The Effects of Hydrophilicity on Water Drainage and Condensate Retention on Air-Conditioning Evaporators

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1 Purdue University Purdue e-pubs International Refrigeration and ir Conditioning Conference School of Mechanical Engineering 26 The Effects of Hydrophilicity on Water Drainage and Condensate Retention on ir-conditioning Evaporators Liping Liu University of Illinois at Urbana-Champaign nthony M. Jacobi University of Illinois at Urbana-Champaign Follow this and additional works at: Liu, Liping and Jacobi, nthony M., "The Effects of Hydrophilicity on Water Drainage and Condensate Retention on ir- Conditioning Evaporators" (26). International Refrigeration and ir Conditioning Conference. Paper This document has been made available through Purdue e-pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at Herrick/Events/orderlit.html

2 R9, Page 1 THE EFFECTS OF HYDROPHILICITY ON WTER DRINGE ND CONDENSTE RETENTION ON IR-CONDITIONING EVPORTORS Liping LIU*, nthony M. JCOBI Department of Mechanical and Industrial Engineering, University of Illinois at Urbana Champaign, 126 W. Green St. MC-244, Urbana, IL 6181 US Phone: (217) , Fax: (217) , lliu9@uiuc.edu *Corresponding uthor BSTRCT n experimental study is conducted to investigate condensate drainage and retention and the attendant thermalhydraulic effect associated with changes in surface hydrophilicity on air-conditioning evaporators. Three heat exchangers that have controlled wettability covering a range of contact angles and a baseline untreated aluminum heat exchanger are tested. Results from dynamic dip-testing as well as wind-tunnel experiments under fully wet conditions are presented. The data show that for the heat exchangers used in this study the Colburn j factor is not strongly influenced by condensate retention, but the friction factor is significantly reduced by the enhancement of surface hydrophilicity. Heat exchangers with improved wettability hold slightly more water than the baseline in a dynamic dip-test, but retain remarkably less than the untreated coil in a wet wind-tunnel experiment. Steady-state mass retention in wind-tunnel tests decreases with increasing air flow rate, and the retention is more sensitive to the air-side Reynolds number for heat exchangers with higher contact angles. 1. INTRODUCTION The evaporator in air-conditioning systems normally operates with the air-handling surface colder than the dewpoint temperature of the conditioned air. Therefore, moisture condenses and accumulates on the surface of the heat exchanger. Condensate retained on the air-side heat transfer surface has a profound impact on the performance of the heat exchanger and on the air quality. Very recently, material processing advances have produced fins with enhanced wettability, which effectively reduce the contact angle of water condensate and improve the condensate drainage. Methods of coating the aluminum fins have been well documented by Hong and Webb (2), and Kim et al. (22) proposed a new method of plasma treating. The effect of a hydrophilic coating on air-side performance of compact heat exchangers has been examined by Wang et al. (22), and Min et al. (2) provide a long-term performance assessment of heat exchangers for commercial coatings. lthough some studies have been performed with the air-side performance under wet conditions, there is limited information about the condensate characteristics and drainage behavior associated with surface hydrophilicity. It is uncommon to find data reported along with condensate retention data, much less a characterization of surface wettability. The objective of this study is therefore focused on investigating the condensate drainage and retention associated with changes in surface wettability. The thermal-hydraulic effect will also be examined, in order to build a complete picture of the relationships between wettability retention and performance. 2. TEST METHODS Four fin-and-tube heat exchangers with the same geometry (6.6 mm diameter tubes, a staggered tube layout, slit fins, and 1.1 mm fin pitch) as given in Figure 1 were used in this study. s shown in Table 1, the wettability of three of the heat exchangers was controlled through a plasma coating process, and the fourth one represents an untreated International Refrigeration and ir Conditioning Conference at Purdue, July 17-2, 26

3 R9, Page 2 aluminum surface. It has been observed by others that the contact angles measured on the uncoated aluminum fins gradually decrease with increasing numbers of wet/dry cycles, but the wettability of the plasma coated aluminum appears to be robust (Kim et al., 22). 29 mm 6.73 mm 313 mm 25 mm Figure 1. Schematic of heat exchanger geometry Table 1: Hydrophilicity description of the tested heat exchangers Specimen dvancing contact angle θ (º) Receding contact angle θ R (º) Surface treatment Plasma coating Plasma coating Plasma coating Un-coated 2.1 Dynamic dip testing Dynamic dip tests were conducted with the apparatus shown in Figure 2. The apparatus consists of a large water reservoir, a smaller submerged air reservoir to control the submersion of coils by displacement of water using compressed air, and a structure to suspend and weigh the heat exchanger. To start an experiment, the balance was turned on and zeroed after the test coil was suspended over the reservoir. The displacement tank was filled with water, and the water level is made to rise and submerge the test specimen using the compressed air supply. Once the specimen was submerged, the air supply was closed and air vent was suddenly opened to allow water lever to drop quickly and at that moment data recording starts. This apparatus and relevant operating procedure are described in more detail by Zhong and Jacobi (23). s water drains from the specimen, the amount of water retained on the heat exchanger surface is recorded by a computer-based data acquisition system with a minimum recording interval of.1 second. It should be noted that only 1 out of 2 data points are shown in the plots of this paper. The balance has a reported uncertainty of less than.1 grams and the difference of results from repeated runs is approximately 1%. 2.2 Wind-tunnel testing closed-loop wind tunnel as shown in Figure 3 was used to test heat exchangers under wet-surface conditions. The air-side temperature was regulated by controlling the power applied to heaters that can deliver up to 8 kw. Humidification was provided by a boiler, capable of providing more than 12 kg/hr of steam, regulated using dewpoint chilled-mirror sensors and a PID controller. ir-side temperatures were measured using a grid of precision thermocouples at the inlet and exit of the heat exchanger. Volumetric airflow rates up to about 2 m 3 /min were provided using a variable speed drive. The air flow was carefully conditioned upstream of the test heat exchanger to ensure that a uniform approach flow with a low free-stream turbulence is provided. water-glycol mixture supplied by a gear pump was used to cool the evaporator during the experiments. The temperature of the coolant was regulated using a chiller system and monitored using immersion RTD s at the inlet and exit of the heat exchanger. International Refrigeration and ir Conditioning Conference at Purdue, July 17-2, 26

4 R9, Page 3 Coriolis-effect flowmeter was used to measure the mass flow rate of the coolant flow, and test conditions were set to simulate the nearly constant-temperature refrigerant side typical to application. During an experiment, the data acquisition system would sample instruments throughout the tests and log data to a text file for subsequent analysis. Figure 2. Schematic diagram of dip-test apparatus Tests were conducted with a coolant inlet temperature of 4.4ºC, and an air inlet temperature of 23.9ºC. The upstream dewpoint temperature was set to be 18.3ºC. These conditions ensured that the coolant temperature remained lower than the air-side dewpoint temperature throughout an experiment, which is known as the fully wet condition. Data reduction and interpretation follow the methods detailed by the RI Standard for condensing heat exchangers. Correlations for air-side Colburn j factors are provided for specific geometry as a function of the Reynolds number. Likewise, a conventional treatment provides the Fanning friction factor as a function of the Reynolds number. The j factors and friction factors are related to the basic thermal-hydraulic and geometrical data through the following equations: where nd where j = StPr 2 PHX ρ a min 2 ρup min ρ a f = (1 ) σ G tot ρdown tot ρup min σ = front min 2/3 Nu h St = = Re Pr GC Cpaµ a Pr = k ma G = & a ρup + ρ ρa = 2 down pa (1) (2) (3) (4) (5) (6) (7) International Refrigeration and ir Conditioning Conference at Purdue, July 17-2, 26

5 R9, Page 4 Figure 3. Schematic diagram of the wind tunnel; () return duct; (B) thermal mixing chamber; (C) Screens and honeycomb flow conditioning; (D) 9:1 flow contraction; (E) test heat exchanger; (F) measurement locations; (G) resistance heaters; (H) steam injection tube; (I) axial blower Figure 4: The installation of the heat exchanger in the wind tunnel; () wind tunnel; (B) strap; (C) balance; (D) coolant supply; (E) heat exchanger; (F) drainage hole 2.3 Real-time retention measurement Real-time retention measurements were conducted separately from the heat transfer tests. In this way the test section could be modified to obtain the highest data fidelity when recording the retention data, or the best insulation when doing the thermal-hydraulic performance tests. s shown in Figure 4, the heat exchanger was hung in the wind tunnel with a strap. The same balance as used in dynamic dip-testing was placed under the test section to measure the condensate retained on the specimen during the experiment. To reduce the measurement uncertainty, a non-absorbing strap with passage for water to go through was used. Inlet and outlet coolant tubes were kept horizontal and as loose as possible, in order to minimize interference with the specimen weight measuring. Two thin wires were tied between the heat exchanger and the wind-tunnel, providing only horizontal forces against the pushing action of the incoming air. Experimental data showed that the measurements obtained in the circulating system gave results lower than the real object mass with an acceptable accuracy. The error was under 5% for moderate face velocities, reaching approximately 1% at an air face velocity of about 3 m/s. Balance calibration was conducted before each test by recording readings for calibration weights. linear correlation was then acquired and used to calibrate the recorded data in subsequent testing. 3. RESULTS ND DISCUSSION The results from dynamic dip testing are shown in Figure 5, which gives the mass retention in decreasing magnitude to be specimen 1 (θ =3 ) > 2 (θ =5 ) > 3 (θ =11 ) > 4 (uncoated). These results may be explained by recalling that hydrophilic surfaces have an affinity for water and therefore should retain more water. lthough specimen 3 has a large advancing contact angle of 11, it differs from conventional water-repellent surface by having a zero receding contact angle it has a very large contact angle hysteresis. During a dynamic dip test, the heat exchanger is flooded with water, which is completely different from the case of having droplet distributions and combined drywet areas as in a wind-tunnel test. Without the existence of droplets, the contact angle hysteresis is not playing an important role. In the dip test, the retention differences between the three plasma treated heat exchangers are quite small (less than 5%) with an average value of approximately 65 g/m 2 after 5 minutes, while the uncoated heat exchanger holds approximately 57 g/m 2 under the same conditions. This is in accordance with expectation. With the same geometry International Refrigeration and ir Conditioning Conference at Purdue, July 17-2, 26

6 R9, Page 5 and identical test conditions, and the advancing contact angle does not have a significant effect, the only thing that matters is the receding contact angle. 11 Mass per rea (g/m 2 ) θ =3 o θ =5 o Α θ =11 o Time (min) Figure 5. Dynamic dip testing results j θ =3 o θ =5 o θ =11 o Re dh Figure 6. Colburn j factors under fully wet test conditions Shown in Figure 6 and 7 are the wind-tunnel test results under fully wet conditions. The data show that for the heat exchangers used in this study, the Colburn j factor is not strongly influenced by condensate retention, but the friction factor is significantly reduced by the enhancement of surface hydrophilicity. This is because the condensation occurring on specimen 1 and specimen 2 is basically filmwise, and that occurring on specimen 3 and specimen 4 are more likely dropwise. Therefore, droplets can stand up on the surfaces of specimen 3 and specimen 4, bridge between adjacent fins, and block the air flow causing pronounced pressure drops. The reason why the f factors for specimen 1 (θ =3 ) are slightly higher than specimen 2 (θ =5 ) is not clear. International Refrigeration and ir Conditioning Conference at Purdue, July 17-2, 26

7 R9, Page f.15.1 θ =3 o θ =5 o.5 θ =11 o Re dh Figure 7. Fanning f factors under fully wet test conditions 12 1 Mass per rea (g/m 2 ) θ =3 o θ =5 o 2 θ =11 o Time (min) Figure 8. Real-time retention measurement results One set of real-time retention measurement results for a fixed fan power is given in Figure 8. Under the same fully wet test conditions as specified before, specimen 1 (θ =3 ) and specimen 2 (θ =5 ) exhibit similar drainage performance and specimen 3 (θ =11 ) and specimen 4 (uncoated) retain much more condensate. This result is counter to the dynamic dip-test results, which may be due to the distinct condensation and drainage mechanisms in these two experimental systems. For hydrophilic specimens (specimen 1 and specimen 2), the water on the surface is likely to take the form of a film, whether it was deposited by dipping or condensation. Therefore, the amount of water should be roughly equal for International Refrigeration and ir Conditioning Conference at Purdue, July 17-2, 26

8 R9, Page 7 these two cases. Considering the existence of shear due to the air flow in addition to gravity drainage, the steady state retention in the wind-tunnel (shown to be 51g/m 2 in Figure 8) should be slightly less than that in a dynamic dip test (65g/m 2 ). For hydrophobic coils, this behavior is different. The heat exchanger is flooded with water in the dynamic dip test, and when the air vent is suddenly opened, the water level drops quickly and a large amount of water leaves the heat exchanger in a very short time. This flow of water sweeps the surface, reducing the retention much less than the steady-state retention of droplets and bridges in a wind-tunnel test Mass per rea (g/m 2 ) θ =3 o θ =5 o 2 θ =11 o Re dh Figure 9. Steady-state mass retention data s shown in Figure 9, the steady-state mass retention in a wind-tunnel test decreases with increasing air side Reynolds number for all of the heat exchangers. The specimens with higher contact angles retain more water than the hydrophilic ones, and their retention is more sensitive to the change of air side Reynolds number, which is also true for f factors as revealed in Figure 7. One possible explanation is that with the increased air shear, the droplet number density as well as its distribution will be significantly affected for dropwise condensation, while the thin stable water film on hydrophilic heat exchangers is largely unaffected. 4. CONCLUSIONS This study reports the effect of surface hydrophilicity on the air-side performance and the retention and drainage behavior of slit fin-and-tube heat exchangers. Experiments were performed for plasma coated heat exchangers and uncoated aluminum heat exchanger. The major findings of this study are summarized as follows: Heat exchangers with improved wettability hold slightly more water in a dynamic dip test. The advancing contact angle does not serve an important role in affecting the mass retention. Hydrophilic coatings reduce the wet pressure drop significantly without decreasing the wet sensible heat transfer coefficient for a heat exchanger. Heat exchangers with improved wettability retain much less water than hydrophobic ones in a wet windtunnel test. Steady-state mass retention in a wind-tunnel test decreases with increasing air flow rate, and the value is more sensitive to the change of air side Reynolds number for heat exchangers with higher contact angles. International Refrigeration and ir Conditioning Conference at Purdue, July 17-2, 26

9 R9, Page 8 NOMENCLTURE front frontal area (m 2 ) Subscripts min minimum flow area (m 2 ) advancing tot total surface area (m 2 ) R receding C p specific heat (J/kg-K) a air f friction factors ( ) up upstream G mass flux at min. flow area (kg-m 2 /s) down downstream h heat transfer coefficient (W-m 2 /K) HX heat exchanger j Colburn j factor ( ) dh hydraulic diameter k conductivity (W-m/K) m& mass flow rate (kg/s) Nu Nusselt number ( ) Pr Prantdl number ( ) Re Reynolds number ( ) St Stanton number ( ) θ dynamic contact angle (º) σ area ratio ( ) ρ density (kg/m 3 ) P pressure drop (Pa) REFERENCES Hong, K., Webb, R. L., 2, Wetting Coatings for Dehumidifying Heat Exchangers, HVC&R Research, vol. 6, no. 3: p Hong, K., Webb, R. L., 1999, Performance of Dehumidifying Heat Exchangers With and Without Wetting Coating, J. Heat Transfer, vol. 121: p Min, J., Webb, R. L., Bemisderfer, C. H., 2, Long-Term Hydraulic Performance of Dehumidifying Heat- Exchangers With and Without Hydrophilic Coatings, HVC&R Research, vol. 6, no. 3: p Kim, G., Lee, H., Webb, R. L., 22, Plasma Hydrophilic Surface Treatments for Dehumidifying Heat Exchangers, Experimental Thermal and Fluid Science, vol. 27: p Wang, C. C., Lee, W. S., Sheu, W. J., Chang, Y. J., 22, Comparison of the irside Performance of the Fin-and- Tube Exchangers in Wet Conditions; With and Without Hydrophilic Coating, pplied Thermal Engineering, vol. 22: p Shin, J., Ha, S., 22, The Effect of Hydrophilicity on Condensation Over Various Types of Fin-and-Tube Heat Exchangers, Int. J. Refrig., vol. 25: p Wang, C. C., Chang, C. T., 1998, Heat and Mass Transfer for Plate Fin-and-Tube Heat Exchangers, With and Without Hydrophilic Coating, Int. J. Heat Mass Transfer, vol. 41: p Zhong, Y., Jacobi,. M., 23, Dynamic Dip-Testing to ssess the Condensate-Drainage Behavior of Dehumidifying Heat Exchangers, 2 nd Int. Conference on Heat Transfer, Fluid Mechanics and Thermodynamics: p CKNOWLEDGEMENT We are grateful for assistance from LG Electronics Company in providing the heat exchanger samples, and for financial support from the ir Conditioning and Refrigeration Center (CRC) at the University of Illinois. International Refrigeration and ir Conditioning Conference at Purdue, July 17-2, 26