Energy losses and threshold conditions for choking in channel contractions

This paper presents a study on the energy losses and threshold conditions for choking in short, lateral contractions in subcritical open channel flows. A theoretical equation to predict the limiting opening ratio for choking is derived from the conservation of energy and continuity principles. This equation accounts for the critical flow conditions in the contraction and the local energy losses. For the computations of energy losses, an expression for the energy loss coefficient is developed based on a total of 186 sets of choking experiments conducted in the past by various researchers. It is shown that the proposed equation for energy loss coefficient represents the experimental data from various sources within 5% discrepancy. The analysis shows that for flows under choking conditions the energy losses between the upstream section and the critical section in the contraction are mainly governed by the opening ratio, sigma, and are also affected by the encroachment structure shape, the inlet angle, alpha, and the relative contraction length, L*. The energy loss coefficient can vary from 1.56 to 0.27 for sigma changing from 0.12 to 0.87. It can also be reduced up to 60% for encroachment structure shape changing from sharp-corner to-rounded-comer conditions; and up to 75% for alpha going from 90 degrees. to 30 degrees and L* from 1.33 to 0.