Intermittent flow regimes near the convection threshold in ferromagnetic nanofluids

Abstract

The onset and decay of convection in a spherical cavity filled with ferromagnetic nanofluid and heated from below are investigated experimentally. It is found that, unlike in a single-component Newtonian fluid where stationary convection sets in as a result of supercritical bifurcation and where convection intensity increases continuously with the degree of supercriticality, convection in a multicomponent ferromagnetic nanofluid starts abruptly and has an oscillatory nature. The hysteresis is observed in the transition between conduction and convection states. In moderately supercritical regimes, the arising fluid motion observed at a fixed temperature difference intermittently transitions from quasiharmonic to essentially irregular oscillations that are followed by periods of a quasistationary convection. The observed oscillations are shown to result from the precession of the axis of a convection vortex in the equatorial plane. When the vertical temperature difference exceeds the convection onset value by a factor of 2.5, the initially oscillatory convection settles to a steady-state regime with no intermittent behavior detected afterward. The performed wavelet and Fourier analyses of thermocouple readings indicate the presence of various oscillatory modes with characteristic periods ranging from one hour to several days.

High Rayleigh number convection in a cubic cell with adiabatic sidewalls

“…In order to check the possibility of LSC reorientations in a cubic cell we carried out a series of temperature measurements. A simple and efficient way to study LSC orientation in a closed cavity is based on a set of differential thermocouples [19,41,42]. In our case temperature measurements were done in the cell A by four differential copper-constantan thermocouples, located in the middle horizontal cross-section (Fig.…”

Colloidal Energetic Systems

“…The possibility of the occurrence of periodic features in thin FF layers, as shown in Figure , has been demonstrated both experimentally and in numerical simulations. Considering now the case of spherical symmetry with no constraints on the thickness of the active FF layer, other researchers reported experimental results that show a harmonic response and invite the scientific community to provide results on other nonmagnetic colloids . As shown in the spherical setup shown in Figure , the onset of thermomagnetic convection was recorded with a temperature difference between the top and bottom of the 16 mm in‐diameter‐sphere of only 1.9 °K, resulting in a typical convection shear rate of ≈10 −1 s −1 .…”

“…Base convection vortex developed in a spherical cell. Reproduced with permission . Copyright 2015, American Physical Society. …”

“…The limiting factor being the number of windings, giving a minimum inner diameter of 2.21 mm, and to maintain the same liquid volume (2.144 mm 3 ) of reported experiments, the outer diameter is 19.93 mm. The vertical speed of the FF may be estimated to reach the order of 1 mm s −1 under a thermal gradient of ≈2 K . The typical magnetization of an iron surfactant‐based spherical NP–FF has been theoretically shown to be in the range of 0.6 T (with 90% filling factor), while relevant increments to this typical value are expected to be reached by nanostructuration, as elongated Janus particles could double the magnetization .…”

О Перераспределении Примеси В Коллоидных Смесях

“…В экспериментальных наблюдениях конвекции наножидкостей существуют течения, которыe не проявляются в однородных средах: бегущие волны [3,5], перебросовые течения [6,7], локализованные структуры [8]. Теоретические исследования показывают, что большинство подобных течений обусловлены концентрационной неоднородностью смеси, которые вызываются гравитационным [9,10] или термодиффузионным [11] разделением компонент, магнетофорезом [12], а также их совместным влиянием [13].…”