Model Turbines

I designed Drag Rotor 3 following all the guidelines from Shapeways. The two guidelines I missed on Drag Rotor 2 were keeping the minimum thickness greater than 0.6mm minimum for bronze and having a sharp edge on a thin section. The blades on Drag Rotor 3 are 0.6mm thick and have a round end. The following picture shows a 3D view of the blades. The drawing for this rotor will be shown in the next post. I should point out that Shapeways minimum thickness is not the same for all materials so should be checked. I may try this rotor if I have a problem with Drag Rotor 2 since it will work with the cast housing and cast cover plate of Drag Turbine 3.

This is the drawing for Drag Rotor 3. I had to make the pockets deeper and reduce the number of blades to open up the flow area with the extra blade thickness.  

Edited By Turbine Guy on 06/01/2021 14:01:08

Am I correct in assuming that the use of the Spencer one sided blower principle to a turbine rotor would result in a bidirectional turbine while your adaptation is uni-directional. Would be a big disadvantage for model applications!

Werner

Hi Werner,

If the Spencer blowers use 90 degree blades they would be truly bidirectional. You are right that using the 40 degree blades makes the turbine almost unidirectional. My design would have some torque running in the opposite directions if the flow in the ports is reversed. You bring up a very interesting point about running in both directions, I have not looked at that. Using the 90 degree blades would make the drag turbine truly reversible but quite a bit less efficient. The maximum efficiencies found for the tests of 90 degree blades and 40 degree blades in 'A STUDY OF HIGH ENERGY LEVEL, LOW POWER OUTPUT TURBINES' by Dr. Balje are 20% and 33% respectively. These were from actual tests so should give an indication of the difference in efficiency. We have both seen that things don't always work out the same for miniature steam models so there might not be this much difference. I may try a rotor with 90 degree blades and see how it does.

Thanks for bringing this up,

Byron

Edited By Turbine Guy on 06/01/2021 15:30:55

My casting for Drag Rotor 2 arrived and looks very good. The following photo shows it sitting on the bag it was shipped in. Putting it in a jewelry bag was a nice touch. The measurement around the OD and across the face of the rotor varied by approximately +/- 0.002" (0.05mm). The actual OD after casting averaged approximately 1.026”. This OD on the 3D model used to make the casting, shown on the drawing in the post of 22/12/2020, is 1.04”. From Shapeways Bronze Guidelines the average shrinkage is given as 2.5%. For a after casting OD of 1.026” and a shrinkage of 2.5%, the mold ID would be approximately 1.05” before casting. This is approximately 0.01” larger than the 3D model, so the accuracy of the mold should also be considered. The machining allowance I assumed for Drag Rotor 2 was for the casting to be +/- 0.020" and is probably about right for this size of casting.

Hey Werner, what model needs a shaft speed of 25000rpm? Almost everything, other than a noise maker, will need a reduction gearbox and that means direction control is possible.

The main problem all small models have is efficiency, so optimising for power is a good start.

Hi Roger,

You are right of course, but then you will have problems with remote controlling the reversing gear because of unsynchronised tooth positions. In addition it will not be possible to dynamically brake the model with counter steam. I built such a gauge 1 turbine loco drive many years ago - that's why I know.

Hi Roger and Werner,

If the drag turbine works well enough to demonstrate the efficiency estimated by Dr. Balje can be approached with my tiny model, that would indicate that an efficiency in the range of 15% to 20% could be obtained with 90 degree blades. The best efficiency I have been able to obtain from my tangential and axial impulse turbines using my airbrush compressor and smallest propeller is approximately 14% as shown in the table below. If an efficiency of over 15% can be obtained with 90 degree blades, it would be higher than I have been able to obtain up to date and have full power in both directions. This would be quite a bit less than the 28% efficiency I hope to get with the 40 degree blades but would still be a major improvement and I think worth testing.

Thanks for the feedback,

Byron

In the post of 10/01/2021 I discussed how the shrinkage on the casting for Drag Rotor 2 did not seem to match Shapeways average of 2.5% for bonze. After discussing this with a Shapeways customer service representative, I found that the shrinkage is not an exact amount. It varies with the size and shape of the casting and can be different for some locations in the casting. For my size and type of casting he suggested 2% shrinkage would be a better estimate. This is about what the shrinkage was on the OD, the longest dimension of this casting. If the shrinkage is critical in your design it might be a good ideal to discuss this with a Shapeways customer service representative.

I finished Drag Rotor 2 as shown in the following photo. Machining the rotor would have been very easy if I had done all the machining with the cast rotor not taken out of the chuck until all the critical machining was finished. Holding the cast rotor by the round part that extended from the back face of the rotor allowed enough room to machine both faces and the OD to their final dimension after reaming the bore. Because I got what I thought would be a strong enough press fit, I decided to cut off the excess length of the extension and then press fit the rotor to the shaft for the final machining. My hacksaw slipped while cutting off the extension and I ended up with not enough length of the hub to run a pin through. I had to rely on the press fit of the rotor without most of the hub. This gave enough strength to machine the front and back face of the rotor but the wave in the OD caused enough force to break the press fit. The holes added were for pins I used to go through the rotor and into a clamping collar that gave me enough grip to turn the OD. I then used part of the cut off extension that had the reamed hole to make a collar the right length to get the hub back to it’s intended length. I ended up with rotor and shaft machined to the dimensions I needed but could have saved myself a lot of work. The cast housing and end plate have been shipped from Shapeways and will arrive soon. Hopefully, I will do a better job with them.

Edited By Turbine Guy on 17/01/2021 21:14:29

The castings for the Drag Turbine 3 Housing and Cover arrived and are shown in the following photos. They came in jewelry bags like the rotor casting. Before setting these up for machining, I made a few measurements to check the shrinkage. The drawing showing these dimensions will added in the next post. Overall, the castings looked very good, and when the cover is placed on the housing all the holes and edges line up nicely. I opted for a natural finish, so the cleanup might have polished some of the surfaces.

This is the drawing mentioned in the previous post. The upper dimension is the size that would be obtained by a uniform 2% shrinkage. The lower dimension in parenthesis is the actual measured dimension. Each actual measured dimension was taken in several places and the average was used. This should give a reasonable indication of what to expect assuming 2% shrinkage with these types and sizes of bronze castings. As can be seen in the drawing the actual shrinkage varies. The worst deviation was on the wide thin portion, the OD of the housing and cover flanges. These dimensions were from 0.009” to 0.011” larger than they would be with a perfect mold and uniform 2% shrinkage. All the other measured dimensions were within +/- 0.003”, which is excellent in my opinion.

After reading about the accuracy of drill chucks I decided to see if I could get a collet holder (chuck) for my Unimat 3. I couldn't find any source in the USA so I made the following thread Unimat 3 Collets. Several people helped me in my decision of what to purchase and I ordered a set of collets that included a collet holder from ebay UK. Anyone with a Unimat 3 and thinking about getting collets may find this thread useful. Since this set is coming from the UK, it will be a few weeks before I can start the machining on the Drag Turbine 3 Housing and Cover. To get the maximum performance with the Drag Turbine, the clearances must be made as small as possible. I thought using collets could help with the reaming of the bearing bores and with centering the cast parts in the 4 jaw chuck.

While waiting for my collet set to arrive, I made the following changes to Drag Rotor 3 shown in the drawing below. I cleaned up my method of forming the blades to utilize the options I have with castings. Instead of modifying the solid model made for machining the rotor with a keyway cutter, I designed the blades from the start for casting. I also changed the end of the blades from 0.3mm radius to a 0.18mm radius with a 1mm radius lead in edge. As you can see from the drawing, the 0.6mm thickness must be used for most of the length of the blade. Shapeways allows a minimum thickness of 0.35mm providing the length is no longer than 0.35mm. I was also able to reduce the maximum blade depth H’ to its optimum value. This allowed me to increase the number of blades. These changes should help if I decide to try another rotor.

The following drawing shows the advantage of the smaller radius at the tip of the blade when cleanup of the casting is required. When the face of the rotor is machined, a flat is formed at the end of the blades. The more the face is machined, the wider the flat becomes. The drawing dimensions are in millimeters and the blades are shown as if they were running in a straight line. The top set of blades is with the 0.3mm radius I originally planned to use on Drag Rotor 3 as shown in the post of 06/01/2021. The bottom set of blades is what is shown on the drawing of the preceding post. I show the amount being taken off the face in 0.05mm increments as the blades proceed from left to right. The width of the flat on the existing Drag Rotor 2 is 0.38mm so only a little over 0.05mm can be taken off the 0.3 radius without exceeding this. Up to 0.15mm can be taken off the 0.15 radius without exceeding the flat width of the existing blades. The efficiency of the drag turbine is reduced significantly if the flat gets too wide so the minimum amount of cleanup of the face of the rotor should be used.

I read through the section on Drag Turbines again in ‘A Study of High Energy Level, Low Power Output Turbines’ done for the Office of Naval Research in 1957. This study concluded that the increased drag on the rotor was due to the corkscrew rotation of the flow into the bottom of the blades and out of the top of the blades into the flow channel as discussed in the 09/12/2020 post in this thread. This study also stated that the drag coefficients were independent of the rotor speed that had been confirmed with tests of the stall torque. I designed Drag Turbine 3 based on the concept used by Spencer in their Vortex Regenerative Blowers as shown in the 09/12/2020 post and below. I thought that the Spencer concept would increase the rotation of the gas due to the flow being guided back into the flow channel by a radius rather than being discharged onto a flat surface like shown for the two-sided impeller. The two-sided impeller was the type used in the study and the drag coefficients were developed for. If the corkscrew rotation of the flow is what causes the higher rotor drag coefficient and the rotor drag coefficient is independent of rotor speed, the corkscrew flow must be established even with the rotor stationary. Comparing the one-sided and two-sided impellers, I can see that any flow from the stationary channel that enters the two-sided impeller is guided towards the OD of the impeller. This would start the flow moving in a corkscrew direction even with the impeller stationary. For the one-sided impeller, any flow from the stationary channel that enters the impeller is guided toward the center of the blades when the impeller is stationary. I assumed that the centrifugal force would start the flow moving out the outer part of the blade when the rotor got to the higher speeds and provide the necessary circulation. In the next post I will show a source with test results for these two concepts.

The following figures and table are copied from ‘A modified theory for the flow mechanism in regenerative flow pump’ by J. W. Song, A Engeda, and M. K. Chung from the proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 2003 found at this Link. The best efficiency for the radial blades like used in Dr. Balje’s reports is shown in Fig. 8 Test 2. The best efficiency for the semi-circle blade like my Drag Turbine 3, is shown in Fig.10. Table 1 gives the dimensions of these pumps. Rtip is the radius to the tip of the blade or channel, Rhub is the radius to the hub of the blade or channel, Ac is the area of flow channels, Bb, is the effective width of the blade channel, β is the blade angle, Zb is the number of blades, and C is the clearance. Q/Qs is the ratio of the flow to the solid rotation flow. You can see from the figures that the peak efficiency of the semi-circle blades is slightly higher, but the estimated performance of the radial blades is much closer to the test values.

Edited By Turbine Guy on 10/02/2021 18:47:16

The collet set I ordered finally arrived. The runout of the collet holder was so bad I had to request a refund. I got a full refund from the seller and found another Ebay seller located in the USA that had a collet set with a genuine EMCO collet holder. I ordered that collet set and it should arrive within a week. When it arrives, I will finally be able to start the machining on the Drag Turbine 3 housing.

In the meantime, I found another article ‘A STUDY OF THE PERIPHERAL COMPRESSOR’ by Phillip Cates issued August 26, 1966 at this link Peripheral Compressor. Since this study deals with gases rather than liquids, it might be a better example of what to expect with semi-circular blades. The following figures copied from page 4 of this study shows the type of blading that was reviewed. Page 37 described what was found from the analysis and testing of a pump with the type of blades shown in Figure 2(b). The maximum overall efficiency for this type of blading was given as 46% when tested with a liquid. Page 111 showed the dimensions for a contoured blading that gave the best efficiency of any of the long blading shown in Figure 2(a). The maximum efficiency was given as 41% from testing with air. Tests with pumps typically show higher efficiency than tests using gases, but the half-torus blading appears to be quite effective.

The attached figure shows the contoured blading that gave the best efficiency of the long blading discussed in the last post. The tests with the Chevron blading (Figure 2(e) in the last post) showed a maximum efficiency of 43% with blade angles of 45 degree. The maximum efficiency for 90 degree blades from these same tests was 40%. This confirmed Balje’s findings that the optimum efficiency was with blade angles from 40 to 45 degree, but had much less difference in efficiency with 90 degree blades as shown in the following table.

The following picture shows the changes to the blades of Drag Rotor 3 in revison 2. I am trying to optimize Drag Rotor 3 based on everything I learn before making a casting. The following changes were made to revision 1. I made the slope from the bottom of the blades to the top of the blades similar to the contoured blades in the last post. The intent of this was to bias the flow direction toward the top of the blades to help start the circulation. I also recessed the front edge of the blades 0.010” back from the front of the rotor so that the major portion of the front could be cleaned up before any contact with the blades. I added a chamfer at the outer edge of the back face of the rotor to give a clearance for the point of the cutting tool when turning the housing bore. I shorted the length of the boss used for adding the metal during casting to reduce the amount of material required. All these changes were made to hopefully improve the performance, make the machining easier, and reduce the cost. Unless I find any further improvements after finishing the machining on the existing castings, this is what I plan to order next from Shapeways.