Lowest Kinematic Viscosity Fluid - High Pressure Gas

[Timtam]

I am trying to determine which fluid to use in an experiment.
An important aspect of the experiment is the ratio of inertial forces (due to mass) vs the viscous forces that the fluid will experience

Amongst common liquids the lowest I can see is Acetone with a kinematic viscosity of 0.41 cSt
(Abs Viscosity0.3mpa.s, Temp 20'c, Density 0.791 g cm−3, Pressure 1 atm)

Amongst gases absolute viscosities are a magnitude less (eg Air 0.005 cSt) but as the densities are also way less. (Again eg: Air 0.00035kg/m3) We actually end up with a higher Kinematic viscosity - the ratio of inertial to viscous is worse.

What interests me is the statement that gas viscosity is independent of pressure ( by the reason that as pressure increases, density increases and the mean free path inversely falls - from good old Wiki

"Maxwell's calculations show that the viscosity coefficient is proportional to the density, the mean free path, and the mean velocity of the atoms. On the other hand, the mean free path is inversely proportional to the density. So an increase in density due to an increase in pressure doesn't result in any change in viscosity"

My question is if I want to improve the inertial forces the fluid flow carries (by way of its mass) without increasing viscosity -Should I use a high pressure Gas (increase its density~mass) as I would only see a weak affect on its Dynamic Viscosity

Higher density for same Dynamic Viscosity would equate to a lower Kinematic Viscosity ?

My fluid carries higher inertia for same viscosity, Is this correct ?

 

[eljose79]

I would first like to commend you for considering the ratio of inertial to viscous forces in your experiment. This is an important factor to consider when choosing a fluid for an experiment.

Based on the information you have provided, it does seem that using a high pressure gas would be a good option for your experiment. As you mentioned, increasing the pressure of a gas would increase its density, which would in turn increase its mass and therefore its inertial forces. This would result in a higher ratio of inertial to viscous forces, which aligns with your goal of improving the inertial forces in your fluid flow.

Additionally, as you pointed out, the viscosity of a gas is independent of pressure, so increasing the pressure would not significantly affect its dynamic viscosity. This means that you would not have to worry about the gas becoming too viscous and affecting the flow in your experiment.

In terms of the kinematic viscosity, it is correct that a higher density for the same dynamic viscosity would result in a lower kinematic viscosity. This is because kinematic viscosity is calculated by dividing the dynamic viscosity by the density.

In summary, it seems that using a high pressure gas would be a good option for your experiment if you want to improve the inertial forces without significantly increasing the viscosity. However, as with any experiment, it is important to carefully consider all factors and conduct thorough testing to determine the best fluid for your specific experiment.