Heat transfer and airflow question

[Ioannis86]

Hello everybody, I would like your comments on my findings and perhaps prompt me to look at relevant literature.

Consider an enclosed airspace 1m length, 1m height, 0.5m width. At the center of its one 0.5 side a fan is drawing air out of the chamber through an aperture(steady, low air extraction) . At its other end a heater is used to produce different thermal conditions (through another aperture). I have placed 3 thermocouples at the fan's side, 1 at the center of the aperture and 2 at the lower corners. For a full heat input, the temperature elevation at the second hour is 4.5 C for the right corner and 1.7C for the left corner. This must mean that the left corner is better ventilated than the right corner, but, if that is the case, why does the elevation at the fan aperture is 8.1C, since it is definitely a point in space always under ventilation??

 

[soothsayer]

I've included a drawing of what I pictured the set up to look like, let me know if I have something wrong.

You're saying the temperature change is greatest at the thermocouple at the fan aperture, and want to know why when this is the point of the most ventilation. I think there could be a couple reasons for that.

1) Your heater is likely a source of radiation, in which case the thermocouple at the fan will receive the most direct heat radiation. At the temperature changes you're talking about, I don't know how much this would influence things, but at a timescale of an hour, it is a possible contributor.

2) Fans do not lower the temperature of an atmosphere, they actually raise it, by contributing energy to the system. Imagine your box full of air molecules. The average kinetic energy of these molecules in a given area is proportional to the temperature of that area. The fan thermocouple is receiving a constant dose of molecules that have been heated on one side of your box and accelerated directly into the fan aperture, thereby hitting your thermocouple and delivering an exchange of energy. There is less "wind energy" in the bottom corners where your other thermocouples are, and they are also further away from the heater.

As to why the two bottom corner thermocouples show a different temperature change, this I cannot say. It perhaps has to do with the construction of the box--the thermocouples are not perfectly spaced, the box is not perfectly rectangular, the heater aperture is not quite centered, air flow is not perfectly isotropic, etc, etc. Likely, there are a lot of sources of error that would contribute to an otherwise theoretically identical temperature change reading.

Even if I have understood your set up a bit incorrectly, hopefully this has provided some insight.

 

[Ioannis86]

Hello Soothsayer,

Thank you for your answer, the drawing you made is representative of what is happening.

2) Fans do not lower the temperature of an atmosphere, they actually raise it, by contributing energy to the system. Imagine your box full of air molecules. The average kinetic energy of these molecules in a given area is proportional to the temperature of that area. The fan thermocouple is receiving a constant dose of molecules that have been heated on one side of your box and accelerated directly into the fan aperture, thereby hitting your thermocouple and delivering an exchange of energy. There is less "wind energy" in the bottom corners where your other thermocouples are, and they are also further away from the heater.

So this means that low temperature change is due to the lack of ventilation and not from its effect?

 

[soothsayer]

So this means that low temperature change is due to the lack of ventilation and not from its effect?

It seems counter-intuitive, and I'm still trying to convince myself one way or another, but what you have is much like a convection oven. Convection is really going to be the #1 source of heat transfer in your system. You are putting heat in through one aperture and pulling air out from another (by the way, how does air get back into the chamber? You're not drawing a vacuum, are you?). Your fan is aiding this convection, and providing the best path for heat flow directly onto itself, and subsequently, the adjacent thermocouple. My thinking is that this thermocouple should then heat up the quickest. The two thermocouples in the corner don't have that same convection path aiding heat flow to their respective regions, and so they won't heat up as quickly.

It would also help me to know where you are pulling air in from (the heater aperture?) and what temperature your heater is at.

 

[soothsayer]

Fyi: I just spoke to a PhD friend of mine about it, and he confirmed my theory. So in a word: yes. Low temperature change is due to poor ventilation, which is not so counter-intuitive, now that I think about it. He also brought up that your heater is likely hot enough for incandescence and so radiation heating, as I mentioned, is also a significant contributor.

 

[Ioannis86]

by the way, how does air get back into the chamber?

There is an inlet and an outlet, essentially the fan is drawing air from the chamber, air that is brought from the lab. I have included an image. http://postimage.org/image/51lemp431/
When the heater is used with full power it does not emit light because there's a thermostat in series to cut off power at 70C. Nonetheless you have answered my main question and gave me some keywords to look for in available literature, so thank you very much.

 

[soothsayer]

Yeah, regardless of radiation, my response should explain the situation. You're welcome!