Twisted Tube Research Articles

Experimental Study of Fouling Resistance in Twisted Tube Heat Exchanger

This paper discusses fouling of a twisted tube heat exchanger under different conditions of fluid velocity and heat input. The fluid velocity was varied from 0.5 to 2.0 m/s, whereas the heat input to the heat exchanger was varied from 200 to 800 W. The experimental results show that for low fluid velocity of 0.5 m/s, the fouling resistance showed noticeable variation with respect to heat input, whereas for high velocity ranges, that is, 1.0–2 m/s, the variation in fouling resistance is less. The fouling in twisted tube steadily increases with time for different values of heat input from 1000 min onward for fluid velocity in the range from 1.0 to 2.0 m/s. It is also observed that fouling resistance curves overlap for various values of heat input. During the initial 1000 min of the test duration, the maximum fouling in a twisted tube heat exchanger decreases with increase in fluid velocity from 1.0 to 2.0 m/s. This behavior of the fouling rate can be attributed to the fact that at higher fluid velocity, flow becomes turbulent, and this in turn flushes the fouling particles. The time-series correlations for the fouling resistance are found to be logarithmic in nature.

Heat transfer and pressure drop performance of twisted oval tube heat exchanger

Twisted oval tube heat exchanger is a type of heat exchanger that aims at improving the heat transfer coefficient of the tube side and also decreasing the pressure drop of the shell side. In the present work, tube side and shell side heat transfer and pressure drop performances of a twisted oval tube heat exchanger has been experimentally studied. The tube side study shows that the tube side heat transfer coefficient and pressure drop in a twisted oval tube are both higher than in a smooth round tube. The shell side study shows that the lower the modified Froude number FrM, the higher the shell side heat transfer coefficient and pressure drop. In order to comparatively analyze its shell side performance of the heat exchanger, a rod baffle heat exchanger with similar size of the twisted oval tube heat exchanger is designed and its performance is calculated with Gentry's method. The comparative study shows that the heat transfer coefficient of the twisted oval tube heat exchanger is higher and the pressure drop is lower than the rod baffle heat exchanger. In order to evaluate the overall performance of the twisted oval tube heat exchanger, a performance evaluation criterion considering both the tube side and shell side performance of a heat exchanger is proposed and applied. The analyze of the overall performance of the twisted oval tube shows that the twisted oval tube heat exchangers works more effective at low tube side flow rate and high shell side flow rate.

Trapped modes in finite quantum waveguides

The Laplace operator in infinite quantum waveguides (e.g., a bent strip or a twisted tube) often has a point-like eigenvalue below the essential spectrum that corresponds to a trapped eigenmode of finite L2 norm. We revisit this statement for resonators with long but finite branches that we call "finite waveguides". Although now there is no essential spectrum and all eigenfunctions have finite L2 norm, the trapping can be understood as an exponential decay of the eigenfunction inside the branches. We describe a general variational formalism for detecting trapped modes in such resonators. For finite waveguides with general cylindrical branches, we obtain a sufficient condition which determines the minimal length of branches for getting a trapped eigenmode. Varying the branch lengths may switch certain eigenmodes from non-trapped to trapped states. These concepts are illustrated for several typical waveguides (L-shape, bent strip, crossing of two stripes, etc.). We conclude that the well-established theory of trapping in infinite waveguides may be incomplete and require further development for being applied to microscopic quantum devices.

Experimental and numerical study of convective heat transfer and fluid flow in twisted oval tubes

Twisted oval tube heat exchanger is a type of heat exchanger aims at decreasing the pressure drop of the shell side. In the present study, heat transfer and pressure drop performances of twisted oval tube have been studied experimentally and numerically. The experimental study of the twisted oval tube shows that heat transfer process can be enhanced but also with an increasing of pressure drop when compared with the smooth round tube. The effects of geometrical parameters on the performance of the twisted oval tube have been analyzed numerically. The result reveals that the heat transfer coefficient and friction factor both increase with the increasing of axis ratio a/b, while both decrease with the increasing of twist pitch length P. The influence of a/b and P on the overall performance of the twisted oval tubes are also studied. Aiming at obtaining the heat transfer enhancement mechanism of the twisted oval tube, secondary flow, total velocity and temperature distributions of flow section are given. From the analysis it can be concluded that the emergence of twist in the twisted oval tube results in secondary flow. It exists in the form of spiral flow when a/b is big, but in the form of up and down when a/b is small. It is this secondary flow that changes the total velocity and temperature distributions of the twisted oval tube when compared with a smooth oval tube with the same sectional geometric parameters. Then the synergy angle between velocity vector and temperature gradient is reduced and the heat transfer process is enhanced.

SIMULATION OF NANOSCALE ETCHING FOR NANOTUBE AND GRAPHENE DEVICES

ABSTRACTIn order to find an efficient method to etch nano-carbon materials by hydrogenation in a controlled manner, we have studied hydrogen-atom adsorption on various deformed nanotubes using computer simulations based on the density-functional theory. The nanotube with an atomic lack is compared to a deformed tube with the Stone-Wales defect and a twisted tube wall. Similar to the known experimental etching condition for graphene, an atomic lack is effective to accumulate hydrogen atoms around the defect. Compared to the flat graphene, however, nanotube walls with curvature allow on-top adsorption of a hydrogen atom and selectivity in the hydrogenated site becomes worse. To achieve a controlled etching process, usage of a tungsten tip which realizes focused hydrogenation is proposed for natotubes and curved graphene.

Effect of twisted tape inserts on pressure drop during R-404A condensation

An experimental investigation was conducted to find the influence of twisted tape inserts on the pressure drop during forced convective condensation of R-404A vapor in a horizontal tube. The test section was a double-pipe counter-flow heat exchanger. The refrigerant flowed in the inner tube and cooling water flowed in the annulus. Experimental tests were carried out for plain tube and four tubes with different twisted tape turbulators and several mass velocities and vapor qualities. Investigation clearly showed that using twisted tapes inside tubes increases pressure drop between 89 and 239% higher than that for plain tube. An empirical correlation was developed to predict the pressure drop during condensation inside twisted tape inserted tubes. Comparing the predicted and experimental data, it was found that the majority of the present data falls within ±20% of the proposed correlation.

Twisted magnetic tubes with field aligned flow

Aims. We study the equilibrium and stability of twisted magnetic flux tubes with mass flows along the field lines. Then, we focus on the stability and oscillatory modes of magnetic tubes with uniform twist B0 = B0(r/peϕ + ez) in a zero-β plasma, surrounded by a uniform, purely longitudinal field. Methods. First we investigate the possible equilibriums, and then consider the linearised MHD equations and obtain a system of two first-order differential equations. These are solved numerically, while analytical approximations involving confluent hypergeometric functions are found in the thin tube limit. Finally, new appropriate boundary conditions are deduced and the outer solution considered (with the apparition of cut-off frequencies). We use this to derive a dispersion relation, from which the frequencies of the normal modes can be obtained. Results. Regarding the equilibrium, the only value of the flow that satisfies the equations for this magnetic field configuration is a super-Alfvenic one. Then, we consider the normal modes of this configuration. The thin-tube approximation proves accurate for typical values, and it is used to prove that the equilibrium is unstable, unless the pitch is large. The stability criteria for twisted tubes are significantly lowered. Conclusions. The twisted tube is subject to the kink instability unless the pitch is very high, since the Lundquist criterion is significantly lowered. This is caused by the requirement of having a magnetic Mach number greater than 1, so the magnetic pressure balances the magnetic tension and fluid inertia. This type of instability might be observed in some solar atmospheric structures, like surges.

Experimental study on convective heat transfer and flow resistance characteristics of water flow in twisted elliptical tubes

Heat transfer and flow resistance characteristics of water flow inside the twisted elliptical tubes (TETs) with different structural parameters were experimentally investigated. Effects of tube structural parameters (aspect ratio and twist pitch) on the performance of TETs were analyzed and the overall thermal-hydraulic performance of TETs was evaluated. Experimental results showed that the TETs can provide considerable heat transfer augmentation and also high pressure drop inside tube. Larger tube aspect ratios and smaller twist pitches resulted in higher heat transfer coefficients and friction factors. The best operating regime for TETs is at lower Reynolds numbers. It was also discovered that the experimental Nusselt numbers/friction factors can be expressed with one unified equation for entire Reynolds number range, which confirms the early flow transition from laminar to turbulent in TETs. Experimental results were compared with some existing correlations, and the causes for the differences between them were analyzed. Heat transfer enhancement mechanism of TETs was discussed from the viewpoint of field synergy. The longitudinal vortex induced by the twisted tube wall improves the synergy between the velocity vector and temperature gradient, which in turn results in a better heat transfer performance. ► Thermal-hydraulic characteristics of water flow in twisted elliptical tubes. ► Effect of tube structural parameter on performance of twisted elliptical tubes. ► Correlations for heat transfer and flow resistance of twisted elliptical tubes. ► Heat transfer enhancement mechanism basing on the field synergy principle.

Dependence of the Magnetic Energy of Solar Active Regions on the Twist Intensity of the Initial Flux Tubes

Abstract We present a series of numerical experiments that model the evolution of magnetic flux tubes with a different amount of initial twist. As a result of calculations, tightly twisted tubes reveal a rapid two-step emergence to the atmosphere with a slight slowdown at the surface, while weakly twisted tubes show a slow two-step emergence waiting longer the secondary instability to be triggered. This picture of the two-step emergence is highly consistent with recent observations. These tubes show multiple magnetic domes above the surface, indicating that the secondary emergence is caused by an interchange mode of magnetic buoyancy instability. In the case of the weakest twist, the tube exhibits an elongated photospheric structure, and never rises into the corona. The formation of the photospheric structure is due to an inward magnetic tension force of the azimuthal field component of the rising flux tube (i.e., tube's twist). When the twist is weak, the azimuthal field cannot hold the tube's coherency, and the tube extends laterally at the subadiabatic surface. In addition, we newly found that the total magnetic energy measured above the surface depends on the initial twist. Strong twist tubes follow the initial relation between the twist and the magnetic energy, while weak twist tubes deviate from this relation, because these tubes store their magnetic energy in the photospheric structure.

Fallopian Tube Torsion: A Rare Emergency

Abstract Background: Fallopian tube torsion is a rare cause of acute lower abdominal pain, occurring more commonly in females of the reproductive age group. It lacks any pathognomonic symptoms, signs, or imaging features, thus causing delay in surgical intervention. Case: This case reports a 19-year-old virgin girl presenting with acute lower abdominal pain, associated with mild tenderness in the lower abdomen and tender cystic mass on rectal examination. Laparoscopy revealed a twisted fallopian tube associated with an ovarian cyst. Right-sided salpingectomy was done as the tube was gangrenous. Conclusions: As fallopian tube torsion is a rare emergency, high clinical suspicion helps in early diagnosis and treatment, leading to salvage of the tube. (J GYNECOL SURG 27:41)

Instability of twisted magnetic tubes with axial mass flows

Context. Recent observations of various kinds of jets in the solar atmosphere motivate studying the influence of mass flow on the stability of solar magnetic structures.Aims. We study the influence of axial mass flows on the stability of twisted magnetic flux tubes.Methods. We use the incompressible magnetohydrodynamic equations to get the dispersion relation governing the behaviour of normal modes in uniformly twisted magnetic tubes with sub-Alfvenic flows. The dispersion relation is then solved analytically and numerically to find stability criteria for twisted tubes with flow.Results. Two main important results are found. First, the axial mass flow reduces the threshold of kink instability in twisted magnetic tubes. Second, the twist of magnetic tubes leads to the Kelvin-Helmholtz instability of sub-Alfvenic flows for the harmonics with a large enough azimuthal wave number –m . Conclusions. The observed mass flow may trigger the kink instability in magnetic configurations that are near their stability threshold, leading to solar flares and coronal mass ejections. The effect is more significant for photospheric magnetic tubes than for coronal ones. Sub-Alfvenic flows undergo the Kelvin-Helmholtz instability in slightly twisted magnetic tubes if the azimuthal wavenumber is big enough.

The Hardy inequality and the heat equation in twisted tubes

We show that a twist of a three-dimensional tube of uniform cross-section yields an improved decay rate for the heat semigroup associated with the Dirichlet Laplacian in the tube. The proof employs Hardy inequalities for the Dirichlet Laplacian in twisted tubes and the method of self-similar variables and weighted Sobolev spaces for the heat equation.

Performance analysis of heat exchangers of an existing naphtha hydrotreating plant: A case study

This paper presents the performance evaluation of heat exchangers of an existing naphtha hydrotreating (NHT) plant. Originally, the NHT plant consisted of six plain tube heat exchangers connected in series. During plant revamps operation, three plain tubes were replaced with the three twisted tube heat exchangers. In this study, the heat exchangers data were collected from the plant before and after installation of the three twisted tube heat exchangers. The data were then analyzed to see the effects of the twisted tube configuration on fouling of heat exchangers and heat transfer. The analysis of the data showed that the twisted tube heat exchangers caused reduction in fouling resistance of tubes and increased the heat transfer. Also, the replacement of the three shells and tube type heat exchangers by the twisted tubes resulted in an increase of feed flow rate by about 7.85%. An economic analysis showed that the simple payback period for the twisted tube heat exchangers is 2.12 years. It can be concluded that considerable benefits in terms of energy and cost savings can be realized through the application of this innovative twisted tube heat exchanger technology in existing or new chemical plants.

On the emergence of toroidal flux tubes: general dynamics and comparisons with the cylinder model

In this paper we study the dynamics of toroidal flux tubes emerging from the solar interior, through the photosphere and into the corona. Many previous theoretical studies of flux emergence use a twisted cylindrical tube in the solar interior as the initial condition. Important insights can be gained from this model, however, it does have shortcomings. The axis of the tube never fully emerges as dense plasma becomes trapped in magnetic dips and restrains its ascent. Also, since the entire tube is buoyant, the main photospheric footpoints (sunspots) continually drift apart. These problems make it difficult to produce a convincing sunspot pair. We aim to address these problems by considering a different initial condition, namely a toroidal flux tube. We perform numerical experiments and solve the 3D MHD equations. The dynamics are investigated through a range of initial field strengths and twists. The experiments demonstrate that the emergence of toroidal flux tubes is highly dynamic and exhibits a rich variety of behaviour. In answer to the aims, however, if the initial field strength is strong enough, the axis of the tube can fully emerge. Also, the sunspot pair does not continually drift apart. Instead, its maximum separation is the diameter of the original toroidal tube.

Computational Simulation of Swirl Enhanced Flow and Heat Transfer in a Twisted Oval Tube

Fully developed swirling Flow in a twisted tube with an oval cross section is considered. The results shown here are for a tube with an elliptical cross section with a twist ratio of 6 and an aspect ratio of 0.7 (see equations above). The wall of the tube is held at a constant temperature. The secondary flows induced by the tube twist promote mixing in the plane normal to the bulk flow direction. This mixing works to maintain a high temperature gradient close to the tube wall and in turn increase the Nusselt number relative to that in a straight tube of the same cross section. The temperature plots show that it is indeed the case that the in‐plane mixing is effective in maintaining the temperature gradient near the tube wall. The isometric view shows flow streamlines (ribbons represent adjacent streamlines) to illustrate the swirling natur eo ft he flow.

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

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

Diagnosis and management of ovarian cyst accidents

Ovarian cyst accidents include cyst rupture, haemorrhage and torsion. Torsion commonly occurs to the whole adnexa and is not necessarily associated with an ovarian cyst. Suspected adnexal torsion should always be managed with early laparoscopy and de-torsion of the twisted tube or ovary. Ovarian cyst rupture and haemorrhage usually occur in association with physiological (functional) cysts and are generally self-limiting. Laparoscopy may be necessary in cases where the diagnosis is in doubt or for haemodynamic compromise. Clinical features of ovarian cyst accidents are nonspecific. Ultrasound is the first-line investigation and is diagnostic in the case of haemorrhage. Typical ultrasound findings have been described for ovarian torsion, including an enlarged oedematous ovary with peripheral displacement of follicles. Doppler blood flow findings are variable and not diagnostic. Recurrent cyst rupture or haemorrhage should be prevented by suppression of ovulation, usually with the combined oral contraceptive. Fixation of the ovary by a variety of techniques should be considered to prevent recurrent torsion.

Heat Transfer of Boiling R134a and R142b on a Twisted Tube with Machine Processed Porous Surface

The objective of this work was to investigate nucleate pool boiling heat transfer performance and mechanism of R134a and R142b on a twisted tube with machine processed porous surface (T-MPPS tube) as well as to determine its potential application to flooded refrigerant evaporators. In the experimental range, the boiling heat transfer coefficients of R134a on a T-MPPS tube were 1.8–2.0 times larger than those of R134a on a plain tube. In addition, the developed experimental correlations verified that the predictions of the heat transfer coefficients of boiling R134a and R142b on a T-MPPS tube at the experimental conditions were considerably accurate.

10.1007/s11509-008-3003-8

Specific features pertinent to the flow of oil and heat transfer in bundles of smooth and profiled twisted tubes are investigated. The effect of neighboring tubes in a bundle on the heat transfer on a working tube is considered. Dependences for estimating drag and heat transfer in bundles of profiled twisted tubes are obtained.

3‐D non‐linear evolution of a magnetic flux tube in a spherical shell: The isentropic case

AbstractWe present recent 3‐D MHD numerical simulations of the non‐linear dynamical evolution of magnetic flux tubes in an adiabatically stratified convection zone in spherical geometry, using the anelastic spherical harmonic (ASH) code.We seek to understand the mechanism of emergence of strong toroidal fields from the base of the solar convection zone to the solar surface as active regions. We confirm the results obtained in cartesian geometry that flux tubes that are not twisted split into two counter vortices before reaching the top of the convection zone. Moreover, we find that twisted tubes undergo the poleward‐slip instability due to an unbalanced magnetic curvature force which gives the tube a poleward motion both in the non‐rotating and in the rotating case. This poleward drift is found to be more pronounced on tubes originally located at high latitudes. Finally, rotation is found to decrease the rise velocity of the flux tubes through the convection zone, especially when the tube is introduced at low latitudes. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)