Precise
traversing with pitot static tube in a circular cross section duct.
Objective:
To
obtain the method factor through precise traversing in the fan testing duct
setup for different flow conditions, and to plot the nature of velocity
profiles.
Introduction:
Precise
traversing is often warranted in a duct particularly if the duct is a part of
the testing setup. The purpose is basically to obtain a precise estimate for
the average air velocity in the duct. Generally about 20 readings are necessary
in the circular cross-section duct to get a proper estimate for average air
velocity. Typically the cross-section is segmented into 5 concentric equal
areas, the measuring stations are marked such that
ri = r [(2i-1)/10]1/2
---------------(for i= 1,2,3,4,5)
where
r is the duct radius ,and ri is the radius of the i th concentric
group of readings from the center.
When a pitot static tube is used for
the precise traversing, the velocity head may be obtained at each velocity
station by connecting the total and static velocity portals from the pitot
static tube to the manometer. The corresponding velocity values are obtained
as:
v = ((2 * g * h)/r)1/2
where
g is the acceleration due to gravity(9.79 m/s2), h is the velocity
head in mm. w.g. and r is the air density in kg/m3.
The velocity profile in the duct is dependent on the flow regime and the
surface roughness. In order to get a representative variation of velocity in
the duct cross-section, several velocity conditions are studied by suitably
throttling the flow at the duct outlet.
Instruments:
Fan
testing duct setup, pitot static tube, Askania minimeter, scale and calipers,
Assmann Psychrometer, Aneroid Barometer and Tachometer.
Procedure:
(1)
Obtain
the diameter of the duct and determine the position of the pitot static tube
measuring stations. Mark the positions correspondingly on the limbs of the
pitot static tube.
(2)
Slide
the pitot static tube into the device holding and traversing the pitt static
tube along the vertical axis.
(3)
Level
the Askania minimeter. Connect it to the total and static pressure portals from
the pitot static tube and take the initial reading.
(4)
Throttle
the duct outlet to create low velocity condition in the duct.
(5)
With
the fan running. Obtain the ten velocity head readings corresponding to the ten
measuring stations along the vertical diagonal by sliding the pitot static
tube, and also the fan speed with the Stroboscope.
(6)
Remove
the pitot static tube and slide it into the device for holding the traversing
along the horizontal diagonal. As in the step 5, obtain 10 velocity head
observations for the 10 measuring points.
(7)
Obtain
a central pitot static tube velocity head measurement.
(8)
By
changing the throttle position, create a different velocity condition in the
duct.
(9)
Repeat
the steps to obtain a new set of values for the changed flow conditions.
(10)
Take
ambient temperature and barometric pressure measurements to obtain a
representative air density for the experimental conditions.
Computations:
Calculate
the velocity for each velocity pressure reading and correct all the velocities
to standard fan speed. For each flow condition the mean of the 20 readings
provides the average velocity. Method factor is essentially the ratio (Vaverage/Vmax),
where Vmax is the velocity observed with the central pitot tube
measurement.
Plot the velocity profiles both
along the vertical and horizontal diagonals for the different flow conditions.
Remarks:
The
method factor obtained will be subsequently used in the fan testing experiment
to determine the quantity flow rtes in the duct from the central Pitot tube
measurements for velocity head.