Numerical simulation of thermal performance for super large-scale wet cooling tower equipped with an axial fan

https://doi.org/10.1016/j.ijheatmasstransfer.2019.01.111Get rights and content

Highlights

  • The super large-scale natural draft wet cooling tower (S-NDWCT) model is established.

  • The water dropping potential energy utilization device is introduced.

  • Two different airflow uniformity coefficients ψvel and ψtem are discussed.

  • Thermal performance for S-NDWCT equipped with an axial fan is analyzed.

Abstract

For the super large-scale natural draft wet cooling towers (S-NDWCTs), the higher rain zone produces water dropping potential energy which can be used to drive an axial fan, meanwhile, the larger diameter deteriorate the whole ventilation performance. Based on these issues, a three dimensional (3D) numerical model for a S-NDWCT equipped with an axial fan was established to analyze the thermal performance at different fan diameters and fan power. In order to evaluate the influence of fan, one dimensionless number m, represents the ratio between the fan diameter and the cooling tower diameter, was introduced in this paper, as well as air velocity uniformity coefficient ψvel and air temperature uniformity coefficient ψtem. Simulation results manifested that, compared with natural draft pattern, the thermal performance and ventilation performance of S-NDWCT with an axial fan improve partly according to these two uniformity coefficient and several thermal performance parameters, and they improve continuously with the increasing of fan diameter and fan power. At the given fan rotate speed (20 rpm), the water temperature drop ΔT, ventilation rate G, Merkel number N and cooling efficiency η enhance persistently as the diameter of the fan increases, while these parameters enhance firstly, and then reduce at the given power (300 kW). Under 15.0 m fan diameter (m = 0.125) and 300 kW fan power conditions, compared with natural draft pattern, ΔT, G, N, and η all reach to the maximum of 9.31 °C, 31,549 kg/s, 1.65 and 53.5%, and enhance by 0.14 °C, 611 kg/s, 0.04 and 0.8%, respectively. It demonstrates that the cooling tower shows out the outstanding thermal and ventilation performance when the diameter ratio m is 0.125.

Introduction

Cooling towers are used to extract heat from hot water to the atmosphere [1], [2], [3], [4]. The natural draft cooling towers (NDCTs) consist of mainly dry cooling towers (NDDCTs) [5], [6], [7] and wet cooling towers (NDWCTs). Nowadays, the geometric volume of NDWCT becomes more and more larger since the large-scale generator unit appears, and it can be called as super large-scale natural draft wet cooling towers (S-NDWCTs) when the bottom diameter exceeds 100 m. In large-scale thermal power plants or inland nuclear power stations, S-NDWCTs are widely used to cool the circulating water from the condenser, and the water temperature can be close to the air wet-bulb temperature which is much lower than the air dry-bulb temperature inside the NDWCT [8]. Improving the thermal performance of NDWCTs can decrease the water temperature entering the condenser, reduce steam turbine back-pressure and finally improve power generation efficiency.

In 1904, the first industrial cooling tower appears in the world. After that, experts from various countries performed many theoretical and experimental researches on them. In the aspect of theoretical study, Lewis [9] deduced the Lewis relation to calculate the heat and mass transfer between air and water, which became the theoretical basis of thermodynamic analysis for the cooling towers, afterwards, a number of researchers began to investigate the heat and mass transfer process and thermal performance of cooling towers. Fisenko [10] presented a mathematical model to evaluate the cooling performance, including water drop cooling in the water-spraying zone and film cooling in the fillings zone, and the results explained that the error between calculated and experimental results was less than 3%. Kloppers and Kröger [11], [12] made a detailed comparison between the Poppe [13], Merkel [14] and e-NTU [15] methods, and found that the Merkel and e-NTU method can give the similar results, but both of them are less accurate than the Poppe method. However, the above three methods which are used for the theoretical calculation of thermal performance are one-dimensional. In fact, the airflow is entirely three-dimensional (3D) [16] and accompanied by complex turbulent vortices, and the airflow and heat transfer process inside the tower are coupled and interactional. As a result, with the development of computational fluid dynamics, two-dimensional (2D) and three-dimensional (3D) methods became dominant and widely implemented. Recently, Ghazani [17] performed the comprehensive analysis of a model wet cooling tower by using the laws of thermodynamics, and calculated the entropy generation of every part, these conclusions can help to choose high quality fillings.

With the development of research in NDWCTs, many scholars studied the thermal performance of wet cooling towers from many aspects, including model experiment [18], [19], [20], [21], [22], [23], numerical simulation [24], [25], [26], [27], [28], [29], [30], [31] and field test [32], [33], [34], [35], [36], [37].

For the experimental research, Lemouari [18] focused on the hydraulic characteristics of the cooling tower, and studied the effect of the air and water flowrates at different inlet water temperature. Pan [19] proposed that the arrangement of water distribution in the pipe and nozzle affected the cooling effect in the design of cooling tower. Similarly, in the structure optimization of cooling tower, Gao [20], [21] studied the influence of non-uniform fillings distribution on the cooling efficiency by the thermal-state model experiment, and obtained the optimal fillings pattern. Additionally, Chen [22] performed the model experiment to investigate the effect of cross walls on the thermal performance under crosswind conditions, and the results showed that cross walls can improve the thermal performance of the NDWCTs. Wang [23] also conducted thermal-state model experiment, and studied the effect of inlet airflow guiding channels on the thermal performance under crosswind conditions. This study found that guiding channels with 70° setting angle lead to better ventilation and cooling performance.

Additionally, as one of the main research methods, the numerical simulation becomes a popular method for studying the thermal performance of cooling towers. Hawlader [24] and Williamson [25], [26] developed a 2D axisymmetric model to investigate the non-uniformity of flow field inside the NDWCTs, and obtained that the two-dimensional model has the ability to resolve radial non-uniformities across the tower which the one-dimensional model only computes as a bulk averaged value. Besides, AL-Waked [27], [28] developed a 3D CFD model to simulate both the water flow in the fillings and droplets in the water-spraying and rain zones, and analyzed the effect of crosswind on cooling performance. The simulation results can guide the design and optimization research in the future. Kalimanek [29] presented a study on numerical modeling of a natural draft wet-cooling tower with flue gas injection, and derived that the injected flue gas has insignificant influence on the cooled water temperature. For optimized design of cooling tower performance, Xia [30] proposed and numerically investigated a closed wet cooling tower with novel design, and evaluated the cooling tower performance under different operating conditions. Chen [31] proposed a novel method for improving the cooling performance of natural draft wet cooling towers (NDWCTs) by installing air ducts in the rain zone for the first time, and demonstrated that air ducts improve both the aerodynamic field and the cooling performance of the NDWCT and that the improvement is quite dependent on the crosswind velocity.

Finally, field test is also an effective method to conduct the academic research since it can overcome the shortcomings of model experiment and numerical simulation. Zou [32] and Gao [33] performed the field test on the high level water collecting wet cooling towers (HWCTs) of a 1000 MW unit to investigate ventilation and thermal performance under crosswind conditions. The test results manifested that crosswind destroys the uniformity of circumferential inflow air, and reduces wind velocity in the lateral and leeward side. Meanwhile, with the rising of crosswind velocity, crosswind appears an increasingly serious adverse effect on the thermal performance and uniformity of air temperature distribution inside tower. Zhang [34], [35] conducted field test on the NDWCT of a 135 MW unit, and proposed the concept of air inlet deflection angle and air inlet uniformity coefficient. The test results showed that crosswind increases ventilation resistance, and destroys the uniformity of circumferential air inlet. Širok [36], [37] manufactured a robot which can move over the drift eliminators to measure the velocity and temperature field above the drift eliminators. Based on this study, they also came up with the thermovision method which enables quick detection of the local efficiency of cooling towers.

In summary, the previous researchers focused mainly on the medium and large-scale NDWCTs to study the heat and mass transfer performance, and rarely involved super large-scale NDWCTs (S-NDWCTs). Moreover, seldom of them discussed the water dropping potential energy of the rain zone, and no one performed the study of strengthening ventilation by using the axial fan which can be driven by the water dropping potential energy of the rain zone. This study focuses on the thermal and ventilation performance improvement by utilizing the water dropping potential energy, and proposes a new method for both the utilization of the water dropping potential energy and the thermal performance improvement of S-NDWCTs, which can guide the further energy-saving research and optimization design of the S-NDWCTs.

Section snippets

Physical model for axial fan driven by water dropping potential energy

This paper mainly studies the influence of forced ventilation on the thermal performance of the S-NDWCT, the driving principle of the axial fan is briefly introduced. The water dropping potential energy is calculated by the theorem of kinetic energy, which is given by,E=12qΔvrd2=12qvbot2-vtop2where E is the water dropping potential energy in the rain zone, kW, q represents the circulating water mass flow rate, kg/s, vrd is the velocity of raindrops in the z direction, m/s, vbot is the final

Results and discussion

In this study, a dimensionless number m represents the ratio between the fan diameter Dfan and the cooling tower diameter Dct in the fan plane, m = Dfan/Dct. For different m, the fan rotate speed n is set to 20 rpm. The values of different m are reported in Table 5.

Conclusions

A three dimensional (3D) numerical model for a super large-scale natural draft wet cooling tower (S-NDWCT) equipped with an axial fan is established in this paper. Based on this model, the numerical calculation is conducted to analyze the thermal performance at different fan diameters and fan power. By the discussion of airflow contours, flow fields uniformity and several performance parameters, the main conclusions are as follows,

  • (1)

    After the forced ventilation is realized by the axial fan, the

Conflict of interest

The authors declared that there is no conflict of interest.

Acknowledgements

This work is supported by National Natural Science Foundation of China (51776111).

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