Experimental investigations have shown that the
enhancement in heat transfer coefficients for air flow in a
channel roughened with low blockage (e/Dh<0.1) angled ribs is
on the average higher than that roughened with 90∘ ribs
of the same geometry. Secondary flows generated by the angled ribs
are believed to be responsible for these higher heat transfer
coefficients. These secondary flows also create a spanwise
variation in the heat transfer coefficient on the roughened wall
with high levels of the heat transfer coefficient at one end of
the rib and low levels at the other end. In an effort to
investigate the thermal behavior of the angled ribs at elevated
Reynolds numbers, a combined numerical and experimental study was
conducted. In the numerical part, a square channel roughened with
45∘ ribs of four blockage ratios (e/Dh) of 0.10,0.15,0.20, and 0.25, each for a fixed pitch-to-height ratio
(P/e) of 10, was modeled. Sharp as well as round-corner ribs
(r/e=0 and 0.25) in a staggered arrangement were studied.
The numerical models contained the smooth entry and exit regions
to simulate exactly the tested geometries. A
pressure-correction-based, multiblock, multigrid,
unstructured/adaptive commercial software was used in this
investigation. Standard high Reynolds number k−ε
turbulence model in conjunction with the generalized wall
function for most parts was used for turbulence closure. The
applied thermal boundary conditions to the CFD models matched the
test boundary conditions. In the experimental part, a selected
number of these geometries were built and tested for heat
transfer coefficients at elevated Reynolds numbers up to
150 000, using a liquid crystal technique. Comparisons between
the test and numerically evaluated results showed reasonable
agreements between the two for most cases. Test results showed
that (a) 45∘ angled ribs with high blockage ratios
(>0.2) at elevated Reynolds numbers do not exhibit a good
thermal performance, that is, beyond this blockage ratio, the
heat transfer coefficient decreases with the rib blockage and (b)
CFD could be considered as a viable tool for the prediction of
heat transfer coefficients in a rib-roughened test section.
