Large eddy simulation of three-dimensional mixed convection on a vertical plate

• Large Eddy Simulation von dreidimensionaler Mischkonvektion an einer vertikalen Platte

Garbrecht, Oliver; Kneer, Reinhold (Thesis advisor); Kabelac, Stephan (Thesis advisor)

Aachen (2017, 2018)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017

Abstract

Convective heat losses on solar central receivers arise from a combination of buoyancy-driven natural and forced convection. This mixed convection is characterized by two factors: First, the direction of gravity. Gravity aligned with the imposed flow results in a two-dimensional flow case. Three-dimensional mixed convection occurs when gravity acts perpendicular to the free stream. The second factor is the relative influence of buoyancy, as described by Gr/Re^2. In this work, three regimes are distinguished: Buoyancy-dominated, interia-dominated, and mixed convection with equipollent mechanisms. This thesis seeks to investigate the thermo-fluid interaction of turbulent three-dimensional mixed convection by using computational fluid dynamics. For that purpose, a large-eddy simulation model has been developed and qualitatively validated. In a series of simulations, a flat, hot, vertical plate was exposed to a horizontal fluid flow. Interpretation of the simulation results proved that buoyancy-induced instabilities are a key factor in the onset of turbulence. Reducing the plate temperature led to a significant reduction in turbulence. It was also found that an increase in free-stream velocity suppresses instabilities, delaying the laminar-turbulent transition. These findings are of high relevance for the heat transfer, as an evaluation of the heat transport mechanisms revealed that most of the transfer is linked to turbulence. Depending on flow conditions, a higher velocity can thus reduce the average heat transfer rate off the plate. In order to understand the underlying mechanisms, boundary layer profiles were evaluated. A three-dimensional mixed convection boundary layer flow can be described by a local flow angle and a local velocity magnitude. In wall proximity, a high-angle region was found, exhibiting a significant vertical updraft. After transition, the turbulent transport of horizontal momentum from the free stream into this region is reflected in a reduction of the angle. Consequently, the highest angles were found in the transition zone. When buoyancy had a high influence, a distinct velocity peak emerged in the velocity magnitude. The turbulent coherent structures showed a characteristic pattern: In the laminar flow, elongated streaks form prior to transition. These streaks soon exhibit secondary instabilities and eventually break up into structures like wall-attached hairpin vortices. Here, the buoyancy-fueled updraft has an impact by partially distorting these vortices. While the simulation results are successful in providing insights into the physics, they also point out challenges in finding physically meaningful modeling approaches. This is true i.e. for heat transfer correlations and numerical wall models.

Institutions

• Chair of Heat and Mass Transfer [412610]