POOL BOILING HEAT TRANSFER OF ETHANOL-WATER MIXTURES AT SUB-ATMOSPHERIC PRESSURES

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1 Proceedings of te 22 t National and t International ISHMT-ASME Heat and Mass Transfer Conference December 28-3, 203, IIT Karagpur, India HMTC POOL BOILING HEAT TRANSFER OF ETHANOL-WATER MIXTURES AT SUB-ATMOSPHERIC PRESSURES Roan M. Nemade Indian Institute of Tecnology Kanpur Kanpur (UP) India nemaderoan@gmail.com Sameer Kandekar Indian Institute of Tecnology Kanpur Kanpur (UP) India samkan@iitk.ac.in ABSTRACT Pool boiling as been carried out on circular aluminum surfaces (D = 30 mm) wit two surface rougness values (0.8 μm and 20 μm, respectively) for operating vapor temperatures of 30 C to 70 C. Te eat transfer coefficient as been estimated as a function of applied eat flux in te range of MW/m 2 for 2.0%, 25.0% and 80.0% molar etanol-water mixtures, and 00% pure forms of bot te liquids, respectively. In addition, bubble dynamics is observed by ig speed camera at 2000 fps. Te eat transfer measurement on bot surfaces sows degradation even wit te presence of small amount of etanol in water. At iger concentrations of etanol, te eat transfer first reaces minimum and ten attains a maximum at 80.0% molar etanol concentration. A comparison of eat transfer coefficient wit predictions obtained from recent mixture boiling correlations indicates marked difference for all compositions studied for te smoot surface. However, in case of roug surface, te values are closer to predictions. Analysis of ig speed images of bubble growt indicate bubble growt on bot surfaces is significantly ampered in case of mixtures, wic is attributed to mass transport limitation of etanol from bulk to interface and non-linear variation of termo-pysical properties wit composition. A unique bubble departure beavior at subatmosperic pressure is observed werein a strong liquid reflux takes place into te base of te very large sized bubbles. NOMENCLATURE A 0 Empirical constant (-) C sf Surface constant (-) Heat transfer coefficient (W/m 2 K) K Correction factor (-) k Termal conductivity (W/mK) p Pressure (Pa) q Heat flux (W/m 2 ) Ra Average rougness number (μm) T Temperature (K) x Liquid mole fraction (-) y Vapor mole fraction (-) δ Tickness (m) T Temperature difference (K) Subscripts,2 Component bp Boling range id Ideal mix Mixture s Surface c Termocouple INTRODUCTION Pool boiling of binary mixtures as been a subject of study since te time wen binary refrigerants became popular. Besides tis, suc mixture systems exist in a variety of industrial applications like petroleum distillation, avionics cooling, and ydrogen production wit etanol/metanol water systems, etc., to name a few. It is known in te literature tat te eat transfer coefficient (HTC) for binary mixtures deteriorates compared to tat of te respective pure components [-4]. Te reasons cited for te same include, amongst oter, rise in te local interfacial temperature due to preferential evaporation of te more volatile component, and te consequent mass transfer limitation for transport from bulk to te interface and non-linear variation of mixture properties wit composition. A variety of refrigerant mixtures, organic to aqueous ave been studied and te general trend in variation of HTC wit composition is known for bot zeotropic and azeotropic type refrigerant mixtures [-3, 5]. In addition, te general trend in te eat transfer coefficient is known for a range of binary mixtures tat ave been studied at atmosperic conditions. For zeotropic mixtures, te HTC for any mixture composition is always less tan tat of te pure components. However, for azeotropic mixtures, te mixture HTC sows a minimum corresponding to te maximum boiling point range [6] and a maximum corresponding to te azeotropic point [5]. Researcers ave also tried to predict te eat transfer caracteristics of mixtures by formulating correlations based on pure fluid predictions. Prediction of binary mixture HTC is based on two possible approaces [6]. One can directly employ existing pure fluid correlations and make appropriate use of binary mixture properties in te same. In suc an approac te binary mixture is approximated as a pseudo pure

2 fluid aving same properties as tat of binary mixture. Wile, tis approac may take care of te non-linear variation of mixtures properties, oter profound effects related to transport mecanisms cannot be addressed. A more popular metod of prediction is to first estimate te binary mixture HTC as a mole fraction average of te individual component HTC. Hence for a given eat flux q, x ( T () T id T x ) were, x is te mole fraction of more volatile component and T and T 2 are wall supereats for pure components. Hence, id x x Te final binary HTC is ten calculated by applying a correction factor tat is based on any one, or a combination of te various oter factors responsible for te deterioration of eat transfer, as indicated earlier. Hence, te eat transfer coefficient for binary mixture is ten given by, mix 2 2 (2) K (3) id Te expression for K as been proposed in varied manner in te literature, depending on te metodology adopted for including te effect of degradation. Following is a very brief account of suc studies: Te correlation by Körner [7] includes te effect of vapor liquid mole fraction difference and mixture density. Jungnikel et al. [8] added a term to account for te eat flux dependence. Stepan and Körner [5] included te effect of pressure in te Körner relation. Tome [9] as formulated a simple expression for binary mixture HTC only to include te effect of boiling temperature range of a mixture. Fujita and Tsutsui [6] modified te Tome correlation to include te effect of eat flux on eat transfer. Tese different K factors available are indicated below: Stepan and Körner [5], Tome [0], K A Fujita and Tsutsui [6], K 0 ( p) y x (4) K (5) T bp T id exp( q" 0 ) T bp T id However, looking at te literature it is noted tat te explicit effect of operating pressure and temperature as not been extensively investigated and te relation wit bubble dynamics is not correlated accordingly. EXPERIMENTAL SETUP AND PROCEDURE Te scematic of te setup is sown in figure, wile figure 2 sows te details of te eater surface and instrumentation. Pool boiling was carried out on 30 mm diameter flat surfaces of aluminum (k = 70 W/mK), aving two surface rougness values of 0.8 μm and 20.0 μm, respectively. Tis surface was mounted in a cylindrical eater block assembly from were te eat was transferred to te surface by conduction. Te eater block was fitted wit four cartridge eaters (50 W eac). Te entire eater block and test surface was surrounded radially and from te bottom by Teflon insulation (k = 0.25 W/mK). (6) Figure. SCHEMATIC DIAGRAM OF THE EXPERIMENTAL SETUP AND PHOTOGRAPH OF THE PRESSURE VESSEL. Figure 2. SCHEMATIC OF THE BOILING TEST SURFACE AND POSITIONS OF THE EMBEDDED THERMOCOUPLES. Te eater and te elical tube condenser assembly were mounted on te two sides of a cylindrical glass pressure vessel, capable of maintaining vacuum better tan 0-4 mpa. Te glass vessel itself was insulated by a 5 mm Polyuretane insulation (k = W/mK), wic ad a small window for visualization. Five termocouples were mounted 0.8 mm below te surface of te test piece one at te centre and remaining four in eac quarter of te surface, as sown in figure 2. One termocouple eac was employed for measuring te bulk liquid pool and vapor temperature, respectively. Additionally, a termocouple was mounted at te base of te eater block to measure maximum safe temperature of te system and also te facilitate estimation of eat flux to te surface by conduction approximation. Te temperature on te boiling surface was estimated by considering -D conduction troug te small δ=0.8 mm aluminum material tat was present above te 5 surface termocouples. Hence, if T c is te average termocouple reading ten te actually surface temperature (T s ) was taken as in eq. 7. T T ( q" ) k (7) s c / Te operating pressure was controlled by circulating cooling water troug te condenser coil mounted in te boiling pressure vessel at te desired temperature by a cryostat

3 (Haake ). In addition, te bubble dynamics were captured by ig speed camera (Potron ) at a frame rate of 2000 fps. An absolute pressure transducer was mounted on te vessel to measure system vapor pressure (Honeywell, 0-3 bar abs.). Te eat flux at te boiling surface was taken from te electrical power supplied and te boiling surface area ( m 2 ). Tis estimation was found to conform to eat flux estimate using a one-dimensional conduction approximation troug te eater block witin 5%-7%. Subsequently, experiments were performed for 2.0% and 25.0% etanol-water mixtures as well as pure etanol, to estimate te eat transfer coefficient at different operating temperatures and eat fluxes for te two test surfaces aving different rougness. Figure 4 sows te results for te smoot surface (Ra = 0.8 μm). RESULTS AND DISCUSSION Te pool boiling experiments were carried out under operating temperatures in te range of 30ºC to 70ºC. Te eat flux was controlled from 0.0 MW/m 2 to 0.5 MW/m 2. Actual wall temperature was estimated from te sub-surface termocouples by -D conduction approximation. At eac eat flux level, te average wall temperature was noted at steady state (te average of five surface mounted termocouples). Te wall supereat ( T) was ten obtained as te difference between te average surface temperature and te bulk saturation temperature (measured by te termocouple placed in te vapor space). Depending on te applied eat flux, te vapor space temperature was controlled by te temperature of te coolant water passing troug te condenser. Te pool boiling caracteristics for water were first bencmarked wit existing popular correlations for pure fluids, i.e. Rosenow correlation [0] and Forster-Zuber correlation []. Figure 3 sows comparison of te obtained experimental data wit tese two correlations, respectively (For te Rosenow correlation te surface constant C sf as been taken to be equal to 0.022). Te experimental results were found to conform to correlations wit a maximum deviation of 6 %. Figure 3: BENCHMARKING DATA SET: COMPARISON OF HEAT FLUX VERSUS WALL SUPERHEAT FOR PURE WATER ON ALUMINUM SURFACE (R a = 0.8 MICRONS) AT SATURATION TEMPERATURES FOR 30ºC, 40ºC AND 50ºC, WITH TWO POPULAR POOL BOILING CORRELATIONS. Figure 4. HEAT TRANSFER COEFFICIENT ON THE ALUMINUM SURFACE (Ra = 0.8 ΜM) FOR 2.0%, 25.0% ETHANOL-WATER MIXTURES AND PURE COMPONENTS FOR OPERATING TEMPERATURES OF (a) 30 C, (b) 40 C, (c) 50 C, (d) 60 C.

4 As a general observation it is noted tat, addition of even very small amount of etanol in water (2.0% molar), te eat transfer coefficient reduced significantly for all operating temperatures. Suc a general beavior as also been noted in te available literature wit two possible reasons: (i) cange in termopysical properties of te mixture [2,3], and, (ii) mass transfer limitation wic prevents water molecules to come closer to te growing bubble interface as preferential evaporation of more volatile component (etanol) takes place []. It must also be noted tat te boiling point of te mixture is not unique. Figure 5. COMPARISON OF THE EXPERIMENTAL HEAT TRANSFER COEFFICIENT AT DIFFERENT ETHANOL- WATER MIXTURE CONCENTRATIONS WITH AVAILABLE CORRELATIONS, FOR 0.8 μm AND 20.0 μm ROUGH SURFACES, RESPECTIVELY, AT (a) T sat = 50 C, (b) T sat = 60 C, (c) T sat = 70 C. (For All CASES q" = 0.23 MW/m) As te etanol concentration is increased from 2.0% to 25.0%, furter reduction in eat transfer coefficient slows down. Comparatively, slower reduction in eat transfer at iger concentration is related to te fact tat te boiling temperature ranges (i. e. te range of temperature over wic boiling takes place at a concentration is maximum at very low concentrations wereas it decreases at iger concentrations [6]. Te eat transfer coefficient of pure etanol was observed to be approximately close to tat of 25.0% mixture but te value was iger or lower tan te later depending on te operating temperature and eat flux. Similar experiments were performed on te roug surface, for operating temperatures of 50 C, 60 C and 70 C. Figure 5, sows comparison of te results for bot te surfaces wit predictions from binary mixtures HTC correlations from eq. 3 to eq. 6. Te results from figure 5 in general, indicate tat te measured values sow qualitative similarity in te trend predicted by correlations for variation of eat transfer coefficient wit composition for bot rougness values alike. However, tere exists a striking difference in te actual experimentally observed magnitude at eac composition. Te values for pure fluids are closer to predictions; owever, for mixtures, tere is a remarkable difference in case of smoot surface (Ra = 0.8 μm) wit values for roug surface being closer to predictions. As te operating temperature is increased, te percentage deviation in te observed and predicted values decreases. As all of te correlations for binary mixtures are formulated from data gatered at atmosperic pressure, tey do not incorporate te effect of pressure on eat transfer. Hence, tey tend to over predict te experimental data reported ere for mixture boiling at sub-atmosperic pressures. However, as can be seen from te previous observation, te experimentally observed values seem to converge towards te predicted values from te correlations, as operating temperature is increased. In fact, te eat transfer caracteristics actually coincides well wit te predictions, for te roug surface at T sat = 70 C. Also, all binary mixture eat transfer correlations used are based on Stepan-Abdelsalam relation, were te surface rougness is assumed to be.0 μm [2]. In addition to eat transfer measurements, ig speed videograpic images of individual bubbles were captured at 2000 fps, for binary mixtures, as well as pure water at low pressures. Figure 6 sows representative isolated bubble growt images for pure water on smoot surface (Ra = 0.8 μm). As expected from classical bubble growt mecanisms, initially te growt is rapid, during te inertia controlled regime, werein te bubble remained nearly emisperical in sape. Eventually, te bubble spread on te surface stops as te buoyant forces overcome te surface tension. As buoyant forces begin to dominate, te bulk of bubble sifts upwards even as te base of te bubble sticks to te surface causing necking. Images post detacment indicated considerable reflux of liquid into te vapor bubble from te bottom as te interface tat was stretced just before detacment, rebounded into te bubble, due to te surrounding liquid gusing from below, as seen in te last two images (after bubble departure). A comparison of bubble departure diameters, as sown in Figure 7, sows considerable reduction from pure water to mixture (2.0% and 25.0% etanol-water mixture) as well as pure etanol. From te figure, te bubble departure diameter reduces from 38.0 mm for water to 27.4 mm for 2.0% mixture (27.9% decrease compared to water) wereas for 25.0% mixture te diameter is 7.6 mm (53.7% decrease compared to water). For etanol, te bubble departure size is in te range of 4 mm to 8 mm.

5 Heat transfer coefficient for roug surface (20.0 μm) is ~30-40 % iger tan te smoot surface (0.8 μm) wic is attributed to iger nucleation site density and large number of smaller bubble sizes in case of roug surface. Experimental data at sub-atmosperic pressures, in te entire range of etanol concentration, only qualitatively matc te trend predicted by te available correlations for binary mixtures. However, quantitatively, eat transfer coefficients for roug surface are closer to te predictions as compared to te smoot surface. Wit increase in operating temperature (and terefore te operating pressure), experimentally obtained eat transfer coefficient values for bot te surfaces tend to be more closer to te predictions. A significant reduction in bubble departure diameters due to bubble growt deterioration in mixtures was observed from ig speed images of bubble growt. A unique penomenon of liquid reflux into te bubble base was observed as large sized bubbles caused cange in relative importance of inertia, buoyancy and surface tension forces. Figure 6. OBSERVED BUBBLE GROWTH BEHAVIOUR FOR Ra = 0.8 μm SURFACE IN PURE WATER AT T sat = 50 C, q = MW/m 2. AFTER THE DEPARTURE THE LIQUID REFLUX IS CLEARLY SEEN. Figure 7. RELATIVE BUBBLE DEPARTURE SIZES FOR T sat = 50 C, q" = MW/m 2 ON SMOOTH SURFACE. THE BUBBLE DEPARTURE SIZE OF PURE WATER is 38.0 mm. Here, te departure diameter is taken as te diameter of a spere aving te same volume as tat of te actual bubble. Hence, considerable reduction in departure diameter takes place for 2.0% mixture as compared to 25.0% mixture. CONCLUSIONS Following conclusions can be drawn from tis study: Tere is a significant reduction in eat transfer coefficient for etanol-water mixture compared to pure water even for very low concentration of etanol. Tis penomenon is observed at all operating pressures. At iger concentrations of etanol, te eat transfer coefficient first decreases to a minimum, ten acieves a maximum, close to te azeotropic composition. In addition, te cange in eat transfer is intensified wit increase in operating vapor pressure wic is attributed to reduction in te boiling temperature range (difference between te bubble point and te dew point temperature) wit pressure. ACKNOWLEDGEMENTS Te researc is supported by Department of Science and Tecnology, Government of India under te IRHPA Sceme (Project No: DST/CHE/ ), titled Micro-devices for Process Applications. Te contribution of Mr. Hemant Ragav (intern from NIT Rourkela under IIT Kanpur SURGE program) in generating additional experimental data sets is acknowledged. REFERENCES [] van Wijk, W.R., Vos, A.S., van Stralen, S.J.D., 956, Heat transfer to boiling binary liquid mixtures, Cem. Eng. Sci., 5, pp [2] Sternling, C.V., Ticacek, L.J., 96, Heat transfer coefficients for boiling mixtures-experimental data for binary mixtures of large relative volatility, Cem. Engg. Sci., 6, pp [3] Stepan, K., Preusser, P., 979, Heat transfer and critical eat flux in pool boiling of binary and ternary mixture, Ger. Cem. Eng., 2, pp [4] Tome, J.R., 982, Latent and sensible eat transport rates in te boiling of binary mixtures, J. Heat Transfer, 04, pp [5] Fujita, Y., Tsutsui, M., 994, Heat transfer in nucleate pool boiling of binary mixtures, Int. J. Heat Mass Transfer, 37, pp [6] Stepan, K., Körner, M., 969, Berecnung des Wärmeübergangs verdampfender binärer Flüssigkeits -gemisce, Cemie Ing. Tecn., 4(7), pp [7] Körner, M., P.D. Dissertation, TU Aacen, 967. [8] Jungnickel, H., Wassilew, P., Kraus, W.E., 980, Investigations on te eat transfer of boiling binary refrigerant mixtures, Int. J. Refrig., 3, pp [9] Tome, J.R., 983, Prediction of binary mixture boiling eat transfer coefficients using only pase equilibrium data, Int. J. Heat Mass Transfer, 26, pp [0] Rosenow, W.M., 952, A metod of correlating eat- transfer data for surface boiling of liquids, Trans. A.S.M.E., 74, pp [] Forster, H.K., Zuber, N., 955, Dynamics of vapour bubbles and boiling eat transfer, A..C.E. Journal, (4), pp [2] Stepan, K., Abdelsalam, M., 978, Heat-transfer correlations for natural convection boiling, Int. J. Heat Mass Transfer, 23, pp