Analysis of droplet dynamic behavior and condensation heat transfer characteristics on rectangular microgrooved surface with CuO nanostructures

International Journal of Heat and Mass Transfer 130 (2019) 1096–1107

Contents lists available at ScienceDirect

International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt

Analysis of droplet dynamic behavior and condensation heat transfer characteristics on rectangular microgrooved surface with CuO nanostructures Qi Peng, Li Jia ⇑, Chao Dang, Zhoujian An, Yongxin Zhang, Liaofei Yin Institute of Thermal Engineering, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China Beijing Key Laboratory of Flow and Heat Transfer of Phase Changing in Micro and Small Scale, Beijing 100044, China

a r t i c l e

i n f o

Article history: Received 24 August 2018 Received in revised form 30 October 2018 Accepted 3 November 2018

Keywords: Dropwise condensation Enhanced heat transfer CuO nanostructures Rectangular microgrooves Droplet dynamics

a b s t r a c t Condensation is a ubiquitous phase-change phenomenon in nature and has been widely adopted in various energy-intensive industrial application. Many efforts have been focused on regulating droplet dynamics to enhance the condensation heat transfer by developing micro/nanostructured surface. In this work, a microgrooved surface with CuO nanostructures was fabricated by combination of simple machining and scalable self-limiting chemical oxidation method for regulating droplet dynamic behavior. The wettability, droplet dynamics and heat transfer characteristics on such surface were compared with that on plain and microgrooved hydrophobic surfaces. The anisotropic wettability was observed on microgrooved surface and enhanced by creating nanostructures. 15–43% higher heat flux was reached on microgrooved hydrophobic surface compared to plain hydrophobic surface due to an increase of effective heat transfer area, the sweeping effect of liquid columns for droplets on adjacent plateaus and cooperation of liquid columns flowing and liquid bridges sliding. Several novel droplet dynamic behaviors that the suspension of large droplets, suction of spindle-shaped droplets and strong self-oscillation of falling droplets were observed on the microgrooved surface with nanostructures and enhanced the condensation heat transfer. Compared with plain hydrophobic surface, the heat flux was enhanced up to 55–102%. Ó 2018 Elsevier Ltd. All rights reserved.

1. Introduction Condensation is a ubiquitous phase-change phenomenon occurring in nature [1,2] and it plays an essential role in various industrial applications including power generation [3], nuclear reactor [4], thermal management [5–7], water desalination and harvesting [8,9], and environmental control [10]. According to the wetting behavior of condensed droplet on surface, usually characterized as contact angle, the vapor condensation is divided into two modes: dropwise and filmwise condensation. Dropwise condensation has attracted significant interest since it was proposed by Schmidt [11], due to its excellent heat transfer performance. It is well known that the heat transfer performance of dropwise condensation can be an order of magnitude higher than that of filmwise condensation [12]. In the past eight decades, the dropwise condensation on conventional heat transfer materials

⇑ Corresponding author at: Institute of Thermal Engineering, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China. E-mail address: [email protected] (L. Jia). https://doi.org/10.1016/j.ijheatmasstransfer.2018.11.012 0017-9310/Ó 2018 Elsevier Ltd. All rights reserved.

has been a significant research topic [13,14]. Many attentions were focused on creating non-wetting surfaces and promoter coatings for easy droplet removal to enhance the heat transfer performance of dropwise condensation [15–17]. While robust coatings with low thermal resistance continue to be a challenge and require more development [18,19]. As rapid development of micro/nano manufacturing technology, many efforts have been concentrated on the development of superhydrophobic surfaces for enhancing condensation heat transfer [20–24]. The micro-droplets can undergo coalescence-induced droplet jumping on suitable designed superhydrophobic surfaces due to the release of excess surface energy [25–27]. Such a self-propelled droplet jumping behavior has offered a new avenue to further enhance heat transfer [28–30]. However, for considerably high nucleation density and large surface subcooling, condensed droplets highly adhered on surface form and lead in a flooding condensation mode, which significantly hindered applicabilities of superhydrophobic surfaces for rapid droplet removal and the condensation heat transfer enhancement. The microgroove was usually adopted as micropatterns on surfaces to enhance the heat transfer of dropwise condensation, due to the reduction of resistance and adhesion work of droplets sliding

Q. Peng et al. / International Journal of Heat and Mass Transfer 130 (2019) 1096–1107

1097

Nomenclature T P q k xi x T DT h re hfg SEM

temperature (K) pressure (Pa) heat flux (W/m2) thermal conductivity (W/m K) the distance between ith hole and condensing surface (m) average distance (m) average temperature (K) surface subcooling (K) heat transfer coefficient (W/m2 K) critical nucleation radius (nm) latent heat (kJ/kg) scanning electron microscope

on surfaces with microgrooves, which implied that the microgroove had a positive effect on heat transfer enhancement of dropwise condensation [31–33]. Thus, many interests have been concentrated on the wettability and droplet dynamic behavior on various microgrooved surfaces [34,35]. Due to the potential energy barrier caused by the crest on microgrooved surface, the wetting perpendicular to the groove direction was suppressed. As a result, the contact angle perpendicular to groove direction was larger than that along groove direction. Recently, there have been some studies demonstrating the anisotropic wettability on microgrooved surfaces and finding that the geometry of microgroove has significant effect on the wettability and droplet dynamics on such surface [36–38]. The anisotropic heat transfer performance caused by the anisotropic wettability on microgrooved surface has also been confirmed, and the heat transfer during dropwise condensation was increased to 30–50% for vertical grooved surface due to the enhancement of sweeping effects of falling droplets [39]. As mentioned above, in most studies, only the geometry of microgroove was considered and the dimensions of microgrooves on surfaces were so tiny (less than 100 lm) that the processes for surface treatment were complicated and expensive, which hinder scalable applications of such surfaces. In this work, for rapid removal of condensed droplets and condensation heat transfer enhancement, the microgrooved surface with CuO nanostructures was fabricated by combination of simple machining and selflimiting chemical oxidation method. When the condensed droplet became larger than the width of microgrooves due to growth and coalescence, the droplets would suspend on the microgrooves rather than permeated into microgrooves to expose the bottom of microgrooves for new nucleation. In addition, suspended droplets had less basal area contacting with surface, which resulted in reducing droplet departure size for accelerating the renewal of surface. The heat transfer tests demonstrated a significant dropwise condensation heat transfer enhancement on microgrooved surface with CuO nanostructures compared to that on plain hydrophobic surface. Furthermore, the fabrication process of such surface combining machining and chemical oxidation method has provided a low cost and scalable avenue to large-scale industrial applications.

2. Experiment 2.1. Preparation of condensing surfaces As known, copper was applied widely in thermal and energy systems due to its low cost and high thermal conductivity. Thus,

Greek symbols h contact angle (°) q density (kg/m3) r surface tension (N/m) Subscripts s saturation v vapor l liquid wat water w wall == direction along to groove \ direction perpendicular to groove

a 3 mm-thick circular copper plate with a diameter of 40 mm was made of high purity copper in this work. As shown in Fig. 1 (a), the rectangular microgroove arrays were fabricated by machining on polished copper plate. The detailed dimension of microgroove was presented in Fig. 1(b). The depth and width of microgroove were 400 and 380 lm, respectively. The width of plateau between microgrooves was 285 lm. Prior to chemical oxidation, the microgrooved surface was cleaned in an ultrasonic bath with acetone for 10 min to remove the organic contaminants, and then rinsed with isopropyl alcohol, ethanol and de-ionized (DI) water. The microgrooved surface was then dipped into a 2.0 M hydrochloric acid solution for 5 min to remove the nature oxide film on the surface, then triple-rinsed with DI water, and dried within clean nitrogen stream. After that, a self-limiting chemical oxidation process in Ref. [21] was used to create CuO nanostructures on microgrooved surface. During the oxidation process, the thin Cu2O layer was formed on the copper surface, then the CuO nanostructures grown from the Cu2O intermediate layer [40]. The self-limiting oxidation method allowed for a low parasitic thermal conduction resistance due to the relatively high thermal conductivity of copper oxides (33 W/m K for CuO and 30 W/m K for Cu2O [41]). The microgrooved surface with CuO nanostructures was subsequently functionalized to achieve hydrophobicity by immersing into a hot (70 °C) ethanol solution of 2.5 mM n-octadecanethiol (98% n-octadecyl mercaptan) for 1 h. A 3 nm-thick self-assemble monolayer coating was deposited on CuO nanostructures and the nanostructures remained original appearance [42]. As the contrast, the plain and microgrooved hydrophobic surface were also fabricated for dropwise condensation heat transfer test.

2.2. Experimental apparatus and procedure The condensation experiments were performed in a custommade chamber which allowed to achieve a board range of condensing conditions. As showed in Fig. 2, the experimental system consisted of an evaporator, a condensing chamber, a high-speed camera, an electronic refrigeration system based on Peltier elements and a data acquisition system. Two cartridge heaters mounted in evaporator were adjusted by the voltage regulator. A pressure transducer (Omega) with an accuracy of ±0.25% and two T-type thermocouples (Omega) with an accuracy of ±0.1 K were installed to monitor the pressure and temperature of vapor and water in evaporator, respectively. To prevent steam condensation on the inside wall, the heating tape was wrapped around connecting pipe and covered with fiberglass for insulation. The test surface,

1098

Q. Peng et al. / International Journal of Heat and Mass Transfer 130 (2019) 1096–1107

Fig. 1. The characteristics of surface with rectangular grooves. (a) The schematic of copper surface with microgrooves; (b) the geometry of microgrooves.

Fig. 2. Schematic of condensation experimental apparatus.

40 mm in diameter, was vertically adhered to the top of the cooling stage in the center of condensing chamber under inlet of vapor, and the thermal grease with a thermal conductivity of 25 W/m K was filled in between text surface and cooling stage to minimize the thermal contact resistance. At the bottom of the condensing chamber, a backflow pipe conducted the condensate going back to the evaporator through a post condenser. In the front of the condensing chamber, a view window was mounted for visual observation of the condensation process. A T-type thermocouple and a pressure transducer were set to measure the temperature and pressure in the condensing chamber. The whole condensing chamber was insulated well with the fiberglass. Four Peltier elements were adhered to the bottom of the cooling stage to create the surface subcooling due to its convenient manipulation and precise temperature control. To prevent condensation on the test surface before stable condensation condition, the Peltier elements first heated the condensing surface to achieve higher temperature than saturation temperature. The cooling stage was machined as three co-axial copper cylinders with different diameters of 40, 98 and 190 mm, respectively. To acquire the heat flux and temperature distribution, four thermocouple holes with 1 mm diameter were drilled into the cylinder with 40 mm

diameter, and ten T-type thermocouples were respectively put in four holes and top of the cooling stage along the axial direction with two thermocouples at each location. The cooling stage was thermally insulated with polytetrafluoroethylene (PTFE) to ensure one-dimensional conduction. Prior to condensation experiments, the condensing chamber was evacuated and the DI water was sufficiently boiled to eliminate non-condensable gases dissolved in the water. During the experiments, all the temperature and pressure data were collected by the data acquisition system to determine the surface subcooling, heat flux and condensation heat coefficient. The dynamic behaviors of droplets were recorded by a high-speed camera. In this work, the condensation pressure remained 60 kPa corresponding to saturation temperature of 85.93 °C, which was general operation condition in heat pipe, low-temperature heat pump and multi-effect evaporation desalination [43]. The range of surface subcooling for experiments operation was from 0 to 17 K. 2.3. Data reduction As mentioned previously, because the cooling stage was insulated well, the heat transfer process in the cylinder could be

Q. Peng et al. / International Journal of Heat and Mass Transfer 130 (2019) 1096–1107

considered as one-dimension steady heat conduction. Combining Fourier’s law of heat conduction and least square method, the heat flux in the cooling stage was expressed as:

P5 q ¼ k



  i¼1 ðxi  xÞ T i  T P5 2  i¼1 ðxi  xÞ

in ith hole. The average temperature T and the average distance x were defined as:

x ¼

 ð1Þ

where q is the heat flux of vapor condensation; k is the thermal conductivity of copper; xi is the distance between ith hole and condensing surface and Ti is the average temperature of two thermocouples

1099

T ¼

P5

i¼1 xi

5

ð2Þ

P5

i¼1 T i

5

ð3Þ

Based on the temperature gradient in the cooling stage, the temperature of condensing surface could be calculated as:

Fig. 3. The morphology and wettability of plain hydrophobic surface. (a) The morphology of plain hydrophobic surface; (b) the contact angle on plain hydrophobic surface.

Fig. 4. The morphology and wettability of microgrooved hydrophobic surface. (a) The morphology of plateau on microgrooved surface; (b) the morphology at the bottom of the groove; (c) the contact angle perpendicular to groove direction; (d) the contact angle along to grooved direction.

1100

Q. Peng et al. / International Journal of Heat and Mass Transfer 130 (2019) 1096–1107

q T w ¼ T þ x k

ð4Þ

According to Eqs. (1) and (4), the heat transfer coefficient was acquired as follows:

h¼

q q ¼ DT T s  T w

ð5Þ

where Ts is the saturation temperature corresponding to experimental pressure. 3. Result and discussion 3.1. The morphology and wettability of test surface The morphologies of all test surfaces were observed by using 3D optical surface profilometer and scanning electronic microscope. The wettability of these surfaces was characterized by the apparent contact angle of a 5 lL deposited droplet measured by optical contact angle measuring device with an accuracy of ±0.25°. Fig. 3 showed that the morphology and contact angle of the plain hydrophobic surface. It was seen that the surface was smooth with the roughness of 0.013 lm, and the measured contact angle was 101.8°. The morphology and wettability of microgrooved hydrophobic surface in Fig. 4. Fig. 4(a) and (b) were the morphologies of the plateau and the bottom of microgroove, respectively. The plateau with roughness of 0.016 lm was smooth due to the precise polish. However, it was considerably difficult to polish the bottom and sidewall of microgrooves. Thus, the roughness of the bottom of microgrooves, Ra = 0.49 lm, was much higher than that of plateau.

Fig. 4(c) and (d) presented the contact angle of droplet perpendicular and along groove direction on microgrooved hydrophobic surface, respectively. It was clearly seen that the droplet kept on the plateau of microgrooves rather than permeate into microgroove, and the contact angle perpendicular to groove direction, h\ = 142.7°, was significantly larger than that on the plain hydrophobic surface, h = 101.8°. While the contact angle along groove direction, hk = 111.5°, was considerably less than that perpendicular to groove direction, which confirmed the anisotropic wettability of microgrooved surface. Fig. 5 showed characteristics of CuO nanostructures and wetting behavior of droplet on microgrooved surface with nanostructures. It was seen in Fig. 5(a) and (b) that sharp, knife-like CuO nanostructures were created on the microgrooved surface and had characteristics of the height of 1 lm, the thickness of 100 nm, and the average widths of 300 nm. The unique knife-like morphology of CuO nanostructures, with a tip dimension less than 10 nm, ensured nucleation within the structures instead of the tips of the structures, due to the increased energy barrier associated with nucleation on features similar in size to the critical

Table 1 The contact angle on different test surfaces. Sample

Plain surface Microgrooved surface Microgrooved surface with nanostructures

Contact angle h\ (°)

h== (°)

101.8 142.7 156.9

111.5 115.8

Fig. 5. The characteristics of nanostructures and wettability of microgrooved surface with CuO nanostructures. (a) SEM image of microgrooved surface with CuO nanostructures; (b) details of CuO nanostructures on microgrooved surface; (c) the contact angle perpendicular to grooved direction on microgrooved surface with CuO nanostructure; (d) the contact angle along groove direction on microgrooved surface with CuO nanostructures.

Q. Peng et al. / International Journal of Heat and Mass Transfer 130 (2019) 1096–1107

nucleation radius [44]. The anisotropic wettability was also observed on such surface in Fig. 5(c) and (d). Similar to microgrooved hydrophobic surface, a 5 lL droplet suspended on microgroove. The contact angle perpendicular to groove direction, h\ = 156.9°, was larger than 150°, which presented the superhydrophobicity. While the contact angle along groove direction, hk = 115.8°, was nearly identical to the contact angle along groove direction on microgrooved hydrophobic surface. It was concluded that nanostructures enhanced the anisotropic wettability on microgrooved surface. In order to compare with each other conveniently, the contact angles on all surfaces were summarized in Table 1. The anisotropic wettability of microgrooved surface depended on the difference of energy barrier for droplet to overcome during the wetting process in different directions. When a droplet wetted along groove direction, only the surface tension was required to overcome, which was similar to the wetting process on smooth surface. However, when wetting perpendicular to groove direction, besides surface tension, a droplet needed to overcome the potential energy barrier caused by the crest on microgrooved surface.

1101

nanostructure, microgrooved and plain hydrophobic surface. The direction of microgrooves on test surfaces was parallel to the vertical direction, the stable dropwise condensation was observed on three surfaces during experiments. Fig. 6 presented that there were significant differences in condensation modes on three test surfaces. Conventional spherical cap shaped droplets were observed on plain hydrophobic surface during condensation. For

3.2. Condensation mode and droplet behavior In order to understand the effects of nanostructures and microgrooves on condensation heat transfer performance, the condensation modes and droplet dynamic behavior were visually investigated on three test surfaces: microgrooved surface with

Fig. 7. The variation of the critical nucleation radius with surface subcooling.

Fig. 6. The condensation modes and morphologies of droplets on three test surfaces. (a) Conventional spherical cap shaped droplets on plain hydrophobic surface; (b) small spherical cap shaped droplets on the plateaus and liquid columns with different length in microgrooves on microgrooved hydrophobic surface; (c) spindle-shaped droplets on plateaus, small droplets in microgrooves and the suspended droplet (orange dash ellipse) with larger size than microgrooves on microgrooved surface with CuO nanostructures; (d) the magnified image of suspended droplet. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

1102

Q. Peng et al. / International Journal of Heat and Mass Transfer 130 (2019) 1096–1107

(a)

t=0s

t=0.018s

t=0s

t=0.028s

(b)

Fig. 8. Typical coalescence behaviors of droplets on microgrooved hydrophobic surface. (a) The large droplets on plateaus were swept through coalescing with liquid columns in adjacent microgrooves (the solid ellipse); (b) the liquid columns in two neighboring microgrooves merged to form a liquid bridge by simultaneously contacting with droplets on the plateau between microgrooves (the solid ellipse). (The dash ellipse highlighted that condensate overflowed from microgroove to inform a spindle-shaped droplet.)

microgrooved hydrophobic surface, the initial droplets forming randomly in microgrooves rapidly grew to a critical size comparable to the dimension of microgrooves. With further growth of droplets, due to confinement of microgrooves, the droplets in microgrooves were stretched along groove direction which formed liquid columns with different length to fill the microgrooves. While the spherical cap shaped droplets were seen on the plateaus on microgrooved hydrophobic surface. The growth of droplets on plateaus was confined by the size of plateaus, which led to the reduction of large droplets with considerably high thermal conduction resistance. For microgrooved surface with CuO nanostructures, as shown in Fig. 6(c), the spindle-shaped droplets were formed on the plateaus, while the small spherical droplets emerged densely at the bottom and side walls of microgrooves during condensation. The reason for formation of spindle-shaped droplets is that the lateral coalescences of droplets were suppressed due to the energy barrier introduced by microgrooves. When the small droplets on the plateaus grew larger than the size of plateaus by merging, the droplets were stretched along vertical direction as spindle shape due to the cooperation of gravity and surface tension. Especially, a novel droplet behavior was observed on microgrooved surface with CuO nanostructures. As shown in Fig. 6(c) and (d), a droplet with larger size than microgroove suspended on microgroove instead of

permeating into the microgroove (large orange dash ellipse). It was seen clearly in Fig. 6(d) that a small droplet (small dash ellipse) grew at the bottom of microgroove under the large suspended droplet. Due to the confinement of microgrooves, the droplets with larger size than microgrooves deformed, the lower part of confined droplets in microgrooves with a small radius possessed a high outward pressure. However, the upper part of the droplets exceeding microgrooves with large radius offered a relatively low inward pressure. As a result, the outward Laplace pressure difference drove confined droplets out of microgrooves. Finally, the droplets would rise on microgrooves by coalescence with surrounding droplets. The suspended droplets helped to expose the surfaces in microgrooves for vapor condensation and facilitated the droplets removal. The highly adhered Wenzel state droplets and flooding condensation mode were seen on the plateaus during condensation. The phenomenon could be explained by nucleation theory [45]. Condensation originated from the formation of the small molecular cluster. When the size of cluster exceeded the critical nucleation radius, the molecular cluster would grow further and form a macroscopic droplet. The critical nucleation radius could be calculated by Kelvin’s classical equation [13],

re ¼

2T v rlv hfg ql DT

ð6Þ

Q. Peng et al. / International Journal of Heat and Mass Transfer 130 (2019) 1096–1107

1103

(a)

t=0s

t=0.018s

t=0s

t=0.066s

(b)

Fig. 9. Typical coalescence behaviors of droplets on microgrooved surface with CuO nanostructures. (a) Droplets suction from the bottom of microgrooves to the spindleshaped droplets on the plateaus (the solid ellipse); (b) several spindle-shaped droplets on the plateaus coalesced each other to generate a large droplet suspending on the microgrooves (the solid ellipse).

where Tv, rlv, hfg, ql and DT represent the temperature of vapor, liquid-vapor surface tension, latent heat, density and subcooling, respectively. Fig. 7 showed the variation of the critical nucleation radius with surface subcooling. As the increase of surface subcooling, the critical nucleation radius decreased sharply from tens of nanometers at low surface subcooling to only a few nanometers at large surface subcooling. Furthermore, the CuO nanostructures with a spacing of 1 lm allowed for initial droplets nucleation in the nanostructures. As a result, the pinned Wenzel state droplets formed. Meanwhile, compared with the other surfaces without nanostructures, these CuO nanostructures offered much higher nucleation density which resulted in flooding condensation mode easily. Compared with plain hydrophobic surface, the different coalescence behaviors were found on microgrooved hydrophobic surface and microgrooved surface with CuO nanostructures. It was seen from the solid ellipse in Fig. 8(a) that the large droplets on plateaus with the size approaching plateaus were swept by merging with liquid columns in adjacent microgrooves, which resulted in accelerating the removal of large droplets and confining the growth of droplets on the plateaus. As a result, the plateaus on the condensing surface were mainly covered with small droplets representing better thermal transfer performance. Especially, the Fig. 8(b) showed that new morphologies of droplets occurred on microgrooved hydrophobic surface due to coalescence of droplets. When

liquid columns in two neighboring microgrooves simultaneously merged with the droplets on the plateau between microgrooves, the liquid bridge formed. With further growth of the liquid bridge, several microgrooves could be covered eventually. The dash ellipse in Fig. 8(b) highlighted another mode to form a liquid bridge. The condensate overflowed from microgroove to form a spindleshaped droplet, after that, the spindle-shaped droplet merged with round droplets and liquid columns to generate the liquid bridge covering the microgrooves. The formation of spindle-shaped droplet may be attributed to the defects created during the fabrication process of microgroove, which hindered the spreading of condensate in microgroove. Some novel dynamic behaviors of droplets were observed on the microgrooved surface with CuO nanostructures. As shown in Fig. 9(a), the droplets at the bottom of microgroove were sucked to the spindle-shaped droplet on the plateaus. The suction behavior of spindle-shaped droplets improved the renewal frequency and impeded the generation of large droplets and spreading of condensate in microgrooves. More interestingly, several spindleshaped droplets on plateaus merged each other to generate a large droplet, and the droplets in microgrooves were also swept during the coalescence process. As shown in Fig. 9(b), the generated droplet suspended on the microgroove instead of permeating into microgroove. The suspension of large droplets enabled the surfaces in microgrooves to expose for vapor condensation and resulted in

1104

Q. Peng et al. / International Journal of Heat and Mass Transfer 130 (2019) 1096–1107

(a)

t=0s

t=0.042s

t=0.078s

t=0s

t=0.034s

t=0.28s

t=0s

t=0.128s

t=0.148s

t=0s

t=0.038s

t=0.056 s

(b)

(c)

(d)

Fig. 10. The departure process of droplets on test surfaces during dropwise condensation. (a) The conventional departure of droplet on plain hydrophobic copper surface; (b) the flowing of liquid column in microgroove and sweeping of droplets on adjacent plateaus on microgrooved hydrophobic surface; (c) the sliding of liquid bridge with liquid columns in microgrooves on microgrooved hydrophobic surface; (d) the departure of suspended droplet on microgrooved surface with CuO nanostructures.

the less contact area with condensing surface which decreased surface adhesion for easy removal of droplet by gravity. The departure behavior of droplets on three test surfaces were compared during condensation. As shown in Fig. 10(a), on the plain hydrophobic copper surface, the conventional departure behavior of droplet was presented that a large droplet slid down due to gravity and swept droplets staying at its pathway by coalescence. The surface covered by droplets was exposed for new nucleation. It was inferred that small departure size and high departure frequency were beneficial for enhancement of heat transfer. Differing from the departure of droplets on plain hydrophobic surface, there

were two different departure modes, liquid column flowing and liquid bridge sliding, being observed on the microgrooved hydrophobic surface during condensation. Fig. 10(b) and (c) indicated the sliding process of liquid column and liquid bridge, respectively. Due to the continuous accumulation of condensate in microgroove, the liquid column grew long enough to overcome surface adhesion to flow down by gravity, which was highlighted by the dash. During the flowing process of the liquid column, the large droplets on adjacent plateaus were swept, which was highlighted by the solid ellipses in Fig. 10(b). Sweeping effect of flowing liquid column caused more exposed surface for vapor

Q. Peng et al. / International Journal of Heat and Mass Transfer 130 (2019) 1096–1107

1105

condensation, which was in favor of improving the heat transfer. It was seen in Fig. 10(c) that a liquid bridge covering four microgrooves overcame the surface adhesion to slide down on microgrooved hydrophobic surface. Especially, several liquid columns in microgrooves (green1 solid) was carried as the tails of the liquid bridge to flow down, which caused the increase of refreshed surface. The cooperation of liquid columns flowing and liquid bridge with tails sliding improved the renewal frequency and sweeping areas. A unique departure behavior of droplet was observed on microgrooved surface with CuO nanostructures. As shown in Fig. 10(d), the interesting phenomenon that large droplets in microgrooves were swept by falling droplet, but a few micro-droplets remained at the bottom of microgrooves confirmed that the large droplets suspended on the microgrooves rather than permeated into the microgrooves. Compared with conventional droplets, the suspended droplets had fewer contact areas with condensing surface which was highly advantageous to the rapid removal of droplets. In addition, the surfaces in microgrooves under suspended droplets were exposed for vapor condensation instead of being covered with condensate. Differing from falling droplet on plain hydrophobic surface, the much stronger self-oscillation of falling droplet was observed on microgrooved surface with CuO nanostructures. The formation of strong self-oscillation was mainly attributed to the energy barrier for lateral coalescence of droplets caused by microgrooves. The strong self-oscillation behavior of falling droplet could augment the perturbation at the liquid-vapor interface to enhance the transfer of heat and mass and improved mobility of droplet to facilitate the rapid removal of droplet.

4. Condensation heat transfer performance Fig. 11(a) and (b) indicate the heat flux and heat transfer coefficient as a function of surface subcooling on three test surfaces at vapor pressure of 60 kPa, respectively. For all test surfaces, the heat flux increased monotonically with increase of surface subcooling due to gravity removal. As the driving force for condensation, the increase of surface subcooling facilitated the growth of condensed droplets, which led to improving renewal frequency of droplets at high heat flux. Compared to the dropwise condensation on plain hydrophobic surface, the overall heat flux and heat transfer coefficient were significantly improved on both microgrooved hydrophobic surface and microgrooved surface with CuO nanostructures. The heat transfer performance of dropwise condensation was significantly dependent on droplet dynamic behaviors under various surface subcooling. For microgrooved hydrophobic surface, the fabrication of microgrooves on condensing surface extended the effective heat transfer areas, and the plateaus were mainly covered with small droplets possessing better heat transfer performance due to confinement of microgrooves. In the meantime, the generated liquid columns in microgrooves timely swept the large droplets on adjacent plateaus. As the drainage pathway of condensate, the cooperation of the liquid columns flowing in microgrooves and liquid bridges sliding with tails facilitated the renewal of surface and extended the sweeping area. As a result, the increase of 15–43% for heat flux was achieved on microgrooved hydrophobic surface under the entire range of surface subcooling. For microgrooved surface with CuO nanostructures, the similar trends in heat flux and heat transfer coefficient with surface subcooling were observed. The heat transfer performance of microgrooved surface with CuO nanostructures always outperformed that of both the plain and microgrooved hydrophobic surface 1 For interpretation of color in Fig. 10, the reader is referred to the web version of this article.

Fig. 11. The condensation heat transfer performance. Overall heat flux as a function of surface subcooling. (b) Heat transfer coefficient as a function of surface subcooling.

under various surface subcooling. The formation of CuO nanostructures on microgrooved surface considerably increased the nucleation density to promote vapor condensation. In addition, the novel dynamic behaviors of droplets on microgrooved surface with CuO nanostructures were significantly conducive to the enhancement of dropwise condensation heat transfer performance. The suspension of large droplets enabled the surface under droplets to expose for nucleation, and the suspended droplets implied smaller contact areas with surface which decreased the departure size of droplets and improved the renewal frequency. Meanwhile, the suction effect of spindle-shaped droplets on plateaus facilitated the renewal of surface. The strong self-oscillation of falling droplets enhanced the perturbation at liquid-vapor interface to facilitate vapor condensation and rapid removal of droplets. Consequently, the significant enhancement of 55–102% for heat flux was attained with respect to that on plain hydrophobic surface under entire range of surface subcooling. 5. Conclusion In summary, a microgrooved surface with CuO nanostructures was fabricated by the combination of simple machining and scalable self-limiting chemical oxidation method. The wettability, droplet dynamic behaviors and condensation heat transfer performance were investigated on microgrooved surface with CuO nanostructures, plain and microgrooved hydrophobic surfaces through visualization experiments and heat transfer tests. The

1106

Q. Peng et al. / International Journal of Heat and Mass Transfer 130 (2019) 1096–1107

anisotropic wettability was observed on two microgrooved surfaces, and nanostructures enhanced the anisotropy of wetting behavior. On the microgrooved hydrophobic surface, the microgrooves increased the effective heat transfer areas and confined the further growth of droplets on plateaus. Meanwhile, the liquid columns in microgrooves swept large droplets on adjacent plateaus to refresh surface. The cooperation of liquid columns flowing and liquid bridge with tails sliding improved the renewal frequency of surface and increased the sweeping areas. Consequently, 15–43% higher heat flux was achieved on microgrooved hydrophobic surface compared with that on plain hydrophobic surface. On microgrooved surface with CuO nanostructures, the best heat transfer performance was attained. Enough high nucleation density was created to facilitate vapor condensation due to CuO nanostructures on the surface. The droplets at the bottom of microgrooves were sucked to spindle-shaped droplets on plateaus which promoted the renewal of surface. Interestingly, the large droplets suspended on microgrooves instead of permeating into microgrooves, which led to more areas in microgrooves for vapor condensation and smaller departure droplet size. In addition, the strong self-oscillation of falling droplets significantly increased the perturbation at liquid-vapor interface which contributed to the enhancement of heat and mass transfer. Ultimately, the tremendous enhancement of 55–102% for heat flux was reached on microgrooved surface with CuO nanostructures compared to that on plain hydrophobic surface.

Conflict of interest None.

Acknowledgement This research was supported by the Fundamental Research Funds for the Central Universities (2018YJS133) and National Key R&D Program of China (2018YFB0604301).

References [1] A.R. Parker, C.R. Lawrence, Water capture by a desert beetle, Nature 414 (2001) 33–34. [2] Y.M. Zheng, H. Bai, Z.B. Huang, X.L. Tian, F.Q. Nie, Y. Zhao, J. Zhai, L. Jiang, Directional water collection on wetted spider silk, Nature 463 (2010) 640–643. [3] J.M. Beér, High efficiency electric power generation: the environmental role, Prog. Energy Combust. Sci. 33 (2) (2007) 107–134. [4] M.K. Yadav, S. Khandekar, P.K. Sharma, An integrated approach to steam condensation studies inside reactor containments: a review, Nucl. Eng. Des. 300 (2016) 181–209. [5] C. Dang, L. Jia, Q.Y. Lu, Investigation on thermal design of a rack with the pulsating heat pipe for cooling CPUs, Appl. Therm. Eng. 110 (2017) 390–398. [6] J.B. Boreyko, Y.J. Zhao, C.H. Chen, Planar jumping-drop thermal diodes, Appl. Phys. Lett. 99 (23) (2011) 234105. [7] J. Oh, P. Birbarah, T. Foulkes, S.L. Yin, M. Rentauskas, J. Neely, R.C.N. PilawaPodgurski, N. Miljkovic, Jumping-droplet electronics hot-spot cooling, Appl. Phys. Lett. 110 (12) (2017) 123107. [8] A.D. Khawaji, I.K. Kutubkhanah, J.M. Wie, Advances in seawater desalination technologies, Desalination 221 (1) (2008) 47–69. [9] H. Kim, S.W. Yang, S.R. Rao, S. Narayanan, E.A. Kapustin, H. Furukawa, A.S. Umans, O.M. Yaghi, E.N. Wang, Water harvesting from air with metal-organic frameworks powered by natural sunlight, Science 356 (2017) 430–434. [10] J. Lee, S. Mahendra, P.J.J. Alvarez, Nanomaterials in the construction industry: a review of their applications and environmental health and safety considerations, ACS Nano 4 (7) (2010) 3580–3590. [11] E. Schmidt, W. Schurig, W. Sellschopp, Versuche über die Kondensation von Wasserdampf in Film- und Tropfenform, Technische Mechanik und Thermodynamik 1 (2) (1930) 53–63. [12] J.W. Rose, Dropwise condensation theory and experiment: a review, Proc. Inst. Mech. Eng., Part A: J. Power Energy 216 (2) (2002) 115–128. [13] J.W. Rose, Further aspects of dropwise condensation theory, Int. J. Heat Mass Transf. 19 (12) (1976) 1363–1370.

[14] R.F. Wen, Z. Lan, B.L. Peng, W. Xu, X.H. Ma, Y.Q. Cheng, Droplet departure characteristics and dropwise condensation heat transfer at low steam pressure, J. Heat Transf. 138 (7) (2016) 071501. [15] Y.L. Lee, T.H. Fang, Y.M. Yang, J.R. Maa, The enhancement of dropwise condensation by wettability modification of solid surface, Int. Commun. Heat Mass Transf. 25 (8) (1998) 1095–1103. [16] L.B. Boinovich, A.M. Emelyanenko, Hydrophobic materials and coatings: principles of design, properties and applications, Russ. Chem. Rev. 77 (7) (2008) 583–600. [17] M.H. Rausch, A.P. Fröba, A. Leipertz, Dropwise condensation heat transfer on ion implanted aluminum surfaces, Int. J. Heat Mass Transf. 51 (2008) 1061– 1070. [18] X.H. Ma, J.W. Rose, D.Q. Xu, J.F. Lin, B.X. Wang, Advances in dropwise condensation heat transfer: Chinese research, Chem. Eng. J. 78 (2) (2000) 87– 93. [19] D.J. Preston, D.L. Mafra, N. Miljkovic, J. Kong, E.N. Wang, Scalable graphene coatings for enhanced condensation heat transfer, Nano Lett. 15 (5) (2015) 2902–2909. [20] N. Miljkovic, R. Enright, E.N. Wang, Effect of droplet morphology on growth dynamics and heat transfer during condensation on superhydrophobic nanostructured surfaces, Acs Nano 6 (2) (2012) 1776–1785. [21] N. Miljkovic, R. Enright, Y. Nam, K. Lopez, N. Dou, J. Sack, E.N. Wang, Jumping droplet enhanced condensation on scalable superhydrophobic nanostructured surfaces, Nano Lett. 13 (1) (2013) 179–187. [22] H.J. Cho, D.J. Preston, Y.Y. Zhu, E.N. Wang, Nanoengineered materials for liquid–vapour phase-change heat transfer, Nat. Rev. Mater. 2 (2016) 16092. [23] R.F. Wen, Q. Li, J.F. Wu, G.S. Wu, W. Wang, Y.F. Chen, X.H. Ma, D.L. Zhao, R.G. Yang, Hydrophobic copper nanowires for enhancing condensation heat transfer, Nano Energy 33 (2017) 177–183. [24] C.S. Sharma, J. Combe, M. Giger, T. Emmerich, D. Poulikakos, Growth rates and spontaneous navigation of condensate droplets through randomly structured textures, ACS Nano 11 (2) (2017) 1673–1682. [25] J.B. Boreyko, C.H. Chen, Self-propelled dropwise condensate on superhydrophobic surfaces, Phys. Rev. Lett. 103 (18) (2009) 184501. [26] F.Q. Chu, X.M. Wu, B. Zhu, X. Zhang, Self-propelled droplet behavior during condensation on superhydrophobic surfaces, Appl. Phys. Lett. 108 (19) (2016) 194103. [27] P. Birbarah, N. Miljkovic, External convective jumping-droplet condensation on a flat plate, Int. J. Heat Mass Transf. 107 (2017) 74–88. [28] M.C. Lu, C.C. Lin, C.W. Lo, C.W. Huang, C.C. Wang, Superhydrophobic Si nanowires for enhanced condensation heat transfer, Int. J. Heat Mass Transf. 111 (2017) 614–623. [29] R.F. Wen, S.S. Xu, X.H. Ma, Y.C. Lee, R.G. Yang, Three-dimensional superhydrophobic nanowire networks for enhancing condensation heat transfer, Joule 2 (2018) 269–279. [30] R.F. Wen, S.S. Xu, D.L. Zhao, Y.C. Lee, X.H. Ma, R.G. Yang, Hierarchical superhydrophobic surfaces with micropatterned nanowire arrays for highefficiency jumping droplet condensation, ACS Appl. Mater. Interfaces 9 (51) (2017) 44911–44921. [31] K. Watanabe, Y. Udagawa, H. Udagawa, Drag reduction of Newtonian fluid in a circular pipe with a highly water-repellent wall, J. Fluid Mech. 381 (1999) 225–238. [32] A.D. Sommers, A.M. Jacobi, Wetting phenomena on micro-grooved aluminum surfaces and modeling of the critical droplet size, J. Colloid Interface Sci. 328 (2) (2008) 402–411. [33] M. Izumi, S. Kumagai, R. Shimada, N. Yamakawa, Heat transfer enhancement of dropwise condensation on a vertical surface with round shaped grooves, Exp. Therm. Fluid Sci. 28 (2004) 243–248. [34] P. Li, J. Xie, J. Cheng, K.K. Wu, Anisotropic wetting properties on a precisionground micro-V-grooved Si surface related to their micro-characterized variables, J. Micromech. Microeng. 24 (7) (2014) 075004. [35] K.O. Zamuruyev, H.K. Bardaweel, C.J. Carron, N.J. Kenyon, O. Brand, J.P. Delplanque, C.E. Davis, Continuous droplet removal upon dropwise condensation of humid air on a hydrophobic micropatterned surface, Langmuir 30 (33) (2014) 10133–10142. [36] Y.F. Zhong, A.M. Jacobi, J.G. Georgiadis, Effects of surface chemistry and groove geometry on wetting characteristics and droplet motion of water condensate on surfaces with rectangular microgrooves, Int. J. Heat Mass Transf. 57 (2) (2013) 629–641. [37] C.H. Lu, M. Beckmann, S. Unz, D. Gloess, P. Frach, E. Holst, A. Lasagni, M. Bieda, Heat transfer model of dropwise condensation and experimental validation for surface with coating and groove at low pressure, Heat Mass Transf. 52 (2016) 113–126. [38] C.H. Ma, S.X. Bai, X.D. Peng, Y.G. Meng, Anisotropic wettability of laser microgrooved SiC surfaces, Appl. Surf. Sci. 284 (Supplement C) (2013) 930–935. [39] B.J. Qi, J.S. Zhou, J.J. Wei, X. Li, Study on the wettability and condensation heat transfer of sine-shaped micro-grooved surfaces, Exp. Therm. Fluid Sci. 90 (Supplement C) (2018) 28–36. [40] Y. Nam, Y.S. Ju, A comparative study of the morphology and wetting characteristics of micro/nanostructured Cu surfaces for phase change heat transfer applications, J. Adhes. Sci. Technol. 27 (20) (2013) 2163–2176. [41] M.S. Liu, C.C. Lin, I.T. Huang, C.C. Wang, Enhancement of thermal conductivity with CuO for nanofluids, Chem. Eng. Technol. 29 (1) (2006) 72–77. [42] M.D. Porter, T.B. Bright, D.L. Allara, C.E.D. Chidsey, Spontaneously organized molecular assemblies. 4. Structural characterization of n-alkyl thiol

Q. Peng et al. / International Journal of Heat and Mass Transfer 130 (2019) 1096–1107 monolayers on gold by optical ellipsometry, infrared spectroscopy, and electrochemistry, JACS 109 (12) (1987) 3559–3568. [43] R.F. Wen, Z. Lan, B.L. Peng, W. Xu, X.H. Ma, Droplet dynamics and heat transfer for dropwise condensation at lower and ultra-lower pressure, Appl. Therm. Eng. 88 (2015) 265–273.

1107

[44] R. Enright, N. Miljkovic, N. Dou, Y. Nam, E.N. Wang, Condensation on superhydrophobic copper oxide nanostructures, J. Heat Transf.-Trans. Asme 135 (9) (2013) 091304. [45] D. Reguera, H. Reiss, Fusion of the extended modified liquid drop model for nucleation and dynamical nucleation theory, Phys. Rev. Lett. 93 (16) (2004) 165701.