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Turbine Guy22/04/2020 12:21:41
348 forum posts
198 photos

I just read Mike Tilby’s very informative Steam Turbines Large and Miniature Article 23. He generously included a reference to this thread. This article is on Stumpf-type (Terry-type) turbines. He correctly stated that I believe the open pockets can be as efficient as pockets with an outer wall, especially when the spacing between the rotor and the casing is very small. To back this up, I will compare the performance of the last test of my Turbine 3 running on air with the performance estimated from the following chart. This chart, also shown in the post of 14/03/2019, is copied from ‘A Study Of High Energy Level, Low Output Turbines’ prepared by Dr. O. E. Balje for the Department of the Navy in December 1957. This chart illustrates the maximum performance he estimated for various types of turbines when optimized. The heavy solid lines in the chart are for axial turbines. The dashed lines are for Terry turbines. The thin solid lines are for Drag turbines. The drag turbines are like turbine pumps. The blades circulate the flow in a way that increases the drag force on the rotor. The units for the volume flow are ft*3/sec and lb/ft^3 for the gas density. The values of Ns and Ds for Turbine 3 for the test of 3/4/2020 are 1.0 and 14.9 respectively. The efficiency of an optimized Terry turbine with these values of Ns and Ds from the chart is approximately 16% and the efficiency of Turbine 3 was 14.3% based on my most conservative values of the power required by the propeller. I’ll get into the Reynolds number effects and how they lower the efficiency of the optimum Terry turbine below that of my turbine for this particular case in the next post.Ns Ds Diagram

Turbine Guy22/04/2020 12:24:32
348 forum posts
198 photos

The Reynolds number of my turbine for the test conditions shown in the table below and mentioned in the last post is 1.4 X 10^4. The minimum Reynolds number for the performance given in the chart shown in the last post is 2 x 10^5. The more viscous running conditions of my test compared to what was assumed making the chart, means that the viscous losses are higher than assumed in the chart. Dr. Balje gives an efficiency correction of 0.87 for the 1.4 X 10^4 Reynolds number. That reduces the efficiency of the optimum Terry turbine to 13.9% compared with Turbine 3’s estimated efficiency of 14.3% for this case with low Reynolds number, low pressure, low temperature, low speed, and low energy.

Turbine Test Results 5

Edited By Turbine Guy on 22/04/2020 12:27:41

Turbine Guy30/04/2020 12:15:36
348 forum posts
198 photos

In the post of 07/03/2020 I mentioned that I had mounted one of Werner Jeggli’s cast axial turbine rotors to a shaft and made a cover plate with a nozzle so that I could test his rotor in my Turbine 3 housing. The following picture shows these parts. I ran this rotor on steam with a clearance of 0.031 in. between the face of the rotor and the face of the cover plate. With this clearance, the cast axial rotor had approximately the same power output as my machined tangential rotor. The updated test sheet shown below shows the results of that test. The next post will have a drawing showing the dimensions of the parts and the placement of the rotor in the housing for this test.

Axial Turbine 2 PartsTurbine Test Results 9

Turbine Guy30/04/2020 12:25:05
348 forum posts
198 photos

The following drawing shows the mounting of the new cover plate and Werner's axial rotor described in the previous post in my Turbine 3 housing. The dimensions show the size of the rotor and cover plate and the position of the rotor for the test shown in the last post.

Axial Turbine 2

Turbine Guy02/05/2020 14:29:43
348 forum posts
198 photos

I ran the following test to estimate the maximum stall torque and rotor velocity coefficient for the axial rotor using my airbrush compressor. This was done for the tangential rotor and shown in the post of 01/02/2020. Comparing the values found for the tangential rotor and the axial rotor, the axial rotor has a larger maximum stall torque (0.20 in-oz vs 0.18 in-oz) but only in the optimum positions. The nozzle was larger for the axial rotor (0.032 vs 0.028) which accounted for most of the increase in torque. The maximum torque stayed the same regardless of the position for the tangential rotor.

Axial Turbine RVC

Turbine Guy03/05/2020 18:11:48
348 forum posts
198 photos

I updated the following test results to include running the cast axial rotor on air. I also updated the performance of my Turbine 1 and Turbine 2 running on air. The only changes to Turbine 1 and Turbine 2 were finding the optimum position of the rotors and setting the collar clearance to 0.004 in. This updated the performance to the maximum values I have found optimizing the position of the rotor for the propeller being used. The ball bearings used in all these tests had the original oil supplied by the manufacturer (AeroShell Fluid 12) and had not been exposed to steam.

Turbine Test Results 10

Turbine Guy22/05/2020 14:01:27
348 forum posts
198 photos

I made an estimate of the performance of my Turbine 3 using a low cost gearbox with a 30:1 ratio. The gearbox shown below is part of a gearmotor and is capable of running with an input speed of up to 30,0000 rpm and can handle the torque of Turbine 3 running on air. All the gear shafts use the brass side plates for bearing surfaces, so the friction is high and the overall efficiency relatively low. I picked this gearbox since it is readily available and very low cost as shown below. It also has a performance sheet that will be shown in the next post. What I am trying to do is show my very simple turbine and a very low cost gearbox can be used at output speeds of model steam engines. I tested two model steam engines, a very simple oscillating cylinder steam engine and a more complex piston valve steam engine using my airbrush compressor. The tests are shown in the following thread https://www.model-engineer.co.uk/forums/postings.asp?th=139899. This thread has complete descriptions of the steam engines tested, the propellers used for the tests, and all the test parameters. The thread jumps back and forth between tests of turbines, steam engines, and many proposals for ways to improve them. If you have the patience to read through the entire thread you can see all the things I explored. In the next post, I will compare the estimated performance of my Turbine 3 using this gearbox with the performance of the steam engines.

Pololu Gearmotor 3

Turbine Guy22/05/2020 15:01:22
348 forum posts
198 photos

I showed the wrong gearmotor in the last post. The following is the gearmotor that should have been shown.

Pololu Gearmotor 5

Turbine Guy23/05/2020 14:39:11
348 forum posts
198 photos

The following chart shows the performance of the gearmotor shown in the last post. I will show the estimated performance of my Turbine 3 with the gearbox from this gearmotor compared with performance of the Stuart ST oscillating cylinder steam engine in the next post.

Pololu Gearmotor 2

Turbine Guy23/05/2020 16:43:20
348 forum posts
198 photos

The following chart compares the performance of Turbine 3 using the 30:1 ratio gearbox described in the preceding posts with the Stuart Turner ST oscillating cylinder steam engine. The data for the Stuart ST comes from the post of 26/10/2019 in the Testing Models thread shown in the following link https://www.model-engineer.co.uk/forums/postings.asp?th=139899&p=4. The data shown for the Stuart ST improved with the changes shown in the chart, so the comparison is between the optimized Turbine 3 and the optimized Stuart ST. The big change in performance of the Stuart St with the larger propeller was the result of the very small port opening area of this engine as described in the Testing Models thread. The propeller used with Turbine 3 was chosen to keep the output power to the maximum shown for the gearbox. Turbine 3 is capable of much higher power with a 30:1 ratio gearbox if the speed and power is not limited.

Stuart ST vs Turbine 3

Turbine Guy24/05/2020 14:35:48
348 forum posts
198 photos

In the last post I showed the estimated power my Turbine 3 could produce with a very low cost 30:1 ratio gearbox. With that gearbox, the estimated power for Turbine 3 running on my airbrush compressor was 1.5 watts. The power of a Stuart Turner ST oscillating cylinder steam engine modified to get the best performance I was able to obtain, reached a power of 1.6 watts with the same air pressure. The output at the low steam engine speeds for these is estimated to be about the same using this low cost speed reducer. The speed reducer shown below is much more expensive, but can run continuously at speeds up to 60,000 rpm and a maximum continuous power of 11 watts. I will show the estimated power of my Turbine 3 using this gearhead in my next post.

Maxom Gearhead

Robert Atkinson 224/05/2020 15:45:43
avatar
976 forum posts
20 photos

Have you though of using a small brushed DC motor as a dynamometer?
With a variable load you can check the power at various speeds and torque. The Torque is directly proportional to current regardless of speed. You can calibrate the motor / Dynamometer torque constant at stall using the weight / lever arrangement while supplying enough power to keep it balanced. A current reading gives you thedivied by the torque give you the constant for that motor. Using a brushed DC motor as a dynamometer you can get a pretty good estimate of the static or running torque and power. Most accurate is to meaure the speed and torque and do the math. Simply measuring the electrical power (V x A) will give a slightly lower figure but you can compensate by allowing for the voltage drop caused by the DC resistance of the motor.

Robert G8RPI.

gerry madden24/05/2020 16:26:44
175 forum posts
73 photos

Excellent to hear Robert, I never realised this was independent of speed ! Now I can check the efficiencies of some clock gear-trains and measure my improvements smiley

Posted by Robert Atkinson 2 on 24/05/2020 15:45:43:

..........The Torque is directly proportional to current regardless of speed. ………….

Turbine Guy24/05/2020 17:37:03
348 forum posts
198 photos
Posted by Robert Atkinson 2 on 24/05/2020 15:45:43:

Have you though of using a small brushed DC motor as a dynamometer?
With a variable load you can check the power at various speeds and torque. The Torque is directly proportional to current regardless of speed. You can calibrate the motor / Dynamometer torque constant at stall using the weight / lever arrangement while supplying enough power to keep it balanced. A current reading gives you thedivied by the torque give you the constant for that motor. Using a brushed DC motor as a dynamometer you can get a pretty good estimate of the static or running torque and power. Most accurate is to meaure the speed and torque and do the math. Simply measuring the electrical power (V x A) will give a slightly lower figure but you can compensate by allowing for the voltage drop caused by the DC resistance of the motor.

Robert G8RPI.

Hi Robert,

Werner Jeggli uses a simiiar scheme using a brushless DC servomotor testing his turbines. I prefer the simplicity of using the propellers that I can get test data for. The propellers are quite inexpensive and readily available and the results I have obtained using them show results I believe are just as accurate. The downside of using the propellers compared to using motors as generators is that you need a propeller correct for your application. The propeller I use testing my turbines with air gives me adequate results to compare changes or different turbines. Werner is way ahead of me testing with steam because he can operate his turbines at a much higher speed and adjust the load to optimum values. I have not been able to obtain a propeller with test data that can operate at speeds as high as the 35,000 rpm. This is the speed Werner is currently able run with adequate ball bearing life. I will use the Maxom gearhead to estimate the performance at this speed running on steam.

Thanks for the input,

Byron

Robert Atkinson 224/05/2020 20:48:39
avatar
976 forum posts
20 photos

Hi,

My background is electronics so I tend to look in that direction for solutions. A brushless DC would act like a 3 phase alternator so a little harder to instrument. Known propellors are a useable solution and are used to check full size aircraft engines. These are normally special item with short blades and are called "club" propellers as they look more like clubs than blades. There is a document on their calibration here:
https://www.casa.gov.au/files/003pdf-13
(I'm in aviation)

Robert G8RPI.

Turbine Guy25/05/2020 17:49:36
348 forum posts
198 photos

Hi Robert,

I followed your link and read the report. This is similar to the way I find the power using propellers with performance given for various speeds. The following is a portion of the performance sheets given by APC for the 4x3.3EP propeller I use for testing with steam. The performance is given for several speeds and, as I noted in my test sheets, the power coefficient is the same for speeds ranging from 5,000 rpm to 10,000 rpm. With the assumed air density, propeller diameter, and power coefficient remaining constant, the power is proportional to the cube of the speed. Since the static power given by the table is approximately 0.77 watts at 5,000 rpm, the power at speeds up to 10,000 rpm can be found by the following equation.

P=0.77(N/5,000)3

Where: P = Power required to turn the propeller at the speed N, watts
N= Propeller speed, rpm.

APC 4x3.3E

Turbine Guy29/05/2020 14:21:27
348 forum posts
198 photos

I decided to try to fix the relief valve on my smallest boiler. I used the methods I discussed in the post of 25/03/2020 in this thread shown in this link https://www.model-engineer.co.uk/forums/postings.asp?th=140195&p=8. The repair of the relief valve was a complete success using these guidelines. The boiler does not leak any steam until the relief valve starts to open at about 32 psig. Before repairing the relief valve it was starting to leak before the boiler reached 20 psig. The following picture shows my Turbine 2 connected to this boiler and the wick type burner I use. As you can see in the photo, there is no stop valve used, so the turbine inlet gets almost the full pressure of the boiler. For the test I ran after fixing the relief valve, I did the following. I put a carefully measured ½ cup of water in the empty boiler. I added the ball bearings I’ve been running with Krytox GPL 105 oil in my turbine after adding a slight amount of this oil to each bearing. I then filled the wick burner with denatured alcohol, lit it, and set it in the boiler. It took a few minutes for the steam to start coming out but then reached the maximum pressure of 25 psig in a short time. The pressure remained constant until the boiler ran out of water in approximately 12 minutes. The turbine speed took several minutes to reach it’s maximum speed of 17,000 rpm but held approximately that speed for the rest of the test. The mass flow is 1.30 lb/hr and the power required for the GWS EP 2508 propeller I used in the test is approximately 0.9 watts at 17,000 rpm. The mass flow was about what I was getting running my turbines on the small boiler before the relief valve started leaking. The power was less running on steam from my small boiler than what I have obtained running on air with Turbine 2 (0.9 watts vs 1.4 watts). I’ll see if I can explain the lower performance in my next post.

Turbine 2 Steam

Turbine Guy30/05/2020 18:08:41
348 forum posts
198 photos

I thought the reason for the drop in performance of Turbine 2 running on steam versus running on air was the viscosity of the Krytox GPL 105 oil I use with steam. This oil is approximately 6 times more viscous than Krytox GPL 102 oil at 212 F (100 C). I ran another test of Turbine 2 identical to the test in the last post except that I did not add any oil to the ball bearings. The turbine obtained the same 17,000 rpm maximum speed of the last test, but the speed slowly dropped to approximately 16,000 rpm at the end of the test. The vibration increased as the speed dropped. I use the stopwatch feature of my watch to time the runs, but I failed to hit the stop button at the end of the run, so I didn’t get the mass flow. I then made a test with Krytox GPL 102 oil added to each ball bearing before starting the test. The maximum speed reached was approximately 16,000 rpm and the speed also decreased slowly as the run continued and the minimum speed was approximately 15,500 rpm. The vibration was very high for the second test. The pressure remained at about 25 psig for the entire length of both tests. I did hit the stop button on the second test, but something started the stopwatch again, so when I looked at the elapsed time it was way over the length of the run. I take the burner out of my boiler immediately after it runs dry, so I didn’t check the elapsed time until I started to record the data. Normally my watch will keep the elapsed time until I restart the stopwatch, so I must have bumped the button. Anyway, I didn’t get the mass flow for either of these runs. These tests indicated that vibration is a major part of the loss in performance. I knew the rotor of Turbine 2 has the worst balance of any of my rotors, but I didn’t realize that it could result in this large of a power drop. When I first put a shaft in the cast rotor Werner Jeggli gave me, I was anxious to test it and ran the test before balancing the rotor. The maximum speed running on air for the unbalanced rotor was 18,000 rpm. The maximum speed running on air for the balanced rotor was 21,500 rpm. This is an increase in power of approximately 0.8 watts due to balancing. I ran my Turbine 2 on air with ball bearings containing the Aeroshell Fluid 12 oil that I normally use. These were the same bearings and oil used in the tests of the cast rotor. The maximum speed was approximately 18,000 rpm and there was very little vibration. Apparently, the Aeroshell Fluid 12 oil has a better balance of power lost due to viscosity and power loss due to rotor unbalance running on air than either of the Krytox oils running on steam. The 18,000 rpm running on air, is not as high as the maximum speed of 19,500 rpm I have already obtained with Turbine 2 and Aeroshell Fluid 12 oil. About all I can confirm with these tests is the importance of the oil in the ball bearings. The oil needs to be correct for the gas and temperatures used and needs to be freshened each run for maximum performance.

Tim Stevens30/05/2020 18:31:44
avatar
1430 forum posts

I wonder. A turbocharger uses 'spare' power from exhaust gas to compress gas to 'overfill' the engine cylinders, using a turbine to drive a turbine. There is not much difference between waste exahaust gas and steam - not so hot, wetter, certainly, but in principle one hot gas is much like another. Is there any reason why the exhaust half of a turbocharger should not serve as a turbine to drive a (small) vehicle? A purist would say that this is not proper model engineering, perhaps - but has anyone tried it?

Cheers, Tim

Turbine Guy30/05/2020 19:15:16
348 forum posts
198 photos

Hi Tim,

The following link shows what a Mechanical Engineering student did trying to accomplish what you suggested.

https://dspace.mit.edu/bitstream/handle/1721.1/32971/62861135-MIT.pdf?sequence=2

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