## Convective heat transfer coefficient vs. gas density (pressure)

## Convective heat transfer coefficient vs. gas density (pressure)

(OP)

Hello,

I am here to ask a simple, basic question about heat exchangers which is puzzling me.

Whenever I look for approximated values for convective heat transfer coefficients, I just find tables depending of phase of matter (liquid/gas), type of flow (laminar/turbulent) or type of gas (monatomic or not).

But I never find any parameter associated to the pressure or density of the gas.

My understanding is that air at 100 bars should get much higher convective heat transfer that air at 1 bar, as the number of molecules hitting the solid barrier is much higher.

Am I missing something? Are there any formulas related to convective heat transfer coefficient which take gas density or pressure as a parameter?

I am here to ask a simple, basic question about heat exchangers which is puzzling me.

Whenever I look for approximated values for convective heat transfer coefficients, I just find tables depending of phase of matter (liquid/gas), type of flow (laminar/turbulent) or type of gas (monatomic or not).

But I never find any parameter associated to the pressure or density of the gas.

My understanding is that air at 100 bars should get much higher convective heat transfer that air at 1 bar, as the number of molecules hitting the solid barrier is much higher.

Am I missing something? Are there any formulas related to convective heat transfer coefficient which take gas density or pressure as a parameter?

## RE: Convective heat transfer coefficient vs. gas density (pressure)

https://www.thermal-engineering.org/what-is-prandt...

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

## RE: Convective heat transfer coefficient vs. gas density (pressure)

You may study the correlations underneath function of Re and Pr numbers .

Good luck

Pierre

## RE: Convective heat transfer coefficient vs. gas density (pressure)

so, If I am understanding ok the Prandt number, Air heat transfer is dominated by thermal diffusivity (as air Prandt's number is 0.71).

And that situation does not change with pressure, as I have seen in the formulas and tables.

So I am checking thermal diffusivity of air at varying pressure, and I find the the values are dramatically lower when the pressure is higher.

Consequence would be (for a given area) heat transfer being lower the higher the gas density, then?

## RE: Convective heat transfer coefficient vs. gas density (pressure)

Helium is used for heat transfer, Krypton is used to insulate double pane windows. High pressure increases gas density an increase convective heat flow. Fiberglass-type insulation is used to reduce convection.

## RE: Convective heat transfer coefficient vs. gas density (pressure)

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

## RE: Convective heat transfer coefficient vs. gas density (pressure)

## RE: Convective heat transfer coefficient vs. gas density (pressure)

https://en.m.wikipedia.org/wiki/Grashof_number

These may also be of interest.

https://en.m.wikipedia.org/wiki/Rayleigh_number

https://en.m.wikipedia.org/wiki/Nusselt_number

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

## RE: Convective heat transfer coefficient vs. gas density (pressure)

ye, I am interested in heat transfer in heat exchangers (forced convection).

I have seen many examples of conductive heat transfer examples here and there, but they never mention pressure, which I find puzzling.

I am not interested in an specific pressure, just in the general correlations (if any) between conductive heat transfer and gas density (using forced convection, both laminar and turbulent).

I am looking at pierreicks's material, but there is a lot there to chew for me. I will keep trying.

## RE: Convective heat transfer coefficient vs. gas density (pressure)

Given a heat exchanger surface and constant mass transfer rate (ie, 1 m2, 1 kg/s of air),

does convective heat transfer coefficient change significantly with air density?

Can a simple rule of thumb be extracted, like "eight times the pressure, double the convective heat transfer coefficient"?

## RE: Convective heat transfer coefficient vs. gas density (pressure)

.

Convective heat transfer is dependent on velocity and temperature difference, but as velocity has dependencies on density and density of a gas is very dependent on pressure, convected heat transfer rate could be said to have at least some indirect dependencies on density, but only as far as providing the motive force required to reach the given velocity; not towards affecting the convective heat transfer rate.

Otherwise the conductive process includes all of the direct effects of pressure-density.

https://www.engineeringtoolbox.com/air-properties-...

If you have "high pressure convection", you really have conduction happening at the same time and need to include that process too.

A black swan to a turkey is a white swan to the butcher ... and to Boeing.

## RE: Convective heat transfer coefficient vs. gas density (pressure)