Studies on Geyser Boiling Characteristics of Sodium Heat Pipes

鈉熱管間歇沸騰特性研究

Student thesis: Doctoral Thesis

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Author(s)

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Detail(s)

Awarding Institution
Supervisors/Advisors
  • Chin PAN (Supervisor)
  • Taosheng LI (External person) (External Supervisor)
  • Ming JIN (External person) (External Supervisor)
Award date23 Sept 2024

Abstract

Amid the rapid advancement of China's deep-sea and deep-space exploration initiatives, traditional energy systems driven by chemical and solar sources are approaching performance thresholds. In response to this challenge, nuclear reactor-based power systems, which are known for their superior environmental adaptability, long-endurance capabilities, and high power density, have emerged as potent solutions to overcome these limitations. Within this scope, heat pipe-cooled nuclear reactors are characterized by their solid-state nature and simplified system architecture, particularly in scenarios requiring rapid load adaptation, making them ideal for deep-sea exploration and terrestrial mobile nuclear power sources. High-temperature heat pipes, which are pivotal for heat transfer in these reactors, are known for their robust thermal conductivity, remarkable isothermality, and passive heat dissipation capabilities. However, under certain operational conditions, the Geyser Boiling (GB) within the heat pipe's liquid pool can compromise the stable heat transfer efficiency of the heat pipe. This research delves into high-temperature sodium heat pipes with a wick structure, examining the unstable two-phase flow characteristics within and dissecting the Geyser Boiling Phenomena (GBP) through experimental and numerical simulation approaches.

Initially, a testing platform was established to investigate the GBP characteristics within the high-temperature heat pipe. Observations from experiments highlighted that during the startup phase, the movement of the frontal face results in temperature fluctuations in the condensation section of the heat pipe. Furthermore, these experiments identified three distinct modes of the GBP, classified based on the oscillation patterns of the wall temperatures. An in-depth analysis of the genesis and oscillatory characteristics of these GB modes leads to a comprehensive understanding of the mechanisms driving GB in heat pipes. Additionally, the study meticulously examines the influence of the inclined angle and heating power on the properties of GBP. The results reveal that, at a constant heating power, increasing the inclined angle initially reduces and then increases both the period and amplitude of the GB. Beyond an inclination angle of 45°, these parameters are stabilized. At the same inclined angle, an increase in heating power causes a minimal change in the amplitude of the GB for angles less than 10°, with a noticeable decrease in the period. For inclined angles greater than 10°, both the period and amplitude initially increase and subsequently decrease.

By leveraging insights from experimental observations, this study develops a Temperature Calculation Model (TCM) and a Geyser Boiling Multiphase Flow Model (GBMFM) to scrutinize the heat transfer dynamics of high-temperature heat pipes and the underlying causes of GBP. The TCM, by integrating the source terms in adjacent meshes at the interface between the vapor chamber and wick, simulates the vaporization and condensation dynamics within the heat pipe. The model employs the Schrage model in conjunction with the Clausius-Clapeyron equation, which enables the dynamic computation of phase-change metrics in response to temperature differentials. The GBMFM, initialized with the derived temperatures of the vapor chamber and wall from the TCM, adopts the Volume of Fluid (VOF) model to simulate the multiphase flow within the heat pipe. Distinguishing between evaporation and boiling, the two disparate vaporization processes, this model meticulously captures the cyclical formation, growth, coalescence, and disruption of slug bubbles during the GB.

Finally, numerical simulations were conducted to probe the GBP within high-temperature sodium heat pipes, concentrating on how the surface tension, contact angle, and nucleate boiling superheat degree influence the fluid flow dynamics during GB. Additionally, the study delves into the impact of external factors such as gravity, pipe diameter, and filling ratio on the periodicity and amplitude of GB. Numerical simulations of bent heat pipes further assessed the bending effects on GBP. The findings reveal that the properties of the working fluid play a pivotal role in the manifestation of GBP. Notably, an increase in the surface tension significantly broadens the extent of GB, leading to larger and more persistent slug bubbles, thereby exacerbating the boiling phenomena. Conversely, a larger contact angle tends to reduce the slug bubble size, thereby shortening the GB periods. Elevating nucleate boiling superheat postpones the commencement of GB without altering its severity. Regarding external factors, the gravity proves to be a crucial inducer of GB; in its absence, the heat transfer capacity of the heat pipe diminishes and GB is suppressed. As the heat pipe diameter is increased, the pipe will delay the GB to be fully-developed, and the resulting slug bubbles became shorter and more prone to rupture. The augmented filling ratio intensifies the temperature oscillation amplitude of the GB while decreasing its frequency. The simulations of bent heat pipes indicate that bending can effectively mitigate GB occurrences.

In conclusion, this study investigates experimentally and numerically the internal dynamic characteristics during GBP and heat transfer mechanisms of high-temperature sodium heat pipes. The results of the study reveal the significant effects of the inclination angle, heating power, working fluid properties, and external parameters on the GB, and suggest effective methods for suppression of GBP. These insights not only enhance the understanding of GB within high-temperature sodium heat pipes but also offer valuable guidelines for designing and optimizing efficient nuclear power systems for deep-sea and deep-space exploration.

    Research areas

  • heat pipe cooled reactor, high-temperature heat pipe, geyser boiling phenomenon, VOF multiphase flow, slug flow