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The effects of the surface roughness on the dynamic behavior of the successive micrometric droplets impacting onto inclined hot surfaces
International Journal of Heat and Mass Transfer 101 (2016) 1217–1226
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
The effects of the surface roughness on the dynamic behavior of the successive micrometric droplets impacting onto inclined hot surfaces Deendarlianto a,b,⇑, Yasuyuki Takata c,d, Masamichi Kohno c,d, Sumitomo Hidaka c, Takaaki Wakui c, Akmal Irfan Majid a,b, Hadiyan Yusuf Kuntoro a,b, Indarto a,b, Adhika Widyaparaga a,b a
Department of Mechanical & Industrial Engineering, Faculty of Engineering, Gadjah Mada University, Jalan Grafika No. 2, Yogyakarta 55281, Indonesia Centre for Energy Studies, Gadjah Mada University, Sekip K-1A Kampus UGM, Yogyakarta 55281, Indonesia Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan d International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b c
a r t i c l e
i n f o
Article history: Received 16 December 2015 Received in revised form 13 May 2016 Accepted 31 May 2016
Keywords: Spray cooling Quenching curves Droplet Spread ratio Dimensional height Sliding distance Solid–liquid contact time
a b s t r a c t The effect of surface roughness on the dynamic behavior and the heat transfer phenomena of multiple successive micrometric water droplets impacting onto inclined heated solid surfaces has been studied experimentally. The inclination angles were 15°, 30°, and 45° from horizontal. The droplet diameters were 500 lm and 700 lm. The solid surface temperatures were decreased from 500 °C to 100 °C. The test material was stainless steel-grade 304 (SUS 304) with different surface roughness ranged from Ra 0.04 up to Ra 10. The droplet dynamics during the impacting onto inclined hot surfaces were investigated by using high-speed video camera. It was found that the surface roughness significantly affects quenching behavior. The higher the surface roughness, the lower the quenching time during the spray cooling. The solid-droplet contact time and the droplet spread diameter increase with the increase of surface roughness. Thus causing the decrease of the quenching time of inclined hot walls. Meanwhile, the critical heat flux and Leidenfrost temperatures are shown to be insensitive to the surface roughness. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction In metal industries, obtaining a more cost effective processing method and enhancing product quality involve the improvement of cooling technique to reduce the energy consumption and manufacturing time. Advanced heat transfer technology by implementing the spray cooling during the quenching process will increase the quality (including of strength to weight and enhanced corrosion resistance properties) of the metal products. During the cooling process, the microstructure of the steel is typically transformed from austenite to pearlite, ferrite, cementite, bainite, or martensite, which affect the level of the material hardness. When large local temperature variations arise during this cooling process, the local thermal stresses and transformation stresses will occur, and cause unwanted change in the products, such as distortions in shape and the crack initiation [1]. In spray cooling, hot materials are cooled by the impingement of the liquid, typically water, from the spray jets, whereas large ⇑ Corresponding author at: Department of Mechanical & Industrial Engineering, Faculty of Engineering, Gadjah Mada University, Jalan Grafika No. 2, Yogyakarta 55281, Indonesia. E-mail address: [email protected] (Deendarlianto). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.05.132 0017-9310/Ó 2016 Elsevier Ltd. All rights reserved.
cooling rates are associated with the large temperature difference between hot and cool conditions. This process involves a complex interplay of several heat-transfer mechanisms including the convection of superheat due to the momentum of the incoming droplets, axial advection, and conduction through the moving solid material. The change in the temperature gradient across the solid material due to an abrupt increase or decrease in the heat extraction rate causes differential thermal expansion in the solidifying metal and the generation of high thermal stress and strain. This can ultimately lead to internal or surface defects, which can severely compromise the quality of the cooled metal product. Understanding the physical of the phenomena is essential in order to build a model capable of predicting the heat transfer with a high accuracy. However, under practical conditions, the dispersion of the liquid results in the generation of numerous droplets which collectively can be difficult to study systematically. To investigate the underlying phenomena of spray cooling transient heat transfer characteristics in a more manageable fashion, multiple successive droplet studies should be applied. Laboratory studies of spray cooling have typically measured the temperature variation and the effect of liquid properties on the evaporation. A brief literature review of this topic was provided
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by Deendarlianto et al. [2]. They noticed that the droplet impact dynamics depend on both substrate temperature and impact velocity [3–8]. Further experiments to identify the fluid-dynamic regimes of the droplet interaction and to quantify the outcome of droplet impact was carried out by Moita and Moreira [9]. The introduction of a special technique to modify the solid surface to alter the contact angle in order to control the boiling characteristics was proposed by Miyauchi et al. [10]. They introduced the ultraviolet irradiation (UV) on the TiO2 coating of a solid surface, and it is shown from their results that heat transfer characteristics of liquid–vapor phase change phenomena like boiling and condensation can be controlled and/or enhanced by UV irradiation making use of TiO2-coated surface. All of the above studies focused on single droplet impact on hot solid normal and hot inclined surfaces. Few studies on the multiple successive droplets impact are found in literature, even though the droplets usually impact upon a surface in the form of successive multiple droplets in real situations. When the multiple successive droplets impact on hot surfaces, a complex interaction takes place between the solid and liquid droplet, i.e., the collision dynamics and the change of it’s boiling regimes. Some relevant results from studies regarding the impacts of multiple successive droplets are briefly introduced in the next section. To understand the physics of the spray cooling heat transfer, Bernardin and Mudawar [11] and Bernardin et al. [12] explained the heat transfer regimes as described as transient quenching regimes, shown in Fig. 1. As shown clearly in the Figure, the quench process progresses slowly at film boiling regime, whereas the
Fig. 1. Transient quenching regimes.
liquid–solid contact is very brief as the liquid becomes separated from the surface due to the appearance of the insulating vapor layer. This is defined as the Leidenfrost temperature. As the surface temperature decreases, the droplet-to-surface contact time increases along with a corresponding increase in the surface heat flux. This regime is defined as the transition-boiling regime. At the lowest temperature limit of this regime, the critical heat flux (CHF) point is encountered. Here the boiling heat fluxes as well as cooling rates have a maximum value. Below this point, the nucleate boiling regime exists, whereas the liquid droplet effectively wet the surface and heat fluxes are large, rapidly decreasing with surface temperature to the lower limit, termed the onset of bubble. The further decrease of surface temperature, the heat transfer occurs by single-phase convection. The effect of the change in surface roughness on the boiling characteristics of metallic surfaces was investigated by [13]. The surface roughness is defined as the surface irregularities which result from the various machining processes or material condition, whereas in practical application, spray cooling or boiling of the liquid droplet is regularly used for the cooling of metal surface from high temperature with an oxide (scale) layer; the initial surface is not clean [14]. Roughness includes the shortest wavelength irregularities of a surface. Next, Bernardin and Mudawar [15] also investigated the changes in surface roughness associated with spray quenching. They used samples of Aluminum alloys with three types of surfaces i.e. polished, particle blasted and extruded finished surfaces. They reported that the surface roughness features up to 25 lm increase the bubble nucleation density during transition and nucleate boiling regimes, while large roughness feature of about 25–1 mm influence the impact and spreading of spray droplets and consequently Leidenfrost temperature. Meanwhile, surface temperature corresponding to critical heat flux was fairly independent of surface roughness [16]. Next, Engel [17] reported that the surface roughness promote the droplet breakup, and is contrary to that of Ganic and Rohsenow [18], and Cumo et al. [19]. They reported that the surface roughness enhances liquid–solid contact, hence it increases the film boiling heat transfer. The above literature review indicates that the effect of surface roughness on the boiling or spray cooling characteristics is not clear and many fundamental questions remain to be answered. One issue which has received inadequate attention is the effect of surface roughness on the fundamental parameters such as the droplet spreading and contact angles during the spray cooling of heated solid surface. This may be due to the difficulties of producing of rough surfaces and the scientific analysis behind of it. Therefore, systematic investigation on the spray cooling in order to develop a deepen understanding on the relevant phenomena from the viewpoint of physic is needed. Here it is considered that the physics of the droplets impacting on heated walls can be used to predict the global heat and the heat transfer characteristics of an entire spray. The objective of the present experimental study was to experimentally investigate the isolated effect of the surface roughness on the collision dynamics and heat transfer phenomena of successive micrometric water droplets impacting onto inclined hot surface. In the present paper, the experimental results of the effect of the surface roughness on the transient quenching curves on the basis of inclination angle will be presented first. These data will be explained in terms of time variation of surface temperature due the surface roughness. Afterwards, the effects of surface roughness on those curves will be discussed followed by the interfacial behaviors of the impacted droplets on the examined surface treatments by means of the visual observation. Finally, the effect of the time function of the impacted droplet diameter ratio and droplet contact time will be evaluated.
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2. Experimental apparatus & procedures The details of the experimental apparatus and procedure used in the present study were described in the previous paper Wakui
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et al. [20] and only the main features are presented here as shown in Fig. 2. The apparatus consists of two micro jet dispenser (MJ020, Mect Co), a droplet frequency controller, a high-speed video camera, an illumination systems and two personal computers for
Fig. 2. The schematic drawing and photo of experimental apparatus.
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the acquisition of the surface temperature data and to control the visualization purposes. The droplets were generated from the nozzles that have inner diameter 0.5 mm by switching the open/ close of the solenoid valve inside the nozzle. A micro jet-dispenser was used to manipulate the timing of the solenoid valve to regulate the droplet diameter. Here each micro dispenser controls four nozzles. In the present experiment, a long distance microscope is attached to the high-speed video camera to observe the droplet behavior; therefore the high-speed image of the impinging droplet can be obtained. The video images were taken at 15,000 frames per second and a shutter speed of 1/100,000 s. To study the effect of surface roughness, we have made the stainless steel surfaces with the differences of surface roughness as the test material as shown in Fig. 3. The test samples used in the present experimental study were made from SUS 304 formed into a disc shape. The diameter and thickness of the disk were respectively 30 mm and 5 mm with different surface roughness. The surface roughness (Ra) was 0.04, 0.2, 3.0, and 10. A Ra of 0.04 was made by polishing the surface by emery paper and a lapping machine. The other surface roughnesses were made by sand blast treatment. The surface roughness amplitude (Ra) was measured by a microscope (ultra deep shape measurement microscope VK-8500 of KEYENCE Corp.). In the present experiments, the test sample surfaces were heated up inside an electric furnace. The temperature inside the furnace was set to 630 °C. During the heating, the nitrogen gas was supplied into the furnace to avoid the oxidation of the sample. After the test sample temperature reached 630 °C, the sample was taken out from the furnace and put it on the sample holder of the experimental apparatus. The test sample temperature was measured by a thermocouple of 0.5 mm in diameter and embedded into the sample center from the side at a location of 2 mm in depth from the surface. The temperature was recorded at every 0.12 s, and spray cooling was started when the test sample reached down to 500 °C. The liquid droplets were injected successively from a determined number of nozzles with preset frequency until the test sample was cooled down to 100 °C. The cooling curve was obtained from the temperature history of the test sample. We have used three inclination angles of the hot surfaces, 15°, 30°, and 45° from horizontal in order to avoid a secondary collision with same droplet. When the surface is inclined from horizontal, the droplet bounced out from the test sample after the collision, therefore, the result obtained from the present experimental study was the cooling ability of the droplet at the primary impingement.
Ra0.04
Next, we have also experimented using nozzles numbering from 6 to 8. In addition, we have carried-out the cooling experiment at the ambient temperature. Finally, the condition of the experimental series conducted in the present experiment is summarized in Table 1. 3. Results and discussions Fig. 4 shows the results regarding the effect of the surface roughness on the transient cooling/ quenching curves. In the figures, the surface temperature is plotted against the time with the surface roughness as a parameter. The droplet diameter is 700 lm, and the droplet velocity is 4 m/s. The figures show that spray cooling was initiated when the temperature of the sample has reduced to 500 °C. When the temperature reaches the Leidenfrost point, the transition boiling regime is present and the temperature reduction (dT/dt) steepens. Observation of Fig. 4 shows also that this transition to begin occurring at around 220 °C. Further temperature reduction brings the boiling regime into the nucleate boiling regime where the temperature reduction initially steepens further until a critical heat flux is obtained where dT/dt is maximum. Below the critical heat flux temperature, the value of dT/dt becomes more gradual. Close observation of Fig. 4 reveals that the cooling time decreases with the increase of the surface roughness. The curves also displays that the Leidenfrost and critical heat flux temperatures are seem to be insensitive to the surface roughness. To further illustrate this effect as described above, high-speed image investigations were carried out as shown in Figs. 5–7. Figs. 5–7 show the effect of the surface roughness on the deformation behavior of the multiple successive water droplets
Table 1 Experimental data set. No.
Surface roughness (Ra)
Droplet diameter (lm)
Impact velocity (m/s)
Droplet frequency (Hz)
Inclination angle (°)
1
0.04 0.2
3
3
4
10
2.5 4.0 2.5 4.0 2.5 4.0 2.5 4.0
88
2
500 700 500 700 500 700 500 700
(15, 45) (15, 45) (15, 45) (15, 45)
Ra0.2
0.5mm
Ra3
Ra10
Fig. 3. The microscope images of the sample surfaces (SUS 304, the enlargement size is 175 times).
88 88 88
30 & 30 & 30 & 30 &
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impacting on the inclines hot surfaces, Figs. 5–7 being for the surface temperature of 200 °C, 250 °C, and 300 °C respectively. The droplet evaporation behavior impacting on the inclined hot surface below, around and above the Leidenfrost temperatures are also observed. The observed phenomena as follows: 1. From Fig. 5, we can see that the behavior of the impacting droplet on the inclined hot surfaces is almost similar for all the test surface roughness. After the impact, the droplets spread until
Fig. 4. The effect of the surface roughness on the time variation of quenching curves (droplet diameter = 700 lm, droplet velocity = 4 m/s).
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the maximum spreading diameter and then shrinks and bounces off from the inclined hot surfaces. During the spreading it is considered that the surface tension balances the inertia force in order to keep the droplet from bouncing off. Close investigation of the figures reveals that also the spreading period is affected by the surface roughness. The fact can be shown at t = 2.60 ms of Ra = 0.04 where the spreading process is stopped and the shape of the droplet is unaxisymmetric. The blow out of the vapor bubbles in the form of the secondary droplets is also observed here. On the other hand at Ra = 3 and Ra = 10 the droplets continue to spread until t = 3.0 ms. This means that the surface roughness have a significant influence on the spreading period of the droplet impacting onto the inclined hot surfaces.
Fig. 5. The effect of surface roughness on the evaporation successive droplet dynamics on a solid surface at 200 °C (inclination angle = 15°, do = 700 lm).
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2. From Figs. 6 and 7, it is shown that the dynamics of the droplets impacting on to inclined heated surfaces are not affected by the surface roughness. After the impact, the droplet spreads to the maximum diameter, then recoils and rebounds. After the maximum spreading diameter is reached, the droplet is elongated
upward in the shape of bowling pin without any secondary droplets production. The observed phenomena confirms to the previous results from [2], who observed the similar topic of the single droplet. Close investigations of the figures reveals also that the contact time between the droplet and heated surfaces seem to be affected by the surface roughness. In case of surface temperature is 250 °C as shown in Fig. 6, at t = 2.0 ms of Ra = 0.04 and Ra = 3 the droplet leaved the heated surface, meanwhile at Ra = 10.0 the droplet still attaches the inclined heated surface. Moreover detailed quantitative informations to explain the observed phenomena in the form of the
Fig. 6. The effect of surface roughness on the evaporation successive droplet dynamics on a solid surface at 250 °C (Inclination angle = 15°, do = 700 lm).
Fig. 7. The effect of surface roughness on the evaporation successive droplet dynamics on a solid surface at 300 °C (Inclination angle = 15°, do = 700 lm).
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Fig. 8. Definition of spread factor, dimensionless height, sliding distance.
spreading diameter and the contact time between liquid droplet and inclined heated surface are given in the next section of this paper.
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3. Fig. 6 shows the droplet dynamics at T = 250 °C. This temperature is slightly above the observed Leidenfrost temperature. Therefore, it can be seen that droplet bounce still occurs as the vapor film is still present at this temperature. As for T = 300 °C, again it is observed that the contact time of the droplet and the heated surface is longer at high surface roughness compared to low surface roughness. For Ra = 10, the droplet is still within contact of the surface at t = 2.00 ms while for Ra = 0.04 and Ra = 3, the droplets have detached at 1.33 ms and 1.53 ms respectively 4. In Fig. 7, for T = 300 °C, it can be seen bouncing occurs at all surface roughness levels tested. This is caused by the limited contact between the droplet and the heat surface due to the presence of the thin vapor layer occurring in the film boiling regime. However, the contact is also influenced by the surface roughness. For Ra = 10, at t = 2.00 ms after droplet impact, the droplet is still within contact of the heated surface, as opposed to Ra = 0.04 and Ra = 3 where the droplets have already clearly detached from the surface at t = 1.33 ms and t = 2.00 ms. This shows that with increased surface roughness, the contact time is increased, most likely due to the enhanced surface area contacting the droplet caused by the asperities. As can be seen in Fig. 4(a), the temperature reduction gradient (dT/dt) at 300 °C, is steeper for high Ra compared to low Ra. Meanwhile, it can be seen the formation of secondary droplets is not prevalent for all tested surface roughness. Since this means that reduction of liquid volume of the droplet cannot account for the steeper
Fig. 9. Time evolution of the spread factor (d/do) during the impact of a droplet on an inclined hot surface (Inclination angle = 15°, droplet diameter = 700 lm, droplet velocity = 4.5 m/s).
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temperature reduction gradient, the steeper gradient must be caused by the increased surface area contact due to increased roughness in combination with the contact time for higher Ra. In the present work, three physical parameters are used to characterize droplet dynamics impacting onto inclined hot surfaces. Those are the diameter of the wetted area d, the droplet height above the surface h, and the sliding distance x as shown in Fig. 8. Normalized these quantities by the initial droplet diameter d0, results the so-called ‘spread factor (d/d0)’ and ‘the dimensionless height (h/do)’ [21]. Figs. 9 and 10 show the evolution measured values of spread factor (d/d0) and the dimensionless height (h/do) with the surface roughness as the parameter. The inclination angle, droplet diameter, and the droplet velocity are 15° from horizontal, 700 lm, and 4.5 m/s, respectively as an example. In the figures, (a), (b), (c), and (d) correspond to the cases of the surface temperature of 250 °C, 300 °C, 400 °C, and 500 °C, respectively. The figures reveals that the d/d0 and h/do are independent of surface roughness in the earlier period of impact, t < 0.5 ms, meanwhile those increase with the increase of the surface roughness. These above facts indicate that the shear stresses as the forces restraining the radial outflow of the droplet from the point of impact are indeed negligible during the early stage of impact under the present experimental conditions. Here the effect of the surface roughness is incorporated in shear stress. Next, the effect of the surface roughness on the h/do after t > 0.5 ms becomes stronger when the surface roughness of Ra = 10.0. The observed facts indicate that the maximum spread ratio for the droplets are insensitive to the change of surface roughness. Although the test surfaces have different roughness, the maximum spread ratio of rough surface is almost the same with the smooth surface, therefore, it can be concluded that the surface roughness has no influence on the spread ratio. This obtained result is in agreement with that of Lee et al. [22], who investigated the maximum spreading of drops impacting on smooth and rough surface in a horizontal surface. They reported that the roughness of the their substrate has a rather small influence on the spreading. In connecting the observed facts to the Fig. 4(a), it can be seen that at T = 250 °C, the dT/dt for higher surface roughness is again steeper than for low surface roughness while the difference is not as large as it is for T = 300 °C. Fig. 9(a) shows that for T = 250 °C, the maximum droplet spread size is the same for all surface roughness levels. Even so, the contact time at high surface roughness was longer compared to low surface roughness, similar to the observation at T = 300 °C. For high temperatures, at T = 400 °C and T = 500 °C, Fig. 9 (c) and (d) show that with higher temperature, droplet spreading diameter decreases. However, again for higher surface roughness, the spreading diameter is higher than for low surface roughness. Nevertheless, the difference in spreading diameter reduces with increased surface temperature. As such, the difference of dT/dt at T = 500 °C is small between Ra = 2, Ra = 3 and Ra = 10. However, the dT/dt for Ra = 0.04 at this temperature is markedly more gradual. This shows that as the surface is smooth for Ra = 0.04, the lack of asperities results in reduced heat transfer despite the similar droplet spread. Fig. 11 shows the measured sliding distance, x, during the impacted droplet onto inclined hot surfaces with the surface roughness and inclination angle as the parameters. The droplet diameter and the droplet velocity are 700 lm, and 4.5 m/s, respectively. In the present experimental study, the sliding distance x is defines as the distance between the droplet’s impact point and point corresponding to the droplet’s bounce-out from the hot surface. From the figure it is depicted that the sliding distance increases with the inclination angle. This is due to the increase of normal force acting on the interface of liquid-droplet with the
increase of the inclination angle, hence, the droplet moves in a longer distance on the inclined hot surface. Close investigation on the figure also reveals that the sliding distance is affected by the surface roughness. That is the higher the surface roughness the longer the sliding distance during the droplet impacting on the inclined hot surface. According to the photographic observations the surface roughness interrupts the shrinking droplet, therefore the sliding distance is longer at a higher surface roughness. The evidence is shown in Fig. 6 at T = 1.53 ms of Ra = 10.0. Moreover to elucidate the effect of surface roughness on the droplet evaporation in a heated inclined wall, the solid–liquid contact time will evaluated. Fig. 12 illustrates the effect of the surface
Fig. 10. Time evolution of the dimensionless height (h/do) during the impact of a droplet on an inclined hot surface (Inclination angle = 15°, droplet diameter = 700 lm, droplet velocity = 4.5 m/s).
Deendarlianto et al. / International Journal of Heat and Mass Transfer 101 (2016) 1217–1226
Fig. 11. The effect of the surface roughness on the droplet sliding distance (droplet diameter = 700 lm, droplet velocity = 4.5 m/s).
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Fig. 13. The effect of the surface roughness on the critical heat flux and Leidenfrost Temperature.
droplet evaporation. Here the solid-droplet contact time decreases as the surface temperature decreases while increasing with the increase of the surface roughness. The principal effect of raising surface roughness is that it traps the droplet resulting a higher resistance force on the droplet to bounce-out from the inclined heated surface. Therefore, the transit time increases with an increase in surface roughness. This means that the stability of the vapor layer during the film boiling is sensitive to the surface condition; therefore, it is a possible way to control the boiling process in this regime. Fig. 13 depicts the effect of surface roughness on the critical heat flux (CHF) and Leidenfrost (LDF) temperatures, obtained from the analysis of quenching curves of Figs. 5–7. The figure indicates that CHF and LDF temperatures seem independent of the change of surface roughness over the tested inclination angle ranges. The obtained results confirms to that of Berenson [22], who performed the experimental study on the heat transfer during pool boiling while partly contradicting the experimental results of Bernardin et al. [23]. They carried the experimental study on the effect of surface roughness on water droplet impacting the hot surfaces, and reported that the CHF temperature seems to be insensitive to the surface roughness and droplet velocity, meanwhile LDF temperature decreases with increasing surface roughness. This apparent contradiction to the results reported here may be the results of fundamental differences in heat transfer mechanisms between normal droplet and micrometric droplet diameters. The present study employed micrometric diameter droplets, in which the surface tension plays an important role to influence the spreading and bouncing processes. These facts illustrate also the complex liquid– solid interaction which occurs during the droplet impaction onto hot solid surface. 4. Conclusion Fig. 12. Solid liquid contact time (droplet diameter = 700 lm, droplet velocity = 4.5 m/s).
roughness on the solid-droplet contact time, in which (a) and (b) correspond to the cases of inclination angle of 15° and 45° respectively. In the figures, the surface temperature is plotted against the solid-droplet contact time with the surface roughness as a parameter. As shown in the figure it is concluded that surface roughness plays an important role on the solid–liquid contact during the
An experimental study to study the effects of surface roughness on the dynamic behavior of multiple successive droplets impacting on inclined hot solid surfaces has been carried-out. Experimental runs were carried out by spraying water droplets on the inclined hot surfaces where the droplet diameter and velocity were independently controlled. The studies included photographic studies of single droplets impacting a hot surface, and quenching of a small surface by a droplet stream. It encompassed three inclination angles and four surface roughness configurations. The major effects
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of the surface roughness are on the quenching curves, interfacial behavior, and the evolution droplet size. The results are summarized as follows: 1. The surface roughness plays an important role on the cooling ability of the spray cooling during the multiple successive droplets impacting onto inclined hot surfaces. The higher the surface roughness, the lower the cooling/quenching time during the multiple successive droplets evaporation onto inclined hot inclined wall. 2. The spread diameter of the droplet impacting onto inclined hot surface at early stage of impact (t < 0.5 ms) was not affected by the surface roughness. At t > 0.5 the spread diameter increases with the increase of surface roughness. In addition, it was found also that the higher the surface roughness, the higher the contact time of solid–liquid contact during this event. 3. Due to the surface roughness interrupting the shrinking droplet, the higher the surface roughness the longer the sliding distance during the droplet impacting on the inclined hot surface. 4. The critical heat flux and Leidenfrost temperatures of multiple successive droplets evaporation impacting onto inclined hot surfaces were insensitive to the surface roughness. 5. From the present experiment it is found that the surface roughness is considered as an important parameter, therefore, a future work on the development of new experimental correlation and numerical model on this topic is needed.
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The effects of the surface roughness on the dynamic behavior of the successive micrometric droplets impacting onto inclined hot surfaces Deendarlianto a,b,⇑, Yasuyuki Takata c,d, Masamichi Kohno c,d, Sumitomo Hidaka c, Takaaki Wakui c, Akmal Irfan Majid a,b, Hadiyan Yusuf Kuntoro a,b, Indarto a,b, Adhika Widyaparaga a,b a
Department of Mechanical & Industrial Engineering, Faculty of Engineering, Gadjah Mada University, Jalan Grafika No. 2, Yogyakarta 55281, Indonesia Centre for Energy Studies, Gadjah Mada University, Sekip K-1A Kampus UGM, Yogyakarta 55281, Indonesia Department of Mechanical Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan d International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan b c
a r t i c l e
i n f o
Article history: Received 16 December 2015 Received in revised form 13 May 2016 Accepted 31 May 2016
Keywords: Spray cooling Quenching curves Droplet Spread ratio Dimensional height Sliding distance Solid–liquid contact time
a b s t r a c t The effect of surface roughness on the dynamic behavior and the heat transfer phenomena of multiple successive micrometric water droplets impacting onto inclined heated solid surfaces has been studied experimentally. The inclination angles were 15°, 30°, and 45° from horizontal. The droplet diameters were 500 lm and 700 lm. The solid surface temperatures were decreased from 500 °C to 100 °C. The test material was stainless steel-grade 304 (SUS 304) with different surface roughness ranged from Ra 0.04 up to Ra 10. The droplet dynamics during the impacting onto inclined hot surfaces were investigated by using high-speed video camera. It was found that the surface roughness significantly affects quenching behavior. The higher the surface roughness, the lower the quenching time during the spray cooling. The solid-droplet contact time and the droplet spread diameter increase with the increase of surface roughness. Thus causing the decrease of the quenching time of inclined hot walls. Meanwhile, the critical heat flux and Leidenfrost temperatures are shown to be insensitive to the surface roughness. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction In metal industries, obtaining a more cost effective processing method and enhancing product quality involve the improvement of cooling technique to reduce the energy consumption and manufacturing time. Advanced heat transfer technology by implementing the spray cooling during the quenching process will increase the quality (including of strength to weight and enhanced corrosion resistance properties) of the metal products. During the cooling process, the microstructure of the steel is typically transformed from austenite to pearlite, ferrite, cementite, bainite, or martensite, which affect the level of the material hardness. When large local temperature variations arise during this cooling process, the local thermal stresses and transformation stresses will occur, and cause unwanted change in the products, such as distortions in shape and the crack initiation [1]. In spray cooling, hot materials are cooled by the impingement of the liquid, typically water, from the spray jets, whereas large ⇑ Corresponding author at: Department of Mechanical & Industrial Engineering, Faculty of Engineering, Gadjah Mada University, Jalan Grafika No. 2, Yogyakarta 55281, Indonesia. E-mail address: [email protected] (Deendarlianto). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.05.132 0017-9310/Ó 2016 Elsevier Ltd. All rights reserved.
cooling rates are associated with the large temperature difference between hot and cool conditions. This process involves a complex interplay of several heat-transfer mechanisms including the convection of superheat due to the momentum of the incoming droplets, axial advection, and conduction through the moving solid material. The change in the temperature gradient across the solid material due to an abrupt increase or decrease in the heat extraction rate causes differential thermal expansion in the solidifying metal and the generation of high thermal stress and strain. This can ultimately lead to internal or surface defects, which can severely compromise the quality of the cooled metal product. Understanding the physical of the phenomena is essential in order to build a model capable of predicting the heat transfer with a high accuracy. However, under practical conditions, the dispersion of the liquid results in the generation of numerous droplets which collectively can be difficult to study systematically. To investigate the underlying phenomena of spray cooling transient heat transfer characteristics in a more manageable fashion, multiple successive droplet studies should be applied. Laboratory studies of spray cooling have typically measured the temperature variation and the effect of liquid properties on the evaporation. A brief literature review of this topic was provided
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by Deendarlianto et al. [2]. They noticed that the droplet impact dynamics depend on both substrate temperature and impact velocity [3–8]. Further experiments to identify the fluid-dynamic regimes of the droplet interaction and to quantify the outcome of droplet impact was carried out by Moita and Moreira [9]. The introduction of a special technique to modify the solid surface to alter the contact angle in order to control the boiling characteristics was proposed by Miyauchi et al. [10]. They introduced the ultraviolet irradiation (UV) on the TiO2 coating of a solid surface, and it is shown from their results that heat transfer characteristics of liquid–vapor phase change phenomena like boiling and condensation can be controlled and/or enhanced by UV irradiation making use of TiO2-coated surface. All of the above studies focused on single droplet impact on hot solid normal and hot inclined surfaces. Few studies on the multiple successive droplets impact are found in literature, even though the droplets usually impact upon a surface in the form of successive multiple droplets in real situations. When the multiple successive droplets impact on hot surfaces, a complex interaction takes place between the solid and liquid droplet, i.e., the collision dynamics and the change of it’s boiling regimes. Some relevant results from studies regarding the impacts of multiple successive droplets are briefly introduced in the next section. To understand the physics of the spray cooling heat transfer, Bernardin and Mudawar [11] and Bernardin et al. [12] explained the heat transfer regimes as described as transient quenching regimes, shown in Fig. 1. As shown clearly in the Figure, the quench process progresses slowly at film boiling regime, whereas the
Fig. 1. Transient quenching regimes.
liquid–solid contact is very brief as the liquid becomes separated from the surface due to the appearance of the insulating vapor layer. This is defined as the Leidenfrost temperature. As the surface temperature decreases, the droplet-to-surface contact time increases along with a corresponding increase in the surface heat flux. This regime is defined as the transition-boiling regime. At the lowest temperature limit of this regime, the critical heat flux (CHF) point is encountered. Here the boiling heat fluxes as well as cooling rates have a maximum value. Below this point, the nucleate boiling regime exists, whereas the liquid droplet effectively wet the surface and heat fluxes are large, rapidly decreasing with surface temperature to the lower limit, termed the onset of bubble. The further decrease of surface temperature, the heat transfer occurs by single-phase convection. The effect of the change in surface roughness on the boiling characteristics of metallic surfaces was investigated by [13]. The surface roughness is defined as the surface irregularities which result from the various machining processes or material condition, whereas in practical application, spray cooling or boiling of the liquid droplet is regularly used for the cooling of metal surface from high temperature with an oxide (scale) layer; the initial surface is not clean [14]. Roughness includes the shortest wavelength irregularities of a surface. Next, Bernardin and Mudawar [15] also investigated the changes in surface roughness associated with spray quenching. They used samples of Aluminum alloys with three types of surfaces i.e. polished, particle blasted and extruded finished surfaces. They reported that the surface roughness features up to 25 lm increase the bubble nucleation density during transition and nucleate boiling regimes, while large roughness feature of about 25–1 mm influence the impact and spreading of spray droplets and consequently Leidenfrost temperature. Meanwhile, surface temperature corresponding to critical heat flux was fairly independent of surface roughness [16]. Next, Engel [17] reported that the surface roughness promote the droplet breakup, and is contrary to that of Ganic and Rohsenow [18], and Cumo et al. [19]. They reported that the surface roughness enhances liquid–solid contact, hence it increases the film boiling heat transfer. The above literature review indicates that the effect of surface roughness on the boiling or spray cooling characteristics is not clear and many fundamental questions remain to be answered. One issue which has received inadequate attention is the effect of surface roughness on the fundamental parameters such as the droplet spreading and contact angles during the spray cooling of heated solid surface. This may be due to the difficulties of producing of rough surfaces and the scientific analysis behind of it. Therefore, systematic investigation on the spray cooling in order to develop a deepen understanding on the relevant phenomena from the viewpoint of physic is needed. Here it is considered that the physics of the droplets impacting on heated walls can be used to predict the global heat and the heat transfer characteristics of an entire spray. The objective of the present experimental study was to experimentally investigate the isolated effect of the surface roughness on the collision dynamics and heat transfer phenomena of successive micrometric water droplets impacting onto inclined hot surface. In the present paper, the experimental results of the effect of the surface roughness on the transient quenching curves on the basis of inclination angle will be presented first. These data will be explained in terms of time variation of surface temperature due the surface roughness. Afterwards, the effects of surface roughness on those curves will be discussed followed by the interfacial behaviors of the impacted droplets on the examined surface treatments by means of the visual observation. Finally, the effect of the time function of the impacted droplet diameter ratio and droplet contact time will be evaluated.
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2. Experimental apparatus & procedures The details of the experimental apparatus and procedure used in the present study were described in the previous paper Wakui
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et al. [20] and only the main features are presented here as shown in Fig. 2. The apparatus consists of two micro jet dispenser (MJ020, Mect Co), a droplet frequency controller, a high-speed video camera, an illumination systems and two personal computers for
Fig. 2. The schematic drawing and photo of experimental apparatus.
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the acquisition of the surface temperature data and to control the visualization purposes. The droplets were generated from the nozzles that have inner diameter 0.5 mm by switching the open/ close of the solenoid valve inside the nozzle. A micro jet-dispenser was used to manipulate the timing of the solenoid valve to regulate the droplet diameter. Here each micro dispenser controls four nozzles. In the present experiment, a long distance microscope is attached to the high-speed video camera to observe the droplet behavior; therefore the high-speed image of the impinging droplet can be obtained. The video images were taken at 15,000 frames per second and a shutter speed of 1/100,000 s. To study the effect of surface roughness, we have made the stainless steel surfaces with the differences of surface roughness as the test material as shown in Fig. 3. The test samples used in the present experimental study were made from SUS 304 formed into a disc shape. The diameter and thickness of the disk were respectively 30 mm and 5 mm with different surface roughness. The surface roughness (Ra) was 0.04, 0.2, 3.0, and 10. A Ra of 0.04 was made by polishing the surface by emery paper and a lapping machine. The other surface roughnesses were made by sand blast treatment. The surface roughness amplitude (Ra) was measured by a microscope (ultra deep shape measurement microscope VK-8500 of KEYENCE Corp.). In the present experiments, the test sample surfaces were heated up inside an electric furnace. The temperature inside the furnace was set to 630 °C. During the heating, the nitrogen gas was supplied into the furnace to avoid the oxidation of the sample. After the test sample temperature reached 630 °C, the sample was taken out from the furnace and put it on the sample holder of the experimental apparatus. The test sample temperature was measured by a thermocouple of 0.5 mm in diameter and embedded into the sample center from the side at a location of 2 mm in depth from the surface. The temperature was recorded at every 0.12 s, and spray cooling was started when the test sample reached down to 500 °C. The liquid droplets were injected successively from a determined number of nozzles with preset frequency until the test sample was cooled down to 100 °C. The cooling curve was obtained from the temperature history of the test sample. We have used three inclination angles of the hot surfaces, 15°, 30°, and 45° from horizontal in order to avoid a secondary collision with same droplet. When the surface is inclined from horizontal, the droplet bounced out from the test sample after the collision, therefore, the result obtained from the present experimental study was the cooling ability of the droplet at the primary impingement.
Ra0.04
Next, we have also experimented using nozzles numbering from 6 to 8. In addition, we have carried-out the cooling experiment at the ambient temperature. Finally, the condition of the experimental series conducted in the present experiment is summarized in Table 1. 3. Results and discussions Fig. 4 shows the results regarding the effect of the surface roughness on the transient cooling/ quenching curves. In the figures, the surface temperature is plotted against the time with the surface roughness as a parameter. The droplet diameter is 700 lm, and the droplet velocity is 4 m/s. The figures show that spray cooling was initiated when the temperature of the sample has reduced to 500 °C. When the temperature reaches the Leidenfrost point, the transition boiling regime is present and the temperature reduction (dT/dt) steepens. Observation of Fig. 4 shows also that this transition to begin occurring at around 220 °C. Further temperature reduction brings the boiling regime into the nucleate boiling regime where the temperature reduction initially steepens further until a critical heat flux is obtained where dT/dt is maximum. Below the critical heat flux temperature, the value of dT/dt becomes more gradual. Close observation of Fig. 4 reveals that the cooling time decreases with the increase of the surface roughness. The curves also displays that the Leidenfrost and critical heat flux temperatures are seem to be insensitive to the surface roughness. To further illustrate this effect as described above, high-speed image investigations were carried out as shown in Figs. 5–7. Figs. 5–7 show the effect of the surface roughness on the deformation behavior of the multiple successive water droplets
Table 1 Experimental data set. No.
Surface roughness (Ra)
Droplet diameter (lm)
Impact velocity (m/s)
Droplet frequency (Hz)
Inclination angle (°)
1
0.04 0.2
3
3
4
10
2.5 4.0 2.5 4.0 2.5 4.0 2.5 4.0
88
2
500 700 500 700 500 700 500 700
(15, 45) (15, 45) (15, 45) (15, 45)
Ra0.2
0.5mm
Ra3
Ra10
Fig. 3. The microscope images of the sample surfaces (SUS 304, the enlargement size is 175 times).
88 88 88
30 & 30 & 30 & 30 &
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impacting on the inclines hot surfaces, Figs. 5–7 being for the surface temperature of 200 °C, 250 °C, and 300 °C respectively. The droplet evaporation behavior impacting on the inclined hot surface below, around and above the Leidenfrost temperatures are also observed. The observed phenomena as follows: 1. From Fig. 5, we can see that the behavior of the impacting droplet on the inclined hot surfaces is almost similar for all the test surface roughness. After the impact, the droplets spread until
Fig. 4. The effect of the surface roughness on the time variation of quenching curves (droplet diameter = 700 lm, droplet velocity = 4 m/s).
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the maximum spreading diameter and then shrinks and bounces off from the inclined hot surfaces. During the spreading it is considered that the surface tension balances the inertia force in order to keep the droplet from bouncing off. Close investigation of the figures reveals that also the spreading period is affected by the surface roughness. The fact can be shown at t = 2.60 ms of Ra = 0.04 where the spreading process is stopped and the shape of the droplet is unaxisymmetric. The blow out of the vapor bubbles in the form of the secondary droplets is also observed here. On the other hand at Ra = 3 and Ra = 10 the droplets continue to spread until t = 3.0 ms. This means that the surface roughness have a significant influence on the spreading period of the droplet impacting onto the inclined hot surfaces.
Fig. 5. The effect of surface roughness on the evaporation successive droplet dynamics on a solid surface at 200 °C (inclination angle = 15°, do = 700 lm).
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2. From Figs. 6 and 7, it is shown that the dynamics of the droplets impacting on to inclined heated surfaces are not affected by the surface roughness. After the impact, the droplet spreads to the maximum diameter, then recoils and rebounds. After the maximum spreading diameter is reached, the droplet is elongated
upward in the shape of bowling pin without any secondary droplets production. The observed phenomena confirms to the previous results from [2], who observed the similar topic of the single droplet. Close investigations of the figures reveals also that the contact time between the droplet and heated surfaces seem to be affected by the surface roughness. In case of surface temperature is 250 °C as shown in Fig. 6, at t = 2.0 ms of Ra = 0.04 and Ra = 3 the droplet leaved the heated surface, meanwhile at Ra = 10.0 the droplet still attaches the inclined heated surface. Moreover detailed quantitative informations to explain the observed phenomena in the form of the
Fig. 6. The effect of surface roughness on the evaporation successive droplet dynamics on a solid surface at 250 °C (Inclination angle = 15°, do = 700 lm).
Fig. 7. The effect of surface roughness on the evaporation successive droplet dynamics on a solid surface at 300 °C (Inclination angle = 15°, do = 700 lm).
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Fig. 8. Definition of spread factor, dimensionless height, sliding distance.
spreading diameter and the contact time between liquid droplet and inclined heated surface are given in the next section of this paper.
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3. Fig. 6 shows the droplet dynamics at T = 250 °C. This temperature is slightly above the observed Leidenfrost temperature. Therefore, it can be seen that droplet bounce still occurs as the vapor film is still present at this temperature. As for T = 300 °C, again it is observed that the contact time of the droplet and the heated surface is longer at high surface roughness compared to low surface roughness. For Ra = 10, the droplet is still within contact of the surface at t = 2.00 ms while for Ra = 0.04 and Ra = 3, the droplets have detached at 1.33 ms and 1.53 ms respectively 4. In Fig. 7, for T = 300 °C, it can be seen bouncing occurs at all surface roughness levels tested. This is caused by the limited contact between the droplet and the heat surface due to the presence of the thin vapor layer occurring in the film boiling regime. However, the contact is also influenced by the surface roughness. For Ra = 10, at t = 2.00 ms after droplet impact, the droplet is still within contact of the heated surface, as opposed to Ra = 0.04 and Ra = 3 where the droplets have already clearly detached from the surface at t = 1.33 ms and t = 2.00 ms. This shows that with increased surface roughness, the contact time is increased, most likely due to the enhanced surface area contacting the droplet caused by the asperities. As can be seen in Fig. 4(a), the temperature reduction gradient (dT/dt) at 300 °C, is steeper for high Ra compared to low Ra. Meanwhile, it can be seen the formation of secondary droplets is not prevalent for all tested surface roughness. Since this means that reduction of liquid volume of the droplet cannot account for the steeper
Fig. 9. Time evolution of the spread factor (d/do) during the impact of a droplet on an inclined hot surface (Inclination angle = 15°, droplet diameter = 700 lm, droplet velocity = 4.5 m/s).
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temperature reduction gradient, the steeper gradient must be caused by the increased surface area contact due to increased roughness in combination with the contact time for higher Ra. In the present work, three physical parameters are used to characterize droplet dynamics impacting onto inclined hot surfaces. Those are the diameter of the wetted area d, the droplet height above the surface h, and the sliding distance x as shown in Fig. 8. Normalized these quantities by the initial droplet diameter d0, results the so-called ‘spread factor (d/d0)’ and ‘the dimensionless height (h/do)’ [21]. Figs. 9 and 10 show the evolution measured values of spread factor (d/d0) and the dimensionless height (h/do) with the surface roughness as the parameter. The inclination angle, droplet diameter, and the droplet velocity are 15° from horizontal, 700 lm, and 4.5 m/s, respectively as an example. In the figures, (a), (b), (c), and (d) correspond to the cases of the surface temperature of 250 °C, 300 °C, 400 °C, and 500 °C, respectively. The figures reveals that the d/d0 and h/do are independent of surface roughness in the earlier period of impact, t < 0.5 ms, meanwhile those increase with the increase of the surface roughness. These above facts indicate that the shear stresses as the forces restraining the radial outflow of the droplet from the point of impact are indeed negligible during the early stage of impact under the present experimental conditions. Here the effect of the surface roughness is incorporated in shear stress. Next, the effect of the surface roughness on the h/do after t > 0.5 ms becomes stronger when the surface roughness of Ra = 10.0. The observed facts indicate that the maximum spread ratio for the droplets are insensitive to the change of surface roughness. Although the test surfaces have different roughness, the maximum spread ratio of rough surface is almost the same with the smooth surface, therefore, it can be concluded that the surface roughness has no influence on the spread ratio. This obtained result is in agreement with that of Lee et al. [22], who investigated the maximum spreading of drops impacting on smooth and rough surface in a horizontal surface. They reported that the roughness of the their substrate has a rather small influence on the spreading. In connecting the observed facts to the Fig. 4(a), it can be seen that at T = 250 °C, the dT/dt for higher surface roughness is again steeper than for low surface roughness while the difference is not as large as it is for T = 300 °C. Fig. 9(a) shows that for T = 250 °C, the maximum droplet spread size is the same for all surface roughness levels. Even so, the contact time at high surface roughness was longer compared to low surface roughness, similar to the observation at T = 300 °C. For high temperatures, at T = 400 °C and T = 500 °C, Fig. 9 (c) and (d) show that with higher temperature, droplet spreading diameter decreases. However, again for higher surface roughness, the spreading diameter is higher than for low surface roughness. Nevertheless, the difference in spreading diameter reduces with increased surface temperature. As such, the difference of dT/dt at T = 500 °C is small between Ra = 2, Ra = 3 and Ra = 10. However, the dT/dt for Ra = 0.04 at this temperature is markedly more gradual. This shows that as the surface is smooth for Ra = 0.04, the lack of asperities results in reduced heat transfer despite the similar droplet spread. Fig. 11 shows the measured sliding distance, x, during the impacted droplet onto inclined hot surfaces with the surface roughness and inclination angle as the parameters. The droplet diameter and the droplet velocity are 700 lm, and 4.5 m/s, respectively. In the present experimental study, the sliding distance x is defines as the distance between the droplet’s impact point and point corresponding to the droplet’s bounce-out from the hot surface. From the figure it is depicted that the sliding distance increases with the inclination angle. This is due to the increase of normal force acting on the interface of liquid-droplet with the
increase of the inclination angle, hence, the droplet moves in a longer distance on the inclined hot surface. Close investigation on the figure also reveals that the sliding distance is affected by the surface roughness. That is the higher the surface roughness the longer the sliding distance during the droplet impacting on the inclined hot surface. According to the photographic observations the surface roughness interrupts the shrinking droplet, therefore the sliding distance is longer at a higher surface roughness. The evidence is shown in Fig. 6 at T = 1.53 ms of Ra = 10.0. Moreover to elucidate the effect of surface roughness on the droplet evaporation in a heated inclined wall, the solid–liquid contact time will evaluated. Fig. 12 illustrates the effect of the surface
Fig. 10. Time evolution of the dimensionless height (h/do) during the impact of a droplet on an inclined hot surface (Inclination angle = 15°, droplet diameter = 700 lm, droplet velocity = 4.5 m/s).
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Fig. 11. The effect of the surface roughness on the droplet sliding distance (droplet diameter = 700 lm, droplet velocity = 4.5 m/s).
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Fig. 13. The effect of the surface roughness on the critical heat flux and Leidenfrost Temperature.
droplet evaporation. Here the solid-droplet contact time decreases as the surface temperature decreases while increasing with the increase of the surface roughness. The principal effect of raising surface roughness is that it traps the droplet resulting a higher resistance force on the droplet to bounce-out from the inclined heated surface. Therefore, the transit time increases with an increase in surface roughness. This means that the stability of the vapor layer during the film boiling is sensitive to the surface condition; therefore, it is a possible way to control the boiling process in this regime. Fig. 13 depicts the effect of surface roughness on the critical heat flux (CHF) and Leidenfrost (LDF) temperatures, obtained from the analysis of quenching curves of Figs. 5–7. The figure indicates that CHF and LDF temperatures seem independent of the change of surface roughness over the tested inclination angle ranges. The obtained results confirms to that of Berenson [22], who performed the experimental study on the heat transfer during pool boiling while partly contradicting the experimental results of Bernardin et al. [23]. They carried the experimental study on the effect of surface roughness on water droplet impacting the hot surfaces, and reported that the CHF temperature seems to be insensitive to the surface roughness and droplet velocity, meanwhile LDF temperature decreases with increasing surface roughness. This apparent contradiction to the results reported here may be the results of fundamental differences in heat transfer mechanisms between normal droplet and micrometric droplet diameters. The present study employed micrometric diameter droplets, in which the surface tension plays an important role to influence the spreading and bouncing processes. These facts illustrate also the complex liquid– solid interaction which occurs during the droplet impaction onto hot solid surface. 4. Conclusion Fig. 12. Solid liquid contact time (droplet diameter = 700 lm, droplet velocity = 4.5 m/s).
roughness on the solid-droplet contact time, in which (a) and (b) correspond to the cases of inclination angle of 15° and 45° respectively. In the figures, the surface temperature is plotted against the solid-droplet contact time with the surface roughness as a parameter. As shown in the figure it is concluded that surface roughness plays an important role on the solid–liquid contact during the
An experimental study to study the effects of surface roughness on the dynamic behavior of multiple successive droplets impacting on inclined hot solid surfaces has been carried-out. Experimental runs were carried out by spraying water droplets on the inclined hot surfaces where the droplet diameter and velocity were independently controlled. The studies included photographic studies of single droplets impacting a hot surface, and quenching of a small surface by a droplet stream. It encompassed three inclination angles and four surface roughness configurations. The major effects
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of the surface roughness are on the quenching curves, interfacial behavior, and the evolution droplet size. The results are summarized as follows: 1. The surface roughness plays an important role on the cooling ability of the spray cooling during the multiple successive droplets impacting onto inclined hot surfaces. The higher the surface roughness, the lower the cooling/quenching time during the multiple successive droplets evaporation onto inclined hot inclined wall. 2. The spread diameter of the droplet impacting onto inclined hot surface at early stage of impact (t < 0.5 ms) was not affected by the surface roughness. At t > 0.5 the spread diameter increases with the increase of surface roughness. In addition, it was found also that the higher the surface roughness, the higher the contact time of solid–liquid contact during this event. 3. Due to the surface roughness interrupting the shrinking droplet, the higher the surface roughness the longer the sliding distance during the droplet impacting on the inclined hot surface. 4. The critical heat flux and Leidenfrost temperatures of multiple successive droplets evaporation impacting onto inclined hot surfaces were insensitive to the surface roughness. 5. From the present experiment it is found that the surface roughness is considered as an important parameter, therefore, a future work on the development of new experimental correlation and numerical model on this topic is needed.
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