- Email: [email protected]
Experimental study on the drag characteristic and thermal performance of non-uniform fillings for wet cooling towers under crosswind conditions
Accepted Manuscript Experimental study on the drag characteristic and thermal performance of nonuniform fillings for wet cooling towers under crosswind conditions Yang Zhou, Kun Wang, Ming Gao, Zhigang Dang, Suoying He, Fengzhong Sun PII: DOI: Reference:
S1359-4311(18)30940-2 https://doi.org/10.1016/j.applthermaleng.2018.05.071 ATE 12210
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
9 February 2018 9 May 2018 18 May 2018
Please cite this article as: Y. Zhou, K. Wang, M. Gao, Z. Dang, S. He, F. Sun, Experimental study on the drag characteristic and thermal performance of non-uniform fillings for wet cooling towers under crosswind conditions, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.05.071
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.
Experimental study on the drag characteristic and thermal performance of non-uniform fillings for wet cooling towers under crosswind conditions Yang Zhou, Kun Wang, Ming Gao*, Zhigang Dang, Suoying He, Fengzhong Sun School of Energy and Power Engineering, Shandong University, Jinan 250061, China; * Correspondence: [email protected]; Tel.: +86-531-88399008
Abstract: A thermal-state model experimental study was performed to investigate the drag characteristic and thermal performance of non-uniform fillings for wet cooling towers under crosswind conditions, several valuable performance parameters, including cooling efficiency, drag coefficient, ventilation rate, heat transfer coefficient and Merkel number, etc. were analyzed in this paper. Experimental researches proved that the non-uniform fillings are not sensitive to the crosswind, and can alleviate the adverse effect of crosswind on wet cooling towers. From the perspective of drag characteristic and thermal performance, the P4 pattern is the optimal non-uniform pattern under crosswind conditions, which has the higher ventilation rate, heat transfer coefficient, Merkel number and lower drag coefficient within the experimental crosswind velocity range. However, the P3 pattern has the lower evaporation loss and the outstanding relatively water-saving performance. Therefore, studies in this paper revealed that the optimal non-uniform pattern should be selected by terms of comprehensive consideration of energy conservation and water-saving. The P4 pattern is the optimal non-uniform pattern if giving the priority to energy conservation, and conversely, the P3 is the optimal pattern if considering water-saving characteristic preferentially. Keywords: Wet cooling tower; non-uniform fillings; drag characteristic; thermal performance; crosswind conditions 1. Introduction The natural draft cooling towers (NDCTs) consist of mainly dry cooling towers (NDDCTs) and wet cooling towers (NDWCTs) [1-3], and the NDDCTs are adopted to some power plants where is lack of water resources due to the water-saving performance[4-6]. Compared with the NDDCTs, the NDWCTs are used widely in thermal power plants (including some nuclear power plants) as one of the main equipments in the cold-end system [7-8], and the overall thermal efficiency of power plants depends on the thermal performance of NDWCTs to a great extent. According to the previous work [9], the fillings zone accounts for 70% of the heat exchange amount for the NDWCTs, and the raining zone and water distribution zone account for 20% and 10%, respectively. Meanwhile, the thermal performance of NDWCTs is highly sensitive to ambient conditions, particularly the ambient crosswind velocity [10]. Therefore, it is indispensable and significant to study the thermal performance of fillings zone for the NDWCTs, especially under crosswind conditions. Fillings used in the cooling towers can be clarified into three types, i.e., film, trickle and splash fillings [11-12], and nowadays, film fillings are the most popular one. In 1940s, Simpson et al. [13] began to the first investigation for the fillings. From then on, many researchers were convinced of the importance of fillings investigation, then carried out this research work in three ways, i.e., theoretical study, experimental research and numerical simulation. First of all, in the aspect of theoretical study, Dreyer et al [14] developed one-dimensional
mathematical model to study the thermal performance of splash fillings, which can be used to guide optimum layout of fillings. Additionally, Milosabljevic et al. [15] also derived a mathematical model according to one-dimensional heat and mass balance equations, and analyzed the thermal performance of different filling materials. Aiming at the fillings in different types of water-cooling towers, Jose et al [16] came up with the concept of thermo-fluid dynamic efficiency which is not a function of the height of the fillings, then validated it by experimental data. Based on three kinds of conventional fillings, Kloppers et al. [17-18] presented a new form of empirical equation, and obtained the loss coefficient correlations for fillings of NDWCTs. Afterwards, they analyzed three mathematical models, and the results reported that Poppe model had high accuracy and reliability. Additionally, Stabat et al. [19] presented a simplified model for indirect cooling towers behavior by using simulation tools, and this model can estimate energy and water consumptions under different operating conditions. In the same year, Khan et al. [20] investigated the fouling characteristic of fillings and proposed a fouling model in the fillings zone. According to the exergy analysis, Muangnoi et al. [21] put forward to a mathematical model which can be used to study the properties of water and air in fillings zone, and the results showed that the lowest exergy destruction is located at the top of the tower. Recently, Ghazani et al. [22] performed the comprehensive analysis of a model wet cooling tower using the laws of thermodynamics, and calculated the entropy generation of every part. The research conclusions can help to choose fillings of high quality. Secondly, for the experimental research, previous work mostly focused on the fillings type, fillings arrangement and fillings materials. Lemouari et al. [23] performed the experimental study for wet cooling towers filled with a VGA (Vertical Grid Apparatus) type fillings which is 0.42 m high, and they focused on mainly the influence of air-water ratio and water temperature on thermal performance and heat exchange amount. About the fillings arrangement, Grobbelaar et al. [24] also studied experimentally the thermal performance for the counter-flow and cross-flow fillings, and reported that the thermal performance for cross-counter flow fillings is between that of purely counter-flow and cross-flow fillings. In addition, Shahali et al. [25] also conducted the experimental research to reveal the thermal performance under different types and arrangements of fillings, including three kinds of PVC fillings which are 7, 9 and 18 ribs. Experimental study obtained that the cooling temperature and efficiency enhances with the increasing of rib numbers of packings. In another work, Li et al. [26] studied heat and mass transport mechanism of film type cooling, which was combined with an on-site test on counter flow film type cooling tower, and developed a mathematical model on the evaporation and cooling efficiency. Mofrad et al. [27] performed the experimental study on the effect of different types of filled beds on the thermal performance of wet cooling towers. By analyzing six kinds of filled beds, they drawn the conclusions that metal reticular bed is the best choice. Moreover, several researchers studied experimentally the fillings performance for forced-draft wet cooling towers. Gharagheizi et al. [28] presented an experimental study for two film-type fillings which are vertical corrugated packings (VCP) and horizontal corrugated packings (HCP), and obtained that compared with the HCP, the tower with VCP has outstanding thermal performance. Lavasani et al. [29] also study the performance of a forced draft wet cooling tower filled with a rotational splash type fillings, and the results showed that rotational splash type packings with higher rotational velocity reject more heat from water considerably. Singh et al. [30] conducted the experiment to research trickle, film and splash fills, and concluded that the wire-mesh (trickle) fills are the most efficient fills for forced draft mechanical cooling tower. Besides, Singla et al. [31] analyzed the changing rules of Merkel number
under different operating conditions based on the expanded wire mesh packings, and the conclusions can guide the operation of forced-draft wet cooling towers. Finally, computational fluid dynamics (CFD) method has being used to research the thermal performance of fillings for wet cooling tower, including two-dimensional axisymmetric two-phase model for fillings which has the capability to represent non-uniformities in fill layout and water distribution [32], CFD model for the fillings material and spraying water in cooling tower [33], Fillings fouling model [34], the two-dimensional model for the optimal fills shape and water distribution [35], and the fillings model of heat/mass transfer for wet cooling tower [36], and so on. Besides, the CFD method also is used to simulate and solve other issues for cooling towers, such as effect of crosswind on thermal performance [37-38], thermal performance research for dry cooling towers [39-40], and the hybrid (dry/wet) cooling system [41-42] etc. Briefly, a lot of literatures analyzed the characteristic of fillings by using theoretical model, experiment research and numerical simulation, and obtained some valuable conclusions for the further study of the fillings, especially Behnia’s research work [32] which discussed the non-uniformities in fill layout by numerical simulation laid the theoretical foundation for the future research of non-uniform fillings. Certainly, most of the research work above-mentioned regarded the fillings as uniform layout, and failed to discuss the issue of non-uniform layout fillings by the thermal-state model experiment method. Additionally, on the basis of model experiment in lab, Gao et al. [43-44] recently studied the thermal performance of different layout patterns under windless conditions and the typical crosswind condition, including uniform and non-uniform fillings. Experimental results demonstrated that the non-uniform layout patterns have the outstanding thermal performance under windless conditions. But Gao et al. did not analyze the adaptability of non-uniform fillings to crosswind, and neglected to study the drag characteristic and thermal performance, failed to obtain the optimal pattern under crosswind conditions. Actually, the NDWCTs have to face to the variable crosswind velocity during the operating process. Consequently, studies regarding the drag characteristic and thermal performance of non-uniform fillings under crosswind conditions are more crucial to the further energy-saving research and optimal design for NDWCTs. Based on this, under various crosswind velocity conditions, the thermal-state model experiment is performed in this study to reveal the drag characteristic and thermal performance for different layout patterns of fillings, and derive the optimal layout pattern under crosswind conditions. This study may provide a new direction to the energy-saving of NDWCTs under crosswind conditions, and furthermore it can guide engineering design of fillings zone if considering the ambient crosswind. 2. Experimental Study Design 2.1. Experimental objectives In this study, the corresponding inlet and outlet parameters inside wet cooling towers with different layout patterns of fillings are measured to calculate and derive the cooling efficiency, cooling temperature difference, drag coefficient, heat transfer coefficient, Merkel number and evaporation loss under various crosswind velocity, and the effect of crosswind on these performance parameters is analyzed. The ultimate objective is to obtain the optimal non-uniform pattern under crosswind conditions.
2.2. Experimental setup and measurement instruments The experimental diagram is shown in Fig. 1. The pictures of model tower and water distributing system are showed in Fig. 2. The model tower is manufactured and installed according to the similarity principle [43, 45, 46], the related measuring instruments are depicted in Table 1. Additionally, the detailed information for similarity criteria can be found in Gao’s paper [43, 45, 46].
Fig. 1. Schematic diagram of experimental cooling tower
Fig. 2. Pictures of model tower and water distributing system Table 1 Monitored parameters and measurement instruments. Items Measuring instruments
Range
Accuracy
Atmospheric pressure
Hot-wire manometer (KA31)
0-4.99 m/s
±3%
Wet bulb temperature
Psychrometer
0-50 ℃
±0.1 ℃
Inlet air temperature
Psychrometer
0-50 ℃
±0.1 ℃
Outlet air temperature
Thermocouple
0-200 ℃
±0.1 ℃
Inlet/outlet water temperature
Mercury thermometer
0-50 ℃
±0.1 ℃
Relative humidity of air
Hygrometer
10-95% RH
±2%
Water flow rate Crosswind velocity (produced by lower fan)
Rotameter
0-60 L/min
±1.5%
Hot-wire manometer (KA31)
0-4.99 m/s
±0.01 m/s
2.3. Experimental operating conditions In order to reveal and obtain relationship between drag characteristic, thermal performance and the layout patterns of fillings under crosswind conditions, it is indispensable to conduct experiment of variable conditions, including the variation of circulating water flow rate, inlet water temperature, crosswind velocity, fillings layout patterns, and the different operating conditions are listed in Table 2. Table 2 Experimental operating conditions. Items
Detailed operating conditions
Circulating water flow rate
4 L/min, 6 L/min, 8 L/min
Circulating water temperature
50 ℃, 55 ℃, 60 ℃
Experimental crosswind velocity (Upper fan)
0, 0.2 m/s, 0.4 m/s, 0.6 m/s, 0.8 m/s
Experimental crosswind velocity (Lower fan)
0, 0.4 m/s, 0.8 m/s, 1.2 m/s, 1.6 m/s
Fillings layout patterns
Five kinds ( Seen in Part 2.4)
In the following discussion, crosswind velocity is the crosswind velocity produced by the lower fan (Seen in Fig.1) 2.4. Fillings layout patterns inside the model tower In this experiment, the fillings inside model tower adopt different height at the different radius, as seen in Fig.3, i.e. non-uniform layout. In order to study the drag characteristic and thermal performance for different layout patterns, all of the layout patterns have the same fillings volume. Fig.4 illustrates the block plan of non-uniform fillings, and the detailed information of filling size is listed in Table 3.
Fig. 3. Fillings layout inside model tower
Fig. 4. Block plan of non-uniform fillings
Table 3 Details of five layout patterns. Item
P1
P2
Uniform fillings
P3
P4
P5
Non-uniform fillings
ra (cm)
29.5
9
11
13
15
rb (cm)
29.5
24.7
23.2
21
19
rc (cm)
29.5
29.5
29.5
29.5
29.5
ra/rc
-
0.31
0.37
0.44
0.51
rb/rc
-
0.84
0.79
0.71
0.64
H1 (cm)
8
4
4
4
4
H2 (cm)
8
8
8
8
8
H3 (cm)
8
10
10
10
10
P1: uniform fillings; P2-P5: non-uniform fillings 3. Thermal performance analysis for uniform and non-uniform fillings under crosswind conditions In this part, the cooling temperature difference t and efficiency are selected as the
performance evaluation indicators, and the expressions of t and are given by, t t1 t2
t1 t2 t t1 tlim t1 tlim
(1) (2)
In above equations, t1 is the circulating water inlet temperature, ℃, t2 is the outlet temperature, ℃, and tlim represents inlet air wet bulb temperature, ℃. Figs. 5-8 illustrate the changing rules of t and for different layout patterns under crosswind
conditions. Among of them, the circulating water inlet temperature and circulating water flowrate are 60 ℃ and 6 L/min in Figs. 5-6, and 50 ℃ and 6 L/min, respectively in Figs. 7-8.
Fig. 5. Relation curves between the cooling temperature difference and crosswind velocity under different layout patterns (60 ℃, 6 L/min)
Fig. 6. Relation curves between the cooling efficiency and crosswind velocity under different layout patterns (60 ℃, 6 L/min)
It can be observed form Figs. 5-6 that the t and for the non-uniform fillings (P2-P5 patterns) are dramatically higher than those of the uniform fillings (P1 pattern) under any crosswind velocity. Under P1-P5 conditions, these two parameters decrease firstly, and then increase with the increasing of crosswind velocity, then reach to the minimum while the crosswind velocity is equal to 0.4 m/s.
In addition, experimental study shows that, for uniform pattern (P1), the t and at 0.4 m/s
crosswind reduce by 7.7% and 4.7%, respectively compared with those of the windless condition. However, for the four non-uniform fillings (P2-P5), their reductions are within 3.5%-5.0% and
1.5%-4.5%, respectively. It demonstrates that the non-uniform fillings can alleviate the adverse effect of crosswind on thermal performance, i.e., the non-uniform fillings are not sensitive to the crosswind. Almost the same phenomena can be found for the different working conditions in Figs. 7-8. For
uniform pattern (P1), the t and at 0.4 m/s crosswind reduce by 9.0% and 3.0%, respectively
compared with those of the windless condition. However, their reductions are only within 2.5%-3.8% and 1.3%-3.0%, respectively for the four non-uniform fillings (P2-P5).
Fig. 7. Relation curves between cooling temperature difference and crosswind velocity under different layout patterns (50 ℃, 6 L/min)
Fig. 8. Relation curves between cooling efficiency and crosswind velocity under different layout patterns (50 ℃, 6 L/min)
According to the analysis of the cooling temperature difference and cooling efficiency, it can be drawn that the non-uniform fillings have the superior thermal performance under crosswind conditions,
even though the crosswind velocity equals to 0.4 m/s which is the extremely adverse crosswind velocity [44-46]. It proves that the uniform air dynamic field under non-uniform fillings is still beneficial to the heat and mass transfer performance even though under crosswind conditions. The heat and mass transfer performance for the wet cooling towers depends on mainly the uniformity of air dynamic field and the synergic match between dynamic and drag field. According to the analysis of Gao et al [43], the uniform fillings produce the non-uniform air dynamic field, disorder the synergic match between dynamic and drag field, which deteriorates the heat and mass transfer course. This phenomena become more severe under crosswind conditions. Compared with the uniform fillings, the non-uniform fillings can balance the air dynamic field inside tower under crosswind conditions, which impels outer air to enter into the tower center and reduces the ventilation resistance. Thus, for the non-uniform fillings, the outer cold air can enter into the center as far as possible, which can improve the heat and mass transfer performance near the center, and set up synergic match between dynamic and drag field. Briefly, the non-uniform fillings generate the uniform relatively air dynamic field inside tower under crosswind conditions, which results in the balance of heat and mass transfer in the whole tower. So the non-uniform fillings has the superior relatively thermal performance under crosswind conditions. 4. Comparative study for four non-uniform fillings under crosswind conditions In order to derive the optimal pattern of non-uniform fillings under crosswind conditions and realize the engineering application, the drag characteristic and thermal performance are analyzed for the four non-uniform fillings, and five performance parameters, including the drag coefficient, ventilation rate, heat transfer coefficient, Merkel number and evaporation loss, are introduced to act as the evaluation indictors in this part. 4.1. Calculation method of evaluation indicators In this paper, the ventilation rate G can be calculated by, G Qw
(3)
where Qw is the circulating water flowrate, m /h, and is the air-water ratio which can be given by, 3
C pw t
(4)
K (i2 i1 )
In Eq. (4), c pw is the specific heat of water, kJ/kg℃, i1 and i2 are the specific enthalpy of inlet air and outlet air, kJ/kg. Additionally, the K in Eq. (4) is called as heat coefficient, and the K value can embody the evaporation loss to a certain degree, which can be written by,
K 1
t2 586 0.56(t2 20)
(5)
The ventilation drag is different due to the various layout patterns of fillings, so the drag coefficient is also an important parameter which can evaluate the drag characteristics of different non-uniform fillings. The balance equation between drag force and draft force is written by,
H e g 1 2 m
v02 2
According to the Eq. (6), the ventilation drag coefficient can be written by,
(6)
2 H e g 1 2
(7)
m v02
where H e is the height of model tower, m, 1 and 2 are the air density of inlet tower and outlet tower, kg/m3, m is the average value of 1 and 2 , kg/m3, g is the gravity acceleration, m/s2, v0 represents the average wind velocity of fillings section, m/s, which can be derived by,
v0 =
4G D2
(8)
D is the diameter of fillings section, m. Besides the ventilation drag and ventilation rate G, the other three parameters, which are heat transfer coefficient v , evaporation loss Qew , and Merkel number n , are adopted to evaluate thermal performance of wet cooling towers. As one of the crucial evaluable indicators, the heat transfer coefficient v can be written by,
v
C pma G 2 1
(9)
(tm m ) V
Here, G is the ventilation rate, m3/h, 1 , 2 and m are the inlet air temperature, outlet air temperature and the average air temperature, ℃. Additionally, V is the fillings area, m2, and C pma is the specific heat of wet air, kJ/kg℃, which can be calculated by,
C pma C pd 1C pv 1.005 1.8421
(10)
What’s more, evaporation loss Qew cannot be ignored due to the growing shortage of water resources, meanwhile, the wet cooling towers bring out the relatively large evaporation loss. Thus, evaporation loss also is introduced to act as an evaluation indicator, which can be given by, Qew G m 2 1
(11)
In Eqs.(10) and (11), 1 and 2 are the humidity ratio of inlet and outlet air, kg/kg,which can be calculated by,
1＝0.622
1 p'' pa 1 p'' 1
(12)
1
2＝0.622
2 p'' pa 2 p'' 2
(13) 2
where 1 and 2 are the relative humidity of inlet and outlet air, pa is the atmosphere pressure, Pa, In addition, p''1 and p'' 2 are the saturated water vapor pressure at the temperature of 1 and 2 , Pa. Finally, another considerable parameter is Merkel number, and the calculation method of Merkel number refers to the enthalpy potential method proposed by Merkel in 1925 [47], and Simpson expansion formula can be written as,
n
c pw t 1 4 1 '' '' '' 6 K i2 i1 im im i1 i2
(14)
In Eq.(14), i1'' , i2'' and im' represent the corresponding saturated enthalpy when the temperature is t1 ,
t2 and tm t1 t2 2 , kJ/kg. i1 , i2 and im i1 i2 2 are the specific enthalpy of inlet air, outlet air and the average value, kJ/kg. c pw represents specific heat of water, kJ/kg℃. In the following text, the above-mentioned five performance paramters would be used to analyze the drag characteristic and thermal performance for the four non-uniform fillings. 4.2. The drag characteristic for the four non-uniform fillings While the circulating water flowrate and inlet temperature keep constant, meanwhile, the other components of cooling towers are not changed, the drag coefficient inside tower mainly depends on the geometry of non-uniform fillings and crosswind conditions. Thus, the changing rules of drag coefficient are studied in this part under different non-uniform fillings and crosswind velocities. Figs. 9-10 depict the changing rules of drag coefficient and ventilation rate under crosswind conditions for the non-uniform fillings when the circulating water flowrate is 6L/min and the inlet water temperature is 60℃.
Fig. 9. Relation curves between drag coefficient and crosswind velocity under non-uniform fillings (60℃, 6L/min) It can be observed in Fig. 9 that, for the four non-uniform fillings, the drag coefficient increases firstly and then decreases with the increasing of crosswind velocity, and reaches to the maximum at 0.4m/s crosswind velocity. Moreover, the drag coefficient of P2-P5 at 0.4m/s crosswind velocity enhances by 29.0%, 28.3%, 17.4% and 28.9%, respectively compared with that of windless condition. Apparently, the drag coefficient for P4 pattern has the relatively smaller increasing amplitude which is only 17.4%, i.e., the P4 pattern has the lower ventilation resistance.
Fig. 10. Relation curves between ventilation rate and crosswind velocity under non-uniform fillings (60 ℃, 6 L/min)
And the almost same rules can be seen in Fig. 10, the ventilation rate decreases firstly and then increases with the increasing of crosswind velocity, and reaches to the minimum when the crosswind velocity equals to 0.4m/s. Additionally generally speaking, the P4 pattern has the relatively higher ventilation rate under crosswind conditions, and the ventilation rate of P2-P5 at 0.4m/s crosswind velocity reduces by 10.4%, 10.3%, 9.5% and 10.2%, respectively compared with that of windless condition. Obviously at the conventional adverse crosswind velocity (0.4m/s), the ventilation rate for P4 pattern has the relatively smaller decreasing amplitude which is only 9.5%. In addition, there are crossovers in Figs. 9-10 due to probably the experimental error, so in order to analyze visually the ventilation rate and drag coefficient under the four non-uniform fillings, the arithmetic average value for the two parameters at five velocity points (0, 0.2m/s, 0.4m/s, 0.6m/s and 0.8m/s) are introduced in Fig. 11. It can be seen obviously in Fig. 11 that the P4 pattern has the higher average ventilation rate and lower average drag coefficient which are 151.6m3/h and 32.6, respectively.
Fig. 11. Average value for ventilation rate and drag coefficient under non-uniform fillings
Based on the analysis of drag coefficient and ventilation rate, the P4 pattern has the lower drag coefficient and larger ventilation rate. It can be derived that the crosswind has the relatively weak influence on the P4 pattern. Therefore, the P4 pattern has the excellent relatively drag characteristic under crosswind conditions, especially under the conventional adverse crosswind velocity (0.4m/s). 4.3 The thermal performance for the four non-uniform fillings Besides the drag characteristic, three thermal performance parameters, including heat transfer coefficient, Merkel number and evaporation loss, are used to evaluate the heat and mass transfer performance for the four non-uniform fillings, and the relation curves between heat transfer coefficient, Merkel number, evaporation loss and crosswind velocity are shown in Figs. 12-14 when the circulating water flowrate is 6L/min and the inlet water temperature is 60℃.
Fig. 12. Relation curves between heat transfer coefficient and crosswind velocity under non-uniform fillings (60 ℃, 6 L/min)
Fig. 13. Relation curves between Merkel number and crosswind velocity under non-uniform fillings (60 ℃, 6 L/min) Figs. 12-13 describe the relation curves between heat transfer coefficient, Merkel number and crosswind velocity under the four non-uniform fillings. It can be seen from these two figures that the heat transfer coefficient and Merkel number also decrease firstly, and then increase with the increasing of crosswind velocity, and the turning-points also appear at 0.4m/s crosswind velocity. It reports in Fig. 12 that the heat transfer coefficient reaches to the minimum value at 0.4m/s crosswind velocity, and the decreasing amplitudes of heat transfer coefficient for P2-P5 patterns come to 7.9%, 10.5%, 5.3% and 8.0%, respectively compared with that of the windless condition. Furthermore, based on Fig. 13, the reductions of Merkel number for P2-P5 patterns are 4.2%, 5.6%, 2.3% and 4.8%, respectively. There are also crossovers in Figs. 12-13 due to probably the experimental error. In a similar way, the arithmetic average value for the heat transfer coefficient and Merkel number at five velocity points are introduced in Fig. 14. Moreover, since the experimental Merkel number is much smaller than the heat transfer coefficient, so the Merkel number in Fig. 14 is 100 times of the original experimental value so as to put the heat transfer coefficient and Merkel number in one figure. It can be seen obviously in Fig. 14 that the P4 pattern has the higher average heat transfer coefficient and average Merkel number which are 75.8 w/m2℃ and 11.9, respectively. Surely, the actual experimental Merkel number equals to 0.119.
Fig. 14. Average value for heat transfer coefficient and Merkel number under non-uniform fillings
According to the heat transfer coefficient and Merkel number, it can be inferred that the crosswind has the relatively slight influence on the P4 pattern, i.e., the P4 pattern has the outstanding thermal performance under crosswind conditions, especially under the conventional adverse crosswind velocity (0.4m/s). As mentioned above (in Fig.11), there are higher ventilation rate and lower drag coefficient for the P4 pattern, additionally the non-uniform fillings provides the uniform relatively aerodynamic field. Thus, the P4 presents the outstanding thermal performance under crosswind conditions. Finally, the evaporation loss is discussed in this paper for the four non-uniform fillings under crosswind conditions. Fig. 15 describes the relation curves between evaporation loss and crosswind velocity under the four non-uniform fillings. In Fig. 15, the evaporation loss also decreases firstly, and then increases with the increasing of crosswind velocity. Additionally, the evaporation loss for P4 pattern has the largest value under crosswind conditions, and the average value of evaporation loss at five velocity points is 1.08 g/s, 1.06 g/s, 1.14 g/s and 1.13g/s, respectively for the P2-P5 patterns. According to the previous analysis, the P4 pattern has the higher heat transfer coefficient and ventilation rate under crosswind conditions, so the larger evaporation loss appears in the P4 pattern. Obviously, the P4 pattern is not the optimal pattern if only considering the evaporation loss, but the P3 pattern has the outstanding water-saving performance.
Fig. 15. Relation curves between evaporation loss and crosswind velocity under non-uniform fillings (60 ℃, 6 L/min) According to the comparative study for the four non-uniform fillings, it points out that the optimal non-uniform pattern should be selected by terms of comprehensive consideration of energy conservation and water-saving. Experimental researches show that the P4 is the optimal pattern if giving the priority to energy conservation, and conversely, the P3 is the optimal pattern if considering water-saving characteristic preferentially. 5. Conclusions In this study, the fillings are divided into three blocks which are inner block, medium block and outer block, and the radius of three blocks are ra, rb and rc, respectively. As a result, there are one uniform and four non-uniform layout patterns due to the different radius. Then the drag characteristic and thermal performance of different layout patterns are analyzed under crosswind conditions. By the discussion of several performance parameters, the main conclusions are as follows, (1) The analysis of model experiment discovers the cooling temperature difference and cooling efficiency for the non-uniform fillings are higher than those of the uniform fillings under crosswind conditions. More importantly, it also demonstrates that the non-uniform fillings are not sensitive to the crosswind, and can alleviate the adverse effect of crosswind on thermal performance for wet cooling towers. (2) Experimental researches prove that the P4 pattern is the optimal non-uniform pattern from the perspective of drag characteristic and thermal performance under crosswind conditions. Within the experimental crosswind velocity range, the P4 pattern has the higher average ventilation rate and lower average drag coefficient which are 151.6m3/h and 32.6, respectively. Additionally, it also has the higher average heat transfer coefficient and average Merkel number which are 75.8 w/m2 ℃ and 0.119, respectively. (3) If only considering the evaporation loss, the P3 pattern has the outstanding relatively water-saving performance, and the average evaporation loss is only 1.06g/s under the experimental conditions, however that of the other three patterns (P2, P4 and P5) is 1.08 g/s, 1.14 g/s and 1.13g/s, respectively.
Briefly, studies in this paper manifest that the optimal non-uniform fillings should be selected by terms of comprehensive consideration of energy conservation and water-saving. The P4 pattern is the optimal non-uniform pattern if giving the priority to energy conservation, and conversely, the P3 is the optimal pattern if considering water-saving characteristic preferentially. Here, ra/rb and ra/rc are 0.37 and 0.79, respectively for P3 pattern, but they are 0.44 and 0.71, respectively for P4 pattern. Acknowledgements This work is supported by National Natural Science Foundation of China (51776111) and Natural Science Foundation of Shandong province (ZR2016EEM35/ZR2017QEE010). References [1]
Kröger DG. ir-cooled Heat Exchangers and Cooling Towers, Thermal-flow Performance Evaluation and Design, Begell House, Inc., New York, 1998.
[2]
Al-Waked R, Behnia M. The performance of natural draft dry cooling towers under crosswind: CFD study, Int J of Energy Research 2004; 28: 147-161.
[3]
Al-Waked R, Behnia M. CFD simulation of wet cooling towers, App Therm Eng 2006; 26: 382-395.
[4]
Lia X, Gurgenci H, Guana Z, Wang X, Duniama S. Measurements of crosswind influence on a natural draft dry cooling tower for a solar thermal power plant. Appl Energy 2017; 206: 1169-1183.
[5]
Calautit JK, Chaudhry HN, Hughes BR, Ghani SA. Comparison between evaporative cooling and a heat pipe assisted thermal loop for a commercial wind tower in hot and dry climatic conditions. Appl Energy 2013; 101: 740-55.
[6]
Li X, Duniam S, Gurgenci H, Guan Z, Veeraragavan A. Full scale experimental study of a small natural draft dry cooling tower for concentrating solar thermal power plant. Appl Energy 2017; 193: 15-27.
[7]
Gao M, Sun F, Wang K, Shi Y, Zhao Y. Experimental research of heat transfer performance on natural draft counter flow wet cooling tower under cross-wind conditions. Int J Therm Sci 2008; 47: 935-941.
[8]
Alavi SR, Rahmati M. Experimental investigation on thermal performance of natural draft wet cooling towers employing an innovative wind-creator setup. Energy Convers Manage 2016; 122: 504-514.
[9]
Williamson N, Behnia M, Armfield S. Comparison of a 2D axisymmetric CFD model of a natural draft wet cooling tower and a 1D model. Int J Heat Mass Transfer 2008; 51: 2227-2236.
[10] Gao M, Sun F, Zhou S, Shi Y. Performance prediction of wet cooling tower using artificial neural network under cross-wind conditions. Int J Therm Sci 2009; 48: 583-589. [11] He S, Gurgenci H, Guan ZQ, Huang X, Lucas M. A review of wetted media with potential application in the pre-cooling of natural draft dry cooling towers. Renew Sust Energ Rev 2015; 44: 407–422 [12] He S, Guan Z, Gurgenci H, Hooman K, Lu Y, Alkhedhair AM. Experimental study of the application of two trickle media for inlet air pre-cooling of natural draft dry cooling towers. Energy Convers Manage 2015; 89: 644-654. [13] Simpson WM, Sherwood TK. Performance of small mechanical draft cooling towers. Refrig Eng 1946; 52: 535-543. [14] Dreyer AA, Erens PJ. Modeling of cooling tower splash pack. Int J Heat Mass Transfer 1996; 39: 109-123. [15] Milosabljevic N, Heikkilä P. A comprehensive approach to cooling tower design. Appl Therm Eng 2001; 21: 899-915. [16] Jose AS. The use of thermo-fluid dynamic efficiency in cooling towers. Heat Transfer Eng 2002, 23: 22-30. [17] Kloppers JC, Kröger DG. Loss coefficient correlation for wet cooling tower fills. Appl Therm Eng 2003; 23:
2201-2211. [18] Kloppers JC, Kröger DG. A critical investigation into the heat and mass transfer analysis of counterflow wet cooling towers. Int J Heat Mass Transfer 2005; 48: 765-777. [19] Stabat P, Marchio D. Simplified model for indirect-contact evaporative cooling-tower behavior. Appl Energy 2004; 78: 433-451. [20] Khan JR, Qureshi BA, Zubair SM. A comprehensive design and performance evaluation study of counter flow wet cooling towers. Int J of Refrig 27(2004) 914-923． [21] Muangnoi T, Asvapoositkul W, Wongwises S. An exergy analysis on the performance of a counter-flow wet cooling tower. Appl Therm Eng 2007; 27: 910-917. [22] Ghazani MA, Hosseini AH, Emami MD. A comprehensive analysis of a laboratory scale counter flow wet cooling tower using the first and the second laws of thermodynamics. Appl Therm Eng 2017; 125: 1389-1401. [23] Lemouari M, Boumaza M. Experimental investigation of the performance characteristics of a counterflow wet cooling tower. Int J Therm Sci 2010; 49: 2049-2056. [24] Grobbelaar PJ, Reuter HCR, Bertrand TP. Performance characteristics of a trickle fill in cross- and counter-flow configuration in a wet-cooling tower. Appl Therm Eng 2013; 50: 475-484. [25] Shahali P, Rahmati M, Alavi SR, Sedaghat A. Experimental study on improving operating conditions of wet cooling towers using various rib numbers of packing. Int J Refrig 2016; 67: 80-91. [26] Li Y, You X, Qiu Q, Li J. The study on the evaporation cooling efficiency and effectiveness of cooling tower of film type. Energy Convers Manage 2011; 52(1): 53-59. [27] Mofrad PI, Saeed ZH, Shanbedi M. Experimental investigation of filled bed effect on the thermal performance of a wet cooling tower by using ZnO/water nanofluid. Energy Convers Manage 2015; 127: 199-207. [28] Gharagheizi F, Hayati R, Fatemi S. Experimental study on the performance of mechanical cooling tower with two types of film packing. Energy Convers Manage 2007; 48: 277-280. [29] Lavasani, AM, Baboli ZN, Zamanizadeh M, Zareh M. Experimental study on the thermal performance of mechanical cooling tower with rotational splash type packing. Energy Convers Manage 2014; 87: 530-538. [30] Singh K, Das R. An experimental and multi-objective optimization study of a forced draft cooling tower with different fills. Energy Convers Manage 2016; 111: 417-430. [31] Singla RK, Singh K, Das R. Tower characteristics correlation and parameter retrieval in wet cooling tower with expanded wire mesh packing. Appl Therm Eng 2016; 96: 240-249. [32] Williamson N, Armfield S, Behnia M. Numerical simulation of flow in a natural draft wet cooling tower-The effect of radial thermofluid fields. Appl Therm Eng 2008; 28: 178-189. [33] Huang D, Du C. Numerical optimization on arrangement of the filling material and spraying water in cooling tower. Chin J Appl Mech 2000; 17: 102-110. (In Chinese) [34] Qureshi BA, Zubair SM. A complete model of wet cooling towers with fouling in fills. Appl Therm Eng 2006; 26: 1982-1989. [35] Williamson N, Behnia M, Armfield SW. Thermal optimization of a natural draft wet cooling tower. Int J Energy Res 2008; 32: 1349-1361. [36] Klimanek A, Bialecki RA. Solution of heat and mass transfer in counterflow wet-cooling tower fills. Int Commun Heat Mass Transfer 2009; 36: 547-553. [37] Zou Z, Gong H, Lie X, Li X, Yang Y. Numerical investigation of the crosswind effects on the performance of a hybrid cooling-tower-solar-chimney system. Applied Thermal Engineering 2017; 126: 661-669.
[38] Klimanek A, Cedzich M, Białecki, R. 3D CFD modeling of natural draft wet-cooling tower with flue gas injection. Appl Therm Eng 2015; 91: 824-833. [39] Goodarzi M, Keimanesh R. Numerical analysis on overall performance of Savonius turbines adjacent to a natural draft cooling tower. Energy Convers and Manage 2015; 99: 41-49. [40] Li X, Xia L, Gurgenci H, Guan Z. Performance enhancement for the natural draft dry cooling tower under crosswind condition by optimizing the water distribution. Int. J. Heat Mass Transf 2017; 107: 271–280. [41] Barigozzi G, Perdichizzi A, Ravelli S. Wet and dry cooling systems optimization applied to a modern waste-to-energy cogeneration heat and power plant. Appl Energy 2011; 88: 1366-1376. [42] Asvapoositkul W, Kuansathan.M. Comparative evaluation of hybrid (dry/wet) cooling tower performance. Appl Therm Eng 2014; 71: 83-93. [43] Gao M, Zhang L, Wang N, Shi Y, Sun F. Influence of non-uniform layout fillings on thermal performance for wet cooling tower. Appl Therm Eng 2016; 93: 549-555. [44] Gao M, Guo C, Ma, C, Sun F. Thermal performance for wet cooling tower with different layout patterns of fillings under typical crosswind conditions. Energies 2017; 10(65): 1-8. [45] Gao M, Sun F, Turan A. Experimental study regarding the evolution of temperature profiles inside wet cooling tower under crosswind conditions. Int J Therm Sci 2014; 86: 284-291. [46] Gao M, Sun F, Wang N. Experimental research on circumferential inflow air and vortex distribution for wet cooling tower under crosswind conditions. Appl Therm Eng 2014; 64: 93-100. [47] Askari S, Lotfi R, Seifkordi A, Rashidi AM, Koolivand H. A novel approach for energy and water conservation in wet cooling towers by using MWNTs and nanoporous graphene nanofluids. Energy Convers Manage 2016; 109: 10-18.
Highlights: The performance of non-uniform fillings for wet cooling towers is studied in lab Non-uniform fillings can alleviate adverse effect of crosswind on cooling towers P4 is optimal non-uniform pattern if giving the priority to energy conservation P3 is the optimal pattern if considering water-saving characteristic preferentially
S1359-4311(18)30940-2 https://doi.org/10.1016/j.applthermaleng.2018.05.071 ATE 12210
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
9 February 2018 9 May 2018 18 May 2018
Please cite this article as: Y. Zhou, K. Wang, M. Gao, Z. Dang, S. He, F. Sun, Experimental study on the drag characteristic and thermal performance of non-uniform fillings for wet cooling towers under crosswind conditions, Applied Thermal Engineering (2018), doi: https://doi.org/10.1016/j.applthermaleng.2018.05.071
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.
Experimental study on the drag characteristic and thermal performance of non-uniform fillings for wet cooling towers under crosswind conditions Yang Zhou, Kun Wang, Ming Gao*, Zhigang Dang, Suoying He, Fengzhong Sun School of Energy and Power Engineering, Shandong University, Jinan 250061, China; * Correspondence: [email protected]; Tel.: +86-531-88399008
Abstract: A thermal-state model experimental study was performed to investigate the drag characteristic and thermal performance of non-uniform fillings for wet cooling towers under crosswind conditions, several valuable performance parameters, including cooling efficiency, drag coefficient, ventilation rate, heat transfer coefficient and Merkel number, etc. were analyzed in this paper. Experimental researches proved that the non-uniform fillings are not sensitive to the crosswind, and can alleviate the adverse effect of crosswind on wet cooling towers. From the perspective of drag characteristic and thermal performance, the P4 pattern is the optimal non-uniform pattern under crosswind conditions, which has the higher ventilation rate, heat transfer coefficient, Merkel number and lower drag coefficient within the experimental crosswind velocity range. However, the P3 pattern has the lower evaporation loss and the outstanding relatively water-saving performance. Therefore, studies in this paper revealed that the optimal non-uniform pattern should be selected by terms of comprehensive consideration of energy conservation and water-saving. The P4 pattern is the optimal non-uniform pattern if giving the priority to energy conservation, and conversely, the P3 is the optimal pattern if considering water-saving characteristic preferentially. Keywords: Wet cooling tower; non-uniform fillings; drag characteristic; thermal performance; crosswind conditions 1. Introduction The natural draft cooling towers (NDCTs) consist of mainly dry cooling towers (NDDCTs) and wet cooling towers (NDWCTs) [1-3], and the NDDCTs are adopted to some power plants where is lack of water resources due to the water-saving performance[4-6]. Compared with the NDDCTs, the NDWCTs are used widely in thermal power plants (including some nuclear power plants) as one of the main equipments in the cold-end system [7-8], and the overall thermal efficiency of power plants depends on the thermal performance of NDWCTs to a great extent. According to the previous work [9], the fillings zone accounts for 70% of the heat exchange amount for the NDWCTs, and the raining zone and water distribution zone account for 20% and 10%, respectively. Meanwhile, the thermal performance of NDWCTs is highly sensitive to ambient conditions, particularly the ambient crosswind velocity [10]. Therefore, it is indispensable and significant to study the thermal performance of fillings zone for the NDWCTs, especially under crosswind conditions. Fillings used in the cooling towers can be clarified into three types, i.e., film, trickle and splash fillings [11-12], and nowadays, film fillings are the most popular one. In 1940s, Simpson et al. [13] began to the first investigation for the fillings. From then on, many researchers were convinced of the importance of fillings investigation, then carried out this research work in three ways, i.e., theoretical study, experimental research and numerical simulation. First of all, in the aspect of theoretical study, Dreyer et al [14] developed one-dimensional
mathematical model to study the thermal performance of splash fillings, which can be used to guide optimum layout of fillings. Additionally, Milosabljevic et al. [15] also derived a mathematical model according to one-dimensional heat and mass balance equations, and analyzed the thermal performance of different filling materials. Aiming at the fillings in different types of water-cooling towers, Jose et al [16] came up with the concept of thermo-fluid dynamic efficiency which is not a function of the height of the fillings, then validated it by experimental data. Based on three kinds of conventional fillings, Kloppers et al. [17-18] presented a new form of empirical equation, and obtained the loss coefficient correlations for fillings of NDWCTs. Afterwards, they analyzed three mathematical models, and the results reported that Poppe model had high accuracy and reliability. Additionally, Stabat et al. [19] presented a simplified model for indirect cooling towers behavior by using simulation tools, and this model can estimate energy and water consumptions under different operating conditions. In the same year, Khan et al. [20] investigated the fouling characteristic of fillings and proposed a fouling model in the fillings zone. According to the exergy analysis, Muangnoi et al. [21] put forward to a mathematical model which can be used to study the properties of water and air in fillings zone, and the results showed that the lowest exergy destruction is located at the top of the tower. Recently, Ghazani et al. [22] performed the comprehensive analysis of a model wet cooling tower using the laws of thermodynamics, and calculated the entropy generation of every part. The research conclusions can help to choose fillings of high quality. Secondly, for the experimental research, previous work mostly focused on the fillings type, fillings arrangement and fillings materials. Lemouari et al. [23] performed the experimental study for wet cooling towers filled with a VGA (Vertical Grid Apparatus) type fillings which is 0.42 m high, and they focused on mainly the influence of air-water ratio and water temperature on thermal performance and heat exchange amount. About the fillings arrangement, Grobbelaar et al. [24] also studied experimentally the thermal performance for the counter-flow and cross-flow fillings, and reported that the thermal performance for cross-counter flow fillings is between that of purely counter-flow and cross-flow fillings. In addition, Shahali et al. [25] also conducted the experimental research to reveal the thermal performance under different types and arrangements of fillings, including three kinds of PVC fillings which are 7, 9 and 18 ribs. Experimental study obtained that the cooling temperature and efficiency enhances with the increasing of rib numbers of packings. In another work, Li et al. [26] studied heat and mass transport mechanism of film type cooling, which was combined with an on-site test on counter flow film type cooling tower, and developed a mathematical model on the evaporation and cooling efficiency. Mofrad et al. [27] performed the experimental study on the effect of different types of filled beds on the thermal performance of wet cooling towers. By analyzing six kinds of filled beds, they drawn the conclusions that metal reticular bed is the best choice. Moreover, several researchers studied experimentally the fillings performance for forced-draft wet cooling towers. Gharagheizi et al. [28] presented an experimental study for two film-type fillings which are vertical corrugated packings (VCP) and horizontal corrugated packings (HCP), and obtained that compared with the HCP, the tower with VCP has outstanding thermal performance. Lavasani et al. [29] also study the performance of a forced draft wet cooling tower filled with a rotational splash type fillings, and the results showed that rotational splash type packings with higher rotational velocity reject more heat from water considerably. Singh et al. [30] conducted the experiment to research trickle, film and splash fills, and concluded that the wire-mesh (trickle) fills are the most efficient fills for forced draft mechanical cooling tower. Besides, Singla et al. [31] analyzed the changing rules of Merkel number
under different operating conditions based on the expanded wire mesh packings, and the conclusions can guide the operation of forced-draft wet cooling towers. Finally, computational fluid dynamics (CFD) method has being used to research the thermal performance of fillings for wet cooling tower, including two-dimensional axisymmetric two-phase model for fillings which has the capability to represent non-uniformities in fill layout and water distribution [32], CFD model for the fillings material and spraying water in cooling tower [33], Fillings fouling model [34], the two-dimensional model for the optimal fills shape and water distribution [35], and the fillings model of heat/mass transfer for wet cooling tower [36], and so on. Besides, the CFD method also is used to simulate and solve other issues for cooling towers, such as effect of crosswind on thermal performance [37-38], thermal performance research for dry cooling towers [39-40], and the hybrid (dry/wet) cooling system [41-42] etc. Briefly, a lot of literatures analyzed the characteristic of fillings by using theoretical model, experiment research and numerical simulation, and obtained some valuable conclusions for the further study of the fillings, especially Behnia’s research work [32] which discussed the non-uniformities in fill layout by numerical simulation laid the theoretical foundation for the future research of non-uniform fillings. Certainly, most of the research work above-mentioned regarded the fillings as uniform layout, and failed to discuss the issue of non-uniform layout fillings by the thermal-state model experiment method. Additionally, on the basis of model experiment in lab, Gao et al. [43-44] recently studied the thermal performance of different layout patterns under windless conditions and the typical crosswind condition, including uniform and non-uniform fillings. Experimental results demonstrated that the non-uniform layout patterns have the outstanding thermal performance under windless conditions. But Gao et al. did not analyze the adaptability of non-uniform fillings to crosswind, and neglected to study the drag characteristic and thermal performance, failed to obtain the optimal pattern under crosswind conditions. Actually, the NDWCTs have to face to the variable crosswind velocity during the operating process. Consequently, studies regarding the drag characteristic and thermal performance of non-uniform fillings under crosswind conditions are more crucial to the further energy-saving research and optimal design for NDWCTs. Based on this, under various crosswind velocity conditions, the thermal-state model experiment is performed in this study to reveal the drag characteristic and thermal performance for different layout patterns of fillings, and derive the optimal layout pattern under crosswind conditions. This study may provide a new direction to the energy-saving of NDWCTs under crosswind conditions, and furthermore it can guide engineering design of fillings zone if considering the ambient crosswind. 2. Experimental Study Design 2.1. Experimental objectives In this study, the corresponding inlet and outlet parameters inside wet cooling towers with different layout patterns of fillings are measured to calculate and derive the cooling efficiency, cooling temperature difference, drag coefficient, heat transfer coefficient, Merkel number and evaporation loss under various crosswind velocity, and the effect of crosswind on these performance parameters is analyzed. The ultimate objective is to obtain the optimal non-uniform pattern under crosswind conditions.
2.2. Experimental setup and measurement instruments The experimental diagram is shown in Fig. 1. The pictures of model tower and water distributing system are showed in Fig. 2. The model tower is manufactured and installed according to the similarity principle [43, 45, 46], the related measuring instruments are depicted in Table 1. Additionally, the detailed information for similarity criteria can be found in Gao’s paper [43, 45, 46].
Fig. 1. Schematic diagram of experimental cooling tower
Fig. 2. Pictures of model tower and water distributing system Table 1 Monitored parameters and measurement instruments. Items Measuring instruments
Range
Accuracy
Atmospheric pressure
Hot-wire manometer (KA31)
0-4.99 m/s
±3%
Wet bulb temperature
Psychrometer
0-50 ℃
±0.1 ℃
Inlet air temperature
Psychrometer
0-50 ℃
±0.1 ℃
Outlet air temperature
Thermocouple
0-200 ℃
±0.1 ℃
Inlet/outlet water temperature
Mercury thermometer
0-50 ℃
±0.1 ℃
Relative humidity of air
Hygrometer
10-95% RH
±2%
Water flow rate Crosswind velocity (produced by lower fan)
Rotameter
0-60 L/min
±1.5%
Hot-wire manometer (KA31)
0-4.99 m/s
±0.01 m/s
2.3. Experimental operating conditions In order to reveal and obtain relationship between drag characteristic, thermal performance and the layout patterns of fillings under crosswind conditions, it is indispensable to conduct experiment of variable conditions, including the variation of circulating water flow rate, inlet water temperature, crosswind velocity, fillings layout patterns, and the different operating conditions are listed in Table 2. Table 2 Experimental operating conditions. Items
Detailed operating conditions
Circulating water flow rate
4 L/min, 6 L/min, 8 L/min
Circulating water temperature
50 ℃, 55 ℃, 60 ℃
Experimental crosswind velocity (Upper fan)
0, 0.2 m/s, 0.4 m/s, 0.6 m/s, 0.8 m/s
Experimental crosswind velocity (Lower fan)
0, 0.4 m/s, 0.8 m/s, 1.2 m/s, 1.6 m/s
Fillings layout patterns
Five kinds ( Seen in Part 2.4)
In the following discussion, crosswind velocity is the crosswind velocity produced by the lower fan (Seen in Fig.1) 2.4. Fillings layout patterns inside the model tower In this experiment, the fillings inside model tower adopt different height at the different radius, as seen in Fig.3, i.e. non-uniform layout. In order to study the drag characteristic and thermal performance for different layout patterns, all of the layout patterns have the same fillings volume. Fig.4 illustrates the block plan of non-uniform fillings, and the detailed information of filling size is listed in Table 3.
Fig. 3. Fillings layout inside model tower
Fig. 4. Block plan of non-uniform fillings
Table 3 Details of five layout patterns. Item
P1
P2
Uniform fillings
P3
P4
P5
Non-uniform fillings
ra (cm)
29.5
9
11
13
15
rb (cm)
29.5
24.7
23.2
21
19
rc (cm)
29.5
29.5
29.5
29.5
29.5
ra/rc
-
0.31
0.37
0.44
0.51
rb/rc
-
0.84
0.79
0.71
0.64
H1 (cm)
8
4
4
4
4
H2 (cm)
8
8
8
8
8
H3 (cm)
8
10
10
10
10
P1: uniform fillings; P2-P5: non-uniform fillings 3. Thermal performance analysis for uniform and non-uniform fillings under crosswind conditions In this part, the cooling temperature difference t and efficiency are selected as the
performance evaluation indicators, and the expressions of t and are given by, t t1 t2
t1 t2 t t1 tlim t1 tlim
(1) (2)
In above equations, t1 is the circulating water inlet temperature, ℃, t2 is the outlet temperature, ℃, and tlim represents inlet air wet bulb temperature, ℃. Figs. 5-8 illustrate the changing rules of t and for different layout patterns under crosswind
conditions. Among of them, the circulating water inlet temperature and circulating water flowrate are 60 ℃ and 6 L/min in Figs. 5-6, and 50 ℃ and 6 L/min, respectively in Figs. 7-8.
Fig. 5. Relation curves between the cooling temperature difference and crosswind velocity under different layout patterns (60 ℃, 6 L/min)
Fig. 6. Relation curves between the cooling efficiency and crosswind velocity under different layout patterns (60 ℃, 6 L/min)
It can be observed form Figs. 5-6 that the t and for the non-uniform fillings (P2-P5 patterns) are dramatically higher than those of the uniform fillings (P1 pattern) under any crosswind velocity. Under P1-P5 conditions, these two parameters decrease firstly, and then increase with the increasing of crosswind velocity, then reach to the minimum while the crosswind velocity is equal to 0.4 m/s.
In addition, experimental study shows that, for uniform pattern (P1), the t and at 0.4 m/s
crosswind reduce by 7.7% and 4.7%, respectively compared with those of the windless condition. However, for the four non-uniform fillings (P2-P5), their reductions are within 3.5%-5.0% and
1.5%-4.5%, respectively. It demonstrates that the non-uniform fillings can alleviate the adverse effect of crosswind on thermal performance, i.e., the non-uniform fillings are not sensitive to the crosswind. Almost the same phenomena can be found for the different working conditions in Figs. 7-8. For
uniform pattern (P1), the t and at 0.4 m/s crosswind reduce by 9.0% and 3.0%, respectively
compared with those of the windless condition. However, their reductions are only within 2.5%-3.8% and 1.3%-3.0%, respectively for the four non-uniform fillings (P2-P5).
Fig. 7. Relation curves between cooling temperature difference and crosswind velocity under different layout patterns (50 ℃, 6 L/min)
Fig. 8. Relation curves between cooling efficiency and crosswind velocity under different layout patterns (50 ℃, 6 L/min)
According to the analysis of the cooling temperature difference and cooling efficiency, it can be drawn that the non-uniform fillings have the superior thermal performance under crosswind conditions,
even though the crosswind velocity equals to 0.4 m/s which is the extremely adverse crosswind velocity [44-46]. It proves that the uniform air dynamic field under non-uniform fillings is still beneficial to the heat and mass transfer performance even though under crosswind conditions. The heat and mass transfer performance for the wet cooling towers depends on mainly the uniformity of air dynamic field and the synergic match between dynamic and drag field. According to the analysis of Gao et al [43], the uniform fillings produce the non-uniform air dynamic field, disorder the synergic match between dynamic and drag field, which deteriorates the heat and mass transfer course. This phenomena become more severe under crosswind conditions. Compared with the uniform fillings, the non-uniform fillings can balance the air dynamic field inside tower under crosswind conditions, which impels outer air to enter into the tower center and reduces the ventilation resistance. Thus, for the non-uniform fillings, the outer cold air can enter into the center as far as possible, which can improve the heat and mass transfer performance near the center, and set up synergic match between dynamic and drag field. Briefly, the non-uniform fillings generate the uniform relatively air dynamic field inside tower under crosswind conditions, which results in the balance of heat and mass transfer in the whole tower. So the non-uniform fillings has the superior relatively thermal performance under crosswind conditions. 4. Comparative study for four non-uniform fillings under crosswind conditions In order to derive the optimal pattern of non-uniform fillings under crosswind conditions and realize the engineering application, the drag characteristic and thermal performance are analyzed for the four non-uniform fillings, and five performance parameters, including the drag coefficient, ventilation rate, heat transfer coefficient, Merkel number and evaporation loss, are introduced to act as the evaluation indictors in this part. 4.1. Calculation method of evaluation indicators In this paper, the ventilation rate G can be calculated by, G Qw
(3)
where Qw is the circulating water flowrate, m /h, and is the air-water ratio which can be given by, 3
C pw t
(4)
K (i2 i1 )
In Eq. (4), c pw is the specific heat of water, kJ/kg℃, i1 and i2 are the specific enthalpy of inlet air and outlet air, kJ/kg. Additionally, the K in Eq. (4) is called as heat coefficient, and the K value can embody the evaporation loss to a certain degree, which can be written by,
K 1
t2 586 0.56(t2 20)
(5)
The ventilation drag is different due to the various layout patterns of fillings, so the drag coefficient is also an important parameter which can evaluate the drag characteristics of different non-uniform fillings. The balance equation between drag force and draft force is written by,
H e g 1 2 m
v02 2
According to the Eq. (6), the ventilation drag coefficient can be written by,
(6)
2 H e g 1 2
(7)
m v02
where H e is the height of model tower, m, 1 and 2 are the air density of inlet tower and outlet tower, kg/m3, m is the average value of 1 and 2 , kg/m3, g is the gravity acceleration, m/s2, v0 represents the average wind velocity of fillings section, m/s, which can be derived by,
v0 =
4G D2
(8)
D is the diameter of fillings section, m. Besides the ventilation drag and ventilation rate G, the other three parameters, which are heat transfer coefficient v , evaporation loss Qew , and Merkel number n , are adopted to evaluate thermal performance of wet cooling towers. As one of the crucial evaluable indicators, the heat transfer coefficient v can be written by,
v
C pma G 2 1
(9)
(tm m ) V
Here, G is the ventilation rate, m3/h, 1 , 2 and m are the inlet air temperature, outlet air temperature and the average air temperature, ℃. Additionally, V is the fillings area, m2, and C pma is the specific heat of wet air, kJ/kg℃, which can be calculated by,
C pma C pd 1C pv 1.005 1.8421
(10)
What’s more, evaporation loss Qew cannot be ignored due to the growing shortage of water resources, meanwhile, the wet cooling towers bring out the relatively large evaporation loss. Thus, evaporation loss also is introduced to act as an evaluation indicator, which can be given by, Qew G m 2 1
(11)
In Eqs.(10) and (11), 1 and 2 are the humidity ratio of inlet and outlet air, kg/kg,which can be calculated by,
1＝0.622
1 p'' pa 1 p'' 1
(12)
1
2＝0.622
2 p'' pa 2 p'' 2
(13) 2
where 1 and 2 are the relative humidity of inlet and outlet air, pa is the atmosphere pressure, Pa, In addition, p''1 and p'' 2 are the saturated water vapor pressure at the temperature of 1 and 2 , Pa. Finally, another considerable parameter is Merkel number, and the calculation method of Merkel number refers to the enthalpy potential method proposed by Merkel in 1925 [47], and Simpson expansion formula can be written as,
n
c pw t 1 4 1 '' '' '' 6 K i2 i1 im im i1 i2
(14)
In Eq.(14), i1'' , i2'' and im' represent the corresponding saturated enthalpy when the temperature is t1 ,
t2 and tm t1 t2 2 , kJ/kg. i1 , i2 and im i1 i2 2 are the specific enthalpy of inlet air, outlet air and the average value, kJ/kg. c pw represents specific heat of water, kJ/kg℃. In the following text, the above-mentioned five performance paramters would be used to analyze the drag characteristic and thermal performance for the four non-uniform fillings. 4.2. The drag characteristic for the four non-uniform fillings While the circulating water flowrate and inlet temperature keep constant, meanwhile, the other components of cooling towers are not changed, the drag coefficient inside tower mainly depends on the geometry of non-uniform fillings and crosswind conditions. Thus, the changing rules of drag coefficient are studied in this part under different non-uniform fillings and crosswind velocities. Figs. 9-10 depict the changing rules of drag coefficient and ventilation rate under crosswind conditions for the non-uniform fillings when the circulating water flowrate is 6L/min and the inlet water temperature is 60℃.
Fig. 9. Relation curves between drag coefficient and crosswind velocity under non-uniform fillings (60℃, 6L/min) It can be observed in Fig. 9 that, for the four non-uniform fillings, the drag coefficient increases firstly and then decreases with the increasing of crosswind velocity, and reaches to the maximum at 0.4m/s crosswind velocity. Moreover, the drag coefficient of P2-P5 at 0.4m/s crosswind velocity enhances by 29.0%, 28.3%, 17.4% and 28.9%, respectively compared with that of windless condition. Apparently, the drag coefficient for P4 pattern has the relatively smaller increasing amplitude which is only 17.4%, i.e., the P4 pattern has the lower ventilation resistance.
Fig. 10. Relation curves between ventilation rate and crosswind velocity under non-uniform fillings (60 ℃, 6 L/min)
And the almost same rules can be seen in Fig. 10, the ventilation rate decreases firstly and then increases with the increasing of crosswind velocity, and reaches to the minimum when the crosswind velocity equals to 0.4m/s. Additionally generally speaking, the P4 pattern has the relatively higher ventilation rate under crosswind conditions, and the ventilation rate of P2-P5 at 0.4m/s crosswind velocity reduces by 10.4%, 10.3%, 9.5% and 10.2%, respectively compared with that of windless condition. Obviously at the conventional adverse crosswind velocity (0.4m/s), the ventilation rate for P4 pattern has the relatively smaller decreasing amplitude which is only 9.5%. In addition, there are crossovers in Figs. 9-10 due to probably the experimental error, so in order to analyze visually the ventilation rate and drag coefficient under the four non-uniform fillings, the arithmetic average value for the two parameters at five velocity points (0, 0.2m/s, 0.4m/s, 0.6m/s and 0.8m/s) are introduced in Fig. 11. It can be seen obviously in Fig. 11 that the P4 pattern has the higher average ventilation rate and lower average drag coefficient which are 151.6m3/h and 32.6, respectively.
Fig. 11. Average value for ventilation rate and drag coefficient under non-uniform fillings
Based on the analysis of drag coefficient and ventilation rate, the P4 pattern has the lower drag coefficient and larger ventilation rate. It can be derived that the crosswind has the relatively weak influence on the P4 pattern. Therefore, the P4 pattern has the excellent relatively drag characteristic under crosswind conditions, especially under the conventional adverse crosswind velocity (0.4m/s). 4.3 The thermal performance for the four non-uniform fillings Besides the drag characteristic, three thermal performance parameters, including heat transfer coefficient, Merkel number and evaporation loss, are used to evaluate the heat and mass transfer performance for the four non-uniform fillings, and the relation curves between heat transfer coefficient, Merkel number, evaporation loss and crosswind velocity are shown in Figs. 12-14 when the circulating water flowrate is 6L/min and the inlet water temperature is 60℃.
Fig. 12. Relation curves between heat transfer coefficient and crosswind velocity under non-uniform fillings (60 ℃, 6 L/min)
Fig. 13. Relation curves between Merkel number and crosswind velocity under non-uniform fillings (60 ℃, 6 L/min) Figs. 12-13 describe the relation curves between heat transfer coefficient, Merkel number and crosswind velocity under the four non-uniform fillings. It can be seen from these two figures that the heat transfer coefficient and Merkel number also decrease firstly, and then increase with the increasing of crosswind velocity, and the turning-points also appear at 0.4m/s crosswind velocity. It reports in Fig. 12 that the heat transfer coefficient reaches to the minimum value at 0.4m/s crosswind velocity, and the decreasing amplitudes of heat transfer coefficient for P2-P5 patterns come to 7.9%, 10.5%, 5.3% and 8.0%, respectively compared with that of the windless condition. Furthermore, based on Fig. 13, the reductions of Merkel number for P2-P5 patterns are 4.2%, 5.6%, 2.3% and 4.8%, respectively. There are also crossovers in Figs. 12-13 due to probably the experimental error. In a similar way, the arithmetic average value for the heat transfer coefficient and Merkel number at five velocity points are introduced in Fig. 14. Moreover, since the experimental Merkel number is much smaller than the heat transfer coefficient, so the Merkel number in Fig. 14 is 100 times of the original experimental value so as to put the heat transfer coefficient and Merkel number in one figure. It can be seen obviously in Fig. 14 that the P4 pattern has the higher average heat transfer coefficient and average Merkel number which are 75.8 w/m2℃ and 11.9, respectively. Surely, the actual experimental Merkel number equals to 0.119.
Fig. 14. Average value for heat transfer coefficient and Merkel number under non-uniform fillings
According to the heat transfer coefficient and Merkel number, it can be inferred that the crosswind has the relatively slight influence on the P4 pattern, i.e., the P4 pattern has the outstanding thermal performance under crosswind conditions, especially under the conventional adverse crosswind velocity (0.4m/s). As mentioned above (in Fig.11), there are higher ventilation rate and lower drag coefficient for the P4 pattern, additionally the non-uniform fillings provides the uniform relatively aerodynamic field. Thus, the P4 presents the outstanding thermal performance under crosswind conditions. Finally, the evaporation loss is discussed in this paper for the four non-uniform fillings under crosswind conditions. Fig. 15 describes the relation curves between evaporation loss and crosswind velocity under the four non-uniform fillings. In Fig. 15, the evaporation loss also decreases firstly, and then increases with the increasing of crosswind velocity. Additionally, the evaporation loss for P4 pattern has the largest value under crosswind conditions, and the average value of evaporation loss at five velocity points is 1.08 g/s, 1.06 g/s, 1.14 g/s and 1.13g/s, respectively for the P2-P5 patterns. According to the previous analysis, the P4 pattern has the higher heat transfer coefficient and ventilation rate under crosswind conditions, so the larger evaporation loss appears in the P4 pattern. Obviously, the P4 pattern is not the optimal pattern if only considering the evaporation loss, but the P3 pattern has the outstanding water-saving performance.
Fig. 15. Relation curves between evaporation loss and crosswind velocity under non-uniform fillings (60 ℃, 6 L/min) According to the comparative study for the four non-uniform fillings, it points out that the optimal non-uniform pattern should be selected by terms of comprehensive consideration of energy conservation and water-saving. Experimental researches show that the P4 is the optimal pattern if giving the priority to energy conservation, and conversely, the P3 is the optimal pattern if considering water-saving characteristic preferentially. 5. Conclusions In this study, the fillings are divided into three blocks which are inner block, medium block and outer block, and the radius of three blocks are ra, rb and rc, respectively. As a result, there are one uniform and four non-uniform layout patterns due to the different radius. Then the drag characteristic and thermal performance of different layout patterns are analyzed under crosswind conditions. By the discussion of several performance parameters, the main conclusions are as follows, (1) The analysis of model experiment discovers the cooling temperature difference and cooling efficiency for the non-uniform fillings are higher than those of the uniform fillings under crosswind conditions. More importantly, it also demonstrates that the non-uniform fillings are not sensitive to the crosswind, and can alleviate the adverse effect of crosswind on thermal performance for wet cooling towers. (2) Experimental researches prove that the P4 pattern is the optimal non-uniform pattern from the perspective of drag characteristic and thermal performance under crosswind conditions. Within the experimental crosswind velocity range, the P4 pattern has the higher average ventilation rate and lower average drag coefficient which are 151.6m3/h and 32.6, respectively. Additionally, it also has the higher average heat transfer coefficient and average Merkel number which are 75.8 w/m2 ℃ and 0.119, respectively. (3) If only considering the evaporation loss, the P3 pattern has the outstanding relatively water-saving performance, and the average evaporation loss is only 1.06g/s under the experimental conditions, however that of the other three patterns (P2, P4 and P5) is 1.08 g/s, 1.14 g/s and 1.13g/s, respectively.
Briefly, studies in this paper manifest that the optimal non-uniform fillings should be selected by terms of comprehensive consideration of energy conservation and water-saving. The P4 pattern is the optimal non-uniform pattern if giving the priority to energy conservation, and conversely, the P3 is the optimal pattern if considering water-saving characteristic preferentially. Here, ra/rb and ra/rc are 0.37 and 0.79, respectively for P3 pattern, but they are 0.44 and 0.71, respectively for P4 pattern. Acknowledgements This work is supported by National Natural Science Foundation of China (51776111) and Natural Science Foundation of Shandong province (ZR2016EEM35/ZR2017QEE010). References [1]
Kröger DG. ir-cooled Heat Exchangers and Cooling Towers, Thermal-flow Performance Evaluation and Design, Begell House, Inc., New York, 1998.
[2]
Al-Waked R, Behnia M. The performance of natural draft dry cooling towers under crosswind: CFD study, Int J of Energy Research 2004; 28: 147-161.
[3]
Al-Waked R, Behnia M. CFD simulation of wet cooling towers, App Therm Eng 2006; 26: 382-395.
[4]
Lia X, Gurgenci H, Guana Z, Wang X, Duniama S. Measurements of crosswind influence on a natural draft dry cooling tower for a solar thermal power plant. Appl Energy 2017; 206: 1169-1183.
[5]
Calautit JK, Chaudhry HN, Hughes BR, Ghani SA. Comparison between evaporative cooling and a heat pipe assisted thermal loop for a commercial wind tower in hot and dry climatic conditions. Appl Energy 2013; 101: 740-55.
[6]
Li X, Duniam S, Gurgenci H, Guan Z, Veeraragavan A. Full scale experimental study of a small natural draft dry cooling tower for concentrating solar thermal power plant. Appl Energy 2017; 193: 15-27.
[7]
Gao M, Sun F, Wang K, Shi Y, Zhao Y. Experimental research of heat transfer performance on natural draft counter flow wet cooling tower under cross-wind conditions. Int J Therm Sci 2008; 47: 935-941.
[8]
Alavi SR, Rahmati M. Experimental investigation on thermal performance of natural draft wet cooling towers employing an innovative wind-creator setup. Energy Convers Manage 2016; 122: 504-514.
[9]
Williamson N, Behnia M, Armfield S. Comparison of a 2D axisymmetric CFD model of a natural draft wet cooling tower and a 1D model. Int J Heat Mass Transfer 2008; 51: 2227-2236.
[10] Gao M, Sun F, Zhou S, Shi Y. Performance prediction of wet cooling tower using artificial neural network under cross-wind conditions. Int J Therm Sci 2009; 48: 583-589. [11] He S, Gurgenci H, Guan ZQ, Huang X, Lucas M. A review of wetted media with potential application in the pre-cooling of natural draft dry cooling towers. Renew Sust Energ Rev 2015; 44: 407–422 [12] He S, Guan Z, Gurgenci H, Hooman K, Lu Y, Alkhedhair AM. Experimental study of the application of two trickle media for inlet air pre-cooling of natural draft dry cooling towers. Energy Convers Manage 2015; 89: 644-654. [13] Simpson WM, Sherwood TK. Performance of small mechanical draft cooling towers. Refrig Eng 1946; 52: 535-543. [14] Dreyer AA, Erens PJ. Modeling of cooling tower splash pack. Int J Heat Mass Transfer 1996; 39: 109-123. [15] Milosabljevic N, Heikkilä P. A comprehensive approach to cooling tower design. Appl Therm Eng 2001; 21: 899-915. [16] Jose AS. The use of thermo-fluid dynamic efficiency in cooling towers. Heat Transfer Eng 2002, 23: 22-30. [17] Kloppers JC, Kröger DG. Loss coefficient correlation for wet cooling tower fills. Appl Therm Eng 2003; 23:
2201-2211. [18] Kloppers JC, Kröger DG. A critical investigation into the heat and mass transfer analysis of counterflow wet cooling towers. Int J Heat Mass Transfer 2005; 48: 765-777. [19] Stabat P, Marchio D. Simplified model for indirect-contact evaporative cooling-tower behavior. Appl Energy 2004; 78: 433-451. [20] Khan JR, Qureshi BA, Zubair SM. A comprehensive design and performance evaluation study of counter flow wet cooling towers. Int J of Refrig 27(2004) 914-923． [21] Muangnoi T, Asvapoositkul W, Wongwises S. An exergy analysis on the performance of a counter-flow wet cooling tower. Appl Therm Eng 2007; 27: 910-917. [22] Ghazani MA, Hosseini AH, Emami MD. A comprehensive analysis of a laboratory scale counter flow wet cooling tower using the first and the second laws of thermodynamics. Appl Therm Eng 2017; 125: 1389-1401. [23] Lemouari M, Boumaza M. Experimental investigation of the performance characteristics of a counterflow wet cooling tower. Int J Therm Sci 2010; 49: 2049-2056. [24] Grobbelaar PJ, Reuter HCR, Bertrand TP. Performance characteristics of a trickle fill in cross- and counter-flow configuration in a wet-cooling tower. Appl Therm Eng 2013; 50: 475-484. [25] Shahali P, Rahmati M, Alavi SR, Sedaghat A. Experimental study on improving operating conditions of wet cooling towers using various rib numbers of packing. Int J Refrig 2016; 67: 80-91. [26] Li Y, You X, Qiu Q, Li J. The study on the evaporation cooling efficiency and effectiveness of cooling tower of film type. Energy Convers Manage 2011; 52(1): 53-59. [27] Mofrad PI, Saeed ZH, Shanbedi M. Experimental investigation of filled bed effect on the thermal performance of a wet cooling tower by using ZnO/water nanofluid. Energy Convers Manage 2015; 127: 199-207. [28] Gharagheizi F, Hayati R, Fatemi S. Experimental study on the performance of mechanical cooling tower with two types of film packing. Energy Convers Manage 2007; 48: 277-280. [29] Lavasani, AM, Baboli ZN, Zamanizadeh M, Zareh M. Experimental study on the thermal performance of mechanical cooling tower with rotational splash type packing. Energy Convers Manage 2014; 87: 530-538. [30] Singh K, Das R. An experimental and multi-objective optimization study of a forced draft cooling tower with different fills. Energy Convers Manage 2016; 111: 417-430. [31] Singla RK, Singh K, Das R. Tower characteristics correlation and parameter retrieval in wet cooling tower with expanded wire mesh packing. Appl Therm Eng 2016; 96: 240-249. [32] Williamson N, Armfield S, Behnia M. Numerical simulation of flow in a natural draft wet cooling tower-The effect of radial thermofluid fields. Appl Therm Eng 2008; 28: 178-189. [33] Huang D, Du C. Numerical optimization on arrangement of the filling material and spraying water in cooling tower. Chin J Appl Mech 2000; 17: 102-110. (In Chinese) [34] Qureshi BA, Zubair SM. A complete model of wet cooling towers with fouling in fills. Appl Therm Eng 2006; 26: 1982-1989. [35] Williamson N, Behnia M, Armfield SW. Thermal optimization of a natural draft wet cooling tower. Int J Energy Res 2008; 32: 1349-1361. [36] Klimanek A, Bialecki RA. Solution of heat and mass transfer in counterflow wet-cooling tower fills. Int Commun Heat Mass Transfer 2009; 36: 547-553. [37] Zou Z, Gong H, Lie X, Li X, Yang Y. Numerical investigation of the crosswind effects on the performance of a hybrid cooling-tower-solar-chimney system. Applied Thermal Engineering 2017; 126: 661-669.
[38] Klimanek A, Cedzich M, Białecki, R. 3D CFD modeling of natural draft wet-cooling tower with flue gas injection. Appl Therm Eng 2015; 91: 824-833. [39] Goodarzi M, Keimanesh R. Numerical analysis on overall performance of Savonius turbines adjacent to a natural draft cooling tower. Energy Convers and Manage 2015; 99: 41-49. [40] Li X, Xia L, Gurgenci H, Guan Z. Performance enhancement for the natural draft dry cooling tower under crosswind condition by optimizing the water distribution. Int. J. Heat Mass Transf 2017; 107: 271–280. [41] Barigozzi G, Perdichizzi A, Ravelli S. Wet and dry cooling systems optimization applied to a modern waste-to-energy cogeneration heat and power plant. Appl Energy 2011; 88: 1366-1376. [42] Asvapoositkul W, Kuansathan.M. Comparative evaluation of hybrid (dry/wet) cooling tower performance. Appl Therm Eng 2014; 71: 83-93. [43] Gao M, Zhang L, Wang N, Shi Y, Sun F. Influence of non-uniform layout fillings on thermal performance for wet cooling tower. Appl Therm Eng 2016; 93: 549-555. [44] Gao M, Guo C, Ma, C, Sun F. Thermal performance for wet cooling tower with different layout patterns of fillings under typical crosswind conditions. Energies 2017; 10(65): 1-8. [45] Gao M, Sun F, Turan A. Experimental study regarding the evolution of temperature profiles inside wet cooling tower under crosswind conditions. Int J Therm Sci 2014; 86: 284-291. [46] Gao M, Sun F, Wang N. Experimental research on circumferential inflow air and vortex distribution for wet cooling tower under crosswind conditions. Appl Therm Eng 2014; 64: 93-100. [47] Askari S, Lotfi R, Seifkordi A, Rashidi AM, Koolivand H. A novel approach for energy and water conservation in wet cooling towers by using MWNTs and nanoporous graphene nanofluids. Energy Convers Manage 2016; 109: 10-18.
Highlights: The performance of non-uniform fillings for wet cooling towers is studied in lab Non-uniform fillings can alleviate adverse effect of crosswind on cooling towers P4 is optimal non-uniform pattern if giving the priority to energy conservation P3 is the optimal pattern if considering water-saving characteristic preferentially