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Post by dpottier on Aug 21, 2014 22:16:12 GMT -8
Blade and Ball Concept The most familiar form of drag is made up of friction forces, which act parallel to the object's surface (burn chamber and bell), plus pressure forces, which act in a direction perpendicular to the object's surface. Did you know that a Formula 1 race car can have a drag coefficient as high as .70, compared to .30 for most cars. Let's have a look at some shapes and their drag coefficients  In the P Tripwire, when facing forward, it has an airfoil profile with a hard leading edge with a 20 degree sweep. In the test Peter conducted he reported little effect. In the Wind Tunnel simulation I conducted I had the same result. However, when Peter reversed the direction it created nice downstream eddies as the air fell over the trailing edge. I had the same results in the wind tunnel simulations. My original Blade and Ball concept was based on an old design for a vortex generator. Notice the blade has a prism profile but is angled towards airflow at an angle to that airflow and a height about 3X the radius of the ball  Which resulted in this "starting point" configuration for the burn chamber ceiling. The half spheres are 20mm in diameter or just a bit over 3/4 inch.  Once the components were design and assembled the concept was ready for testing (wind tunnel simulation). What follows is Part One of the testing process. Part Two will see the addition of side bricks to emulate Peter's 4" test unit. The design will be tested with and without the blade as in Part One. Tests will also be done to see what happens when adding the half spheres to the side walls and omitting the blade. Great or any designs do not mean easy implementation and what works in theory (the drawing board) does not always survive the real world implementation. More than a few aircraft, ships and automobiles have gone this rout. All do, however, advance our knowledge base and stimulate thought, possibly even become "mind candy" for the adventurous few. As an afterthought I want to add 2 or 3 spheres to ramp portion Peters great tripwire design in the 5mm and 10mm configuration. As Peter's tripwire mold was carved from a foam block, if making a tripwire negative for casting a single brick, a few taps with piece of wood on a ballpeen hammer will create the depressions (dents) needed to cast the spheres. This would work well for those intending to cast their own insulating fire bricks from clay and sawdust mixtures. David |
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Post by peterberg on Aug 22, 2014 4:43:37 GMT -8
The surprise from the 10mm test was a ramp effect. The air went to the floor of the burn chamber and had no effect on the laminar layer at the top. This ramp effect is strange indeed. But taking a closer look, I think the trip wire ridge is serving as the leading edge instead of the trailing edge. When that's the case, then this ramp effect does seem logical. Are you sure you updated the correct file, with the trip wire facing the riser? |
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Post by photoman290 on Aug 22, 2014 8:51:52 GMT -8
like the mind candy analogy. think as winter is not that far away i should stop eating candy and get on with finishing my winter RMH project. deeply into geopolymer research as well as this. keep up the good work i am finding it really interesting.
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Post by dpottier on Aug 22, 2014 20:15:32 GMT -8
This ramp effect is strange indeed. But taking a closer look, I think the trip wire ridge is serving as the leading edge instead of the trailing edge. When that's the case, then this ramp effect does seem logical. Are you sure you updated the correct file, with the trip wire facing the riser? Video files are correct - tripwire angle is facing towards burn chamber and not the feedtube. 20 degree angle is the trailing edge. Sorry the videos are a bit dark. Should the angled side be towards the P Channel / feedtube or facing towards the burn tube? I just finished a test sequence. First, I re-ran the test that showed the ramp effect and will post the video. In the second test I added 2 - 20mm diameter half spheres to the ramp part of the tripwire. I liked the test results. Drag Force=0.156(N) and the Drag Coefficient= 0.67.  |
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Post by peterberg on Aug 23, 2014 2:21:14 GMT -8
Should the angled side be towards the P Channel / feedtube or facing towards the burn tube? The angled side should be directed to the burn tunnel/heat riser side, so that is correct. Maybe it would be a good move to run the test using the 5 mm wire again, complete with sides of the tunnel. In the second test I added 2 - 20mm diameter half spheres to the ramp part of the tripwire. I liked the test results. Drag Force=0.156(N) and the Drag Coefficient= 0.67. Hmm... The drag force is much higher than testing the bare 5 mm trip wire but the coefficient is lower. I don't know how to interpret these figures. All the numbers aside, there happened to be a significant difference running the stove with or without a trip wire. It's possible we are missing something and my reasoning that the progress was due to disturbing the boundary layer could be utterly wrong. Still, the effect is there, this small ridge has a positive effect on combustion quality. There might well be other, maybe combined reasons, why this is the case. |
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Post by dpottier on Aug 23, 2014 6:45:32 GMT -8
All the numbers aside, there happened to be a significant difference running the stove with or without a trip wire. It's possible we are missing something and my reasoning that the progress was due to disturbing the boundary layer could be utterly wrong. Still, the effect is there, this small ridge has a positive effect on combustion quality. There might well be other, maybe combined reasons, why this is the case. These tests have been with the 10mm tripwire. Next group of tests will be with the 5mm tripwire, with sides and then sides with the spheres. That should give you a more realistic picture. I also want to try adding a couple of spheres on the sides at the entrance to the burn tunnel just to see what happens. I was also surprised at the increase in Drag Force and lowered Drag Coefficient. When you see the video you will have a better idea of what is happening inside. The airflow went directly to the deck (ceiling) of the chamber and stayed there. The air flow between the 2 spheres takes a jump over the edge. By jump I mean it curves upwards then cascades over the point. Also the increased pressure points seen previously were not evident.  |
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Post by peterberg on Aug 23, 2014 23:57:10 GMT -8
Gents,
I've moved the discussion about Ulaan Baatar etcetera to a new thread "Ulaan Baatar heating problems". Since this is off topic it's now in the board with this name.
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Post by dpottier on Aug 24, 2014 4:10:30 GMT -8
Gents, I've moved the discussion about Ulaan Baatar etcetera to a new thread "Ulaan Baatar heating problems". Since this is off topic it's now in the board with this name. Thank you Peter. I totally agree. My bad for taking this thread off-topic David |
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Post by dpottier on Aug 24, 2014 10:37:04 GMT -8
Hmm... The drag force is much higher than testing the bare 5 mm trip wire but the coefficient is lower. I don't know how to interpret these figures. Peter Here are the 10mm P Tripwire plus 10 - 20mm Spheres (2 on tripwire + plus 4 more per side), in a completely enclosed burn chamber, test results: Drag Force - 0.002(N); Drag Coefficient - 0.65 Best regards David |
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Post by dpottier on Aug 26, 2014 22:12:56 GMT -8
I wonder, would the stream be different when you implemented the sides of the tunnel as well? Peter I thought you might like to have some base line data as well. Let's first go back in time to 1965 and the Dane, Piet Hein and his ellipse also known as the superelipse or super egg. Reported to be the perfect combination of the square and the circle. One of my reasons for looking at this shape is I have noticed large, transient (numerically) pressure ridges along the corners of the square Burn Chamber (see video). For those not around in those days, the super egg was designed by the Danish poet and scientist Piet Hein, in 1965. It is obtained by rotating a super ellipse around one of its axes. The super ellipse belongs to the Lamé curves.  Design tested:  Three Burn Chamber shapes were tested: Circle, Square/Rectangular and the Superelipse. Round - 6" diameter X 18" long: (a 6" diameter is the diagonal on a 4" square) Drag Force - 0. 0(N) Drag Coefficient - 0.61Superelipse - as in image above X 24" long: Drag Force - 0. 0(N) Drag Coefficient - 0.68Square - 4"X4" X 18" long (approx) : Drag Force - 0. 002(N) Drag Coefficient - 0.74The video result: Peter, if there is any test I have run that you would like to see longer recordings of, or a test you would like me to run that I have missed and peaks your curiosity, I am happy to oblige. All tests to date have basically been straight burn chamber tests. I have also modeled and tested the 5mm tripwire with the addition of the Feed Tube (4"1X4"X12" and P-Chanel (4"X11"X1" with 1/2" curve at base) All the air flow went rushing down the Feed Tube so I will add a few "bricks" to close the bottom of the Feed Tube and connect it to the Burn Chamber so the airflow will, hopefully, follow the shape of those components and give us an idea of what is going on inside and the drag generated by the Feed Tube and Burn Chamber combined. best regards David |
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Post by peterberg on Aug 26, 2014 23:14:01 GMT -8
I have also modeled and tested the 5mm tripwire with the addition of the Feed Tube (4"1X4"X12" and P-Chanel (4"X11"X1" with 1/2" curve at base) All the air flow went rushing down the Feed Tube so I will add a few "bricks" to close the bottom of the Feed Tube and connect it to the Burn Chamber so the airflow will, hopefully, follow the shape of those components and give us an idea of what is going on inside and the drag generated by the Feed Tube and Burn Chamber combined. David, All the air is rushing down the feed tube, yes that's true. In particular when the feed is EMPTY. But when running this is not the case, there's always fuel which blocks the airflow partly. When on the other hand the p-channel always stays open. The more fuel is in the feed, the higher the combustion rate, so the flow in the p-channel automagically will adjust up or down according to the stage of the burn. I'd suggest you run the same test with the top of the feed tube blocked for, say, 60% or even more. That super elipse sure looks interesting, I'll try to react to that in the evening. |
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Post by Deleted on Aug 27, 2014 0:59:10 GMT -8
David, could you try an egg shape with the smaller end at the bottom ?
Would be better for the coal stage.
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Post by dpottier on Aug 27, 2014 8:18:20 GMT -8
I'd suggest you run the same test with the top of the feed tube blocked for, say, 60% or even more. That super ellipse sure looks interesting, I'll try to react to that in the evening. This is the model as tested and when creating this image I realized I had made a big mistake. The curl at the bottom of the P-Channel is 1/2" radius and I think 1/4" is what it should have been. Easy to correct for next test sequence.  Test Tunnel airflow went down the Feedtube and straight through the tunnel and not into the Burn Chamber. A base brick, 2 glass side bricks and a glass rear brick should get all the air into the burn chamber. Easy to add fuel "sticks" later. The tunnel tests "push" airflow. I wish there was a way to add a "draw" force element as in a real world burn situation, such as the Feedbox being full of wood and the airflow coming from the P-Channel to meet the demand (draw). I think the tests so far have added to the knowledge base of some parts and their effects. So many scenarios to test, i.e. 2 -20mm spheres at first brick and/or in the 4 corners and the Tripwire one or two bricks closer to your end curve before the Bell Chamber; 2 - 20mm spheres on floor of Burn Chamber before the Bell Chamber curve; etc. and I am sure you can think of many more for testing. Once I have the components drafted they can be assembled in any configuration you can think of. I guess my next bit of work is on the Bell Chamber curve. In the 60's a friend owned a Danish Modern Furniture store and gave me a brass Piet Hein superellipse or "squircle" as a gift. Had I not owned and played with one I would have never thought of that shape for a Burn Tunnel design. best regards David |
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Post by dpottier on Aug 27, 2014 8:20:33 GMT -8
David, could you try an egg shape with the smaller end at the bottom ? Would be better for the coal stage. Hi Karl Sure, give me a day or two and I'll post the results. Thanks for the idea. David |
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Post by peterberg on Aug 27, 2014 11:34:28 GMT -8
This is the model as tested and when creating this image I realized I had made a big mistake. The curl at the bottom of the P-Channel is 1/2" radius and I think 1/4" is what it should have been. Easy to correct for next test sequence. David, I think something is definitely wrong as the drawing you showed in your post is far off a real life setup. There never has been a curl at the bottom of the p-channel, the trip wire should be situated at the ceiling of the burn tunnel which happened to be combustion chamber. The feed tube should end at the floor of the burn tunnel and the p-channel ending a mere 3/8" below the ceiling of the tunnel. This whole thing is looking quite mixed up if you ask me. |
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