Conference Agenda

The heat transfer mechanisms of flow boiling are still unclear. Recent research shows that evaporation of microlayer contributes to bubble growth in pool boiling. In order to investigate the microlayer heat transfer and dynamics, the 650nm-thick indium-tin-oxide (ITO) film is deposited on a 1mm-thick sapphire substrate to heat the fluid. The thickness of the microlayer underneath the bubble was measured using high-speed laser interferometry (LIF) and the transient temperature distribution on the wall was measured by an infrared (IR) camera. Another camera was used to capture the side bubble image. The three devices worked synchronously. The experiment was conducted at 0.11MPa with deionized water as the fluid, covering heat fluxes of 110-174.4 kW/m², subcooling degrees of 0-11.2 °C, and liquid flow velocity of 0.12-0.27 m/s. The typical bubble behavior was analyzed, including bubble growth, sliding, departure and wall temperature distribution. The inverse heat conduction problem (IHCP) of the boiling surface was solved based on conjugate gradient method (CGM) for wall heat flux partitioning. The wall heat flux partitioning outcome has revealed that the formation and evaporation of microlayer have an important effect on the growth of flow boiling bubbles.

Annular two-phase flow occurs in multiple types of nuclear reactors under normal operating conditions and accident scenarios. Annular flow typically features high flow quality and relatively thin liquid film around the fuel elements in nuclear reactors. This flow structure has significant safety implications since the liquid film could break into rivulets due to vaporization and entrainment. Important parameters to characterize annular flow consist of liquid film thickness, wave velocity on the surface of the liquid film, the size and velocities of entrained liquid droplets, and entrainment fraction and rate. This study contributes to existing database of annular flow with both liquid film and droplet measurements conducted with an air-water test facility. The experimental test section consists of a vertical pipe with an inner diameter of 9.525 mm and a length of 2.9 meters, as well as two measurement ports designed for the measurement of liquid film thickness and surface wave velocity by two sets of parallel-wire conductance probes placed at each port. To capture liquid droplet size and velocity, two high-speed cameras are used to capture the droplet field as they exit the test section outlet after extraction of liquid film. The annular flow testing matrix consists of an array of inlet conditions with superficial gas velocity ranging from 7.80 m/s to 34.91 m/s and superficial liquid velocity ranging from 0.09 m/s to 0.44 m/s, spanning the entrainment and non-entrainment annular flow regimes. Acquired data has been used to validate and improve existing correlations or closure models for annular flow.

The experimental characterization of thin liquid films in annular flow regime is central in many engineering applications, ranging from chemical industry to refrigeration systems, and in particular to cooling of light water nuclear reactors, where it is crucial for safety thermal analysis of boiling water reactors and for validation of system codes as well as CFD codes. However, characterization of thin films in annular flow is particularly challenging given the flow turbulence and the high non-linearity of the free-surface behaviour. Particularly, film thickness and wave characteristics are very challenging to be measured, with many correlations from the literature showing substantial offsets in predicting the same quantities under seemingly close boundary conditions. In this paper, results of simultaneous measurements of vertical upward annular flow film in an adiabatic test section is presented using three different techniques. These consist in: a) total internal reflection method, providing highly resolved local thickness measurements; b) X-ray attenuation method, providing interfacial topology and void fraction that can be converted into thickness information; and c) a conductivity film sensor providing high speed thickness and wave information with a spatial resolution of 2 mm. By performing independent calibrations, the three techniques are cross-validated within the corresponding uncertainties. To the authors’ knowledge, this is the first time that the three measurement techniques for film thickness are combined, thus constituting a unique benchmark. The three techniques complement each other and provide highly reliable measurements of annular flows, which are also compared to existing correlations available in the open literature.

Safety evaluation of offshore floating nuclear plants requires boiling two-phase flow data under heaving conditions with long wave period (10–20s). Void feedback effect necessitates investigating the mutual influence of oscillating coolant flow and thermal power.

This study conducted forced convection boiling experiments with sinusoidally oscillating coolant flow velocity and thermal power in an annular double wall channel with an electrically heated rod at atmospheric pressure. The inner diameter of flow channel was 20 mm, and outer diameter of the heater rod was 10 mm, with a heated length of 2000 mm. The axial power profile of heater rod was uniform. The time-averaged inlet flow velocity and linear power density were maintained 2 m/s and 15.7 kW/m respectively. Experiments considered no oscillation and 0.05 and 0.1 Hz sinusoidally oscillating inlet flow velocities and thermal powers with ±15% zero-peak amplitude. The distributions of two-phase flow parameters and flow regimes were identified using an optical void probe with radial traverse at two heights and stereo high-speed camera. An optical fiber sensor with a sheath tube was mounted on the heater rod surface to obtain the axial temperature distribution and capture heat transfer characteristics.

The void fraction oscillated corresponding to the inlet oscillations, significantly at the top of the heated area and slightly in the middle, and its amplitude was affected by the oscillation frequency. This behavior may be attributed to the oscillatory acceleration of the fluid flow. The radial distribution showed less bubble formation in the middle rather than at the top.

The experimental characterization of thin liquid films in multiphase annular flows is particularly relevant to the safety analysis of the cooling channels of light water reactors and for validation of best-estimate system codes as well as CFD codes. The total internal reflection method (TIRM) is an optical method known for decades for being able to non-intrusively measure film thickness of a wide range of fluids flowing over a transparent wall. This measurement is performed by recording with a camera the reflected circular pattern of a laser beam pointed to the flow. The transparent wall is often curved (such as in a pipe), which leads to a potential loss of information, since part of the reflected pattern has to be discarded from each frame because of optical distortions from the curved wall surface. This is also the case for the TIRM experiments performed in our laboratory on adiabatic vertical upward annular flows in a circular section pipe. However, in this work, an innovative approach is developed to use the information on the shape of the distortion rather than discarding it, thus maximizing the value of the measurement and improving accuracy. This is achieved thanks to a thorough data analysis backed up by a previously validated optical ray tracing simulation that replicates our TIRM experiments including the distorted patterns. From the combination of simulation and experiments, new insights are gained into the potential and the limits of standard TIRM film thickness measurements applied to curved pipes.