Experiment studies on impact of siphon gradient on gas-liquid two-phase flow characteristic

Abstract: With the constant development and application of a large number of surface water resources, the Karez type underground reservoir has become the key of the water conservancy project in Xinjiang in recent years. The siphon pipeline with longer distance and larger vacuum is the most important part of the Karez type underground reservoir. This study explored the impact of gradient change on the hydraulic characteristics of siphon pipeline with inclined arrangement. A total of 11 gradients were designed at the installation height of 4 and 6 m. The waterhead changed from 5 to 135 cm. The experiment was carried out in organic glass pipes. The pipe length was 18.15 m. The observations and measurements included the gas-liquid two-phase flow phenomenon, void fracture, kinematic velocity of bubble, discharge capacity and total head loss inside the siphon. The experimental result shows that in the flat slope pipe, air bubbles were rich with diameter about 4-5 mm in the head of the pipe and the bubbles in diameter of 1 mm were on the wall of pipe. During the movement, the bubbles was clustered into big bubbles and moved in the different directions from the flow. In inverse slope pipe, many small bubbles were on the wall but the air movement direction was same with the flow direction. Different the flat slope, the airbag was concentrated near downstream when it moved downstream. Different from inverse slope pipe, the airbag moved upstream slowly in the opposite direction from the flow direction. With the gradual increasing of gradient, the fake cavitation phenomenon inside the pipe weakened little by little, the volume of bubble or airbag diminished and the quantity dropped off. With the gradual increasing of the gradient, the void fracture in the pipe diminished, the kinetic velocity of bubble accelerated, the water delivery flow in the siphon strengthened gradually, the total head loss also increased gradually, the maximum flow increasing percentage was 23.8% and the total head loss increased by 42.86%. When the gas rate was larger than 11%, flow type in pipe was transitional and air mass type and the effects of gradient on flow rate could not be ignored. When the gas rate was smaller than 30%, the siphon in the pipeline was unstable. The gas-liquid two-phase flow phenomenon induced by the gradient change under such the conditions above made the effect of gas rate on flow resistance different from the liquid phase flow. Thus, based on the experimental data at installation height of 4 m on inverse slope, a formula for flow rate estimation was derived under the condition of transitional and air mass flow with gas rate of 11%-30% on gradient of 1/60-1/10. The flow rate formula was validated by using data at the installation height of 6 m. The validation results showed the relative error of measured and calculated flow rate in the inverse slope pipe was within 6% and it in the positive slope pipe was within 7%. It suggests that the formula is reliable. The results above provide valuable information for the pipe arrangement in the practical engineering.