Heat can cool near critical fluids
- Paper number
IAC-08.A2.4.5
- Author
Dr. Daniel Beysens, CEA, France
- Coauthor
Dr. Yves Garrabos, CNRS, France
- Coauthor
Dr. Thomas Fröhlich, Astrium GmbH, Germany
- Year
2008
- Abstract
Keywords: Critical phenomena, Adiabatic heating, Piston effect. We report experiments in very compressible fluids (as e.g. fluids near their critical point) performed under earth gravity and under weak acceleration in a spatial environment. The experimental configuration is similar to an industrial heat exchanger. The fluid is enclosed in a cell with its walls maintained at constant temperature. A heat pulse is sent in the fluid by a point source (thermistor). The fluid is confined in a temperature-controlled cylindrical sample that is one arm of an optical interferometer. Two or three thermistors, depending of the set-up, are used to measure the local fluid temperature. One thermistor is also used to send a heat pulse with power in the range 10-100 mW. The pulse duration can be monitored. The density variations are visualized by the interferograms. During heating, the diffusive thermal boundary layer around the thermistor expands and pressurizes the outer bulk fluid, resulting in a homogeneous and very fast adiabatic heating. In general, without convection, the thermal behaviour is governed by the balance of heat flux between the heat source boundary layer, which compresses the bulk fluid, and the (thermostated) wall boundary layer, which depresses the bulk fluid. This phenomenon is due to the thermo-compressible nature of the “Piston effect” in this very expandable and compressible medium. In the present experiments where heating pulses are performed under acceleration, at the end of the pulse, a long transient is observed where the bulk fluid temperature goes significantly below the initial temperature. This phenomenon lasts for an appreciable period of time, corresponding to the diffusive destruction of the wall boundary layer. The balance between the heating boundary layer at the heat source and the cooling boundary layer at the thermostated wall is thus modified. In this sense, heating the near critical fluid results in cooling, which appears in contradiction with the 2nd principle of thermodynamics (at least during the transient). It is therefore a spectacular demonstration of the thermo-mechanical nature of the heat exchange by the piston effect. This cooling transient effect, which is all the more pronounced that the acceleration is large, is analyzed by using a simple 1-dimensional model with empirical heat losses.
- Abstract document
- Manuscript document
IAC-08.A2.4.5.pdf (🔒 authorized access only).
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