yield (UMY) surface is used. UMY approach reduces the Mises-based cylindrical yield surface by Mises stress of the cyclic stress amplitude. UMY method was slightly conservative, and sometimes overly conservative, especially at high ratio of cyclic load to primary steady load. Conservatism, caused by the assumption that the modified yield surface remains isotropic, is eliminated by considering anisotropic Load Dependent Yield Modification approach, LDYM. This approach reduces yield strength based on relative orientation of steady primary and cyclic stress tensors. This work assumed elastic perfect plastic material behavior, with no strain hardening for both original and modified yield surfaces. Ratchet boundaries of several structures, published in literature, were obtained using UMY and LDYM approaches and verified against published data and results of conventional methods. Numerical procedures for UMY and LDYM approaches are extremely fast relative to conventional numerical schemes, and are not restricted by complex geometry or loading.     

An Improvement on the Method for Calculating the Capillary Limit of Axial—Grooved Heat Piper

A new model has been developed to predict the capillary limit of axial-grooved heat pipe.In the model the concepts of liquid saturation or liquid fraction of the cross-sectional area of groove,the modified relative permeability,absolute permeability of groove and Leverrt‘s function are used.The Leverrt‘s function is well represented by the functionf(s)=1(1/√5)(1/2-)^0.175.In the model the effects of gravitational force,capillary force and viscous force are considered.The calcuated results are in good agreement with existing experimental data reported in the literature.     

Net-section limit load approach for failure strength estimates of pipes with local wall thinning

The net-section limit load approach is typically used in assessment of pipes with local wall thinning, based on which a maximum load carrying capacity is easily estimated from one equation that includes two terms associated with the effect of the defect geometry and the material's resistance (strength). To better understand the applicability of the net-section limit load approach to pipes with local wall thinning, four different limit load expressions for pipes with local wall thinning under pure bending are considered, together with two different definitions of the material's resistance. Estimated failure moments are then compared with full-scale pipe test data. It is found that the use of an appropriate limit load solution reduces not only the degree of conservatism but also the dependence of the assessment results on the wall thinning geometry, and thus gives the best results. Therefore, finding such solutions for pipes with local wall thinning is an important issue.     

Effects of attached straight pipes on finite element limit analysis for pipe bends

The effect of the length of an attached straight pipe on the plastic limit load of a 90° pipe bend under combined pressure and bending is quantified, based on finite element (FE) limit analyses using elastic–perfectly plastic materials with the small geometry change option. Systematic FE limit analyses of pipe bends with various lengths of the attached pipe are performed. It is shown that the effect of the length of the attached straight pipe on plastic limit loads can be significant, and the limit loads tend to decrease with decrease of the length of the attached straight pipe. In the limiting case of no attachment, the limit loads are found to be close to existing analytical solutions.     

Influences of some materials on J-estimation methods for pipes with circumferential surface cracks

The approximate calculation methods (SC.ENG1 and SC.ENG2) for the J-integral for pipes with circumferential surface cracks are discussed and three-dimensional elastic–plastic finite element models for circumferentially surface-cracked pipe are conducted to evaluate the accuracy of these methods for different pipe materials used in China. The numerical studies verify that the SC.ENG2 method provides more accurate estimates of J than SC.ENG1. Based on three-dimensional elastic–plastic fracture analysis, the distribution of the local J-integral along the front of a circumferential constant-depth internal surface crack is investigated and the influences of different pipe materials with different yield plateaux on J-integral values are discussed. The validity of SC.ENG1 and SC.ENG2 J-integral estimation methods for pipe steel materials with different yield plateaux used in China are examined in detail and the SC.ENG2 method is found to provide reasonable estimates of J for materials with yield plateaux.     

Experimental investigation of capillary-assisted evaporation on the outside surface of horizontal tubes

Capillary-assisted evaporation is a typical heat transfer method in heat pipes which is characterized by high evaporation coefficient due to extremely thin liquid film. This paper introduces such a micro-scale heat transfer method into normal-scale applications. A series of enhanced heat transfer tubes with circumferential rectangular micro-grooves on the outside surfaces have been experimentally investigated. The aim is to investigate the influence of the tubes’ geometries and operating parameters on the evaporation heat transfer coefficients. In the experiment, the tested tubes are hold horizontally and the bottom surfaces are immersed into a pool of liquid. The heat is added to the thin liquid film inside the micro-grooves through the heating fluid flowing inside the tubes. The factors influencing the capillary-assisted evaporation performance, such as the immersion depth, evaporation pressure, superheating degree, etc. are considered. The experimental results have indicated that there is a positive correlation between the evaporation heat transfer coefficient and evaporation pressure, and negative for the superheating and immersion depth. For water, under the evaporation saturated temperature of 5.0 ± 0.1 °C, the superheating of 4.0 ± 0.1 °C and the dimensionless liquid level of 1/2, the film side evaporation heat transfer coefficients are 3100–3500 W/m² K, which are equivalent to those of the falling film evaporator in LiBr–water absorption machine (2800–4500 W/m² K [Y.Q. Dai, Y.Q. Zheng, LiBr–water Absorption Machine, first ed., Chinese National Defence Industry Press, Beijing, China, 1980.]).     

Effect of vibration loading on the fatigue life of part-through notched pipe

A systematic experimental and analytical study has been carried out to investigate the effect of vibration loading on the fatigue life of the piping components. Three Point bend (TPB) specimens machined from the actual pipe have been used for the evaluation of Paris constants by carrying out the experiments under vibration + cyclic and cyclic loading as per the ASTM Standard E647. These constants have been used for the prediction of the fatigue life of the pipe having part-through notch of a/t = 0.25 and aspect ratio (2c/a) of 10. Predicted results have shown the reduction in fatigue life of the notched pipe subjected to vibration + cyclic loading by 50% compared to that of cyclic loading. Predicted results have been validated by carrying out the full-scale pipe (with part-through notch) tests. Notched pipes were subjected to loading conditions such that the initial stress-intensity factor remains same as that of TPB specimen. Experimental results of the full-scale pipe tests under vibration + cyclic loading has shown the reduction in fatigue life by 70% compared to that of cyclic loading. Fractographic examination of the fracture surface of the tested specimens subjected to vibration + cyclic loading have shown higher presence of brittle phases such as martensite (in the form of isolated planar facets) and secondary micro cracks. This could be the reason for the reduction of fatigue life in pipe subjected to vibration + cyclic loading.     

A numerical study of flow and heat transfer in internally finned rotating straight pipes and stationary curved pipes

In this article the effects of internal fins on laminar incompressible fluid flow and heat transfer inside rotating straight pipes and stationary curved pipes are numerically studied under hydrodynamically and thermally fully developed conditions. The fins are assumed to have negligible thickness with the same conditions as the pipe walls. Two cases, constant wall temperature and constant heat flux at the wall, are considered. First the accuracy of the numerical code written by a finite volume method based on SIMPLE algorithm is verified by the available data for the finless rotating straight pipes and stationary curved pipes, and then, the numerical results for those internally finned pipes are investigated in detail. The numerical results for different sizes and numbers of internal fins indicate that the flow and temperature field analogy between internally finned rotating straight pipes and stationary curved pipes still prevail. The effects of Dean number (K_L) versus friction factor, Nusselt number, and other non-dimensional parameters are studied in detail. From the numerical results obtained, an optimum fin height about 0.8 of pipe radius is determined for Dean numbers less than 100. At this optimum value, the heat transfer enhancement is maximum, and the heat transfer coefficient appears to be 6 times as that of corresponding finless pipes.     

The net-section stress associated with the extension of a part-through full-circumference crack in a pipe

Earlier workers have used a simple net-section stress approach, based on collapse-type analyses, to predict the size and shape of a part-through circumferential crack that will cause failure of a pipe fabricated from a ductile material: stainless steel. The equations of equilibrium are applied, assuming that the cracked section behaves like a plastic hinge, with a region of uniform tensile stress, σ*, acting above the neutral axis and a region of uniform compressive stress, — σ*, acting below the neutral axis; σ* is the average of the yield and ultimate stresses. Both experimental and fracture mechanics calculations have hither to shown that crack extension occurs at net-section stresses that are approximately the same as that used in the collapse analysis, when the crack is of the through-wall type, thus providing partial vindication of the net-section stress approach. The present paper extends this work and a fracture mechanics analysis shows that the net-section stress associated with the extension of a part-through full-circumference crack can be appreciably higher than that for a through-wall crack. The paper therefore provides further support for the usefulness of the simple net-section stress approach for predicting the failure of a stainless steel pipe containing a circumferential crack, since its predictions should be conservative in this situation.     

A first-principles study on the hydrogenation of acetone on HxMoO3 surface

Hydrogenation of acetone on the (010) surface of hydrogen molybdenum bronzes was investigated by density functional theory (DFT) calculations with periodic slab models. The formation of H-bond between the carbonyl oxygen of acetone and the terminal OH group of the surface leads to a stable adsorption of acetone. The effect of hydrogen concentration in the bronzes on the hydrogenation of acetone was systematically investigated, indicating the hydrogenation reaction is a one-step concerted and exothermic process regardless of the hydrogen contents in the bronzes surface. The 8H surface with increased H-content shows a significantly exothermic reaction process and exhibits the smallest kinetic barrier compared with 4H or 6H surfaces. Additionally, the selectivity for hydrogenation acetone could increase owing the absence of CC bond activation. The findings in this study can help with designing of high-efficient and low-cost metal oxide catalysts for hydrogenation of unsaturated substances.     

A parametric study of residual stresses in multi-pass butt-welded stainless steel pipes

Multipass circumferential butt-welding of stainless steel pipes is simulated numerically in a non-linear thermo-mechanical FE-analysis. In particular, the through-thickness variation at the weld and heat affected zone, of the axial and hoop stresses and their sensitivity to variation in weld parameters are studied. Recommendations are given for the through thickness variation of the axial and hoop stresses to be used when assessing the growth of surface flaws at circumferential butt welds in nuclear piping systems.     

A shock tube study of methyl decanoate autoignition at elevated pressures

A shock tube study of the autoignition of methyl decanoate, a candidate surrogate for biodiesel fuels containing large methyl esters, has been carried out. Ignition delay times were measured in reflected-shock-heated gases by monitoring electronically-excited OH chemiluminescence and pressure. Methyl decanoate/air mixtures were studied at equivalence ratios of 0.5, 1.0, and 1.5, at temperatures from 653 to 1336 K, and for pressures around 15–16 atm. The experimental results illustrate negative-temperature-coefficient behavior characteristic of alkanes, with ignition delay times very similar at high temperatures and somewhat longer at low temperatures than those for n-decane. Experimental results are compared to the kinetic modeling predictions of Herbinet et al. [Combust. Flame 154 (2008) 507–528] with remarkable agreement. Both reaction flux analysis and the comparison of experimental methyl decanoate and n-decane ignition delay times illustrate the importance of the long alkyl chain in controlling methyl decanoate overall reactivity and the subtle role the methyl ester group has on inhibiting low-temperature reactivity.     

Nucleate pool-boiling enhancement outside a horizontal bank of twisted tubes with machined porous surface

Nucleate pool boiling of refrigerants is of important application in the flooded evaporator of refrigeration and air-conditioning system. Many surface geometries involve machined porous surface have been adopted to enhance the nucleate pool boiling heat transfer of refrigerants. Nucleate pool-boiling performance of R134a and R142b outside a horizontal bank of twisted tubes with machined porous surface (T-MPS tubes) was investigated in this paper. The experimental results showed that the T-MPS tube bank could enhance boiling heat transfer evidently. The enhancement ratios of R134a from the T-MPS tube bank were 1.4–1.7 and the maximum enhancement ratio of R142b could reach up to 4.4. Analyzing the tube bank effects of boiling heat transfer for R134a and R142b, the overall trend showed that the boiling heat transfer performance of the T-MPS tube bank was inferior to that of single T-MPS tube slightly.     

A global limit load solution for plates with surface cracks under combined end force and cross-thickness bending

A global limit load solution for rectangular surface cracks in plates under combined end force and cross-thickness bending is derived, which allows any combination of positive/negative end force and positive/negative cross-thickness moment. The solution is based on the net-section plastic collapse concept and, therefore, gives limit load values based on the Tresca yielding criterion. Solutions for both cases with and without crack face contact are derived when whole or part of the crack is located in the compressive stress zone. From the solution, particular global limit load solutions for plates with extended surface cracks and through-thickness cracks under the same loading conditions are obtained. The solution is consistent with the limit load solution for surface cracks in plates under combined tension and positive bending due to Goodall & Webster and Lei when both the applied end force and bending moment are positive. The solution reduces to the limit load solution for plain plates under combined end force and cross-thickness bending when the crack vanishes.     

Effects of hydrophilic surface treatment on evaporation heat transfer at the outside wall of horizontal tubes

An experimental study has been conducted to investigate the effects of hydrophilic surface treatment on evaporation heat transfer at the outside wall of various kinds of copper tubes. Plain, spiral, corrugated, and low-finned tubes were selected as test tubes. In this work, to increase the wettability of distilled water on copper tubes, a novel hydrophilic surface treatment method using plasma was employed. The experiments show that every kind of hydrophilic surface treated tube tested in the work exhibits superior evaporation heat transfer performance as compared with that of the same kind of untreated tube. It is found out that during the evaporation process, the high wettability of the surface obtained through hydrophilic treatment induces film flow on the tubes while sessile drops are formed on untreated tubes. The film has a smaller thickness as well as a greater heat transfer area than the sessile drops, and this yields higher heat transfer rate for hydrophilic surface treated tubes than that for untreated tubes.     

Pool boiling heat transfer of water and nanofluid outside the surface with higher roughness and different wettability

In order to investigate the effect of surface wettability on the pool boiling heat transfer, nucleate pool boiling experiments were conducted with deionized water and silica based nanofluid. A higher surface roughness value in the range of 3.9 ~ 6.0μm was tested. The contact angle was from 4.7° to 153°, and heat flux was from 30kW/m² to 300kW/m². Experimental results showed that hydrophilicity diminish the boiling heat transfer of silica nanofluid on the surfaces with higher roughness. As the increment of nanofluid mass concentration from 0.025% to 0.1%, a further reduction of heat transfer coefficient was observed. For the super hydrophobic surface with higher roughness (contact angle 153.0°), boiling heat transfer was enhanced at heat flux less than 93 kW/m², and then the heat transfer degraded at higher heat flux.     

Numerical study on the cavitating flow in liquid hydrogen through elbow pipes with a simplified cavitation model

Cavitation is usually caused by the pressure difference between the static pressure and the saturated vapor pressure under the local temperature and may result in huge damage to the pipelines. This paper developed a simplified cavitation model based on Rayleigh–Plesset bubble equation and Zwart cavitation model, and conducted a series of numerical simulations with the process of phase change and latent heat added to the solver by UDFs (User Defined Functions). The aim of the paper is to study the affecting factors on the cavitation process of liquid hydrogen in elbow pipes. The results show that the thermal effect can suppress the occurrence and development of cavitation. As the process of cavitation goes on, the suppression of thermal effect is more remarkable. Before the cavitating flow reaches its steady state, cavitation process is very sensitive to the changes of inlet velocity and outlet pressure. Increasing the inlet velocity or decreasing the outlet pressure can both strengthen the cavitation process. The turbulent viscosity ratio has little effects on cavitation process of liquid hydrogen, but the increase of turbulent viscosity ratio can enhance the thermal effect and lower the temperature gradient in the cavity. In addition, the structure of the cavity is found to be related to the bend angles. The cavitation process is enhanced with the decrease of the angles since the duration of centrifugal force is longer.     

An experimental study on the effect of the flaring rate on flame dynamics in open pipes

Under the condition that the gas composition constant equivalence ratio is Φ = 1, and the initial temperature and initial pressure are T₀ and P₀, respectively, the experimental study of the premixed gas flames with different hydrogen doping ratios (φ = 10%–40%) is different. The behavior and shape change of propagation in the flaring rate pipe (? = 1.0–0.25). The study found that the pre-mixed gas flame in the flared pipe has undergone more complicated shape changes than other studies. One of the outstanding findings is that the tulip flame appeared twice in this open pipe experiment. And through the high-speed camera and high-frequency pressure sensor to obtain the tulip flame picture and the pressure change in the combustion chamber, comprehensive analysis of the experimental results, and the results show that every appearance of the tulip flame is accompanied by the deceleration of the flame front and the increase of overpressure in the combustion chamber.     

Pyrolysis and oxidation of decalin at elevated pressures: A shock-tube study

Ignition delay times and ethylene concentration time-histories were measured behind reflected shock waves during decalin oxidation and pyrolysis. Ignition delay measurements were conducted for gas-phase decalin/air mixtures over temperatures of 769–1202 K, pressures of 11.7–51.2 atm, and equivalence ratios of 0.5, 1.0, and 2.0. Negative-temperature-coefficient (NTC) behavior of decalin autoignition was observed, for the first time, at temperatures below 920 K. Current ignition delay data are in good agreement with past shock tube data in terms of pressure dependence but not equivalence ratio dependence. Ethylene mole fraction and fuel absorbance time-histories were acquired using laser absorption at 10.6 and 3.39 μm during decalin pyrolysis for mixtures of 2200–3586 ppm decalin/argon at pressures of 18.2–20.2 atm and temperatures of 1197–1511 K. Detailed comparisons of these ignition delay and species time-history data with predictions based on currently available decalin reaction mechanisms are presented, and preliminary suggestions for the adjustment of some key rate parameters are made.