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BUILD SIMUL (2015) 8: 179 – 188 DOI 10.1007/s12273-014-0201-8

Evaluation of exhaust performance of cooling towers in a super high-rise building: A case study

1. Department of Architecture, Green Building and New Energy Research Center, Tongji University, 1239 Siping Road, 200092 Shanghai, China 2. Department of Urban Planning, Green Building and New Energy Research Center, Tongji University, 1239 Siping Road, 200092 Shanghai, China 3. College of Mechanical & Energy Engineering, Tongji University, 1239 Siping Road, 200092 Shanghai, China

Abstract

Keywords

The exhaust performance of a cooling tower placed in the interior space of a building is crucial

super high-rise building,

due to limited space and stochastic ambient wind conditions. Improper design of the cooling tower could lead to a reduction in thermal efficiency and could also deteriorate the operational

cooling tower,

performance of the chillers. In this paper, the exhaust performance of cooling towers in a super

exhaust recirculation ratio

high-rise building considering both side exhaust and interlayer exhaust methods is investigated using CFD simulations. The results show that the exhaust performance of cooling towers under interlayer exhaust is better than that under side exhaust. However, the exhaust recirculation phenomenon of the cooling towers on the windward side caused by outdoor wind is still obvious because the outdoor wind speed is low. The total pressure differences between the inlet and outlet of the tower units under interlay exhaust become larger with increases in wind speed in

computational fluid dynamics (CFD),

Article History Received: 10 June 2014 Revised: 13 September 2014 Accepted: 16 September 2014 © Tsinghua University Press and

each district. The fan total head should be carefully determined to overcome the surplus pressure

Springer-Verlag Berlin Heidelberg

drop caused by the wind. This study helps to guide other similar cases utilizing the interior space of buildings for the cooling towers.

2014

1

Introduction

E-mail: [email protected]

in practical applications, exhaust recirculation in the cooling towers often occurs. The recirculation in cooling towers is defined as an adulteration of the air entering the tower by a portion of the air leaving the tower. This adulteration by the exhaust air raises the wet-bulb temperature of the entering air above that of the ambient air, reducing the overall tower performance. It has been found that a mere two-degree Fahrenheit increase in the entering wet-bulb temperature could result in a degradation of 12%–16% in tower capability, and for optimum cooling tower performance and enhanced safety, a 0.5 to 2℉ recirculation allowance has been loaded on the design for the wet-bulb temperature (Bhatia 2001; Evapco Bulletin 1999; Wang 2010). The exhaust recirculation that occurs in cooling towers is caused by many things, most of which are due to the limited space surrounding cooling towers or improper tower design. For example, some cooling towers are installed in wells or located adjacent to the walls of their serving buildings due to limited space. This will cause unfavorable flow interaction

Indoor/Outdoor Airflow and Air Quality

Air-conditioning systems in buildings are the major electricity users, accounting for almost half of the total energy consumption, especially in super high-rise buildings. Cooling chillers with evaporative cooling towers have been widely used for central air-conditioning in commercial buildings. It is essential to maintain and improve the efficiency of cooling towers in buildings for energy savings and normal operation of air-conditioning systems. Cooling towers are economic and effective devices for heat rejection using direct evaporative cooling technology, which extracts wasted heat from air-conditioning equipment in buildings (Xuan et al. 2012). In common mechanical draft cooling towers, fans at the outlets are used to draw air through the towers. The wet-bulb temperature of the entering air at the inlet of the cooling tower is one of the key factors affecting cooling tower performance, which is influenced by the wind environment around the cooling tower. However,

Research Article

Zhi Zhuang1 (), Chun-Ming Hsieh2, Bin Wang3

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nearby the cooling towers, resulting in the recirculation of the hot, moist exhaust air. In addition, in order to pursue architectural esthetics, decorative enclosures are likely utilized to surround cooling towers. These can easily cause exhaust recirculation, especially when the enclosures are higher than the cooling towers themselves. Furthermore, when a number of cooling towers are installed centrally, such as on the top of a building, the exhaust from upstream cooling towers may be drawn into the inlets of downstream cooling towers, which can aggravate recirculation (Lee et al. 2014). During recent years, with the development of computer technology, computational fluid dynamics (CFD) has been widely applied as a method to guide engineering design due to its advantages of being quick, accurate and effective, low-cost, etc. (Patankar 1980). It can be used to predict the airflow distribution around buildings and air conditioning systems (Liu et al. 2013; Rahmatmand et al. 2014; Hsieh et al. 2007, 2011). The CFD tool assists with cooling tower layout and air inlet positioning to improve the exhaust performance of the cooling tower. There have been some numerical research works conducted to evaluate the effects of wind on the exhaust performance of cooling towers without surrounding enclosures, such as in power plant applications. For example, Becker et al. (1989) adopted a numerical model to study the plume recirculation in a cooling tower. It was found that the intake velocity, discharge velocity, crosswind speed and wind direction, etc. had significant influences on the amount of recirculation. Gu et al. (2007) reported that the wind perpendicular to a tower’s major axis was significantly correlated with the recirculation. In addition, field measurements indicated that recirculation increased as the wind speed increased until a maximum value of recirculation occurred, and then further increases in the wind speed resulted in a decrease in the recirculation of the tower under consideration. Al-Waked and Behnia (2006) investigated numerical heat and mass transfer inside a natural draft wet cooling tower under different operating and crosswind conditions. Meroneya (2006) proposed a protocol to predict plume rise and surface drift deposition from mechanical draft cooling towers and presented a specific urban cooling tower situation with and without the urban buildings surrounding a cooling tower complex. Moon et al. (2008) compared the averaged reentering rates of all the cooling towers on a building roof in Seoul, Korea according to two wall types to study the effect of the intake outdoor air louver on the recirculation rates using a numerical method. Williamson et al. (2008) presented a two-dimensional axisymmetric two-phase simulation of the heat and mass transfer inside a natural draft wet cooling tower with particular emphasis on determining the extent of nonuniformities across the tower.

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In a building using cooling towers for chillers, the best place to locate any cooling tower is on the roof. However, in practice, there are many applications in which the cooling towers are placed on a roof with surrounding enclosures, and some studies on recirculation in cooling towers with surrounding enclosures have been reported to guide tower design. For example, Kaiser et al. (2005), Al-Waked and Behnia (2007) and Lee et al. (2014) conducted numerical simulations of wind environments around the cooling towers in buildings and proposed technical measures to improve both the effective operation and the proper layout of these cooling towers. Ge et al. (2012) evaluated the amount of recirculation in a counter-flow cooling tower under conditions of different enclosure structures and crosswind conditions using a CFD simulation and investigated the effects of recirculation in cooling towers on the energy performance of a chilling plant and plume potential. Currently, the presence of super high-rise buildings in China is increasing rapidly due to the significant space efficiency and financial benefits associated with these structures (Sev and Özgen 2009). There were almost 1000 super high-rise buildings in China by the end of 2012. The large amount of building area results in the need for large-capacity HVAC units in these buildings. However, the roof space for placing the cooling towers in super highrise buildings is usually not adequate. The utilization of the interior space of buildings for installing the cooling towers has been attracting more and more attention. However, there are restrictions associated with limited space and stochastic outdoor wind, and because few studies have focused on the exhaust performance of a cooling tower placed in the interior space of a super high-rise building, the exhaust performance of such cooling towers needs more investigation. It is necessary to simulate and evaluate the exhaust performance of these cooling towers so as to provide guidelines for optimizing the overall design plans for cooling towers. This paper discusses the exhaust performance of a cooling tower placed in the interior space of a super high-rise building using a case study. The research objective of this paper is to analyze recirculation phenomena caused by a cooling tower cluster installed in the interior space of a super high-rise building. The research findings also help to guide other similar cases. 2 Case descriptions 2.1

Project overview

This project is a newly built super high-rise green building located in Nanjing, China. With a building height of 370 m, it consists of three tower buildings (named towers A, B and

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C) and a five-floor podium. Its gross floor area is nearly 1 million m2. Among the three towers, a six-floor overhead platform is designed at the height of the 41st – 47th floors. Due to the fact that the roof area is not adequate and is also used for other specific functions, it has been proposed that clusters of the air-conditioning system be installed in the bottom floor of the overhead platform (as shown in Fig. 1).

2.2

Exhaust types of cooling towers

There are 44 cooling towers installed in total, including 13 for district B-C, 18 for district A-C and 13 for district A-B. In this study, numerical computation was conducted on the two exhaust types of cooling towers provided by the designer, including side exhaust and interlayer exhaust type cooling systems. The exhaust performance of cooling towers by the above two systems were compared and analyzed. 2.2.1 Side exhaust type Figure 2 shows the preliminary design plan for the side exhaust type cooling system. The side exhaust type of cooling towers has the following characteristics: 1) The inlet and outlet of the cooling tower is set on the outside of the serving building, and the airflow path for exhausting hot air to the outdoors is short. 2) Under side exhaust, the cooling tower occupies a small area, accounting for about 25% of the building area of the whole floor. 3) There exists a phenomenon where the exhausted hot air is hampered by the incoming wind, and the flow direction is disturbed, especially on the windward side. It is necessary to judge the occurrence possibility and the consequences of this phenomenon. 2.2.2 Interlayer exhaust type

Fig. 1 Project appearance and cooling tower site plan

Figure 3 shows the preliminary design plan for the interlayer

Fig. 2 Preliminary design plan for cooling towers by side exhaust: (a) sectional view; (b) plan view

Fig. 3 Preliminary design plan for cooling towers by interlayer exhaust: (a) sectional view; (b) plan view

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exhaust type cooling systems. The interlayer exhaust type of cooling towers has the following characteristics: 1) An interlayer is added and used for air exhaust. The exhausted hot air from the cooling towers can either flow outdoors directly or flow through the interlayer to the opposing outdoor location. Thus, the airflow path may be lengthened. 2) Under interlayer exhaust conditions, the cooling towers occupy almost 50% of the whole floor area. There is also extra effort to construct the interlayer space. 3) The exhausted hot air from the cooling tower may flow directly outdoors or flow through the exhaust interlayer to the outdoors, which is determined by the wind direction and wind speed, especially on the windward side. 3

Numerical model and evaluation methods

3.1 Simulation models A geometric physical model was built based on the design drawings for the buildings and cooling towers. Figure 4 shows the physical model and computational domain. The computational domain for the CFD simulation was 1500 m × 1500 m × 800 m. The domain settings were based on recommendations from COST Action 732 (Schatzmann and Britter 2011) and the Working Group of the Architectural Institute of Japan (Tominaga et al. 2008). The large eddy simulation (LES) model was used in the airflow model since the LES resolves large scale unsteady motions and requires only small-scale modeling in contrast to Reynoldsaveraged Navier–Stokes equations (RANS). Dynamic properties such as the fluctuations of wind pressure on buildings, which are primarily due to large-scale motions, can be directly reproduced in LES. The results comparing three turbulence models, zero equation, standard k–ε two equations and LES are discussed in Section 3.4. In additions, grid independence tests were conducted for the present study by analyzing the

same model with finer grids until the calculated results yielded only very small changes in the simulation results. The final mesh number was 7 563 000 for the analyses. The CFD code Windperfect DX is used in this study, which has been widely approved in outdoor wind environment simulation (http://www.env-simulation.com/). 3.2 Boundary conditions In China, the local meteorological stations usually measure the wind speeds at the height of 10 m as the reference. According to local meteorological conditions in Nanjing and based on data in typical meteorological years (Meteorological Information Center of CMA 2005), the outdoor designed wet-bulb temperature is 28.3℃, and the wind direction and wind speed distribution at the height of 10 m are shown in Fig. 5. The predominant wind direction is south-east, and the outdoor wind speeds range from 0 to 11.8 m/s with the average of 2.8 m/s. In this study, four representative wind speeds U0 at the height Z0=10 m including 0 m/s, 2 m/s, 5 m/s and 10 m/s were chosen for case study. For the wind boundary conditions, the distribution of wind speeds along altitudes complies with the rule of power exponent and can be derived by the following formula (MOHURD 2012): Uz Z α =( z) U0 Z0

(1)

where Uz is the wind speed at height Z. U0 is the reference wind speed at the height Z0 of 10 m. α is an empirical constant depending on atmospheric conditions, flow stabilities and the surface configuration. For isothermal conditions over typical urban areas, α is recommended to be 0.22 (MOHURD 2012). The cooling tower parameters are defined in Table 1 according to the product sample. The air rate of a single

Fig. 4 Geometric physical model: (a) computational domain; (b) sectional view

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Fig. 5 Wind direction and wind speed distribution at the height of 10 m: (a) wind direction; (b) wind speed

perature at the cooling tower outlet (℃); T¥ is the ambient air temperature (℃). In fact, the cooling towers more or less have some recirculation. During the engineering design, the smaller the ERR value is, the better the exhaust performance of the cooling tower is. The quota for ERR still lacks systemic analysis. The maximum ERR of 20% is usually required in the cooling tower design manual, and the design wet-bulb temperature is usually increased by 0.2–0.5℃ to compensate for the shortage caused by the exhaust recirculation (ECADI 2002).

Table 1 Cooling tower parameters for the case study Item

Value

Cooling capacity (kW)

4644

Water flow (LPS)

222.2

Entering fluid temperature (℃)

37.0

Leaving fluid temperature (℃)

32.0

3

Airflow rate (m /s)

32.4

Leaving air wet-bulb temperature (℃)

36.8

Evaporated water rate (L/s)

0.53

Inlet pressure drop (kPa)

17.1

3.4 unit is 32.4 m3/s, and the wet-bulb air temperature at the exhaust outlet is predicted to be 36.8℃ by heat and mass balance equations based on the outdoor climate, design cooling capacity and product sample. The detailed cooling tower design calculation process can be found in literature (ECADI 2002). 3.3

Exhaust performance evaluation index

Generally, the exhaust recirculation ratio (ERR) index can be used to evaluate the degree of the recirculation or the short cut for the humid, hot airflow from the cooling towers. The ERR can be simply estimated by the Eq. (2) (Zhao and Liu 2008): φ=

Tin - T¥ ´100% Tout - T¥

(2)

where  is the average exhaust recirculation ratio at the cooling tower inlet (%); Tin is the inflow air temperature at the cooling tower inlet (℃); Tout is the exhaust air tem-

Model validation

Experimental validation of the models is required, although this may be expensive and time-consuming. Several studies comparing RANS and LES for dispersion modeling around buildings, LES shows good agreement with experimental data in terms of distributions of mean velocity and turbulent energy around buildings (Tominaga and Stathopoulos 2013). The object in this study is a real project under construction, and lacks test data for model validation. In order to select the appropriate turbulence model, the wind-tunnel experiment data of flow field around a high-rise building within a model city block (Yoshie et al. 2005), which is similar to our study case, is used for airflow model selection. The zero equation, standard k–ε model and LES are adopted for simulation, and compared with the wind-tunnel results, as shown in Fig. 6. It can be seen that the modeled wind speeds by LES are more consistent with measurements. Similar findings can also be found by Gousseau (2012). Therefore, the LES model is assumed to be applicable for airflow analyses. The model in this study will be further validation by test data in the future.

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Fig. 7 Outlet streamlines of cooling towers under various wind speeds (side exhaust): (a) V=0 m/s; (b) V=2 m/s; (c) V=5 m/s; (d) V=10 m/s Fig. 6 Comparisons of simulation results by three turbulence models

4 Results and discussion 4.1

Airflow and temperature distribution

4.1.1 Side exhaust type Figures 7 and 8 illustrate the outlet air streamlines and the inlet temperature distribution of the cooling towers by side exhaust at different wind speeds. Figure 7(a) indicates that when there is no wind, the hot air discharged from each cooling tower in the three districts flows upward naturally due to a buoyancy effect and exhausts to the outdoors well. However, some recirculation was observed in district A-C, which results in higher air temperature there as shown in Fig. 7(a). The reason for this is that the big velocity at the inlet of the cooling tower enhances this local negative pressure, and part of the hot air exhausted from outlets is sucked into the inlet again. Figures 7(b)–(d) and Figs. 8(b)–(d) show the results as there is wind blowing. For the windward side (district B-C), the incoming wind obstructed by the building exterior facade flows upwards and downwards. Some exhausted hot air from each cooling tower will flow downwards along with the downward wind. Because the cooling tower clusters are installed at the bottom of the platform, and the inlets of the cooling tower are below the outlets, the exhausted hot air is sucked into the inlets of the cooling tower on the windward

Fig. 8 Inlet temperature distribution of cooling towers under various wind speeds (side exhaust): (a) V=0 m/s; (b) V=2 m/s; (c) V=5 m/s; (d) V=10 m/s

side, and recirculation occurs. On the other two districts A-C and A-B, some streamlines of the hot air exhausted from the cooling towers are slightly scattered by the incoming wind, most of the exhaust flows downward. When the outdoor wind speed is greater than 5 m/s, the outlet air of the cooling tower on the windward side is well mixed and diluted by the high speed incoming wind. Then, only a little

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of hot air is sucked into the inlets of cooling towers, and most flows out from the bottom of the building platform. So it can be seen that the inlet air temperature gradually decreases as the wind speed gets larger.

4.1.2 Interlayer exhaust type Figures 9 and 10 illustrate the outlet air streamlines and inlet temperature distribution of the cooling towers by interlayer exhaust at different wind speeds. When there is

Fig. 9 Outlet streamlines of cooling towers at different wind speeds (interlayer exhaust): (a) V=0 m/s; (b) V=2 m/s; (c) V=5 m/s; (d) V=10 m/s

no wind, as shown in Fig. 9(a), the hot air exhausted from each cooling tower on three sides flows upward naturally due to a buoyancy effect and exhausts well to the outdoors. However, exhaust air recirculation also exists in some areas of district A-B. The main reason for this is the same as that of the side exhaust type (see Fig. 7(a)). Figures 9(b)–(d) and Figs. 10(b)–(d) show the results as there is wind blowing. The incoming wind obstructed by the building exterior facade flows upwards and downwards. When the wind speed is small, as shown in Fig. 9(b) and Fig. 10(b), the outlet air of the cooling tower on the windward side (district B-C) flows with the downwards airflow towards the air inlets and is drawn in again so that the recirculation is obvious. As for the cooling towers in district A-B and district A-C, the inflow is obstructed by the building, and a zone of negative pressure is formed on the leeward side at both sides of the building so that the inflowing wind forms an eddy that disturbs the exhaust air of the cooling towers and increases the recirculation ratio. When the wind speed is greater than 5 m/s, as shown in Fig. 9(c) and Fig. 10(c), more exhausted air from the cooling tower on the windward side (district B-C) flows into the interlayer, and the rate of exhausted air against the wind direction reduces gradually. When the outdoor wind pressure is great enough, as shown in Fig. 9(d) and Fig. 10(d), the exhausted air from the cooling tower on the windward side completely flows downwards into the interlayer so that the exhausted hot air barely flow back into the inlets of the cooling tower, and the inlet air temperature is close to the ambient wind temperature. As for the cooling towers in district A-B and district A-C, the exhaust air is well mixed and diluted quickly by the incoming wind. Even if part of the hot air is drawn into the inlets again, the air temperature rise in these two areas is not obvious due to limited recirculation air. 4.2

Fig. 10 Inlet temperature distribution of cooling towers at different wind speeds (interlayer exhaust): (a) V=0 m/s; (b) V=2 m/s; (c) V=5 m/s; (d) V=10 m/s

Exhaust performance evaluation

Figure 11 compares the ERRs by side exhaust and interlayer exhaust at different wind speeds in each district. In Figs. 11(a) and (b), it can be seen that the ERRs in almost all districts for both side exhaust and interlayer exhaust are relatively small when there is no wind, and get bigger when the inflow wind speed is between 2 and 5 m/s, and then become smaller when the wind speed is larger than 10 m/s. These phenomena can be explained by the followings. When there is no wind, the hot air from the cooling towers is exhausted to outdoor (by side exhaust) or to both the interlayer and outdoor (by interlayer exhaust). However, the recirculation still exists in some areas. The main reason for this lies in the fact that due to the inductive effect caused by the local negative pressure with the big air velocity at the

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Figure 11(c) indicates the comparison of the ERRs for these two exhaust types. Most of the ERRs in the three districts by interlayer exhaust can be reduced about 5%–25% smaller than those by side exhaust except that when there is no wind. When there is no wind, the air velocity at the outlets by side exhaust is relatively stronger than that by interlayer exhaust, and can overcome the inductive effect at the inlets of the cooling towers. So the ERR in district B-C by side exhaust is smaller than that by interlayer exhaust. But in fact, the probability of no wind blowing is very small in about 200 meters high upper the air. Therefore, in general, the heat rejection performance by interlayer exhaust is better than that by side exhaust. Furthermore, more complicated scenarios, such as the design of the interlayer height, the inlet and outlet sizes, as well as adding overhangs at the outlet of cooling towers by interlayer exhaust can be optimized to further reduce exhaust recirculation, especially the obvious recirculation on the windward side when the wind speed is low. 4.3

Fig. 11 Comparisons of ERR at different wind speeds: (a) side exhaust; (b) interlayer exhaust; (c) ERR difference (interlayer–side)

inlets, part of the hot air exhausted from the outlets is sucked into the inlets again so that the ERR increases. When there is wind blowing, the wind flows upwards and downwards after being obstructed by the exterior facade of the building. When the wind blows at low speed, the exhausted air from the cooling towers on the windward side (district B-C) flows toward the inlets with the downward wind and is sucked so that the ERR is large and the recirculation is obvious. As for the cooling towers on the other two sides in district A-B and district A-C, the negative pressure zones occur on the leeward side due to the wind being obstructed by the building, which induces the exhausted air into the inlets of the cooling towers and increases the ERR. However, when the wind blows at a very high speed, such as greater than 10 m/s, the exhausted hot air is well adulterated and is diluted quickly by the wind. Even if it is sucked into the inlets again, the ERR is still very low.

Fan head evaluation

From above analysis, it can be seen that interlayer exhaust type has the advantages of reducing the possibility of recirculation in the cooling tower; however heat rejection performance of the cooling tower may be deteriorated by the outdoor wind with a decrease in pressure at the inlet or an increase in pressure at the outlet of the cooling tower unit. Therefore, it is necessary to explore the total pressure difference between the inlet and outlet of the cooling tower unit, which helps to select the appropriate total fan head for the cooling towers. The average total pressure at the inlet and outlet of each tower sector at different wind speeds are shown in Fig. 12. In Fig. 12(a), it can be seen that the average total pressure at the inlet of each district is negative. In the windward district B-C, the average total pressure gradually decreases with the increase in wind speed of no more than 5 m/s, and then significantly increases as the wind becomes stronger. In the leeward districts B-C and A-C, the average total pressure gradually decreases with the increase in wind speed. The total average pressure greatly decreases as the wind speed becomes greater than 10 m/s in district A-C. The main reason for this is that the wind flows through the sky platform and produces a negative pressure zone in district A-C located on the leeward side, which decreases the static pressure at the inlet of the local cooling towers. In Fig. 12(b), it can be seen that the average total pressure at the outlet of each district is positive. In the windward district B-C, the average total pressure slightly decreases with the increase in wind speed of no more than 2 m/s, and then significantly increases as the wind becomes

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total pressure difference slightly increases with the increase in wind speed of no more than 5 m/s, and then significantly increases as the wind becomes stronger. When the wind speed is 10 m/s, the average total pressure differences in districts B-C, A-B and A-C are 69, 74 and 155 Pa, respectively, which are higher than those with no wind by 21%, 24% and 227%. The significant increase in total pressure difference in district A-C is due to the negative pressure zone produced on the leeward side by the wind flowing through the sky platform, which increases pressure drop between the inlet and outlet of the cooling towers. Therefore, the total fan head should be carefully determined under windy conditions. 5 Conclusions

Fig. 12 Comparisons of static pressure difference under different wind speeds: (a) total pressure at the inlet; (b) total pressure at the outlet; (c) total pressure difference (outlet–inlet)

stronger. This is due to the increasing pressure drop in the interlayer as the wind flows across it when the wind speed is greater than 2 m/s (as shown in Fig. 9). The greater the wind speed is, the greater the total pressure increases are. However, in the leeward districts B-C and A-C, the average total pressure decreases with the increase in wind speed, especially when the wind speed is greater than 10 m/s. The main reason for this is that the wind flowing through the district A-C located on the leeward side produces a negative pressure zone, which decreases the static pressure at the outlet of the local cooling towers. Figure 12(c) shows the average total pressure difference between the inlet and outlet of each district. In the windward district B-C and leeward district A-B, the average total pressure difference slightly increases with the increase in wind speed. While, in the leeward district A-C, the average

The exhaust performance for a cooling tower placed in the interior space of a building is crucial due to limited space and stochastic ambient wind conditions. Correct layout must be investigated to provide satisfactory installation. In this study, the exhaust performance of the cooling towers in a super high-rise building using both side exhaust and interlayer exhaust methods is evaluated using numerical simulations. Although this study only analyzed some limited cases that cannot cover all situations, this provides a first step towards a comprehensive understanding of the exhaust performance for cooling towers placed in the interior space of a super high-rise building. The main findings from this study are listed as follows: 1) When there is no wind, the exhaust recirculation ratios are low in the cases of both side exhaust and interlayer exhaust. In some areas, part of the exhausted air is still sucked into the inlets due to the inductive effect at the inlets. When there is wind blowing, most of the ERRs for the cooling towers in the three districts by interlayer exhaust are smaller than those by side exhaust, which is more obvious especially when the wind speed is between 2 and 5 m/s. 2) The average total pressure difference between the inlet and outlet of the cooling towers using interlay exhaust increases with the increase in wind speed in each district. When the wind speed is 10 m/s, the average total pressure difference in leeward district A-C is higher than those with no wind by 227% due to the negative pressure zone produced on the leeward side by the wind flowing through the platform. The total fan head should be carefully determined in the design to overcome the surplus pressure drop due to wind. 3) By comparing the evaluation results of side exhaust and interlayer exhaust methods for cooling towers, the interlayer exhaust method for cooling tower design is determined to be better than the side exhaust with the lower exhaust recirculation ratio. The preliminary results indicate that

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interlayer exhaust can be considered a design priority because of big reduction in the possibility of discharge recirculation. Acknowledgements This research is supported by special fund of Key Laboratory of Eco Planning & Green Building, Ministry of Education (Tsinghua University), China. Special thanks to Mr. Weiqun Zhang from Golden Eagle Group for cooperation. References Al-Waked R, Behnia M (2006). CFD simulation of wet cooling towers. Applied Thermal Engineering, 26: 382–395. Al-Waked R, Behnia M (2007). Enhancing performance of cooling tower. Energy Conversion and Management, 48: 2638–2648. Becker BR, Stewart WE Jr, Walter TM (1989). A numerical model of cooling tower plume recirculation. Mathematical and Computer Modelling, 12: 799–819. Bhatia A (2001). Cooling Towers. Continuing Education and Development, Inc. Evapco Bulletin 311G (1999). Equipment Layout Manual. Evapco, Inc. ECADI (2002). Industrial water treatment. In: Water Supply & Drainage Design Handbook, 2nd edn. Beijing: China Building Industry Press. (in Chinese) Ge G, Xiao F, Wang S, Pu L (2012). Effects of discharge recirculation in cooling towers on energy efficiency and visible plume potential of chilling plants. Applied Thermal Engineering, 39: 37–44. Gousseau P, Blocken B, Van Heijst GJF (2012). On validation and solution verification of large-eddy simulation of wind flow around a high-rise building. In: Proceedings of 7th International Colloquium on Bluff Body Aerodynamics and Applications (BBAA7), Shanghai, China. Gu Z, Chen X, Lubitz W, Li Y, Luo W (2007). Wind tunnel simulation of exhaust recirculation in an air-cooling system at a large power plant. International Journal of Thermal Sciences, 46: 308–317. Hsieh CM, Aramaki T, Hanaki K (2007). The feedback of heat rejection to air conditioning load during the nighttime in subtropical climate. Energy and Buildings, 39: 1175–1182. Hsieh CM, Aramaki T, Hanaki K (2011). Managing heat rejected from air conditioning systems to save energy and improve the microclimates of residential buildings. Computers, Environment and Urban Systems, 35: 358–367. Kaiser AS, Lucas M, Viedma A, Zamora B (2005). Numerical model of evaporative cooling processes in a new type of cooling tower. International Journal of Heat Transfer, 48: 986–999. Lee JH, Moshfeghi M, Choi YK, Hur N (2014). A numerical simulation on recirculation phenomena of the plume generated by obstacles around a row of cooling towers. Applied Thermal Engineering, 72: 10–19.

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