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The effect of surface roughness on dynamic behaviour of the successive multiple droplets impacting onto aluminium hot surfaces

The effects of hot surface roughness on dynamic behaviour of multiple droplets impacting hot solid surfaces were investigated in this research. The test material is Aluminium 6061 with surface roughness variation of 0.08 μm, 0.18 μm and 0.52 μm. The surface temperatures were 100 °C, 120 °C and 140 °C. The droplet dynamics during the impact have been observed using high-speed video camera then the results were characterized by using image processing technique. It could be concluded that each temperature shows different thermodynamic impact regime. The results also reveal that at surface temperature lower than 140 °C, the increase of surface roughness leads the increase of spreading diameter. In contrast, the spreading factor decreases with the increasing of surface roughness at 140 °C due to incomplete wetting of droplet on the surface. Meanwhile, the maximum spreading factor is relatively independent to the change of surface roughness.


Introduction
The high demands of industry for products with enhanced material properties which is easy to manufacture and low cost are challenging processing techniques. Processing of aluminum alloys has gained considerable attention from the automobile and aerospace industries because it could improve the quality of material i.e. strength to weight ratio and corrosion resistance. Spray cooling is one of the method in metal processing frequently applied in aluminum and steel industries, electronic devices and emergency cooling application which has powerful heat management technique to control microstructural development [1]. Spray cooling involves the liquid drops i.e. water hitting to a hot metal surface. On the other hand, the limited knowledge of the heat transfer phenomena during spray cooling have aroused some researcher to examine the complete picture of the mechanism.
Deendarlianto have proposed a clear method to investigate the surface wettability effect on the phenomena of heat transfer and dynamics behavior of droplet. The single water droplet was dropped onto the aluminium heated surface [2]. They found that the increasing of static contact angle decreased the wetting limit temperature. The results also showed that the increasing of static contact angle increased the maximum spreading diameter for the temperature below the wetting limit.  The effects of surface roughness on the evaporation successive droplet dynamics on inclined heated stainless steel SUS 304 surface were also investigated by Deendarlianto [3]. The results show that quenching behavior was significantly influenced by varying the surface roughness. The increase of surface roughness results in faster quenching time, higher contact time of solid-droplet and higher spreading diameter of droplet Kim have examined the effect of surface roughness on pool boiling heat transfer which using water as the fluid on aluminum surface. They concluded that boiling heat transfer increases with the increase of surface roughness (Ra) [4]. It is because of the improvement of aluminum surface wettability which deteriorates boiling heat transfer performance caused by nucleation cavities.
The research about heat transfer phenomena of droplet impacting hot surface on some thermodynamic regimes i.e. single phase, nucleate boiling and transition boiling is limited. Therefore, the investigation about the effect of surface roughness at dynamic behavior of multiple water droplets impinging aluminum heated surface will be revealed in present study.

Methodology
The illustration of the experimental apparatus and procedure used in the present study were described in previous paper by Wibowo [5] where the schematic diagram of the experiment is shown in Figure 1.  A heat source or heater is placed directly under the test material. K-type thermocouples which have an uncertainty of 0.75% were used as temperature sensors. The multiple droplets were injected from reservoir to aluminum hot surface through a droplet injector which is controlled by an automatic valve and located 70 mm above hot surface as done by previous research [6]. The droplet diameter was 3.1 mm with the frequency of 7 drops/second. The surface temperature was varied from 100 °C up to 140 °C. The test material is Aluminum 6061 with surface roughness variation of 0.08 µm, 0.18 µm and 0.52 µm ( Figure 2). A high speed video camera which is set at 4000 frame per second and 1024x768 of resolution were utilized to investigate the droplet behavior during impact phenomena on the aluminum hot surface. Furthermore, an image processing technique was applied to analyze the video images by converting them become binary images ( Figure 3). The outcomes are the droplet spreading diameters thus the spreading ratio could be obtained by dividing droplet spreading diameter with initial droplet diameter. Next, the results is pictured in a time series diagram which compares every surface roughness.

Results and discussions
Surface roughness plays a very important role in transferring the heat from metallic surfaces to the other materials or fluids during boiling. Predicting or correlating the boiling heat transfer data with micro surface geometry and finding the effective method in enhancing heat removal are challenging some researchers. Three aluminum surfaces were prepared in order to investigate the effects of surface roughness on boiling heat transfer at aluminum surfaces which dynamic behaviors could be seen in Figs. 4-6 which belong to surface temperature of 100 ºC, 120 ºC and 140 ºC, respectively. Fig. 4 shows effect of surface roughness on droplet dynamics impacting aluminum hot surface at 100 ºC. It was observed that there is no presence of nucleate in droplet impact mechanism at 100 ºC. The behavior of the impacting droplet is almost similar for all surface roughness. On the other hand, surface roughness highly influences the spreading maximum period where Ra = 0.52 μm was the fastest time required to reach the maximum spreading. In this case the heat transfer occurs by single-phase convection.   Fig. 5 it is shown that after impacting the solid surfaces, the droplets spread until reaching the maximum diameter and afterwards shrink. Even though there is no linear correlation between surface roughness and spreading maximum period as previous temperature, the effect of the geometry could be taken into consideration. The small vapor bubbles formed at droplet surface interface at temperatures slightly exceeding the saturation temperature indicated that the liquid heat transfer developed in nucleate boiling regime [7].
The increasing of temperature leads to regime alteration where nucleate boiling regime changes into transition regime. In transition regime, vapor bubbles are generated by heat transfer from the wall developed and break the liquid lamella forming some secondary drops as observed in Fig. 6. This regime is characterized as the transition-boiling regime [8]   Two physical parameters are employed in order to characterize droplet dynamics which impacts the aluminum hot surfaces. Those are droplet wetting diameter d and the droplet height above the surface. By dividing these quantities by initial droplet diameter d0 we obtain 'spreading factor' (d/d0) and 'dimensionless height (h/d0)' [9].
In favor of examining the effect of surface roughness on spreading factor, the wetted diameter was measured by using image processing techniques. The evolution of spreading factor as a function of time is presented in Fig. 7. There are oscillations with the same pattern found for all variations of surface roughness at 100 ºC as a result of unbalanced force or energy inside the droplet produced by pressure differential between inside and outside the droplet and/or momentum [10]. Close investigation of the figure reveals that the effect of surface roughness on the spreading factor become stronger within t = 100 ms -180 ms. The higher the surface roughness, the higher the spreading factor. Contact angle increases with the increase of surface roughness [11] thus provides low damping coefficient [12] causing larger droplet spreading diameter.  Figure 8 shows the transformation of spreading factor (d/d0) on different surface roughness as the parameter at 120 ºC. Spreading factor is significantly affected by the surface roughness at t > 20 ms where spreading factor increases with the increase of surface roughness. The spreading diameter for rough surface is higher than the smooth one on account of large number of cavities on bubble nucleation which enhances the wettability [4]. The evidence is shown in Fig. 5 at t = 125 ms of Ra = 0.52 μm.    The transformation of spreading factor as a function of time is demonstrated in Fig. 9. It could be seen that spreading factor decreases with the increasing of surface roughness. The increasing of surface roughness causes the heat transfer increases [4] due to fast vapor produced near the contact line. The peripheral part of the lamella levitates in that vapor layer made the solid surface was not completely wetted. This is supported by visual examination in Fig. 6 at t = 168.5 ms of Ra = 0.52 μm. Figure 10 illustrates the comparison of maximum spreading ratio at various temperatures with surface roughness as parameter. The result reveals that the maximum spreading factors are unresponsive to the surface roughness changing. Although the aluminum surfaces have different surface roughness, the maximum spreading factor of the rough surface is almost the same with the smooth surface, hence, it can be summarized that the surface roughness does not affect the maximum spreading factor. This conclusion confirms to that of Deendarlianto [3] who investigated the spreading ratio maximum of water droplet which impacted onto inclined heated surface with different surface roughness and reported that the ratio is not influenced by the surface roughness. Lee at al. [13] also performed the experimental study of drop impact on smooth and rough surface where the conclusion leads to the small influence of surface roughness on the spreading.