Superfluid plug as a control device for helium coolant

New results have been obtained on the characteristics of the superfluid plug as a nonmechanical control device for supplying cold helium vapour on demand from a container of superfluid helium. The superfluid plug is a device which has been proposed for space applications to serve as a phase separator for liquid helium in the superfluid state. Typical plugs are made of a porous material having pores of one to ten μm in diameter. The experimental arrangement is such that one side of the plug is in contact with the superfluid liquid helium while vapour at a low pressure (of order 1 to 10 torr) is maintained on the other side.The data reported here are for a plug with approximately 5 μm diameter pores. Temperatures, pressures, and flow rate were monitored during the experiment. A theoretical background and steady state data are presented on mass flow rates and pressures as a function of liquid temperature.The typical response of the flow rate to a change in heat input from a heater is an exponential rise or fall to an equilibrium flow rate which is proportional to the amount of heat input. The time constants of the exponential changes were measured for two heater control modes under study. The study has included an investigation of the important parameters effecting the dynamic response of the plug including the superfluid properties, plug material properties, plug pore size and plug permeability. Operating temperatures from 1.5 K to the lambda point were investigated and heating rates up to two watts were applied. These tests serve to demonstrate that the superfluid plug can be employed as a flow control device in a control system designed to provide coolant on demand.     

He II phase separaton with slits and porous plugs for space cryogenics

Superfluid helium (He II) is an excellent coolant for space instruments like infrared detectors which need temperatures down to 2 K A He II phase separator is a crucial part of the cryogenic system at zero-gravity: it keeps the liquid phase separated from the vapour, by using the fountain effect in narrow flow channels between the He II storage under a heat load and the (cooler) vent line pumped by the space vacuum. The boil-off gas is used as a heat intercept and leaves the system at the ambient temperature which is close to 300 K on the Space Shuttle. A number of research groups are now in the middle of investigations of so called porous plugs as phase separators which are fine pore sinter bodies of cm dimensions.This paper compares and discusses the different research efforts and tries to unify seemingly contradictory results within a simple theory deduced from the phase separating properties of single straight slits. According to the two-fluid model including Gorter-Mellink friction, the flow properties depend strongly on the internal geometry, which makes quantitative predictions for porous plugs difficult.Suggestions are made with regard to room temperature measurements of permeability, effective porosity and the like and with regard to a low temperature experimental programme.     

Porous plug and superfluid helium film flow suppressor for the soft X-ray spectrometer onboard Astro-H

Suppression of superfluid helium flow is critical for the Soft X-ray Spectrometer (SXS) onboard Astro-H, to achieve a life time of the liquid helium over 5 years. The superfluid film flow must be sufficiently small, compared to a nominal helium gas flow rate of the SXS . For this purpose, four devices composed of a porous plug, an orifice, a heat exchanger, and knife edge devices will be employed based on the experience of the X-ray microcalorimeter (XRS for X-Ray Spectrometer) onboard Suzaku. The porous plug is a phase separator of the liquid and gas helium. A potential film flow leaking from the porous plug is suppressed by the orifice. Almost all the remaining film flow evaporates at the heat exchanger. The knife edge devices stop the remaining film flow by using atomically sharp edges. In this paper, we describe the principle and design of these four devices.     

Numerical simulation of cavitating flow of liquid helium in venturi channel

The fundamental characteristics of the two-dimensional cavitating flow of liquid helium through a venturi channel near the lambda point are numerically investigated to realize the further development and high performance of new multi-phase superfluid cooling systems. First, the governing equations of the cavitating flow of liquid helium based on the unsteady thermal nonequilibrium multi-fluid model with generalized curvilinear coordinates system are presented, and several flow characteristics are numerically calculated, taking into account the effect of superfluidity. Based on the numerical results, the two-dimensional structure of the cavitating flow of liquid helium though venturi channel is shown in detail, and it is also found that the generation of superfluid counterflow against normal fluid flow based on the thermomechanical effect is conspicuous in the large gas phase volume fraction region where the liquid-to-gas phase change actively occurs. Furthermore, it is clarified that the mechanism of the He I to He II phase transition caused by the temperature decrease is due to the deprivation of latent heat for vaporization from the liquid phase.     

Analysis of helium II thermal links for instrument cooling in low gravity

The Low Temperature Microgravity Physics Facility (LTMPF) is a reusable, cryogenic facility that will accommodate a series of low temperature experiments to be conducted at the International Space Station. The facility will use a He II cryostat to cool the instruments. Some configurations of the science instruments in the cryostat will require an enhanced thermal link between the He II bath and parts of the instruments. Such an enhanced link can be made with plumbing filled with He II. This paper reports the results of analysis that was performed using the BATC proprietary helium flow software called SUPERFLO, on four different concepts for this link. The four concepts analyzed were: a simple tube with the heated end closed, a closed end tube with a porous plug at its entrance, a closed end tube filled with capillary tubes, and a porous plug driven flow loop. It was found that the concepts that used a porous plug were more robust since they were much less prone to boiling. This is due to the low gravity which causes all of the liquid in helium tank and plumbing to be very close to saturated conditions unless a porous plug is used to create a thermomechanical pressure. The effects of varying system parameters such as a acceleration, heat flux, pore size and tube size were also investigated and the results are reported.     

Study of the design limits of porous plug type helium phase separators

The design limits of five porous plugs developed for use in He II phase separation were studied experimentally. The performance graphs are presented in the range of bath temperatures 1.5 – 2.0 K and for a pressure drop up to 12 mbar. The data are compared with test results at ambient temperatures and will help to design porous plug type phase separators more precisely.     

Effect of property variations on the operation of phase separators for superfluid helium

Many low-gravity space missions have used porous plugs to keep He II in a dewar while venting the vaporized gas to space. The operational state of a He II phase separator is determined by the interplay of several competing physical processes. It appears that one of these phenomena has not been included in past published models of a separator. When He II flows through a porous material that sustains a thermal gradient, large pressure maxima and moderate temperature maxima occur in the profiles. The transition of the superfluid component to the normal component causes the maxima. The ratio of the density of the two components is a thermodynamic property and the transition between the components adjusts the ratio to the local value of the pressure and temperature. This can be a large effect with the pressure maxima reaching ten times the saturation pressure. We present a model for a He II phase separator, similar to previous models, but with the property variation process included. We derive formulas that show the conditions when the effect is significant and the extent of the changes from constant property models. The property variation phenomenon is modeled using a modified Green’s function approach with step functions. It is iterative on the pressure, temperature and velocity profiles starting with the constant property profiles. This process generates a power series expansion of the pressure, temperature and velocity profiles. Two iterations are sufficient for nearly all applications. The predictions of the model are compared with reported measurements and the agreement is good.     

He II heat transfer through random packed spheres: Pressure drop

Heat flow induced pressure drop through superfluid helium (He II) contained in porous media is examined. In this experiment, heat was applied to one side of a He II column containing a random pack of uniform size polyethylene spheres. Measured results include steady state pressure drops across the random packs of spheres (nominally 35 μm, 49 μm, and 98 μm diameter) for different heat inputs. Laminar, turbulent, and transition fluid flow regimes are examined. The laminar permeability and equivalent channel shape factor are compared to our past studies of the temperature drop through He II in the same porous media of packed spheres. Results from the pressure drop experiments are more accurate than temperature drop experiments due to reduced measurement errors achieved with the pressure transducer. Turbulent results are fitted to models with empirically derived friction factors. A turbulent model considering only dynamic pressure losses in the normal fluid yields the most consistent friction factors. The addition of the laminar and turbulent heat flow equations into a unifying prediction fits all regimes to within 10%.     

Helium II Confinement with Aerogel

After a proposal to use capillarity and Van der Waals forces in aerogel to prevent superfluid helium motion in space missions that carry sensitive gradiometers, we have investigated the behaviour of superfluid helium when only partially filling an aerogel sample. We discuss here the effect of gravity on He II distribution in aerogel. We present a way to investigate it, based on measurements of the tortuosity of liquid–vapour interface and the adsorption isotherm, together with the results of an experiment performed by means of a torsion pendulum. The observed high tortuosity of the liquid–vapour interface for pressure values below saturation, shows that He II in aerogel assumes a configuration where capillary forces are indeed able to bar even the liquid motion driven by 1-g gravity.     

Fixed slit geometry passive phase separator for superfluid helium

A fixed slit geometry passive separator for superfluid helium (He II) was developed and tested. Six different phase separator test models were designed and built, three of them with 10 μm slit width and three with 8 μm width. The slit flow length was 8 mm for all test phase separators. The slit forming materials were stainless steel, copper and Zerodur. The phase separator flow behaviour was tested with He II in the temperature range 1.6–2.0 K. The flow characteristic was found to depend on the slit width, the channel geometry and the channel forming material, with copper yielding a much higher flow rate than stainless steel and Zerodur. This behaviour is attributed to the much higher thermal conductivity of copper. One phase separator was also tested for its dynamic behaviour with transient temperature change of the He II bath.     

Characterization of superfluid helium transfer

Steady state flow rate and friction factor of superfluid helium (He II) were measured under controlled experimental conditions. The driving pressure difference was due to the saturated vapour pressure difference and a small hydrostatic pressure head; it ranged from 1 to 35 mm Hg (0.13 to 4.7 kPa). Previous studies were restricted primarily to isothermal flow and small He II column height difference. In this study the temperature ranged from 1.5 K to T_γ. The stainless steel transfer tubes with 0.87 mm and 1.18 mm internal diameters were under an adiabatic condition. Several tube lengths were studied, the results were universal. For comparison, the transfer of He I was also investigated with the same apparatus. Characteristics on the flow rate and friction factor are presented and discussed.     

Characterisation of porous ceramic plugs for use in electrochemical sensors

In electrochemical sensors like pH-, reference- or ion-selective electrodes a porous ceramic plug (or diaphragm) maintains the conducting junction with the test solution. These liquid junctions should have a resistance as low as possible meanwhile avoiding leakage through the junction. Five porous magnesium stabilised zirconium oxide plugs with different porosity's and pore size (distribution) were investigated as liquid junctions. The physical properties of these porous plugs were investigated with SEM and Mercury Intrusion Porosimetry. Important working conditions of these porous plugs are the resistance of the porous plug filled with an electrolyte and the contamination speed through these porous plugs, both for the test solution as the reference solution. The first property was measured by a 4-wire resistance measurement. The second property was measured by measuring the flow through rate of the reference electrolyte through the plug. It was shown that an optimal plug i.e., low leakage and high conductivity through the membrane, had a high porosity and relative small pores (0.25 m). A simple mathematical model based on the Hagen-Poiseuille equation was developed to describe the porous plug characteristics. It was shown that mathematical calculation of the porous plug resistance was in good agreement with experimental results.     

Temperature dependence of gas conductance through porous silicon carbide plugs

A simple technique for gas flow stabilization in vacuum systems is described. The gas conductance through porous silicon carbide plugs is shown to fall by a factor of 2.5 for the gases He and Ne as the plug temperature is increased from 77K to 800 K. The conductance falls by a factor of 2 for Ar, Kr, N₂ and CO₂ between room temperature and 800 K. The negative temperature coefficient of gas conductance through the plugs is satisfactorily explained by a simple analytical expression. It is suggested that the presence of clay binder material used in the manufacture of the plugs is a major factor in determining the conductance variations.     

Non-linear vapour—liquid phase separation including microgravity effects

Vapour—liquid phase separation, by means of porous plugs, for He II vessels has been investigated with emphasis on the non-linear transport regime. The data are indicative of a normal fluid flow ‘bottleneck’ which controls in the pore size range of the order of 1 μm. The results are described by a phenomenological equation for large pores, suitably modified to take care of the pore-size dependent entropy transport. The latter is coupled, via a two-fluid model and first law constraints, to the mass throughput of the plug. Zero gravity constraints suggest that this mode is dominant once the terrestrial gravity force has been removed.     

Tortuosity and Dissipation of Liquid 4He in Aerogel

No Heading Flow of liquid ⁴He through silica aerogel has been studied by means of a torsional oscillator. Preliminary results on the tortuosity and dissipation of the ⁴He flow in 88%-porous aerogel are compared with earlier measurements on a 92%-porous sample where a transverse sound resonant technique was used. A hydrodynamic model for saturated superfluid helium in porous media is presented and its predictions are compared with the experimental results.PACS numbers: 67.40.HF, 67.40 Pm, 61.43.Gt     

Development of porous plug phase separator and superfluid film flow suppression system for the Soft X-ray Spectrometer onboard ASTRO-H

ASTRO-H is the sixth Japanese astronomy satellite scheduled for launch in 2014. The Soft X-ray Spectrometer instrument is onboard ASTRO-H. This is a 6 × 6 array of X-ray microcalorimeters with an energy resolution of <7 eV at 0.5–10 keV. Superfluid liquid helium is utilized as a part of the cooling system. To retain the liquid helium in the tank under zero-gravity, a porous plug phase separator made of sintered stainless is used. Since the vapor mass flow rate is only 29 μg/s, any additional superfluid film loss influences the lifetime of the liquid helium. Therefore, a film flow suppression system consisting of an orifice, a heat exchanger, and knife edge devices is adopted based on the design used for the X-ray Spectrometer onboard Suzaku. The film flow will be suppressed to <2 μg/s, sufficiently smaller than the vapor flow rate. In the present investigation, the design and ground experiments of a helium vent system composed of the porous plug and film flow suppression system are presented. The results show that the phase separation and the film flow suppression are satisfactorily achieved.     

Comparison of centrifugal and fountain effect pumps

The efficiency of a pumping system is defined in terms of energy flows into and out of a control volume surrounding the pump. It is shown that the centrifugal pump power requirement is affected little by the heat leaks expected in a planned He II transfer system. In contrast the power requirement for a superfluid fountain effect pump is greatly dependent on the thermal conduction through both the porous plug and the downstream transfer line. If the downstream conduction is negligible, the efficiency of a fountain effect pump will be significantly less than that of available centrifugal pumps.     

Review paper: The hydrodynamics of helium II film

After an introduction to the hydrodynamical properties of He II, a survey is given of the flow properties of superfluid helium in the film. Special attention is given to some unsolved problems, such as: has rotating He II (ω<ω _c1 ) a shrunken temperature-dependent parabolic meniscus and is a stationary moving He II film thinner than a static film? A review will be given of the present state of affairs both from an experimental and theoretical point of view.     

Compression of3He by refluxing4He: A model for computing HEVAC effects in3He-4He mixtures

This article comprises the first part of a study concerning the effect that a flow of gaseous⁴He has on the concentration distribution of³He atoms in the presence of a super fluid film. We present a simple model in which hydrodynamic effects in the gas phase (diffusion and viscosity), and local thermodynamic equilibrium with a superfluid film are considered. Results are derived and discussed for the simple case in which a heat flow is sustained along a cylindrical tube lined with a helium film. This heat flux drives a superfluid flow in the film, and a corresponding counterflow of⁴He in the vapour. The pressure, temperature,³He concentration, and film thickness profiles along the tube are computed. Over a wide range of conditions, a dramatic reduction of the³He concentration, a large temperature increase, and a spectacular film thinning towards hotter regions are predicted to result from a heat flow. The results of a series of experiments supporting this model will be presented in a forthcoming article.Unité de recherche de l'Ecole Normale Supérieure et de l'Université Pierre et Marie Curie, associée au CNRS (URA 18).     

Numerical study of flow and heat transfer of superfluid helium in capillary channels

A modified two-fluid model is adopted to study flow and heat transfer of superfluid helium in a microchannel with a diameter as small as that of a superleak in a fountain effect pump. Variable properties of superfluid helium and energy dissipations due to the two-fluid mutual friction and the friction at the channel wall are fully taken into consideration. It is found that the normal fluid component flow is not trivial even in a channel with diameter of a micrometre, and that there exists an optimum diameter for the maximum mass flow rate. The flow of superfluid helium through a channel with different temperatures at the ends differs considerably from that of a Newtonian fluid. The strong dependence of the thermodynamic properties on temperature and pressure, as well as the internal-convection mechanism are found to be the causes of the unique flows.