Model Turbines

The drill I mentioned in the last post is called a quick-change drill. An example is shown below. This type of drill gets it’s name from having the same larger diameter so that one collet can be used for many small drills. What is helpful for drilling nozzles, is the relatively short cantilever length of the drill. Since I use a 0.125 diameter end mill for the inlet hole, the large diameter portion can extend into the milled hole and give the drill additional support. I tried one of these on the last nozzle I drilled and it seemed to work well.

The following drawing shows the dimensions for the Turbine 3 SD base plate and cover plate. This completes the drawings for Turbine 3 SD showing the cleaned up dimensions for the best performing version of my tangential turbines.

I have tried everything I planned to do with the tangential flow impulse turbines so I am going to try designing what Dr. Balje called a drag turbine. His document ‘A Study of High Energy Level, Low Power Output Turbines’ done for the Office of Naval Research in 1957 gives guidelines for designing drag turbines. His guidelines for designing Terry Turbines in this document, were what I used designing my tangential flow impulse turbines. The drag turbine operates on the same principle as a regenerative blower or turbine pump. The following page from the Spencer Vortex Regenerative Blower catalog explains this principle and shows a one-sided version and a two-sided version. I prefer the one-sided version for the reasons they give plus it will be so much easier to make.

The chart shown below and in some earlier posts 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 units for the volume flow are ft3/sec and lb/ft3 for the gas density. Dr. Balje intended this chart be used by determining the volume flow at the turbine exhaust (Q3) and expansion head (Had) of the supply gas. My airbrush compressor can supply a volume flow of 0.011 ft3/sec at a pressure of 10 psig (25 psia). The expansion head for air at a pressure of 25 psia is 14,800 ft-lb/lb. Once the expansion head and exhaust volume flow are found, you choose the rotor OD and speed you would like to run and calculate Ns and Ds. I decided to try a rotor OD of 0.927” and a speed of 25,000 rpm for Drag Turbine 1. With this rotor diameter and speed, Ns = 2 and Ds = 8.1. From the chart, the maximum efficiency of an optimized drag turbine is estimated to be approximately 30%.

To obtain the efficiency shown in the last post, my Drag Turbine 1 must meet certain requirements. I will list some of these requirements below and some in the next post. The requirements that apply to the cover and housing shown in the following drawing are:

The channel diameter/width ratio, D/H needs to be approximately 17.5. The channel diameters ratio, D/d, needs to be between 1.3 to 1.5 The rotor clearance to channel width ratio, h/H, needs to be less than 0.02

Edited By Turbine Guy on 10/12/2020 20:51:15

Edited By Turbine Guy on 10/12/2020 20:52:05

The requirements of the rotor shown in the following drawing to obtain the efficiency shown in the post of 09/12/2020 are:

The rotor pocket depth to cover channel depth ratio, H’/H, should be approximately 1. The angle of the rotor pockets should be 40 degrees. The blade thickness to cover channel depth ratio, s/H should be less than 0.20. The pocket spacing to cover channel depth ratio, l’/H, should be between 1 and 2.

In the last posts I gave the drawings showing the dimensions for Drag Turbine 1. I also gave the requirements that had to be met to achieve an estimated efficiency of 30%. The following is a copy of the requirements and at the end of each requirement the corresponding values of Drag Turbine 1 are shown in parentheses.
The channel diameter/width ratio, D/H needs to be approximately 17.5 (17.5). The channel diameters ratio, D/d, needs to be between 1.3 to 1.5 (1.3). The rotor clearance to channel width ratio, h/H, needs to be less than 0.02 (0.02). The rotor pocket depth to cover channel depth ratio, H’/H, should be approximately 1 (1.0). The angle of the rotor pockets should be 40 degrees (40 degrees). The blade thickness to cover channel depth ratio, s/H should be less than 0.20 (0.17 min 0.62 max) The pocket spacing to cover channel depth ratio, l’/H, should be between 1 and 2 (1.8). In addition to these requirements the ratio of the inlet gas density to the outlet gas density needs to be less than 2. For air at an inlet pressure of 10 psig and exhausting to atmosphere pressure, the ratio is approximately 1.7. Drag Turbine 1 meets all the requirements except for the blade thickness. Dr. Balje’s report assumes a constant blade thickness and has a chart showing the loss of efficiency with s/H ratios greater than 0.2. Since Drag Turbine 1 has varying blade thickness, I make a few assumptions to use the chart. Because the flow enters the bottom of the pockets and flows out the top of the pockets, I assume that the blade thickness on the lower half should be used. The average blade thickness on the lower half is approximately 0.015”. This makes the s/H ratio for the lower half 0.015/0.053 = 0.28. For a s/H ratio of 0.28, Dr. Balje’s chart gives an efficiency correction of 0.95. With this correction, the estimated efficiency of Drag Turbine 1 is approximately 28%.

The estimated efficiency for Drag Turbine 1 is based on what my airbrush compressor is capable of. When I determined that I needed the air pressure to be 10 psig, I ran tests with a nozzle to see what throat size took all the flow at 10 psig. I found that a nozzle bore of 0.047 diameter could pass all the flow my airbrush compressor was capable of at 10 psig. The mass flow for 10 psig inlet pressure, atmospheric exhaust, and a nozzle efficiency of 95% is estimated to be 3.0 lb/hr. The enthalpy drop for these pressures, assuming isothermal flow, is approximately 19 btu/lb. For this mass flow and enthalpy drop the input energy of the air is approximately 16.6 watts. With the estimated efficiency of 28% given in the last post, the estimated power at 25,000 rpm is 4.6 watts. The maximum power of any of my turbines or steam engines running on my airbrush compressor was the 4.7 watts my Chiltern steam engine was able to produce Testing Models. The Chiltern is a very nice engine with a fully supported crankshaft, piston valve, and Teflon piston rings. If Drag Turbine 1 can be built and is able to reach the estimated efficiency, it would be pretty impressive performance for a small turbine running at only 25,000 rpm. Probably the biggest advantage of the drag turbine over the impulse turbines running on very low input energies, is the effect of Reynolds number. The Reynolds number tends to drop dramatically with very low input energies. This causes a corresponding loss in efficiency for impulse turbines but has almost no effect on drag turbines. The drag turbines were given their name since they relied on increasing the drag force on the rotor to much higher levels than the drag force on the flow channel. Even though the increased drag on the rotor is caused by the dynamic effects of the spiral flow, it is treated like a viscous drag in the analysis.

The following photo is a mockup of the best setup I have been able to come up with to make the pockets in the rotor of Drag Turbine 1. The rotor used in the mockup is approximately the size of the drag rotor. The keyway mill is the only one I have and is much bigger than the size needed. Also, the indexer is just set at the approximate angle. I used this mockup to determine If I had room to get everything in position. A 45 degree angle bracket shown in the next post is available and could be used for mounting the indexer to the carriage mounting plate. This bracket could be used instead of the 40 degree optimum angle with just a slight loss in drag coefficient.

The following is a copy of the bracket I mentioned in the last post. This has a hole spacing that I think will fit in the space available. I placed an order for this set of brackets since I want to make sure they will work before purchasing the expensive keyway cutter needed to make the pockets in the drag rotor.

Byron,

Your having a go at a drag turbine is very commendable and I wish you success. But instead of machining the rotor it would be much simpler to have it lost wax cast by shapeways. It really does not cost that much!

Hi Werner,

You make a good point. The pockets in the drag rotor are much easier to cast than the blades in your rotors and are more accessible for cleanup, so this probably would be good to try. By casting the rotor, the thickness at the entrance to the pockets could be made the same all the way across and that would help the flow. I'll take a look at designing for casting both the rotor and housing. Thanks for the idea.

Hope your projects are going well,

Byron

I designed a wax cast rotor that would work in place of the machined rotor in Drag Turbine 1. I added a couple of what I think are improvements. I added a boss on the rotor to replace the spacer in Drag Turbine 1. This extends the grip on the shaft and would allow a pin to be inserted through the rotor and rotor shaft. I also extended the shaft so that it passes through the rotor. This would allow balancing the rotor using Werner’s method described in the post of 29/11/2020. I made a 3D model of the rotor adding an allowance for machining the critical surfaces to their final dimension. I then scaled up the solid model 2% to allow for shrinkage. The following drawing of the solid model was made to show enough dimensions to confirm the scale. I went to the Shapeways Website and followed their instructions for getting a quote. The instructions were very straightforward except they always call it 3D printing even if you select materials like the bronze I requested, that is only available with wax casting. To get a quote you need a 3D solid part file. I made a step file for my drag rotor and dragged and dropped it where they requested. Their program responded that the file was very complex and would take a little time to analyze. After a short time, their program showed a 3D model that looked correct, but their assumed scale was wrong. I used the dimensions in the following drawing to correct the scale. The only thing left to do was select the material. After selecting the material, the price, expected ship date, and shipping cost were shown. My rotor cost $63.46 including shipping and has an estimated ship date of 13/01/2021. I thought the cost was reasonable for a quantity of 1, so I placed an order. The keyway cutter I required for making the machined rotor cost $52. Werner was right, this is a better way to go.

Edited By Turbine Guy on 22/12/2020 19:49:25

The following drawing shows the dimensions of the cast drag rotor after machining. This drawing assumes the casting will clean up to these dimensions. These dimensions assume the minimum amount I think will have to be taken off. There is quite a bit of additional allowance for cleanup if needed. The changes from Drag Rotor 1 are reducing the shaft diameter from 5/32” to 1/8” and adding a boss to extend the contact surface of the cast rotor to the shaft. The extra contact length also provides enough room to add a pin through the rotor and shaft. The change in shaft diameter was made since the ball bearings for the smaller diameter had higher load capacity and a higher allowable speed. Also, shims were available in 0.001” thickness where 0.004” was the minimum thickness for the larger shaft. The drag turbine requires the minimum clearance of the rotor and housing to get the best performance and the 0.001” shims will be very helpful in lining up the rotor with small clearances. The cast rotor allowed the pockets to have a full radius on each side, whereas the shank of the keyway cutter limited the depth of cut. Also, the blades of the casting have the same thickness from the bottom to the top of the pocket.

Edited By Turbine Guy on 24/12/2020 15:10:22

The following drawing is the assembly with the cast rotor, cast housing, and cast cover. After seeing the cost of the cast rotor, I thought I would look at using castings for the cover and housing. If Shapeways can cast them and the cost is reasonable, I will probably place an order. The castings save a lot of machining and simplify adding fillets and rounding surfaces which reduces the mass and improves the appearance.

Merry Christmas,

Byron

I requested a quote for casting the housing and cover plate shown in the last post. The cost was higher than I thought it would be, so I asked Shapeways for some guidelines to reduce the cost. This is the link to the guidelines they sent me Shapeway Guidelines. They also said bronze models are first printed in wax and then cast in bronze. After reading their guidelines, I found that I needed to reduce the volume and overall length of the castings as much as possible. I got quotes on a few changes to the hexagonal shape and then tried reducing the volume even further by changing the shape to what is shown in the following drawing. My goal was to get the cost for casting the rotor, housing, and cover plate down to around $200. The castings for the parts shown in the following drawing cost a total of $229 including taxes and shipping. This was close enough to my goal, so I placed an order for the housing and cover plate that are expected to be shipped 21/01/2020.

While working on the cast housing and cover plate, I asked Shapeways what the minimum allowable thickness was and was given this link Shapeways Materials. Since I had already looked at the materials, I took a closer look and found there was a separate tab called ‘Design Guidelines’. This section gave the minimum thickness as 0.6mm for supported or unsupported sections with a natural finish. Since the minimum thickness of the blades of Drag Rotor 2 is approximately .01” (.3mm), I was concerned that the rotor would not be able to be cast as designed even though it passed the initial review by Shapeways. I mentioned what I had discovered to Shapeways and Mitchell Jetten of Shapeways Customer Service sent me the following response.

“The minimum acceptable wallthickness for this material is 0.6mm. For details, parts that stick out, you should be able to go down to as low as 0.35mm thick and 0.35mm long. If the detail sticks out more than 0.35mm this will be classified as wall instead and would require to be 0.6mm thick.
The main reason we have this guidelines is because we need to actually be able to 3D print the wax model, clean this and create a negative mold around this before we are able to cast the model in brass. Since the wax material is really fragile, going below this guideline greatly increases the risk of parts breaking or warping.
On our materials page by clicking on the Design Guidelines tab you can read more about this:

That said, our checking process is manual, it does happen from time to time that a model that is below the design guidelines gets approved. In your case the model was successfully printed and cast, however it often also happens that a model gets accidentally accepted and then gets printed or cast but fails at either of these stages, which will result in the model being rejected".

After receiving this response I was notified by Shapeways that Drag Rotor 2 was cast and cleaned up successfully and has been shipped. I thought I would share Mitchell's explanation of the importance of the minimum thickness, The reason the blades of Drag Rotor 2 were able to be cast was probably because their unsupported length is so short.

Edited By Turbine Guy on 03/01/2021 20:11:36

That is really useful feedback on Shapeways. Their customer support & turn-around time are impressive, especially over the feastive season. I am intending to try their 3d printing in aluminium shortly, based on the input from Werner & yourself.

Dave

Its amazing they work but I just love it.

The 3D Printing is wonderful too.

Keep up the good work Turbine Guy.

Thanks for the responses Dave and Roger. I see a lot of possibilities for having parts made by Shapeways. It's interesting the different ways they can make the parts, like binder jetting, wax casting, and selective laser melting. Also the different materials they have available for the parts. I am also very pleased with the way they respond to questions and how quick they can make a quote.

Happy New Year

Byron