Testing Models

I made the following chart to summarize the testing of my turbines. I lowered the estimated powers in this chart slightly from what I have previously given to be more conservative. These values of power are approximately the power I calculate using Dr. Balje’s guidelines. The first test with velocity staging resulted in lower power than with the reversing chamber removed. This is comparing an optimized single stage with the first try of velocity staging. Hopefully by optimizing the reversing chamber I can improve the performance of the velocity staging. The next post gives the updated drawing that is the first revision for turbine 3 with the velocity staging. As you can see there are quite a few changes from the drawing shown in the post of 11/07/2019. Some changes were made to improve the design while others were to repair errors in machining. This drawing shows the dimensions important for setup and analysis and shows the actual part dimensions at the time of this revision.

The followings is the drawing I described in the last post.

I ran several tests with air to find the maximum performance. These tests are described in the Model Turbines thread and summarized in this and the following posts. The improvement in performance running on air was very small. The speed increased by 500 rpm without the reversing chamber and 600 rpm with the reversing chamber with the improvements. The turbine still made slightly more power without the velocity staging running on air. Apparently, the back pressure caused by the second stage was enough to keep the performance from getting better than with a single stage. I ran the velocity staging with steam on 14/10/2019 and added the performance of this test to the table. The extra energy of the steam allowed the velocity staging to work better. The boiler pressure was the same as the test of 11/7/2019 without velocity staging, so the energy supplied should be approximately the same. The velocity staging with steam got a 1.8 watt increase in power. I planned on running on air immediately after running on steam to see if that would keep the ball bearings from gumming up. Even after releasing the hose clamp, the silicon hose I used for steam would not come off. I pressed the hose from my airbrush compressor against the silicon hose and got enough pressure to spin the turbine and blow the water out. I then tried pulling as hard as I could on the silicon hose. The stainless steel (ss) tube that was Loctited to my turbine housing popped out. I had to cut the silicon hose to remove it from the ss tube. I don’t know what caused the silicon hose to bond to the ss tube. I Loctited the ss tube back on my turbine housing and will run the turbine on air after the Loctite cures. Hopefully, the ball bearings will not gum up.

I ran Turbine 3 on air after Loctiting the stainless steel inlet tube back on the housing. The speed never got back to the speed the turbine was running with the reversing chamber installed but my airbrush compressor was shutting down periodically. This indicated the turbine nozzle was partially blocked. I cleaned out the nozzle and ran the turbine on air again. The speed of the turbine went back to the same maximum speed it had before with velocity staging. Running the turbine on air immediately after running on steam appears to keep the ball bearing oil from gumming up. I wasn’t sure if the blockage in the nozzle occurred before or after the run on steam so I ran the turbine on steam again. After cleaning out the nozzle, Turbine 3 reached a maximum speed of 32,500 rpm with velocity staging. The resulting power of 12.4 watts was about what I hoped I would be able to achieve with velocity staging. This is an increase in power of approximately 4.5 watts with velocity staging. This increase in power is a little misleading. It should be pointed out that if the turbine was turning a load that could be adjusted, the power without velocity staging of Turbine 3 would be around 11 watts at 32,500 rpm. The power added by the velocity staging would be approximately 1.5 watts compared with the power that could be achieved at this speed without velocity staging. Using a propeller for the load results in the speed being set by the torque required to turn the propeller. The speed was very consistent during the short run on steam. Also, the relief valve on the boiler was constantly releasing steam so the pressure of 50 psig (3.4 bar) was the maximum the boiler could produce. I ran Turbine 3 on air after the run on steam and the maximum speed obtained was 22,600 rpm so the friction of the ball bearings has not changed. I am very pleased with the performance of Turbine 3 running on steam with velocity staging. The following chart is updated to show all the tests.

The following drawing shows some of the important dimensions of my Turbine 3 after getting the best performance I was able to obtain with velocity staging. Making the outer edge of the reversing chamber as thin as possible made the biggest improvement. This opened up the space for the steam or air exiting the second stage. I was able to meet all my goals with these last tests so I don't plan on any further testing of Turbine 3.

Since I was able to achieve the performance I hoped to obtain with my turbines, I decided to work on my Stuart Turner St oscillating steam engines again. The following photo shows the machining done on the cylinder and standard of the parts on the right and the same parts without modification on the left for comparison. I machined the mating faces of the standard and cylinder on the parts on the right. As I suspected, the mating surfaces were worn conical by the tilting back and forth of the cylinder as the engine ran. You can also see in the photo how much larger the cylinder support shaft is on the parts on the right side. The following is the speeds I posted 1/7/2019 after running the APC 8x6 propeller with the enlarged cylinder support shaft. The maximum speeds for 5, 10, 15, and 20 psig were 450, 655, 765, and 851 rpm respectively. After machining the faces, the maximum speeds for 10, 15, 20, and 24 psig were 1,000, 1,125, 1,215, and 1,450 rpm respectively. I was able to maintain a pressure of 24 psig after machining the faces. 20 psig was the maximum I could maintain before machining the faces. After testing I took the cylinder off and tried moving the piston with the ports open and closed. There was only a slight increase in force moving the piston with the port closed so the packing loosened up from the previous tests. For propellers, the power goes up by the cube of the speed so the increase in performance was substantial. In the next post I’ll add a chart similar to the one I used for documenting the testing of turbines.

The following chart shows the maximum estimated powers my Stuart Turner ST oscillating steam engine has produced in all the tests I have made so far. The dates in the table are with the month first, day second, and year third as we use in the USA. This chart gives only the maximum estimated powers obtained on the given dates for the changes to the engine shown. Posts in this thread near the dates shown have quite a bit of additional information. I am still trying to find out why the amount of power obtained is so low for this engine. I have tried to be very careful in the changes I've made but maybe I have done something that causes this. The loose packing is allowing more leakage but the power has increased over what was produced with the tight packing due to the mating faces of the cylinder and standard being flat now. I'm going to try using the floating O-ring next.

The following is the speeds I posted 10/25/2019 running the APC 8x6 propeller after machining the faces. The maximum speeds for 10, 15, 20, and 24 psig were 1,000, 1,125, 1,215, and 1,450 rpm respectively. After switching to the floating O-ring, the maximum speeds for 10, 15, 20, and 24 psig were 1,000, 1,250, 1,400 and 1,500 rpm respectively. I was able to maintain a pressure of 24 psig after machining the faces, 20 psig was the maximum I could maintain before machining the faces. After testing I took the cylinder off and tried moving the piston with the ports open and closed. The force to move the piston was very large with the port closed when moving toward the top cylinder cover. The force was quite a bit less with the port closed and moving toward the bottom of the cylinder due to leakage around the connecting rod. There was very little force required to move the piston in either direction with the ports open. Even with the much tighter clearance on the cylinder support shaft, the cylinder is pried away from the standard if the pressure is above 15 psig (1 bar). This only happens with the tight packing or floating O-ring. Tightening the spring to the maximum doesn’t keep the cylinder from being pried away. I believe the biggest weakness of the simple oscillating cylinder engines is the cylinder pin riding in a hole on one side only. If the seal is good on the piston, the force cocking the cylinder can be very large. If you make the spring large enough to keep the face of the cylinder from being pried away from the face of the standard, the friction loss becomes too large. From my testing with a relatively tight clearance on the cylinder pin, the best power came with the spring compressed less than the maximum. The flow lost by the cylinder cocking was still relatively low because of the tight clearance. The cylinder would tilt away from the standard each time the pressure was above 15 psig. This caused a leakage at higher pressures that limited the power. My airbrush compressor had to run continuously at pressures above 15 psig but could maintain pressures below 15 psig by running intermittently. Apparently, the friction and binding caused by the increased pressure partially offsets the gain in performance from having less leakage by the piston.

The following chart has been updated to show the tests with the floating O-ring and shows the maximum performance was about the same for the floating O-ring and bedded in packing.

I kept looking for reasons my Stuart ST oscillating steam engine was putting out such low power. I analyzed the flow through the ports with the small opening area. The following diagram shows the ports in the maximum opening position. The circles with the solid lines represent the holes in the standard. The dashed circles represent the holes in the rotating cylinder. As you can see the maximum overlap is very small. The hole in the rotating cylinder is 1/16 inch in diameter and the full flow area is 0.0031 square inches. The maximum opening area of the overlapped ports is 0.0013 square inches. The average opening area during the intake cycle is 0.00084 square inches. I estimated the pressure drop in the inlet and exhaust ports to fill and empty the cylinder at 1500 rpm with the average opening area. The estimated inlet pressure drop was 3.7 psi and the estimated exhaust pressure drop was 8.3 psi. With half the available pressure lost in filling and emptying the cylinder at 1500 rpm, I thought running at a lower speed would be more efficient. I removed the APC 8 x 6 propeller and mounted an APC 15 x 10 propeller. The maximum speed I was able to obtain with the 15 inch propeller was 950 rpm. The power required by the APC 15 x 10 propeller at that speed is 1.6 watts. This is a major increase in power from my previous tests and is more like I expected from the Stuart ST steam engine. Apparently, it is best to run at a low speed due to the very small port opening area. I will add an updated chart including this test in my next post.

Edited By Turbine Guy on 29/11/2019 21:13:22

The following chart shows the test described in the last post. 22 psig was the maximum pressure I could obtain with my airbrush compressor running continuously with the APC 15 x 10 propeller.

Thanks for these Turbine Guy, interesting stuff.

Aeromodeller used to have a special set of props they used to test engines!

Neil

Thanks for your kind remarks. I love using the APC propellers. They're not very expensive, have performance charts for each propeller, and are designed to use the same adapter. I'm so used to trying to run my model turbines as fast as I can, it took me a while to realize steam engines normally work best at low speeds. I think Stuart Turner might have purposely limited the intake opening on their oscillating engine to keep it from running fast enough to hurt itself. With the relatively high friction and very small port opening area, I doubt the runaway speed without a load is very high.

The following photo shows the test setup I used to find the stall torque of my turbine 3. The description of the test and the results are given in this link. https://www.model-engineer.co.uk/forums/postings.asp?th=140195. This is a very simple and accurate way to find the stall torque.

The following jpeg files show the description of the test and results mentioned in the last post. Since so many things effect the rotor velocity coefficient, I thought a full description of this simple accurate method to determine it's actual value would be useful.

Delete post

Edited By Turbine Guy on 23/12/2019 18:24:53

The hydraulic torque and power can be calculated as shown below. The values used are from the test to find the stall torque in the preceding post. The table gives my best estimate of the power of my turbine 3 running on air from my airbrush compressor.

I purchased the Tyne steam engine described below from Miniature Steam Models (MSM). This engine comes with a displacement lubricator and a small bottle of steam oil and is intended to be run on steam. It was only available as a partially assembled version at the time of purchase. I decided that I would do my first tests with all the installed parts in exactly the position they arrived in. This included the position of the nut that compresses the spring that pushes the cylinder against the standard. That is the most critical adjustment. I liberally oiled everything on the engine and drew oil into the cylinder by putting a few drops of oil into the exhaust and turning the engine backward. I mounted the APC 15x10E propeller that gave the best performance with my Stuart Turner ST oscillating cylinder steam engine. I then attached the hose from my airbrush compressor and carefully increased the pressure until I found the minimum pressure of 12 psig that the Tyne would keep running with. I made many runs of only a couple of minutes long between reoiling. After several minutes of doing this, the Tyne would keep running with a pressure of 4 psig. MSM gives 25 psig as the operating pressure for their boilers recommended for use with this engine. I kept increasing the pressure until the airbrush compressor reached the maximum pressure of 22 psig it could supply with existing engine adjustments. The maximum speed was about 825 rpm for this inlet pressure. I made several short runs at this pressure with reoiling between each run and the speed stayed approximately the same. I noticed that leakage started at the interface of the cylinder with the standard before reaching this pressure. I also noticed that the speed increased when I turned off the airbrush compressor and the pressure started to drop. This indicated that the optimum pressure for the initial settings is lower than 22 psig. I will find the pressure I get the maximum speed with the initial settings in my next test.

I tried running the MSM Tyne steam engine with different pressures with all the assembled parts in the same position as when they were received. I could not find any pressure that the speed increased over the 825 rpm maximum of the first test. 22 psig was still the pressure that obtained the maximum speed. The run lengths were only a couple of minutes for each pressure with reoiling after each pressure change. I then tried tightening and loosening the nut that adjusts the spring force pushing the cylinder against the standard. The position of the nut for maximum speed was the initial as received position. Tightening or loosening the nut caused the speed to drop. An air pressure of 22 psig is the highest my airbrush compressor is capable of for this engine and the optimum adjustments. The operating characteristics for this engine are similar to what I found for the Stuart Turner ST oscillating cylinder engine. The adjustment of the spring force needs to be lower than required to stop all leakage for maximum performance. I never tried the APC 15x10E propeller on the ST oscillator until I made several modifications, so I don’t know how the as received performance would have been. The modifications I made based on the guidelines given by K. N. Harris in the second edition of Model Stationary and Marine Steam Engines did improve the performance of the ST as shown in the 29/11/2019 post. I updated the following chart from that post to include the test of the Tyne steam engine.

The performance of the MSM Tyne steam engine started to vary quite a bit after I tried adjusting the spring tension that pushes the cylinder against the standard. I noticed that the shaft the cylinder pivots on was moving with the nut so the adjustment was actually turning the shaft into and out of the cylinder. I backed the shaft all the way out and removed the nut from the shaft. I noticed that the thread length on the pivot shaft was just long enough to pass through the nut. It appears that MSM intended the nut to be fully tightened to set the spring force. I first tried adding my highest strength Loctite to the threads of the pivot shaft and screwing it into the cylinder. This didn’t work since there is no shoulder on the pivot shaft and threads on the end of the shaft set the alignment. I then tried holding the cylinder tightly against the standard as I screwed the pivot shaft through the standard into the cylinder. The second method worked well enough that the engine reached a top speed of approximately 800 rpm after the pivot shaft was reinstalled. This speed was very close to the 825 rpm maximum speed I was able to obtain with the initial settings. I never planned on removing the pivot shaft from the cylinder but the nut that compresses the spring needs to be able to be removed without turning the pivot shaft out of the cylinder. Hopefully my repair will last. I measured the pivot shaft when I removed it, and the diameter is 0.1562 in. This is the same diameter that I increased the pivot shaft of the Stuart Turner ST oscillating cylinder to as one of the improvements. The fit of the pivot shaft in the standard is also better than the fit of the ST’s original pivot shaft but much looser than the fit of my increased diameter pivot shaft in the ST. Both manufacturers have more clearance between the pivot shaft and the standard than the ideal. I will try making a run with steam next.

I decided to run the MSM Tyne on air with a smaller propeller before installing the displacement lubricator and running on steam. I wanted to see if the Tyne ran better at low speeds like the Stuart Turner ST. The maximum speed I reached with the APC 8x6 propeller was approximately 1,300 rpm. I added this test to the chart shown below. You can see from the chart that the Tyne had a little more power than the ST before modifications, but less power after the improvements. Both engines are very speed limited due to the tiny size of the ports and only partial port opening as explained in the 29/11/2019 post. I doubt the maximum speed without any load would be fast enough to damage the engines. Both springs that press the cylinder against the standard limit the pressure to less than 25 psig. At any higher pressure the cylinders will move far enough away from the standards to relieve the extra pressure. The soft springs also protect the oscillators from water trapped in the cylinder doing any damage.