Another dumb question – some experimental results

Bingo for @itchy.

 

The combustion process being very rapid producesss shock wavea, audible in the roarer burner. This isn't cinfined to a singke direction. The back pressure is the result of the actual process feeding back through the vapour stream.

 

The whole issue is much more interesting than it might appear to be at first glance. My thinking (always suspect) did a complete one-eighty as a result of the hard work of @Twoberth.

 

If I am interpreting the latest posts on this topic correctly, the consensus so far is that the heat transfer into the 210 burner tubes is higher than into the No.1 burner tubes (higher surface area to volume ratio, and thinner walls). So the vapour gets hotter, and its density is lower than in the No.1 tubes, and this is responsible for the lower mass flux out of the 210 burner jet.

Every good hypothesis must stand up to scrutiny when applied to real life situations, and so I did some calculations on three burner options; a 210 burner, a No.1 burner, and a partially blocked No.1 burner (a common occurence). The dimensions I used are as in the diagram below.



For a fixed length of tube in all cases, the heat flux inwards through the outer wall is proportional to
A/r, where A is the outer surface area and r is the wall thickness. Since A = circumference x length, then the flux is proportional to OD/r.

For the three tubes, these ratios work out approximately to be 6:5:4, with the highest heat flux through the thinner 210 tube.

But the middle tube is heating the largest volume of vapour in the bore. The bore volumes are in the ratio of (ID)2, which is 16:25:16

And so the heat flux per unit volume is in the ratio of
(6÷16): (5÷25): (4÷16) or roughly 6:3:4

So the middle tube (No.1) has the lowest heat input into the vapour and so its vapour has the highest density as predicted.

However I was a bit uneasy, as this simple analysis assumes that the carbon gunk layer in the third burner has the same thermal conductivity as the tubes, which it doesn't. However it is not a solid layer of carbon, and although it is tenacious, it is porous and not a very good insulator.

As you may expect, this is not the only process where the coking up of tubes is an issue. Carbon deposits in chemical process and heat exchanger tubes is a common problem and much published research is available. If you type 'thermal and hydraulic effects of carbon deposits' into Google, you get pages and pages of hits.

Being a nerd, I read several papers by Gascoin, Abraham and Gillard, three scientists at the University of Orleans. These are some of the conclusions from their various papers on coke deposits in tubes. (English is not their first language, so the translation from French is not perfect)

‘The thermal insulating effect of coke is not significant...' .

‘The thermal insulating effect by coke deposits is proved to have a negligible impact on the system’.

‘its (carbon deposit) insulating effect is of minor importance’.

So I am relaxed about the carbon deposits (coke) in stove burners not being a major insulator, and that my simple assumption on the thermal conductivity through the tubes is not too far out.

Based on the simple heat flow analysis above, the hypothesis holds water (or kerosene).

Well done to all those who contributed. I had a totally wrong explanation initially until @itchy @Radler and others drew attention to the vapour temperature effect.

This may not be the full explanation or even the correct explanation, and I look forward to additional contributions.

 

@Twoberth You do dig into these ideas with more attention to detail than I ever could.

Agree with the last sentence above. I am a little uncomfortable with lower temperature/higher density explanation but it sort of makes sense, and I am out of other ideas. Will just have to wait for a light bulb to go on with someone esle.

 

With your above "heat exchanger" details I know there is plenty I'm missing.

Nonetheless, considering a stove burner at full roar.
The sizzling / boiling / magic sweet spot,
the precise area/level where the *liquid surface changes to vapor has to occur at some level between points A & B:
A. the surface level of liquid fuel (the level is dynamic) and
B. below the top of the 4 burner tubes (halfway ish?). Complete vaporization has to have occurred by the time vapor crosses the top of the burner (liquid fuel present at the top of the burner = fail).
In my view, A to B and all points in between is the vapor chamber.
---------------------------------------------

Evaporative exchangers and hands-on observations of heat exchangers i've serviced aside. It suffices to say, the design of "the piping":
of any heat exchanger vs
any stove's vapor chamber differs significantly.

An aside, convection heat: can factor into heat exchangers. Although also present with our stoves convection is not a factor with our vaporization.

Your arrows seem to indicate the (full length? of) pipe exposed to external heat. Is that your intent with the diagram?

If so, I add some basics with certainty:
so, present below the level of our stove's flame, are 2 heat sources, both from above:
radiant heat &
conductive heat.
There is no reason to completely block the radiant heat (which also contributes to vaporization) but
the self pressurizing self vaporizing function is reliant on the conductive heat (if radiant & convection heat were absent nearest the vapor chamber, the burner will still function).
Below the flame the, from the top --> downward --> conductive heat is not external to the pipe... it is the pipe (sourced from the directed flame super-heating the top of the burner and the flame spreader).
That said, your explanation may still hold (if my input amounts only to a question / minor tweek of the diagram?).

Thermal transfer / heat exchange is present on our stoves but the burner design is to maximize vaporization within the vapor chamber resulting in rapid vapor expansion.
The conductive heat also heats / expands the air in the tank.
The 1870s/1880s related inventions intrinsic to our stoves (and to blow lamps) is the development of liquid fuel vaporization! <-- that is the key, that is the magic sweet spot imo.

At full roar within the vapor chamber is, the magic sweet spot, a chamber that maximizes the internal sizzling surfaces, the boiling point / vaporization which results in *rapid vapor expansion.

*Re kerosene vaporization (expansion rate liquid to vapor), was that answered and I missed it?: " … how much it [kerosene] expands (liquid to vapor) I've not found." … "... and the rate of expansion of kerosene liquid to vapor" ??
IE: steam (from water heated to 212F) expands to 1600x it's liquid volume.

I've seen the vaporization / boiling temperature range for kerosene is between 300 and 575F.

 

@OMC
Thanks for the feedback.

I used a simplified scenario to compare three different burner tube dimensions.

I ignored radiation and convection, and I also assumed the whole outer surface area as being exposed to the heat, which is clearly not the case. However, since this is a comparison and these are 'systematic errors' in attempting to simulate the real situation, the effect of these 'errors' applies equally to each of the three options, and the effect cancels out when comparing them to each other.

It was not meant to be an accurate calculation, but a simple comparison to test the premise that it could be the vapour temperature difference that causes the difference in mass flow.

It doesn't prove that it is the temperature difference, it just shows that it could be.

 

Old thread resurrection alert.

Read all this thread with great interest, although not professing to understand all the details.
What I was hoping to work out was in what part(s) of the burner tubes does the carbon build up?
In reading @OMC 's contribution

.
If that is correct, does that mean that the carbon will have been created before the fuel hits the top of the burner, as well?
Would that mean that the carbon build up would be more likely in the outer rising tubes rather than on the underside of the burner head or in the U tube dropping down to the jet?

My interest stems from wanting to find a safe way of unblocking choked burners. As a cyclist, there are always pieces of frayed cables available to me - I have used single strands for replacement of jet prickers (gear strands for smaller and brake strands for larger jets), then I thought of scraping the insides of the outer 'riser' tubes with the end of a frayed brake cable, using a twisting action to scour it out. I have pulled out a fair bit of carbon this way but clearly cannot get around the bend to across the burner top or the U tube. So to distill the question further, if I scour out the outer tubes is there much of a risk that i have left the remainder still quite choked?
Another observation on the decoking process - back in the day we used to use Redex with our petrol to decarbonise our cylinder heads - the efficacy was always a bit suspect, but would a soak in Redex or a shot or two added to the fuel help to cleanse the internal pipework?

Has anyone tried this (and lived to tell the tale)?

 

Hello Dean,
Welcome!
I just deleted a lengthy reply, that was in a nutshell,
re > my: "Complete vaporization has to have occurred by the time vapor crosses the top of the burner" My impression still holds . -and-
> your: "If that is correct, does that mean that the carbon will have been created before the fuel hits the top of the burner, as well?"
^^^ This is not a conclusion I sign onto then/now.
For myself it does not also mean that, there are many variables (incl. usage, dynamic pressure & liquid level/boiling location, fuel & residue) and I do not have an answer.
===================

Before experimenting with chemicals or you go at soft brass with frayed steel cable. Your request is:
"My interest stems from wanting to find a safe way of unblocking choked burners."

That is not a focus of this thread and THERE ARE many threads regarding your topic.

Re choked burners, stating the obvious, remove burner and jet, see what you see from two open ends.
Blasts of carb cleaner is mild start.
Next a good soak and repeat but thorough cleaning of choke buildup is a process, if you need it.
FYI re that: more recently the heat and quench method is not being recommended.
Might I suggest a renewed search but in the fettling forum.
Here is a sample of on-topic comments, credit @kerophile :

... Air fed carbon burning (Original Primus method) ...
Remove the burner from stove, remove jet. Connect burner to air supply and blow through whilst heating the burner. The coke will ignite, Stop heating, and allow coke to burn away to ash. Blow through to remove cracked coke and ash, Replace jet.
Be careful not to overheat the burner or you will melt the braze holding it together. source

carry on

edit: Dean myself and more than a few members may follow your current effort with interest. You came to the right place with the right question. I welcome if you resurrect another thread on-topic or create anew. thank you

 

The observations made, and postulated reasons given are rather fascinating. I wonder if there are any combustion engineers from Rolls-Royce, Pratt and Whitney, Allison etc who would give us a clear scientific and engineering explanation?
Fluid flow, heat transfer, flame velocity and similar factors are surely "bread and butter" for such wizards, when designing modern, fuel efficient, jet engines for large aircraft?
Cheers
Simon Foxxx

 

Constrictions upstream of the jet can only have an impact if they are similar in size to the jet, which seems very doubtful. The length of the jet (rather than diameter) is the most likely candidate here.

However, one burner has far more surface area exchanging heat with the flame, so the gas stream will be hotter.

P1V1/T1 = P2V2/T2

P is the same in both cases so the equation becomes:

V1/T1 = V2/T2

If T2>T1 then also V2>V1. The same amount of gas in the hotter burner therefore takes up more space (volume), which would cause reduced flow and lower power through the same orifice.

So we are back to the length of the jet to explain what we are seeing here. The burner must get just hot enough to complete vaporisation- any hotter than that and power will start to reduce rather than increase. This is why it’s a stable, self correcting system. Jet size is all about achieving stoichiometry with the available air flow, knowing that fuel flow has an inherent stability, once gaseous.

If hotter fuel delivered more power then the system would run away into melt down...