Your thoughts on the enormous new hole in my bumper.
02-26-2009, 03:52 PM
#181
Quotes from another source: Contrails ! Radiator aerodynamics
The radiator in a free stream
Photo GTH
It is often wrongly believed that a radiator or finned barrel directly exposed in a free airstream will achieve efficient cooling.
Actually, less than one third of the air arriving in front of the radiator will effectively flow across the core. The rest will flow around the obstacle, not without violent turbulence.
The efficiency is poor and the drag is prohibitive.
Photo GTHIt is often wrongly believed that a radiator or finned barrel directly exposed in a free airstream will achieve efficient cooling.
Actually, less than one third of the air arriving in front of the radiator will effectively flow across the core. The rest will flow around the obstacle, not without violent turbulence.
The efficiency is poor and the drag is prohibitive.
It is the pressure difference between the two faces of the radiator core, which forces air to flow through it : the ΔP. Without this pressure difference, no flow through the core, no cooling.
Minimizing drag
To minimize internal drag, we thus need a core with sufficient area (volume flow), an airstream considerably slower than the aircraft airspeed, and as high a Delta T as possible.
There is thus some advantage in running the engine as hot as compatible with safe operating limitations.
To minimize internal drag, we thus need a core with sufficient area (volume flow), an airstream considerably slower than the aircraft airspeed, and as high a Delta T as possible.
There is thus some advantage in running the engine as hot as compatible with safe operating limitations.
Diffuser
Doc Küchemann & Weber
It is the most important part of the duct. It comprises a diverging portion (not funnel shaped) in order to slow down the airflow without causing any separation.
According to Bernoulli's law, slowing air with no separation raises the static pressure. The presence of the radiator aft of a well designed diffuser renders flow separation still less likely.
The gain is twofold : the increased pressure facilitates the passage through the core, and this pressure on the diverging walls of the diffuser results in a forward force, opposing the drag of the core.
Doc Küchemann & WeberIt is the most important part of the duct. It comprises a diverging portion (not funnel shaped) in order to slow down the airflow without causing any separation.
According to Bernoulli's law, slowing air with no separation raises the static pressure. The presence of the radiator aft of a well designed diffuser renders flow separation still less likely.
The gain is twofold : the increased pressure facilitates the passage through the core, and this pressure on the diverging walls of the diffuser results in a forward force, opposing the drag of the core.
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02-26-2009, 06:22 PM
#187
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I suppose it doesn't help to point out that in Savington's case the issue was that he had an oil cooler up there that was getting little if any flow at all, hence the hole. Others like myself who have a large intercooler (same one as you have I think pat) might do something like this to get some flow to the top half of the intercooler. But as to the radiator, I concur that ducting is the most essential element to make the radiator work more efficiently.
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02-26-2009, 10:07 PM
#189
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Pat, great stuff. It's all relevant for our cars - without huge fuckoff FMICs in front of the radiators. Drop an FMIC and it slows the airflow considerably, heats the air considerably, and changes the game.
I did my hole because I added an oil cooler and I didn't want to stack another heat exchanger in the mouth of the car.
I did my hole because I added an oil cooler and I didn't want to stack another heat exchanger in the mouth of the car.
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02-26-2009, 10:38 PM
#190
Pat, great stuff. It's all relevant for our cars - without huge fuckoff FMICs in front of the radiators. Drop an FMIC and it slows the airflow considerably, heats the air considerably, and changes the game.
I did my hole because I added an oil cooler and I didn't want to stack another heat exchanger in the mouth of the car.
I did my hole because I added an oil cooler and I didn't want to stack another heat exchanger in the mouth of the car.
Suppose you seal the exit of the IC to the entrance of the radiator, like what I did. Air first goes through the condenser (we will assume A/C is off) and then through the IC where it rejects some amount of heat, causing the air to increase in temperature. PV= RT. Guess what happens when T goes up? So does pressure. It's plausible that the IC adding heat to the air is actually increasing the flow through the radiator, possibly to the point that it's higher flow rate than no IC at all....
Granted the temperature of said air will be above ambient as it exits the IC and presumably the specific volume of air will be lower than if there was no IC in front. But the larger delta P means more air moving faster through the radiator which causes turbulence, which would improve the heat exchangers efficiency, somewhat offsetting the loss in efficiency due to the decrease in delta T. Point is it's not quite as cookie-cutter simple as people take it to be.
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02-27-2009, 12:08 AM
#191
I don't think a fan can move enough air to cool an engine at speed. The hood bulges from pressure at high-speeds...which tells me that we should focus on getting the air out, rather than adding fans.
I'm going to try making a little cup into my shrouding for the first 1/4 of the fmic...maybe this **** will work.
I'm going to try making a little cup into my shrouding for the first 1/4 of the fmic...maybe this **** will work.
Next, the reason you see the hood bulge @ speed is partially based on Bernoulli's principle. Air flowing over a convex surface, like your hood, when @ speed has high ram pressure, but low static pressure. The air under the engine bay is moving slower than the air going over the top of the hood which has less ram pressure but more static pressure. This actually induces lift, just like on a winged aircraft. In the case of the hood though, it will bulge giving people the impression that there is high pressure under the engine bay. Take a look at a 90 some convertible Mustang or Camero when they're on the freeway (they have really poor convertible top bracing). With the top up @ 60+ mph, it bulges quite a bit. Would you suggest that the passenger compartment has high pressure? No, but is does have higher static pressure than the air going over the convex shape of the top.
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02-27-2009, 08:54 PM
#193
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Regarding cutting a big hole in the front bumper of your car.
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