Peterberg Batch Box Dimensions

[satamax]

I wonder what would be the effect of a proper airstreamed venturi port.

I mean the back of the firebox would be rounded in a shape which would concentrate the flames on the port, then the port sides would be shaped properly for the air to flow as best as it can; and the gasses expansion after that would suck what's on the firebox side.

I won't try, i have no means of testing nor the time. But it would be cool to know.

And may be a longer port, i mean, narrow, venturi shaped, to concentrate all the flames in a little area, insulated like mad.

Technicaly the rapid expansion of gasses after a venturi is what creates the suction effect. like, increasing the flow of compressed air.

www.youtube.com/watch?v=Na9ORhYjvJU

[Donkey]

Aug 27, 2013 11:13:31 GMT -8 peterberg said:

I've tried smaller and wider ports as well, results were not as good. So yes, the constant could be 72.36% but the question of why isn't answered.

Tantalizing, don't you think?

Yes, quite.
Peter, you know more about this than I do.. Where else in the stove world do we see ratios like this? Is there some corollary to another type of stove? Something well known.. ?
Perhaps a connection can be inferred somewhere. ?

[peterberg]

@donkey,
To my knowledge, nowhere in the stove world is something like this known, sorry. During all these years of digging through stove information I've never come across a constant, let alone this particular one.

@max,
Shaping the firebox to optimize the venturi effect could lead to a less usable fire box. Maybe I could try a shaped port, sometime in the years ahead...

[Deleted]

Aug 28, 2013 2:08:13 GMT -8 peterberg said:

@max,
Shaping the firebox to optimize the venturi effect could lead to a less usable fire box. Maybe I could try a shaped port, sometime in the years ahead...

There is not much shaping needed to minimize the "Head Loss Coefficient".

The Thermal Wizard
www.thermal-wizard.com/tmwiz/default.htm

Klick on Head Loss Coefficients and then on Duct Entrance.

For cast boxes it is easy to make a big radius at the port entrance.

[peterberg]

Very interesting. Since you seem to be the one who's knowledgable, what radius at the port entrance would you recommend? And what is the hydraulic diameter of a port of say, 240 x 54 mm?

[Deleted]

Aug 28, 2013 2:54:19 GMT -8 peterberg said:

Very interesting. Since you seem to be the one who's knowledgable, what radius at the port entrance would you recommend? And what is the hydraulic diameter of a port of say, 240 x 54 mm?

The radius should be as big as feasible.
At a given widh the height of the port influences the flow velocity in the port,
which depends on the velocity before the port and the CSA ratio of box and port.
I think the height of the port will influence the head loss mainly by the flow velocity,
which is neglible by a radius to diameter ratio > 0.25.
A ratio of 0.4 could be used for safety.
54 mm x 0.25 = 13.5 mm.
54 mm x 0.4 = 21.6 mm.
I would recommend a radius of 15-20 mm.

[peterberg]

OK, I've recalculated the rectangle of 240 x 54 mm to a circle of 128.45694 mm or 0.1284569 m diameter. Inserting this dia in the Thermal Wizard, flow velocity of 2 meter/sec. and a minimum of 0.03 m for the bellmouth radius, this would result in a Head Loss Coefficient of 0. Anything smaller radius would result in a slight loss.

On the other hand, only counting the sides of the port to be rounded off, this sides combined could be seen as the circumference of a circle with a diameter of 0.1527888 m. Using a radius of 0.04 m, this would result in no loss again.

This Thermal Wizard is maybe not fit for calculating a rectangle duct, I don't know.

Alright, maybe someone who's gearing up for the Woodstove Design Challenge could use this!

[Donkey]

Thanks Karl..

[satamax]

Thanks a lot guys.

Karl, Peter, i was thinking about the airfoil shape you see in some high end carburetors. Tho, not having seen one for a fair while, i can't remember if it's normal a normal airfoil or a reversed one. I have seen a flame in such a venturi, gas flame, which created a horendous amount of suction. The gas jet has to be placed perfectly to create the best suction, it was like a ramjet of some kind but meant for sucking air or throwing flames. Can't remember exactly. Had a light thrust, but it had thrust. If we could implement that effect into a rocket, that would be mad I'm imagining this in the burn tunel of a J tube. I've been searching images for a while. But can't seem to find a rendering of this.

[Donkey]

Ok.. So I'm flying by the seat of my pants here but in looking about a little, I've maybe found something interesting.

From Wikipedia, the formula for stack effect


SI units:

where:
Q = stack effect draft (draught in British English) flow rate, m³/s
A = flow area, m²
C = discharge coefficient (usually taken to be from 0.65 to 0.70)
g = gravitational acceleration, 9.81 m/s²
h = height or distance, m
Ti = average inside temperature, K
To = outside air temperature, K

U.S. customary units:

where:
Q = stack effect draft flow rate, ft³/s
A = area, ft²
C = discharge coefficient (usually taken to be from 0.65 to 0.70)
g = gravitational acceleration, 32.17 ft/s²
h = height or distance, ft
Ti = average inside temperature, °R
To = outside air temperature, °R

This equation assumes that the resistance to the draft flow is similar to the resistance of flow through an orifice characterized by a discharge coefficient C.

Notice that there IS a constant here AND it's in the "ballpark" of our 72% -- discharge coefficient.

I'm not sure how (or even if) this relates, though it FEELS at least close to the right direction for further inquiry.

[Donkey]

From Wikipedia again:

In a nozzle or other constriction, the discharge coefficient (also known as coefficient of discharge) is the ratio of the actual discharge to the theoretical discharge, i.e., the ratio of the mass flow rate at the discharge end of the nozzle to that of an ideal nozzle which expands an identical working fluid from the same initial conditions to the same exit pressures.

OK, maybe NOT a direct correlation...

[ukdan]

Nice thread, very useful. Thanks Peter and Donkey

Can I ask a (potentially stupid) question please?

Where exactly should the port sit in terms of height relative to the batch box? Should the top of the port sit level with the top of the batch box, or bottom of port level with bottom of batch box, or somewhere in between?

Thanks
Dan

[peterberg]

That's an easy one, the bottom of the port should be level with the firebox floor. Which, incidentally, is only as wide as the port. The rest is sloped sides, left and right.

[ukdan]

Great, thanks very much Peter. Easy answer

Did you experiment with this port position in any of your prototypes.

[peterberg]

Yes, I did. In fact, I've spent the best part of 2012 doing that. See this thread, it's all about these batch box thingies.