I'm too am surprised at the low results from what is getting called the distanced PP core. One question that pops to mind is has anybody tried adding some air turbulators (on an aircraft's wing they are vortex generators) to the open channels? I know in some solar hot air collectors gains are observed on extracting the heat from the absorber sheet if a few (~10-20) turbulators are added to the back of a 4'x8' collector. My understanding is that in reducing laminar airflow the temperature gradient of the air at the heat exchanger interface skin is reduced. For these scale heat exchangers running course grit sand paper over the PP might suffice. Although that might increase dust buildup across the entire surface negating any positive change. These should also help to balance the air flow/pressure restriction between the two heat exchanger channels as well.
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