Air inlet optimization (5” batch box)

[fiedia]

I built a 5” batch prototype with air inlet in the door frame as per Peterberg design for his DSR2. The primary air flows through the frame and penetrates the firebox on each side of the door. The air flow along the door minimizes deposit on the glass. Details can be found: here

On Peterberg advice, I did some trials reducing the primary air inlets to improve burn stability and reduce thermal overdrive occurrences.

Four different setups were tested:
- no tape on the inlet slots (20.4cm², i.e. 16.6% of HR CSA)
- 50mm tape on the inlet slots (14.4cm², i.e. 11.7% of HR CSA)
- 70mm tape on the inlet slots (12.0cm², i.e. 9.8% of HR CSA)

- 90mm tape on the inlet slots (9.6cm², i.e. 7.8% of HR CSA)

the tape is stuck at the bottom of the slot.

The secondary air inlet (floor channel) is not changed (11.96cm² i.e. 9.7% of HR CSA).

Results :

With usual wood loads (medium size logs), the burns were more stable with 50mm and 70mm tapes. For 90mm tapes, it would burn badly and create dirty smoke unless I left the door slightly open.

A potential explanation is:

- With full inlet opening (24.4cm²), smokes come from thermal overdrive. The wood load is getting too hot. HR temperature is high enough to create post combustion but there is not enough air to burn all gases generated in the firebox.

- With 90mm tape: there is not enough air to generate high flames. The HR temperature is too low to start post combustion.

- with 50mm tape: the fire is more stable than with no tape but thermal overdrive occurred at the end of the burning phase.

- with 70mm tape: the fire was stable and clean during the whole burning phase.

The curves below show the temperature at the hottest point of the HR (bottom) for each inlet setting.

The 70mm tape setting gives the best results. I tested this setup with different types of wood.

- mix of medium size hard wood (50mm) and split pine boards (20mm): the fire lasts 40mn with temperatures between 600 and 700°C and absolutely no smoke.

- exclusively big logs of hard wood (80mm): temperature stays around 500°C during almost one hour. It is quite low but sufficient to start post combustion since no smoke is generated.

- exclusively split pine boards (20mm): temperature stays just below 800°C during 30mn. Thermal overdrive occurs after 15mn.

In conclusion, the 12cm² setting (10% of HR CSA) is much more stable than the initial opening (20cm²). Big logs burn without generating smoke. Thermal overdrive occurs only with thin split pine boards.

Now I wonder where is the best place for the inlet slots. It seems that the fire quality is improved when there is more room between the firebox ceiling and the wood load. Does it mean that it is better to have the air flow above the fire ?

peterberg , you have tried with a slot on top of the frame. Did you notice an improvement ?

[peterberg]

Some space above the fuel seems to be better for secondary combustion, yes. One of the goals regarding the DSR3 development is to be able to leave out secondary air provision altogether. All air comes in through the door frame with strong emphasis on the upper part of the firebox.

Open systems are able to burn remarkably clean without separate secondary air provision at all. But: keeping distance from the rear wall and the ceiling of the firebox is all-important in this. So in the latest test runs I tried to mimic this in the DSR3 and it works, yes. In general: the top slot is as large as the size of the air slots that are taped off in my setup.

[fiedia]

peterberg ,     

I need once more your advises before modifying my air inlets (batch 8"/200mm this time). It heats well but I would like to improve two points: HR temperature and dirt on the firebox window.

HR temperature: it is difficult to get temperatures above 400°C. Starting a fire, the door is left slightly open until the fire is burning well and HR temperature reaches 600°C. Once the door closed, the HR temperature drops around 400°C again and remains there unless I reopen the door, and again, and again may be 2 or 3 times. There is an adjustable air inlet on the top of the frame. Opening it does not really help.

Firebox window: the window is getting black on the left and right sides facing the inlet lower halves. It looks like incoming air flows mainly through the upper half of the slots. Did I make a mistake positioning the slots at the top of the door frame ? Does it lead the air to flow through the firebox along the ceiling directly to the HR port?

May be I should make the slots narrower and longer. It could force the air to flow lower behind the glass and inside the firebox. It could keep the window cleaner and improve the combustion on the bottom of the firebox. What is your experience?

Present slot dimensions are 20mm x 160mm (2 slots of 0.1 x HR CSA). New dimensions would be 12mm x 320mm (2 slots of 0.12 x HR CSA).

       

[peterberg]

Feb 18, 2023 23:54:27 GMT -8 fiedia said:

I need once more your advises before modifying my air inlets (batch 8"/200mm this time). It heats well but I would like to improve two points: HR temperature and dirt on the firebox window.

Reading your post I am wondering whether or not your chimney is up to specs for a 200 mm system. Taking this a step further: are you sure there isn't any restriction downstream in the system?

How high the riser temperature can get is depending on the spot where you measure it. At the top of the riser, measured in the center: normally about 750ºC running at full bore. Just a couple of occasions I measured 900ºC, using hardwood species and a refill back to back.
When measured in the riser itself: halfway the height of the port, at the back of the riser, just one inch inside a temperature of 1000 to 1100ºC is possible. All done in a 150 mm system.

When the temperature in the riser drops while the door is closed could be an indication there isn't enough fresh air available. Since air provision is a combination of opening and air velocity you have to look at the whole of the air system, nothing is been said how the trajectory is done.

You didn't say there's a floor channel. The air system could be done without, in the door frame there should be a slit in the upper half left and right. The measurements you showed is less than that, plus there should be slits in the door frame lintel, together as large as the gaps left and right. Please don't try to calvulate how wide the slit would be, relative to the riser's CSA. All gaps 20 mm wide, left and right as long as half the height of the door frame, in the upper half.
From the central inlet the air need to go through a 90 degrees bend on both sides, this transition should be twice as wide as the duct itself in order to avoid friction. Which means the air inlet itself is the restricting factor, nothing else. Using this system in the DSR3 yielded very good results, firebox door stayed clean throughout 20 or more test runs.

[fiedia]

peterberg ,

Thank you for your insights

A view of the door and heat riser:


Fumes go directly from HR to the flue when the bypass is open:

HR octagonal 0.2m x 1.2m. Bypass 0.2m x 0.2m. Flue 0.2m diameter x 4m insulated pipe.
Thermocouples at HR bottom + 11cm, 22cm, 33cm, 44cm and 55cm.
Floor channel 50mm x 80mm with a 50mm x 50mm duct facing the port.
Door frame primary air entry bottom left and right 50mm x 80mm.
Door frame primary air exit upper half left and right 20mm x 160mm.
Door height 400mm.

If I understood well your post, I just need to make my upper slots a little bit longer: 20mm x 200mm.

“From the central inlet the air need to go through a 90 degrees bend on both sides, this transition should be twice as wide as the duct itself in order to avoid friction.“

Sorry but I do not understand what you mean by transition. Does it mean that my bottom inlets should be 100mm x 80mm?

“You didn't say there's a floor channel. The air system could be done without”

Should I try to run the batch rocket with closed floor channel?

[peterberg]

Sorry for the long delay, I haven't been well the past weeks.

Feb 20, 2023 11:33:38 GMT -8 fiedia said:

If I understood well your post, I just need to make my upper slots a little bit longer: 20mm x 200mm.

That's what I meant, yes.

Feb 20, 2023 11:33:38 GMT -8 fiedia said:

“From the central inlet the air need to go through a 90 degrees bend on both sides, this transition should be twice as wide as the duct itself in order to avoid friction.“

Sorry but I do not understand what you mean by transition. Does it mean that my bottom inlets should be 100mm x 80mm?

All the air that's going left and right, in horizontal direction need to change direction to vertically upwards. In order to avoid friction at that point you'll need a wider bend. Because the air velocity is high there, I would recommend a bend that's twice as wide.

Feb 20, 2023 11:33:38 GMT -8 fiedia said:

“You didn't say there's a floor channel. The air system could be done without”

Should I try to run the batch rocket with closed floor channel?

It looks like you mixed a floor channel system with the one of the DSR3. The latter one doesn't incorporate a floor channel, all the air is going through the door frame. This also means there shouldn't be an opening from the air inlet directly into the firebox. In your drawing it looks like there is such an opening, plugging the floor channel won't help in that case, rather the opposite.
What you have now is the air system in a normal floor channel arrangement, plus aspects of the DSR2 air system and one aspect of the DSR3 arrangement.
It's one or the other, I didn't test this particular combination, certainly not with a sidewinder batchrocket.