Pulse characteristics and its influence of negative pressure feedback jet sprinkler

Abstract: The negative pressure feedback jet sprinkler is a new type of irrigation sprinkler independently developed by China. Its design principle is based on the Coanda Effect. Pulse (including pulse frequency and pressure amplitude) is an important feature of the sprinkler, which has a significant impact on the sprinkler's range, rotation characteristics and spray uniformity. At present, the mechanism of its influence on pulse characteristics has not been systematically studied. The main factors affecting the pulse characteristics of the sprinkle are ratio of the shortest distance between inlet and side wall to inlet width (SW), ratio of distance from inlet to concave to inlet width (HW) and sidewall inclination. In this paper, firstly, design interval of the pulse characteristic structure was obtained through 125 sets of numerical simulations. Secondly, by studying the influence of different factors on the pulse characteristics, the effects of main factors on the pulse frequency and pressure amplitude were obtained. The pulse frequency decreased with the increase of SW and HW, and the values of SW at the turning point of the falling rate was 0.525 and 9 respectively; The inclination of the sidewall inclination had little effect on the pulse frequency; The pressure amplitude increased with the increase of SW, and the value of SW at the point where the growth rate became smaller was 0.450. The pressure amplitude increased first and then decreased with the increase of sidewall inclination and HW, and the value of sidewall inclination and HW at the turning point was 12° and 9, respectively. Based on the above studies, a better design interval for the pulse characteristic structure was further determined. Finally, through four-factor three-level orthogonal test, pulse frequency and pressure amplitude were used as evaluation parameters, and range analysis and comprehensive scoring methods were used to obtain the sprinkler structure with optimal pulse characteristics and they were the shortest distance between inlet and side wall of 1.80 mm, the sidewall inclination of 10°, and the distance from inlet to concave of 28.0 mm. Furthermore, the computer simulation of the sprinkler under the optimal structural parameters was carried out. The numerical simulation obtained the pressure flow diagram during the working process of the sprinkler. A periodic dynamic analysis of the pulse formation process was performed through the pressure flow diagram. At the same time, the pressure difference between the two sides of the main jet was measured during the simulation. The results showed that the maximum positive pressure on both sides of the main jet appeared near 63 mm from the nozzle cross section at different working pressures. This was because the entrainment of the high-speed water flow when the jet was attached to the wall created a strong vortex flow near the wall of the attachment point, which caused a large pressure difference. The maximum negative pressure appeared at a distance of 40 mm from the sprinkler inlet, which was caused by the movement of the deflection in the control tube caused by the uneven pressure difference between the two sides of the jet when the jet was attached to the wall. The pressure difference across the jet at a distance of 70 mm from the sprinkler inlet was also negative, which was mainly caused by the jet deflected into the nozzle, hitting the inner wall of the sprinkler and forming a local vortex. In sum, with the same internal structure design, the position of the attachment point of the sprinkler under different pressure conditions remained basically unchanged. This study provides guidance to optimization of sprinkler structure and to improvement of sprinkler performance according to the characteristics of the high-speed jet Coanda.