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Experimental investigation of water spraying in an indirect evaporative cooler from nozzle type and spray strategy perspectives
Energy & Buildings 214 (2020) 109871
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Experimental investigation of water spraying in an indirect evaporative cooler from nozzle type and spray strategy perspectives Tiezhu Sun a, Xiang Huang a, Yi Chen b,∗, Hong Zhang a a b
School of Urban Planing and Municipal Engineering, Xi‘An Polytechnic University, China Division of Science, Engineering and Health Studies, College of Professional and Continuing Education (CPCE), The Hong Kong Polytechnic University, China
a r t i c l e
i n f o
Article history: Received 7 October 2019 Revised 9 January 2020 Accepted 12 February 2020 Available online 14 February 2020 Keywords: Indirect evaporative cooling Water spray Spray nozzle Distribution uniformity Intermittent spray strategy
a b s t r a c t Indirect evaporative cooling is a refrigerant-free and low-energy-consumed cooling technology. In practical cases, degraded cooling efficiency of indirect evaporative coolers (IEC) can be observed owning to poor wettability of the wet channels. Although many efforts had been made to improve the unevenly distribution of water film inside an IEC, existing research focus on developing novel materials for better diffusion and optimizing the nozzle arrangement. The influences of nozzle types and water spray strategy (continuous or intermittent) on water distribution performance had seldom been discussed. In this study, the IEC is experimentally investigated from the point of water spraying. Firstly, the water distribution performance of five commonly used spray nozzles has been quantitatively investigated by three proposed indicators (coverage ratio, uniformity coefficient and water volume in distribution regions) under different water rate. Secondly, the optimal nozzles are selected to be used in an advanced porous IEC prototype with good water storage and diffusion characters, which can facilitate the intermittent spray strategy. A series of intermittent spray strategies were tested to recommend a combination of spraying period and intermittent period for higher wet-bulb efficiency and less water consumption. The results show that the spiral type nozzle is the optimal for IEC application because of its high coverage ratio, good uniformity and acceptable water volume in the distribution area. The recommended intermittent spray strategy for porous ceramic IEC is a combination of 10 s–12 s spraying period and 1 min intermittent period. © 2020 Elsevier B.V. All rights reserved.
1. Introduction In recent decade, the number of data centers are increasing rapidly all over the world as the boost development of information technology [1]. The cooling solution of data centers, aimed at reduction of primary energy consumption, becomes a hot research topic [2]. In particular, many researchers focus on the development and application of an energy-less and pollution-free sustainable cooling technology named indirect evaporative cooling [3]. The Indirect Evaporative Cooler (IEC) sensibly cools the air with the aid of water evaporation in the adjacent working air channels. It is an ideal cooling solution for the data center because it specializes in handling sensible cooling load with low energy consumption and no environmental harmful refrigerant [4]. The fundamental heat and mass transfer modeling of IEC had been extensively reported. The finite differential method (FDM) [5], analytical method [6], modified ɛ-NTU method [7,8] and Computational Fluid Dynamics (CFD) [9] had been widely used to solve the
∗
Corresponding author. E-mail address: [email protected] (Y. Chen).
https://doi.org/10.1016/j.enbuild.2020.109871 0378-7788/© 2020 Elsevier B.V. All rights reserved.
temperature and humidity distribution inside the two channels. In the modeling, the wall wettability of the secondary air channels is often assumed to be 1, indicating that the wall is fully covered by the water film [10]. However, it is reported that the wettability is greatly influenced by the operating conditions, such as the nozzle type, spraying water flow rate, nozzle arrangement and air velocity [11,12]. The wettability ratio is only 1/3 to 2/3 under real operation [13]. The numerical simulation results show that the wetbulb efficiency improves significantly when the wettability ratio improves from 0 to 0.4 [14]. Because the wettability of secondary air channels plays a crucial role on IEC cooling performance, many researchers had made great efforts to improve the wettability ratio by utilizing novel materials, surface coating and novel water distribution and spray strategy. In terms of novel materials and surface coating, Guilizzoni et al. [15] investigate the effect of two kinds of coatings on the performance of IEC system. One is a standard epoxy coating (STD) while another one is a novel hydrophilic lacquer (HPHI). The results show that in case of water distribution from the top of the IEC system, wet-bulb effectiveness of HPHI device is higher than that of STD unit by 10%. Xu et al. [16] experimentally test a range of fabrics
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Nomenclatures A Xi N Aw X¯ t
Total surface area of water collection grid, m2 Collected water volume in ith grid cell, ml Number of grid cells Water coverage area, m2 Average collected water volume, ml Temperature, ◦ C
Greek symbols α coverage ratio ηwb wet-bulb effectiveness ζ uniformity coefficient Greek symbols α coverage ratio ηwb wet-bulb effectiveness ζ uniformity coefficient Subscripts p primary air s secondary air in inlet out outlet (textiles) weaved from various fibers which can be used as a wet surface medium in an evaporative cooler. The wicking ability, diffusivity and evaporation ability of some fabrics are found to be better than Kraft paper. Wang et al. [17] studied the feasibility of using porous ceramics for IEC application because of its hydrophilic property and water storage behavior. By using the novel material, intermittent spray strategy is proposed as a better way for water distribution compared with traditional continuous spraying in terms of energy saving. There are various forms water distribution and spray strategy in an IEC, including drip type, crack or hole type, rotating type, spray type, flexible tube type and in-depth type, as shown in Fig. 1. The drip type water distribution is realized by splitting the water flow uniformly through the equally distributed shunt grooves. The drip type water distribution is often used in an evaporative cooler, where water distribution pressure is unnecessary [18]. The crack or
hole type water distribution is realized by creating crack or punching on the water pipe directly. The manufacture process is easy, but precision is poor. The rotating water distribution is realized by either pipe rotation or nozzle rotation [19]. The nozzle rotation is a form of water flow that forced to pass the internal guiding structure. The water coverage area is large and evenly distributed, however, the investment and maintenance cost are high. The flexible tube type water distribution method is to locate several flexible tubes on the top of the heat exchanger. The water flow can directly contact the heat exchanger. Therefore, the space occupied by the spray height is unnecessary. This water distribution method is usually used when the heat exchanger is made of hydrophilic material, such as fibers that can cause capillary phenomenon inside [20]. Because the channel gap in an IEC is very small, usually ranging from 3 mm to 5 mm, the water is not easily distributed evenly at the bottom of an IEC. To solve the problem, in-depth water distribution method is invented. The water distributor is located deep inside the air channels by using comb-type structure. Among all the water distribution methods, the spray type dominates the market, including traditional IEC and dew-point IEC. Ahmed et al. [21] investigate the effect of three water spray configurations (external spray, internal spray and mixed spray) on the performance of a ventilation energy recovery IEC. The results show that the mixed mode performs the best and internal spraying performs better than the external spray by increasing wettability. De Antonellis et al. [22] experimentally investigated five different water nozzles and airflow arrangement of a cross-flow IEC system in order to find the highest wet-bulb efficiency under a limited water consumption. But the quantified indicators for evaluating the water uniformity is not analyzed. De Antonellis et al. [23] studied the effects of nozzles numbers, size, water flow rate and arrangement. Experimental results show that the cooling performance of an IEC is strongly influenced by the water flow rate but slightly affected by the nozzle number and size. In addition, nozzles in counter flow arrangement is superior to cross flow arrangement. For spray water distribution, the nozzles play a crucial role in water distribution uniformity and wettability of the channels, in particular the nozzle type, water flow rate and nozzle position. The different types of nozzle are shown in the Fig. 2. From the left to right, they are solid cone type, spiral type, sector type, mixed flow type, pipe type, square type, blow type, wide-angle solid cone type,
Fig. 1. Real photo of (a) drip type (b) crack/hole type (c) rotating type (d) spray type (e) flexible tube type and (f) in-depth type water distribution.
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Fig. 2. Photos of different types of nozzle.
sputter type, flower basket type, striker type and target impact nozzle. Different nozzle has different characteristic, which would result in diverse water distribution effect. In real application, unevenly water distribution is an urgent problem to be solved. It can be always observed that little water covers the corner of the heat exchanger, but flooding takes place in the overlap area of the adjacent nozzle. However, the water distribution performance under different nozzle type has not been quantitatively evaluated for IEC application in open literature. The broad literature review shows that current effort in water distribution research of an IEC focuses on optimization of nozzle arrangement, flow direction and water flow rate. There is a lack in researches deals with quantitative evaluation of water distribution performance (coverage ratio and uniformity) under different nozzle types. The suitability of various types of spray nozzle for IEC application is urgently needed to be explored. It is expected that higher water coverage ratio and better uniformity can be achieved with less overlap area and total water consumption. Moreover, it can be found that the spray strategy is dominated by continuous spraying in order to form steady water film. However, with the emergence of increasing novel materials and coatings with super wicking ability, diffusivity and evaporation ability applied in IEC, intermittent spray strategy is regarded as a better way for water spraying strategy in terms of energy saving. In an effort to narrow the research gap related to the IEC water distribution research, the study is carried out from two aspects. Firstly, we quantitatively investigate the water distribution performance under various types of spray nozzles and water flow rate in order to select the most suitable one for IEC application. Secondly, the optimal nozzles are used in an advanced porous IEC prototype, which is able to store the water and facilitate the intermittent spray strategy. A series of intermittent spray strategy of porous IEC is tested to recommend a combination of spraying period and intermittent period for higher wet-bulb efficiency. 2. Methodology The water spraying of an IEC is investigated from two aspects. Firstly, the water distribution performance of five types of spray
Fig. 4. Water distribution test system.
nozzles under different water flow rate is investigated. Secondly, different intermittent spray strategy is tested for porous ceramic IEC. 2.1. Methodology of studying water distribution performance under different nozzles The main function of water distribution system of an IEC is to spray the water onto the core heat and mass transfer unit, so that the wetted heat exchanger can have fully contact with the working air. The study experimentally investigates the performance of five commonly used spray nozzles, including spiral type, conical type, square type, sector type and target impact type as shown in the Fig. 3. The water distribution test system is shown in Fig. 4. It is consisted of a one-meter single pipe with inner diameter of 30 mm, three identical spray nozzles, circulated pump and water collection grid tank. The three spray nozzles are arranged every 330 mm. The nozzles are located 350 mm height from the water collection grid tank. To quantitatively evaluate the water distribution at each location, a water collecting grid tank is placed right under the spray nozzles. The total number of grid cell is 17 × 9 = 153. The dimensions of each grid cell is 60 mm(length) × 60 mm(width) × 50 mm
Fig. 3. Five types of commonly used spray nozzle.
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Fig. 5. Photos of test instrument. Table 1 Specifications of each instrument.
Fig. 6. Photo of porous ceramics IEC prototype.
Instrument
Parameter
Range
Accuracy
Plastic tube rotor flowmeter
Water flow rate
Measuring cylinder
Water volume
Pressure gage
Water pressure
0–10m³/h 0–10mL 0–100mL 0–250mL 0–1MPa
±4% 0.1 mL 2 mL 5 mL ±1%
(height). Comprehensive statistics can be made by measuring the collected water volume in each grid cell. The pump used in the test is a small submersible pump (Model: WP-7500) with rated power of 105 W and rated flow rate of 5500 L/h. The other experimental instruments include a flow meter, a pressure gage and a measuring cylinder to measure the water flow rate, the spray pressure and amount of water in each water collection grid cell. The test instruments are shown in Fig. 5. The specifications of each instrument are listed in the Table 1. In the test, each spraying nozzle is tested under five water flow rate scenarios (70 L/h, 120 L/h, 170 L/h, 220 L/h and 270 L/h). For each scenario, the spraying time is controlled to be 8 s. The amount of water in each water collection grid is then measured after each testing. Therefore, the water distribution layout can be obtained for all testing cases. To evaluate the water distribution effect, three indicators are proposed, including uniformity coefficient, coverage ratio and water volume in distribution regions. To describe the uniformity of water distribution, the uniformity coefficient is introduced as an indicator, calculated as Eq.(1). The smaller the uniformity coefficient, the more uniform the water distribution.
ξ=
N X − X2 i 1 i
X
N
(1)
where, ζ is the uniformity coefficient; Xi is the collected water volume in the ith collection grid cell, ml; X¯ is the average collected water volume of N collection grid cells, ml. The coverage ratio is defined as the water coverage area divided by the total surface area of water collection grid, ranging from 0 to 100%, calculated as:
α=
Aw A
(2)
where, α is the coverage ratio; Aw is the water coverage area, m2 ; A is the total surface area of water collection grid, m2 . Water volume in distribution regions is another consideration because enough water needs to be guaranteed for effective water evaporation. 2.2. Methodology of studying water spraying strategy The material for manufacturing IEC has been developed for many years. The traditional material of early-generation IEC products is metal foil and polymer. Later, more and more novel materials were proposed to improve the surface hydrophilicity, such as
fibers and ceramics. Previous study indicates that porous ceramics can improve cooling efficiency of IEC by increasing the surface wettability and reducing the pump power consumption because of its capillary action and water storage feature. Intermittent water spray strategy is used for porous ceramics IEC, on one hand, to create evenly distributed water film by capillary action, on the other hand, to save pump power consumption. This study experimentally investigates different intermittent spray strategy for porous ceramics IEC by using the optimal spray nozzle concluded from Section 2.1. The real photo of dew-point IEC prototype is shown in the Fig. 6. It is a tubular IEC made of porous ceramics. The rated air flow rate is 10 0 0 m3 /h and wet-bulb efficiency is 110%. There are 10 rows of porous ceramics tubes vertically arranged in staggered pattern. Ten tubes for odd rows and nine tubes for even rows, which gives a total number of 95 tubes. The diameter of each porous ceramic tube is Ф30 × 5 and length is 0.6 m. The water spray system consists of a water tank, a submersible pump, a rotor flowmeter, a regulating valve, pipes, spiral spray nozzles and timer. The pump with rated flow rate of 1500 L/h and head of 16 m is selected for water distribution. The intermittent spray strategy is tested according to the following steps. Firstly, turn on the spray system and start timing. Secondly, observe the water diffusion process of porous ceramic tubes. Thirdly, stop timing and turn off the pump to observe the water droplets overflow and drip from the tube wall. Record the time span between the start time and stop time. The time span is called spraying period, which determines the water consumption. Then, waiting for the tube surface drying out under the working air flow. Record the drying out time span, which is called intermittent period. The intermittent period determines the energy saving of the pump. The inlet and outlet air temperature and humidity are collected under different combination of spraying period and intermittent period. The recommended spray strategy is concluded when the IEC achieves highest average wet-bulb efficiency over the time, consumes less water and less pump energy consumption. The wet-bulb effectiveness is the most commonly used rating indicator for an IEC. It is a parameter describing the extent of approach of the outlet primary air temperature against the wet-bulb temperature of the inlet secondary air, expressed as:
ηwb =
t p,in − t p,out t p,in − twb ,s
(3)
The water stored in porous structure can be estimated by the total supply water volume minus the collected overflow water volume at the bottom tank. Five water spraying strategies were tested as listed in Table 2. The spraying period plays an important role on water distribution. If it is too short, the tube surface cannot be fully wetted. If it is too long, the excess water would form thick water film on the surface and drip downward, resulting in lower wet-bulb efficiency and water waste. In the experiment, the initial spraying lasts for 3 min to fully wet the IEC when the cooler is turned on. The IEC is allowed
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Fig. 7. The photos of water distribution effect of five spray nozzles.
Table 2 Intermittent water spraying strategy for porous ceramics IEC. No.
Spraying period (s)
Intermittent period (min)
1 2 3 4 5
8 10 12 15 45
1 1 1 1 3
to work stably for 15 min before the intermittent spray algorithm is set by the control panel. During the operation of IEC, the inlet and outlet air temperature and humidity, and water consumption are collected. The specifications of different measuring instruments are listed in Table 3. 3. Results and analysis 3.1. Analysis on water distribution performance 3.1.1. Influence of nozzle types The photos of water distribution effect under five spraying nozzles (spiral type, conical type, square type, sector type and target impact type) are shown in Fig. 7. It can be seen from the figures that the spiral nozzle can distribute a large amount of water in a wide range. This is because the spiral nozzle has a compact structure with streamlined inner spiral flow path. The water flow is divided into two parts. A small part of the sprayed water is broken into small water droplets and a large part of the water is sprayed
out as column shape due to small resistance. The solid cone nozzle has smaller water distribution amount and distribution region. The spray region is roughly circular but concentrates at the center of the circle. The square nozzle has a moderate amount of water distribution and distribution range. The spray area is roughly square. Therefore, there is little overlap region if the nozzles are properly installed. The sector type nozzle has even smaller distribution region. The spray region is a line-shape. Therefore, it is suitable for installation in the corner of the water distribution system or in the blank area of the water. Due to the special structure of target impact nozzle, the water hits the target plate when it flows out at high speed. The reaction force causes the water flow to break into small water droplets. The droplets are scattered to surroundings at very large angle (close to 180°), so they are called wide-angle nozzles. During the test, the water distribution by target impact nozzle is very uniform, however, the amount of water in each collection grid is too limited. The water distribution of five spray nozzles under the same water flow rate and pressure are shown in Fig. 8. The water spray period is 8 s. The collected water volume (ml) at each grid cell was measured and indicated by red color. The deeper the color, the more water collected. If no water in the cell, it is left as blank. It can be clearly seen that the spiral type nozzle and target impact type nozzle can cover the largest area; while the target impact type shows the best uniformity as small transition of color difference between the cells can be observed. It can be seen that the spray region covered by the conical type nozzle is very limited. Although the collected water volume in one cell can reach as high as 98 ml, only two to three grid cells can be sprayed by a single nozzle. It obviously cannot meet the require-
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Fig. 8. Water distribution volume of five spraying nozzles (220 L/h).
Table 3 Specifications of different measuring instruments. Parameter
Location
Device
Range
Accuracy
Dry bulb temperature Relative humidity Air velocity
Primary air inlet & outlet Working air inlet & outlet Primary air & working air inlet
RHLOG-T-H
−25–55◦ C 15–85% RH 0.1–30 m/s
±0.3 ◦ C ±3% RH ±0.1 m/s
SwemaAir300
Table 4 Experimental results of water distribution with different spray nozzle. Nozzle type
Flow rate (L/h)
Pressure (MPa)
Test area (m2 )
Covered area (m2 )
Coverage ratio (%)
Spray angle (°)
Uniformity
Spiral Conical Square Sector Target impact
220 220 220 220 220
0.05 0.05 0.05 0.05 0.05
0.5 0.5 0.5 0.5 0.5
0.392 0.144 0.342 0.126 0.446
78.4 28.8 68.4 25.2 89.2
110 44 85 42 168
1.35 3.42 1.60 3.53 0.96
ment of IEC. The spray region by sector type nozzle is more or less the same with that of conical type nozzle. There is almost no water on the sides and corners of the test area. In terms of spray region, the square type nozzle performs better than conical type and sector type nozzle. The spray coverage ratio of square type nozzle is around 68.4%, i.e., 68.4% of area can be covered by the spray water. The coverage ratio can be further improved to 78.4% when using spiral type nozzle. The highest coverage ratio is 89.2% provided by target impact nozzle. However, apart from the coverage ratio, the uniformity of water distribution is another important consideration for nozzle selection, which can be calculated by Eq. (1). The smaller the uniformity coefficient, the more uniform the water distribution. The
testing condition, coverage ratio, spray angle and uniformity of the five nozzles are summarized in Table 4. In terms of coverage ratio, the performances of spray nozzles from the best to the worst are: target impact > spiral > square > conical > sector. In terms of uniformity, the performance of spray nozzles from the best to the worst is: target impact > spiral > square > conical > sector. It can be seen from the Table 4 that the target impact nozzle can provide highest coverage ratio and best uniformity, but the sprayed water volume in many regions are too limited. The statistic results show that the largest water volume in the grid cell is only 12 ml. The water volume in most of the grid cell ranges from 1 ml to 2 ml, which can hardly meet the requirement of wetting the channel surface of IEC. The reason for target impact nozzle
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Fig. 9. Water coverage ratio under different water flow rate.
Fig. 10. Uniformity coefficient of water distribution under different water flow rate.
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increases from 70 L/h to 270 L/h. The spiral type can provide the second highest water coverage ratio ranging from 57.4% to 81.3%. The sector type performs the worst with the water coverage ratio ranging from only 15.6% to 25.9%. Fig. 10 shows the uniformity coefficient of water distribution under different water flow rate. It can be seen that the water flow rate has limited influence on the water distribution uniformity coefficient for all types of spray nozzles. The target impact nozzle can provide the most uniform water distribution with the uniformity coefficient fluctuates around 1.0 under all flow rates. The spiral type and square type perform similar in terms of water distribution coefficient. However, the conical type and sector type nozzles can hardly provide uniform water distribution, therefore, it is not recommended to be applied in an IEC. Considering the above two indicators of water distribution, the target impact type and spiral type nozzles can be the possible choice for IEC application. However, the third indicator, water volume in distribution regions is another consideration to ensure enough water for effective water evaporation. Fig. 11 shows the comparative water distribution volume of spiral nozzle and target impact nozzle under the largest water flow rate (270 L/h). It can be seen that although the target impact nozzle can cover 97% of the total spraying area, the sprayed water volume in many regions are too limited. The statistic results show that the largest water volume in the grid cell is only 14 ml. The water volume in most of the grid cells ranges from 1 ml to 3 ml even under the water flow rate as large as 270 L/h. The water amount can hardly meet the requirement of wetting the wall surface of IEC. On the contrary, the largest water volume in the grid cell is 74 ml under spiral nozzle spraying. The water volume in most of the grid cells ranges from 20 ml to 50 ml, many times higher than that of target impact nozzle. By comprehensive analyzing the three water distribution indicators (coverage ratio, uniformity coefficient and water volume in distribution regions), the spiral type nozzle is the optimal for IEC application because of its high coverage ratio, good uniformity and acceptable water volume in the distribution area. 3.2. Analysis on spray strategy
can provide high coverage area and good uniformity, but limited amount of water volume lies in its unique structure and operating principle. During the operation, the high speed supplied water strikes on the plate of target impact nozzle and spurts into very small water droplets. A large proportion of the small droplets will disperse around in the air rather than descent into the collected grid cell. As a result, a large quantity of water droplets will splash onto the shell of IEC, and then flow down into the IEC channels along the shell at the sides and corners. That is why the water volume collected on the side is even more than that of in the center of test area. In sum, the target impact nozzle is not suitable for IEC application, but a good choice for spray chamber where small size droplets are needed to increase the contact area between the water and air. Therefore, among the five spray nozzles, the spiral type nozzle can be the optimal choice for IEC application because of its high coverage ratio (78.4%), second smallest uniformity coefficient (1.35) and acceptable water volume in the distribution area. 3.1.2. Influence of water flow rate The influence of water flow rate on water distribution performance under different spray nozzles is also investigated. Fig. 9 shows the water coverage ratio under different water flow rate. It can be seen that the coverage ratio increases with the increase of spraying water flow rate for all the nozzle types. The trend is more obvious under smaller water flow rate. Among the five nozzles, the target impact nozzle can achieve the highest water coverage ratio from 64.1% to 90.3% when the water flow rate
Intermittent spray strategy is proposed for a porous ceramic IEC. The prototype IEC is equipped with spiral type nozzles. A series of tests were conducted to recommend an intermittent spray strategy with a combination of spraying period and intermittent period. Fig. 12 shows the wetting process of porous ceramics IEC under spraying period. It can be seen that the tube surface is dry at the beginning. After 3 s, local wetting under the spraying region is formed. The wetting area gradually diffuses along the tubes by capillary action. After 15 s spraying, the water stored in the upper part of the porous ceramics tubes reaches the limit, then the excess water dripped downward to the lower part of the tubes. Columnar water flow is formed on the whole heat exchanger. Different combination of spraying period and intermittent period is tested to the porous ceramics IEC prototype. The Fig. 13 shows the variation of wet-bulb efficiency and temperature drop under different intermittent spray strategy when the inlet air temperature is 34 ◦ C. It can be seen that when the intermittent period keeps at 1 min, the wet-bulb efficiency rapidly improves from 76% to 108% with the spraying period increases from 8 s to 10 s. Then, it decreases slowly when the spraying period extends further from 10 s to 15 s. Not surprisingly, the average wet-bulb efficiency drops dramatically when the spraying period extends to 45 s and intermittent period increases to 3 min. It can be explained as follows. The lengthen of spraying period can improve the wetting conditions (wetting area and stored water amount) of porous ceramics surface. The surface is not totally dry out in the intermittent pe-
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Fig. 11. Comparative water distribution volume of spiral nozzle and target impact nozzle under 270 L/h flow rate.
Fig. 12. Wetting process of porous ceramics IEC under spraying period.
riod so that the evaporative cooling process is not been deteriorated. However, if the spray period is too long, the stored water in the porous structure is beyond its limit, then excess water will drip down or retains on the tube surface to form thick water film. The thick water film will deteriorate the heat transfer by adding extra thermal resistance. Besides, if the intermittent period is correspondingly lengthened, the tube surface will dry out but cannot be replenished in time, which affects the evaporation process. Based on the test results, the recommended intermittent spray strategy is a combination of spraying period of 10 s and intermittent period of 1 min under 34 ◦ C and 53% RH ambient conditions. Another test was conducted when the inlet air temperature is 40 ◦ C, as shown in Fig. 14. It shows that the supply air temperature drop and wet-bulb efficiency largely increase when the spray
Fig. 13. IEC performance under different intermittent spray strategy (t = 34◦ C).
period increases from 8 s to 12 s. However, they decrease dramatically when the spray period further extends. The highest webbulb efficiency is achieved when the spraying period is 12 s under t = 40 ◦ C rather than 10 s under t = 34 ◦ C. The reason can be explained as follows. The water consumption of IEC increases with the increase of inlet air temperature under the same moisture content. Under 40 ◦ C inlet air temperature, the water stored in the porous structure evaporates faster than that of 34 ◦ C. Therefore, longer spraying period is needed to enable the wetting conditions of the tube surface during the intermittent period.
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Table 5 Water consumption under different intermittent water spray strategy. No
Intermittent spray strategy
Initial water height in the tank (mm)
Final water height in the tank (mm)
Water consumption (L/h)
1 2 3 4 5
Spraying Spraying Spraying Spraying Spraying
100 65 96 90 95
82 50 73 62 74
4.51 4.81 6.92 8.43 7.22
period: period: period: period: period:
8 sIntermittent period: 1 min 10 sIntermittent period: 1 min 12 sIntermittent period: 1 min 15 sIntermittent period: 1 min 45 sIntermittent period: 3 mins
tally investigates the water spraying of IEC from nozzle type and spray strategy perspectives. Firstly, the water distribution performance of five commonly used spray nozzles has been quantitatively investigated by three proposed indicators (coverage ratio, uniformity coefficient and water volume in distribution regions). Secondly, intermittent spray is tested as a novel spray strategy for a porous ceramic IEC prototype. The main conclusions are summarized as follows.
Fig. 14. IEC performance under different intermittent spray strategy (t = 40◦ C).
To recommend a better intermittent spray strategy, the water consumption is another consideration apart from wet-bulb efficiency. Table 5 lists out the water consumption under five intermittent spray strategies. It can be seen that the water consumption increases with the spraying period. Based on the above analysis, the recommended spraying period is 10 s–12 s considering the average wet-bulb efficiency. The corresponding water consumption is only 4.81 L/h–6.92 L/h. According to open literature, the optimal spray water consumption of tubular porous IEC is 8.8 kg/(m h) [24]. Therefore, the optimal water consumption of this IEC prototype is calculated to be 5.3 L/h, which is within the range (4.81 L/h– 6.92 L/h) of experimental results. It indicates that intermittent spray strategy has almost the same water consumption with continuous spray. The intermittent spray shows no advantage in water saving, however, it can contribute to the pump energy saving [17]. Of interesting, the water consumption under spraying period of 45 s and intermittent period of 3 min is less than that of spraying period of 15 s and intermittent period of 1 min. It may be attributed to the unsteady output flow rate of spray nozzles under relatively long operating time. In sum, the recommended intermittent spray strategy for porous ceramic IEC is a combination of 10 s–12 s spraying period and 1 min intermittent period. Longer spraying period is needed at higher ambient air temperature. 4. Conclusion The water distribution plays an important role on the performance of indirect evaporative cooler (IEC). This study experimen-
1 In terms of coverage ratio, the performances of spray nozzles from the best to the worst are: target impact > spiral > square > conical sector. In terms of uniformity, the performances of spray nozzles from the best to the worst are: target impact > spiral > square > conical > sector. 2 The target impact nozzle can provide highest coverage ratio and best uniformity, but the sprayed water volume in many regions are too limited. Therefore, it is not suitable for IEC application, but a good choice for spray chamber. 3 Among the five spray nozzles (spiral type, conical type, square type, sector type and target impact type), the spiral type nozzle can be the optimal choice for IEC application because of its high coverage ratio, good uniformity and acceptable water volume in the distribution area. 4 The intermittent spray strategy is proposed for porous ceramic IEC. A series of test have been conducted under different combination of spraying period and intermittent period. The recommended intermittent spray strategy for porous ceramic IEC is a combination of 10 s–12 s spraying period and 1 min intermittent period. Longer spraying period is needed at higher ambient air temperature.
Declaration of Competing Interest None CRediT authorship contribution statement Tiezhu Sun: Conceptualization, Methodology. Xiang Huang: Funding acquisition, Supervision. Yi Chen: Writing - original draft, Writing - review & editing, Supervision, Visualization. Hong Zhang: Investigation. Acknowledgement The research presented in the paper is financially supported by (1) the National Natural Science Foundation of China (Grant no. 51676145); (2) National Key Research and Development Program (Grant no. 2016YFC0700404).
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Appendix Table Water distribution indicators of five types of nozzles under different flow rate Nozzle type
Flow rate (L/h)
Pressure (MPa)
Test area (m2 )
Covered area (m2 )
Coverage ratio (%)
Spray angle (°)
Uniformity
Spiral
70 120 170 220 270 70 120 170 220 270 70 120 170 220 270 70 120 170 220 270 70 120 170 220 270
0.005 0.015 0.03 0.05 0.07 0.005 0.015 0.03 0.05 0.07 0.005 0.015 0.03 0.05 0.07 0.005 0.015 0.03 0.05 0.07 0.005 0.015 0.03 0.05 0.07
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
0.287 0.326 0.348 0.392 0.406 0.102 0.121 0.141 0.144 0.149 0.217 0.256 0.289 0.342 0.35 0.078 0.088 0.116 0.126 0.130 0.365 0.365 0.423 0.446 0.451
57.4 65.2 69.6 78.4 81.3 20.3 24.2 28.2 28.8 29.9 43.5 51.3 57.8 68.4 70.1 15.6 17.5 23.3 25.2 25.9 64.1 73.1 84.6 89.2 90.3
96 101 106 110 113 40 43 44 44 45 81 84 84 85 86 38 39 41 42 42.7 151 152 166 168 169
1.61 1.53 1.41 1.35 1.37 3.53 3.51 3.49 3.42 3.46 1.51 1.49 1.45 1.60 1.67 3.71 3.66 3.57 3.53 3.61 1.01 0.99 0.95 0.96 0.97
Conical
Square
Sector
Target impact
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Energy & Buildings journal homepage: www.elsevier.com/locate/enbuild
Experimental investigation of water spraying in an indirect evaporative cooler from nozzle type and spray strategy perspectives Tiezhu Sun a, Xiang Huang a, Yi Chen b,∗, Hong Zhang a a b
School of Urban Planing and Municipal Engineering, Xi‘An Polytechnic University, China Division of Science, Engineering and Health Studies, College of Professional and Continuing Education (CPCE), The Hong Kong Polytechnic University, China
a r t i c l e
i n f o
Article history: Received 7 October 2019 Revised 9 January 2020 Accepted 12 February 2020 Available online 14 February 2020 Keywords: Indirect evaporative cooling Water spray Spray nozzle Distribution uniformity Intermittent spray strategy
a b s t r a c t Indirect evaporative cooling is a refrigerant-free and low-energy-consumed cooling technology. In practical cases, degraded cooling efficiency of indirect evaporative coolers (IEC) can be observed owning to poor wettability of the wet channels. Although many efforts had been made to improve the unevenly distribution of water film inside an IEC, existing research focus on developing novel materials for better diffusion and optimizing the nozzle arrangement. The influences of nozzle types and water spray strategy (continuous or intermittent) on water distribution performance had seldom been discussed. In this study, the IEC is experimentally investigated from the point of water spraying. Firstly, the water distribution performance of five commonly used spray nozzles has been quantitatively investigated by three proposed indicators (coverage ratio, uniformity coefficient and water volume in distribution regions) under different water rate. Secondly, the optimal nozzles are selected to be used in an advanced porous IEC prototype with good water storage and diffusion characters, which can facilitate the intermittent spray strategy. A series of intermittent spray strategies were tested to recommend a combination of spraying period and intermittent period for higher wet-bulb efficiency and less water consumption. The results show that the spiral type nozzle is the optimal for IEC application because of its high coverage ratio, good uniformity and acceptable water volume in the distribution area. The recommended intermittent spray strategy for porous ceramic IEC is a combination of 10 s–12 s spraying period and 1 min intermittent period. © 2020 Elsevier B.V. All rights reserved.
1. Introduction In recent decade, the number of data centers are increasing rapidly all over the world as the boost development of information technology [1]. The cooling solution of data centers, aimed at reduction of primary energy consumption, becomes a hot research topic [2]. In particular, many researchers focus on the development and application of an energy-less and pollution-free sustainable cooling technology named indirect evaporative cooling [3]. The Indirect Evaporative Cooler (IEC) sensibly cools the air with the aid of water evaporation in the adjacent working air channels. It is an ideal cooling solution for the data center because it specializes in handling sensible cooling load with low energy consumption and no environmental harmful refrigerant [4]. The fundamental heat and mass transfer modeling of IEC had been extensively reported. The finite differential method (FDM) [5], analytical method [6], modified ɛ-NTU method [7,8] and Computational Fluid Dynamics (CFD) [9] had been widely used to solve the
∗
Corresponding author. E-mail address: [email protected] (Y. Chen).
https://doi.org/10.1016/j.enbuild.2020.109871 0378-7788/© 2020 Elsevier B.V. All rights reserved.
temperature and humidity distribution inside the two channels. In the modeling, the wall wettability of the secondary air channels is often assumed to be 1, indicating that the wall is fully covered by the water film [10]. However, it is reported that the wettability is greatly influenced by the operating conditions, such as the nozzle type, spraying water flow rate, nozzle arrangement and air velocity [11,12]. The wettability ratio is only 1/3 to 2/3 under real operation [13]. The numerical simulation results show that the wetbulb efficiency improves significantly when the wettability ratio improves from 0 to 0.4 [14]. Because the wettability of secondary air channels plays a crucial role on IEC cooling performance, many researchers had made great efforts to improve the wettability ratio by utilizing novel materials, surface coating and novel water distribution and spray strategy. In terms of novel materials and surface coating, Guilizzoni et al. [15] investigate the effect of two kinds of coatings on the performance of IEC system. One is a standard epoxy coating (STD) while another one is a novel hydrophilic lacquer (HPHI). The results show that in case of water distribution from the top of the IEC system, wet-bulb effectiveness of HPHI device is higher than that of STD unit by 10%. Xu et al. [16] experimentally test a range of fabrics
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T. Sun, X. Huang and Y. Chen et al. / Energy & Buildings 214 (2020) 109871
Nomenclatures A Xi N Aw X¯ t
Total surface area of water collection grid, m2 Collected water volume in ith grid cell, ml Number of grid cells Water coverage area, m2 Average collected water volume, ml Temperature, ◦ C
Greek symbols α coverage ratio ηwb wet-bulb effectiveness ζ uniformity coefficient Greek symbols α coverage ratio ηwb wet-bulb effectiveness ζ uniformity coefficient Subscripts p primary air s secondary air in inlet out outlet (textiles) weaved from various fibers which can be used as a wet surface medium in an evaporative cooler. The wicking ability, diffusivity and evaporation ability of some fabrics are found to be better than Kraft paper. Wang et al. [17] studied the feasibility of using porous ceramics for IEC application because of its hydrophilic property and water storage behavior. By using the novel material, intermittent spray strategy is proposed as a better way for water distribution compared with traditional continuous spraying in terms of energy saving. There are various forms water distribution and spray strategy in an IEC, including drip type, crack or hole type, rotating type, spray type, flexible tube type and in-depth type, as shown in Fig. 1. The drip type water distribution is realized by splitting the water flow uniformly through the equally distributed shunt grooves. The drip type water distribution is often used in an evaporative cooler, where water distribution pressure is unnecessary [18]. The crack or
hole type water distribution is realized by creating crack or punching on the water pipe directly. The manufacture process is easy, but precision is poor. The rotating water distribution is realized by either pipe rotation or nozzle rotation [19]. The nozzle rotation is a form of water flow that forced to pass the internal guiding structure. The water coverage area is large and evenly distributed, however, the investment and maintenance cost are high. The flexible tube type water distribution method is to locate several flexible tubes on the top of the heat exchanger. The water flow can directly contact the heat exchanger. Therefore, the space occupied by the spray height is unnecessary. This water distribution method is usually used when the heat exchanger is made of hydrophilic material, such as fibers that can cause capillary phenomenon inside [20]. Because the channel gap in an IEC is very small, usually ranging from 3 mm to 5 mm, the water is not easily distributed evenly at the bottom of an IEC. To solve the problem, in-depth water distribution method is invented. The water distributor is located deep inside the air channels by using comb-type structure. Among all the water distribution methods, the spray type dominates the market, including traditional IEC and dew-point IEC. Ahmed et al. [21] investigate the effect of three water spray configurations (external spray, internal spray and mixed spray) on the performance of a ventilation energy recovery IEC. The results show that the mixed mode performs the best and internal spraying performs better than the external spray by increasing wettability. De Antonellis et al. [22] experimentally investigated five different water nozzles and airflow arrangement of a cross-flow IEC system in order to find the highest wet-bulb efficiency under a limited water consumption. But the quantified indicators for evaluating the water uniformity is not analyzed. De Antonellis et al. [23] studied the effects of nozzles numbers, size, water flow rate and arrangement. Experimental results show that the cooling performance of an IEC is strongly influenced by the water flow rate but slightly affected by the nozzle number and size. In addition, nozzles in counter flow arrangement is superior to cross flow arrangement. For spray water distribution, the nozzles play a crucial role in water distribution uniformity and wettability of the channels, in particular the nozzle type, water flow rate and nozzle position. The different types of nozzle are shown in the Fig. 2. From the left to right, they are solid cone type, spiral type, sector type, mixed flow type, pipe type, square type, blow type, wide-angle solid cone type,
Fig. 1. Real photo of (a) drip type (b) crack/hole type (c) rotating type (d) spray type (e) flexible tube type and (f) in-depth type water distribution.
T. Sun, X. Huang and Y. Chen et al. / Energy & Buildings 214 (2020) 109871
3
Fig. 2. Photos of different types of nozzle.
sputter type, flower basket type, striker type and target impact nozzle. Different nozzle has different characteristic, which would result in diverse water distribution effect. In real application, unevenly water distribution is an urgent problem to be solved. It can be always observed that little water covers the corner of the heat exchanger, but flooding takes place in the overlap area of the adjacent nozzle. However, the water distribution performance under different nozzle type has not been quantitatively evaluated for IEC application in open literature. The broad literature review shows that current effort in water distribution research of an IEC focuses on optimization of nozzle arrangement, flow direction and water flow rate. There is a lack in researches deals with quantitative evaluation of water distribution performance (coverage ratio and uniformity) under different nozzle types. The suitability of various types of spray nozzle for IEC application is urgently needed to be explored. It is expected that higher water coverage ratio and better uniformity can be achieved with less overlap area and total water consumption. Moreover, it can be found that the spray strategy is dominated by continuous spraying in order to form steady water film. However, with the emergence of increasing novel materials and coatings with super wicking ability, diffusivity and evaporation ability applied in IEC, intermittent spray strategy is regarded as a better way for water spraying strategy in terms of energy saving. In an effort to narrow the research gap related to the IEC water distribution research, the study is carried out from two aspects. Firstly, we quantitatively investigate the water distribution performance under various types of spray nozzles and water flow rate in order to select the most suitable one for IEC application. Secondly, the optimal nozzles are used in an advanced porous IEC prototype, which is able to store the water and facilitate the intermittent spray strategy. A series of intermittent spray strategy of porous IEC is tested to recommend a combination of spraying period and intermittent period for higher wet-bulb efficiency. 2. Methodology The water spraying of an IEC is investigated from two aspects. Firstly, the water distribution performance of five types of spray
Fig. 4. Water distribution test system.
nozzles under different water flow rate is investigated. Secondly, different intermittent spray strategy is tested for porous ceramic IEC. 2.1. Methodology of studying water distribution performance under different nozzles The main function of water distribution system of an IEC is to spray the water onto the core heat and mass transfer unit, so that the wetted heat exchanger can have fully contact with the working air. The study experimentally investigates the performance of five commonly used spray nozzles, including spiral type, conical type, square type, sector type and target impact type as shown in the Fig. 3. The water distribution test system is shown in Fig. 4. It is consisted of a one-meter single pipe with inner diameter of 30 mm, three identical spray nozzles, circulated pump and water collection grid tank. The three spray nozzles are arranged every 330 mm. The nozzles are located 350 mm height from the water collection grid tank. To quantitatively evaluate the water distribution at each location, a water collecting grid tank is placed right under the spray nozzles. The total number of grid cell is 17 × 9 = 153. The dimensions of each grid cell is 60 mm(length) × 60 mm(width) × 50 mm
Fig. 3. Five types of commonly used spray nozzle.
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Fig. 5. Photos of test instrument. Table 1 Specifications of each instrument.
Fig. 6. Photo of porous ceramics IEC prototype.
Instrument
Parameter
Range
Accuracy
Plastic tube rotor flowmeter
Water flow rate
Measuring cylinder
Water volume
Pressure gage
Water pressure
0–10m³/h 0–10mL 0–100mL 0–250mL 0–1MPa
±4% 0.1 mL 2 mL 5 mL ±1%
(height). Comprehensive statistics can be made by measuring the collected water volume in each grid cell. The pump used in the test is a small submersible pump (Model: WP-7500) with rated power of 105 W and rated flow rate of 5500 L/h. The other experimental instruments include a flow meter, a pressure gage and a measuring cylinder to measure the water flow rate, the spray pressure and amount of water in each water collection grid cell. The test instruments are shown in Fig. 5. The specifications of each instrument are listed in the Table 1. In the test, each spraying nozzle is tested under five water flow rate scenarios (70 L/h, 120 L/h, 170 L/h, 220 L/h and 270 L/h). For each scenario, the spraying time is controlled to be 8 s. The amount of water in each water collection grid is then measured after each testing. Therefore, the water distribution layout can be obtained for all testing cases. To evaluate the water distribution effect, three indicators are proposed, including uniformity coefficient, coverage ratio and water volume in distribution regions. To describe the uniformity of water distribution, the uniformity coefficient is introduced as an indicator, calculated as Eq.(1). The smaller the uniformity coefficient, the more uniform the water distribution.
ξ=
N X − X2 i 1 i
X
N
(1)
where, ζ is the uniformity coefficient; Xi is the collected water volume in the ith collection grid cell, ml; X¯ is the average collected water volume of N collection grid cells, ml. The coverage ratio is defined as the water coverage area divided by the total surface area of water collection grid, ranging from 0 to 100%, calculated as:
α=
Aw A
(2)
where, α is the coverage ratio; Aw is the water coverage area, m2 ; A is the total surface area of water collection grid, m2 . Water volume in distribution regions is another consideration because enough water needs to be guaranteed for effective water evaporation. 2.2. Methodology of studying water spraying strategy The material for manufacturing IEC has been developed for many years. The traditional material of early-generation IEC products is metal foil and polymer. Later, more and more novel materials were proposed to improve the surface hydrophilicity, such as
fibers and ceramics. Previous study indicates that porous ceramics can improve cooling efficiency of IEC by increasing the surface wettability and reducing the pump power consumption because of its capillary action and water storage feature. Intermittent water spray strategy is used for porous ceramics IEC, on one hand, to create evenly distributed water film by capillary action, on the other hand, to save pump power consumption. This study experimentally investigates different intermittent spray strategy for porous ceramics IEC by using the optimal spray nozzle concluded from Section 2.1. The real photo of dew-point IEC prototype is shown in the Fig. 6. It is a tubular IEC made of porous ceramics. The rated air flow rate is 10 0 0 m3 /h and wet-bulb efficiency is 110%. There are 10 rows of porous ceramics tubes vertically arranged in staggered pattern. Ten tubes for odd rows and nine tubes for even rows, which gives a total number of 95 tubes. The diameter of each porous ceramic tube is Ф30 × 5 and length is 0.6 m. The water spray system consists of a water tank, a submersible pump, a rotor flowmeter, a regulating valve, pipes, spiral spray nozzles and timer. The pump with rated flow rate of 1500 L/h and head of 16 m is selected for water distribution. The intermittent spray strategy is tested according to the following steps. Firstly, turn on the spray system and start timing. Secondly, observe the water diffusion process of porous ceramic tubes. Thirdly, stop timing and turn off the pump to observe the water droplets overflow and drip from the tube wall. Record the time span between the start time and stop time. The time span is called spraying period, which determines the water consumption. Then, waiting for the tube surface drying out under the working air flow. Record the drying out time span, which is called intermittent period. The intermittent period determines the energy saving of the pump. The inlet and outlet air temperature and humidity are collected under different combination of spraying period and intermittent period. The recommended spray strategy is concluded when the IEC achieves highest average wet-bulb efficiency over the time, consumes less water and less pump energy consumption. The wet-bulb effectiveness is the most commonly used rating indicator for an IEC. It is a parameter describing the extent of approach of the outlet primary air temperature against the wet-bulb temperature of the inlet secondary air, expressed as:
ηwb =
t p,in − t p,out t p,in − twb ,s
(3)
The water stored in porous structure can be estimated by the total supply water volume minus the collected overflow water volume at the bottom tank. Five water spraying strategies were tested as listed in Table 2. The spraying period plays an important role on water distribution. If it is too short, the tube surface cannot be fully wetted. If it is too long, the excess water would form thick water film on the surface and drip downward, resulting in lower wet-bulb efficiency and water waste. In the experiment, the initial spraying lasts for 3 min to fully wet the IEC when the cooler is turned on. The IEC is allowed
T. Sun, X. Huang and Y. Chen et al. / Energy & Buildings 214 (2020) 109871
5
Fig. 7. The photos of water distribution effect of five spray nozzles.
Table 2 Intermittent water spraying strategy for porous ceramics IEC. No.
Spraying period (s)
Intermittent period (min)
1 2 3 4 5
8 10 12 15 45
1 1 1 1 3
to work stably for 15 min before the intermittent spray algorithm is set by the control panel. During the operation of IEC, the inlet and outlet air temperature and humidity, and water consumption are collected. The specifications of different measuring instruments are listed in Table 3. 3. Results and analysis 3.1. Analysis on water distribution performance 3.1.1. Influence of nozzle types The photos of water distribution effect under five spraying nozzles (spiral type, conical type, square type, sector type and target impact type) are shown in Fig. 7. It can be seen from the figures that the spiral nozzle can distribute a large amount of water in a wide range. This is because the spiral nozzle has a compact structure with streamlined inner spiral flow path. The water flow is divided into two parts. A small part of the sprayed water is broken into small water droplets and a large part of the water is sprayed
out as column shape due to small resistance. The solid cone nozzle has smaller water distribution amount and distribution region. The spray region is roughly circular but concentrates at the center of the circle. The square nozzle has a moderate amount of water distribution and distribution range. The spray area is roughly square. Therefore, there is little overlap region if the nozzles are properly installed. The sector type nozzle has even smaller distribution region. The spray region is a line-shape. Therefore, it is suitable for installation in the corner of the water distribution system or in the blank area of the water. Due to the special structure of target impact nozzle, the water hits the target plate when it flows out at high speed. The reaction force causes the water flow to break into small water droplets. The droplets are scattered to surroundings at very large angle (close to 180°), so they are called wide-angle nozzles. During the test, the water distribution by target impact nozzle is very uniform, however, the amount of water in each collection grid is too limited. The water distribution of five spray nozzles under the same water flow rate and pressure are shown in Fig. 8. The water spray period is 8 s. The collected water volume (ml) at each grid cell was measured and indicated by red color. The deeper the color, the more water collected. If no water in the cell, it is left as blank. It can be clearly seen that the spiral type nozzle and target impact type nozzle can cover the largest area; while the target impact type shows the best uniformity as small transition of color difference between the cells can be observed. It can be seen that the spray region covered by the conical type nozzle is very limited. Although the collected water volume in one cell can reach as high as 98 ml, only two to three grid cells can be sprayed by a single nozzle. It obviously cannot meet the require-
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Fig. 8. Water distribution volume of five spraying nozzles (220 L/h).
Table 3 Specifications of different measuring instruments. Parameter
Location
Device
Range
Accuracy
Dry bulb temperature Relative humidity Air velocity
Primary air inlet & outlet Working air inlet & outlet Primary air & working air inlet
RHLOG-T-H
−25–55◦ C 15–85% RH 0.1–30 m/s
±0.3 ◦ C ±3% RH ±0.1 m/s
SwemaAir300
Table 4 Experimental results of water distribution with different spray nozzle. Nozzle type
Flow rate (L/h)
Pressure (MPa)
Test area (m2 )
Covered area (m2 )
Coverage ratio (%)
Spray angle (°)
Uniformity
Spiral Conical Square Sector Target impact
220 220 220 220 220
0.05 0.05 0.05 0.05 0.05
0.5 0.5 0.5 0.5 0.5
0.392 0.144 0.342 0.126 0.446
78.4 28.8 68.4 25.2 89.2
110 44 85 42 168
1.35 3.42 1.60 3.53 0.96
ment of IEC. The spray region by sector type nozzle is more or less the same with that of conical type nozzle. There is almost no water on the sides and corners of the test area. In terms of spray region, the square type nozzle performs better than conical type and sector type nozzle. The spray coverage ratio of square type nozzle is around 68.4%, i.e., 68.4% of area can be covered by the spray water. The coverage ratio can be further improved to 78.4% when using spiral type nozzle. The highest coverage ratio is 89.2% provided by target impact nozzle. However, apart from the coverage ratio, the uniformity of water distribution is another important consideration for nozzle selection, which can be calculated by Eq. (1). The smaller the uniformity coefficient, the more uniform the water distribution. The
testing condition, coverage ratio, spray angle and uniformity of the five nozzles are summarized in Table 4. In terms of coverage ratio, the performances of spray nozzles from the best to the worst are: target impact > spiral > square > conical > sector. In terms of uniformity, the performance of spray nozzles from the best to the worst is: target impact > spiral > square > conical > sector. It can be seen from the Table 4 that the target impact nozzle can provide highest coverage ratio and best uniformity, but the sprayed water volume in many regions are too limited. The statistic results show that the largest water volume in the grid cell is only 12 ml. The water volume in most of the grid cell ranges from 1 ml to 2 ml, which can hardly meet the requirement of wetting the channel surface of IEC. The reason for target impact nozzle
T. Sun, X. Huang and Y. Chen et al. / Energy & Buildings 214 (2020) 109871
Fig. 9. Water coverage ratio under different water flow rate.
Fig. 10. Uniformity coefficient of water distribution under different water flow rate.
7
increases from 70 L/h to 270 L/h. The spiral type can provide the second highest water coverage ratio ranging from 57.4% to 81.3%. The sector type performs the worst with the water coverage ratio ranging from only 15.6% to 25.9%. Fig. 10 shows the uniformity coefficient of water distribution under different water flow rate. It can be seen that the water flow rate has limited influence on the water distribution uniformity coefficient for all types of spray nozzles. The target impact nozzle can provide the most uniform water distribution with the uniformity coefficient fluctuates around 1.0 under all flow rates. The spiral type and square type perform similar in terms of water distribution coefficient. However, the conical type and sector type nozzles can hardly provide uniform water distribution, therefore, it is not recommended to be applied in an IEC. Considering the above two indicators of water distribution, the target impact type and spiral type nozzles can be the possible choice for IEC application. However, the third indicator, water volume in distribution regions is another consideration to ensure enough water for effective water evaporation. Fig. 11 shows the comparative water distribution volume of spiral nozzle and target impact nozzle under the largest water flow rate (270 L/h). It can be seen that although the target impact nozzle can cover 97% of the total spraying area, the sprayed water volume in many regions are too limited. The statistic results show that the largest water volume in the grid cell is only 14 ml. The water volume in most of the grid cells ranges from 1 ml to 3 ml even under the water flow rate as large as 270 L/h. The water amount can hardly meet the requirement of wetting the wall surface of IEC. On the contrary, the largest water volume in the grid cell is 74 ml under spiral nozzle spraying. The water volume in most of the grid cells ranges from 20 ml to 50 ml, many times higher than that of target impact nozzle. By comprehensive analyzing the three water distribution indicators (coverage ratio, uniformity coefficient and water volume in distribution regions), the spiral type nozzle is the optimal for IEC application because of its high coverage ratio, good uniformity and acceptable water volume in the distribution area. 3.2. Analysis on spray strategy
can provide high coverage area and good uniformity, but limited amount of water volume lies in its unique structure and operating principle. During the operation, the high speed supplied water strikes on the plate of target impact nozzle and spurts into very small water droplets. A large proportion of the small droplets will disperse around in the air rather than descent into the collected grid cell. As a result, a large quantity of water droplets will splash onto the shell of IEC, and then flow down into the IEC channels along the shell at the sides and corners. That is why the water volume collected on the side is even more than that of in the center of test area. In sum, the target impact nozzle is not suitable for IEC application, but a good choice for spray chamber where small size droplets are needed to increase the contact area between the water and air. Therefore, among the five spray nozzles, the spiral type nozzle can be the optimal choice for IEC application because of its high coverage ratio (78.4%), second smallest uniformity coefficient (1.35) and acceptable water volume in the distribution area. 3.1.2. Influence of water flow rate The influence of water flow rate on water distribution performance under different spray nozzles is also investigated. Fig. 9 shows the water coverage ratio under different water flow rate. It can be seen that the coverage ratio increases with the increase of spraying water flow rate for all the nozzle types. The trend is more obvious under smaller water flow rate. Among the five nozzles, the target impact nozzle can achieve the highest water coverage ratio from 64.1% to 90.3% when the water flow rate
Intermittent spray strategy is proposed for a porous ceramic IEC. The prototype IEC is equipped with spiral type nozzles. A series of tests were conducted to recommend an intermittent spray strategy with a combination of spraying period and intermittent period. Fig. 12 shows the wetting process of porous ceramics IEC under spraying period. It can be seen that the tube surface is dry at the beginning. After 3 s, local wetting under the spraying region is formed. The wetting area gradually diffuses along the tubes by capillary action. After 15 s spraying, the water stored in the upper part of the porous ceramics tubes reaches the limit, then the excess water dripped downward to the lower part of the tubes. Columnar water flow is formed on the whole heat exchanger. Different combination of spraying period and intermittent period is tested to the porous ceramics IEC prototype. The Fig. 13 shows the variation of wet-bulb efficiency and temperature drop under different intermittent spray strategy when the inlet air temperature is 34 ◦ C. It can be seen that when the intermittent period keeps at 1 min, the wet-bulb efficiency rapidly improves from 76% to 108% with the spraying period increases from 8 s to 10 s. Then, it decreases slowly when the spraying period extends further from 10 s to 15 s. Not surprisingly, the average wet-bulb efficiency drops dramatically when the spraying period extends to 45 s and intermittent period increases to 3 min. It can be explained as follows. The lengthen of spraying period can improve the wetting conditions (wetting area and stored water amount) of porous ceramics surface. The surface is not totally dry out in the intermittent pe-
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Fig. 11. Comparative water distribution volume of spiral nozzle and target impact nozzle under 270 L/h flow rate.
Fig. 12. Wetting process of porous ceramics IEC under spraying period.
riod so that the evaporative cooling process is not been deteriorated. However, if the spray period is too long, the stored water in the porous structure is beyond its limit, then excess water will drip down or retains on the tube surface to form thick water film. The thick water film will deteriorate the heat transfer by adding extra thermal resistance. Besides, if the intermittent period is correspondingly lengthened, the tube surface will dry out but cannot be replenished in time, which affects the evaporation process. Based on the test results, the recommended intermittent spray strategy is a combination of spraying period of 10 s and intermittent period of 1 min under 34 ◦ C and 53% RH ambient conditions. Another test was conducted when the inlet air temperature is 40 ◦ C, as shown in Fig. 14. It shows that the supply air temperature drop and wet-bulb efficiency largely increase when the spray
Fig. 13. IEC performance under different intermittent spray strategy (t = 34◦ C).
period increases from 8 s to 12 s. However, they decrease dramatically when the spray period further extends. The highest webbulb efficiency is achieved when the spraying period is 12 s under t = 40 ◦ C rather than 10 s under t = 34 ◦ C. The reason can be explained as follows. The water consumption of IEC increases with the increase of inlet air temperature under the same moisture content. Under 40 ◦ C inlet air temperature, the water stored in the porous structure evaporates faster than that of 34 ◦ C. Therefore, longer spraying period is needed to enable the wetting conditions of the tube surface during the intermittent period.
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Table 5 Water consumption under different intermittent water spray strategy. No
Intermittent spray strategy
Initial water height in the tank (mm)
Final water height in the tank (mm)
Water consumption (L/h)
1 2 3 4 5
Spraying Spraying Spraying Spraying Spraying
100 65 96 90 95
82 50 73 62 74
4.51 4.81 6.92 8.43 7.22
period: period: period: period: period:
8 sIntermittent period: 1 min 10 sIntermittent period: 1 min 12 sIntermittent period: 1 min 15 sIntermittent period: 1 min 45 sIntermittent period: 3 mins
tally investigates the water spraying of IEC from nozzle type and spray strategy perspectives. Firstly, the water distribution performance of five commonly used spray nozzles has been quantitatively investigated by three proposed indicators (coverage ratio, uniformity coefficient and water volume in distribution regions). Secondly, intermittent spray is tested as a novel spray strategy for a porous ceramic IEC prototype. The main conclusions are summarized as follows.
Fig. 14. IEC performance under different intermittent spray strategy (t = 40◦ C).
To recommend a better intermittent spray strategy, the water consumption is another consideration apart from wet-bulb efficiency. Table 5 lists out the water consumption under five intermittent spray strategies. It can be seen that the water consumption increases with the spraying period. Based on the above analysis, the recommended spraying period is 10 s–12 s considering the average wet-bulb efficiency. The corresponding water consumption is only 4.81 L/h–6.92 L/h. According to open literature, the optimal spray water consumption of tubular porous IEC is 8.8 kg/(m h) [24]. Therefore, the optimal water consumption of this IEC prototype is calculated to be 5.3 L/h, which is within the range (4.81 L/h– 6.92 L/h) of experimental results. It indicates that intermittent spray strategy has almost the same water consumption with continuous spray. The intermittent spray shows no advantage in water saving, however, it can contribute to the pump energy saving [17]. Of interesting, the water consumption under spraying period of 45 s and intermittent period of 3 min is less than that of spraying period of 15 s and intermittent period of 1 min. It may be attributed to the unsteady output flow rate of spray nozzles under relatively long operating time. In sum, the recommended intermittent spray strategy for porous ceramic IEC is a combination of 10 s–12 s spraying period and 1 min intermittent period. Longer spraying period is needed at higher ambient air temperature. 4. Conclusion The water distribution plays an important role on the performance of indirect evaporative cooler (IEC). This study experimen-
1 In terms of coverage ratio, the performances of spray nozzles from the best to the worst are: target impact > spiral > square > conical sector. In terms of uniformity, the performances of spray nozzles from the best to the worst are: target impact > spiral > square > conical > sector. 2 The target impact nozzle can provide highest coverage ratio and best uniformity, but the sprayed water volume in many regions are too limited. Therefore, it is not suitable for IEC application, but a good choice for spray chamber. 3 Among the five spray nozzles (spiral type, conical type, square type, sector type and target impact type), the spiral type nozzle can be the optimal choice for IEC application because of its high coverage ratio, good uniformity and acceptable water volume in the distribution area. 4 The intermittent spray strategy is proposed for porous ceramic IEC. A series of test have been conducted under different combination of spraying period and intermittent period. The recommended intermittent spray strategy for porous ceramic IEC is a combination of 10 s–12 s spraying period and 1 min intermittent period. Longer spraying period is needed at higher ambient air temperature.
Declaration of Competing Interest None CRediT authorship contribution statement Tiezhu Sun: Conceptualization, Methodology. Xiang Huang: Funding acquisition, Supervision. Yi Chen: Writing - original draft, Writing - review & editing, Supervision, Visualization. Hong Zhang: Investigation. Acknowledgement The research presented in the paper is financially supported by (1) the National Natural Science Foundation of China (Grant no. 51676145); (2) National Key Research and Development Program (Grant no. 2016YFC0700404).
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Appendix Table Water distribution indicators of five types of nozzles under different flow rate Nozzle type
Flow rate (L/h)
Pressure (MPa)
Test area (m2 )
Covered area (m2 )
Coverage ratio (%)
Spray angle (°)
Uniformity
Spiral
70 120 170 220 270 70 120 170 220 270 70 120 170 220 270 70 120 170 220 270 70 120 170 220 270
0.005 0.015 0.03 0.05 0.07 0.005 0.015 0.03 0.05 0.07 0.005 0.015 0.03 0.05 0.07 0.005 0.015 0.03 0.05 0.07 0.005 0.015 0.03 0.05 0.07
0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
0.287 0.326 0.348 0.392 0.406 0.102 0.121 0.141 0.144 0.149 0.217 0.256 0.289 0.342 0.35 0.078 0.088 0.116 0.126 0.130 0.365 0.365 0.423 0.446 0.451
57.4 65.2 69.6 78.4 81.3 20.3 24.2 28.2 28.8 29.9 43.5 51.3 57.8 68.4 70.1 15.6 17.5 23.3 25.2 25.9 64.1 73.1 84.6 89.2 90.3
96 101 106 110 113 40 43 44 44 45 81 84 84 85 86 38 39 41 42 42.7 151 152 166 168 169
1.61 1.53 1.41 1.35 1.37 3.53 3.51 3.49 3.42 3.46 1.51 1.49 1.45 1.60 1.67 3.71 3.66 3.57 3.53 3.61 1.01 0.99 0.95 0.96 0.97
Conical
Square
Sector
Target impact
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