Radiator efficiency

[spicmart]

The radiator of the Spitfire Mk V was only 55 % of the size of te Me 109G's yet was only 4 % less efficient.
It could sustain higher pressure cooling than the German radiators hence the smaller size.
It used copper, which was about 3 times heavier, instead of aluminium.


View: https://www.youtube.com/watch?v=ImEpk1s-Vk0 (duration about 4 minutes into the link)

View: https://twitter.com/i/web/status/1407448574401581057

I asked about this issue on flugzeugforum.de and if the Germans ever managed to catch up but would not get a definite response.

One wonders how much faster German fighters would have gone when equipped with smaller (less draggy?) radiators.

 

[pbehn]

The calculations and science involved in this make my head hurt, eyes pop out and ears bleed. If it was 55% of the size in some way it must have been more efficient, so how do you define efficiency. Weight is one thing normally science uses mass and density. The density of copper may be 3 times that of aluminium, but copper is a much better conductor of heat than aluminium. If you can make a higher pressure system it will have less water and be smaller. The water you put in a cooling system has far more mass or weight than the heat exchange matrix in the radiator.

 

[spicmart]

The calculations and science involved in this make my head hurt, eyes pop out and ears bleed. If it was 55% of the size in some way it must have been more efficient, so how do you define efficiency. Weight is one thing normally science uses mass and density. The density of copper may be 3 times that of aluminium, but copper is a much better conductor of heat than aluminium. If you can make a higher pressure system it will have less water and be smaller. The water you put in a cooling system has far more mass or weight than the heat exchange matrix in the radiator.

Sorry, if it makes you uncomfortable. I was just presenting.

 

[swampyankee]

Pedantically, "efficiency" doesn't apply to heat exchangers, of which radiators -- which don't work by radiation -- are an example. as heat exchangers do no work.

One possibility is that the Merlin's coolant temperature was higher, which means that there is a greater temperature difference between the coolant and the air, so there can be greater heat transfer per unit area. A second is that the Spitfire's coolant system's designers accepted a greater pressure loss in the radiator[1], giving more heat transfer surface. For must gas-liquid heat exchangers, the material that the tubes are made of doesn't really matter; the heat transfer rate is determined by the boundary layer between the gas flow and the fluid-carrying tubes.

I'm tempted to editorialize here, but I shan't: designing cooling systems for high-powered aircraft, piston-engined aircraft is hard. A lot of people got systems that worked by were far from optimal.


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[1] Alas, we're stuck with the term. Radiators use convection, not radiation. "Liquid-air heat exchangers" is a bit clumsy.

 

[Shortround6]

Basically they had about 20 years to go design water cooled engines and their radiators from 1910 to 1930.
Goals were keeping the engine from overheating and not leaking too badly. Occasional attempts at streamlining. And streamlining could be very hap-hazard. Beautifully faired engine cowl with a large radiator with no cowling/duct hanging from the upper wing precisely positioned to shower the exposed pilot with steam/hot water from a leak.
Around 1930 they discovered Prestone (ethylene glycol) which not only kept the water from freezing. It allowed for higher temperatures without boiling. The higher temperature difference allowed for a smaller radiator for the same heat transfer. However pure ethylene glycol has about 1/2 the heat capacity of water at the same temperatures. Does mean you don't have to drain the water out over night in cold weather Pure ethylene glycol has a boiling point of 197.3 degrees C. Which allows for quite a bit of latitude in cooling of engines as local hot spots will not flash to steam with all the problems that entails. The US jumped on the Prestone band wagon with thoughts of running cooling systems at 300 Degrees F. for really small cooling systems. Unfortunately it turned out that at around 250 degrees F a lot of heat went into the oil and you needed a larger oil system and oil cooler. By around 1939-40 engine companies had lost some of their faith in Prestone (and steam cooling) and were trying to use pressurized water to get higher temperatures and/or using water/Prestone blends.
Some pressurized water systems used up to 50% Prestone for cold weather protection. Some used 30%? Some Prestone gave some protection from "hot spots" in certain parts of an engine. Some companies designed their engines to tolerate a bit of "steam" which got separated out in the header tank.

Now this back and forth from pure water to Prestone and back to mostly water all happened in around 10 years. Which also caused a lot of changes in radiator design (size needed for a certain amount of cooling) and air flows to maximize heat transfers at different temperatures of the liquid coolant and/or expected air temperatures and densities. Air was certainly colder at 20,000ft but the air was 1/2 as dense at 20,000ft so you needed more air flow. And air that was passing through the radiator at 400mph instead of 200mph didn't pick up quite as much heat from the same thickness radiator.

Ethylene glycol was also a major component in the manufacture of some explosives (like Dynamite) and there may have been worries about supply in case of war?

 

[Shortround6]

I will also note that radiators themselves were undergoing some radical changes.

Hurricane radiator. There are no fins like on a car. There is a "tank" with hundreds of hexagonal tubes running from front to back.
Oil coolers were made the same way.

So there was a lot of variation on the amount of area exposed to air vs the amount air exposed to the liquid coolant from radiator to radiator which was going to affect the "efficiency". Not all car makers could make aircraft radiators.

 

[don4331]

The couple that you missed S Shortround6 are:

Pure ethylene glycol burns. (you sort of hint that it might with it being a feedstock for dynamite).​

Ethylene glycol is much more 'slippery' than water. So, seals that would hold in water, didn't keep mixtures in.​

​

And having ethylene glycol weeping out of seals, and across red hot exhaust manifolds was a bad combination.​

​

To the Ta154 picture: Is it a material issue e.g. too weak/too brittle? Is it a manufacturing issue e.g. inconsistent thickness/bad bend/welding (soldering) heat stress? Is it installation issue: e.g. too much vibration?

 

[Simon Thomas]

The tubes are not hexagonal all the way through the radiator. There obviously needs to be room for coolant to pass around.

https://www.touraine-radiateurs.com/medias/FAISCEAUX/tubes-hexagonaux-faisceau-radiateur-04.jpg

 

[Shortround6]

Thank you.

 

[z42]

These kinds of "honeycomb" style radiators were quite common in old cars, but were subsequently replaced by mostly "tube and fin" style radiators which provided improved heat transfer.

Not sure if there's an analogue here with the transition from fire tube boilers to water tube boilers?

 

[z42]

As for the material of the radiator core, I'm not sure that makes much difference for the heat transfer. Copper (and brass AFAIU) used to be a common radiator material, but has largely been replaced by aluminum due to it being lighter and cheaper. Aluminum however requires additional anti-corrosion additives in the coolant.

I think the big advantage of copper is that it's easier to join the parts together with brazing or soldering.

 

[Shortround6]

Not sure if there's an analogue here with the transition from fire tube boilers to water tube boilers?

I don't think so.
Fire tube boilers had hot gases flowing though the tubes in the 'water tank/boiler'.
Water tube boilers had a collection of water/steam filled tubes suspended in the 'fire' or pathways the hot gases took from the firebox/burners.

There are some advantages to each type. The number of railroad locomotives that used water tube boilers can be counted on one hand.

 

[MIflyer]

Pedantically, "efficiency" doesn't apply to heat exchangers, of which radiators -- which don't work by radiation -- are an example. as heat exchangers do no work.

Heat exchanger efficiency is very well defined in my branch of mechanical engineering. I recall running tests on F-4 and F-106 environmental control system heat exchangers well into the nighttime hours to determine if the repair procedures employed decreased their capabilities..

Fin-plate heat exchangers are known to be significantly more efficient than tube-shell heat exchangers. That increased efficiency means that you need a smaller heat exchanger for the job at hand. Shell-tube heat exhangers are easier to construct and to repair but fin-plates (like a car radiator) are more efficient. I recall one company telling us that they could use the techniques employed to repair F-106 shell-tube heat exchangers to replace the fin-plate types used on other fighters and that they had done so for Boeing 727 units. But airliners have a lot more margin than jet fighters and we told them No.

The greatest heat exhanger triumph of WW2 was the Aftercooler for the two stage Merlins. Without that compact lightweight unit the Spitfire would have been virtually out of business by 1943 and the P-51 never developed into a long range escort fighter. An air to air aftercooler would never have worked. The fact that Stanley Hooker recognized this and did not instead pursue his own expertise at cajoling air to behave better shows just how brilliant he was.

 

[don4331]

Do we have any efficiency numbers for the Merlin aftercooler? Both thermal and air resistance. (Any pictures of the matrix inside the aftercooler).

Equally, do we have numbers for the efficiency of the "radiators" for the big 3 - Spitfire, Mustang and Mosquito?

Having an" aftercooler" after 2nd stage where air is hottest - has advantages over "intercoolers" say between turbo and engine compressor...but was it greatest triumph for both efficiency and packaging? (I'm sort thinking an Allison with remote 1st stage to an air to air exchanger where the 'radiator' was on P-51 - ala XP-72 could have done the same thing).​

My work with aircraft is limited; so I'm sticking to what I know: 70% efficient air to water aftercooler and 70% efficient water to air radiator on my truck gives a system efficiency of only 49% and then there is power loss to drive the cooling fluid. While an air to air intercooler of even 60% does a better job.

Aside: Should we be including oil cooling in our radiator efficiency discussions - as noted if you let water/glycol get too warm minimizing the 'radiator' size, you increase the size of the oil cooler.

And oil coolers come with their own set of issues - mainly what I know as 'coring' where the oil in the outer tubes gets cooled so much that its viscosity increases and flow reduces - which quick becomes a feedback loop. The result is the only oil flowing through the oil cooler is going very fast through limited tubes and not efficiently cooling.​

 

[swampyankee]

Heat exchanger efficiency is very well defined in my branch of mechanical engineering. I recall running tests on F-4 and F-106 environmental control system heat exchangers well into the nighttime hours to determine if the repair procedures employed decreased their capabilities..

Fin-plate heat exchangers are known to be significantly more efficient than tube-shell heat exchangers. That increased efficiency means that you need a smaller heat exchanger for the job at hand. Shell-tube heat exhangers are easier to construct and to repair but fin-plates (like a car radiator) are more efficient. I recall one company telling us that they could use the techniques employed to repair F-106 shell-tube heat exchangers to replace the fin-plate types used on other fighters and that they had done so for Boeing 727 units. But airliners have a lot more margin than jet fighters and we told them No.

The greatest heat exhanger triumph of WW2 was the Aftercooler for the two stage Merlins. Without that compact lightweight unit the Spitfire would have been virtually out of business by 1943 and the P-51 never developed into a long range escort fighter. An air to air aftercooler would never have worked. The fact that Stanley Hooker recognized this and did not instead pursue his own expertise at cajoling air to behave better shows just how brilliant he was.

View attachment 787828

Pedantically, "efficiency" is energy in divided by work out. Heat exchangers don't do work, so, pedantically, "efficiency" is not correct. This is not a hill I'm going to die on, although my engineering thermo and HX instructors would yell at me for using "efficiency."

Of course, this may be one of those engineering terms that differs regionally and by school, like which direction of work is positive or negative.

 

[Simon Thomas]

You are correct. For heat exchangers, effectiveness is used to measure performance.

From "Fundamentals of Heat Exchanger Design", Shah et al

subscripts: h = hot fluid, c = cold fluid, i = inlet, o = outlet.

For intercoolers, they generally talk about efficiency as a measure of how much temperature was decreased by the intercooler compared to the temperature rise caused by compression. For the Merlin, the efficiency was about 40% (Lovesey's Merlin paper). The inlet temp rise at FS at max revs was 205°C (Lovesey's Merlin paper) and temp at the intercooler exit was 100°C (Hooker's autobiography).

 

[don4331]

You are correct. For heat exchangers, effectiveness is used to measure performance.

From "Fundamentals of Heat Exchanger Design", Shah et al
View attachment 787853
subscripts: h = hot fluid, c = cold fluid, i = inlet, o = outlet.

For intercoolers, they generally talk about efficiency as a measure of how much temperature was decreased by the intercooler compared to the temperature rise caused by compression. For the Merlin, the efficiency was about 40% (Lovesey's Merlin paper). The inlet temp rise at FS at max revs was 205°C (Lovesey's Merlin paper) and temp at the intercooler exit was 100°C (Hooker's autobiography).

Those numbers don't sound right...

(205-100)/.4 = 262.5, as output temperature can't be more than 205C, that says input had to be -57.5C And there is no way the "radiator" could be cooling the glycol water to that temperature.

 

[Daggerr]

Those numbers don't sound right...

(205-100)/.4 = 262.5, as output temperature can't be more than 205C, that says input had to be -57.5C And there is no way the "radiator" could be cooling the glycol water to that temperature.

The ".... 40 per cent intercooling ...." that Lovesey mentions in his Merlin paper is not an exact number, but merely a rough indication. 35 to 40 % was their objective when they designed the intercooler.

Note also that Lovesey does not call it efficiency of effectiveness.
The efficiency of any heat exchanger is normally defined by the formula that Simon Thomas posted.

The intercooler does not know and does not care what the temperature rise over the supercharger is.
Its performance is only determined by the inlet temperatures of the hot and cold fluids, its heat exchange area A, and its overall heat transfer coefficient U (which depends on fluid velocities and physical properties). When these are all known one can easily calculate the outlet temperatures of the hot and cold fluids.

According to p169 of "The Merlin in Perspective" (RRHT HS2) the Merlin 61 has a full throttle altitude of 23500 ft while delivering +15 psi boost.
So that means that the supercharger must be delivering a pressure ratio of 5.0
At -32 oC ambient and an adiabatic efficiency of about 69 % that would result in a supercharger outlet temperature of 173 oC, which corresponds with a temperature rise of 205 oC.
At an intercooler outlet temperature of 100 oC (according to Hooker) the "percent intercooling" (as defined by Lovesey) is then (173 - 100)/205 = 36 %.

I used an assumed supercharger efficiency of about 69 % to get the 205 oC rise quoted by Lovesey. Probably a few percent too high but the above calculation is just to check whether the numbers quoted by Lovesey and Hooker are plausible.

If the Merlin 61 flies at a different altitude than 23500 ft the ambient temperature will be different, and so will the supercharger outlet temperature, as well as the intercooler outlet temperature, and the newly calculated "percent intercooling" will likely also differ from the 36 % above.

 

[z42]

For some pretty impressive radiator tech check the precooler on the SABRE air breathing rocket engine concept. Cooling the intake air from 1000C at Mach 5 to - 100C before it enters the engine compressor. Using helium gas as the coolant, with the heat ultimately dumped into the liquid hydrogen fuel via another heat exchanger.

IAC-20-C4-7-1 the Synergetic Air-Breathing Rocket Engine

71st International Astronautical Congress (IAC) – The CyberSpace Edition, 12-14 October 2020. Copyright ©2020 by the International Astronautical Federation

docslib.org

 

[z42]

I don't think so.
Fire tube boilers had hot gases flowing though the tubes in the 'water tank/boiler'.

Exactly. So somewhat analogous to a honeycomb radiator where you have a tank of liquid, and then a series of tubes going through the tank where gas (in this case, ambient air) flows and exchanges heat with the liquid in the tank.

Water tube boilers had a collection of water/steam filled tubes suspended in the 'fire' or pathways the hot gases took from the firebox/burners.

Yes. So somewhat analogous to a finned tube style radiator where liquid flows through a set of tubes exchanging heat with the gas passing through the array of tubes.

What I was going after was that while there are superficial similarities, the analogy breaks down in the sense there were different factors behind the transitions.

There are some advantages to each type. The number of railroad locomotives that used water tube boilers can be counted on one hand.

Sure, and similarly fire tube boilers were used in low performance applications (say a typical tramp steamer) long after they had been replaced by water tube boilers in more demanding applications (like a warship or fast passenger liner).