Abstract
In order to have a more comprehensive grasp of the performance of the rim-driven thruster, the external characteristics are studied by experiment and carry out numerical simulation to study inner flow characteristics. The test results for the rim-driven thruster find that the head curve had a hump shape. The head is the largest while the flow rate Q = 600 m3/h. The numerical simulation is carried out to reveal the cause of the hump of head. The results show that large-scale backflows gradually appear near the wall in front of the impeller inlet, the central area of the impeller outlet, and the two sides of the central low-pressure zone with the reducing of the flow rate, which can cause a large flow loss and result in a drop in head. The discrepancy between the pressure surface and the suction surface of blade decreases rapidly in the range of r/R = 0.2–0.5, which is another major factor leading to the drop in head under small-flow conditions. Structurally, there is no blade in the impeller center and there are the large backflows in the middle of impeller, which causes much volume loss and is a main cause of the decreasing of head under small flow rate.
Introduction
The rim-driven propeller cancels the shafting structure, making the propeller more compact, which makes the thruster with high propulsion performance, vibration, and noise reduction. The rim driven has been an important development direction of submarine propulsion technology.1,2 The prototype of rim-driven thruster was proposed by Kort 3 of Germany in 1940. In 2003, General Dynamics of the United States designed and manufactured a five-blade shaftless propeller and its main feature is the rotating part does not need to be sealed and waterproof, and the seawater enters the motor through the characteristic flow channel to cool the motor, and improves the power density of the motor.
Cao studied the impact of abandoning the drive shaft on the hydrodynamic performance of the water jet propulsion pump, and found that the loss coefficient and wake coefficient of the water inlet channel were significantly reduced, and the thrust increased by 20% in the high-speed conditions, and the propulsion efficiency increased by 15%.4,5 Su 6 canceled the drive shaft to reduce the disturbance of the water jet’s velocity field by the uneven inflow and improved the propulsion performance of the propeller, and thrust increased by approximately 9%. The above research shows that the rim-driven propeller has obvious advantages in performance because of structural characteristics.
In 2011, Yakovlev et al. 7 and Song et al. 8 studied the effects of hub and hubless propellers on the performance of propellers, and found that the two types of propellers have similar performance curves, but hubless propellers have greater thrust and torque. In 2014, Andersen 9 designed a rim-driven propeller and analyzed the impact of hub and hubless on the performance of the propeller by CFD method, and the results show that the hubless blade can reduce the energy loss by 10%. It can be seen that the rim drive thruster avoids the existence of the hub vortex behind of the hub, reduces the overcurrent velocity of the propeller, and improves the propulsion performance of the thruster. Freeman and Marshall 10 studied the natural frequency, mode shape, flow trajectory, blade surface pressure, and thrust performance of a shaftless propeller with a diameter of 280 mm by FEA and CFD methods. Du used the low-order panel method based on the velocity potential combined with the equal-pitch wake vortex model to predict the steady hydrodynamic performance of the rim-driven thruster, and found that the pressure difference on the blade is mainly distributed on the leading edge. In the vicinity, the load is reduced by the influence of the rim edge at the large radius. 11
At present, there are few researches on the internal flow mechanism of rim-driven propellers. The axial flow water jet propulsors are both axial flow fluid machinery, and the internal flow law has mutual reference value. Verbeek has found that the water inlet channel will cause a shaft power loss of 7%–9% through experiments.12,13 And compared with the numerical simulation, it is found that the existence of the water inlet channel causes the uneven pressure distribution, and the non-uniform inflow is a major factor in the reduction of the propulsion efficiency. The impact of the secondary flow of the elbow, the disturbance of the drive shaft, the flow separation at the lip or slope, and the ship’s bottom boundary layer on the water jet pump is successively analyzed by Bulten, 14 Duerr, 15 Brander and Walker. 16
It can be seen from the above that the rim-driven propeller avoids the disturbance of the shafting structure to the inlet flow field by cancelling the shafting structure. Secondly, the performance of the hubless propeller is better. Finally, the special flow structure, such as the secondary flow and boundary separation, is the main factor of thruster loss. This manuscript will study the performance characteristics of the rim driven thruster by experimental methods and carry out the numerical simulation to explore the internal flow characteristics and reveal the flow loss mechanism.
Introduction to the test
The external characteristics of the rim driven thruster are tested to verify the accuracy of the numerical simulation.
Test rig
The rim driven thruster performance test system was built to test the energy performance characteristics of the rim-driven thruster. The Figure 1 shows the research model of the rim driven thruster and the Figure 2 shows the schematic diagram of the rim drive thruster performance test rig. The main performance parameters of rim driven thruster are the rotating speed n = 1450 rpm, the rated flow rate Qd = 1300 m3/h, the rated head H = 6.5 m. And the geometric parameters are the diameter of impeller D = 300 mm, the blade number Z = 7, virtual hub ratio dh/D = 0.2.
Physical image of rim driven thruster: (a) physical image of impeller and (b) physical image of research model.
Schematic diagram of the rim drive thruster performance test rig.
Measuring point layout
As shown in Figure 3, the inlet pressure measuring point is located at 2 × D before the thruster inlet, and the outlet pressure measuring point is located at after the thruster outlet.
The test rig of the rim driven thruster.
The test equipment
The external characteristics acquisition system mainly includes measuring instrument and acquisition system, the inlet and outlet pressure sensor, flowmeter, and so on. The measuring instrument is applied to collect the inlet pressure, outlet pressure, power, and efficiency. The head is calculated by the inlet and outlet pressure. The flow rate can be measured by the electromagnetic flowmeter.
The measuring of the inlet and outlet pressure
The pressure sensors shown in Figures 4 and 5 are applied to the measuring of inlet and outlet pressure. The measuring range of inlet pressure sensor is ±100 kPa and the measuring range of outlet pressure sensor is 0–1 MPa. Their measuring accuracies are 0.5% FS.
The inlet pressure sensor.
The outlet pressure sensor.
The measuring of the flow rate
The electromagnetic flowmeter is applied to measure the flow rate and the working pressure is 1.6 MPa. The measuring accuracy is 0.5%.
The rotating speed and power
The input power and rotating speed can be acquired by the frequency converter and its components.
Therefore, the system error calculated by above is 0.67%.
Results and analysis
The Figure 6 shows the external characteristics curve of the rim driven thruster and the head H is 5.95 m, the power P is 39.1 kW, and the efficiency η is 54.1% under rated condition.
The external characteristics of test: (a) the flow rate-head performance curve, (b) the flow rate-power performance curve, and (c) the flow rate-efficiency performance curve.
It can be seen from Figure 6(a) that the head of the propeller first increases and then decreases as the flow rate increases and the head curve has a hump. When the flow rate is 600 m3/h under design rotating speed, the head of the thruster is the highest. As the speed decreases, the flow rate corresponding to the hump peak gradually decreases.
It can be seen from Figure 6(b) that the power first increase and then decreases with the decreasing of flow rate, and the power is the largest under Q = 500 m3/h at design rotating speed. As the speed decreases, the power consumed by the thruster also gradually decreases.
The Figure 6(c) shows that the efficiency first increases and then decreases with the increasing of flow rate, and the efficiency of the propeller is the highest when Q = 1400 m3/h at design rotating speed. With the decrease of the speed, the flow rate corresponding to the highest efficiency becomes lower and lower, but the highest efficiency slightly increases, which is due to the flow loss is smaller when the rotating speed is lower and causes the increasing of efficiency.
Numerical simulation and result analysis
This manuscript will study the inner flow characteristic of the rim driven thruster to reveal the hump characteristics of head curve.
Research model
The Figure7 shows the numerical simulation model for rim-driven thruster, which has the same parameters with the Figure 1(a).
Three-dimensions diagram of the rim-drive blades.
The Figure 8 shows that the numerical calculation model of the rim-driven thruster consists of three parts: inlet extension, impeller, and outlet extension. The length of the inlet extension is 3 × D, and the length of the outlet extension is 4 × D.
The calculation model of rim-drive thruster.
Mesh division and calculation settings
The structured grid is used for grid division of the rim-drive thruster. The partial encryption of the boundary layer are made at pressure surface, suction surface, the tip of blade, and the wall of inlet and outlet. The thickness of the first layer is 0.1 mm to ensure that the calculated Y+value is between 10 and 100 and the Figure 9 shows grid schematic.
Structured grid schematic: (a) inlet extension, (b) impeller domain, and (c) outlet extension.
Calculation settings and grid independence test
The inlet boundary condition is flow speed and the outlet boundary condition is 1 atm. The near wall area adopts the non-slip wall, and the turbulence model is SST model.
As shown in Table 1, taking the head of the propeller as the standard, this manuscript divides the water body of the propeller into five sets of different numbers of grids to check the independence of the number of grids. The Table 1 shows that the influence of the number of grids on the calculation results gradually decreases, and the deviation of the head prediction is within 3% while the total of grids is 4,885,507. Choosing Scheme 3 for subsequent calculations after considering the calculation time and accuracy.
Grid independence test.
The calculation of the external characteristics and inner flow
The external characteristics of calculation
The Figure 10 shows the comparison of numerical simulation and experiment of external characteristics.
The result comparison of numerical simulation and experiment: (a) flow rate—head curve and (b) flow rate—efficiency curve.
The head of the numerical simulation is larger than the experiment under the same flow rate condition. Meanwhile the efficiency has similar trend with the head. The head relative deviation of each flow rate condition is within 5% between the numerical simulation and experimental results. Therefore, the model established in this research is accurate and reliable. The head under design conditions of thruster is 6.25 m. The head curve of the rim-driven thruster has a peak at Q = 700 m3/h with the increasing of flow rate. As the flow rate increases, the efficiency of the thruster gradually increases, and the efficiency under design conditions is 55.5%.
Analysis of the inner flow
The internal flow of the rim driven thruster under five flow conditions of 1300, 1000, 700, 400, and 0 m3/h is studied, and then reveals the cause for hump of the head -flow rate curve.
Analysis of pressure distribution and streamline
Figure 11 shows the pressure and streamline distribution of the middle section of the inlet extension under different flow rate conditions. A high-pressure zone is gradually generated near the wall from the interface between inlet extension and impeller. With the decreasing of flow rate, the radial and axial dimensions of the high-pressure zone on the wall become larger, and the axial length of the high-pressure zone is basically the same as the high-speed zone. The axial length of the wall high pressure zone is greater than 2 × D while the flow rate is 400 m3/h. In addition, streamline distortion and backflow of different scales appeared before the impeller inlet under every operating conditions. When the flow rate Q = 1300 m3/h and Q = 1000 m3/h, there is a small-scale backflow near the wall of the interface between the inlet extension and the impeller inlet. When the flow rate Q = 1300 m3/h and Q = 1000 m3/h, there is a small-scale backflow near the wall. While the flow rate is reduced to Q = 700 m3/h, the backflow area near the wall becomes larger and larger, forming two accompanying backflow areas of different sizes. When the flow rate is reduced to Q = 400 m3/h, the scale of the backflow increases significantly. As the flow rate decreases, the scale of the backflow gradually increases near the wall before impeller inlet, which destroys the stability of the impeller inlet flow, causes the flow of the impeller inlet to block, and reduces the working power of the blades on the rim side.
The pressure and streamline distribution of middle section in inlet: (a) 1300 m3/h, (b) 1000 m3/h, (c) 700 m3/h,(d) 400 m3/h, and (e) 0 m3/h.
Analysis of inner flow in the impeller
The Figure 12 shows the pressure and streamline distribution in middle section of impeller. As the flow rate decreases, the pressure in the center of the impeller gradually decreases. The pressure at tip side of blade also decreases. Especially the pressure drops faster in small flow conditions. Otherwise, the pressure at rim side of blade firstly increases, and while the flow rate Q = 700 m3/h, the pressure level is the highest. As the flow rate continues to decrease, the pressure at rim side of blade gradually decreases. The reason lies in the flow loss caused by the inlet return flow, which causes to the decrease of the pressure on the blade rim side under the small flow condition.
The pressure and streamline distribution on the middle section of impeller: (a) 1300 m3/h, (b) 1000 m3/h, (c) 700 m3/h, (d) 400 m3/h, and (e) 0 m3/h.
The Figure 12 shows that the steady backflow is generated near the impeller rim. As the flow rate decreases, the pressure gradient near the blade rim increases, resulting in a slight increase in the scale of the backflow. When the flow rate is reduced from Q = 700 m3/h to Q = 400 m3/h, the radial dimension of backflow does not increase significantly, and the flow pattern in the central of the flow channel is stable.
The pressure analysis on the blade surface
The Figures 13 and 14 show the pressure distribution in the pressure surface and suction surface of blade and the pressure distribution of the blade can reflect the ability of the blade to work.
The pressure distribution on the suction surface of the blades: (a) 1300 m3/h, (b) 1000 m3/h, (c) 700 m3/h, (d) 400 m3/h, and (e) 0 m3/h.
The pressure distribution on the pressure surface of the blades: (a) 1300 m3/h, (b) 1000 m3/h, (c) 700 m3/h,(d) 400 m3/h, and (e) 0 m3/h.
It can be seen from Figures 13 and 14 that the pressure on the pressure surface and suction surface of the blade gradually decreases from the rim to the tip, and the pressure on the pressure surface is higher than that on the suction surface. While the flow rate is reduced to Q = 700m3/h, a small area of low pressure is gradually generated near the inlet edge on the suction surface of the blade. As the flow rate decreases, the low-pressure area becomes larger, which affects the flow around the airfoil section under small flow conditions and reduces the uniformity of the flow around, and then the blade’s working capacity decreases.
The intersection line between the pressure surface and the suction surface of the blade and the transverse interface through the origin is selected as the analysis curve to quantitatively analyze the pressure distribution along the radial direction of blade surface. As shown in Figure 15, the pressure distribution along the radial distance on the blade surface analysis curve is extracted and normalized.
The pressure distribution of analysis curve on the pressure and suction surface.
The Figure 15 shows that the pressure on the pressure and suction surface of blades gradually decreases with the increasing of flow rate while the radial distance r is within 0.11–0.15 m and the pressure within 0.03–0.08 m increases with the increasing of flow rate. Meanwhile, the pressure of the blade decreases faster under small flow conditions. Therefore, the poor work done by the blades in the range of 0.03–0.08 m is a main factor that causes the hump of the head curve under the small flow rate.
The flow analysis of outlet
The Figure 16 shows that the pressure and streamline distribution on the middle section of outlet. The pressure on the middle section is large on both sides and small in the middle. The fluid from the outlet of impeller has an axial velocity, a circumferential velocity, and a relatively large pressure. The high-pressure fluid is subjected to centrifugal force and moves to the wall under the action of the circumferential velocity. However, the fluid in the center is not subjected to the work of the impeller and has a relatively low pressure. The fluid in the high-pressure area of the wall flows to the low-pressure area in the center of the impeller under the action of the pressure difference, which causes backflow structures of different scales in the outlet. As the flow rate decreases, the area of the low-pressure zone gradually increases, and the backflow gradually expands downstream. When the flow rate is less than Q = 400 m3/h, the area of the low-pressure zone increases significantly, and the scale of the backflow on both sides of the low-pressure zone increases significant, which will cause greater flow loss. On the other hand, there is a backflow of the streamlines in the middle domain of the impeller while the flow rate is less than 700 m3/h, which means the volume loss. Therefore, the flow rate is the smaller, the backflow is more serious and the volume loss is greater.
The pressure and streamline distribution on the middle section of outlet: (a) 1300 m3/h, (b) 1000 m3/h, (c) 700 m3/h,(d) 400 m3/h, and (e) 0 m3/h.
Conclusion
This manuscript studied the performance characteristics of the rim-driven thruster by experimental methods, and then took the rim-driven thruster as the research object to explore the internal flow characteristics by numerical simulation methods and to reveal the flow loss mechanism.
The test results of the external characteristics for the rim-driven thruster found that the head curve had a hump shape under the flow rate Q = 600 m3/h. On the other side, the highest efficiency of the thruster slightly increases with the increasing of rotating speed. The numerical simulation results shows that the head curve has the same trend with the test results.
Large-scale backflows gradually appear near the wall in front of the impeller inlet, the central area of the impeller outlet and the two sides of the central low-pressure zone, causing a large flow loss, resulting in a drop in the head of the rim-driven thruster. The discrepancy between the pressure surface and the suction surface of blade decreases rapidly in the range of r/R = 0.2–0.5, which is another major factor leading to the drop in head under small-flow conditions.
Structurally, there is no blade in the center of the impeller and there are the large backflow in the middle domain of impeller, which cause much volume loss and is a main cause of the decreasing of head under small flow rate conditions.
Footnotes
Handling Editor: Chenhui Liang
Author contributions
Conceptualization, Z.Z. and H.L.; methodology, Z.Z.; software, Z.Z.; validation, Z.Z. and H.L.; formal analysis, Z.Z.; investigation, Z.Z.; resources, Z.Z.; data curation, Z.Z.; writing—original draft preparation, Z.Z.; writing—review and editing, Z.Z.; visualization, H.L.; supervision, H.L.; project administration, H.L.; funding acquisition, H.L.
Declaration of conflicting interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Project support by the National Science Foundation (Grant Nos. 1601440040 and 51779108) and the second level of scientific research funding for the fifth phase of “333 Project” in Jiangsu.
