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Rewetting velocity in quenching at reduced gravity
International Journal of Thermal Sciences 49 (2010) 1567e1575
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International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts
Rewetting velocity in quenching at reduced gravity G.P. Celata a, *, M. Cumo b, F. D’Annibale a, L. Saraceno a, G. Zummo a a b
Institute of Thermal-Fluid Dynamics, ENEA, Rome, Italy University of Rome La Sapienza, Corso Vittorio Emanuele II, 244, Rome, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 July 2009 Received in revised form 2 February 2010 Accepted 17 April 2010 Available online 7 June 2010
The results of quenching tests in tubes at microgravity and normal gravity conditions are presented. The aim of the experimental campaign was to gather both quantitative data and direct observations regarding phenomena involved in the rewetting of tubes. The rewetting velocity at different refrigerant flow-rate and subcooling conditions is obtained from experimental data. Test sections are made of Pyrex tubes with three different inner diameters: 2.0 mm, 4.0 mm and 6.0 mm. They are coated with IndiumTin-Oxide (ITO), which is transparent but allowing the direct heating of the Pyrex tube. Tests are performed with vertical test section and downward liquid flow. The working fluid is FC-72, a fluorinert liquid. Measurements included wall temperatures along the flow channel, inlet and outlet temperatures, pressures and mass flow-rate. The 6.0 and 2.0 mm tubes show a significant decrease in the quenching velocity at reduced gravity and a dependence on the coolant mass flow-rate, especially for ground experiments. The results of the 4.0 mm tube show a contradictory behaviour in micro/normal gravity comparison. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Quenching Microgravity Rewetting velocity FC-72 ITO coating
1. Introduction Due to its high-performance in heat removal and transport, flow boiling regime is likely to be employed in a wide range of future space applications or systems such as satellites for communications, thermal management of the International Space Station, cooling of electronic devices subjected to high thermal load, cooling of nuclear space reactors, etc. In order to conveniently develop and design these microgravity thermal systems (in particular heat exchangers), it is therefore necessary to understand main flow boiling phenomena at low gravity conditions. Quenching of high temperature surfaces is one of relevant aspects related to flow boiling heat transfer. In space quenching occurs, for instance, during the transfer of liquid oxygen and hydrogen, typical rocket fuels. Basically, quenching is encountered when a cold liquid flows on a dry and hot surface and the surface temperature is sufficiently higher than a certain limit. This limit is called rewetting temperature, the highest temperature at which the direct contact between the cold liquid and the hot surface is possible. Under these conditions, heat is removed from the hot surface to the coolant through various modes of heat transfer: film boiling, nucleate boiling, convection, radiation, and conduction through the channel walls.
* Corresponding author. E-mail address: [email protected] (G.P. Celata). 1290-0729/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ijthermalsci.2010.04.012
When the wall temperature falls down the rewetting limit, the liquid coolant comes in contact with the hot surface, rewetting it and heat transfer is mainly between the liquid and the wall. The extreme portion of the surface rewetted by the coolant is called quench front. The complex heat transfer mechanisms occurring during quenching phenomena are still far to be understood at normal gravity conditions. At reduced gravity the situation is even worst, because of the small amount of experimental data available. To date only few experimental works dedicated to quenching at reduced gravity are available [1e4]. Antar and Collins [1] obtained flow pattern visualizations and wall temperature measurements during quenching of hot tube aboard the NASA KC-135 aircraft. The coolant fluid is liquid nitrogen, with saturation conditions at the inlet. Two test sections are used, 10.5 mm i.d. and 600 mm in length, and 4.32 mm i.d. and 700 mm in length. Authors describe a new two-phase flow pattern at reduced gravity indicated as filamentary flow. The filamentary flow is a sort of inverted annular flow characterised by a long liquid filaments flowing in the centre of the channel and surrounded by vapour. The filaments have a diameter of approximately one third of the pipe diameter. The thermal analysis of the quenching shows that at reduced gravity the pipe rewetting time was longer than that at normal gravity (lower rewetting velocity). Therefore, gravity affects the total duration of the quenching process. Westbye et al. [2] performed quenching experiments of hot tube at microgravity aboard the NASA KC-135. The liquid coolant was
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Nomenclature D G g g0 p Q t T u W z
tube diameter [m] refrigerant mass flux [kg m2 s1] gravitational acceleration [m s 2] gravitational acceleration at earth [m s2] pressure [MPa] volumetric flow-rate [ml/min] time [s] temperature [ C] velocity [m s1] electrical power [W] axial coordinate [m]
Subscripts el electrical in inlet condition rew rewetting condition w channel wall wi initial channel wall condition
subcooled R113. The test section is a stainless steel tube, 11.3 mm i.d. and 914 mm in length. Tests at reduced gravity are compared with tests at terrestrial gravity with a horizontal test section. Rewetting temperatures at low gravity are 15e25 C lower than those recorded at normal gravity. Heat transfer coefficients during film boiling at low gravity are lower (up to 50%) than those at normal gravity. Therefore, the total duration of quenching process is found significantly longer at microgravity (lower rewetting velocity) than that at 1-g. The observed flow pattern at 0-g is inverted annular flow and dispersed flow, with a thicker vapour film thickness in inverted annular flow than that at 1-g. Adham-Khodaparast et al. [3] conducted transient quenching experiments on a hot flat surface under microgravity conditions aboard the NASA KC-135 parabolic aircraft for an inlet flow of Freon-113 with 20 C subcooling. They have found that: i) initial rewetting of the hot surface occurs for surface superheats well above the expected rewetting superheats for Freon-113; ii) the wall
superheat at the onset of rewetting increases with mass velocity, inlet subcooling and the gravity level; iii) heat transfer coefficient during film boiling is considerably reduced in microgravity due to the thickening of the vapour film; iv) the maximum heat flux during rewetting increases with an increase in mass velocity, inlet subcooling and gravity. Kawaji et al. [4] carried out quenching experiments of a hot tube at microgravity aboard the NASA KC-135. The liquid coolant was R113, under subcooled conditions. The test section is a quartz tube, heated externally by a nichrome heating tape wound around the tube and connected to an AC power source, 14.0 mm I.D. and 1.2 m long. The two-phase flow patterns encountered in a transient quenching process in microgravity are different from those found under 1-g conditions, and, as a consequence, the heat transfer rate is substantially reduced. Kawanami et al. [5] conducted an experiment at JAMIC (drop shaft, 10 s of microgravity) using LN2 as a test fluid, and a transparent heated tube for observing fluid behaviour and measuring heat transfer during tube quenching. Experiments were performed in a low mass velocity region (100e300 kg/m2 s) that is easily influenced by gravity. They have found that: (1) The heat transfer and quench front velocity increase in microgravity; (2) The heat transfer and quench front velocity under microgravity conditions increase up to 20% from those under 1-g, and the difference in the heat transfer characteristics under 1-g and 0-g decreases with increasing mass velocity. It is concluded from these results that the increase in the heat transfer under microgravity conditions is caused by the increase in the quench front velocity. Celata et al. [6] have recently published new results on microgravity experiments carried out in parabolic flight, using FC-72 as fluid, with a 6.0 mm I.D. Pyrex pipe, heated with an electric tape wrapped around the test section, in upward flow. They have found that the quench front velocity at low gravity is slightly affected by the mass flow-rate and resulting much lower than the value obtained at terrestrial gravity under same conditions (about 1/3 of the 1-g values). With the aim of improving the knowledge of quenching under microgravity conditions, the present paper is mainly focused on the rewetting velocity determination from experimental data at reduced gravity. This velocity represent the velocity of the quench (or rewetting) front propagation along the surface during the
Fig. 1. A typical parabola of the Zero-G aircraft.
G.P. Celata et al. / International Journal of Thermal Sciences 49 (2010) 1567e1575 Table 1 Test matrix for parabolic flight campaign. PF 48
March 2008
number of parabolas D [mm] G [kg/m2 s] DTsub,in [K] p [bar] TWi [ C]
16
16
26
2.0 180e660 26 to 47 1.6; 2.3; 3 230
4.0 70e520 30 to 45 1.7; 2.3; 3 230
6.0 50e460 40 to 60 1.7; 2.3; 3 236
cooling and, together with the correlated rewetting time, represents the main parameter to quantify the quenching phenomenon. The effect of gravity on rewetting velocity is analysed through the comparison with corresponding tests performed at terrestrial gravity. The study intends also to clarify the effect of tube diameter, mass flow-rate and refrigerant subcooling on quenching in microgravity. 2. Experimental facility The experiments at reduced gravity were conducted onboard of a modified Airbus A-300, called Zero-G, maintained by Novespace, which, among others, is used by ESA, the European Space Agency, for parabolic flight campaigns. This modified aircraft can perform special manoeuvres (parabolic flights), according to the schematisation shown in Fig. 1, in order to allow a period of reduced gravity of 20e22 s. The level of vertical acceleration reached during the flight in microgravity is about 102 g. Present manuscript describes parabolic flight campaign results carried out in March 2008 (ESA PF48) with test sections of 2.0, 4.0 and 6.0 mm in inner diameter (thickness of 2.0, 1.0 and 1.5 mm, respectively). During the experimental campaign (3 flight days, 31 parabolas for day) an amount of 58 tests were dedicated to quenching according the test matrix described in Table 1. The other parabolas have been dedicated to flow boiling heat transfer experiments which analysis is in progress. The experimental facility used for the experiments was designed in order to fulfil the requirements imposed by Novespace (and by CEV, the French safety company for flight) for the experimental facilities to be hosted aboard the Zero-G Airbus. Special attention was devoted to the test protocol of the facility in order to obtain steady flow boiling conditions at the beginning of each parabola. Therefore, all the regulations of the experimental loop, electrical power steps, mass flow-rate, inlet pressure, and inlet temperature, were regulated to successfully perform flow boiling experiments in such critical conditions. We have used three different system pressures to obtain different liquid subcooling degree without changing the fluid inlet temperature. As a matter of fact, parabolic flight tests are carried out with very tight time schedule between adjacent parabolas (1 min) or blocks of parabolas (3e8 min). Changing the system pressure is faster than changing the inlet fluid temperature. The experimental facility, schematised in Fig. 2, and named MICROBO (MICROgravity BOiling), consists of a gear pump (Qmax ¼ 500 ml/min), a filter, a flow-meter, an electric pre-heater, the test section, a condenser, a bellows and a tank for the storage of the process fluid (FC-72). The test section, shown in details in Fig. 3, is made of a Pyrex tube and is 180 mm in length. An ITO (Indium-Tin-Oxide) transparent layer of less than 100 nm thickness is coated by sputtering on the external surface in order to allow the tube heating through the electrical resistance of the ITO layer (Joule effect) as well as
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simultaneous visualization of the phenomena inside the pipe. Flow pattern visualization is performed with a digital video camera. The wall temperature is measured by a set of ten thermocouples attached to the outer tube wall. All the thermocouples and the electrical connectors are glued on the external surface of the tube with a special epoxy resin. This special technique is developed in the ENEA laboratories. An evaluation of the experimental uncertainty is reported in Table 2. The refrigerant fluid is FC-72, perfluorohexane C6F14, a fluorinert liquid manufactured by 3M, used in electronic cooling and widely used for experiments of boiling on parabolic flights. The test section is confined in a special box made of ERTACELÒ to avoid any leakage in the cabin in case of its break. For the same reason all the piping of the facility is under a double confinement made of Plexiglas. The picture in Fig. 4 shows the experimental facility in flight configuration in the cabin of the A-300 Zero-G. During each parabola video sequences of the flow structure are recorded with an ultra rapid shutter video camera on a digital video recorder with a frequency of 50 fps. The movie is taken near the entrance section of the flow channel. Video images obtained are able to provide information on the flow pattern at the exit of the heated tube and can be analysed to determine the vapour bubble dimensions and velocities for different values of the typical gravity level. From movies it is possible to compare flow structures in microgravity and in normal gravity under the same operational conditions (mass flow-rate, inlet fluid temperature, system pressure, and electrical power). For these tests, the test section is vertically oriented with downward flow.
3. Experimental results The typical quenching test is carried out heating the test section, without liquid flow-rate, during the hypergravity phase of the parabola, which lasts about 22 s. The electrical power during this drying period is selected in order to obtain the desired initial wall
Fig. 2. Schematic of the experimental facility.
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Fig. 3. ITO test section.
temperature T Wi approximately at the end of the hypergravity phase. Once the wall temperature reaches the fixed temperature, the liquid flow-rate is injected in the test section thus starting the rewetting process. This starts as close as possible to the beginning of the microgravity period. Simultaneously to liquid flow-rate injection, the thermal power is also switched off. Mass flux value obtained under microgravity conditions, which depends both on the pump head and the hydrostatic pressure in the test loop, and will be used for the terrestrial reference tests. Keeping the loop layout in mind, variations in the hydrostatic head due to different gravity conditions are completely negligible (o.m. 0.01 bar) and always less than the accuracy of the pressure transducers used. Fig. 5 shows a plot of typical results of transient wall temperatures, for all thermocouples, during quenching experiments, for a generic parabola. The left vertical axis indicates the wall temperature, the right vertical axis indicates the acceleration level, and the horizontal axis indicates time in seconds. After the parabola, and the total quenching of the pipe, the test tube is heated again in order to restore the test conditions for the next parabola. Accordingly, the temperature of the tube wall increases again to the prefixed value. Differences in the maximum wall temperatures before the electric power switch off are due to copper clamps (electric heating) which act as heat sinks and make the axial wall temperature profile non uniform. The maximum wall temperature range was about 230e250 C. When the liquid is injected in the hot tube, at first only a stream of vapour is in contact with the channel wall. Vapour heat transfer is not very effective but, as the thermal power is switched off, an initial reduction in the wall temperature is observed, after the peak value. In this phase, the heat transfer regime is governed by film boiling and the liquid stream cannot touch the hot wall due to its high temperatures. When the wall temperature is reduced to the rewetting temperature, the liquid is able to come into direct contact with the channel walls and nucleate boiling can take place (Figs. 6 and 7). At this time the heat transfer rate from the hot wall to the liquid is significantly increased as shown clearly on the graph by the rapid variation of the wall temperature slope. According to Westbye et al. [2], Chen et al. [3], Kim and Lee [4], and Barnea et al. [7], the rewetting temperature Trew is defined as the apparent rewetting temperature obtained by drawing the tangents on the transient temperature curves in the regions where
Table 2 Evaluation of the experimental uncertainty. Parameter
Uncertainty
p [MPa] T [ C] Q [ml/min] Wel [W] z [m]
1.04% 0.625% 0.93% 2.9% 0.5%
the significant change of the curve slope occurs, as schematically drawn in Fig. 7. The point of intersection of the two tangents identifies the apparent rewetting temperature and the rewetting time along the tube axial distance. In particular, the apparent rewetting temperature is identified by the intersection between the horizontal line, drawn from the intersection of the two tangents, and the temperature curve. The corresponding rewetting time trew is defined on the horizontal axis of the graph. Form the knowledge of the rewetting time along the tube finally it is possible to determine the rewetting velocity, urew. The rewetting front velocity is hence calculated considering the distance between two adjacent thermocouples and the time needed for the quench front to pass from one thermocouple position to the adjacent one, the velocity being obtained by the ratio of the two parameters. Although in this way it would be possible to calculate the ‘local’ velocity corresponding to each thermocouples couple, we decided to consider as the reference velocity for each test the one calculated between the two thermocouples located at the inlet (T W1) and at the outlet (T W10) of the test section. An even more convenient way would be using thermocouples T W2 and T W9 for the evaluation of trew, as these thermocouples are less affected by the cold sink represented by the copper clamp (especially after the electric supply shutdown). However, it is necessary to make a further premise with regard to the calculation of rewetting time and temperature. Their values are obtained from the wall temperature traces versus time, where the wall temperature is the external wall temperature. If we correct the readings of the external wall temperature making use of the Fourier equation along the wall thickness under transient conditions employing the finite differences method (2-dimensional), we can found the typical trend shown in Fig. 8. The time shifting in Tw versus time is linked to heat conduction along the radius of the wall thickness and therefore to thermal inertia, as the channel heating occurs on the external surface (ITO layer), while the heat is delivered to the cooling through the internal surface of the tube. One can notice how the difference is quite small in the rewetting zone, also taking into account the experimental uncertainty, while the slopes are the same. Also considering that this difference is practically the same for any wall thermocouple along the test section, the rewetting time and velocity calculated with the external wall temperatures (T W1 and T W10) are effectively coincident with those obtained using the internal wall temperatures (T W2 and T W9). Normal gravity tests, carried out under same thermal hydraulic conditions for comparison with microgravity data, have been performed on ground before and after the parabolic flight. The results of the rewetting velocity urew as a function of mass flux for terrestrial and reduced gravity are shown in Figs. 9e12 for D ¼ 6.0 mm tube diameter tests. The rewetting velocity, as already said, indicates the speed at which the quench front moves from the inlet along the hot tube, and is responsible of the time necessary for the rewetting of the whole tube. The quench front velocity is significantly lower than the liquid velocity, typically an order of
G.P. Celata et al. / International Journal of Thermal Sciences 49 (2010) 1567e1575
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Fig. 4. MICROBO facility in flight configuration.
magnitude lower. Figures clearly show that urew at reduced gravity, for D ¼ 6.0 mm tube diameter, is lower than that at normal gravity, other conditions being equal, reducing to even one third of the terrestrial gravity value and more. For low liquid mass flux the effect of the gravity level on the quench front velocity is less evident. These results also show that at 1-g the rewetting velocity is strongly dependent on the mass flow-rate, while at 0-g is definitely less sensible to the variation of the coolant mass flow-rate. Results obtained with the 2.0 mm tube diameter are shown in Figs. 13 and 14. Quenching tests are performed at the similar fluid inlet conditions as the 6.0 mm pipe tests. The quench front velocity
appears less sensible to the variation of the coolant mass flow-rate, both in microgravity and at terrestrial gravity conditions. Microgravity tests still show values of velocity lower then corresponding ones performed on ground. The 6.0 mm pipe exhibits a rewetting velocity higher than the 2.0 mm pipe, as clearly shown in Fig. 15 (microgravity conditions). Comparing Figs. 12 and 15 it is quite evident, as expected, that the rewetting velocity is directly related to the pipe diameter, being higher for the larger diameter pipe also at terrestrial gravity. From the figure it is also evident the influence of the liquid subcooling on the rewetting velocity, also under microgravity conditions. For both
Fig. 5. Typical wall temperature and gravity level versus time traces for one generic parabola.
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100 T
March08 80
sub
[K]
-53 -52
Day1 d = 6 mm TWi = 236°C
p [bar]
g
2.3 2.3
0 1
u
rew
[mm/s]
downward flow
60
40
20
0 0
Fig. 6. Quenching flow pattern in microgravity.
100
200
300
400
500
G [kg/m2s] Fig. 9. Rewetting velocity versus mass flux for 1-g and 0-g tests, D ¼ 6 mm.
Fig. 7. Method for the determination of the rewetting temperature and time.
240
Fig. 10. Rewetting velocity versus mass flux for 1-g and 0-g tests, D ¼ 6 mm.
50
Tout - exp Tin - cal
220
T
March08 40
200
sub
-39 -42
Day1 d = 6 mm T = 236°C
[K]
p [bar] 1.7 1.7
g 0 1
[mm/s]
downward flow
160
rew
Twall [°C]
Wi
180
u
140
30
20
120 100
10
Day 1 - parabola 0 - TW4
80 55
60
65
70
75
80
time [s]
0 0
100
200
300
400
500
2
G [kg/m s] Fig. 8. Comparison between experimental (outer) and calculated (inner) wall temperature.
Fig. 11. Rewetting velocity versus mass flux for 1-g and 0-g tests, D ¼ 6 mm.
G.P. Celata et al. / International Journal of Thermal Sciences 49 (2010) 1567e1575
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120 T
March08 100
Wi
[mm/s]
80
[K]
sub
-46 -46 -53 -52 -48
D1 d = 6 mm T = 236°C downward flow
p [bar]
g
2.3 2.3 2.3 2.3 2.3
0 1 0 1 0
u
rew
60
40
20
0 0
100
200
300
400
500
G [kg/m2s] Fig. 12. Rewetting velocity versus mass flux for 1-g and 0-g tests, D ¼ 6 mm.
Fig. 15. Comparison between 6 and 2 mm data at zero gravity conditions.
20
20 T
March08
-47 -45
D3 d = 2 mm T = 230°C
15
sub
[K]
p [bar] 3 3
g
March08
0 1
D3 d = 2 mm T = 230°C
15
T
sub
Wi
Wi
[K]
p [bar]
g
-47 -45
3 3
0 1
downward flow downward flow
-47
3
1
upward flow
[mm/s]
10
rew
10
u
u
rew
[mm/s]
downward flow
5
5
0
0 0
100
200
300
400
500
600
700
0
800
100
200
300
G [kg/m2s]
400
500
600
700
800
2
G [kg/m s]
Fig. 13. Rewetting velocity versus mass flux for 1-g and 0-g tests, D ¼ 2 mm.
Fig. 16. Downward and upward flow comparison.
20
20 T
March08 15
sub
-34 -35
D3 d = 2 mm T = 230°C
[K]
p [bar] 2.3 2.3
g
March08
0 1
D3 d = 2 mm T = 230°C
15
T
Wi
Wi
p [bar]
g
-34 -35
[K]
2.3 2.3
0 1
downward flow downward flow
-37
2.3
1
upward flow
sub
[mm/s]
10
rew
10
u
u
rew
[mm/s]
downward flow
5
5
0
0 0
100
200
300
400
500
600
700
800
2
G [kg/m s] Fig. 14. Rewetting velocity versus mass flux for 1-g and 0-g tests, D ¼ 2 mm.
0
100
200
300
400
500
600
2
G [kg/m s] Fig. 17. Downward and upward flow comparison.
700
800
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G.P. Celata et al. / International Journal of Thermal Sciences 49 (2010) 1567e1575
40
25
[
T
March08
35
Wi
p [bar]
g
March08
3 2.3 1.6
0 0 0
D2 d = 4 mm T = 230°C
20
[mm/s] rew
20
u
u
p [bar]
g
-36
2.3
0
-34
2.3
1
sub
[K]
downward flow
downward flow
25
T
Wi
rew
[mm/s]
[K]
-45 -36 -30
D2 d = 4 mm T = 230°C
30
sub
15
15
10
10 5
5 0
0 0
100
200
300
400
500
600
0
100
200
Fig. 18. Rewetting velocity versus mass flux for 0-g tests, D ¼ 4 mm.
300
400
500
600
G [kg/m2s]
G [kg/m2s]
Fig. 19. Rewetting velocity versus mass flux for 1-g and 0-g tests, D ¼ 4 mm.
Fig. 20. Video sequence frame for 1-g test, D ¼ 4 mm; G ¼ 400 kg m2 s1; p ¼ 3 bar.
G.P. Celata et al. / International Journal of Thermal Sciences 49 (2010) 1567e1575
the 2.0 and the 6.0 mm pipe the rewetting velocity urew is an increasing function of the liquid inlet subcooling. Such an evidence can be explained keeping in mind that a higher liquid subcooling corresponds to a lower bulk temperature of the fluid; then, during the quenching, the heat transfer rate between the vapour layer adjacent to the wall and the liquid core increases with subcooling, leading to lower quenching time and, consequently, higher quenching velocity. In the frame of the normal gravity experimental campaign, further tests with refrigerant injection from the bottom of the test section have been performed. Figs. 16 and 17 show the comparison between urew values found. As expected, rewetting velocity values in microgravity conditions are still lower than corresponding values in both terrestrial gravity tests series. On the other hand, as expected, there is a clear influence of the refrigerant flow direction on urew: downward flow tests show rewetting velocity values always greater than corresponding upward flow tests, other conditions being equal. Although there are not enough evidences to clarify the physics of quenching in microgravity, the lower values of quenching velocity in a low gravity environment, with respect to terrestrial gravity, would seem to be linked to the enlargement of the vapour layer under reduced gravity. Because of this phenomenon, under microgravity conditions the quench front velocity would seem to be less dependent on mass flux than at terrestrial gravity evidencing the smaller vales shown in Figs. 9e17. Data analysis of the 4.0 mm pipe has to be discussed separately. Fig. 18 shows the rewetting velocity as a function of mass flux, for microgravity tests. The rewetting velocity looks to be less affected by the mass flux with respect to the 2.0 and 6.0 mm pipes. The comparison with ground tests plotted in Fig. 19 shows that the rewetting velocity at terrestrial gravity is lower than the corresponding value in microgravity, contrarily to the 2.0 and 6.0 mm pipes evidences. This apparent contradiction may be attributed to an anomalous trend of the temperature-versus-time curve linked to a specific flow pattern taking place in the flow, which is typical of this particular geometry. Indeed, watching at the movies of the 4.0 mm pipe tests we observe significant and continuous flow instabilities, as it is possible to see in frame sequence of Fig. 20. These instabilities appear with the presence of vapour blanket which only partially fills the tube cross section allowing the liquid to reach the end of the test channel before the actual quenching of the initial part of the pipe (see frames 47 and 86 in Fig. 20). Such behaviour leads to a kind of two-fold quenching occurring almost simultaneously at the two extreme ends of the test section (liquid inlet and outlet), as it is possible to see in frames 186 and 211 of Fig. 20. This makes impossible to get a correct evaluation of the rewetting velocity for this type of tests.
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4. Conclusion An experimental investigation on quenching of tubes to study the effects of gravity level, fluid velocity, and tube diameter has been performed. The objective of the experiment is to develop a data base with microgravity tests and to collect observations on the rewetting velocity at the two different gravity conditions. Test tubes are made of Pyrex, coated with a transparent layer of Indium-Tin-Oxide (ITO), having three different inner diameters: 2.0, 4.0 and 6.0 mm. Tests are performed with vertical test section with liquid flowing downwards and using FC-72. The 6.0 mm tube diameter results show a significant decrease in the quenching front velocity at reduced gravity, especially for high mass flux. Same behaviour is exhibited by the 2 mm tube but less dependence on the refrigerant mass flow-rate. At terrestrial gravity conditions upward flow tests provide lower rewetting velocity values than corresponding downward flow tests (data available only for D ¼ 2 mm). The 4.0 mm tube shows large instabilities in the flow pattern that makes a correct evaluation of the rewetting velocity experimental data impossible. Acknowledgements Thanks are due to M. Morlacca, V. Pietrelli, A. Scotini, and M. Sica for their contribution in design and assembling the test sections and the experimental facility, and for their technical assistance. Authors are strongly indebted to A. Lattanzi and L. Simonetti who flew on the parabolic flights and performed the experiments. Authors would also like to express their thanks to the personnel of Novespace for their collaboration during the parabolic flights. The work is carried out in the frame of the ESA MAP Contract 14227/02/ NL/SH, with the additional financial support of Snecma Moteurs. References [1] B.N. Antar, F.G. Collins, Flow boiling during quench in low gravity environment. Microgravity Sci. Technol. 10 (1997) 118e128. [2] C.J. Westbye, M. Kawaji, B.N. Antar, Boiling heat transfer in the quenching of a hot tube under microgravity. J. Thermophys. Heat Transfer 9 (AprileJune, 1995) 302e307. [3] W.J. Chen, Y. Lee, D.C. Groeneveld, Measurement of boiling curves during rewetting of a hot circular duct. Int. J. Heat Mass Transfer 22 (1979) 973e976. [4] A.K. Kim, Y. Lee, A correlation of rewetting temperature. Lett. Heat Mass Transfer 6 (1979) 117e123. [5] O. Kawanami, H. Azuma, H. Ohta, Effect of gravity on cryogenic boiling heat transfer during tube quenching. Int. J. Heat Mass Transfer 50 (2007) 3490e3497. [6] G.P. Celata, M. Cumo, M. Gervasi, G. Zummo, Quenching experiments inside 6.0 mm tube at reduced gravity. Int. J. Heat Mass Transfer 52 (2009) 2807e2814. [7] Y. Barnea, E. Elias, I. Shai, Flow and heat transfer regimes during quenching of hot surfaces. Int. J. Heat Mass Transfer 37 (1994) 1441e1453.
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International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts
Rewetting velocity in quenching at reduced gravity G.P. Celata a, *, M. Cumo b, F. D’Annibale a, L. Saraceno a, G. Zummo a a b
Institute of Thermal-Fluid Dynamics, ENEA, Rome, Italy University of Rome La Sapienza, Corso Vittorio Emanuele II, 244, Rome, Italy
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 July 2009 Received in revised form 2 February 2010 Accepted 17 April 2010 Available online 7 June 2010
The results of quenching tests in tubes at microgravity and normal gravity conditions are presented. The aim of the experimental campaign was to gather both quantitative data and direct observations regarding phenomena involved in the rewetting of tubes. The rewetting velocity at different refrigerant flow-rate and subcooling conditions is obtained from experimental data. Test sections are made of Pyrex tubes with three different inner diameters: 2.0 mm, 4.0 mm and 6.0 mm. They are coated with IndiumTin-Oxide (ITO), which is transparent but allowing the direct heating of the Pyrex tube. Tests are performed with vertical test section and downward liquid flow. The working fluid is FC-72, a fluorinert liquid. Measurements included wall temperatures along the flow channel, inlet and outlet temperatures, pressures and mass flow-rate. The 6.0 and 2.0 mm tubes show a significant decrease in the quenching velocity at reduced gravity and a dependence on the coolant mass flow-rate, especially for ground experiments. The results of the 4.0 mm tube show a contradictory behaviour in micro/normal gravity comparison. Ó 2010 Elsevier Masson SAS. All rights reserved.
Keywords: Quenching Microgravity Rewetting velocity FC-72 ITO coating
1. Introduction Due to its high-performance in heat removal and transport, flow boiling regime is likely to be employed in a wide range of future space applications or systems such as satellites for communications, thermal management of the International Space Station, cooling of electronic devices subjected to high thermal load, cooling of nuclear space reactors, etc. In order to conveniently develop and design these microgravity thermal systems (in particular heat exchangers), it is therefore necessary to understand main flow boiling phenomena at low gravity conditions. Quenching of high temperature surfaces is one of relevant aspects related to flow boiling heat transfer. In space quenching occurs, for instance, during the transfer of liquid oxygen and hydrogen, typical rocket fuels. Basically, quenching is encountered when a cold liquid flows on a dry and hot surface and the surface temperature is sufficiently higher than a certain limit. This limit is called rewetting temperature, the highest temperature at which the direct contact between the cold liquid and the hot surface is possible. Under these conditions, heat is removed from the hot surface to the coolant through various modes of heat transfer: film boiling, nucleate boiling, convection, radiation, and conduction through the channel walls.
* Corresponding author. E-mail address: [email protected] (G.P. Celata). 1290-0729/$ e see front matter Ó 2010 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ijthermalsci.2010.04.012
When the wall temperature falls down the rewetting limit, the liquid coolant comes in contact with the hot surface, rewetting it and heat transfer is mainly between the liquid and the wall. The extreme portion of the surface rewetted by the coolant is called quench front. The complex heat transfer mechanisms occurring during quenching phenomena are still far to be understood at normal gravity conditions. At reduced gravity the situation is even worst, because of the small amount of experimental data available. To date only few experimental works dedicated to quenching at reduced gravity are available [1e4]. Antar and Collins [1] obtained flow pattern visualizations and wall temperature measurements during quenching of hot tube aboard the NASA KC-135 aircraft. The coolant fluid is liquid nitrogen, with saturation conditions at the inlet. Two test sections are used, 10.5 mm i.d. and 600 mm in length, and 4.32 mm i.d. and 700 mm in length. Authors describe a new two-phase flow pattern at reduced gravity indicated as filamentary flow. The filamentary flow is a sort of inverted annular flow characterised by a long liquid filaments flowing in the centre of the channel and surrounded by vapour. The filaments have a diameter of approximately one third of the pipe diameter. The thermal analysis of the quenching shows that at reduced gravity the pipe rewetting time was longer than that at normal gravity (lower rewetting velocity). Therefore, gravity affects the total duration of the quenching process. Westbye et al. [2] performed quenching experiments of hot tube at microgravity aboard the NASA KC-135. The liquid coolant was
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Nomenclature D G g g0 p Q t T u W z
tube diameter [m] refrigerant mass flux [kg m2 s1] gravitational acceleration [m s 2] gravitational acceleration at earth [m s2] pressure [MPa] volumetric flow-rate [ml/min] time [s] temperature [ C] velocity [m s1] electrical power [W] axial coordinate [m]
Subscripts el electrical in inlet condition rew rewetting condition w channel wall wi initial channel wall condition
subcooled R113. The test section is a stainless steel tube, 11.3 mm i.d. and 914 mm in length. Tests at reduced gravity are compared with tests at terrestrial gravity with a horizontal test section. Rewetting temperatures at low gravity are 15e25 C lower than those recorded at normal gravity. Heat transfer coefficients during film boiling at low gravity are lower (up to 50%) than those at normal gravity. Therefore, the total duration of quenching process is found significantly longer at microgravity (lower rewetting velocity) than that at 1-g. The observed flow pattern at 0-g is inverted annular flow and dispersed flow, with a thicker vapour film thickness in inverted annular flow than that at 1-g. Adham-Khodaparast et al. [3] conducted transient quenching experiments on a hot flat surface under microgravity conditions aboard the NASA KC-135 parabolic aircraft for an inlet flow of Freon-113 with 20 C subcooling. They have found that: i) initial rewetting of the hot surface occurs for surface superheats well above the expected rewetting superheats for Freon-113; ii) the wall
superheat at the onset of rewetting increases with mass velocity, inlet subcooling and the gravity level; iii) heat transfer coefficient during film boiling is considerably reduced in microgravity due to the thickening of the vapour film; iv) the maximum heat flux during rewetting increases with an increase in mass velocity, inlet subcooling and gravity. Kawaji et al. [4] carried out quenching experiments of a hot tube at microgravity aboard the NASA KC-135. The liquid coolant was R113, under subcooled conditions. The test section is a quartz tube, heated externally by a nichrome heating tape wound around the tube and connected to an AC power source, 14.0 mm I.D. and 1.2 m long. The two-phase flow patterns encountered in a transient quenching process in microgravity are different from those found under 1-g conditions, and, as a consequence, the heat transfer rate is substantially reduced. Kawanami et al. [5] conducted an experiment at JAMIC (drop shaft, 10 s of microgravity) using LN2 as a test fluid, and a transparent heated tube for observing fluid behaviour and measuring heat transfer during tube quenching. Experiments were performed in a low mass velocity region (100e300 kg/m2 s) that is easily influenced by gravity. They have found that: (1) The heat transfer and quench front velocity increase in microgravity; (2) The heat transfer and quench front velocity under microgravity conditions increase up to 20% from those under 1-g, and the difference in the heat transfer characteristics under 1-g and 0-g decreases with increasing mass velocity. It is concluded from these results that the increase in the heat transfer under microgravity conditions is caused by the increase in the quench front velocity. Celata et al. [6] have recently published new results on microgravity experiments carried out in parabolic flight, using FC-72 as fluid, with a 6.0 mm I.D. Pyrex pipe, heated with an electric tape wrapped around the test section, in upward flow. They have found that the quench front velocity at low gravity is slightly affected by the mass flow-rate and resulting much lower than the value obtained at terrestrial gravity under same conditions (about 1/3 of the 1-g values). With the aim of improving the knowledge of quenching under microgravity conditions, the present paper is mainly focused on the rewetting velocity determination from experimental data at reduced gravity. This velocity represent the velocity of the quench (or rewetting) front propagation along the surface during the
Fig. 1. A typical parabola of the Zero-G aircraft.
G.P. Celata et al. / International Journal of Thermal Sciences 49 (2010) 1567e1575 Table 1 Test matrix for parabolic flight campaign. PF 48
March 2008
number of parabolas D [mm] G [kg/m2 s] DTsub,in [K] p [bar] TWi [ C]
16
16
26
2.0 180e660 26 to 47 1.6; 2.3; 3 230
4.0 70e520 30 to 45 1.7; 2.3; 3 230
6.0 50e460 40 to 60 1.7; 2.3; 3 236
cooling and, together with the correlated rewetting time, represents the main parameter to quantify the quenching phenomenon. The effect of gravity on rewetting velocity is analysed through the comparison with corresponding tests performed at terrestrial gravity. The study intends also to clarify the effect of tube diameter, mass flow-rate and refrigerant subcooling on quenching in microgravity. 2. Experimental facility The experiments at reduced gravity were conducted onboard of a modified Airbus A-300, called Zero-G, maintained by Novespace, which, among others, is used by ESA, the European Space Agency, for parabolic flight campaigns. This modified aircraft can perform special manoeuvres (parabolic flights), according to the schematisation shown in Fig. 1, in order to allow a period of reduced gravity of 20e22 s. The level of vertical acceleration reached during the flight in microgravity is about 102 g. Present manuscript describes parabolic flight campaign results carried out in March 2008 (ESA PF48) with test sections of 2.0, 4.0 and 6.0 mm in inner diameter (thickness of 2.0, 1.0 and 1.5 mm, respectively). During the experimental campaign (3 flight days, 31 parabolas for day) an amount of 58 tests were dedicated to quenching according the test matrix described in Table 1. The other parabolas have been dedicated to flow boiling heat transfer experiments which analysis is in progress. The experimental facility used for the experiments was designed in order to fulfil the requirements imposed by Novespace (and by CEV, the French safety company for flight) for the experimental facilities to be hosted aboard the Zero-G Airbus. Special attention was devoted to the test protocol of the facility in order to obtain steady flow boiling conditions at the beginning of each parabola. Therefore, all the regulations of the experimental loop, electrical power steps, mass flow-rate, inlet pressure, and inlet temperature, were regulated to successfully perform flow boiling experiments in such critical conditions. We have used three different system pressures to obtain different liquid subcooling degree without changing the fluid inlet temperature. As a matter of fact, parabolic flight tests are carried out with very tight time schedule between adjacent parabolas (1 min) or blocks of parabolas (3e8 min). Changing the system pressure is faster than changing the inlet fluid temperature. The experimental facility, schematised in Fig. 2, and named MICROBO (MICROgravity BOiling), consists of a gear pump (Qmax ¼ 500 ml/min), a filter, a flow-meter, an electric pre-heater, the test section, a condenser, a bellows and a tank for the storage of the process fluid (FC-72). The test section, shown in details in Fig. 3, is made of a Pyrex tube and is 180 mm in length. An ITO (Indium-Tin-Oxide) transparent layer of less than 100 nm thickness is coated by sputtering on the external surface in order to allow the tube heating through the electrical resistance of the ITO layer (Joule effect) as well as
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simultaneous visualization of the phenomena inside the pipe. Flow pattern visualization is performed with a digital video camera. The wall temperature is measured by a set of ten thermocouples attached to the outer tube wall. All the thermocouples and the electrical connectors are glued on the external surface of the tube with a special epoxy resin. This special technique is developed in the ENEA laboratories. An evaluation of the experimental uncertainty is reported in Table 2. The refrigerant fluid is FC-72, perfluorohexane C6F14, a fluorinert liquid manufactured by 3M, used in electronic cooling and widely used for experiments of boiling on parabolic flights. The test section is confined in a special box made of ERTACELÒ to avoid any leakage in the cabin in case of its break. For the same reason all the piping of the facility is under a double confinement made of Plexiglas. The picture in Fig. 4 shows the experimental facility in flight configuration in the cabin of the A-300 Zero-G. During each parabola video sequences of the flow structure are recorded with an ultra rapid shutter video camera on a digital video recorder with a frequency of 50 fps. The movie is taken near the entrance section of the flow channel. Video images obtained are able to provide information on the flow pattern at the exit of the heated tube and can be analysed to determine the vapour bubble dimensions and velocities for different values of the typical gravity level. From movies it is possible to compare flow structures in microgravity and in normal gravity under the same operational conditions (mass flow-rate, inlet fluid temperature, system pressure, and electrical power). For these tests, the test section is vertically oriented with downward flow.
3. Experimental results The typical quenching test is carried out heating the test section, without liquid flow-rate, during the hypergravity phase of the parabola, which lasts about 22 s. The electrical power during this drying period is selected in order to obtain the desired initial wall
Fig. 2. Schematic of the experimental facility.
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Fig. 3. ITO test section.
temperature T Wi approximately at the end of the hypergravity phase. Once the wall temperature reaches the fixed temperature, the liquid flow-rate is injected in the test section thus starting the rewetting process. This starts as close as possible to the beginning of the microgravity period. Simultaneously to liquid flow-rate injection, the thermal power is also switched off. Mass flux value obtained under microgravity conditions, which depends both on the pump head and the hydrostatic pressure in the test loop, and will be used for the terrestrial reference tests. Keeping the loop layout in mind, variations in the hydrostatic head due to different gravity conditions are completely negligible (o.m. 0.01 bar) and always less than the accuracy of the pressure transducers used. Fig. 5 shows a plot of typical results of transient wall temperatures, for all thermocouples, during quenching experiments, for a generic parabola. The left vertical axis indicates the wall temperature, the right vertical axis indicates the acceleration level, and the horizontal axis indicates time in seconds. After the parabola, and the total quenching of the pipe, the test tube is heated again in order to restore the test conditions for the next parabola. Accordingly, the temperature of the tube wall increases again to the prefixed value. Differences in the maximum wall temperatures before the electric power switch off are due to copper clamps (electric heating) which act as heat sinks and make the axial wall temperature profile non uniform. The maximum wall temperature range was about 230e250 C. When the liquid is injected in the hot tube, at first only a stream of vapour is in contact with the channel wall. Vapour heat transfer is not very effective but, as the thermal power is switched off, an initial reduction in the wall temperature is observed, after the peak value. In this phase, the heat transfer regime is governed by film boiling and the liquid stream cannot touch the hot wall due to its high temperatures. When the wall temperature is reduced to the rewetting temperature, the liquid is able to come into direct contact with the channel walls and nucleate boiling can take place (Figs. 6 and 7). At this time the heat transfer rate from the hot wall to the liquid is significantly increased as shown clearly on the graph by the rapid variation of the wall temperature slope. According to Westbye et al. [2], Chen et al. [3], Kim and Lee [4], and Barnea et al. [7], the rewetting temperature Trew is defined as the apparent rewetting temperature obtained by drawing the tangents on the transient temperature curves in the regions where
Table 2 Evaluation of the experimental uncertainty. Parameter
Uncertainty
p [MPa] T [ C] Q [ml/min] Wel [W] z [m]
1.04% 0.625% 0.93% 2.9% 0.5%
the significant change of the curve slope occurs, as schematically drawn in Fig. 7. The point of intersection of the two tangents identifies the apparent rewetting temperature and the rewetting time along the tube axial distance. In particular, the apparent rewetting temperature is identified by the intersection between the horizontal line, drawn from the intersection of the two tangents, and the temperature curve. The corresponding rewetting time trew is defined on the horizontal axis of the graph. Form the knowledge of the rewetting time along the tube finally it is possible to determine the rewetting velocity, urew. The rewetting front velocity is hence calculated considering the distance between two adjacent thermocouples and the time needed for the quench front to pass from one thermocouple position to the adjacent one, the velocity being obtained by the ratio of the two parameters. Although in this way it would be possible to calculate the ‘local’ velocity corresponding to each thermocouples couple, we decided to consider as the reference velocity for each test the one calculated between the two thermocouples located at the inlet (T W1) and at the outlet (T W10) of the test section. An even more convenient way would be using thermocouples T W2 and T W9 for the evaluation of trew, as these thermocouples are less affected by the cold sink represented by the copper clamp (especially after the electric supply shutdown). However, it is necessary to make a further premise with regard to the calculation of rewetting time and temperature. Their values are obtained from the wall temperature traces versus time, where the wall temperature is the external wall temperature. If we correct the readings of the external wall temperature making use of the Fourier equation along the wall thickness under transient conditions employing the finite differences method (2-dimensional), we can found the typical trend shown in Fig. 8. The time shifting in Tw versus time is linked to heat conduction along the radius of the wall thickness and therefore to thermal inertia, as the channel heating occurs on the external surface (ITO layer), while the heat is delivered to the cooling through the internal surface of the tube. One can notice how the difference is quite small in the rewetting zone, also taking into account the experimental uncertainty, while the slopes are the same. Also considering that this difference is practically the same for any wall thermocouple along the test section, the rewetting time and velocity calculated with the external wall temperatures (T W1 and T W10) are effectively coincident with those obtained using the internal wall temperatures (T W2 and T W9). Normal gravity tests, carried out under same thermal hydraulic conditions for comparison with microgravity data, have been performed on ground before and after the parabolic flight. The results of the rewetting velocity urew as a function of mass flux for terrestrial and reduced gravity are shown in Figs. 9e12 for D ¼ 6.0 mm tube diameter tests. The rewetting velocity, as already said, indicates the speed at which the quench front moves from the inlet along the hot tube, and is responsible of the time necessary for the rewetting of the whole tube. The quench front velocity is significantly lower than the liquid velocity, typically an order of
G.P. Celata et al. / International Journal of Thermal Sciences 49 (2010) 1567e1575
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Fig. 4. MICROBO facility in flight configuration.
magnitude lower. Figures clearly show that urew at reduced gravity, for D ¼ 6.0 mm tube diameter, is lower than that at normal gravity, other conditions being equal, reducing to even one third of the terrestrial gravity value and more. For low liquid mass flux the effect of the gravity level on the quench front velocity is less evident. These results also show that at 1-g the rewetting velocity is strongly dependent on the mass flow-rate, while at 0-g is definitely less sensible to the variation of the coolant mass flow-rate. Results obtained with the 2.0 mm tube diameter are shown in Figs. 13 and 14. Quenching tests are performed at the similar fluid inlet conditions as the 6.0 mm pipe tests. The quench front velocity
appears less sensible to the variation of the coolant mass flow-rate, both in microgravity and at terrestrial gravity conditions. Microgravity tests still show values of velocity lower then corresponding ones performed on ground. The 6.0 mm pipe exhibits a rewetting velocity higher than the 2.0 mm pipe, as clearly shown in Fig. 15 (microgravity conditions). Comparing Figs. 12 and 15 it is quite evident, as expected, that the rewetting velocity is directly related to the pipe diameter, being higher for the larger diameter pipe also at terrestrial gravity. From the figure it is also evident the influence of the liquid subcooling on the rewetting velocity, also under microgravity conditions. For both
Fig. 5. Typical wall temperature and gravity level versus time traces for one generic parabola.
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100 T
March08 80
sub
[K]
-53 -52
Day1 d = 6 mm TWi = 236°C
p [bar]
g
2.3 2.3
0 1
u
rew
[mm/s]
downward flow
60
40
20
0 0
Fig. 6. Quenching flow pattern in microgravity.
100
200
300
400
500
G [kg/m2s] Fig. 9. Rewetting velocity versus mass flux for 1-g and 0-g tests, D ¼ 6 mm.
Fig. 7. Method for the determination of the rewetting temperature and time.
240
Fig. 10. Rewetting velocity versus mass flux for 1-g and 0-g tests, D ¼ 6 mm.
50
Tout - exp Tin - cal
220
T
March08 40
200
sub
-39 -42
Day1 d = 6 mm T = 236°C
[K]
p [bar] 1.7 1.7
g 0 1
[mm/s]
downward flow
160
rew
Twall [°C]
Wi
180
u
140
30
20
120 100
10
Day 1 - parabola 0 - TW4
80 55
60
65
70
75
80
time [s]
0 0
100
200
300
400
500
2
G [kg/m s] Fig. 8. Comparison between experimental (outer) and calculated (inner) wall temperature.
Fig. 11. Rewetting velocity versus mass flux for 1-g and 0-g tests, D ¼ 6 mm.
G.P. Celata et al. / International Journal of Thermal Sciences 49 (2010) 1567e1575
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120 T
March08 100
Wi
[mm/s]
80
[K]
sub
-46 -46 -53 -52 -48
D1 d = 6 mm T = 236°C downward flow
p [bar]
g
2.3 2.3 2.3 2.3 2.3
0 1 0 1 0
u
rew
60
40
20
0 0
100
200
300
400
500
G [kg/m2s] Fig. 12. Rewetting velocity versus mass flux for 1-g and 0-g tests, D ¼ 6 mm.
Fig. 15. Comparison between 6 and 2 mm data at zero gravity conditions.
20
20 T
March08
-47 -45
D3 d = 2 mm T = 230°C
15
sub
[K]
p [bar] 3 3
g
March08
0 1
D3 d = 2 mm T = 230°C
15
T
sub
Wi
Wi
[K]
p [bar]
g
-47 -45
3 3
0 1
downward flow downward flow
-47
3
1
upward flow
[mm/s]
10
rew
10
u
u
rew
[mm/s]
downward flow
5
5
0
0 0
100
200
300
400
500
600
700
0
800
100
200
300
G [kg/m2s]
400
500
600
700
800
2
G [kg/m s]
Fig. 13. Rewetting velocity versus mass flux for 1-g and 0-g tests, D ¼ 2 mm.
Fig. 16. Downward and upward flow comparison.
20
20 T
March08 15
sub
-34 -35
D3 d = 2 mm T = 230°C
[K]
p [bar] 2.3 2.3
g
March08
0 1
D3 d = 2 mm T = 230°C
15
T
Wi
Wi
p [bar]
g
-34 -35
[K]
2.3 2.3
0 1
downward flow downward flow
-37
2.3
1
upward flow
sub
[mm/s]
10
rew
10
u
u
rew
[mm/s]
downward flow
5
5
0
0 0
100
200
300
400
500
600
700
800
2
G [kg/m s] Fig. 14. Rewetting velocity versus mass flux for 1-g and 0-g tests, D ¼ 2 mm.
0
100
200
300
400
500
600
2
G [kg/m s] Fig. 17. Downward and upward flow comparison.
700
800
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40
25
[
T
March08
35
Wi
p [bar]
g
March08
3 2.3 1.6
0 0 0
D2 d = 4 mm T = 230°C
20
[mm/s] rew
20
u
u
p [bar]
g
-36
2.3
0
-34
2.3
1
sub
[K]
downward flow
downward flow
25
T
Wi
rew
[mm/s]
[K]
-45 -36 -30
D2 d = 4 mm T = 230°C
30
sub
15
15
10
10 5
5 0
0 0
100
200
300
400
500
600
0
100
200
Fig. 18. Rewetting velocity versus mass flux for 0-g tests, D ¼ 4 mm.
300
400
500
600
G [kg/m2s]
G [kg/m2s]
Fig. 19. Rewetting velocity versus mass flux for 1-g and 0-g tests, D ¼ 4 mm.
Fig. 20. Video sequence frame for 1-g test, D ¼ 4 mm; G ¼ 400 kg m2 s1; p ¼ 3 bar.
G.P. Celata et al. / International Journal of Thermal Sciences 49 (2010) 1567e1575
the 2.0 and the 6.0 mm pipe the rewetting velocity urew is an increasing function of the liquid inlet subcooling. Such an evidence can be explained keeping in mind that a higher liquid subcooling corresponds to a lower bulk temperature of the fluid; then, during the quenching, the heat transfer rate between the vapour layer adjacent to the wall and the liquid core increases with subcooling, leading to lower quenching time and, consequently, higher quenching velocity. In the frame of the normal gravity experimental campaign, further tests with refrigerant injection from the bottom of the test section have been performed. Figs. 16 and 17 show the comparison between urew values found. As expected, rewetting velocity values in microgravity conditions are still lower than corresponding values in both terrestrial gravity tests series. On the other hand, as expected, there is a clear influence of the refrigerant flow direction on urew: downward flow tests show rewetting velocity values always greater than corresponding upward flow tests, other conditions being equal. Although there are not enough evidences to clarify the physics of quenching in microgravity, the lower values of quenching velocity in a low gravity environment, with respect to terrestrial gravity, would seem to be linked to the enlargement of the vapour layer under reduced gravity. Because of this phenomenon, under microgravity conditions the quench front velocity would seem to be less dependent on mass flux than at terrestrial gravity evidencing the smaller vales shown in Figs. 9e17. Data analysis of the 4.0 mm pipe has to be discussed separately. Fig. 18 shows the rewetting velocity as a function of mass flux, for microgravity tests. The rewetting velocity looks to be less affected by the mass flux with respect to the 2.0 and 6.0 mm pipes. The comparison with ground tests plotted in Fig. 19 shows that the rewetting velocity at terrestrial gravity is lower than the corresponding value in microgravity, contrarily to the 2.0 and 6.0 mm pipes evidences. This apparent contradiction may be attributed to an anomalous trend of the temperature-versus-time curve linked to a specific flow pattern taking place in the flow, which is typical of this particular geometry. Indeed, watching at the movies of the 4.0 mm pipe tests we observe significant and continuous flow instabilities, as it is possible to see in frame sequence of Fig. 20. These instabilities appear with the presence of vapour blanket which only partially fills the tube cross section allowing the liquid to reach the end of the test channel before the actual quenching of the initial part of the pipe (see frames 47 and 86 in Fig. 20). Such behaviour leads to a kind of two-fold quenching occurring almost simultaneously at the two extreme ends of the test section (liquid inlet and outlet), as it is possible to see in frames 186 and 211 of Fig. 20. This makes impossible to get a correct evaluation of the rewetting velocity for this type of tests.
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4. Conclusion An experimental investigation on quenching of tubes to study the effects of gravity level, fluid velocity, and tube diameter has been performed. The objective of the experiment is to develop a data base with microgravity tests and to collect observations on the rewetting velocity at the two different gravity conditions. Test tubes are made of Pyrex, coated with a transparent layer of Indium-Tin-Oxide (ITO), having three different inner diameters: 2.0, 4.0 and 6.0 mm. Tests are performed with vertical test section with liquid flowing downwards and using FC-72. The 6.0 mm tube diameter results show a significant decrease in the quenching front velocity at reduced gravity, especially for high mass flux. Same behaviour is exhibited by the 2 mm tube but less dependence on the refrigerant mass flow-rate. At terrestrial gravity conditions upward flow tests provide lower rewetting velocity values than corresponding downward flow tests (data available only for D ¼ 2 mm). The 4.0 mm tube shows large instabilities in the flow pattern that makes a correct evaluation of the rewetting velocity experimental data impossible. Acknowledgements Thanks are due to M. Morlacca, V. Pietrelli, A. Scotini, and M. Sica for their contribution in design and assembling the test sections and the experimental facility, and for their technical assistance. Authors are strongly indebted to A. Lattanzi and L. Simonetti who flew on the parabolic flights and performed the experiments. Authors would also like to express their thanks to the personnel of Novespace for their collaboration during the parabolic flights. The work is carried out in the frame of the ESA MAP Contract 14227/02/ NL/SH, with the additional financial support of Snecma Moteurs. References [1] B.N. Antar, F.G. Collins, Flow boiling during quench in low gravity environment. Microgravity Sci. Technol. 10 (1997) 118e128. [2] C.J. Westbye, M. Kawaji, B.N. Antar, Boiling heat transfer in the quenching of a hot tube under microgravity. J. Thermophys. Heat Transfer 9 (AprileJune, 1995) 302e307. [3] W.J. Chen, Y. Lee, D.C. Groeneveld, Measurement of boiling curves during rewetting of a hot circular duct. Int. J. Heat Mass Transfer 22 (1979) 973e976. [4] A.K. Kim, Y. Lee, A correlation of rewetting temperature. Lett. Heat Mass Transfer 6 (1979) 117e123. [5] O. Kawanami, H. Azuma, H. Ohta, Effect of gravity on cryogenic boiling heat transfer during tube quenching. Int. J. Heat Mass Transfer 50 (2007) 3490e3497. [6] G.P. Celata, M. Cumo, M. Gervasi, G. Zummo, Quenching experiments inside 6.0 mm tube at reduced gravity. Int. J. Heat Mass Transfer 52 (2009) 2807e2814. [7] Y. Barnea, E. Elias, I. Shai, Flow and heat transfer regimes during quenching of hot surfaces. Int. J. Heat Mass Transfer 37 (1994) 1441e1453.