ICMF2013-558

8th International Conference on Multiphase Flow ICMF 2013, Jeju, Korea, May 26 - 31, 2013

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High-speed visualization of droplet impingement on high temperature solid and liquid

metal surfaces

Nejdet Erkan1, Tetsui Yasui1, Koji Okamoto1

1Nuclear Professional School, The University of Tokyo, Japan

Keywords: Droplet, impingement, sub-atmospheric pressure, boiling, liquid metal, visualization

Abstract Liquid droplet impingement boiling on the hot surfaces has crucial importance for the industries employing spray cooling applications Although numerous experimental and computational efforts exist in the literature, several contradictory approaches, pertaining to the thermal-hydraulic behaviour of droplets, are encountered due to the parametric sensitivity of the phenomena. Complex and uncontrollable boundary conditions, such as surface roughness and ambient pressure, may affect the results in variety of ways that cause one to end up with irrelevant conclusions in every occasion. In order to understand the physical mechanism behind some series of experiments are performed under atmospheric and sub-atmospheric pressures with heated cupper plate and liquid metal which eliminates the effect of surface roughness. A distinct boiling behaviour is observed on the surfaces with/without surface roughness. On the polished cupper surface droplet keeps its integrity with intermittent touching to the surface and bouncing back from the surface at 313 oC, in contrast, trapped vapour layer bursting from periphery breaks the droplet suddenly into small flying out liquid fragments on liquid metal surface. In addition to the surface roughness, ambient pressure is decreased to 50 kPa at which time delay of boiling incipience follows different tendencies in both ambient pressures at similar super heat values suggesting that lesser air molar concentration in the environment cause remarkable delays in the incipience of boiling.

Introduction

Hydrodynamic and thermodynamic characteristics of liquid droplet impingement onto hot-surfaces have profound significance for various industrial applications, which utilize spray cooling, such as ex-vessel cooling for in-vessel melted core retention in case of a nuclear power plant (NPP) severe accident, hot-core reflooding by the emergency cooling systems of an NPP, cooling of electronic systems, combustion inside an engine, metallurgical quenching and spacecrafts heat rejection systems etc.

Spray cooling is an effective method of cooling employing liquid droplets fragmented by a spray nozzle and directed to a targeted hot surface. The droplets spread on the surface and evaporate or form a thin liquid film removing large amounts of energy due to the latent heat of evaporation. Heat transfer rates, much higher than in pool boiling, can be obtained with sprays since less resistance exists against the removal of vapor film formed in between the hot-surface and liquid (Celata et al 2009, Mohapatra et al 2012, Kim 2007 and so on). In essence, that film layer is speculated to be a mixture of vapor and ambient gas, and it degrades the heat flux in wall-liquid interface severely. Although the spray systems include multiple droplets and an individual droplet behavior cannot be extrapolated to the dynamics of large spray systems, the physics behind still demands fundamental investigations in simplified scales to develop efficient cooling systems.

Although numerous studies, dedicated to the understanding of thermal-hydraulic mechanism of the droplet impingement onto hot-surfaces, exists in the literature, the number of investigations still continues to increase due to the recent availability of more advanced and high-speed experimental and computational tools. Even though those noteworthy amounts of progresses in this branch of research available, contradictory approaches related to the boiling physics of a droplet have arisen due to the parametric sensitivity the phenomena which are originated from the complex uncontrollable boundary conditions (BC) (e.g. surface properties, ambient conditions and so on) of the problem (Moreira et al 2010). Heat transfer from the hot-surface to the droplet is dictated by several parameters which are needed to be identified for most of the cooing applications due to the variety of surface properties determining the critical parameters such as critical heat flux, Leidenfrost boiling point etc.

In this study, we presented a parametric study addressing significant factors of droplet impingement boiling phenomena with the experimental results obtained from high-speed visualization tests performed under well-controlled BCs. Polished and etched cupper plate and a liquid metal, contained in a stainless steel pot, services as a hot surface under atmospheric pressure and sub-atmospheric pressure. Liquid metal (U-Alloy) as a hot-surface provides a smooth surface due to its high surface tension that discards the major effects of surface roughness. Additional to the elimination of surface roughness effect, droplet impingement on liquid metal

surface under sub-atmospheric pressure is investigated to elucidate interference of air molecules with interface which forms a heat flux resistanceboiling behavior.

Experimental Facility

A schematic layout of experimental setupatmospheric and sub-atmospheric pressures is Figure 1. Precision flow controller drives the distilled water flow with a syringe pump. A plastic microtube, having inner and outer diameters of 150 respectively, is connected to the syringedispatch and impinge onto hot-surface from a height when it reaches to maximum size supported by the surface tension forcesUtilizing surface tension and gravity force balanceuniform dispatching droplet diameter of around produced at the tip of the tube. Temperature of the hot plate is controlled according to thermocouple data which is recorded 2 mm below the liquid metal surface(an alloy of Bi, Pb, Cd, and Sn, having melting point of 70 oC, density of 9946.27 kg/m3) is used as a hot surface and assumed to be non-deformable due to the droplet impact since its density and surface tension are much higher than those of water. Liquid metal surface temperature agitated to lover temperatures due to the droplet impingement; however, it recovers to 95% of settled initial temperature in 35s. As a solid hot-surface, polished cupper plate (30x30 mm, 0.5 mm thick) is employed. Time resolved images are recorded with a high-speed camera operating at spatial and temporal resolutions of 256x256 pixels and 10 kHz respectively. 200 mm Nikon Micro Nikkor lens is adapted to the camera to have larger spatial resolutions.

Figure 1: Schematic view of experimental setup.

Results and Discussions Effect of surface roughness

One of the important parameter in boiling phenomena whose effects still cannot be resolved sufficiently due to its

8th International Conference on Multiphase Flow ICMF 2013, Jeju

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atmospheric pressure is investigated to elucidate interference of air molecules with vapor-liquid interface which forms a heat flux resistance controlling the

xperimental setup for the atmospheric pressures is shown in

Precision flow controller drives the distilled . A plastic microtube,

150 µm and 360 µm syringe pump. Droplet

surface from a decided size which can be

supported by the surface tension forces against gravity. Utilizing surface tension and gravity force balance, nearly

droplet diameter of around 2.5 mm is Temperature of the hot plate

is controlled according to thermocouple data which is recorded 2 mm below the liquid metal surface. U-alloy 70

g melting point of 70 ) is used as a hot surface and

deformable due to the droplet impact since its density and surface tension are much higher than

Liquid metal surface temperature agitated lover temperatures due to the droplet impingement;

however, it recovers to 95% of settled initial temperature in surface, polished cupper plate (30x30

mm, 0.5 mm thick) is employed. Time resolved images are mera operating at spatial and

temporal resolutions of 256x256 pixels and 10 kHz respectively. 200 mm Nikon Micro Nikkor lens is adapted to the camera to have larger spatial resolutions.

experimental setup.

One of the important parameter in boiling phenomena whose effects still cannot be resolved sufficiently due to its

tremendously varying characteristictype of material and surface chemistry.3 show temporal evolution contact with the cupper plate and with inverted (a) and filtered images (b)conditions with similar Weber numbers. surface roughness effects, relativelare chosen in order to prevent the splashing effects, resulted from energetic impacts, from suppressing the surface roughness contribution. When the droplet impinge onto temperature of 313 oC which is well above Leidenfrost point (LFP) by regarded as lower temperature boundary of film boiling regime, wavy structures on droplet surface can be discerned from varying non-uniformity of image intensities2a). Disturbances observed after 1.3 ms are characterized with unordinary random motionsare made more explicit with Sobel filteringTwo modes of surface waves are distinguished during droplet collision onto the hot surfaceis created due to the droplet impact kinetic energy which is partially transferred to the droplet and generates energetic waves encircling the droplet and the bottom of the droplet to the upwardhydrodynamic waves are visibleand 1.3 ms after droplet’s contact with the surface2a). Parallel horizontal lines and of the droplet (Figure 2b) until the 2.3 ms structures from another perspectiveThe second mode is characterized asfluctuations which begin to aafter 2.3 ms that indicates nontaking place beneath the droplet due to the vapor generation from the nucleation spotshaving intermittent pressure fluctuations which disturbs lower surface of droplet, promote unstable waves propagating whole across the liquid surface. Pressure variations are likely to be caused by generation rate attributed to the nonstructures (surface roughness), since indented uneven structures lead to non-uniform heat transfer to the liquid surface with intermittent direct contact conduction by rupturing the vapor film. Elevated heat transferenhance vapor production rate resulting in droplet bounce back from the surface with keeping its integrityin Figure 2 ).

International Conference on Multiphase Flow Jeju, Korea, May 26 - 31, 2013

tremendously varying characteristics associated with the of material and surface chemistry. Figure 2 and Figure

of water droplet after initial cupper plate and liquid metal respectively

with inverted (a) and filtered images (b) under ambient conditions with similar Weber numbers. To amplify the surface roughness effects, relatively low impact velocities are chosen in order to prevent the splashing effects, resulted from energetic impacts, from suppressing the surface

When the droplet impinge onto the cupper plate having a C which is well above 225 oC estimated

by Bernardin et al (1997) and regarded as lower temperature boundary of film boiling regime, wavy structures on droplet surface can be discerned

uniformity of image intensities (Figure after 1.3 ms on droplet surface

unordinary random motions which are made more explicit with Sobel filtering in Figure 2b. Two modes of surface waves are distinguished during droplet collision onto the hot surface above LFP. First mode

due to the droplet impact kinetic energy which is partially transferred to the droplet and generates uniform

encircling the droplet and propagating from the bottom of the droplet to the upward direction. Those hydrodynamic waves are visible to some extent from 1 ms and 1.3 ms after droplet’s contact with the surface (Figure

. Parallel horizontal lines and smooth pulses on the edge ) until the 2.3 ms also confirm those

from another perspective. he second mode is characterized as nonuniform random

which begin to appear on the droplet surface 2.3 ms that indicates nonuniform pressure transients

taking place beneath the droplet due to the instantaneous from the nucleation spots. Vapor layer is

having intermittent pressure fluctuations which disturbs lower surface of droplet, promote unstable waves

across the liquid surface. Pressure variations are likely to be caused by non-uniform vapor

the non-uniformity of surface structures (surface roughness), since indented uneven

uniform heat transfer to the liquid surface with intermittent direct contact conduction by rupturing the vapor film. Elevated heat transfer rates

por production rate resulting in droplet bounce back from the surface with keeping its integrity (7.6 ms later

(a)

(b)

Figure 2: Droplet impingement on cupper plate inverted images b) filtered with Sobel, (313 u=0.39 m/s).

On the contrary, in the case of zero surface roughness (liquid metal surface) first mode of waves continued to be monitored until droplet reaches its largest diameter up to 2.9 ms, any notion of uneven disturbances on the droplet surface cannot be discerned (Figure Soon after at 3.1 ms a liquid spurt come into sightwater and liquid metal interface suggesting that spread on the surface as much as surface tension forces allowed and vapor layer reached the maximupressure for disrupting the liquid layer on it and eruptedThis blast of vapor wieldy destroys and fragments water the proximity of interface resulting in flying out of small secondary droplets. Unlike to cupper surface, no mnon-structured disturbances developed on the droplet surface until a sudden blast taking place interface up to 3.1 ms. Based on this observation, we may speculate that a stable vapor layer is formed under the droplet in contrast to cuppesurface case in which localized pressure disturbances associated with spotted vapor production are discarded by localized strains of water surface without any fragmentation. However, absence of surface roughness on the liquid metal surface leads to a uniform vapor generation that degraded the heat transfer rate from surface to the droplet equally for every spatial position beneath the water


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Droplet impingement on cupper plate a) original (313 oC, We=5.2,

zero surface roughness first mode of waves continued to be

droplet reaches its largest spreading diameter up to 2.9 ms, any notion of uneven disturbances on

Figure 3a-b). urt come into sight from the

liquid metal interface suggesting that droplet on the surface as much as surface tension forces

ched the maximum necessary pressure for disrupting the liquid layer on it and erupted.

destroys and fragments water in resulting in flying out of small

secondary droplets. Unlike to cupper surface, no major disturbances developed on the droplet

surface until a sudden blast taking place through the

Based on this observation, we may speculate that a stable in contrast to cupper

localized pressure disturbances vapor production are discarded by

water surface without any lumped liquid fragmentation. However, absence of surface roughness on

leads to a uniform vapor generation degraded the heat transfer rate from surface to the

equally for every spatial position beneath the water

droplet without creating any noninterface (from 0 ms to 2.9 ms in layer reaching sufficient pressureentrapment, but not sufficient for lifting terupt from the periphery instead of 2.9 and 3.1 ms in Figure 3).

Figure 3: Droplet impingement on liquid metal a) original inverted images b) filtered with Sobel, (307u=0.37 m/s).

Contact angle and boiling incipience dependence on surface roughness

The contact angle between a liquid and a solid surface is

regarded as one of the important parameterboiling phenomena since it characterizes wettability of a certain solid surface by a specific fluid (Tong During droplet impingement onto hot we observed a sudden stepwise declinewhereas on cupper plate such a not be detected. As mentioned and discussed previous section, droplet on the hot liquid metal surface sits on a vapor/air cushion for a while then suddenly wetted with the liquid metal surface which initiaboiling. A description of that rapid change in is demonstrated in Error! Reference source not found.


droplet without creating any non-uniform strain on the water interface (from 0 ms to 2.9 ms in Figure 3). Upon vapor layer reaching sufficient pressure for breaking out its

, but not sufficient for lifting the droplet yet, it instead of lifting the droplet off (at

(a)

(b)

Droplet impingement on liquid metal a) original images b) filtered with Sobel, (307oC, We=4.7,

and boiling incipience dependence on

The contact angle between a liquid and a solid surface is one of the important parameters governing

since it characterizes wettability of a certain solid surface by a specific fluid (Tong et al 1990). During droplet impingement onto hot liquid metal surface,

erved a sudden stepwise decline in contact angle er plate such a rigorous variation could

not be detected. As mentioned and discussed partially in previous section, droplet on the hot liquid metal surface sits on a vapor/air cushion for a while then suddenly wetted with the liquid metal surface which initiate the

g. A description of that rapid change in contact angle Error! Reference source not found.,

from those time sequential images transient variation of contact angle is detected and

Figure 4: Stepwise change of contact angle.

Figure 5 Variation of contact angle after droplets to the liquid metal surface with 0.37 m/s velocity

Figure 6 Variation of contact angle after droplets to the liquid metal surface with 0.41 m/s velocity

plotted in Error! Reference source not found.Reference source not found. for the impact velocities of 0.37 m/s and 0.41 m/s respectively. At a first glance on these graphs, droplet is wetted with the liquid metal surface earlier when it impinge on with a higher veReference source not found.) compared to the lower velocity case (Error! Reference source not found.). On the other hand, impingement velocity does not affect gradually the time evolution of contact angle with cupper surface which has 108 oC surface temperature. It increases in first 1 ms then reaches a maximum value and retain this valrelatively longer time for both impingement speeds that is likely to be due to the surface roughness of cupper plate. When the shape of two sequential images of one droplet

1 2 30

50

100

150

Time[ms]

θ[d

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ree

]

Temperature307202106108

1 2 30

50

100

150

Time[mc]

θ[d

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Temperature309202105110


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transient variation of

Stepwise change of contact angle.

after the collision of

with 0.37 m/s velocity.

after the collision of

with 0.41 m/s velocity.

Error! Reference source not found. and Error! for the impact velocities of 0.37

m/s and 0.41 m/s respectively. At a first glance on these graphs, droplet is wetted with the liquid metal surface earlier when it impinge on with a higher velocity (Error!

) compared to the lower velocity ). On the other hand,

impingement velocity does not affect gradually the time evolution of contact angle with cupper surface which has

C surface temperature. It increases in first 1 ms then reaches a maximum value and retain this value for a relatively longer time for both impingement speeds that is likely to be due to the surface roughness of cupper plate.

When the shape of two sequential images of one droplet

captured at the instants of 2.7 and 2.9 ms in clearly visible that sudden change in contact angle is followed by incipience of boiling. Based on this finding a parameter regarded as delay timeelapsed until a drastic change of contact angle takes place is employed to investigate the droplet impingement boiling on a surface.

Effect of droplet velocity on boiling incipience Zero surface roughness changes the drople

characteristics due to the effect of wettability of droplet with the liquid metal. Delay time for the surface wetting and incipience of boiling is explored for different impact velocities and illustrated in Error! Reference source not found.. For all velocities unchanged up to the 80K superheat (surface temperature of around 180 having different magnitudes tendency of delay time variation, especially for the case of 0.37 m/s impact velocity, with respect to the wall super heat resembles the drop evaporation curve introduced by Bernardin and Mudawar (1999). In their study, sessile drop evaporation time decreases in nucleate boiling regime until the CHF temperature corresponding to minimum drop life time, thereafter droplet life time start to increase in transition to film boiling regime which continues up to the LFP point temperature. From that perspedelay times for the case of 0.37 m/s corresponds to an implicit evidence of boiling mode transition point which can be speculated to be critical temperature for the transition to film boiling regime. However, onset of transition to film boiling regime occurs in stepwise manner that differs from the case of sessile droplet. Transition boiling continues up the surface temperature after that temperature droplet and surface contact could not be observed that implies the LFP point iexceeded.

Figure 7: Delay time or boiling incipience due to the surface temperature for several droplet velocities.

Similarly, smaller peaks detected at 180 temperature for the higher they may not be attributed to a transition point due to the fact that delay times continues to decrease after that point

4 5

Temperature307℃202℃106℃108℃(Copperplate)

4 5

Temperature309℃202℃105℃110℃(Copperplate)


captured at the instants of 2.7 and 2.9 ms in Figure 3, it is clearly visible that sudden change in contact angle is followed by incipience of boiling. Based on this finding a

delay time and identified as the time elapsed until a drastic change of contact angle takes place is employed to investigate the droplet impingement boiling

Effect of droplet velocity on boiling incipience

Zero surface roughness changes the droplet boiling characteristics due to the effect of wettability of droplet

Delay time for the surface wetting and incipience of boiling is explored for different impact

Error! Reference source not delay time remains nearly

80K superheat (corresponding to a surface temperature of around 180 oC), later on a peak having different magnitudes are observed. General tendency of delay time variation, especially for the case of 0.37 m/s impact velocity, with respect to the wall super heat resembles the drop evaporation curve introduced by Bernardin and Mudawar (1999). In their study, sessile drop

me decreases in nucleate boiling regime until the CHF temperature corresponding to minimum drop life time, thereafter droplet life time start to increase in transition to film boiling regime which continues up to the LFP point temperature. From that perspective, the peak in delay times for the case of 0.37 m/s corresponds to an implicit evidence of boiling mode transition point which can be speculated to be critical temperature for the transition to film boiling regime. However, onset of

boiling regime occurs in stepwise manner that differs from the case of sessile droplet. Transition boiling continues up the surface temperature of 238 oC,

droplet and surface contact could not t implies the LFP point is already

Delay time or boiling incipience time variation

due to the surface temperature for several droplet

maller peaks detected at 180 oC surface higher impact velocities, however,

may not be attributed to a transition point due to the fact that delay times continues to decrease after that point

as well. That decrement in delay times turn to incrementwith a steep climb at around 280 oC (at around super heof 180K) and any wettability event cannot be observed after around 200K wall superheat. 180K superheat value is likely to be a minimal point for the transition regime to film boiling.

Bernardin et al (1997) reported critical heat flux temperature on a polished cupper surface droplets having a 3 mm diameter and 0.7 m/s velocity 130 oC. They concluded that CHF and LFP temperatures130 oC and 225 oC respectively, seems fairly insensitive to droplet impact velocity over tested range. On the other hand, they demonstrated that while CHF temperature is insensitive to surface roughness, LFP temperature shows higher sensitivity against surface roughness.Considering the boiling behavior of droplets impinging on solid metallic surfaces, which has some particular level of surface roughness even though surface treatmentwould not be inappropriate to come into such a conclusion that under the zero surface roughness LFP and CHF temperatures are likely to be varying drastically according to the surface superheat and droplet impingementas it is evidenced from the surface wettability transients

Effect of pressure The noncondensible gas and ambient press

important parameter governing the boiling phenomena. Emmerson (1975) investigated sessile dropletfound that droplet evaporation time is reduced as the pressure increases. He explained that tendency associatingwith the decrease in latent heat of vaporization due to increasing pressure. He also noted that whereas LFP increases with pressure, droplet evaporation time or dropletlifetime remain shorter. Buchmuller et aldroplet impingement boiling under high pressures, up tbars, with water droplets having 2.4 mm diameter and 0.77 m/s impact velocity. They observed shifts in boiling transition points including LFP due to the increase in saturation temperature.

Error! Reference source not found. pretime variation versus wall superheat underpressure and sub-atmospheric pressures hitting to surface at the speeds of 0.41 m/s and 0.43 m/sRed line denotes the surface temperaturesvalues at which peaks are detected for both casessignificant delay in wettability due to the boiling observed for the case of sub-atmospheric pressure in between 0-60 oK superheat regions, a particudelay is detected under atmospheric pressure range of superheat. If we extrapolate the Emmerson (1975)’s point of view (valid for the pressures higher than the atmospheric one) to the sub-atmospheric pressure; since the latent saturated water at 50 kPa (2305 kJ/kg) higher than that of atmospheric pressure (2257 kJ/kg), apparently more heat energy is needed for the creation of vapor layer in sub-atmospheric pressure, hence, the reason of no delay in boiling around low super-heats (below 25K) the lack of sufficiently thick vapor layer formation.

On the other hand, we observed an intriguing sharp increase in delay time under the sub-atmospheric pressure


5

as well. That decrement in delay times turn to increment C (at around super heat

and any wettability event cannot be observed after around 200K wall superheat. 180K superheat value is likely to be a minimal point for the transition regime to

reported critical heat flux (CHF) surface for the impinging

having a 3 mm diameter and 0.7 m/s velocity as hey concluded that CHF and LFP temperatures,

seems fairly insensitive to droplet impact velocity over tested range. On the other hand, they demonstrated that while CHF temperature is insensitive to surface roughness, LFP temperature shows higher sensitivity against surface roughness.

e boiling behavior of droplets impinging on metallic surfaces, which has some particular level of

surface roughness even though surface treatment applied, it would not be inappropriate to come into such a conclusion

ess LFP and CHF rying drastically according

impingement velocity as it is evidenced from the surface wettability transients.

pressure is another verning the boiling phenomena.

droplet boiling and droplet evaporation time is reduced as the

lained that tendency associating ease in latent heat of vaporization due to

increasing pressure. He also noted that whereas LFP increases with pressure, droplet evaporation time or droplet

et al (2012) studied pressures, up to 16

droplets having 2.4 mm diameter and 0.77 m/s impact velocity. They observed shifts in boiling transition points including LFP due to the increase in

presents the delay heat under atmospheric

for the droplets hitting to surface at the speeds of 0.41 m/s and 0.43 m/s.

the surface temperatures and superheat at which peaks are detected for both cases. While no

in wettability due to the boiling is atmospheric pressure in

a particular level of delay is detected under atmospheric pressure in the same

If we extrapolate the Emmerson (1975)’s point of view (valid for the pressures higher than the atmospheric one) to

ince the latent heat of kJ/kg) higher than that of

atmospheric pressure (2257 kJ/kg), apparently more heat energy is needed for the creation of vapor layer in

atmospheric pressure, hence, the reason of no delay in (below 25K) is likely to be

the lack of sufficiently thick vapor layer formation. , we observed an intriguing sharp

atmospheric pressure

beginning earlier than those of atmospheric pressure caand rises up to elevated levels at a superof 80K. If we took into account solely the differences in latent heats of both cases and the time to reach a stable film boiling, the peak in boiling delay in subpressure must have been developed at higher superheats and it must have not been much diverged from the atmospheric pressure. From this point of view, another factor must come into playregime by generating more stable blockage against heat transfer and prevents droplet’s wetting during boiling in sub-atmospheric pressures.

Figure 8: Delay time versus sup(0.41 m/s) and sub-atmospheric (0.43 m/s

Air bubble entrapment beneath the impinging droplets

on solid surfaces in atmospheric pressures is known from some researches in the literature. investigated the air bubble entrapment under the impacting droplets onto cold solid surfaces. They explored dynamicsof air bubble for several time periods elapsed after the first contact of the droplet. They observed air bubble for various impact velocities of the water droplets having a diameter of 4-5.5 mm and showed that air occurs in wide range of Weber numbers. by Driscoll et al (2012) that at suba ring of highly populated microbubblesdiameter silicon oil droplet impacting with 3.19 m/s velocity, in contrast to the atmospheric pressure conditions, encircling a larger bubble are entrapped and their sizesgrow with decreasing pressure.

Under the light of aforementioned can be hypothesized that larger delayssub-atmoshperic pressure isentrapped air bubbles and accompanying which are blocking the droplet wettability and blanketing conductive heat transfer to the droplet leading to larger delays. Even this is the case, quantification is essential for the confirmation of this effect.

Conclusions


beginning earlier than those of atmospheric pressure case and rises up to elevated levels at a superheat temperature

. If we took into account solely the differences in latent heats of both cases and the time to reach a stable film boiling, the peak in boiling delay in sub-atmospheric

been developed at higher superheats and it must have not been much diverged from the atmospheric pressure. From this point of view, another

and govern the film boiling regime by generating more stable blockage against heat

er and prevents droplet’s wetting during boiling in

Delay time versus super-heat at atmospheric atmospheric (0.43 m/s) pressures.

Air bubble entrapment beneath the impinging droplets on solid surfaces in atmospheric pressures is known from some researches in the literature. Thoroddsen et al (2005) investigated the air bubble entrapment under the impacting

faces. They explored dynamics of air bubble for several time periods elapsed after the first

They observed air bubble for various impact velocities of the water droplets having a

and showed that air entrapment ccurs in wide range of Weber numbers. Recently reported

that at sub-atmospheric pressures, a ring of highly populated microbubbles beneath a 3.1 mm diameter silicon oil droplet impacting with 3.19 m/s

tmospheric pressure conditions, encircling a larger bubble are entrapped and their sizes grow with decreasing pressure.

aforementioned previous findings it can be hypothesized that larger delays under

atmoshperic pressure is likely to be caused by entrapped air bubbles and accompanying microbubbles which are blocking the droplet wettability and blanketing conductive heat transfer to the droplet leading to larger delays. Even this is the case, further investigation and

is essential for the confirmation of this

8th International Conference on Multiphase Flow ICMF 2013, Jeju, Korea, May 26 - 31, 2013

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Effective parameters including surface roughness and ambient pressure in droplet impingement boiling phenomena are investigated utilizing polished cupper plate and liquid metal. Boiling behaviour of water droplets demonstrated completely different manner on the liquid metal surface, it is characterized by sudden vapour burst and droplet fragmentation instead of bounce back from the surface. Wetting behavior of droplet shows different tendency with respect to surface roughness. When the droplet hits the liquid metal surface, it remains nonwetted for a while then a rapid wetting is observed, whereas on the cupper surface contact angle increases monotonically and remained constant. With zero surface roughness boiling transition temperatures are likely to be varying drastically according to the surface superheat and droplet impingement velocity as it is evidenced from the surface wettability transients. Under sub-atmospheric pressure boiling behavior of the droplet indicates harsh difference such that delay time increases while the pressure decreases. The reason of this tendency is likely to be associated with the differences in air entrapment dynamics in subatmospheric pressure. References Bernardin J.D. Stebbins C.J. and Mudawar I. Effects of surface roughness on water droplet impact history and heat transfer regimes, Int. J. Heat Mass Transfer, 40-1: 73-88 (1997). Bernardin J.D. Mudawar I. The Leidenfrost point: Experimental study and assessment of existing models. Journal of Heat Transfer, 121:894-903 (1999). Buchmuller I. Roisman I, Tropea C. Influence of elevated pressure on impingment of a droplet upon a hot surface. ICLASS 2012, Heidelberg, Germany (2012). Celata G.P, Cumo M, Mariani A., Saraceno L. A comparison between spray cooling and film boiling during the rewetting of a hot surface. Heat Mass Transfer, 45:1029-1035 (2009). Driscoll M.M. Stevens C.S. and Nagel S.R. Thin film formation during splashing of viscous liquids, Ph. Rev. E, 82:036302 (2010). Emmerson G.S: The effects of pressure and surface material on the Leidenfrost point of discrete dropes of water, Int. J. Heat Mass Transfer, 18:381-386 (1975). Kim J. Spray cooling heat transfer: The state of the art, Int. J. of Heat and Fluid Flow, 28:753-767 (2007). Mohapatra S.S. Chakraborty S. and Pal K. Experimental studies on different cooling process to achieve ultra-fast cooling rate for hot steel plate. Exp. Heat Transfer, 25:111-126 (2012). Moreira A.L.N. Moita A.S. Panao M.R. Advance and challenges in explaining fuel spray impingement: How much of single droplet impact research is useful?, Prog. in Energy and Combustion Sci., 36:554-580 (2010).

Tong W. Bar-cohen A. Simon T.W and You S.M. Contact angle effects on boiling incipience of highly wetting liquids. Int. J. Heat Mass Transfer, 33-1:91-103 (1990). Thoroddsen S.T. Etoh T.G. Takehara K. Ootsuka N. And Hatsuki Y. The air bubble entrapment under a drop impacting on a solid surface. J. Fluid Mech. 545:203-2012 (2005).

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ICMF2013-558