Contents lists available at ScienceDirect International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt Buoyancy-driven instabilities and particle deposition in a Taylor-Couette apparatus Mohamed Aksouh a, ⁎ , Rachida Chemini b , Amina Mataoui a , Sébastien Poncet c a Faculty of Physics, USTHB, Algiers, Algeria b Faculty of Mechanical and Process Engineering, USTHB, Algiers, Algeria c Mechanical Engineering Department, Université de Sherbrooke, Sherbrooke, QC, Canada ARTICLE INFO Keywords: Taylor-Couette flow Natural convection Buoyancy-driven instability Lagrangian particle tracking Numerical simulation ABSTRACT The present paper aims at studying the particle trajectories and sedimentation inside Taylor-Couette buoyancy- driven flows. The dynamical and thermal features of Taylor–Couette-flows inside a three-dimensional differ- entially heated cavity are investigated for Reynolds numbers Re ranging from 67.3 to 392.7 and Grashof numbers Gr between 764.4 ≤ Gr ≤ 3822.1. The results indicate a strong interaction between natural convection and the base Taylor-Couette flow due to rotation for a weak radial temperature gradient. A spectral analysis allows to identify different flow regimes. For discrete particle simulations, the Lagrangian Particle Tracking method is used to follow the particle trajectories inside the Taylor-Couette apparatus. Water droplets are con- sidered as solid spherical particles with different diameters (10 ≤ D p ≤ 35 μm). The time analysis of suspended and deposited particles along different walls shows that the rotation of the inner-cylinder coupled to the natural convection influences significantly the time and location of the particle deposition. 1. Introduction A viscous fluid confined in an annular space of two rotating coaxial cylinders is called Taylor–Couette flow. The Taylor-Couette configura- tion is characterized by height-to-gap aspect ratio Γ = H/(r o - r i ) and the radius-ratio η = r i /r O . For ideal case, the height-to-gap aspect ratio tends to infinity (Γ → ∞) and the radius ratio is equal to unity (η = 1). However, for rotating machinery, such as rotating compressors, turbo- machines, electrical motors, rotating heat exchangers, the height of the cylinders is finite inducing thus end-wall effects on the flow structure. As an example, Lalaoua and Naït Bouda [1] recently performed a nu- merical investigation on the onset of axisymmetric and wavy Taylor- Couette flows between three combinations of cylinders and spher- ocylinders, which modify the end-cap conditions of the classical Taylor- Couette configuration. Their results showed that the transition from one regime to another is delayed compared to the base case between con- centric cylinders. Similarly, imposing a radial temperature gradient between the two cylinders engenders an axial flow due to natural convection. Since many decades, several studies were performed experimentally [2–4], theoretically [5,6] and numerically [7,8]. These studies were carried out to understand the stability and flow structures for combined Taylor- Couette and natural convection flow. Apart from being a canonical flow to understand the route to turbulence, Taylor-Couette flows are also relevant in many industrial applications: chemical vapor deposition techniques, crystal growth, cooling of rotating electrical motors, reactor fuel rods and rotating tube heat exchangers [9], among other examples. By heating the rotating inner cylinder and keeping the outer cy- linder stationary (RHISCO), the buoyancy effects modify significantly the Taylor vortices which are altered inside the annular space [7]. The same flow behavior was detected experimentally by Lepiller et al. [4] on the stability of Taylor-Couette flow subjected to a weak radial temperature gradient. They found that the radial temperature gradient destabilizes the Taylor-Couette flow leading to a pattern of moving helical vortices only near the bottom below the threshold of the Taylor vortices. The size of the pattern increases as the rotation frequency of the cylinder raises. However, Sorour and Coney [3] showed experi- mentally that the stability curve is independent of fluid properties, but varies versus the annular radius-ratio, Taylor number and temperature difference. In a tall Taylor-Couette apparatus (Γ = 80 and η = 0.8), the formation of instabilities was investigated numerically by Viazzo and Poncet [10]. Seven instability regimes were highlighted by DNS cal- culations as spiral rolls being regular (SPI), modulated (MSPI), wavy (WSPI) or with defects (SPI + D), Taylor vortex flow (TVF) or wavy vortex flow (WVF) or a combination of both (SPI + WVF), confirming the experimental results of Guillerm [11]. In a recent work, Lopez et al. https://doi.org/10.1016/j.icheatmasstransfer.2020.104518 ⁎ Corresponding author. E-mail address: maksouh@usthb.dz (M. Aksouh). International Communications in Heat and Mass Transfer 113 (2020) 104518 0735-1933/ © 2020 Elsevier Ltd. All rights reserved. T