Helical flow

Since there are no dead spaces in a SHE, the helical flow pattern combines to entrain any solids and create high turbulence creating a self-cleaning flow passage. 

Description These exchangers are typically a series of stacked helical-coiled tubes connected to manifolds, then inserted into a casing or shell. They have many advantages hke spiral-plate designs, such as avoiding differential expansion problems, acceleration effects of the helical flow increasing the heat transfer coefficient, and compactness of plot area. They are typically selected because of their economical design. 

The most common form has both sides in helical flow patterns, pure countercurrent flow is followed and the LMTD correction factor approaches 1.0. Temperature crosses are possible in single units. Like the spiral-plate unit, different configurations are possible for special apphcations. 

The density change on polymerization is typically about 20%, and this density gradient can cause significant secondary flows and natural convection effects. The experiments cited above for vinyl acetate polymerization were performed in a helical reactor. The centrifugal force in helical reactors induces secondary flows as well. The effects of helical flow have been analyzed, but were found to be less significant than the effects of natural convection [14]. 

ScHONEELD, F., Haedt, S., Simulation of helical flows in microchannels, AlChE J. (2003) accepted for publication. 

A perturbed helical flow structure of the advection field is indicated by the data. 

Merchuk, J. C., Ladwa, N., and Bulmer, M., Improving the Airlift Reactor The Helical Flow Promoter, 3rd Int. Conf. on Bioreactor and Bioprocess Fluid Dynamics, (A. W. Nienow, ed.), p. 61, Information Press, Ltd., Oxford (1993)... 

One of the benefits of static mixers like those mentioned above are the secondary flow patterns set up by the swirling helical flow. 

Let us point out that the impossibility of a dynamo in the spherical and plane cases has already been noted in [6, 9, 10]. We have shown that these cases are the only ones (for a stationary dynamo this fact was noted in [11]), and have generalized the assertion of boundedness of one of the field components to the general case. As an illustration of the second conclusion we point out the dynamo effect of a helical flow with v = 0 in cylindrical coordinates [13, 14]. The leading term in the maximal increment of this dynamo is proportional to for vm = 0 the dynamo effect is absent. Moreover, here the most effective generation is for angular harmonics with d/dtp 0, which is an additional indication of the decisive role of the coupling term 2s 2dHtp/d(p in the equation for Hs. 

Numerical calculations of helical flow in non-linear viscoplastic fluid have been... 

The results of the latest research into helical flow of viscoplastic fluids (media characterized by ultimate stress or yield point ) have been systematized and reported most comprehensively in a recent preprint by Z. P. Schulman, V. N. Zad-vornyh, A. I. Litvinov 15). The authors have obtained a closed system of equations independent of a specific type of rheological model of the viscoplastic medium. The equations are represented in a criterion form and permit the calculation of the required characteristics of the helical flow of a specific fluid. For example, calculations have been performed with respect to generalized Schulman s model16) which represents adequately the behavior of various paint compoditions, drilling fluids, pulps, food masses, cement and clay suspensions and a number of other non-Newtonian media characterized by both pseudoplastic and dilatant properties. 

A. N. Prokunin and M. L. Fridman 23 24) seem to be the first investigators to have done the first adequately accurate calculations of power-function fluid helical flow in a circular (tubular) head with a rotary core. At the same time a series of experiments... 

The Theory of Spiral (Helical) Flow of Molten Thermoplastic Polymers... 

Helical flow being analyzed as resultant from two independent flows (axial and circular), we may well assume that stable flow parameters (at least the flow rate) are determined primarily by viscous (flow) properties of the system, and the highelasticity effects (at superimposition of two flows) can be neglected in this case with a sufficient degree of accuracy which is reasonable from the point of view of engineering. The above assumtion was checked for correctness in 28,29) in a specific model of a viscoelastic fluid. 

Mixing by Helical Flows in Curved and Meander Micro Channels Most Relevant Citations... 

The generation of secondary helical flows in suitably curved channels, known as Dean vortices, is not an entirely new fluidic phenomenon discovered at the micro scale actually it was found already also for wound tubings of conventional diameter (see a summary in [152]). In-depth studies concerning Dean vortices in curved channels were made in the framework of various applications such as filtration, heat exchange, friction and mixing. 

Helical flows are not only found for 2-D mixer designs. In analogy with findings for macro-scale alternating helical coils, microstructured 3-D designs were proposed [49, 153], For reasons of limitations of today s micro fabrication, not real helices were made, but easier to fabricate structures such as complex micro channels based of L-shaped elements arranged in a 3-D fashion. 

Figure 1.143 Model geometry of the curved square channel used for simulating helical flows. Only half the geometry is shown due to reflection symmetry a and R denote the channel dimension and the radius of curvature, respectively [152].

Flow patterns of helical flow, depicted by velocity vectors... 

M 70a] [P 62] Computational flow simulation of the secondary flow, depicted by velocity vectors, was performed for Dean numbers of 10 and 100 [47]. The helical flow is weak for the smaller Dean number. The center of rotation is located close to the midpoint of the patch. For a Dean number of 100, a notable increase in the relative strength of the helical flow is observed the center of the vortex is shifted towards the outer channel wall. 

M 69] [P 61] In a curved channel, helical flows can be produced with four vortices, composed of two times two types, a small and large one (see Figure 1.147) [152], The total vorticity of the small vortices was integrated over the relevant part of the cross-section. It is found that these vortices start to develop at Dean numbers around 200. The strongest increase in the vorticity is observed at Dean numbers between 300 and 400. 

Benchmarking of helical flow strength in curved and bas-relieved channels... 

Periodic switching of helical flow in curved channels - chaotic advection... 

M 70a] [P 61] It is known that stirring and chaotic mixing can be achieved by an unsteady potential flow [154], This can also be utilized for helical flows in curved... 

The results obtained on mixing time also give experimental evidence for the existence of a threshold value around K = 140, which is associated with a change in vorticity [47]. At this point, the helical flow pattern switches from a two- to the four-vortex fluid system. Besides qualitatively confirming such a threshold value, the experiments are also in very good quantitative accord with the value predicted by the simulations. 

Simulation of helical flows for channel with oblique ridges - relative transverse velocity... 

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