Here is a list of all the postings Turbine Guy has made in our forums. Click on a thread name to jump to the thread.
|Thread: Model Turbines|
Adding an insert as referenced in the last text gives the working gas a place to expand before contact with the rotor. This is important for higher pressures with resulting higher Mach numbers. I tried to get the Nozzle Block Casting shown in the following drawing from Shapeways since it would do the same thing as adding the insert. The file I sent to Shapeways the first time was rejected because it was too complex. The Nozzle Block I show below was initially accepted but then rejected. It appears that the very small steps in diameter at the exit of the nozzle are the problem. I was hoping I could get this made for two reasons. It would allow me to test the effect of the stepped nozzle without the possibility of destroying the existing nozzle. It would also allow me to test the increase in efficiency of using two nozzles or the effect of running at a higher pressure using just one nozzle. Unfortunately, the very tiny dimensions of small model turbines make meeting the requirements for casting difficult. I made a request to Shapeways to make the casting as submitted and I will take responsibility for the cleanup or failure if I cannot make it work. I have submitted a second design that I think eliminates aIl the problems but will require more machining after casting. I asked Shapeways to review the second design and if they agree that it can be made, add it to the order. The following drawing shows how I plan to open up the area in the existing Tangential Turbine 3 SD housing and mount the insert. All the dimensions of the Nozzle Block are after casting assuming 2% shrinkage. After I hear from Shapeways what they are willing to do, I will make an update to this thread.
Edited By Turbine Guy on 07/05/2021 19:16:16
My problems of getting more power from the drag turbines might be to a large extent due to the characteristics of small boilers and airbrush compressors. Neither of these does well in supplying energy at low pressures. I looked at the energy available to turbines as the pressure is lowered in the table shown in the 09/04/2021 post. The power of Drag Turbine 3 was much higher than Tangential Turbine 3 SD at very low pressures even though the available energy was almost the same. The drag turbine appears to need a large mass flow at a very low pressure to reach it’s potential. I have tried everything I can think of to increase the performance of Drag Turbine 3 without any success, so I plan to work on my impulse turbines again. I want to try using an insert like I described starting with the 07/04/2020 Post. I think I can make this work since I have a new nozzle without any of the obstructions that caused the failure in my first attempt.
I eliminated the binding problem on Drag Turbine 4 but the power is considerably lower than I get with Drag Turbine 3. I thought it was due to leakage but when I press the face of Drag Rotor 4 against the face of the cover plate the pressure only goes up slightly. The maximum pressure when running for Drag Turbine 3 is 4.0 psig and for Drag Turbine 4 is only 2.0 psig. The following drawing shows the existing dimensions of Drag Rotor 2 and Drag Rotor 4. I’m not sure if the reduction in channel size caused by the extra blades and thicker blades prevents the flow from circulating properly. The thicker blades are required to meet Shapeways minimum thickness requirements. The final dimensions of Drag Rotor 4 show the results of taking more off the face of the rotor to try to minimize leakage. Removing more from the face only increased the running pressure slightly, from about 1.5 to 2.0 psig.
I lapped the mating faces of the cover plate and the housing using an oilstone. The lapping removed enough material that I had to change the number of shims. I ran Turbine 3 again and the pressure was back to 4 psig but the maximum speed turning the GWS EP 2.5x0.8 propeller was only 14,500 rpm. After the test running on steam described in the post of 10/04/2021 the maximum speed running on air was 17,000 rpm with the turbine still hot. The power at 14,500 rpm is approximately 0.6 watts with this propeller and 0.9 watts at 17,000 rpm. The viscosity of the Krytox GPL 105 oil is high enough at the low temperatures running on air to lose approximately 0.3 watts with oil recently added. I found with the oil tests shown in the 13/03/2020 Post that this oil will run a long time when using air only and gradually have less resistance as the oil gets thinner from use. I ran Drag Turbine 4 again on air but was having contact with the rotor regardless of the number of shim washers. I lapped the face of Drag Rotor 4 like I had done with Drag Rotor 2 but still had interference with any number of shims. I was about to measure Drag Rotor 2 and Drag Rotor 4 and try to see where the clearance was less but dropped my electronic caliper and broke it. I ordered a new caliper that is scheduled to arrive next Monday. I will see if the new caliper helps me find what is wrong with Drag Rotor 4.
I finished the machining on Drag Rotor 4 using the methods mentioned in the last post. The following picture shows the new rotor. I tried my first tests with Drag Turbine 4 but could not get a pressure above 2.5 psig even with the face of the rotor pressed against the face of the cover plate. I tried Drag Rotor 3 and the pressures was much lower than I was getting before. I checked for external leaks and couldn’t find any. I plugged the line going to the turbine inlet and turned the airbrush compressor on and the pressure immediately went up to the set pressure of 30 psig and stopped. I turned off the airbrush compressor and the pressure didn’t drop after several minutes. This confirmed that there was no leakage in the lines going to the turbine. I tried blocking the exhaust tube and the pressure only went up slightly. The one positive thing of this test was that the pressure didn’t change with the position of the rotor. I will give an update when I find what the problem is.
My Drag Rotor 4 casting finally arrived. The following picture shows a front view of it. The drawing below shows the design dimensions based on 2% shrinkage with the actual measured dimension show in parenthesis. The actual dimensions were very close to the design values again. The larger diameters had the maximum deviation. The 2% shrinkage appears to be a good assumption for these types and sizes of bronze castings from Shapeways. I will start the machining of Drag Rotor 4 on Monday. I have already finished the machining of the rotor shaft. I plan on lining up the casting in the 4 jaw chuck and then reaming the shaft bore with the reamer held in the collet holder using a very accurate collet. I will then remove the casting from the 4 jaw chuck and shrink it onto the rotor shaft. All of the remaining machining will be done with the rotor shrunk on the rotor shaft and the rotor shaft held in the very accurate collet and collet chuck mounted on the lathe head stock. This should ensure that the faces of the rotor are as perpendicular to the rotor shaft axis and as parallel with each other as I can make them. If anyone has a better way to ensure the highest accuracy, I would appreciate them giving it.
I ran Drag Turbine 3 on steam from my Stuart 504 boiler with the GWS EP 2.5x0.8 propeller. I used the single wick burner from my Stuart Twin Drum boiler for heat so that the energy input would be the same as if I ran the much smaller Twin Drum boiler. The pressure gage had stopped working on the Twin Drum boiler and that is the reason I didn’t use it for the test. The tiny pressure gage on the 504 boiler is so small and has so few scale divisions that reading low pressures is almost a guess. The pressure stayed approximately constant at the lowest part of the gauge scale so was below 10 psig. The total time of the run was 7 minutes and 19 seconds to empty the boiler that had a carefully measured ½ cup of water added. The mass flow was approximately 2.1 lb/hr. The enthalpy drop for 10 psig saturated steam and atmospheric exhaust is approximately 40 btu/lb. With this mass flow and enthalpy drop, the available energy to the turbine was approximately 25 watts. The speed of the turbine remained constant at about 15,000 rpm for most of the run. The power required for the propeller running at 15,000 rpm is approximately 0.64 watts. I ran the turbine on air shortly after the run with steam to blow out any condensed moisture. The maximum speed running on air was approximately 17,000 rpm with a corresponding power of about 0.93 watts. The power running on air was lower than shown on the table of the 09/04/2021 post because of the Krytox GPL 105 oil I use when running on steam. This oil has very high viscosity at room temperature but is resistant to steam and works well at the higher temperatures. Like my impulse turbines, the drag turbine performed better at low pressures with air than with steam even though the steam had much more available energy. I’m not sure if this is a result of the loss of energy from condensing of the steam or the rotational losses spinning at high speed through wet surfaces.
Thank you for sharing your very well done video. I hope it will encourage others to show what they have done or plan to do with model turbines.
I updated the following table to include the performance of Drag Turbine 3 running on air. It is interesting that it had one of the lowest power outputs but was the highest in efficiency. The higher efficiency of the drag turbine compared with the impulse turbines at very low pressures and speeds is what attracted me to this concept. I still believe it can outperform my impulse turbines running on my airbrush compressor, if I can reduce the leakage. My airbrush compressor can't supply enough air to get the pressure up to the design pressure of 10 psig with the existing leakage. My new cast rotor is scheduled to arrive late today. I’ll see if what I believe to be design improvements and hopefully better machining will get me closer to my goal with Drag Turbine 4.
I tried adding a sleeve between the ball bearings as shown in the following drawing. The sleeve was reamed to a very close sliding fit with the rotor shaft. Even the slightest bending of the rotor shaft would cause contact with the sleeve. The sleeve diameter of ¼” is twice the diameter of the rotor shaft so the effective moment of inertia of the rotor and sleeve increases by a factor of 16. I thought that this would stop the shaft from deflecting without increasing the bearing loads. The ball bearings were starting to make noise indicating they needed lubrication, so I added a drop of Krytox GPL102 oil to each ball bearing. This oil has a higher viscosity than the Aeroshell 12 oil that comes with the ball bearings but is the best replacement I have found. The maximum speed with a total shim washer thickness of 0.007” and a GWS EP 2.5x0.8 propeller was 15,500 rpm with or without the sleeve. The extra stiffness added by the sleeve appeared to make no difference. This was a much larger drop in speed from the 18,500 rpm that I was able to reach before I heard the bearing noise than oil viscosity would cause. I don’t know if the axial load the ball bearings get with the flow channel on only one side has shortened the life and the ball bearings are worn out, but the noise is gone.
I tried running Drag Turbine 3 with the flangeless ball bearing between the existing ball bearings as shown in the following drawing and using a GWS EP 2.5x0.8 propeller. With a total shim thickness of 0.007” the maximum speed was 14,000 rpm and the pressure was 4.0 psig. With the third ball bearing added, there was slight contact of the rotor with the cover plate with the 0.007” total shim thickness. With a total shim thickness of 0.006” there was no contact of the rotor with the cover plate or housing and the maximum speed was 13,500 rpm and the pressure was 3 psig. The performance was about the same for the 0.007” and 0.006” total shim thicknesses. Moving the rotor 0.001” further away from the cover plate eliminated the contact but dropped the pressure 1 psig. When I removed the third bearing, the maximum speed with 0.006” total shim thickness was 15,000 rpm and the pressure was 3.3 psig. With a total shim thickness of 0.007” the speed was 18,500 rpm and the pressure was 4.0 psig. Adding the third bearing reduced the performance. Apparently any improvement for reducing deflection was more than offset with the extra friction caused by the increased bearing load.
I looked at the friction torque of the three ball bearings that will fit in my existing Drag Turbine 3 housing. Dynaroll is the supplier of the ball bearings and gave the guidelines shown below for the resistance torque. The 1/8” shaft size ball bearing has a total of 6, 1/16” diameter balls. The 5/32” and 3/16” shaft sizes ball bearings have a total of 10, 0.039” diameter balls. The load on the ball bearing closest to the rotor is the largest and is primarily axial with bending moment. The smallest shaft size has fewer but larger diameter balls so the reduction in torque for a smaller number of balls is offset by the larger ball size. Likewise, the larger shaft sizes have more but smaller diameter balls so the reduction in torque for smaller diameter balls is offset by the larger quantity. The pitch diameter of the balls in the bearing with the 1/8” diameter shaft is 0.227” and for the bearings with the larger shafts is 0.253” so that is not much of a factor. It appears that the resistance torque of the ball bearings will be about the same regardless of shaft size. The resistance torque of ball bearings is normally considered insignificant, but I found in the testing of my other turbines it can affect performance. I was able to gain an additional 0.8 watt of power just by balancing the rotor as discussed in the post of 30/05/2020 Link. The following was copied from that post.
"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".
Edited By Turbine Guy on 23/03/2021 14:42:58
I looked at what you suggested. The following drawing shows the way I think you intended to move the bearing. This would be the best solution but would require a new cover plate, since a ball bearing won't fit in the existing cover plate. I ordered the flangeless ball bearing and it is supposed to arrive tomorrow. I'm going to try using it between the existing bearings first since it doesn't require a change to any of the existing parts.
Thanks for the feedback,
Thank you for your very kind remarks. We all like encouragement, and comments like yours give us the incentive to try new things.
The following drawing is a section view of Drag Turbine 3 showing the dimensions needed for evaluating deflection. The diameter of the shaft (D) and the spacing between bearings (L) are two things that can be changed on this turbine without changing the cover plate or housing. The deflections change in direct proportion to the change of the moment of inertia (I) and for shafts I = πD^4/64. Three ball bearings of the type used in Drag Turbine 3 are available that will fit in the existing bearing bore and have the following nominal shaft sizes 1/8”, 5/32”, and 3/16”. Since the moment of inertia varies with the fourth power of shaft diameter, changing from the 1/8” shaft to a 5/32” shaft would increase the moment of inertia by a factor of 2.44. Likewise, changing from the 1/8” shaft to a 3/16” shaft would increase the moment of inertia by a factor of 5.06. Changing the shaft size will require modifying the existing rotor but could find what shaft size works best for this turbine. The downside of increasing the shaft size is the torque required to turn the ball bearings goes up with increased shaft sizes. The spacing between the bearings (L) could be changed by inserting a ball bearing without the flange in between the two existing ball bearings. The reduction in deflection is directly proportional to L, so placing the added ball bearing right next to the ball bearing closest to the rotor could reduce the deflection considerably. The downside to shortening the distance between bearing centers is that the load on the bearings increases in direct proportion to the reduction in center spacing. Having the extra ball bearing may result in the stiffest arrangement being with the center bearing moved more to the center.
|Thread: Airbrush Compressors|
This Wobble Piston Link illustrates the operation very well. You hit the nail on the head when you mentioned the Wobble Piston. This appears to be what is used in my airbrush compressor. I appreciate learning more about this concept. I think the link found by Michael that I showed in my last post answered my question about drop in mass flow with higher pressure. After restoring the airbrush compressor the man in the video ran two nozzles. The continuous running pressure for one was approximately 20 psig and for the other was approximately 30 psig. The two nozzle sizes for the set are 0.8mm and 0.3mm. The mass flow for these nozzle sizes and pressures are approximately 1.7 lb/hr and 0.24 lb/hr respectively using my assumed nozzle efficiency.
Thanks all of you for your help,
Thank you for showing the link. The video on This Link confirmed that this type of air brush compressor does use a piston seal described as being plastic and the parts looked like what was shown in my last post. I agree with what Dave pointed out, that the piston is called a link and no connecting rod is shown. If the piston and connecting rod are actually one part and the cylinder does not rock, the piston would need to tilt back and forth as it moves up and down.. There is only one very brief part of the video where the piston is pushed down slightly and it looked to me like the piston pivoted in the cylinder as it moved down. The piston seal would need to be very flexible and there would need to be enough clearance between the piston and cylinder to do this. This would put all the load on the piston seal and help to explain the large amount of seal particles that had to be wiped off.
Thanks for sharing this link,
I copied the following parts list shown in the Master TC-20 manual. It appears this airbrush compressor uses a single piston ring so leakage by the piston could occur and possibly slowing down of the motor with higher pressures. I am hoping someone can confirm that there is a drop in mass flow with increased pressure for this airbrush compressor.
Thanks for your feedback,
I have a Master TC-20 airbrush compressor like shown in the picture below that I use for running my model turbines and steam engines When I started testing my model turbines and engines. I calculated the mass flow as described in Testing Models. Because all the flow was going through nozzles with published efficiencies and air acts like a perfect gas, I trusted these calculated mass flows. I have always assumed that the mass flow was constant for a positive displacement air compressor when using different pressures. I just noticed this compressor listed a flow of 0.8 cfm which I assumed to be at atmospheric pressure (scfm). This volume flow at standard conditions would be a mass flow of approximately 3.6 lb/hr. The mass flow I recently calculated for a 0.030” diameter nozzle throat and a pressure of 23 psig was approximately 1.8 lb/hr. I used a pressure gauge of 0-30 psig with a +/- 0.3 psi accuracy for this last calculation. After seeing this difference in mass flow, I made a nozzle with an 0.080” diameter throat and found that the pressure was approximately 2.0 psig to pass all the flow from my airbrush compressor with it running constantly. The mass flow for this pressure and nozzle throat diameter is estimated to be approximately 3.1 lb/hr which is fairly close to the mass flow shown for atmospheric pressure. This somewhat confirms my method of calculating the mass flow but shows that the mass flow is not constant for this compressor. The only three reasons I could think of for a drop in mass flow with an increase in pressure was leakage by the piston, change in speed, or external leakage. I ran the compressor with all my fittings and a plug in the hose where it would be clamped to the nozzle. I set the regulator to a pressure of 30 psig and then shut of the airbrush compressor. The pressure didn’t drop after waiting several minutes, so I assume no external leaks. I don’t know how much a AC motor like used to drive this compressor varies with load but would appreciate any input on this. I also don’t know what type of piston seal is used in this airbrush compressor but I assume even with a piston ring, there could be some leakage. I would appreciate any feedback about loss in mass flow with increase in pressure for this simple type of airbrush compressor.
Edited By Turbine Guy on 17/03/2021 19:10:55
|Thread: Model Turbines|
I bought a 0-30 psig pressure gauge with increments of 0.5 psi in order to get more accurate readings at the very low pressures. The following picture shows my test setup with the new pressure gauge. The large 3.5” diameter dial and +/- 0.3 psi accuracy will really help. I made the following four pressure measurements. First, I measured the pressure with the cover plate removed and pressed tight against a flat surface. The pressure was approximately 2.5 psig. That is the pressure required to push all the flow through the existing channel size with smooth walls on all sides. The second measurement was with the cover plate bolted tight to the turbine housing with the rotor supported by the ball bearing but pushed tight against the cover plate. The pressure was approximately 9.5 psig. That is the pressure required to push all the flow through the existing channel size with blades and without leakage. The 7 psig increase in pressure was due to the resistance to flow that the blades cause. The third measurement was with the 0.007” total thickness of shim washers keeping the face of the rotor as close to the face of the cover plate as I was able to do without any contact. The pressure was approximately 5.0 to 6.5 psig dependent on the rotor position. That is the pressure required to push all the flow through the existing channel size with blades and with leakage. The flow channel area was sized per Dr. Balje’s guidelines to require 10 psig air pressure with very small leakage. The forth pressure measurement was with everything the same as the third measurement except with the turbine running at a speed of 18,500 rpm. I was able to reach that speed again with the GWS EP 2.5x0.8 propeller. The pressure was approximately 4.0 psig. Dr. Balje’s testing indicated the pressure should stay almost constant for speeds from stalled to design speed for his type of drag turbine. My tests showed approximately a 2.0 psi drop in pressure from average stall pressure. The leakage is the most significant problem needing to be resolved and the drop in pressure with rising speed is the second concern.
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