Water spray cooling an aircraft radiator?

Evaporating a liquid without delta heat is against the laws of thermodynamics.

The process is exactly the same, be it “boiling” or “evaporation”.

Simply injecting air droplets into the flow will not remove the energy required to evaporate the water from the engine – not anywhere near it.

Assuming for the moment that the water remains liquid after being injected into the air stream, here is what will happen:

1. some of the water droplets will evaporate into the air stream before it enters the radiator. This will slightly lower the cooling air temperature, but not by much because the air stream is quite cold already and very quickly saturated.
2. some of the water droplets will hit the radiator, stick to the surface, evaporate there and remove energy from the radiator
3. some of the water droplets will hit the radiator, flow back on the surface to be then again being blown off by the air stream, being a little bit warmer at the end
4. some of the water droplets will simply be carried through the radiator and evaporate further downstream, cooling the airflow but too late
5. some of the water droplets will make it until they leave the aircraft as droplets

(2) is the process which is likely to cool the radiator the most, but it will be nowhere near the energy required to evaporate all the water injected into the air flow that way.

T28 wrote:

Spraying water on the rad isn’t such an outlandish idea – P51 racers like Strega do it.

Exactly! But those work with supercharger pressure ratios of 5:1 resulting in compressor discharge = intercooler induction air= 200C , when a properly designed spray system will definitely boil water. I doubt you can easily do so when the radiator is only at 100C.

BTW, those inject water/methanol inside the induction too

Peter wrote:

Evaporating water is well worth doing:

The specific heat of water is 4.2 kJ / kg / K.
The latent heat of vapourisation of water is 2260 kJ/kg.

That was my point: if you cannot make it boil, the effect is minimal. Thx for looking up the numbers

Get a high pressure spray bar and start testing in a mockup of the cowl while running hot coolant through the rad?

If anything, point the bar at the “second class citizen” cooling fins.

Antonio wrote:

But those work with supercharger pressure ratios of 5:1 resulting in compressor discharge = intercooler induction air= 200C , when a properly designed spray system will definitely boil water.

And they only operate at low altitudes with respective temperatures so that icing downstream is not an issue

The Extra radiator scoop is way below wing or stab level. I see how icing in general may be a concern but in this particular case it is sprayed nowhere near lifting surfaces.

Ultranomad wrote:

A classical solution used on aircraft around the WWII era was to inject water into the engine intake. This would provide a much more efficient cooling. It is still used in some turbine engines for hot-and-high operation.

It also works on our PT6. When we do a compressor wash we let most of the water drain and then the start a few minutes later is as cool as it gets.

T28 wrote:

The process is exactly the same, be it “boiling” or “evaporation”.

No, it is definitely not exactly the same. Evaporation happens exclusively on the surface. Water molecules leaving the water, transitioning into gaseous state in the air. This process cools down the liquid because the fastest molecules leaves. It happens at all temperatures, and whenever there is a liquid-gas interface. Boiling is a forced transition from liquid state to gaseous state. It cannot happen unless the liquid reaches the boiling point temperature at the given pressure, and the surface has nothing to do with it. Boiling requires an external heat source, evaporation requires a gas liquid interface.

The water molecules themselves don’t care if they transition to gas form due to an external heat source or because they pop out of the surface. For them it is the same