Critical Gap of air in heat shielding

[tmusliah]

I was told during a recent design review of mine, that there was a certain distance (thickness) of air which if not maintained as a minimum would make any metal or composite heat shield manufactured not work as effectively as it was designed for.
Does this "critical gap" (as it was refered to) exist?
Does someone out there know if there is a graph showing the relationship between this minimum gap and input heat?

I am trying to shield a turbo charger from the firewall of an operator cabin, in an articulated dump truck.

Tx in advance...

Trev

 

[IRstuff]

Not sure if this is relevant, but the building thermal analysis pack for Mathcad has this reference:
El-Sherbiny, et al. 1982. ASME Journal of Heat Transfer,Vol. 104, pp. 96-102. which supposedly shows that the convective heat transfer for a vertical cavity reaches a minimum at 13 mm. This is based on cranking Nusselt numbers as a function of gap.

TTFN

 

[MintJulep]

Intuitively it makes some sense. Certainly it would be a function of much more than input heat.

Figure your heat transfer paths:

From the surface of your hot thing to the heat shield by radiation and convection, less what is lost to everything that is not the heat shield.

Conduction through the shield.

From the heat shield to the rest of the world:

Radiation. Convection to the air. Convection out of the air to other stuff.

The other stuff depends on how much air flow, which in turn is a function of the gap, among other things. The air flow would in-turn affect those dimensionless numbers that in part dictate convection coefficients.

So it seems possible that there is an "optimum" gap, for any set of conditions. Not sure of a critical gap.

 

[tmusliah]

Thanks for the info guys...

IRstuff > Although the vertical cavity in the window is not really the same as the heat shield, it does share one characteristics, namely: Low air flow to remove hot air.

It does however see much higher temperatures. 100 deg celcius under bonnet ambient temperatures, and the turbo is around 450 deg celcius.

I wonder if increasing the gap to 20mm may improve the shielding characteristics of the air.

MintJulep > The airflow on the machine around the affected area is not all that good. The machine has a top speed of 52km/h, so the affects of ram air in cooling (or removing hot air) is little to none.

I would prefer to keep these area's open to the firewall and the bonnet (we will see much less failure to our fibreglass shields), however the legislation (and remote chance that someone decides to put his finger onto the turbo [The Stella Awards bunch]) prevent me from doing this.

But I am all ears (and eyes) for new ideas.

 

[IRstuff]

If the rest of the compartment is relatively cooler than the turbo, then the wider the gap the better, since that would allow more cooler air to flow in the gap, pulling heat from the turbo and exchanging to the entire compartment, rather than to just the firewall.

Additionally, one would suppose that reflective coatings on both the turbo and firewall should be implemented to minimize emission and absorption of radiated heat.

TTFN

 

[MintJulep]

IR,

While wider allows potentially more airflow, wouldn't it also tend to be at a lower velocity, lower Reynolds #, and thus lower convection coefficient?

All of course dependant on lots of other factors.

 

[IRstuff]

Sure, but the object is to keep the firewall cooler, so a low htc is good there, as is the lowered temperature of the immediate ambient air.

TTFN

 

[MintJulep]

Hum.

But at the same time the low htc reduces the convective component leaving the heat shield, hence its temperature will increase, and the radiant component will increase, resulting in a higher radient transfer to the firewall.

Interesting problem.

 

[IRstuff]

that's why you'd need the aluminum foil on the firewall side ;-)

TTFN