Search

This paper presents the experimental results and analyses of the structure velocity of air-water two-phase flow in a 3 × 3 rod bundle channel. A total of 56 flow conditions were tested and investigated for rod-gap, sub-channel, rod-wall and global regions of rod bundle geometry. The experimental tests were carried out under bubbly and cap-bubbly flow regimes with superficial gas and liquid velocities of 0–1 m/s and 1–1.7 m/s, respectively. The conductivity probes were set at different heights to measure the global and local void fractions. The structure velocity of air-water two-phase flow is the average bubble velocity calculated by the method in this study. The structure velocity were determined by utilizing the cross-correlation technique to analyze the time lags of the bubbles passing through the conductivity probes. The results of this study indicated that the structure velocity may increase with increasing superficial gas and liquid velocities. In low superficial gas velocity region, the structure velocity may first slightly increase and follow by a sudden jump which appear in most regions. After the sudden jump, the structure velocity may keep increasing mildly. The present structure velocities have been compared with the area-averaged gas velocities predicted by the drift flux model, and it appears that most structure velocities show a good agreement with the averaged gas velocities from the drift flux model after the jump.

In this study, upward air–water two-phase flow tests were carried out in a 3 cm diameter pipe under atmospheric pressure, and over 3000 data points were collected from a wide range of superficial gas and liquid velocities (⟨jg⟩ ≈ 0.02–30 m/s and ⟨jf⟩ ≈ 0.02–2 m/s) for the investigation of flow regime identification. The probability density function (PDF) of transient void fraction signals and its full-width at half-maximum (FWHM) were obtained and used for analysis and data classification. Considering the features of PDF profiles, the flow conditions can be classified into four regions, which show a fair agreement with the existing flow regime maps in general trends. Furthermore, by examining the FWHM distributions, two more regions with high-FWHM (HF) values were identified as the transitions of higher-flow bubbly-to-slug and slug-to-churn flows as well as most portion of churn flow, and a valley region next to the HF regions can express the transition of churn-to-annular flows. Overall, six groups of flow conditions can be classified based on the present methodology, and each group can be corresponding to specific flow regimes or transition regions. This study can provide a simple and efficient way for flow regime identification.

In order to examine and predict the enhancement of heat transfer performance under ultrasonic vibration, experimental tests were carried out and empirical correlations have been developed in this study. A stainless-steel circular heater rod was first tested for heat transfer performance with and without ultrasonic vibration in a thermostat water tank. The loaded heat flux was about 7.6 × 103–7.1 × 104 W/m2, which covered the heat transfer regimes from natural convection to subcooled boiling. The size of water tank was 165 × 310 × 100 mm, and the liquid subcooling was controlled as 50–70 K during the tests. The ultrasonic vibration was generated by three transducers attached to the tank bottom with a total power of 150 W and an ultrasonic frequency of 40 kHz. Effects of height, heat flux and liquid subcooling were investigated and analyzed. In the present test ranges, the maximum heat transfer increment can be up to 1557 W/m2 K, and the maximum h-increment ratio can be about 3.01 (301%). Furthermore, the present data were used to develop empirical correlations for heat transfer enhancement, and the present correlations can predict the experimental database with an averaged error of 14.1%. The present study can provide the experimental evidences and a prediction method for heat transfer enhancement under ultrasonic vibration.

This study investigated the void fraction characteristics under low superficial gas velocity (Jg) and low superficial fluid velocity (Jf) conditions in a 3x3 rod bundle geometry. Three arrangements of conductivity probes were set to measure the void fraction at various cross-sectional regions, including rod-gap, sub-channel and rod-wall regions. The experimental tests were performed under the flow conditions of Jg = 0-0.236 m/s and Jf = 0-0.142 m/s, and the time-averaged void fractions were recorded at each flow condition. It was observed that while the superficial gas velocity increases, the small bubbles started to cluster together and become big bubbles. As the superficial fluid velocity increases, the local void fractions of the three test regions will get closer and the bubble distribution will be more uniform across the cross section.

In this paper, an experimental facility of a subcooled boiling flow loop with a vertical exciter has been constructed and tested to investigate the transient variations of subcooled boiling flow parameters under forced excitations. Various heat fluxes (q"= 6204 and 11029 W/m2) were loaded to the test section under stationary and different forced excitation conditions f= 0, 0.51, 1.06 and 1.62 Hz). The transient signals of differential pressure (DP) and fluid temperature were recorded during the tests, and the fast Fourier transform (FFT) was utilized to analyze the transient data. It was observed that under low frequency excitations (f= 0.51 Hz), the variations of pressure and temperature due to forced excitations were relatively minor and may not be clearly identified by the FFT. Whereas in higher frequency excitations f= 1.06 and 1.62 Hz), the excitation frequencies can be clearly shown in FFT results, which suggests that the flow can be oscillated and the transient variations were observable under the higher frequency forced excitations.