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DOI: 10.1615/ICHMT.2008.CHT.100
page 24

Jan Wissink
Brunel University

Wolfgang Rodi
Institute for Hydromechanics,University of Karlsruhe, D-76128 Karlsruhe, Germany


An overview is provided of some Direct Numerical Simulations (DNS) carried out by the authors of situations related to transitional flow over and heat transfer to turbine blades. The flow cases considered are 1) Separating flow over and heat transfer to a flat plate with oscillating external flow, 2) flow around and heat transfer from a heated turbine blade with and without periodically oncoming wakes and background turbulence and 3) flow around and heat transfer from the stagnation region of a heated cylinder with and without incoming free-stream fluctuations.
In the first DNS, every inflow oscillation period a new separation bubble is formed which eventually disintegrates when a big roll of recirculating flow is shed. Once the roll has been shed, the flow inside it very rapidly turns turbulent as the roll is convected downstream. The recirculating flow inside the rolls transports hot fluid from the free-stream towards the surface of the plate causing a local maximum in the heat transfer rate, m the dead-air region of the separation bubble the heat transfer rate is found to be very low as there is very little convection of heat towards the flat plate. Although the turbulent Prandtl number, σT, is found to vary significantly over the turbulent flow region, the phase-averaged values over the entire area where the flow is (mildly) turbulent are found to be about σT= 0.9, which is exactly the value commonly used in RANS calculations.
In the second DNS the boundary layer around the model turbine blade considered is found to remain attached at all times. Along the entire pressure side and the favourable-pressure-gradient-part of the suction side, the flow is found to remain laminar in all simulations. In the simulations with incoming wakes and background turbulence, by-pass transition was found to take place in the downstream half of the suction side boundary layer causing an increase in heat transfer. Compared to experiment, only a modest increase in the heat transfer from the model blade in the laminar parts of the boundary layer was observed.
In the third DNS, the heat transfer from the stagnation region of a circular cylinder with incoming free-stream fluctuations was found to be driven by jets in the fluctuating velocity field. The impinging jets transport cold fluid from the outer flow towards - and subsequently along - the surface of the cylinder causing large regions with a relatively high Nusselt number. The hot fluid that is subsequently convected from the surface of the cylinder into the free-stream by outward moving jets gives rise to narrow, low-Nusselt-number regions.
With the calculations an impression is given of the present capability of DNS in simulating the details of flow and heat transfer processes in turbine-related situations.

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