Maurice-Xavier Francois

He II heat transfer through superconducting cables electrical insulation

Abstract For NbTi magnets cooled by superfluid helium (He II), the most severe heat barrier comes from the electrical insulation of the cables. Tests on electrical multi-layer insulations, made of Kapton®, dry fiber and epoxy resin impregnated fiberglass tapes, indicate that heat transfer is influenced by He II contained in the insulation. Electrical insulation can be considered as a composite material made of a solid matrix with a complicated helium channels network. For several insulations, this network is characterized by steady-state heat transfer experiment through an elementary insulation pattern. Measurements in Landau regime for low temperature difference (10−5–10−3 K) and in Gorter–Mellink (GM) regime for higher temperature differences permit to determine an equivalent He II channel cross-section (10 −6 m 2 ) with an equivalent channel thickness (25 μm). We use the assumptions that He II heat transfer through the channels network and conduction in the insulation are decoupled and that the channels length is determined from the insulation overlap. It is observed that He II heat transfer is competing with conduction in the insulation. Furthermore, the measurements reveal an anomaly of heat transfer in the vicinity of the λ temperature which is associated to the phenomenon of λ-point depression.

Response of a thermoacoustic refrigerator to the variation of the driving frequency and loading

The performance of a thermoacoustic refrigerator subjected to variable loading was analyzed experimentally and the data were compared with those obtained using a computational model. The computational model relies on one-dimensional cross-section-averaged equations discretized using the network analogy. The thermoacoustic refrigerator was modeled by dividing it into 1 mm long slices in the direction of the acoustic axis. The hot heat exchanger of the thermoacoustic refrigerator was maintained at ambient temperature and the temperature of the cold heat exchanger was varied to achieve temperature differences of ΔT=0, 5 and 10 K along the stack. The cooling load was measured and calculated for these temperature differences while varying the driving frequency between 30 and 65 Hz. The contribution of the progressive and stationary waves and the losses on the thermoacoustic heat flow was computed and discussed.

Steady-state transition boiling on thin wires in liquid nitrogen. The role of Taylor wavelength

Transition boiling on a thin wire in liquid nitrogen is investigated. Results presented in the form of a classical boiling curve exhibit some discontinuous branches in the mixed boiling region, located between the negative slope area and the film boiling regime. A model is proposed in order to explain this new feature. It is based on steady coexistence of two areas, one in nucleate and one in film boiling. The main hypothesis concerns the wire length covered with film boiling which is supposed to be a multiple of the Taylor wavelength, this last quantity being commonly regarded as the interval separating two neighbouring vapour pockets in the film boiling regime. The temperature of each area is allowed to vary in a range compatible with the respective regime. The comparison with experimental results shows that some branches develop with a constant proportion of each regime. Furthermore, this model shows that no steady branch is observed when the wire length covered with film boiling is less than one Taylor wavelength. The only experimental points occurring in this case are those located in the negative slope area.

Steady-State Heat Transfer in He II through Porous Superconducting Cable Insulation

The LHC program includes the study of thermal behavior of the superconducting cables wound in the dipole magnet cooled by superfluid helium (He II). Insulation of these superconducting cables forms the major thermal shield hindering the He II cooling. This is particularly a problem in magnets which are subjected to thermal loads. To investigate He II heat transfer processes an experimental model has been realized which creates a one-dimensional heat transfer in such media. Insulation is generally realized by wrapping around the superconducting cable a combination of different kind of Kapton® tapes, fiber-glass impregnated by epoxy resin or Kevlar® fiber tapes. Steady-state heat transfer in He II through these multi-layer porous slabs has been analyzed. Experimental results for a range of heat flux show the existence of different thermal regimes related to He II. It is shown that the parameters of importance are a global geometrical factor which could be considered as an equivalent “permeability” related to He II heat transfer, the transfer function f(T) of He II and the thermal conductivity of the slab. We present and analyze results for different insulations as a function of the temperature.

Experiments and Valve Modelling in Thermoacoustic Device

In a so called heat driven thermoacoustic refrigerator, using either a pulse tube or a lumped boost configuration, heat pumping is induced by Stirling type thermodynamic cycles within the regenerator. The time phase between acoustic pressure and flow rate throughout must then be close to that met for a purely progressive wave. The study presented here relates the experimental characterization of passive elements such as valves, tubes and tanks which are likely to act on this phase relationship when included in the propagation line of the wave resonator. In order to carry out a characterization — from the acoustic point of view — of these elements, systematic measurements of the acoustic field are performed varying various parameters: mean pressure, oscillations frequency, supplied heat power. Acoustic waves are indeed generated by use of a thermoacoustic prime mover driving a pulse tube refrigerator. The experimental results are then compared with the solutions obtained with various one‐dimensional linear...

TRANSIENT BOILING ON THIN WIRES IN LIQUID NITROGEN

Transient boiling experiments are carried out on a small horizontal brass wire (O=25μm) immersed in a liquid nitrogen bath under atmospheric pressure. Heat pulses of variable intensity ranging between 60% of the peak heat flux value and the peak heat flux are supplied to the wire in order to investigate the onset of a premature transition to film boiling. Various configurations are observed depending on the power level. At low heat flux, steady nucleate boiling is achieved in a few hundreds of milliseconds preceeded by a large temperature overshoot. Steady nucleate boiling is still observed on the wire at high heat flux but the transient period is drastically extended and more than ten seconds are sometimes required to achieve steady state. This feature is peculiar to the pulse heating procedure, which induces large wire temperature overshoots. Since the geometry is a thin wire, these conditions are favorable for the occurrence of mixed boiling states. Due to the important lifetime of these particular states, revealed by this transient heating procedure, this phenomenon has frequently been confused with a premature transition to film boiling.

Experimental validations of a new thermoacoustic simulation software CRISTA

A new simulation software CRISTA has been developed at LIMSI‐CNRS. It is based on the Rotts equations approximation. It computes all thermal and acoustic parameters of a given thermoacoustic device whose geometry is previously designed with another program TADESIGN. To realize the simulation, the user needs only to define a drive ratio at some point of the system and the heat exchanger temperatures. Note that for a prime mover the hot heat exchanger temperature is a simulation result. Every converged solution guarantees the physical principles. Moreover, CRISTA allows computing the quality factor of the resonator. The experimental validations have been successfully performed on different devices coupled to the same prime mover: a simple RLC load, an acoustic amplifier, a pulse tube refrigerator and a lumped boost pulse tube refrigerator.

The VALTA project: Full scale conversion of CHP engine flue gas heat into electricity

The VALTA project originated in France and lead by HEKYOM aimed to design and build such a downstream energy converter to be applied to a commercial genset, converting 70kW heat flue gas heat into 15 kW electricity. The equipment is supposed to convert exhaust gas enthalpy in the temperature range of 350°C to 200°C into acoustic energy and from there into electricity. The VALTA project was partially funded by ADEME, www.ademe.fr, in a specific program for ”efficiency improving in industrial processes.

Numerical simulation of a thermoacoustic wave amplification

We performed a numerical study of the thermal and physical phenomena occurring in thermoacoustic wave generators. The goal of the simulation is to predict the amplification due to thermoacoustics of a wave initially of small amplitude. Therefore, we focus on the stack and the two heat exchangers, which we call the active cell, which is acoustically compact. The resonator area is split into two parts: the active cell, in which heat transfer takes place, and a resonator, in which the flow is acoustic. The flow in the two‐dimensional active cell can be approximated as a low Mach number viscous, conductive flow, subjected to spatially uniform pressure fluctuations. This model is formally derived using asymptotic expansions in terms of Mach number. The focus here is heat transfer between two successive stack plates. The two‐dimensional time‐dependent problem resulting from this model is solved numerically. Outside the active cell, flow in the resonator is described by a reversible acoustic one‐dimensional mode...

PIV contribution for measuring acoustic and streaming flow in thermoacoustic systems, using phase average dynamics

The present study deals with the experimental challenge of the measurement of the velocity field generated by a thermoacoustic wave. The system consists in a cylindrical standing‐wave resonator, filled with gas confined at high mean pressure, driven by a thermoacoustic prime‐mover. The axial and radial components of fluid velocity are measured using Particle Image Velocimetry (PIV) with an optical flow technique. The average cycle of acoustic oscillation of the velocity field is reconstructed from a temporally under‐sampled set of PIV snapshots, using an embedding method for building out a suitable phase space based on Singular Value Decomposition (SVD). This reconstruction allows us to extract both oscillation component of the velocity field (with the harmonic content) and time‐averaged component of velocity (streaming flow). The measurements are confirmed using a second experimental procedure, based on a classical phase‐averaged method: velocity measurements are synchronized with the pressure signal, th...