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A new theoretical model of plastic behavior of circular metal tubes during the contraction process as a metal forming and energy absorber processes is presented in this study. Some theoretical relations are developed to predict the axial load versus the axial displacement of the contraction process, based on the theoretical model. Some quasi-static experiments were performed on circular aluminum and brazen tubes to validate the theoretical analysis. In this research, effects of tube radius, contraction ratio, tube wall thickness and angle of conical die on energy absorption characteristics of circular tubes under axial quasi-static loading are investigated by the experimental and theoretical methods. Results show that there is an admissible agreement between the theoretical results and experimental measurements. Experimental results show that there is a steady force during the end zone of the process which is an important factor in viewpoint of the energy absorption. Also, the steady force increases by increment of die semi-angle, tube radius, friction coefficient and the contraction ratio. [ABSTRACT FROM AUTHOR]

By emphasising on the axial compressive resistance of circular concrete-filled double-skin carbon steel short (CFDST) columns, this paper proposes a direct design that takes into account the confinement effect of both outer and inner tubes. First, available test results of axially-loaded CFDST short columns were collected from the literature. Then, the results obtained from available resistance models, including design specifications, were compared with the test results. The comparisons revealed that most design models are conservative, some being too conservative, and all of them do not own the same accuracy along the entire slenderness range of the circular tubes, a fact attributed to the absence of confinement effect provided by the inner tube. Accordingly, the test resistances were used to obtain the lateral pressure (f r p) due to the confinement effect provided not only by the outer tube (as in the previous studies) but rather by both outer and inner tubes of CFDST short columns with circular–circular and circular–square configurations. The calibrated formulae for f r p were used to propose a design resistance formula for axially-loaded CFDST short columns. The results showed that these formulae provide much better resistances than the available predictors for any tube slenderness and for columns manufactured from different grades of steel and concrete materials. • Test results of axially-loaded CFDST short columns were collected from the literature. • Available resistance models were found to be conservative compared to the test results. • Formulae for the lateral pressure provided by both outer and inner tubes of CFDST is proposed. • Confinement-Based Direct Design of CFDST short Columns is proposed. [ABSTRACT FROM AUTHOR]

In this paper, the deformation and energy absorption of thin-walled circular tubes in splitting and expansion–splitting modes are investigated by experiments and theory modeling. First, axial compression experiments of both steel and aluminum alloy tubes were conducted. The results show that the initial peak force in splitting mode could be reduced by increasing the number of initial cracks, and the steady force and the stroke efficiency of tubes could be greatly improved by introducing splitting deformation to the expansion mode in series. Afterwards, theoretical models for these two modes are developed by considering the stress coupling and stress equilibrium, which can well predict the experimental results. Based on the theoretical models, it is found that the initial peak force increases with the decrease of the number and the length of the initial cracks. However, when these two parameters are smaller enough, the initial peak force will not increase. In expansion–splitting mode, the splitting effect can weak the force induced by the expansion deformation, and there is a critical expansion die angle to make the steady force minimum. Moreover, the plastic dissipation is always larger than the tearing dissipation, while the friction dissipation can account for the largest proportion when the expansion die angle is small. • A modified model of tube in splitting mode is developed by considering the stress coupling and the stress equilibrium. • A novel theoretical model of tube in expansion–splitting​ mode is proposed. • Compared to the experiments, the theoretical models are accurate to predict the initial peak force and the steady force. • In expansion–splitting mode, the splitting effect can weak the compressive force induced by the expansion deformation. [ABSTRACT FROM AUTHOR]

• Heat transfer and flow behavior of mixed tube bundles were studied numerically. • Friction factor of mixed tube bundles is lower than that for the circular ones. • Case IV has a high ratio of N u ¯ f at Re=1780, which was 14.6 % greater than case II. • At Re>1780, case VII has the best N u ¯ f , which was 2.1 –4.5% greater than case II. • Correlations for the thermal and hydraulic performance were developed. Flow and heat transfer across six-row staggered mixed tube bundles composing of circular and drop-shaped tubes have been investigated numerically. The Reynolds number Re was within the range of 1.78 × 103–18.72 × 103. Six cases of the mixed tube bundle were discussed. The streamlines, velocity, turbulent kinetic energy, temperature, and static pressure contours for the studied cases were predicted using Ansys Fluent software package. The results of the mixed tube bundles were compared with those of the circular and drop-shaped ones. The best thermal performance was obtained for case IV (circular tubes in the 1st, 3rd, 5th rows, and drop-shaped tubes in the 2nd, 4th, 6th rows). However, it was found that case VII (drop-shaped tubes in the 1st, 2nd, 4th, 6th rows, and circular tubes in the 3rd, 4th rows) has the best hydraulic performance compared to the other cases of circular and mixed tube bundles. Besides, the average Nusselt number N u ¯ , friction factor f , and efficiency ε were observed to be positively correlated to the Reynolds number. As the Re increased, both N u ¯ and ε increased, while f decreased. Results showed that the maximum values of the thermal-hydraulic performance were achieved for case IV at Re = 1.78 × 103 and case VII at Re>1.78 × 103, which were about 14.55% and (2.08–4.49)%, respectively, higher than those obtained for the drop-shaped tube bundle. Finally, generalized correlations of N u ¯ , f , and ε (in terms of Re) were developed, offering a useful reference for the design of such heat exchangers. [ABSTRACT FROM AUTHOR]

The single deformation mode of splitting tube significantly restricts its energy absorption capability. To address this limitation, in this paper we propose a new hybrid absorber with an external die added to the outside of the expanding–splitting tube, thus the circular tube undergoes sequential expanding, splitting, and bending deformations. We fabricate samples and perform quasi-static compressive experiments on the splitting tube, expanding–splitting tube, and proposed hybrid splitting tube. Experimental results show that the hybrid tube deforms in a controlled and ordered way as designed. Remarkably, the steady crushing force of the hybrid tube is 52.1% and 221.6% higher in comparison to that of the same tube with splitting mode, and expanding–splitting mode, respectively. The crushing force responses and deformation processes are well captured by finite element simulations. The relative errors of the steady crushing forces and energy absorption are within 5.0%. A parametric analysis is performed. We show that the structure parameters play an important role in the crashworthiness performance. In particular, the tube with thicker walls has a larger crushing force and a higher specific energy absorption. The small cracking number and spacing between the inner and outer die can trigger unstable deformation. Our results can advance the engineering application of splitting tubes and the lightweight design of energy absorbers. [Display omitted] • A new hybrid energy absorber of expanding–splitting–bending​ circular tube is proposed. • Quasi-static experiments demonstrate the feasibility of the proposed new energy absorber. • Comparisons of experimental and FE results show excellent agreement. • Effect of structure parameter on crashworthiness performance is investigated by a parametric analysis. [ABSTRACT FROM AUTHOR]

Pada sebuah pembangkit listrik tenaga gas, sistem pelumasan turbin sangat diperlukan. Pelumas yang telah digunakan didinginkan kembali menggunakan lube oil cooler. Lube oil cooler merupakan compact heat exchanger tipe circular tubes, continuous fins yang berfungsi sebagai pendingin oli dengan udara sebagai fluida pendingin. Pada kondisi operasional didapatkan bahwa temperatur oli keluar lube oil cooler masih cukup tinggi. Hal ini dapat menyebabkan turbin gas shut down. Berdasarkan kondisi tersebut, maka dilakukan analisa performa lube oil cooler existing dan melakukan redesign untuk meningkatkan effectiveness dari lube oil cooler. Analisa performa lube oil cooler existing meliputi perpindahan panas actual dan effectiveness. Sedangkan redesign dilakukan dengan variasi laju aliran massa fluida dingin (udara) dan surface designation berdasarkan standard Compact heat exchangers untuk tipe circular tubes, continuous fins. Dengan batasan yang digunakan dalam perancangan lube oil cooler adalah volume ruang penempatan heat exchanger. Perancangan menggunakan metode LMTD dan NTU meliputi perhitungan perpindahan panas pada sisi tubes dan fins, area perpindahan panas, heat transfer actual, overall heat transfer coefficient serta effectiveness. Dari perhitungan yang telah dilakukan didapatkan effectiveness dari lube oil cooler existing adalah sebesar 13.6%. Berdasarkan analisa redesign, hasil yang memiliki performa paling baik adalah surface designation 8.0-3/8 T dengan laju aliran massa udara 7.5 kg/s dengan temperatur keluar oli sebesar 342.14 K, effectiveness 29%. Adapun detail dimensi redesign adalah jumlah tubes 245, diameter tube 0.0102 m, jumlah fins/ meter 315, transverse pitch 0.022 m dan longitudinal pitch sebesar 0.0254 m.

The effect of nanofluids on heat transfer inside circular tubes under uniform constant heat flux boundary condition was investigated. The working nanofluid was a suspension of γ-AlO nanoparticles of average diameter 20 nm. The heat transfer coefficients were calculated experimentally in the range of 1057 < Re < 2070 with different particle volume concentrations of 0.1%, 0.3% and 0.9%. Increasing the particle volume fraction led to enhancement of the convective heat transfer coefficient. The results show that the average heat transfer coefficient increased 16.8% at 0.9% volume concentration and Reynolds number of 2070 compared with distilled water. In addition, the enhancement of the convective heat transfer was particularly significant in the entrance region and decreased with axial distance. Finally, an empirical correlation for Nusselt number has been proposed for the present range of nanofluids. The mean deviation between the predicted Nusselt number and experimental values for the new correlation is 3.57%. [ABSTRACT FROM AUTHOR]

This work investigated the energy absorption behavior of expansion circular tubes. This kind of energy absorption device dissipates the impact of kinetic energy through plastic deformation and friction. A finite element model (FEM) was established with reference to the size of the coupler, which was used for connecting two vehicles. A special case was designed to validate the accuracy of the FEM. Furthermore, a theoretical prediction model that took into consideration the additional shear deformation and expansion ratio enlargement was compared with the steady compressional force of the experiment and numerical simulation. The theoretical prediction was accurate. Based on this valid FEM, a series of parameter studies was developed. Specifically, a list of conical mandrels with different angles from 5° to 40° was established, and the coefficient of friction varied from 0 to 0.3. Both had great influence on energy absorption. The relationship between the expansion angle and steady compressional force is nonlinear, while the friction coefficient and steady compressional force are linear. With friction, the inflection point of force curve existed between 10° and 20°. [ABSTRACT FROM AUTHOR]

Theoretical and numerical analyses are carried out to reveal the role of metal tube reinforcement in the enhancement of the energy absorption capacity of foam materials with different densities and strength. A theoretical model is proposed to estimate the strength increase of the reinforcement due to the confined tube, which buckles. Using this model, the contribution of the reinforcement is obtained to the mean quasi-static tube force and also to the total quasi-static average force of reinforced materials comprising different foam and reinforcing components. The proposed model is also used to estimate the efficiency of the reinforced foam materials in the quasi-static loading regime. FE simulations are carried out to verify the theoretical quasi-static model and to examine some essential dynamic effects due to the inertia of the foam and reinforcing tubes on the energy absorption of the material when subjected to impact loading. Attention is paid to the force enhancement at the impacted end and to the dynamic load transfer to the distal end of blocks of reinforced materials. [ABSTRACT FROM AUTHOR]

In this study, indentation of circular metal tubes during lateral compression process between a V-shape indenter and a rigid platen is investigated by the experimental and theoretical methods. A new analytical model of plastic deformation in the circular metal tubes during the process is introduced and the theoretical analysis is performed based on the energy method and several relations are derived to predict the average lateral load and absorbed energy by the tubes as functions of lateral displacement, geometrical characteristics, and material properties of the tube. Also, to verify the theoretical analysis, some lateral compression tests were performed on circular metal tubes and the results were compared with the corresponding theoretical predictions. The comparison shows a good agreement between the theoretical predictions and experimental results and so, it confirms general form of the present theoretical formula. Also, effects of each geometrical dimension and material properties of the tubes on the average lateral load are investigated using the experimental measurements and the theoretical predictions. [ABSTRACT FROM AUTHOR]

Computational Fluid Dynamics (CFD) analysis of turbulent convection heat transfer of NaK-78, liquid metal alloy in a uniformly heated circular tube are performed and the results are compared to reported experimental measurements. Also presented are the results for a tri-lobe channel, of the same heated length, equivalent hydraulic diameter, wall heat flux, liquid mass flux, setup and the inlet and exit bulk temperatures as in the circular tube experiments. The tri-lobe channel has 28% larger cross-sectional and surface areas than the circular tube. Examined are the effect of using the two-layer realizable k–ε, the Shear Stress Transport (SST) k–ω, and the SST Detached Eddy Simulation (DES) turbulence models on the computation time and the accuracy of the results. The calculated axial distribution of the wall temperature for the circular tube compares favorably to reported measurements at Pe = 163 and 796. This agreement confirms the soundness of the implemented methodology and the numerical meshing grid. The calculated and experimental Nu values for fully developed flows in the uniformly heated circular tube, covering a wide range of Pe values, are correlated to within ±10%, as: Nu = 5.6 + 0.013 Pe 0.863 Although the calculated Nu values using the different turbulence models are well within the ±10% of the experimental data, the computation time using the DES turbulence model is 13 and 14 time that using the k–ω and the k–ε model, respectively. Unlike for the circular tube, the local Nu in the tri-lobe channel varies azimuthally with the values in the corners are much lower than in the concavities of the tri-lobe channel. The calculated average values of Nusselt number, N ¯ u for the fully developed flows of liquid NaK-78 in the tri-lobe channel are correlated, to within ±5% as: N ¯ u = 6.6 + 0.007 Pe 0.94 For Pe < 100, the N ¯ u values for the tri-lobe channel are ∼15% higher than for the uniformly heated circular tube, but the difference gradually decreases with increasing Pe . [ABSTRACT FROM AUTHOR]

The elastic limit analysis for elliptical and circular tubes under lateral force is conducted in the present paper, and elastic-perfect plastic material model is employed. An analytical expression for describing the elastic limit load of elliptical and circular tubes under lateral force is obtained. It shows that the critical load for elliptical and circular tubes under lateral force increases with the plastic yielding strength of the tube material, the ratio of the wall thickness to radius, the wall thickness and the length of the tube, as well as the degree of deviation from circular tube monotonously. The experimental results from available literature for both steel and aluminum tubes are cited to verify the proposed expression, and it shows that the expression reflects the loading capacity of elliptical and circular tubes quite well, which indicates the proposed expression is a reasonable formula. [ABSTRACT FROM AUTHOR]

Computer simulations are becoming increasingly important for the optimisation of various processes since they can significantly reduce the development time and costs. The discrete element method (DEM) offers a powerful tool for simulating the interactions of large numbers of particles. The method has proven its usefulness for many applications where granular and spherical particles are handled. Very recently DEM, was used to simulate the interactions of tubular particles. While the existing linear elastic models could describe the reversible bending and denting of these particles in a realistic way for small deformations, they are inadequate for describing the bending and compression behaviour at higher forces. In this region, buckling phenomena become important. Therefore, new phenomenological models, inspired by the collapse of steel tubes due to ovalisation of the cross section and the formation of plastic hinges, have been implemented for metal and synthetic plastic tubes. Using these models virtual, bendable tubes were created in the DEMeter++ software. The DEM parameters were calibrated for copper and PVC tubes based on combined bending and denting tests conducted using an Universal Testing System. The simulations were in good agreement with the measurements ( R 2 > 0.91 ). This realistic DEM model of bending can, in future research, be used to simulate the processing of large numbers of tubular particles (e.g. the processing of crop stems). [ABSTRACT FROM AUTHOR]

6082 is a relatively new alloy that currently provides the best combination of properties in 6xxx series alloys. A series of tests was conducted on the stability of heat-treated aluminium alloy 6082-T6 circular tube columns to check the reliability of buckling strength predictions of 6082-T6 alloy circular tube columns using current design rules. First, nine stub columns were tested to obtain stress–strain curves and three parameters of the Ramberg–Osgood expression ( E , σ 0.2 and n ). The experimental stress–strain curves were in good agreement with the Ramberg–Osgood expression, and the mean value of σ 0.2 / n obtained from the tests was close to the Steinhardt assumption. Second, prior to column tests, the initial out-of-straightness of 15 circular tubes was accurately measured. Third, these 15 tubes, with five nominal slenderness ratios varying from 20.4 to 69.6, were tested between two pinned ends under axial compression to obtain failing modes and buckling strengths. Finally, the experimental buckling strengths were compared with the buckling strengths predicted by several current aluminium structure design codes, including the American Aluminium Design Manual (AA), Australian/New Zealand Standard 1664 (AS/NZS), and Eurocode 9 (EC9), as well as the general column curve formulation proposed by Rasmussen and Rondal [13] . These comparisons shows that the AA predictions are too conservative at small slenderness ratios, the AS/NZS predictions are unsafe at large slenderness ratios, the EC9 predictions are conservative, and the Rasmussen–Rondal formulation provides the closest and generally conservative strength predictions. [ABSTRACT FROM AUTHOR]

Despite being characterised by a reduction in compressive, tensile and flexural strengths when compared to standard concrete, rubberised concrete (RuC) is being used as a way to conserve natural resources and reduce the amount of tires entering landfills. An excellent solution to limit the RuC's compressive strength drop is to incorporate this type of concrete into steel tubes. However, up-to-date there is a lack of lateral confining pressure formulae for square and circular rubberised concrete-filled steel tubular (RuCFST) columns and rubberised concrete-filled double-skin steel tubular (RuCFDST) columns. Hence, the main aim of this technical note is to provide new design formulae based on lateral confining pressure and calibrated by means of test results available in the literature. These formulae are then used to suggest more accurate design strength models compared to those obtained from European, American and Australian specifications. [Display omitted] • Lateral confining pressure of steel tubes for rubberised concrete is investigated. • Square and circular RuCFST and RuCFDST columns are considered. • New lateral confining pressure formulae are proposed. • More accurate design strength models compared to those of EC4, AISC and AS2327 are proposed. [ABSTRACT FROM AUTHOR]

This paper researches a type of absorber of kinetic collision energy that works on the principle of shrinking and splitting a tube of circular cross section. During collision, a seamless tube is thrust through a cone bush, squeezing the tube. Energy is absorbed by the plastic deformation, and through the friction between the tube and the bush. After passing through the cone bush, the tube presses against a splitter, and further energy is lost to friction and plastic deformation during the splitting process. Grooves on the inner wall of the tube prevent uncontrolled longitudinal tearing of the wall during the splitting. This new combined method of energy absorption enables greater absorption power with compact dimensions. Scaled samples have been tested in the laboratory. The influence of geometry and manufacturing technology of the samples, as well as the benefits of using such an absorber, are presented and discussed in this paper. The results show that the combined absorber has approximately 60% higher absorption power than the shrinking absorber by itself. [ABSTRACT FROM AUTHOR]

The fountain flow of a viscous liquid during the filling of a circular tube has been studied. The mathematical basis of the considered process is formed by motion, continuity, and energy equations with corresponding initial and boundary conditions with allowance for temperature-dependent viscosity, free boundaries, and the dissipation of mechanical energy. The finite-difference method is used for the numerical solution of the problem. Some resullts of the parametric studies of the kinematic characteristics of a flow depending on the determinative dimensionless parameters are given. The flow patterns for the formulation of problems with different boundary and initial conditions are shown. The temperature and velocity distributions in different tube cross sections and the shape characteristics of the free surface are given. The separation of the flow into a three-dimensional flow zone near the free surface and a one-dimensional flow zone far away from it and the shape transformation of the free boundary depending on the level of dissipative heating are demonstrated. [ABSTRACT FROM AUTHOR]

Laminar convective heat transfer of water-alumina nanofluid in a circular tube with uniform heat flux is investigated numerically on the basis of two-component model, which takes into account nanoparticle transport by diffusion and thermophoresis. A new expression for thermophoretic mobility is suggested on the basis of existing experimental results and theoretical concepts. It is shown that thermophoresis leads to a significant reduction of nanoparticle volume fraction in the boundary layer near the wall. The corresponding viscosity reduction causes the velocity increase near the wall and flattening of velocity profile near the tube axis to keep the mass flow rate constant. The decrease of wall shear stress leads to the decrease of the required pressure drop. The calculations for two-component model provide higher values of the local and average heat transfer coefficients in comparison with the one-component model. The difference does not exceed 10% and decreases with increasing the thermal Peclet number. The calculations for one-component model show the independence of local and average Nusselt numbers on the nanoparticle volume fraction. The results for two-component model predict the increase of Nusselt number when the thermophoretic effect becomes stronger. The effectiveness of water-alumina nanofluid is analyzed by plotting the average heat transfer coefficient against the required pumping power. It is shown that the nanofluid shows better performance than the base fluid in the range of low pumping power and, correspondingly, low inlet velocity. [ABSTRACT FROM AUTHOR]

Turbulent convective heat transfer coefficients of 9 wt% Al2O3/water and TiO2/water nanofluids inside a circular tube were investigated independently at the Royal Institute of Technology, KTH (Sweden) and at University of Birmingham (UK). The experimental data from both laboratories agreed very well and clearly show that Nusselt numbers are well correlated by the equations developed for single phase fluids with the thermophysical properties of nanofluid. The heat transfer coefficients of nanofluids can be compared with those of the base fluids at the same Reynolds number or at the same pumping power. As the same Reynolds number requires higher flow rate of nanofluids therefore such comparison shows up to 15% increase in heat transfer coefficient. However, at equal pumping power, the heat transfer coefficient of Al2O3 nanofluid was practically the same as that of water while that of TiO2 was about 10% lower. Comparing performance at equal Reynolds number is clearly misleading since the heat transfer coefficient can always be increased by increased pumping power, accordingly, the comparison between the fluids should be done at equal pumping power. [ABSTRACT FROM AUTHOR]

Highlights: [•] Molten salt convection in circular tube with nonuniform heat flux was measured. [•] Reynolds number, Prandtl number and heat flux ratio were further considered. [•] Nusselt number difference on different sides increased with the heat flux ratio. [•] Average Nusselt number had little relation with nonuniform heat flux. [•] Experimental correlation very well fit with experiments and classical correlations. [ABSTRACT FROM AUTHOR]

Abstract: Multi-cell columns are highly efficient energy absorbing components under axial compression. However, the experimental investigations and theoretical analyses for the deformation modes and mechanisms of them are quite few. In this paper, the axial crushing of circular multi-cell columns are studied experimentally, numerically and theoretically. Circular multi-cell columns with different sections are axially compressed quasi-statically and numerical analyses are carried out by nonlinear finite element code LS-DYNA to simulate the experiments. The deformation modes of the multi-cell columns are described and the energy absorption properties of them are compared with those of simple circular tube. Theoretical models based on the constituent element method are then proposed to predict the crush resistance of circular multi-cell specimens. The theoretical predictions are found to be in a good agreement with the experimental and numerical results. [Copyright &y& Elsevier]

The characteristics of the longitudinal vortex induced by trapezoid-winglets in a circular tube are investigated by the Particle Image Velocimetry (PIV) Technique with flow Reynolds number in the range of 500-13 000. In the experimental test section, four trapezoid-winglets are fixed symmetrically on the tube wall in two different ways: up-flow and down-flow. The results show that a counter-rotating vortex pair is formed behind each winglet and they distribute as a symmetrical vortex array in the transverse section. Between the two vortexes in a vortex pair the fluid flows towards the wall in the up-flow winglet case and away from the wall in the down-flow winglet case, corresponding also to the regions of peak values of the velocity components normal to the mainstream. Both of the flow patterns enhance the velocity in the near wall region, leading to the intensification of the transverse mixing and the mass transfer in the tube. With Reynolds number increasing, the flow maintains the vortex pattern in the case of the up-flow winglets, indicating better persistence of the longitudinal vortex, while the vortexes in the case of the down-flow winglets are more scattered and tend to breaking into small eddies. The trapezoid winglet shows the preferable turbulent disturbance characteristics in the tube and the experimental results provide benchmark data for further CFD studies. [ABSTRACT FROM AUTHOR]

This paper demonstrates the quasi-static axial compression and high-speed axial compression tests of extruded magnesium alloy circular tubes for evaluating the crash and fracture behavior of mg parts. To capture the buckling and fracture behavior of Mg tube during the axial compression tests, FE simulation adopts different types of flow curves depending on the deformation mode such as tension and compression with LS-DYNA software. The Mg tube undergoes compressive plastic strain prior to buckling while according to the model based on Hill yield criterion only bulging along the radial direction is predicted. Due to the tension-compression asymmetry of Mg alloys, diameter of Mg tube expands largely at the initial plastic strain before having bulging or folding while only a bulging mode typical for materials with cubic crystal structure can be predicted. Simulation results such as punch load and deformation mode are compared with experimental results in the axial crushing test with AZ61 alloy. [ABSTRACT FROM AUTHOR]

Highlights: [•] The effects of the volume fraction and the Reynolds number on heat transfer and pressure drop are determined. [•] The heat transfer increased with an increase in volume concentration. [•] The highest heat transfer enhancement was achieved at Re =8000 and ф =0.005. [•] The friction factor increased with the increase of the particle volume fraction. [•] A correlation based on the Reynolds number, Prandtl number and the volume fraction is developed. [Copyright &y& Elsevier]

Nano fluid is considered to be a class of high efficient heat transfer fluid created by dispersing some special solid nanoparticles (normally less than 100 nm) in traditional heat transfer fluid. The present experiment was conducted aiming at investigating the forced heat transfer characteristics of aqueous copper (Cu) nanofluid at varying concentration of Cu nano-particles in different flow regimes (300< Re⩽16 000). The forced convective heat transfer enhancement is available both in the laminar and turbulent flow with increasing the concentration. Especially, the enhancement rate increases dramatically in laminar flow regime, for instance, the heat transfer coefficient of Cu/water nanofluid increases by two times at around Re=2 000 compared with that of base fluid water, and averagely increases by 62% at 1% volume fraction. However, the heat transfer coefficient of Cu/water decreases sharply in the transition flow regime. Furthermore, it has the trend that the heat transfer coefficient displays worse with increasing the concentration. [ABSTRACT FROM AUTHOR]

Abstract: A parametric study on the axial cutting deformation of circular AA6061-T6 extrusions was completed under both dynamic and quasi-static loading conditions. Parameters which would influence the response of the cutting deformation mode, including tube diameter, tube wall thickness, number of cutter blades, and loading conditions, were investigated in the experimental tests. For the impact tests completed within this research the peak cutting load and the mean cutting force ranged from 14.45 kN to 31.31 kN and from 8.28 kN to 20.56 kN, respectively. The quasi-static peak cutting load and the mean cutting force ranged from 9.07 kN to 26.93 kN and from 7.06 kN to 24.92 kN, respectively. Generally, the axial cutting force increased with the increase of the number of cutter blades, the extrusion diameter, and the extrusion wall thickness. Comparison of the experimental results to the theoretical predictions using an analytical model developed by the authors showed that the predicted steady-state axial cutting forces agreed well with the experimental results. [Copyright &y& Elsevier]

Abstract: Complexation of dextran with the active sites of dextransucrase was used to produce dextran on a tube in a laminar flow. A dextransucrase solution was passed through a Teflon tube at flow rates of 10 and 50mL/h to immobilize the enzyme on the inner surface of the tube, and then sucrose solution was passed through the enzyme-immobilized tube to form giant dextran particles. The Reynolds number during the procedure was 4 or 20. The distribution of dextran along the flow direction in the tube was determined colorimetrically and showed that the dextran formed was distributed such that the amounts of dextran were higher near the ends than in the middle section of the tube. Direct observation of the dextran after colorization with phloxine showed that giant dextran particles, up to approximately 20μm in size, were formed. The distribution of dextran produced by the enzymatic reaction in a laminar flow was determined quantitatively. [Copyright &y& Elsevier]

Abstract: In this article, the multi-objective optimization of cylindrical aluminum tubes under axial impact load is presented. Absorbed energy and specific absorbed energy are considered as objective functions while the mean crush load should not exceed allowable limit. The geometric dimensions of tubes including diameter, length and thickness are chosen as design variables. The Non-dominated Sorting Genetic Algorithm –II (NSGAII) is applied to obtain the Pareto optimal solutions. A back-propagation neural network (ANN) is constructed as the surrogate model to formulate the mapping between the variables and the objectives. The finite element software ABAQUS/Explicit is used to generate the training and test sets for the ANNs. Validating the results of finite element model, several impact tests are carried out using drop hammer. [Copyright &y& Elsevier]

Abstract: A numerical study of the crushing of thin-walled circular aluminum tubes has been carried out to investigate their behaviors under axial impact loading. These kinds of tubes are usually used in automobile and train structures to absorb the impact energy. A Multi-Objective Optimization of circular aluminum tubes undergoing axial compressive loading for vehicle crash energy absorption is performed for five crushing parameters using the weighted summation method. To improve the accuracy of the optimization process, artificial neural networks are used to reproduce the behavior of the crushing parameters in crush dynamics conditions. An explicit finite element method (FEM) is used to model and analyzed the behavior. A series of aluminum cylindrical tubes are simulated under axial impact condition for the experimental validation of the numerical solutions. A finite element code, capable of evaluating parameters crush, is prepared of which the outputs are used for training and testing the developed neural networks. In order to find the optimal solution, a genetic algorithm is implemented. With the purpose of illustrating optimum dimensional ratios, numerical results are presented for thin-walled circular aluminum AA6060-T5 and AA6060-T4 tubes. Multi-Objective Optimization of circular aluminum tubes has been performed in the basis of different priorities to create the ability for designer to select the optimum dimension ratio. Also, crush parameters of two aluminum alloys has been compared. [Copyright &y& Elsevier]

Abstract: The oscillating flow and temperature field in an open tube subjected to cryogenic temperature at the cold end and ambient temperature at the hot end is studied numerically. The flow is driven by a time-wise sinusoidally varying pressure at the cold end. The conjugate problem takes into account the interaction of oscillatory flow with the heat conduction in the tube wall. The full set of compressible flow equations with axisymmetry assumption are solved with a pressure correction algorithm. Parametric studies are conducted with frequencies of 5–15Hz, with one end maintained at 100K and other end at 300K. The flow and temperature distributions and the cooldown characteristics are obtained. The frequency and pressure amplitude have negligible effect on the time averaged Nusselt number. Pressure amplitude is an important factor determining the enthalpy flow through the solid wall. The frequency of operation has considerable effect on penetration of temperature into the tube. The density variation has strong influence on property profiles during cooldown. The present study is expected to be of interest in applications such as pulse tube refrigerators and other cryocoolers, where oscillatory flows occur in open tubes. [Copyright &y& Elsevier]

Abstract: The paper investigates collapse mechanisms and energy absorption capacity during the axial compression of the end-capped thin-walled circular aluminum tubes which are hollow or filled with polyurethane foam. An experimental technique is used to evaluate the crushing behavior of the circular tubes under compressive quasi-static strain rate. A numerical model is presented based on finite element analysis to simulate the crushing of circular tubes considering nonlinear response due to material behavior, contact boundary conditions and large deformation. The validated model using existing experimental results is used to evaluate the dynamic response in order to determine the dynamic amplification factor relating the quasi-static results to dynamic response. The experimental and numerical results are used to determine energy absorption capacity due to the plastic deformation of thin-wall tube and crushable foam. The performance of end-capped tubes is compared with non-capped tubes and it is found that maximum initial peak load can be controlled and convenient crash protection systems can be obtained using end-capped circular tubes. [Copyright &y& Elsevier]

Abstract: This paper presents the results of a global comparison between high strength steel and normal steel circular tube used to build steel and composite columns submitted to static loading, as regards the economic aspects. The comparison is based on an optimum design taking into account the strength, stability and stiffness conditions of Eurocode 3 and 4. The automatic implementation of the algorithms allows achieving a high amount of case studies, covering the realistic possibilities of building columns. The investigations are realized on simple columns, columns included in braced or un-braced frames and whole frames. The field of application of high strength steel (vs normal steel), regarding the total cost of the member, is provided in a chart clearly indicating where the use of high strength steel becomes economic. [Copyright &y& Elsevier]

Abstract: An analytical model for the steady-state axial cutting of circular tubes by a cutter with multiple blunt blades and without/with the presence of a curved surface profile deflector was developed. Experimental observations have indicated that there exist six different dissipation mechanisms. These include the outward bending of cut sidewalls, the formation of cylindrical flaps in the transient and stable cutting regions, circumferential membrane stretching, chip formation and friction. In the analytical model development, rigid-perfectly plastic material behaviour obeying the von Mises yield criterion was employed to determine the steady-state stresses and fully plastic bending moments for each deformation zone which were assumed to be uncoupled to each other. The effect of the friction force was included to the proposed solution and the total axial cutting force was determined through use of the principle of virtual power. The proposed analytical model was validated by comparing the predicted cutting force to experimental data and the effects of tube wall thickness, number of cutter blades, and extrusion diameter were investigated. A good correlation was found between the theoretical predictions and experimental observations. [Copyright &y& Elsevier]

Abstract: Laminar convective heat transfer of nanofluids in a circular tube under constant wall temperature condition is studied numerically using a CFD 1 [1] Computational Fluid Dynamics. approach. Single-phase and two-phase models have been used for prediction of temperature, flow field, and calculation of heat transfer coefficient. Effects of some important parameters such as nanoparticle sources, nanoparticle volume fraction and nanofluid Peclet number on heat transfer rate have been investigated. The results of CFD simulation based on two-phase model were used for comparison with single-phase model, theoretical models and experimental data. Results have shown that heat transfer coefficient clearly increases with an increase in particle concentration. Also the heat transfer enhancement increases with Peclet number. Two-phase model shows better agreement with experimental measurements. For Cu/Water nanofluid with 0.2% concentration, the average relative error between experimental data and CFD results based on single-phase model was 16% while for two-phase model was 8%. Based on the results of the simulation it was concluded that the two-phase approach gives better predictions for heat transfer rate compared to the single-phase model. [Copyright &y& Elsevier]

Axial crash of thin-walled circular aluminum tubes is investigated in this study. These kinds of tubes usually are used in automobile and train structures to absorb the impact energy. An explicit finite element method (FEM) is used to model and analyse the behaviour. Formulation of the energy absorption and the mean crash force in the range of variables is presented using design of experiments (DOE) and response surface method (RSM). Comparison with experimental tests has been accomplished in some results for validation. Also, comparison with the analytical aspect of this problem has been done. Mean crash force has been considered as a constraint as its value is directly related to the crash severity and occupant injury. The results show that the triggering causes a decrease in the maximum force level during crash. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a study of the effectiveness of adding a buckling initiator which is used to reduce the initial peak force of a thin-walled circular tube under axial impact loadings. The buckling initiator is installed near the impact end of the circular tube and is composed of a pre-hit column along the axis of the tube and several pulling strips uniformly distributed around the top edge of the tube. This device functions just before the impact happens and does not affect the structural stiffness under its normal working conditions. By using two kinds of aluminum-alloy circular tubes, a series of quasi-static compression tests were conducted. The deformation mode, the initial peak force and the mean crushing force of the tubes with different number of pulling strips N, pre-hit height h and inclined angle of the pulling strips θ 0 were studied in the experiments. The results reveal that by using this buckling initiator, the large progressive deformation of the axially crushed circular tube switches from ring mode or mixed mode to diamond mode. Although specimens with N=2, 3 and 4 were tested, the stable deformation tended to diamond mode with lobe number N=3. With suitable selection of pre-hit height h, the initial peak force could be reduced by more than 30%. In addition, a simplified theoretical analysis is conducted to illustrate the reduction of the initial force as well as the energy dissipation mechanisms, leading to good agreements with the experimental results. [Copyright &y& Elsevier]

Abstract: Circular tubes have been widely used as structural members in many engineering applications. Therefore, its collapse behavior has been studied for many decades, focusing on its energy absorption characteristics and collapse mechanism. In order to predict the collapse behavior of members, one could rely on the use of finite element codes or experiments. These tools are helpful and have high accuracy but are costly and require extensive running time. Therefore, an approximate model of tubes collapse mechanism is an alternative especially for the early step of design. This paper is also aimed to develop a closed-form solution to predict the moment–rotation response of circular tube subjected to pure bending. The model was derived based on the principle of energy rate conservation. The collapse mechanism was divided into three phases. New analytical model of ovalisation plateau in phase 2 was derived to determine the ultimate moment. In phase 3, the Elchalakani et al. model [Int. J. Mech. Sci. 2002; 44:1117–1143] was developed to include the rate of energy dissipation on rolling hinge in the circumferential direction. The 3-D geometrical collapse mechanism was analyzed by adding the oblique hinge lines along the longitudinal tube within the length of the plastically deformed zone. Then, the rates of internal energy dissipation were calculated for each of the hinge lines which were defined in terms of velocity field. Inextensional deformation and perfect plastic material behavior were assumed in the derivation of deformation energy rate. In order to compare, the experiment was conducted with a number of tubes having various D/t ratios. Good agreement was found between the theoretical prediction and experimental results. [Copyright &y& Elsevier]

A transverse magnetic field (TMF) perpendicular to the flow direction can control the magnetogasdynamic (MGD) flow and heat transfer characteristics in circular tubes, which has broad application prospects in many fields. However, the influence law of magnetic field on the MGD flow and heat transfer characteristics are not clear. Considering the Joule heating effect and the transition gradient of the magnetic field, this study utilized numerical calculations to analyze the variations of parameters including the current, electromagnetic force, and Joule heat in a partially magnetized circular tube and reveal the influence law and regulation mechanism of an applied TMF on flow and heat transfer characteristics. The results show that the flow and heat transfer of the Hartmann and Roberts layers exhibit anisotropy, which becomes prominent as the Hartmann number (Ha) increases. The heat transfer exhibits suppression resulting from the effect of the magnetic field on turbulence in the MGD flow. Moreover, the average Nusselt number (Nu) in the magnetized region increases initially and then decreases as Ha increases. The varying gradient of the magnetic field causes high-velocity jets near the Roberts layer and generates a large amount of Joule heat, thus enhancing the heat transfer in the magnetic field transition region. • Effects of an applied TMF on MGD flow and heat transfer characteristics are analyzed. • Numerical simulation showed anisotropic behaviors near Hartmann and Roberts layers. • Within a certain Hartmann number range, TMF has a heat-transfer-inhibiting effect. • Joule heat and turbulence suppression combined regulate the convective heat transfer. • High velocity flow and heat transfer enhancement appear in the magnetic field transition region. [ABSTRACT FROM AUTHOR]

Abstract: This investigation develops a numerical model of laminar film condensation from a downward-flowing steam-air mixture onto a horizontal circular tube. The significant non-similarity of the coupled two-phase flow laminar film condensation is such that the boundary layer governing conservations of momentum, species and energy in the mixture and liquid phases are solved by finite-volume methods. Numerical analysis of both the local condensate film thickness and heat transfer characteristics elucidated the simultaneous effects of inlet-to-wall temperature difference and inlet air concentration, the Reynolds number of the mixture, and the dimensionless parameter, F, by adopting a unified condensate parameter. The local Nusselt number and liquid film thickness increase as both the non-condensable air mass fraction and the tube temperature decreases. The numerical results on local heat transfer and film thickness for low/no non-condensable gas (air) agree closely with the theoretical results of Yang [Sheng-An Yang, Superheated laminar film condensation on a nonisothermal horizontal tube, J. Thermophys. Heat Transfer 11(4) (1997) 526–532], Fuji [T. Fujii, H. Uehare, C. Kurata, Laminar filmwise condensation of flowing vapor on a horizontal tube, Int. J. Heat Mass Transfer 15 (1972) 235–246] and Homescu and Panday [D. Homescu, P.K. Panday, Forced convection condensation on a horizontal tube: Influence of turbulence in the vapor and liquid phases, J. Heat Transfer Trans. ASME 121(4) (1999) 874–885]. Meanwhile, the comparison of average heat transfer coefficient with the experimental data from Rose and Lee [W.C. Lee, J.W. Rose, Forced convection film condensation on a horizontal tube with and without non-condensing gases, Int. J. Heat Mass Transfer 27(4) (1984) 519–528] demonstrates reasonably good agreement with the parameter F (, the ratio of gravity to mixture velocity) and G (=, the suction effect). [Copyright &y& Elsevier]

The solution of the problem of hydrostatic compression of an elastomeric tube is based on the nonlinear theory of elasticity for incompressible materials employing the method of Ritz variations and the ANSYS finite-element solutions. The main purpose of the investigations is to obtain the loading diagrams in the total range of elastomeric tube thicknesses, without any kinematical and physical restrictions on the behavior of elastomer, e.g., neglecting the Kirchhoff and Timoshenko hypotheses or Hooke's law. [ABSTRACT FROM AUTHOR]

Abstract: A new type of structure, called retractable tube, is introduced in the present work to overcome the drawbacks of two traditional types of invertubes. The inversion processes of the proposed tubes under axial compression are simulated by using the non-linear finite element code LS-DYNA and the features of the force–displacement curves and the deformation modes of the proposed tubes are analyzed. A comparative study is conducted to compare the energy absorption characteristics of the new proposed tubes with the plain circular tubes based on the performance indices used most commonly for energy-absorbing devices. The results show that the whole efficiencies of the new proposed tubes are significantly higher than those of the corresponding plain circular tubes. In addition, a parametric study is carried out to investigate the effects of geometric parameters on the behavior of retractable tubes. [Copyright &y& Elsevier]

Abstract: In the study of impact attenuation devices, rings/tubes have received a large amount of research due to their adaptability, i.e. they are low in cost and are readily available for selection in the design process. They also exhibit desirable force-deflection responses which is important in the design of energy absorbing devices. The function of such a device is to bring a moving mass to a controlled stop and ideally cause the occupant ride down deceleration to be within acceptable limits so as to avoid injuries or to protect delicate structures. An energy absorbing device may consist of a system of rings/tubes that are compressed laterally which absorbs the kinetic energy upon impact and dissipating it as plastic work. In this work, the quasi-static analysis of nested circular and elliptical (non-circular) type energy absorbers is examined using experimental and numerical techniques. Although these devices are usually exposed to much higher velocities, it is common to analyse the quasi-static response first, since the same pre-dominant geometrical effects will also occur under dynamic loading conditions. Nested systems consist of circular tubes or elliptical shaped tubes of different diameters which are placed within each other and their axes being parallel. Compression of these systems is achieved via two rigid flat platens, one placed above and below the absorber to be analysed. Discussion is made on how the circular tubes were formed into elliptical shapes and how such a modified shape can exhibit greater crush efficiencies than their circular shaped counterparts. A numerical technique via the finite element method is used to simulate the loading and response of such devices and hence, comparison of numerical and experimental force-deflection response is presented. [Copyright &y& Elsevier]

Abstract: Peristaltic motion of an incompressible non-Newtonian fluid in a deformable tube is studied under long wavelength and low Reynolds number. The effect of magnetic field on the motion of a Johnson–Segalman fluid has been modelled. Analytical solutions are obtained for the stream function, axial velocity and axial pressure gradient under the small Weissenberg number. The pressure rise and frictional force per wavelength have been computed through numerical integration. The study shows that, in addition to Weissenberg number and the effect of magnetic field, the occlusion and volume flux also affect the peristaltic flow. These effects are noticeable in the stream function, axial velocity, axial pressure gradient, the pressure rise and the frictional force. Graphs are plotted for axial pressure gradient, pressure rise and frictional force and discussed in detail. [Copyright &y& Elsevier]