Combining an obstacle and electrically driven vortices to enhance heat transfer in a quasi-two-dimensional MHD duct flow

Abstract

The design of vortex promoters in a heated-wall duct is often limited by the considerations of practicality, especially in complex systems such as fusion blankets. The present study investigates the use of current injection to invoke a street of vortices in quasi-two-dimensional high transverse magnetic field magnetohydrodynamic duct flows to enhance instability behind a cylinder. The intent is to generate intensive flow vorticity parallel to a magnetic field downstream of a field-aligned cylinder. Electric current enters the flow through an electrode embedded in one of the Hartmann walls, radiates outward, imparting a rotational forcing around the electrode due to the Lorentz force. The quasi-two-dimensional nature of these flows then promotes a vortical rotation across the interior of the duct with axis aligned to the magnetic field. The hot and cold walls are parallel to the magnetic field. Electric current amplitude and pulse width, excitation frequency and electrode position are systematically varied to explore their influences on the convective heat transport phenomenon. This investigation builds on a recommendation from previous work of Bühler (J. Fluid Mech., vol. 326, 1996, pp. 125-150) dedicated to understanding of the flow stability in a similar configuration. This study provides supportive evidence for the use of current injection as an alternative to the conventional mechanically actuated turbuliser, with heat transfer almost doubled for negligible additional pumping power requirements.

From three-dimensional to quasi-two-dimensional: transient growth in magnetohydrodynamic duct flows

“…The present numerical scheme is built upon an existing solver that has been rigorously validated for two-and three-dimensional incompressible Navier-Stokes flows and Q2D MHD flows (Hussam et al 2012b,a;Hussam & Sheard 2013;Vo et al 2015;Cassells et al 2016;Hamid et al 2016;Leigh et al 2016;Ng et al 2016;Sheard et al 2016). Nonetheless, a careful eye was kept on the conservation properties of the numerical algorithm; specifically, the discretised divergence of the velocity and current fields.…”

Linear stability of horizontal, laminar fully developed, quasi-two-dimensional liquid metal duct flow under a transverse magnetic field and heated from below

“…[15][16][17][18]) and heat transfer properties of a liquid metal flow is of great importance to the future designs of experimental fusion reactors where maximising heat transfer is a key criterion. The enhancement of heat transfer and instability growth in MHD flows can be achieved by implementing turbulent promoters such as bluff bodies [19][20][21] and current injection [22]. However, physical modifiers are not always practicable and therefore the characterisation of flow instabilities is required.…”

Current Injection Vortex Promoter for Heat Transfer Enhancement in a Magnetohydrodynamic Duct Flow

“…Hamid, et al [13] explained that this is due to the competition between the driving Lorentz force and the Hartmann damping. In the low-H regime, an increase in friction parameter yield stronger electrically driven vortices.…”

“…u, p and θ are the velocity, pressure and temperature fields, respectively, projected onto a plane orthogonal to the magnetic field,  is the gradient operator and 0 u is the force vector field [13]. The dimensionless parameters Reynolds number, Hartmann friction parameter and Peclet…”

“…Here length is scaled by L, velocity by 0 U , pressure by An advanced, high-order, in-house solver based on a spectral element method for spatial discretization is employed to simulate the cases. The implementation of the SM82 model within the solver has previously been validated in [5,6,13,14], and those papers should be consulted for further details.…”