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Microturbine Exhaust Cooling

by Jonsp
Tags: cooling, exhaust, microturbine
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Apr20-12, 10:57 AM
P: 2
Hi guys, not sure if this is in the correct section but I'll go ahead anyway.

I'm working on an automotive design project. The car that I'm designing has twin micro jet turbines, like the Jaguar CX-75, to charge and power the electric motors propelling the car.

The problem that I'm trying to solve is that the heat exhausted from the turbines is incredibly hot and obviously isn't safe to shoot around in pedestrian areas. Apparently it can melt bumpers of vehicles behind and the tarmac below.

I'm wondering whether there would be a way to rapidly cool the air being exhausted to a heat which isn't capable of melting people. I don't know if it's even possible to cool air that quickly, but I'm stuck for ideas. Any help would be appreciated.

Thanks in advance.
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Apr20-12, 12:36 PM
P: 490
Mix it with a huge volume of ambient air.
Apr22-12, 06:51 AM
P: 2
How would I do that in the back of a car?

Apr22-12, 08:03 AM
P: 490
Microturbine Exhaust Cooling

I once had to do a design for a customer who wanted a large air-cooled turbocharged diesel mounted in the bilge of a wood boat, with a dry stack and no seawater cooling. We had to keep the temperatures in the engine room much lower than I at first thought possible. I found very excellent ideas on the Deutz website. I’ve since seen the same concepts applied to jet engine design. I would imagine that could be adapted for your design.

My time for writing right now is very limited. But if you like, we can talk more about this later. But I thought that this little tidbit might give your something to contemplate until then.
Apr22-12, 10:34 AM
Topher925's Avatar
P: 1,672
Depending on the material of the body, you may want to consider sinking the heat to the body of the car as it will have a very large surface area.
Apr22-12, 01:53 PM
P: 490
I rather like the idea of using the car body as a heat sink and a heat exchanger. My boat design did just about everything except that. If I had used a heat exchanger in the keel, then I guess that would have been similar.

This was an air cooled diesel engine with no water based coolant at all. It had fins on the cylinder jugs, much like an old VW bug engine. It also had a high capacity oil cooler for cooling the lower end. It had a shroud around the top end that served as an air duct to force the cooling air to where it would be most effective. The oil cooler was located inside that shroud. Cooling air was blown through the shroud by a powerful belt-driven ducted fan that looked sort of like the front end of a turbofan jet engine. All this was mounted inside of a small closed room.

I therefore had three different gas flows to deal with. From hottest to coolest, they were: the exhaust gas, the engine cooling air, and the air cooling the air space around the engine. Air to cool the engine room was drawn in through vents from the ambient sea air, without supplying another fan to drive it. The exhaust gas and the engine cooling air were forced flows, but the engine room cooling air was drawn in my natural convection.

For most of the duty cycle, the engine only supplied 50 hp or less; but it was rated at 120 hp for continuous operation with a 50,000 hour life. It could also supply as much as 400 hp for short durations, which was occasionally required by this duty cycle. The length of these high power time periods was limited only by my ability to get rid of the surplus heat, which the following design did very well.

Those three different gas flows went up a vertical stack in three different concentric pipes. The exhaust went through the center. The engine cooling air went through the first annular space, and the engine room cooling air went through the second annular space. The first two were forced at high speed, but the third was drawn in by natural convection. The momentum of the first two was sufficient to induce a momentum into the third because the first two were several feet shorter than the third. During those last several feet, all three gas flows mixed together, coming out at a reasonably low temperature not to cause problems above. That induced momentum was sufficient to draw fresh air into the engine room through the vents mentioned above. The customer retrofitted another device that provided supplemental cooling during his high power runs. It sprayed a water mist into the engine cooling air intake, which absorbed much heat. Similar water cooling was designed into some of the earlier jet engines, and has been used on some automotive engines as an aftermarket device.
Apr22-12, 03:32 PM
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Danger's Avatar
P: 8,964
Well, you could always tow around a weenie-wagon and use your exhaust to cook the hotdogs for sale.
Pkruse, I love the approach that you took with the boat, but could it be miniaturized enough to fit in a car?
My first thought was to employ old-style suppressor technology, adapted to this purpose. Either a folded-path or conch-shell design, wrapped in baffles and absorbent materials such as brass or stainless steel mesh while still offering minimal back-pressure could be effective. Wrap-around tubes or even a surrounding jacket for liquid cooling would add to the efficiency.
That's all that I've got for now, but I can think on it some more.
Apr23-12, 04:40 AM
P: 478
Rover produced a prototype gas turbine car in the 1950s (60s) it might be worth looking about to see how they got around the problem.
Apr23-12, 08:05 AM
P: 490
In the gas turbine industry, a very hot exhaust is the first indicator that we have done a poor job of extracting the available energy, but somethimes cannot be avoided. Keep in mind that a simple diffuser will drop the temperature a great deal. As for fitting my boat ideas into a car, I can think of several ways of doing that. The same ideas have been put into aircraft engines.
Apr23-12, 09:34 AM
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Quote Quote by Pkruse View Post
a very hot exhaust is the first indicator that we have done a poor job of extracting the available energy
Agreed. I assume, however, that such is an inherent problem with the engines involved. Hmmm... come to think of it, perhaps OP can find more efficient units to employ.
Apr23-12, 02:59 PM
P: 490
That would normally mean increasing the number of turbine stages.
Apr23-12, 05:38 PM
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Quote Quote by Jobrag View Post
Rover produced a prototype gas turbine car in the 1950s (60s)
There were some studies of gas turbine powered railway engines done in the UK in the 1960s or early 70s as well.

IIRC they came to nothing for a similar reason - what to do with the engine exhaust when travelling through tunnels.
Apr24-12, 05:09 AM
P: 478
You could point the exhausts up.

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