1. Introduction
Along with the increasing energy demand, it is urgent to develop renewable and environment-friendly energy. Since the oil crisis of the 1970s, small hydropower has gained great attraction. Compared to the huge hydraulic turbines, pumps are easy to manufacture and maintain, and easily available all over the world. Experiments have shown that, at relatively low power outputs [
1], pumps with high technological standards can compete with conventional turbines in terms of maximum efficiency in reverse operation [
2].
An experimental study of district heating in Sweden shows that pump as turbine (PAT) can recover energy and supply power, thus reducing operating costs and providing temporary heating as a pump in case of failure of the main pump [
3,
4]. In another study, PAT was applied to the regional heating system in Poland for experimental study. The experimental results show that venue operating system (VOS) needs to be introduced for stable energy recovery. In addition, by using PAT instead of the pressure-reducing valve, the noise and vibration of the pipe network can be reduced and the damage of the pipe can be prevented or reduced [
5,
6].
The hydraulic losses in PAT are due to the long flow passage and unmatched fluid flow across the turbine, as it is not designed to run in reverse flow [
7]. Modifications to optimize the performance were proposed to enhance the hydraulic characteristics and consequently increase the efficiency [
8]. The proposed modifications include trimming the impeller blades, adjusting the blade number, adding splitter blades, installing guide vanes, and rounding impeller leading edges [
9,
10,
11,
12]. Experimental and numerical investigation into impeller trimming to the influence of PAT was carried out on a single-stage centrifugal PAT by Yang et al. [
13,
14]. As impeller diameter is cut down, its geometrical parameters of impeller diameter, blade wrap angle, impeller inlet width, and blade inlet angle are changed.
In [
14], it was proposed to add splitter blades between the original blades of PAT. Results show that splitter blades have a positive impact on PAT performance. With the increase in splitter blades, its required pressure head is dropped and its efficiency is increased. In the study of Patel et al. [
15], guide vanes were added at the entrance of the impeller in volute of PAT. The impeller and eight fixed guide vanes provided at 75 degrees led to improved performance of PAT. Although the inflow angle at the entrance of impeller has improved, these two methods undoubtedly increase the manufacturing cost of PAT. Zeng et al. [
16,
17] proposed the concept of “Impulse Pump—as—Turbine (Impulse PAT)”, where the centrifugal pump impeller is used as a hydro turbine by pairing with the spear valve injector from an impulse hydro turbine. The spear valve injector regulates water inlet flow rate and, thus, regulates power output of the new concept turbine. The validation methods from 1D calculation based on Euler’s turbine equation and numerical simulation by commercial CFD package shows that this new proposed concept is feasible, with efficiency around 40% [
18,
19].
Among all modifications, rounding the leading edge of the impeller is the simplest way to increase PAT efficiency. This modification can be achieved using simple tools. The purpose of rounding the impeller leading edges is to reduce the jet wake produced by sharp edges that caused flow separation. Ashish et al. [
20] tested four kinds of centrifugal pumps with different rotating speed, and tested the effect of blade rounding and shroud rounding separately. Studies on rounding the impeller leading edges show enhanced hydraulic performance. There is an increase in efficiency between 1% and 2.5%. Experimental results show that, on most of the testing PATs, the efficiency of inner and outer shroud rounding is improved but, compared with blade rounding, the effect is not obvious. The effect of blade rounding plays an important role in the whole effect of the impeller rounding. Impeller rounding effectively reduces the loss within the flow region [
18]. Shingh et al. [
21] conducted experimental tests on a variety of PATs in a wider specific speed range and obtained positive results, and proposed to standardize the modification of impeller rounding on all PATs.
For the effect of impeller rounding on PAT, many researchers have conducted experiments with higher precision. In [
10,
11], theoretical models have been established, then a sophisticated experimental platform has been constructed to test the performance of various PATs, and the author proposes a comprehensive application of the modified method impeller rounding on all PATs. However, the flow conditions inside the PAT, such as pressure distribution and flow separation area, cannot be observed only through experimental methods.
Computational fluid dynamics (CFD) is a research method that aims to discretize the Navier–Stokes equations of nonlinear partial differential equations describing fluid phenomena into algebraic equations and obtain approximate solutions using numerical methods. The optimization process of PAT internal hydraulic performance can be accomplished through a comprehensive understanding of the flow visualization. The visualization of interfaces between fluid flow, rotating impeller, and pump casing can be achieved through simulation modeling. In addition, simulation modeling has the ability to investigate the fluid interaction, and pinpoint the root cause of the performance in the turbo machinery study. The objective of this paper is to study the effect of rounding the leading edges of impellers on PAT performance through computation simulation modeling. This enables internal hydraulic analysis that can determine the hydraulic changes causing the variations in PAT performance, which provides more comprehensive evidence for the study of theoretical models and experimental methods.
2. Structure of Impeller Rounding
The experimental evaluation of mass transfer distributions and oil flow visualization in long and short 90° elbows were conducted in a water tunnel [
22]. It consists of a tank, a pump, and a settling chamber. Recently, in the study of PAT performance optimization, the effect of impeller rounding has been paid attention by many researchers. The authors of [
23] compare three rounding schemes: (1) taking two times the gap width as the rounding radius; (2) taking one half of the blade thickness as the rounding radius; (3) on the basis of scheme (1), using small radius rounding for the back outer edge of the blade. Considering that the hydraulic efficiency is greatly improved and the head drop is small, the rounding method is adopted in this paper. In this study, impeller rounding can be divided into two modifications of impeller, namely, blade rounding and shroud rounding, and the fillet radius is one half of the blade thickness. The extent of the rounding of the peripheral edges of the impeller is shown in
Figure 1, which comprises of ‘bullet shaped’ rounding of the blade edges in the front view and the shrouds in the side view. In order to understand the modification of blade rounding more intuitively,
Figure 2 shows a schematic diagram of the changes before and after blade rounding.
In order to study the influence of impeller rounding on the flow region, the variable to be considered is the radial velocity [
24]. The flow rate can be represented by the radial flow velocity component at the inlet impeller area, as shown in Equation (1):
where
is the radial velocity at the impeller inlet, and
is the flow area of the impeller inlet.
As seen from Equation (1), radial flow velocity depends on the flow rate and the flow area under consideration. It can be seen from
Figure 1 that the impeller rounding increases the flow area at the impeller inlet. The change to the radial flow velocity component causes a change to the inlet vortex angle as well. However, as seen from the velocity triangles in
Figure 3, the change to radial flow velocity could affect the relative flow entry (β1) at the inlet, which could, in turn, have an influence on the hydraulics in zone I and zone II.
5. Conclusions
In this study, the effect of the optimization method ‘Impeller rounding’ on PAT was simulated by CFD numerical analysis. Simulation results show that impeller rounding has improved the efficiency of PAT, and, through the observation of the internal flow region, the reason for the performance improvement has been analyzed. First, compared with the original model, the efficiency of the rounded impeller PAT is improved under all test conditions. In addition, the increase is more obvious at higher rotating speeds. This shows that impeller rounding is very beneficial for the common PAT with small diameter and high rotating speed.
The reason for the increase in efficiency is that, in terms of shaft power, the head is reduced in almost all operating conditions. In addition, when both the shaft power and the head are reduced, the head is reduced more, which leads to an increase in efficiency. The reduced flow obstruction at the entrance of the modified impeller is the main reason for the head drop, which shows that water passes better when passing through the rounded impeller. It can be observed from the visualization results that the pressure distribution inside the rounded impeller is more uniform, and the wakes and secondary flow are also reduced. CFD and flow visualization results have important reference significance for the study of the effect of impeller rounding on PATs.