NEW low cost 4 fan KidWind wind tunnel

 

This NEW wind tunnel design has the same 48” by 48” opening and it powered by 4 ValuTek 3-Speed Floor Fans 20” from FarmTek for $303.80 plus tax and shipping. The 4 mil vinyl can be purchased (4’ wide 25’ role for $30) at Ace hardware and is held to the 1’ PVC pipe by 1” black hook and loop. The front and back frames stay together with the 3 Way PVC 1” elbow corner Side Outlet Tee fittings from Amazon for $21.  Just the four pipes connecting the front and back frames are removed for storage against the main fan holding section. The storage footprint for this tunnel is 12” X 48” X 48”. It takes less than 5 minutes to set up or take down. 

And here is the wind tunnel loaded for transport or storage storage in your classroom. 

This is a project that can be made in a day and requires only common tools and machines to cut the pieces.

Lets begin…

The first step is to cut the 4’ X 8’ sheet of 1/2” 3 ply plywood in half. This is best done by using a straight edge to guide the saw and get a straight and accurate cut. Use of proper PPE will be important thought out this project. 

This next step might seem a bit odd but you will come to see that saving and taping the four boxes that the fans were shipped in will be a good idea. Be sure to go all the way around the four boxes and make this a strong stable work surface.

Now we all know that the layers of plywood have the grain running at right angles to each other. This gives the plywood its strength in both directions, To maximize this take the half sheet you just cut and turn it 90 degrees like shown in the picture. Doing this will also make sure that there is a factory cut edge on all four sides.

Clamp the two halves together and even up the edges. Then use some screws to temporarily hold the halves together. Keep the screws close to the edges. Now use a straight edge to draw lines from each of the corners. 

Now might be a good time to look over and study the plan drawing for this project. The centers of the fan holes are measured out on the diagonal lines you have just drawn. The location and size of these four holes is the most critical part of this project because the clearances are less than 1/4”.

Just as a double check of my measurements of 1’ - 5 1/2” on the diagonal I also used a framing square to measure up and over from the edges of the plywood and verify the center locations of the four fan holes. I hope you have also noticed that I have now placed the two pieces of plywood on top of the four shipping boxes that the fans came in.

You are going to use a large dividers or make one out of a stick with a screw in one end and a hole for a pencil EXACTLY 10  - 3/4” away from the screw for the RADIUS of the fan holes to be cut. Drawing the circles for the holes is a must. They must be just right and the diameter MUST be 21 - 1/2” ! Ok you are going to have a choice here as to how you cut out the holes. A jigsaw will work but I prefer a Roto zip with a radius guide. Either way you will be cutting through the two pieces of plywood and the cardboard top of the shipping boxes below (not the table). 

OK one down and three to go! See how the boxes help out?

Now you are going to want to check your work and now is the time to do it. If the hole is to big and the fan drops through you are going to change that for the next three. Making the hole smaller will require some added material. On the other hand if the hole is to small you will have to make it bigger and that take some hand work you will not want to repeat on the next three.

Did I mention that when you get the fans you won’t have to assemble the stand frame and will have to remove the hardware that the stand frame bolts to so that only the rim of the fan wire grills is left. This rim stops the fan from going through the holes you are cutting in the plywood.

Four holes cut and looking good sitting on top of the cardboard box working surface.

The fans have two sides. The air flows in one side and out the other. You will want to get it right because in a wind tunnel the air is drawn into the tunnel and then hits the turbine blades that are being tested. This is different than when you use a single 20” box fan in your classroom. You have a fifty fifty chance of getting this right. Don’t blow it. For these fans the air comes in on the motor side. Now if you haven’t figured it out yet the fans are going to be “sandwiched” between the two pieces of plywood you cut. In the picture the fans are laying on the first player of plywood. The 1/4” thick pieces you see are spacers that equal the thickness of the rim of the fans grill.

There are also some little tabs along the edge of the grill and some wood will need to be removed so the 1/4” thick spacer fits flat to the plywood. I used a 1” forstner bit to drill this semi-circle of wood out.

The fans are pretty heavy and the two layers of plywood do a good job holding them. But just for a little extra I put a small cross of the 1/4” material in the center of the fans. All this spacer material should be tacked in place with short nails or screws so they don’t move when the top piece of plywood is added.

So now the top layer has been added and the fans are locked in place. Now all you have to do is fasten them together, right?

Not so fast. You need to add a rim of 1 by 2  (actual size is 3/4” by 1 - 1/2”)  to both sides of the “sandwich”. This will give it strength and thickness for longer screws. 

Now the the box ides was pretty good, right? Well then you are going to like this for the 1 X 2 pieces. If you look ate the plans carefully you can see how each of the 8 pieces are the same length.

This save some extra measuring. But the lapping at the corners is reversed from one side to the other. This gives the joint more strength. Trust me on this.

You are going to use 2 - 1/2” GRK screws to put the “sandwich” together. You will clamp the 5 layers that make up the “sandwich” together and then drill pilot holes for the screws. Yes I know that the screws are self drilling but again trust me when I tell you drilling pilot holes will make for a better job. Also you will drill the pilot hole ALL THE WAY THROUGH the 3” of material. This will leave a small hole on the other side so when you screw that side you can go in between the screws of the first side.

IF you did all the steps correctly you have a four fan “sandwich” like this. Congratulations!

Again on the plans PVC cutting list it calls for 2 - 30” support legs. Screw them into the back side 1 X 2  rim pieces (drill some pilot holes for these screws) and you will have a nice stand for the fans while you finish the work and when you set up you wind tunnel in the future..

CABLE MANAGEMENT - A big deal when you are moving a tunnel like this around and setting it up. Lets see how we do it…

First we are going to go 2” in on the diagonal lines we drew. The location for each 1 - 1/4” hole is shown on the plan drawing.

Use a 1 - 1/4” hole saw to cut the hole to run each fans plug through.

Whoa that is a big hole and will leak a lot of air. We need to seal the hole around the plug wire.

Remember those 8 big circles of plywood you cut out earlier. Well here is a small use for one of them. Use a 2 - 1/2” hole saw and cut four discs out. Then saw a 1/4” slot from the edge to the center hole in each of the disc’s. This is for the wire to slide into.

And here is how it works. Put a small screw in to hold the disc in place and you have the holes covered.

You are looking into the tunnel and this is what you will see when you put a turbine in to the tunnel for testing. The 3 speed control switches and fan motors are all here and ready to go. Now if you only had the testing tunnel part made. So let’s get on it.

From the PVC cutting list on the plans you have cut the eight 45” long pieces that will form the front and back frames of the tunnel. 3 way Tees will be attached the the end of 4 of them. Lay the assembly on a flat surface to make sure the Tee’s line up. 

Then you can add the other two pieces. Be sure to hammer everything together with a rubber hammer to make sure the pipe’s are all the way into the Tee’s. Then put a 3/4” lath screw into each pipe and Tee. These frames are never taken apart.

Now the four shorter 43 - 1/2” pipes can be add to set the distance from front to back of the tunnel. This distance needs to be less than the 48” so the 48” vinyl can be wrapped around the pipe and attach to the black hook and loop material. One full length piece of hook and loop material is needed on each of the bottom sides of the tunnel.

This is the part where you secure a piece of 20ga annealed stove pipe to the PVC frame in four places so that the tunnel can be attached to the fans. The 5/32 X 7/8” fender washers are held inside of the frame with screws and the wire is twisted around the screw.

Two #10  - 1” sheet metal screws are put into the back frame pieces for the wire from the tunnel to be fastened to. Use a figure 8 motion to secure the wire and it will hold better and be easier to remove when taking the tunnel down for storage.

Here you can see how the rubber hammer is used to to make sure the joints are together. You can also see the strip of hook and loop that will be on the bottom edge of each side of the tunnel.

A word about attaching the 4mil vinyl. The hook and loop material has an adhesive backing. I had pretty good success if I put a the hook and loop material together. Then I adhered the loop side to the PVC frame where I wanted it. Then I started in the top center of the vinyl and pulled off the tape so I could stick the vinyl to that piece. Then I worked from the center out doing the same thing. Removing the tape from the hook and loop stuck to the PVC and then sticking the vinyl to it.

Another ting to do is to place a tape marking the TOP FRONT cross piece frame and another marking the TOP BACK cross piece of the frame. Remember these frames a never taken apart. Only the shorter pipes connecting them together come out. Taking the pipes apart is best done using the rubber hammer.

As a little bonus I cut two of the scrap plywood discs and made some plates that can be set on the top of the tunnel. The back one holds the power strip the the fans plug into and had a switch that can be easily reaches to start and stop the fans. The front piece is where you can set the Lab Quest data collection equipment.

So there you have it. A KidWind wind tunnel that will get you wind speeds of 4 m/s or less for less and $500. If you have questions or comments post them on the blog and I will try to help you out.

Dick Anderson

Wisconsin KidWind Advisor

 

 

 

 

  

  

Roll 99 for the Duct Tape collection

 

Steve and Sandy K. Are world travelers and here is the list of countries they have visited and worked in. Each time they have worked tirelessly to seek out a roll of Duct Tape to add to the World Duct Tape collection. Each roll has a story and if you want to read about them check out my blog at ducttape2thefuture.blogspot 

This time Steve and Sandy were on a fun educational trip to learn about Mongolia. They flew to South Korea first. This gave Steve the opportunity to seek out a roll for the collection. Then they went on to Mongolia for a two week stay with a group of 10 like minded people to experience life there living in a yert! Getting a roll of duct tape was difficult until they were taken to the “Black Market”. This was sort of a very large area where many vendor gathered to sell and trade just about everything you can think of.

So Steve adds roll number 99 to the collection. Only 101 more rolls to go. I will be working of getting some national attention about the World Duct Tape collection in the near future . Keep watching.

 

The things I discover along the way

 KidWind is where a team of students put their minds and skills together to design, build and test a model wind turbine to see how many Joules of electrical energy it will produce in 30 seconds while running in a 4 m/s (8 mph) wind velocity. This is an example of some of the thinking that goes on during the process. 

Let’Look at the chart above showing the testing data collected.  

Here are some things that change when you adjust the resistance load on a generator.

- As the ohms of load in (black numbers) went up the generator rpm in (gold bars) went up. Why?

    The resistance is placed in parallel with the armature windings. This results in a lower resistance in the circuit. This allows the armature to turn faster in the same wind speed.

- As the generator rpm in (gold bars) went up the voltage on (green line) went up. Why?

    When the armature turns faster the wires cut through the magnetic lines of force of the magnets faster and this causes the voltage to increase.

- As the Ohms in (black numbers) went up the Amps on (red line) went down. Why?

    The Ohm Load is in parallel with the armature wire where the amps are being created. In a parallel circuit the amperage is divided over each resistance. Some going to the Ohm Load and some coming through the armature wire. There is a bit more to this that needs to be explained.  

- Most watts on top (blue line) was produced where the rpm in (gold bars) crossed the Amps on (red line) 

    Why? I am going to call this crossing point the “Goldie Locks Spot” Not to hot. Not to cold. Just right! Not to much resistance for the flow of electricity (electrons) through the generator armature wire turning through a magnetic field at this speed. Which is caused by this much wind velocity. 

So let’s look at what we know:  

Generator - 40 watt 12 volt dc

Generator Resistance - 6 ohm

Number of blades - 16

Blade material - 20 gauge galvanized steel

Blade size - 1.75”  (4.4 cm) root 6” (15.2 cm) tip 17” (43.2 cm) long

Rotor radius - 18” (.45 m )

Wind velocity - 4 m/s (8 mph)

Free spinning rotor RPM - 4500

Generator pulley - 1.75” diameter

Bicycle rim diameter - 17.5”

Belt - 5mm poly material

What happened? Let’s see…

30 ohms of resistance load was placed in parallel with the generators 6 ohms of resistance. The total load result was (R1 X R2) / (R1 + R2) = 5 ohm (Note: actual measured circuit resistance in test was 4.4 ohms) .

The rpm reached with this load was 3180. At this point data was collected for 30 seconds. The median voltage was 22.4 vdc and the median amperage produces was .753 Adc. Calculating watts as V x A this resulted in 16.86 watts times 30 seconds run time  = 506 Joules of electrical energy.

Comments and questions are welcome.

Dick Anderson

Wisconsin KidWind Advisor

Bicycle wheel wind turbine rotor

The purpose of this post is to inform and demonstrate the thinking and problem solving S.T.E.M. skills used by KidWind teams in the design, building and testing of a model wind turbines to produce electric power. 

This design will be modeled after the common water pumping, wind mills, used for years on many farms. 

With that goal in mind the team must decide on the size and type of blades that will make up the rotor. They must decide on the generator size and drive train that will connect to the rotor. And lastly they need to decide how the rotor and generator will be supported and held in the wind tunnel for testing.

 

Starting with this 18” bicycle wheel rim. Now the “wheels” of a KidWind team can start turning using their S.T.E.M skills to solve problems they encounter along the way. 

What will the blades be made of? I chose galvanized sheet metal because that is what the farm windmills used and I could get this out of the local heating contractor’s scrap bin for nothing and recycle it.

 

How many blades and what size will the blades be? This design was going to follow that of a water pumper so I wanted a high solidity number. Note:Turbines with high solidity around 0.80 produce high torque at lower speeds.  Keeping in mind how I was going to keep the blades from bending. My 20 gauge scrap material was 16” wide so that would be the length of each blade. 

What diameter circle could I get with 16” long blades? I would calculate the circumference ( cir = 2 Pi X radius ) to be 100.48 inches. If I choose to make the blade tip 6” wide. Then dividing 6” into 100.48 gave me 16.74 blades. I would round this down to 16 blades and have a slight space between blades (resulting in a very high Solidity). 

How would I layout the blades on the sheet metal and then cut them to size? I determined that if I alternated tops and bottoms of the blades on the 16” wide sheet there would be no waste. My 20 gauge sheet metal was to thick to cut by hand with an aviation tin snips so I would need to find a friend in the heating industry with a mechanical squaring shears cut them.  Note: for some reason I only cut 8 blades but in hindsight this will prove to be a good thing as it will allow for more testing and comparison when I add in the other 8 blades. 

 I found a 6” collar in the stove pipe department at Menards as an anchor for the root ends of the blades. The inside diameter of the collar hole was 3.5” so I could calculate the circumference (cir = 2 Pi X radius) to be 11”. Thinking that clipping the point off would provide a wider root end to attach each blade with 1/4” pop rivets to the 6” collar. By chance I tried thought about making them 2” wide but knew they would not fit the 11” circumference. I would divide 11” by 8 blades and come up with a 1.375” wide blade at the root end. Call it “dumb luck”.

After clipping off the points of each blade I needed to begin the process of attaching the blades to the bicycle rim. The root ends would be attached with 1/4” steel pop rivets. Using two rivets per blade. The mid-point of each blade would be attached with a 6-32 NFT (National Fine Thread) 3/4” long machine screw on the fixed side and a 8-32 NFT  2” long screws on the pitch adjustable blade edge.

Here you can see the mid-blade attachment screws. The 8-24 NCT  2” long machine screw has a nut that holds it to the bicycle rim.  The 2” long one goes into the blade and the nuts can be positioned to set and hold the necessary pitch of each blade. Note: holes for these machine screws could be drilled but a better method is to use a sheet metal punch. This works like a paper punch but is much heavier duty. Any local heating contractor will have this tool.

How will the robot be supported? With the rotor assembled it is now time to think about that. There are several materials and structures that could be considered. Think about what materials and skills you have. Think about the strength, weight and costs. I choose plastic PVC pipe because I had a lot on hand from previous KidWind turbine projects. The idea was to run the 5/16 - 24 NF (National Fine)  threaded axles of the bicycle wheel through the centers of the four way fittings. 

A  problem  soon became apparent. The 5/16 - 24 NF (National Fine) threaded axles were to short to go through the 4 - way fittings

This new problem would require some new learning and machine skills. Drilling and threading of a 1/2 - 13  NC (National Course) threaded piece of Ready rod. The drilling could be done on a drill press or with a hand drill but would be best done using an engine lathe. The use of a Tap Drill chart would be needed to determine the correct size of tap drill to use so that enough material would be left for thread cutting. Also a 5/16 - 28 NF tap and tap wrench would be needed. Then the process of cutting the threads would be performed. 

How will the turbine support system be built? The idea for the design of the turbine stand would be to place the 1 - 1/2” PVC pipe legs at 45 degree angles as shown. Each leg would have Tee fitting on the end. Two identical front and back frames would be made.

How long will the legs be cut? Critical math calculations would need to be made to determine the correct length for cutting the legs. The depth the PVC pipe goes into each fitting needs to also be considered. You know the height and width of the wind tunnel. You know the location inside of the wind tunnel you  want the center of the turbine rotor to be at when it is placed into the wind tunnel. Nice application for the Pythagorean Theorem here I would say. 

How far apart should the two frames be spaced? With the front and back halves made and the rotor mounted between them it was a matter of determining the rotor blade clearance I wanted. Then calculating and cutting the PVC pipe spacers to put between the Tee ends.

The design was made to have the rotor blades to be as close as possible to the face of the fan in the wind tunnel. This was done to maximize the amount of air that would strike the blades.

How would the blades be made strong enough to not bend at the tips? This was done by coping what the windmill industry did by lacing a No. 9 fence wire through holes made near the tips of each blade.

How would the No. 9 fence wire be able to go through the blade and not bend the blade? This was done by punching 3 holes to form a slot that the wire could pass through and allow for the 15 degree pitch of each blade and allow for changing the pitch to change the speed and torque of the rotor.

.   

Rotor supported and placed into the 48” by 48” KidWind wind tunnel as close to the fan as possible. 

(Note: I just noticed that in the picture above the rim of the bicycle wheel is between the fan and the blades. This increases the distance between the blades and the fan. A test will be made by reversing this and see if it increases the output of the turbine.)

How will the generator be attached to the frame and rotor? In this design the bicycle wheel will act at a pulley. A belt made out of green 5mm Poly Belt stock bought on Amazon will drive the 2.5 “ V-pulley on the 40 watt 12 volt DC generator. The generator will be attached to a piece of 1/2 plywood with automotive hose clamps and the board is also held to the PVC frame with hose clamps. This arrangement allows for tension adjustment of the drive belt.

     

How will the pulley be attached to the generator shaft? This problem will be solved by making a bushing with a 1/2” O.D. (Outside Diameter) to fit the 2.5” pulley and a 5/16” I.D. (Inside Diameter) to fit the generator shaft. A cross hole will need to be drilled to allow the set screw in the pulley to pass and seat on the generator shaft. (Note: Hexagon material was used for this bushing to make it easier to drill the cross hole.)

One final touch will be to add a sensor that will be used to measure the RPM of the turbine rotor during testing. This sensor uses the Hall Effect with a magnet taped to a blade. Here you can see the output display, the magnet on a blade and the sensor that is taped to the PVC frame.

Originally the bicycle rim was between the rotor blades and the fan making the distance about 6”. Reversing the set-up so the rotor blades are on the other side of the bicycle rim reducing the distance between the rotor blades and fan to about 3”. Will reversing the set-up so the rotor blades are closer to the fan make any difference? The answer to that is a resounding YES! 

The rotor RPM went up from 243 to 293. With the 1”10 ratio the generator RPM went up from 2,430 to 2,930 and with a 45 ohm load the Joule out put went up from 200J to 305J !

Now let’s imagine that for an instant challenge you gave a KidWind team 10 minutes to look and study this turbine, and go over it with this handout sheet, protractor, a ruler, a multimeter and a calculator . The task is to write down as many statements about this turbine they can make.

EXAMPLE 

1. It has 8 blades

2. The blades are made out of sheet metal 

3.

4.

5…

Below is my list organized by turbine part,

ROTOR

1. 8 Blades

2. Sheet metal blades

3. 16” long blades

4. Area for one blade is  ((1.375 + 6) 16 ))/ 2 = 59 sq. In.

5. Total blade area is 59 x 8 = 472 sq. In.

6. Rotor radius is 17.5”

7. Rotor swept area is pi x 17.5 x 17.5 = 961.6 sq. In. (.619 sq m)

8 Solidity is 472 / 961.6 =0.491 

9. The blade pitch angle is 15 degrees

10. This is a HAWT (Horizontal Axis Wind Turbine)

11. The rotor circumference is 2 pi x 17.5 =109.956 inches (2.79 m)

12. In a 4 m/s wind at 293 RPM (4.88 RPS) 4.88 x 2.79 = 13.6 m / 4 m = 3.4 TSR

DRIVE TRAIN SYSTEM

13. Pulley and belt 

14. 17” diameter drive pulley (bicycle wheel rim

15. 1.5” diameter pulley on generator shaft

16.  Ratio  17.5 / 1.5 = 1 to 11.66

17. 1/4” round belt

GENERATOR  Name Plate

18. 40 watt

19. 25 volts at 3,000 RPM

20. 25 volts and .186 amps = 4.66 watt 

21. Turbine Theoretical power output = ( 1.22 x .619 x 64 ) 2 = 96 watts

22. Power with Betz’s Limit = .58 x 96 = 55 watts

23. System efficiency = 10 w /  55 w = .18 = 18%

STAND or TOWER

24. 8 Tees at $3.59 each = $29

25. 2 Crosses @ $6.11 each = $12

26. 8  -  28” long 1-1/4” PVC pipe = 18.7 feet

27. 4. -   6” long 1-1/4” PVC pipe =    2.0 feet

28. 20.7 feet of 1-1/4” PVC at $1 per foot = $20.70

29. 14” long 1/2” threaded rod

30. I  18” bicycle wheel rim

31. 16    1/4” diameter 1/8” draw steel pop rivets

32. 8    6-32 NF 3/4” long machine screws and nuts

33. 8    8-24 NC 2” long machine screws with 24 nuts 

 

Is this applied S.T.E.M. ?

 

.