Experimental Study of the Air Side Performance of Fin-and-Tube Heat Exchanger with Different Fin Material in Dehumidifying Conditions

the Air Side Performance of Fin-and-Tube Heat Exchanger with Different Fin Abstract: Under dehumidifying conditions, the condensed water will directly affect the heat transfer and resistance characteristics of a ﬁn-and-tube heat exchanger. The geometrical form of condensed water on ﬁn surfaces of three different ﬁn materials (i.e., copper ﬁn, aluminum ﬁn, and aluminum ﬁn with hydrophilic layer) in a ﬁn-and-circular-tube heat exchanger was experimentally studied in this paper. The effect of the three different ﬁn materials on heat transfer and friction performance of the heat exchanger was researched, too. The results show that the condensation state on the surface of copper ﬁn and aluminum ﬁn are dropwise condensation. The condensation state on the surface of the aluminum ﬁn with the hydrophilic layer is ﬁlm condensation. For the three different material ﬁns, increasing the air velocity ( u a,in ) and relative humidity ( RH in ) of the inlet air can enhance the heat transfer of the heat exchanger. Friction factor ( f ) of the three different material ﬁns decreases with the increase of u a,in , however, increases with the increase of RH in . At the same u a,in or RH in , Nusselt number ( Nu ) of the copper ﬁn heat exchanger is the largest and Nu of the aluminum ﬁn with hydrophilic layer is the smallest, f of the aluminum ﬁn heat exchanger is the largest and f of the aluminum ﬁn with hydrophilic layer is the smallest. Under the identical pumping power constrain, the comprehensive heat transfer performance of the copper ﬁn heat exchanger is the best for the studied cases.


Introduction
Fin-and-tube heat exchanger (FTHE) is a common structural form of a heat exchanger. The heat transfer between the cold and hot fluid is carried out, the refrigerant is in the tube, and the air is outside the tube. This type of heat exchanger has the characteristics of simple structure, easy processing, and assembly and is widely used in the petrochemical industry, aviation, vehicle, power machinery, air conditioning, refrigeration, etc. [1][2][3][4]. When the fin surface temperature of the FTHE is lower than the dew point temperature of the airflow through the heat exchanger, the moisture in the air will be condensed on the fin surface. Thus, heat and mass transfer occur simultaneously during the process of dehumidification [5].
The condensed water will not only directly affect the characteristics of heat transfer and pressure drop of the FTHE but also provide a humid environment for the growth of bacteria and microorganisms, which will cause corrosion to the heat exchanger and bring a series of health problems. Therefore, it is necessary to study the heat transfer and resistance performances of the FTHE under the conditions of dehumidification, as well as the condensation and movement characteristics of the condensed water.
The variable speed centrifugal fan can provide the circulating air for the experimental test system. The electrode humidifier was used to control the relative humidity of the inlet air. The temperature of the inlet air was monitored using a dry-bulb thermometer and the air was heated by an electric heater. The nozzles chamber based on ASHRAE41.2 standard [23] was used to measure the airflow rate. Two pressure difference transducers with a ±3.0 Pa precision were used to measure the pressure differences across the nozzles and the heat exchangers, respectively. The grading ring, which connects the four static pressure holes of the same section through an organic plastic tube and plays a role in stabilizing the pressure of the test section, was connected with the corresponding pressure difference transducer. The precisions of the dry and wet bulb temperature transducer and humidity transducer were ±0.2 • C and ±2%, respectively. under the given conditions of relative humidity, inlet temperature, and inlet air velocity. The moist air handling system comprises 7 parts: variable speed centrifugal fan, flow equalization board, grading ring, mixer, nozzles, electrode humidifier, and electric heater. The variable speed centrifugal fan can provide the circulating air for the experimental test system. The electrode humidifier was used to control the relative humidity of the inlet air. The temperature of the inlet air was monitored using a dry-bulb thermometer and the air was heated by an electric heater. The nozzles chamber based on ASHRAE41.2 standard [23] was used to measure the airflow rate. Two pressure difference transducers with a ±3.0 Pa precision were used to measure the pressure differences across the nozzles and the heat exchangers, respectively. The grading ring, which connects the four static pressure holes of the same section through an organic plastic tube and plays a role in stabilizing the pressure of the test section, was connected with the corresponding pressure difference transducer. The precisions of the dry and wet bulb temperature transducer and humidity transducer were ±0.2 °C and ±2%, respectively. The test section was the core of the whole experimental system. In order to ensure the uniformity and stability of the airflow, a flow equalization board, a grading ring, and a mixer were, respectively, installed at the inlet and outlet of the test section. The test heat exchangers were made of copper tubes and 3 fin materials (copper fin, aluminum fin, and aluminum fin with hydrophilic layer). The photos of test specimens with 3 different fin materials are given in Figure 2. Their detailed configurations are tabulated in Table 1.  The test section was the core of the whole experimental system. In order to ensure the uniformity and stability of the airflow, a flow equalization board, a grading ring, and a mixer were, respectively, installed at the inlet and outlet of the test section. The test heat exchangers were made of copper tubes and 3 fin materials (copper fin, aluminum fin, and aluminum fin with hydrophilic layer). The photos of test specimens with 3 different fin materials are given in Figure 2. Their detailed configurations are tabulated in Table 1.  The visual data acquisition system is the brain of the whole experimental test system, collecting all kinds of experimental data. In order to observe the geometry, condensation position, formation, growth, and movement characteristics of condensation water on the fin surface, 2 cameras were, respectively, placed on the side and top of the  The visual data acquisition system is the brain of the whole experimental test system, collecting all kinds of experimental data. In order to observe the geometry, condensation position, formation, growth, and movement characteristics of condensation water on the fin surface, 2 cameras were, respectively, placed on the side and top of the heat exchanger, as shown in Figure 3. Two T-type thermocouples were used to measure the water temperature difference between the inlet and outlet of the heat exchanger. A total of 9 T-type thermocouples were arranged on the fin surface of the heat exchanger to measure the fin surface temperature. All the thermocouples were calibrated with an accuracy of ±0.2 • C.
The low temperature circulating water system was used to control the inlet temperature of circulating water into the test heat exchangers. The visual data acquisition system is the brain of the whole experimental test system, collecting all kinds of experimental data. In order to observe the geometry, condensation position, formation, growth, and movement characteristics of condensation water on the fin surface, 2 cameras were, respectively, placed on the side and top of the heat exchanger, as shown in Figure 3. Two T-type thermocouples were used to measure the water temperature difference between the inlet and outlet of the heat exchanger. A total of 9 T-type thermocouples were arranged on the fin surface of the heat exchanger to measure the fin surface temperature. All the thermocouples were calibrated with an accuracy of ±0.2 °C.
The low temperature circulating water system was used to control the inlet temperature of circulating water into the test heat exchangers.

Data Processing Method
The data processing method used in this paper was the Threlkeld method [24] and ASHRAE41.2 standard [23].
Heat transfer of air side: a a a,in a,out Heat transfer of water side: w

Data Processing Method
The data processing method used in this paper was the Threlkeld method [24] and ASHRAE41.2 standard [23].
Heat transfer of air side : Q a = m a (i a,in − i a,out ) Heat transfer of water side : The average heat transfer rate : The overall surface efficiency : where A tc is the outside surface area of tubes, A af is the fin surface area, η f,wet is the wet fin efficiency obtained according to the method from Liang et al. [25], A 0 is the total air side surface area. Thus, the air side sensible heat transfer coefficient is The overall average Nu is defined by : Nu = hsde/λ The equivalent diameter of the air side is defined by : d e = 4A min L x /A 0 where d e is the equivalent diameter of the air side, A min is the minimum free flow area of the heat exchanger, L x is the fin length along the air flow direction. The friction factor of the air side, according to Kays and London [26], is defined as: where σ = A min /A fr , G = m a /A min , A fr is the frontal area of the heat exchanger, G is the mass flow of air at the minimum free flow area. Under identical pumping power constrain, JF (i.e., thermal performance factor) is used to evaluate comprehensive heat transfer performance of the three different fin material cases, which is defined as:   Figure 4. It can be seen that with the start-up of the fan, the air flows through the heat exchanger, the vapor in the air gradually condenses into tiny droplets on the fin surface near the circular tubes (as shown in Figure 4a,b). With the increase of ventilation time, the size and quantity of condensed droplets increase gradually (as shown in Figure 4c). Moreover, with the increase of the volume of condensation droplets, some adjacent droplets gradually merge into large droplets (as shown in Figure 4d). With the continuous condensation, the volume of the condensed droplets increases further. If the adhesion force of the condensate droplet to the fins is larger than the resultant force of the drag force produced by the airflow and the gravity of the condensate droplet, the condensed droplet will still adhere to the fin surface perfectly and gradually form a flat hemispherical shape. When the adhesion force of the condensate droplet to the fins is smaller than the resultant force of the drag force produced by the airflow and the gravity of the condensate droplet, the condensate droplet will slide on the fin surface. In the process of sliding, the condensate will merge with some other condensate droplets and discharge from the fin together (as shown in Figure 4e-h). The condensate water will continue to condense on the fin surface, then merge and discharge. of the condensate droplet, the condensate droplet will slide on the fin surface. In the process of sliding, the condensate will merge with some other condensate droplets and discharge from the fin together (as shown in Figure 4e-h). The condensate water will continue to condense on the fin surface, then merge and discharge. The geometry form and movement characteristics of condensate droplets on the aluminum fin surfaces are shown in Figure 5. Comparing Figure 4 with Figure 5, we can see that the condensation process of condensate droplets on the copper fin surface and aluminum fin surface is similar. They are all dropwise condensation, and they all experience the process of nucleation, growth, coalescence, and discharge from the fin surface. The difference is that the size of condensate droplets on the copper fin surface is generally larger than that on the aluminum fin surface. Figure 6 shows the geometry form and movement characteristics of condensed water on the surface of the aluminum fin with a hydrophilic layer. It can be seen from Figure  6a-c that with the air flowing through the heat exchanger, the condensate water on the surface of the aluminum fin with hydrophilic layer firstly appears near the circular tubes and presents film condensation. With the increase of ventilation time, the size of tiny condensate film gradually increases and merges with the adjacent condensate film (as shown in Figure 6d,e). Moreover, the thickness of condensate film increases gradually. When the resultant force of the drag force produced by the airflow and the gravity of the condensate film is larger than the adhesion force of the condensate film to the fins, the condensate film will slide down the fin surface. In the process of sliding down, it will gradually merge with other condensation films and discharge the fin together (as shown in Figure 6f-h). Then, the next cycle of the condensation process will be carried out.   of the condensate droplet, the condensate droplet will slide on the fin surface. In the process of sliding, the condensate will merge with some other condensate droplets and discharge from the fin together (as shown in Figure 4e-h). The condensate water will continue to condense on the fin surface, then merge and discharge. The geometry form and movement characteristics of condensate droplets on the aluminum fin surfaces are shown in Figure 5. Comparing Figure 4 with Figure 5, we can see that the condensation process of condensate droplets on the copper fin surface and aluminum fin surface is similar. They are all dropwise condensation, and they all experience the process of nucleation, growth, coalescence, and discharge from the fin surface. The difference is that the size of condensate droplets on the copper fin surface is generally larger than that on the aluminum fin surface. Figure 6 shows the geometry form and movement characteristics of condensed water on the surface of the aluminum fin with a hydrophilic layer. It can be seen from Figure  6a-c that with the air flowing through the heat exchanger, the condensate water on the surface of the aluminum fin with hydrophilic layer firstly appears near the circular tubes and presents film condensation. With the increase of ventilation time, the size of tiny condensate film gradually increases and merges with the adjacent condensate film (as shown in Figure 6d,e). Moreover, the thickness of condensate film increases gradually. When the resultant force of the drag force produced by the airflow and the gravity of the condensate film is larger than the adhesion force of the condensate film to the fins, the condensate film will slide down the fin surface. In the process of sliding down, it will gradually merge with other condensation films and discharge the fin together (as shown in Figure 6f-h). Then, the next cycle of the condensation process will be carried out.      The geometry form and movement characteristics of condensate droplets on the aluminum fin surfaces are shown in Figure 5. Comparing Figure 4 with Figure 5, we can see that the condensation process of condensate droplets on the copper fin surface and aluminum fin surface is similar. They are all dropwise condensation, and they all experience the process of nucleation, growth, coalescence, and discharge from the fin surface. The difference is that the size of condensate droplets on the copper fin surface is generally larger than that on the aluminum fin surface. Figure 6 shows the geometry form and movement characteristics of condensed water on the surface of the aluminum fin with a hydrophilic layer. It can be seen from Figure 6a-c that with the air flowing through the heat exchanger, the condensate water on the surface of the aluminum fin with hydrophilic layer firstly appears near the circular tubes and presents film condensation. With the increase of ventilation time, the size of tiny condensate film gradually increases and merges with the adjacent condensate film (as shown in Figure 6d,e). Moreover, the thickness of condensate film increases gradually. When the resultant force of the drag force produced by the airflow and the gravity of the condensate film is larger than the adhesion force of the condensate film to the fins, the condensate film will slide down the fin surface. In the process of sliding down, it will gradually merge with other condensation films and discharge the fin together (as shown in Figure 6f-h). Then, the next cycle of the condensation process will be carried out. Figure 7 shows the geometry form of condensate water on the fin surfaces of three different materials under different air inlet velocities (u a,in ) at τ = 300 s, RH in = 50%, T a,in = 27 • C, T w,in = 12 • C. It can be found that the condensate droplets on the copper fin surface and aluminum fin surface are dropwise condensation. However, the condensate water on the aluminum fin with the hydrophilic layer is film condensation. In addition, we can see that with increasing of u a,in , the diameter of condensate water distributed on the fin surfaces of three different materials all gradually decrease. Especially when the air inlet velocity reaches 4 m/s, there is almost no large-diameter condensate water on the fin surfaces. This is because, with the increase of u a,in , the drag force acting on the surface of the condensate water increases, the combined force of drag force and gravity will increase, the condensate water with a larger diameter will be quickly discharged from the fin surface.

The Effect of u a,in and RH in on the Geometry Form of Condensate Water on the Fin Surfaces
The effect of air inlet relative humidity (RH in ) on the geometry form of condensate water on the fin surfaces of three different materials at τ = 300 s, u a,in = 2 m/s, T a,in = 27 • C, T w,in = 12 • C is given in Figure 8. It can be seen from Figure 8a,b that with the changing of RH in , the condensation and movement characteristics of the condensation droplets on the surfaces of copper fin and aluminum fin are similar. When RH in = 40%, there are many condensate droplets on the surfaces of copper fin and aluminum fin, and they are evenly distributed on the fin surfaces. When RH in increases from 50% to 70%, the number of condensate droplets on the surfaces of copper fin and aluminum fin all decrease. Moreover, when RH in reaches 80%, there are almost no condensate droplets on the surfaces of copper fin and aluminum fin. This is because when RH in is very large, the condensation rate is very fast, the condensate droplets will be quickly discharged from the fin surfaces. For the aluminum fin with a hydrophilic layer, as shown in Figure 8c, with the increase of RH in , the thickness of condensation film becomes thinner, and the distribution of condensate film on the fin surface is more uniform. This also shows that with the increase of RH in , the condensation rate is accelerated. The thicker condensate film will be quickly discharged from the surface of the aluminum fin with a hydrophilic layer.
surfaces of copper fin and aluminum fin are similar. When RHin = 40%, there are many condensate droplets on the surfaces of copper fin and aluminum fin, and they are evenly distributed on the fin surfaces. When RHin increases from 50% to 70%, the number of condensate droplets on the surfaces of copper fin and aluminum fin all decrease. Moreover, when RHin reaches 80%, there are almost no condensate droplets on the surfaces of copper fin and aluminum fin. This is because when RHin is very large, the condensation rate is very fast, the condensate droplets will be quickly discharged from the fin surfaces. For the aluminum fin with a hydrophilic layer, as shown in Figure 8c, with the increase of RHin, the thickness of condensation film becomes thinner, and the distribution of condensate film on the fin surface is more uniform. This also shows that with the increase of RHin, the condensation rate is accelerated. The thicker condensate film will be quickly discharged from the surface of the aluminum fin with a hydrophilic layer.      T a,in = 27 • C and 20 • C. When u a,in ranges from 0.5 m/s to 4.0 m/s, comparing with the case of T a,in = 20 • C, Nu of T a,in = 27 • C increases about 0.06-0.15 times, however Nu of T a,in = 35 • C increases about 0.30~0.73 times compared with that of T a,in = 27 • C. As shown in Figure 9b, f decreases with the increase of u a,in . At the same u a,in , f of T a,in = 20 • C is the smallest and f increases with increasing T a,in . The differences of f between T a,in = 35 • C and 27 • C is larger than that between T a,in = 27 • C and 20 • C. When u a,in ranges from 0.5 m/s to 4.0 m/s, comparing with the case of T a,in = 20 • C, f of T a,in = 27 • C increases about 0.02-0.05 times, f of T a,in = 35 • C increases about 0.34-0.54 times compared with that of T a,in = 27 • C. This shows that increasing the air inlet temperature will not only improve the heat transfer performance of the heat exchanger but also increase the flow resistance.

The Effect of Ta,in on Nu and f of Heat Exchanger
For the aluminum fin case, as shown in Figure 9c,d and aluminum fin with hydrophilic layer case as shown in Figure 9e,f, the similar trends of the effect of T a,in on Nu and f can be observed. Just the differences of Nu and f of the aluminum fin with a hydrophilic layer between T a,in = 27 • C and 20 • C is not obvious.

The Effect of Tw,in on Nu and f of Heat Exchanger
The effect of water inlet temperature Tw,in on Nu and f of three different fin materials at RHin = 50%, Ta,in = 27 °C and ua,in = 0.5 m/s-4.0 m/s are shown in Figure 10. It can be seen from Figure 10a

The Effect of T w,in on Nu and f of Heat Exchanger
The effect of water inlet temperature T w,in on Nu and f of three different fin materials at RH in = 50%, T a,in = 27 • C and u a,in = 0.5 m/s-4.0 m/s are shown in Figure 10. It can be seen from Figure 10a,b that Nu of the three different T w,in (12 • C, 15 • C, and 18 • C) all increase with increasing u a,in , however, f all decrease with increasing u a,in . Under the same air inlet velocity, Nu of T w,in = 18 • C is the largest, and then gradually decreases with decreasing of T w,in . The differences of Nu between T w,in = 15 • C and 12 • C is larger than that between T w,in = 15 • C and 18 • C. When u a,in ranges from 0.5 m/s to 4.0 m/s, comparing with the case of T w,in = 12 • C, Nu of T w,in = 15 • C increases about 0.16-0.75 times, however Nu of T w,in = 18 • C increases about 0.01-0.1 times compared with that of T w,in = 15 • C. At the same u a,in , f of T w,in = 12 • C is the largest and f decreases with increasing T w,in , however, the differences of the three different T w,in are not obvious. This shows that the heat transfer performance of the copper fin heat exchanger can be improved by increasing the water inlet temperature. The reason is that when the water inlet temperature is low, the quantity of condensate generated on the fin surface will increase; at the same inlet air velocity, the thickness of condensate will increase. The purpose of destroying the airflow boundary layer cannot be achieved thus that the heat transfer capacity is weakened, and the pressure drop is increased.
For the aluminum fin and aluminum fin with hydrophilic layer cases, similar trends of the effect of T w,in on Nu and f are observed in Figure 10c Figure 11 shows the effect of ua,in on Nu and f of the three different fin materi RHin = 50%, Ta,in = 27 °C, Tw,in = 12 °C. It can be found in Figure 11a that Nu of the different fin materials all increase with the increase of ua,in. This is because the boun layer becomes thinner with the increase of ua,in. The discharged speed of condensat ter from the fin surface is accelerated, which further enhances the disturbance o fluid, thus the heat transfer capacity is enhanced. Compared with 0.5 m/s, Nu of co fin, aluminum fin, and aluminum fin with hydrophilic layer increase about 1.99, 1.30  Figure 11 shows the effect of u a,in on Nu and f of the three different fin materials at RH in = 50%, T a,in = 27 • C, T w,in = 12 • C. It can be found in Figure 11a that Nu of the three different fin materials all increase with the increase of u a,in . This is because the boundary layer becomes thinner with the increase of u a,in . The discharged speed of condensate water from the fin surface is accelerated, which further enhances the disturbance of the fluid, thus the heat transfer capacity is enhanced. Compared with 0.5 m/s, Nu of copper fin, aluminum fin, and aluminum fin with hydrophilic layer increase about 1.99, 1.30, and 2.62 times, respectively, at 4.0 m/s. Under the same air inlet velocity, Nu of the copper fins is larger than aluminum fins, and Nu of the aluminum fins is larger than that of aluminum fins with a hydrophilic layer.

The Effect of u a,in on Nu and f of Heat Exchanger
As shown in Figure 11b, f decreases with increasing u a,in for all three fin materials. The reason is that with the increase of u a,in , the velocity of airflow increases, and the increase of kinetic energy of airflow is greater than that of pressure drop, and the condensate water can be discharged from the fin surface faster, which makes the air passage smoother and pressure drop decreases. Compared with 0.5 m/s, f of copper fin, aluminum fin, and aluminum fin with hydrophilic layer decrease about 34.46%, 38.81%, and 50.55%, respectively, at 4.0 m/s. At the same air inlet velocity, f of the aluminum fin is the largest, and f of the aluminum fin with hydrophilic layer is the smallest. This is because the condensation water produced on the fin surface will not gather on the fin surface after the hydrophilic layer is attached to the fin surfaces; the discharged speed of condensate water from the fin surface is accelerated, which will reduce the pressure loss of the air side channel. the condensation water produced on the fin surface will not gather on the fin surface after the hydrophilic layer is attached to the fin surfaces; the discharged speed of condensate water from the fin surface is accelerated, which will reduce the pressure loss of the air side channel.

The Effect of RHin on Nu and f of Heat Exchanger
The effect of RHin on Nu and f of three different fin materials at ua,in= 2 m/s, Ta,in = 27 °C, Tw,in = 12 °C is given in Figure 12. It can be seen from Figure 12a that the Nu of the three different fin materials all increases with the increase of RHin. It shows that increasing RHin can enhance the convective heat transfer intensity of the heat exchanger. This is because with the increase of RHin, the condensation rate on the low-temperature fin surface will accelerate, thus the proportion of latent heat transfer in the heat and mass transfer process will gradually increase. Moreover, with the increase of RHin, the discharge process is accelerated, which increases the disturbance of the air stream, thus the heat transfer performance of heat transfer is enhanced. Compared with RHin = 40%, Nu of the copper fin, aluminum fin, and aluminum fin with hydrophilic layer increased about 0.93, 1.23, and 3.12 times, respectively, at RHin = 80%. Under the same RHin, Nu of the copper fins is higher than that of aluminum fins, and Nu of the aluminum fins is higher than that of aluminum fins with a hydrophilic layer.
As shown in Figure 12b, f increases with the increase of RHin for all three fin materials. The main reason is that with the increase of RHin, the condensation and discharge process is accelerated, which enhances the disturbance of the air stream. Thus, the pressure drop of the air side increases. Compared with RHin = 40%, f of the copper fin, aluminum fin and aluminum fin with hydrophilic layer increases about 1.21, 1.38, and 1.03 respectively at RHin = 80%. At the same RHin, f of the aluminum fin is largest, and f of the aluminum fin with hydrophilic layer is smallest.

The Effect of RH in on Nu and f of Heat Exchanger
The effect of RH in on Nu and f of three different fin materials at u a,in = 2 m/s, T a,in = 27 • C, T w,in = 12 • C is given in Figure 12. It can be seen from Figure 12a that the Nu of the three different fin materials all increases with the increase of RH in . It shows that increasing RH in can enhance the convective heat transfer intensity of the heat exchanger. This is because with the increase of RH in , the condensation rate on the low-temperature fin surface will accelerate, thus the proportion of latent heat transfer in the heat and mass transfer process will gradually increase. Moreover, with the increase of RH in , the discharge process is accelerated, which increases the disturbance of the air stream, thus the heat transfer performance of heat transfer is enhanced. Compared with RH in = 40%, Nu of the copper fin, aluminum fin, and aluminum fin with hydrophilic layer increased about 0.93, 1.23, and 3.12 times, respectively, at RH in = 80%. Under the same RH in , Nu of the copper fins is higher than that of aluminum fins, and Nu of the aluminum fins is higher than that of aluminum fins with a hydrophilic layer.
As shown in Figure 12b, f increases with the increase of RH in for all three fin materials. The main reason is that with the increase of RH in , the condensation and discharge process is accelerated, which enhances the disturbance of the air stream. Thus, the pressure drop of the air side increases. Compared with RH in = 40%, f of the copper fin, aluminum fin and aluminum fin with hydrophilic layer increases about 1.21, 1.38, and 1.03 respectively at RH in = 80%. At the same RH in , f of the aluminum fin is largest, and f of the aluminum fin with hydrophilic layer is smallest.

The Effect of the Fin Material on JF
In order to comprehensively evaluate the heat transfer and resistance characteristics of the three different fin materials cases under dehumidifying conditions, the thermal performance factor JF shown in Equation (9) is used as the evaluation criterion. As given in Equation (9), Nu and f of the aluminum fin are used as reference values. Figure 13 shows the effect of ua,in on JF at RHin = 50%, Ta,in = 27 °C, Tw,in = 12 °C. As shown in Figure 13, the range of JFC,f/Al,f was 1.24-1.53 at ua,in = 0.5 m/s-4 m/s, they were all larger than 1.0. This shows that the comprehensive heat transfer performance of copper-fin-and-circular-tube heat exchanger is better than that of the aluminum fin heat exchanger. Moreover, with the increase of ua,in, the comprehensive heat transfer advantages of the copper fin heat exchanger are more obvious than that of the aluminum fin heat exchanger. However, the range of JFAl,f,h,l/Al,f was 0.48-0.96 at ua,in = 0.5 m/s-4 m/s, they were all smaller than 1.0. That is to say that the comprehensive heat transfer performance of the aluminum fin with hydrophilic layer heat exchanger was worse than that of the aluminum fin heat exchanger. With the increase of ua,in, the difference of comprehensive heat transfer capacity between aluminum fin with hydrophilic layer heat exchanger and aluminum fin heat exchanger becomes smaller.

The Effect of the Fin Material on JF
In order to comprehensively evaluate the heat transfer and resistance characteristics of the three different fin materials cases under dehumidifying conditions, the thermal performance factor JF shown in Equation (9) is used as the evaluation criterion. As given in Equation (9), Nu and f of the aluminum fin are used as reference values. Figure 13 shows the effect of u a,in on JF at RH in = 50%, T a,in = 27 • C, T w,in = 12 • C. As shown in Figure 13, the range of JF C,f/Al,f was 1.24-1.53 at u a,in = 0.5 m/s-4 m/s, they were all larger than 1.0. This shows that the comprehensive heat transfer performance of copper-fin-and-circular-tube heat exchanger is better than that of the aluminum fin heat exchanger. Moreover, with the increase of u a,in , the comprehensive heat transfer advantages of the copper fin heat exchanger are more obvious than that of the aluminum fin heat exchanger. However, the range of JF Al,f,h,l/Al,f was 0.48-0.96 at u a,in = 0.5 m/s-4 m/s, they were all smaller than 1.0. That is to say that the comprehensive heat transfer performance of the aluminum fin with hydrophilic layer heat exchanger was worse than that of the aluminum fin heat exchanger. With the increase of u a,in , the difference of comprehensive heat transfer capacity between aluminum fin with hydrophilic layer heat exchanger and aluminum fin heat exchanger becomes smaller.

The Effect of the Fin Material on JF
In order to comprehensively evaluate the heat transfer and resistance characteristics of the three different fin materials cases under dehumidifying conditions, the thermal performance factor JF shown in Equation (9) is used as the evaluation criterion. As given in Equation (9), Nu and f of the aluminum fin are used as reference values. Figure 13 shows the effect of ua,in on JF at RHin = 50%, Ta,in = 27 °C, Tw,in = 12 °C. As shown in Figure 13, the range of JFC,f/Al,f was 1.24-1.53 at ua,in = 0.5 m/s-4 m/s, they were all larger than 1.0. This shows that the comprehensive heat transfer performance of copper-fin-and-circular-tube heat exchanger is better than that of the aluminum fin heat exchanger. Moreover, with the increase of ua,in, the comprehensive heat transfer advantages of the copper fin heat exchanger are more obvious than that of the aluminum fin heat exchanger. However, the range of JFAl,f,h,l/Al,f was 0.48-0.96 at ua,in = 0.5 m/s-4 m/s, they were all smaller than 1.0. That is to say that the comprehensive heat transfer performance of the aluminum fin with hydrophilic layer heat exchanger was worse than that of the aluminum fin heat exchanger. With the increase of ua,in, the difference of comprehensive heat transfer capacity between aluminum fin with hydrophilic layer heat exchanger and aluminum fin heat exchanger becomes smaller.  The effect of RH in on JF at u a,in = 2 m/s, T a,in = 27 • C, T w,in = 12 • C is given in Figure 14. From Figure 14, we can see that the range of JF C,f/Al,f was 1.14-1.46 when RH in increased from 40% to 80%. JF C,f/Al,f were all larger than 1.0. Moreover, with the increase of RH in , JF C,f/Al,f gradually approached 1.0. The range of JF Al,f,h,l/Al,f was 0.45~0.88 at RH in = 40-80%. JF Al,f,h,l/Al,f was smaller than 1.0. Moreover, with the increase of RH in , JF Al,f,h,l/Al,f increased gradually and tended to be stable. This shows that with increasing of RH in , the differences in comprehensive heat transfer performance of the three heat exchangers with different fin materials become smaller and smaller.
For the three heat exchangers with different fin material, the above research show that the comprehensive heat transfer performance of the copper fin heat exchanger was the best, and the comprehensive heat transfer performance of aluminum fin with hydrophilic layer was the worst under dehumidifying conditions. gradually and tended to be stable. This shows that with increasing of RHin, the differences in comprehensive heat transfer performance of the three heat exchangers with different fin materials become smaller and smaller. For the three heat exchangers with different fin material, the above research show that the comprehensive heat transfer performance of the copper fin heat exchanger was the best, and the comprehensive heat transfer performance of aluminum fin with hydrophilic layer was the worst under dehumidifying conditions.

Conclusions
For a fin-and-circular-tube heat exchanger, we experimentally studied the effect of three different fin materials: copper fin, aluminum fin, and aluminum fin with hydrophilic layer on heat transfer and resistance performance, and the geometry form and movement characteristics of condensate water on the three fin surfaces were studied, too.
JF was used to screen the optimum fin material at different ua,in and RHin. The main conclusions can be summarized as follows: (1) The condensation state on the surface of copper fin and aluminum fin is dropwise condensation. They all experience the process of nucleation, growth, coalescence, and discharge from the fin surface. The condensation state on the surface of the aluminum fin with the hydrophilic layer is film condensation.

Conclusions
For a fin-and-circular-tube heat exchanger, we experimentally studied the effect of three different fin materials: copper fin, aluminum fin, and aluminum fin with hydrophilic layer on heat transfer and resistance performance, and the geometry form and movement characteristics of condensate water on the three fin surfaces were studied, too. JF was used to screen the optimum fin material at different u a,in and RH in . The main conclusions can be summarized as follows: (1) The condensation state on the surface of copper fin and aluminum fin is dropwise condensation. They all experience the process of nucleation, growth, coalescence, and discharge from the fin surface. The condensation state on the surface of the aluminum fin with the hydrophilic layer is film condensation. (2) Under the same air inlet velocity, Nu and f of T a,in = 35 • C is largest, and then gradually decreases with the decrease of T a,in . Nu decreases and f increases with the decrease of T w,in . (3) At the same u a,in or RH in , for the three different material fins, the heat transfer performance of copper fin heat exchanger is the best, and heat transfer performance of aluminum fin with hydrophilic layer is the worst. f of the aluminum fin is the largest, and f of the aluminum fin with hydrophilic layer is the smallest. (4) Under identical pumping power conditions, the comprehensive heat transfer performance of the copper fin heat exchanger is the best for the studied three different fin materials.