## 4 Discussion
### Response Law of HFI Fertilizer Injection Performance to AFT Structure Design  
Since an HFI is a combination of an AFT and diaphragm pumps, \(Q_{\rm inj}\) is influenced primarily by the AFT output power, the energy conversion efficiency of the diaphragm pump, and the mechanical transfer between the two. Therefore, when \(H\) is 0.12\(-\)0.18 MPa, the \(Q_{\rm inj}\) of the HFI composed of D33 is, on average, 1.3%, 5.6%, and 7.0% higher than those of D34, D43, and D44, respectively, which is smaller than the differences in \(P_{\rm max}\) and \(\overline{\Delta BP}\)among the four participants in the numerical simulation. The venturi fertilizer injector is widely used in the integration of drip irrigation with water and fertilizer because of its advantages, such as its simple structure and low cost [53]. Both domestic and international researchers, as well as microirrigation equipment manufacturers, have conducted extensive research aimed at reducing \(H_{\text{min}}\) and improving \(Q_{\text{inj}}\) under the same \(H\)[15; 16; 42; 54; 55]. During the operation of the venturi fertilizer injector, the collision and mixing of high-pressure, high-speed water flow with low-pressure, low-speed fertilizer liquid easily results in the formation of whirlpools and low-energy fluid clusters. This disrupts the normal flow of internal water, resulting in significant pressure loss and turbulent kinetic energy loss at the fertilizer suction port [15; 56]. Therefore, the \(H_{\text{min}}\) of optimized or mass-produced venturi fertilizer injectors is typically \(\geq\)0.12 MPa, which is higher than the \(H_{\text{min}}\) in this study. Under an \(H\) of 0.12-0.18 MPa, the \(Q_{\text{inj}}\) of the venturi injector ranges from 30.7 to 119.9 L/h, which is lower than the \(Q_{\text{inj}}\) in this paper. The main reason for this phenomenon is that the flow driven by the AFT in the HFI does not interfere with the flow path through which the diaphragm pump draws the fertilizer. Therefore, the applicability of the HFI proposed in this paper--specifically, its \(H_{\text{min}}\) and fertilizer injection efficiency (i.e., \(Q_{\text{inj}}\) under the same \(H\))--can meet engineering requirements and enable the integration of water and fertilizer for surface irrigation or sprinkler/drip irrigation systems. This holds true for both low-pressure pipeline irrigation systems (water supply pressure \(<\) 0.15 MPa) and high-pressure sprinkler irrigation systems. Considering both the fertilizer injection efficiency and service life, the \(Q_{\text{inj}}\) for the HFI corresponding to D34 and D44 is 1.3% and 7.0% lower, respectively, than that of D33, and the \(NP\) of the second impeller is 24.7% and 37.9% lower, respectively, than that of D33. Considering the additional production and installation costs associated with two sets of molds for the HFI based on D34, D44 was ultimately chosen as the finalized impeller structure for the HFI. Compared with a venturi injector, an HFI has smaller pressure loss and higher \(Q_{\text{inj}}\). However, an HFI has many overall components, high manufacturing costs, and requires more thorough cleaning after each irrigation. So, it is more suitable for low-pressure conditions where a venturi injector cannot work normally. When the irrigation system pressure is high, the use of a venturi fertilizer injector will have higher economic benefits.