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Ken
Here is all I have been able to
find at this time regarding "wedge" diffusers. There are two postulated
approaches in K&W. One assumes that the wedge or oblique duct losses
are the same as a straight duct until a certain inlet/core area (or angle) ratio
point is reached at which point an additional (and severe) loss factor
occurs. The second approach assumes that the loss due to obliqueness is
there for any angle (or area ratio) , it just becomes significant at a critical
angle or ratio. Regardless, they both refer to the critical area ratio as Ai/Ab
critical = 1/(1+Kpo)^(1/2). Kpo is the pressure drop for your specific
core and Ai is inlet area and Ab is core area. So unfortunately you need
to do some more digging to estimate the Kpo for your partcicular
core.
It appears that the more severe the angle is the
more inlet area is required to reduce these severe losses. However,
again use of the streamline duct profile appears to reduce this requirement.
Attached is a rather poor quality diagram of the comparison of a oblique (wedge)
duct with straight walls which shows the opening as 0.44 of the core area or
0.44Ab. This is compared to the use of a 0.20Ab ratio for streamline
ducting (note the small "kick out" of the duct at the upper corner of the
duct/core interface). The angle noted at the top is 64 degrees (or 90-64 =
26 deg between duct wall and core). Note when using the streamline
duct profile the inlet can apparently be reduced to 0.20Ab vice 0.44Ab (for the
straight wall duct) with "....about the same losses". So using the
streamline duct profile permits an inlet of nearly 1/2 the size of the straight
duct.
Further a quote regarding the diagarm "..In the
case shown, the upper wall has a slight inflection so that the excessively high
angle of deflection at the upper part of the duct/core interface is
avoided. It was found that the losses were again about the same as the
symmetrical streamline diffuser in figure 12.8". So if I am interpreting
this correctly, it appears that you can have an oblique duct without severe
losses provided attention is paid to the critical Ai/Ab ratio. Lacking the
specific KPO value for your radiator, the only thing I can suggest is to keep
your duct angle to 26 deg (or more) and your Ai/Ab ratio at 0.20 or
above.
Now, whatever study there is out there (I could not
find anything on the web) about Wedged ducts may have a different spin on the
topic than I presented here, but this is all I could come up with at the
moment.
Will let you know if I find any more on it. Hope
this helps some.
Ed
Ed Anderson
RV-6A N494BW Rotary
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