What is direction of the convection during ventilation?

Answer 1

Convection is the movement of air (or another fluid) in respond to differetial densities, differential pressures, and similar "setups" that might drive mass movement. Convection can be forced or natural, and this makes answering this question much more difficult. That said, consider the instance of natural convection during ventilation.

In a ventilation lineup powered by natural convection, we'd see hoter, less dense air rising, and cooler, more dense air slipping in underneath it. That's the general way a convection cycle begins. The location of a heat source will help determine what's happening. If an oven is on and hot, it will heat air in its vicinity. This hot air will rise and be displaced by cooler air. The heated air will cool in the area higher up and sink, and the cooler air that moved in near the oven will be heated and rise. A "circle" or cycle of convection will have been set up.

There are countless other convection scenarios that can be scripted for a given ventilation situation. But with natural convection, there will be rising air (which is less dense) near the heat source, and cooler (more dense) air will slide in to replace it. The cooler air will have to come from somewhere, and as the hot air will eventually cool, it will sink to "complete the circle" of convection. With something like a simple diagram and a bit of thought based on what has been set down here, an investigator could summarize a convection cycle for ventilation using natural convection.