flyrotary@lancaironline.net Mailing List Archive

Hi Mike,

I agree, You certainly don't have to keep the cooling system stock - after all this IS experimentation. My point was that I did not believe that NPG was suited to a 13B with a stock cooling system. You can certainly disagree with that statement - but, I (and perhaps others) would like to know why.

To better deal with the properties of NPG, I pointed out that you would likely need to make some modifications such as increasing the flow rate (pump speed) of the NPG as well as providing more power (clog/serpentine belts) to the water pump to address the added resistance due to its much higher viscosity that the 50/50 mixture. Also as others have pointed out, you would probably not want to use the GM cores (that many of us use) due to their smaller passages.

So could the 13B coolant system be modified to better deal with some of the NPG properties, no doubt in my mind that it is certainly possible to do so. However, I am not convinced that there is sufficient benefit derived from the use of NPG that modification over the stock system designed for a 50/50 mixture would be justified (in my mind at least).

I am eager for some one do this experiment (other than me {:>)) - however, based on what happened to Dave's engine after he replace the 50/50 mixture with NPG - I would suggest proceeding cautiously. Not saying NPG was the cause - we simply don't know at this time (and may never), but that was the only change he made before having his severe engine problems.

Also, one might care that should a 100% pure glycol (ethylene or propylene) liquid land on a hot exhaust - the probability of ignition and fire is fairly high and something that should be considered. Having had a brake fire while on the ground, I do not care to have a coolant fire in the air.

However , I will retract my earlier statement that we ALL agree that NPG is not suited for a 13B with a stock cooling system. It was mostly made in jest in the first place.

Best Regards

----- Original Message -----

From: Michael LaFleur

To: Rotary motors in aircraft

Sent: Wednesday, February 01, 2006 10:36 PM

Subject: [FlyRotary] Re: sutability of NPG for rotary engine use

Well, I'm not ready to agree with that conclusion yet. And who say's we need to keep the cooling system stock?

Mike

Ed Anderson <eanderson@carolina.rr.com> wrote:

Thanks for your assessment, Monty.

Scares me a bit that we all (more or less) agree that the NPG is probably not well suited to the stock rotary coolant system {:>)

Ed

----- Original Message -----

From: M Roberts

To: Rotary motors in aircraft

Sent: Wednesday, February 01, 2006 8:35 PM

Subject: [FlyRotary] sutability of NPG for rotary engine use

I wouldn't worry about the theory behind mdot*Cp*deltaT too much. This has been tested thoroughly. As long as your Cp number is correct the calculation will be nuts on. The place you get into trouble is measuring the constants for Cp or for a heat transfer coefficient. Your analysis looks good to me Ed. The point Ernest made is a valid one. Cp is per unit mass. A more dense fluid will transfer more heat per volume flow than a less dense fluid. Bill S. also makes some good points.

The main thing I have to add is: it would be nice to know what the convection coefficient is for NPG. That is what gives the heat transfer between the hot metal and the coolant. A more viscous fluid would tend to have a thicker boundary layer and less turbulence. That could cause problems. The turbulence and mixing of the boundary layer help to transfer heat.

I also would be cautious about the vapor pressure. Boiling is not a bad thing. It is a good thing. The heat transfer coefficient for a phase change (liquid to gas) is infinite. This helps to cool a hot spot. We are talking about sub cooled boiling here where the bulk liquid is cooler than the boiling point. Locally the liquid boils and transfers all the heat the metal can move. The limiting factor is actually the metal conduction for this case. The bubbles of vapor are cooled by the surrounding coolant and collapse. Put a pot on the stove and watch as you transfer from sub-cooled to nucleate and finally bulk boiling. You can see the process happen.

Both bulk and nucleate boiling are to be avoided. Sub cooled boiling, a thin boundary layer and turbulence are all good things. NPG strikes out on all three. In addition it requires more power to pump and the pressure drop through the evaporator core type coolers at low temps is suspect.

In short:

The 13b was developed to use water/glycol as a coolant.

To properly validate NPG you need a dyno and a lot of thermocouples, plus a way to measure the mass flow of the coolant, pressure drops, pump power, and the heat transfer coefficient.

Anybody got that laying around in their hangar?

Do you want to be a guinea pig?

I would not use NPG.

Monty