Di Muoio, E

Jäger, B

Puech, L

Rousset, B

Thibault, P

Vallcorba, R

Van Weelderen, R

Wolf, P E

CERN Document Server

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHEuropean Laboratory for Particle PhysicsHE II CO-CURRENT TWO PHASE FLOW AT HIGH VAPOR VELOCITIESB. Rousset1, B. Jager1, E. di Muoio1, L. Puech2, P. Thibault2, R. Vallcorba1,R. van Weelderen3 and P.E. Wolf2In the framework of LHC studies, we have performed several experiments on He II co-current two-phaseflow. It was found that for high vapor velocities, the heat exchange capacity between the He II flow andthe pipe wall is significantly better than what can be accounted for by the liquid to wall interface of astratified two-phase flow pattern. This seems to indicate a transition from a pure stratified two-phaseflow into either a partially annular two-phase flow or a stratified two-phase flow including liquiddroplets in the vapor flow or a combination of the two. In the last two cases, it is assumed that liquiddroplets which get dispersed on the tube wall increase the wetted surface. A new facility has beendesigned to analyze this flow behavior. High sensitivity capacitive liquid level sensors glued onto theinner wall of the pipe were used in order to detect a possible semi-annular flow pattern whereas lightdiffraction and scattering were used to detect liquid droplets. Finally, in addition to a circumferentialheat exchange box, local heat exchange boxes located at different azimuth positions are added.Description of this new facility, calibration of the local heat exchange boxes and first results arepresented.1 DRFMC/SBT/CEAG, Grenoble, France2 CRTBT/CNRS, Grenoble, France3 CERN,  LHC DivisionPresented at the 2001 Cryogenic Engineering Conference and International Cryogenic Materials ConferenceCEC/ICMC 200116-20 July 2001, Madison, Wisconsin, USAGeneva, Large Hadron Collider ProjectAdministrative SecretariatLHC DivisionCERNCH - 1211 Geneva 23SwitzerlandLHC Project Report 517Abstract19 October 2001HE II CO-CURRENT TWO PHASE FLOW AT HIGHVAPOR VELOCITIESB. Rousset1, B. Jager1, E. di Muoio1, L. Puech2, P. Thibault2, R. Vallcorba1,R. van Weelderen3, P. E. Wolf21DRFMC/SBT/CEAGGrenoble, 38054, France2CRTBT/CNRSGrenoble, 38042, France3CERN, European Organization for Nuclear Research, LHC Division,C1211 Geneva, 23, SwitzerlandABSTRACTIn the framework of LHC studies, we have performed several experiments on He IIco-current two-phase flow. It was found that for high vapor velocities, the heat exchangecapacity between the He II flow and the pipe wall is significantly better than what can beaccounted for by the liquid to wall interface of a stratified two-phase flow pattern. Thisseems to indicate a transition from a pure stratified two-phase flow into either a partiallyannular two-phase flow or a stratified two-phase flow including liquid droplets in the vaporflow or a combination of the two. In the last two cases, it is assumed that liquid dropletswhich get dispersed on the tube wall increase the wetted surface. A new facility has beendesigned to analyze this flow behavior. High sensitivity capacitive liquid level sensors[1]glued onto the inner wall of the pipe were used in order to detect a possible semi-annularflow pattern whereas light diffraction and scattering[2] were used to detect liquid droplets.Finally, in addition to a circumferential heat exchange box, local heat exchange boxeslocated at different azimuth positions are added. Description of this new facility,calibration of the local heat exchange boxes and first results are presented.INTRODUCTIONIn the framework of LHC cooling scheme studies, we have investigated thethermohydraulic behavior of He II two-phase flow[3,4]. The huge amount of data acquiredon previous Cryoloop experiments confirmed the pressure loss predictions given by theAndristos/Hanratty[5] model. However, apparent excess heat transfer (with respect to purestratified two-phase flow) occurred at high Vgs (superficial vapor velocity), possibly due toan onset of change in flow pattern. The more commonly encountered transition is toannular flow with liquid beginning to creep on the wall. The other one is a transition tomist flow which appears for high Vgs and high qualities. In order to discriminate betweenthese two possible transitions and build a physical model for wall heat transfer, our“Cryoloop” facility was upgraded. Specific devices designed to characterize mist andannular flow were added, whereas heat transfer measurements were maintained andimproved.DESCRIPTION OF THE EXPERIMENTTest LoopT4W2W1T5T14LHe-4.5KPumping unitDPSlope 0 to 1.4%Bellows10 mLTLTJT ValveInlet boxD3 D4 recovery unit HeaterThermometerFlowmeterLevel gaugeCCD cameraLTLaserVisualizationcapacitive levelgauge Heat exchange boxes1.5 mOutlet bathFIGURE 1. Test facility flow schemeLocal heat exchange boxesPressurized HeIItube for  pressure and fill inHeater ThermometerW4W5W6T10T9T8Capacitive level gaugeCCD cameraPyrex tubeLaser AngulardetectionLASER detection300 and 80 KwindowsLaserCross section enligtheningor diffuse enlighteningW3T7Circumferential heat exchange boxT6AngulardetectionFIGURE 2. Details of the new instrumentationThe "Cryoloop II" (figure 1) has already been described previously[4] and mainlyconsists of an inlet box used to create the inlet quality, a 10 m inclinable straight line ofdimensions 42x43 mm, and an outlet saturated bath where excess liquid is evaporated.During a run, total mass flow ( 1m ) is kept constant using a fixed aperture for the J.T. valve.The inlet power ( 2W ) is used to increase step by step the quality. Finally, liquid dry out isobtained at a power doW _2  and used as a means to calculate heat losses ( ossesW1 ) :dosatosses WLmW _211 −=  .                                                      (1)The global measurements include the total mass flow rate (taken at roomtemperature), the saturated temperature, heating powers, temperature increase in thecircumferential heat exchange box when power is on (which gives access to the thermalheat exchange). In addition, three local He II subcooled boxes located at different azimutalpositions (figure 2) have been introduced in order to measure wall heat transfer at thebottom, the middle and the top of the pipe.Four capacitance level gauges glued inside the inner pipe (see ref. 1 for details) wereused to detect liquid film on the pipe wall. Furthermore, interfacial area of liquid dropletsis determined by the analysis of light scattering (see ref. 2 for details).Calibration of the local heat exchange boxesCalibrations were performed in a separate cryostat, where boxes are placed vertically.The boxes were filled with He II at 1 bar. The inner tube was entirely filled with saturatedHe II, so that the wetted perimeter was always equal to 100 %. The heat transfer coefficientof each box was measured for various temperatures (figure 3a). We measured separatelythe thermal conductivity of a copper sample taken from the central tube. Using a ANSYSthermal model, the Kapitza resistance between copper and He II was then adjusted to givean accurate prediction of the experimental data (see figure 3b).Calculation of the wetted perimeter when the inner tube is only partly wetted bysaturated He II is not straightforward and the numerical model (ANSYS code) was used todetermine the relation between wetted inner perimeter and heat removed on each box(figure 4). This model takes into account the contribution of both the liquid and the vaporphase and of the radial conduction through the copper tube wall to determine the heatexchange capacity of a two-phase flow configuration. Due to conduction through thecopper pipe wall, heat flux streamlines are not strictly radial (figure 4). Consequently heatflux on the bottom box varies over 20 % of inner wetted perimeter while 15 % is sufficientto cover surface in front of the box (figure 5). On the other hand heat deposited on middlebox became to be extracted before liquid level on inner tube reached the low part of thebox.It was found that for vapor flow rates involved, heat exchange due to vapor flow wasalways negligible.00.020.040.060.080.10.120 50 100 150 200 250 300 350Heat flux inside one heat exchange box (Watt/m2)∆∆ ∆∆T (K)_T for  a saturated bath at 1.86 KLinear interpolationh=∆W/∆T∆505560657075801.7 1.8 1.9 2 2.1Temperature (K)Heat flux (W/m2 ) ExperimentsAnsys code∆T  pressurized HeII and saturated HeII : 0.02 KFIGURE 3(a). Calibration of thermal heat coefficient for the local boxes (b). Heat flux removed for a fullywetted perimeter00.10.20.30.40.50.60.70.80.910 10 20 30 40 50 60 70 80 90 100Inner wet perimeter (%), case of horizontal interfaceHeat flux exchange /Maximal heat flux exchangeBottom heat exchange boxMiddle heat exchange boxTop heat exchange boxBottom heat exchange boxMiddle heat exchange Top heat exchange boxFIGURE 5. Relation between heat flux removed in local boxes and inner wetted perimeterProcedure for circumferential wall heat transfer capacity measurementIn case of “over wetting” due to liquid droplets impinging on the pipe wall, thethickness of the liquid layer is probably thin and can be dried out by sufficient heat flux. Inorder to check the dry out crisis, heat flux must be varied. Waiting to reach steady state istime consuming and a signal response procedure was adopted. The system is similar to aRC circuit, R being the thermal resistance between annular box and two phase flow and Cthe heat capacity of the box. The describing equation isWShtTShcMTwetwetP 1'' =∂∂+ , where satbox TTT −='                                   (2)boxT  represents the temperature inside the He II pressurized box, satT  the temperatureof the He II saturated flow, M is the mass of superfluid inside the box, Pc  thecorresponding specific heat and h the heat transfer coefficient calculated after calibration.With the assumption of constant Pc  and h, the resolution of equation (1) in the case ofa sinusoidal power injected ( ( )tW ωcos ) leads to( ) ( ) ( )ϕωω ++= tWShatT wet cos111'20, where wetPShcMa =0 and ωϕ 0tan a−=       (3)ϕ  corresponds to the phase shift and the term ( )2011ωa+characterizes thedamping of the system.00.050.10.150.20 20 40 60 80 100 120Heat flux  W3 (W/m2)∆∆ ∆∆T (K)Power injected step by stepSinusoidal power00.050.10.150.20 20 40 60 80 100 120Heat flux W3 (W/m2)∆∆ ∆∆T (K)Power injected step by stepSinusoidal power compensatedfor time lag (18 s)FIGURE 6 Inlet power W2 100 Watt, Total mass flow 6.8 g/s, T° 1.8 K (a). Comparison between steady“step by step” heat flux increases and sinusoidal heat flux (b). Same comparison after phase shift adjustmentTypical values in our case are the following:Volume of the box: 1.048 liter, pressurized He II temperature :1.85 K, and wettedperimeter of 22.5 %, one obtains for a period of 500 seconds a damping factor of 0.976 anda phase shift of 17.6 seconds. As the data acquisition period was equal to one second, the∆T corrected from the shift phase was directly derived from temperature measurementstaken 18 acquisition periods after power measurements.Figures 6a and 6b show comparison between “step by step” heat flux increases andsinusoidal heat flux without and with rough phase adjustment. Agreement for the last oneallows us to calculate the local slope, from which wetting is calculated.RESULTSCircumferential heat exchange boxThis device was first used to check our previous measurements, i.e. the increase ofwall heat transfer at high Vgs. Figure 7 shows the reproducibility of our results. Thediscrepancy for low Vgs is explained by slight differences in slope between the twoexperiments. This point having been established, the box was used to determine whetherthe “over-wetting” could be burnt and the corresponding heat flux needed for this.In order to do this, the wetting was calculated using the local slope of the curve( )3WfT =∆ . For a given heat flux we can assume that a part of it is used to burn thedroplets, while the remaining heat flux acts as if there were no droplets.At 0 Watt, as there was no “extra-wetting”, the given wetting shown on figure 7b isalmost constant and equal to the value indicated by the capacitive measurements. On theother hand, at high velocity (107 Watt and 1.8 K), the wetting decreases drastically as theheat flux goes from a few W/m2 to 80 W/m2 after which it reaches a plateau. However, thevalue of this plateau is not compatible with that obtained by capacitive measurements.2530354010 30 50 70 90 110Inlet power (W)Wet perimeter (%)7.2 g/s,  1.77 K Cryoloop 19986.9 g/s et 1.8 K cryoloop II20253035400 50 100Heat flux (W/m2)Wetting (% )Inlet power : 0 WattInlet power : 107.5 WattFIGURE 7 (a). Repeatability of heat transfer measurements after the installation of the new Cryoloop (10 mlong instead of 20 m) Case of horizontal pipe, with heat flux W3 equal to 20 W/m2 (b). Influence of heat fluxW3 on heat transfer measurements Case of 6.8 g/s, 1.8 K and 1.4 % slope.Local heat exchange boxesFor all our steady state measurements, the liquid level inside the pipe corresponds to awetted perimeter higher than 15 % and the bottom heat exchange box remains always fullywetted. At low Vgs due to heat losses, the middle and top boxes always remain in normalhelium state, which prevents any heat transfer measurements. For high vapor velocities, thefluid in the middle box comes into the superfluid state, whereas the top box remains in thenormal state. This indicates the increase in wetted surface when vapor velocity is increasedand the stratified nature of the process involved. The value found on the middle box for aninlet power of 100 Watt, a total mass flow of 6.8 g/s and a temperature of 1.8 Kcorresponds to an inner wetted perimeter equal to 30 %.Correlation of results coming from the different sensorsFigure 8 shows calculated wetting derived from different measurement methods.Capacitive measurements show a constant decrease of wetting as inlet power grows. Thisindicates that no significant thick film creeps along the inner pipe wall. The capacitivemeasurements are representative of liquid level inside the pipe. Shown also on the samefigure is the difference between thermal measurements and  capactive measurements,which we count as “over wetting”.In ref 2 it has been shown that it is possible to convert the optical measurementsperformed on mist flow at the mid-height of the tube into the liquid-vapor interfacialdensity (Σ ). 22 rn π=Σ , where n represents the number of droplets per unit of volume andr its radius. Laser light scattered (Σ ) is also plotted on figure 8 after scaling all the data byan appropriate factor. It is noticeable that applying a simple coefficient on optical raw datagives such a good correlation with “over-wetting”.Another run is plotted on figure 8b corresponding to a new slope of 1.4 %. Behavioris similar to that observed at zero slope. Residual discrepancies could be due to differencesin total mass flow and saturated temperature.0102030400 50 100Inlet power W2 (W)Wetting (%)Optical measurementsthermal measurements at 20 W/m2Capacitive measurements triangle - square = over wetting0102030400 50 100Inlet power W2 (W)Wetting (%)Optical measurementsThermal measurements at 20 W/m2Capacitive measurementstriangle -squareFIGURE 8 (a). Comparison between different ways to estimate the wetting in the case of horizontal line, 6.9g/s and 1.8 K (b). Case of 1.4 % slope, 6.6 g/s and 1.8 KCONCLUSIONLiquid droplets were found present at high vapor velocities while no thick film wasdetected. Behavior of the droplet flow is consistent with the increase of wall heat transferas both phenomena scale with the vapor velocity. However, there are still some apparentcontradictions with an extra heat exchange due to liquid droplets, e. g. the difficulty tocompletely dry out the droplets. Analysis of all the data acquired is still in progress,especially concerning optical measurements.ACKNOWLEDGEMENTSWe would take the opportunity of this paper to thank Philippe Lebrun at CERN forfruitful discussions.REFERENCES1. Thibault P.and al., "Probing the Wetted Perimeter in a Pipe-Flow Experiment Using a Capacitive Sensor"to be published in the proceeding of this conference2. Di Muoio E. and al., "Optical Investigations of HeII Two Phase Flow " to be published in the proceedingof this conference)3. Rousset B. and al., "Stratified Two-phase Superfluid Helium Flow-Part: I" Cryogenics 37 (1997)4. Rousset B.and al., "heII two phase flow in an inclinable 22 m long line" Advances in CryogenicsEngineering 45 (2000)5. Andritsos, N. & Hanratty, T.J., "Influence of Interfacial Waves in Stratified Gaz-liquid Flows", AIChEJournal (1987), Vol. 33, 444-454

He II Co-Current Two Phase Flow at High Vapor Velocities

He II Co-Current Two Phase Flow at High Vapor Velocities

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