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I have a rectangular tent that I'm planning to use as a mushroom fruiting chamber. It measures approximately 120 x 60 x 150 cm and has two duct fans installed--an intake fan near the top of the left side and an exhaust fan near the bottom of the right side. (Both fans are the same size and type.) I placed a cool air humidifier at the bottom left of the tent and was expecting the water vapor to flow towards the exhaust fan. Instead, it moves in the opposite direction--towards the intake fan and seemingly against the direction of air flow. This happens even when the exhaust fan is on the highest power setting and the intake fan is on the lowest setting. Only when I turn the intake fan off entirely does the vapor from the humidifier start moving in the direction of the exhaust fan. Here is a rough illustration.

What could be the explanation for this? Does it have something to do with the fact that water vapor is less dense than dry air and therefore travels toward the intake fan where the pressure is higher? (Appologies for the crude diagram).

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The flow in such a system is probably not what you expect. I am speculating here, because I didn't do any calculations, but I have done research on flow patterns in jets entering thin enclosures. This seems similar to me.

There is not a flow going directly from inlet to exhaust. Because of your inlet fan, you have a jet of fluid going directly into your enclosure. The jet will drag some of the air inside with it. The air starts to recirculate and in the enclosure you will get a big cell of circulating air. The flowrate of the recirculating cell can be bigger than your inlet flowrate (depending on dissipation). (Note: the inlet flow velocity will always be bigger than your recirculating flow velocity). So there can be big draft towards your inlet. I expect that you can observe the vapor reaching the inlet, and then being blown away with the incoming yet.

I don't think this has to do with the density of the vapor. You can verify this by switching the inflow and outflow. I expect you to see something similar (but to a bit lesser degree because your recirculating flow pass the outlet first, and then the inlet).

There are things that you can change to your set-up, but it depends on the goal you want to reach. If you want to keep the humidified air inside, this is not the worst set-up.

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