[Stoves] Superficial Velocities experiment in WoodGas Stoves

[frank]

Stovers,
Loosing sleep over my experiment, I think I might have the answer.
The rate of flow was 0.5 liters per minute. Yet it took only 16 seconds 
for the CO2 to reach the detector in the empty chamber. When half the 
chamber void space was filled with  'fuel', and the air had to move 
around through the open spaces, the time was about a minute - as expected.

What I think happens is when the air is sucked from a small 1/4" dia. 
hole from the bottom of the empty liter chamber and air is drawn in from 
a small hole in the top  it must form a 'pipe' of flow down the center 
of the chamber with a lot of dead space along the sides. Hard to believe 
but that is the only answer I can come up with. The shape of the peak 
was sharp at the front and a long tail.

That could be what others are seeing when they determined that air 
mixing in the secondary air-pyrolysis gases produce much better 
combustion. It seems air flowing through a pipe does very little mixing.

I wonder if a layer of large 2 cm dia glass beads just above the 
secondary air and just below the combustion zone would be enough to mix 
in a TLUD. So many things to check out.

Frank

 





frank wrote:

>Dear Penn,
>
>Robert Penn Taylor wrote:
>
>  
>
>>Frank,
>>
>>I have a few questions for you about your setup:
>>
>>1. What are the details of the flow measurement, especially what type 
>>of flowmeter?
>>    
>>
>
>It is a needle valve below a meter with a small ball that floats  in the 
>air flow (sorry-forgot the name of the type meter)
>
>  
>
>>2. Are you monitoring pressures in the system? If so, where and what 
>>are they?
>>    
>>
>
>No - only keeping the flow constant. The fish aquarium pump pushes more 
>than enough air and I reduce it with the needle valve to what I want.
>
>  
>
>>3. When the CO2 is injected, is it via a pressurized pulse or are you 
>>relying on a venturi effect to pull a measured volume of CO2 into the 
>>system passively? If it's a pulse, what is the timing on it? Either 
>>way, what is its pressure?
>>    
>>
>
>I inject using a syringe from a pure CO2 gas flow from a CO2 tank 
>bleeding out air.  The intergrater on the computer is going. I wait and 
>inject at a specific time ( being measured every second) Record that 
>start time and then record the time when the peak height is at the 
>highest.  No pressure.
>
>  
>
>>4. I assume your analyzer measures CO2 concentration. If you integrate 
>>the concentration the analyzer reads over the time of the experiment, 
>>do you end up with the same amount of CO2 you put into the system?
>>    
>>
>
>I think the amount of CO2 is not important. As long as I use the same 
>amount for both (A) and (B). I am looking for Time of Travel.
>
>  
>
>>I suspect you've set up a constant volume flow rather than a constant 
>>mass flow. Your hypothesis of faster movement in the restricted setup 
>>is based on the assumption of constant mass.
>>    
>>
>
>Over my head! 
>
>Thanks
>Frank
>
>
>  
>
>>-Penn Taylor
>>
>>-- 
>>
>>Robert Penn Taylor
>>Graduate Research Assistant
>>Department of Mechanical Engineering
>>Iowa State University
>>(515) 294-5311
>>
>>frank wrote:
>>
>>    
>>
>>>Stovers,
>>>
>>>Trying to understand the movement of air through small fuel I set up 
>>>a simple experiment (see below) where I measured the time air took to 
>>>pass through an empty one liter chamber and again with the chamber 
>>>filled with plastic balls (fuel). Thinking the time of travel would 
>>>be faster with the balls due to reduction of void space by 
>>>~50%.causing the air to rush faster past the balls.
>>>Flow was constant at 0.5 LPM.
>>>
>>>I found just the opposite! It took about 18 seconds without the 
>>>'fuel' and about 56 seconds with the 'fuel' between the injection of 
>>>CO2 and the peak height. ????
>>>
>>>Crispin mentioned awhile back he thought Surface Area was an 
>>>important factor to know.
>>>A) I am wondering if this increased surface with the balls is the 
>>>cause of the increased time it took for the air to pass?
>>>B) Can we determine the surface area of a small fuel doing this 
>>>experiment (standardized against beads).
>>>C) Can we calculate the speed air is going past the fuel surface with 
>>>this experiment?
>>>D) Am I (once again!) way off base?
>>>
>>>
>>>Thanks for any enlightenment.
>>>
>>>Frank
>>>
>>>
>>>
>>>      
>>>
>>>>EXPERIMENT
>>>>Purpose:
>>>>To show that when fuel fills a container the air travels faster 
>>>>through the fuel than in the same container without fuel due to the  
>>>>reduced void space.
>>>>
>>>>Set-up:
>>>>I used a one liter squeeze water bottle for the fuel container and 
>>>>140 plastic spheres that each are 3.7657 cm3 for the fuel.
>>>>
>>>>Train from back end to front:
>>>>flow meter and needle valve set for 0.5 LPM
>>>>Aquarium air pump sucking air from up stream and flow controlled by 
>>>>needle valve. Steady flow.
>>>>CO2 analyzer
>>>>Out flow of the water (fuel) container piped to analyzer. Fuel 
>>>>container (A) not filled and (B) filled with total 527 cm3 plastic 
>>>>balls (fuel) leaving 47.3 % of total void space.
>>>>Two hole stopper fitted where the cap was. One hole for injecting in 
>>>>CO2 and the other with continued flow of CO2 free air being drawn 
>>>>through the train
>>>>CO2 free air comes from head space at the top of a large bottle of 
>>>>water with NaOH to remove the CO2 from supply line
>>>>Compressor on roof supplying air to the NaOH scrubber.
>>>>
>>>>Procedure:
>>>>(A)
>>>>System run until CO2 meter indicated a steady baseline with CO2 free 
>>>>air and *no fuel in the fuel container*.
>>>>250uL CO2 injected
>>>>Time from injection to high point of peak.
>>>>
>>>>(B)
>>>>System run until CO2 meter indicated a steady baseline with CO2 free 
>>>>air and fuel container *filled with fuel* .
>>>>250uL CO2 injected
>>>>Time from injection to high point of peak
>>>>
>>>>Results:
>>>>(A)
>>>>Time (seconds) from injection to high point of peak (NO Fuel)
>>>>18
>>>>17
>>>>18
>>>>16
>>>>20
>>>>
>>>>(B)
>>>>Time (seconds) from injection to high point of peak (Fuel)
>>>>63
>>>>56
>>>>58
>>>>56
>>>>50
>>>>
>>>>Conclusion:
>>>>It takes longer for air to pass through the restricted flow (due to 
>>>>fuel) than with an open fuel container.
>>>>
>>>>
>>>>Frank
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>        
>>>>
>>    
>>
>
>  
>

-- 
Frank Shields
Soil Control Lab
42 Hangar way
Watsonville, CA  95076
(831) 724-5422 tel
(831) 724-3188 fax
frank at compostlab.com
www.compostlab.com