Introduction

Because the increasing pressure resulted from carbon emissions of the traditional vapor compressor air-conditioning on worldwide environment, the evaporative cooler could become a promise alternative one to the compression refrigeration system due to its characteristics of energy-saving, economy and environmentally friendly, especially in the dry areas1,2,3,4. Because the produced air in dry channels can be cooled without changing its absolute humidity by evaporating the water film into the working air in wet channels, the indirect evaporative cooling (IEC) has been extensively studied by scholars to improve its performance shortfalls in efficiency and cool air supply temperature5, in which, the water film with high quality is the key precondition. The vertical shell-tube IEC (VTIEC) has the advantages in its area occupancy, anti-clogging, pressure bearing capacity, and performance improvement6,7,8, but the available VTIEC consisted of smooth heat exchange tubes is difficult to achieve a continuous, stable and thin water film on its smooth tube inner wall9, which lowers its performance and attraction to people10. Therefore, the formation of water film with a high quality in VTIEC and its effect on VTIEC performance are needed to explore.

By using the vertical tube indirect-target nozzle to distribute water, Huang Xiang,et al.11,12 obtained the VTIEC consisted of smooth tubes with the wet bulb efficiency more than 54%. Sun Tiezhu, et al.13 showed the comprehensive convective heat transfer coefficient of VTIEC with smooth aluminum tubes by experimental methods, which ranges from 36.10 to 437.4 W/(m2⋅K) when the outdoor dry bulb temperature is 16 ℃ ~ 18 ℃ and 2 ℃ ~ 4 ℃ larger than the wet bulb temperature. Chang Jianpei, et al.14 predicted the annual applicable hours of VTIEC with composite refrigeration used in a typical urban data center and found its maximum wet-bulb efficiency is 83% at the air-dried bulb temperature of 30.7 ~ 31 ℃. Fan Lijuan, et al.10 and Sun Tiezhu, et al.15 obtained a better water film by using porous ceramic tubes in VTIEC due to larger specific surface area of tubes, which improved the optimal cooling efficiency up to 78%, but the thermal resistance of ceramic tube is larger than the metal tube. Muhammad Ahmad Jamil, et al.16 proposed a new VTIEC consisted of smooth tubes and a humidifier, which can achieve a temperature drop of about 20℃, the overall heat transfer coefficient of 30 W/(m2·K) and the maximum cooling capacity of 180W in the conditions of higher temperature and airflow velocity, but there is more energy consumption and water loss.

By using piezoelectric ceramics in the wet channel, Wenchao Shi, et al.17 improve water distribution due to large water storage capacity, and the wet-bulb efficiency of VTIEC up to 67% and COP(coefficient of performance) up to 37 with little temperature fluctuation. By the three-dimensional numerical method, Xiaochen Ma, et al.18 analyzed the nozzle configuration influence on the water distribution in a diamond-shaped IEC and showed the positive effects of hydrophilic coatings and fiber coatings on the water film and COP. By using moisture-guiding fibers, Xin Cui, et al.19 improved water distribution, migration and diffusion outside smooth tubes, and obtained COP up to 41 and the wet-bulb efficiency up to 51.3% respectively. But the porous filling material in wet channels means a thick water film and large thermal resistance. Mohammed A. Sulaiman, et al.20 introduced the shell-tube structure of DPEC(dew point evaporative cooler) than the flat plate structure to improve the energy efficiency by about 12.2% and lower the produce air temperature by about 3.1℃. By incorporating VTIEC with a solar-assisted HDH (Humidification- dehumidufucation) desalination unit, Mohamed Abdelgaied, et al.21 designed an innovative hybrid system for air conditioning and desalination, and its cooling capacity is up to 941.7W.

Table 1 summarizes the water distribution methods used in IECs mentioned above and their effects on Wet bulb(WB), Dew point efficiency(DPE) and produced air temperature drop(PATD). The high quality water film along the tube inner wall is one of the key factors on VTIEC performance, which should be a continuous, stable and uniform thin water film. However, the water film formed by the traditional VTIEC with smooth tubes is usually unsatisfactory because it’s usually thick, discontinuous and uneven.

Table 1 Water distribution methods used in IECs and IEC performances.
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By the research on the heat pipe, Literatures22,23 indicated that the micro-grooved tubes can improve liquid distribution uniformity on the tube surface and its heat transfer efficiency. As the heat exchange tubes, literatures24,25,26,27 shows the positive effects of the micro-grooved tubes on the performance of evaporative condenser, heat dissipation of high-power electronic equipment, etc. But the research blank in the effects of replacing the smooth tubes with the inner grooved tubes in VTIEC to improve the water film, and then the performance, has always been there. This study will propose a new VTIEC with inner grooved tubes to improve the water film quality and show its performance by experimental method.

VTIEC and its evaluating index

The traditional VTIEC

Figure 1a and b show a VTIEC with vertical shell-tube structure and the vertical view of its tubes staggered along the produced airflow direction respectively. In the traditional VTIEC consisted of smooth tubes, the water is sprayed into the smooth tubes from the top nozzles and flows downward the tube inner wall, and the working air vertically flows in the tubes from the bottom to the top accompanied by the heat and mass transferred with the water film. This process in wet channel decreases temperature of the water film and the working air, and increases humidity of the working air. At the same time, the produced air horizontally flows outside the tubes in the dry channel and is cooled. By study, literatures28,29 suggested both reasonable structures of traditional VTIECs and their operating parameters shown in Table 2.

Fig. 1
figure 1

Structure of VTIEC (a) Overall structure, (b) Vertical view of tubes, (c) Detailed inner grooves.

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Table 2 Parameters of structure and operating in literature28,29.
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In the working process, the water downward the tube inner wall is mainly forced by the gravitational force, the surface tension of itself and the capillary force applied by the tube surface. When the smooth tubes are used in traditional VTIEC, the surface tension of water on tube inner wall is usually larger than the capillary force from tube surface. And so, the water flow tends to shrink into water droplets or columns than thin film, which tends the dry spots everywhere unless the water volume is large. The large water flow will result in a thick water film, which is against the VTIEC performance due to larger thermal resistance from water film.

The new VTIEC

If the smooth tubes of VTIEC are replaced by the inner grooved tubes, the heat transfer capacity and capillary force to the water can be effectively increased by the larger inner surface area of the inner grooved tube22,23,27, which could help to form a high quality water film in a continuous, stable, uniform and thin condition. The structure of the new VTIEC provided in this paper is the same as Fig. 1a, but it consisted of inner grooved tubes than smooth tubes. The vertical view of the inner grooved tubes is also the same as Fig. 1b. Figure 1c details the inner grooves on the inner wall of tube, which can direct the water flow and make sure the water film evenly.

To indicate the effect of the inner grooved tube on the water film, and then, on the performance improvement of VTIEC, a new VTIEC with inner grooved tubes is fabricated as shown in Fig. 1. Table 3 gives the structure and operating parameters of new VTIEC, which is referenced to Table 2 obtained in literatures5,14,28,29.

Table 3 Structure and operating parameters of new VTIEC.
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Evaluation index

(1) Usually, the low temperature of the produced air at outlet is expected and its ideal value is the wet bulb temperature corresponding to its inlet condition without pre-cooling30. The wet bulb efficiency ηIEC defined by Eq. (1) is usually used to evaluated VTIEC performance.

$$ \eta_{{{\text{IEC}}}} = \frac{{t^{\prime}_{{{\text{g1}}}} - t^{\prime\prime}_{{{\text{g1}}}} }}{{t^{\prime}_{{{\text{g1}}}} - t^{\prime}_{{{\text{s1}}}} }} $$
(1)

(2) COP (Coefficient of Performance) defined by Eqs. (2)–(4) represents the ratio of VTIEC cooling capacity to its power consumption.

$$ {\text{COP}} = \frac{\varphi }{P} $$
(2)
$$ \varphi = \rho_{1} Q_{1} C_{{{\text{P1}}}} \left( {t^{\prime}_{{{\text{g1}}}} - t^{\prime\prime}_{{{\text{g1}}}} } \right)/3600 $$
(3)
$$ P = \frac{{Q_{1} \eta_{1} \Delta p_{1} + Q_{2} \eta_{2} \Delta p_{2} }}{{3.6 \times 10^{6} }} + \frac{{Q_{{\text{p}}} H\gamma }}{{1000\eta_{{\text{p}}} }} $$
(4)

where η1 and η2 is the fan efficiency of the produced air and working air respectively, 0.875 in this study, γ is the specific weight of water, 9800 N/m3, ηp is the pump efficiency, 0.778 in this study.

3) The cooling capacity per unit area φu is defined is as follows.

$$ \varphi_{{\text{u}}} = \frac{\varphi }{{\text{S}}} \times 10^{ - 3} $$
(5)
$$ {\text{S}} = n\pi Dl $$
(6)

Experimental system and methods

Experimental system and parameters

Located at Lanzhou Jiaotong University of China, the experimental bench consists of the VTIEC shown in Fig. 1, a constant temperature and humidity air processor shown in Fig. 2, and a measurement system. Figure 1 also shows the test parameters and their positions at VTIEC inlet and outlet, mainly including the dry bulb temperature, the relative humidity, and the pressure drop. The air flow is obtained by a nozzle flowmeter. The instruments and their specifications are shown in Table 4.

Fig. 2
figure 2

The constant temperature and humidity air processor.

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Table 4 Measuring instrument models and specifications.
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The constant temperature and humidity air processor can supply the produced airflow and the working airflow for the test need respectively. The rated cooling capacity of the processor is 62 kW, and the air volumes of the produced air and the working air can be adjusted from 0 to 4000m3/h, whose power consumption is not included in VTIEC COP.

Uncertainty analysis

Followed the method of R. J. Moffa et al.31, the relative error of the experiment can be defined by Eqs. (7)–(10).

$$ \frac{{\left( {{\updelta }COP} \right)}}{COP} = \left\{ {\left( {\frac{{{\updelta }\varphi }}{\varphi }} \right)^{2} + \left( {\frac{{{\updelta }P}}{P}} \right)^{2} } \right\}^{0.5} $$
(7)
$$ \frac{{\left( {{\updelta }\varphi } \right)}}{\varphi } = \frac{{\left( {{\updelta }\varphi_{{\text{u}}} } \right)}}{{\varphi_{{\text{u}}} }} = \left\{ {\left( {\frac{{{\updelta }Q_{1} }}{{Q_{1} }}} \right)^{2} + \left( {\frac{{{\updelta }t^{\prime}_{{{\text{g}}1}} }}{{t^{\prime}_{{{\text{g}}1}} - t^{\prime\prime}_{{{\text{g}}1}} }}} \right)^{2} + \left( {\frac{{{\updelta }t^{\prime}_{{{\text{g}}1}} }}{{t^{\prime}_{{{\text{g}}1}} - t^{\prime\prime}_{{{\text{g}}1}} }}} \right)^{2} } \right\}^{0.5} $$
(8)
$$ \frac{{\left( {{\updelta }p} \right)}}{p} = \left\{ {\left( {\frac{{{\updelta }Q_{1} }}{{Q_{1} }}} \right)^{2} + \left( {\frac{{{\updelta }\Delta p_{1} }}{{\Delta p_{1} }}} \right)^{2} + \left( {\frac{{{\updelta }Q_{2} }}{{Q_{2} }}} \right)^{2} + \left( {\frac{{{\updelta }\Delta p_{2} }}{{\Delta p_{2} }}} \right)^{2} + \left( {\frac{{{\updelta }Q_{{\text{p}}} }}{{Q_{{\text{p}}} }}} \right)^{2} } \right\}^{0.5} $$
(9)
$$ \frac{{\left( {{\updelta }\eta_{{{\text{IEC}}}} } \right)}}{{\eta_{{{\text{IEC}}}} }} = \left\{ {\left( {\frac{{{\updelta }t^{\prime}_{{{\text{g}}1}} }}{{t^{\prime}_{{{\text{g}}1}} - t^{\prime\prime}_{{{\text{g}}1}} }}} \right)^{2} + \left( {\frac{{{\updelta }t^{\prime}_{{{\text{g}}1}} }}{{t^{\prime}_{{{\text{g}}1}} - t^{\prime\prime}_{{{\text{g}}1}} }}} \right)^{2} + \left( {\frac{{{\updelta }t^{\prime}_{{{\text{g}}1}} }}{{t^{\prime}_{{{\text{g}}1}} - t^{\prime}_{{{\text{s}}1}} }}} \right)^{2} + \left( {\frac{{{\updelta }t^{\prime}_{{{\text{s}}1}} }}{{t^{\prime}_{{{\text{g}}1}} - t^{\prime}_{{{\text{s}}1}} }}} \right)^{2} } \right\}^{0.5} $$
(10)

Experimental scheme

The experiment has been conducted from July 15th, 2023 to September 15th, 2023. When the variations of the air temperature and the relative humidity are within 0.3℃ and 3% respectively, the stable state is thought to achieve, usually after 5–10 min of the experiment start28. The experiment is conducted in groups using the control variable method. Each group is tested three times. After the stability is reached, the data is recorded in real time and the average value is obtained as the final result. Relative errors of the test results are listed in Table 5 and the maximum error is less than 3.26%.

Table 5 The uncertainties of test values.
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Results and discussion

The relationship between air velocity and air flow rate can be defined by Eqs. (11),(12).

$$ u_{1} = \frac{{Q_{1} }}{{3600n_{1} l\left( {S_{1} - d} \right)}} $$
(11)
$$ u_{2} = \frac{{4Q_{2} }}{{3600n\pi D^{2} }} $$
(12)

In the wet channel, the complex heat and mass transfer process happens between the water film and the working air, which is pushed forward by the working air enthalpy difference between outlet and inlet. The working air enthalpy at inlet is defined by Eq. (13).

$$ h_{2} = 1.01t_{{{\text{g}}2}} + \left( {2500 + 1.84t_{{{\text{g}}2}} } \right)d_{2} $$
(13)

The air parameter influence at inlet

The latent heat of evaporation in the wet channel will be assigned to the cooling working air, water film and produced air respectively based on structural factor and operational factor effects. According to Eq. (13), when the inlet humidity of the working air increases, its corresponding enthalpy also increases, which decrease the enthalpy difference between outlet and inlet, and the evaporation rate of the water film, and then, resulting in a lower cooling capacity of VTIEC, a lower COP, a higher outlet temperature of the produced air and a lower wet bulb efficiency.

Figures 3a–c respectively indicate the effects of the produced air inlet temperature and working air inlet enthalpy on the cooling capacity per unit area, COP and the wet bulb efficiency. During the experiment, the produced air inlet relative humidity is 25% and the flow rate is 3000m3/h(5.21 m/s), and the working air inlet temperature is 26 °C and its flow rate is also 3000m3/h(3.43 m/s).

Fig. 3
figure 3

The inlet enthalpy effect of working air and the inlet temperature effect of produced air, (a) on the cooling capacity (b) on the COP (c) on the wet efficiency.

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When the wet channel is in the same condition, the cooling capacity from water evaporation is the same. When the produced air temperature at inlet increases from 25℃ to 33℃, the cooling capacity per unit area and COP based on the dry channel side sharply increase firstly, and then ease up as shown in Fig. 3a,b respectively. When the produced air inlet temperature and the working air inlet specific enthalpy are 33 °C and 45.36 kJ/kg respectively, the cooling capacity per unit area and COP reaches its maximum of 292.57 W/m2 and 33.65. This is because, when the temperature difference between the produced air and the tube wall increases, the cooling capacity share resulted from evaporation obtained by the produced air significantly increases firstly, but this trend slows down subsequently. Due to the same reason, the wet bulb efficiency firstly increases, and then decreases as shown in Fig. 3c. The wet efficiency at 30 °C is the maximum and the temperature drop maximum of the produced air is 10.1 °C.

The velocity influence of airflow

To indicate the velocity effects of the produced air and the working air on VTIEC performance, the tests are carried out according to A, B, and C conditions respectively. In these conditions, the corresponding flows and velocities of the produced air and the working air are shown in Table 6. During the experiment, the temperature and the relative humidity of the produced air at inlet are 31℃ and 25% respectively, and the temperature and the relative humidity of the working air at inlet are 26℃ and 30%. Figures 4a–c respectively indicate the velocity effects of the airflows on the cooling capacity per unit area, COP and the wet bulb efficiency. Because the maximum air volume of the constant temperature and humidity air processor is less than 4000m3/h, the velocity ranges in three conditions are not the same.

Table 6 Air flows and velocities in test.
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Fig. 4
figure 4

The air velocity effects (a) on the cooling capacity (b) on the COP (c) on the wet efficiency.

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The performance parameters of VTIEC depends on the total cooling capacity from evaporation, and the cooling capacity shared by the working air, the water film and the produced air respectively. When the air velocities in both channels increase until the produced air velocity is about 4.05 m/s, the total cooling capacity from evaporation, and the cooling capacity shared by the working air, the water film and the produced air are improved respectively. When the air velocities in both channels continually increase after the produced air velocity of 4.05 m/s, because the air velocity effects on the total cooling capacity from evaporation and the cooling capacity shared by the produced air are weaken compared with that on the cooling capacity shared by the working air, the cooling capacity per unit area decreases as shown in Fig. 4a. Another reason for this could be the water film is disrupted due to a lager working air velocity. The maximum cooling capacity per unit area is 307.74 W/m2 when the produced air velocity is 6.07 m/s and the working air velocity is 3.99 m/s.

When the air velocities in both channels increase, the increase of the air pressure losses cause a larger power consumption of fans in both channels, which is more significant than that of the cooling capacity shared by the produced air, and so, COP decreases as shown in Fig. 4b. The low velocity of the produced air and the lower velocity of the working air is beneficial for COP. The maximum COP is 44.97 when the produced air velocity is 2.60 m/s and the working air velocity is 1.71 m/s.

When the air velocities in both channels increase, the total cooling capacity from evaporation, and the cooling capacity shared by the working air, the water film and the produced air are all improved, but the velocity effects on these parameters trend weaken as shown in Fig. 4c. At the same cooling capacity shared by the produced air, the produced air temperature at outlet increases due to the airflow increase, which results a low of COP. The maximum wet bulb efficiency is 81.7% when the produced air velocity is 4.05 m/s and the working air velocity is 4 m/s corresponding to the velocity ratio of 1.01 between the produced air and the working air.

Conclusion

To improve the water film quality, and then performance of VTIEC, this paper provides a new VTIEC with inner grooved tubes replacing smooth tubes, and indicates its performance by experimental method based on the cooling capacity per unit heat exchange area (φu), the coefficient of performance (COP) and the wet bulb efficiency (ηIEC). The conclusions are drown as follows.

  • (1) In the conditions of this paper, The maximum unit cooling capacity, wet-bulb efficiency and COP are 307.74W/m2, 81.7% and 44.97. The lower working air inlet enthalpy significantly increases the total cooling capacity of the evaporation inside the wet channel, thereby improving the performance of the VTIEC. The produced air velocity is positive effect on the cooling capacity per unit area until the produced air velocity of about 4.05 m/s and is negative effect on COP. the velocity ratio of 1.52 of the produced air to the working air is better one condition compared to 1.01 and 2.28.

  • (2) Compared with previous studies on the improvement of water film, the COP, cooling capacity and wet-bulb efficiency of the new VTIEC consisted of inner grooved tubes are significantly improved. The results will promote the application of the new VTIEC.

  • The water scaling could be common in the inner grooves if Ca2+ and Mg2+ concentrations in water are large, which will fill in the inner grooves, and so, the soften water is encouraged to use in the new VTIEC. The effects of various inner grooved structure could be different, the combined action of inner grooves and hydrophilic coating could be better, and a more comprehensive research could be carried out by numerical method than experiment, but these contents are not involved in this paper, all of which will be focused on in the next work.