(流体力学和 与传热英文课件）Pressure drop and loss due to friction.pptVIP

2. Friction loss from sudden contraction of cross section The friction loss from sudden contraction is proportional to the velocity head in the smaller conduit and can be calculated by the equation 1.4-31 V is the average velocity in the smaller , or downstream section . Experimentally , for laminar flow , Kc0.1, and the contraction loss hc is negligible . For turbulent flow, Kc is given by the empirical equation 3. Effect of fittings and valves Fittings and valves disturb the normal flow lines and cause friction. In short lines with many fittings, the friction loss from the fittings may be greater than that from the straight pipe. 1.4-32 Factor Kf is found by experiment and differs for each type of connection. Fitting Kf Globe valve, wide open 10.0 Angle valve, wide open 5.0 Gate valve, wide open 0.2 half open 5.6 Return bend 2.2 Tee 1.8 Elbow, 90° 0.9 45° 0.4 Sometimes, the losses in fittings is indicated as an equivalent pipe length. The data given in Table are presented as Le/D, where Le is the equivalent length of straight pipe having the same frictional loss as the fitting. The K values can be converted to Le/D values. The Le values for the fittings are simply added to the length of the straight pipe to get the total length of equivalent straight pipe . 4. Frictional losses in mechanical-energy-balance equation Form-friction losses are incorporated in the hf term of mechanical energy balance equation. They are combined with the skin-friction losses of the straight pipe to give the total friction loss. (1.4-34 ) Friction Loss in Noncircular Conduits The friction in long straight channels of constant noncircular cross section may be estimated by use of the equation used for circular pipe if the diameter in the Re and i