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The thermal-hydraulic performance of plain tubes with and without wire coils in turbulent regimes is investigated experimentally and numerically. The effects of wire coil distribution (circular cross section) within the tube were explored experimentally, and water was employed as the working fluid. The numerical simulation was carried out using software programmer ANSYS Fluent 2019 R3 using the finite-volume approach. In the turbulent regime, six cross-sectioned wire coils were analyzed, including: circular, rectangular, hexagonal, square, star shape, and triangle. The utilization of a tube with a wire coil has been shown to increase heat transfer rate and pump consumption. The results indicate a high level of concurrence, as the deviations are all below 8%. Compared with plain tube, the wire coils, according to the arrangement (TWD), gave the best PEC. The heat transfer enhancement ability of different cross sections follows the following order: StCS > RCS > HCS > SqCS > CCS > TCS. Also, the sequence of pump consumption for each cross section is as follows: RCS > StCS > SqCS > HCS > CCS > TCS. [ABSTRACT FROM AUTHOR]

This study evaluates the effect of a Rectangular Vortex Generator (RWVG) on the thermo-hydraulic performance of tubular heat exchangers for the flow Reynolds number range of 6000-2000). The geometric parameters, such as the flow attack angle (22.5°-135°) and normalized height (h/D=0.0392-0.3137), are varied for a fixed normalized width of 0.137 and longitudinal pitch distance of 60mm. Three-dimensional Computational Fluid Dynamics methodology is used to carry out the analysis. The results show that the presence of a rectangular vortex generator produces a pair of longitudinal vortices for a lower attack angle of flow and larger normalized height. The highest enhancement in Nusselt number is about 2.03 for a flow attack angle of 22.5° for a given h/D=0.0784 and b/D=0.137. The highest enhancement in friction factor is about 9.82 for h/D=0.3137 and b/D=0.137. The highest thermal enhancement factor ranges from 1.4-1.16 for α=22.5° for a given h/D=0.0784 and b/D=0.137. [ABSTRACT FROM AUTHOR]

This article presents turbulent flow patterns and thermal behaviors in a circular tube mounted with 30° multiple V–baffles. The influences of V–baffle numbers (N) of 2, 3 and 4 with pitch ratios (PR) of 1.0, 1.5 and 2.0 for Reynolds number (Re) between 3000 and 20,000 were investigated numerically at a fixed blockage ratio, BR= 0.05. These simulated results were proposed in four sections: numerical validations, flow and thermal characteristics and thermal performance. It was visible that the multiple V–baffles can help induce the impinging jet leading to disrupting the boundary layer, increasing the fluid mixing, and then enhancing the heat transfer above the plain tube alone. The rises of the number of V–baffles and the lower baffle pitch length led to raising the heat transfer and friction loss. The maximum thermal performance was obtained around 2.06 at N= 4, PR= 1.0 and lower Re.

In this research, the effects of different channel cross-section shapes on the flow, heat transfer and pyrolysis characteristics of n-decane were analyzed and compared based on CFD simulations. The interactions between cracking, heat transfer and flow field in a circular tube and a rectangular tube were studied. The results showed that the mean pressure drop in the rectangular channel is 1.18 times as high as that in the circular channel with pyrolysis due to its smaller equivalent diameter. The maximum value of the chemical heat sink in the rectangular channel is 1.6 times as high as that in the circular channel. The high temperature zone of any cross section in the rectangular channel is much larger than that in the circular channel due to the superposition of the boundary layer and lower turbulent kinetic energy in the corners of the rectangular channel. The maximum value of the Nu in the circular channel is 1.3 times as high as that in the rectangular channel with pyrolysis due to larger heat capacity, lower viscosity and higher wall shear stress. [ABSTRACT FROM AUTHOR]

The automobile industry is focused on optimizing the structure of vehicles to enhance crashworthiness and energy absorption capabilities. This optimization involves utilizing highly nonlinear computational analysis and design techniques that consider various materials and structural characteristics. The objective of this research has been to propose a crashworthiness design for the circular side door beam that connected to side door structures. In order to address the complex issue of crashworthiness design optimization, the Non-dominated Sorting Genetic Algorithm II (NSGA-II) has been employed. Additionally, the Radial Basic Function (RBF) algorithm has been used to create surrogate models, which has improved the effectiveness of optimization process. The analysis has focused on two side door beam by adjusting their crosssectional shape and material properties in order to enhance their energy-absorbing performance. In order to determine the maximum energy absorbed, the optimization problem has been formulated by considering the maximum peak load as a constraint, as well as beam cross-section, thickness, and density as variables for achieving the maximum specific energy absorption. The Finite Element Analysis (FEA) has been also utilized in order to achieve the optimal design and conduct a simulation analysis of two side door beam in an automobile. [ABSTRACT FROM AUTHOR]

This article presents turbulent flow patterns and thermal behaviors in a circular tube mounted with 30° multiple V-baffles. The influences of V-baffle numbers (N) of 2, 3 and 4 with pitch ratios (PR) of 1.0, 1.5 and 2.0 for Reynolds number (Re) between 3000 and 20,000 were investigated numerically at a fixed blockage ratio, BR = 0.05. These simulated results were proposed in four sections: numerical validations, flow and thermal characteristics and thermal performance. It was visible that the multiple V-baffles can help induce the impinging jet leading to disrupting the boundary layer, increasing the fluid mixing, and then enhancing the heat transfer above the plain tube alone. The rises of the number of V-baffles and the lower baffle pitch length led to raising the heat transfer and friction loss. The maximum thermal performance was obtained around 2.06 at N = 4, PR = 1.0 and lower Re. [ABSTRACT FROM AUTHOR]

Based on the two-arc profile assumption, the expansion deformation and energy absorption of circular tubes compressed by conical-cylindrical dies were reconsidered. First, the deformation of the two arcs was analyzed independently and an improved model denoted as Model-I was established. Then, by further involving the coupling between the bending moment and membrane forces, a more elaborate model, i.e., Model-II was developed. Afterwards, experiments and simulations were conducted to verify the models, which show that, compared with previous theoretical models, Model-II could not only capture the prominent features of the deformation, but also improve the prediction accuracy of the steady driving force significantly. By means of this model, it was found that the critical semi-conical angle, which makes the driving force minimum, increases with the increase of the friction coefficient, expansion ratio as well as the radius/thickness ratio of the tube. And, the energy dissipation due to stretching is always greater than that of bending, while the friction dissipation can account for the largest proportion at small semi-conical angle or large friction coefficient. At a certain friction and die conditions, the specific energy absorption of expanded tubes can be much higher than that under progressive collapse mode.

Flame spread over liquid fuel possibly occurs in a long and narrow confined space, i.e., tunnel, pipeline or corridor. The spread of liquid fire in a tube belongs to the ventilation-controlled combustion. The 100 cm-long cylindrical quartz glass tube was built to examine flame spreading behavior on butanol surface in the tube. The inner diameters of the tube were 10 and 20 cm, respectively. The sealing ratio and sealing end position were controlled by the height and position of glass sheets at both ends. Three blocking methods were conducted: ignition-end blocking, non-ignition-end blocking and two-ends blocking. The effects of sealing ratio and sealing end position on flame spreading characteristics such as gas-liquid temperature distribution, oxygen concentration, preheating time and heat transportation of surface flow were revealed. The flame separation behavior was observed due to the insufficient oxygen supply. The non-ignition end blocking could effectively reduce the surface flow velocity and restrain the flame spread. The gas- and liquid-phase transfer involving surface flow of flame spread were revealed. For ignition-end blocking at D = 200 mm, when the blocking ratio increases from 10 % to 40 %, the radiant heat flow from front flame to surface flow area decreases by 17.6 %. [ABSTRACT FROM AUTHOR]

In order to address the energy absorption (EA) issue caused by limited installation space, a circular tube EA structure with axial cutting coupled radial extrusion (ACCRE) deformation modes is proposed and constructed. The FE model of cutting and extrusion die, bottom plate, cross rib and circular tube was established. The accuracy of the FE model was verified by two repeated tests. Finally, the peak force was predicted by theoretical model. Under the influence of dies, the circular tube produces chips and grooves and exhibits multi-level EA characteristics, with specific EA of 43.38 kJ/kg and first and second order platform force of 107.53 kN and 515.22 kN. With a maximum error of only 6.78 %, the FE model and experimental data offer accurate simulations of deformation patterns, steady-state loads, and EA. The specific EA of the ACCRE tube is increased by 120.2 % and 223.7 %, respectively, when compared to the single deformation modes of cutting and extrusion. This finding indicates that the ACCRE deformation mode can adequately cause the tube to undergo plastic deformation, improve the material utilization rate to better EA effect. [Display omitted] • Finite element analysis is performed and compared with experimental results. • Circular tube with ACCRE deformation modes is suggested and constructed. • The crashworthiness of ACCRE circular tube was analyzed. • Formula for calculating the stable force of ACCRE circular tubes. [ABSTRACT FROM AUTHOR]

In this study the impact of a novel enhanced tube on the hydrothermal performance of a working fluid flow is investigated. The behavior of the turbulent flow is analyzed numerically considering new designs of turbulators in a circular tube for applications in solar thermal collectors. Also, the three-dimensional model of the current problem is simulated using Computational Fluid Dynamics (CFD). The steady-state governing equations, of the heat transfer fluid flowing in the circular tube, are solved through a turbulence model. The influences of the angle and pitch of the flow turbulators on the Nusselt number and friction factor are studied. The findings indicate that the Nusselt number increases by adding the flow turbulators in the tubular tube. Additionally, a decrease in the pitch of the flow turbulators leads to larger heat transfer coefficient values, while the friction factor reduces by increasing the distance between the turbulators. [ABSTRACT FROM AUTHOR]

The heat transfer of fluids in pipelines under conditions with variable thermophysical parameters is ubiquitous in engineering applications, but effective methods to predict the heat transfer parameters are scarce. To address this issue, an experimental method was proposed to measure the heat transfer parameters in a circular tube by considering the effect of the insufficient development of turbulence in the inlet region and a Robin boundary on the wall. The variation of the Nusselt number of the inner wall was calculated by measuring the temperature distribution of the outer wall, and a temperature-related Gnielinski correlation was developed for conditions under which the thermophysical parameters vary with the fluid temperature. The results demonstrated that the deviations between the experimental Nusselt numbers and the values predicted using the traditional Gnielinski correlation became larger and larger as the flow distance increased. The temperature-related correlation generally improved the prediction accuracy by more than 25%, and the accuracy was better with increasing inlet temperature and decreasing airflow velocity. This study provides an important empirical reference for the prediction of the convective heat transfer parameters in pipelines and related engineering applications under strong heating or cooling conditions.

In this paper, the large deflection of a circular mesh tube was investigated by using a finite element method. During this investigation, it is assumed that the tube is deformed elastically, and its structural nonlinearity due to large deflection was considered. In addition, a theoretical model of an arc-shaped beam is proposed in order to predict the equivalent stiffness. The equivalent stiffness can be used to estimate the nonlinear large deflection response of the tube. In particular, the effects of the number of spirals and the number of turns on the prediction accuracy was discussed.

In the present research, the crushing response and crashworthiness characteristics of aluminum circular tubes with and without vertical corrugations were studied. In this regard, crashworthiness characteristics and collapsing behaviors of these structures are analyzed. The tubes of higher diameter under quasi-static axial compression show greater values of crashworthiness indexes compared with those of the tubes with smaller diameter, except SEA of a small tube with 6 vertical corrugations. The energy absorption capacity is enhanced by the combination between small diameter and the number of vertical corrugations. The peak force of the vertically corrugated tubes is greater than that of non-corrugated ones; however, their force-displacement curves are smoother. The influence of the number of vertical corrugations on the crashworthiness performance is studied. An analytical relationship is developed for mean force based on the experimental results and the 'plastic hinge line' concept. A comparison of the analytical results with the experimental ones indicates the reasonable accuracy of the analytical models. Improved Technique of Order Preference by Similarity to Ideal Solution (ITOPSIS) is employed to select the best structure for energy absorption purposes. [ABSTRACT FROM AUTHOR]

Based on the two-arc profile assumption, the expansion deformation and energy absorption of circular tubes compressed by conical-cylindrical dies were reconsidered. First, the deformation of the two arcs was analyzed independently and an improved model denoted as Model-I was established. Then, by further involving the coupling between the bending moment and membrane forces, a more elaborate model, i.e., Model-II was developed. Afterwards, experiments and simulations were conducted to verify the models, which show that, compared with previous theoretical models, Model-II could not only capture the prominent features of the deformation, but also improve the prediction accuracy of the steady driving force significantly. By means of this model, it was found that the critical semi-conical angle, which makes the driving force minimum, increases with the increase of the friction coefficient, expansion ratio as well as the radius/thickness ratio of the tube. And, the energy dissipation due to stretching is always greater than that of bending, while the friction dissipation can account for the largest proportion at small semi-conical angle or large friction coefficient. At a certain friction and die conditions, the specific energy absorption of expanded tubes can be much higher than that under progressive collapse mode. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Donghua University (Natural Science Edition) is the property of Journal of Donghua University (Natural Science) Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Paraffin melting is widely used in the fields of phase change materials energy storage, gathering and transportation pipe-line paraffin removal, etc. Analysis of the phase change mechanism and influencing factors of paraffin melting in the circular tube deeply has important guiding significance for improving the heat storage capacity by changing the structure of phase change material storage device and ensuring the safe transportation of crude-oil in the pipe-line. A double distribution lattice Boltzmann model based on enthalpy method is established to simulate the temperature field and the flow field of paraffin melting in a circular tube in this paper. The influence of different Rayleigh and Prandtl numbers on the paraffin melting process in a circular tube is analyzed. The results show that the natural-convection process is strengthened with the increase of the Rayleigh number, and the decrease of the average Nusselt number on the wall is smooth in the transition stage of wax melting due to the existence of fuzzy zone. The melting rate of paraffin can be accelerated or reduced by controlling the Prandtl number, so as to meet the relevant requirements of engineering. [ABSTRACT FROM AUTHOR]

Correlations for the Nusselt number for the fully-developed laminar flow of Carreau fluids through circular pipe subject to a uniform heat flux have been sought. Based on the mathematical expression, the Nusselt number could be obtained by numerical integration. To evaluate the Nusselt number for many conditions, an efficient integration method has been proposed. Using the obtained Nusselt number for different material constants and flow conditions, an improved correlation method has been proposed. The proposed correlation could reduce the maximum error from 3% to 0.9%. [ABSTRACT FROM AUTHOR]

This work proposes a method for evaluating the Darcy friction factor and the Nusselt number for fully developed laminar convective flow of Carreau fluids through straight circular tube under a constant heat flux. General mathematical formula for the Darcy fiction factor and the Nusselt number has been derived. This work has found that both the Darcy friction factor and the Nusselt number vary along with the flow rate from their Newtonian to Power law values in a similar way. Then, they have been evaluated for the Carreau fluids with various constants. [ABSTRACT FROM AUTHOR]

Collision of structure of a vehicle is not limited to the axial direction but it can occur laterally. The purpose of this paper is to present a study of the energy absorption behavior of different length of the circular mild steel tube under lateral crushing. A ring/tube (length of 10 mm, 35 mm, and 60 mm), 60 mm diameter and 1.5 mm thickness is compressed quasi-statically. Maximum loading setup to Instron machine was 50 kN. The speed of compression is 5mm/min. Finite Element Analysis (FEA) it used to validate the experimental work to ensure of getting accurate results. Numerical results of energy absorption and collapse load showed respectively 96.52% and 94.36% agreement with experimental results. The theoretical results showed 14.37% deviation with experimental and 15.5% with numerical results. The specimen with 60 mm length leads to better energy absorption than the other specimens. The results obtained numerically and experimentally in addition to theoretically showed the energy absorbed and collapse load varies with the length of the tube.

Copyright of Journal of Engineering for Thermal Energy & Power / Reneng Dongli Gongcheng is the property of Journal of Engineering for Thermal Energy & Power and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The thermal enhancement and pressure drop in a circular tube with radially-arrayed winglet vortex generator (VG) mounted inside at different orientations were experimentally studied. A series of four winglet rings containing VGs were on the inner surface of the tube at different sections. The effects of winglet attack angles β (0–45°), pitch ratios PR (1.6–4.8), porosity ratio γ (0–20%), winglet length L (10–20 mm), and inclination angle α (0–30°) on heat transfer and pressure drop characteristics were carefully examined. The study was carried out at Reynolds numbers (Re) ranging from 6 × 103 to 2.7 × 104 nestling in the turbulent flow regime. Results showed a significant effect of the winglets on the heat transfer enhancement and pressure penalty compared to the smooth tube. Experiments further revealed that as the length or attack angle of winglets increased, both Nusselt number (Nu) and friction factor (f) were intensified. When it turned to pitch ratio and inclination angle of winglets, the trend became adverse. By comparing the contribution of different winglet parameters, it is preferable to optimize the pitch ratio (PR) other than length, inclination angle (α) nor attack angle (β) for a higher thermal enhancement. The case of small porosity ratio (γ = 10%) at a low Re yields the maximum thermal enhancement of 1.26. Empirical correlations for Nu and f were generated for the winglets based on experimental data. [ABSTRACT FROM AUTHOR]

In this current study, the heat‐transfer augmentation mechanism and pressure drop with flow field structures over different dimple arrays in turbulent flow of three‐dimensional circular tube are investigated based on computational fluid dynamics numerical methods using the standard k − e turbulent model. The water is the working fluid over the range of Reynolds numbers from 1500 to 24,000, with three different tubes dimpling arrangements are analyzed. The numerical simulation results are validated through available experimental data, and they show good agreement results between them. The numerical analysis results showed that the flow fields are symmetric within the middle cross‐sectional direction in the pipe with and without dimples. Also, the flow near or close to the dimples is chaotic and including small vortexes and eddies. The results found that the temperature difference in the dimple tube with 2mm diameter at low flow range 0.56 L/min was higher than that of smooth pipe, dimple tube 0.75 and 1.5mm by 26.8%, 10.57%, and 3.68%, respectively. It can be concluded that using dimples in heat‐exchanger tubes can provide rates of heat transfer that is higher than that without dimpled tubes at the same operating conditions. Hence, this is an important enhancement in process industries for the energy conversion applications. [ABSTRACT FROM AUTHOR]

Summary: Helium‐Xenon (He‐Xe) gas mixture featured with low Prandtl number is increasingly used in various energy devices as heat‐carrying agent. To figure out the heat transfer characteristics of He‐Xe gas mixture in uniformly heated circular and tri‐lobe channels, numerical investigations are conducted and presented in the article. The adopted calculation models are validated by comparison between simulation results and experimental data. The local and total heat transfer characteristics are obtained, and new temperature correction factors for the circular and tri‐lobe channels are proposed and recommended. The simulation results obtained by using the SST k − ω turbulent model agree well with the experimental data, this kind of turbulent model matched with appropriate turbulent Prandtl number model could provide satisfactory temperature predictions for He‐Xe gas mixture in a wide range of temperature ratio Tw/Tb. The simplified Petukhov correlation with temperature correction can give conservative predictions with relative error less than 10%. For the tri‐lobe channel, due to the local restricted flow, the gas temperature at the corners is much higher than that at the concavities. The local wall temperature and local Nu number vary azimuthally, showing a large temperature gradient. The tri‐lobe channel has better heat transfer performance than that of the circular tube when the temperature ratio Tw/Tb is no more than 1.5, but it deteriorates significantly with the increasing temperature ratio. This article may contribute to the numerical simulation and thermal design of heat transfer components concerning He‐Xe gas mixture. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Harbin University of Science & Technology is the property of Journal of Harbin University of Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Liquid film thickness is an important parameter for predicting phase change heat transfer in pulsating heat pipes (PHPs). This study experimentally evaluated the liquid film thickness formed during the oscillation of the liquid column within PHPs. The liquid column was oscillated sinusoidally under various oscillating conditions to simulate the flow phenomena. The average thickness of the liquid film formed during the oscillations was determined by comparing the oscillation amplitudes of the tip of the liquid column with and without the liquid film. It was confirmed that the average thickness of the liquid film varied with the capillary number for each working liquid (water, ethanol, and FC-40). The characteristic velocity of the capillary number was the time-average velocity of the tip of the liquid column, which was derived by assuming that the tip of the liquid column oscillated sinusoidally. Additionally, the average liquid film thicknesses during the liquid column oscillations were evaluated according to the correlations for various flow conditions proposed in previous studies. In this evaluation, the velocity and acceleration values obtained by assuming that the tip of the liquid column oscillated sinusoidally were determined based on the experimental results. This evaluation demonstrated the acceleration effect, thereby indicating that the acceleration made the liquid film thinner when the capillary number was higher than the threshold of the capillary number for each working liquid. Finally, empirical correlations with and without the acceleration effect were proposed for the average liquid film thickness of oscillating flows in terms of capillary and Laplace numbers. It was demonstrated that the proposed correlations could predict the average liquid film thickness within the range of approximately ±15 % accuracy.

This article presents the heat transfer and friction factor characteristics of circular tube fitted with an array of novel semi-elliptical vortex generator inserts using air as working fluid. The study makes use of computational fluid dynamics methodology and the influence of varying aspect ratio (AR = 1, 2, 4 and 6), flow attack angle (45°, 60°, 75° and 90°) and longitudinal pitch (P = 30 mm, 60 mm and 90 mm) of vortex generator is investigated for the turbulent flow Reynolds number regime of 8000–26000. The numerical results reveal that the presence of semi-elliptical vortex generator has significant influence on both Nusselt number and friction factor characteristics of circular tube. The vortex generators produce strong longitudinal vortices which intensifies fluid mixing near the tube wall region thereby augmenting the heat transfer to the flowing air stream. In addition, the presence of flow impingement effect on the upstream side and flow reattachment zones on the downstream side of vortex generator is found to contribute to enhanced heat transfer. The influence of aspect ratio on heat transfer is found to be more significant than that of flow attack angle. However, both parameters affect the friction factor significantly. On the other hand, the pitch distance of insert significantly affects both heat transfer and friction factor. The maximum rise in Nusselt number and friction factor is about 2.1 times and 6.34 times higher than plain tube, respectively, for P = 30 mm. The thermal enhancement factor (TEF) is found to decrease with increasing flow attack angle and aspect ratio for all flow rates considered in the study. However, the TEF increases with decreasing pitch distance of inserts and the overall TEF is found to be in the range of 0.86–1.19.

AIM: To explore the effect of anastomosis surgery for lacrimal canaliculi laceration combined with circular tube implantation.METHODS: A retrospective study was conducted on 59 cases(59 eyes)from June 2018 to December 2018 with lower lacrimal canalicular laceration. The nasal cut end of lacerated lacrimal canaliculi was found under the microscope, the circular tube was implanted as a support. The tube was removed 3mo after the surgery. All the patients were followed additional 3mo. The therapeutic effect and complications were observed.RESULTS: All patients were successfully operated. 46 cases(46 eyes)were cured, 5 cases(5 eyes)were improved, 8 cases(8 eyes)had no effect. The overall successful rate was 86%(51/59). No serious complications and eyelid deformity were found.CONCLUSION: The anastomosis surgery combined with circular tube implantation is an effective treatment for the lacrimal canaliculi laceration.

This article presents the heat transfer and friction factor characteristics of circular tube fitted with an array of novel semi-elliptical vortex generator inserts using air as working fluid. The study makes use of computational fluid dynamics methodology and the influence of varying aspect ratio (AR = 1, 2, 4 and 6), flow attack angle (45°, 60°, 75° and 90°) and longitudinal pitch (P = 30 mm, 60 mm and 90 mm) of vortex generator is investigated for the turbulent flow Reynolds number regime of 8000–26000. The numerical results reveal that the presence of semi-elliptical vortex generator has significant influence on both Nusselt number and friction factor characteristics of circular tube. The vortex generators produce strong longitudinal vortices which intensifies fluid mixing near the tube wall region thereby augmenting the heat transfer to the flowing air stream. In addition, the presence of flow impingement effect on the upstream side and flow reattachment zones on the downstream side of vortex generator is found to contribute to enhanced heat transfer. The influence of aspect ratio on heat transfer is found to be more significant than that of flow attack angle. However, both parameters affect the friction factor significantly. On the other hand, the pitch distance of insert significantly affects both heat transfer and friction factor. The maximum rise in Nusselt number and friction factor is about 2.1 times and 6.34 times higher than plain tube, respectively, for P = 30 mm. The thermal enhancement factor (TEF) is found to decrease with increasing flow attack angle and aspect ratio for all flow rates considered in the study. However, the TEF increases with decreasing pitch distance of inserts and the overall TEF is found to be in the range of 0.86–1.19. [ABSTRACT FROM AUTHOR]

Introducing nonuniform thickness has shown promising potential in enhancing the energy absorption of thin-walled tubes. However, existing studies were focused on the axial loading, with little attention being paid to the oblique loading condition. In this paper, the energy absorption performance and the deformation modes of the circular tubes with gradient thicknesses under oblique loads are investigated. Finite element simulations and experiments were carried out for both uniform-thick and gradient-thick tubes under the axial and oblique loads, and satisfactory agreement was achieved betweent the numerical and the experimental results. The validated finite element models were used to investigate the effects of the thickness gradient and loading angle on the deforamtion modes and the energy absorption. The results highlight the advantages of the gradient-thickness tubes in improving the energy absorption performance under the oblique loading condition, especially at a larger loading angle. A novel progressive bending deformation mode was observed for the tube with large thickness gradient at a loading angle larger than 15°, which is beneficial for the energy absorption performance. [ABSTRACT FROM AUTHOR]

Behaviors of a prolate ellipsoid inside circular tube Poiseuille flow are studied experimentally. In the study, Reynolds number Re ∈ (100,700) and the confinement ratio D/A ∈ (1.2,2.8) are considered, where D is the diameter of the tube and A is the length of the major axis of the ellipsoid. Two typical stable motion modes are identified, namely, the horizontal, and inclined modes. Then another inclined mode (inclined mode II) is found at high Reynolds number (Re ∈ (1000,3200)) and small D/A, and the inclined angle of ellipsoid increases with the increase of Re. The possible mechanism is explained. Our experiment shows that the lagging velocity U increases as Re increases. Further numerical analysis using FLUENT shows that due to the increase of U, the moment acting on the particle would make the inclined angle of the particle increase. [ABSTRACT FROM AUTHOR]

In the present study, turbulent flow and heat transfer inside a three-dimensional tube with elliptic and circular cross sections are simulated using finite volume method. The main aim of this study is to investigate the effects of ellipse aspect ratio and twisting of the tube wall on the flow characteristics and heat transfer. According to the obtained results, by converting the cross section from circular to elliptical and reducing the aspect ratio of elliptic tube with smooth wall, the friction factor and heat transfer increase. Investigating on the effect of wall twisting shows that, due to generation of secondary flow, the heat transfer and friction factor increase. By reducing the twist pitch, the generated vortices get merged into a large vortex and affect the flow across the whole cross section, causing the mixing to improve and heat transfer to increase. According to the obtained results, in the elliptical tube with an aspect ratio of 1.65, at twist pitches of 0.4 and 0.2, the heat transfer increases compared to a smooth tube with a similar cross section by 5% and 20%, respectively; and by a sudden reduction of twist pitch from 0.2 to 0.1 along the path, the heat transfer improves up to 30%. The effect of addition of multi-wall carbon nanotube nanoparticles to the base fluid inside the twisted elliptical tube with an aspect ratio of 1.65 on the heat transfer and flow characteristics is also analyzed, 28–32% in heat transfer for Reynolds number of 4000 and 16,000. According to the obtained results, the performance evaluation criterion (PEC) of nanofluid with low volume fraction and at lower Reynolds numbers is less than 1; however, for the flow of nanofluid with a volume fraction of 0.03, the heat transfer enhancement prevails over the increase in the pressure drop and the PEC is more than 1. [ABSTRACT FROM AUTHOR]

A comparison of optimization values for empty aluminum circular tubes and double circular tubes containing foam under flexible loading is presented in this paper. Thin-walled tube is widely used in the vehicle structure for impact mitigation especially on the vehicle door. In this study, AA6063 T6 aluminum alloy tubes were filled with foam with both ends supported and subjected to quasi-static bending. Finite Element (FE) modeling and analysis has been performed using finite element software. The developed FE model has been experimentally validated and good agreements have been observed between experiment and simulation results. The deformation of the tube under bending load was studied with variations in thickness (t) and diameter (d) of the tube wall. The design of various objective optimization (multi objective optimization) is developed with considering two crashworthy performance that conflicting each other namely Peak Crush Force (PCF) and energy absorption per unit mass (SEA). In order to fulfill these purposes, Radial Base Function (RBF) and Non-dominated Sorting Genetic Algorithm (NSGA II) are adopted. It was observed that double circular tube containing foam exhibit higher energy absorption than that of an empty double circular tube. As such, this structure could be recommended as a power absorbent component such as a side beam on a vehicle door. [ABSTRACT FROM AUTHOR]

As computational fluid dynamics (CFD) advances, entropy generation minimization based on CFD becomes attractive for optimizing complex heat-transfer systems. This optimization depends on the accuracy of CFD results, such that accurate turbulence models, such as elliptic relaxation or elliptic blending turbulence models, become important. The performance of a previously developed elliptic blending turbulence model (the SST k–ω–φ–α model) to predict the rate of entropy generation in the fully developed turbulent circular tube flow with constant heat flux was studied to provide some guidelines for using this class of turbulence model to calculate entropy generation in complex systems. The flow and temperature fields were simulated by using a CFD package, and then the rate of entropy generation was calculated in post-processing. The analytical correlations and results of two popular turbulence models (the realizable k–ε and the shear stress transport (SST) k–ω models) were used as references to demonstrate the accuracy of the SST k–ω–φ–α model. The findings indicate that the turbulent Prandtl number (Prt) influences the entropy generation rate due to heat-transfer irreversibility. Prt = 0.85 produces the best results for the SST k–ω–φ–α model. For the realizable k–ε and SST k–ω models, Prt = 0.85 and Prt = 0.92 produce the best results, respectively. For the realizable k–ε and the SST k–ω models, the two methods used to predict the rate of entropy generation due to friction irreversibility produce the same results. However, for the SST k–ω–φ–α model, the rates of entropy generation due to friction irreversibility predicted by the two methods are different. The difference at a Reynolds number of 100,000 is about 14%. The method that incorporates the effective turbulent viscosity should be used to predict the rate of entropy generation due to friction irreversibility for the SST k–ω–φ–α model. Furthermore, when the temperature in the flow field changes dramatically, the temperature-dependent fluid properties must be considered.

Electric heat tracing is the main measure for cold protection of the polar transfer coefficient in marine engineering equipment, but thermal equilibrium is the key problem this technology faces. In this paper, the circular tube was the research object. We studied the influence of convective heat transfer by Fluent software and experiments with a wind speed of 0–40 m/s and temperature of −40–0 °C by constant heat flux heating. The results show that the convective heat transfer increases with increased wind speed and decreased temperature. When the temperature is below −30 °C, the effect of temperature is increased; when the wind speed is greater than 25 m/s and the temperature is lower than −20 °C, the effect of temperature on the convective heat transfer coefficient of the circular tube increases. Based on the simulation data, we established a prediction model, and the rationality of the prediction model was verified by tests. The model provides reference for the design of electric heat tracing of circular tubes on polar ships.

In order to gain a better understanding of the latent heat transfer mechanism in pulsating heat pipes (PHPs), this study experimentally investigated the thickness characteristics of a liquid film that forms during oscillations of the liquid column within PHPs. Accordingly, the liquid column was oscillated sinusoidally under different oscillating conditions to simulate the flow phenomena. A circular tube with an inner diameter of 2 mm was used as the test channel; ethanol and FC-40 were used as working liquids. The average thickness of the liquid film formed due to the liquid column oscillations on the channel wall was determined by comparing the oscillation amplitudes of the tip of the liquid column obtained with and without the liquid film. It was confirmed that the average thickness of the liquid film varied based on the average capillary number for each working liquid. The time-average velocity of the tip of the liquid column, which was derived based on the assumption that the tip of the liquid column oscillates sinusoidally, was used as the characteristic velocity to calculate the average capillary number. The experimental results were compared with various correlations proposed in previous studies. The results show that the acceleration of the oscillating liquid column affected the liquid film thickness for a high average capillary number. For this high average capillary number, the rate of increase in liquid film thickness with the average capillary number decreased, and the liquid film thicknesses during the liquid column oscillation approached those under the steady condition. Furthermore, the liquid film thicknesses for the vertical channel were compared with those for the horizontal channel, and it was made clear that the effect of gravity on liquid film thickness is significant. Thus, the thickness characteristics of a liquid film during the liquid column oscillation within PHPs were clarified.

In the present study, the heat transfer and hydrodynamic analysis of flow through single-pipe heat exchangers of circular and square cross-sectional configurations were performed. The experimental and numerical investigations were conducted to evaluate the performance of two metallic oxides (Al2O3 and SiO2) and two carbon-based nanostructured nanofluids (KRG and GNP) in comparison with the distilled water (DW). The data obtained from the experimental runs with DW as a working fluid in both test sections were used to validate the 3-D numerical models for the square and circular pipe heat exchangers. The flow in both test sections is considered as a fully developed turbulent flow with the Reynolds number range of 6000–11,000, and both the test sections were subjected to a uniform heat flux at their outer surfaces. The concentrations of all nanofluids used in the present study were in the range of (0.025–0.1 mass%). The test rig was firstly validated during the water run by using different empirical correlations for the evaluation of pressure drop and Nusselt number and showing a very good agreement, and then, the numerical models were validated with the data obtained experimentally and the errors were less than 10% for both models. For the square tube flow, the average errors between the numerical and experimental findings of Nusselt number and pressure drop were 6.8% and 2.49%, respectively, and for the circular pipe flow, the evaluated errors were 9.34% and 5.92% for Nusselt number and pressure drop, respectively. The performance index for all the nanofluids was calculated to obtain the convective heat transfer coefficients and friction losses of the fluids in both the tubes. The results showed that the non-covalent graphene–DW is not suitable for heat transfer applications due to its higher viscosity. The results also showed a different enhancement of heat transfer for the same nanofluid in circular and square tube flows, whereas the performance index of the same nanofluid appears nearly the same for flow through both the cross sections. [ABSTRACT FROM AUTHOR]

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The paper investigates the thermal properties of Al2O3 water-based nanofluid in a circular copper tube fitted with tight spring insert. The nanoparticles were dispersed in distilled water. Thermal conductivity, Nusselt number, force convection heat transfer coefficient, viscosity, pressure drop and friction factor at different particle volume fractions (0.5%, 1.0% and 1.5%) were analyzed. The heat transfer test section was fabricated to supply constant heat flux. The results showed that Nusselt number continuously increased with the volume fraction and temperature of the nanofluids. However, the friction factor was found to be minimum at a volume fraction of 1.0 vol.%. The Nusselt number of nanofluid was approximately 43.2% greater than that of the base fluid at a volume fraction of 1.0% vol. [ABSTRACT FROM AUTHOR]

Abstract Winglet pairs are promising longitudinal vortex generators which can be used to produce streamwise vortices that do not decay until further downstream and consequently increase heat transfer rate with comparatively lower pressure penalty. This paper deals with the effect of delta winglet vortex generator (DWVG) pairs on thermal and flow behaviors in a circular tube for Reynolds numbers (Re) range of 5000–25000. The DWVG pairs involved are the pitch ratio (PR = 9.6), four attack angles (α = 10°, 20°, 30° and 40°), three winglet height (h = 5 mm, 7.5 mm and 10 mm) and three spacing between leading edges (s = 10 mm, 15 mm and 20 mm). The experimental results indicate that the Nusselt number (Nu) increases with Re while friction factor (f) decreases with Re. Nusselt number and friction factors both are increasing with attack angle and winglet height, while the middle spacing yields the highest Nu and f. Maximum Nusselt number increment (Nu / Nu 0) with the DWVG pairs was observed as being 73% larger than that of smooth tube, while the maximum friction factor increment (f / f 0) was 2.5 times larger. Thermal enhancement factor (TEF) decreases with Re. The largest TEF obtained, 1.44, is with the combination of α30°s15 h 7.5 at Re = 5000. Compared with other types of VGs in published experimental research papers, the current DWVG pairs show better thermal performance than many of them. Vortices downstream of the DWVG are visualized with smoke flow for better understanding of the flow behavior. [ABSTRACT FROM AUTHOR]

Highlights • Two-phase flow of blood is considered in a circular tube with magnetic properties. • Uniform magnetic field and an external oscillating pressure gradient are considered. • Motions of blood and magnetic particles both are discussed. • Laplace transform is used for finding exact solution and then plotted graphically. • Tzou's algorithm is used for numerical solution and compared in table with exact solution. Abstract In this paper, two-phase flow of blood is considered through a circular tube along with magnetic properties. The tube is considered as a circular cylinder form and the blood is flowing through it under the influence of uniform magnetic field and an external oscillating pressure gradient. Exact solutions for the fluid and magnetic particles velocities are obtained by means of integral transforms. The velocity of the fluid is presented as a sum of post transient and transient solutions. Moreover, a semi-analytical solution based on the Bessel equation and Tzou's algorithm for the inverse Laplace transform is obtained. A comparison among the profiles of the fluid's velocity determined with both solutions is also made. Furthermore, in order to study the influence of the material parameters, numerical simulations and graphical illustrations are used and useful conclusions are summarized. [ABSTRACT FROM AUTHOR]