Hydrogen as a rotor cooler in generators

[JohnCU]

We use hydrogen in our main generators to cool the rotor... because it has a high thermal conductivity (which I understand) but it also has a low density...which helps with what? I read about it helping with windage losses, but I can't quite connect the two, since I'm not familiar with the actual design of generators, just the schematics.

 

[Mark R]

Lower density -> lower viscosity -> less friction

 

[JohnCU]

Originally posted by: Mark R
Lower density -> lower viscosity -> less friction

ahh simple enough! thanks.

 

[CycloWizard]

Originally posted by: Mark R
Lower density -> lower viscosity -> less friction

Not necessarily true... Oils are generally less dense than water and generally much more viscous.

Without seeing the schematic, I will throw this out there. Density is directly important in transient heat transfer operations since it is a variable in thermal diffusivity. The simplest way to look at this is the transient 1-D diffusion equation, dT/dt=-a*d^2T/dx^2, where T is temperature, x is the spatial coordinate, t is time, and a is the thermal diffusivity. The thermal diffusivity is equal to (IIRC) the thermal conductivity divided by the product of density and heat capacity. Thus, the lower the density, the higher the thermal diffusivity. Since thermal diffusivity is the proportionality constant between the temporal thermal gradient and the spatial thermal gradient, a lower density increases the rate of heat transfer (that is, dT/dt is generally larger for larger a, and a is generally larger when density is smaller).

 

[JohnCU]

Originally posted by: CycloWizard

Originally posted by: Mark R
Lower density -> lower viscosity -> less friction

Not necessarily true... Oils are generally less dense than water and generally much more viscous.

Without seeing the schematic, I will throw this out there. Density is directly important in transient heat transfer operations since it is a variable in thermal diffusivity. The simplest way to look at this is the transient 1-D diffusion equation, dT/dt=a*dT/dx, where T is temperature, x is the spatial coordinate, t is time, and a is the thermal diffusivity. The thermal diffusivity is equal to (IIRC) the thermal conductivity divided by the product of density and heat capacity. Thus, the lower the density, the higher the thermal diffusivity. Since thermal diffusivity is the proportionality constant between the temporal thermal gradient and the spatial thermal gradient, a lower density increases the rate of heat transfer (that is, dT/dt is generally larger for larger a, and a is generally larger when density is smaller).

yeah, you are correct. i did a whole damn project on the heat equation and still didn't remember this, oh well.