How to calculate the pressure ratio of a fan ?

[kemo1988]

Hi everyone

 

I am working on my graduation project which is hovercraft, and i am using integerated system. The fan i got is 80 cm dia. and has 5 blades and the available power is 13 hp @ 3600 rpm.

 

I am sure that it is going to produce a good thrust, but here i am worried about the lift, i want a pressure of about 0.0091 bar ( gauge ), and i dont know any data about this fan, so i was asking: how can i calculate the pressure ratio of this fan in terms of power, speed, blade cross section and diameter to know if the lift will be succsesful or not.

 

I am also sure that it will produce the required flow rate, but here i am concern about the pressure.

 

please need help

[Mr Skeptic]

I think you'd be better off measuring the pressure than trying to calculate it.

[kemo1988]

I think you'd be better off measuring the pressure than trying to calculate it.

 

yes you are right, but i havent buy this fan yet, i want to be sure that when i buy the fan it will match the required pressure, i dont want to spend money and then through it away.

[Mr Skeptic]

Ah. Maybe if you look up the fan on the internet, you might find this information. Model number, or whatever.

 

The trouble with trying to calculate it is that you'd need to know the shape of the blades, and that sort of nonsense. It would be a lot of trouble to get an exact answer. You might be able to estimate it from the amount of airflow that it produces.

[forufes]

or thermodynamically(flow energy) using mass rate and energy rate(where the input plus the work should equal the output )..

[kemo1988]

Ah. Maybe if you look up the fan on the internet, you might find this information. Model number, or whatever.

 

The trouble with trying to calculate it is that you'd need to know the shape of the blades, and that sort of nonsense. It would be a lot of trouble to get an exact answer. You might be able to estimate it from the amount of airflow that it produces.

 

it doesent have to be an exact answer, and i dont know how to estimate the pressure from the flowrate, i can get its discharge at a certain speed and use similarity to get the new discharge at the operating speed.

 

if you know how to estimate the pressure from the flow rate i would be grateful

[ewmon]

This webpage looks like a tutorial of sorts on fans, blowers, compressors, etc. Offhand, I'd think there'd be a way to calculate pressure differential from power, "disk area" (which is 0.5 m² for your fan), flow rate, etc.

[Mr Skeptic]

it doesent have to be an exact answer, and i dont know how to estimate the pressure from the flowrate, i can get its discharge at a certain speed and use similarity to get the new discharge at the operating speed.

 

if you know how to estimate the pressure from the flow rate i would be grateful

 

Well let's see:

(don't trust me, I've never done anything remotely similar to this before)

Power = 13 HP = 9,694 watts (that's pretty impressive for a fan!)

If 100% efficiency, Power --> kinetic energy/s ==> 9,694 W = 9,694 J/s

KE/s = (1/2) M v^2/s = 9,694 J/s ==> v = sqrt(9,694 J * 2 / M) where v is air velocity

Flow Rate = M / s = density * volume / s = density * area * v

Density (air, 25 C) = 1.184 kg/m^3

Area = pi * r^2 = pi * (0.8 m)^2 = 2 m^2

Flow Rate = density * area * v = 2.38 kg/m * v = 2.38 kg/m * sqrt(9,694 J * 2 / M) = 331.39 kg/s * sqrt(kg/M) = M/s

==> (M/kg)^(3/2) = 331.39 ==> M/kg) = 331.39^(2/3) = 47.89 ==> M = 47.89 kg

So Flow rate = 47.89 kg/s,

v = sqrt(9,694 J * 2 / M) = 20.12 m/s

 

P = F/A = (M * v / s)/A = 47.89 kg * 20.12 m/s / s / 2 m^2 = 482 pascals = 0.0048 bar

 

So half of what you need if my half-guesswork calculations are right. The last bit in particular seems dubious, since the air would come to a stop and I'd need different numbers.

 

So, how close was I? Hand over the model number and company name, etc.

[kemo1988]

I think this calculation can not be applied for a fan

 

I think this calculations can be applied if we have a source has a 2 m^2 area and blows air which has an energy equivelant to 13 hp (like wind), or we can assume a fan with infinit number of blades.

 

There is some important prarameters must taken into acount which are: number of blades, blade shape and blade angle, it seems to be complicated to calculate.

 

anyway, on tuesday i will meet someone has phd in turbomachinery, i will talk him about this subject and let you know.

 

thank you very much.

[Mr Skeptic]

Oh, I messed something up. The area I calculated is 4 times too big, because I used diameter as radius.

 

After recalculating correcting for that, I get 0.012 bar, so just over what you need. If you can give the info on the fan, I might be able to look up the actual numbers.

Edited April 30, 2010 by Mr Skeptic

[kemo1988]

yes you are right, i was just about to correct that, what fan info do you need?

[Mr Skeptic]

The name, model number, manufacturer, that sort of thing.

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Merged post follows:

Consecutive posts merged

Also, you might consider using a fan that produces more pressure but less airflow; such a fan will have less surface area. Think about it: how big a hole do you want on the upper surface of your craft? If you have an inflatable skirt at the bottom of your craft, this will give you a wide margin of airflow you can use (excess airflow lifts the skirt and allows more air to leave, too little airflow lowers the skirt and provides a tighter seal. For example, if the skirt is 1 mm off the ground and your craft has a circumference of 8 m, then the surface area under the craft will be about 60 times less than that of your 0.5 m^2 fan.

Edited May 1, 2010 by Mr Skeptic
Consecutive posts merged.

[kemo1988]

i am using integerated system, it is one fan for thrust and lift

 

 

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[Mr Skeptic]

Hm, searching for that fan only gave 2 results, and neither of them were lists of fans. This site seems to have fans, but they are separate from the engine. It does also have a lift calculator. http://www.hoverhawk.com/

[Charlatan]

Hi everyone

 

I am working on my graduation project which is hovercraft, and i am using integerated system. The fan i got is 80 cm dia. and has 5 blades and the available power is 13 hp @ 3600 rpm.

 

I am sure that it is going to produce a good thrust, but here i am worried about the lift, i want a pressure of about 0.0091 bar ( gauge ), and i dont know any data about this fan, so i was asking: how can i calculate the pressure ratio of this fan in terms of power, speed, blade cross section and diameter to know if the lift will be succsesful or not.

 

I am also sure that it will produce the required flow rate, but here i am concern about the pressure.

 

please need help

 

Helicopter project? Oh boy oh boy! To make your air lift work you will need to applly more force to the fan than there is resistance from grvity. Gravity is what is keeping it down, so, use your abacus to work out what the pull is and then over power it related to the mass. If the mass is [x] then make the force [y] greater than the pull of gravity [z] times the mass. So, [x] + [z] needs to be about half or so [y].

[Cap'n Refsmmat]

I think kemo1988 knows that, seeing as he's already calculated the pressure he needs.

[kemo1988]

Hm, searching for that fan only gave 2 results, and neither of them were lists of fans. This site seems to have fans, but they are separate from the engine. It does also have a lift calculator. http://www.hoverhawk.com/

 

I know this site, it sells only fans and give a certain operating point.

 

maybe if i get a point for my fan (flowrate, pressure, power), and by using similarity i can get its chart at certain speed, but i need to know the equation for calculating the power consumed by the fan for a given required thrust.

[Charlatan]

I know this site, it sells only fans and give a certain operating point.

 

maybe if i get a point for my fan (flowrate, pressure, power), and by using similarity i can get its chart at certain speed, but i need to know the equation for calculating the power consumed by the fan for a given required thrust.

 

The power required by the fan is equal to the applied energy for it to lift at a progressive rate. This means you need to wiegh the actual blade of the fan and then calculate the required energy to make it spin by applying kinetic energy caused by the engine, supplying you with horse power or something like that. But, the equation will be [horse power] divided into [mass] equals [rpm] required. [Power] needed is equal to [rpm] per volt, or, you need to get practical with it if information is lacking.

[Cap'n Refsmmat]

Power is not the same thing as energy. None of that makes mathematical sense.

[kemo1988]

Now, after thinking, i will buy the fan and make some measurments to get its chart.

 

A fan chart is a relation between volume flow rate, pressure and power consumed at different blade angles. All i need is to construct a short duct for the fan and pitot tube for pressure and flow rate and discharge air velocity measurments and a tool to measure the torque.

 

anyway, the buyer have now a 75cm & 110cm diameter fan, and as i stated i am using integerated system so both fans must give a static pressure of 1000 Pa, but here i want to make comparison between them for thrust, which will give more thrust for the same engine (13 hp @ 3600 rpm), i attached a PDF for calculating a fan thrust, and you can see that its mainly function of: air discharge velocity from the fan and the diameter, it is also stated that when the diameter increase the discharge velocity decrease, but which parameter will have the most effect (area or air velocity)....

 

waiting for your reply..

thrustcalc[1].pdf