Elsevier

Renewable Energy

Volume 153, June 2020, Pages 175-181
Renewable Energy

Numerical estimation of prototype hydraulic efficiency in a low head power station based on gross head conditions

https://doi.org/10.1016/j.renene.2020.01.113Get rights and content

Highlights

  • Hydraulic performance estimating method based on gross head conditions for low-head prototype turbines.

  • Net head change as regards the flow interaction between the turbine unit, intake, tailrace.

  • A concept of Plant Hydraulic Efficiency was introduced.

Abstract

Low-head hydraulic power units typically have a short intake pipe or no that structure. For this reason, the upstream and downstream flows in reservoirs could influence the internal flow characteristics in the turbine, which sometimes increase the flow non-uniformity and the local hydraulic losses. Meanwhile, the efficiency estimation methods for prototype turbines still have a limitation for determining the unit discharge, which is generally predicted by using a Winter-Kennedy method. This paper proposes a hydraulic performance estimating method focusing on low-head turbine units, which is conducted by using CFD analysis and based on site test results. A bulb type turbine unit was adopted as a research object, which is operating in a low-head run-of-the-river power station. The flow behaviors in turbine were simulated based on two water levels under single unit operation, with two reservoir modeling. The simulation results agreed well with test ones for the turbine power as well as the relative discharge curve. The prototype efficiency in operation was estimated at the same time. In addition, this paper analyzed the relationship between the gross head and net head, and introduced the concept of plant hydraulic efficiency, as regards the flow interaction between the turbine unit, intake and tailrace.

Introduction

Run-of-the-river plant commonly utilizes low-head hydraulic resources, in which the river continuously flows from the plant upstream into downstream. It has relatively low environmental impacts, but requires higher project costs relative to the turbine output, in comparison with high-head plant projects. The water levels in plant vary in response to seasonal river flows, and the change is large relatively to the turbine head. As a result, low-head Kaplan turbines or bulb turbines are generally adopted to operate with the high hydraulic efficiency in a wide range of operating heads.

Low-head hydraulic turbines are characterized by low Froude number and high specific speed, in this regard the flow field in turbine is more governed by the vertical pressure gradient generated by the gravity force rather than the horizontal one, by two water levels at the unit upstream and downstream. The vertical distribution is more obvious with decreasing the turbine discharge, at a constant operating head. It has been a major issue in horizontal type turbines as well as vertical ones [[1], [2], [3]]. For the horizontal units, it can increase the pressure pulsation on runner blades, which could more influence the dynamic response behavior of the rotating runner as the diameter increases. For the vertical turbines, it could more influence the cavitation performance of unit as the blade chord length and the installation angle increase.

Low-head hydropower plants are generally designed with short penstock structures or without ones, as a result, the intake design is quite important to minimize the minor hydraulic losses. Since the available head and the discharge are variable but also in a wide range, the intake channel would have different performances at each operating condition. In addition, the flow diffusion performance at the draft tube as well as into the tailrace would be also important for low-head hydropower units. The interaction between the swirling flow in draft tube and the open-channel flow at tailrace aggravates flow non-uniformity, depending on each tail water level.

Furthermore, free surface fluctuations and vortices, at the intake upstream and the unit downstream, could have an effect on the flow characteristics in turbine due to the short inlet channel and draft tube, which increase not only flow non-uniformity but also flow instability [4]. Sometimes, they even increase the air content in the turbine unit, and lead to complicated flow behaviors characterized by two-phase flow of water liquid and air as well as water vapor.

Various numerical methods have been proposed to predict the turbine hydraulic performances of the prototype units as well as the scaled model units, for those low-head units [[5], [6], [7], [8], [9]]. One of them is to use an extended turbine inlet or draft tube channel, which is conducted to consider the effects of the flow non-uniformity at turbine inlet and outlet boundaries, due to short channel lengths. Another one is to model the intake channel but without the upstream reservoir side. Enomoto [10] established a full flow passage model consists of a turbine unit and other model test equipment. They showed that the simulated results agreed well with the test ones. These numerical modeling indicate that the flow conditions at the turbine inlet and outlet boundaries have an important role in predicting flow behaviors in the low-head units.

Prototype turbine performances are difficult to measure, especially for low-head units. The prototype discharge is typically estimated by pressure difference methods such as the Winter-Kennedy method [11]. However, those methods are influenced by changing inflow conditions in turbine [12]. It should be noted that the low-head unit is more sensitive to the inflow conditions [2,[13], [14], [15], [16], [17]]. It means that the discharge coefficient is sometimes not constant, and it could depend on different installation positions for measuring pressures. It would result in significant difference between the prototype and scaled model units. In addition, higher project cost is usually required for low-head power plants [18]. Therefore, hydraulic performance estimating method for low-head unit prototype should be improved with the turbine inflow effects, experimentally or numerically.

Various researchers introduced the numerical models for predicting flow behaviors in prototype units, with reservoir modeling as the computational domain [2,[19], [20], [21]]. Furthermore, the flow interactions in between the reservoir and a turbine were studied with two-phase flow phenomena, such as the surface vortex or fluctuation. Ahn [4] numerically investigated the effect of surface vortices on operating conditions of a tidal power turbine, which was conducted based on air-core vortex simulations [22]. They indicated that the reservoir modeling plays an important role in predicting flow characteristics in turbine and estimating prototype hydraulic performances. However, most of them employed a simplified reservoir model as a computational domain, to simulate full passage flows of the unit.

The geometrical factors could have a significant influence on the simulation results, for example, the minor losses differ by those factors, such as a sudden or gradual change in flow cross-sectional area, the inlet edge shape and the bend shape, not only at the reservoir side but also at the turbine intake and tailrace. Therefore, all of the geometrical factors should be considered as much as possible when modeling the 3D reservoir computational domain, to more reliably predict and assess turbine prototype performances.

In this paper, a hydraulic performance estimating method is proposed for low-head prototype turbines. A bulb turbine is adopted as the research object, which is operating in a low-head run-of-the-river power plant. A simulation method is proposed to predict turbine flow behaviors based on the water level conditions. The computational domain is modeled with considering most of the geometrical factors of the intake and tailrace channels. Hydraulic performances are estimated in the case of single unit operation, by using the simulation combined with the site test. Furthermore, this paper analyzes the relationship between the gross head and net head, in terms of the flow interaction between the turbine unit, intake and tailrace. Finally, a hydraulic efficiency concept is introduced to analyze low-head unit operations, which is called the plant hydraulic efficiency.

Section snippets

Bulb turbine prototype

This paper adopted a bulb turbine unit as the research object, which is operating in a low-head run-of-the-river power plant. The prototype hydraulic performances will be assessed by using the proposed method described in section 3. The turbine unit consists of a bulb body, 3 runner blades, 16 guide vanes, an inlet channel and a draft tube, as shown in Fig. 1. The runner blades and guide vanes are adjustable. The turbine design parameters are as follows: runner diameter of 4.3 m, rated head of

Method of hydraulic performance estimation

The prototype turbine power and the water level conditions can be obtained from the site test. The net head of turbine and discharge conditions are still unknown. The pressure difference for estimating discharge could be also measured at each operating condition, simultaneously. However, the surrounding flow conditions of a turbine unit sufficiently influence prototype operating conditions as described in section 1. It means that the existing method for estimating the turbine discharge has

Numerical result validation

Fig. 7, Fig. 8 show the comparison result between the simulation and site test for the relative turbine power and discharge. The rated power was recorded at the runner blade opening of 70.59% and the guide vane opening of 74% for the lowest water level condition. It was estimated as a high efficiency point than other rated conditions of different blade openings, during the performance test. The relative power in the figure is divided by the rated power, and the difference between them is within

Plant hydraulic efficiency

The present paper proposes an efficiency concept for low-head units. They are significantly influenced by inflow and outflow conditions, as the prototype unit head decreases. Fig. 12 shows the flow behavior at the turbine intake under the rated operating condition. There are two vortex structures, which have similar behaviors to the upstream flow in single unit operation, as shown in Fig. 2. The generated vortex flows into the turbine unit and obviously increases the flow non-uniformity. In

Conclusions

The present paper proposed a hydraulic performance estimating method for low-head prototype turbines. A bulb turbine was adopted as the research object, which is operating in a low-head run-of-the-river power plant.

A simulation method was established to predict internal flow behaviors in turbine, based on the gross head condition. Most of the geometrical factors were considered to model a turbine unit and the intake and tailrace channels with the 3D reservoir modeling. The numerical results

Acknowledgments

This research was funded by National Natural Science Foundation of China (No. 51876099 and No. 51779122).

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