The mechanism of vortex instability in electromagnetically driven flow in an annular thin layer of electrolyte

A circumferential flow of a conducting fluid in an annular channel can be created by the action of a Lorentz force arising as a result of the interaction between an applied vertical magnetic field and a radial electric current flowing through the electrolyte. Quite unexpectedly, experiments revealed that a robust vortex system appears near the outer cylindrical wall in such flows. McCloughan and Suslov (J. Fluid Mech. 887:A23, 2020) (McCS) reported comprehensive linear stability results of such a flow for variable Lorentz forcing. Here we complement that study by investigating the flow structure as a function of the channel aspect ratio. Remarkably, despite the completely different physical nature of parametric dependences, dynamic in McCS and purely geometric here, we show that in both scenarios vortices appear on a background of a steady axisymmetric flow at the boundary between two counter-rotating toroidal structures and have a similar energy distributions. The two studies demonstrate the robustness of the mechanism responsible for the vortex formation: Rayleigh's inviscid centrifugal instability aided by radial shear in the boundary layer near the outer cylindrical wall. References P. A. Davidson. An introduction to magnetohydrodynamics. Cambridge University Press, 2nd edition, 2017. doi:10.1017/CBO9780511626333. J. McCloughan and S. A. Suslov. Linear stability and saddle–node bifurcation of electromagnetically driven electrolyte flow in an annular layer. J. Fluid Mech., 887:A23.1–30, 2020. doi:10.1017/jfm.2020.29. J. Perez-Barrera, J. E. Perez-Espinoza, A. Ortiz, E. Ramos, and S. Cuevas. Instability of electrolyte flow driven by an azimuthal Lorentz force. Magnetohydrodynamics, 51(2):203–213, 2015. http://mhd.sal.lv/contents/2015/2/MG.51.2.4.R.html. S. A. Suslov, J. Perez-Barrera, and S. Cuevas. Electromagnetically driven flow of electrolyte in a thin annular layer: Axisymmetric solutions. J. Fluid Mech., 828: 573–600, 2017. doi:10.1017/jfm.2017.551.

Rayleigh–Taylor Instability of Swirling Annular Layer With Mass Transfer

The interfacial instability of Rayleigh–Taylor type at the cylindrical boundary involving the liquid phase and vapor phase of a fluid has been considered when the vapor is warmer than the liquid. We use viscous potential flow theory to include the viscosity at the interface. To examine the stability of the arrangement, the normal-mode analysis is performed together with the effect of heat as well as mass transfer and free swirl. The physical system consists of an annular fluid layer restricted in a cylinder with vapor phase in the core. This work investigates the effect of a variety of variables on the instability of the interface. It is found that when the heat transfer constant increases, the range of stability increases. Also, the range of stability increases faster in the presence of swirling.

Steel-free hybrid reinforcing bars for concrete structures

Extensive research has been conducted on the replacement of steel rebars with fibre-reinforced polymer rebars to eliminate the steel corrosion problem in conventional steel bar–reinforced concrete structures. However, as the performance of fibre-reinforced polymer rebars is substantially inferior in compression (due to issues such as fibre micro-buckling) than in tension, their use in concrete columns is generally not recommended; this poses a significant challenge when a steel-free structure is needed. This article presents a novel steel-free hybrid rebar developed at The Hong Kong Polytechnic University that overcomes the above-mentioned problem. Such a hybrid rebar typically consists of a central fibre-reinforced polymer rebar, an external fibre-reinforced polymer confining tube and an annular layer of high-strength cementitious material such as ultrahigh-performance concrete. To demonstrate the performance of these hybrid rebars, results from a series of preliminary tests and associated modelling work are presented in the article. These results indicate that (1) the fibre-reinforced polymer rebar at the centre is well supported against bar buckling and fibre micro-buckling, (2) the compressive strength of the fibre-reinforced polymer material can be fully mobilized and (3) the stress–strain response of hybrid rebars can be designed to resemble an elastic–plastic response with some post-yielding hardening.

Variation principle in calculating the flow of a two-phase mixture in the pipes of the cooling systems in high-rise buildings

The analytical solution of one of the urgent problems of modern hydromechanics and heat engineering about the distribution of gas and liquid phases along the channel cross-section, the thickness of the annular layer and their connection with the mass content of the gas phase in the gas-liquid flow is given in the paper.The analytical method is based on the fundamental laws of theoretical mechanics and thermophysics on the minimum of energy dissipation and the minimum rate of increase in the system entropy, which determine the stability of stationary states and processes. Obtained dependencies disclose the physical laws of the motion of two-phase media and can be used in hydraulic calculations during the design and operation of refrigeration and air conditioning systems.

Electromagnetically driven flow of electrolyte in a thin annular layer: axisymmetric solutions

Experimental observations of an azimuthal electrolyte flow driven by Lorentz force in a thin annular fluid layer placed on top of a magnet show that it develops a robust vortical system near the outer cylindrical wall. It appears to be a result of instabilities developing on a background of steady axisymmetric flow. Therefore, the goal of this paper is to establish a scene for a future comprehensive stability analysis of such a flow. We discuss popular depth-averaged and quasi-two-dimensional approximate solutions that take advantage of the thin-layer assumption first, and argue that they cannot lead to the observed flow patterns. Thus, three-dimensional toroidal flows are considered. Their similarities to various other well-studied rotating flow configurations are outlined, but no close match is found. Multiple axisymmetric solutions are detected numerically for the same governing parameters, indicating the possibility of subcritical bifurcations, namely type 1, consisting of a single torus, and type 2, developing a second counter-rotating toroidal flow near the outer cylinder. It is suggested that the transition between these two axisymmetric solutions is likely to be caused by the centrifugal instability, while the shear-type instability of the type 2 solution may be responsible for the observed vortex structures. However, a dedicated stability analysis which is currently underway and will be reported in a separate publication is required to confirm these hypotheses.

THE METHOD OF CALCULATION AND ANALYSIS OF HEAT TRANSFER COEFFICIENT OF BIMETALLIC FINNED TUBES OF AIR COOLING UNITS WITH IRREGULAR EXTERNAL CONTAMINATION

The article focuses on a new method of calculating heat transfer coefficient of bimetallic finned tubes of air coolers taking into account external operational pollution. In contrast to wellknown methods that use the assumption of a uniform distribution of operational contamination layer with a constant thickness over the entire surface of the fins in the present method being introduced it is assumed that the thickness of the pollution layer during long-term operation is changed irregularly. Under such conditions the thickness of the pollution layer at the base of the fins becomes much greater than at the rest of the finned surface. The suggested method is based on a mathematical model developed with the use of the method of electrothermal analogy, whereby the heat flow through the wall of the finned tube is considered as divided into two components, viz. through the annular layer of outside contamination adjacent to the base of the ribs, and through the remaining part of the external ribbed surface covered with a thin layer of pollution. Within the framework of the developed methodology a new method for determining the thermal resistance of the pollution layer, which is based on analytical solution of two dimensional problem of heat conduction in the annular layer has been created. With the use of this technique the influence of the degree of contamination of the intercostal space of the industrially manufactured bimetallic finned tubes on the heat transfer coefficient has been studied taking into account the intensity of heat transfer of air and the properties and composition of the pollutant for industrial manufactured bimetallic finned tubes. It is established that a layer thickness of the pollutant at the base of the ribs has the greatest influence on the heat transfer coefficient. This is due primarily to the change of actual coefficient of the fins. It is demonstrated that the heat conductivity of the external pollutant has a significant impact on the heat transfer coefficient when the heat exchanger functions in the mode of forced convection of air.