Cooling fan, suck or blow?

Transcription

1 Cooling fan, suck or blow? During construction of a vacuum pump the question arises which method to use for cooling of the vacuum pump. Should the fan blow or suck the air. Which method is most energy efficient? A simple test setup was arranged to answer the question. A: Setup The cooling fan was setup to transport the air thorough a narrow passage that would give considerable resistance for the air to pass. The fan used was a HK EDF70 brushless fan, see picture below. Some specifications for the Dusted Fan: Size: 70mm Weight: 139g Dimensions: 90x83mm Motor: 2839 Brushless outrunner Blade type: 6 No.Of cells: 4S(14.8V) Shaft diameter: 4.0mm RPM/V: 2800 Output Power: 550w Max Current(<60s): 36A Thrust: 1200g (42.33oz.) To answer the question, suck or blow, a simple test jig was setup. A principle sketch can see in the figure below. The dusted fan was mounted in a pipe with an inner diameter 82 mm. To get a god laminar flow it was 162 mm long. Thereafter a smaller tube, simulating the air resistance, was Cooling fan.docx, :33, page: 1 (9)

2 mounted. The smaller tube was 98 mm long and had an inner diameter of 24 mm. Thereafter it was connected to tube of the same type as the ducted fan was connected to. A photo of the setup can be seen below: Below is a close up picture of the smaller tube and the mounting brackets. The smaller tube was in the centerline of the bigger tubes. The fan motor was connected to a 3-phase converter that supplied the current to the brushless motor. The speed of the fan could be adjusted by a standard servo PWM signal of 1 to 2 ms. The power supply had a nominal voltage of 13.8 volts. A DVM meter was connected to measure the current as well as the supply voltage. Cooling fan.docx, :33, page: 2 (9)

3 B: Measurements The measurement of the airflow was done by a Mastech precision Digital Anemometer. All values where noted and thereafter transferred to an Excel sheet. The sound level was also measured from a reference point relative to the test setup. About 60 cm from the test jig. Three tests were done: 1. Blow test without constriction 2. Blow test with constriction 3. Suck test with constriction Below is a table with the values recorded during the test 1, without the tube mounted. The two bigger tubes were connected in series. Cooling fan.docx, :33, page: 3 (9)

4 1. Blow test without constriction I[A] I[mV] U[V] P [W] Af[m/s] Sound [db] 0,79 2,7 13,76 10,9 11,82 77,7 1,49 4,98 13,75 20,5 15,81 81,8 2,2 7,58 13,74 30,2 18,07 88,2 3,41 11,56 13,72 46,8 21,33 92,7 5,09 17,73 13,7 69,7 25,12 93,0 6,66 22,64 13,69 91,2 28,32 95,5 8,64 30,75 13,66 118,0 31,28 98,5 10,95 36,5 13,63 149,2 34,33 100,0 14, ,59 194,1 36,94 104,9 16,48 57,6 13,54 223,1 38,07 105,6 Air Speed [m/s] 40 Blow Air speed without constriction Af[m/s] db 120,0 100,0 80,0 60,0 40,0 Sound without constriction Sound [db] 20,0 0,0 The next test was to insert the constriction and make a blow test. I got the following figures during the measurements: Cooling fan.docx, :33, page: 4 (9)

5 2. Blow test with constriction I[A] I[mV] U[V] P [W] Af[m/s] Sound [db] Remark 0,58 1,91 13,75 8,0 2,98 74,0 Start, min 1,05 3,45 13,74 14,4 3,47 78,9 1,95 6,61 13,74 26,8 4,6 84,2 3,2 10,85 13,72 43,9 5,2 90,1 4,45 15,2 13,7 61,0 5,92 90,7 Sound protection 6,45 22,3 13,68 88,2 6,8 93,2 8,52 30,5 13,64 116,2 7,68 95,9 Oscillating sound 11,1 37,9 13,61 151,1 7,98 98,0 14,4 49,8 13,55 195,1 8,6 106,3 15,96 56,3 13,54 216,1 8,81 102,9 Max Air Speed [m/s] 10 Blow Airspeed with constriction Af[m/s] db 120,0 Sound, blow with constriction 100,0 80,0 60,0 40,0 Sound [db] 20,0 0,0 The final test was to do a suck test with the constriction mounted. I got the values below: Cooling fan.docx, :33, page: 5 (9)

6 I[A] I[mV] U[V] P [W] Af[m/s] Sound [db] Remark 1,03 3,57 13,75 14,2 3,62 78,1 Min 2,25 7,75 13,73 30,9 4,65 84,1 3,5 12,2 13,72 48,0 4,9 103,9 5,9 20,38 13,68 80,7 5,82 95,7 7, ,66 105,5 6,59 99,8 Oscillating current 9,51 33,93 13,62 129,5 7,42 99,8 14,38 50,57 13,56 195,0 8,71 101,6 14,89 52,17 13,54 201,6 8,71 102,1 Max Suck Airspeed with construction Air Speed [m/s] Af[m/s] db 120 Sound, suck with constriction Sound [db] 20 0 Cooling fan.docx, :33, page: 6 (9)

7 C: Analyze A seven degree polynomial was calculated for the blow and suck test case. For the blow case we get: Yblow = -2,71132E-12x6 + 1,89511E-09x5-5,07829E-07x4 + 6,57925E-05x3-4,41585E-03x2 + 1,84518E-01x + 1,69064E+00 And for suck we get: Ysuck = -4,00300E-12x6 + 2,81086E-09x5-7,73359E-07x4 + 1,04305E-04x3-7,05485E-03x2 + 2,46359E-01x + 1,28757E+00 We use the polynomial above to calculate new table for the same level of power: Compare Blow vs. Suck Blow [m/s] Suck [m/s] Delta 10,0 3,16 3,14 0,4% 20,0 4,07 4,11-1,1% 30,0 4,66 4,58 1,7% 40,0 5,10 4,82 5,8% 50,0 5,48 4,99 9,8% 60,0 5,84 5,18 12,8% 70,0 6,21 5,43 14,4% 80,0 6,57 5,73 14,7% 90,0 6,92 6,08 13,8% 100,0 7,23 6,45 12,2% 110,0 7,49 6,80 10,1% 120,0 7,69 7,13 7,9% 130,0 7,83 7,41 5,7% 140,0 7,93 7,66 3,6% 150,0 8,00 7,88 1,6% 160,0 8,07 8,09-0,2% 170,0 8,17 8,30-1,5% 180,0 8,31 8,50-2,2% 190,0 8,49 8,67-2,0% 200,0 8,68 8,71-0,3% 210,0 8,82 8,50 3,7% Now we can make a graph to see the relations between blow and suck. We get the graph as below. As we can see blow vs. suck is the same at low power levels but above 30 Watts the start to separate. From this point blow starts to be better. The max deviation is about 15% in favor for blow. However above 160 Watts suck passes blow and become slightly better for a short while Cooling fan.docx, :33, page: 7 (9)

8 Airspeed m/s 10,00 9,00 8,00 7,00 6,00 5,00 4,00 3,00 2,00 1,00 0,00 Blow [m/s] Suck [m/s] If we make the difference between the two curves (blow and suck) we get the graph as below: Delta % [m/s] Delta Blow vs. Suck Air Flow 16,0% 14,0% 12,0% 10,0% 8,0% 6,0% 4,0% Delta 2,0% 0,0% -2,0% -4,0% This is a nice almost sinus shaped curve. The sound data is not so much to discuss. All cases ended up in about 100 db level. Some slight variations in the curves were noted. The moist remarkable is the start of the suck graph. Cooling fan.docx, :33, page: 8 (9)

9 D: Discussion It s interesting to speculate why the graphs looks like they do. Why is the blow setup a better performer most of the time? Why does the suck catch up at high power levels? At high power levels you can imagine that the air molecules have a so high speed that when they bounce of the constriction wall they start to interfere with the fan blow flow. This effect will be more pronounced at high airspeeds equal high power levels. In the suck case you can argue that it will be more difficult for the fan wings to get a high efficiency due to the fact that it will operate in under pressure (slightly vacuum). There are simply less air molecules to work with. My brother has promised to make a computer simulation of this setup in SolidWorks. I look forward to see how that simulation matches the measurement results above. With SolidWorks it s possible to even see an individual particle and do animations of the flow. See picture below: Look forward to see that later on. /BG Keywords: Airflow, constriction, air, suck, blow, cooling, efficiency, fan, ducted, contraction, reduction Cooling fan.docx, :33, page: 9 (9)