If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
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. |
#2
|
|||
|
|||
On Wed, 02 Jul 2003 05:09:00 GMT, "Bruce A. Frank"
wrote: I'll get into this a bit more. 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. Me too. I've got a video at home that was produced by Jerry Schweitzer. He used to build Ford V-6's for people to use for auto conversions, don't know if he does that anymore. He had two airplanes that were Ford powered, a Pacer and an RV-4. The video I have is about the Pacer installation. In it, the radiator is mounted flat underneath the engine. It's a pretty big sized radiator. The air is drawn through the stock openings in the chin cowl, passes by the PSRU, which is good because it needs to have cooling air going over it. Then the air has to get around the engine and the hot exhaust pipes and THEN pass through the radiator and remove heat from it's fins. But we aren't done yet. After that convoluted journey, the air then has to bend 90 degrees to head out the exit, which is just a hole in the bottom of the cowling underneath the firewall. The hot air exit had a lip in the front of the opening to promote a low pressure area right around the opening. Despite all the problems with this layout, it not only worked, it worked well. Corky Scott |
#3
|
|||
|
|||
And several more who have just sealed the radiator to the bottom of the
cowl. Cut a hole in the cowling for the radiator exhaust air to exit; placed a lip at the front edge or several rear facing louvers over the opening and it cools....even bringing with it all that hot air radiating from the engine exhaust pipes (though temps are higher than setups using a dedicated duct system). Many different styles of radiator arrangement have worked...radiators behind the engine, under the engine, above the engine, even in front of the engine...the common element is exhausting that hot radiator air into a low pressure area. I have talked to builders who were absolutely vexed and ready to abandon the project. In most cases simple installation of a 2" lip on the leading edge of the exit opening completely solved the problems. Let me add that the dedicated duct offers a better opportunity to control temps and drag, but many arrangements have worked exceedingly well...even radiators mounted behind the baggage compartment (Blanton's original design with the V-6 STOL) Corky Scott wrote: Me too. I've got a video at home that was produced by Jerry Schweitzer. He used to build Ford V-6's for people to use for auto conversions, don't know if he does that anymore. He had two airplanes that were Ford powered, a Pacer and an RV-4. The video I have is about the Pacer installation. In it, the radiator is mounted flat underneath the engine. It's a pretty big sized radiator. The air is drawn through the stock openings in the chin cowl, passes by the PSRU, which is good because it needs to have cooling air going over it. Then the air has to get around the engine and the hot exhaust pipes and THEN pass through the radiator and remove heat from it's fins. But we aren't done yet. After that convoluted journey, the air then has to bend 90 degrees to head out the exit, which is just a hole in the bottom of the cowling underneath the firewall. The hot air exit had a lip in the front of the opening to promote a low pressure area right around the opening. Despite all the problems with this layout, it not only worked, it worked well. Corky Scott -- 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. |
#4
|
|||
|
|||
The fans that wear out seem not able to take the thrust loading of being
spun by the air load, but as I said, the one in Bayard's Defiant had no problems. You may be correct that those other systems had too high air velocity. Morgans wrote: "Bruce A. Frank" wrote in message ... 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. Seems to me that if the fan was spinning so fast that it is wearing out bearings, the air speed past the radiator is waaaaaay to high. Design the inlet (read small and divergent ducts) so that it keeps the air speed past the radiator low at cruise speed, and then plan on using the fan for ground operations, and climb, if need be. As far as pushing air, I know that it needs to be drawn through to an area of lower pressure; that's what the fan on the back side of the radiator is all about. See any flaws in my reasoning? BOb, I know what you have to say; that the idea of using anything other than a lycosarus is a flaw! ;-) -- 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.| |
#5
|
|||
|
|||
The sides of the duct angle away, diverge from the intake opening, at an
angle of 7 degrees. A round duct would assume a section conical shape with the walls tilted 7 degrees from the centerline. The problem is that the usual distance needed for the duct to expand from the intake opening to the size of the radiator takes more length than is available in the typical prop to firewall distance. With a square or rectangular cross section duct the sides would diverge at 7 degrees relative to the centerline. What works is left and right sides diverging, or top and bottom diverging or both top, bottom and sides diverging. Most installations expand the duct more rapidly because one is moving from an in intake opening of about 55 sq. inches to a radiator of 400 to 500 sq. inches in a distance of only 18 to 24 inches (this if aprox for a 200 hp installation). There is more to this. The opening of the intake, if nearly flush to the nose bowl surface, should have rounded edges to split the air stream as an airfoil would with smooth flow both down the throat and along the cowling. An intake that sticks forward of the surface of the nose bowl can also have a rounded leading edge, but relatively sharp sheet metal edges seem to work well also...spliting the air stream before the interference of the surface of the nose bowl. There have been articles over the years, I believe, in both Sport Aviation and Kitplanes on air duct divergence and leading edge shape. but I cannot point you to a specific article. Ernest Christley wrote: Bruce A. Frank wrote: 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 Very informative post, Bruce. Thank you. But could you explain this? I've heard this 7 degree figure knocked around a couple of times, but have no idea what it refers to. Does it mean that if I have a square opening in the XY plane and a line extending through the middle of it in the Z plane, then an 'extrusion' of the square will form a box that moves away from the line at a rate of 7 degrees? Then the biggest question is, how does this correspond to the rounded intake plenums that I keep seeing? -- 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. |
Thread Tools | |
Display Modes | |
|
|