Numerical simulation and verification of hydraulic performance and energy dissipation mechanism of two-ways mixed flow emitter

Abstract: The two-ways mixed flow emitter is a new kind of drip irrigation emitter. The main working principle is forming the mixed phenomenon of forward and backward flow to increase more local head loss and eliminate extra inlet pressure. Computational Fluid Dynamics (Fluent software) plays an important role in analyzing hydraulic performance, flow field characteristic and energy dissipation mechanism of drip irrigation emitter. In order to study the hydraulic performance and the energy dissipation mechanism, we chose 5 key geometric parameters as factors and designed 25 sets of experimental schemes according to the orthogonal experimental design method. The flow rate and flow index under different pressures were determined by testing and simulating. Laboratory experiments were carried out in State key laboratory Base of Eco-hydraulic Engineering in Arid Area, Xi'an University of Technology. In this paper, 5 turbulence models including standard k-ε model, RNG k-ε model, realizable k-ε model, standard k-ω model and SST k-ω model were chosen and compared, respectively. Based on the flow rate of each experimental scheme within the range of 50-250 kPa, the relative errors of the experimental and simulated flow rate of these 5 models were compared, respectively. The more precision turbulence model was chosen. The flow index, the flow ratio of the forward flow to the backward flow, and the flow field distribution in different pressure were calculated and analyzed, and the relationship between them was explored. The results showed that the simulated results of RNG k-ε model were better than that of the other 4 models. The relative error between the test value and the simulated value of RNG k-ε model was from 1.656% to 3.151%, which was the minimum error among these 5 models. Especially, when the pressure was in the range of 50-150 kPa, the simulated values were much closer to the test values with the determination coefficient of 0.998 and the relative error of 1.28%-1.89% in the pressure of 50-150 kPa, which could accurately reflect the flow rate of the emitter. So RNG k-ε model could be used to accurately simulate the flow rate and flow mechanism of two-ways mixed flow emitter. In addition, the simulated flow index ranged from 0.414 to 0.483 under different pressures, indicating excellent hydraulic performance. In the low pressure range, the flow index was from 0.414 to 0.456, the flow ratio of the forward flow to the backward flow tended to 1, and the hydraulic performance was more prominent. However, for the two-ways mixed flow emitter, the high pressure could lead to the fast increase amplitude of the backward flow, resulting in the less flow ratio and the poor hydraulic performance. The flow velocity distribution showed the forward flow and the backward flow joined together and strongly mixed near blocking water device tooth, meanwhile, produced large local head loss and eliminate more fluid energy. In the high pressure range, the greatly uneven distribution of the forward flow and the backward flow would reduce the effect of energy dissipation. Therefore, the appropriate increase of the forward flow would help to speed up the flow mixing, enhance effect of energy dissipation, and improve irrigation quality. In order to change flow direction, setting up a number of blocking water device in the flow channel side wall of emitter could increase forward flow rate, optimize the flow ratio of the forward flow to the backward flow, and improve hydraulic performance. Flow field analysis revealed that after increasing blocking water device of side wall, the forward and backward flow rate were similar and the flow index under high, medium and low pressures was 0.445, 0.431 and 0.422, respectively. It verified the optimum of emitter based on mechanism of energy dissipation was reliable. These conclusions can provide the method guidance for structure optimization and hydraulic performance improvement.