## 4 Conclusions  
In this study, the URANS model was solved and the entropy theory was applied to analyze the distribution of energy dissipation in full flow pump device with four kinds of backflow clearance radii. The simulation results were validated by experimental data, and the following conclusions were obtained.  
(1) The rotation of the motor rotor causes extra shaft power, and the backflow clearance produces backflow effect in the impeller. Thus, compared with conventional axial flow pumps (\(R_{\text{b}}\) = 0 mm), the design efficiency and head decrease by 9.5% and 7.2%, and the design shaft power increase by 2.5% in the tubular pump with \(R_{\text{b}}\) = 2 mm. In addition, \(R_{\text{b}}\) has the most obvious influence on the hydraulic performance of the full tubular pump under 1.2\(Q_{\text{des}}\). Compared with \(R_{\text{b}}\) = 0 mm, the efficiency and head reduce by 18.8% and 13%, and the shaft power increase by 7.1%, respectively, under \(R_{\text{b}}\) = 2 mm;  
(2) Hydraulic losses in the full tubular pump are mainly from the total dissipation of the impeller, backflow clearance, rear guide vane and outlet passage. Under 0.8\(Q_{\text{des}}\) and 1.0\(Q_{\text{des}}\), the total dissipation of the impeller increases with the increase of \(R_{\text{b}}\) (\(R_{\text{b}}\) = 0.5-2 mm), but the total dissipation of the rear guide vane and outlet passage decrease with the increase of \(R_{\text{b}}\). Therefore, \(R_{\text{b}}\) has no obvious influence on the efficiency of the full tubular pump. Under 1.2\(Q_{\text{des}}\), the total dissipation of both the impeller and outlet passage increase with the increase of \(R_{\text{b}}\). Thus, the efficiency of the full tubular pump declines with the increase of \(R_{\text{b}}\);  
(3) Under 1.0\(Q_{\text{des}}\), the total dissipation rate on the pressure side of the impeller blade decreases with the increase of \(R_{\text{b}}\), and the total dissipation rate on the suction side and near the impeller inlet increases with the increase of \(R_{\text{b}}\). Since the kinetic energy of the impeller outflow is reduced due to backflow effect, the total dissipation rate in the guide vane and on the outlet passage wall decrease with the increase of \(R_{\text{b}}\).  
Insufficient \(R_{\text{b}}\) may lead to the collision between the motor rotor and motor shell, which will affect the safe operation. However, excessive \(R_{\text{b}}\) would lead to a significant decline of hydraulic performance for the full tubular pump. Therefore, the results show the internal relationship between the internal dissipation and \(R_{\text{b}}\). In addition, the spatial distribution of total dissipation rate in each hydraulic component can provide suggestions for the targeted optimization of a full tubular pump.  
Conceptualization: F.M. and Y.L.; methodology: F.M. and J.P.; software: F.M.; validation: F.M. and Y.L.; formal analysis: F.M.; investigation: F.M.; resources: F.M.; data curation: F.M. and Y.L.; writing--original draft preparation: F.M.; writing--review and editing: F.M.; visualization: F.M.; supervision: [PERSON]; project administration: Y.L.; funding acquisition: J.P. All authors have read and agreed to the published version of the manuscript.  
This research was funded by Natural Science Foundation of China (grant no. 51879121), Primary Research and Development Plan of Shandong Province (grant no. 2019 TSLH0304).  
Not applicable.  
The authors declare no conflict of interest.