Vortices and turbulence in square vs round pipe

[JackAubrey]

TL;DR Summary

Desire a a vortex in a vertical combustion tube, square vs round.

Greetings,

I am about to start experimenting with misnamed rocket stoves and rocket mass heaters. I say misnamed because I think the velocities are too slow to be rocket science. (Why would I experiment with these? Because I have cement, perlite, and reinforcement materials at hand and I just had hip surgery so I cannot get around so spryly at the moment. TMI, sorry.)

Anyway I seem to recall from circa 1976 that at low velocities a square pipe and a round pipe of the same dimension (8") would serve equally well for axial flow but I want a swirling flow in a vertical pipe. So I wonder how well a vortex can be maintained in a square tube especially where the Reynolds number of the cob like material is so horrible. I may find out empirically but would like to know what to expect.

This is another occasion where I wish I had elected fluid dynamics as an option course. I sure did enjoy 2 semesters of physics though. This is my first post to your forum so please forgive my awkward style, etc.

Thank you.

 

[Lnewqban]

Welcome, Jack!

How tall is that 8-inch vertical pipe?
What material?
What flow velocity?

 

[JackAubrey]

Thanks for posting. I was hesitant to post as I have too many unknowns but was still was hopeful for some generalities Height is 36-48 inches. Material will be cob-like, rough like concrete pipe. I can only guess the velocity. What is the velocity of a wood stove draft, I am guessing 10-15 fps straight line before the vortex inducement. I suppose I should invest in a cheap anemometer.

Source https://www.physicsforums.com/threads/vortices-and-turbulence-in-square-vs-round-pipe.1002425/

 

[berkeman]

I want a swirling flow in a vertical pipe

Welcome to PhysicsForums.

Sorry if I'm missing the obvious, but why do you want a swirling exhaust gas pattern?

I have cement, perlite, and reinforcement materials at hand

What's the perlite for? It's non-combustible and usually used for growing plants...

Perlite MSDS: http://www.schundler.com/msdsperl.htm

I just had hip surgery

Hope it went well. My wife went through that a couple years ago, and is very happy overall with the recovery and results.

 

[JackAubrey]

Regarding swirl: I want high velocity but I want the gases to spend some time in the pipe for the secondary burn. There is some smoke in the stream when it leaves the horizontal burn chamber the vertical pipe is well insulated and the high temperature will complete the burn, eliminating almost all of the smoke.

The perlite is a good high temp insulator for mixing into cob and for lightning hypertufa. I stumbled onto hpertufa when I started searching for something to replace plastic plant pots which are now $15+ in the size I want.

Seems I cannot insert a jpg file but since a picture/schematic is worth 1024 words maybe this will give you an idea.

OFF TOPIC:
Regarding surgery fortunately I did some research. The first doc said "we will need to lift this muscle". As I suspected "lift" meant "detach". He bristled when I asked him about SuperPATH method and he said he had been doing that way for 20 years. Over my shoulder I told him things had progressed since then. Dr Elsharkawy in Plano, TX used a minimally invasive technique and I left the hospital on a walker the same day. With some good essential oils applied the incision was completely closed in 21 days and I started working out in a pool. Don't have full strength in the Sartorius but otherwise get around real well.

 

[JackAubrey]

Inserting images is a booger here. Here is a Rocket Mass Heater diagram.

 

[berkeman]

said he had been doing that way for 20 years. Over my shoulder I told him things had progressed since then.

Yeah, hip surgery has progressed a lot in the last 10-20 years. We were lucky and my wife got linked up with a surgeon who is on the leading edge of research and hip surgeries. She actually had to have both hips replaced within 2 years (due to inherited arthritis), and Dr. Sah was already certified in and using an even newer technique for her 2nd surgery:
https://sahortho.com/

Seems I cannot insert a jpg file

Inserting images is a booger here.

It should be pretty easy. Click the "Attach files" link below the Edit window.

Here is a Rocket Mass Heater diagram.

So is the intent of the vortex to slow down the exhaust flow and facilitate more heat transfer out of the exhaust gasses to the piping? Can you comment on what the vortex is supposed to help? And if heat transfer is a goal, why are you using concrete for the heat conducting medium?

Thanks.

 

[jrmichler]

Your idea looks a lot like this from Liberator LLC (https://www.rocketheater.com/), except with added thermal mass and without the pellet feeder:

I saw their heater in operation at an energy fair a few years ago. They were getting clean combustion and good heat transfer, which was evidenced by low stack temperature. Their demo model was burning sticks of wood, not pellets.

The amount of heat transfer in this system is almost entirely due to the heat transfer area, and only slightly due to artificially induced turbulence. Keep in mind that anything that induces a vortex or turbulence also increases friction loss in the exhaust gases. Also, you need a minimum stack temperature in order to get combustion air and exhaust gases to flow through the system. The result is that, if you artificially induce a vortex or turbulence, you will need increased stack temperature to get the necessary gas flow rate. The thermal efficiency will decrease.

The inventor told me that their patent only covered a design feature in the grate, but they needed a patent in order to get financing to put it into production. The basic rocket heater design with the downflow heat exchanger is prior technology.

 

[JackAubrey]

Yeah, hip surgery has progressed a lot in the last 10-20 years. We were lucky and my wife got linked up with a surgeon who is on the leading edge of research and hip surgeries. She actually had to have both hips replaced within 2 years (due to inherited arthritis), and Dr. Sah was already certified in and using an even newer technique for her 2nd surgery:
https://sahortho.com/It should be pretty easy. Click the "Attach files" link below the Edit window.

So is the intent of the vortex to slow down the exhaust flow and facilitate more heat transfer out of the exhaust gasses to the piping? Can you comment on what the vortex is supposed to help? And if heat transfer is a goal, why are you using concrete for the heat conducting medium?

Thanks.

Glad her surgeries went well. Hopefully I only need one as I fell hard on that hip years ago damaging the socket and it finally caught up with me. Got to where I could not even ride a bike and I gained 40 pounds.

I never was good at attention to details and I was trying to use the insert image tool. Then Mr Duh saw "Attach files" after popping off.

You are right about slowing down the flow. However, the concrete is not the transfer medium, it insulates the vertical pipe to keep the temperature up for a more complete burn. Per the image after the hot gases leave the top of the inner tube they go back down the larger diameter barrel giving up heat there and then in a long almost level section of pipe in a cob thermal mass and finally up the stack where the temp is still about 200 degrees.

 

[JackAubrey]

Your idea looks a lot like this from Liberator LLC (https://www.rocketheater.com/), except with added thermal mass and without the pellet feeder:
View attachment 282063
I saw their heater in operation at an energy fair a few years ago. They were getting clean combustion and good heat transfer, which was evidenced by low stack temperature. Their demo model was burning sticks of wood, not pellets.

The amount of heat transfer in this system is almost entirely due to the heat transfer area, and only slightly due to artificially induced turbulence. Keep in mind that anything that induces a vortex or turbulence also increases friction loss in the exhaust gases. Also, you need a minimum stack temperature in order to get combustion air and exhaust gases to flow through the system. The result is that, if you artificially induce a vortex or turbulence, you will need increased stack temperature to get the necessary gas flow rate. The thermal efficiency will decrease.

The inventor told me that their patent only covered a design feature in the grate, but they needed a patent in order to get financing to put it into production. The basic rocket heater design with the downflow heat exchanger is prior technology.

Thanks for the reply.

Dang if that doesn't look like the front end of a rocket mass heater. This looks a lot like one of my designs but his is much better. Don't know why he has fins on the side of the burn box though.

The reason I want to induce a vortex is to increase burn efficiency. If the gas travels at the same speed but in a spiral it seems would take longer to get out of the pipe. There have been demonstrated cases in rocket stoves and rocket mass heaters where swirling reduced smoke and increased heat transfer. Is this increase due to slower speed or does the vortex cause more mixing and a better burn? I do not know. For that matter is there a more efficient heat transfer or is more fuel being consumed. It has been very hard to find anything but empirical evidence for these devices. Some of the well documented stoves and some examples on Youtube, built by people who have designed and built tens or hundreds of rocket mass stoves, develop almost no ash, no detectable creosote and have a 200 degree stack temperature with no inducers.

Some of the most accomplished (and published) designers declare that the vertical tube shall be square but don't offer much justification. I think it is because they like to build the tube from thin fire brick. So although I am a sparky and not a real physicist I still want ot now why. But then these folks say the area of the square feed hole where air and fuel are introduced must be the same area as the pipes in the cob mass. Apparently no one has pointed out that the cross sectional area of the feed tube is greatly reduce by the wood stuffed into it.

I really appreciate all the replies as y'all are making me think and I need to do that as I think I was overdosed on anesthetic to the point that I am quite self aware of cognitive dysfunction. Please forgive spelling and grammar errors. And excessive babbling.

Best regard,
John