Model Turbines

[Turbine GuyParticipant @turbineguy]

I compared the tilting of the rotor shaft of Tangential Turbine 5B and Axial Turbine 4C by moving the propeller end of each perpendicular to the axis of the ball bearings. Tangential Turbine 5B had almost no movement and Axial Turbine 4C had about the same movement even with the extra ball bearing. Since both turbines have the same very close sliding fit between the ball bearing bore and the rotor shaft, the extra play of Axial Turbine 4C is primarily due to the extra clearance between the ball bearing OD and the housing bore. When I made Axial Turbine 4 I found that the fit between the ball bearing OD and the housing bore was a little loose but I thought it was tight enough to work. When I made Tangential Turbine 5 I used a reamer for the housing bore that was 0.0005 smaller and got a tighter fit. I decided to try filling the clearance of Axial Turbine 4 between the ball bearings OD and the housing bore with Loctite 290. Because Loctite 290 is designed to flow into very tight spaces and tends to get into spaces where it is not desired, I didn’t apply it to the ball bearing ODs for fear it would get inside the ball bearings and lock them up. Instead I applied it to the housing bore and waited until it got a little tacky and then installed the ball bearings in the housing. I waited a couple hours for the Loctite to partially cure and tried tilting the rotor. The tilt is almost entirely gone and the rotor rotates freely and will spin a few seconds after spinning it by hand. I will wait 24 hours until it is fully cured and then run a test to see if this improves the performance.

[Turbine GuyParticipant @turbineguy]

The dental ball bearings of Axial Turbine 4A shown on the following drawing were Loctited with the rotor in the same position as the last test of Axial Turbine 4C so the performance could be compared with everything the same except for the extra bearing. The air pressure required to run Axial Turbine 4A at 28,000 rpm was 11 psig as shown by the 11/30/2024 test in the following spreadsheet. Eliminating the tilting of the rotor shaft due to excess clearance on the OD of the ball bearings improved the efficiency of Axial Turbine 4A to 28.8% compared with the efficiency of 27.3% before the Loctite was tried. This showed that with the proper bearing clearances, or the bearings fixed to the housing with Loctite, the efficiency of Axial Turbine 4A with two ball bearings is better than Axial Turbine 4C with three ball bearings.

[Turbine GuyParticipant @turbineguy]

The rotor in Axial Turbine 4 was designed for the housing used by Axial Turbine 3 that required the sleeve in the rotor to project out toward the inner ball bearing. This creates quite a bit of cantilever that I eliminated in the design of Tangential Turbine 5. I am going to try pushing the sleeve in the rotor of Axial Turbine 4 so that it projects out the other side as shown in the following drawing. I am also going to put a longer sleeve in the housing bore that will get the face of the inner ball bearing closer to the face of the rotor as also shown on the following drawing. Since these changes are significant, I will call this Axial Turbine 5. I also show below the last revision of Axial Turbine 4A for comparison. Reducing the amount of cantilever should reduce the deflection and the load on the ball bearings and increase the efficiency.

[Turbine GuyParticipant @turbineguy]

I finished the machining on Axial Turbine 5 and got a very good fit between the ball bearing OD and the sleeve bore. This eliminated almost all tilting of the rotor. The temperature is below freezing in our area and my shop is uninsulated and the temperature is 46 F. My first test at this temperature almost matched the performance of Axial Turbine 4A with the ball bearings OD Loctited to the sleeve bore. With my electric space heater It would take about 4 hours to heat my shop up to the 70 F temperature that all the tests in the spreadsheet shown in the 30 November 2024 post were run at. Because it takes a lot of time to heat my shop and increases our electric bill significantly, I prefer to do the final setup and testing in warmer weather. The reason the temperature is important in the test results is the efficiency of the turbine drops significantly at low temperatures. A test I ran a while ago started with a shop temperature at 50 F. I tested the turbine at that temperature and then again at 70 F. There was an increase in performance of about 9.5% running at the higher temperature. The first test of Axial Turbine 5 looks promising, I will find the optimum settings and run a test at 70 F when the weather gets warmer.

[Turbine GuyParticipant @turbineguy]

It got warm enough in our area for me to test Axial Turbine 5 at 70F after finding the optimum position of the rotor. I was not able to match the last performance of Axial Turbine 4A even though I felt I had made significant improvements. I checked everything I changed and could not find anything I had done wrong. I knew the rotor for Axial Turbine 5 had a very slight imbalance causing a little vibration at high speeds but could not balance it by Werner Jeggli’s method shown in the 29 November 2020 post on page 9 because the diameter was not the same on each side of the rotor. I decided to make a revision to Axial Turbine 5 that has the diameter the same size on both sides of the rotor as shown on the following drawing. I then balanced the rotor and ran a test of Axial Turbine 5 R1 and it did significantly worse than before the change. After checking everything I could think of and not finding a reason the performance kept getting worse, I tried the propeller used on the last test of Tangential Turbine 5B and the performance increased quite a bit. I compared the two propellers and the one I first used on Axial Turbine 5 R1 appeared to have more pitch on one blade like it had been twisted from the original position. I was curious how easy it would be to twist the blades into a new position so I tried twisting slightly by hand the blades on the second propeller I used on Axial Turbine 5 R1. It did not take much force to twist them slightly from the original position. I turned both blades in the direction that reduced the pitch by about the same slight amount and then installed the propeller on Axial Turbine 5 R1 and ran a test. With the pitch reduced slightly, Axial Turbine 5 R1 exceeded the best performance of all my turbines even though the test was at 52 F. This showed how much the test results could change if the propeller blades get twisted from their original position and how little force it takes to twist them. The test results with the GWS EP 2508 propeller have been very consistent up to the tests of Axial Turbine 5 so I looked for what could have twisted the blades on this turbine. The only thing I could think of was that the press fit of the propeller on the rotor shaft was larger than I have had before and required extra force to push the propeller onto the shaft. Apparently I was not as careful as I needed to be in removing and reinstalling the propeller several times while finding its optimum position. I ordered some new propellers and will try to be more careful in the amount of force I use and how I push the propeller onto the shaft.

[Turbine GuyParticipant @turbineguy]

I received the new GWS EP2508 propellers and carefully installed one on Axial Turbine 5. I warmed my shop up to 70 F and ran a test and added the results to the following spreadsheet. The results were as I expected due to the reduction in the amount of cantilever that should have resulted in a slight increase in efficiency. Axial Turbine 5 performed slightly better than any of the Axial Turbine 4 configurations. There was no detectable vibration at the top speed of 28,000 rpm, so the balancing of the rotor was successful. This also helped raise the efficiency.

[Turbine GuyParticipant @turbineguy]

I explained the method I used to find the rotor velocity coefficient (RVC) by turning a large propeller at a very low speed in the second 23 March 2023 post on page 19. I like this method better than calculating the RVC from the maximum stall torque that was explained in the first 23 March 2023 post because it gives the effective RVC directly instead of estimating it from the maximum RVC. In my latest spreadsheet for estimating the RVC from testing with a propeller at low speeds I added the option of using propeller power coefficients since that is the most accurate when using APC propellers for small power outputs. Their power is given in horsepower shown in three decimal places so it rounds to about 0.001 Hp at the low power levels of my tests. Their torques are given in in-lbs and shown in three decimal places so that is a little better but using the power coefficients gets the most accurate result. The following spreadsheet shows the results for the last test of Axial Turbine 5. I chose the APC 5 x 3EP propeller since it is the largest propeller that is in its normal operating range at the speeds shown in the following spreadsheet that was copied from the Static Testing of Micro Propellers by Robert W. Deters and Michael S. Seleg. Since the rotational losses are proportional to the speed cubed, the losses at the 4,300 rpm test speed would be a very small percent of the losses at the top speed of 28,000 rpm (4.300/28.000)^3 = 0.0036).

[Turbine GuyParticipant @turbineguy]

The reason I updated the rotor velocity coefficient (RVC) in the last post was because I have been trying to find why the rotational losses for the turbines that use the axial flow rotors were so much higher than for the tangential flow rotors. The blades on the axial rotors raise the losses over the pockets of the tangential rotors but are quite a bit larger than the estimates given by most sources I have found. The RVC found in the last post is what I used in the following spreadsheet to find the rotational losses. These rotational losses are within the estimations of the sources I have found. The rotational losses found in this spreadsheet are still a little higher than the estimates of some of the sources but less than others. None of the sources include the effect of moisture on the rotational losses for air but the sources showing the rotational losses for steam indicate it is significant. When I run my tests with air at 70 F that I use for spreadsheets, the moisture in the air increases due to the expansion of the air in the nozzle so might account for the rotational losses being on the high side.

[Turbine Guy]

On Turbine Guy Said:

On Turbine Guy Said:

The following test sheet shows the performance of Axial Turbine 4D and Tangential Turbine 5C running on air I said I was going to show after the 27/03/2023 post that also included the following drawings. I will look at the reasons for this performance in the next posts.

I took a second look at the way the nozzle velocity coefficients were found

On Turbine Guy Said:

I estimated the nozzle velocity coefficient of Axial Turbine 4D to be approximately 0.69 and 0.73 for Tangential Turbine 5C for the tests shown in the last post. These velocity coefficients were determined by starting with the velocity coefficient of 0.93 for the conical nozzle as explained in the 17/03/2023 post then finding the reduction for the pressure drop due to friction. The reduction due to pressure drop was 0.94 for both turbines since they had the same nozzle throat length. This reduced the nozzle velocity coefficient to 0.87. I then found the reduction for expanding to supersonic velocities with a converging only nozzle shown in the following diagram as line A. In the tests shown in the previous post, the Mach number for Axial Turbine 4D was 1.47 and for Tangential Turbine 5C was 1.43. The reduction for expanding supersonic is approximately 0.97 for both turbines when rounded to two decimal places. This reduces the nozzle velocity coefficient to 0.84 for both turbines. The last reduction was for the distance between the nozzle outlet to the rotor. This was 0.186” for Axial Turbine 4D and 0.113” for Tangential Turbine 5C as shown on the two drawings of the last post. The reduction in nozzle velocity coefficient for this was 0.82 for Axial Turbine 4D and 0.87 for Tangential Turbine 5C. This reduced the nozzle velocity coefficient to 0.69 for Axial Turbine 4D and to .73 for Tangential Turbine 5C as stated at the first of this post. The reduction in rotor velocity coefficient for supersonic velocity was 0.98 for both turbines. The rotor velocity coefficient found from the static torque test explained in the 23/03/2023 post were 0.68 for Axial Turbine 4A and 0.27 for Tangential Turbine 5B. Neither of these turbines had the insert and the discharge velocity of the nozzles were approximately sonic, so using the correction for supersonic velocities reduces the rotor velocity coefficient to 0.67 for Axial Turbine 4D and to 0.26 for Tangential Turbine 5C. I will show in the next post how the test results in the last post compare with the velocity diagrams like shown in the 23/03/2023 post.

Edited By Turbine Guy on 29/03/2023 21:20:09

I looked at the way I determined the nozzle velocity coefficients in this post and believe the corrections I made for the distance from the nozzle outlet to the rotor are not correct. I believe the methods used for the other corrections are valid and the corrected value for the nozzle velocity coefficient of 0.84 for both turbines should be used.  The distance from the nozzle outlet to the rotor is space needed for the gas to expand to supersonic Velocities.  I will try to find the distance required for the gas to expand to supersonic velocities and the effect of having too short or too long of a distance.

I tried adding an insert with a throat diameter of 0.025” in the nozzle of Axial Turbine 5 to see if the improvements I made changing from Axial Turbine 4 to Axial Turbine 5 would increase the efficiency using the insert. The views shown in the post I quoted show approximately the dimensions of the insert I added in Axial Turbine 5 and how it fits in the nozzle since the nozzle was the same for both Axial Turbine 4 and Axial Turbine 5. Instead of the performance improving, the maximum speed of the propeller for the pressure of 44.0 psig shown for Axial Turbine 4D in the table decreased from 28,000 rpm to 22,000 rpm. This was a substantial decrease in performance so I tried a test running on steam and got similar results. I removed the insert and had the same performance running on air as shown in the table of the 17 December 2024 post so there wasn’t a problem with any of the parts or the placement of the rotor in the housing. The only reason I could find for this drop in performance using the insert was the air not expanding to supersonic velocity. I estimated the performance with sonic velocity and it was still much higher than found in the test. The last thing I could think of was to estimate the performance based on the velocity that would pass the mass flow at the discharge diameter of 0.042”. In other words, expanding to a lower velocity instead of to a supersonic velocity. The estimated performance based on expanding to a lower velocity was close to what was found in the test so this appears to be what happened. I have got performance with the inserts that could only be obtained with the flow expanding to supersonic velocities like shown in the quoted post so I am not sure why I only had this problem with this last configuration. When I remove an insert, I have to remove all the Loctite that held it in place. When I did the cleanup after removing the insert used in this last test, I noticed that I had to remove more Loctite than normal. If the flow had to pass through this extra Loctite it might have been what caused the flow to go subsonic. If you use Loctite 290 to hold a nozzle or insert in place you must be very careful not to let the clearance get too large or it can block part of the flow. Loctite 290 is designed to work in very tight clearances. When I have kept the clearances small Loctite 290 has worked very well for me running with air or steam.

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