Abstr:Natural and mixed convection boundary layers on vertical heated walls (B)
Underlying Flow Regime 3-07
The effects of buoyancy on heat transfer can be predicted readily for laminar flows. When the flow is in the upward direction past a heated surface (or downwards past a cooled surface) heat transfer is enhanced, whereas in the opposite cases heat transfer is impaired. These influences are the consequence of the distortion of the velocity field.
However, the situation is different with turbulent flows, as heat transfer is dictated by changes in turbulent diffusion. In configurations with forced and free convection aligned, local heat transfer coefficients significantly lower than those for forced flow alone can result. In contrast, for downward flow in heated tubes buoyancy forces cause a general enhancement of the turbulent diffusion properties of the flow, with the result that wall temperature distributions are well behaved and heat transfer coefficients are higher than those for forced flow alone. Eventually as the free convection component becomes more and more dominant, heat transfer for upward flow also becomes enhanced and in the limit the heat transfer coefficients for the two cases are the same.
This UFR is of particular relevance to British Energy’s and Magnox Electric’s Application Challenges.
Contributors: Mike Rabbitt - British Energy