Experimental study regarding the effects of forced ventilation on the thermal performance for super-large natural draft wet cooling towers

Applied Thermal Engineering 155 (2019) 40–48

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Research Paper

Experimental study regarding the effects of forced ventilation on the thermal performance for super-large natural draft wet cooling towers

T

⁎

Yang Zhoua, Ming Gaoa, , Guoqing Longb, Zhengqing Zhanga, Zhigang Danga, Suoying Hea, Fengzhong Suna a b

School of Energy and Power Engineering, Shandong University, Jinan 250061, China China Energy Engineering Group, Guangdong Electric Power Design Institution, Guangzhou 510000, China

H I GH L IG H T S

performance is experimentally studied for wet cooling tower with axial fan. • Thermal ventilation improves the inlet air uniformity under crosswind condition. • Forced temperature drop enhances by 6.46–13.35% at forced ventilation pattern. • Water • Merkel number enhances by 0.69–5.62% at forced ventilation pattern.

A R T I C LE I N FO

A B S T R A C T

Keywords: Super-large wet cooling tower Forced ventilation Axial fan Crosswind Thermal performance

In this paper, an axial fan was introduced for thermal performance improvement of super-large natural daft wet cooling towers (S-NDWCTs), and the model experiment was performed to study the thermal performance of SNDWCTs installed with an axial fan under windless and crosswind conditions. The experimental results manifested that, compared with traditional natural ventilation pattern, the thermal performance of forced ventilation is outstanding by analyzing the inlet air uniformity coefficient, cooling water temperature drop, Merkel number, etc. Moreover, the cooling water temperature drop is proportional to fan power under windless condition, and it enhances approximately by 12.06% at 3.77 W fan power, compared with natural ventilation pattern. Under crosswind conditions, the inlet air uniformity coefficient (ψ) and the water temperature difference on the water basin surface at forced ventilation pattern are more uniform than those of natural ventilation pattern, and ψ at 2.67 W condition increases by 8.08% compared with natural ventilation pattern while the crosswind velocity reaches to 0.6 m/s. Additionally, the cooling water temperature drop and Merkel number at forced ventilation pattern are also higher than those of natural ventilation pattern. Compared with natural ventilation pattern, these two parameters enhance by 6.46–13.35% and 0.69–5.62%, respectively within the experimental crosswind velocity ranges (0–0.6 m/s).

1. Introduction Since the cooling tower was invented, it was widely used for extracting heat from warm water to atmosphere in thermal power plants. As the mainstream types of cooling tower, natural draft dry cooling towers (NDDCTs) [1–5] and natural draft wet cooling towers (NDWCTs) [6–9] were used extensively in industry. Due to the high cooling efficiency of NDWCTs, they were used more widely than NDDCTs in power plants. Nowadays, to meet the cooling requirement of the large-scale power plants, the geometric dimension of NDWCTs is getting more enormous, which is called as super-large natural draft wet cooling

⁎

towers (S-NDWCTs). For example, the diameter of tower bottom exceeds 130 m for the S-NDWCTs, which impede the entrance of outside air and affect circumferential inlet air, finally deteriorate the thermal performance. Hence, it is necessary to study the way to enhance the ventilation rate, and improve thermal performance of the S-NDWCTs, especially under crosswind conditions. In recent years, numerous research efforts have been devoted to improve thermal performance of NDWCTs by the optimization of different zones, including fillings zone [10–16], water distribution zone [17–21] and raining zone [22]. Furthermore, many scholars found that the thermal performance of NDWCTs is affected by ambient conditions,

Corresponding author. E-mail address: [email protected] (M. Gao).

https://doi.org/10.1016/j.applthermaleng.2019.03.149 Received 7 January 2019; Received in revised form 27 March 2019; Accepted 28 March 2019 Available online 29 March 2019 1359-4311/ © 2019 Elsevier Ltd. All rights reserved.

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Nomenclature Fr P M ρ0 Δρ g L v0 vc v A ρ Q CW i1′ ′ im′ ′ i2 ψ

j vri v¯r Δt t1 t2 Δt c − o

Froude number prototype tower model tower the air density of the outlet (kg/m3) the air density difference between the inlet and outlet of tower (kg/m3) the acceleration of gravity (m/s2) the characteristic size (m) the outlet air velocity (m/s) the crosswind velocity at the tower outlet (m/s) the air velocity above the fillings (m/s) the cross-sectional area of fillings (m2) the air density above the fillings (m2) the water circulating flow rate (m3/h) the water specific heat (J/(kg·K)) the specific enthalpy of saturated air for t1 (J/kg) the specific enthalpy of saturated air for tm (J/kg) the outlet air specific enthalpy (J/kg) the inlet air uniformity coefficient

nc no tc to N K i 2′ ′ i1 im

the number of velocity measuring points around the tower the inlet air velocity at measuring point of number i (m/s) the average value of vri (m/s) the cooling water temperature drop (K) the inlet water temperature (K) the outlet water temperature (K) the cooling water temperature difference on the water basin surface (K) the number of center measuring points on the water basin surface the number of outside measuring points on the water basin surface the temperature at water basin center measuring points (K) the temperature at water basin outside measuring points (K) Merkel number the evaporation coefficient the specific enthalpy of saturated air for t2 (J/kg) the inlet air specific enthalpy (J/kg) the specific enthalpy of air average temperature (J/kg)

Secondly, the water distribution zone and the raining zone respectively accounted for 10% and 20% of the total cooling capacity for NDWCTs [33]. Hence, a lot of researches concerning the raining zone and water distribution zone have been done to improve the thermal performance of NDWCTs. On the basis of three-dimensional numerical simulation, Jin et al. [17] presented a comparative study for different water-distribution methods, and they found that the outlet water temperature drop exceeds 0.5 °C if adopting the two-subarea water distribution structure. Li [18] divided water distribution system into a three-area water distribution structure and analyzed the influence of non-uniform water distribution on the cooling performance of NDWCTs. The results showed that the outlet water temperature can be effectively reduced by adopting the three-area water distribution structure. According to the studies of fillings zone [11–16] and water distribution zone [17,18], these researchers usually focused on the performance improvement by one aspect, like fillings materials, fillings heights and water spray rates, but they seldom studied the joint effects by optimizing different zones. Based on this, Williamson and Behnia [19] studied the thermal performance of the cooling tower using the Poppe model and the results showed that the water temperature reduce by 0.04 °C after optimizing the fillings zone and the arrangements of water distribution. Additionally, Huang and Du [20] described the comprehensive effects regarding non-uniform arrangement of the fillings and water spray distribution in NDWCTs. Smrekar et al. [21] also studied how the efficiency of a natural draft cooling tower can be improved by optimizing the heat transfer along the cooling tower fillings using a suitable water distribution across the plane area of the cooling tower. Compared with lots of studies for water distribution zone, few researches were devoted to improving thermal performance in raining zone. Chen [22] came up with a new method that the air ducts are installed in raining zone to reconstruct aerodynamic field and improve thermal performance. The consequences demonstrated that installing air ducts reduce air temperature in the central zone, improve cooling efficiency of NDWCTs. Previous researches put forward to different optimization schemes in different zones, and the thermal performance of the whole tower improves significantly to some extent. Definitely, most of the aforementioned literatures ignored the effects of ambient conditions, while in fact, the thermal performance of NDWCTs was highly sensitive to ambient crosswind conditions [23–29]. Wang et al. [26] developed a

especially the ambient crosswind [23–29]. As a result, different scholars came up with different methods to weaken the adverse effects of crosswind [30–32]. Firstly, according to the literature [33], the fillings zone accounted for about 70% of the total cooling capacity for NDWCTs, so the optimization for fillings zone had the most pronounced effects compared with other zones. Milosabljevic and Heikkilä [10] conducted an experiment to study the heat and mass transfer coefficients for different filling materials, but they did not further analyze the thermal performance for different type fillings. Also by experimental work, Shahali et al. [11] investigated the wet cooling tower performance to explain effects of type and arrangement of fillings, and three different types of PVC fillings (7, 9 and 18 ribs) were studied. The results manifested that the temperature drop of water and the cooling efficiency increase through enhancement of rib numbers. Besides experimental research, Klimanek and Bialecki [12] presented a fillings model of heat and mass transfer for wet cooling tower by computational fluid dynamics (CFD) method. They applied shooting technique with self-adaptive RungeKutta step control to solve the resulting model equations, and this technique worked out the spatial distributions of all flow parameters. Still by CFD method, Blain et al. [13] described the heat and mass transfers in fillings zone through 3D numerical simulation, and the numerical simulation results showed high accuracy. In fact, according to the research [14], the cooling capacity was non-uniform along the direction of fillings radius, meaning that cooling capacity gradually reduces from the outside to the center of fillings and the central zone has the lowest cooling capacity. From above-mentioned articles [10–13], many scholars mainly concentrated on the uniform layout fillings, but they ignored the influence caused by nonuniform layout fillings. Based on the non-uniform layout fillings, Gao et al. [14,15] conducted a series of thermal-state model experiments to study the thermal performance of NDWCTs under windless condition and the typical crosswind conditions. They found that the thermal performance of non-uniform layout patterns can enhance by 30% at maximum compared with the uniform layout fillings. However, the influence of the crosswind conditions was under-investigated and they failed to obtain the optimal pattern under crosswind conditions. Recently, Zhou et al. [16] also studied the thermal performance for nonuniform layout fillings and obtained the optimal layout pattern under crosswind conditions. Their findings provided guideline for engineering design of fillings zone if the crosswind conditions to be taken into consideration. 41

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numerical model to study the flow field at the inlet air zone under crosswind conditions, and received that crosswind seriously destroys the axially symmetrical distribution. On this basis, Gao et al. [27,28] studied the effects of crosswind on the thermal performance of NDWCTs by thermal-state model experiments and artificial neural networks technology, and they arrived at some similar conclusions. Furthermore, Gao et al. [29] concluded that the unsymmetrical circumferential inlet air and vortex under crosswind conditions destroy the air dynamic field inside cooling tower, affecting seriously the whole airflow rate, which in turn deteriorates the heat and mass transfer performance. Consequently, some scholars proposed effective countermeasures against the crosswind adverse effects. Alavi and Rahmati [30] investigated the heat transfer performance of NDWCTs under crosswind by virtue of an innovative wind-creator setup. Wang et al. [31] reported experimental research on the guiding channel effects regarding the thermal performance of NDWCTs subjected to crosswind. Based on the principle of wind control and diversion, Chen and Sun [32] studied the internal aerodynamic field of the cooling tower by thermal-state experiment. Through the comparison and analysis of the results under different experimental conditions, it was proposed that the aerodynamic field can be optimized and reconstructed by the combination of air duct and deflector. From above brief review, considerable research efforts had been conducted to improve the cooling performance of NDWCTs, including water distribution zone, raining zone, fillings zone, and utilization of crosswind. It can be seen that most of these studies were based on sufficient ventilation rate inside tower, however, few researches focused on the ways to increase the ventilation rate, especially for the SNDWCTs. Therefore, a thermal-state model experiment is conducted in this study, and during the experiment, an axial fan is arranged inside the model tower to enhance the ventilation rate and improve the thermal performance. In this research, the main innovation is that a novel method was initially put forward to improve the thermal performance of traditional NDWCTs. That is, an axial fan was introduced to install inside the wet cooling towers, and several performance parameters were studied by thermal-state model experiment. Furthermore, at the end of the paper, we proposed the utilization of the water dropping potential energy to drive axial fan in the future research. Under this condition, the axial fan does not consume external energy, which is benefit for both energysaving and performance promotion of wet cooling towers.

experiment, the working conditions of model tower simulate that of the actual S-NDWCTs in power plant in terms of the engineering similarity theory. The whole experiment course is as follows: the circulating water is heated up to the specified temperature in the lower tank and piped to the upper tank by the circulating water pump. Then, the heated water flows through the water distribution zone, fillings zone and raining zone from top to bottom with a certain flow rate, while the cold air flows from bottom to top in model tower. Therefore, the circulating water is cooled in tower and returns to the lower tank. After the system startup, making system runs a period of time (generally it is around 20 munities) until all the monitored parameters reach to reasonable values and keep stable, which ensures the steady state before collecting experimental data. In model tower, the air is driven by both natural draft and the forced ventilation, here, the natural draft is caused by density difference and forced ventilation is induced by the axial fan. During the process of experiment, the temperature data measured by thermocouples are acquired by Agilent 34970A HP data acquisition instrument which collects data every 30 s, and it is connected to the computer to monitor and save data in real time. What’s more, the radial inlet air velocity is collected by using the hot-wire anemometer. The detailed parameters of measurement instruments are listed in Table 2. 2.2. Similarity criterion To be more accurate and reliable, the thermal-state model experiment must comply with the following similarity criterion, such as the geometric similarity which has been explained in Part.2.1, the dynamic similarity and kinematic similarity. For dynamic similarity, the density Froude number must be satisfied because the buoyancy and the inertial force are the main factors to be concerned, and the density Froude number is as follows [34],

v0 v0 ⎛ ⎞ ⎛ ⎞ Fr = ⎜ ⎟ = ⎜ ΔρgL/ ρ ⎟ Δ ρgL / ρ 0 0 ⎝ ⎠P ⎝ ⎠M

(1)

where subscript P denotes prototype tower and subscript M denotes model tower; ρ0 is the density of the outlet air; Δρ is the air density difference between the inlet and outlet of tower, and L is the characteristic size. Besides the density Froude number, according to kinematic similarity, the air velocity scale between the model and prototype tower should be equal, and is given as [34],

2. Experimental parts

⎛ v0 ⎞ = ⎛ v0 ⎞ ⎝ vc ⎠P ⎝ vc ⎠ M

2.1. Experiment descriptions

⎜

An actual S-NDWCT applied to 1000 MW unit is chosen as a prototype in the present experiment. According to the geometric similarity, the ratio of similarity is selected as 1:180 and the main geometric parameters of prototype and model tower are listed in Table 1. What’s more, the rated power and rated speed of fan is 30 W and 1500 r/min respectively, and the environmental temperature and pressure is respectively constant 28 °C, and 102.3 kPa during the experimental process. Fig. 1 describes the experimental schematic diagram in this study. As is shown in Fig. 1, the experiment device consists of five systems, including cooling water cycle, crosswind simulation, data acquisition and storage and circuit control systems. The axial fan, which is driven by DC power source in the experiment, is installed above the drift eliminators in the center of the tower to realize the forced ventilation. The upper and lower fans produce crosswind to simulate the ambient crosswind velocity. In our experiment, there is a flat channel at the fan outlet to ensure the homogeneity of crosswind on the exposed side of tower. Meanwhile, the outlet width of fans is larger than the inlet diameter of model cooling tower. Thus, the homogeneity of crosswind can be ensured on the exposed side of the tower. In the course of the

⎟

⎜

⎟

(2)

where v0 is the outlet air velocity and vc is the crosswind velocity at the tower outlet. Based on the Eqs. (1) and (2), crosswind velocity in the experimental model is 3/40 of actual velocity. In this experiment, the actual crosswind velocity values are chosen to be 0 m/s for the windless state, 2 m/s, 4 m/s, 6 m/s and 8 m/s, respectively, and the corresponding experimental crosswind velocity values are 0 m/s, 0.15 m/s, 0.3 m/s, 0.45 m/s and 0.6 m/s. In order to meet the cooling condition of the model tower and the prototype tower, air and water ratio must also be consistent, and is Table 1 Parameters of prototype and model tower.

42

Items

Prototype tower

Model tower

Diameter of tower bottom (m) Total height of tower (m) Diameter of tower outlet (m) Height of tower inlet (m) Water spray area (m2)

133.2 170.0 82.8 12.0 11,500

0.74 0.94 0.46 0.07 0.36

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Fig. 1. Schematic diagram of experimental cooling tower.

defined as [34],

Table 3 Experimental operating conditions.

⎛ ρvA ⎞ = ⎛ ρvA ⎞ ⎝ Q ⎠P ⎝ Q ⎠M ⎜

⎟

⎜

⎟

(3)

here v represents air velocity above the fillings; A is cross-sectional area of fillings; ρ is the air density above the fillings; Q is water circulating flow rate. In the prototype tower, the actual circulating water flow rates are 94,000 m3/h and 78,000 m3/h, respectively. According to Eqs. (1) and (3), the circulating water flow rate in the experimental model can be calculated and its values are 0.22 m3/s and 0.18 m3/s, respectively. Furthermore, the water inlet temperature and crosswind velocity are determined by field measurement in power plant, and the fan diameter and power are chosen by actual industrial fan according to the similarity criterion. The experimental operating conditions are shown in Table 3.

Items

Unit

Value

Circulating water flow rate Circulating water inlet temperature Crosswind velocity Fan diameter Experimental fan power

m3/s °C m/s m W

0.18, 0.22 45 0, 0.15, 0.3, 0.45, 0.6 0.08 0, 0.63, 1.69, 2.67, 3.77

2.3. Measurement points layout of the model tower In this study, a number of measurement points of radial inlet air velocity and water temperature are arranged in this model experiment system in order to analyze the ventilation performance and thermal performance. For the measurement of radial inlet air velocity, there are 8 hot-wire anemometers placed at the bottom of tower circumferential inlet, and the corresponding measurement points are arranged uniformly at the central position of vertical along the air intake. The schematic diagram of hot-wire anemometers layout is shown in Fig. 2, where No. 1 represents the measurement point for the exposed side of the tower, and No. 5 indicates the measurement point for the leeward side.

Fig. 2. Arrangement of inlet air velocity measurement points.

In order to evaluate the water temperature distribution uniformity on the water basin surface, two laps of temperature measurement points are arranged on the water basin surface, and the detailed drawing of thermocouple layout is shown in Fig. 3.

Table 2 Monitored parameters and measurement instruments. Items

Measuring instruments

Range

Accuracy

Atmospheric pressure Crosswind velocity Dry and wet bulb temperature Outlet air temperature Inlet and outlet water temperature Air humidity Circulating water flow rate Data collection

DPM3 type barometer Hot-wire anemometer (KA31) Psychrometer Copper-constantan thermocouple Mercury thermometer Hygrometer Rotameter Agilent 34970A HP data acquisition instrument

80–106 kPa 0–4.99 m/s 0–50 °C 0–200 °C 0–50 °C 10–95% RH 0–3.6 m3/s _

± 0.1 kPa ± 2% ± 0.1 °C ± 0.1 °C ± 0.1 °C ± 2% ± 1.5% _

43

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uniformity coefficient (ψ) is adopted in this study to evaluate effects of crosswind after installing an axial fan inside tower, and it is defined as [35],

1

ψ= 1+

1 j

j

∑ (vri − v¯r )2 i=1

(4)

where j represents the number of measuring points around the tower, which equals to 8 in this study; vri stands for velocity at measuring point of number i, and v¯r represents the average value of vri. Under windless condition, the values of inlet air velocity (vri) are almost equal at different velocity measurement points around the tower. Thus, vri is invariably equal to v¯r , and ψ = 1 at windless condition. While at crosswind conditions, the uniformity of circumferential inlet air is destroyed, and ψ is always less than 1 in this case. Based on the above analysis, ψ ranges between 0 and 1, and the smaller value of ψ represents worse uniformity of circumferential inlet air. Fig. 5 describes the changing rules of inlet air uniformity coefficient with crosswind velocity. In this case, circulating water flow rate is 0.22 m3/s and the inlet water temperature is 45 °C. What’s more, the fan power values are selected to be 0 W, 0.63 W and 2.67 W. The environmental crosswind velocities are 0 m/s, 0.15 m/s, 0.3 m/s, 0.45 m/s and 0.6 m/s, respectively. It is obvious in Fig. 5 that ψ decreases with the increasing of crosswind velocity under different power values. When the velocity exceeds 0.3 m/s, ψ starts to decrease rapidly. Additionally, the lower fan power is, the more obvious the decreasing trend becomes. The major reason is that forced ventilation improves the ventilation rate of lateral and leeward under crosswind conditions, so the inlet air uniformity around the tower improves to a great extent. Besides, as the crosswind velocity comes to 0.6 m/s, compared with windless condition, the ψ at natural ventilation pattern drops by 14.02%, while ψ only reduces by 7.06% under 2.67 W. Obviously, the circumferential inlet air becomes uniform relatively under forced ventilation pattern. At 0.6 m/s crosswind velocity, compared with natural ventilation pattern, ψ increases by 8.08% under 2.67 W. In conclusion, forced ventilation weakens the adverse effects of crosswind, and improves the uniformity of the circumferential inlet air according to the above analysis, further, the improvement effect becomes more remarkable with the increasing of fan power values.

Fig. 3. Arrangement of temperature measurement points on the water basin surface.

3. Effects of forced ventilation on the circumferential inlet air velocity 3.1. Effects of forced ventilation on the inlet air velocity under windless condition The circumferential inlet air velocity can reflect the ventilation performance for wet cooling tower, and it should be uniform under windless condition due to the axial symmetry structure of cooling tower. Thus, in this part, the changing rules of circumferential inlet air velocity are studied under different fan power conditions. Under windless condition, the mean inlet air velocity, which is the arithmetic mean value of eight velocity measurement points around the tower, represents the ventilation performance around the tower to a great extent. The high inlet air velocity indicates the high ventilation rate. Fig. 4 depicts the variations of mean inlet air velocity with fan power under windless condition. In this section, the circulating water flow rates are 0.18 m3/s and 0.22 m3/s, and the inlet water temperature is chosen to be 45 °C. The fan power values are 0 W for natural ventilation, 0.63 W, 1.69 W, 2.67 W and 3.77 W, respectively. It is clear in Fig. 4 that with the increasing of the fan power, the mean inlet air velocity increases gradually and a minor increasing appears under high power conditions, which can be explained that the geometric dimensions of tower inlet fail to provide adequate ventilation rate under high power conditions, leading to the decreasing slope of inlet air velocity. In addition, the mean inlet air velocity at forced ventilation condition is higher than that of natural ventilation under the same water flow rate condition. When the power varies from 0 W to 1.69 W, the mean velocity improves by 48.80% and 38.43% under 0.18 m3/s and 0.22 m3/s. The distribution water density and ventilation resistance increase with the increasing of circulating water flow rate, which leads to the decreasing of mean inlet air velocity, so the mean velocity at 0.18 m3/s is higher than that of 0.22 m3/s at the same fan power. When the power reaches to 1.69 W, the velocity of 0.18 m3/s increases by 20.17% compared with 0.22 m3/s flow rates. Briefly, the mean inlet air velocity increases significantly under experimental power conditions. That is to say, the ventilation performance increases greatly after installing an axial fan inside wet cooling tower.

4. Effects of forced ventilation on the thermal performance Based on the analysis of Figs. 4 and 5, the forced ventilation obviously increases inlet air velocity and improves the uniformity of inlet

3.2. Effects of forced ventilation on the inlet air uniformity under crosswind conditions Based on the analysis in Part.3.1, the effects caused by axial fan on the ventilation performance are very demonstrable under windless condition. However, according to the previous researches about the influence of ambient conditions [21,27], crosswind severely destroys the uniformity of circumferential inlet air. As a result, using the mean inlet air velocity, like Part.3.1, is unsuitable to evaluate ventilation performance under crosswind conditions. Hence, the inlet air

Fig. 4. Development of mean inlet air velocity with fan power under windless condition. 44

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according to the analysis of Part.3. Thus, in order to obtain the effects of forced ventilation, it is necessary to study the thermal performance at forced ventilation pattern, including temperature difference of water basin surface, cooling water temperature drop and Merkel number. Firstly, to reflect the heat transfer uniformity inside tower under forced ventilation pattern, the temperature difference between the average temperature of center measuring points and average temperature of outside measuring points on the water basin surface is introduced, and it is defined as,

Δtc − o =

nc

∑ tkc − k=1

1 no

no

∑ tko k=1

(6)

here nc and no represent the number of center and outside measuring points on the water basin surface, which equals to 8 in this study; tc and to stand for temperature at center and outside measuring points, respectively. Based on the Eq. (6), the smaller value of temperature difference means that heat transfer is more uniform relatively inside tower and the effect of forced ventilation is more remarkable. Different fan power represents different forced ventilation pattern. Fig. 7 depicts the changing rules of the water temperature difference with crosswind velocity under different fan powers. In this case, the circulating water flow rate is 0.22 m3/s and the inlet water temperature is 45 °C. It can be observed in Fig. 7 that as the increasing of the crosswind velocity, the water temperature differences at different power conditions decrease after an initial increase, and its variations show the symmetrical distributions under the natural and forced ventilation patterns. Under the natural ventilation pattern, the variation trends of water temperature difference are nearly the same between Gao et al. [37] and this study, and Gao et al. had studied in detail the variations of water and air temperature with crosswind velocity. Besides, under the forced ventilation pattern, the water temperature difference is lower than that of natural ventilation pattern. Obviously, the reason is that forced ventilation improves the ventilation rate and facilitates the cold air into the center of tower, therefore, the heat transfer is more uniform inside the tower. Furthermore, under the windless condition, the water temperature difference at 2.67 W reduces by 22.73% compared with natural ventilation pattern, while it reduces by 13.64% at 0.63 W. Then, when the crosswind velocity comes to 0.3 m/s, the water temperature differences reach to the maximum at different power conditions. In this case, compared with natural ventilation pattern, the biggest reduction of temperature difference is 20.69% at 2.67 W. Finally, as the crosswind velocity reaches to 0.6 m/s, the water temperature difference at

Fig. 5. Changing rules of uniformity coefficient ψ with crosswind velocity.

air velocity. In this part, the cooling water temperature drop, water basin temperature difference and Merkel number are introduced to evaluate the thermal performance under windless and crosswind conditions. 4.1. Effects of forced ventilation on the thermal performance under windless condition In the evaluation standards of cooling tower, the cooling water temperature drop directly reflects on the heat transfer performance. Thus, under windless condition, the cooling temperature drop is introduced to evaluate the thermal performance after installing axial fan, which is given as,

Δt = t1 − t2

1 nc

(5)

here Δt represents the cooling water temperature drop; t1 is inlet water temperature and t2 is outlet water temperature. Fig. 6 illustrates the variations of cooling water temperature drop with fan power under windless condition when the inlet water temperature is 45 °C. Obviously, with the increasing of fan power, the cooling water temperature drop rises continually, but when the power exceeds 1.69 W, the increasing amplitude is gradually getting slight. Furthermore, the rising trend of temperature drop is basically identical under different flow rates. According to Fig. 6, the cooling water temperature drop at forced ventilation pattern is higher than that of natural ventilation pattern. At 0.22 m3/s water flow rates, the temperature drop under 1.69 W increases by 4.88% compared with natural ventilation, and when the power varies from 1.69 W to 3.77 W, it increases by about 3.01%. Due to the smaller water spraying density which leads to the lower ventilation resistance, the cooling water temperature drop at 0.18 m3/s flow rate is higher than that of 0.22 m3/s. In this case, the temperature drop improves by 7.39% from 0 W to 1.69 W, while it is 4.35% when the power varies from 1.67 W to 3.77 W. In addition, compared with natural ventilation pattern, the temperature drop at 3.77 W condition improves 12.06% and 8.03% under 0.18 m3/s and 0.22 m3/s flow rates. Obviously, according to the variations of the cooling temperature drop, the thermal performance under windless condition improves obviously with the increasing of fan power. 4.2. Effects of forced ventilation on the thermal performance under crosswind conditions Under crosswind conditions, crosswind destroys axisymmetric distribution of aerodynamic field inside the tower [36], however, forced ventilation with the axial fan relieves the adverse effects of crosswind

Fig. 6. Variations of water temperature drop with fan power under windless condition. 45

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Table 4 Uncertainty analysis of experimental measurement. Parameter

Maximum standard deviation

Relative uncertainty

Mean inlet air velocity Inlet air uniformity coefficient Cooling water temperature drop Water temperature difference Merkel number

± 0.01 m/s ± 0.01 ± 0.1 °C

2.37% 0.79% 0.14%

± 0.2 °C ± 0.2

0.28% 3.06%

reflect thermal performance improvement of the whole tower. As a result, cooling water temperature drop and Merkel number are employed to evaluate thermal performance improvement. Fig. 8 explains the variations of cooling water temperature drop with crosswind velocity under different fan powers. It is known that when the crosswind velocity varies from 0 m/s to 0.3 m/s, the cooling water temperature drop decreases gradually, and it reaches a minimum under 0.3 m/s crosswind velocity. Beyond this velocity, the cooling water temperature drop starts to increase with the increasing of crosswind velocity. Additionally, under the forced ventilation pattern, the temperature drop is higher than that of natural ventilation pattern, which is in line with the descriptions in Fig. 7. In other words, more cold air flows into the center of tower, brings about the uniformity of heat transfer under forced ventilation pattern. Consequently, the higher cooling water temperature drop can be obtained at the higher fan power condition, especially for 2.67 W fan power in this study. Moreover, the cooling water temperature drop under high power has a relatively small change with the increasing of crosswind velocity. When crosswind velocity varies from 0 m/s to 0.3 m/s, the temperature drop only reduces by 1.62% under 2.67 W, while it decreases by 7.60% under natural ventilation pattern. It indicates that forced ventilation weakens the adverse effects of crosswind in terms of cooling water temperature drop. At the turning-point, the temperature drop under 2.67 W enhances by 13.35% compared with natural ventilation. In a word, under different crosswind velocity within experimental ranges, the higher fan power is, the bigger temperature drop becomes. Finally, as one of the evaluation indicators of wet cooling towers, Merkel number reflects the cooling efficiency, and its formula is as follows [34],

Fig. 7. Changing rules of temperature difference Δtc − o with crosswind velocity.

Fig. 8. Variations of cooling water temperature drop with crosswind velocity.

N=

CW Δt ⎛ 1 1 1 ⎞ + ′ + ′ ⎜ ⎟ 6K ⎝ i′ ′2 − i1 i′ m − im i′ 1 − i2 ⎠

(7)

where CW is the water specific heat, i1′ ′, i 2′ ′ and im′ ′ are the specific enthalpy of saturated air for t1, t2 and tm (tm = (t1 + t2)/2 ) respectively. i1, i2 and im are the air inlet, outlet and average temperature specific enthalpy respectively. K is evaporation coefficient. Fig. 9 represents the changing rules of N with crosswind velocity under different fan powers. Similarly, N decreases gradually when the crosswind velocity changes from 0 m/s to 0.3 m/s, and it reaches a minimum under 0.3 m/s crosswind velocity. After the crosswind velocity is more than 0.3 m/s, N enhances gradually with the increasing of crosswind velocity. It can be seen in Fig. 9 that N under the higher power condition is greater than that of lower power condition. The reason is also that forced ventilation improves ventilation rate, leads to the improvement of heat transfer inside the tower. As a result, the cooling efficiency enhances and the Merkel number increases. Compared with natural ventilation pattern, N at forced ventilation pattern enhances by 0.69–5.62% within the experimental crosswind velocity ranges (0–0.6 m/s). Under windless condition, compared with natural ventilation pattern, N at 2.67 W condition increases by 5.62%. Then, compared with natural ventilation, N at 2.67 W increases by 2.20% at the turning-point. Finally, when the crosswind velocity reaches to 0.6 m/s,

Fig. 9. Variations of Merkel number N with crosswind velocity.

different power conditions is basically same as that of windless condition. Therefore, according to the variations of the water basin temperature difference, the heat transfer uniformity of forced ventilation pattern is better than that of natural ventilation pattern. On the basis of above analysis, the temperature difference only represents the uniformity of heat transfer inside the tower, but it cannot 46

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Acknowledgement

N under different power condition is basically close to the windless condition. All in all, according to the above analysis for the forced ventilation, variation trends of several performance evaluation indicators are obtained through the thermal-state model experiment, and the variation, for one certain performance indicator, is roughly identical under different fan power condition. Therefore, the experimental results in this study can provide a guideline for further research and practical application for thermal performance improvement of S-NDWCTs.

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5. Uncertainty analysis Based on the accuracy of the experimental instruments listed in the Table 2, an uncertainty analysis is conducted by using theoretical procedures [38]. The analysis results are given in Table 4. In Table 4, the uncertainty analysis of experimental results shows that the error is within the allowable range for the experimental research. 6. Conclusions For the super-large natural draft wet cooling towers (S-NDWCTs) with an axial fan installed above the drift eliminators, a thermal-state model experiment was established to study the effects of forced ventilation on thermal performance for S-NDWCTs. By way of the variable conditioning experiments, the variations of performance evaluation indicators are obtained under different fan power conditions. The following conclusions can be drawn from the present study. (1) Forced ventilation enhances the uniformity of circumferential inlet air and heat transfer inside the tower. Compared with natural ventilation pattern, inlet air uniformity coefficient (ψ) at 0.6 m/s crosswind velocity increases by 8.08% under 2.67 W. By analyzing the water temperature difference between center and outside of water basin surface, under windless and 2.67 W fan power conditions, it reduces by 22.73% compared with natural ventilation pattern. (2) The cooling temperature drop under forced ventilation pattern improves significantly at windless and crosswind conditions. At windless condition, the temperature drop at 3.77 W condition improves by 8.03% under 0.22 m3/s flow rate, compared with natural ventilation pattern. At crosswind conditions, compared with natural ventilation, the cooling temperature drop under the forced ventilation pattern enhances by 6.46–13.35% within the experimental crosswind velocity ranges. (3) Forced ventilation pattern has more advantages in cooling efficiency over the natural ventilation pattern, and Merkel number (N) improves obviously under the high power conditions. Moreover, compared with natural ventilation pattern, N at forced ventilation pattern enhances by 0.69–5.62% within the experimental crosswind velocity ranges. Explanations: In this research, we studied the influence rules of forced ventilation for S-NDWCTs by model experiment, and obtained some above-mentioned valuable conclusions. In the course of experiment, the fan was driven by DC power source. While for actual SNDWCTs, the average velocity of raindrops can reach to about 5 m/s, which produces water dropping potential energy. It is estimated that the water dropping potential energy reaches to approximately 400 kW for the actual S-NDWCTs. As a result, in next research, we intend to select a suitable fan for actual cooling tower which can be driven by rain zone potential energy based on this study. Thus, this experimental research can lay a theoretical foundation, and guide the engineering practice of utilization for water dropping potential energy in the future. 47

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