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Ventilation enhancement for a natural draft dry cooling tower in crosswind via windbox installation
Applied Thermal Engineering 137 (2018) 93–100
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Ventilation enhancement for a natural draft dry cooling tower in crosswind via windbox installation
T
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Weiliang Wang , Hai Zhang, Junfu Lyu, Qing Liu, Guangxi Yue, Weidou Ni Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China
H I GH L IG H T S
consists of windbreaks, an enclosure, a top and a barricade is proposed. • AA windbox modelling was conducted for the effect of windbox on the flow field of a NDDCT. • FLFCFDis adopted to quantify the affecting process of the windbox on NDDCT flow field. • The suggested windbox configuration is effective in enhance the NDDCT performance. • A reduction of annual unit coal consumption of 30,000–45,000 tons is estimated. •
A R T I C LE I N FO
A B S T R A C T
Keywords: NDDCT Crosswind Windbox Windbreaks Flow loss factor
A natural draft dry cooling tower (NDDCT) is demanded to save water for power generation in arid area, but its performance could be degraded greatly by the crosswind. To overcome the degradation, this paper proposes the installation of a windbox, which consists of windbreaks, an enclosure, a back barricade and a wind top to improve the pressure distribution outside the heat exchanger bundle of a NDDCT, so as to increase the ventilation rate under crosswind condition. Full dimensional computational fluid dynamics (CFD) modelling was conducted for the windbox installed around the NDDCT of a large scale coal-fired power plant. Different configurations of the windbox were studied. The flow characteristics along the streamline in the NDDCT were analyzed and quantified by adopting the concept of flow loss factor (FLF). Based on simulation and experiments, the installation of the windbox is shown to be effective. The windbox with a 120 m radius enclosure and a full size louver-type top could improve the ventilation rate of a NDDCT by ∼60% in gale crosswind condition, and keep high performance in breeze crosswind condition. Consequently, an annual reduction of coal consumption of 30,000–45,000 tons could be achieved on a 1000 MW unit, which is ∼3000,000–4500,000 $/a.
1. Introduction A large scale thermal power plant equipped with wet cooling towers consumes 10–20 M tons of water per year [1–3]. Such amount of water is often unacceptable for arid countries and regions [4,5]. Thus, indirect dry cooling technology, with its merits of water saving, low operation and maintenance cost, and long service time, has being increasingly used [6]. However, the performance of the main component of the indirect dry cooling system, namely the natural draft dry cooling tower (NDDCT) is found to be sensitive to ambient crosswind [7]. Previous studies found that a crosswind at 20 m/s could decrease the ventilation rate of a NDDCT by 36%. Besides, crosswind could increase the air temperature inside the tower up to 7.5 °C [8], and thereby
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decrease the heat transfer efficiency by more than 25% [9]. In another word, the crosswind could significantly reduce the efficiency, reliability and increase the operation cost of a power plant [10]. To prevent the performance degradation caused by the crosswind, a few measures were suggested to take [11–13]. Among them, installation of windbreaks is a common one due to its competent effectiveness [9,14–16], which was validated through numerical studies and wind tunnel experiments [17,18]. Besides, computational fluid dynamics (CFD) simulations showed the arrangement of [16]an enclosure around the radiators bundle was an effective way [19,20]. Straightforwardly, the combination windbreaks and enclosure was also suggested [21]. However, the size of the enclosure introduced in previous studies is too large to install.
Corresponding author. E-mail address: [email protected] (W. Wang).
https://doi.org/10.1016/j.applthermaleng.2018.03.059 Received 12 February 2018; Received in revised form 18 March 2018; Accepted 19 March 2018 Available online 20 March 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature
Superscripts
D NDDCT P q T U v z
∗
difference natural draft dry cooling tower pressure kPa mass flow rate kg/s temperature K potential flow kg/(s·m2) average velocity m/s the vertical height m
Subscripts 0 bottom chimney f inlet m outlet r radiator t total
Greek letters Δ ρ ξ Ω
total value
differential error air density kg/m local resistance coefficient flow resistance 1/m2
the baseline value the area inside the radiator the area right inside the tower chamber flow the area prior to the inlet of the NDDCT mass the area above the outlet of the NDDCT reference value the area between the radiator fins total value the overall streamline field
simplified as a constant heat source in numerical modelling, and mimicked by evenly assembled heating rods in the experiments. Due to symmetric characteristics of the flow field, the CFD model is developed in a half-cylinder configuration, with a dimension of 1200 m (in diameter) × 1700 m (in height), ∼10 times of the tower. The large space allows the crosswind to develop a reasonable velocity profile from a constant one at the inlet. The surfaces including the side walls, the ground, the inside/outside cooling tower shells and the support and joint faces between adjacent radiators are all set as adiabatic walls with no slip condition. The pressure-based solver built in FLUENT with pressure-velocity coupling SIMPLEC method is used. The governing equations of the momentum, energy, turbulent kinetic energy and dissipation rate are discretized using the second-order upwind differencing scheme. The air flow is assumed to be in fully developed turbulent regime, with negligible air density variation. Boussinesq approximation is used in the vertical momentum equation to consider the buoyancy force [26]. Governing equations for steady, buoyant, and turbulent flow including heat transfer are continuity, momentum, energy, and turbulence modelling equations and use standard k−ε model to describe the turbulent flow [27]. Based on grid checking, the model of hexahedral meshing with a grid number of 13.8 M was adopted. More detailed descriptions about the CFD model can be referred to our previous study [20].
This paper proposes a more compact and effective approach by optimizing the structure based on the combination of an enclosure and windbreaks. The flow fields around the NDDCT under crosswind condition were obtained using CFD simulation. The concept of flow loss factor (FLF) [22] was adopted to quantify the effect of the local flow field on the overall performance of a NDDCT. Besides, a set of hot state experiments were carried out in a wind tunnel with a NDDCT model to validate the simulation results [22]. 2. Numerical and experimental approaches 2.1. Numerical approach The NDDCT investigated in this paper is the one installed in a large scale coal-fired power plant in China. It is 170 m tall. The heights of the extension platform and radiators are 27.5 m and 24 m respectively, and the length of radiator support is 2 m. The outlet, throat and base radiator diameters of the tower are 84.466 m, 82 m and 146.17 respectively. Provided the power load is constant, the steam rate per unit power generation increases slightly when back pressure increases in a certain range, and the latent heat of the saturated vapour decreases slightly [23–25]. As a result, the variation of heat rejection could be negligible. Indeed, it was found that a 5 kPa increment of back pressure resulted in only ∼2% increment of the exhaust heat. Hence, in this study, the heat release in NDDCT radiators is approximated as constant under different crosswind conditions. Namely, the heat release of the radiator bundle is
(a) Photo of the actual NDDCT model
(b) System diagram of test rig
Fig. 1. Experimental system. (a) Photo of the actual NDDCT model; (b) System diagram of test rig. 94
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induces flow separation at the leeward side, resulting in correspondent low pressure zones as Fig. 3(a) shows. These results confirm many previous studies [29–31]. The circumferential pressure distribution at radiator outlet is relatively more even. A slight increment at the leeward section can be found in Fig. 3(b), as the result of asymmetric air intake between the windward and the leeward. Although a micro pressure transmitter with an accuracy of ± 0.0625 Pa is adopted, the measuring error is considerable considering the target value falls in the range of 0.1 Pa – 1 Pa. It can be seen that despite several deviated measured points, the results of CFD calculation and experiment have similar trends. Considering the measuring error of such low pressure and system errors existing in experiment, the errors between experiment and CFD calculation are well accepted, i.e., the CFD calculation results are well confirmed by the experimental data. The following discussion are then mainly based on the results of CFD calculation.
2.2. Experimental approach The experiments were conducted in a wind tunnel using a NDDCT model in the scale of 1/200 of the simulated one. The schematic diagram of the experimental system is shown in Fig. 1. The NDDCT model, mainly consisting of a chimney (1), resistance bundle (2), heating rods (3), is built up according to the geometric scaling law of geometry. The resistance bundle has a series of parallel zigzag iron pieces to mimic the resistance characteristic of the radiators according to Euler number. Three tunable electrical heating rods are mounted around the resistance pieces to mimic the heat rejection from the radiators according to Froude number. The wind tunnel can supply a crosswind (4) up to 30 m/s. Detailed descriptions of the experimental system are introduced in previous publications [28]. The tunnel wind velocity is set according to momentum law. The basic parameters and experimental conditions are shown in Table 1. As exhibited in Fig. 1(b), the pressure is measured through pressure sampling tubes (5), the temperature is measured by thermocouples (6), and the velocity is measured by a pitot tube (7) equipped on a Rocker (8). The pressure signals from sampling tubes and pitot tube are transformed by micro pressure transmitter (9). Both pressure and temperature signals are converted into digital signals through A/D converter (10) and recorded by a computer (11).
3.2. The total FLF and ventilation loss of a baseline NDDCT As investigated previously [22,32], a general description of Bernoulli equation [33] is expressed in Eq. (1), where P ∗ represents the total pressure including the dynamic pressure and altitude pressure, a flow resistance Ωf and a potential flow Uf are defined as Eqs. (2) and (3). Consequently, an inverse proportional relationship between the flow resistance and mass flow rate is obtained as Eq. (4). By a differential method, a FLF is deducted as Eq. (5) to depict the effect of the pressure loss on the overall dimensionless mass flow rate. The subscripts –r and –t denote the reference and the total conditions respectively.
2.3. Windbox installation In this study, the installation of a windbox is proposed to prevent the performance degradation. The windbox, as shown in Fig. 2, contains a set of windbreaks, an enclosure, a back barricade and a wind top as shown in Fig. 2(a). The windbreaks are set to break the side acceleration, and the enclosure is set to improve the pressure distribution outside the radiator bundle. Meanwhile, a wind top is set inside the upper ring margin of the enclosure to form a windbox configuration, so as to prevent the upward flow of the collected air at the rear side; a back barricade of 5 m width is also set inside the enclosure to reduce the flow separation after the windbreaks. The windbreak inside the windbox remains a size of 26 m (in height) × 25 m (in width) as proposed in previous study [21]. For construction feasibility, the height of the enclosure is reduced from 62 m to 27.5 m. To optimize the configuration and dimension, five different windboxes were assessed, first, the radius of the enclosure is studied on cases of 160 m, 120 m and 100 m as shown respectively in Fig. 2(a–c). Besides, the width of wind top is studied on cases of 5 m (by default), 10 m and 25 m with an enclosure radius of 100 m as shown in Fig. 2(d and e). A 25 m width top is a full size top which precisely seals the space between the up margins of radiator and enclosure at side and back sections.
1 P1∗ = P2∗ + ξ · ρ2 v22 2
Ωf = Uf =
(1)
ξ S
(2)
2ρΔP ∗
(3)
Uf = qm ·Ωf
FLF=
(4)
ΔΩf Ωf− r−t
= FLF0 +
qm− r−t qm
ΔP∗ ΔPr∗− t
⎡ ⎣
d(ΔP∗) d(qm) − q ⎤ 2ΔP∗ m ⎦
(5)
With regarding to the flow characteristics along the streamline, five pressure surfaces are selected as shown in Fig. 4 [22,32], including far field inlet, radiator inlet, radiator outlet, chimney inlet, and tower outlet respectively. A Cartesian coordinate is set up, where the origin locates at the centre of the radiator circle on the ground, X axis direction is reverse to that of the crosswind, Y axis is perpendicular to the crosswind, and Z axis is vertical. The total FLF and ventilation loss of a baseline NDDCT under different crosswind conditions are exhibited in Fig. 5, where the ventilation loss refers to the reduced mass flow rate compared to crosswind free condition. The variation trend of the FLF-total shows a good agreement with that of the ventilation loss, i.e. a FLF is proportional to the correspondent ventilation loss. Consequently, FLF is adopted to quantitatively study the effect of the flow change in each region on the overall ventilation rate.
3. Results and discussion 3.1. The pressure distributions of the baseline NDDCT Fig. 3 compares the pressure distributions of baseline NDDCT between the experiment and simulation, where Fig. 3(a) and (b) exhibit circumferential pressure distribution at the radiator inlet and outlet at Vt of 1–1.5 m/s respectively. Under crosswind condition, the crosswind stagnates at the inlet centre of the windward radiator bundle (0°/360°), forming a high pressure zone; while accelerates at the side sections, and Table 1 The designed working conditions of the NDDCT model.
Baseline Experiment
Pa kPa
RH %
Q MW
Dm
Vin m/s
qm kg/s
Tout K
Ta K
ΔP kPa
Re
92.7 101
71.0 74.0
817 1.56
82.0 0.41
3.19 0.23
3.57e4 0.071
328 328
303 306
1.09e-1 5.20e-4
2.94e7 1.04e4
95
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(a) 160 m radius + 5 m top
(b) 120 m radius + 5 m top
(d)100 m radius10 m top
(c) 100 m radius + 5 m top
(e) 100 m radius 25 m top
Fig. 2. The schematic of different windbox configurations (crosswind blows from front).
3.3. Enclosure effect on overall flow field
suggested.
As 10 m/s is a typical crosswind velocity, the flow fields of the NDDCT installed with different windbox sizes is analyzed at 10 m/s as shown in Fig. 6(a–c) respectively. It can be seen that the swirling intensity of mainstream vortex decreases greatly as the enclosure radius changes from 160 m to 120 m. While it rebound to be even higher as the enclosure radius continues to decrease to 100 m. An enclosure is set to collect the accelerated side flow and increase the back side static pressure, while when the windbox at radius of 100 m is too small, the air flow outside the radiator bundle is forced to accelerate at the rear side, resulting an even lower pressure condition. Consequently, the the intensity of the mainstream vortex become to increase. Because mainstream vortex is found to be the main degrading factors arising from the inlet in previous studies [22], a smaller enclosure radius as 100 m is not
3.4. Enclosure effect on total FLF and ventilation loss Crosswind affects the flow characteristics by changing the inlet and outlet field of the NDDCT. The windboxes influence the inlet field of the NDDCT under crosswind conditions, hence to influence the flow characteristics along the streamline of the NDDCT. The FLF’s of each flow section on different cases are calculated as shown in Fig. 7. It can be seen that as the enclosure radius decreases from 160 m to 120 m, most of the FLF’s decrease at 10 m/s, while increase at 20 m/s. As the enclosure radius continues to decrease to 100 m, the FLF’s at all sections increase obviously in all crosswind range. Comparatively, the windbox at radius of 120 m shows a good performance as related to the FLF. Since the FLF has a linear relationship to the decrement of
1.0
0
Radiator inlet pressure, Pin Pa
Simulation
0.0
0
60
120
180
240
-0.5 -1.0 -1.5
300
360
Radiator outlet pressure, Pout Pa
0
experiment
0.5
-2.0
60
120
180
240
-0.15
Experiment
Simulation
-0.3
-0.45
-0.6
-0.75
Circumferential angle (°)
Circumferential angle (°)
Fig. 3. Validation of the pressure distribution for the CFD model. 96
300
360
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that, through a structure optimization of back barricades and an enclosure top, approaches with smaller enclosures of 160 m radius and 120 m radius achieve similar ventilation performances to the referenced large enclosure one in all investigated crosswind range. Compared to the approach with an enclosure of 160 m, the ventilation rate at the approach with an enclosure of 120 m radius prevails in wide range of breeze crosswind (∼0–14 m/s). Meanwhile, the enclosure with a radius of 120 m costs only about three quarters of the construction fee, and save more than 40 percent of the construction area. Compared to the baseline case, the optimized approach with 120 m radius enclosure could enhance the NDDCT ventilation rate by ∼15–20%. 3.5. Effect of the enclosure top on overall flow field Fig. 4. The locations of the pressure surfaces selected.
The width of the top on 100 m radius enclosure case further studied on different cases of 10 m and 25 m respectively. The streamlines of different top cases at crosswind of 10 m/s are shown in Fig. 9. Compared to 5 m width top shown in Fig. 6(c), it is found that as the width of the top increases to 10 m, the swirling intensity of the mainstream vortex decreases greatly, while as the width continues to increase to 25 m, the swirling intensity of the mainstream vortex reverses to increase to be even higher. In comparison, the top with of 10 m exhibits a good performance considering the flow field inside the NDDCT.
1.4
Flow loss factor, FLF
FLF-total
1
20
Ventilation loss
0.8
15
0.6
10
0.4 5
0.2 0 0
5
10 15 20 Crosswind velocity, V m/s
25
Ventilation loss, qm 103 kg/s
25
1.2
3.6. Effect of the enclosure top on FLF and ventilation loss The FLF’s of each flow section along the streamline on different enclosure tops are illustrated in Fig. 10. On comparison to the 5 m width top case shown in Fig. 7(c), under crosswind ranging from ∼0–10 m/s, most of the FLF’s decrease on 10 m width top case, while increase greatly on the 25 m width top case. However, as the crosswind increase to be above 15 m/s, the FLF’s on 10 m width top case begin to exceed those in 5 m width top case, while most of the FLF’s on 25 m width top case decrease greatly to a very relatively lower level. Generally, the FLF performance on 10 m top cases prevail the other ones under the overall crosswind range. Along with the changing of top width, the variations of FLF’s are reflected in the ventilation rate of each case, as shown in Fig. 11. In consistent with the variation of FLF’s, it can be see that, as the top width
0 30
Fig. 5. The variation trend of ventilation loss along with different FLF’s.
ventilation rate, the ventilation rates of the NDDCT on different cases are influenced accordingly. Fig. 8 shows the ventilation rate of the NDDCT on baseline case, referenced[21] case and optimized cases, where the enclosure-230 m case denotes the referenced model with a combination of windbreaks and an enclosure of 230 m in radius. Others refer to each case with correspondent enclosure radius. It can be found
(a) 160 m radius
(b) 120 m radius Fig. 6. The streamlines of NDDCT with different enclosures at 10 m/s. 97
(c) 100 m radius
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(a)160 m radius
(b) 120 m radius
(c) 100 m radius Fig. 7. The FLF’s on different enclosure radius cases.
3.7. Evaluation of the windbox performance On the ground of a conjoint analysis of the enclosure radius and top width, a combination of an enclosure with 120 m radius, a top with 25 m width, including back barricades and windbreaks are recommended. Considering the drawbacks of the top with 25 m width in breeze crosswind, a louver like structure is proposed for the top, which can be opened up in low crosswind condition. As a result, a ventilation enhancement of ∼60% in gale crosswind condition and no degradation in breeze crosswind condition is estimated by installing such optimized windbox structure as shown in Fig. 12 (dash line of “Enclosure120+top-25). By adopting the calculation method introduced in our previous publication [20], the initial temperature difference (ITD) of the NDDCT could be obtained. The calculation results show that ITD could be reduced to 30 °C by adopting the proposed windbox at crosswind of 20 m/ s, which is 9 °C less than that of the baseline case. By consulting the operating parameters, a 9 °C reduction of ITD could reduce the back pressure of the steam turbine by 3.5–4.5 kPa, which is correspondent to a reduction of the unit net coal consumption rate by 6–9 g/kWh. Given an annual operation time of 5000 h, the proposed windbox could reduce the overall coal consumption of a 1000 M unit by 30,000–45,000 tons annually. Taking a standard coal price of 100 $/ton, an operation cost reduction of 3000,000–4500,000 $ could be achieved by taking such method. Since the experimental results of the modelling NDDCT shows a good similarity to the full scale ones, it is assumed that the research results in this paper could be generalized to applied to other NDDCT by adopting scaling law of geometry.
Fig. 8. The ventilation rates on different enclosure cases.
increases from 5 m to 10 m, the ventilation rate increases slightly in low crosswind and decreases slightly in high crosswind. However, as the top width continues to increase to 25 m, the ventilation rate decreases greatly in breeze crosswind, while increase greatly in gale crosswind. As introduced in Section 2.3, a 25 m width top is a full size top which precisely seals gap between the radiator and enclosure at side and back sections. Then the windward opening of the enclosure become the only entrance of the NDDCT. As the result, under breeze condition, as the ventilation rate through the windward opening of the enclosure is less than the baseline ventilation, the windbox acts as a baffle of the inlet air on other sides; while under gale condition, as the ventilation rate through the windward opening is high than the baseline ventilation, the windbox acts as a channel, which directs all of the inlet air via the windward opening to the radiators, hence greatly increase the ventilation rate. 98
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(a)10 m top
(b) 25 m top
Fig. 9. The streamlines of NDDCT with different tops at 10 m/s.
4. Conclusions As the performance of a natural draft dry cooling tower (NDDCT) degrades greatly under crosswind condition, a windbox configuration consisting of windbreaks, an enclosure, a back barricade and a wind top is proposed to be installed around the NDDCT to increase its ventilation rate. By adopting a full dimensional computational fluid dynamic (CFD) modelling, the flow field around NDDCT was studied. With the help of flow loss factor (FLF), the effects of windbox on the flow characteristics of each flow field sections were also studied. By investigating different sizes and components of the windbox, the configurations of the windbox were optimized. Simulation results show that a windbox with a 120 m radius enclosure and a full size louver top is effective in improving the flow field around a NDDCT under crosswind condition. By installing such structure, the performance of a NDDCT is enhanced greatly at all crosswind range. The degradation of the ventilation rate is avoided under breeze condition, and reversed to be an enhancement on gale condition. An ∼60% increment of ventilation rate is estimated at ∼20 m/s crosswind. In case of a 1000 MW unit, the suggested windbox could reduce the standard coal consumption by 30,000–45,000 tons annually, which is correspondent to ∼3000,000–4500,000 $/a. The approach could be generalized according to geometry similarity.
Fig. 11. The ventilation rates on different top cases.
Acknowledgement This research is supported by China Postdoctoral Science Foundation (2017M620758), Special Funds of the National Natural Science Foundation of China (No. L1522032), and the Consulting Project of Chinese Academy of Engineering (No. 2015-ZCQ-06).
(a) 10 m top
(b) 25 m top Fig. 10. The FLF’s on different enclosure radius cases. 99
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[12] Y. Kong, W. Wang, X. Huang, L. Yang, X. Du, Y. Yang, Wind leading to improve cooling performance of natural draft air-cooled condenser, Appl. Therm. Eng. (2018). [13] W. Wang, X. Huang, L. Yang, X. Du, Performance recovery of natural draft dry cooling systems by combined air leading strategies, Energies 10 (12) (2017) 2166. [14] A.F. Du Preez, D.G. Kroger, Effect of wind on performance of a dry-cooling tower, Heat Recovery Syst. CHP 13 (1993) 139–146. [15] H. Gu, H. Wang, Y. Gu, J. Yao, A numerical study on the mechanism and optimization of wind-break structures for indirect air-cooling towers, Energ. Convers. Manage. 108 (2016) 43–49. [16] Y. Lu, Z. Guan, H. Gurgenci, A. Alkhedhair, S. He, Experimental investigation into the positive effects of a tri-blade-like windbreak wall on small size natural draft dry cooling towers, Appl. Therm. Eng. (2016). [17] R. Al-Waked, M. Behnia, The effect of windbreak walls on the thermal performance of natural draft dry cooling towers, Heat Transfer Eng. 26 (8) (2005) 50–62. [18] L. Chen, L. Yang, X. Du, Y. Yang, Performance improvement of natural draft dry cooling system by interior and exterior windbreaker configurations, Int. J. Heat Mass Tran. 96 (2016) 42–63. [19] W. Wang, P. Liu, Z. Li, W. Ni, J. Liu, Y. Li, A numerical study on the improving techniques of the cooling performance of a natural draft dry cooling tower under crosswind, Chem. Eng. Trans. 45 (2015) 1081–1086. [20] W. Wang, H. Zhang, P. Liu, Z. Li, J. Lv, W. Ni, The cooling performance of a natural draft dry cooling tower under crosswind and an enclosure approach to cooling efficiency enhancement, Appl. Energ. 186 (2017) 336–346. [21] W. Wang, H. Zhang, Z. Li, J. Lv, W. Ni, Y. Li, Adoption of enclosure and windbreaks to prevent the degradation of the cooling performance for a natural draft dry cooling tower under crosswind conditions, Energy 116 (2016) 1360–1369. [22] W. Wang, The Reconfiguration of the Flow Field of a Natural Draft Dry Cooling Tower on Crosswind Condition for Cooling Enhancement [Doctor], Tsinghua University, Beijing, 2017. [23] Y. Zhao, F. Sun, Y. Li, G. Long, Z. Yang, Numerical study on the cooling performance of natural draft dry cooling tower with vertical delta radiators under constant heat load, Appl. Energ. 149 (6) (2015) 225–237. [24] G. Barigozzi, A. Perdichizzi, S. Ravelli, Performance prediction and optimization of a waste-to-energy cogeneration plant with combined wet and dry cooling system, Appl. Energ. 115 (2014) 65–74. [25] G. Barigozzi, A. Perdichizzi, S. Ravelli, Wet and dry cooling systems optimization applied to a modern waste-to-energy cogeneration heat and power plant, Appl. Energ. 88 (4) (2011) 1366–1376. [26] M. Goodarzi, R. Ramezanpour, Alternative geometry for cylindrical natural draft cooling tower with higher cooling efficiency under crosswind condition, Energ. Convers. Manage. 77 (1) (2014) 243–249. [27] M. Goodarzi, A proposed stack configuration for dry cooling tower to improve cooling efficiency under crosswind, J. Wind Eng. Ind. Aerod. 98 (12) (2010) 858–863. [28] W. Wang, Y. Wang, H. Zhang, G. Lin, J. Lu, G. Yue, et al., Fresh breeze cuts down one-third ventilation rate of a natural draft dry cooling tower: a hot state modelling, Appl. Therm. Eng. 131 (2018) 1–7. [29] K. Tanimizu, K. Hooman, Natural draft dry cooling tower modelling, Heat Mass Transfer 49 (2) (2013) 155–161. [30] B. Sheng, Nmerical Research of Crosswind Effect on the Performance of Indirect Air Cooling Tower and Exploration of the Anti-Wind Measures [Master], Southeast University, Nanjing, 2015. [31] C. Huang, S. Zhao, Research on resistance performance of indirect -dry cooling tower, J. China Instit. Water Resources Hydropower Res. 09 (03) (2011) 195–199. [32] W. Wang, J. Lv, H. Zhang, Q. Liu, G. Yue, W. Ni, A quantitative approach identifies the critical flow characteristics in a natural draft dry cooling tower, Appl. Therm. Eng. 131 (2018) 522–530. [33] Q. Zheng, Z. Lu, Fluid Mechanics, China Machine Press, Beijing, 1979.
Fig. 12. The ventilation rates on proposed cases.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.applthermaleng.2018. 03.059. References [1] Q. Shun-yi, H.U. Yang, The present situation and applied prospective of the air cooling technology in power plant, Power Station Auxil. Equipm. 32 (4) (2011) 1–4. [2] W. Wang, W. Ni, Z. Wang, Z. Li, Y. Li, J. Liu, A review on the research of a dry cooling tower affected by cross wind, Proceedings CSEE 35 (04) (2015) 882–890. [3] W. Li, J. Chai, J. Zheng, Investigation of natural draft cooling tower in China, Heat Transf. Eng. 38 (11–12) (2017) 1101–1107. [4] E. Al-Bassam, G.P. Maheshwari, A new scheme for cooling tower water conservation in arid-zone countries, Energy 36 (7) (2011) 3985–3991. [5] R. Ghosh, T.K. Ray, R. Ganguly, Cooling tower fog harvesting in power plants - A pilot study, Energy 89 (2015) 1018–1028. [6] Y. Sun, Z. Guan, H. Gurgenci, K. Hooman, X. Li, Investigation on the influence of injection direction on the spray cooling performance in natural draft dry cooling tower, Int. J. Heat Mass Tran. 110 (2017) 113–131. [7] Y. Kong, W. Wang, L. Yang, X. Du, Y. Yang, A novel natural draft dry cooling system with bilaterally arranged air-cooled heat exchanger, Int. J. Therm. Sci. 112 (2017) 318–334. [8] Q. Wei, B. Zhang, K. Liu, X. Du, X. Meng, A study of the unfavorable effects of wind on the cooling efficiency of dry cooling towers, J. Wind Eng. Ind. Aerod. 54 (94) (1995) 633–643. [9] M. Goodarzi, R. Keimanesh, Heat rejection enhancement in natural draft cooling tower using radiator-type windbreakers, Energ. Convers. Manage. 71 (2013) 120–125. [10] D. Zhang, P. Liu, L. Ma, Z. Li, W. Ni, A multi-period modelling and optimization approach to the planning of China's power sector with consideration of carbon dioxide mitigation, Comput. Chem. Eng. 37 (2012) 227–247. [11] H.T. Liao, L.J. Yang, X.Z. Du, Y.P. Yang, Triangularly arranged heat exchanger bundles to restrain wind effects on natural draft dry cooling system, Appl. Therm. Eng. (2016).
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Research Paper
Ventilation enhancement for a natural draft dry cooling tower in crosswind via windbox installation
T
⁎
Weiliang Wang , Hai Zhang, Junfu Lyu, Qing Liu, Guangxi Yue, Weidou Ni Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China
H I GH L IG H T S
consists of windbreaks, an enclosure, a top and a barricade is proposed. • AA windbox modelling was conducted for the effect of windbox on the flow field of a NDDCT. • FLFCFDis adopted to quantify the affecting process of the windbox on NDDCT flow field. • The suggested windbox configuration is effective in enhance the NDDCT performance. • A reduction of annual unit coal consumption of 30,000–45,000 tons is estimated. •
A R T I C LE I N FO
A B S T R A C T
Keywords: NDDCT Crosswind Windbox Windbreaks Flow loss factor
A natural draft dry cooling tower (NDDCT) is demanded to save water for power generation in arid area, but its performance could be degraded greatly by the crosswind. To overcome the degradation, this paper proposes the installation of a windbox, which consists of windbreaks, an enclosure, a back barricade and a wind top to improve the pressure distribution outside the heat exchanger bundle of a NDDCT, so as to increase the ventilation rate under crosswind condition. Full dimensional computational fluid dynamics (CFD) modelling was conducted for the windbox installed around the NDDCT of a large scale coal-fired power plant. Different configurations of the windbox were studied. The flow characteristics along the streamline in the NDDCT were analyzed and quantified by adopting the concept of flow loss factor (FLF). Based on simulation and experiments, the installation of the windbox is shown to be effective. The windbox with a 120 m radius enclosure and a full size louver-type top could improve the ventilation rate of a NDDCT by ∼60% in gale crosswind condition, and keep high performance in breeze crosswind condition. Consequently, an annual reduction of coal consumption of 30,000–45,000 tons could be achieved on a 1000 MW unit, which is ∼3000,000–4500,000 $/a.
1. Introduction A large scale thermal power plant equipped with wet cooling towers consumes 10–20 M tons of water per year [1–3]. Such amount of water is often unacceptable for arid countries and regions [4,5]. Thus, indirect dry cooling technology, with its merits of water saving, low operation and maintenance cost, and long service time, has being increasingly used [6]. However, the performance of the main component of the indirect dry cooling system, namely the natural draft dry cooling tower (NDDCT) is found to be sensitive to ambient crosswind [7]. Previous studies found that a crosswind at 20 m/s could decrease the ventilation rate of a NDDCT by 36%. Besides, crosswind could increase the air temperature inside the tower up to 7.5 °C [8], and thereby
⁎
decrease the heat transfer efficiency by more than 25% [9]. In another word, the crosswind could significantly reduce the efficiency, reliability and increase the operation cost of a power plant [10]. To prevent the performance degradation caused by the crosswind, a few measures were suggested to take [11–13]. Among them, installation of windbreaks is a common one due to its competent effectiveness [9,14–16], which was validated through numerical studies and wind tunnel experiments [17,18]. Besides, computational fluid dynamics (CFD) simulations showed the arrangement of [16]an enclosure around the radiators bundle was an effective way [19,20]. Straightforwardly, the combination windbreaks and enclosure was also suggested [21]. However, the size of the enclosure introduced in previous studies is too large to install.
Corresponding author. E-mail address: [email protected] (W. Wang).
https://doi.org/10.1016/j.applthermaleng.2018.03.059 Received 12 February 2018; Received in revised form 18 March 2018; Accepted 19 March 2018 Available online 20 March 2018 1359-4311/ © 2018 Elsevier Ltd. All rights reserved.
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Nomenclature
Superscripts
D NDDCT P q T U v z
∗
difference natural draft dry cooling tower pressure kPa mass flow rate kg/s temperature K potential flow kg/(s·m2) average velocity m/s the vertical height m
Subscripts 0 bottom chimney f inlet m outlet r radiator t total
Greek letters Δ ρ ξ Ω
total value
differential error air density kg/m local resistance coefficient flow resistance 1/m2
the baseline value the area inside the radiator the area right inside the tower chamber flow the area prior to the inlet of the NDDCT mass the area above the outlet of the NDDCT reference value the area between the radiator fins total value the overall streamline field
simplified as a constant heat source in numerical modelling, and mimicked by evenly assembled heating rods in the experiments. Due to symmetric characteristics of the flow field, the CFD model is developed in a half-cylinder configuration, with a dimension of 1200 m (in diameter) × 1700 m (in height), ∼10 times of the tower. The large space allows the crosswind to develop a reasonable velocity profile from a constant one at the inlet. The surfaces including the side walls, the ground, the inside/outside cooling tower shells and the support and joint faces between adjacent radiators are all set as adiabatic walls with no slip condition. The pressure-based solver built in FLUENT with pressure-velocity coupling SIMPLEC method is used. The governing equations of the momentum, energy, turbulent kinetic energy and dissipation rate are discretized using the second-order upwind differencing scheme. The air flow is assumed to be in fully developed turbulent regime, with negligible air density variation. Boussinesq approximation is used in the vertical momentum equation to consider the buoyancy force [26]. Governing equations for steady, buoyant, and turbulent flow including heat transfer are continuity, momentum, energy, and turbulence modelling equations and use standard k−ε model to describe the turbulent flow [27]. Based on grid checking, the model of hexahedral meshing with a grid number of 13.8 M was adopted. More detailed descriptions about the CFD model can be referred to our previous study [20].
This paper proposes a more compact and effective approach by optimizing the structure based on the combination of an enclosure and windbreaks. The flow fields around the NDDCT under crosswind condition were obtained using CFD simulation. The concept of flow loss factor (FLF) [22] was adopted to quantify the effect of the local flow field on the overall performance of a NDDCT. Besides, a set of hot state experiments were carried out in a wind tunnel with a NDDCT model to validate the simulation results [22]. 2. Numerical and experimental approaches 2.1. Numerical approach The NDDCT investigated in this paper is the one installed in a large scale coal-fired power plant in China. It is 170 m tall. The heights of the extension platform and radiators are 27.5 m and 24 m respectively, and the length of radiator support is 2 m. The outlet, throat and base radiator diameters of the tower are 84.466 m, 82 m and 146.17 respectively. Provided the power load is constant, the steam rate per unit power generation increases slightly when back pressure increases in a certain range, and the latent heat of the saturated vapour decreases slightly [23–25]. As a result, the variation of heat rejection could be negligible. Indeed, it was found that a 5 kPa increment of back pressure resulted in only ∼2% increment of the exhaust heat. Hence, in this study, the heat release in NDDCT radiators is approximated as constant under different crosswind conditions. Namely, the heat release of the radiator bundle is
(a) Photo of the actual NDDCT model
(b) System diagram of test rig
Fig. 1. Experimental system. (a) Photo of the actual NDDCT model; (b) System diagram of test rig. 94
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induces flow separation at the leeward side, resulting in correspondent low pressure zones as Fig. 3(a) shows. These results confirm many previous studies [29–31]. The circumferential pressure distribution at radiator outlet is relatively more even. A slight increment at the leeward section can be found in Fig. 3(b), as the result of asymmetric air intake between the windward and the leeward. Although a micro pressure transmitter with an accuracy of ± 0.0625 Pa is adopted, the measuring error is considerable considering the target value falls in the range of 0.1 Pa – 1 Pa. It can be seen that despite several deviated measured points, the results of CFD calculation and experiment have similar trends. Considering the measuring error of such low pressure and system errors existing in experiment, the errors between experiment and CFD calculation are well accepted, i.e., the CFD calculation results are well confirmed by the experimental data. The following discussion are then mainly based on the results of CFD calculation.
2.2. Experimental approach The experiments were conducted in a wind tunnel using a NDDCT model in the scale of 1/200 of the simulated one. The schematic diagram of the experimental system is shown in Fig. 1. The NDDCT model, mainly consisting of a chimney (1), resistance bundle (2), heating rods (3), is built up according to the geometric scaling law of geometry. The resistance bundle has a series of parallel zigzag iron pieces to mimic the resistance characteristic of the radiators according to Euler number. Three tunable electrical heating rods are mounted around the resistance pieces to mimic the heat rejection from the radiators according to Froude number. The wind tunnel can supply a crosswind (4) up to 30 m/s. Detailed descriptions of the experimental system are introduced in previous publications [28]. The tunnel wind velocity is set according to momentum law. The basic parameters and experimental conditions are shown in Table 1. As exhibited in Fig. 1(b), the pressure is measured through pressure sampling tubes (5), the temperature is measured by thermocouples (6), and the velocity is measured by a pitot tube (7) equipped on a Rocker (8). The pressure signals from sampling tubes and pitot tube are transformed by micro pressure transmitter (9). Both pressure and temperature signals are converted into digital signals through A/D converter (10) and recorded by a computer (11).
3.2. The total FLF and ventilation loss of a baseline NDDCT As investigated previously [22,32], a general description of Bernoulli equation [33] is expressed in Eq. (1), where P ∗ represents the total pressure including the dynamic pressure and altitude pressure, a flow resistance Ωf and a potential flow Uf are defined as Eqs. (2) and (3). Consequently, an inverse proportional relationship between the flow resistance and mass flow rate is obtained as Eq. (4). By a differential method, a FLF is deducted as Eq. (5) to depict the effect of the pressure loss on the overall dimensionless mass flow rate. The subscripts –r and –t denote the reference and the total conditions respectively.
2.3. Windbox installation In this study, the installation of a windbox is proposed to prevent the performance degradation. The windbox, as shown in Fig. 2, contains a set of windbreaks, an enclosure, a back barricade and a wind top as shown in Fig. 2(a). The windbreaks are set to break the side acceleration, and the enclosure is set to improve the pressure distribution outside the radiator bundle. Meanwhile, a wind top is set inside the upper ring margin of the enclosure to form a windbox configuration, so as to prevent the upward flow of the collected air at the rear side; a back barricade of 5 m width is also set inside the enclosure to reduce the flow separation after the windbreaks. The windbreak inside the windbox remains a size of 26 m (in height) × 25 m (in width) as proposed in previous study [21]. For construction feasibility, the height of the enclosure is reduced from 62 m to 27.5 m. To optimize the configuration and dimension, five different windboxes were assessed, first, the radius of the enclosure is studied on cases of 160 m, 120 m and 100 m as shown respectively in Fig. 2(a–c). Besides, the width of wind top is studied on cases of 5 m (by default), 10 m and 25 m with an enclosure radius of 100 m as shown in Fig. 2(d and e). A 25 m width top is a full size top which precisely seals the space between the up margins of radiator and enclosure at side and back sections.
1 P1∗ = P2∗ + ξ · ρ2 v22 2
Ωf = Uf =
(1)
ξ S
(2)
2ρΔP ∗
(3)
Uf = qm ·Ωf
FLF=
(4)
ΔΩf Ωf− r−t
= FLF0 +
qm− r−t qm
ΔP∗ ΔPr∗− t
⎡ ⎣
d(ΔP∗) d(qm) − q ⎤ 2ΔP∗ m ⎦
(5)
With regarding to the flow characteristics along the streamline, five pressure surfaces are selected as shown in Fig. 4 [22,32], including far field inlet, radiator inlet, radiator outlet, chimney inlet, and tower outlet respectively. A Cartesian coordinate is set up, where the origin locates at the centre of the radiator circle on the ground, X axis direction is reverse to that of the crosswind, Y axis is perpendicular to the crosswind, and Z axis is vertical. The total FLF and ventilation loss of a baseline NDDCT under different crosswind conditions are exhibited in Fig. 5, where the ventilation loss refers to the reduced mass flow rate compared to crosswind free condition. The variation trend of the FLF-total shows a good agreement with that of the ventilation loss, i.e. a FLF is proportional to the correspondent ventilation loss. Consequently, FLF is adopted to quantitatively study the effect of the flow change in each region on the overall ventilation rate.
3. Results and discussion 3.1. The pressure distributions of the baseline NDDCT Fig. 3 compares the pressure distributions of baseline NDDCT between the experiment and simulation, where Fig. 3(a) and (b) exhibit circumferential pressure distribution at the radiator inlet and outlet at Vt of 1–1.5 m/s respectively. Under crosswind condition, the crosswind stagnates at the inlet centre of the windward radiator bundle (0°/360°), forming a high pressure zone; while accelerates at the side sections, and Table 1 The designed working conditions of the NDDCT model.
Baseline Experiment
Pa kPa
RH %
Q MW
Dm
Vin m/s
qm kg/s
Tout K
Ta K
ΔP kPa
Re
92.7 101
71.0 74.0
817 1.56
82.0 0.41
3.19 0.23
3.57e4 0.071
328 328
303 306
1.09e-1 5.20e-4
2.94e7 1.04e4
95
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(a) 160 m radius + 5 m top
(b) 120 m radius + 5 m top
(d)100 m radius10 m top
(c) 100 m radius + 5 m top
(e) 100 m radius 25 m top
Fig. 2. The schematic of different windbox configurations (crosswind blows from front).
3.3. Enclosure effect on overall flow field
suggested.
As 10 m/s is a typical crosswind velocity, the flow fields of the NDDCT installed with different windbox sizes is analyzed at 10 m/s as shown in Fig. 6(a–c) respectively. It can be seen that the swirling intensity of mainstream vortex decreases greatly as the enclosure radius changes from 160 m to 120 m. While it rebound to be even higher as the enclosure radius continues to decrease to 100 m. An enclosure is set to collect the accelerated side flow and increase the back side static pressure, while when the windbox at radius of 100 m is too small, the air flow outside the radiator bundle is forced to accelerate at the rear side, resulting an even lower pressure condition. Consequently, the the intensity of the mainstream vortex become to increase. Because mainstream vortex is found to be the main degrading factors arising from the inlet in previous studies [22], a smaller enclosure radius as 100 m is not
3.4. Enclosure effect on total FLF and ventilation loss Crosswind affects the flow characteristics by changing the inlet and outlet field of the NDDCT. The windboxes influence the inlet field of the NDDCT under crosswind conditions, hence to influence the flow characteristics along the streamline of the NDDCT. The FLF’s of each flow section on different cases are calculated as shown in Fig. 7. It can be seen that as the enclosure radius decreases from 160 m to 120 m, most of the FLF’s decrease at 10 m/s, while increase at 20 m/s. As the enclosure radius continues to decrease to 100 m, the FLF’s at all sections increase obviously in all crosswind range. Comparatively, the windbox at radius of 120 m shows a good performance as related to the FLF. Since the FLF has a linear relationship to the decrement of
1.0
0
Radiator inlet pressure, Pin Pa
Simulation
0.0
0
60
120
180
240
-0.5 -1.0 -1.5
300
360
Radiator outlet pressure, Pout Pa
0
experiment
0.5
-2.0
60
120
180
240
-0.15
Experiment
Simulation
-0.3
-0.45
-0.6
-0.75
Circumferential angle (°)
Circumferential angle (°)
Fig. 3. Validation of the pressure distribution for the CFD model. 96
300
360
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that, through a structure optimization of back barricades and an enclosure top, approaches with smaller enclosures of 160 m radius and 120 m radius achieve similar ventilation performances to the referenced large enclosure one in all investigated crosswind range. Compared to the approach with an enclosure of 160 m, the ventilation rate at the approach with an enclosure of 120 m radius prevails in wide range of breeze crosswind (∼0–14 m/s). Meanwhile, the enclosure with a radius of 120 m costs only about three quarters of the construction fee, and save more than 40 percent of the construction area. Compared to the baseline case, the optimized approach with 120 m radius enclosure could enhance the NDDCT ventilation rate by ∼15–20%. 3.5. Effect of the enclosure top on overall flow field Fig. 4. The locations of the pressure surfaces selected.
The width of the top on 100 m radius enclosure case further studied on different cases of 10 m and 25 m respectively. The streamlines of different top cases at crosswind of 10 m/s are shown in Fig. 9. Compared to 5 m width top shown in Fig. 6(c), it is found that as the width of the top increases to 10 m, the swirling intensity of the mainstream vortex decreases greatly, while as the width continues to increase to 25 m, the swirling intensity of the mainstream vortex reverses to increase to be even higher. In comparison, the top with of 10 m exhibits a good performance considering the flow field inside the NDDCT.
1.4
Flow loss factor, FLF
FLF-total
1
20
Ventilation loss
0.8
15
0.6
10
0.4 5
0.2 0 0
5
10 15 20 Crosswind velocity, V m/s
25
Ventilation loss, qm 103 kg/s
25
1.2
3.6. Effect of the enclosure top on FLF and ventilation loss The FLF’s of each flow section along the streamline on different enclosure tops are illustrated in Fig. 10. On comparison to the 5 m width top case shown in Fig. 7(c), under crosswind ranging from ∼0–10 m/s, most of the FLF’s decrease on 10 m width top case, while increase greatly on the 25 m width top case. However, as the crosswind increase to be above 15 m/s, the FLF’s on 10 m width top case begin to exceed those in 5 m width top case, while most of the FLF’s on 25 m width top case decrease greatly to a very relatively lower level. Generally, the FLF performance on 10 m top cases prevail the other ones under the overall crosswind range. Along with the changing of top width, the variations of FLF’s are reflected in the ventilation rate of each case, as shown in Fig. 11. In consistent with the variation of FLF’s, it can be see that, as the top width
0 30
Fig. 5. The variation trend of ventilation loss along with different FLF’s.
ventilation rate, the ventilation rates of the NDDCT on different cases are influenced accordingly. Fig. 8 shows the ventilation rate of the NDDCT on baseline case, referenced[21] case and optimized cases, where the enclosure-230 m case denotes the referenced model with a combination of windbreaks and an enclosure of 230 m in radius. Others refer to each case with correspondent enclosure radius. It can be found
(a) 160 m radius
(b) 120 m radius Fig. 6. The streamlines of NDDCT with different enclosures at 10 m/s. 97
(c) 100 m radius
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(a)160 m radius
(b) 120 m radius
(c) 100 m radius Fig. 7. The FLF’s on different enclosure radius cases.
3.7. Evaluation of the windbox performance On the ground of a conjoint analysis of the enclosure radius and top width, a combination of an enclosure with 120 m radius, a top with 25 m width, including back barricades and windbreaks are recommended. Considering the drawbacks of the top with 25 m width in breeze crosswind, a louver like structure is proposed for the top, which can be opened up in low crosswind condition. As a result, a ventilation enhancement of ∼60% in gale crosswind condition and no degradation in breeze crosswind condition is estimated by installing such optimized windbox structure as shown in Fig. 12 (dash line of “Enclosure120+top-25). By adopting the calculation method introduced in our previous publication [20], the initial temperature difference (ITD) of the NDDCT could be obtained. The calculation results show that ITD could be reduced to 30 °C by adopting the proposed windbox at crosswind of 20 m/ s, which is 9 °C less than that of the baseline case. By consulting the operating parameters, a 9 °C reduction of ITD could reduce the back pressure of the steam turbine by 3.5–4.5 kPa, which is correspondent to a reduction of the unit net coal consumption rate by 6–9 g/kWh. Given an annual operation time of 5000 h, the proposed windbox could reduce the overall coal consumption of a 1000 M unit by 30,000–45,000 tons annually. Taking a standard coal price of 100 $/ton, an operation cost reduction of 3000,000–4500,000 $ could be achieved by taking such method. Since the experimental results of the modelling NDDCT shows a good similarity to the full scale ones, it is assumed that the research results in this paper could be generalized to applied to other NDDCT by adopting scaling law of geometry.
Fig. 8. The ventilation rates on different enclosure cases.
increases from 5 m to 10 m, the ventilation rate increases slightly in low crosswind and decreases slightly in high crosswind. However, as the top width continues to increase to 25 m, the ventilation rate decreases greatly in breeze crosswind, while increase greatly in gale crosswind. As introduced in Section 2.3, a 25 m width top is a full size top which precisely seals gap between the radiator and enclosure at side and back sections. Then the windward opening of the enclosure become the only entrance of the NDDCT. As the result, under breeze condition, as the ventilation rate through the windward opening of the enclosure is less than the baseline ventilation, the windbox acts as a baffle of the inlet air on other sides; while under gale condition, as the ventilation rate through the windward opening is high than the baseline ventilation, the windbox acts as a channel, which directs all of the inlet air via the windward opening to the radiators, hence greatly increase the ventilation rate. 98
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(a)10 m top
(b) 25 m top
Fig. 9. The streamlines of NDDCT with different tops at 10 m/s.
4. Conclusions As the performance of a natural draft dry cooling tower (NDDCT) degrades greatly under crosswind condition, a windbox configuration consisting of windbreaks, an enclosure, a back barricade and a wind top is proposed to be installed around the NDDCT to increase its ventilation rate. By adopting a full dimensional computational fluid dynamic (CFD) modelling, the flow field around NDDCT was studied. With the help of flow loss factor (FLF), the effects of windbox on the flow characteristics of each flow field sections were also studied. By investigating different sizes and components of the windbox, the configurations of the windbox were optimized. Simulation results show that a windbox with a 120 m radius enclosure and a full size louver top is effective in improving the flow field around a NDDCT under crosswind condition. By installing such structure, the performance of a NDDCT is enhanced greatly at all crosswind range. The degradation of the ventilation rate is avoided under breeze condition, and reversed to be an enhancement on gale condition. An ∼60% increment of ventilation rate is estimated at ∼20 m/s crosswind. In case of a 1000 MW unit, the suggested windbox could reduce the standard coal consumption by 30,000–45,000 tons annually, which is correspondent to ∼3000,000–4500,000 $/a. The approach could be generalized according to geometry similarity.
Fig. 11. The ventilation rates on different top cases.
Acknowledgement This research is supported by China Postdoctoral Science Foundation (2017M620758), Special Funds of the National Natural Science Foundation of China (No. L1522032), and the Consulting Project of Chinese Academy of Engineering (No. 2015-ZCQ-06).
(a) 10 m top
(b) 25 m top Fig. 10. The FLF’s on different enclosure radius cases. 99
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[12] Y. Kong, W. Wang, X. Huang, L. Yang, X. Du, Y. Yang, Wind leading to improve cooling performance of natural draft air-cooled condenser, Appl. Therm. Eng. (2018). [13] W. Wang, X. Huang, L. Yang, X. Du, Performance recovery of natural draft dry cooling systems by combined air leading strategies, Energies 10 (12) (2017) 2166. [14] A.F. Du Preez, D.G. Kroger, Effect of wind on performance of a dry-cooling tower, Heat Recovery Syst. CHP 13 (1993) 139–146. [15] H. Gu, H. Wang, Y. Gu, J. Yao, A numerical study on the mechanism and optimization of wind-break structures for indirect air-cooling towers, Energ. Convers. Manage. 108 (2016) 43–49. [16] Y. Lu, Z. Guan, H. Gurgenci, A. Alkhedhair, S. He, Experimental investigation into the positive effects of a tri-blade-like windbreak wall on small size natural draft dry cooling towers, Appl. Therm. Eng. (2016). [17] R. Al-Waked, M. Behnia, The effect of windbreak walls on the thermal performance of natural draft dry cooling towers, Heat Transfer Eng. 26 (8) (2005) 50–62. [18] L. Chen, L. Yang, X. Du, Y. Yang, Performance improvement of natural draft dry cooling system by interior and exterior windbreaker configurations, Int. J. Heat Mass Tran. 96 (2016) 42–63. [19] W. Wang, P. Liu, Z. Li, W. Ni, J. Liu, Y. Li, A numerical study on the improving techniques of the cooling performance of a natural draft dry cooling tower under crosswind, Chem. Eng. Trans. 45 (2015) 1081–1086. [20] W. Wang, H. Zhang, P. Liu, Z. Li, J. Lv, W. Ni, The cooling performance of a natural draft dry cooling tower under crosswind and an enclosure approach to cooling efficiency enhancement, Appl. Energ. 186 (2017) 336–346. [21] W. Wang, H. Zhang, Z. Li, J. Lv, W. Ni, Y. Li, Adoption of enclosure and windbreaks to prevent the degradation of the cooling performance for a natural draft dry cooling tower under crosswind conditions, Energy 116 (2016) 1360–1369. [22] W. Wang, The Reconfiguration of the Flow Field of a Natural Draft Dry Cooling Tower on Crosswind Condition for Cooling Enhancement [Doctor], Tsinghua University, Beijing, 2017. [23] Y. Zhao, F. Sun, Y. Li, G. Long, Z. Yang, Numerical study on the cooling performance of natural draft dry cooling tower with vertical delta radiators under constant heat load, Appl. Energ. 149 (6) (2015) 225–237. [24] G. Barigozzi, A. Perdichizzi, S. Ravelli, Performance prediction and optimization of a waste-to-energy cogeneration plant with combined wet and dry cooling system, Appl. Energ. 115 (2014) 65–74. [25] G. Barigozzi, A. Perdichizzi, S. Ravelli, Wet and dry cooling systems optimization applied to a modern waste-to-energy cogeneration heat and power plant, Appl. Energ. 88 (4) (2011) 1366–1376. [26] M. Goodarzi, R. Ramezanpour, Alternative geometry for cylindrical natural draft cooling tower with higher cooling efficiency under crosswind condition, Energ. Convers. Manage. 77 (1) (2014) 243–249. [27] M. Goodarzi, A proposed stack configuration for dry cooling tower to improve cooling efficiency under crosswind, J. Wind Eng. Ind. Aerod. 98 (12) (2010) 858–863. [28] W. Wang, Y. Wang, H. Zhang, G. Lin, J. Lu, G. Yue, et al., Fresh breeze cuts down one-third ventilation rate of a natural draft dry cooling tower: a hot state modelling, Appl. Therm. Eng. 131 (2018) 1–7. [29] K. Tanimizu, K. Hooman, Natural draft dry cooling tower modelling, Heat Mass Transfer 49 (2) (2013) 155–161. [30] B. Sheng, Nmerical Research of Crosswind Effect on the Performance of Indirect Air Cooling Tower and Exploration of the Anti-Wind Measures [Master], Southeast University, Nanjing, 2015. [31] C. Huang, S. Zhao, Research on resistance performance of indirect -dry cooling tower, J. China Instit. Water Resources Hydropower Res. 09 (03) (2011) 195–199. [32] W. Wang, J. Lv, H. Zhang, Q. Liu, G. Yue, W. Ni, A quantitative approach identifies the critical flow characteristics in a natural draft dry cooling tower, Appl. Therm. Eng. 131 (2018) 522–530. [33] Q. Zheng, Z. Lu, Fluid Mechanics, China Machine Press, Beijing, 1979.
Fig. 12. The ventilation rates on proposed cases.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.applthermaleng.2018. 03.059. References [1] Q. Shun-yi, H.U. Yang, The present situation and applied prospective of the air cooling technology in power plant, Power Station Auxil. Equipm. 32 (4) (2011) 1–4. [2] W. Wang, W. Ni, Z. Wang, Z. Li, Y. Li, J. Liu, A review on the research of a dry cooling tower affected by cross wind, Proceedings CSEE 35 (04) (2015) 882–890. [3] W. Li, J. Chai, J. Zheng, Investigation of natural draft cooling tower in China, Heat Transf. Eng. 38 (11–12) (2017) 1101–1107. [4] E. Al-Bassam, G.P. Maheshwari, A new scheme for cooling tower water conservation in arid-zone countries, Energy 36 (7) (2011) 3985–3991. [5] R. Ghosh, T.K. Ray, R. Ganguly, Cooling tower fog harvesting in power plants - A pilot study, Energy 89 (2015) 1018–1028. [6] Y. Sun, Z. Guan, H. Gurgenci, K. Hooman, X. Li, Investigation on the influence of injection direction on the spray cooling performance in natural draft dry cooling tower, Int. J. Heat Mass Tran. 110 (2017) 113–131. [7] Y. Kong, W. Wang, L. Yang, X. Du, Y. Yang, A novel natural draft dry cooling system with bilaterally arranged air-cooled heat exchanger, Int. J. Therm. Sci. 112 (2017) 318–334. [8] Q. Wei, B. Zhang, K. Liu, X. Du, X. Meng, A study of the unfavorable effects of wind on the cooling efficiency of dry cooling towers, J. Wind Eng. Ind. Aerod. 54 (94) (1995) 633–643. [9] M. Goodarzi, R. Keimanesh, Heat rejection enhancement in natural draft cooling tower using radiator-type windbreakers, Energ. Convers. Manage. 71 (2013) 120–125. [10] D. Zhang, P. Liu, L. Ma, Z. Li, W. Ni, A multi-period modelling and optimization approach to the planning of China's power sector with consideration of carbon dioxide mitigation, Comput. Chem. Eng. 37 (2012) 227–247. [11] H.T. Liao, L.J. Yang, X.Z. Du, Y.P. Yang, Triangularly arranged heat exchanger bundles to restrain wind effects on natural draft dry cooling system, Appl. Therm. Eng. (2016).
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