An exploratory research on performance improvement of super-large natural draft wet cooling tower based on the reconstructed dry-wet hybrid rain zone

International Journal of Heat and Mass Transfer 142 (2019) 118465

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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt

An exploratory research on performance improvement of super-large natural draft wet cooling tower based on the reconstructed dry-wet hybrid rain zone Zhengqing Zhang, Ming Gao ⇑, Zhigang Dang, Suoying He, Fengzhong Sun School of Energy and Power Engineering, Shandong University, Jinan 250061, China

a r t i c l e

i n f o

Article history: Received 23 April 2019 Received in revised form 14 June 2019 Accepted 21 July 2019 Available online 27 July 2019 Keywords: Super-large natural draft wet cooling tower Dry-wet hybrid rain zone Exploratory research Ventilation performance Thermal performance

a b s t r a c t The ambient cold air cannot reach the center area of the super-large natural draft wet cooling towers (S-NDWCTs) due to the large diameter and the large water droplets resistance in the rain zone. In order to solve this issue, the reconstructed dry-wet hybrid rain zone was proposed to improve the performance of S-NDWCTs in this paper. Some split-flow plates were arranged inside the rain zone which can divide the conventional rain zone into the dry zones and wet zones, and then produced the dry-wet hybrid rain zone. The 3D numerical model of S-NDWCTs was established and validated by the actual design data, and then the ventilation and thermal performance of the S-NDWCTs with the reconstructed dry-wet hybrid rain zone were investigated. The numerical simulation results demonstrated that, after adopting the reconstructed dry-wet hybrid rain zone, the airflow velocity inside the tower becomes larger significantly, and the high temperature areas of both the moist air and the circulating water in the central area of the tower disappear. Additionally, by comparing with the S-NDWCTs with a conventional rain zone, it was discovered that, under the conditions of split-flow plates dimension parameters used in this paper, the ventilation rate and the Merkel number increase by about 2.38% and 9.89% at most, respectively; the outlet water temperature decreases by approximately 0.48 °C at most. The investigations in this paper could provide a new idea and a novel method for the in-depth energy-saving research and structural optimal design of the S-NDWCTs. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction According to the heat transfer mode between cooling air and circulating water, the natural draft cooling towers can be classified into the natural draft wet cooling towers (NDWCTs) [1–3] and the natural draft dry cooling towers (NDDCTs) [4,5]. NDWCTs are widely used in thermal plants and some nuclear plants for the high cooling efficiency and low operating cost [6–8]. For the NDDCTs, their main advantage is saving water, and they are widely used in arid areas [9]. The efficiency of the cooling towers affects the vacuum of the condenser in the thermodynamic system and further affects the total cycle efficiency of the generating unit [10]. The studies on the cooling tower efficiency and energy saving have received much attention. Various methods, including experiment [3], numerical simulation [11], artificial neural network [8], exergy analysis [10,12], and so on, were proposed by researchers to investigate the performance of the cooling tower. In recent years, the ⇑ Corresponding author. E-mail address: [email protected] (M. Gao). https://doi.org/10.1016/j.ijheatmasstransfer.2019.118465 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.

main research focuses on the NDWCTs are the ambient crosswind effects and the internal resistance to airflow [13,14]. And on the other hand, for NDDCTs, the main research focus is anti-freezing [9,15]. A lot of works have been done by researchers to investigate the effects of ambient crosswind on the cooling tower, and further to improve the performance of the cooling tower. By the crosswind tunnel and simulation method, Bender et al. [16–18] investigated the effects of the ambient crosswind on the operating performance of the NDWCT, and then the reductions of the windward intake flow rate were examined after using the protective wind walls. The results show that the crosswind has a significant effect on the cooling tower and the protective wind walls are effective to some extent. To reduce the adverse effects of the crosswind, Chen et al. [19,20] investigated the cross wall (solid and porous) based on the experimental method. The results indicate that the solid walls are more effective under the low crosswind velocities, and the porous walls are more effective under the high crosswind velocities. Kong et al. [21] investigated the thermos-flow performances of NDDCTs under the crosswind conditions. In their paper,

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Z. Zhang et al. / International Journal of Heat and Mass Transfer 142 (2019) 118465

Nomenclature c D E f G g h Ka Kh k N P Q q R r Sm Se T t V

specific heat, J/(kg
°C) diffusion coefficient, m2/s energy, J resistance force, N ventilation rate, kg/s gravity acceleration, m/s2 enthalpy, J/kg coefficient of mass transfer, m2/s thermal conductivity, W/(m
K) heat coefficient Merkel number pressure, Pa circulating water flow, m3/h water-spraying density, kg/(m2
s) gas constant, J/(mol
K) latent heat of vaporization, kJ/kg mass source term, kg/(m3
s) energy source term, W/m3 air dry-bulb temperature, °C circulating water temperature, °C volume, m3

the horizontally arranged heat exchanger bundles (HAHEBs) and vertically arranged heat exchanger bundles (VAHEBs) are contrastive analyzed. And the results indicated that the cooling performances of NDDCTs with VAHEBs are superior to that with HAHEBs in any case, especially at the high wind speeds. For reducing the resistance to airflow in the center area of the NDWCTs and improving the thermal performance of the tower, the non-uniform layout fillings were proposed by Gao et al. [22,23]. The experimental results show that the non-uniform fillings can enhance the thermal performance of the NDWCTs either under the windless conditions or under the crosswind conditions. The heat transfer coefficient and total heat rejection of the circulating water can be enhanced by about 40% and 28%, respectively, compared with the uniform layout pattern cooling tower under windless conditions. Based on the simulation method, Lye et al. [24] also studied the effects of the non-uniform layout fillings on the performance of the tower and found that the non-uniform fillings can enhance the performance of the tower both under windless and crosswind conditions. Nowadays, with the increasing of the unit power, the design size of the cooling tower becomes larger and larger. For the NDWCTs, the super-large natural draft wet cooling towers (SNDWCTs) have become mainstream cooling tower type. In general, the bottom diameter of the S-NDWCTs is more than 120 m, and the height is more than 170 m. Moreover, the spraying-water rate generally exceeds 100,000 m3/h which produces a larger ventilation resistance inside the rain zone. For the S-NDWCTs, it is highly difficult for the ambient cold air to reach the center area of the tower due to the larger bottom diameter and water droplets resistance, which deteriorates the ventilation and thermal performance of the towers. To solve the problem that the ambient cold air cannot reach the center area of the tower due to the larger bottom diameter and water droplets resistance, many methods, such as adding air duct in the rain zone, equipping fan upper the fillings, flue gas injection in the high position and so on, were proposed in the previous research works. Chen et al. [25,26] investigated the effects of adding air ducts in the rain zone on the thermal and ventilation performance by numerical simulation and experimental methods. In their papers,

v Y

velocity, m/s mass fraction

Greek symbols q density, kg/m3 l viscosity, kg/s v humidity ratio, g/kg e turbulent dissipation rate, m2/s3 r relative errors, % u air relative humidity, % Subscripts 1 inlet variable 2 outlet variable a airflow d design value; water droplets m mass t test value w water

the temperature uniformity coefficient, the airflow rate, and the outlet water temperature were calculated to evaluate the effects of air ducts on the aerodynamic field of the tower. They derived that both the ventilation performance and thermal performance are improved after adding the air ducts in the rain zone. However, the airflow inside the ducts cannot conduct the heat and mass transfer, which cannot produce the density difference, and affects the ventilation performance. Dang et al. [27] and Zhou et al [28]. proposed a novel method that an axial fan driven by the water dropping potential energy is arranged inside tower to enhance the ventilation rate of the S-NDWCTs. Based on the numerical simulation and the thermal state model experimental method, they obtained that the ventilation rate in the center area of the tower with the axial fan improves largely compared with the usual tower. The NDWCTs equipped with flue gas injection acquired successful application in Europe and have drawn wide attention in China in recent years [29]. The previous research results indicated that the flue gas injection inside the cooling tower can enhance the ventilation rate, and the emissions are environmentally acceptable [30–32]. However, the previous methods for reducing the resistance to airflow in the rain zone of the S-NDWCTs failed to change the conventional structure of rain zone, and cannot improve thoroughly the ventilation performance for the S-NDWCTs. In this paper, a new method was proposed to improve the ventilation performance. Some split-flow plates were arranged inside the rain zone which can divide the conventional rain zone into the dry zones and wet zones, and then produced the dry-wet hybrid rain zone. The 3D numerical model of the S-NDWCTs with a dry-wet hybrid rain zone and the usual wet cooling tower were established and validated by comparing with the design data. Based on the comparison analysis, the effects of the dry-wet hybrid rain zone on the ventilation and thermal performance were investigated in this study. 2. Reconstructed dry-wet hybrid rain zone The structural sketch of an S-NDWCT with the dry-wet hybrid rain zone is shown in Fig. 1. Split-flow plates are installed at the

Z. Zhang et al. / International Journal of Heat and Mass Transfer 142 (2019) 118465

3

represents the width of the split-flow plate. While the L is the length of the split-flow plate. The a stands for the installing angle between the split-flow plate and the horizontal surface, which is beneficial for collecting the cooling water. 3. Numerical simulation 3.1. Geometric description In this paper, a super-large natural draft wet cooling tower in China is taken as the research object. The geometry sizes of the cooling tower are shown in Fig. 3. The actual water-spraying area of the tower is 12,944 m2. The S-wave type fillings are adopted in the tower. The model of the cooling tower with a dry-wet hybrid rain zone is also created based on this tower size, and the split-flow plates are arranged in the rain zone as shown in Fig. 2. In this study, the installing parameters of the split-flow plates are determined by the pre optimal calculation. And the w, L, and a are 13.5 m, 43.3 m, and 15°, respectively. 3.2. Governing equations Fig. 1. Structural sketch of the NDWCTs with the dry-wet hybrid rain zone.

bottom of the fillings as shown in Fig. 1. The water droplets dripping from the fillings are blocked by the split-flow plates, therefore, there are no water droplets in the area under the split-flow plates. So, the areas under the split-flow plates are called as the dry zones and the other areas are the wet zones, as shown in Fig. 2. The external ambient air could reach the central position due to the lower water droplets resistance under the split-flow plates. That is to say, the airflow channels with lower resistance to airflow appear under the split-flow plates. Additionally, on the way to the inside of the tower, the external ambient air under the split-flow plates can also flow through both outsides of the channel, which makes the wet zones also have more ventilation rate. The exploratory research is conducted in this paper, so only four rectangular split-flow plates are installed in the four quadrants of the rain zone. Obviously, the number or shape of the split-flow plates should be optimized according to the practical conditions, which would be investigated in the further research. The detailed structural sketch of the dry-wet hybrid rain zone and the installing parameters of the split-flow plates are shown in Fig. 2. The dry zones and the wet zones appear alternately in the rain zone, as shown in Fig. 2. The h is the installing height of the split-flow plate, which represents the height of the lower edge of the plates to the top surface of the collecting water basin. The w

Fig. 2. Detailed structural sketch of the dry-wet hybrid rain zone (Range of the wet zones: Between the split-plates; above the split-plates) (Range of the dry zones: Under the split-plates).

3.2.1. Governing equations for airflow The mass conservation equations, momentum conservation equations, energy conservation equations, and species transport equations are used to describe the airflow. The air around the cooling tower can be assumed to be incompressible gas due to the small variations in pressure and temperature. Thus, the steadystate Reynolds averaged Navier-Stokes equations are used to describe the airflow momentum conservation inside and outside the cooling tower. The previous research [33] shows that the Rayleigh number calculated based on the tower size and the typical operating conditions is about 1014, which is far larger than the turbulence transition Rayleigh number of about 108–1010. The airflow in the cooling tower is in a turbulent state. The standard k-e turbulence models are used to close the N-S equations in this paper. The governing equations used to describe the airflow are as follows. (i) mass conservation equation






r
qa ! v ¼ Sm

Fig. 3. Geometry size (m) of the cooling tower.

ð1Þ

4

Z. Zhang et al. / International Journal of Heat and Mass Transfer 142 (2019) 118465

! ! ! ! where the v ¼ v x i þ v y j þ v z k stands for the air velocity vector; the qa represents the air density. (ii) momentum conservation equation




 !!

r
qa v

v

¼  rP þ r



l þ lt

 
!

 !T

rv þ rv

! ! þ qa g þ F

2 !  r
vI 3

C 3e



ð2Þ

! ! where P is the static pressure; qa g and F are the gravitational body force and external body forces, respectively; l is the molecular viscosity; I is the unit tensor. In the direction z, the pressure gradient term  @P and the mass @z force term qa g can be described by,



 @P @P0
 qa g ¼  þ qa;1  qa g @z @z

ð3Þ

where the residual pressure P 0 ¼ P  P s , and the Ps ¼ P 0  qa;1 gz; the P0 represents the pressure at the 0 m height; the qa,1 represents the operating density.

r qa v

Z

!

T

ca dT T ref

X ! ¼ r ðk1 þ k2 ÞrT  hj J j

! þ Sae

ð4Þ

RT T ref

cP;j dT is the

sensible enthalpy of the species j, J/kg; the k1 and k2 represent the ! laminar and turbulence thermal conductivity, respectively; J j is the diffusion flux of species j; Sae is the energy source.

3.2.2. Governing equations for circulating water Ignoring the inclination movement of the cooling water droplets, only the radial and circumferential direction deference are considered. Thus, the one-dimensional motion model of the water droplets was assumed [35,36] in this paper. The final control volume as shown in Fig. 4 is used to describe the derivation of the energy and mass conservation equations of the circulating water. The governing equations for the circulating cooling water are as follow, (i) mass conservation equation

dq ¼ Sm dðzÞ

ð11Þ

(ii) momentum conservation equation



r
qa ! v Y v ¼ r
½qa ðD þ Dt ÞrY v  þ Sm

ð5Þ

where the D and Dt represent the laminar and turbulence diffusion coefficient of the water vapor, respectively, m2/s. (v) state equation of moist air

P0 Rg ð1  Y v Þ þ Rv Y v T

qa ¼ 

ð6Þ

(vi) standard k-e turbulence models















r
qa ! vk ¼r
q lþ

e2 k



lt rk þ Gk þ Gb  qa e rk

r
qa ! v e ¼ r
qa l þ  C 2e q a





dv wz ðq  qÞg fz  ¼ w dðzÞ qw
v wz mw
v wz

where the vwz is the falling velocity of the water droplets, the fz represents the resistance force from the airflow, N.

ð7Þ



d ðcw tw qÞ ¼ Swe dðzÞ

Sm ¼ K a





v00w  v

ð8Þ

2

e

ð9Þ

where the Cl is constant. And the constant C1e, C2e, Cl, rk, and re of the standard turbulent flow are confirmed by Launder and Spalding

ð14Þ

where the Ka represents the convective mass transfer coefficient, kg/(m3
s); v00w and v are the humidity ratio of the saturated air membrane at the vapor-water interface and the humidity ratio of airflow, respectively. The source term of the energy conservation equation for airflow is as follow,

Sae ¼ ðK h þ Sm cv Þðtw  T a Þ

lt e re þ C 1e ðGk þ C 3e Gb Þ re k

ð13Þ

3.2.3. Source terms of governing equations The source term for mass conservation equations is derived based on the single membrane theory of convective mass transfer [37], and is as follow,

where the Gk represents the turbulent kinetic energy generation term based on the average velocity gradient. Gb stands for the turbulent kinetic energy generation term based on the buoyancy force, the C1e, C2e, and C3e are constant. The turbulence viscosity coefficient lt is calculated as follow,

k

ð12Þ

(iii) energy conservation equation

(iv) species transport equations

lt ¼ q a C l

ð10Þ

j

where Tref represents the referent temperature; hj ¼








v

z ¼ tanh qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

v2 þ v 2

x y

where the Sm is the water vapor mass transferred from circulating water to moist air in the form of convection.

(iii) energy conservation equation

!

[34] based on the experimental method. The C3e is calculated by the Eq. (10).

Fig. 4. Finite control volume for circulating water.

ð15Þ

Z. Zhang et al. / International Journal of Heat and Mass Transfer 142 (2019) 118465

5

where the cv is the specific heat of water vapor, J/(kg
°C). The source term of the energy conservation equation for circulating water is as follow,

Swe ¼ K h ðt w  T a Þ þ Sm r w

ð16Þ

In the rain zone and water-spraying zone, Km and Kh can be computed as follows [38,39],

K a ¼ qa N wd Awd K h ¼ Nwd Awd

c
dwd

1=2

2:0 þ 0:6Rewd Sca1=3



 k
1=3 2:0 þ 0:6Re1=2 wd Pr a dwd

ð17Þ ð18Þ

where the Nwd is the number of the water droplets per unit volume; the Awd represents the surface area of one droplets, m2; the Rewd is the Reynolds number corresponding to dwd; the Sca and the Pra are the Schmidt number and the Prandtl number of the moist air, respectively; the c is the water vapor diffusion coefficient; the k is the air thermal conductivity. In the fillings, based on the experimental correlations for filling characteristics, Km and Kh can be computed as Eqs. (19) and (20).

K a ¼ Bqm g na

ð19Þ

K h ¼ Lef K a

ð20Þ

where B, m, and n are constants determined by the material and structure of the fillings; q and ga are the water-spraying density and ventilation density of the tower, respectively; the Lef represents the Lewis factor derived by the Bosnjakovic formula [40]. In this paper, B, m, and n are 1.69, 0.23, and 0.7, respectively. The resistances to airflow in x, y, and z directions in waterspraying and rain zone are calculated by the Eq. (21) and appear as the momentum source term of the airflow.

Fi ¼ 

6q

qw pjv wz jd3p

fi

ð21Þ

where the i represents the coordinates x, y, and z; vwz stands for the vertical falling velocity of the water droplets; f i ¼ C d Re pd8w l ðv z þ v wz Þ is the interactive force between airflow and water droplet. The resistance to airflow in the fillings can be calculated by the Eq. (22), and appears as the momentum source term of the airflow in the fillings.

Fz ¼

DP Hfill

ð22Þ

where the DP is the pressure drop of the air through the fillings and compute by the Eq. (23); the Hfill represents the height of the fillings.

DP ¼ q a A v M z

ð23Þ

A ¼ 0:0016q2 þ 0:00177q þ 1:07

ð24Þ

M ¼  0:001q2  0:0033q þ 1:94

ð25Þ

Fig. 5. Sketch of the calculation domains and boundary types.

mass transfer zone, in which the governing equations for circulating water are calculated, includes the fillings part, the waterspraying part, and the rain zone part. For the steady conditions, the mass flow rate and temperature of the circulating water are initial parameters of inlet water. In this paper, the water droplets are sprayed down at an initial velocity of 3.5 m/s and with the diameter of 3 mm in the water-spraying zone. Additionally, below the fillings, the water droplets with the initial velocity of 0.4 m/s and with the diameter of 5 mm. 3.4. Grid system and independence The whole domain adopts the hexahedron cell. The grids for the ambient domain and the tower domain were meshed separately, and merged together by the interface, as shown in Fig. 6. The grids for mass and heat transfer zone are densified so as to present the accurate results. For ambient gird, from near the tower shell to the inlet or outlet boundary, the grid size gradually increases. In order to certify the grid independence, five grid systems with the grid number of 963252, 1302768, 1620617, 2089958, and 2228692, respectively, are proposed to calculate the same design operating condition. The calculation results based on different grid systems are shown in Table 1. The relative errors, as shown in Eq. (26), are defined to evaluate the calculation accuracy. From Table 1, the grids with the cell number greater than 2,089,958 meet the requirements of independence.

r¼

t2;d  t 2;c  100% t 1;d  t2;d

ð26Þ

where the t2,d is the design outlet water temperature, °C; the t2,c represents the calculated outlet water temperature, °C; and the t1,d stands for the design inlet water temperature, °C. 3.5. Calculation method

3.3. Calculation domain and boundary conditions The calculation domain and boundary types are shown in Fig. 5. The whole calculation domain is a cylinder with a height of 1000 m and a radius of 700 m. The pressure inlet and the pressure outlet boundary type are used as shown in Fig. 5. The ground of ambient and the tower shell are the boundary type of wall. For this tower, the calculation domain is divided into the heat and mass transfer zone and the usual airflow zone. The heat and

The simulation processes are based on the CFD software Fluent. The species transport model composed of air and water vapor is employed to calculate the interactive between circulating water and airflow. The air pressure and the velocity are coupled based on the SIMPLE method. The air pressure is discretized by the body force weighted scheme, and other variables of the air and the circulating water are discretized by the second-order upwind scheme. The governing equations used to describe the circulating

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Z. Zhang et al. / International Journal of Heat and Mass Transfer 142 (2019) 118465

Fig. 6. Sketch of the grid system.

Table 1 Verification of grid independence. Item

Grid 1

Grid 2

Grid 3

Grid 3

Grid 4

Cell number Design inlet water temperature t1d (°C) Design outlet water temperature t2d (°C) Calculated outlet water temperature t2c (°C) Relative errors r (%)

963252 31.54 21.11 20.12 9.8

1302768 31.54 21.11 20.56 5.3

1670617 31.54 21.11 20.85 3.3

2089958 31.54 21.11 20.85 2.5

2228692 31.54 21.11 20.87 2.3

water are added in Fluent by UDF. The variables of the circulating water, including the water temperature, the water-spraying rate, and the velocity of the water droplets, are defined as UDS in Fluent. The under-relaxation method was used to help the stable convergence of the iteration solution. The iterations can be considered convergent when the calculation residual of energy equation less than 106 and the calculation residuals of other equations less than 104, and also the variations of outlet water temperature less than 0.01 °C during at least 100 iteration steps.

Table 2 Main geometrical dimensions of the towers. Items

Tower 1

Tower 2

Tower 3

Water-spraying area (m2) Filling height (m) Tower height (m) Tower top inner diameter (m) Air inlet height (m) Tower throat height (m) Tower throat inner diameter (m)

12,944 2 171.28 84.77 11.37 132.29 79.462

13,000 2 177.2 79.3 12.0 141.1 77.9

8500 1 149 71.418 9.7 112.5 62.716

3.6. Validation In this paper, the geometric structure of the cooling tower changed after the split-flow plates were arranged in the rain zone, thus it is necessary to analyze the adaptability of the numerical model. To validate the numerical model, three natural draft wet cooling towers with different geometric dimensional were calculated. The main geometrical dimensions of the three towers are listed in Table 2. The calculated outlet water temperatures from three different towers are listed in Table 3. And the research object in this study is the tower 1. By the comparisons between the design values and the calculation values, it can be seen that the calculation relative errors are less than 2.5% for these three towers. Certainly, the errors from this model are acceptable, that is to say, the numerical model and the calculation method proposed in this paper meet the requirements of the simulation for the natural draft wet cooling tower. 4. Results and discussion The aerodynamic field inside the S-NDWCTs will change when the usual rain zone is replaced by the dry-wet hybrid rain zone, which further affects the heat and mass transfer performance. In this paper, the effects of the dry-wet hybrid rain zone on the aerodynamic field inside the cooling tower were researched at first. And then, the temperature field and airflow behaviors inside the S-NDWCTs, especially around the dry zone, were investigated. By

Table 3 Comparisons between design and calculated outlet water temperature values. Item

Tower 1

Tower 2

Tower 3

Atmospheric pressure Pa (kPa) Dry-bulb temperature h (°C) Wet-bulb temperature s (°C) Circulating water rate Q (m3/h) Inlet-water temperature t1 (°C) Design out-let water temperature t2d (°C) Calculated outlet water temperature t2c (°C) Relative errors r (%)

100.14 16.3 14.3 90,720 31.54 21.11 20.85 2.5

100.33 32.28 25.78 91,680 39.33 31.27 30.61 2.11

100.4 31.7 27.4 38,286 41.0 31.9 31.79 1.2

the comparisons between the usual cooling tower and the drywet hybrid rain zone cooling tower, the changing rules of the ventilation rate, the Merkel number, and the outlet water temperature with the water-spraying rate Q, the inlet water temperature t1, the inlet air dry-bulb temperature T1, and the inlet air relative humidity u were analyzed finally. 4.1. Aerodynamic field inside the cooling tower Heat and mass transfer are carried out in the cooling tower by airflow caused by density difference. The heat and mass transfer performances are greatly related to the air velocity. For the SNDWCTs, the resistance to airflow is large due to the water droplets resistance in the rain zone and the large bottom diameter

Z. Zhang et al. / International Journal of Heat and Mass Transfer 142 (2019) 118465

of the tower, thus, it is difficult that the cold ambient airflow reaches the center area of the tower. After arranging the splitflow plates to form a dry-wet hybrid rain zone, the resistance to airflow in the rain zone would decrease significantly. Therefore, heat and mass transfer performance will be enhanced. The cooling capacity proportion of the fillings zone is in the range of 60–80%, thus, the cooling performance of the fillings plays a decisive role in the cooling performance of the cooling tower. The airflow velocity magnitude fields under the fillings are shown in Fig. 7. The velocity magnitude fields are obtained under the operating condition with a circulating rate of 90,720 m3/h, an inlet water temperature of 31.54 °C, an inlet air dry-bulb temperature of 16.3 °C, and an inlet air relative humidity of 80%. From the Fig. 7, the velocity magnitude in the cooling tower with the drywet hybrid rain zone is significantly larger than that in the usual cooling tower, especially in the central area of the tower. Fig. 8 shows the airflow velocity magnitude at the surface y = 0 (longitudinal section of the tower) around the rain zone. The velocity magnitude fields indicate that the airflow reaches a more inward position in the rain zone of the cooling tower after arranging the split-flow plates. And the high-velocity area in the cooling tower barrel also becomes larger relatively. The air velocity near the fillings zone becomes greater after installing the split-flow plates. The high velocity inside the fillings will enhance the heat exchange between the moist air and the circulating water. The path-lines of the airflow under the split-flow plates, the dry zone, are shown in Fig. 9. There are not droplet resistance to airflow in the dry zone, thus, the airflow velocity is larger and the air can reach the inner zone of the tower. From the Fig. 9, it can be seen that, in the process of moving inward, the airflow in the dry zone can also blow into the wet zone on both sides of the dry zone channel. Thereby, the ventilation rate would increase in the whole rain zone.

4.2. Heat and mass transfer inside the cooling tower In general, a high air temperature area exists in the center of the tower, as shown in Fig. 10(b) due to the low airflow rate. The air temperature field is centrosymmetric distributed under the windless condition. The moist air temperature in the outer-ring is lower than that in the center area. The low airflow rate and the high air temperature make the heat and mass transfer performance deteriorated, especially in the S-NDWCTs. As described in Section 4.1, the moist air velocity increases after installing the split-flow plates. The increase of the moist air velocity enhances the heat and mass transfer between the moist air and

7

the circulating water. From the Fig. 10(a), the air temperature in the center of the tower has decreased due to a higher air velocity after adding the split-flow plates. Additionally, the moist air temperature field is still centrosymmetric distributed under the windless condition after installing the split-flow plates because that the split-flow plates are centrosymmetric distributed. Fig. 11 shows the air temperature fields at the surface y = 0 (longitudinal section of the tower) in the tower. It can be found that the moist air temperature decreases significantly after adding the split-flow plates. Especially, the low air temperature area in the rain zone becomes larger relatively. From the Fig. 11(a), the air temperature in the dry zone is increasing in the process of moving inward, it indicates that the air in the dry zone is also constantly absorbing heat. Fig. 12 shows the mass fraction of H2O at the surface y = 6.5, which is the central longitudinal section of the dry zone. The mass fraction of the H2O is increasing in the process of moving inward, which indicates that there is a mass transfer between hot circulating water and air. The evaporation of circulating water brings heat of the circulating water into the air and causes the air temperature to increase. The circulating water temperature fields under the fillings are shown in Fig. 13. The distribution rules of the circulating water temperature are quite similar to that of the air temperature. For the usual cooling tower, the circulating water in the center of the cooling tower has not been fully cooled due to the low airflow rate and the high air temperature, as shown in Fig. 13(b). In contrast, the high water temperature area disappeared after adding the split-flow plates. The comparisons of the water temperature drop in different zones between the usual tower and the dry-wet hybrid rain zone tower are shown in Fig. 14. From the figure, the effects of drywet hybrid rain zone on the water temperature drop in different zones are significant because the air velocities in rain zone, fillings, and water-spraying zone all increase. After adopting the dry-wet hybrid rain zone, the water temperature drops in the waterspraying, fillings, and rain zone increase by about 0.06, 0.14, and 0.24 respectively, under the design operating condition.

4.3. Comparative analysis of ventilation rate The changing curves of ventilation rate with water-spraying rate are shown in Fig. 15(a). It can be observed in Fig. 15(a) that the ventilation rate of the tower with a dry-wet hybrid rain zone is larger than that of the usual tower. For both towers, with the increasing water-spraying rate, the ventilation rate increases initially, followed by a decrease. Under the smaller water-spraying

Fig. 7. Air velocity (m/s) fields under the fillings (Q = 90,720 m3/h, t1 = 31.54 °C, T1 = 16.3 °C, u = 80%).

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Z. Zhang et al. / International Journal of Heat and Mass Transfer 142 (2019) 118465

Fig. 8. Air velocity (m/s) around the rain zone at y = 0 (Q = 90,720 m3/h, t1 = 31.54 °C, T1 = 16.3 °C, u = 80%).

Fig. 9. The path-lines colored by air velocity magnitude (m/s) under the split-flow plates (Q = 90,720 m3/h, t1 = 31.54 °C, T1 = 16.3 °C, u = 80%).

Fig. 10. The air temperature (K) fields at the upper surface of the drift eliminator (Q = 90,720 m3/h, t1 = 31.54 °C, T1 = 16.3 °C, u = 80%).

Z. Zhang et al. / International Journal of Heat and Mass Transfer 142 (2019) 118465

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Fig. 11. The air temperature (K) fields at the surface y = 0 (Q = 90,720 m3/h, t1 = 31.54 °C, T1 = 16.3 °C, u = 80%).

Fig. 12. The mass fraction of H2O at the surface y = 6.5 in dry zone (Q = 90,720 m3/h, t1 = 31.54 °C, T1 = 16.3 °C, u = 80%).

rate condition, the increase of the water-spraying rate makes the heat transfer surface between moist air and water droplets increase. However, with the increase of the water-spraying rate, the resistance to airflow from the water droplets increase, which reduces the ventilation rate. Actually, the ventilation rate changes little within the given range of the water-spraying rate. The turning point of the curve appears in a larger water-spraying rate after using the dry-wet hybrid rain zone, which means that under the condition of guaranteeing maximum ventilation rate, more circulating water can be cooled.

Fig. 15(b) shows the changing rules of ventilation rate with inlet water temperature. From the figure, the ventilation rate increases obviously with the increasing inlet water temperature. Under the constant spraying-water rater condition, the resistance to airflow from water droplets is constant. While the inlet water temperature increases, the final state temperature of the moist air increases. A higher moist air temperature causes a greater air density difference, thus the air pumping force increases. From the Fig. 15(c), the ventilation rate of both the usual tower and the dry-wet rain zone tower decrease with the increasing inlet air dry-bulb temperature. The decrease of the temperature deference between the circulating water and the airflow leads to the decrease of the heat transfer amount. Thus, this leads to a decrease in ventilation rate. The changing curves of ventilation rate with the inlet air relative humidity is shown in Fig. 15(d). As the inlet air relative humidity increases, the ventilation rates of both the usual tower and drywet rain zone tower remain essentially the same. That is to say, the effect of the inlet air relative humidity on the ventilation rate is slightly. It is worth noting that the ventilation rate of the tower with a dry-wet hybrid rain zone is larger than that of the usual tower in the whole range of the water spraying rate, the inlet water temperature, the inlet air dry-bulb temperature, and the inlet air relative

Fig. 13. The circulating water temperature (°C) fields under the fillings (Q = 90,720 m3/h, t1 = 31.54 °C, T1 = 16.3 °C, u = 80%).

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c w Dt 1 4 1 N¼ þ 00 þ 00 00 6 h1  h2 hm  hm h2  h1

! ð28Þ

00

Fig. 14. Column graphs of water temperature drop in different zone.

humidity. The detailed calculation discovered that the ventilation rate increases by about 2.38% at most after adopting the dry-wet hybrid rain zone in the cooling tower. 4.4. Comparative analysis of Merkel number Merkel number is an important and common used parameter to evaluate the thermal performance of the cooling tower. Assuming that the latent heat of vaporization of water is constant, and ignoring the evaporation of the circulating water, the Merkel number can be calculated by Eq. (27). The Eq. (27) can be expanded to Eq. (28) according to Simpson expansion integration.

Z N¼

t1

t2

cw dt 00 h h

ð27Þ

where the h is the enthalpy of saturated moist air on the water 00 00 00 surface; the h is the enthalpy of the moist air; the h1 , h2 , hm are saturated enthalpy value of moist air with temperature of t1, t2, and tm = (t1 + t2)/2 respectively; the h1, h2, hm are inlet, outlet, and average enthalpy value of the moist air, respectively. The changing curves of Merkel number with the water-spraying rate, the inlet water temperature, the inlet air dry-bulb temperature, and the inlet air relative humidity are shown in Fig. 16. The data shows that the increase of the water-spraying rate, the inlet water temperature, the inlet air dry-bulb temperature, and the inlet air relative humidity all can cause the decreases of the Merkel number. By comparisons, the Merkel number of the cooling tower with the dry-wet hybrid rain zone is greater than that of the usual cooling tower. That is, the thermal performance has been enhanced after using the dry-wet hybrid rain zone. Additionally, the Merkel number of the tower with dry-wet hybrid rain zone is greater than that of the usual tower in all range of the inlet water temperature. Through analysis, the Merkel number increases by about 9.89% at most. That is to say, the thermal performance has been enhanced by about 9.89% after using the dry-wet hybrid rain zone. 4.5. Comparative analysis of outlet water temperature The outlet water temperature can reflect the thermal performance of the cooling tower directly. The lower the outlet water temperature is, the better the heat transfer of the condenser is, and the higher the unit efficiency is. The changing curves of the outlet water temperature with the water-spraying rate, the inlet water temperature, the inlet air dry-bulb temperature, and the

Fig. 15. The changing curves of ventilation rate.

Z. Zhang et al. / International Journal of Heat and Mass Transfer 142 (2019) 118465

Fig. 16. The changing curves of Merkel number.

Fig. 17. The changing curves of outlet water temperature.

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inlet air relative humidity are shown in Fig. 17. The curves indicate that the outlet water temperature increase with the increasing corresponding variables. From the figure, it can be observed that the outlet water temperature of the cooling tower has decreased after using the dry-wet hybrid rain zone. As described in Section 4.3, as the increasing of the waterspraying rate, the ventilation rate changes little, which causes the insufficient cooling of the circulating water, as shown in Fig. 17(a). The Fig. 17(b) show that the relationship between inlet and outlet water temperature is not linear. The higher the inlet water temperature is, the greater the cooling temperature difference is, when other parameters remain constant. This is because higher intake water temperature will cause greater ventilation, as described in Section 4.4. The increase of the air inlet dry-bulb temperature and relative humidity leads to the decrease of the heat and mass transfer between circulating water and moist air, which cause the increase of the outlet water temperature, as shown in Fig. 17(c) and (d). The direct purpose of the researches on the thermal performance of the cooling tower is to reduce the outlet water temperature. After calculation and analysis, it is found that the outlet water temperature decreases about 0.48 °C at most after using the drywet hybrid rain zone. 5. Conclusions This study changes the conventional geometric construction of the rain zone for the super-large natural draft wet cooling towers (S-NDWCTs), and puts forward the new idea of dry-wet hybrid rain zone. Base on the exploratory research on ventilation and thermal performance under the dry-wet hybrid rain zone conditions, the main conclusions are as follows. (1) The 3D numerical model for the S-NDWCTs was established and validated. And the calculation relative error of the numerical model is less than 2.5%. (2) The numerical simulation results showed that, after arranging the split-flow plates, the airflow velocity magnitude inside the tower, especially in the central area, becomes larger significantly. Moreover, the high temperature areas of both the moist air and the circulating water in the central area of the tower disappear under the dry-wet hybrid rain zone conditions. (3) After adopting the dry-wet hybrid rain zone, the ventilation and thermal performance of the natural draft wet cooling tower improves to a certain extent, the ventilation rate and the Merkel number increase by about 2.38% and 9.89% at most, respectively, and the outlet water temperature decreases by around 0.48 °C at most. The dry-wet hybrid rain zone proposed and the research results obtained in this study could provide a new direction in the optimal design of the super-large natural draft wet cooling tower. The next work is the comprehensive optimization research of the dry-wet hybrid rain zone. Declaration of Competing Interest The authors declared that they have no conflicts of interest to this work. Acknowledgement This paper is supported by the Key Research and Development Project of Shandong Province (2019GSF109084), the National Nat-

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