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Experiment number 09

Determination of bend shock loss

 

Objective:

To determine the shock factors for a 45 split and a double bend from the measurement of pressure losses due to shock, and quantities in the duct.

 

Introduction:

Shock losses occur in the fluid flow whenever there is a change in the direction of flow, or change in the cross-sectional for the flow is encountered. The loss of pressure due to the formation of eddies and flow separation at the shock sources is expressed in terms of velocity pressure for the fluid. If

Ps is the shock pressure loss in mm. w.g. then,

                          Ps = X * Pv

Where Pv is the velocity head (mm w.g.) for the flow specified either before or after the shock source. The shock factor ‘X’ is unique for the flow regime encountered defined by the Reynolds number). However, for the general flow conditions encountered in the underground mines, the shock factor may be treated constant for each shock source. The value for ‘X’ may be derived essentially through the scale model studies.

            The experimental duct setup has a 45 split and a double bend. Three shock factors are determined from the setup:

Ø      Shock factor due to split with respect to the flow in the straight branch.

Ø      Shock factor due to split with respect to the floe in the deflected branch.

Ø      Shock factor due to the double bend.

 

Instruments:

Experimental duct setup, Askania minimeter, four inclined tube manometers, two pitot static tubes, scale and calipers.

 

Procedure:

(1)   Measure the diameters of the main branch and the deflected branch.

(2)   Place one pitot static tube in one portion of each split such that its nose is at the center of the air duct and facing the air stream.

(3)   Connect the  manometer as follows:

·        Askania minimeter to the Pitot-static tube in deflected branch to measure the velocity head at the center of the duct.

·        One inclined manometer to the Pitot-static tube in the main branch to measure the velocity head.

·        One inclined manometer between the static pressure portals across the double bend.

·        One inclined manometer across the split between the inlet and the straight branch static pressure portals.

·        One inclined manometer across the split between the inlet and deflected branch static pressure portals.

 

(4)   Level the Askania minimeter and the four manometers and take their initial readings.

(5)   With the fan running, once the flow is stabilized, take the first set of observations.

(6)   Obtain 6 to 8 sets of readings for varying flow conditions. Adjusting the throttles on the outlet of the splits varies the flow in the splits.

 

 

Computations:

Calibrate the inclined tube observations with the ‘calibration charts’ available. The average velocity head values are obtained in the splits as follows:

            Average velocity head = velocity head on the center where ‘m’ is the method factor assumed to be 0.85. Plot three graphs as follows:

Ø      Static pressure drop across the split with respect to the straight branch against average velocity pressure in the straight branch.

Ø      Static pressure drop across the split with respect to the deflected branch against average velocity pressure in the deflected branch.

Ø      Static pressure dorp across the double bend against the average velocity pressure in the deflected branch.

 

Pass a best-fit straight line through the points marked for each case, and the sope of this line indicates the respective shock factor.

            The shock factors for the split are given by empirically derived relationships as follows:

 For the straight branch

            X = 0.5{(Q/Qm)-1}2

For the deflected branch

            X = (Q/2Qd)2 + {(Q/Qd)-1}+ Xd

 

Where Q, Qm and Qd refer to the total, straight branch and deflected branch airflow rates, and Xd is computed from the shock loss formulae.

Convert the average velocity heads obtained into quantity flow rates (assuming the sir density of 1.15 kg/m) and verify the applicability of the above empirical relationships.

 

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