POOL BOILING IN REDUCED GRAVITY P. Di Marco And W.
Multiphase Science and Technology, Vol. 13, No. 3, pp. 179-206, 2001POOL BOILING IN REDUCED GRAVITYP. Di Marco and W. GrassiDipartimento di Energetica, Università di Pisa, via Diotisalvi 2,56126 Pisa, ItalyAbstract. The main outcomes of the worldwide experimental activitydealing with pool boiling in reduced gravity are summarized. Thecurrently available experimental facilities and experimental opportunitiesare examined, the main results obtained by the various experimentalteams are reviewed, and highlights of current and future applications ofboiling in space systems are given. The work initiated by several groupsaround the world seems to indicate that pool boiling (especially thesubcooled one) may be safely sustained in micro-g conditions withappropriate measures and that improvements in performances (e.g. byapplication of other force fields) are possible. However, due to the highcost and low availability of flight opportunities, and to their limitations inspace and time, a final assessment has yet to be completed, and someresults are still controversial.In the second part of the paper, a review of the main pool boilingfeatures and of the related models is carried out. The main effects ofgravity and other force fields are stressed and compared with the abovementioned experimental results on earth and under reduced gravityconditions. The most commonly accepted viewpoints are reported foreach aspect. The empirical correlations developed for boiling heat transferin terrestrial conditions do not trivially extend their validity outside theirrange of application. Thorough experimentation in microgravity is thusneeded to assess the performance of boiling heat transfer in suchconditions.It is believed that, after further experimental activity, it will bepossible to design efficient boiling systems for future spacecrafts. Theresearch in microgravity, by eliminating the dominant effect of thebuoyancy forces, may also help clarify the role played by the variousmechanisms in the boiling phenomenon.
180P. DI MARCO and W. GRASSI1. AVAILABLE MICRO-g FACILITIESThe state of microgravity is not the result of suppression of the gravity force, but ratherof being in free fall. In such conditions, in fact, the inertia force counterbalances gravity.Microgravity conditions can thus be created by letting an object fall freely in a so-calleddrop tower or by flying a ballistic trajectory (parabolic flight) with an aircraft, the onlylimitation being that in these facilities, micro-g conditions can only be established for alimited time. Longer duration can be obtained in an orbital flight. More precisely, totalgravity suppression is impossible, due to the mechanical perturbations in the system, sothat, depending upon the technique, values of mean residual gravity acceleration rangingfrom 10-2 to 10-5 g are attained. Thus, it would be more appropriate to speak about“reduced gravity” rather than “microgravity”, although the latter term has gainedpopularity. Several kinds of facilities are currently available for microgravityexperimentation and boiling experiments have been carried out in all the facilitiesdescribed in the following.Attention should be drawn not only to the duration and to the mean level ofmicrogravity phase, but also to its quality: the so-called g-jitter, i.e. the small oscillations(which may also imply changes in direction) of the acceleration of gravity around itsmean value, which can substantially affect bubble behavior. An example of g-jitter isgiven in Fig.1.Droptowers and Dropshafts (dropshafts are wells in the ground) can provide a goodquality microgravity (10-5 g) for a limited time. From the very simple physicalrelationship of free fall motion, the drop height required to have a duration t ofmicrogravity is given by H 0.5 g t2. This means that several hundreds meters are0.040.030.02g0.010.00-0.01-0.02-0.030510t (s)1520Figure 1 Example of g-jitter in parabolic flight (Di Marco & Grassi, 19991a)
POOL BOILING IN REDUCED GRAVITY181necessary to have an appreciable duration of microgravity. Additional decelerationspaces should be also provided at the bottom, to allow for a safe recovery of the facility.As an example, the droptower of ZARM, Bremen, Germany, provides 4.7 s ofmicrogravity with a free fall of 110 m, and the dropshaft of JAMIC in Hokkaido, Japan,yields 10 s of free fall, with a total depth, including the braking zone, of 790 m. Shorterfacilities are operated in some laboratories. According to the formula above, a facility of20 m height is able to provide 2s of microgravity. Partial values of gravity can beobtained with appropriate braking systems.Ballistic Flights encompass both parabolic flights and sounding rockets flights. Inparabolic flight, an aircraft describes a (parabolic) free-fall trajectory in which inertiaforce counterbalances gravity. Around 20 s of relatively poor quality microgravity(around 10-2g, with a strong jitter, due to both atmospheric conditions and pilot’s skill)are available. The main advantage is that a number of repetitions are possible(classically, 30 parabolas per day for three days), allowing extensive testing, parameteradjustments and even failure recovery. Besides, scientists can control the experimentdirectly aboard the aircraft, more or less in the same way as in their laboratory. For thesereasons, parabolic flights are generally considered as the first step in setting up anexperimental program. Currently available aircrafts may accommodate very largeexperiments (up to 20m in the European Airbus 300). A particular technique is the socalled free-floating one, in which the apparatus is free to move inside the cabin to reducethe influence of the g-jitter. Weight and time of micro-g are more limited in this case,due to the eventual impact of the apparatus on the walls. Different values of gravity canalso be obtained by flying particular trajectories.In sounding rocket flights, a rocket is launched in an almost vertical trajectory, whichcan provide from 6 to 20 minutes of very good quality microgravity (10-4-10-5 g),depending on the height reached. Generally, the experiment can be controlled fromground via telemetry, and even video images, which are very useful in boilingexperiments, can be obtained. Costs are however high, and space and weight limitationsare more stringent. The payload mass is around 300 kg, shared among severalexperimental facilities. Some examples: the NASA carrier Orion yields 200 s ofmicrogravity with an apogee of 170 km; the German carrier, TEXUS, the SwedishMASER, and the Japanese TR-1A can provide 6 min of microgravity (with an apogee ofabout 250 km), while 15 minutes can be attained with the Swedish MAXUS.Orbital Flights can provide a good level of microgravity: 10-4 to 10-5 g, with the gjitter depending mainly on the crew movements inside the spacecraft and on operation ofon-board systems. Although the time duration is potentially very long, a limit is oftenfound in the available power. Generally, the experimental facilities have to be selfpowered, and space and weight limitations may be substantial. It must be taken intoaccount that for most satellites the visibility from ground is limited to a few minutes perday, and the possibility of interaction with the experiment is thus reduced. Somefacilities, like GetAway Special (GAS) are flown on the NASA Space Shuttle in place ofballast. In such a case, the experiment is simply switched on and off by the crew andeverything, from the automated control of the experimental sequence to the energyneeded, must be pre-loaded into the container. Data are available only upon retrieval ofthe container. The possibility of experimentation in orbital flight will greatly improvedwith the placing into service of the International Space Station, whose operation is
182P. DI MARCO and W. GRASSIforeseen to start in the near future. Several multi-purpose facilities for the study of fluidbehavior and heat transfer will be placed in it, e.g. the Fluid Science Laboratory (FSL)by ESA.2. EXPERIMENTAL RESULTSIn the following, highlights of the experimental activity carried out on pool boiling inmicrogravity are given. Their main features are summarized in table 1. A commonfeature of all the experimental facilities must be stressed: due to the absence of gravity, afree surface separating liquid and vapor must be avoided. All the experimentalcontainers were thus initially filled with liquid, and connected to a bellows to allow forthermal dilatation of the fluid and for volume compensation due to the formation ofbubbles. In this way, pressure and subcooling conditions could also be varied during theexperiments.The first experiments of pool boiling in microgravity were initiated in the late 50s.Siegel and coworkers (Siegel and Usiskin, 1959; Usiskin and Siegel, 1961; Siegel andHowell, 1965) used a 2.5 m-high droptower. Merte and Clark (1964) studied transientboiling of nitrogen on a sphere in a 10 m-high droptower. Studies up to 1990 weresurveyed by Straub (Straub et al., 1990).Merte and coworkers (Lee et al., 1997, 1998) reported on experiments carried out(adopting the same hardware) in five different missions in a GAS facility on SpaceShuttle, in the period 1992-96. Pool boiling of R113 on a rectangular plate (19x38 mm)was investigated at heat flux up to 80 kW/m2, for different subcoolings (up to 22 K) anda duration of up to 280 s. The authors claim to have attained steady state conditions in 27of 45 runs. The mechanism of steady state boiling was described, and there is asubstantial agreement on these observations also by other authors, as detailed insubsequent sections of the paper. A large bubble resides a short distance from the heaterand acts as a reservoir, engulfing bubbles forming on the surface, see Fig. 2. This largebubble maintains its size due to balance of condensation at its cap and coalescence ofnew, small bubbles at the base. Lateral coalescence of bubbles along the surface wasobserved, with consequently induced motion in the fluid, causing small oscillations inthe heater temperature. It is inferred that the dimensions of the surface may affect theliquid renewal under the large bubble and thus the possibility of maintaining steady stateconditions. Subcooling was found effective in enhancing boiling performance, and theprobability of having steady state conditions increased with subcooling. In theseexperiments, the heat transfer coefficient was increased up to about 30% with respect toterrestrial conditions at low and intermediate heat flux (40 kW/m2), while at higher heatflux degradation took place, see Fig.3.The group leaded by Abe (Oka et al., 1992, 1995, 1996) performed several tests onsquare plates with R113, water and pentane both in droptowers and in parabolic flight inthe period 1989-1996. A boiling mechanisms quite analogous to the one described byLee et al (1997) is reported. However, for R113, the bubbles tended to maintain ahemispherical shape with a large contact area with the surface, they remained attached tothe surface at higher heat fluxes, and as a consequence, a situation of surface dryout wasgradually attained (no sudden transition was observed, however). In contrast, water
POOL BOILING IN REDUCED GRAVITY183Table 1 Main features of the experimental activitiesREFERENCESiegel & Usiskin,1959, 1961DTFLUIDWaterSiegel & Keshock1964Zell, Straub et al.1984, 1986Straub et al., 1991DTWaterSRR113PFR12Oka et al., 1992PFAbe et al. 1994,1999Oka; Abe, Mori &Nagashima, 1995,1996Tokura et al., 1995Shatto et al. ,pentaneMethanolWaterStraub, Steinbichleret al., 1996, 1998Straub & Picker,1996, 1998, 1999OFR134aOFR123Di Marco & Grassi,1996, 1999PFSRR113,FC72Lee, Merte &Chiaramonte, 1997,1998Ohta, Kawaji et al.,1998OFR113Flat plate,19x38 mmSREthanolFlat plate, 50mm diameterMotoya et al., 1999DTWaterSuzuki et al, 1999PFWaterWire, 0.2 mmdiameterRibbon 0.1 mmthick, 20x5 mmDTPFHEATERWires, 0.4 - 4mmRibbon, 5x63mm22 mm roundnickel heaterFlat plate 20x40mmWire, 0.2 mm0.05 mmPipe, 8 mm o.d.Flat plate 40x20mmFlat plate 40x40mmNOTESVarious gravity levelstested by means of acounterweightSingle bubble studiedSaturated and subcooledconditionsSeveral flightsSaturated and subcooledconditionsSeveral flightsperformedbinary mixtureFlat squareplate, 30, 40and 80 mmWireFlat andcylindricalheatersWire, 0.2 and0.05 mmHemisphericalheater 1.41 mmdiameterWire, 0.2 mmdiameterReduced pressure testedSaturated and subcooledconditionsSaturated and subcooledconditionsPartial g-level testedElectrostatic fieldappliedSaturated andsubcooled conditions.Duration up to 5 minLocal temperature andfilm thicknessmeasurementsEffect of scale foulingstudiedSubcooled conditionsCHF only reported
184P. DI MARCO and W. GRASSITable 1 (Cont’d)REFERENCEQiu et al., 1999Ahmed & Carey,1998Snyder & Chung,2000Kim et al., TERFlat plate with asingle,controlled-sizenucleation siteFlat plate, 12mm diameterFlat plate,25x25 mmFlat plate,2.7x2.7 mmNOTESSingle bubble studiedBinary mixtureElectrostatic fieldappliedDuration 2 sMulti array heater withimposed surfacetemperature.Legend: OF: orbital flight; SR: sounding rocket; PF: parabolic flight; DT: gure 2 Saturated or quasi-saturated boiling pattern encountered in microgravity over a flat plate.bubbles exhibited “necking”, and were readily detached by the surface. The difference isattributed to the different consumption rate of the liquid film underlying the bubbles andto the forces that deform the bubbles from hemispherical shape with a large contact areainto a nearly spherical shape, which in turn allows detachment due to the motion of thesurrounding fluid. Latent heat of vaporization and surface tension are mainly responsibleof these effects. Thus, it appears that surface and fluid properties play an even greaterrole in micro-g boiling than in normal gravity. In parabolic flight on horizontal plates
POOL BOILING IN REDUCED GRAVITY18530002α (W/m K)20001 g, sub. 22 K10001 g, sub. 11 K1 g, sub. 0-2.7 K0 g, sub. 22 K0 g, sub. 11 K0 g, sub. 0-2.7 K002040602q" (kW/m )80Figure 3 Heat transfer coefficient in microgravity (a 10-4 g) on a flat plate (19x39 mm), fluid:R113. (from Lee and Merte, 1998).(Oka et al., 1995) a significant effect of g-jitter on surface temperature was observed.Abe et al. (1994) investigated boiling in binary mixtures in a dropshaft: enhancement ofboiling heat transfer was reported.Ahmed and Carey (1998) who conducted experiments on boiling of subcooled ornearly saturated binary mixtures (water/2 propanol) on a flat copper surface in parabolicflight. Nucleate boiling heat transfer and critical heat flux were found to be independentof gravity level in the range 10-2 g - 2 g, and this was attributed to the strong contributionof favourable Marangoni convection. Currently available correlations for binarymixtures were found to be acceptable at twice the normal gravity, but inadequate whenthe gravity is reduced of orders of magnitude.Some remarkable discrepancies are present between the results of Lee at al. (1997)and those of Oka et al. (1995, 1996). In the former case, bubbles of R113 were observedto detach from the surface, and enhancement of heat transfer was reported, while Oka etal. claimed degradation of heat transfer in pool boiling. The different nature of theheating surface and its contamination may play a role in this frame.Di Marco and Grassi (1996, 1997, 1999a) conducted experiments in parabolic flightof pool boiling of R113 and FC-72 on a 0.2-mm platinum wire, in slightly subcooled
186P. DI MARCO and W. GRASSIconditions. High values of heat flux, up to CHF, were tested. Data were recorded also inthe enhanced gravity phase of trajectory and during special trajectories resulting in aconstant gravity value of 1.5 g for 40 s. Pool boiling data at Martian gravity level (0.4 g)were also collected. The experiments have been recently repeated on a sounding rocketflight (MASER-8) with results in good agreement with the former ones. Despite a veryevident change in bubble size and velocity, no effect of gravity acceleration on the heattransfer coefficient in nucleate boiling was found. Critical heat flux (CHF) was clearlydetected (Fig. 4) and it was found to be reduced of about 50%. A cylindrical electrostaticfield around the wire was also imposed in some parabolas. No significant effect wasdetected on the heat transfer coefficient, but the imposition of an electric field was foundto be effective in drastically reducing bubble size and in increasing CHF also inmicrogravity. At high values of applied voltage, the same value of CHF as in terrestrialconditions was measured, thus demonstrating the dominance of the electric force onbuoyancy in these conditions. It is also important to point out that the detachment of thebubbles from the heated surface took place in both the presence and absence of anelectric field, the difference being that in the former case the bubbles slowed down andstopped at a small distance from the surface and started to coalesce.Snyder and Chung (2000) reported on experiments carried out in a droptower (1.5 sof free fall) for pool boiling of FC-72 (5 K of subcooling) on a flat heater (25x25 mm).An electrostatic field was also generated by imposing a voltage drop up to 23 kV in threeFigure 4 Critical heat flux transition in parabolic flight (a 10-2 g) on a wire (0.2 x 45 mm),fluid: FC-72 (Di Marco & Grassi, 1991).
POOL BOILING IN REDUCED GRAVITY187electrode geometries: flat parallel plate, flat diverging plate, perpendicular pin. The heattransfer coefficient was found to decrease drastically in microgravity in the absence ofan electric field. The application of the electric field increased the heat transfercoefficient from 40% to 70% in microgravity; in some cases, at low heat fluxes, a heattransfer coefficient even larger than in normal gravity conditions was measured. Themost efficient geometrical configuration was the diverging plate. The electric field wasfound to also decrease the average bubble size (about 1 mm with respect to 70 mm in theabsence of EF).Tokura et al.(1995) performed pool boiling experiments in JAMIC dropshaft onboiling of methanol on thin platinum wires (0.1 and 0.05 mm diameter). Two boilingregimes were observed: in the first, bubbles of relatively small size sprung out from thewire and quasi-steady-state conditions were attained, and in the second the bubblescoalesced laterally along the wire to eventually form large spherical bubbles thatenclosed the wire without detaching. The first regime took place typically in low heatflux conditions, the second at higher ones. Due to the near absence of liquid, the wireglowed white inside the large bubble. Coexistence of the two regimes was also observed.In the first regime, the heat transfer coefficient was quite close to the expected terrestrialvalue. The authors concluded that it is not possible to attain steady state boiling at highheat flux in microgravity.Shatto et al. (1999) experimented in parabolic flight with water at reduced pressures.Some data at Martian gravity level (0.4 g) were also collected. Two heaters, a flat and acylindrical one, were employed. Nucleate boiling heat transfer was found to be increasedin reduced gravity. A significant reduction in critical heat flux under reduced gravity wasmeasured.Motoya et al. (1999) studied boiling of saturated water on a platinum wire (0.2 mmdiameter) in the JAMIC dropshaft. The effect of scale deposition (calcium carbonate) onthe wire was also studied. In the bare wire, the same boiling mechanism described byTokura above was observed, with a large coalescing bubble causing burnout at high heatflux. Conversely, on the scaled surface, the bubbles kept on detaching even at high heatflux. The pool boiling performance in microgravity was almost the same as in normalgravity for the bare wire, wit
MASER, and the Japanese TR-1A can provide 6 min of microgravity (with an apogee of about 250 km), while 15 minutes can be attained with the Swedish MAXUS. Orbital Flights can provide a good level of microgravity: 10-4 to 10-5 g, with the g-jitter depending mainly on the crew movements in
I2 NWI Sox Fossey I4 Team PA Larcinese J2 NKY Bandits Premier J4 Team Ohio Gray K2 Indy Dreams Gold Lewis K4 Steel City Cyclones Cox L2 Motor City Madness Collins L4 WV Firestix Haynes 9:30 POOL 14UP POOL GAME 1 POOL 14UP POOL GAME 1 POOL 14UP POOL GAME 1 POOL 14UP POOL GAME 1 POOL 14UP POOL GAME 1 POOL 14UP POOL GAME 1 POOL 14UP POOL GAME 1 .
Pool deck equipment Pool main drain piping Pool filter system Pool surge tank Pool water treatment equipment Pool area HVAC system This Report provides a summary of concerns noted and improvement considerations. Photographs may be found in Appendix A for reference. Photos 1 and 2 provide an overall view of the pool room.
control design for an indoor swimming pool Pool water temperature The pool water requires to be heated to an acceptable temperature. Within a typical indoor pool, the pool water is normally heated to a temperature between 27 C and 30 C. The majority of the heat loss from the pool water is by evaporation from the pool surface into the pool .
7.3.2 Flow Boiling: Pressure Drop Characteristics 335 7.3.3 Flow Boiling: Heat Transfer 336 7.3.4 Natural Convection Boiling 339 7.3.5 Explosive Boiling 339 7.4 Selected Properties of Liquids Used for Cooling Micro-Devices . 3
For In Ground Pools up to 40,000 gallons. Part 3 FROG BAM Use with the POOL FROG System for an Algae-Protection Guarantee. Part 4 POOL FROG Bac Pac Pre-fi lled with chlorine tablets and fi ts inside the POOL FROG Mineral Reservoir. (POOL FROG Mineral Reservoir sold inside POOL FROG Mineral Cycler and separately for replacements. POOL .
o type of question: e.g. Pool 1 (Multiple Choice), Pool 2 (Essay) Use whatever terminology will allow you to distinguish your pools by content area AND pool value. Examples of a pool names: Civil War Pool (hard) Ch 1-5 Pool (50 pts) Civil War Pool (50%) Civil War Pool (Multiple Choice)
Standard Hill North Mannville Sands Pool. Revised PO. Standard Hill West McLaren Sand Pool. Revised PO. Storthoaks Tilston Beds Pool. Revised PO. Verendrye Viking Sand Pool. Revised PO. Wauchope Central Tilston Beds Pool. Revised PO. White Bear Tilston-Souris Valley Beds Pool. Revised PO. Workman Frobisher Beds Pool. Revised PB.
An experimental technique for precise and systematic measurements of entire boiling curves under steady-state and transient conditions has been developed. Pool boiling experiments for well wetting uids and uids with a larger contact angle (FC-72, isopropanol, water) yield single and reproducible boiling curves if the system is clean.
