Gas volume at different temperatures throughout the system
Jan 22, 2021 8:16:58 GMT -8
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Post by Solomon on Jan 22, 2021 8:16:58 GMT -8
I'm trained as an Civil Engineer. I have a Master's in Civil Engineering, though I'm not currently practicing. It only has a few related concepts to rocket mass heaters (RMHs), fluid dynamics, chemistry, physics, etc. I took a class on air pollution control and that familiarized me with the different volumes of air at different temperatures.
So to see such emphasis put on a constant cross sectional area throughout the system (CSA) doesn't track for me.
So I calculated a quick linear correction factor for the volume of air at different temperatures. I don't know if this holds for the high temperatures seen in a RMH, but it should be in the neighborhood.
70 degrees will be assumed to be room temperature at which the volume of the air is 1. So to find the volume of air at a temperature, you just take the volume you have, (or flow rate in this case) and multiply it by the correction factor.
(1 CF air)(1)=1 cubic foot of air at 70 degrees.
1 CFM*2.86=2.86 CFM at 1000 degrees.
1 CF*4.86=4.86 CF at 2000 degrees
The equation is F=0.002T+0.86 where F is the correction factor and T is the temperature (F).
As you can see, at the operating temperatures claimed, the volume of air should be around 5 times normal inside the heat riser. Much of this flow rate is handled by the velocity of the gas as it travels up through the heat riser.
One of the things that got me to thinking about this is seeing some builders talking about using oversized heat risers with success. This tracks. It is assumed that somewhere low in the heat riser is the maximum temperature of the system. Therefore, that should be the largest CSA of the system. If we are reaching 2000 as is claimed, that means that the volume of air at that temperature is 4.86.
Another thing to think about is the fact that by burning wood, we have not only increased the temperature by like 1000 degrees or more immediately, we have also increased the the amount of gas because we are turning oxygen and solid hydrocarbons into carbon dioxide, carbon monoxide, water vapor, plus the water vapor from the moisture in the wood which also expands. Hard to tell without doing some deeper chemistry, but the volume of gas is instantly increased by a significant amount.
Another thing to keep in mind is that we are trying to keep the heat concentrated to burn the fuel completely, so we can't just upsize everything to match the gas flow. As volume increases, temperature decreases.
At any rate, I'm no expert, I've only built one brick core. I'm just seeing some things in the traditional book design that don't make a terrible lot of sense. I also realize that the present pinnacle of design is specifically designed to be cheap to construct and there are going to be tradeoffs, some significant, in reaching that goal.
I'm interested to hear your thoughts.
So to see such emphasis put on a constant cross sectional area throughout the system (CSA) doesn't track for me.
So I calculated a quick linear correction factor for the volume of air at different temperatures. I don't know if this holds for the high temperatures seen in a RMH, but it should be in the neighborhood.
70 degrees will be assumed to be room temperature at which the volume of the air is 1. So to find the volume of air at a temperature, you just take the volume you have, (or flow rate in this case) and multiply it by the correction factor.
(1 CF air)(1)=1 cubic foot of air at 70 degrees.
1 CFM*2.86=2.86 CFM at 1000 degrees.
1 CF*4.86=4.86 CF at 2000 degrees
The equation is F=0.002T+0.86 where F is the correction factor and T is the temperature (F).
As you can see, at the operating temperatures claimed, the volume of air should be around 5 times normal inside the heat riser. Much of this flow rate is handled by the velocity of the gas as it travels up through the heat riser.
One of the things that got me to thinking about this is seeing some builders talking about using oversized heat risers with success. This tracks. It is assumed that somewhere low in the heat riser is the maximum temperature of the system. Therefore, that should be the largest CSA of the system. If we are reaching 2000 as is claimed, that means that the volume of air at that temperature is 4.86.
Another thing to think about is the fact that by burning wood, we have not only increased the temperature by like 1000 degrees or more immediately, we have also increased the the amount of gas because we are turning oxygen and solid hydrocarbons into carbon dioxide, carbon monoxide, water vapor, plus the water vapor from the moisture in the wood which also expands. Hard to tell without doing some deeper chemistry, but the volume of gas is instantly increased by a significant amount.
Another thing to keep in mind is that we are trying to keep the heat concentrated to burn the fuel completely, so we can't just upsize everything to match the gas flow. As volume increases, temperature decreases.
At any rate, I'm no expert, I've only built one brick core. I'm just seeing some things in the traditional book design that don't make a terrible lot of sense. I also realize that the present pinnacle of design is specifically designed to be cheap to construct and there are going to be tradeoffs, some significant, in reaching that goal.
I'm interested to hear your thoughts.