Development of a compact batchrocket core.

Fair enough Peter. Thank you for the quick reply.

Our normal approach for everything is "form follows function" but the ranch boss lady really preferred the idea of stainless steel barrels for the intended location.  I was fortunate and found some that had been used for transporting cabernet and the guy was selling them cheap.  I have time to think about my design, the build will be this summer.

Peter van den Berg wrote:The effect of the thinner firebox' back wall kept nagging me. I reinstated the superwool on four sides of the riser box, a bit over an inch (30 mm) higher and also crossing the port.



I tried four testruns in total, some were really nice but half the runs were not cutting the cake. All were loaded with the same fuel and the same pattern was mimicked.



The following diagram happened to be a good one, the results of the one next day were disappointing, to put it mildly.





What the heck was going on? Still, the thought of why the less deeper port was yielding bad results kept following me. But today it dawned on me: since the shallower port generated bad results, would a deeper port generate better ones? After all, such an effect would be a sliding scale, rather than an on-off switch. So the inplementation of a deeper port was suddenly a thing to do right now.
In order to keep the bottom of the riser box as a square, I took out the wool at the rear wall and added the same amount at both sides of the riser port. What actually happened was that the bottom half of the afterburner shifted one inch (25.4 mm) to the back. Now the depth of the port rose to 8 cm (3.15"), exactly what Independentenergy ( a poster on Donkey's forum) was asking for, although the riser floor remained a square.



I used the exact same amount and type of fuel as before, arranged in the same manner. There was more wind today, 5 Bft with strong gusts, normally not a good omen for a succesful run.
And this were the results, surprising to say the least.



As can be seen, the O² went down rather quick. I closed the stove's door at three minutes into the diagram and refrained from doing anything else than looking at it, biting my nails (figuratively speaking) and hoping for the best. At the 3.5 minutes mark the CO dropped lower than 500 ppm and stayed there until 45 minutes. A run with a length of 57 minutes and CO below 500 ppm for 41.5 minutes is something that I rarely see, I tell you that. On top of that, the white Testo filter came out very light grey instead of very black!
Average numbers for this run: O² 12.09%, eff. 87.47%, CO 307.9 ppm, TR 146.27 ºC. The CO level for an entire run, ended according to the EU norm and compensated for to the 13% oxygen level would be 286.35 ppm. Now waiting for a day with calmer weather conditions in order to verify whether this was a one-off or new, stable behaviour.

Hi Peter,

For everyone's information, Peter has been privately coaching Thomas and I at dry stacking the shorty core (see Casting large refractory slabs) and thought I would instead ask on the forums so everyone can also see and benefit from Peters responses.
So, this question for Peter has to do with the depth of the riser port and function of the liner at the base of the riser.
In the final sketchup file of the shorty core, Peter has the depth of the port twice as deep as a regular batch box depth. It was unclear to me what effect this has on the burn other than moving the afterburner 1" back (as Peter mentioned). How does it increase velocity or flame characteristics with more depth?
Also, the liner only goes part way up the riser, opening up the top end to a greater csa. What effect does that also have on the burn? A venturi effect?

Gerry Parent wrote:
So, this question for Peter has to do with the depth of the riser port and function of the liner at the base of the riser.
In the final sketchup file of the shorty core, Peter has the depth of the port twice as deep as a regular batch box depth. It was unclear to me what effect this has on the burn other than moving the afterburner 1" back (as Peter mentioned). How does it increase velocity or flame characteristics with more depth?

In fact, at the time the total depth of the port ended up as twice the width of the same. As far as I can see, the double vortex is right against the backwall of the riser, quite far away from the front of the port. And therefore also quite far away from the end port.

Gerry Parent wrote:Also, the liner only goes part way up the riser, opening up the top end to a greater csa. What effect does that also have on the burn? A venturi effect?

The liner in the lower end of the riser is there to form the double vortex properly. Without this liner, the riser box proved to be too wide, only a straight flame was formed. The wider top half is there to provide the gases with room to expand, slowing the gas velocity quite a bit.
So, the double vortex is rising up over the entire height of the rear wall since there's no liner there. Then, the stream curls to horizontal in the direction of the end port which is system size again. In order to reach the end port, the stream has to bend down a bit. This effect will heat up the closed top of the riser more than it normally would do, so the temperature will rise quicker there. The complete path approaches the length of the normal straight riser. The volume of the entire riser box is very close, if not more, to the volume of a normal length straight riser.
The expansion space and the restricted end port together will limit the maximum burn rate of the core, that's the general idea.

Time will tell whether this holds true or not. This is all theoretics at this time, based on experiments with one single, small development model.

Very fascinating information Peter.
To know the why and hows of your design helps me to see the functioning aspect of each component and know its purpose for being there.

Peter van den Berg wrote:The liner in the lower end of the riser is there to form the double vortex properly. Without this liner, the riser box proved to be too wide, only a straight flame was formed. The wider top half is there to provide the gases with room to expand, slowing the gas velocity quite a bit.
So, the double vortex is rising up over the entire height of the rear wall since there's no liner there. Then, the stream curls to horizontal in the direction of the end port which is system size again. In order to reach the end port, the stream has to bend down a bit. This effect will heat up the closed top of the riser more than it normally would do, so the temperature will rise quicker there. The complete path approaches the length of the normal straight riser. The volume of the entire riser box is very close, if not more, to the volume of a normal length straight riser.
The expansion space and the restricted end port together will limit the maximum burn rate of the core, that's the general idea.

Time will tell whether this holds true or not. This is all theoretics at this time, based on experiments with one single, small development model.

Thus far, with about a weeks worth of careful observation of the shorty cores performance, I would say Peters description of what is going on under the hood is very similar to what I have observed also.

The flames rarely have come out the end port even with our biggest fires so far are strongly suggesting that burn rates are being automatically controlled as Peter mentioned.

The bricks we cast are doing very well. No signs of cracking or shrinking. I would feel confident now to light a big fire with no worries of anything happening to them.

The only thing I would change thus far is to use either insulated bricks or superwool to form the entire heat riser. The superwool could possibly be made similar to the 5 minute riser encased in a metal skin, either square or round (while maintaining proper csa)  however, the metal compression frame would need to be redesigned to accommodate this much softer material.

The new door is complete and in full operation.

very interesting