Case Studies in Thermal Engineering 35 (2022) 102116
Available online 13 May 2022
2214-157X/© 2022 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Effect of forced ventilation on the thermal performance of wet cooling towers
Deying Zhang
a, Nini Wang
b, Jinpeng Li
c, Jinheng Li
c, Suoying He
a, Ming Gao
a,*aShandong Engineering Laboratory for High-efficiency Energy Conservation and Energy Storage Technology & Equipment, School of Energy and Power Engineering, Shandong University, Jinan, 250061, Shandong, China
bShandong Electric Power Engineering Consulting Institute Corp. LTD, Jinan, 250013, Shandong, China
cShandong Beinuo Cooling Equipment Corp, LTD, Dezhou, Shandong, 253000, China
A R T I C L E I N F O Keywords:
Wet cooling tower Forced ventilation Crosswind environment Thermal performance
A B S T R A C T
To enhance the thermal performance of the wet cooling tower under crosswind environment, a 3D numerical model of a wet cooling tower is proposed based on the method of utilizing the water-dropping potential energy to achieve the purpose of forced ventilation of the wet cooling tower. Under the condition of an annual average crosswind speed of 5 m/s, the effects of the number, power, and installation layouts of the fan groups on the thermal performance of the tower are investigated. The results show that the forced ventilation method can effectively alleviate the unfavorable impacts of the crosswind, and the vortices area on the leeward is reduced. Within the scope of this paper, the number of fans has a slight effect on the performance when the power of the fan is less than 200 kW. When the power of the fans exceeds 200 kW, the thermal performance of the forced ventilation tower with the fan groups is obviously better than that with an axial fan. The research indicates that the cooling tower equipped with four fans has better performance. Under the power of 200 kW and 1000 kW, the outlet water temperature is about 0.12 ◦C and 0.34 ◦C lower than that of the conventional tower.
Nomenclature
c specific heat, J/(kg ·◦C) d diameter of water droplets, mm E water-dropping potential energy, kW f resistance force, N
K evaporation coefficient N Merkel number
q Water-spraying density, kg/(m2⋅s) R Radius, m
t Temperature, ◦C
→u air velocity, m/s Sφ source term
* Corresponding author. Shandong Engineering Laboratory for High-efficiency Energy Conservation and Energy Storage Technology & Equipment, School of Energy and Power Engineering, Shandong University, Jinan, 250061, Shandong, China.
E-mail address: [email protected] (M. Gao).
Contents lists available at ScienceDirect
Case Studies in Thermal Engineering
journal homepage: www.elsevier.com/locate/csite
https://doi.org/10.1016/j.csite.2022.102116
Received 4 March 2022; Received in revised form 3 May 2022; Accepted 9 May 2022
Spφ additional source term Γφ diffusion coefficient Greek symbols
χ moisture content of the air, kg/kg ρ density, kg/m3
ε relative error
α circumferential positions of fans βx Mass transfer coefficient, kg/(m3⋅s) Subscripts
a air
w cooling water i inlet variable o outlet variable z z-direction
1. Introduction
A large amount of carbon dioxide emissions from all countries in the world has made human being face the climate-changing issues.
As the largest energy consuming country, China has actively participated in the work of reducing carbon emissions and put forward the goal of “Emission Peak and Carbon Neutrality”. The power industry is the foundation of the national economy, and the reduction of emissions is urgent for it.
The cooling tower is an important part of the cold end of the thermal system. According to the contact mode of air and water, cooling towers can be classified as wet cooling towers [1,2], dry cooling towers [3], and hybrid cooling towers [4]. Because of the stable operation and high efficiency, wet cooling towers are widely used in power plants. However, the efficiency of the wet cooling tower is less than 60% and the performance is closely related to the environment, especially for the crosswind. Thus, it has huge energy-saving potential.
To reveal the influence mechanism of crosswind and explore the performance change rules of wet cooling towers, scholars had conducted lots of works. Rahmati [5] et al. obtained the temperature distribution in the wet cooling tower by infrared thermal images and the impact of operating parameters on the thermal performance. In a windless environment, the air intake and the aerodynamic field inside the tower are distributed symmetrically [6]. The crosswind changes the pressure field around the air inlet, resulting in the ambient air being difficult to enter the tower [7]. Additionally, in a crosswind environment, the air velocity in the windward is depend on the crosswind speed, and the large crosswind speed hinders the ventilation and form vortices on the leeward inside the tower [8].
Al-Waked et al. [9] found out that the water temperature drops first decreases and then increases as the crosswind speed increases. The water temperature drops decrease by 1.7 ◦C at most while the crosswind speed is 5 m/s. Through thermal model tests, Gao [10] and Rahmati et al. [11,12] obtained the same variation law of water temperature drops as in literature [9]. However, most crosswind speeds are in the unfavorable range for wet cooling towers performance.
For enhancing the performance of wet cooling towers, a lot of scholars had conducted massive studies on adjusting the uniform of air intake and relieving the adverse effects of crosswind. Through experiment results, Rahmati [13] found that the more layers of the fillings, the better the cooling performance of the cooling tower. Kumar [14] et al. calculated various operating conditions of a tower and proposed the idea of installing fillings in multiple zones inside the tower to improve the thermal performance. By thermal model tests and numerical simulation respectively, Gao [15,16] et al. and Wang [17,18] et al. showed that the tower has a better thermal performance when the fillings zone adopts a non-uniform arrangement. The aerodynamic field and the air-water ratio inside the tower are more uniform, and the outlet water temperature is significantly reduced. Chen [19] et al. took the 660 MW unit as the research object and adopted unequal space fillings. The results showed that the outlet water temperature decreased by about 0.2 ◦C. Based on the data of the model test, Dmitriev [20] et al. analyzed the thermal resistance of four types of fillings and obtained the pressure drop formula of different fillings. Shahali [21] et al. investigated the operation of the wet cooling tower under various conditions and proposed a guideline to reach the optimum thermal performance of the wet cooling tower.
Aiming at reducing the adverse effect of crosswind, Liu [22] and Zhou et al. [23] put forward the method of installing the air deflector at the air inlet, which extremely enhanced the uniformity of the cooling tower’s circumferential air intake. Al-Waked [24]
et al. investigated a series of cross-walls and found that the cooling tower using porous cross-walls has better thermal performance than solid cross-walls, and less affected by crosswind. Zhang [25] et al. altered the rain zone structure of the traditional cooling tower and put forward a new method of installing splitter plates in the rain zone to form dry and wet zones. And the results showed the outlet water temperature reduced by 0.48 ◦C. Wang [26] et al. optimized the angle and size of the splitter plates based on the research of Zhang [25].
Through the numerical method and the thermal model test, Dang [27] and Zhou [28] et al. conducted preliminary research on the wet cooling tower equipped with a fan. The results showed that the air velocity inside the tower is accelerated, and the high tem- perature zone in the center of the tower is reduced after adopting the forced ventilation method. However, the impact of crosswind and
the number of fans on the performance were not considered before.
Based on the research of literature [27,28], this paper takes the wet cooling tower of a 1000 MW unit as the research object, the effects of the number, power, and installation layouts of the fan groups on the thermal performance under crosswind environment are analyzed.
The originality of this paper can be clarified as.
(1) Based on FLUENT software, the three-dimensional numerical model of the wet cooling tower equipped with fan groups under crosswind conditions is established.
(2) Under crosswind conditions, the impact of the number and layouts of the fan groups on the thermal performance is revealed.
(3) Based on the optimized layouts of the fan groups, the change rules of the thermal performance with the power are analyzed.
Thus, the results of this paper can guide the in-depth energy-saving of the wet cooling towers.
2. Modeling
2.1. Utilizing of the water-spraying potential energy
The area of the water-spraying zone of the tower equipped for a 1000 MW unit is about 13 000 m2, which contains a tremendous amount of water-spraying potential energy. This paper is based on the following assumptions: the water turbine is set up in the water collecting basin and driven by the water droplets, and the axial fan above the eliminator is driven by the water turbine to realize the forced ventilation of the wet cooling tower (referred as the forced draft tower in this paper), as shown in Fig. 1. It should be noticed that this paper is only a preliminary theoretical investigation. It does not consider the collection of water-spraying potential energy, and the cooperation between the components of water-spraying potential energy utilization and the fans. This paper focuses on the ability of fans to raise the thermal performance of wet cooling towers in a crosswind environment. The energy loss during the transmission of each component is considered in the fan power calculation in the form of an energy loss coefficient.
Fig. 1.Schematic of spraying potential energy utilization.
Table 1
Geometric parameters of the cooling tower.
Parameters Value
Height of the tower 171.28 m
Area of fillings zone 12 944.00 m2
Height of air inlet 11.37 m
Top height of the fillings zone 13.87 m
Diameter of the bottom 137.00 m
Height of the throat 132.29 m
Diameter of the outlet 84.77 m
2.2. Physical model
According to the results of the literature [29], the optimized installation height of the fan is 18 m. Under the power of the fan is 200 kW, the thermal performance of the cooling tower is less affected by the diameter of the fan. To save the computation cost, this paper selects the fan with an installation height of 18 m and a diameter of 5 m to study the thermal performance of the tower equipped with fan groups. The effects of the number, power, and installation layouts of fan groups on thermal performance are studied at the crosswind speed of 5 m/s (the annual average crosswind speed at the location of the cooling tower). The geometric parameters and the Table 2
Design condition and environmental parameters.
Parameters Value
Inlet water temperature 31.54 ◦C
Outlet water temperature 21.11 ◦C
Air dry-bulb temperature 16.3 ◦C
Air wet-bulb temperature 14.3 ◦C
Ambient pressure 100 140 Pa
Cooling water rate 90 720 m3/h
Fig. 2.Sketch of installation positions of fans.
Table 3
Installation layouts of the fan groups.
(a) Installation layouts of 2 fans
Radial position Installation layouts
R1 12 14 16 23 26 34 36 46 56
R2 12 14 16 23 26 34 36 46 56
R3 12 14 16 23 26 34 36 46 56
(b) Installation layouts of 3 fans Radial positions Installation layouts
R1 123 126 134 135 136 156 234 235 236 256 346 356
R2 123 126 134 135 136 156 234 235 236 256 346 356
R3 123 126 134 135 136 156 234 235 236 256 346 356
(c) Installation layouts of 4 fans
Radial positions Installation layouts
R1 1234 1236 1246 1256 1345 1356 2345 2356 3456
R2 1234 1236 1246 1256 1345 1356 2345 2356 3456
R3 1234 1236 1246 1256 1345 1356 2345 2356 3456
(d) Installation layouts of 5 fans
Radial position Installation layouts
R1 12346 12456 13456 23456
R2 12346 12456 13456 23456
R3 12346 12456 13456 23456
Note: The positions of fans (1–6) in Table 3 are corresponding to that in Fig. 2.
design condition of the tower are shown in Tables 1 and 2.
In our previous research [29], three radial positions and five circumferential positions were selected to research the thermal performance of the tower with a fan. For the fan groups, there are still three radial positions, but eight circumferential positions. Thus, the layouts of the fan groups are complex and diverse. To simplify the calculation, based on the results of the previous study, five circumferential positions (α =0◦, 45◦, 135◦, 225◦, 315◦) and the center of the tower are obtained. Then the representative layouts of the fan groups are analyzed by the method of orthogonal analysis. In detail, the radial installation positions are still the radius 1/4 (R1
=15.73 m), 2/4 (R2 =31.463 m), and 3/4 (R3 =47.193 m). And the installation angle is calculated counter clockwise and the α =0◦is in the flow direction of the crosswind. The installation positions are depicted in Fig. 2. The layouts of 2, 3, 4, and 5 fans are shown in Table 3.
2.3. Mathematical model
In order to simplify the geometric model, the details of the geometric structure are ignored, for instance, herringbone columns, internal voids of the fillings, and so on. The loss of air momentum generated by these structures is expressed in the form of source terms in the governing equations. Only the main equations are illustrated in this paper, the detailed descriptions of the air and cooling water can be found in the literature [25].
2.3.1. Governing equations
The airflow is described by Equation (1), which can represent the mass, momentum, energy, and composition equations of the airflow.
∇(ρ→uφ) = ∇(Γφ∇φ) +Sφ+Spφ (1)
where ρ represents the density of humidity air, kg/m3; φ is a general variable, representing 1, air velocity, temperature, and water vapor mass fraction in the mass, momentum, energy, and composition equations; Sφ, Spφ are the source term and the additional source term of the air.
The movement of water droplets is assumed as one-dimensional, the mass, momentum, and energy equations describing the flow of cooling water are depicted in equation (2) ~ (4),
dq
d( − z)= − Sm (2)
dvwz
d( − z)=(ρw− ρ)g ρwvwz
− fz
mw·vwz (3)
d
d( − z)(cwtwq) = − Swe (4)
Fig. 3.Mesh system and boundary conditions.
where Sm, Swe are the mass source term and energy source term; vwz represent the falling velocity of water droplets, m/s; fz is the air resistance, N; tw is the water temperature, ◦C.
2.3.2. Source terms
According to the literature [30], the volumetric mass transfer rate between air and water can be expressed by formula (5),
Sm=βx(χ′′− χ) (5)
where χ′′, χ are the moisture content of the saturated air layer on the water surface and ambient air, respectively, kg/kg.
The energy source terms of humid air and cooling water are calculated by formulas (6) and (7),
Sae= (h+Smcv)(tw− ta) (6)
Swe=h(tw− ta) +Smrw (7)
where h represents heat transfer coefficient, W/(m2⋅◦C); cv is the vapor specific heat at constant pressure, J/(kg⋅◦C); rw is the latent heat of vaporization of water, kJ/kg.
2.4. Validation
2.4.1. Mesh independence
The geometric model of the cooling tower is established by ICEM, and an ambient column zone with a height of 500 m and a diameter of 600 m is established to eliminate the influence of the tower body on the computational boundary. The mesh is generated in the external environment, tower shell, and main heat and mass transfer zone respectively. In the crosswind environment, the mesh system and the boundary conditions are depicted in Fig. 3. The ground and the wall of the cooling tower are dealt as stationary walls and processed by standard wall functions; the finite volume method is used to discretize the governing equations, and the SIMPLE algorithm performs coupling iterations of pressure and velocity. In the forced draft tower, the fan is simulated by the MRF model, and the blades are set as rotating walls.
The relative error of the water temperature drops (as shown in Equation (8)) is taken as the index for the mesh independence, and the mesh system of 1 403 916, 2 137 207, 2 884 724, 4 091 812 grids are established. Table 4 shows the relative error of the design and the simulated value of the water temperature drops.
ε=|tto− to| ti− tto
×100% (8)
Where ti, to, tio are the values of inlet, simulated outlet, and designed outlet water temperature, ◦C.
From Table 4, when the mesh number is 2 884 724, the relative error of the water temperature drops is 3.4%, and the numerical result is basically unchanged with the increase of the grids. Thus, the mesh system with 2 884 724 grids is taken as a reference for the following meshing of forced draft mode.
2.4.2. Model validation
The method of model verification in Ref. [31] is adopted. Based on the design condition, only the air dry-bulb temperature is changed to obtain the design outlet water temperature. Table 5 gives the design and simulation value of the outlet water temperature (to). From Table 5, the maximum deviation of the to is 3.60%, indicating that this model can be used in the following research.
Table 4
Results of mesh independence.
Mesh number 1 403 916 2 137 207 2 884 724 4 091 812
Inlet water temperature (◦C) 31.54 31.54 31.54 31.54
Designed outlet water temperature (◦C) 21.11 21.11 21.11 21.11
Simulated outlet water temperature (◦C) 21.682 21.670 21.465 21.475
Relative error (%) 5.5 5.36 3.4 3.5
Table 5
Results of validation.
Operating condition 1 2 3 4
Air dry-bulb temperature (◦C) 14 15 17 19
Relative humidity (%) 81 81 81 81
Inlet water temperature (◦C) 31.54 31.54 31.54 31.54
Designed outlet water temperature (◦C) 19.84 20.38 21.52 22.69
Simulated outlet water temperature (◦C) 19.95 20.59 21.71 23.07
Relative error (%) 0.94 1.88 1.80 3.60
2.5. Estimation of water-spraying potential energy
The water-spraying potential energy is estimated by the diameter distribution of water droplets in Refs. [32,33], the empirical formula for the falling velocity of water droplets [34,35], and the formula of kinetic energy.
v= − 17.8951+448.9498d+16.3719d2− 45.9516d3·0.1mm≤d≤1.4mm (9) v=24.1660+448.8336d− 75.6265d2+4.2659d3·1.4mm<d≤5.8mm (10)
v= (17.20− 0.844d) × (0.1d)0.5·5.8mm<d (11)
E=1
2m•v2 (12)
where d represents the diameter of the water droplets, mm; m• is the cooling water rate, kg/s.
According to the above formulas, the available water-dropping potential energy is about 550 kW. Considering the loss in the process of energy collection and utilization, only 200 kW is used for the subsequent study.
In this paper, the wet cooling tower equipped with the fan groups (2, 3, 4, and 5 fans) under the fixed total power of 200 kW is studied at first, and the speed of the fan groups is shown in Table 6. Then, the effect of the total power on the thermal performance is conducted, and the rotational speed of the fan groups at various power is shown in Table 7. In Table 7, the diameter of 1 fan is 20 m and the fan groups are 5 m.
3. Results and discussion
First of all, the airflow field of the entire tower with and without fans is studied. Then the thermal performance variation of the forced draft tower with the different number of fans are investigated, and the optimized layouts of the fan groups are obtained. At last, the variation of thermal performance with the power is researched based on the optimized layouts of the fan groups and a fan [29].
3.1. Flowfield of the air
The fan groups inside the tower have a similar effect on the performance of the forced draft mode. The tower equipped with 5 fans is taken as an example. Fig. 4 depicts the air streamlines of the entire tower inside the conventional tower and the forced draft tower equipped with 5 fans. The radial position of the fans is R3 =41.793 m, and the circumferential positions are α =0◦, 45◦, 135◦, 225◦, and 315◦. From Fig. 4(a), the conventional tower has two symmetrical vortices on the leeward under the crosswind condition. In the forced draft tower, the fans at α =135◦and 225◦are located around the vortices. The rotation of the fans changes part of the airflow direction, reduces the amount of air that forms the vortices, and diminishes the area affected by the vortices. In addition, the air velocity is tremendously accelerated by the fans, and the air with high temperature and humidity is quickly discharged out of the tower, which is favorable to enhance the thermal performance of the wet cooling tower. In the conventional tower, the airflow on the windward is difficult to move upwards due to the inertia, and a large area of low air velocity is formed above the air inlet. In the forced draft tower, the fan located at α =0◦forces part of the air to turn upwards because of its suction effect. Thus, the utilization rate of fillings is enhanced in the forced draft tower.
3.2. Air temperature field
The air temperature field directly reflects the heat transfer intensity inside the tower. To illustrate the advantages of the tower equipped with fan groups under crosswind environment, the air temperature field of the conventional tower and the forced draft tower is compared. Take the fan group with 5 fans as an example, the air temperature fields are shown in Fig. 5. From Fig. 5(a), it can be observed that the uniformity of the air temperature field is destroyed and the high temperature area on the leeward is expanded. In the forced draft tower, the fans accelerate the air velocity and reduce the area of the maximum temperature zone. The fan installed at α = Table 6
Rotational speed of fan groups at the fixed power of 200 kW.
Fan number 2 3 4 5
Rotational speed (rpm) 282 246 224 208
Table 7
Rotational speed of the fan groups at various power (rpm).
Fan number/Power (kW) 100 200 300 400 500 700 900 1000
1 28 35 40 44 48 53 58 60
2 224 282 323 355 383 428 466 482
3 196 246 282 310 334 374 407 421
4 178 224 256 282 304 340 370 383
5 165 208 238 262 282 315 343 355
0◦increases air circulation in the air inlet, improves the heat and mass transfer of the air and water, and the air temperature on the windward was reduced. Thus, the overall temperature inside the tower is diminished, which means the thermal performance of the tower is enhanced.
Fig. 4.Comparison of streamlines between conventional and forced ventilation tower.
3.3. Thermal performance
The outlet water temperature, cooling efficiency, and the Merkel number are taken as the evaluation indices of thermal perfor- mance. The change laws of thermal performance with the number, layouts, and power of the fan groups are analyzed.
3.3.1. Outlet water temperature
The airflow field changes inside the forced draft tower and the thermal performance changes accordingly. The outlet water temperature to of the forced draft tower equipped with two, three, four, and five fans in a crosswind environment (V =5 m/s) are shown in Fig. 6.
As demonstrated in Fig. 6, the change rules of the to under the same layouts of fans are similar. The to increases with the increase of the fan radial distance, that is, the cooling tower with the fans installed near the tower center has better thermal performance. On the one hand, the reason is the fans gathered in the center of the tower have cohesion, which has a better effect on improving the ventilation of the tower center. On the other hand, when the installation radius is large, the fans are located close to the air inlet with large inertia of crosswind, which causes the forced ventilation effect to be greatly weakened. The cooling tower equipped with 2 fans achieves a lower to at the R2-12. That is because when the total available power is fixed, the rotational speed of 2 fans is relatively large, and a fan installed at the R2-2 location reduces the low speed area of air on the windward, the other fan at position R2-1 increases the ventilation of the tower center. Thus, the combined effect of this layout is greater than the layout of R1-12.
At the same radial position, the fans installed at positions 0◦, 45◦, and 135◦effectively reduce the area of the low air speed and prevent part of the air from being drawn into the vortices. Thus, the tower has better thermal performance when the fan is located in the windward. In the range of this paper, the cooling tower equipped with 2, 3, 4, and 5 fans achieved better thermal performance when the layouts of fan groups are R2-12、R1-234、R1-2345, and R1-23456, respectively. Compared with the conventional tower, the water temperature drops increased by 0.12, 0.12, 0.12 and 0.11 ◦C respectively.
Fig. 5.Air temperature field of the conventional tower and forced draft tower.
3.3.2. Cooling efficiency
The cooling efficiency is the ratio of the actual cooling temperature difference to the maximum cooling temperature difference, as shown in formula (13). The greater the cooling efficiency, the better the thermal performance of the cooling tower.
η=ti− to
ti− τ (13)
Where, τ is the wet-bulb temperature, ◦C.
Fig. 7 illustrates the change of the cooling efficiency at various numbers and layouts of fans under the crosswind speed of 5 m/s.
From Fig. 7, the change rules of the cooling efficiency is opposite to that of the outlet water temperature. Under the same layouts, the cooling efficiency decreases with the increase of installation radius. Under the same installation radius, the tower has better cooling efficiency when the fans installed in the windward. In the range of this research, the cooling tower has better thermal performance when the tower equipped with 2, 3, 4, and 5 fans and the fans layouts are R2-12、R1-234、R1-2345, and R1-23456, respectively.
Compared with the conventional tower, the relative increment of the cooling efficiency is 1.39%, 1.36%, 1.43%, and 1.31%
respectively.
3.3.3. Merkel number
Merkel number is a vital parameter, which represents the cooling capacity of the tower. The calculation formula as below, N=cwΔt
6K ( 1
i˝1− i1
+ 4
i˝m− im
+ 1 i˝2− i2
)
(14)
where K is the coefficient of the heat taken away by evaporated water, K=1− 586−0.56(ttw2
w2−20); i1, i2, im represent the enthalpy of inlet and outlet air, and the average enthalpy of inlet and outlet air, kJ/kg; i1′′, i2′′, im’’ are the saturated enthalpy corresponding to tw1, tw2, tm, kJ/kg.
Fig. 6.Outlet water temperature of the wet cooling tower equipped with fan groups.
Fig. 8 demonstrates the change rules of the Merkel number of the forced draft tower under crosswind speed of 5 m/s. The variation of the Merkel number is similar to water temperature drops. At the same radial position, the Merkel number is larger when the fans are located in the windward. At the same layout, the change rules of the Merkel number is slightly different with the water temperature drops. This is because the thermal performance of the tower from R1 to R2 changes little, and the temperature of air and water are linearized to obtain the average temperature. Therefore, the more accurate calculation method of the average temperature for air and water should be investigated in the future study.
3.4. Effect of power on thermal performance for forced draft tower
From the above analysis, it can be obtained that under the fixed power of 200 kW, the thermal performance of the tower equipped with different number of fans only has slight differences under the optimized layouts of the fan groups. Therefore, the influence of the number of fans can be ignored under fixed fan power of 200 kW. In our previous research of the cooling tower equipped with a fan [29], the optimized position of the fan is R1-2 at the crosswind speed of 5 m/s. Combined with the optimized installation layouts of the fan groups which have been obtained in section 3.3, the influence of the power on the performance for the optimized forced ventilation modes is studied.
Figs. 9–11 depict the change rules of the outlet water temperature (to), cooling efficiency (ɳ), and Merkel number (N) with power at optimized forced ventilation mode. The change rules of the thermal performance have the same trend. With the increase of the fan power, the to decreases, the ɳ and the N increase. Additionally, the thermal performance of the forced draft tower equipped with the fan groups are approach, which is better than that of a fan. When the power is less than 200 kW, the maximum difference of the to for different forced ventilation modes is about 0.03 ◦C, and the influence of the number of fans can be ignored. However, the advantages of forced draft towers equipped with fan groups are gradually obvious with the increase of the fan power. When the power changes from 100 kW to 1000 kW, the to of the tower with 1, 2, 3, 4, and 5 fans reduce by about 0.08, 0.24, 0.25, 0.26, and 0.25 ◦C; the ɳ increased by about 0.54%, 1.43%, 1.44%, 1.54%, and 1.44%; the N increased by about 0.02, 0.06, 0.05, 0.06, and 0.05.
At the same power, the thermal performance of the forced draft tower increases first and then decreases with the increase of fans Fig. 7.Cooling efficiency of the wet cooling tower equipped with fan groups.
Fig. 8.The change curves of the Merkel number of forced draft tower under crosswind speed of 5 m/s.
Fig. 9.The change rules of outlet water temperature with power at optimized forced ventilation layout.
number. In the scope of this research, the tower equipped with 4 fans has a better performance. When the fan power is 1000 kW, the to
of the forced draft tower equipped with 4 fans is about 0.18 ◦C lower than that of a fan, and about 0.34 ◦C lower than the conventional tower.
4. Conclusions
Based on the three-dimensional numerical model of a 1000 MW wet cooling tower, the influence of the number, power, and layouts of fans on the thermal performance is investigated in the annual average crosswind speed at the location of the cooling tower. The main conclusions are as follows:
(1) Under the crosswind condition, the thermal performance of the forced draft tower is better than the conventional tower.
Compared with the conventional tower, the low air speed area on the windward and the vortices area on the leeward are reduced.
(2) With the same number of fans and the layouts, the closer the fans are to the center of the tower, the better the thermal per- formance of the forced draft tower. Within the range of this paper, the optimized layouts for 2, 3, 4, and 5 fans are R2-12, R1-234, R1-2345, and R1-23456, respectively. Compared with the conventional tower, the outlet water temperature is reduced by 0.12, 0.12, 0.12 and 0.11 ◦C.
(3) When the power is less than 200 kW, the influence of the fans number can be ignored. The advantages of the forced draft tower equipped with fan groups become more obvious as the power increases. In the range of this paper, the forced draft tower equipped with 4 fans has better thermal performance, the outlet water temperature is about 0.34 ◦C lower than the conventional tower when the fan power is 1000 kW.
Fig. 10.The change rules of cooling efficiency with power at optimized forced ventilation layout.
Fig. 11.The change rules of Merkel number with power at optimized forced ventilation layout.
Author statement
Deying Zhang: Software, Data curation, Writing - Original draft preparation, Writing - Review & Editing Nini Wang: Software, Data support Jinpeng Li: Data curation, Data support Jinheng Li: Data curation, Data support Suoying He: Project administration, Supervision Ming Gao: Conceptualization, Methodology, Project administration, Supervision.
Funding
This paper is supported by the National Natural Science Foundation of China (51776111).
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This paper is supported by the National Natural Science Foundation of China (51776111).
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