Tuesday, 29 January 2008

Gas velocity measurements

The blower I made from an old vacuum cleaner is quite powerful, in fact it gives a noticeable thrust on it's own.
By mounting it vertically on one end of a balance (a length of Al shelf support) and hanging a known weight on the other ( a 1lb hammer) I was able to get an estimate of the thrust.

That came out as 0.62 Newtons.

Knowing the diameter of the outlet (45mm : 0.0016m2), I calculate massflow to be 0.035kg/sec, which corresponds to a velocity of 18m/sec. (Volume flow is 0.029 m3/sec )

This piece of information gives me a handle on a couple of things that I have been trying to find out

Turbine behaviour

The turbine cross-section area is also 0.0016 m2 (happy coincidence) so velocity at the turbine should also be 18m/sec also when the blower is used to drive the engine cold.
This corresponds to a calculated peripheral velocity of 26m/sec and angular velocity 10,000rpm (NGV and turbine blade angle both 35o).

This calculated speed is in good agreement with that measured, when the blower is placed directly against the air intake. (Chamber pressure measured : 1.0026 bar)

N.B. A small 'flag' held aft of the turbine shows no residual rotation of the airflow.

This gives me a lot of confidence in the turbine itself.

Pitot tube

Fed the blower output into a 50mmm dia pipe (0.00196m2). That gives an expected flow velocity of 0.029/0.00196 = (about)15 m/sec

Using a couple of pieces of small diameter plastic pipe (3mm), I measured the 'ram' pressure with my water 'U' tube manometer. It came out at 1.4 cm H2O which is equivalent to 1.4 millibar.
1.4 millibar is 1.4*101.325 = (about)140 Pascal (ie kg per sq m)

Bernoulli states that pressure = density * v2/2
so v2 = 2 * pressure / density = 2 * 140 / 1.2 = 233
so v = 15.275 - which agrees well with the expected value.

This means I can measure flow speed reasonably accurately using a simple manometer.
That's a bit of a breakthrough.

Measurement of the airflow at intake and exhaust gave the same figures but restricting airflow to give different velocities gave results that suggested that the volume flow rate dropped with the restriction (which makes sense)


I have a motor that I can use to drive the compressor at about 15000rpm.
If the turbine were actually working and turning at that speed, the massflow should be 0.017 kg/sec. (calculation not shown - from my spreadsheet)
So if I connect a pitot tube to the outlet of my test rig, I should be able to examine the behaviour of the compressor with that goal in mind.

No comments: