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Experiment number 8

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.