Mechanics of airflow

[RedZep]

Always one of the main variables that is discussed for upcoming vapes, the airflow.
As I'm fairly new to the hobby, I had presumed until recently that the more open the airflow the better. However I have seen many instances of people preferring less open.

I'm curious as to the mechanics of airflow. Does a more open airflow require a stronger pull? Does a tighter airflow kill a bowl faster, and in less hits? If so what are the potential physics behind this?

When a vape is too open/tight what are the negative variables associated with that?

What is an example vape that has your perfect airflow and why?

I'm open to any and all discussion as I'm very keen to learn more about this.

 

[Farid]

In all vapes, the air that flows through can either cool the weed or it can heat the weed.
Colloquially on this forum - when the air has a net heating effect on the weed the device is classified as a "convection" vaporizer. When the air has a net cooling effect it is classified as a "conduction" vaporizer.

Even in instances where the air has a net cooling effect many vapes are able to "preheat" the incoming air, thus reducing the cooling effect. Examples of this include the air that goes through the grooves on a vapcap tip's grooves, or the air that travels around an Arizer's oven

Whenever you have the air being heated, the speed at which you draw will impact how much the air is heated. Draw very quickly, and the air may not have time to heat up.

There are other factors at play as well. The speed at which you inhale will cause the Reynolds number to increase and if it goes high enough, the airflow will become turbulent. When turbulence occurs, the properties of the airflow change. Turbulence can cause more contact between the walls of the heater and the air, increasing the heat tranfer coefficient. It will also change the velocity profile of the air, with a more even velocity, versus the parabolic velocity profile you have with laminar flow (which can result in a hot spot in the center of a load). See below:

Turbulence will also cause draw resistance, so that's something to consider - for that reason it's not necessarily something that is desired.

 

[Cheebsy]

Often new users struggle to control thier draw effectively and a restricted set up makes it easier to do a long slow draw and much harder to overwhelm the heater. I prefer a less restricted setup relying on my ability to control draw speed when needed.

 

[vapviking]

Not exactly on-topic, but related...
I'm no scientist, but I believe my acid reflux gets worse when I use more restricted vapes; that sucking on a tight straw action, no bueno for me!
Some vapes just need time to make the vapor, as a lot of it is made during the draw. Intentionally slowing the draw by restricting can compensate.

 

[Farid]

From a user perspsctive, one major benefit to a very restricted flow on many conduction vapes is that it allows you to do mouth to lung hits. This doesn't apply to convection vapes because the airflow is needed to produce the vapor.

From a technical perspective, what vapviking said is true for many conduction vapes - if you draw too quickly, you will cool the load and prevent it from producing vapor.

With convection vapes the speed at which you draw can have a variety of effects depending on how powerful the heater is. With relatively weak heaters, drawing too fast can overpower them, as the incoming cool air needs time to make contact with the heater. With very powerful heaters, they can keep up with harder hits. But many convection vapes (some logs for instance) also allow cold air to enter after the heater. This makes things challenging to evaluate since variation in draw speeds may result in a different ratio of hot air and cold air, resulting in a different temperature at the load. With my log, for example, I find it will combust if I draw too slowly or too quickly.