Drop impact cooling enhancement on nano-textured surfaces. Part II: Results of the parabolic flight experiments [zero gravity (0g) and supergravity (1.8g)]

International Journal of Heat and Mass Transfer 70 (2014) 1107–1114

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Drop impact cooling enhancement on nano-textured surfaces. Part II: Results of the parabolic flight experiments [zero gravity (0g) and supergravity (1.8g)] Suman Sinha-Ray a, Sumit Sinha-Ray a, Alexander L. Yarin a,b,⇑, Christina M. Weickgenannt c, Johannes Emmert c, Cameron Tropea c a

Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 W. Taylor St., Chicago, IL 60607-7022, United States College of Engineering, Korea University, Seoul, South Korea c Institute of Fluid Mechanics and Aerodynamics, Technische Universität Darmstadt, 64287 Darmstadt, Germany b

a r t i c l e

i n f o

Article history: Available online 6 December 2013 Keywords: Spray cooling Nanotextured surface Zero gravity Supergravity

a b s t r a c t This article extends the results of the experiments at the earth gravity described in Part I to the case of the parabolic flight experiments conducted in Bordeaux, France, in June 2013 on a Novespace plane during a parabolic flight campaign that was supported by NASA and ESA. This second part details the droplet generator, heating system and the experimental rig developed for the flight experiments at zero gravity (0g) and supergravity (1.8g). Even though the setup used in the flights was an offshoot of the setup developed in the ground experiments of Part I, it had a number of modifications dictated by safety and space restrictions. After the experimental Section 1, the results of the heat flux measurements during the parabolic flights are described and discussed in Section 2, followed by conclusions. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The drop/jet generator developed in the ground experiment and described in Part I [1] was used in the parabolic flight experiments conducted in Bordeaux, France, in June 2013 on a Novespace plane during the 57th ESA parabolic flight campaign. Parabolic flights were performed onboard an Airbus 300 that followed a flight pattern which alternated ascents and descents with short level flight breaks. Each of those maneuvers, called parabolas, provided up to 22 s of reduced gravity or weightlessness. For each parabola, there were also two periods of increased gravity (1.5–1.8g) which lasted for 20 s immediately prior to and following the 22 s of weightlessness. During the campaign 93 parabolas were provided to perform the experiments. The introduction of Part I [1] is also valid for Part II of this work; therefore this article begins directly with a description of the experimental section. 2. Experimental The experimental setup used in the parabolic flights was essentially the one developed and tested in the ground experiments in ⇑ Corresponding author at: Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 W. Taylor St., Chicago, IL 60607-7022, United States. Tel.: +1 (312) 996 3472; fax: +1 (312) 413 0447. E-mail address: [email protected] (A.L. Yarin). 0017-9310/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.11.008

Part I [1]. However, it should be emphasized that there were several experimental modifications resulting from the safety constraints imposed by the operating agency. One result of these restrictions was that not all possible parameter variations conducted on the ground could also be performed during the parabolic flights. 2.1. Droplet generator The schematic of the drop generator used in parabolic flights is shown in Fig. 1(a). Due to the safety related reasons we had to guarantee that during the zero gravity parts of the flight, water does not escape from the reservoir. To achieve that, the water reservoir was re-designed in such a way (Fig. 1(b)) that water can never leave the reservoir. The reservoir was made out of Lexan instead of Plexiglas because all flammable and/or breakable materials are prohibited in an experimental setup by Novespace. Water in the reservoir was separated from the high pressure line by a piston made of Teflon. It should be emphasized that boring a perfectly cylindrical bore in a Lexan tube is not possible. To avoid the problem posed by a non-perfect circularity of the bore cross-section, an O-ring was added to the Teflon piston, which also facilitated the piston motion. The high pressure air line is connected to the reservoir through a solenoid valve (ASCO- 8262H022 12 V DC solenoid valve 2-way NC 1/400 ). The valve opens and closes following a rectangular control signal with 12–18 ms duration and 12 VDC. This results in 40–60 psi pressure bursts of a controlled duration,

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Fig. 1. Schematic of the setup for cooling experiments in parabolic flights. (a) Overall and (b) water reservoir.

which in turn, are transmitted to the reservoir of working fluid (distillated water) through the piston. In Fig. 1b some of the functional parts are denoted by characters, which are explained in detail in the following Table 1 (see also Fig. 4). Note, that the only coolant used in parabolic flights was water, since Novec fluid FC7300 had to be excluded due to the safety reasons. 2.2. Heating system A FPGA-based measurement system from National Instruments, which incorporates a 100 ms P temperature controller and protocoling functions was used. An Ultramic600 heater from Watlow with an integrated type K thermocouple was used under the copper plates with a thickness of 1.5 mm, with or without copper-plated nanofibers. The heater (Watlow CER-1-01-00002) has a power rating of 967 W at 240 VAC. The heating power is controlled by multiple switches in series, in that way all safety devices are able to interrupt heating one by one (cf. Table 2). A schematic of the heated target system is shown in the top part of Fig. 2. The operation temperature of the surface was 125 °C. The thermal cut-off fuses were chosen at the temperature of 240 °C, in order to retain flexibility in the experiment and to be able to heat the heater over the desired surface temperature, when there is a high temperature gradient because of the expected high heat flux. A Teflon target holder with mounted thermal fuses, the ceramic

insulation with aluminum target clamps and the heater are shown near the bottom in Fig. 2. It should be mentioned that the onboard (inside the aeroplane) regulation allowed usage upto 8 A max. If the heater were used to its full potential, there could be a risk of overdrawing current. Besides the potentially aggressive nature of the heater, it could have happened that the heater could have melted the thermal fuse. That would result in a complete stop of the experiment (as it was mounted in the Teflon block holding the heater and the sample as shown in Fig. 2) if the heater were running at its full power capability. These two reasons did not allow us to use the heater to its full capacity, and its output was limited to 200–250 W at maximum (the P temperature controller does not allow us to measure precisely the actual heat that was supplied but yields a range of operation). The heater was controlled by the surface temperature of the sample (denoted as T1) and shut off when the heater temperature exceeds a threshold temperature which was set to 30 °C higher than the desired surface temperature (denoted as T2). 2.3. Enclosure used in the flight experiments One of the largest safety concerns for this experiment (in addition to the usually feared explosion and fire) was to avoid any possible leakage of water or water vapor inside the aeroplane during flight. In this regard, special care was taken to keep the sample

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S. Sinha-Ray et al. / International Journal of Heat and Mass Transfer 70 (2014) 1107–1114 Table 1 Functional description of the droplet generator. Ref. Items

Function

Functional description table The short term actuation of the solenoid D ASCOvalve creates an instant push and 8262H022 vaccum, which results in production of 12V DC drop/jet on demand at different solenoid valve 2-way frequencies NC 1=4 00

Related parameters

Expected values

Admissible range

The actuation voltage-12 V, Frequency varies between 1 and 10 HZ, the active time of opening 10–14 ms

The solenoid The solenoid valve is valve will be rated for 130 Psi of air subjected to pressure 40–60 Psi

Remarks

The valve parameters are chosen in such a manner that the valve is always within the safe limit of operation

The burst strength of the tubing is 300– 400 Psi

The tube strength is well above the operation parameters and is not flammable

The piston is subjected to pressure of 40–60 Psi

The compressive strength of piston relates to a pressure 2100 Psi

The piston is subjected to compressive stress, which is lower than the admissible strength

The copper tubing acts as the supply line Pressurized at a repeated pulse of of water actuation provided by the system

Maximum pressure of 40–60 Psi

The copper tubes are rated for 980 Psi

The copper tube will be working well within the safety limit

Providing air for droplet generator

Tank pressure: 116 Psi, air input: 110 l/min, air output: 90 l/min, noise LWA/db: 85/66; motor: 0.7 kW/1HP, V/Hz: 230/50, dimension: 530  210  540 mm, weight 15 kg, oil free

43 Psi needed for droplet generator

Overpressure Opens, when the pressure in the spray release valve chamber rises above critical value

Pressure limit 14 Psi, limit not changeable by the operator

14 Psi

0-14 Psi (operation), 165 m3/h (survival)

J

Pressure regulators

Reduce the pressure to experimental need

Working pressure in the droplet generator pulses

43 Psi

14-72 Psi (operation), 0-232 Psi (survival)

K

Spray chamber

Experimental chamber, heater and droplet generator are installed inside

Tested maximum pressure 36 Psi

14 Psi

0-36 Psi

E

Lexan tubing The tubing performs as the reservoir of water

The air pushed by the piston creates The air the pulse for delivery of water. creates pulse at a pressure of 40–60 Psi

F

Teflon piston The piston on being actuated by pressure lined with O- pulse pushes the air column, which in turn pushes water. The piston also ring separates water in the reservoir from the rest of the droplet generator

The piston gets actuated at a frequency of 1–10 Hz for a time frame of 10–14 ms at a pressure of 40–60 Psi

G

Copper tubing(1/ 400 OD)

H

Compressor

I

Table 2 Functional description of the heater. Ref.

Type

Description

Function

A

Powerflex 4 M 200/240 V 0. 75 kW 4.2A Allen Bradley 22F-A4P2N113

Solid state relays variable power switch, internal ‘‘S Type’’ filters for EMC filtering

Will be given the command to shut off the heater, if the temperature of the heater is above 200 °C

B

Coupling relays module, series 49 by Finder ‘‘49.61.8.230.0060’’ 1  16 A 250 VAC, or similar

Electromechanical relay

Relay, which will be switched off by the FPGA controller by the following rule. There were two thermocouples. One of them was measuring the temperature of sample surface, and the other was measuring the heater temperature. The sample surface temperature was kept practically constant at 120–125 °C and the maximum possible temperature of the heater was 150 °C. If the temperature of the heater exceeded 150 °C, the heater was shut off, and the heater surface temperature was controlled using a cascade control algorithm

C

Panasonic EYP2BN143

Non-reversal thermal cut-off-device

Shuts down the heater as soon as the temperature of the mounting position reaches 240 °C. The mounting position is directly under the heater installation (ceramic).

and the heater assembly inside a closed chamber (Fig. 3), which was located inside a chamber made of Lexan. A schematic diagram picturing the entire setup and the image of the actual setup used on the flights are shown in Fig. 4 (a) and (b), respectively. The setup also ensured that during takeoff, hard landing and turbulence during flight, none of the equipment loosens or is dislocated. It is also emphasized that these restrictions did not permit any changes and fine tuning of the setup during a flight. Such changes and fine tuning were possible only on the ground. 2.4. Heat flux measurements The heat flux removed was measured by either one or (simultaneously) two of the methods developed in the ground experiments

and described in Part I [1]. Here we use the same notations as in Part I [1] for the heat fluxes measured by those methods: qccd and qaxis. It should be mentioned that the third method of heat flux measurement, resulting in the value, qexperimental, was not used due to the safety concerns under flight conditions. However, as it was shown in the ground experiments, all three methods provide the values of the same order, which means that the results of the two methods employed in flight experiments, qccd and qaxis, should be in agreement with qexperimental. It should be emphasized that owing to the stiffness of the enclosures, short time frames of operation, and limited number of trials possible (there were 3 days of flights and on each day there were 31 parabolas available), the experiments were conducted on 3 copper-plated nanofiber samples and one bare copper sample, since the sample exchange was

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possible only on ground. Each and every case was repeated in 2–3 similar cases to evaluate a possible data scatter, and the measurement results are presented below. 3. Results of the parabolic flight experiments and discussion 3.1. Performance of the droplet generator under different gravity conditions

Fig. 2. Overview of the heated target system (top), and heated target assembly (bottom).

The working principle of the droplet generator remained the same as in the ground experiments. However, the operating voltage for the operation under the flight conditions was changed in comparison to the ground operation. On ground the operating voltage was varied between 14.8 and 15.2 V. This ensured faster charging and discharging of the solenoid valve. This faster charging and discharging resulted in quick opening and closing of the valve resulting in smaller droplets and better control of droplet production. It should be emphasized that the solenoid valve was rated for 12 V. However, under the higher voltage used on the ground, the solenoid valve was charged for 7.8–9.2 ms at a time gap of 0.2– 1.0 s. This short time of actuation never posed any trouble or safety concern related to the solenoid valve, even under continuous operation over hours. However, due to the flight safety regulations, we were not allowed to use the solenoid valve at voltages beyond 12 V. This resulted in longer jets generated at zero gravity than on ground (at 1g) and at the supergravity (1.8g). Moreover, it was found that under 1.8g the droplet generator produced single drops under actuation, whereas under 0g the droplet generator produced a stream of small droplets. This can be explained as follows. In the present droplet generator pressure produced by air pushes water to form a jet. Then, by releasing air through the exhaust vent and by closing the solenoid valve, a negative pressure is produced in the system. The negative pressure results in a pull,

Fig. 3. Schematic of the spray chamber.

Fig. 4. Droplet generator, the spray chamber and enclosure: (a) schematic of droplet generator. (b) actual drop generator assembled in the rig used in the flight experiments.

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which stretches the water jet in the form of droplet train. The pull imposed on the jet interferes with its breakup into droplets which basically stems from the interplay of two opposing forces–surface tension and gravity. Whereas surface tension tends to minimize the droplet size, the gravity force tends to increase the droplet size and stabilizes the breakup. The faster the opening and closing of the valve is done, the more efficient is the ‘‘push and pull’’ mechanism. The in-flight decrease of the solenoid valve voltage to 12 V resulted in the 16–20 ms of charging time, which is much longer than the charging time of 7.8–9.2 ms at 14.8–15.2 V in the ground experiments. The corresponding doubled charging time in the parabolic flight experiments (due to safety concerns) resulted in a less efficient charging and discharging, and thus in longer jets at 0g. Also, under 0g conditions, in the absence of gravity, surface tension becomes dominant and the protruding water jet breaks up into multiple tiny drops (cf. Fig. 5). On the other hand, on ground and under 1.8g conditions, no such breakup can is observed, and a single droplet is formed from a wrapped-on released jet.

1111

3.2. Heat flux removed in control ground experiments using the flight rig installed at the aeroplane Prior to the in-flight experiments, the cooling rate was measured using the setup on the grounded aeroplane. Bare copper surfaces and copper surfaces covered with copper nanofiber mats were used as targets. In the experiments, a single drop was issued onto the targets from the droplet generator (cf. Fig. 6). The drop impact, spreading and evaporation were recorded using the high speed camera. Using the measured time of evaporation and the effective wetted area, the heat flux removed from the surface was calculated. In Fig. 6 three different snapshots of a single water drop impacting onto a bare copper (Fig. 6a) and copper nanofiber mat surface (Fig. 6b) are shown at t = 0, t = 400 ms and t = 679 ms. It can be seen that at t = 679 ms the water drop evaporated on the copper nanofiber mat, but the water drop did not evaporate on the bare copper. The heat removal rate corresponding to these images was found as 61.31 W/cm2 for bare

Fig. 5. Water drops produced by the droplet generator under different gravity conditions. It can be seen that under the ground and 1.8g conditions the generator produced single drops, whereas under zero gravity a stream of tiny water droplets is was produced. The droplets are highlighted by arrows.

Fig. 6. High speed images of water drop impacting and evaporating on: (a) bare copper, and (b) copper nanofiber at 125 °C. Three different time instants are shown. It can be seen that water drop evaporates much faster on the copper nanofiber mat. The impinging drops are highlighted by arrows and the wetted areas are traced by dashed lines.

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copper and 124.45 W/cm2 for copper nanofiber mat. It should be emphasized that in the present experiment, in contrast to Ref. [2], the target was not located on top of a large hotplate kept at a constant temperature. The large size of the hotplate in Ref. [2] provided a practically infinite heat source for a single drop experiment. In the present experiment, however, the heater used was 1 in2 in area, which was the same as the sample area, and the heater thickness was small. As a result, in the present experiment the heater did not serve as an infinite heat source like the hotplate in Ref. [2]. Also, as it was mentioned earlier, for safety reasons in the present experiments the heater was kept at a low output (with the maximum output being 200–250 W) and the temperature sampling frequency was  40–60 ms. All these factors resulted in a smaller heat supply rate in comparison with that in Ref. [2], and correspondingly, to a smaller heat removal rate. It is also worth noting that no Leidenfrost effect was observed on bare copper in distinction to Ref. [2]. This is definitely related to the fact that the heater had low thermal capacity and limited output and the sampling was slower in the present case. However, the trend of the main result was the same in the present control experiments and in the experiments in Ref. [2]. Namely, the heat removal rate was much higher at the copper nanofiber mat than on bare copper.

to those in Fig. 7(b) and (c) (here and hereinafter the notation for the heat fluxes measured by different methods, qaxi and qccd, is the same as in Part I [1] where it is explained). It can be seen that in Fig. 7a and Fig. 7b a continuous water supply is observed during the course of the experiment, while the wetted area did not increase and there was no spill over. The latter shows that the oncoming water had evaporated. This clearly manifests the fact that the heat removal rate can be also measured using the method described as qccd in Part I [1]. It was found that for situations similar to the one in Fig. 7(b), the heat removal rate found by both of these methods (qaxi and qccd) was almost the same. Summarizing, we state that, depending on the situation recorded by the high speed camera, the heat removal rate was measured by using one or two methods. It can be seen from the data discussed in Part I [1] that sometimes the values of qccd and qaxis did not match closely in the ground experiments; this can be explained as follows. Under the ground conditions experiments, the heater was used at its full capacity. As a result, a slight mismatch in the temperature sampling frequency and heater delivery rate can produce some variations. Still, even then the values of qccd and qaxi were sufficiently close. It should be emphasized that under the flight conditions the heater was used only in a conservative way (at lower voltage and reduced capacity), which eliminated such possibility, and qaxi and qccd were almost the same.

3.3. Methodology followed for in heat removal measurements in parabolic flights 3.4. Heat removal rates In the parabolic flight experiments, three different situations were encountered (Fig. 7). (i) There was a continuous supply of water droplets to the surface during the experiment, while the wetted area did not touch the thermocouple, or (ii) there was continuous supply of water droplets to the surface and the wetted area touched the thermocouple, or (iii) there was a pre-existing water pool (a leftover from the previous experiment), which touched the thermocouples. Because of turbulence during the flight the aeroplane was sometimes vibrating, which resulted in falling of water droplets at different locations at the sample surface. The sample size was 1 in2. Sometimes water droplet fell in the middle of the sample (Fig. 7(a)), and sometimes water droplets touched the clamp on the right-hand side of the sample (Fig. 7b and c). The thermocouple measuring T1 was held in place between the clamp and the sample. This gives a direct temperature measurement of the wetted spot. The temperature of the sample surface was set at 125 °C, but it was found to be constant at 122 °C, since some heat was lost to the surrounding air. This temperature measurement provided us with the direct measurement of T1. That, in turn, allowed us to measure the heat flux qaxi using the data similar

Under the flight conditions, four different scenarios were encountered and studied: (i) A single droplet impacting the surface under the supergravity at 1.8g; (ii) A pool boiling with continuous supply of water droplets under the supergravity at 1.8g; (iii) A pool boiling of a pre-existing liquid film without supply of water at the supergravity at 1.8g; and (iv) A pool boiling regime under zero gravity (at 0g) with continuous supply of water droplets. A set of the corresponding representative images is shown in Fig. 8. In particular, Fig. 8a shows a single drop impact and evaporation on a bare copper sample, and Fig. 8(b) a single drop impact and evaporation on a copper nanofiber mat. Fig. 8 shows that at t = 200 ms, water has already completely evaporated from the nanofiber mat, whereas water delivered by the drop is still remaining on the bare copper surface. Since the initial size of water droplets is almost the same in both cases, the results shown in Fig. 8 indicate a low heat removal rate in the latter case (on the bare copper surface) than in the former one (on the nanofiber mat). The resulting heat removal rates measured in all cases studied are listed in Table 3. It can be seen that under the supergravity

Fig. 7. Different scenarios of drop impact onto the sample surface. (a) Continuous water supply of water droplets to the surface during the experiment, while the wetted area did not touch the thermocouple. (b) A continuous supply of water droplets to the surface, and the wetted area touched the thermocouple. (c) There is a pre-existing water pool (a leftover from the previous experiment), which touched the thermocouples. The wetted spots are marked by dashed lines.

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Fig. 8. Impact of a single water drop onto: (a) bare copper and (b) copper nanofiber mat at t = 0 and 200 ms under the supergravity (1.8g) conditions. It can be seen that on the nanofiber mat the drop evaporated in 200 ms, whereas it still did not evaporate on bare copper. The oncoming drops are highlighted by arrows and the wetted spot on the bare copper is marked by white dotted line.

conditions the heat removal rate on the nanofiber mat is almost 5 times larger than that on the bare copper surface. For comparison, similar experiments under the ground conditions (1g) revealed that the heat removal rate on the copper nanofibers was only 2 times larger than that on the bare copper surface. This can be explained as follows. According to [2–6], water drops easily penetrate into the nanofiber mat pores. Indeed, the penetration velocity U  (D/d)V0, i.e. U is very high compared to the impact velocity V0, with the ratio of the drop size to the pore size D/d >>1. This results in pinning of the water droplets on nanofiber mats as reported in [2–6]. In [7] it was shown that a copper nanofiber mat subjected to pool boiling promotes bubble nucleation and growth by increasing the average fluid temperature around the bubbles. That is the mechanism responsible for the heat transfer enhancement. Under the supergravity conditions at 1.8g, the higher gravity facilitates water penetration into the pores of the nanofiber mat, which also results in a higher average fluid temperature. This, in turn, facilitates faster evaporation and increases the heat removal rate in comparison with the ground conditions. The scenarios (ii)–(iv) are shown in Fig. 9(a)–(c) and the corresponding heat removal rates are listed in Table 3. The table shows that for all the conditions the heat removal rate for copper nanofiber mat is higher than that of the bare copper. In Table 3 for scenario (iv) the heat removal rate for bare copper is given by two

Fig. 9. Pool boiling of water on bare copper and copper nanofibers for different scenarios: (a) the supergravity (1.8g) and continuous supply of water droplets, (b) the supergravity (1.8g) and a pre-existing liquid pool on the surface, (c) zero gravity 0g and continuous supply of water droplets. The oncoming drops are highlighted by arrows and the wetted spots are marked by dotted lines.

values: the value in italic is qaxi, whereas the other one is qccd. It is seen that the two values measured by two different methods closely agree with each other. Table 3 also shows that the heat removal rate for bare copper is higher under 0g than for 1.8g. The reason for that is the following. For pool boiling the heat removal rate is diminished due to the presence of liquid at the surface. Under the supergravity, at 1.8g, the vapor film can remain intact, whereas under zero gravity at 0g it is partially disrupted, which results in a better contact of

Table 3 The measured heat removal rates for bare copper and copper nanofiber mat for different experimental conditions. Scenario

Heat removal rate for bare copper (W/cm2)

Heat removal rate for copper nanofiber mat (W/cm2)

(i) 1.8g and single drop (ii) 1.8g, pool boiling with continuous supply of water droplets (iii) 1.8g, pool boiling with no supply of water (iv) 0g, pool boiling with continuous supply of water droplets

60.16 13 31 113 (116)

338 128.43 168 190

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the oncoming liquid with the heated substrate. However, it should be emphasized that under all the conditions copper nanofiber mats perform better than the bare copper. The nanofiber mats are beneficial because they increase the average fluid temperature and promote bubble growth, thus increasing the heat flux [7]. Although copper nanofiber mats performed much better than the bare copper under the supergravity (1.8g), their benefits at zero gravity (0g) are less pronounced. It can be explained by the fact that the heater was supplying maximum of 200–250 W at its peak operation, and the maximal heat removal rate has already been reached on the nanofibers. 4. Conclusion Due to the safety concerns, the heater could not be used to its full capacity during the parabolic flights, therefore the heat removal rate could not be as high as 0.9 kW/cm2 demonstrated in the ground experiments described in Part I [1]. The maximum achievable heat removal rate was therefore diminished by this heater limitation. Under these conditions the heat removal rate was in the range 200–338 W/cm2 for zero gravity (0g), normal gravity (1g) [the flights setup] and supergravity (1.8g). In all cases the copper nanofibers removed heat much better than the bare copper. The heat removal rate achieved due to the copper nanofibers during the parabolic flights was 2 to 5 times higher than that for the bare copper. It should be emphasized that in several cases (at 1.8g; see Fig. 9) we observed regimes with surface flooding which is worth of comparison with pool boiling regimes studied in the other works. For example, in [8–10] pool boiling of water over superhydrophobic (as a means of heat removal enhancement) smooth surfaces was studied, and the measured heat removal rates were in the range 1–4.5 W/cm2. In Ref. [7] pool boiling of water over electrospun copper-plated nanofibers was studied, with the

maximum heat removal rate reached (because of the surface deterioration), close to 65 W/cm2. These values are significantly lower than the values of 128–190 W/cm2 achieved in the present work. Acknowledgements The authors are grateful to NASA for the support of this work through the Grant No. NNX10AR99G and to ESA through support of the Topical Team DOLFIN. References [1] S. Sinha-Ray, A.L. Yarin, Drop impact cooling enhancement on nano-textured surfaces. Part 1: theory and results of the ground (1g) experiments, Int. J. Heat Mass Transfer (2013). Submitted for publication. [2] S. Sinha-Ray, Y. Zhang, A.L. Yarin, Thorny devil nanotextured fibers: the way to cooling rates on the order of 1 kW/cm2, Langmuir 27 (2011) 215–226. [3] R. Srikar, T. Gambaryan-Roisman, C. Steffes, P. Stephan, C. Tropea, A.L. Yarin, Nanofiber coating of surfaces for intensification of drop or spray impact cooling, Int. J. Heat Mass Transfer 52 (2009) 5814–5826. [4] A. Lembach, H.B. Tan, I.V. Roisman, T. Gambaryan-Roisman, Y. Zhang, C. Tropea, A.L. Yarin, Drop impact, spreading, splashing, and penetration into electrospun nanofiber mats, Langmuir 26 (26) (2010) 9516–9523. [5] C.M. Weickgenannt, Y. Zhang, S. Sinha-Ray, I.V. Roisman, T. GambaryanRoisman, C. Tropea, A.L. Yarin, Inverse-Leidenfrost phenomenon on nanofiber mats on hot surfaces, Phys. Rev. E 84 (2011) 036310. [6] C.M. Weickgenannt, Y. Zhang, A.N. Lembach, I.V. Roisman, T. GambaryanRoisman, A.L. Yarin, C. Tropea, Nonisothermal drop impact and evaporation on polymer nanofiber mats, Phys. Rev. E 83 (2011) 036305. [7] S. Jun, S. Sinha-Ray, A.L. Yarin, Pool boiling on nano-textured surfaces, Int. J. Heat Mass Transfer 62 (2013) 99–111. [8] R. Rioboo, M. Marengo, S. Dall’Olio, M. Voue, J. De Coninck, An innovative method to control the incipient flow boiling through grafted surfaces with chemical patterns, Langmuir 25 (2009) 6005–6009. [9] B. Bourdon, R. Rioboo, M. Marengo, E. Gosselin, J. De Coninck, Influence of the wettability on the boiling onset, Langmuir 28 (2012) 1618–1624. [10] B. Bourdon, P. Di Marco, R. Rioboo, M. Marengo, J. De Coninck, Enhancing the onset of pool boiling by wettability modification on nanometrically smooth surfaces, Int. Commun. Heat Mass Transfer 45 (2013) 11–15.