Length-diameter Ratios Research Articles

Bubble Dynamics and Heat Transfer Characteristics of Flow Boiling in a Single Pentagonal Rib Channel

Abstract Two-phase flow boiling is the preferred method for efficient heat dissipation in electronic equipment, so the bubble dynamics and heat transfer characteristics of flow boiling in a single pentagonal rib channel are numerically analyzed by the volume of fluid (VOF) model. Three typical rib structures are obtained, whose block ratios are 0.4, 0.75, and 0.25, and the corresponding length-diameter ratios are 1.72, 0.59, and 12.70, respectively. The Reynolds number is 14,000 and 28,000. The heat flux is set from 50 to 150 kW/m2. The results show that the starting position of the bubble is at the cone and sides of the rib, and its moving direction is related to the configuration of the rib, the heat flux, and fluid velocity. Additionally, the bubbles are shedding at the corner of the rib, and the short rib does not appear to be the vortex shedding phenomenon. The wall temperature and temperature fluctuation increase with the increase of heat flux and with the decrease of fluid velocity. The heat transfer coefficient decreases with the increase of heat flux and with the decrease of fluid velocity. The short rib channel has a higher heat transfer coefficient, approximately 14% and 50% higher than that of the median and long rib channels, respectively. Compared with the single-phase flow, the main frequency of the lift coefficient increases with the increase of the fluid velocity while decreasing with the increase of heat flux. The main frequency of the median rib without harmonic signal keeps constant, and that for long rib decreases by 38%. The study of the instantaneous characteristics of bubble growth in pentagonal rib channels is instructive to the efficient heat dissipation in electronic equipment.

Research on bar deflection of low-stress cropping process under bending-shear coupling effect

In view of the influence of shear effect on bar deflection in low-stress cropping, which makes it difficult to control the loading die and seriously affects the surface quality of cropping bar, based on the energy balance theory in this paper, the stiffness equivalent transformation of the bar with annular V-shaped notch and dynamic crack is carried out, and the coupling effect of bending moment and shear force on the bar deflection before and after crack initiation is quantified. Based on it, the deflection characterization formula for the whole cropping process of the bar with different length-diameter (L/2R) ratios is established. The change laws of bar deflection with different L/2R ratios are analyzed by XFEM method. Especially for the bar with L/2R ≤ 2.84, it is pointed out that it is necessary to consider the influence of shear deflection on the basis of bending deflection, and its influence degree decreases with the increase of L/2R. The real-time synchronous dynamic deflection test platform is built on the cropping machine of rotary bending. The research shows that the calculated values obtained by the deflection characterization formula proposed are in good agreement with the corresponding experimental results. The evaluation parameter, maximum contact stress reduction ratio ξ(p-l)/p, is proposed to characterize the degree of surface contact stress reduction under line contact loading relative to point contact loading. It is pointed out that the variation range of ξ(p-l)/p of the bar with different L/2R ratios under the influence of deflection is as high as 49.8%-64.1%. And further application of double bearings outer ring loading is also beneficial to obtain high cropping quality.

Investigations of the static and dynamic performances of an annular seal with a tilted rotor by transient CFD methods with dynamic mesh

A transient computational fluid dynamics (CFD) method based on dynamic mesh is adopted to investigate the static and dynamic performances of an annular seal with a tilted rotor. The reliability of the CFD method is verified by the experimental data, and then the variations of dynamic performance and sealing performance under different length-diameter ratios and misalignment angles are investigated by the method. The results show that the leakage flowrate of the tilted annular seal decreases with the increasing length-diameter ratio, while the whirl frequency ratio increases with the length-diameter ratio. The increase of misalignment angle has little detrimental effect on the stability of long shaft system in the three misalignment modes. In addition, the misalignment mode for the tilted position at the half-length of the annular seal can reduce the leakage flowrate.

Forming limit of 6061 aluminum alloy tube at cryogenic temperatures

Cryogenic medium pressure forming has been proposed to fabricate aluminum alloy complex tubular components. It is urgently needed to evaluate and predict the cryogenic forming limit of aluminum alloy tubes for the novel process. Cryogenic bulging test using different length-diameter ratios was proposed to test the forming limit in the tension-tension zone. A modified M-K model was established to rapidly predict the cryogenic forming limit, based on the dynamic hardening with deformation. Theoretical analysis and fracture characterization were implemented to reveal the improvement mechanism of cryogenic forming limit from the macroscopic and microscopic perspectives, respectively. The forming limit of AA6061 tube at −196 °C is approximately double that at room temperature, especially in the tension-tension zone, which contributes significantly to the forming of complex features under biaxial tension. The cryogenic limiting deformation ability increases gradually as the strain state changes from plane strain to biaxial tension. Under the biaxial tension condition of λ=1.2, the limiting strain can reach 0.52 at −196 °C. The cryogenic forming limit can be successfully predicted by the modified M-K model with a prediction deviation of only 4.35% at −196 °C. The enhanced cryogenic hardening ability promotes the improvement of deformation uniformity and delays the occurrence of uncoordinated deformation. Ductile fracture is main fracture mechanism at cryogenic temperature. The significantly increased ductile fracture characteristics contribute to the improved cryogenic forming limit. This research can provide important guidance for the cryogenic forming of aluminum alloy tubular components.

Evaluation of the Energy Consumption and Fractal Characteristics of Different Length-Diameter Ratios of Coal under Dynamic Impact

Coal samples having the same diameter (50 mm) and different length-diameter ratios (l/d), i.e., 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 were tested under dynamic uniaxial impact compression using the Split Hopkinson Pressure Bar (SHPB) experimental system. This study evaluates: (a) The effects of l/d on the energy consumption law and fractal characteristics of coal crushing; (b) The effects of l/d and stress balance on energy dissipation; (c) The effects of l/d and energy consumption density on the fractal characteristics of coal crushing. The findings under different l/d are as follows: (1) The coal samples show similar stress–strain curve shapes in stages including elastic, plastic, and failure stage, which is an “open” shape, the proportion of plastic stage increases, and strain-softening occurs; (2) The dynamic compression dissipation energy and energy consumption ratio of coal shows the same trend, showing two stages with the increase of length-diameter ratio, which increases linearly in the first stage and overall decreases step-by-step; (3) The average particle size increases while fractal dimension of fragmentation decreases linearly, which endorses the decreasing trend of fragmentation degree; (4) It is determined that there is a power relationship between fractal dimension and energy dissipation density; (5) A new index Crushing Density Energy Efficiency (CDEE) is proposed, which can be used to characterize the rock-breaking efficiency of crushing energy consumption under different conditions. This index is inversely proportional to l/d. The research results can provide a basis for the design of top coal caving mining, and the determination of blasting parameters.

Mechanical behavior of steel-concrete interface and composite column for circular CFST in fire

A refined numerical model was proposed to simulate the mechanical behavior of circular concrete-filled steel tubular (CFST) columns under fire conditions. The model was calibrated against the thermal and mechanical results from physical testing and showed good reliability. Based on the numerical analyses, the mechanical properties of steel-concrete interface and composite column for circular CFST with different confinement coefficients during the entire fire process were investigated in depth. Moreover, a series of parametric analyses were undertaken to investigate the influence of main factors, such as the load ratio, the steel tube ratio, the steel strength, the cross-sectional diameter and the concrete strength, on the mechanical behavior of the circular CFST column. Finally, a theoretical formulation and a simplified formulation for evaluating the ultimate bearing capacity of circular CFST columns with different length-diameter ratios under fire conditions were proposed, which showed an acceptable accuracy.

Comprehensive Comparison of a High-Speed Permanent Magnet Synchronous Motor Considering Rotor Length–Diameter Ratio

For high-speed permanent magnet machines (HSPMMs), many design schemes of rotor length–diameter ratios can satisfy the constraints of multiple physical fields during the motor design period. The rotor length–diameter ratio greatly impacts the comprehensive performances of multiple physical fields. However, these analyses are missing in the existing literature. Therefore, this paper focuses on the influence of the rotor length–diameter ratio on comprehensive performances. Firstly, finite element models (FEM) of multiple physical fields are built by ANSYS Workbench platform and Motor-CAD software. Then, the comprehensive performances of multiple physical fields are comparatively analyzed. Finally, the designed HSPMM is implemented, based on one prototype of 60 kW, 30,000 rpm to verify the results of comparative analysis. Based on the comparative analysis above, the influent laws of rotor length–diameter ratios on comprehensive performances of multiple physical fields are discussed and summarized, which can be used as a reference for the rotor structural design of HSPMMs.

Adjusting the γ/α2 and Ti2AlC phase on microstructure evolution and improving mechanical properties at room and elevated temperatures with the addition of Ta

Alloys of Ti-46Al-6Nb-2.5C-xTa (at. %) were prepared to adjust the γ/α2 phase, refine microstructures, and improve mechanical properties at room and elevated temperatures simultaneously. Phase contents, lamellar colony size, Ti2AlC morphology, compressive and tensile properties, and related mechanisms were discussed. The results show that the relative content of the γ phase increases and the relative content of the α2 phase decreases when Ta content increases from 0 to 2.0 at. %. The solid solution effect of Ta in reinforcing phases contributes to the absence of brittle B2 phase. The content of small length-diameter ratios of Ti2AlC particles increases. The lamellar colony size decreases from 29.9 to 21.6 μm. The constitutional undercooling and heterogeneous nucleation are increased by Ta during the solidification process. Compressive strength increases from 1974 to 2330 MPa when Ta content increases to 0.8 at. %, and the strain increases from 19.8 to 28.2% when Ta content increases to 1.2 at. %. The tensile strength of the Ti-46Al-6Nb-2.5C-2.0Ta alloy increases from 460 to 503 MPa and its strain increases from 4.1 to 8.3%, when temperature increases from 750 to 950 °C. The refinement of lamellar colonies and Ti2AlC, solid solution of Ta, and the ratio of γ/α2 are the main reasons for improving mechanical properties of Ti-46Al-6Nb-2.5C-2.0Ta alloy.

Facile synthesis of porphyrin-MOFs with high photo-Fenton activity to efficiently degrade ciprofloxacin

The slow conversion of Fe(Ⅱ)/Fe(III) cycle was largely limited the degradation efficiency of many photo-Fenton systems. Herein, four Fe-MOFs nanorods (namely Fe-TCPP-1, Fe-TCPP-2, Fe-TCPP-3, Fe-TCPP-4) with decreasing length-diameter ratios were synthesized in a household microwave oven, using photosensitizer porphyrin and iron ions with Fenton activity as building blocks. Among them, the Fe-TCPP-3 exhibited high photogenerated electron-hole (e−-h+) separation efficiency and largest pore structure, endowing Fe-TCPP-3 with superior photo-Fenton property. In addition, Fe-TCPP-3 based photo-Fenton system was applied to efficiently degrade antibiotic ciprofloxacin (CIP) under neutral condition, due to the continuously generated reactive species (h+, e−, OH·, O2·–, 1O2) in Fe-TCPP-3 under visible-light irradiation. With irradiation for 30 min, the degradation efficiency of the system could reach about 73 %, which was about 26-fold towards the system without light irradiation. This study paved a way to modulating the photo-Fenton activity of MOF-based catalysts.

Study on equivalent fracture toughness of recycled steel fiber-reinforced concrete based on digital image correlation technology

Recycled steel fiber-reinforced concrete is a new hybrid high performance environmental protection composite material, which can be widely used in the construction industry and provide a new research direction for national green and sustainable development. In this study, three-point bending beams with a prefabricated incision were used to evaluate the fracture performance of plain concrete, steel fiber-reinforced concrete and recycled steel fiber-reinforced concrete with different fiber volume contents, fiber length-diameter ratios and relative incision depths based on digital image correlation technology. The results show that recycled steel fibers can improve the crack resistance of concrete beams better than ordinary steel fibers; The crack resistance of specimens increases with the increases of volume ratio of recycled steel fibers, and when the fiber content is from 0.0 % to 1.0 %, the increased amplitude of the crack resistance of specimens increases gradually, when the fiber content is from 0.5 % to 1.0 %, the increased amplitude of the crack resistance of specimens reaches the maximum, and when the fiber content is from 1.0 % to 1.5 %, the increased amplitude of the crack resistance of specimens decreases; The crack resistance of specimens increases with the increases of length-diameter ratio of recycled steel fibers, and when the fiber length-diameter ratio is from 30 to 50, the crack resistance of specimens increases relatively large, when the fiber length-diameter ratio is from 50 to 70, the crack resistance of specimens increases relatively small; The crack resistance of specimens decreases with the increases of relative incision depth of specimens, and when the relative incision depth is from 0.2 to 0.3, the crack resistance of specimens decreases relatively large, when the relative incision depth is from 0.3 to 0.4, the crack resistance of specimens decreases relatively small; The equivalent fracture toughness of concrete beams does not change with the changes in the relative incision depth. The simulated results correlated well with the experimental results, except some macroscopic phenomena during the cracking process. This study can provide references for the research of fracture behavior of steel fiber-reinforced concrete materials and the recycling of steel wire from waste tires.

Stiffness and natural frequency effects on a hollow truncated conical shaft

Larger tool length-diameter ratios generate undesired mechanical vibrations during machining processes. Previous studies show that natural frequency increases by adding a central hole and reducing the mass. Using FEA, the static stiffness and natural frequency response for different hole geometries, applied to conical shafts of various slenderness ratios were determined. The results show a convergence between an increase of natural frequency and decrease of static stiffness. In some cases, it was found that the working frequency range increases without considerably decreasing the rigidity, enhancing the response against vibrations. These findings could be implemented to develop reliable machining tools.

The influence of incident angles and length-diameter ratios on the round-ended cylinder under regular wave action

The round-ended cylinder is commonly applied in coastal engineering as foundation. While researches related to the round-ended cylinder are pretty limited. In the present study, the influence of the incident wave angles and length-diameter (LD) ratios is investigated through numerical simulation. An equation is then proposed to calculate the maximum horizontal wave force based on numerical solutions and verified by random cases. Additionally, results of numerical simulation, proposed equation, the Chinese design code and the American design code are compared with each other. The results show that the proposed equation has good accuracy and the product of a safety factor, in the range of 1.5–2.0, with the proposed equation result is recommended in practical engineering. Besides, small LD ratios are suggested to be adopted in the structure design.

Study on the Rock Size Effect of Quasistatic and Dynamic Compression Characteristics

To study the size effect of rock under quasistatic and dynamic conditions, the changes in compressive strength with the change in specimen size are measured. Cylindrical granite specimens with length‐diameter ratios in the range of 0.5∼1 are used for uniaxial compression tests using an RMT testing machine and an SPHB impact testing machine. Under quasistatic loading, the failure modes of the specimens with different length‐diameter ratios are different. The larger the size of the specimen structure is, the greater the probability of defects such as joints and micro cracks is and the smaller the influence of the specimen on the distribution of a three‐dimensional stress state is. The rock strength decreases with increasing length‐diameter ratio. Using the improved Weibull formula, the size of the specimen is expressed by the volume, and the calculated rock strength of different volumes is similar to the compressive strength from the quasistatic tests. Under dynamic loading, the dynamic compressive strengths of the specimens with different length‐diameter ratios are similar, and the failure mode of the specimens is different from that under quasistatic loading. Soon after a crack appears in a specimen, the specimen splits. As the size of the specimens decreases, the fragments size to approach the millimeter scale. By improving the Weibull distribution formula and considering variation in strain rate caused by the size of the specimen, the dynamic compressive strength of rocks of different volumes is calculated by introducing the critical strain rate and related parameters, and the results are similar to the experimental dynamic compressive strength obtained. The improved Weibull formula based on the strength size effect can accurately describe the quasistatic and dynamic compressive strength laws.

Deformation and Fracture Failure of a High-Speed Long Rod Intercepted by Linear Explosively Formed Penetrators Sequence.

The fracture failure of a high-speed long rod has historically been a challenge. Since the flying plate and flying rod have a relatively low velocity, it is challenging to achieve a multi-stage fracture of the high-speed long rod within the range of existing technology. In this paper, the linear explosively formed penetrators (LEFPs) sequence with a stable flight velocity of 850 m/s were used to cut a high-speed long rod. We investigated the deformation and fracture of Φ10 mm tungsten alloy long rods having different length-diameter ratios (20, 26, 35) and different speeds (1200, 1400, 1600 m/s) by employing the LEFPs sequence with different spacings (0–40 mm) and different interception angles (30°, 60°). In the meantime, the fractured rods movement pattern was recorded with a high-speed camera to elucidate the change law of the length, speed, linear momentum, and angular momentum of fractured rods. It was found that the length loss rate of the fractured rods is as high as 27%. The fractured rods rotated around the center of mass, and the vertical speed change could reach up to 18% of the muzzle velocity of the long rod, and the greatest reduction of horizontal speed and momentum could reach 37%. The longer the interaction time between LEFPs sequence and the long rod, the more beneficial the failure of the long rod. The application of LEFPs sequence solved the difficult problem of disabling the high-speed long rod, and the quantitative analysis of the fracture failure of the long rod had an important sense for studying the terminal penetration effect of the fractured rods.

Study on mechanical properties and wear resistance of aluminum alloy composites reinforced by basalt fiber

Basalt fiber is a new type of environmentally friendly materials with excellent performance in the 21st century. Basalt fiber (BF) reinforced aluminum alloy composites with different length-diameter ratios (200:1, 400:1), different volume fraction (1%, 4%, 8%) was fabricated by room-temperature pressing and vacuum sintering method. It was found that the BF treated with electroless copper plating on the surface enhanced the mechanical properties and wear resistance of the composites, while the BF without copper plating on the surface weakened the hardness. With increasing the volume fraction of BF with copper plating, the hardness and the wear resistance of the composites increased. The length-diameter ratio ∼400:1 is superior enhancement aspect ratio of 200:1 of the reinforcing effect.

Preparation of monodisperse SiO2 nanorods with hollow structure and parameters affecting the length-diameter ratio

Monodisperse SiO2 nanorods with hollow structure were prepared using FeOOH as a template. Powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption-desorption isotherms were utilized to characterize the products. With a FeCl3 concentration of 0.1 M, the average length and width of the prepared hollow SiO2 nanorods were 397 nm and 116 nm, respectively, and the average length-diameter ratio was calculated as 3.4. The average size of the nanopores of the amorphous SiO2 shell were measured as 2.5 nm. Furthermore, increasing the concentration of Cl− ions improved length-diameter ratios of the FeOOH nanorod template and hollow SiO2 nanorods.

Influencing Factors of Load Carrying Capacity and Cooperative Work Laws of Metro Uplift Piles

The buoyancy of groundwater can reduce the foundation bearing capacity and cause the metro tunnels to float as a whole, which threatens the safety of structures seriously. Therefore, uplift piles are set up to improve the structural stability. In this paper, FLAC3D software is used to establish the calculation models of pile foundation. The bearing failure process of uplift piles was simulated to study the influencing factors on single pile load bearing capacity as well as the cooperative work laws of pile groups. The load-displacement curves of pile top under different length-diameter ratios, pile-soil interface characteristics and pile types are obtained, respectively. The results showed that, increasing the lengthdiameter ratio and the pile-soil interface roughness properly can improve the bearing capacity of uplift piles. Besides, changing the shapes of constant pile section can also improve it, which has the most significant effect concerning of saving material cost. In the loading process of pile groups, the ultimate bearing capacity of corner pile is the biggest, the side pile is the next, and the center pile is the smallest. The deformation characteristics of pile top are as follows: the center pile is the biggest, the side pile is the next, and corner pile is the smallest. Combined with the results, the uplift resistance of group piles can be enhanced pertinently, and the conclusions provide guidance for the design and construction of uplift piles in the actual engineer.

Static instability of the smooth annular seals with choked/unchoked flow

Abstract A computational fluid dynamics (CFD) method is applied to investigate the flow characteristics of a smooth annular seal for various eccentricities(e), length-diameter ratios (L/D) and taper coefficients(ƒ) under choked and unchoked flow conditions. For the unchoked flow condition, the viscous effect is the dominant factor for the negative static direct stiffness coefficient K of the experimental seal (L/D = 1.0) with high eccentricities. For the choked flow, the dominant viscous and inertial effect produce the negative K even at low eccentricities. Increasing L/D leads to a negative K eventually. The K of the divergent and convergent annular seal is negative and positive, respectively. The viscous effect in the minimum clearance of the tapered seal results in a negative K.

Effect of tungsten content on microstructure and mechanical properties of PCBN synthesized in cBN-Ti-Al-W system

Polycrystalline cubic boron nitride (PCBN) compacts were prepared using the mixture of cubic BN and Ti-Al-W powders at 5.5 GPa and 1550 °C for 10 min. The influence of different Tungsten (W) content on composition, microstructure, porosity, mechanical property and cutting performance of the PCBN is investigated. The results show that, with the addition of tungsten, the cubic boron nitride (cBN) crystals are connected with each other by the new product phases TiB2, TiN, Al3Ti and W2B under the pressure of 5.5 GPa and the temperature of 1550 °C. The rod-shaped crystals in the PCBN are expanded from the surface portion of the cBN. As the W content increases, the amount of rod-shaped crystals and the length-diameter ratios decrease in the system. When the tungsten content is 6 wt%, PCBN presents the best comprehensive performance and cutting performance, the porosity, the hardness, the flexural strength and the flank wear are 0.55%, 30.71 GPa, 972.3 MPa and 292 μm, respectively.

Experimental Study on Axial Compression Behavior and Bearing Capacity Analysis of High Titanium Slag CFST Columns

In order to study the axial compression behavior of concrete-filled steel tubular (CFST) columns filled with high titanium slag, a total of 32 specimens, including normal CFST columns, half-high titanium slag CFST columns, and full-high titanium slag CFST columns, were used as experimental samples in this study. The axial compression behaviors of high titanium slag CFST columns and normal CFST columns with various parameters such as length–diameter ratio, strength grade of concrete, strength grade of steel tube, steel content ratio, etc., were evaluated and compared through axial compression testing under monotonic static loading. The results showed that the axial compressive behaviors of high titanium slag CFST columns with various length–diameter ratios were not significantly different from those of normal CFST columns, both of which showed good axial compression performance. In addition, the length–diameter ratio limit between short and medium long column was from 3.5 to 4.4. The length–diameter ratio was the main factor influencing the shape of load–deformation curve of CFST columns. The casing hoop coefficient also had a great influence on the bearing capacity of short columns, while the influence on that of middle and long columns was not obvious. In the end, the bearing capacities of all specimens were calculated by bearing capacity formulas in European EC4, American AISC360-10, and Chinese GB50936-2014 standards. The calculated values were in good agreement with the test results.