Transparent heater for study of the boiling crisis near the vapor–liquid critical point
Introduction
One of the most efficient ways to transfer heat from solid to gas is boiling phenomena in heated liquid in contact with a solid heater. Such a heating process attracts strong attention from engineers, especially when boiling crisis occurs. Boiling crisis is a transition from a regime where vapor bubbles nucleate separately on the heater wall to a regime where the heater wall is entirely covered by a continuous vapor film. When formed, the vapor film reduces drastically the heat transfer at the heater wall, because of the low gas thermal conduction. The boiling crisis can then produce irremediable damage of the exchanger by melting of the solid heater. This transition phenomenon appears when the heat flux exceeds a threshold value, called the critical heat flux and denoted CHF in the following. The estimation of CHF is essential in the industrial heat exchanger design and management.
In a two-phase critical fluid, a bubble spreading over the heater surface was initially observed [1] while crossing the critical point to reach the homogeneous domain, thanks to the microgravity conditions offered by the Russian MIR station. We have then proposed to investigate by experimental observation while approaching the gas–liquid critical point if the recoil force mechanism can be relevant to give a better understanding of the triggering mechanism at the origin of the boiling crisis. Indeed, we have successively shown by modeling and numerical simulation [2], [3] that the thrust of production of vapor can support this gas spreading. This recoil force mechanism was also evidenced by the analysis of the results of a preliminary ground-based experiment where gravity magnetic compensation was produced in a small volume of para-hydrogen near its critical point. With remaining steady gravity forces of order (where is the constant acceleration on Earth's), this last experiment have confirmed that the microgravity conditions which cancel buoyancy forces and generate three-dimensional spheroidal shape of the gas bubble are irreplaceable powerful tools for studying the heat transfer near the gas–liquid–solid contact line.
Our microgravity experimental program to investigate the boiling crisis was planned for the CNES-DECLIC facility [4] that will be used on board the International Space Station (ISS). For that purpose, we have developed new optical pressurized cells designed for future experimental observations of the boiling crisis at low heat fluxes, very close to the CHF. In particular, a new pressurized optical cell integrates an in situ device as a form of a transparent resistive layer ( area), appropriate for light transmission observation of the boiling in liquid films.
High-resolution ( pixels) and high-speed optical diagnostics are synchronized with in situ temperature measurements, and adjusted to the selected monitoring rate of the thermal stimuli produced by the transparent resistive heater. This optical cell is implemented in the Alice-like insert (ALI) of the CNES-DECLIC facility [4], [5], which allows the injection of a thermal power 0–3.4 mW and simultaneous data acquisition at high frequency (2 kHz) from 3 thermistors (THERMOMETRICS B10) located inside the fluid. The heat power per unit area can then be controlled in the range –. Heating period can be adjusted, with a minimum of 43 ms. Resolution from the B10 sensors is better than , corresponding to a temperature accuracy around 0.1 mK. In the unexplored range of low heat fluxes, we will expect simplified analysis of the boiling observations, thanks to the low temperature gradients and critical phenomena universality [6] near the critical point of sulfur hexafluoride, using the high capabilities of the CNES-DECLIC instrument [5].
The paper is organized as follows. Section 2 presents the new design for the pressurized optical cell (called DOC for direct observation cell) dedicated to study the boiling crisis in non-homogeneous critical fluids, during the first increment planned for DECLIC utilization on board the Japanese module of ISS. 3 Heating in the vertical wall configuration, 4 Heating in the pressurized “cooker” configuration show some results obtained during preliminary tests performed on Earth's with both the engineering and the flight models of the ALI Inserts, using the heating device either in vertical (Section 3) or horizontal (Section 4) position.
Section snippets
Direct observation cell (DOC)
Pictures of the flight model (FM) and engineering model (EM) of the optical cells observed by light transmission are given in (a, b) and (c, d) parts of Fig. 1, respectively. Each fluid sample volume corresponds to a cylindrical volume of inner diameter and respective inner thickness for the FM cell and for the EM cell. Such a fluid sample configuration being observed mainly by wide field light transmission, the cell is called “Direct Observation Cell” (DOC).
Heating in the vertical wall configuration
Fig. 3(a) gives a schematic configuration of the optical cell where the light transmission is made along the horizontal axis of the fluid cylinder while the vertical transparent resistive layer is then parallel to the earth gravity acceleration. We use the label “VC” for this vertical heater configuration which corresponds to the nominal up–down position of the DECLIC instrument on Earth. In the two-phase regime, the film heater is directly wet by the (film and bulk) liquid and can initially
Heating in the pressurized “cooker” configuration
Fig. 8 (left) gives the second schematic configuration of the optical cell where the light transmission is made along the vertical axis of the fluid cylinder, while the horizontal transparent resistive layer acts at the bottom of the vertical fluid cylinder. The heating device is then perpendicular to the earth gravity acceleration, only in contact with the liquid part of the two-phase cell configuration. This pressurized “cooker” configuration which is obtained for one specific right angle
Conclusion
We have proposed to investigate the triggering mechanism at the origin of the boiling crisis by direct observation of a gas bubble spreading over a transparent heater surface, during the heating of a critical two-phase sample cell filled by . The main originalities of these new investigations are provided by monitoring of low heat fluxes and control of the liquid–vapor properties, by fine adjustment of the distance to the critical point. Especially using an in situ heating device as a form
Acknowledgements
The authors acknowledge the financial support from CNES (Centre National d’Etudes Spatiales). They gratefully thank Eric Georgin for his assistance during the cell integration and Jean-Pierre Manaud, Iyad Saadeddin and Guy Campet for development and processing of the thin resistive layer made of Sn alloy oxide. They also thank the CNES-DECLIC project team, especially project manager Gerard Cambon, and the associated DECLIC industrial teams (ASTRIUM-ST, COMAT, EREMS, IDEAS, SODERN,
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