Helical Profile Research Articles

Impact of Hybrid Heat Transfer Enhancement Techniques in Shell and Tube Heat Exchanger Design

Despite the advantages of shell and tube heat exchangers, one of their major problems is low thermal efficiency. This problem can be improved by using heat transfer enhancement techniques such as adding nanoparticles to the hot or cold fluids, and/or using tube inserts as turbulators on tube side as well as changing baffles to a helical or twisted profile on the shell side. Although all of these techniques increase the thermal efficiency; however, engineers still need a quantitative approach to assess the impact of these technologies on the shell and tube heat exchangers. This study attempts to provide a combination of such techniques to increase the impact of these improvements quantitatively. For this purpose, at first stage the thermal and hydraulic characteristics of pure fluid, Al2O3/water nanofluid in a plain tube equipped with and without twisted tape turbulator is evaluated based on a developed rapid design algorithm. Therefore, the impact of using enhanced techniques either in form of individual or in hybrid format and the increase of nanoparticle concentration in base fluid have been studied. The results show that using turbulators individually and in hybrid format with nanofluid can be effected on design parameters of a typical heat exchanger by reducing the required heat transfer area up to 10 %.

Two start and Three Start Helical Abrasive Flow Machining for Brittle Materials

In the experimental investigation on drill bit assisted AFM setup, a developed spline, two start helical profile and three start helical profile have been studied. It has been found that the use of new developed profile led to an improvement in the response parameter of percentage improvement in surface finish and material removal.Drill bit assisted abrasive flow machining is a non-conventional finishing operation. Before the advent of drill bit assisted abrasive flow machining simple abrasive flow machining was used. In simple abrasive flow machining media is simply move up and down through the work piece. In drill bit assisted abrasive flow machining a drill bit is used which passes through the work piece which helps to define the path of media responsible for finishing operation. In the present work three types of drill bits are used spline, two star helical and three star helical.For the design of experiment, the taguchi method approach has been employed. L9 OA has been used for the plan of experiments.The three process parameters viz drill bit type, number of cycle, extrusion pressure all have significant effect for the response parameter of MR .The result shows that type of rod have 33.4%, extrusion pressure have 23.46% and number of cycle have 19.53% contribution for response parameter of MR. At selected parameters a maximum improvement of 69.06% has been observed in the Surface finish. With the initial roughness 6.2 microns, an improvement to 1.91 has been observed on the inner cylindrical surface of the cast iron work piece.

Comparison Analysis and Verification on Spur and Helical Three-lobe Rotors with Novel Tooth Profile in Roots Blower

This paper proposes a novel three-lobe rotor tooth profile with the cross section of a combination of eccentric arcs and cycloidal curves for the Roots blower. Based on the geometric modelling of this new tooth profile, a helical tooth profile and a comparative study between the spur and the helical three lobes have been presented. The study reveals the advantages of the new helical tooth profile design which creates greater air flow and reduces the peak of the blower pressure. The tooth profile machining and testing are provided at the end of this paper.

Statistical analysis of material removal rate and surface roughness in electrical discharge turning of titanium alloy (Ti-6Al-4V)

Electrical discharge turning is a unique form of electrical discharge machining process, which is being especially developed to generate cylindrical forms and helical profiles on the difficult-to-machine materials at both macro and micro levels. A precise submerged rotating spindle as a work holding system was designed and added to a conventional electrical discharge machine to rotate the workpiece. A conductive preshaped strip of copper as a forming tool is fed (reciprocate) continuously against the rotating workpiece; thus, mirror image of the tool is formed on the circumference of the workpiece. The machining performance of electrical discharge turning process is defined and influenced by its machining parameters, which directly affects the quality of the machined component. This study presents an investigation on the effects of the machining parameters, namely, pulse-on time, peak current, gap voltage, spindle speed and flushing pressure, on the material removal rate (MRR) and surface roughness (Ra) in electrical discharge turning of titanium alloy Ti-6Al-4V. This has been done by means of Taguchi’s design of experiment technique. Analysis of variance as well as regression analysis is performed on the experimental data. The signal-to-noise ratio analysis is employed to find the optimal condition. The experimental results indicate that peak current, gap voltage and pulse-on time are the most significant influencing parameters that contribute more than 90% to material removal rate. In the context of Ra, peak current and pulse-on time come up with more than 82% of contribution. Finally, the obtained predicted optimal results were verified experimentally. It was shown that the error values are all less than 6%, confirming the feasibility and effectiveness of the adopted approach.

Assessment of the Sampling Performance of Multiple-Copy Dynamics versus a Unique Trajectory.

The goal of the present study was to ascertain the differential performance of a long molecular dynamics trajectory versus several shorter ones starting from different points in the phase space and covering the same sampling time. For this purpose, we selected the 16-mer peptide Bak16BH3 as a model for study and carried out several samplings in explicit solvent. These samplings included an 8 μs trajectory (sampling S1); two 4 μs trajectories (sampling S2); four 2 μs trajectories (sampling S3); eight 1 μs trajectories (sampling S4); 16 0.5 μs trajectories (sampling S5), and 80 0.1 μs trajectories (sampling S6). Moreover, the 8 μs trajectory was further extended to 16 μs to have reference values of the diverse properties measured. The diverse samplings were compared qualitatively and quantitatively. Among the former, we carried out a comparison of the conformational profiles of the peptide using cluster analysis. Moreover, we also gained insight into the interchange among these structures along the sampling process. Among the latter, we computed the number of new conformational patterns sampled with time using strings defined from the conformations attained by each of the residues in the peptide. We also compared the locations and depths of the obtained minima on the free energy surface using principal component analysis. Finally, we also compared the helical profiles per residue at the end of the sampling process. The results suggest that a few short molecular dynamics trajectories may provide better sampling than one unique trajectory. Moreover, this procedure can also be advantageous to avoid getting trapped in a local minimum. However, caution should be exercised since short trajectories need to be long enough to overcome local barriers surrounding the starting point and the required sampling time depends on the number of degrees of freedom of the system under study. An effective way to gain insight into the minimum MD trajectory length is to monitor the convergence of different structural features, as shown in the present work.

Enhancement of polyimide and 3D graphene-polyimide through thermoforming and its effect on mechanical properties and associated creep phenomenon

Polyimides (PIs) are a class of polymers used in diverse applications due to their excellent thermal stability, outstanding chemical resistance, and superb mechanical properties. One of the most widely used PIs is Pyromellitic Dianhydride-Oxydianiline (PMDA-ODA) PI (Kapton®), which can be purchased as thin films or tubes, thus limiting its use to specific applications. In this work, a novel thermoforming process is proposed which facilitates forming of planar PI and novel three-dimensional graphene PI (3D-C/PI) shapes into 3D contours. Our thermoforming process allows the formation of various shapes, such as helical, angular, or cylindrical profiles, thus expanding PIs' usage to new applications. The effect of the thermoforming process on the PI's chemical composition, mechanical properties, and chemical resistance to oxidizing plasma environment are studied. Also investigated is the creep resistance of the thermoformed PI compared to pristine PI at various temperatures; an analytical creep model has been fitted to the experimental data. Our results show that accelerated creep tests of thermoformed and pristine PI result in the same deformation and that the thermoforming process has no influence on the PI's chemical composition or chemical resistance. We also found that 3D-C/PI presents excellent thermoformability and that thermoformed PI exhibits higher elasticity compared to pristine PI.

GADIS: Algorithm for designing sequences to achieve target secondary structure profiles of intrinsically disordered proteins.

Many intrinsically disordered proteins (IDPs) participate in coupled folding and binding reactions and form alpha helical structures in their bound complexes. Alanine, glycine, or proline scanning mutagenesis approaches are often used to dissect the contributions of intrinsic helicities to coupled folding and binding. These experiments can yield confounding results because the mutagenesis strategy changes the amino acid compositions of IDPs. Therefore, an important next step in mutagenesis-based approaches to mechanistic studies of coupled folding and binding is the design of sequences that satisfy three major constraints. These are (i) achieving a target intrinsic alpha helicity profile; (ii) fixing the positions of residues corresponding to the binding interface; and (iii) maintaining the native amino acid composition. Here, we report the development of a G: enetic A: lgorithm for D: esign of I: ntrinsic secondary S: tructure (GADIS) for designing sequences that satisfy the specified constraints. We describe the algorithm and present results to demonstrate the applicability of GADIS by designing sequence variants of the intrinsically disordered PUMA system that undergoes coupled folding and binding to Mcl-1. Our sequence designs span a range of intrinsic helicity profiles. The predicted variations in sequence-encoded mean helicities are tested against experimental measurements.

Effect of Different Rotational Directions of BJUT-II VAD on Aortic Swirling Flow Characteristics: A Primary Computational Fluid Dynamics Study.

BackgroundThe BJUT-II VAD is a novel left ventricular assist device (LVAD), which is thought to have significant effects on the characteristics of aortic swirling flow. However, the precise mechanism of the rotational direction of BJTU-II VAD in the aortic swirling flow is unclear.Material/MethodsA patient-specific aortic geometric model was reconstructed based on the CT data. Three pump’s output flow profiles with varied rotational direction, termed “counterclockwise”, “flat profile”, and “clockwise”, were used as the boundary conditions. The helicity density, area-weighted average of helicity density (Ha), localized normalized helicity (LNH), wall shear stress (WSS), and WSS spatial gradient (WSSG) were calculated to evaluate the swirling flow characteristics in the aorta.ResultsThe results demonstrated that the swirling flow characteristics in the aorta and 3 branches are directly affected by the output blood flow of BJUT-II VAD. In the aortic arch, the helicity density, supported by the clockwise case, achieved the highest value. In the 3 branches, the flat profile case achieved the highest helicity density, whereas the maximum WSS and WSSG generated by clockwise case were lower than in other cases.ConclusionsThe outflow of the BJUT-II VAD has significant effects on the aortic hemodynamics and swirling flow characteristics. The helical blood profiles can enhance the strength of aortic swirling flow, and reduce the areas of low WSS and WSSG regions. The clockwise case may have a benefit for preventing development of atherosclerosis in the aorta.

Shaping of a helical surface in rotary machining

Besides traditional cutting tools, rotary tools are now being used. In view of their benefits, further research is advisable, with a view to their widespread use — in particular, in the machining of complex surfaces. The possibility of producing a helical profile by means of rotary tools in a traditional cutting configuration is mathematically established, with slip of the chip over the face (beginning at the larger diameter of a tapered tool).

A New Method of Diminishing Errors of Form Generated by the Imprecision of CNC Machines

Abstract During the processing of the helical surfaces on CNC machines, independent of the number of axes of the machine, there will certainly appear geometric deviations from the theoretical profile. Errors of form, which appear in this situation, can be detected through successive measurements. In order to diminish them, it is possible to use a method of compensation the deviations previously measured, which consists in adapting the CNC code used to obtain the helical profiles to the conditions required by the imprecision generated by the geometry of the machine.

Helicity inversion in spherical convection as a means for equatorward dynamo wave propagation

We discuss here a purely hydrodynamical mechanism to invert the sign of the kinetic helicity, which plays a key role in determining the direction of propagation of cyclical magnetism in most models of dynamo action by rotating convection. Such propagation provides a prominent, and puzzling constraint on dynamo models. In the Sun, active regions emerge first at mid-latitudes, then appear nearer the equator over the course of a cycle, but most previous global-scale dynamo simulations have exhibited poleward propagation (if they were cyclical at all). Here, we highlight some simulations in which the direction of propagation of dynamo waves is altered primarily by an inversion of the kinetic helicity throughout much of the interior, rather than by changes in the differential rotation. This tends to occur in cases with a low Prandtl number and internal heating, in regions where the local density gradient is relatively small. We analyse how this inversion arises, and contrast it to the case of convection that is either highly columnar (i.e., rapidly rotating) or locally very stratified; in both of those situations, the typical profile of kinetic helicity (negative throughout most of the northern hemisphere) instead prevails.

Computed tomography of the chest: I. Basic principles

### Key points The conventional chest radiograph superimposes a three-dimensional image onto a two-dimensional surface, so limiting its clinical usefulness. Since its introduction in 1971, X-ray computed tomography (CT) has rapidly evolved into an essential diagnostic imaging tool that forms a cross-sectional image, avoiding the super-imposition of structures that occurs in conventional chest imaging, with a >10-fold increase in attenuation sensitivity. Although CT imaging is reported by radiologists, it is important for both anaesthetists and intensivists to be able to interpret the scans, as reporting facilities may not be immediately available. Furthermore, the radiologist may not fully report all facets of a detailed scan and further information may be acquired by a physician with the ability to interpret CT scans. This is the first in a series of two articles written for anaesthetists and intensivists covering both thoracic anatomy and pathology as it relates to CT. A CT scanner makes many measurements, from different rotational angles, of X-ray attenuation ( weakening in force or intensity ) through the cross-sectional plane of the thorax. It then uses these data to reconstruct a digital representation of the cross-section with …

Spiral phase plates with radial discontinuities for the generation of multiring orbital angular momentum beams: fabrication, characterization, and application

A design of spiral phase plates for the generation of multiring beams carrying orbital angular momentum (OAM) is presented. Besides the usual helical profile, these phase plates present radial π-discontinuities in correspondence of the zeros of the associated Laguerre polynomials. Samples were fabricated by electron beam lithography over glass substrates coated with a polymethylmethacrylate resist layer. The optical response was analyzed and the purity of the generated beams was investigated in terms of Laguerre-Gaussian modes contributions. The far-field intensity pattern was compared with theoretical models and numerical simulations, while the expected phase features were confirmed by interferometric analysis with a Mach-Zehnder setup. The high quality of the output beams confirms the applicability of these phase plates for the generation of high-order OAM beams with nonzero radial index. An application consisting of the design of computer-generated holograms encoding information for light beams carrying phase singularities is presented and described. A numerical code based on an iterative Fourier transform algorithm has been developed for the computation of phase-only diffractive optical element for illumination under OAM beams. Numerical analysis and preliminary experimental results confirm the applicability of these devices as high-security optical elements for anticounterfeiting applications.

Design Method for Screw Forming Cutter Based on Tooth Profile Composed of Discrete Points

The forming method has been widely used for manufacturing screw rotors with helical profile. This paper takes the manufacturing of screws for screw pumps as an example and uses the cubic spline interpolation method to obtain the tooth profile of the screw forming cutter according to the tooth profile at any end section of screws composed of discrete points and based on the principle of gearing mesh. Furthermore, this paper studies the space enveloping and geometric characteristics between the screw and cutter during the manufacturing process, combines the shape of the contact line, which is generated due to the cooperative motion of the machine tool, screw, and cutter, with spatial location parameters, and thus innovatively proposes a design method for the screw forming cutter based on discrete points, namely, the form-position geometric method (FPGM). It can be seen after comparing the proposed method with the principle of gear meshing that the cutter-workpiece enveloping solution model, simplified by the FPGM, can overcome the key technical difficulty, i.e., it is difficult to accurately calculate the cusp of the tooth curve; meanwhile, the proposed method can improve the precision of the cutter tooth profile design. Finally, the feasibility and superiority of FPGM are verified by experiments.

Stability of compressible streamwise vortices

A sufficient condition for instability of full compressible inviscid streamwise vortices has been derived using asymptotic analysis with large wavenumbers. Based on the nature of instability, the obtained growth rate is described using an entropy instability term, a spiral instability term, a helicity instability term, and an acoustic instability term. The entropy instability term correlates with compressibility. The spiral instability term is reasonably expressed by the axial and azimuthal velocities and their wavenumbers. The helicity instability term depends on the helicity profiles. In particular, the unstable condition for wake-type flows is satisfied when they possess a negative helicity in the profiles. The acoustic instability term depends on the local speed of sound. Linear and nonlinear numerical simulations are used to validate the instability conditions. The effect of the helicity instability is shown to be significantly strong by the simulations.

DIFFERENTIAL ROTATION IN MAGNETIZED AND NON-MAGNETIZED STARS

Effects of magnetic field on stellar differential rotation are studied by comparing magnetohydrodynamic (MHD) models and their hydrodynamic (HD) counterparts in the broad range of rotation rate and in varying initial rotation profile. Fully-compressible MHD simulations of rotating penetrative convection are performed in a full-spherical shell geometry. Critical conditions for the transition of the differential rotation between faster equator (solar-type) and slower equator (anti-solar type) are explored with focusing on the "Rossby number (${\rm Ro}$)" and the "convective Rossby number (${\rm Ro}_{\rm conv}$)". It is confirmed that the transition is more gradual and the critical value for it is higher in the MHD model than the HD model in the view of the ${\rm Ro}_{\rm conv}$-dependence. The rotation profile shows, as observed in earlier studies, the bistability near the transition in the HD model, while it disappears when allowing the growth of magnetic fields except for the model with taking anti-solar type solution as the initial condition. We find that the transition occurs at ${\rm Ro} \simeq 1$ both in the MHD and HD models independently of the hysteresis. Not only the critical value, the sharpness of the transition is also similar between the two models in the view of the ${\rm Ro}$-dependence. The influences of the dynamo-generated magnetic field and/or the hysteresis on convective motion are reflected in the ${\rm Ro}$. This would be the reason why the transition is unified in the view of the ${\rm Ro}$-dependence. We finally discuss the ${\rm Ro}$-dependence of magnetic dynamo activities with emphasis on its possible relation to the kinetic helicity profile.

The fixity of prostate seed implants: The impact of the strand surface on its ability to migrate inside oil and gel medium.

Radioactive seed implants are widely used to treat cancer patients, most commonly those with prostate cancer. However, the seeds have a tendency to migrate after placement in patients, a phenomenon that can result in unfavorable outcomes. The ability of the seed strand to migrate was investigated by examining the impact of the strand surface on the velocity of its movement inside oil and gel media. We investigated the motion of smooth surface strands and strands with different grooved helical profiles after they were placed in oil and gel media. Three patterns of grooved helices were studied (60, 140, and 300 rotations per meter). The movement of the strands through a tube filled with the medium was recorded by the motion sensor, and the drag forces on the individual strands were calculated and compared for the oil and gel media. The strands with 60, 140, and 300 rotations/meter grooved helical surfaces demonstrated less mobility in both oil and gel than the strands with a smooth surface. The strand with the highest number of helical grooves per meter recorded the largest drag force and moved more slowly in both media. The differential in the motion of the smooth strand and the strands with grooved surfaces can be attributed to the increased surface area of the grooved strands. This finding is significant since it will impact, theoretically, the design, and thus the migration of seed implants that are used to treat cancer patients, particularly those with prostate cancer.

A factor involved in efficient breakdown of supersonic streamwise vortices

Spatially developing processes in supersonic streamwise vortices were numerically simulated at Mach number 5.0. The vortex evolution largely depended on the azimuthal vorticity thickness of the vortices, which governs the negative helicity profile. Large vorticity thickness greatly enhanced the centrifugal instability, with consequent development of perturbations with competing wavenumbers outside the vortex core. During the transition process, supersonic streamwise vortices could generate large-scale spiral structures and a number of hairpin like vortices. Remarkably, the transition caused a dramatic increase in the total fluctuation energy of hypersonic flows, because the negative helicity profile destabilizes the flows due to helicity instability. Unstable growth might also relate to the correlation length between the axial and azimuthal vorticities of the streamwise vortices. The knowledge gained in this study is important for realizing effective fuel-oxidizer mixing in supersonic combustion engines.

Model dynamo may solve Mercury mystery

Simulations suggest that the planet’s top-heavy magnetic field derives from the unusual chemistry of its core.

Conical light sword optical beam and its healing property

A new diffractive optical element, named as a conical light sword optical element, is presented. In the focal volume, this element produces a helical amplitude profile that can be used as an optical twister. We have experimentally demonstrated the optical healing property of the conical light sword optical beam (CLSOB). This healing property comes from the transverse helical energy flow, due to the evolution of multiple unipolar vortices in the propagation of CLSOB. We envisage that this spiral intensity profile and optical healing property of the beam find potential applications in propagation through a scattering and turbulent media, imaging with extended depth of field, and in optical tweezers.