Model Turbines

[Turbine GuyParticipant @turbineguy]

I decided to try to insulate Tangential Turbine 5B by adding a 1/16” thick layer of Nylon over most of the outside surfaces of the housing and cover as shown in the following drawing. I call the new assembly with the added insulation Tangential Turbine 5D.
Since the insulators did not have to be very precise or need much strength I asked a friend to print them from Nylon on a home printer. The following photos show the printed insulators installed on Tangential Turbine 5B. As you can see from the photos, the prints came out very good on the first attempt. I thought the insulation would raise the performance significantly by reducing the heat lost to the cold air being blown over the turbine by the propeller. I have not got any improvement in efficiency running on steam regardless of the pressure tried after several tests even though the performance running on air after the test is always as good as before each test. I don’t see any logical reason that I shouldn’t get at least a little better performance by adding the insulation. I haven’t seen any indication of rubbing or binding. I will see if I can find if there is a problem.

[Turbine GuyParticipant @turbineguy]

Mike Tilby told me that I might be expecting too much from the insulation I added as described in the last post. He shared with me a very interesting 2 part article that explained how adding insulation with too high conductivity could actually increase the heat loss. The nylon I am using for the insulation is a poor enough insulator that this might be my problem. I decided to see if I could get a good estimate of the actual heat loss. I ran a test of Tangential Turbine 5D and measured the temperature of the housing and outer surface of the insulation using the Stuart Twin Drum boiler, turning the GWS EP 2508 propeller, and keeping the boiler at a output pressure of 40 psig with the globe valve. The housing quickly warmed up to a temperature of 163 F and the outside of the insulation had a temperature of 106 F with the propeller blowing air over the turbine. These temperatures stayed almost the same for the remainder of a 11.5 minute run. When steam ran out and the turbine started to lose speed, the temperature of the outside of the insulation started to rise very quickly since the convective heat transfer changed from forced to natural convection. The outside temperature of the insulation went up so quickly and then started to drop very slowly that I wasn’t ready to get the highest temperature when the propeller stopped turning. I have to remove the wick burner immediately after the water is boiled out since it has almost full flame at the end of the run. I purposely start with ½ cup of water since I know it will run out before the wick burner starts to run out of alcohol. This allows me to have approximately maximum output of the wick burner for the full length of the run. This also allows me to know exactly how much water was used. The turbine speed, housing temperature, and outside temperature of the insulation all stayed about the same after the turbine warmed up with the globe valve at the same opening for the rest of the run. This length of run with very stable conditions should give me the data I need to find a good estimate of the heat lost from the turbine housing and cover with the 1/16” thick nylon insulation. The following equation is what I used to find the heat loss through the insulation. I used the value of 0.144 W/mK given by Shapeways for the thermal conductivity of their printed nylon assuming the printed nylon of my insulation would be about the same. I used the heat flow average area of the insulation since the following reference indicated that if the ratio of the outer radius to the inner radius of the insulation was below 2, the error in using the average area is less than 4%. The largest ratio on any part of the insulators was 1.45 for a very small portion with the remainder being less than 1.2. Since the shape of the insulators was relatively complex I used my 3D Cad program Onshape to find the areas. This program will give the total area of all the surfaces that are highlighted which made getting accurate areas very easy.

Q=kA(T1-T2)/s = (0.0833)(0.049)(163-106)/(0.0052) = 45 Btu/hr = 13 W

Where:
Q= Heat Transfer, Btu/hr
k= Thermal Conductivity = 0.0833 (Btu Ft)/ (hr degree F Ft^2)
s= Insulation thickness = 0.0052 Ft
A= Insulation Average Area = 0.049 Ft^2
T1= Housing and Cover Temperature = 163 degree F
T2= Insulation Outside Temperature = 106 degree F

The run time was about the same as the 4/8/2024 test shown in the following table that was discussed in the 9 April 2024 post on page 21. This test was run the same way so the input power to the turbine of 38.8 watts was approximately the same and is an example of how consistent the power available for the same settings has been. The heat lost through the insulation estimated to be approximately 13 watts is about 34% of the energy input to the turbine but a large part of that probably occurs after the steam has flowed through the rotor.

I ran another test with everything the same except with the insulators removed. The run time was almost identical with the boiler pressure held at a constant 40 psig, so the energy output of the boiler was approximately the same. The housing temperature stayed at approximately 176 F for the rest of the run after a short warmup. Mike was right, the nylon insulation actually added slightly more heat loss.

Reference: An Introduction To Heat Transfer by M. Fishenden and O. A Saunders published July 1950.

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