We study non-isothermal buoyancy-driven exchange flow of two miscible Newtonian fluids in an inclined pipe experimentally. The heavy cold fluid is released into the light hot one in an adiabatic small-aspect-ratio pipe under the Boussinesq limit (small Atwood number). At a fixed temperature, the two fluids involved have the same viscosity. Excellent qualitative and quantitative agreement is first found against rather recent studies in literature on isothermal flows where the driving force of the flow comes from salinity as opposed to temperature difference. The degree of flow instability and mixing enhances as the pipe is progressively inclined towards vertical. Similar to the isothermal limit, maximal rate of the fluids interpenetration in the non-isothermal case occurs at an intermediate angle, . The interpenetration rate increases with the temperature difference. The degree of fluids mixing and diffusivity is found to increase in the non-isothermal case compared to the isothermal one. There has also been observed a novel asymmetric behavior in the flow, never reported before in the isothermal limit. The cold finger appears to advance faster than the hot one. Backed by meticulously designed supplementary experiments, this asymmetric behavior is hypothetically associated with the wall contact and the formation of a warm less-viscous film of the fluid lubricating the cold more-viscous finger along the pipe. On the other side of the pipe, a cool more-viscous film forms decelerating the hot less-viscous finger. Double diffusive effects associated with the diffusion of heat and mass (salinity) are further investigated. In this case and for the same range of inclination angles and density differences, the level of flow asymmetry is found to decrease. The asymmetric behaviour of the flow is quantified over the full range of experiments. Similar to the study of Salort et al. [“Turbulent velocity profiles in a tilted heat pipe,” Phys. Fluids 25(10), 105110-1–105110-16 (2013)] for tilted heat pipes, a small Richardson number of is found, above which flow laminarization occurs. In terms of the dimensionless numbers of the problem, it is found that the interpenetrative speeds of the heavy and light fluid layers in non-isothermal and double-diffusive cases increase with the dimensionless temperature difference, rT, Atwood number, At, Grashof number, Gr, Reynolds number, Re, Nahme number, Na, and Péclet number, Pe but decreases with Prandtl number, Pr, and Brinkman number, Br.
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Research Article| June 22 2017
Non-isothermal buoyancy-driven exchange flows in inclined pipes
B. Eslami, S. Shariatnia, H. Ghasemi, K. Alba; Non-isothermal buoyancy-driven exchange flows in inclined pipes. Physics of Fluids 1 June 2017; 29 (6): 062108. https://doi.org/10.1063/1.4986561
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