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  • 1. Ma, Y.
    et al.
    Tian, W.
    Su, G.
    Qian, L.
    Zhang, Youjia
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
    Huang, Y.
    Wang, Y.
    Qiu, S.
    Theoretical research on flow instability in parallel channels under motion conditions2013In: Proceedings of the 21st International Conference on Nuclear Engineering --2013: presented at 2013 21st International Conference on Nuclear Engineering, July 29-August 2, 2013, Chengdu, China, ASME Press, 2013Conference paper (Refereed)
    Abstract [en]

    In motion conditions, in addition to gravitational acceleration, a new acceleration was developed and it was added to the thermal hydraulics characteristics in flow channels. The additional acceleration leads to the different thermal hydraulic characteristics and will trigger the flow oscillation and even flow instability in parallel channels. In order to study the effect of the additional acceleration on the flow oscillation, the corresponding physical models are established in this work. Through the deduction of the mathematical model, the code for flow instability under motion conditions with Gear algorithm is developed. The flow oscillation curves, critical power, marginal stability boundary (MSB) are obtained. After comparison and analysis, it is found that some motion conditions lead to flow periodic oscillation. Different flow passage position results in different oscillation amplitudes. The marginal stability boundaries (MSB) under different motion conditions fit well, that is, the effect of motion conditions on MSB is small. Number of channels has little effect; however, channel arrangement influences the flow in every channel. These conclusions are of great significance in marine reactor design.

  • 2. Wang, Ke
    et al.
    Zhang, Youjia
    KTH, School of Engineering Sciences (SCI), Physics.
    Gong, S.
    Bai, B.
    Ma, W.
    Dynamics of a thin liquid film under shearing force and thermal influences2017In: Experimental Thermal and Fluid Science, ISSN 0894-1777, E-ISSN 1879-2286, Vol. 85, p. 279-286Article in journal (Refereed)
    Abstract [en]

    Study of liquid film dynamics promotes understanding the critical heat flux (CHF) of boiling heat transfer, which occurs as the liquid layers (micro-layer and macro-layer) near the heater wall lose their integrity. Since the measurement at micro-scale is a challenge, and further complicated by the chaotic nature of the boiling process, profound knowledge on the thin liquid film dynamics is not well documented in the existing literature. In the present paper, we employ a confocal optical sensor system to study the dynamics and the integrity of a thin liquid film sheared by the co-flowing air from above and heated from below in a horizontal aluminum channel. The results indicate that the entrainment governs the liquid film thinning process under adiabatic or lower heat flux conditions, whereas the evaporation becomes more pronounced in a higher heat flux system. The detailed evolution of liquid film is discussed. Based on our experimental observations, the critical film thickness of an integral film is related to the condition of the heating surface and the heat flux. For a specific surface, the critical film thickness remains constant under a defined heat flux and increases with the increasing heat flux. A spectrum analysis is also implemented to analyze the film instability. It is concluded that the heat flux is the dominant factor to govern the film instability compared with the effect of differential velocities of gas and liquid flow.

  • 3.
    Wang, Ke
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
    Zhang, Youjia
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
    Ma, Weimin
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
    An experimental study on the dynamics of a liquid film under shearing force and thermal influence2015In: International Topical Meeting on Nuclear Reactor Thermal Hydraulics 2015, 2015, Vol. 5, p. 3668-3677Conference paper (Refereed)
    Abstract [en]

    A profound knowledge of dynamics and instability of a liquid film is crucial for the thorough study on the physical mechanisms of boiling crisis (i.e., departure from nucleation boiling - DNB, and dryout), which is important to the performance and safety of light water reactors. To address this, a series of tests were carried out at various water and air flow rates under atmospheric pressure and different heat fluxes. A confocal optical sensor system was employed to investigate the dynamics of a liquid film in a horizontal aluminum channel. The liquid film was sheared by co-flowing air from above and heated from below. We obtained the experimental data about the variation of thickness of the liquid film under different flow conditions and tried to analyze how it is affected by liquid/gas flow rates, shearing force and heat flux. We found that the shear force, evaporation and the generation of the bubbles enhanced the instability of the liquid film. We also found that the occurrence of the rupture was random and the critical thickness at the rupture increased with increasing heat flux. The spectrum analysis indicated that the effect of the shear force on the liquid film instability became weak when the liquid film is very thin, but the heat flux always enhanced the instability of the liquid film.

  • 4.
    Zhang, Youjia
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
    Ma, Weimin
    KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
    Gong, S.
    An experimental study on liquid film dynamics and interfacial wave of air-water two-phase flow in a horizontal channel2013In: 2013 21st International Conference on Nuclear Engineering, ICONE 2013: Volume 4: Thermal Hydraulics, ASME Press, 2013Conference paper (Refereed)
    Abstract [en]

    This study is concerned with liquid film dynamics and stability of annular flow, which plays an important role in understanding film rupture and dryout in boiling heat transfer. The research work starts from designing and making a test facility which enables the visualization and measurement of liquid film dynamics. A confocal optical sensor is applied to track the evolution of film thickness. A horizontal rectangular channel made of glass is used as the test section. Deionized water and air are supplied into that channel in such a way that an initial stratified flow forms, with the liquid film on the bottom wall. The present study is focused on characterization of liquid film profile and dynamics in term of interfacial wave and shear force induced film rupture under adiabatic condition. Based on the experimental data and analysis, it is found that given a constant water flowrate, the average thickness of water film decreases with increasing air flowrate, while the interfacial wave of the two-phase flow is intensified. As the air flowrate reaches a critical value, a localized rupture of the water film occurs.

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