Performance Evaluation of a Maisotsenko Cycle Cooling Tower with Uneven Length of Dry and Wet Channels in Hot and Humid Conditions

: The use of the Maisotsenko cycle (M-Cycle) in traditional wet cooling towers (TWCTs) has the potential to reduce the costs of electricity generation by cooling water below the inlet air’s wet-bulb temperature. TWCTs cannot provide sufﬁcient cooling capacity for the increasing demand for cooling energy in the power and industrial sectors—especially in hot and wet climates. Due to this fact, an experimental system of an M-Cycle cooling tower (MCT) with parallel counter-ﬂow arrangement ﬁlls was constructed in order to provide perspective on the optimal length of dry channels ( l dry ), thermal performance under different conditions, and pressure drops of the MCT. Results showed that the optimal value of l dry was 2.4 m, and the maximum wet-bulb effectiveness was up to 180%. In addition, the impact of air velocity in wet channels on the pressure drops of the novel ﬁlls was also summarized. This study conﬁrms the great potential of using the M-Cycle in TWCTs, and provides a guideline for the industrial application and performance improvement of MCTs.


Introduction
The traditional wet cooling tower (TWCT) is considered to be one of the key components of electricity generation systems, as well as an important factor affecting the total efficiency of thermal power plants [1,2]. TWCTs are used to decrease the temperature of cooling water, before it can be sent to a condenser for conversion of the exhaust steam discharged by the steam turbine into water [3]. Efficient operation of TWCTs is determined by the outlet water temperature. In Rankine-cycle-based thermal power plants, the coal consumption decreases and the thermal efficiency increases with the decrease in the outlet water temperature of the cooling towers [4,5]. It is therefore necessary to improve the thermal performance of TWCTs.
Methods for improving the thermal performance of TWCTs have been proposed by many authors in the past. Chen et al. [6,7] suggested that installing air ducts in the rain area could improve the aerodynamic field of the tower and alleviate the adverse crosswind effect, so as to significantly improve the performance of TWCTs. Structural improvement measures-including adding air deflectors, using non-uniform fillings, and adding air ducts-were also implemented in one large-scale TWCT in order to weaken the adverse influence of crosswind [8]. However, the outlet water temperature of TWCTs remained limited by the inlet air's wet-bulb temperature, which is a major disadvantage, despite many useful methods having been proposed [9][10][11]. Due to this fact, TWCTs cannot provide cooling water with a sufficiently low temperature to meet the increasing water (process 2i-2o), and is finally discharged into the atmosphere. The water is cooled from the state point 3i to the state point 3o, and then returns to its circuit to absorb heat from the heat source. In this paper, a one-dimensional ε-NTU model was used to analyze the heat and mass transfer in MCTs [26][27][28][29]. The possibility of uneven length of the dry and wet channels of MCTs was considered by the model. As shown in Figure 2, the primary air is marked as 1, the working air is marked as 2, and the water is marked as w. The temperature difference between the primary air t1 and the plate tp1 is the driving force of the sensible heat transfer between the dry and wet channels. Hence, the heat balance equations for primary air in the dry channels are expressed as: The humidity ratio gradient between the working air x2 and the saturated air layer at the air/water interface x′w is the driving force of the mass transfer between the working air and the water. Hence, working air mass balance equations are expressed as: In this paper, a one-dimensional ε-NTU model was used to analyze the heat and mass transfer in MCTs [26][27][28][29]. The possibility of uneven length of the dry and wet channels of MCTs was considered by the model. As shown in Figure 2, the primary air is marked as 1, the working air is marked as 2, and the water is marked as w.
Energies 2021, 14, x FOR PEER REVIEW 3 of 15 water (process 2i-2o), and is finally discharged into the atmosphere. The water is cooled from the state point 3i to the state point 3o, and then returns to its circuit to absorb heat from the heat source. In this paper, a one-dimensional ε-NTU model was used to analyze the heat and mass transfer in MCTs [26][27][28][29]. The possibility of uneven length of the dry and wet channels of MCTs was considered by the model. As shown in Figure 2, the primary air is marked as 1, the working air is marked as 2, and the water is marked as w. The temperature difference between the primary air t1 and the plate tp1 is the driving force of the sensible heat transfer between the dry and wet channels. Hence, the heat balance equations for primary air in the dry channels are expressed as: The humidity ratio gradient between the working air x2 and the saturated air layer at the air/water interface x′w is the driving force of the mass transfer between the working air and the water. Hence, working air mass balance equations are expressed as: The temperature difference between the primary air t 1 and the plate t p1 is the driving force of the sensible heat transfer between the dry and wet channels. Hence, the heat balance equations for primary air in the dry channels are expressed as: The humidity ratio gradient between the working air x 2 and the saturated air layer at the air/water interface x w is the driving force of the mass transfer between the working air and the water. Hence, working air mass balance equations are expressed as: The water film mass balance equation is as follows: The heat balance equation for working air includes the sensible heat transfer and the latent heat transfer. The sensible heat transfer is driven by the temperature difference between the working air t 2 and the water t w , and the latent heat transfer is connected with the evaporation of water: The energy balance equation for the water in the wet channels consists of heat transfer from water to air in dry and wet channels, along with the evaporation of water: where: If the wet channels are longer than the dry channels, the energy balance equations for the water are expressed as: By comparing Equations (9) and (10), it can be predicted that if the temperature of water was much higher than that of the primary air, the water would not heat the primary air, allowing for more effective pre-cooling of the primary air. This means that the uneven length of the dry and wet channels could further improve the cooling performance of MCTs. Hence, it is very important to study the effect of l dry on the performance of MCTs. At the same time, l dry directly affects NTU 1 , according to Equation (3). Hence, the present study also focused on the relationship between the two parameters. It can also be stated that the main operating parameters (including inlet air temperature (t 1i ) and humidity ratio (x 1i ), inlet water temperature (t wi ), and heat capacity ratio between air and water (W 1 /W w )) also affect the performance of the MCT significantly. Due to this fact, an experimental MCT system was constructed, and the impact of the structural parameters (l dry , NTU 1 ) and operating parameters (t 1i , x 1i , t wi , and W 1 /W w ) on the performance of the MCT and the resistance characteristics of the novel fills was studied experimentally.

The Experimental System of MCTs
As shown in Figure 3, the experimental system consisted of the MCT, the air supply system, the water circulation system, and the measurement devices. The MCT consisted of the tower, the novel fill, and the cold-water reservoir, with a total height of 12 m. The novel fill consisted of vortex tubes and a wind shield, as shown in Figure 4. The outer diameter, wall thickness, and length of the vortex tubes were 13, 0.8, and 3000 mm, respectively. Unlike fills in TWCTs, the novel fill in the MCT had both dry and wet channels. In the novel fill, the primary air first flowed through gaps between vortex tubes from top to bottom and became the working air. The working air then flowed through the inner channels of the vortex tubes from bottom to top, and made contact with water flowing from top to bottom. Hence, the gaps between the vortex tubes were dry channels, and the inner channels of the vortex tubes were wet channels. In order to investigate the impact of uneven length of dry and wet channels on the cooling performance of the MCT, the length of the dry channels   The air supply system was equipped with a frequency conversion fan installed on top of the MCT, which could control the air flow rate by changing the frequency. The water supply system was equipped with a cold-water reservoir, a frequency conversion pump, a plate heat exchanger, a gas-fired boiler, and a water distributor. The water was heated to the temperature required for the experimental conditions by the plate heat exchanger, was then evenly distributed on the novel fills by the water distributor and, finally, was collected in the cold-water reservoir to complete the water circulation. The water flow velocity was controlled by changing the frequency, and the inlet water temperature was controlled by the plate heat exchanger. The measurement devices consisted of T-type thermocouples, an ultrasonic flowmeter, a hot-wire anemometer, a Testo 645 humidity meter, and differential pressure meters, whose details are listed in Table 1.   The air supply system was equipped with a frequency conversion fan installed on top of the MCT, which could control the air flow rate by changing the frequency. The water supply system was equipped with a cold-water reservoir, a frequency conversion pump, a plate heat exchanger, a gas-fired boiler, and a water distributor. The water was heated to the temperature required for the experimental conditions by the plate heat exchanger, was then evenly distributed on the novel fills by the water distributor and, finally, was collected in the cold-water reservoir to complete the water circulation. The water flow velocity was controlled by changing the frequency, and the inlet water temperature was controlled by the plate heat exchanger. The measurement devices consisted of T-type thermocouples, an ultrasonic flowmeter, a hot-wire anemometer, a Testo 645 humidity meter, and differential pressure meters, whose details are listed in Table 1. The air supply system was equipped with a frequency conversion fan installed on top of the MCT, which could control the air flow rate by changing the frequency. The water supply system was equipped with a cold-water reservoir, a frequency conversion pump, a plate heat exchanger, a gas-fired boiler, and a water distributor. The water was heated to the temperature required for the experimental conditions by the plate heat exchanger, was then evenly distributed on the novel fills by the water distributor and, finally, was collected in the cold-water reservoir to complete the water circulation. The water flow velocity was controlled by changing the frequency, and the inlet water temperature was controlled by the plate heat exchanger. The measurement devices consisted of T-type thermocouples, an ultrasonic flowmeter, a hot-wire anemometer, a Testo 645 humidity meter, and differential pressure meters, whose details are listed in Table 1. T-type thermocouples were precalibrated and verified before the tests, so as to accurately measure the water temperature along the wet channels. The Testo 645 humidity meter measured the air temperature and relative humidity along the dry channels and at the outlet of the wet channels. Parameters such as the water flow velocity, the inlet air velocity, and pressure drops were also measured. The details of the experimental conditions are listed in Table 2, allowing for the examination of the impact of the structural and operational parameters on the performance of the MCT and the resistance characteristics of the novel fill. The spray water rate was here defined as the ratio of the mass flow of water to the total cross-sectional area of the fills, while the air velocity in the wet channels was defined as the ratio of the mass flow of air to the total cross-sectional area of the wet channels.

Impact of Structural Parameters on the Performance of the MCT
The impact of structural parameters on the performance of the MCT is investigated in the following section. Results showed that the length of the dry channels (l dry ) affected the number of heat transfer units of primary air (NTU 1 ) by changing the pre-cooling process in dry channels. Figure 5a,b show the primary air and water temperature profiles along the wet channels. It can be seen that the MCT achieved the effective pre-cooling of the primary air in the dry channels (even if the inlet water temperature was much higher than the inlet air temperature, as shown in Figure 5b), and finally reduced the water temperature below the wet-bulb temperature of the primary air. The theoretical limit of TWCTs is the wet-bulb temperature of the inlet air. Due to this fact, reducing the inlet air's wet-bulb temperature in the wet channels (t 1WBo = t 2WBi ) can lower the theoretical limit of the MCT. Pre-cooling of the primary air without changing its humidity ratio allows for the reduction in its wet-bulb temperature; hence, the MCT can finally reduce the water temperature below the wet-bulb temperature of the primary air (t 1WBi ). As shown in Figure 5a, the t 1WBi was 28.3 • C, and the wet-bulb temperature after pre-cooling in dry channels (t 1WBo = t 2WBi ) was 25.4 • C, which was about 3 • C lower than t 1WBi . The final outlet water temperature was 27.1 • C, which was~1.2 • C lower than t 1WBi . It can be stated that MCTs can overcome the theoretical limit of TWCTs, and have significant potential for application. As shown in Figure 6a,c, NTU1 increased with of the decrease in the air temperature at the outlets of the dry channels (t1o) and decreased with the increase in t1o, meaning that the NTU1 obtained via the calculation method proposed in this paper can well represent the pre-cooling process in dry channels. Trends in Figure 6a,c also show that the length of the dry channels (ldry) affected NTU1 more significantly than the temperature of the inlet water and the inlet air. NTU1 increased from ~1.0 to 1.5 with the increase in ldry from 1.5 to 2.4 m, but decreased from ~1.5 to 1.45 with the increase in ldry from 2.4 to 2.7 m. Compared with TWCTs, the fills of the MCT had a smaller heat transfer surface where the air made direct contact with the water, because there were dry channels for pre-cooling of the primary air. The outlet water temperature of the MCT seemed to be higher than that of TWCTs; however, the results showed that the MCT obtained a lower outlet water temperature. This was caused by the fact that the MCT provided working air with a lower wet-bulb temperature for the wet channels, via the effective pre-cooling of the primary air in the dry channels. Therefore, NTU1 initially increased with the increase in the heat transfer surface (length) of the dry channels. When the dry channels were too long, the primary air was heated at the inlets to the dry channels, which was detrimental to the pre-cooling of the primary air (as shown in Figure 5b). Due to this fact, excessively long dry channels resulted in a decrease in NTU1. This indicates that finding the position where the water temperature was equal to the inlet air temperature was necessary in order to determine the optimal value of ldry for MCTs.
As shown in Figure 6b,d, the dew-point effectiveness of the primary air (εDP1) decreased with the increase in the temperature of both the inlet water (twi) and the inlet air (t1i) in the area of high NTU1, but trends of εDP1 changing with twi and t1i were different in the area of low NTU1. It can be seen that εDP1 decreased with the increase in twi, but increased with that of t1i in the area of low NTU1. This was also caused by heating of the primary air at the inlets to the dry channels. When twi was higher than t1i, increasing twi enhanced the heating of the primary air, but increasing t1i weakened this process. With the increase in NTU1, the MCT overcame the heating effect and achieved effective pre-cooling of the primary air. This indicates that a change in NTU1 of 0.5 resulted in ~30% higher εDP1. Trends in Figure 6e,f also indicate that the outlet water temperature decreased and the wet-bulb effectiveness increased with increasing NTU1. A change of 0.5 in NTU1 resulted in the wet-bulb effectiveness (εWB) being ~10% higher. It can be stated that MCTs should maximize NTU1 within the optimal value of pressure drops. It can be also observed that the key to achieving higher effectiveness of the M-Cycle is to enhance the pre-cooling of the primary air in the dry channels. According to Equation (3), one of the most effective methods to enhance the pre-cooling is to increase the number of heat transfer units of the primary air (NTU 1 ). How to calculate NTU 1 was the first problem to be solved. As shown in Figure 5b, water heated both the primary and working air at the upper part of the wet channels. Thus, the temperature of the primary air first increased until it was equal to the water temperature; after this point, the water evaporated effectively due to contact with the low-temperature and unsaturated working air. Hence, the temperature of the primary air continuously decreased along the remaining dry channels. Due to the complex heat and mass transfer process, the logarithmic mean temperature difference (LMTD) method could not be used to directly calculate NTU 1 . Hence, a calculation method for NTU 1 was proposed in this paper.
The equation for the heat transfer of primary air: The equation for the maximum heat transfer of primary air: The equation for the dew-point effectiveness of primary air: The equation for the wet-bulb effectiveness of the MCT: The equation for the heat transfer of the primary air can also be expressed as: Due to the fact that ∆t m could not be calculated by using the temperature difference between the water and the primary air at the inlets and outlets of the dry channels-which was based on the LMTD method-the heat transfer process of the MCT was regarded as two separate parts for the purposes of this paper: first, evaporation transferred heat from the water to the working air, and then the water cooled the primary air. Hence, the auxiliary outlet water temperature (t wo ) was calculated by using the parameters of the working air and water, and then ∆t m was calculated by the auxiliary line, as shown in Figure 5a,b.
Equations for calculating NTU 1 : As shown in Figure 6a,c, NTU 1 increased with of the decrease in the air temperature at the outlets of the dry channels (t 1o ) and decreased with the increase in t 1o , meaning that the NTU 1 obtained via the calculation method proposed in this paper can well represent the pre-cooling process in dry channels. Trends in Figure 6a,c also show that the length of the dry channels (l dry ) affected NTU 1 more significantly than the temperature of the inlet water and the inlet air. NTU 1 increased from~1.0 to 1.5 with the increase in l dry from 1.5 to 2.4 m, but decreased from~1.5 to 1.45 with the increase in l dry from 2.4 to 2.7 m. Compared with TWCTs, the fills of the MCT had a smaller heat transfer surface where the air made direct contact with the water, because there were dry channels for pre-cooling of the primary air. The outlet water temperature of the MCT seemed to be higher than that of TWCTs; however, the results showed that the MCT obtained a lower outlet water temperature. This was caused by the fact that the MCT provided working air with a lower wet-bulb temperature for the wet channels, via the effective pre-cooling of the primary air in the dry channels. Therefore, NTU 1 initially increased with the increase in the heat transfer surface (length) of the dry channels. When the dry channels were too long, the primary air was heated at the inlets to the dry channels, which was detrimental to the pre-cooling of the primary air (as shown in Figure 5b). Due to this fact, excessively long dry channels resulted in a decrease in NTU 1 . This indicates that finding the position where the water temperature was equal to the inlet air temperature was necessary in order to determine the optimal value of l dry for MCTs.
As shown in Figure 6b,d, the dew-point effectiveness of the primary air (ε DP1 ) decreased with the increase in the temperature of both the inlet water (t wi ) and the inlet air (t 1i ) in the area of high NTU 1 , but trends of ε DP1 changing with t wi and t 1i were different in the area of low NTU 1 . It can be seen that ε DP1 decreased with the increase in t wi , but increased with that of t 1i in the area of low NTU 1 . This was also caused by heating of the primary air at the inlets to the dry channels. When t wi was higher than t 1i , increasing t wi enhanced the heating of the primary air, but increasing t 1i weakened this process. With the increase in NTU 1 , the MCT overcame the heating effect and achieved effective pre-cooling of the primary air. This indicates that a change in NTU 1 of 0.5 resulted in~30% higher ε DP1 . Trends in Figure 6e,f also indicate that the outlet water temperature decreased and the wet-bulb effectiveness increased with increasing NTU 1 . A change of 0.5 in NTU 1 resulted in the wet-bulb effectiveness (ε WB ) being~10% higher. It can be stated that MCTs should maximize NTU 1 within the optimal value of pressure drops.

Impact of Operating Parameters on the Performance of the MCT
The impact of operating parameters on the performance of the MCT is investigated in the following section. Results showed that the εWB of MCTs can reach the maximum value of 180%, which is not achievable by TWCTs. Even under conditions of t1i ≥ 35 °C and x1i ≥ 16 g/kg, the εWB of MCTs can still be higher than 100%, indicating that MCTs are still applicable under very high inlet air temperatures and humidity ratios. Figure 7a,b show the impact of the inlet air temperature (t1i) on the performance of the MCT. It can be seen that higher t1i led to a higher outlet water temperature (two), but the change in t1i scarcely affected two. Changing t1i from 30 to 40 °C resulted in two changing by ~2.2 °C. Because increasing the temperature of the inlet air without changing its humidity ratio led to an increase in water evaporation in the wet channels, the value of two was reduced to a lower level in order to achieve the effective pre-cooling of the primary air. The wet-bulb temperature of the air at the outlets of the dry channels, which was the theoretical limit of a TMCT, rarely changed under various conditions. Due to this

Impact of Operating Parameters on the Performance of the MCT
The impact of operating parameters on the performance of the MCT is investigated in the following section. Results showed that the ε WB of MCTs can reach the maximum value of 180%, which is not achievable by TWCTs. Even under conditions of t 1i ≥ 35 • C and x 1i ≥ 16 g/kg, the ε WB of MCTs can still be higher than 100%, indicating that MCTs are still applicable under very high inlet air temperatures and humidity ratios. Figure 7a,b show the impact of the inlet air temperature (t 1i ) on the performance of the MCT. It can be seen that higher t 1i led to a higher outlet water temperature (t wo ), but the change in t 1i scarcely affected t wo . Changing t 1i from 30 to 40 • C resulted in t wo changing by~2.2 • C. Because increasing the temperature of the inlet air without changing its humidity ratio led to an increase in water evaporation in the wet channels, the value of t wo was reduced to a lower level in order to achieve the effective pre-cooling of the primary air. The wet-bulb temperature of the air at the outlets of the dry channels, which was the theoretical limit of a TMCT, rarely changed under various conditions. Due to this fact, t wo showed no significant change. This was also why ε WB increased rather than decreasing with the increase in the value of t 1i . Effective pre-cooling of the primary air made the wet-bulb temperature of the air at the outlets of the dry channels much lower than that at the inlets to the dry channels. Hence, ε WB increased from~104% to 109% with t 1i changing from 30 to 40 • C, as shown in Figure 6b. Therefore, it can be stated that MCTs have the potential to be used in hot climates, where TWCTs often show lower efficiency. fact, two showed no significant change. This was also why εWB increased rather than decreasing with the increase in the value of t1i. Effective pre-cooling of the primary air made the wet-bulb temperature of the air at the outlets of the dry channels much lower than that at the inlets to the dry channels. Hence, εWB increased from ~104% to 109% with t1i changing from 30 to 40 °C, as shown in Figure 6b. Therefore, it can be stated that MCTs have the potential to be used in hot climates, where TWCTs often show lower efficiency.  Figure 8a,b show the impact of the inlet air humidity ratio (x1i) on the performance of the MCT. Changing x1i from 16 to 20 g/kg resulted in two increasing by ~2.4 °C. The trends indicate that the change in two with x1i changing by 4 g/kg was twofold greater than that with t1i changing by 5 °C, meaning that x1i affected two more significantly than t1i, even under a very high humidity ratio. Unlike the impact of increasing t1i on the pre-cooling process in the dry channels, increasing the humidity ratio of the inlet air without changing its dry-bulb temperature resulted in an increase in the dew-point temperature and a decrease in the ability to absorb water vapor. Hence, higher x1i led to higher two by increasing the theoretical limit of the MCT. However, changing x1i scarcely affected εWB. Figure 6d shows that εWB changed by less than 2% when x1i increased from 16 to 20 g/kg. This was due to the fact that both the inlet wet-bulb temperature (t1WBi) and two increased in proportion with the increase in x1i. Therefore, it can be stated that MCTs could still retain high εWB even in very wet climates.    Figure 8a,b show the impact of the inlet air humidity ratio (x 1i ) on the performance of the MCT. Changing x 1i from 16 to 20 g/kg resulted in t wo increasing by~2.4 • C. The trends indicate that the change in t wo with x 1i changing by 4 g/kg was twofold greater than that with t 1i changing by 5 • C, meaning that x 1i affected t wo more significantly than t 1i , even under a very high humidity ratio. Unlike the impact of increasing t 1i on the precooling process in the dry channels, increasing the humidity ratio of the inlet air without changing its dry-bulb temperature resulted in an increase in the dew-point temperature and a decrease in the ability to absorb water vapor. Hence, higher x 1i led to higher t wo by increasing the theoretical limit of the MCT. However, changing x 1i scarcely affected ε WB . Figure 6d shows that ε WB changed by less than 2% when x 1i increased from 16 to 20 g/kg. This was due to the fact that both the inlet wet-bulb temperature (t 1WBi ) and t wo increased in proportion with the increase in x 1i . Therefore, it can be stated that MCTs could still retain high ε WB even in very wet climates. fact, two showed no significant change. This was also why εWB increased rather than decreasing with the increase in the value of t1i. Effective pre-cooling of the primary air made the wet-bulb temperature of the air at the outlets of the dry channels much lower than that at the inlets to the dry channels. Hence, εWB increased from ~104% to 109% with t1i changing from 30 to 40 °C, as shown in Figure 6b. Therefore, it can be stated that MCTs have the potential to be used in hot climates, where TWCTs often show lower efficiency.
(a) (b) Figure 7. Impact of the inlet air temperature on the performance of the MCT: (a) t1i vs. two; (b) t1i vs εWB. Figure 8a,b show the impact of the inlet air humidity ratio (x1i) on the performance of the MCT. Changing x1i from 16 to 20 g/kg resulted in two increasing by ~2.4 °C. The trends indicate that the change in two with x1i changing by 4 g/kg was twofold greater than that with t1i changing by 5 °C, meaning that x1i affected two more significantly than t1i, even under a very high humidity ratio. Unlike the impact of increasing t1i on the pre-cooling process in the dry channels, increasing the humidity ratio of the inlet air without changing its dry-bulb temperature resulted in an increase in the dew-point temperature and a decrease in the ability to absorb water vapor. Hence, higher x1i led to higher two by increasing the theoretical limit of the MCT. However, changing x1i scarcely affected εWB. Figure 6d shows that εWB changed by less than 2% when x1i increased from 16 to 20 g/kg. This was due to the fact that both the inlet wet-bulb temperature (t1WBi) and two increased in proportion with the increase in x1i. Therefore, it can be stated that MCTs could still retain high εWB even in very wet climates.   Figure 9a,b show the impact of the inlet water temperature (t wi ) on the performance of the MCT. It can be seen that increasing t wi led to lower efficiency of the MCT. Similar to increasing t 1i , t wo increased with the increase in t wi , but showed no significant change. Changing t wi from 30 to 50 • C resulted in t wo increasing by~0.6 • C. This was also caused by effective pre-cooling, which reduced the impact of increasing t wi . Unlike increasing t 1i , ε WB decreased with the increase in t wi . The maximum ε WB was~180% when t wi was 30 • C, and sharply reduced to~105% when t wi was 40 • C. However, t wi scarcely affected ε WB when t wi was higher than 40 • C. Due to the fact that the temperature difference between t wi and t 1WBo was too small under conditions of low temperature and high humidity ratio, the small drop in the water temperature (by~5.2 • C, Figure 6e) still led to a large ε WB . With the increase in t wi , the temperature difference between t wi and t 1WBo -and that between t wi and t wo -increased in proportion, due to the fact that t wi had no significant effect on t wo . Therefore, the impact of increasing t wi on ε WB was not significant. increasing t1i, two increased with the increase in twi, but showed no significant change.
Changing twi from 30 to 50 °C resulted in two increasing by ~0.6 °C. This was also caused by effective pre-cooling, which reduced the impact of increasing twi. Unlike increasing t1i, εWB decreased with the increase in twi. The maximum εWB was ~180% when twi was 30 °C, and sharply reduced to ~105% when twi was 40 °C. However, twi scarcely affected εWB when twi was higher than 40 °C. Due to the fact that the temperature difference between twi and t1WBo was too small under conditions of low temperature and high humidity ratio, the small drop in the water temperature (by ~5.2 °C, Figure 6e) still led to a large εWB.
With the increase in twi, the temperature difference between twi and t1WBo-and that between twi and two-increased in proportion, due to the fact that twi had no significant effect on two. Therefore, the impact of increasing twi on εWB was not significant.
(a) (b) Figure 9. Impact of the inlet water temperature on the performance of the MCT: (a) twi vs. two; (b) twi vs. εWB. Figure 10a,b show the impact of the heat capacity ratio between the air and the water (W1/Ww) on the performance of the MCT. It can be seen that W1/Ww significantly affected the efficiency of the MCT. With W1/Ww changing from 0.1 to 0.4, two decreased by approximately 7.7 and 9.1 °C when t1i was equal to 40 and 30 °C, respectively, and εWB increased from ~41% to 109%. This was also connected with the pre-cooling process in the dry channels. When the heat capacity of the air was much lower than that of the water (W1/Ww = 0.1), the evaporation of water in the wet channels dramatically decreased, and the primary air was heated for a longer distance in the dry channels. With the increase in W1/Ww (W1/Ww =0.4), the evaporation of water increased, and the MCT overcame the heating effect and achieved effective pre-cooling of the primary air, which resulted in high efficiency of the MCT. It can be stated that in order to ensure that MCTs have high wet-bulb efficiency (εWB ≥ 0.9), W1/Ww should be maintained at a high value (W1/Ww ≥ 0.3).
(a) (b) Figure 9. Impact of the inlet water temperature on the performance of the MCT: (a) t wi vs. t wo ; (b) t wi vs. ε WB . Figure 10a,b show the impact of the heat capacity ratio between the air and the water (W 1 /W w ) on the performance of the MCT. It can be seen that W 1 /W w significantly affected the efficiency of the MCT. With W 1 /W w changing from 0.1 to 0.4, t wo decreased by approximately 7.7 and 9.1 • C when t 1i was equal to 40 and 30 • C, respectively, and ε WB increased from~41% to 109%. This was also connected with the pre-cooling process in the dry channels. When the heat capacity of the air was much lower than that of the water (W 1 /W w = 0.1), the evaporation of water in the wet channels dramatically decreased, and the primary air was heated for a longer distance in the dry channels. With the increase in W 1 /W w (W 1 /W w =0.4), the evaporation of water increased, and the MCT overcame the heating effect and achieved effective pre-cooling of the primary air, which resulted in high efficiency of the MCT. It can be stated that in order to ensure that MCTs have high wet-bulb efficiency (ε WB ≥ 0.9), W 1 /W w should be maintained at a high value (W 1 /W w ≥ 0.3).

Resistance Characteristics of the Novel Fill
With the aim of applying the M-cycle in TWCTs, the fill of MCTs must have adjacent dry and wet channels. Through the special channel arrangement, the air flows through the dry channels and then enters the wet channels, meaning that the pressure drops can be very high. Hence, it is necessary to study the resistance characteristics of the novel fill. Figure 11 shows the variation in pressure drops (∆p) with different air velocities in the wet channels (v 2 ). dry channels. When the heat capacity of the air was much lower than that of the water (W1/Ww = 0.1), the evaporation of water in the wet channels dramatically decreased, and the primary air was heated for a longer distance in the dry channels. With the increase in W1/Ww (W1/Ww =0.4), the evaporation of water increased, and the MCT overcame the heating effect and achieved effective pre-cooling of the primary air, which resulted in high efficiency of the MCT. It can be stated that in order to ensure that MCTs have high wet-bulb efficiency (εWB ≥ 0.9), W1/Ww should be maintained at a high value (W1/Ww ≥ 0.3).
(a) (b) Figure 10. Impact of the heat capacity ratio between air and water on the performance of the MCT: (a) w 1 /w w vs. t wo ; (b) w 1 /w w vs. ε WB .
With the aim of applying the M-cycle in TWCTs, the fill of MCTs must have adjacent dry and wet channels. Through the special channel arrangement, the air flows through the dry channels and then enters the wet channels, meaning that the pressure drops can be very high. Hence, it is necessary to study the resistance characteristics of the novel fill. Figure 11 shows the variation in pressure drops (Δp) with different air velocities in the wet channels (v2).
As shown in Figure 11a, when v2 was equal to 4.62 m/s, Δp increased by ~16.74% with the increase in ldry from 0 to 1.5 m, but increased by ~7.65% with the increase in ldry from 1.5 to 2.7 m. Because the MCT had dry and wet channels, the air changed direction at least twice in the novel fills. Due to this fact, Δp increased dramatically compared with the traditional fills. Therefore, the novel fill should be designed to minimize the angle of the change in the air's direction.
As shown in Figure 7b, the fitting equation for Δp vs. v2 is expressed as: where: This indicates that v2 affected Δp more significantly than changing ldry according to the fitting equation; it also provides a guideline for the optimization of resistance characteristics in MCTs. As shown in Figure 11a, when v 2 was equal to 4.62 m/s, ∆p increased by~16.74% with the increase in l dry from 0 to 1.5 m, but increased by~7.65% with the increase in l dry from 1.5 to 2.7 m. Because the MCT had dry and wet channels, the air changed direction at least twice in the novel fills. Due to this fact, ∆p increased dramatically compared with the traditional fills. Therefore, the novel fill should be designed to minimize the angle of the change in the air's direction.
As shown in Figure 7b where: A = −0.122l 2 dry + 1.341l dry + 11.309 n = −3.446 × 10 −4 l 2 dry + 2.8 × 10 −3 l dry + 1.974 This indicates that v 2 affected ∆p more significantly than changing l dry according to the fitting equation; it also provides a guideline for the optimization of resistance characteristics in MCTs.

Conclusions
An experimental system for MCTs was constructed, and detailed theoretical analysis and experimental research were carried out. A calculation method of NTU 1 was proposed, and the impact of structural (l dry and NTU 1 ) and operational parameters (t w , t 1 , x 1i , and W 1 /W w ) on the performance of MCTs was investigated. Resistance characteristics of the novel fill were also studied. The main conclusions can be summarized as follows: • The water was cooled below the inlet air's wet-bulb temperature the MCT, which is not achievable by TWCTs; • NTU 1 increased from~1.0 to 1.5 with an increase in l dry from 1.5 to 2.4 m, but decreased from~1.5 to 1.45 with an increase in l dry from 2.4 to 2.7 m. Therefore, it is important to ensure the optimal value of l dry for MCTs; • The wet-bulb effectiveness increased by~10% with an increase in NTU 1 of~0.5, indicating that MCTs should maximize NTU 1 within the optimal value of pressure drops; • The inlet air's humidity ratio affected wet-bulb effectiveness more significantly than its temperature; however, the MCT still obtained wet-bulb effectiveness of up tõ 180%, even under conditions of very high inlet air temperature and humidity ratio (t 1i ≥ 35 • C, x 1i ≥ 16 g/kg); • In order to ensure that MCTs have high wet-bulb efficiency (ε WB ≥ 0.9), W 1 /W w should be maintained at a high value (W 1 /W w ≥ 0.3); • The results of this study have significance for the guidance of the industrial application and performance improvement of MCTs.
Author Contributions: Conceptualization, X.L. and X.F.; investigation, X.F., X.L. and Q.W.; methodology, X.L. and X.F.; writing-original draft preparation, X.F.; writing-review and editing, Z.D. and R.Z.; validation, J.W. and Z.L. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data that support the findings of this study are available from the corresponding author upon reasonable request.