FILM BOILING HEAT TRANSFER ON A HIGH TEMPERATURE SPHERE IN NANOFLUID

Proceedings of HT-FED04 004 ASME Heat Transfer/Fluids Engineering Summer Conference July 11-15, 004, Charlotte, North Carolina USA HT-FED04-56471 FILM BOILING HEAT TRANSFER ON A HIGH TEMPERATURE SPHERE IN NANOFLUID HYUN SUN PARK, DEREJE SHIFERAW, BAL RAJ SEHGAL Department of Energy Technology Royal Institute of Technology Stockholm, SE-10044, Sweden (Email) sun@energy.kth.se, dereje@energy.kth.se, bsehgal@egi.kth.se DO KYUNG KIM AND MAMOUN MUHAMMED Department of Material Chemistry Royal Institute of Technology Stockholm, SE-10044, Sweden (Email) kyung@matchem.kth.se, mamoun@metchem.kth.se ABSTRACT Quenching experiments of a high temperature sphere in Al nanofluids are conducted to investigate the characteristics of film boiling and compared to those in pure water tests. One stainless steel sphere of 10 mm in diameter at the initial temperatures of 1000~1400 K was tested in the nanofluids of the volume concentrations from 5 to 0 % and the degrees of subcooling from 0 to 80 K. The test results show that film boiling heat fluxes and heat transfer rates in nanofluids were lower than those in pure water. The differences of the film boiling heat transfer rates between pure water and nanofluids become larger when the liquid subcooling decreases. Those results suggest that the presence of nanoparticles in liquid enhances vaporization process during the film boiling. The effects of nanoparticle concentrations of more than 5 vol. % on film boiling appear to be insignificant. However, the minimum heat fluxes tend to decrease when the concentration increases. Direct quenching without film boiling was repeatedly observed when an unwashed sphere was employed for quenching tests in nanofluids. It suggests that nanoparticle deposition on the sphere surface prevents the sphere from forming film around the sphere, which consequently promotes the rapid quenching of the hot sphere. Bi Biot number [=hl C /k] C Coefficient in Film Boiling Correlations C Micyoshi Coefficient in Micyoshi Film Boiling Correlation Cp Heat Capacity D Diameter I.D. Inner Diameter g Gravitational Acceleration h Heat Transfer Coefficient h fg Heat of Vaporization k Thermal conductivity L C Characteristic Length m Mass NF Nanofluid Nu Nusselt Number [=hd/k] O.D. Outer Diameter Pr Prandtl Number [=Cpµ/k] q " Heat Flux R Radius T Temperature t time Wall Superheat (=T w T sat ) T sub Liquid Subcooling (=T sat T l ) Vol% Volume Percentage Greek NOMENCLATURE µ Viscosity Ar Archimides Number [=g(ρ l -ρ v )D 3 /(ρ v ν v )] ν Kinematic Viscosity Address all correspondence to this author. Phone: +46-8-790-956; fax +46-8-790-9197; email sun@energy.kth.se. 1 Copyright 004 by ASME

ρ σ Density Surface Tension Subscripts film Film Boiling l Liquid MFB Minimum Film Boiling NF Nanofluid rad Radiation S Sphere sat Saturation sub Subcooling v Vapor w Wall INTRODUCTION Suspension of micrometer or millimeter sized particles in fluids has been reported to have enhancement effect in heat transfer. Unfortunately the industrial application of the particle suspension technology was limited by problems associated with erosion, rapid settlement, clogging and pressure drop due to the presence of particles in fluids. The recent development in material technology enables to produce particles in nanoscale. Suspension of nanoparticles, or solid particles with sizes less than 100nm, exhibit better properties than those of conventional micro or larger sized particles, which provide less erosion, stable and long suspension and negligible extra pressure loss. Recently, U. S. Choi at Argonne National Laboratories in USA [1] found that so-called nanofluid, or a liquid solution uniformly mixed with small concentrations of nanoparticles, enhanced the fluid thermal conductivity up to 40 % with small amount of nanoparticle suspension []. Since then, increasing number of research is currently underway to understand the heat transfer characteristics of nanofluids. Initial effort was dedicated to investigate the single-phase convection heat transfer (CHT) of nanofluids [3][4][5] motivated by this remarkable enhancement of thermal conductivity of nanofluids with very low concentration (<10% in volume). Their results showed that the convective heat transfer enhanced with a low concentration of nanoparticles in liquids [5]. The main reasons for the CHT enhancement are reportedly believed that: (a) the increase of the surface area, the heat capacity and the effective thermal conductivities of the fluid due to the suspended nanoparticles, (b) the intensification of interaction and collision among the particles, fluid and the flow passage surface, (c) the enhancement of mixing fluctuation and turbulence of the fluid, and (d) the flattening of the transverse temperature gradient of the fluid due to the dispersion of nanoparticles. Very recently, the investigation on the heat transfer characteristics of nanofluids has been extended to the boiling heat transfer (BHT) regime. Up to the present as far as the authors knowledge, only two studies were reported for the BHT in nanofluids. Their studies were conducted in a poolboiling configuration. First Das et al. [6] performed their pool boiling experiments with a 0 mm cylindrical cartridge heater in a nanofluid, in which alumina nanoparticles with an average size of 38nm were suspended in pure water and investigated the nucleate boiling characteristics in terms of a volume concentration of up to 4%. They observed that the nucleate BHT was deteriorated due to the deposition of nanoparticles on the heated surface, which changed the surface characteristics due to the particle trapping on the surface. Unfortunately thus, this work provided the limited explanation on the exclusive capability of nucleate BHT of the nanofluid. Second, Vassallo et al. [7] performed the pool boiling experiments with a 0.4 mm diameter NiCr wire in 15 and 50 nm silica nanofluids. They reported that the maximum heat flux without the wire failure achieved remarkably up to about 3 times higher than in pure water. Heat flux at the wall superheat where the critical heat flux occurs in pure water becomes about 60% higher. However, no significant enhancement was observed in nucleate boiling regime. The stable film boiling at temperature closed to the wire melting temperature was achieved without wire failure. They also reported that the wire was coated with 0 to 50 µm thick silica observed after the experiments. Both works imply that the practical application of nanofluids for the boiling applications still faces with a problem of the particle deposition on the heated surface. However, there is a noticeable result on critical heat flux enhancement. As can be seen, the current understanding of the boiling process in nanofluid is in a primitive stage and requires more theoretical and experimental works are needed to evaluate the BHT characteristics of the nanofluids. The present study is motivated by the lack of investigation on film BHT in nanofluid, which will provide an data set to construct a preliminary pool boiling curve of nanofluids in full ranges of heat transfer regimes. The work is carried out to investigate the film boiling heat transfer characteristics of nanofluid during the quenching process of a high temperature spherical ball in a nanofluid, or aluminum-oxide nanoparticles with small volume concentration in pure water. Transient cooldown quenching experiment was chosen by two reasons: first to investigate the quenching ability of nanofluids by observing the entire transient quenching boiling processes from film boiling to natural convection heat transfers and second to minimize nanoparticle deposition on the heated surface due to the immediate vapor formation during the boiling process, which enables exclusively to investigate the BHT capability of nanofluids. For the comparison with the previous experimental results in the CHT and BHT, the similar size, concentration and material of the nanoparticles are chosen as, mostly 5 volume percent, about 30 nm and Al nanoparticles, respectively. In this paper, the experimental results of the initial film boiling process during the entire quenching process of a sphere are described. The effects of the nanoparticle concentration, nanofluid subcooling, nanoparticle deposition on the sphere Copyright 004 by ASME

during film boiling heat transfer are investigated and compared with the film boiling characteristics in pure water. EXPERIMENTAL Experimental set-up Figure 1 shows the schematic of the quenching boiling test facility. The experimental setup consists of a 10 mm stainless steel ball, a RF induction furnace, a test chamber which contains de-ionized distilled water or nanofluids. The sphere is drilled, as shown in Fig. 1, to install a 0.5 mm O.D. K-type thermocouple supported by a 0.9 mm O.D stainless tube at the center of the sphere. The thermocouple support to sphere diameter ratio should be minimized to reduce the heat loss through the support. In the present experiments, the ratio is 0.09 (the corresponding area ratios is 0. %) which is smaller than other previous experiments (mostly larger than 0.15) [8][9][10] [11]. The induction furnace consists of a RF induction power supply (HeatTek GT-6) with a peak induction power of 6 kw at nominal frequency of 100 khz and a 4 turns of 6 mm O.D copper tube coil with an inner coil diameter of 50 mm. The sphere is normally heated at maximum 1100 ºC in the induction coil to ensure the formation of steady film boiling during the cool-down transients. The test chamber is a 100x100 mm rectangular Lexan chamber with a height of 150 mm. The surface of liquid in the chamber is 70 mm below the initial position of the sphere inside the induction coil. The final location of the sphere in the chamber is 50 mm below the liquid surface. The pneumatic cylinder delivers the sphere from the furnace to the chamber at a near constant speed of 1m/s, which corresponds to about 10 ms travel time. A data acquisition system (HP SCXI-110) records transient temperatures of the heated sphere at the center and of the liquid inside the test chamber at the sampling rate of 50 Hz. The measured temperatures data are smoothed to 100 Hz sampling rate by the adjacent averaging technique to reduce the measurement noise. A piezoelectric pressure transducer (PCB Piezotronics 10A04) is flush-mounted on the center of a test section wall to measure the acoustic pressure signals produced during the vapor film collapse. However, the pressure measurement was not successful during the present tests reported in this paper. The experiments employed a normal camcorder or high-speed camera to visualize the quenching boiling phenomena in the case of the water tests. Unfortunately, the milky-colored nanofluids even with 5 vol. % are not transparent to visualize the boiling phenomena on the sphere during the tests. (a) 18 16 14 Population (%) 1 10 8 6 4 Figure 1. The schematics of the quenching boiling test facility. 0 0 10 0 30 40 50 60 70 80 90 100 110 Particle size (nm) (b) FIG.. Figure. Transmission electron microscopy (TEM) image (a) and size histogram (b) of Al nanoparticles in nanofluids. Preparation of Nanofluid 3 Copyright 004 by ASME

Nanofluid was prepared by the dispersion of nano size Al particle powders into de-ionized, distilled water using an ultrasonic vibrator. Figure shows a TEM (Transmission Electron Microscopy) image for the Al nanoparticles and the corresponding particle size distribution used in the present tests. The average particle size and size distribution were obtained from the TEM images for the samples using an image analysis program by measuring the diameters of at least 500 particles. Perfectly spherical Al nanoparticles with an average diameter of 33.1 nm were found. In this study, the Al nanoparticle was chosen because the majority of the previous works on heat transfer in nanofluids were carried out employing this nanoparticle. The nanofluids with the 5, 10, 0 volume percent of the nanoparticles were tested at various nanofluid temperatures ranged from 0~70 o C. Most tests were performed with the 5 volume percent nanofluid. However, higher concentrations of nanoparticles (10 and 0 % in volume) in nanofluids were also tested since we presumed that the number of nanoparticles in nanofluid might play an important role on quenching boiling process of the sphere. In prior to the nanofluid tests, a series of pure water tests was conducted at different thermal conditions to confirm the test facility and eventually to compare with the nanofluid tests. Pre-test with Pure Water In prior to the nanofluids tests, a series of quenching tests was performed in pure water and the results were compared with existing film boiling correlations. These tests provided the pure water data for the comparison as well as verified the procedures and post-analysis of the experiments. For the comparison of film boiling heat transfer rates between the pure water and nanofluids, a set of the pure water tests was perform to evaluate the existing film boiling correlation developed by the previous investigators such as Michyoshi [8], Sakurai [9], Dhir [10], Siviour [11] and Liu [11]. It was found that the Michyoshi correlation as shown in Eqs. 1 and agreed well with our pure water experimental data showing agreement within ±18% [13] since the Michyoshi correlation well employed the effect of liquid subcooling and most of our experimental data were obtained in the highly subcooled conditions. This comparison verifies that the present pure water tests for the film boiling represents the general film boiling behavior. 1 4 Ar 1 4 film = C M c Nu Sp where C=C Micyosh =0.696 for pure water and (1) 4 B = Sc 7 1 1 1 3 1 3 1 B A B Sc * α +, E = A + α + 3 1 * 3 1 * 1 A = Sc + β Sp Prl Sc + β Sp Prl, 7 3 4 * + Sp Pr 3 3 7 1 4 * l Sc Sp Prl β + Sp Prl + 1 α = β Sp Prl, ( µρ) β ( µρ ) l υ * pl =, Sc * fg 7 Sc C Tsub =, h * 1 = hfg + Cpv Tsat, Sp Cp ( * v Tsat h Prυ ) fg = h fg RESULTS AND DISCUSSION Quenching of the high temperature sphere Figure 3 shows a typical image of the sphere in highly subcooled pure water during film boiling. The film boiling starts immediately after the sphere immerged into subcooled liquid. The image shows the optically deformed sphere due to the vapor film outside the sphere. The actual sphere is also indistinctly shown in the image. Very thin (order of 100 micrometers or less) vapor film covered the sphere during the film boiling in highly subcooled pure water. The image showed the thickness of the vapor film increases along the sphere surface from the stagnant point. The vapor dome above the separation point of vapor film around the sphere is also shown in the images. It is shown that the shape of the vapor dome at the position of thermocouple and its supporting structures is slightly deformed due to the heat loss through the structural materials. However, this heat loss was not significant to influence on the overall quenching process of the heated sphere. Figure 4 shows the center temperatures of the heated sphere in pure water and nanofluids with different volume Support Sphere (optically deformed) Sphere (actual) β * 3 M c 3 E = E 1 + Sp Prl ( β Pr Sp ) l () Vapor Pure Water Figure 3. The image of the 10mm heated sphere during film boiling in pure water at the liquid subcooling of 70~81 K. 4 Copyright 004 by ASME

concentrations during the transient cool-down process. For typical quenching behavior in highly subcooled pure water at 0 o C, a washed or clean sphere at initially 1063 o C quenched rapidly to about 1000 o C due to the direct contact between the high temperature sphere and the highly subcooled water and subsequently in the film-boiling regime. The vapor film around the sphere abruptly collapsed at approximately 360 o C and produced a high acoustic noise. Thereafter the quenching boiling mode of the sphere turned from the film boiling to the transition boiling, in where the sphere temperature rapidly decreases. Finally single-phase natural convection heat transfer takes over the quenching process after experiencing through the maximum heat flux and nucleate boiling regions. In the Al nanofluids, the quenching behavior of the same sphere was similar to the pure water case, clearly showing the various boiling modes as indicated in Figure 4. For the highly subcooled nanofluids, the center temperatures of sphere in the nanofluids were slightly higher than that in the pure water, which indicated lower heat transfer rates. Center Temerature of Sphere ( o C) 100 1100 1000 900 800 700 600 500 400 300 00 100 Direct contact Boiling Film Boiling Region 10% Al = 70 K Unwashed Spheres Transition Boiling Region Distilled = 80 K Washed Sphere Nucleate Boiling Natural Convection Region 0% Al = 70 K Washed Sphere 10% Al = 70 K Washed Sphere 5% Al = 76 K Washed Sphere 0 0 4 6 8 10 1 14 16 18 0 4 6 8 30 Time (s) Figure 4. Temperature profiles at the center of a 10 mm diameter sphere in pure water and Al nanofluids at the liquid subcooling of about 70~80 K. Effect of nanoparticle deposition on sphere surface In the quenching tests with fresh or washed spheres at about 1000 o C, film boiling was consistently observed. On the other hand, tests with unwashed spheres (spheres used in the tests with nanofluids) repeatedly showed no film boiling at the same experimental conditions during quenching process. The unwashed spheres quenched more rapidly through the transition boiling bypassing the film-boiling mode. These temperature histories at the sphere center are shown in Figure 4. It is clear that this change in quenching was resulted from the presence of residue of nanofluids on the sphere surface. It is noted that the cleaning procedure of the sphere was rather simply; only applying a relatively high-speed pure water jet on the sphere surface. This indicates that the nanoparticles on the surface of the sphere were loosely attached. Probably the thickness of the loosely accumulated nanoparticles on the sphere surface becomes sufficiently large to destabilize the very thin vapor film at highly subcooled liquid and thus to prevent stable vapor film formation. This observation suggests that the rapid quenching of the heated sphere can be achieved by avoiding the formation of film boiling when the surface is prewetted with low concentration nanofluids. In all tests presented in this paper, however, the fresh or washed spheres were used not only to ensure the formation of film boiling during the quenching but also to exclude the effect of the nanoparticle deposition on the quenching process. Film boiling heat fluxes To evaluate the heat transfer rates during the quenching process of the sphere, the surface heat flux on the sphere should be obtained. More often, the task of converting the measured transient center temperature to the surface temperature and corresponding surface heat flux can be simplified by assuming constant temperature through out the material, if the Biot number, Bi, or the ratio of internal (conductive) to external (convective) resistances to heat transfer less than 0.. For a sphere of radius R, the lumped sphere Biot number becomes LC h hr Bis = = < 0.. (3) k 3k The simple energy balance, assuming constant thermophysical properties, can obtain the total surface heat flux, which is split into the film boiling heat flux and the radiation heat flux, mscp dt q s = qs + J q measured film s = rad (4) πd dt where q s, rad =εσ(t s 4 - T sat 4 ), ε=0.6 for the moderately oxidized stainless steel sphere [11] and the radiation factor J is 7/8 for sphere from Bromley at al. [14]. For the evaluation of film boiling heat transfer, the film boiling heat flux was obtained by subtracting radiation contribution from the total surface heat flux. The lumped parameter assumption is valid for the present tests for the film boiling since the Biot numbers during the film boiling are less than 0.1 for the low liquid subcooling and less than 0. for the high liquid subcooling when the surface superheat of the sphere is larger than 300 K. However other boiling heat transfer modes like direct-contact boiling, transition boiling, nucleate boiling etc., in where the heat transfer coefficients are considerably large, the inverse heat transfer analysis should be conducted to predict the accurate surface heat flux and temperature 5 Copyright 004 by ASME

In Figure 5(a), the measured film boiling heat fluxes in nanofluids with different concentration are plotted together with those in pure water at highly subcooled conditions. The heat fluxes range from 0.1 MW/m at the sphere wall superheat of 50 K to 0.31 MW/m at the sphere super heat of 800 K. It shows that the heat fluxes in nanofluids are slightly lower than those in pure water. The difference of the heat fluxes between the nanofluids and the pure water increases when the sphere wall superheat decreases. 0.34 water becomes larger. Similarly it shows larger difference of the heat fluxes at the lower sphere superheat. Initially we postulated that individual nanoparticles or a cluster of nanoparticles in the vapor-liquid interface promote the vaporization during the film boiling. Therefore beside the 5% concentration of nanoparticles which was normally used in other studies, two more concentrations of nanoparticles in pure water, i.e., 10 and 0 % were tested. As shown in Figure 5, however, no noticeable effect of the volume concentration of nanoparticles on the film boiling heat flux at the high and low liquid subcooling cases was observed. q" film, MW/m 0.3 0.30 0.8 High Subcooling 0.6 =76K 10% 0.4 0% =75K =80K 0. =81K =81K =80K 0.0 00 300 400 500 600 700 800 900 1000, K q" MFB, kw/m 60 40 0 00 180 160 Distilled =70~81K 5%, Al =76K 10%, Al 0%, Al (a) 140-5 0 5 10 15 0 5 Concentration, % q" film, MW/m 0.6 0.4 0. 0.0 0.18 0.16 Low Subcooling =31K 0.14 =31K =3K =3K 0.1 400 500 600 700 800 900 1000, K (b) Figure 5. Film boiling heat fluxes of a 10 mm diameter sphere in pure water and nanofluids at (a) high liquid subcooling of about 70~80 K and (b) low liquid subcooling of about 30 K. The film boiling heat fluxes of the nanofluids and the pure water at low subcooling are shown in Figure 5(b). The film boiling heat fluxes in the nanofluids covers from 0.3 MW/m at = 900 K down to 0.1 MW/m at = 40 K. Comparing to the highly subcooled case, the difference of the film boiling heat fluxes between the nanofluid and the pure Figure 6. Minimum film boiling heat fluxes in pure water and Al nanofluids with different concentration. The minimum film boiling heat fluxes in pure water and nanofluids with different concentration at the liquid subcooling of 70~80K are shown in Figure 6. The minimum film boiling heat flux in pure water at the subcooling of 70~80K ranges from 190 to 40 kw/m. In the nanofluids at 70 K subcooling the heat flux shows in the ranges from 150 to 180 kw/m. However, the minimum film boiling heat fluxes at highly subcooled liquids tends to decrease as the nanoparticle concentration increases. Film boiling heat transfer coefficients From the heat flux data, the film boiling Nusselt number was evaluated and shown in Figure 7. In this figure, Nusselt numbers of pure water and nanofluids with different concentration for film boiling in high and low subcooling liquids respectively are compared with various correlations for pure water. The Nusselt numbers for the film boiling were obtained by the subtraction of the Nusselt number for the radiation heat transfer from the total Nusselt numbers measured. 6 Copyright 004 by ASME

It is clearly shown that the film boiling Nusselt numbers for nanofluids were lower than those for pure water. For the liquid subcooling of 70~80K the film boiling Nusselt numbers in nanofluids vary from about 50 to 30 for the sphere wall superheat from 85K down to 50K, respectively. For the low nanofluid subcooling of 30K, the Nusselt numbers decrease to the range of 40 to 80 for the wall superheat from near 900K down to 400K. The differences between the Nusselt numbers of the pure water and nanofluids for low subcooled film boiling are larger than those for highly subcooled film boiling. Nu film Nu film 60 40 0 00 180 160 140 10 100 80 60 40 0 00 300 400 500 600 700 800 900 180 160 140 10 100 80 60 40 (K) (a) =76K 10% 0% =80K =80K Nu Michyosi ( Nu Liu ( Nu Sakurai ( Nu Dhir ( Nu Savior (Water, T sub 0 00 300 400 500 600 700 800 900 (K) (b) =31K =31K =3K =3K Nu Michyosi ( Nu Liu ( Nu Sakurai ( Nu Dhir ( Nu Savior (Water, T sub Figure 7. Film boiling heat transfer for a 10 mm diameter stainless steel sphere in pure water and nanofluids of different concentrations at (a) high liquid subcooling of 70~80 K and (b) low liquid subcooling of 30 K. Figure 8 shows the data comparison obtained from the pure water and nanofluid tests with the Michyoshi correlation as shown in Eqs. 1 and. The correlation coefficient, C Michyoshi, of 0.695 for our pure water tests as an indicator of heat transfer rate becomes nearly the same value, i.e., 0.695, to the original value of 0.696. The coefficient became constant with respect to the liquid subcooling, since the effect of liquid subcooling on the film boiling for pure water was separately taken account into the correlation as shown in Eqs. 1 and. For the nanofluids, however, the Michyoshi coefficient tends to be a week function of the subcooling of the nanofluids, showing the higher heat transfer rate at higher liquid subcooling as shown below C 4 NF =.56 + 9.63 10 T sub 0 (5) with the standard deviation of 0.01. It may illustrate that the presence of nanoparticles in pure water during the film boiling provides additional effect similar to that of liquid subcooling on the film boiling. It may suggest that the vapor film around the sphere during film boiling becomes stable and possibly thicker in the nanofluids. If this dependency of the coefficient on the liquid subcooling is ignored, the average coefficient for the nanofluids becomes 0.619 with the standard deviation of 0.08, which is approximately 10% lower than the coefficient for pure water. C Michyoshi =Nu film /(Ar M c /Sp') 1/4 1.00 0.95 0.90 0.85 0.80 0.75 0.70 0.65 0.60 Pure water Al Nanofluids =400,500,600,700K C Michyoshi, Water =0.696(Michyoshi) =0.695(Present data) 0.55 C NF = 0.6188 C NF = 0.564+9.63x10-4 T sub 0.50 0 10 0 30 40 50 60 70 80 90 100 T sub (K) Figure 8. Coefficient of Michyoshi film boiling correlation in pure water and Al nanofluids. It is still unclear whether the nanoparticles with dilute concentration in pure water enhance the vapor generation and consequently thicken the vapor film on the heated sphere to further reduce the heat transfer rates. It can be confirmed if the vapor production rate in the saturated nanofluid can be measured. However, because of the opaqueness of the nanofluid at the concentrations used in this study, the visual quantification of the vapor generation becomes challenging and may require advanced measurement techniques such as X-ray radiography [15]. More experiment at near saturation condition is still underway to conclude the trend of the film boiling behavior in the nanofluids. 7 Copyright 004 by ASME

CONCLUSIONS A set of quenching experiments of a heated stainless steel sphere in Al nanofluids are conducted to study film boiling heat transfer by comparing with those in pure water. One sphere of 10 mm in diameter at the initial temperatures of 1000~1400 K was tested in the nanofluids of the volume concentrations from 5 to 0 % and the degrees of subcooling from 0 to 80 K. The following results are obtained: Film boiling heat fluxes and heat transfer coefficients in nanofluids were always lower than those observed in pure water. The differences of the film boiling heat transfer coefficients between pure water and nanofluids become larger as the liquid subcooling decreases. It suggests that the presence of nanoparticles with dilute concentration in subcooled liquid enhances vaporization process as a similar way of the effect of liquid subcooling. The effects of nanoparticle concentrations of more than 5 vol% on film boiling heat transfer appear to be negligible. Direct quenching of the sphere without film boiling in nanofluids was repeatedly observed when an unwashed sphere with nanoparticle deposition was quenched. It suggests that nanoparticle deposition on the sphere surface prevents the sphere from forming stable vapor film around the sphere, which consequently promotes the rapid quenching of the hot sphere. REFERENCES [1] U. S. Choi, Enhancing thermal conductivity of fluids with nanoparticles, American Society of Mechanical Engineers, Fluids Engineering Division (Publication) FED, 31: 99-105, 1995. [] J. A. Eastman, S. U. S. Choi, S. Li, W. Yu, and L. J. Thompson, Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles, Applied Physics Letters Vol. 78(6) pp. 718-70. February 599, 001. [3] Xuan, Y., and Roetzel, W., Conceptions for Heat Transfer Correlation of Nanofluids, Int. J. of Heat and Mass Transfer, 43. pp 3701~3707, 000. [4] J.A. Estmann et al., 000, Research Briefs, Argonne National Laboratories, USA. [6] Das, S. K., Putra, N. Roetzel, W., Pool Boiling Characteristics of Nano-fluids, Int. J. of Heat and Mass Transfer, 46, pp 851~86, 003. [7] Vassallo, P., Kumar, P., and D Amico, S., Pool Boiling Heat Transfer Experiments in Silica-Water Nanofluids, Int. J. of Heat and Mass Transfer 47, pp 407~411, 004. [8] Michyoshi, I., Takahashi, O., and Kikuchi, Y., Heat Transfer and the Low Limit of Film Boiling, Proc. Of the First World Conf. On Experimental Heat Transfer, Fluid Mechanics and Thermodynamics, Dubrovnik, Yugoslavia, pp 1404~1415, 1988. [9] Sakurai, A., Shiotsu, M. And Hata, K., A General Correlation for Pool Film Boiling Heat Transfer From a Hrizontal Cylinder to Subcooled Liquid: Part Experimental Data for Various Liquids and Its Correlation, J. of Heat Transfer, Transactions of ASME, Vol. 11, pp 441~450, May, 1990. [10] Dhir, V.K., and Purohit, G.P., Subcooled Film-Boiling Heat Transfer From Spheres, Nuclear Engineering and Design, Vol. 47, pp49~66, (1978). [11] Liu, C., Film Boiling on Spheres in Single- and Two- Phase Flows, Ph.D. Thesis, University of California, Santa Barbara, California, USA, 1995. [1] Siviour, J.B. and Ede, A.J., Heat Transfer In Subcooled Pool Film Boiling, Proc. 4 th Int., Heat Transfer Conf. Vol. 5, B 3.1, Paris-Versailles, 1970. [13] Shiferaw, D., Quenching Boiling Heat Transfer in Pure Water and Nanofluid, M.S. Thesis, Royal Institute of Technology, Stockholm, Sweden, 004. [14] Bromley, L. A., Leroy, N. R., and Robbers, J. A., Heat Transfer in Forced Convection Film Boiling, Industrial and Engineering Chemistry, Vol. 45, No. 1, pp 639-646, 1953. [15] Park, H. S., Hansson, R.C., and Sehgal, B.R., Fine Fragmentation of Molten Droplet in Highly Subcooled Water due to Vapor Explosion observed by X-ray Radiography, in print at Experimental Thermal and Fluid Science, 004. [5] Xuan, Y., and Li, Q., Investigation on Convective Heat Transfer and Flow Features of Nanofluids, J. of Heat Transfer, Vol. 15, pp151~155, 003. 8 Copyright 004 by ASME