Critical heat flux enhancement in microgravity conditions coupling microstructured surfaces and electrostatic field

We run pool boiling experiments with a dielectric fluid (FC-72) on Earth and on board an ESA parabolic flight aircraft able to cancel the effects of gravity, testing both highly wetting microstructured surfaces and plain surfaces and applying an external electric field that creates gravity-mimicking body forces. Our results reveal that microstructured surfaces, known to enhance the critical heat flux on Earth, are also useful in microgravity. An enhancement of the microgravity critical heat flux on a plain surface can also be obtained using the electric field. However, the best boiling performance is achieved when these techniques are used together. The effects created by microstructured surfaces and electric fields are synergistic. They enhance the critical heat flux in microgravity conditions up to 257 kW/m2, which is even higher than the value measured on Earth on a plain surface (i.e., 168 kW/m2). These results demonstrate the potential of this synergistic approach toward very compact and efficient two-phase heat transfer systems for microgravity applications

Physical quantities relevant to characterize the flow in transient subcooled flow boiling crisis investigations

International audienceThe BORAX-type accident is a key scenario in the safety design of pool-type experimental reactors. It considers a reactivity insertion large enough to initiate an exponential power escalation with a period as small as a few milliseconds. This has motivated new investigations of the transient boiling crisis under thermal-hydraulic conditions relevant to such reactors: in the channels formed by planar fuel plates, the highly subcooled water flows at high velocities under moderate pressures. In the literature addressing the flow boiling crisis, authors generally present the value of the critical heat flux of their investigations as a function of the mass flux, the bulk velocity, the flow rate or the Reynolds number. It usually induces representations which are complicated to draw generalizable conclusion. In this communication, we propose to investigate the transient flow boiling crisis as a function of the different parameters and to evaluate the most suitable one to represent the results when different geometries are involved. In order to address this concern, we analyzed experiments diagnosed with high speed visible and infrared cameras conducted at moderate pressure (1 MPa), high subcooling (100 K), different mass fluxes (from 0 to 8,000 kg.m−2.s−1) and values of power escalation period (5, 20 and 100 ms), and two channel widths. This investigation shows that the representations of these tests with the critical heat flux as a function of the different flow parameters do not give clear graphs and no generalizable conclusion can be drawn. However, using the concept of the mantle thickness introduced in the model of Nop et al. [1] permits to propose clearer representations of the transient flow boiling crisis and enables to identify two regimes. For the low shear velocities, the system is boiling-driven and depends on the power escalation period. For the high shear velocities, the system is flow-driven and does not depend on the power escalation period

Physical quantities relevant to characterize the flow in transient subcooled flow boiling crisis investigations

International audienceThe BORAX-type accident is a key scenario in the safety design of pool-type experimental reactors. It considers a reactivity insertion large enough to initiate an exponential power escalation with a period as small as a few milliseconds. This has motivated new investigations of the transient boiling crisis under thermal-hydraulic conditions relevant to such reactors: in the channels formed by planar fuel plates, the highly subcooled water flows at high velocities under moderate pressures. In the literature addressing the flow boiling crisis, authors generally present the value of the critical heat flux of their investigations as a function of the mass flux, the bulk velocity, the flow rate or the Reynolds number. It usually induces representations which are complicated to draw generalizable conclusion. In this communication, we propose to investigate the transient flow boiling crisis as a function of the different parameters and to evaluate the most suitable one to represent the results when different geometries are involved. In order to address this concern, we analyzed experiments diagnosed with high speed visible and infrared cameras conducted at moderate pressure (1 MPa), high subcooling (100 K), different mass fluxes (from 0 to 8,000 kg.m−2.s−1) and values of power escalation period (5, 20 and 100 ms), and two channel widths. This investigation shows that the representations of these tests with the critical heat flux as a function of the different flow parameters do not give clear graphs and no generalizable conclusion can be drawn. However, using the concept of the mantle thickness introduced in the model of Nop et al. [1] permits to propose clearer representations of the transient flow boiling crisis and enables to identify two regimes. For the low shear velocities, the system is boiling-driven and depends on the power escalation period. For the high shear velocities, the system is flow-driven and does not depend on the power escalation period

Physical quantities relevant to characterize the flow in transient subcooled flow boiling crisis investigations

International audienceThe BORAX-type accident is a key scenario in the safety design of pool-type experimental reactors. It considers a reactivity insertion large enough to initiate an exponential power escalation with a period as small as a few milliseconds. This has motivated new investigations of the transient boiling crisis under thermal-hydraulic conditions relevant to such reactors: in the channels formed by planar fuel plates, the highly subcooled water flows at high velocities under moderate pressures. In the literature addressing the flow boiling crisis, authors generally present the value of the critical heat flux of their investigations as a function of the mass flux, the bulk velocity, the flow rate or the Reynolds number. It usually induces representations which are complicated to draw generalizable conclusion. In this communication, we propose to investigate the transient flow boiling crisis as a function of the different parameters and to evaluate the most suitable one to represent the results when different geometries are involved. In order to address this concern, we analyzed experiments diagnosed with high speed visible and infrared cameras conducted at moderate pressure (1 MPa), high subcooling (100 K), different mass fluxes (from 0 to 8,000 kg.m−2.s−1) and values of power escalation period (5, 20 and 100 ms), and two channel widths. This investigation shows that the representations of these tests with the critical heat flux as a function of the different flow parameters do not give clear graphs and no generalizable conclusion can be drawn. However, using the concept of the mantle thickness introduced in the model of Nop et al. [1] permits to propose clearer representations of the transient flow boiling crisis and enables to identify two regimes. For the low shear velocities, the system is boiling-driven and depends on the power escalation period. For the high shear velocities, the system is flow-driven and does not depend on the power escalation period

The boiling crisis of water under exponentially escalating heat inputs in subcooled flow boiling at atmospheric pressure

© 2020 Elsevier Ltd We investigated the boiling crisis of water under exponentially escalating heat inputs using a specially-designed experimental apparatus featuring high-speed infrared thermography to measure the time-dependent temperature and heat flux distributions on the boiling surface, and high-speed video diagnostics to image the bubble growth process. We conducted flow boiling experiments with water at atmospheric pressure on a plate-type heater installed in a 3 × 1 cm2 cross section channel. We tested various values of flow velocity (corresponding to Reynolds numbers from 0 to 35,000), water subcooling (from 10 to 75°C) and rate of power rise (corresponding to exponential power escalation periods, τ, from 1.5 to 500 ms). At long periods critical heat flux (CHF) is independent of the power escalation period and the boiling processes appear to be physically similar to steady state. As the period decreases, the CHF values tend to increase following an asymptotic 1/τ trend. The mechanism that determines this trend depends on subcooling. For high subcooling, CHF monotonically increases as the period decreases, and the DNB occurs through a fully developed nucleate boiling process. By contrast, for low subcooling, CHF first increases as observed for high subcoolings, then decreases, and finally increases again as the period decreases. We observe that such non-monotonic transition is due to a change in the boiling crisis mechanism. Specifically, for very short periods and low subcooling, the boiling crisis happens during the growth of the very first generation of bubbles, which never detach from the heated surface, shortly after the onset of nucleate boiling. In such cases, fully-developed boiling is not achieved

Geopolitical games in Eurasian regionalism: ideational interactions and regional international organisations

This paper analyses the ideational interaction underlying attempts at regional integration and cooperation in Eurasia. While the ideas and values of the European Union have been relatively well-studied within the theory of Europeanisation, the key concepts, ideas, values and principles driving Eurasian regionalism have remained out of the main focus of Western scholarship. This paper aims to shed more light on this ideational basis of Eurasian regionalism by unveiling the discourse developed in Russian scholarship and available only in Russian. Understanding interactions between institutions will always remain partial as long as the ideational interaction is not addressed. Such concepts as ‘integrative mentality’, as a segment of the wider category ‘foreign policy mentality’, and the theory of neo-Eurasianism have been incorporated into Russian political discourse and therefore affect public opinion through specific interpretation of economic, political and cultural processes in the EU’s near neighbourhood and the EU as an actor. The analysis presented in this paper indicates the development of new ideational competition, in addition to the well-documented geopolitical one. The paper also aspires to contribute to emerging research on public support to governmental strategic choices and self-legitimation of international organisations in Eurasia