# Is vortex tube reversible?

If you inject cold and hot air in outlets of a vortex tube, do you obtain a greater pressure at inlet?

• This is a good question, but not really about engineering, so you might get better answers on the physics stackexchange. – fibonatic Jun 1 '15 at 14:04
• @fibonatic Do not advise users to cross-post to other sites. If the question is off-topic, flag it as off-topic -> belongs on another site. If the question is on-topic, but might be better answered somewhere else, you can advise the user to flag their post for migration. – Air Jun 1 '15 at 15:26
• @Air flagging for belonging to another stackexchange only allows for choosing meta-engineering, at least on the android app. – fibonatic Jun 1 '15 at 16:37
• @fibonatic We can look into adding more options, but if the site you want doesn't show up in the list, just use a custom flag. We have plenty of capacity to handle those given the present levels of traffic. – Air Jun 1 '15 at 16:39
• @fibonatic I disagree; while it may be on-topic at physics, this is definitely an engineering question. – Chris Mueller Jun 1 '15 at 20:23

No; air will not flow from low pressure to high pressure, at least not in a vortex tube. The momentum equation for an inviscid fluid flowing at steady-state in one dimension is $$\frac{\partial \left( \rho u^2 \right)}{\partial x} = -\nabla p$$ where $\rho$ is the fluid density, $u$ is velocity, and $\nabla p$ is the pressure gradient. The negative sign in front of the pressure gradient signifies that, in the absence of any other forces, the flow direction will always be opposite that of the pressure gradient.