Enhancing heat transfer in a high Hartmann number magnetohydrodynamic channel flow via torsional oscillation of a cylindrical obstacle
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Abstract:
An approach is studied for side-wall heat transfer enhancement in the magnetohydrodynamic flow of fluid in a rectangular duct that is damped by a strong transverse magnetic field. The mechanism employs the rotational oscillation of a cylinder placed inside the duct to encourage vortex shedding, which promotes the mixing of fluid near a hot duct wall with cooler fluid in the interior. The effectiveness of the heat transfer enhancement is investigated over a wide range of oscillation amplitudes and forcing frequencies. The motivation for exploring this mechanism is inspired by the transient growth response of this flow, which indicates that the optimal disturbances feeding the vortex shedding process are localized near the cylinder, and are characterized by an asymmetrical disturbance with respect to the wake centreline. The results show that a considerable increase in heat transfer from the heated channel wall due to rotational oscillation of the cylinder can be achieved, with the maximum enhancement of more than 30% over a zone extending 10d downstream of the cylinder. As the angular velocity amplitude of oscillation is increased, the range of oscillation frequencies for effective enhancement is widened, and the frequency at which the peak Nusselt number occurs is shifted slightly to lower frequencies. As the amplitude is increased, the formation of strong discrete wake vortices draws fluid from the wall boundary layers into the wake, enhancing heat transfer. The effect of oscillation amplitude on the distribution of local Nusselt number \documentclass[12pt]{minimal}\begin{document}$\textit {Nu}_w$\end{document}Nuw along the heated wall is significant. With an increase in Reynolds number, scope for additional heat transfer enhancement is possible.Keywords:
Oscillation (cell signaling)
Heat transfer enhancement
Vortex shedding
In order to better understand the von Kármán vortex shedding on the basis of the concept of absolute instability, the vortex shedding from a blunt-ended flat-plate is examined experimentally through controlling the wake development. The stability analysis is also made for the velocity distributions in the near wake region to obtain their local instability characteristics. When the vortex shedding is controlled by another flat-plate in the wake, the shedding frequency is remarkedly changed. Such change in frequency is found to be closely related to the change in the absolute instability characteristics of the wake velocity distributions immediately behind the plate base.
Vortex shedding
Kármán vortex street
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The article [10] has presented the necessity of the classification into two types: Isolated wakes and coupled wakes. In this report, different types of momentum defect diffusion at the beginning of wake are analyzed for the two types of coupled wake and isolated wake. According to ·numerical results obtained, we analyses the existence - long or short - of the wake establishment zone before the wake established zone, and this zone is very different between isolated wake and coupled wake.
Momentum (technical analysis)
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Heat transfer, friction factor and enhancement efficiency characteristics in an elliptic tube fitted with elliptic rings have been investigated experimentally. In the experiments, air was used as the tested fluid with a Reynolds number range of 10000 to 32464. The experimental results show a considerable increase in friction factor and heat transfer over the plain tube under the same operation conditions. Over the range investigated, the Nusselt numbers for both employed enhancement devices with different pitches are found to be higher than that of the plain tube. It was found that the best overall enhancement was achieved with pitch = 3d. The results obtained are correlated in the form of Nusselt number and friction factor as a function of Reynolds number and pitches. The results were compared with circular tubes have the same test conditions to show the difference between the circular and elliptic tubes. Key words : Heat transfer enhancement; Elliptic tube
Friction Factor
Heat transfer enhancement
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This article considers the effect of duct shape on the Nusselt number. In order to show the effects of duct shape on the Nusselt number, four illustrative examples are given: the heat transfer in a duct of rectangular cross-section, heat transfer in a duct of semicircular cross-section, the heat transfer in a duct of circular crosssection and heat transfer between two parallel plates. The method used in this paper is general and it can be applied al flows. It is not necessary to use thin plate analogy, first, the velocity distribution is obtained and then the temperature distribution is found. It is shown that the Nusselt numbers calculated for the four examples cited depend not only on duct shape but also on wall friction.
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Vortex shedding
Wake turbulence
Kármán vortex street
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Kármán vortex street
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Kármán vortex street
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Kármán vortex street
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A passive control is numerically investigated in this paper to
effectively suppress the vortex shedding from bluff bodies. To
implement the control, thr
ee sinusoidal leading edge
configurations are considered and compared to a straight square
cylinder case. Large Eddy Simulati
on (LES) is used to model the
wall effects, as well as the near wake.
Numerical velocity fluctuations in the near wake compare well
with experimental data. The LES observations provide an accurate
prediction of wake instability and near wake topology, otherwise
not provided experimentally. This advantage can be used for
further investigation of
a thorough understanding and
enhancement of control.
Numerical case studies are presented using the software package
FLUENT
and the observations are pres
ented in the form of design
charts, as well as velocity spectra and near wake flow details.
Vortex shedding
Bluff
Flow Control
Kármán vortex street
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Vortex shedding
Bluff
Wake turbulence
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