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Field test on ventilation performance for high level water collecting wet cooling tower under crosswind conditions
Accepted Manuscript Field test on ventilation performance for high level water collecting wet cooling tower under crosswind conditions Jian Zou, Suoying He, Guoqing Long, Fengzhong Sun, Ming Gao PII: DOI: Reference:
S1359-4311(17)37840-7 https://doi.org/10.1016/j.applthermaleng.2018.01.065 ATE 11719
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
11 December 2017 18 January 2018 18 January 2018
Please cite this article as: J. Zou, S. He, G. Long, F. Sun, M. Gao, Field test on ventilation performance for high level water collecting wet cooling tower under crosswind conditions, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.01.065
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Field test on ventilation performance for high level water collecting wet cooling tower under crosswind conditions Jian Zou1, Suoying He1, Guoqing Long2, Fengzhong Sun1, Ming Gao1* 1
School of Energy and Power Engineering, Shandong University, Jinan 250061, China 2
China energy engineering group Guangdong electric power design institute, Guangzhou 510000, China *Correspondence authors: M. Gao, associate professor, [email protected]
Abstract：Field test was performed on the high level water collecting wet cooling towers (HWCTs) of a 1000MW unit to investigate ventilation performance under crosswind conditions, the circumferential inflow air distribution rules and ventilation rate were analyzed in this paper. The test results manifest that crosswind destroys the uniformity of circumferential inflow air, increases the wind velocity in the windward side, and reduces wind velocity in the lateral and leeward side. Moreover, the uniformity coefficient of circumferential inflow air and ventilation rate continuously decrease with the increasing of crosswind velocity. In this study, θ represents the angle between cross walls and crosswind direction. When crosswind velocity reaches to 3.74m/s, the uniformity coefficient decreases to 0.61 and 0.49 under θ1=5° and θ2=35°. Compared with 0.28m/s condition, the ventilation rate reduces by 30.13% under θ1=5° and 34.36% under θ2=35°. Additionally, at the same crosswind velocity, the smaller the θ is, the better the ventilation performance becomes. Compared with θ2=35°, the uniformity of circumferential inlet air is better and the ventilation rate is larger than that under θ1=5° condition. Keywords: high level water collecting wet cooling tower, field test, inlet air uniformity, ventilation performance, crosswind
Nomenclature Vc
Ambient crosswind velocity (m/s)
Vr
Effective radial wind velocity (m/s)
v ri
Vr at measuring point of number i (m/s)
vr
Average value of Vr (m/s)
cw
Specific heat of water (kJ·kg-1·K-1)
dTw
Water temperature drop (℃)
G
Ventilation rate (t/h)
Q
Circulating water flowrate (t/h)
m
Mass of water (t)
i1
inlet air enthalpy (kJ·kg-1)
i2
outlet air enthalpy (kJ·kg-1)
K
Evaporation coefficient
t1
Inlet water temperature (℃)
t2
Outlet water temperature (℃)
Greek letters θ
Angle between cross walls and crosswind direction (°)
ψ
Inlet air uniformity coefficient
Subscripts c
crosswind
r
radial
w
water
1. Introduction
Natural draft wet cooling towers are widely used in thermal power plants or in some nuclear power plants to extract spare heat from circulating water. As a primary component of the cool-end system, its cooling efficiency has a direct impact on the condenser vacuum degree and power generation efficiency [1]. Based on the structure of usual wet cooling towers (UWCTs), the high level water collecting wet cooling towers (HWCTs) added the water collecting devices and canceled the rain zone, it is a kind of energy-saving and noise-reduction cooling tower which can decreases the circulating water supply height and reduces pumping power [2]. In recent years, some scholars have carried out researches on the HWCTs. Zhao and Li [3, 4] conducted comparative study on the economic efficiency between the HWCTs and the UWCTs, and received that the HWCTs have higher initial cost but less annual operating cost, so the HWCTs is more cost-effective in long-term operation. Lyn et al. [5] performed the experimental research on the airflow field in the water collecting device, and discovered that the Venturi effects of water collecting devices could enlarge the air mass flow rate and enhance the cooling performance. Besides, Lyn et al. [6] manifested that the non-uniform layout fillings lead to better cooling performance for the HWCTs by terms of numerical computation. Generally speaking, research work on the HWCTs had done on the water collecting device, fillings and economic analysis, but these researches did not focus on the ventilation performances under crosswind conditions. As the cooling performance of cooling towers is very sensitive to the environmental conditions and crosswind continuously influences the ventilation performance during the operating course of cooling towers and further affects its heat and mass transfer efficiency [7, 8]. Hence, many studies in the past can be found concentrated on the cooling performance of cooling towers under crosswind conditions and suggested countermeasures against crosswinds which mainly divided into three aspects: numerical simulation, experimental research and field test. Al-Waked and Behnia [9-10] developed CFD model of UWCTs, and found that environmental crosswind led to the uneven distribution of air and water inside the cooling tower and deteriorated the cooling efficiency. The CFD studies showed that
the crosswind that has a velocity less than 7.5m/s is adverse to the cooling performance. Furthermore, Al-Waked [11] investigated the effect of crosswinds on the thermal performance of a stand-alone NDWCT and two NDWCTs surrounded by power plant building structures, and found that regardless of the cross-winds direction, an increase of 1.3 K or more could be predicted at crosswinds speeds greater than 4 m/s. Wang et al. [12] developed a numerical model to study the flow field at the air inlet zone under crosswind conditions, and received that crosswind seriously destroyed the axially symmetrical distribution. Wang et al. [13] adopted a computational fluid dynamics approach with validation to investigate the cooling performance of a natural draft dry cooling tower at various wind speeds and came to the conclusion that the circumferential non-uniform ventilation and the vortices inside are the main factors to the degrading of the cooling performance under crosswind conditions. Hooman [14] conducted theoretical prediction to predict crosswind effects on the performance of cooling towers and validated against available numerical and experimental data. This work found a closed-form solution for the airflow rate at the tower exit under given velocity crosswind conditions, and the results showed that the total heat rejected under crosswind condition will be affected by the ratio of crosswind velocity to draft velocity at the tower exit. Gao et al. [15, 16] also conducted thermal-state model experiments to investigate the effect of crosswind on the thermal performance of UWCTs, and received that crosswinds had a serious adverse effect on the circumferential inflow air and generate vortex zones in both windward side and leeward side inside the tower, further destroys the uniformity of air/water temperature distribution and deteriorates the thermal performance. Chen et al. [17] studied the effect of cross walls on the thermal performance of UWCTs under crosswind conditions by experimental researches, and the results showed: at all crosswind velocity, the cross wall at a setting angle of 0° resulted in better performance than that at 45°, regardless of the cross wall shapes. Zhang et al. [18] analyzed the influence of crosswind on the outlet airflow pattern of UWCTs and the resultant changes of its cooling range and ventilation drag coefficient by hot model experiments, results showed that the outlet airflow is extruded by upper
crosswind and the ventilation drag coefficient increased by 25%-35% within the crosswind velocity of 0.5m/s-0.8m/s. Mondal et al. [19] investigated the heat transfer performance of UWCTs under crosswind conditions by experiment research, and received that under 0.6m/s crosswind condition, the temperature drop and effectiveness decrease by 4% and 4.5% compared to windless condition, but this decreasing trend is up to the critical Froude number. So far, very few scholars focused on the field test researches on cooling towers, especially the HWCTs. Zhang et al. [20, 21] investigate the ventilation performance of UWCT by field test, and the test results showed that crosswind increases ventilation resistance, and destroys the uniformity of circumferential inflow air, further deteriorates the thermal performance. Duan [22] performed field test to studied the three-dimensional thermal performance test method for large UWCTs, and found that the crosswind adds the additional resistance in air inlet zone, the additional draft in air outlet zone and direct effects the heat transfer performance. Ardekani et al. [23] conducted field test on Heller cooling tower under wind conditions, and obtained that air suction at the tower prevents flow separation at its periphery. Besides, the tower front cooling sectors experience better airflow distribution compared to sectors parallel to wind direction, which can improve thermal performance by about 20% compared to still-air conditions. As can be seen from the above review, previous literatures demonstrated that crosswind leads to adverse effects on the ventilation performance of cooling towers. Some research work had conducted on the HWCTs concerning about numerical simulation and economic analysis, but these researches failed to discuss the ventilation performance of HWCTs under crosswind conditions, especially lack of field test method. Actually, the structure differences between the UWCTs and HWCTs may lead to different ventilation performance under crosswind conditions. In order to reveal the impact of crosswind on the ventilation performance of HWCTs and lay the theoretical foundation for further energy-saving research and optimization design of the HWCTs, the change rules of ventilation rate and circumferential inflow air under various crosswind conditions are researched by field test in this paper.
2. Field test parts 2.1. Test objectives In this study, the influence of crosswind on ventilation performance was studied by field test for high level water collecting wet cooling towers (HWCTs). By measuring circumferential inlet wind velocity at different position, the uniformity of flow field at the air inlet could be described. Besides, the ventilation rate G could be calculated by measuring meteorological parameters and temperature of different cooling zones under crosswind conditions. Based on the researches above, the ventilation performance could be further analyzed. This work could provide theoretical guidance for the optimizing design and energy-saving reconstruction of HWCTs. 2.2. Monitored parameters and measurement instruments This field test was conducted for a 1000MW unit in spring season, and the circulating water flowrate Q remains 69553t/h during the whole test period. The main geometrical dimensions of the cooling tower are listed in Table 1. The monitored parameters and measuring instruments are shown in Table 2, including the circumferential inlet wind velocity and direction, environmental meteorological parameters, circulating water flowrate and temperature of different cooling zones. Table 1. Main geometrical dimension of the HWCT
subject
value
unit
Fillings area
12800.00
m2
Height of the tower
190.00
m
Height of the air inlet
14.85
m
Height of the throat
142.50
m
Diameter of the throat
84.04
m
Diameter of the outlet
86.87
m
Table 2. Monitored parameters and measuring instruments
Item
Measuring instrument
Accuracy
Atmospheric pressure
Small-type meteorological station
±1.5%
Crosswind velocity
Small-type meteorological station
±0.1m/s
Small-type meteorological station
±0.1℃
Platinum resistance thermometer
±0.1℃
Circulating water flowrate
FLEXIM ultrasonic flowmeter
±0.5%
Outflow air temperature
Platinum resistance thermometer
±0.1℃
Anemoscopes
±0.1m/s
Anemoscopes
±0.05°
Inlet dry and wet bulb temperature Inlet and outlet water temperature
Circumferential inlet wind velocity Circumferential inlet wind direction 2.3. Measuring point arrangement and monitoring system The environmental meteorological parameters, including inlet dry and wet bulb temperature, crosswind velocity and atmospheric pressure. These parameters were tested by the small-type meteorological station which settled at a height of 2.5meters and 30-50 meters away from the cooling tower. During this whole field test period, according to the local meteorological conditions, the environmental crosswind velocity was 0.28m/s, 0.70m/s, 1.38m/s, 2.11m/s and 3.74m/s, respectively. The circumferential inlet wind velocity and direction were tested by eight anemoscopes which arranged evenly around the tower, seen in Fig.1.
Fig. 1. Schematic diagram of measuring points around the HWCT
The manhole door was selected as the starting point, α represents the layout angle of inlet wind velocity measuring points, α=0° represents the manhole door position. There were eight groups of measuring points which were settled according to anti-clockwise direction. At each measuring point there were two anemoscopes which settled in the vertical direction to measure the circumferential inlet wind velocity and direction at different heights, which can be seen in Fig.2. Actually the air which entering the tower for heat and mass exchange progress basically comes from the higher position, therefore, in the following discussion, the wind velocity at 6m is used as the studied value, and the wind velocity at 2m is used as the reference value.
Fig.2. Schematic diagram of circumferential measuring points at different heights
According to the conclusion from Gao et al. [15], the crosswind changes the circumferential inlet wind velocity and direction. Actually only the airflow along the radial direction can enter into the tower and take part in the heat and mass transfer course. Therefore, in this paper, the effective radial wind velocity Vr is introduced to analyze the uniformity of circumferential inlet wind, and Vr is the radial velocity component of circumferential inlet wind velocity at the height of 6m, shown in Fig.1. The temperature inside the tower mainly includes the outflow air temperature above the drift eliminators, inlet water temperature, and outlet water temperature. The outflow air temperature measuring points were settled above the drift eliminators, and there are twenty-four measuring points in six laps which are shown in Fig.3.
Fig.3. Sectional view schematic diagram of temperature measuring points above the eliminators
When calculating the ventilation rate, the average value of twenty-four measuring points is used as the outflow air temperature. In addition, the inlet and outlet water temperature were tested in the vertical shaft and water collection tanks, respectively. The schematic diagrams of partial measuring points are shown in Figs.4-5.
Anemoscopes
Measuring points above the drift eliminators
Fig.4. Scene picture of measuring points above the drift eliminators
Fig.5. Scene picture of measuring points around the tower
In order to collect data in real time and reduce the measuring errors caused by the time difference, this test process adopts wireless transmission to record all the data. And the monitoring system is shown in Fig.6.
Fig.6. Schematic diagram of data system
The measuring instruments at each measuring point transform the real time temperature value into the standard current signal which is transmitted to the data acquisition device by wire transmission. Then the data acquisition device collects the current signal and transmits it to the central master station which is connected with computer. At last, the data processing software in computer accepts and stores the related data for further analysis and research.
3. Impact of crosswind on the uniformity of circumferential inlet wind 3.1 Impact of crosswind on inlet air flow field around the tower During the design of cooling towers, the cross walls were installed to lessen the cross-ventilation of HWCTs, and the crosswind direction affects the air flow inside the tower. In this paper, θ represents the angle between cross walls and crosswind direction. According to the local environmental crosswind conditions, in this study, two angles (θ1=5° and θ2=35°) are investigated to analyze the ventilation performance under crosswind conditions, seen in Fig.7.
Fig.7. Schematic diagram of angle θ between crosswind direction and cross walls
The relation curves between crosswind velocity and Vr are shown in Fig.8 (a) and (b) under θ1=5° and θ2=35° conditions, respectively. It can be seen that the distribution of Vr is nearly axially symmetrical and uniform under 0.28m/s crosswind condition, basically the same as that of UWCTs under windless condition [16].
（a）distribution of
Vr under θ1=5° condition
（b）distribution of
Vr under θ2=35° condition
Fig.8. Relation curves between crosswind velocity and Vr
With the increasing of crosswind velocity, Vr in the windward side raised rapidly, leads to the rise of wind pressure in the windward side, which increases inlet resistance in the leeward side, further gives rise to the continuous decline of Vr in the leeward side. As the crosswind velocity reaches to 3.74m/s, compared with 0.28m/s crosswind condition, when θ1=5°, the Vr value in the windward side (α=0°, 45°, 315°) increases by 41.23%, 32.12% and 28.37%, respectively, the average increasing amplitude reaches to 33.91%. However, this value decreases by 33.57%, 18.01% and 14.92% in the leeward side (α=180°, 135°, 270°), and the average decreasing amplitude reaches to 22.17%. When θ2=35°, the Vr value in the windward side (α=135°, 90°, 180°) raises by 38.23%, 30.22% and 25.09%, respectively, the average increasing amplitude reaches to 31.18%, but it declines by 27.89%, 24.39% and 21.16% in the leeward side (α=315°, 0°, 270°), and the average reduction amplitude reaches to 24.48%. Meanwhile, the Vr value in the lateral side also reduced slightly due to the rising of wind pressure in the windward side. Hence, it can be obtained that crosswind mainly affects the ventilation performance of the windward and leeward side. At the velocity of 3.74m/s, the
increasing amplitude of Vr in the windward side (33.91%) under θ1=5° condition exceeds that under θ2=35° condition (31.18%). But under θ2=35° condition, the decreasing amplitude of Vr in the leeward side (24.48%) is greater than that under θ1=5° condition (22.17%). These phenomena indicate that at the same crosswind velocity, under θ1=5° condition, crosswind has a more serious impact on the inflow air in the windward side. But under θ2=35° condition, crosswind leads to relatively more severe influence on the inflow air in the leeward side. 3.2 Impact of crosswind on inlet air uniformity coefficient In this paper, the inlet air uniformity coefficient ψ was introduced to act as evaluation criteria to estimate the impact of crosswind on the uniformity of circumferential inflow air, it can be defined as [20], ψ
1 1 1 n
n
i 1
（1）
(vri vr )
2
where n is the number of measuring points around the tower, and n equals to 8 in this
paper, v ri stands for the Vr at measuring point of number i, and v r represents the average value of Vr . According to the accuracies of the measuring devices listed in Table 2, an uncertainty analysis on ψ was conducted using theoretical procedures [24], the results showed that the average measurement standard deviation of ψ is less than 0.14 or 1.47%. Under windless condition, the values of Vr are equal at all angles, and ψ=1 in this case. As can be seen from the analysis above, the crosswind destroys the uniformity of circumferential inflow air, which leads to ψ<1. Thus, under crosswind conditions, the smaller the ψ is, the worse the uniformity becomes. Fig.9 depicts the changing rules of ψ with crosswind velocity under θ1=5° and θ2=35°conditions. It can be seen that ψ is less than 1 under all kinds of crosswind conditions. With the increasing of crosswind velocity, ψ continuously decreases,
which finally results in the deterioration of the uniformity.
Fig.9. Changing rules of inlet air uniformity coefficient (ψ) with crosswind velocity Vc (m/s)
Meanwhile, the reduction amplitude of ψ increases when the crosswind velocity exceeds 0.70m/s, and at the same velocity, compared with θ1=5°condition, there is a greater decreasing amplitude under θ2=35°condition. When the crosswind velocity comes to 3.74m/s, ψ decreases to 0.61 and 0.49 under θ1=5° and θ2=35°conditions, respectively. Therefore, in terms of circumferential inflow air, the ventilation performance is better under θ1=5°condition. In conclusion, crosswind destroys the uniformity of circumferential inflow air, and increases the effective radial wind velocity Vr in the windward side, causes the rise of wind pressure in the windward side, which augments the ventilation resistance in the lateral and leeward side. At the same crosswind velocity, under θ1=5° condition, crosswind has a more serious impact on the inflow air in the windward side, but under θ2=35° condition, crosswind causes relatively more severe influence on the inflow air in the leeward side. Additionally, compared with θ2=35° condition, the ψ value is relatively higher than that under θ1=5° condition. It indicates that the uniformity of circumferential inflow air is better under θ1=5° condition.
4. Research on the influence of crosswind on the ventilation rate G The ventilation rate G is an important parameter to evaluate the ventilation
performance of cooling tower. As mentioned above, the distribution of circumferential inflow air is symmetrical and the air flow inside the tower is uniform under windless condition, so the ventilation rate G is relatively stable. According to the previous analysis, the crosswind destroys the uniformity of circumferential inflow air, affects the ventilation performance of HWCTs, which leads to a great impact on the ventilation rate G. In this paper, the ventilation rate G is calculated by heat balance method. According to the energy balance theory, the amount of heat which sent out from water equals to that absorbed by the air, and it can be defined as, dQ cw m
dTw G(i2 i1 ) K
(2) where c w is the specific heat of water; and i1 , i2 are the inlet, outlet air enthalpy value, respectively, which can be calculated by the data obtained in the test. Here, the K in Eq. (2) is a heat coefficient that represents the heat carried by evaporation loss, and it can be written as [25],
K 1
t2 586 0.56(t 2 20)
(3) Thus, the ventilation rate G is obtained by the combination of Eq. (2) and Eq. (3),
G
cw Q(t1 t 2 ) K (i2 i1 )
(4) where t1 and t 2 are the inlet and outlet water temperature, respectively. According to the uncertainty analysis on G, the results showed that the average measurement standard deviation of G doesn’t exceed 1738.83t/h or 3.53%. Fig. 10 illustrates the changing rules of G with crosswind velocity under θ1=5° and θ2=35°conditions. It can be seen that G is quite dependent on the crosswind
velocity, and G decreases gradually with the increasing of crosswind velocity. Test results manifest that there is little change in the ventilation rate G when the crosswind velocity is less than 0.70m/s. It means that the crosswind has little influence on the ventilation performance under relatively smaller crosswind velocity.
Fig.10. Changing rules of ventilation rate (G) under crosswind conditions
When the crosswind velocity exceeds 1.38m/s, the ventilation rate G reduces rapidly with the increasing of crosswind velocity. And as the crosswind velocity reaches to 3.74m/s, compared with 0.28m/s condition, the ventilation rate reduces by 30.13% under θ1=5° and decreases by 34.36% under θ2=35°. Thus, in terms of ventilation rate G, it is also proved that the ventilation performance is better under θ1=5°condition. Consequently, field test results demonstrate that crosswind has an adverse effect on the ventilation performance of HWCTs, the ventilation rate G continuously decreases with the increasing of crosswind velocity, but the influence level is different under various θ conditions. At the same crosswind velocity, there is a greater ventilation rate G under θ1=5°condition.
5. Conclusions Based on the field test and mathematical calculation for HWCTs, the principal results are as follows. (1) Crosswind destroys the uniformity of circumferential inflow air, and
increases the effective radial wind velocity in the windward side and the ventilation resistance in the lateral and leeward side. At the same crosswind velocity, under θ1=5° condition, crosswind had a more serious impact on the inflow air in the windward side, however, under θ2=35° condition, crosswind causes relatively more severe influence on the inflow air in the leeward side. (2) The inlet air uniformity coefficient ψ continuously decreases with the increasing of velocity, and the reduction amplitude of ψ increases gradually when the crosswind velocity exceeds 0.70m/s. When crosswind velocity reaches to 3.74m/s, the uniformity coefficient decreases to 0.61 and 0.49 under θ1=5° and θ2=35° conditions. At the same crosswind velocity, compared with θ1=5°condition, there is a greater decreasing amplitude under θ2=35°condition. It proves that the uniformity of circumferential inflow air is better under θ1=5° condition. (3) The ventilation rate has a sustained to decline with the rising of crosswind velocity. As the crosswind velocity reaches to 3.74m/s, compared with 0.28m/s condition, the ventilation rate reduces by 30.13% under θ1=5° and decreases by 34.36% under θ2=35°. It demonstrates that crosswind leads to the deterioration in the ventilation performance, and at the same crosswind velocity, the ventilation performance is superior under θ1=5° condition.
Acknowledgement This paper is supported by National Natural Science Foundation of China (No. 51776111)
and
Shandong
Province
Natural
Science
Foundation
(No.
ZR2016EEM35).
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Highlights
Field test was conducted on high-level cooling tower under crosswind conditions.
Inflow air uniformity coefficient decreases with the rising of crosswind velocity.
The uniformity coefficient decreases to 0.61 and 0.49 under θ1=5° and θ2=35°.
In 3.74m/s, the ventilation rate reduces by 30.13% under 5° condition.
The ventilation performance is better under 5° condition.
S1359-4311(17)37840-7 https://doi.org/10.1016/j.applthermaleng.2018.01.065 ATE 11719
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
11 December 2017 18 January 2018 18 January 2018
Please cite this article as: J. Zou, S. He, G. Long, F. Sun, M. Gao, Field test on ventilation performance for high level water collecting wet cooling tower under crosswind conditions, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.01.065
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Field test on ventilation performance for high level water collecting wet cooling tower under crosswind conditions Jian Zou1, Suoying He1, Guoqing Long2, Fengzhong Sun1, Ming Gao1* 1
School of Energy and Power Engineering, Shandong University, Jinan 250061, China 2
China energy engineering group Guangdong electric power design institute, Guangzhou 510000, China *Correspondence authors: M. Gao, associate professor, [email protected]
Abstract：Field test was performed on the high level water collecting wet cooling towers (HWCTs) of a 1000MW unit to investigate ventilation performance under crosswind conditions, the circumferential inflow air distribution rules and ventilation rate were analyzed in this paper. The test results manifest that crosswind destroys the uniformity of circumferential inflow air, increases the wind velocity in the windward side, and reduces wind velocity in the lateral and leeward side. Moreover, the uniformity coefficient of circumferential inflow air and ventilation rate continuously decrease with the increasing of crosswind velocity. In this study, θ represents the angle between cross walls and crosswind direction. When crosswind velocity reaches to 3.74m/s, the uniformity coefficient decreases to 0.61 and 0.49 under θ1=5° and θ2=35°. Compared with 0.28m/s condition, the ventilation rate reduces by 30.13% under θ1=5° and 34.36% under θ2=35°. Additionally, at the same crosswind velocity, the smaller the θ is, the better the ventilation performance becomes. Compared with θ2=35°, the uniformity of circumferential inlet air is better and the ventilation rate is larger than that under θ1=5° condition. Keywords: high level water collecting wet cooling tower, field test, inlet air uniformity, ventilation performance, crosswind
Nomenclature Vc
Ambient crosswind velocity (m/s)
Vr
Effective radial wind velocity (m/s)
v ri
Vr at measuring point of number i (m/s)
vr
Average value of Vr (m/s)
cw
Specific heat of water (kJ·kg-1·K-1)
dTw
Water temperature drop (℃)
G
Ventilation rate (t/h)
Q
Circulating water flowrate (t/h)
m
Mass of water (t)
i1
inlet air enthalpy (kJ·kg-1)
i2
outlet air enthalpy (kJ·kg-1)
K
Evaporation coefficient
t1
Inlet water temperature (℃)
t2
Outlet water temperature (℃)
Greek letters θ
Angle between cross walls and crosswind direction (°)
ψ
Inlet air uniformity coefficient
Subscripts c
crosswind
r
radial
w
water
1. Introduction
Natural draft wet cooling towers are widely used in thermal power plants or in some nuclear power plants to extract spare heat from circulating water. As a primary component of the cool-end system, its cooling efficiency has a direct impact on the condenser vacuum degree and power generation efficiency [1]. Based on the structure of usual wet cooling towers (UWCTs), the high level water collecting wet cooling towers (HWCTs) added the water collecting devices and canceled the rain zone, it is a kind of energy-saving and noise-reduction cooling tower which can decreases the circulating water supply height and reduces pumping power [2]. In recent years, some scholars have carried out researches on the HWCTs. Zhao and Li [3, 4] conducted comparative study on the economic efficiency between the HWCTs and the UWCTs, and received that the HWCTs have higher initial cost but less annual operating cost, so the HWCTs is more cost-effective in long-term operation. Lyn et al. [5] performed the experimental research on the airflow field in the water collecting device, and discovered that the Venturi effects of water collecting devices could enlarge the air mass flow rate and enhance the cooling performance. Besides, Lyn et al. [6] manifested that the non-uniform layout fillings lead to better cooling performance for the HWCTs by terms of numerical computation. Generally speaking, research work on the HWCTs had done on the water collecting device, fillings and economic analysis, but these researches did not focus on the ventilation performances under crosswind conditions. As the cooling performance of cooling towers is very sensitive to the environmental conditions and crosswind continuously influences the ventilation performance during the operating course of cooling towers and further affects its heat and mass transfer efficiency [7, 8]. Hence, many studies in the past can be found concentrated on the cooling performance of cooling towers under crosswind conditions and suggested countermeasures against crosswinds which mainly divided into three aspects: numerical simulation, experimental research and field test. Al-Waked and Behnia [9-10] developed CFD model of UWCTs, and found that environmental crosswind led to the uneven distribution of air and water inside the cooling tower and deteriorated the cooling efficiency. The CFD studies showed that
the crosswind that has a velocity less than 7.5m/s is adverse to the cooling performance. Furthermore, Al-Waked [11] investigated the effect of crosswinds on the thermal performance of a stand-alone NDWCT and two NDWCTs surrounded by power plant building structures, and found that regardless of the cross-winds direction, an increase of 1.3 K or more could be predicted at crosswinds speeds greater than 4 m/s. Wang et al. [12] developed a numerical model to study the flow field at the air inlet zone under crosswind conditions, and received that crosswind seriously destroyed the axially symmetrical distribution. Wang et al. [13] adopted a computational fluid dynamics approach with validation to investigate the cooling performance of a natural draft dry cooling tower at various wind speeds and came to the conclusion that the circumferential non-uniform ventilation and the vortices inside are the main factors to the degrading of the cooling performance under crosswind conditions. Hooman [14] conducted theoretical prediction to predict crosswind effects on the performance of cooling towers and validated against available numerical and experimental data. This work found a closed-form solution for the airflow rate at the tower exit under given velocity crosswind conditions, and the results showed that the total heat rejected under crosswind condition will be affected by the ratio of crosswind velocity to draft velocity at the tower exit. Gao et al. [15, 16] also conducted thermal-state model experiments to investigate the effect of crosswind on the thermal performance of UWCTs, and received that crosswinds had a serious adverse effect on the circumferential inflow air and generate vortex zones in both windward side and leeward side inside the tower, further destroys the uniformity of air/water temperature distribution and deteriorates the thermal performance. Chen et al. [17] studied the effect of cross walls on the thermal performance of UWCTs under crosswind conditions by experimental researches, and the results showed: at all crosswind velocity, the cross wall at a setting angle of 0° resulted in better performance than that at 45°, regardless of the cross wall shapes. Zhang et al. [18] analyzed the influence of crosswind on the outlet airflow pattern of UWCTs and the resultant changes of its cooling range and ventilation drag coefficient by hot model experiments, results showed that the outlet airflow is extruded by upper
crosswind and the ventilation drag coefficient increased by 25%-35% within the crosswind velocity of 0.5m/s-0.8m/s. Mondal et al. [19] investigated the heat transfer performance of UWCTs under crosswind conditions by experiment research, and received that under 0.6m/s crosswind condition, the temperature drop and effectiveness decrease by 4% and 4.5% compared to windless condition, but this decreasing trend is up to the critical Froude number. So far, very few scholars focused on the field test researches on cooling towers, especially the HWCTs. Zhang et al. [20, 21] investigate the ventilation performance of UWCT by field test, and the test results showed that crosswind increases ventilation resistance, and destroys the uniformity of circumferential inflow air, further deteriorates the thermal performance. Duan [22] performed field test to studied the three-dimensional thermal performance test method for large UWCTs, and found that the crosswind adds the additional resistance in air inlet zone, the additional draft in air outlet zone and direct effects the heat transfer performance. Ardekani et al. [23] conducted field test on Heller cooling tower under wind conditions, and obtained that air suction at the tower prevents flow separation at its periphery. Besides, the tower front cooling sectors experience better airflow distribution compared to sectors parallel to wind direction, which can improve thermal performance by about 20% compared to still-air conditions. As can be seen from the above review, previous literatures demonstrated that crosswind leads to adverse effects on the ventilation performance of cooling towers. Some research work had conducted on the HWCTs concerning about numerical simulation and economic analysis, but these researches failed to discuss the ventilation performance of HWCTs under crosswind conditions, especially lack of field test method. Actually, the structure differences between the UWCTs and HWCTs may lead to different ventilation performance under crosswind conditions. In order to reveal the impact of crosswind on the ventilation performance of HWCTs and lay the theoretical foundation for further energy-saving research and optimization design of the HWCTs, the change rules of ventilation rate and circumferential inflow air under various crosswind conditions are researched by field test in this paper.
2. Field test parts 2.1. Test objectives In this study, the influence of crosswind on ventilation performance was studied by field test for high level water collecting wet cooling towers (HWCTs). By measuring circumferential inlet wind velocity at different position, the uniformity of flow field at the air inlet could be described. Besides, the ventilation rate G could be calculated by measuring meteorological parameters and temperature of different cooling zones under crosswind conditions. Based on the researches above, the ventilation performance could be further analyzed. This work could provide theoretical guidance for the optimizing design and energy-saving reconstruction of HWCTs. 2.2. Monitored parameters and measurement instruments This field test was conducted for a 1000MW unit in spring season, and the circulating water flowrate Q remains 69553t/h during the whole test period. The main geometrical dimensions of the cooling tower are listed in Table 1. The monitored parameters and measuring instruments are shown in Table 2, including the circumferential inlet wind velocity and direction, environmental meteorological parameters, circulating water flowrate and temperature of different cooling zones. Table 1. Main geometrical dimension of the HWCT
subject
value
unit
Fillings area
12800.00
m2
Height of the tower
190.00
m
Height of the air inlet
14.85
m
Height of the throat
142.50
m
Diameter of the throat
84.04
m
Diameter of the outlet
86.87
m
Table 2. Monitored parameters and measuring instruments
Item
Measuring instrument
Accuracy
Atmospheric pressure
Small-type meteorological station
±1.5%
Crosswind velocity
Small-type meteorological station
±0.1m/s
Small-type meteorological station
±0.1℃
Platinum resistance thermometer
±0.1℃
Circulating water flowrate
FLEXIM ultrasonic flowmeter
±0.5%
Outflow air temperature
Platinum resistance thermometer
±0.1℃
Anemoscopes
±0.1m/s
Anemoscopes
±0.05°
Inlet dry and wet bulb temperature Inlet and outlet water temperature
Circumferential inlet wind velocity Circumferential inlet wind direction 2.3. Measuring point arrangement and monitoring system The environmental meteorological parameters, including inlet dry and wet bulb temperature, crosswind velocity and atmospheric pressure. These parameters were tested by the small-type meteorological station which settled at a height of 2.5meters and 30-50 meters away from the cooling tower. During this whole field test period, according to the local meteorological conditions, the environmental crosswind velocity was 0.28m/s, 0.70m/s, 1.38m/s, 2.11m/s and 3.74m/s, respectively. The circumferential inlet wind velocity and direction were tested by eight anemoscopes which arranged evenly around the tower, seen in Fig.1.
Fig. 1. Schematic diagram of measuring points around the HWCT
The manhole door was selected as the starting point, α represents the layout angle of inlet wind velocity measuring points, α=0° represents the manhole door position. There were eight groups of measuring points which were settled according to anti-clockwise direction. At each measuring point there were two anemoscopes which settled in the vertical direction to measure the circumferential inlet wind velocity and direction at different heights, which can be seen in Fig.2. Actually the air which entering the tower for heat and mass exchange progress basically comes from the higher position, therefore, in the following discussion, the wind velocity at 6m is used as the studied value, and the wind velocity at 2m is used as the reference value.
Fig.2. Schematic diagram of circumferential measuring points at different heights
According to the conclusion from Gao et al. [15], the crosswind changes the circumferential inlet wind velocity and direction. Actually only the airflow along the radial direction can enter into the tower and take part in the heat and mass transfer course. Therefore, in this paper, the effective radial wind velocity Vr is introduced to analyze the uniformity of circumferential inlet wind, and Vr is the radial velocity component of circumferential inlet wind velocity at the height of 6m, shown in Fig.1. The temperature inside the tower mainly includes the outflow air temperature above the drift eliminators, inlet water temperature, and outlet water temperature. The outflow air temperature measuring points were settled above the drift eliminators, and there are twenty-four measuring points in six laps which are shown in Fig.3.
Fig.3. Sectional view schematic diagram of temperature measuring points above the eliminators
When calculating the ventilation rate, the average value of twenty-four measuring points is used as the outflow air temperature. In addition, the inlet and outlet water temperature were tested in the vertical shaft and water collection tanks, respectively. The schematic diagrams of partial measuring points are shown in Figs.4-5.
Anemoscopes
Measuring points above the drift eliminators
Fig.4. Scene picture of measuring points above the drift eliminators
Fig.5. Scene picture of measuring points around the tower
In order to collect data in real time and reduce the measuring errors caused by the time difference, this test process adopts wireless transmission to record all the data. And the monitoring system is shown in Fig.6.
Fig.6. Schematic diagram of data system
The measuring instruments at each measuring point transform the real time temperature value into the standard current signal which is transmitted to the data acquisition device by wire transmission. Then the data acquisition device collects the current signal and transmits it to the central master station which is connected with computer. At last, the data processing software in computer accepts and stores the related data for further analysis and research.
3. Impact of crosswind on the uniformity of circumferential inlet wind 3.1 Impact of crosswind on inlet air flow field around the tower During the design of cooling towers, the cross walls were installed to lessen the cross-ventilation of HWCTs, and the crosswind direction affects the air flow inside the tower. In this paper, θ represents the angle between cross walls and crosswind direction. According to the local environmental crosswind conditions, in this study, two angles (θ1=5° and θ2=35°) are investigated to analyze the ventilation performance under crosswind conditions, seen in Fig.7.
Fig.7. Schematic diagram of angle θ between crosswind direction and cross walls
The relation curves between crosswind velocity and Vr are shown in Fig.8 (a) and (b) under θ1=5° and θ2=35° conditions, respectively. It can be seen that the distribution of Vr is nearly axially symmetrical and uniform under 0.28m/s crosswind condition, basically the same as that of UWCTs under windless condition [16].
（a）distribution of
Vr under θ1=5° condition
（b）distribution of
Vr under θ2=35° condition
Fig.8. Relation curves between crosswind velocity and Vr
With the increasing of crosswind velocity, Vr in the windward side raised rapidly, leads to the rise of wind pressure in the windward side, which increases inlet resistance in the leeward side, further gives rise to the continuous decline of Vr in the leeward side. As the crosswind velocity reaches to 3.74m/s, compared with 0.28m/s crosswind condition, when θ1=5°, the Vr value in the windward side (α=0°, 45°, 315°) increases by 41.23%, 32.12% and 28.37%, respectively, the average increasing amplitude reaches to 33.91%. However, this value decreases by 33.57%, 18.01% and 14.92% in the leeward side (α=180°, 135°, 270°), and the average decreasing amplitude reaches to 22.17%. When θ2=35°, the Vr value in the windward side (α=135°, 90°, 180°) raises by 38.23%, 30.22% and 25.09%, respectively, the average increasing amplitude reaches to 31.18%, but it declines by 27.89%, 24.39% and 21.16% in the leeward side (α=315°, 0°, 270°), and the average reduction amplitude reaches to 24.48%. Meanwhile, the Vr value in the lateral side also reduced slightly due to the rising of wind pressure in the windward side. Hence, it can be obtained that crosswind mainly affects the ventilation performance of the windward and leeward side. At the velocity of 3.74m/s, the
increasing amplitude of Vr in the windward side (33.91%) under θ1=5° condition exceeds that under θ2=35° condition (31.18%). But under θ2=35° condition, the decreasing amplitude of Vr in the leeward side (24.48%) is greater than that under θ1=5° condition (22.17%). These phenomena indicate that at the same crosswind velocity, under θ1=5° condition, crosswind has a more serious impact on the inflow air in the windward side. But under θ2=35° condition, crosswind leads to relatively more severe influence on the inflow air in the leeward side. 3.2 Impact of crosswind on inlet air uniformity coefficient In this paper, the inlet air uniformity coefficient ψ was introduced to act as evaluation criteria to estimate the impact of crosswind on the uniformity of circumferential inflow air, it can be defined as [20], ψ
1 1 1 n
n
i 1
（1）
(vri vr )
2
where n is the number of measuring points around the tower, and n equals to 8 in this
paper, v ri stands for the Vr at measuring point of number i, and v r represents the average value of Vr . According to the accuracies of the measuring devices listed in Table 2, an uncertainty analysis on ψ was conducted using theoretical procedures [24], the results showed that the average measurement standard deviation of ψ is less than 0.14 or 1.47%. Under windless condition, the values of Vr are equal at all angles, and ψ=1 in this case. As can be seen from the analysis above, the crosswind destroys the uniformity of circumferential inflow air, which leads to ψ<1. Thus, under crosswind conditions, the smaller the ψ is, the worse the uniformity becomes. Fig.9 depicts the changing rules of ψ with crosswind velocity under θ1=5° and θ2=35°conditions. It can be seen that ψ is less than 1 under all kinds of crosswind conditions. With the increasing of crosswind velocity, ψ continuously decreases,
which finally results in the deterioration of the uniformity.
Fig.9. Changing rules of inlet air uniformity coefficient (ψ) with crosswind velocity Vc (m/s)
Meanwhile, the reduction amplitude of ψ increases when the crosswind velocity exceeds 0.70m/s, and at the same velocity, compared with θ1=5°condition, there is a greater decreasing amplitude under θ2=35°condition. When the crosswind velocity comes to 3.74m/s, ψ decreases to 0.61 and 0.49 under θ1=5° and θ2=35°conditions, respectively. Therefore, in terms of circumferential inflow air, the ventilation performance is better under θ1=5°condition. In conclusion, crosswind destroys the uniformity of circumferential inflow air, and increases the effective radial wind velocity Vr in the windward side, causes the rise of wind pressure in the windward side, which augments the ventilation resistance in the lateral and leeward side. At the same crosswind velocity, under θ1=5° condition, crosswind has a more serious impact on the inflow air in the windward side, but under θ2=35° condition, crosswind causes relatively more severe influence on the inflow air in the leeward side. Additionally, compared with θ2=35° condition, the ψ value is relatively higher than that under θ1=5° condition. It indicates that the uniformity of circumferential inflow air is better under θ1=5° condition.
4. Research on the influence of crosswind on the ventilation rate G The ventilation rate G is an important parameter to evaluate the ventilation
performance of cooling tower. As mentioned above, the distribution of circumferential inflow air is symmetrical and the air flow inside the tower is uniform under windless condition, so the ventilation rate G is relatively stable. According to the previous analysis, the crosswind destroys the uniformity of circumferential inflow air, affects the ventilation performance of HWCTs, which leads to a great impact on the ventilation rate G. In this paper, the ventilation rate G is calculated by heat balance method. According to the energy balance theory, the amount of heat which sent out from water equals to that absorbed by the air, and it can be defined as, dQ cw m
dTw G(i2 i1 ) K
(2) where c w is the specific heat of water; and i1 , i2 are the inlet, outlet air enthalpy value, respectively, which can be calculated by the data obtained in the test. Here, the K in Eq. (2) is a heat coefficient that represents the heat carried by evaporation loss, and it can be written as [25],
K 1
t2 586 0.56(t 2 20)
(3) Thus, the ventilation rate G is obtained by the combination of Eq. (2) and Eq. (3),
G
cw Q(t1 t 2 ) K (i2 i1 )
(4) where t1 and t 2 are the inlet and outlet water temperature, respectively. According to the uncertainty analysis on G, the results showed that the average measurement standard deviation of G doesn’t exceed 1738.83t/h or 3.53%. Fig. 10 illustrates the changing rules of G with crosswind velocity under θ1=5° and θ2=35°conditions. It can be seen that G is quite dependent on the crosswind
velocity, and G decreases gradually with the increasing of crosswind velocity. Test results manifest that there is little change in the ventilation rate G when the crosswind velocity is less than 0.70m/s. It means that the crosswind has little influence on the ventilation performance under relatively smaller crosswind velocity.
Fig.10. Changing rules of ventilation rate (G) under crosswind conditions
When the crosswind velocity exceeds 1.38m/s, the ventilation rate G reduces rapidly with the increasing of crosswind velocity. And as the crosswind velocity reaches to 3.74m/s, compared with 0.28m/s condition, the ventilation rate reduces by 30.13% under θ1=5° and decreases by 34.36% under θ2=35°. Thus, in terms of ventilation rate G, it is also proved that the ventilation performance is better under θ1=5°condition. Consequently, field test results demonstrate that crosswind has an adverse effect on the ventilation performance of HWCTs, the ventilation rate G continuously decreases with the increasing of crosswind velocity, but the influence level is different under various θ conditions. At the same crosswind velocity, there is a greater ventilation rate G under θ1=5°condition.
5. Conclusions Based on the field test and mathematical calculation for HWCTs, the principal results are as follows. (1) Crosswind destroys the uniformity of circumferential inflow air, and
increases the effective radial wind velocity in the windward side and the ventilation resistance in the lateral and leeward side. At the same crosswind velocity, under θ1=5° condition, crosswind had a more serious impact on the inflow air in the windward side, however, under θ2=35° condition, crosswind causes relatively more severe influence on the inflow air in the leeward side. (2) The inlet air uniformity coefficient ψ continuously decreases with the increasing of velocity, and the reduction amplitude of ψ increases gradually when the crosswind velocity exceeds 0.70m/s. When crosswind velocity reaches to 3.74m/s, the uniformity coefficient decreases to 0.61 and 0.49 under θ1=5° and θ2=35° conditions. At the same crosswind velocity, compared with θ1=5°condition, there is a greater decreasing amplitude under θ2=35°condition. It proves that the uniformity of circumferential inflow air is better under θ1=5° condition. (3) The ventilation rate has a sustained to decline with the rising of crosswind velocity. As the crosswind velocity reaches to 3.74m/s, compared with 0.28m/s condition, the ventilation rate reduces by 30.13% under θ1=5° and decreases by 34.36% under θ2=35°. It demonstrates that crosswind leads to the deterioration in the ventilation performance, and at the same crosswind velocity, the ventilation performance is superior under θ1=5° condition.
Acknowledgement This paper is supported by National Natural Science Foundation of China (No. 51776111)
and
Shandong
Province
Natural
Science
Foundation
(No.
ZR2016EEM35).
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Highlights
Field test was conducted on high-level cooling tower under crosswind conditions.
Inflow air uniformity coefficient decreases with the rising of crosswind velocity.
The uniformity coefficient decreases to 0.61 and 0.49 under θ1=5° and θ2=35°.
In 3.74m/s, the ventilation rate reduces by 30.13% under 5° condition.
The ventilation performance is better under 5° condition.