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Pressure Differential in heat Exchangers
Morgans wrote: Why not use a little electric fan to draw air past the radiator for extended ground operations, like cars do? It has been done. Bayard DuPont did it on the rear engine in his Defiant and it worked well. Others who tried it found problems with ram air in flight spinning the fans and wearing out the bearings. I'll get into this a bit more. A primary consideration to get enough air flow is to locate the radiator exhaust air opening in a low pressure area. It has proven virtually impossible to push air through a radiator. When there is no low pressure area into which the radiator exhaust air can flow, regardless of the size of the intake opening, the engine doesn't cool. An advantage to the radiator in the cowling is that prop wash will provide enough air movement to cool the engine during extended holds. Bayard found that he never had cooling problems with the Defiant's front engine once he had created a lip on the bottom firewall edge of the cowl. The lip on the trailing edge of the cowl creating the opening through which the radiator air exhausted dropped the pressure enough to "suck" the air through the radiator. Bayard has sat for as long as 30 minutes with no over heating. A little bit about drag through the radiator. The air going through the radiator has to slow down a lot in the duct from the cowl opening to the radiator. This requires some consideration of the divergence of the duct from the intake opening to the radiator. In a perfect world proper divergence of the walls of the duct should be about 7 degrees. In practical application up to 15 degrees works well. The air expands and slows and the air pressure differential from the front side to the back side of the radiator is what gets the air through. The duct should then converge to re-accelerate the air before it dumps into the slipstream again. To create a mental image, too large an intake opening sort of tries to gulp too big a chunk of air. The plenum created by the divergent duct can't flow that much and the radiator acts like a closed window because it cannot pass that high speed air. The air spills back out of the intake opening creating swirls and burbles like a cup under a wide open faucet...or like trying to drink out of a garden hose shot straight from the nozzle into your mouth (you know, you try to pucker you lips to slow the flow to keep it from squirting out of your nose). This is where the high drag is created. The route through the radiator is a relatively easy one, even making a right angle turn through the fins, because it is a pressure thing rather than an air speed thing easily navigated by the air that has traded its velocity for pressure through the correctly divergent plenum/duct. Hanging a radiator out in the slip stream works poorly except on the slowest planes. The air just piles up in front of it and spills around it. Even a short straight wall duct leading into and out of such an air dam will improve the efficiency of the radiator. Bruce A. Frank, Editor "Ford 3.8/4.2L Engine and V-6 STOL Homebuilt Aircraft Newsletter" | Publishing interesting material| | on all aspects of alternative | | engines and homebuilt aircraft.| *------------------------------**----* \(-o-)/ AIRCRAFT PROJECTS CO. \___/ Manufacturing parts & pieces / \ for homebuilt aircraft, 0 0 TIG welding While trying to find the time to finish mine. |
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