2 3 Correcting Measured Losses By Calculating Ah As Described Above In Section 2 1 The Loss Measured Directly Between 1 (86.36 KiB) Viewed 30 times
2 3 Correcting Measured Losses By Calculating Ah As Described Above In Section 2 1 The Loss Measured Directly Between 2 (20.3 KiB) Viewed 30 times
2 3 Correcting Measured Losses By Calculating Ah As Described Above In Section 2 1 The Loss Measured Directly Between 3 (28.12 KiB) Viewed 30 times
2 3 Correcting Measured Losses By Calculating Ah As Described Above In Section 2 1 The Loss Measured Directly Between 4 (15.9 KiB) Viewed 30 times
2.3 Correcting measured losses by calculating Ah. As described above in section 2.1, the loss measured directly between two piezometers should be corrected by removing the loss that would be present if those two piezometers were connected by sec- tions of straight pipe, rather than the fitting. Remember, this correction is expressed by the equation (6). The basic idea is • Work out the length of the fictitious pipe (the distance from the pizeometer to the centre of the fitting) on the upstream side. In this experimental rig, the piezometers are 4 pipe diameters from the centre of the fitting for the mitre, expansion and contraction, and 4 pipe diame- ters from the entry and exit of the small and large radius bends. • Get the friction factor for the fictitious pipe from Moody chart, using the Reynolds number Re and the roughness. . • Calculate the change in head Ahy, using the friction factor f from the Darcy-Weisbach equation (11). Repeat this process on the downstream side to get Alfa- • Sum these losses to get the total correction Ah-Ah + Alza. Note that if the pipe diameter is the same size on each side of the fitting, the process above reduces to using the length of fictitious pipe between the two piezometers to calculate Ah, directly. 3 DESCRIPTION OF EXPERIMENTAL TASK This experiment consists of measuring the piezometric pressure at a series of points in a pipe network, and using the changes in piezometric pressure across a component or along a pipe to calculate the head loss, and therefore the loss coefficient K or the friction factor /. A schematic of the test rig is provided in figure 4. The rig consists of a length of plastic pipe of diameter 22.5mm with a number of fittings. These fittings are: . • A 90° mitre bend • A straight section of pipe 525mm long • A 90° elbow bend of radius 12.5mm . . • A 90° large radius bend of radius 53.4mm • A sudden enlargement to a pipe of diameter 29.6mm • A sudden contraction Water is pumped through the pipe network supplied from the hydraulic bench on which the pipe network sits, and is discharged into the measuring tank at the bottom of the bench. Piezometer tappings are provided in the pipe walls, 4 pipe diameters before and after each component (measured from the centre of the mitre, the point of the change of diameter in the expansion and contraction, and the entry and exit of the small and large radius bends). The tappings are connected
Large valve A Ara 10- Figure 4. A schematic of the test rig M (b) Sudden enlargement AH (a) 90 bend (c) Sudden contraction Figure 2. Bustrations of typical flows in some components, showing that regions where recirculations occur leading to losses, as well as the fact that there can be a gain in piezometric head but a reduction in overall head in an expansion
Elbow Sudden en- Sudden Large largement contraction elbow 0 0 Table 6. Head loss correction Al; for various flow rates for each component. Large Straight Elbow pipe Sudden en Sudden largement contraction elbow Table 7. Total head loss AH for various flow rates for each component. (L/s) Mitre (L/s) Mitre Straight pipe 0 0 0 0