Investigations of augmented heat transfer due to elastic instabilities within low-inertia flows with streamline curvature
Date of Award
Doctor of Philosophy (PhD)
Mechanical and Aerospace Engineering
Heat--Transmission, Viscous flow, Reynolds number, Fluid mechanics
The present investigation considers thermal transport variations from the presence of elastic instabilities within internal flows. A miniature Viscous Disk Pump (VDP) and a Couette Rotating Flow (RCF) environment are utilized to investigate flow and heat transfer characteristics for very low Reynolds numbers. The viscous disk pump heat transfer results are based upon energy balance measurements which utilize the mixed-mean temperature at the inlet and outlet of the VDP flow passage. The overall heat transfer rate is determined based upon a constant surface temperature thermal boundary condition, and upon a log-mean-temperature difference approach. Significant enhancements of mixing and transport are observed, which are associated with the onset and development of elastic instabilities. Such behavior is verified, relative to Newtonian Boger fluid solvents, using flow visualization results, pressure rise data, overall shear stress variations, increases of overall magnitudes of convective Nusselt numbers, increases of time-averaged magnitudes of mean-square temperature fluctuations, and increases of spectral energy content of unsteady temperature fluctuations. Results associated with RCF are presented which further demonstrate the remarkable ability of viscoelastic fluids to augment surface heat transfer characteristics. With this flow arrangement, the overall heat transfer rate is determined based upon a constant surface heat flux thermal boundary condition. Flow visualization results and Nusselt number data show that the onset of what is believed to be elastic instabilities is well represented using a constant Weissenberg number of 30 criterion. Variations of normalized and time-averaged magnitudes of the mean-square of flow static temperature fluctuations are also consistent with this criterion, since those values also increase substantially as Weissenberg number becomes greater than 30. Power spectral distributions of static temperature fluctuations, for the highest polymer concentrations and highest disk rotation speeds and the same experimental conditions, show broad-band characteristics, with continuous power content present at frequencies from just greater than 0.01 Hz to values of approximately 1 Hz. These spectral distributions are as much as three orders of magnitude higher than distributions which are associated either with the sucrose solvent flow, where ρ=0 ppm, or when no rotation is present and ω=0 rad/s. Magnitudes of Nusselt number enhancement, associated with increased polymer concentration and increased shear rate, are as high as 3.0, compared to zero shear rate values (obtained with zero rotation), which are in the vicinity of 1.0.
Su, Mengying, "Investigations of augmented heat transfer due to elastic instabilities within low-inertia flows with streamline curvature" (2019). Dissertations. 191.