4.1. Pump Unit Energy Characteristic Analysis
Following the numerical computation results, Equations (1) and (2) were used to compute the numerical computation head and efficiency of the axial flow pump units, as shown in
Table 4. The test parameters under different flow conditions were obtained through the test, and the test head and efficiency of the axial flow pump unit were calculated using Equations (6)–(8), as illustrated in
Table 4. A comparison of the numerical calculations and the test energy characteristics is displayed in
Figure 5.
According to the test results shown in
Table 4, the highest efficiency working condition of the pump unit is
Q1 = 333.27 L/s, head
H1 = 6.718 m, and efficiency
η1 = 80.38%. After the origin interpolation analysis is conducted, we can obtain the design working condition
Q2 = 350 L/s, head
H2 = 5.065 m, and efficiency
η2 = 79.56% as outlined in
Table 1. Further, we can obtain the axial flow pump design working condition
Q3 = 350 L/s, head
H3 = 5.0 m, and efficiency
η3 = 80.0% at the time the difference in the head is Δ
H = 0.065 m and the efficiency difference is Δ
η = 0.44%. This indicates that the design point is accurate. Moreover, the difference between the efficiency of the highest efficiency working point and the efficiency of the design working point Δ
η = 0.82% indicates that the design working condition is within the high-efficiency operating condition, thereby meeting the design requirements. The maximum working head is at the beginning of the saddle area—corresponding to a head of
H4 = 9.027 m—which is about 1.78 times that of the design head; this, therefore, indicates that the pump unit can operate in a wider range of conditions, which is more conducive to the efficient, stable, and safe operation of the pump unit.
The comparative analysis displayed in
Table 4 and
Figure 5 shows that the test head is slightly higher than is shown in the numerical calculations in respect to the low-flow condition (
Q = 100~330 L/s). Although the test efficiency is slightly lower than the numerical calculation in the high-flow condition (
Q = 330~457 L/s), the difference between the efficiency in the flow condition (
Q = 100~330 L/s and
Q = 400~457 L/s) is not significant. In addition, the difference in the flow condition (
Q = 330~400 L/s) shows the difference increases; further, the numerical calculation efficiency is higher than the test value and the error margins of both the numerical calculation and test comparison are within 5.0%. As such, in summary, the error values of the numerical calculation and test measurement established in this study is small and, therefore, the numerical calculation results are credible.
According to the numerical calculation results, the axial flow velocity distribution uniformity, velocity weighted average angle, and inlet channel hydraulic loss at the impeller inlet of the axial flow pump unit are calculated using Equations (3)–(5), respectively. These results are displayed in
Table 5, according to which
Figure 6 and
Figure 7 can be drawn.
The design of the inlet channel should take into account the small hydraulic loss, while also providing uniform flow inlet conditions for the impeller. The outlet of the inlet channel is the inlet of the impeller chamber, and its axial velocity distribution uniformity,
Vzu, reflects the advantages and disadvantages of the inlet channel design. The closer
Vzu is to 100%, the more uniform the axial velocity distribution of the inlet channel outlet water flow, and the more uniform the water flow into the impeller in the same direction. As shown in
Table 5 and
Figure 6, we can see that the uniformity of the flow velocity at the impeller inlet of the open inlet channel increases gradually with the flow rate. It was recorded as 80.675% for the design working condition (
Q = 350 L/s) and the streamline can enter the impeller domain evenly. The open inlet channel axial flow pump rig, when compared to the axial flow pump, increased the open inlet channel and inlet flare, resulting in the axial flow pump impeller inlet conditions becoming worse. Additionally, as displayed in
Table 5, it can be seen that in the axial flow pump rig impeller inlet the flow velocity uniformity is only 80.675%, which is the ideal state for when the axial flow pump impeller inlet flow velocity uniformity is close to 100%. When compared to the axial flow pump, the pump rig not only increased the open inlet channel hydraulic losses, but also the part, impeller, guide leaf, and outlet channel due to the increase in the bad flow state. Further, hydraulic losses will also increase and ultimately lead to a lower head and lower efficiency.
In addition, the axial velocity weighted average angle,
θ, reflects the design quality of the inlet channel Moreover, the closer it is to 90°, the better the directional velocity of the outlet flow of the inlet channel is. From
Table 5 and
Figure 6, it can be seen that the velocity-weighted average angle at the impeller inlet is as low in the low-flow condition (
Q = 100~330 L/s); however, this gradually improves with the increase in the flow rate. In regard to the design condition (
Q = 350 L/s), the velocity-weighted average angle reaches 79.223°, and the curve increment decreases, also the value of velocity-weighted average angle at the high-flow condition (
Q = 350~457 L/s) remains essentially flat. The velocity-weighted average angle at the outlet of the inlet channel, under the design working condition (
Q = 350 L/s), could indicate that the inlet channel can provide good water inlet conditions for the impeller.
The Inlet and outlet channel hydraulic losses is shown in
Table 6, through
Table 6 and
Figure 7, we can see that the hydraulic loss of the inlet channel satisfies the quadratic function and that the hydraulic loss curve of the inlet channel can be obtained by fitting
hf = 0.5517
Q2 (fit:
R = 0.9958,
Q unit: m
3/s, and
hf unit: m). Further, the hydraulic loss of the inlet channel is 0.0676 m when at the design condition (
Q = 350 L/s). In addition, when the hydraulic loss of the outlet bend meets the opening upward parabola, through which the fitting can be derived from the outlet bend, the hydraulic loss curve is
hf = 29.439
Q2 − 22.27
Q + 4.4992 (fit:
R = 0.9599,
Q unit: m
3/s, and
hf unit: m). Moreover, the outlet bend hydraulic loss is 0.3183 m and the design conditions (
Q = 350 L/s) near the outlet channel hydraulic loss curve are located at the bottom of the parabola, indicating the smallest instance of hydraulic loss.
4.2. Analysis of the Internal Flow Characteristics of the Pump Unit
In the numerical calculation results of the open inlet channel axial flow pump unit, the flow rates of
Q = 250 L/s (0.714
Qd),
Q = 300 L/s (0.857
Qd),
Q = 350 L/s (1.0
Qd),
Q = 400 L/s (1.143
Qd), and
Q = 450 L/s (1.223
Qd) show that five conditions were selected for the analysis of the internal flow characteristics of the pump unit. In order to better analyze the internal flow characteristics of the open inlet channel, three typical sections were selected, as illustrated in
Figure 8. Here, A1 is the horizontal section over the center of the impeller, A2 is the longitudinal section perpendicular to the incoming flow direction over the center of the impeller, and A3 is the longitudinal section parallel to the incoming flow direction over the impeller center.
The streamlines and velocity distribution of the open inlet channel axial flow pump unit at flow conditions
Q = 250 L/s (0.714
Qd),
Q = 300 L/s (0.857
Qd),
Q = 350 L/s (1.0
Qd),
Q = 400 L/s (1.143
Qd), and
Q = 450 L/s (1.223
Qd) for sections A1–A3 are illustrated in
Figure 9,
Figure 10 and
Figure 11.
As demonstrated in
Figure 9, it can be seen that the open inlet channel has a more uniform distribution for each flow streamline on the left side of the impeller domain. Moreover, there is a vortex at the center of the rear wall due to the backflow of water hitting the rear wall between the right side of the impeller domain and the rear wall. The side wall of the flow channel, near the wall surface due to the side wall effect, the wall velocity is close to zero, whereas the side wall flow velocity stratification is greater, therefore illustrating uniform gradient changes. The streamline and velocity distribution of the open inlet channel illustrate the axisymmetric distribution along the central axis of the inlet channel, which indicates that the numerical calculation results are in accordance with the fluid mechanics theory.
As shown in
Figure 9, in the low-flow working condition (
Q = 250~350 L/s), the stratification effect of flow velocity in the inlet channel near the impeller domain is more obvious. However, the flow velocity distribution is not uniform as there is a semi-circular low-velocity area in front of the impeller domain and there are two symmetrical semi-circular high-velocity areas on the left and right sides of the impeller domain. In addition, both sides of the back wall of the flow channel is a slow velocity zone, the back wall zone has less water movement, and is approximately a stagnant water zone. Under the design condition (
Q = 350 L/s), the stratification effect of flow velocity in the inlet channel near the impeller domain is improved, and there is only a low-velocity semicircular region in front of the impeller domain. There is no sudden change in flow velocity in the region on the left and on the right sides of the impeller domain. Further, the flow velocity in the back wall region increases, but it is still small. In the high-flow working condition (
Q = 350~428 L/s), the stratification effect of flow velocity in the inlet channel near the impeller domain is further improved. In addition, the area of the low-velocity semi-circular region in front of the impeller domain is reduced, but there are two symmetrical high-velocity semi-circular regions on the left and right sides of the impeller domain, and the low-velocity region in the back wall area is further reduced. There is a vortex area at the center of the back wall under each working condition; however, the location and size of the vortex area remain essentially similar as the flow rate increases.
As seen in
Figure 10, it can be determined that the flow velocity is higher near the inlet flare, under each working condition, and the high-velocity area is distributed in a ring shape, which decreases in a gradient from the center of the flare to the surroundings. With the increase in the flow, the ring area of the high-speed area gradually increases. From the distribution of streamlines in
Figure 10, it can also be demonstrated that the streamlines contract towards the flare mouth and that the top streamline of the inlet channel is more uniform compared to the bottom.
As per
Figure 11, it can be seen that the internal flow speed of the open inlet channel is lower, the streamline before the flare inside the inlet channel is more uniformly distributed, the streamline inside the inlet channel is gathered from all around to the flare, and the flow velocity near the flare is obviously increased in a gradient. Further, the water obtains kinetic energy at the impeller; the flow velocity reaches the maximum; and the guide lobe recovers the ring volume and converts part of the kinetic energy into pressure energy. Moreover, the flow velocity at the guide lobe is reduced compared with that at the impeller and, until the water flow into the outlet bend, the flow rate is further reduced. One of the reasons for this is because the internal flow line of the outlet channel is more complex. In addition, the flow pattern is poor and the fluid masses hit one another. However, on the other hand, the kinetic energy is reduced because the kinetic energy is further transformed into pressure energy and position potential energy, which also means that the velocity is reduced.
At low flow rates (
Q = 250~350 L/s), there is significant outflow and backflow at the outlet of the guide leaf due to the low flow rate. In regard to the design conditions (i.e,
Q = 350 L/s) and at high flow rates (
Q = 350~428 L/s) the outflow phenomenon is improved and almost disappears; further, the streamlines inside the impeller and guide leaf are more uniform. In the small flow condition (
Q = 250~350 L/s) and the design working condition (Q = 350 L/s), flow velocity distribution at the outlet of the bend outlet channel is more uniform compared with the high-velocity condition (
Q = 350~428 L/s), and there is no vortex in the bend outlet channel, which means that the flow pattern of the outlet is comparatively reasonable. In addition, there is an obvious high-velocity and low-velocity interaction zone inside the bend outlet channel in the high-flow condition (
Q = 350~428 L/s). Moreover, the flow velocity distribution is not uniform, which seriously affects the conversion of kinetic energy and the recovery of pressure energy of the outlet. In summary, the flow pattern of the discharge water under the design condition (
Q = 350 L/s) is relatively good, which can also be illustrated by the hydraulic loss curve of the outlet channel, as shown in
Figure 7.
The 3D streamlines of the open inlet axial flow pump unit at flow conditions
Q = 250 L/s (0.714
Qd),
Q = 300 L/s (0.857
Qd),
Q = 350 L/s (1.0
Qd),
Q = 400 L/s (1.143
Qd), and
Q = 450 L/s (1.223
Qd) are illustrated in
Figure 12.
From
Figure 12, it can be seen that the streamlines distribution of the open inlet channel under various flow conditions is relatively uniform. With the increase in flow, the streamlines located on both sides of the flare section converges toward the middle. The streamlines of the inlet channel along the inlet direction on the left and right sides are generally symmetrical with respect to the A3 surface. Under small flow conditions (
Q = 250~350 L/s), the streamline inside the outlet bend is generally divided into two streamlines close to the inside and outside of the bend. The intertwining phenomenon of the internal streamlines is obvious, which is the main reason for the large hydraulic losses under this condition. With the increase in the flow rate, the uniformity of streamline distribution in the outlet channel of bend under design conditions (
Q = 350 L/s) and large flow conditions (
Q = 350~428 L/s) is improved; further, the best distribution is achieved at
Q = 350 L/s.