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Optimization of fin surface wettability for promoting the dust removal in heat exchangers under frosting-defrosting conditions
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Optimization of fin surface wettability for promoting the dust removal in heat exchangers under frosting-defrosting conditions Dawei Zhuang , Yifei Yang , Guoliang Ding , Feilong Zhan PII: DOI: Reference:
S0140-7007(19)30432-3 https://doi.org/10.1016/j.ijrefrig.2019.10.010 JIJR 4548
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
27 August 2019 28 September 2019 13 October 2019
Please cite this article as: Dawei Zhuang , Yifei Yang , Guoliang Ding , Feilong Zhan , Optimization of fin surface wettability for promoting the dust removal in heat exchangers under frosting-defrosting conditions, International Journal of Refrigeration (2019), doi: https://doi.org/10.1016/j.ijrefrig.2019.10.010
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Highlights
The effect of wettability on dust removal by frosting-defrosting is investigated.
Dust removing rate on hydrophobic surface is higher than that on other samples.
Remnant dust weight on hydrophobic surface is far less than that on other samples.
Hydrophobic surface is identified as the optimal choice for promoting dust removal.
Optimization of fin surface wettability for promoting the dust removal in heat exchangers under frosting-defrosting conditions Dawei Zhuang, Yifei Yang, Guoliang Ding *, Feilong Zhan (Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China) * Corresponding Author, Tel: 86-21-34206378; Fax: 86-21-34206814; E-mail: [email protected]
Abstract: Dust removal by frosting-defrosting is a novel way to keep the cleanness of heat exchangers of air conditioners during long-term operation, and the effect depends on the fin surface wettability. The purpose of this study is to obtain the optimal choice of fin surface wettability for dust removal by testing dust removal characteristics on various surfaces, covering bare aluminum surface, bare copper surface, hydrophilic surface and hydrophobic surface. The temperatures of the air dry-bulb, air wet-bulb, the frosting surface and the defrosting surface in the experiments are 7 oC, 6 o
C, -15 oC and 5 oC, respectively. The results show that, the hydrophobic surface has the best dust
removal effectiveness; the remnant dust rates on the surfaces are 28.4%, 28.0%, 8.69% and 0.54% for bare aluminum plate, bare copper plate, hydrophilic copper plate and hydrophobic copper plate, respectively; the dust removal time on hydrophobic surface is much less than that on the other ones. Keywords: Dust removal; Defrosting; Frosting; Heat exchanger; Surface wettability.
1. Introduction Fin-and-tube heat exchangers are the commonly used type of outdoor unit heat exchangers in air-conditioning systems (Webb, 1990; Wang et al., 1996; Wang et al., 1997; Gu et al., 2017), and they are exposed to the atmosphere for heat transfer. The dust particles involved in the atmosphere may partly deposit and form the dust layer on fin surfaces of heat exchangers after long-term operation (Zhan et al., 2016; Zhan et al., 2018; Lin et al., 2019). The thermal conductivity of dust particles is far less than that of fins, and the formed dust layer on fins may block the air flow channel (Waring and Siegel, 2008; Pu et al., 2009), resulting in a significant deterioration of the heat exchangers performance (Li et al., 2019), e.g., Ahn et al (2003) found that the heat transfer capacity of heat exchangers may decrease by 10~15% and the air side pressure drop may increase by more than 44% after 7 years. In order to avoid the long-term performance deterioration of outdoor unit heat exchangers, the dust particles deposited on the fin surfaces should be removed timely from the fins, and a convenient and effective method for dust removal is needed. Utilizing the melting frost on the fins of outdoor unit heat exchangers during the frosting-defrosting cycles to peel the deposited dust particles is a convenient way of dust removal. The frosting-defrosting cycle occurs regularly on outdoor unit heat exchangers when the air-conditioning systems operate under heating conditions (Zhang et al., 2019). For the frosting stage, the water vapor in the moist air may penetrate through the dust layer and freeze to form frost on the fin surfaces when the fin surfaces temperature is lower than the freezing point, and the frost may absorb the dust particles to form the dusty frost. For the defrosting stage of the cycle, the dusty frost will melt and discharge from the fin surfaces under the effect of gravity, resulting in the dust removal of heat exchangers.
The effectiveness of the dust removal by frost depends on the mass of dust particles carried and removed with frost. The mass of dust particles carried by frost is strongly affected by the structure of frost formed at the frosting stage; the mass of dust particles removed with frost is determined by the discharge of melting dusty frost at the defrosting stage. In order to maximize the mass of dust particles carried and removed with frost, the frost structure as well as the discharge of melting dusty frost should be optimized. In both the frost structure and the discharge of melting dusty frost, the surface wettability of the fins plays a key role, and the hydrophobic surfaces may have different performances from those of hydrophilic surfaces (Lin and Kedzierski, 2019). When the fin surface is hydrophilic, the water vapor penetrating through the dust layer may condense into water film on the fin surface (Min et al., 2011) and then freeze to form the frost with dense structure, which is prone to adsorb lots of dust particle at the frosting stage; however, the dense frost is difficult to flow down completely at the defrosting stage due to the strong adhesion between the dense frost and the hydrophilic surface. When the fin surface is hydrophobic, the water vapor may freeze to form the frost with loose structure (Wu et al., 2007), and only a few of dust particles can be adsorbed in the frost; however, the loose frost is much easier to fall off the fin surface during the melting process compared to the dense frost (Chu et al., 2016). In order to choose the optimal surface wettability for promoting the dust removal, two effects of the surface wettability should be known, i.e., (1) effect of surface wettability on the frost structure at the frosting stage; (2) effect of surface wettability on the discharge of melting dusty frost at the defrosting stage. The effect of surface wettability on the frost structure has been obtained by the current researchers, including theoretical models and experimental studies. The structure of frost can be
considered as the porous media consisting of small ice particles and air, and the diffusion of water vapor through the frost layer increases the frost density (O’Neal and Tree, 1984). Sami and Duong (1989), Lee et al. (2003) and Yao et al. (2004) develop the model for predicting the average thickness of frost layer without considerations of surface’s geometrical structure. Lenic et al. (2009) simulate the frost deposition by calculating the water vapor diffusion in the porous media. Consequently, it is required to give the frost layer thickness and density at the early stage as the initial condition. A semi-empirical quasi-steady model is proposed by Padki et al. (1989) to predict the frost growth process based on the heat transfer rate correlations ( Holman et al. ,1972) and the mass transfer rate. Lee and Ro (2005) propose a simple model which was built on the assumption that the water vapor concentration at the frost surface is saturated, and that the gradient of vapor pressure is the same as the value obtained from the Clausius Clapeyron equation. The effect of surface wettability on the discharge of melting frost at the defrosting stage are only focus on dust-free frost. The defrosting efficiency is high on a hydrophobic surface with a short frost layer melting time (Jhee et al., 2002). Kim and Lee (2011) compare defrosting characteristics with various surface wettability by fabricating hydrophilic, bare, and hydrophobic surfaces on small test samples. The results show that the differences of defrosting time between various samples were not significant; moreover, retained water is lower in the hydrophilic surface as it appeared only as a film on that surface, whereas droplets formed on the hydrophobic surface. Rahman and Jacobi (2012a; 2012b; 2015) performed defrosting experiments by fabricating a hydrophobic surface with microgrooves. These microgroove structures were found to be very effective in reducing the amount of retained water and lowering the tilt angle of the droplet on the surface. Liang et al. (2015; 2016) and Wang et al. (2015a) analyzed defrosting characteristics for hydrophilic, bare, hydrophobic, and
super-hydrophobic surfaces. A super-hydrophobic surface with a high contact angle resulted in outstanding defrosting performance because of the short defrosting time and less water retention. Wang et al. (2015b) propose a defrosting method using forced convection on a low adhesion super-hydrophobic surface. As the dusty frost is a mixture of frost and dust particles, the frost structure and the physical properties of dusty frost are different from those of dust-free frost, leading to entirely different adhesion characteristics between frost and fin surfaces. The effect of surface wettability on the discharge of melting dusty frost at the defrosting stage is different from that of dust-free frost, which cannot be evaluated by the prediction methods for dust-free frost developed by the existing researches. The purpose of the present study is to experimentally investigate the effect of surface wettability on the discharge of melting dusty frost from fin surfaces, and obtain the optimal surface wettability for promoting the dust removal.
2. Design of experiment 2.1 Objective and technical route
The objective of the present experimental study is to investigate the effect of surface wettability on the removal characteristics of dust fouling layer by frosting-defrosting of heat exchangers, including the dust removing rate and remnant dust weight during the defrosting process. The dust layer removal by defrosting of heat exchangers consists of four physical processes: 1) operation with the clean fin surfaces, 2) dust fouling layer formation on the fin surfaces, 3) frost formation on the dust-covered fin surfaces, 4) dust removal with the defrosting water from the fin
surfaces, as shown in Fig. 1. Correspondingly, four procedures are performed in the experiments. Firstly, prepare the clean fin samples with various surface wettability employed in the heat exchangers; secondly, cover these fin samples by the dust fouling layer with the same mass and thickness; thirdly, adjust the temperature of the samples below the freezing point and form the frost on the dusty samples; fourthly, melt the frost on the samples and test the dust removing rate and the remnant dust weight. The entire technical route is shown in Fig. 1. The details for the above 4 procedures are given in Sections 2.2, 2.3, 2.4, and 2.5, respectively. Clean fin surface Accumulated droplets
Accumulated frost layer
Frost layer blocking
Actual physical processes of
Dust layer
Dust layer
Clean fin
dust removal
Experimental procedures of dust removal
Dust layer
Frost
t=0s
Process 1: Operation on clean fins
Process 2: Dust deposition on fins
Process 3: Frost formation on dusty fins
Procedure 1: Preparation of clean test samples
Procedure 2: Pre-deposition of dust layer on test samples
Procedure 3: Frosting on dust predeposited test samples
Bare copper plate
Dusty air flow
Bare aluminum plate
t = 100 s
Process 4: Dust removal with the defrosting water
Procedure 4: Defrosting and tests of dust removal
Moist air
Frost melting on dust sample Dust layer on fin sample
Defrosting t = 750 s water
Test of remnant dust weight
Frost on dusty sample
Hydrophilic copper plate Hydrophobic copper plate
Clean fin samples
Cool capacity supplied by semiconductor
Heat capacity supplied by semiconductor
Test of dusty removing rate
Fig. 1 Schematic diagram of the experimental technical route
2.2 Preparation of clean test samples with various surface wettability
Various surface wettability of test samples is achieved by the surface modification treatment on substrates. Aluminum and copper, which are widely used as the fin materials in the air conditioners, are chosen as the substrate materials. The hydrophilic and hydrophobic coatings are needed to be manufactured on the substrate surface to investigate the effect of surface wettability on the dust
t = 1300 s
removal; as the surface modification treatment on copper surface is more convenient than on aluminum surface, copper is used as the substrate material to make hydrophilic and hydrophobic surfaces in this study. Thus, four test samples with different surface wettability are chosen in the experiment, i.e. bare aluminum plate, bare copper plate, hydrophilic copper plate and hydrophobic copper plate; the sizes of test samples are 50 mm×50 mm×0.5 mm, as shown in Fig. 2(a). The bare aluminum plate and bare copper plate both are prepared by polishing the plate surfaces with 3000-grit sandpaper. The hydrophilic copper plate and hydrophobic copper plate are fabricated by the micro/nanostructured modification on bare copper plates. The hydrophilic surface modification is performed by immersing the bare copper plate into an aqueous solution of NaOH and (NH 4)2S2O8 for 30 min, and the concentrations of NaOH and (NH4)2S2O8 in the aqueous solution are 2.5 mol L-1 and 0.1 mol L-1, respectively. The hydrophobic surface modification is performed by immersing the hydrophilic copper plate into an aqueous solution of C12H26S for 15 min, and the concentration of C12H26S is 0.1 mol L-1. The SEM photos of all test sample surfaces are shown in Fig. 2(b). To represent the surface characteristics of prepared test samples, the contact angles and the roughness are tested. The contact angles are measured through photographing the contours of droplets on test samples by CCD camera, and the static contact angles of the bare aluminum plate, the bare copper plate, the hydrophilic and hydrophobic copper plate are obtained as 74o, 88o, 10o, and 162o, respectively, as shown in Fig. 2(c); the roughness is measured by Atomic Force Microscope. The detailed values of contact angles and roughness of clean test samples are listed in Table 1.
Bare aluminum plate
Bare copper plate
Hydrophilic plate
Hydrophobic plate
(a) Photos of test samples
(b) SEM photos of microstructure on test samples
(c) CCD photos of water droplets on test samples
θst= 74o±1o
θst= 10o±1o
θst= 88o±1o
θst= 162o±1o
Fig. 2 Test samples employed in the experiments Table 1 Surface wettability and roughness of test samples. Static contact angle
Contact angle
Experimental sample
Surface roughness θst
hysteresis θH
Aluminum plate
74o
31o
0.08 μm
Copper plate
88o
24o
0.15 μm
Hydrophilic copper plate
10o
14o
2.74 μm
Hydrophobic copper plate
162o
7o
2.06 μm
2.3 Pre-deposition of dust layer on test samples
The pre-deposition of dust layer on test samples is achieved by blowing dusty air flow on the test samples and compelling the dust particles to adhere on the surface. To perform the pre-deposition of dust layer, an apparatus with 9 components is established, including air compressor, volume flow meter, mixing box, dust container, electric motor, air duct, dust concentration sensor, test sample and fixture. The schematic and photo of the apparatus are shown as Figs. 3(a) and 3(b), respectively.
The pre-deposition experiment consists of four procedures: 1) supplying the clean air by the air compressor flow into the mixing box; 2) generating the dust particles by the dust container with an electric motor and mixing the particles with the clean air; 3) inducing the dusty air flow through the test sample; 4) depositing the dust particles onto the test sample surface. The experimental conditions of the inlet air velocity, the dust particle concentration of the dusty air, the dust components and the blowing time are adjusted in the dust pre-deposition process. The inlet air velocity is controlled by a flow regulating valve, and is measured by a volume flow meter with a precision of ±2 L min-1; the dust particle concentration is controlled by the speed of the electric motor, and is measured by a dust concentration sensor with a precision of ±0.5 g m-3. The measurement ranges and uncertainties of all the experimental instruments are listed in Table 2. The dust is premixed by two components, including SiO2 and carbon powder (Zhan et al., 2016). The operation of the experimental rig and the experimental conditions are also illustrated by Zhan et al. (2016). The detailed experimental conditions are shown in Table 3. The SEM photos of the dust layers on various samples are obtained to ensure that the deposited dust clusters have the similar structure, as shown in Fig. 4. The scale of deposited dust clusters is at least an order of magnitude greater than that of the microstructures of the test samples. Therefore, the surface microstructures have little effect on the structure of pre-deposited dust clusters. The repeatability tests of the weight of the pre-deposited dust layers are conducted to ensure the reliability of the pre-deposition method. Twelve sets of experimental data of dust layer weight are obtained, including four test samples with three times of dust pre-deposition. The results show that the maximum relative deviation of dust weights on various test samples is 4.9%.
1-Air compressor
5-Electric motor
4-Dust container
7-Dust concentration sensor 8-Test sample
2-Volume flow meter
3-Mixing box
6-Air duct
9-Fixture
a) Schematic of the apparatus
b) Photo of the apparatus Fig. 3 Apparatus for dust pre-deposition on test samples
(a) Bare aluminum plate
(b) Bare copper plate
(c) Hydrophilic copper plate
(d) Hydrophobic copper plate
Fig. 4 SEM photos of deposited dust clusters on various samples
Table 2 Measurement ranges and uncertainties of experimental instruments. Instrument
Model
Range
Uncertainty
Volume flow meter
HTLZD-15/F10 (GuangJi)
1.2 to 12 m3 h-1
±2 L min-1
Thermocouple
5TC-TT-T-20-36 (Omega)
-40 to 200 oC
±0.1 oC
-50 to 100 oC
±0.1 oC
0 to100%
±0.8%RH
Temperature and humidity HC2-SH (ROTRONIC) sensor Speed controller
3IK15RGN-C (OuGuan)
9 to 135 r min-1
±1 r min-1
Dust concentration sensor
ZHKD-FG (Zhonghang Electric)
0 to 20g m-3
±0.5 g m-3
Table 3 Experimental conditions of dust pre-deposition. Parameters
Value
Inlet air velocity
2 m s-1
Dust particle concentration
10.8 g m-3 75% SiO2,
Dust components 25% carbon powder Dust pre-deposition time
30 minutes
2.4 Frosting on dust pre-deposited test samples
The method for frosting on dust pre-deposited test samples is to cool the test samples below the freezing point, and make the water vapor in the air condense and freeze on sample surfaces. The cooling of test samples is conducted in an experimental apparatus composed of a cooling module and a linear guideway, as shown in Figs. 5(a) and 5(b). The cooling module is assembled by a thermoelectric element (TEC), a type-T thermocouple, a heat sink, an aluminum plate and a piece of insulated cotton. TEC is used to supply the cooling capacity; the temperature controller is used to control the temperature of the sample surface by adjusting the input power of TEC; the aluminum plate immersed with the type-T thermocouple covers TEC for the fixation and temperature monitoring of the test sample; the insulated cotton is used to cover on the aluminum plate around the test sample for avoiding the frost formation around the test sample. The linear guideway assembled by a guideway, a slide block with speed-adjustable stepper motor and a paper tape is used to fasten the cooling module. The experimental conditions of the frosting on test samples cover the temperature of sample surfaces of -15 oC, the air dry-bulb temperature of 7 oC, the air wet-bulb temperature of 6 oC and the frosting time of 2 hours. The air dry-bulb temperature and wet-bulb temperature in the experiments are referred to the Chinese National Standard for Room Air Conditioners GB/T 7725-2004 (AQSIQ, 2004); the temperature of sample surfaces and the frosting time are referred to the actual frosting conditions of air-conditioners. The detailed experimental conditions are shown in Table 4.
Cooling module 1 2 3 4
5
6
7
8
1-Thermoelectric element (TEC), 2-Aluminum plate with type-T thermocouple, 3-Insulated cotton, 4-Heat sink, 5-Test sample, 6-Guideway, 7-Slide block with speed-adjustable stepper motor, 8-Paper tape,
(a) Schematic of experimental apparatus
(b) Photo of experimental apparatus Fig. 5 Experimental apparatus for frosting on dust pre-deposited test samples Table 4 Experimental conditions of frosting. Parameters
Value
Temperature of sample surfaces
-15 oC
Air dry-bulb temperature
7 oC
Air wet-bulb temperature
6 oC
Frosting time
2 hours
2.5 Defrosting and measurements of dust removal characteristics
Defrosting experiments are carried out by increasing the temperature of test samples through switching electrodes of TEC shown in the Fig. 5. When the temperature of test samples increases up to 5 oC and the defrosting process begins, the measurements of dust removal characteristics by the melted frost are performed. The measured parameters for dust removal characteristics include remnant dust weight and dust removing rate. For obtaining the remnant dust weight, four procedures are performed in the experiments: 1) measure the weight of the clean test sample Mt; 2) dry the test sample when the dust removal process is completed; 3) measure the total weight of the test sample and the residuum Mtotal; 4) calculate the remnant dust weight Mr by subtracting Mt from Mtotal. For obtaining the dust removing rate, a paper tape with scale is laid on the slide block of the linear guideway and is compelled to move below the test sample forced by the speed-adjustable stepper motor during the defrosting process, as shown in Fig. 5. When the defrosting process begins, the paper tape and the slide block move along the linear guideway with a certain velocity v controlled by the stepper motor, and the dust removed by the melt frost continue to drop down from the test sample onto the moving paper tape. When the defrosting process is completed, the paper tape is dried and clipped into 30 scraps with certain width w, and the weight of the dust on each scrap is measured as Mi (i ranges from 1 to 30). And the dust removing rate can be calculated by the moving velocity v, the width of scraps w and the weight of the dust on each scrap Mi. The measurement uncertainty of dust removing rate and remnant dust weight is determined by the instrument deviation of the analytical balance used in the experiments, which is 0.0001g.
3. Effect of wettability on defrosting behaviors on dust pre-deposited surfaces The dust removing rate and remnant dust weight both depend on the defrosting behaviors on the test samples. In order to choose the surface wettability with the best dust removal performance, the defrosting behaviors on test samples with various wettability are observed, and the mechanism of dust removal by defrosting is analyzed.
3.1 Defrosting behaviors on test samples
The defrosting process on clean test samples with various wettability are shown in Fig. 6. For the bare aluminum plate and the bare copper plate, the frost with porous structure may absorb the adjacent defrosting water, and a frost-water mixture layer forms on the surface; then the size of the mixture layer becomes smaller and smaller due to the melt and removal of the frost, and a large amount of water droplets retain on the surface when the defrosting process is completed. For the hydrophilic copper plate, the melted frost forms the water layer on the surface and flow downward along the direction of gravity during the defrosting process, and only a few water droplets remain at the bottom part of the test sample. The observed frost removal pattern on the hydrophilic surface is different from that mentioned in the previous study (Kim et al., 2018). In this study, the frost on the hydrophilic surface first entirely melts into water and then the water will flow off; while in the study of Kim et al., the frost may directly slide down from the surface. The reason for the difference of the frost removal pattern is that, the frosting time employed in the previous research is twice longer than that applied in this study, and the longer frosting time may result in a heavier frost layer which is prone to slide down before the frost is able to melt completely.
For the hydrophobic copper plate, the frost layer falls off as a whole piece with little water remaining on the surface. The falling frost layer maintains a whole piece due to the loose structure of the frost layer on the hydrophobic surface. The loose frost layer has a small contact region with the surface. Once the frost at the contact region melts, the adhesion of the surface is too weak to support the frost layer, and the frost layer may fall as a whole piece. The frost removal pattern on the hydrophobic surface observed in this study is consistent with that obtained by other researchers (Liang et al., 2015; Kim et al., 2018). The defrosting process on dusty test samples with various wettability are shown in Fig. 7. For the bare aluminum plate and the bare copper plate, the pre-deposited dust is carried by water droplets from the melted frost and is removed through the discharge of water droplets; the dust on the moving path are swept away by the droplets, causing a nonuniform distribution of the remnant dust on plates. For the hydrophilic copper plate, the dust is covered by the water film from the melted frost and is removed as the water film flows away from the surface; the remnant dust is almost uniform-distributed on the surface during the defrosting process. For the hydrophobic copper plate, the dust is involved in the formed frost-water mixture layer and is removed with the falling of the mixture layer; almost no remnant dust is observed on the surface after the defrosting process. According to the observation results shown in Figs. 6 and 7, the surface wettability has a significant effect on the dust removal performance during the defrosting process. Among all the four test samples prepared in this study, the remnant dust on the two bare plates is much more than that on the hydrophilic copper plate, and only the dust on the hydrophobic copper plate can be removed thoroughly.
Defrosting time (s) Test samples 0s
30 s
60 s
120 s
Bare aluminum plate
Bare copper plate
Hydrophilic copper plate
Hydrophobic copper plate
Fig. 6 Defrosting behaviors on clean test samples with various wettability
Defrosting time (s) Test samples 0s
30 s
60 s
120 s
Bare aluminum plate
Bare copper plate
Hydrophilic copper plate
Hydrophobic copper plate
Fig. 7 Defrosting behaviors on dusty test samples with various wettability
3.2 Mechanism of dust removal by defrosting
The mechanism of dust removal by defrosting depends on the removal carrier of dust particles, which is influenced by surface wettability, as shown in Fig. 8. For the bare aluminum plate and the bare copper plate, the removal carrier of dust particles is water droplets from the melted frost. The water droplets may move downwards under the gravity and gather at the bottom part of the surface. The dust particles which are not located at the move path of the water droplets still remain on the surface after the defrosting process. Moreover, the dust particles involved in the hanged droplets at the bottom part may redeposit on the test sample when the hanged droplets are dried, which is consistent with the phenomenon observed by Yang et al. (2019). For the hydrophilic copper plate, the removal carrier of dust particles is the water film from the melted frost. The dust particles mix with water film and become the wet dust particles. Part of the wet dust particles fall off the surface along with the flow of water film, and the other particles may redeposit on the hydrophilic surface as the water film is dried. The remnant dust particles are distributed uniformly on the hydrophilic surface, which is consistent with the observation results reported by other researchers (Yang et al., 2019). For the hydrophobic copper plate, the removal carrier of dust particles is the entire frost layer. The porous structure of the frost layer on the hydrophobic copper plate is much more loosen than that on the other samples. When the frost close to the surface is melted to water film, the adhesion force between the water film and the surface is not able to sustain the weight of the dust frost layer due to the extremely low contact angle hysteresis, and the entire frost layer with the dust particles involved in it will directly fall off the surface. The dust layer on the hydrophobic surface is almost cleared up due to the separation action of
frosting, as shown in Fig. 9. A lot of small gaps exist between the dust particles and the hydrophobic surface, and water vapor may permeate into the gaps and condenses. As the condensate in the gaps freezes into frost, the gaps are expanded, and the dust particles are separated with the surface. The separation of dust particles and surface may weaken the surface adhesion and make the dust particles fall off with the whole frost layer.
Fig. 8 Mechanism of dust removal by defrosting on test samples with various wettability
Hydrophobic surface
Dust particle
Defrosting water
Frost
Water vapor Condensate
Permeation and condensation of water vapor
Separation of dust particles and surface
Fall of dust particles with frost layer
Fig. 9 Destructive action of frosting on dust layer on hydrophobic surface
4. Effects of surface wettability on dust removing characteristics The dust removal characteristics includes of the dust removing rate and the final remnant dust weight. In order to quantitively compare the effect of surface wettability on dust removal characteristics, the measurements of the dust removing rate and the final remnant dust weight and are needed.
4.1 Effect of surface wettability on dust removing rate
Figure 10 shows the variation of remnant dust weight along with the time during the defrosting process for the test samples with various wettability. It can be seen that, the remnant dust weights on the four test samples remain constant at the beginning of the defrosting process, and then decrease with the time until the steady state is reached. Moreover, the steady time of dust removal process for the hydrophobic copper plate is much less than that for the other three plates. The steady time of dust removal process is determined by the fall velocity of dust removal carrier. For the hydrophobic copper plate, the dust removal carrier is the frost layer with loose porous structure, and the adhesion force between the carrier and the plate surface is too weak to remain adhesion on sample surface. For the other three plates, the dust removal carrier is liquid water, and the dust particles are carried by the water and fall off gradually, which will take a long time for the dust removal process to reach the steady state.
Weight of remnant dust, W (g)
0.35 Bare aluminum plate Bare copper plate Hydrophilic copper plate Hydrophobic copper plate
0.30 0.25 0.20 0.15 0.10 0.05 0.00 0
20
40
60
80
100
120
Time, t (s)
Fig. 10 Variation of remnant dust weight along with the time during the defrosting process
4.2 Effect of surface wettability on final remnant dust weight
Figure 11 shows the final remnant dust weights on the test samples with various wettability after the dust removal process. It can be seen that, the remnant dust rates on the surfaces are 28.4%, 28.0%, 8.69% and 0.54% for bare aluminum plate, bare copper plate, hydrophilic copper plate and hydrophobic copper plate, respectively. The remnant dust weight on the hydrophilic surface is nearly 70% less than that on bare surfaces. The possible reason is that, as the carrier of dust particles on the hydrophilic surface, the water film can adhere and remove much more dust particles compared to the discrete water beads on bare surfaces. The remnant dust weight on the hydrophobic surface is nearly 93% less than that on the hydrophilic surface. This is because the residue of defrosted water on the hydrophobic surface is far less than that on the hydrophilic surface, and less residue of water will lead to less particles redeposited on the surface.
Compared to the remnant dust weight on the hydrophobic surface measured by Yang et al. (2019), the remnant dust weight on hydrophobic surface obtained in this study reduces by nearly 94%. The reason for the reduction of the remnant dust weight is that, the frost used as the carrier in this study can remove much more dust particles than the water beads used as the carrier in the study of Yang et al. (2019). The frost on hydrophobic surfaces is porous; the surface area of porous frost is far larger than that of water beads so that the frost can adhere and remove more dust particles. In the actual heat exchangers, the tubes are inserted into the fins, which may deteriorate the dust removal performance of the surfaces since the tubes will impede the fall of the dusty frost and the dusty water from the heat exchangers. Moreover, the performance deterioration of the hydrophobic surface is less than that of the bare surfaces and the hydrophilic surface, because the dusty frost falling from the hydrophobic surface is harder to adhere and redeposit on the tubes compared to the dusty water on other three surfaces. remnant dust on test samples
removal dust from test samples
0.0802 g
Bare aluminum 0.0776 g
Bare copper 0.0253 g
Hydrophilic copper 0.0015 g
Hydrophobic copper
0.05
0.1
0.15
0.2
0.25
0.3
Dust weight, W(g)
Fig. 11 Final remnant dust weight on test samples with various wettability
5. Conclusions The optimal choice of fin surface wettability for promoting the dust removal is obtained experimentally through testing dust removal characteristics on various surfaces, covering bare aluminum surface, bare copper surface, hydrophilic surface and hydrophobic surface. The temperatures of the air dry-bulb, air wet-bulb, the frosting surface and the defrosting surface in the experiments are 7oC, 6 oC, -15 oC and 5 oC, respectively. The following conclusions are obtained. 1) The dust removing rate on the hydrophobic surface is much higher than that on the other three surfaces. The dust particles on the hydrophobic surface is prone to fall off with the entire frost in an instant, and the dust particles on the other three surfaces is removed by the defrosting water slowly. 3) The remnant dust weight on the hydrophobic surface is far less than that on the other ones. The remnant dust rates on the surfaces are 28.4%, 28.0%, 8.69% and 0.54% for bare aluminum plate, bare copper plate, hydrophilic copper plate and hydrophobic copper plate, respectively. 3) The hydrophobic surface with highest dust removing rate and least remnant dust weight is identified as the optimal choice of fin surface wettability for promoting the dust removal among all the four test samples in this study. 4) The dust removal performance of the surfaces when inserted between tubes will deteriorate since the tubes may impede the fall of the dusty frost and the dusty water in the actual heat exchangers. The performance deterioration of the hydrophobic surface caused by the tubes is less than that of the bare surfaces and the hydrophilic surface due to the weaker adhesion between the tube and the dusty frost compared to the dusty water.
Acknowledgements The authors gratefully acknowledge the supports from the National Natural Science Foundation of China (Grant No. 51606119 & 51906135), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51521004).
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Optimization of fin surface wettability for promoting the dust removal in heat exchangers under frosting-defrosting conditions Dawei Zhuang , Yifei Yang , Guoliang Ding , Feilong Zhan PII: DOI: Reference:
S0140-7007(19)30432-3 https://doi.org/10.1016/j.ijrefrig.2019.10.010 JIJR 4548
To appear in:
International Journal of Refrigeration
Received date: Revised date: Accepted date:
27 August 2019 28 September 2019 13 October 2019
Please cite this article as: Dawei Zhuang , Yifei Yang , Guoliang Ding , Feilong Zhan , Optimization of fin surface wettability for promoting the dust removal in heat exchangers under frosting-defrosting conditions, International Journal of Refrigeration (2019), doi: https://doi.org/10.1016/j.ijrefrig.2019.10.010
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Highlights
The effect of wettability on dust removal by frosting-defrosting is investigated.
Dust removing rate on hydrophobic surface is higher than that on other samples.
Remnant dust weight on hydrophobic surface is far less than that on other samples.
Hydrophobic surface is identified as the optimal choice for promoting dust removal.
Optimization of fin surface wettability for promoting the dust removal in heat exchangers under frosting-defrosting conditions Dawei Zhuang, Yifei Yang, Guoliang Ding *, Feilong Zhan (Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China) * Corresponding Author, Tel: 86-21-34206378; Fax: 86-21-34206814; E-mail: [email protected]
Abstract: Dust removal by frosting-defrosting is a novel way to keep the cleanness of heat exchangers of air conditioners during long-term operation, and the effect depends on the fin surface wettability. The purpose of this study is to obtain the optimal choice of fin surface wettability for dust removal by testing dust removal characteristics on various surfaces, covering bare aluminum surface, bare copper surface, hydrophilic surface and hydrophobic surface. The temperatures of the air dry-bulb, air wet-bulb, the frosting surface and the defrosting surface in the experiments are 7 oC, 6 o
C, -15 oC and 5 oC, respectively. The results show that, the hydrophobic surface has the best dust
removal effectiveness; the remnant dust rates on the surfaces are 28.4%, 28.0%, 8.69% and 0.54% for bare aluminum plate, bare copper plate, hydrophilic copper plate and hydrophobic copper plate, respectively; the dust removal time on hydrophobic surface is much less than that on the other ones. Keywords: Dust removal; Defrosting; Frosting; Heat exchanger; Surface wettability.
1. Introduction Fin-and-tube heat exchangers are the commonly used type of outdoor unit heat exchangers in air-conditioning systems (Webb, 1990; Wang et al., 1996; Wang et al., 1997; Gu et al., 2017), and they are exposed to the atmosphere for heat transfer. The dust particles involved in the atmosphere may partly deposit and form the dust layer on fin surfaces of heat exchangers after long-term operation (Zhan et al., 2016; Zhan et al., 2018; Lin et al., 2019). The thermal conductivity of dust particles is far less than that of fins, and the formed dust layer on fins may block the air flow channel (Waring and Siegel, 2008; Pu et al., 2009), resulting in a significant deterioration of the heat exchangers performance (Li et al., 2019), e.g., Ahn et al (2003) found that the heat transfer capacity of heat exchangers may decrease by 10~15% and the air side pressure drop may increase by more than 44% after 7 years. In order to avoid the long-term performance deterioration of outdoor unit heat exchangers, the dust particles deposited on the fin surfaces should be removed timely from the fins, and a convenient and effective method for dust removal is needed. Utilizing the melting frost on the fins of outdoor unit heat exchangers during the frosting-defrosting cycles to peel the deposited dust particles is a convenient way of dust removal. The frosting-defrosting cycle occurs regularly on outdoor unit heat exchangers when the air-conditioning systems operate under heating conditions (Zhang et al., 2019). For the frosting stage, the water vapor in the moist air may penetrate through the dust layer and freeze to form frost on the fin surfaces when the fin surfaces temperature is lower than the freezing point, and the frost may absorb the dust particles to form the dusty frost. For the defrosting stage of the cycle, the dusty frost will melt and discharge from the fin surfaces under the effect of gravity, resulting in the dust removal of heat exchangers.
The effectiveness of the dust removal by frost depends on the mass of dust particles carried and removed with frost. The mass of dust particles carried by frost is strongly affected by the structure of frost formed at the frosting stage; the mass of dust particles removed with frost is determined by the discharge of melting dusty frost at the defrosting stage. In order to maximize the mass of dust particles carried and removed with frost, the frost structure as well as the discharge of melting dusty frost should be optimized. In both the frost structure and the discharge of melting dusty frost, the surface wettability of the fins plays a key role, and the hydrophobic surfaces may have different performances from those of hydrophilic surfaces (Lin and Kedzierski, 2019). When the fin surface is hydrophilic, the water vapor penetrating through the dust layer may condense into water film on the fin surface (Min et al., 2011) and then freeze to form the frost with dense structure, which is prone to adsorb lots of dust particle at the frosting stage; however, the dense frost is difficult to flow down completely at the defrosting stage due to the strong adhesion between the dense frost and the hydrophilic surface. When the fin surface is hydrophobic, the water vapor may freeze to form the frost with loose structure (Wu et al., 2007), and only a few of dust particles can be adsorbed in the frost; however, the loose frost is much easier to fall off the fin surface during the melting process compared to the dense frost (Chu et al., 2016). In order to choose the optimal surface wettability for promoting the dust removal, two effects of the surface wettability should be known, i.e., (1) effect of surface wettability on the frost structure at the frosting stage; (2) effect of surface wettability on the discharge of melting dusty frost at the defrosting stage. The effect of surface wettability on the frost structure has been obtained by the current researchers, including theoretical models and experimental studies. The structure of frost can be
considered as the porous media consisting of small ice particles and air, and the diffusion of water vapor through the frost layer increases the frost density (O’Neal and Tree, 1984). Sami and Duong (1989), Lee et al. (2003) and Yao et al. (2004) develop the model for predicting the average thickness of frost layer without considerations of surface’s geometrical structure. Lenic et al. (2009) simulate the frost deposition by calculating the water vapor diffusion in the porous media. Consequently, it is required to give the frost layer thickness and density at the early stage as the initial condition. A semi-empirical quasi-steady model is proposed by Padki et al. (1989) to predict the frost growth process based on the heat transfer rate correlations ( Holman et al. ,1972) and the mass transfer rate. Lee and Ro (2005) propose a simple model which was built on the assumption that the water vapor concentration at the frost surface is saturated, and that the gradient of vapor pressure is the same as the value obtained from the Clausius Clapeyron equation. The effect of surface wettability on the discharge of melting frost at the defrosting stage are only focus on dust-free frost. The defrosting efficiency is high on a hydrophobic surface with a short frost layer melting time (Jhee et al., 2002). Kim and Lee (2011) compare defrosting characteristics with various surface wettability by fabricating hydrophilic, bare, and hydrophobic surfaces on small test samples. The results show that the differences of defrosting time between various samples were not significant; moreover, retained water is lower in the hydrophilic surface as it appeared only as a film on that surface, whereas droplets formed on the hydrophobic surface. Rahman and Jacobi (2012a; 2012b; 2015) performed defrosting experiments by fabricating a hydrophobic surface with microgrooves. These microgroove structures were found to be very effective in reducing the amount of retained water and lowering the tilt angle of the droplet on the surface. Liang et al. (2015; 2016) and Wang et al. (2015a) analyzed defrosting characteristics for hydrophilic, bare, hydrophobic, and
super-hydrophobic surfaces. A super-hydrophobic surface with a high contact angle resulted in outstanding defrosting performance because of the short defrosting time and less water retention. Wang et al. (2015b) propose a defrosting method using forced convection on a low adhesion super-hydrophobic surface. As the dusty frost is a mixture of frost and dust particles, the frost structure and the physical properties of dusty frost are different from those of dust-free frost, leading to entirely different adhesion characteristics between frost and fin surfaces. The effect of surface wettability on the discharge of melting dusty frost at the defrosting stage is different from that of dust-free frost, which cannot be evaluated by the prediction methods for dust-free frost developed by the existing researches. The purpose of the present study is to experimentally investigate the effect of surface wettability on the discharge of melting dusty frost from fin surfaces, and obtain the optimal surface wettability for promoting the dust removal.
2. Design of experiment 2.1 Objective and technical route
The objective of the present experimental study is to investigate the effect of surface wettability on the removal characteristics of dust fouling layer by frosting-defrosting of heat exchangers, including the dust removing rate and remnant dust weight during the defrosting process. The dust layer removal by defrosting of heat exchangers consists of four physical processes: 1) operation with the clean fin surfaces, 2) dust fouling layer formation on the fin surfaces, 3) frost formation on the dust-covered fin surfaces, 4) dust removal with the defrosting water from the fin
surfaces, as shown in Fig. 1. Correspondingly, four procedures are performed in the experiments. Firstly, prepare the clean fin samples with various surface wettability employed in the heat exchangers; secondly, cover these fin samples by the dust fouling layer with the same mass and thickness; thirdly, adjust the temperature of the samples below the freezing point and form the frost on the dusty samples; fourthly, melt the frost on the samples and test the dust removing rate and the remnant dust weight. The entire technical route is shown in Fig. 1. The details for the above 4 procedures are given in Sections 2.2, 2.3, 2.4, and 2.5, respectively. Clean fin surface Accumulated droplets
Accumulated frost layer
Frost layer blocking
Actual physical processes of
Dust layer
Dust layer
Clean fin
dust removal
Experimental procedures of dust removal
Dust layer
Frost
t=0s
Process 1: Operation on clean fins
Process 2: Dust deposition on fins
Process 3: Frost formation on dusty fins
Procedure 1: Preparation of clean test samples
Procedure 2: Pre-deposition of dust layer on test samples
Procedure 3: Frosting on dust predeposited test samples
Bare copper plate
Dusty air flow
Bare aluminum plate
t = 100 s
Process 4: Dust removal with the defrosting water
Procedure 4: Defrosting and tests of dust removal
Moist air
Frost melting on dust sample Dust layer on fin sample
Defrosting t = 750 s water
Test of remnant dust weight
Frost on dusty sample
Hydrophilic copper plate Hydrophobic copper plate
Clean fin samples
Cool capacity supplied by semiconductor
Heat capacity supplied by semiconductor
Test of dusty removing rate
Fig. 1 Schematic diagram of the experimental technical route
2.2 Preparation of clean test samples with various surface wettability
Various surface wettability of test samples is achieved by the surface modification treatment on substrates. Aluminum and copper, which are widely used as the fin materials in the air conditioners, are chosen as the substrate materials. The hydrophilic and hydrophobic coatings are needed to be manufactured on the substrate surface to investigate the effect of surface wettability on the dust
t = 1300 s
removal; as the surface modification treatment on copper surface is more convenient than on aluminum surface, copper is used as the substrate material to make hydrophilic and hydrophobic surfaces in this study. Thus, four test samples with different surface wettability are chosen in the experiment, i.e. bare aluminum plate, bare copper plate, hydrophilic copper plate and hydrophobic copper plate; the sizes of test samples are 50 mm×50 mm×0.5 mm, as shown in Fig. 2(a). The bare aluminum plate and bare copper plate both are prepared by polishing the plate surfaces with 3000-grit sandpaper. The hydrophilic copper plate and hydrophobic copper plate are fabricated by the micro/nanostructured modification on bare copper plates. The hydrophilic surface modification is performed by immersing the bare copper plate into an aqueous solution of NaOH and (NH 4)2S2O8 for 30 min, and the concentrations of NaOH and (NH4)2S2O8 in the aqueous solution are 2.5 mol L-1 and 0.1 mol L-1, respectively. The hydrophobic surface modification is performed by immersing the hydrophilic copper plate into an aqueous solution of C12H26S for 15 min, and the concentration of C12H26S is 0.1 mol L-1. The SEM photos of all test sample surfaces are shown in Fig. 2(b). To represent the surface characteristics of prepared test samples, the contact angles and the roughness are tested. The contact angles are measured through photographing the contours of droplets on test samples by CCD camera, and the static contact angles of the bare aluminum plate, the bare copper plate, the hydrophilic and hydrophobic copper plate are obtained as 74o, 88o, 10o, and 162o, respectively, as shown in Fig. 2(c); the roughness is measured by Atomic Force Microscope. The detailed values of contact angles and roughness of clean test samples are listed in Table 1.
Bare aluminum plate
Bare copper plate
Hydrophilic plate
Hydrophobic plate
(a) Photos of test samples
(b) SEM photos of microstructure on test samples
(c) CCD photos of water droplets on test samples
θst= 74o±1o
θst= 10o±1o
θst= 88o±1o
θst= 162o±1o
Fig. 2 Test samples employed in the experiments Table 1 Surface wettability and roughness of test samples. Static contact angle
Contact angle
Experimental sample
Surface roughness θst
hysteresis θH
Aluminum plate
74o
31o
0.08 μm
Copper plate
88o
24o
0.15 μm
Hydrophilic copper plate
10o
14o
2.74 μm
Hydrophobic copper plate
162o
7o
2.06 μm
2.3 Pre-deposition of dust layer on test samples
The pre-deposition of dust layer on test samples is achieved by blowing dusty air flow on the test samples and compelling the dust particles to adhere on the surface. To perform the pre-deposition of dust layer, an apparatus with 9 components is established, including air compressor, volume flow meter, mixing box, dust container, electric motor, air duct, dust concentration sensor, test sample and fixture. The schematic and photo of the apparatus are shown as Figs. 3(a) and 3(b), respectively.
The pre-deposition experiment consists of four procedures: 1) supplying the clean air by the air compressor flow into the mixing box; 2) generating the dust particles by the dust container with an electric motor and mixing the particles with the clean air; 3) inducing the dusty air flow through the test sample; 4) depositing the dust particles onto the test sample surface. The experimental conditions of the inlet air velocity, the dust particle concentration of the dusty air, the dust components and the blowing time are adjusted in the dust pre-deposition process. The inlet air velocity is controlled by a flow regulating valve, and is measured by a volume flow meter with a precision of ±2 L min-1; the dust particle concentration is controlled by the speed of the electric motor, and is measured by a dust concentration sensor with a precision of ±0.5 g m-3. The measurement ranges and uncertainties of all the experimental instruments are listed in Table 2. The dust is premixed by two components, including SiO2 and carbon powder (Zhan et al., 2016). The operation of the experimental rig and the experimental conditions are also illustrated by Zhan et al. (2016). The detailed experimental conditions are shown in Table 3. The SEM photos of the dust layers on various samples are obtained to ensure that the deposited dust clusters have the similar structure, as shown in Fig. 4. The scale of deposited dust clusters is at least an order of magnitude greater than that of the microstructures of the test samples. Therefore, the surface microstructures have little effect on the structure of pre-deposited dust clusters. The repeatability tests of the weight of the pre-deposited dust layers are conducted to ensure the reliability of the pre-deposition method. Twelve sets of experimental data of dust layer weight are obtained, including four test samples with three times of dust pre-deposition. The results show that the maximum relative deviation of dust weights on various test samples is 4.9%.
1-Air compressor
5-Electric motor
4-Dust container
7-Dust concentration sensor 8-Test sample
2-Volume flow meter
3-Mixing box
6-Air duct
9-Fixture
a) Schematic of the apparatus
b) Photo of the apparatus Fig. 3 Apparatus for dust pre-deposition on test samples
(a) Bare aluminum plate
(b) Bare copper plate
(c) Hydrophilic copper plate
(d) Hydrophobic copper plate
Fig. 4 SEM photos of deposited dust clusters on various samples
Table 2 Measurement ranges and uncertainties of experimental instruments. Instrument
Model
Range
Uncertainty
Volume flow meter
HTLZD-15/F10 (GuangJi)
1.2 to 12 m3 h-1
±2 L min-1
Thermocouple
5TC-TT-T-20-36 (Omega)
-40 to 200 oC
±0.1 oC
-50 to 100 oC
±0.1 oC
0 to100%
±0.8%RH
Temperature and humidity HC2-SH (ROTRONIC) sensor Speed controller
3IK15RGN-C (OuGuan)
9 to 135 r min-1
±1 r min-1
Dust concentration sensor
ZHKD-FG (Zhonghang Electric)
0 to 20g m-3
±0.5 g m-3
Table 3 Experimental conditions of dust pre-deposition. Parameters
Value
Inlet air velocity
2 m s-1
Dust particle concentration
10.8 g m-3 75% SiO2,
Dust components 25% carbon powder Dust pre-deposition time
30 minutes
2.4 Frosting on dust pre-deposited test samples
The method for frosting on dust pre-deposited test samples is to cool the test samples below the freezing point, and make the water vapor in the air condense and freeze on sample surfaces. The cooling of test samples is conducted in an experimental apparatus composed of a cooling module and a linear guideway, as shown in Figs. 5(a) and 5(b). The cooling module is assembled by a thermoelectric element (TEC), a type-T thermocouple, a heat sink, an aluminum plate and a piece of insulated cotton. TEC is used to supply the cooling capacity; the temperature controller is used to control the temperature of the sample surface by adjusting the input power of TEC; the aluminum plate immersed with the type-T thermocouple covers TEC for the fixation and temperature monitoring of the test sample; the insulated cotton is used to cover on the aluminum plate around the test sample for avoiding the frost formation around the test sample. The linear guideway assembled by a guideway, a slide block with speed-adjustable stepper motor and a paper tape is used to fasten the cooling module. The experimental conditions of the frosting on test samples cover the temperature of sample surfaces of -15 oC, the air dry-bulb temperature of 7 oC, the air wet-bulb temperature of 6 oC and the frosting time of 2 hours. The air dry-bulb temperature and wet-bulb temperature in the experiments are referred to the Chinese National Standard for Room Air Conditioners GB/T 7725-2004 (AQSIQ, 2004); the temperature of sample surfaces and the frosting time are referred to the actual frosting conditions of air-conditioners. The detailed experimental conditions are shown in Table 4.
Cooling module 1 2 3 4
5
6
7
8
1-Thermoelectric element (TEC), 2-Aluminum plate with type-T thermocouple, 3-Insulated cotton, 4-Heat sink, 5-Test sample, 6-Guideway, 7-Slide block with speed-adjustable stepper motor, 8-Paper tape,
(a) Schematic of experimental apparatus
(b) Photo of experimental apparatus Fig. 5 Experimental apparatus for frosting on dust pre-deposited test samples Table 4 Experimental conditions of frosting. Parameters
Value
Temperature of sample surfaces
-15 oC
Air dry-bulb temperature
7 oC
Air wet-bulb temperature
6 oC
Frosting time
2 hours
2.5 Defrosting and measurements of dust removal characteristics
Defrosting experiments are carried out by increasing the temperature of test samples through switching electrodes of TEC shown in the Fig. 5. When the temperature of test samples increases up to 5 oC and the defrosting process begins, the measurements of dust removal characteristics by the melted frost are performed. The measured parameters for dust removal characteristics include remnant dust weight and dust removing rate. For obtaining the remnant dust weight, four procedures are performed in the experiments: 1) measure the weight of the clean test sample Mt; 2) dry the test sample when the dust removal process is completed; 3) measure the total weight of the test sample and the residuum Mtotal; 4) calculate the remnant dust weight Mr by subtracting Mt from Mtotal. For obtaining the dust removing rate, a paper tape with scale is laid on the slide block of the linear guideway and is compelled to move below the test sample forced by the speed-adjustable stepper motor during the defrosting process, as shown in Fig. 5. When the defrosting process begins, the paper tape and the slide block move along the linear guideway with a certain velocity v controlled by the stepper motor, and the dust removed by the melt frost continue to drop down from the test sample onto the moving paper tape. When the defrosting process is completed, the paper tape is dried and clipped into 30 scraps with certain width w, and the weight of the dust on each scrap is measured as Mi (i ranges from 1 to 30). And the dust removing rate can be calculated by the moving velocity v, the width of scraps w and the weight of the dust on each scrap Mi. The measurement uncertainty of dust removing rate and remnant dust weight is determined by the instrument deviation of the analytical balance used in the experiments, which is 0.0001g.
3. Effect of wettability on defrosting behaviors on dust pre-deposited surfaces The dust removing rate and remnant dust weight both depend on the defrosting behaviors on the test samples. In order to choose the surface wettability with the best dust removal performance, the defrosting behaviors on test samples with various wettability are observed, and the mechanism of dust removal by defrosting is analyzed.
3.1 Defrosting behaviors on test samples
The defrosting process on clean test samples with various wettability are shown in Fig. 6. For the bare aluminum plate and the bare copper plate, the frost with porous structure may absorb the adjacent defrosting water, and a frost-water mixture layer forms on the surface; then the size of the mixture layer becomes smaller and smaller due to the melt and removal of the frost, and a large amount of water droplets retain on the surface when the defrosting process is completed. For the hydrophilic copper plate, the melted frost forms the water layer on the surface and flow downward along the direction of gravity during the defrosting process, and only a few water droplets remain at the bottom part of the test sample. The observed frost removal pattern on the hydrophilic surface is different from that mentioned in the previous study (Kim et al., 2018). In this study, the frost on the hydrophilic surface first entirely melts into water and then the water will flow off; while in the study of Kim et al., the frost may directly slide down from the surface. The reason for the difference of the frost removal pattern is that, the frosting time employed in the previous research is twice longer than that applied in this study, and the longer frosting time may result in a heavier frost layer which is prone to slide down before the frost is able to melt completely.
For the hydrophobic copper plate, the frost layer falls off as a whole piece with little water remaining on the surface. The falling frost layer maintains a whole piece due to the loose structure of the frost layer on the hydrophobic surface. The loose frost layer has a small contact region with the surface. Once the frost at the contact region melts, the adhesion of the surface is too weak to support the frost layer, and the frost layer may fall as a whole piece. The frost removal pattern on the hydrophobic surface observed in this study is consistent with that obtained by other researchers (Liang et al., 2015; Kim et al., 2018). The defrosting process on dusty test samples with various wettability are shown in Fig. 7. For the bare aluminum plate and the bare copper plate, the pre-deposited dust is carried by water droplets from the melted frost and is removed through the discharge of water droplets; the dust on the moving path are swept away by the droplets, causing a nonuniform distribution of the remnant dust on plates. For the hydrophilic copper plate, the dust is covered by the water film from the melted frost and is removed as the water film flows away from the surface; the remnant dust is almost uniform-distributed on the surface during the defrosting process. For the hydrophobic copper plate, the dust is involved in the formed frost-water mixture layer and is removed with the falling of the mixture layer; almost no remnant dust is observed on the surface after the defrosting process. According to the observation results shown in Figs. 6 and 7, the surface wettability has a significant effect on the dust removal performance during the defrosting process. Among all the four test samples prepared in this study, the remnant dust on the two bare plates is much more than that on the hydrophilic copper plate, and only the dust on the hydrophobic copper plate can be removed thoroughly.
Defrosting time (s) Test samples 0s
30 s
60 s
120 s
Bare aluminum plate
Bare copper plate
Hydrophilic copper plate
Hydrophobic copper plate
Fig. 6 Defrosting behaviors on clean test samples with various wettability
Defrosting time (s) Test samples 0s
30 s
60 s
120 s
Bare aluminum plate
Bare copper plate
Hydrophilic copper plate
Hydrophobic copper plate
Fig. 7 Defrosting behaviors on dusty test samples with various wettability
3.2 Mechanism of dust removal by defrosting
The mechanism of dust removal by defrosting depends on the removal carrier of dust particles, which is influenced by surface wettability, as shown in Fig. 8. For the bare aluminum plate and the bare copper plate, the removal carrier of dust particles is water droplets from the melted frost. The water droplets may move downwards under the gravity and gather at the bottom part of the surface. The dust particles which are not located at the move path of the water droplets still remain on the surface after the defrosting process. Moreover, the dust particles involved in the hanged droplets at the bottom part may redeposit on the test sample when the hanged droplets are dried, which is consistent with the phenomenon observed by Yang et al. (2019). For the hydrophilic copper plate, the removal carrier of dust particles is the water film from the melted frost. The dust particles mix with water film and become the wet dust particles. Part of the wet dust particles fall off the surface along with the flow of water film, and the other particles may redeposit on the hydrophilic surface as the water film is dried. The remnant dust particles are distributed uniformly on the hydrophilic surface, which is consistent with the observation results reported by other researchers (Yang et al., 2019). For the hydrophobic copper plate, the removal carrier of dust particles is the entire frost layer. The porous structure of the frost layer on the hydrophobic copper plate is much more loosen than that on the other samples. When the frost close to the surface is melted to water film, the adhesion force between the water film and the surface is not able to sustain the weight of the dust frost layer due to the extremely low contact angle hysteresis, and the entire frost layer with the dust particles involved in it will directly fall off the surface. The dust layer on the hydrophobic surface is almost cleared up due to the separation action of
frosting, as shown in Fig. 9. A lot of small gaps exist between the dust particles and the hydrophobic surface, and water vapor may permeate into the gaps and condenses. As the condensate in the gaps freezes into frost, the gaps are expanded, and the dust particles are separated with the surface. The separation of dust particles and surface may weaken the surface adhesion and make the dust particles fall off with the whole frost layer.
Fig. 8 Mechanism of dust removal by defrosting on test samples with various wettability
Hydrophobic surface
Dust particle
Defrosting water
Frost
Water vapor Condensate
Permeation and condensation of water vapor
Separation of dust particles and surface
Fall of dust particles with frost layer
Fig. 9 Destructive action of frosting on dust layer on hydrophobic surface
4. Effects of surface wettability on dust removing characteristics The dust removal characteristics includes of the dust removing rate and the final remnant dust weight. In order to quantitively compare the effect of surface wettability on dust removal characteristics, the measurements of the dust removing rate and the final remnant dust weight and are needed.
4.1 Effect of surface wettability on dust removing rate
Figure 10 shows the variation of remnant dust weight along with the time during the defrosting process for the test samples with various wettability. It can be seen that, the remnant dust weights on the four test samples remain constant at the beginning of the defrosting process, and then decrease with the time until the steady state is reached. Moreover, the steady time of dust removal process for the hydrophobic copper plate is much less than that for the other three plates. The steady time of dust removal process is determined by the fall velocity of dust removal carrier. For the hydrophobic copper plate, the dust removal carrier is the frost layer with loose porous structure, and the adhesion force between the carrier and the plate surface is too weak to remain adhesion on sample surface. For the other three plates, the dust removal carrier is liquid water, and the dust particles are carried by the water and fall off gradually, which will take a long time for the dust removal process to reach the steady state.
Weight of remnant dust, W (g)
0.35 Bare aluminum plate Bare copper plate Hydrophilic copper plate Hydrophobic copper plate
0.30 0.25 0.20 0.15 0.10 0.05 0.00 0
20
40
60
80
100
120
Time, t (s)
Fig. 10 Variation of remnant dust weight along with the time during the defrosting process
4.2 Effect of surface wettability on final remnant dust weight
Figure 11 shows the final remnant dust weights on the test samples with various wettability after the dust removal process. It can be seen that, the remnant dust rates on the surfaces are 28.4%, 28.0%, 8.69% and 0.54% for bare aluminum plate, bare copper plate, hydrophilic copper plate and hydrophobic copper plate, respectively. The remnant dust weight on the hydrophilic surface is nearly 70% less than that on bare surfaces. The possible reason is that, as the carrier of dust particles on the hydrophilic surface, the water film can adhere and remove much more dust particles compared to the discrete water beads on bare surfaces. The remnant dust weight on the hydrophobic surface is nearly 93% less than that on the hydrophilic surface. This is because the residue of defrosted water on the hydrophobic surface is far less than that on the hydrophilic surface, and less residue of water will lead to less particles redeposited on the surface.
Compared to the remnant dust weight on the hydrophobic surface measured by Yang et al. (2019), the remnant dust weight on hydrophobic surface obtained in this study reduces by nearly 94%. The reason for the reduction of the remnant dust weight is that, the frost used as the carrier in this study can remove much more dust particles than the water beads used as the carrier in the study of Yang et al. (2019). The frost on hydrophobic surfaces is porous; the surface area of porous frost is far larger than that of water beads so that the frost can adhere and remove more dust particles. In the actual heat exchangers, the tubes are inserted into the fins, which may deteriorate the dust removal performance of the surfaces since the tubes will impede the fall of the dusty frost and the dusty water from the heat exchangers. Moreover, the performance deterioration of the hydrophobic surface is less than that of the bare surfaces and the hydrophilic surface, because the dusty frost falling from the hydrophobic surface is harder to adhere and redeposit on the tubes compared to the dusty water on other three surfaces. remnant dust on test samples
removal dust from test samples
0.0802 g
Bare aluminum 0.0776 g
Bare copper 0.0253 g
Hydrophilic copper 0.0015 g
Hydrophobic copper
0.05
0.1
0.15
0.2
0.25
0.3
Dust weight, W(g)
Fig. 11 Final remnant dust weight on test samples with various wettability
5. Conclusions The optimal choice of fin surface wettability for promoting the dust removal is obtained experimentally through testing dust removal characteristics on various surfaces, covering bare aluminum surface, bare copper surface, hydrophilic surface and hydrophobic surface. The temperatures of the air dry-bulb, air wet-bulb, the frosting surface and the defrosting surface in the experiments are 7oC, 6 oC, -15 oC and 5 oC, respectively. The following conclusions are obtained. 1) The dust removing rate on the hydrophobic surface is much higher than that on the other three surfaces. The dust particles on the hydrophobic surface is prone to fall off with the entire frost in an instant, and the dust particles on the other three surfaces is removed by the defrosting water slowly. 3) The remnant dust weight on the hydrophobic surface is far less than that on the other ones. The remnant dust rates on the surfaces are 28.4%, 28.0%, 8.69% and 0.54% for bare aluminum plate, bare copper plate, hydrophilic copper plate and hydrophobic copper plate, respectively. 3) The hydrophobic surface with highest dust removing rate and least remnant dust weight is identified as the optimal choice of fin surface wettability for promoting the dust removal among all the four test samples in this study. 4) The dust removal performance of the surfaces when inserted between tubes will deteriorate since the tubes may impede the fall of the dusty frost and the dusty water in the actual heat exchangers. The performance deterioration of the hydrophobic surface caused by the tubes is less than that of the bare surfaces and the hydrophilic surface due to the weaker adhesion between the tube and the dusty frost compared to the dusty water.
Acknowledgements The authors gratefully acknowledge the supports from the National Natural Science Foundation of China (Grant No. 51606119 & 51906135), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51521004).
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