Heat flow around a pot

[Tony]

This posting is partly in response to what is being discussed on the “pimp my heiny’ thread but as I have been working on it for a while and it includes other pots I decided to start a new thread.

Heat flow around the base and up the side of a pot

These pictures where taken by a technique called shadow graph, it is a way of seeing the difference in density caused by the hot gases from the stove and therefore the difference in refractive index. Shadow graph method has limits of sensitivity and really only shows fairly strong differences in refractive index. Basically the hotter the gas the stronger the visualization on the tracing paper and the cooler gasses are hard to see. The photos have had the brightness and contrast played with to make it easier to see the flows.

The technique uses a bright straight light source like a slide projector (yes I have kept mine) and a piece of tracing paper attached to a sheet of Plexiglas. Unfortunately the piece of tracing paper attached to my piece of Plexiglas is a bit dirty.

The first series of shadow graph test where with a cat can side burner stove that I call the Blast Furnace and three different pots, a 150 mm aluminum pot, a GCS 1.5l JetBoil pot and a Heineken pot.


Picture 1: the 150mm pot note the thickness of the hot gas layer, the flame out of the jet can easily be seen, the hot gas flow around the corner of the pot and then the hot gasses start mixing with the surrounding air and becomes turbulent.


Picture 2:The 1.5l JetBoil pot, note the flame jet is visible but he heat flow up the sides is outside of the range of the shadow graph.


Picture 3: The Heiny pot, note the thickness of the hot gas layer and how soon after the flame leaves the stove that the hot gasses become turbulent, it obvious to me that some of the hot gasses do not come into contact with the pot at all.

The next series of photo’s are of the heat flow around a Heiny pot using three different stoves, the first stove is a center flame cat can of open flame chimney design, the second is my Volcano stove set to a fairly strong fame and the third is a Starlyte.


Photo 4: The chimney stove note the nice laminar flow up the sides


Photo 5: The Volcano stove, note the hot gasses are fairly strong and turn turbulent at the bottom rib.


Photo 6: The Starlyte, note that the hot gas layer is slightly stronger that the volcano and the hot gas layer is thin near the bottom rib and then turns turbulent.


Photo 7: Shows the rest of the hot gas flow up the side of the Heiny pot with the Starlyte stove, Note the turbulence increases as the hot gasses travel up the side, the same happens with my Volcano stove.

You can make your own conclusions about stove size vs pot size from these pictures.

I plan to do some more work in this area and am designing some tests to look at the effectivness of a properly designed windscreen

Tony

[zelph]

Tony, fantastic technique and results. I need to study the photos more and gather my thoughts and post my questions on Monday. The photos are totaly interesting and thought provoking. Now I want to make a shadow graph Is the light intensity and distance to subject constant? Bightness of the top photos vary is why I ask. The turbulence that we can see going completely up to the top of the pot, is it visible flame or is it hot gasses? Some questions came sooner than expected

[Tony]

Hi Zelph,

Tony, fantastic technique and results. I need to study the photos more and gather my thoughts and post my questions on Monday. The photos are totaly interesting and thought provoking. Now I want to make a shadow graph Is the light intensity and distance to subject constant? Bightness of the top photos vary is why I ask. The turbulence that we can see going completely up to the top of the pot, is it visible flame or is it hot gasses? Some questions came sooner than expected

Shadow graph has been around for a long time it has been used by puppeteers.

The light source has to be at least several meters away from the subject, it depends on what type and if a projector what the focal length of the lens is. My projector has 300w bulb we use 150-250W at work. We put the tracing paper on the camera side of the plexiglas, normal glass will also work. My projector is about about 3 meters from my stove and my camera is about 1 meter from the shadowgraph but that depends on camera and lens.

The The brightness/contrast of the photos have been modified I have done this to make it easier to see in the photos, it is hard to make them all look the same. Shadowgraph is easily seen by eye but the camera is not as sensitive as the eye and with my cheap point and shoot it does not come out as well.

Unfortunately the shadowgraph method only shows shadows not colors and we cannot tell the difference between the flame and hot gasses, I can do this by other method but they are much much more complicated and expensive to do.

I will post some more shadowgraph photo's soon.


Tony

[zelph]

Tony, thank you for the details on the shadow graph technique. I may rig up something small and quick to do a small scale visual graph of a candle flame to star and work my way up.

Photo 4 shows very little flame? Photo 5 a little bit more showing? photo 6 alot more flame showing? and photo 7 mega flames showing? All turbulent gasses are clean, no soot being deposited?


In some other post made elsewhere I said the heat rises up to the top of the pot and over the top edge of the pot towards the center of the pot. Have you been able to see that occur?

Turbulence directly under the pot caused by the concaved bottom of the pot seems to have soot and a bad odor associated with it. Are you able to use your scientific pot holder to raise and lower the heiny pot over your volcano stove until it causes turbulence on the bottom of the pot to get it to the point of causing soot and a bad odor? At the point of burning sooty, maybe the shadow graph will show the soot combined within the hot gasses and or flames. At this point maybe we can think of a candle with a very long wick burning full bore, sooty blackish type of burn. Now if we trim the wick to 1/8" it burns nice a clean.

The shadow graphs clearly show the rising gases being held in close proximity of the heiny can. It does not take on a inverted cone shape. We keep in mind the statement in the "boilermakers Handbook" about warm temps being emitted in the direction of cooler temps. The hot gasses on the outside of the pot are wanting to emit their heat into the cooler water contained in the pot via the thin walls of the pot. Flames on the out side of the pots sides are beneficial, not detrimental. Many would have us believe that flames must not go beyond the bottom of the pot to be an efficient backpacking stove. We need to keep this geared to backpacking stoves as we know them. Keeping in mind the conditions that the stoves are used under.

I love the science connected to fire and how it works for us. Can't thank you enough Tony for your insight into stove making and the time you put into your research and for sharing it with us!!!!

Last min. addon:; chimney effect will have big influence on rate of travel of gasses on side of pot when windscreen is placed too close to pot. A windscreen that has holes in it's base spaced around it's circumfrence. Windscreen design very important to it's function.

[Tony]

Hi Zelph

Tony, thank you for the details on the shadow graph technique. I may rig up something small and quick to do a small scale visual graph of a candle flame to star and work my way up.
Photo 4 shows very little flame? Photo 5 a little bit more showing? photo 6 alot more flame showing? and photo 7 mega flames showing? All turbulent gasses are clean, no soot being deposited?


In some other post made elsewhere I said the heat rises up to the top of the pot and over the top edge of the pot towards the center of the pot. Have you been able to see that occur?

Yes I have and I will post some pictures of it happening.

Turbulence directly under the pot caused by the concaved bottom of the pot seems to have soot and a bad odor associated with it. Are you able to use your scientific pot holder to raise and lower the heiny pot over your volcano stove until it causes turbulence on the bottom of the pot to get it to the point of causing soot and a bad odor? At the point of burning sooty, maybe the shadow graph will show the soot combined within the hot gasses and or flames.

I have not done many height tests and have not done them with the shadowgraph, I only have so much time in the day and I have been doing what I can. I will try and make my volcano stove burn dirty other than using IPA.

At this point maybe we can think of a candle with a very long wick burning full bore, sooty blackish type of burn. Now if we trim the wick to 1/8" it burns nice a clean.

The shadow graphs clearly show the rising gases being held in close proximity of the heiny can. It does not take on a inverted cone shape. We keep in mind the statement in the "boilermakers Handbook" about warm temps being emitted in the direction of cooler temps. The hot gasses on the outside of the pot are wanting to emit their heat into the cooler water contained in the pot via the thin walls of the pot. Flames on the out side of the pots sides are beneficial, not detrimental. Many would have us believe that flames must not go beyond the bottom of the pot to be an efficient backpacking stove. We need to keep this geared to backpacking stoves as we know them. Keeping in mind the conditions that the stoves are used under.

You are right “about warm temps being emitted in the direction of cooler temps” I have measured the temperatures of the hot gasses going up the side of a pot and the more efficient stove/pot combinations the temperatures are below the boiling point of water I have measured temps down to 35C-40C I have posted a graph below showing the blue line is the temperature of the water as it is being heated and the red line is the hot gas temperature measured 1.5cm from the base and 1cm from the side, not that the hot gas temperature is rising slightly as the water temperature rises, in my view I think that this means that the pot has absorbed the heat from the flame before it goes up the sides and when the cooled gas goes up the sides it can actually suck heat away from the water when the water is above the gas temperature. In this situation a shinny sided pot works the best.




On stove/pots that are blasting away the temperature of the hot gasses going up the side can reach 300-400C (I unable to find any graphs that show this at the moment they are on a computer that I have at home I will try and find one tonight) In this case then the water in the pot can absorb some heat through the sides and a dull black sides are the best but it is still not any ware as efficient as capturing most of the heat through the base.

I love the science connected to fire and how it works for us. Can't thank you enough Tony for your insight into stove making and the time you put into your research and for sharing it with us!!!!

I would like to thank the bplite community for sharing their views, ideas and designs with me and I very much enjoy sharing my views, ideas and designs with others and I especially enjoy a good debate.

Last min. addon:; chimney effect will have big influence on rate of travel of gasses on side of pot when windscreen is placed too close to pot. A windscreen that has holes in it's base spaced around it's circumfrence. Windscreen design very important to it's function.

A properly design windshield can help the efficiency greatly, that is why the Caldera cone works efficiently as it traps the hot gasses. I have done some testing on the Caldera cone design and it is important to get the holes sizes right but that might have to be some other time.

Tony

[Tony]

Hi Zelph,

in some other post made elsewhere I said the heat rises up to the top of the pot and over the top edge of the pot towards the center of the pot. Have you been able to see that occur?

I have an explanation of why the hot gasses join together after going up the ides of the pot.

When the hot gas plume rises up the side of the pot there is some mixing with the surrounding air, the side of the hot gas plume that is in contact with the pot has little or no air to entrain causing a difference of pressure with the side that is in contact with the air which entrains much more air causing the pressure to be higher than the inside of the plume, in the laminar and transitional flow stages this keeps the hot gases against the side of the pot and to a lesser extent with very turbulent hot gasses.

This also happens when the hot gasses reach the top of the pot the mixing can happen on both sides of the plume but on the insides of the plume there is much less air to entrain and therefore a lower pressure difference than with the higher pressure created by the extra mixing on the outer side of the hot gasses causing the hot gasses to join up above the pot.

I hope you can understand this.

I would like to that Emeritus Professor J. S. Turner (one of the world leading experts on turbulent mixing) for this explanation.

Tony

[zelph]

I hope you can understand this.

I would like to that Emeritus Professor J. S. Turner (one of the world leading experts on turbulent mixing) for this explanation.

Tony

I understand perfectly!!!

My thanks to you and Emeritus Professor J. S. Turner for the explanation. It's nice to know why things act the way they do. Now help me understand why my wife does the things that I think are weird

[Tony]

HI Zelph,

Now help me understand why my wife does the things that I think are weird

I am unable to help you with this one, the problem that I have is that my wife thinks that I am weird for playing with backpacking stoves.

maybe I am.

?????How can a technical thread like this one become a thread in how weird I am???????

Tony

[zelph]

HI Zelph,

Now help me understand why my wife does the things that I think are weird

I am unable to help you with this one, the problem that I have is that my wife thinks that I am weird for playing with backpacking stoves.

maybe I am.

?????How can a technical thread like this one become a thread in how weird I am???????

Tony

My wife thinks I'm "simple". I've often told her I try to "KISS" OFF when I design a stove. and she laughs when I tell her the KISS principle, Keep It Super Simple That's why she calls me simple. Glad she doesn't tell me to kiss off.

[zelph]

I bumped this thread up because of recent discussion about Laminar Flow.