A CFD analysis on using a standardized blade in different mechanical draft cooling towers for geothermal power plants

Highlights

• A blade was proposed as a standard model for cooling towers of different sizes.

• A blade designed for a fan shroud diameter of 8.6 m was simulated in a one of 9.2 m.

• The analysis relied on an experimentally validated CFD model of the tower.

• An irregular flow below the blade and recirculation reduced the efficiency.

• The use of additional appendices made this blade suitable for a larger fan shroud.

Abstract

The spare parts standardization without excessively sacrificing energy efficiency may reduce the maintenance costs of components in different geothermal power plants of the same company.

This strategy may apply to the blades of the fan used in mechanical draft cooling towers, which usually come in different sizes and are numerous.

The case study taken as a reference is a standardized blade, designed for a fan diameter of 8.6 m, used in a 9.2 m diameter fan.

The results indicated a decrease in efficiency for the fan with standard blades. Nevertheless, some appendices proved to be effective in limiting the efficiency loss.

Introduction

Cooling towers are essential facilities in geothermal power plants as they cool the mixture from the condenser composed of the condensed steam and the cooling water. Their use is required in arid areas to allow the operation of the powerplants. amongst available technologies, geothermal powerplants generally employ cooling towers in which the draft can be natural or induced.

In the natural draft towers, the airflow depends on the density difference between the heated air in the stack and the ambient air. This working principle requires no electrical consumptions and limits the maintenance costs as the lack of moving parts. Nevertheless, it needs a large volume and a considerable height of the tower to gather sufficient airflow.

In the induced draft tower, one fan creates a controlled airflow that ensures more stable operating conditions. This working principle reduces the tower size despite increases maintenance costs and energy consumptions. In literature, Dehaghani et al. published a study in which they proposed a retrofitting process of wet cooling towers to reduce water and fan power consumptions (Taghian Dehaghani and Ahmadikia, 2017).

The rise of maintenance costs and energy consumptions becomes critical considering different geothermal powerplants in which a high number of induced draft towers operate in different sizes and working conditions. For this reason, their reduction is a desirable aspect to increase electrical production and minimizing production costs.

The standardization of spare parts can limit the operating costs of refrigeration units without excessively penalizing energy efficiency.

Amongst the spare parts, the blade is the component that is responsible for airflow and energy consumption.

Nevertheless, its use can be adapted to different towers to provide the specified airflow under a designed pressure rise without considerably increasing the fluid-dynamic losses.

Consequently, the evaluation of these aspects is pivotal to estimate the performance of the tower.

For this purpose, the CFD technique represents a suitable tool as it provides a numerical description of the blade by varying the fan shroud.

The literature presents a series of works based on CFD models that analysed the flow through fans and cooling towers. Al-Waked et al. numerically investigated the heat and mass transfer in a natural draft tower for different operating conditions and crosswind effects (Al-Waked and Behnia, 2006). Xia et al. (2016) proposed a pre-cooling water spray system for a natural draft tower, showing that a vertical arrangement of the nozzle resulted in better performances.

Venter et al. used a CFD model that represented a test facility to ascertain how the peripheral windscreens affect the performances of air-cooled condenser (Venter et al., 2021). They found that windscreen can be beneficial or detrimental depending on the operating conditions. Blain et al. developed a CFD model that incorporated the equation proposed by Poppe and Merkel to predict the aerothermal performances in a natural draft cooling tower (Blain et al., 2016). The possibility to employ the water dropping potential for the drive of fan in a cooling tower was analysed by Dang et al. (2019). They developed a CFD model to discuss the resulting airflow contours produced by fans with different diameters. Their results confirmed that an axial fan might improve within certain limits the thermal performances of a natural draft tower.

Chen et al. proposed a layout of an air-cooled condenser containing an axial fan (Chen et al., 2018). In their CFD modelling, the fan was simulated by a pressure rise depending on a polynomial function. A prior work about the CFD modelling of an axial fan employed in a mechanical draft tower was provided by Thiart and von Backström (1993). The results captured how the distortion of the inlet flow affected the fan performances. Other studies investigated numerically the mutual influence of axial fans placed along rows in an air-cooled condenser. In these configurations, the induced cross-draft flow separated and distorted at the inlets (Bredell et al., 2006; Meyer, 2005; Heinemann and Becker, 2018).

The analysis of the fan geometry was also considered in the literature. A CFD model was introduced by Galloni et al. to ascertain how the fan shape affected its performances in the cooling of electric motor (Galloni et al., 2018). Jung et al. modified the geometry of the hub of an axial fan to assess the performance variation (Jung and Joo, 2019). They observed how the pressure distribution changed on the outflow of the fan and concluded that the efficiency was maximized by a threshold value of the entrance hub length. In another work (Jung and Joo, 2018), they analysed how to reduce tip leaks flow in an axial fan. For this aim, they considered the presence of a winglet and the variation of the fan shroud height. They suggested that an optimum value of the tip clearance existed to maximize the efficiency for the considered fan geometry. Li et al. (2014) provided relations to evaluate the effects caused by the tip clearances and impeller trimming on the total pressure rise and flow rate. Ye et al. stated that the blade tip pattern determines the sound pressure (Ye et al., 2017). They found this result by numerically analysing the behaviour of a twin-stage variable-pitch axial fan. Luo et al. (2020) investigated the tip loss influence on the aeroacoustic mechanism in an axial fan and indicated that the blade tip vortex resulted in a broadband noise below 600 Hz.

These studies provided a framework to develop a numerical model that simulates the behaviour of an axial in a cooling tower. This model allowed investigating the possibility to employ a standardized blade in a larger fan size. As a case study, a blade with a fan shroud diameter of 8.6 m was chosen as the standard blade to analyse how its performance varies in a fan with a larger external diameter of 9.2 m.