Hi Dave,

Sure had me going for a spell, however, I got out the equations and believe I can point out a different view point.

If I understood you correctly, your basic assertion is that  the same mass flow is required for both thin and thick radiators and since the thicker radiator has a smaller frontal area  it must therefore have a higher velocity air flow to generate the same mass flow to remove the same  heat.  Furthermore the higher velocity also translates into more drag (even with the reduced frontal area due to the drag being proportional to the square of the velocity) - but all the above is not necessarily true.

  In fact I found a NACA study where they looked at the effects of using thicker radiators and I have worked out the equations on a spreadsheet which I believe sheds some concrete facts on the old thin Vs Thick debate - but, it is complex and I'll wait a bit before springing it {:>). 

However  back to your contention that both radiators the thin and the thick required the same mass flow to remove the same amount of heat - it just isn't so and here is why. 

First, we have two radiators one is 1" thick and 1 square ft in frontal area, the second one is 1/2 square feet of frontal area and twice (or more) as thick.  Now turning to our trusty equation for heat rejection and mass flow.

Q = m*Cp*DeltaT is the basic equation that tells us how much heat we remove for a mass flow "m", a specific heat (air = 0.24) and temperature increase in the medium (air) or DeltaT. 

Taking a specific example of say - 5000 Btu/min (which is about the amount of heat an NA 13B generates at 175 HP that needs to be rejected by the coolant).  We know the Cp so that leaves the DeltaT and that is what makes the difference.  We have to assume a DeltaT, lets say 50F (yes, it could easily be different but bear with me)  then we have

m = 5000/(0.24)*(50)/60  = 6.94  lbm/sec of mass flow  . and lets say we have a 1 square foot radiator to get rid of that heat.  Then the velocity requires V1 = m/(p1A1) = 6.94 lbm/min/(.0765*1) = 90 ft/sec = 61.36 mph through the 1 square foot radiator.  Perhaps a bit higher than desirable but that's what we get.

  Now if I understood you correctly your point is that  the same mass flow is also required for the smaller radiator (1/2 sq ft) to remove the same amount of heat and therefore since frontal area is 1/2 the size,  the velocity must be double that of the larger radiator to get the same mass flow and remove the same quantity of heat.  But, it just isn't necessarily so.

Taking the same conditions as before, except this time I use a DeltaT of 100F (hey! its permitted as I'm using a different core here{:>) see further discussion on effects of thickness on DeltaT).  Now we have m = 5000/(0.24)*100/60 = 3.47 lbm/sec of mass flow is required.  That is 1/2 of the mass flow required with a DeltaT of 50F.

Therefore even with 1/2 the frontal area, I can use the same air velocity as before and remove the same amount of heat with 1/2 the mass flow and with LESS drag because my frontal area is now 1/2 that of the thinner larger radiator and the velocity is the same.  Now you can say I cheated by having a different radiator, but that is certainly what you would do - as that is what we are discussing are the relative merits of thinner vs thicker for our application.

But,  If you reduce the frontal area of the radiator,  then you must increase the thickness (or add more fins, turbulators, etc) to increase its Heat transfer coefficient to continue to reject sufficient  heat to the air flow.  Therefore, The air temperature coming out of a thicker radiator is going to be higher than a thin radiator.  The reason is both radiators are flowing at the same velocity (remember I did used  the same velocity for both radiators), and since the velocity of the flow is the same for both radiators, the air spends more time (twice, three, four times depending on the thickness) in the thicker core of the smaller radiator.  The longer duration of the air in the thicker core causes it to be absorb more heat and be raised to a higher temperature than the thinner radiator, therefore the higher deltaT (for the same velocity air).

This probably did not/and will not convince you of the merits of the thicker vs thinner and besides I know your reservations about my deductive reasoning {:>).  So I am working on understanding fully the Naca study I found that addresses the effect of thickness on required mass flow and heat rejection.  I believe it would be considered a fairly credible source and will hopefully enable all to reach their own conclusion.  I think its going to blow the socks off this thick vs thin debate - but, then I've been wrong before {:>)

Boy, this is fun!!!  Sure keeps the old brain working (hopefully).

Anyhow, Dave, I respectively disagree with your assertion (see above) {:>)

Best Regards

Ed

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