Experimental study of the effect of the input and output key angles on the discharge coefficient and efficiency of the PKWs

ABSTRACT The results showed that the discharge coefficient (Cd), efficiency (Ra) and, consequently, the increase in the passing flow discharge have a direct relationship with the inclination angle of the input and output keys, and for this reason, the weir of the piano key weir (PKW) with an angle of 90 degrees has the highest Cd and Ra. An increase in the HT/P ration in all angles leads to a decrease in Cd and Ra. However, the decrease rate of these parameters is less for 15-degree angle compared to other threshold angles of the input and output keys. In the PKWs with different input and output key angles and at low HT/P ratios (total head to weir height), the dropping flow nappe needs aeration due to its bonding. However, by increasing this ratio to the submergence condition and transforming into a linear weir, the dropping flow nappe is aerated. At low ratios of HT/P, the dropping flow nappes do not interfere and the highest discharge coefficient and the highest weir efficiency are obtained. While by increasing the HT/P ratio, the dropping flow nappes start to collide and interfere with each other in the output keys, causing the flow rise and the weir submergence, and consequently, the discharge coefficient and the weir efficiency is decreased.