Experimental Investigations on Thermal Hydraulics and Bubble Dynamics of Subcooled Flow Boiling with Artificial Seawater

海水次冷態流動沸騰的熱水力和氣泡動力研究

Student thesis: Doctoral Thesis

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Award date16 Sep 2021

Abstract

Water is a common fluid employed as reactor coolant for most of nuclear power plants due to low cost, wide availability, and high heat capacity. In the Fukushima Daiichi accident caused by the tsunami following a powerful earthquake on 11th March 2011, the loss of heat sink occurred. Seawater was eventually injected into the nuclear reactor for more than one week as an emergency measure. In this dissertation, thermal hydraulics and bubble dynamics of subcooled flow boiling with 3.5 wt% artificial seawater under different conditions are investigated experimentally.

An experimental loop mainly consisting of 1-m vertical annulus and 1-m riser is designed and constructed. The hydraulic diameter of the annulus and riser is 0.01 and 0.02 m, respectively. Subcooled flow boiling of 3.5 wt% artificial seawater are investigated in this loop and compared with that for de-ionized water to see the effects of salt solution. Boiling behaviors through a high-speed video camera reveals significantly different two-phase flow patterns in the heated annulus between artificial seawater and de-ionized water at the inlet temperature (Tin) of 85℃ and 90℃. Bubbles in de-ionized water tend to merge each other to form large bubbles or slug bubbles, while bubble coalescence is absent in seawater and bubble foam may eventually be formed. However, the rupture of foam flow and the following development of slug bubble at Tin of 55℃, 65℃, and 75℃ for boiling number ranging from 3×10-4 to 11×10-4 together with flow instability is revealed as well.

The subcooled flow boiling heat transfer coefficients with the mass flux of 564, 874, and 1200 kg/m2s in artificial seawater are slightly higher than those in de-ionized water at Tin of 90℃, possibly due to strong bubble agitation. Similar effect is found for cases at Tin of 85℃. The heat transfer coefficients of artificial seawater at Tin of 90℃ agree well with the predictions of the correlation of Papel in the early literature. However, such coexistence of foam flow and slug flow at the inlet temperature of 55-75℃ enhances significantly on subcooled flow boiling heat transfer, leading to a large wall temperature drops up to 10℃, compared with 1℃ for those cases with stable foam flow in artificial seawater and slug flow in de-ionized water.

Subcooled boiling results in the quick rise of void fraction in the flow direction downstream of the net vapor generation point, as evident through flow visualizations and the evaluation of Saha and Zuber’s model and Pan’s model. Fluid temperature measurements at the inlet and exit of the annulus enable the evaluation of qualities and corresponding void fractions at the exit of the annulus through energy balance. Calculated void fraction as high as 50-70% is demonstrated and is consistent with the prediction of models for cases at the Tin of 90℃. Correspondingly, the pressure drop rises very quickly after the heat flux exceeding the initiation of boiling two-phase flow and reaches a plateau of constant pressure drop in artificial seawater, indicating the appearance of bubble foam. Generally, the pressure drops of artificial seawater under the same operating condition are much higher than those of de-ionized water in two-phase boiling region due to its unique bubbly flow pattern.

Bubble characteristics are important to the two-phase flow dynamics and two-phase flow simulation. Different from the known slug flow in subcooled flow boiling of de-ionized water, fined bubbly flow and foam flow are found to be dominant in artificial seawater at high inlet temperatures. Through image analysis, the present study conducts bubble characteristics measurements of such bubbly/foam flow of subcooled flow boiling in artificial seawater at Tin of 55-85℃, including bubble Sauter mean diameter, bubble equivalent diameters and distributions of bubble size, bubble shapes and bubble orientations. The data set is compared with that in de-ionized water.

The images for analyses and measurements of Image J are acquired from a high-speed camera with 4000 frame/s. The results indicate the Sauter mean diameter of de-ionized water is larger than that of artificial seawater by 16-30%. Good agreements with two existing correlations prove the accuracy of current measurements and provide a way to predict the bubble characteristic in such systems with fined bubbles or bubble foam. Further, the effects of inlet temperature, mass flux and heat flux on distributions of bubble size, bubble aspect ratios and bubble orientations are explored as well. The results reveal the two-phase foam flow structure in artificial seawater is comparatively rigid and inflexible, compared with that in the de-ionized water. Increasing the inlet temperature to 85℃ may keep the Sauter mean diameter of bubbles in artificial seawater at a stable value. The only peak of the unimodal bubble size distributions of artificial seawater is at 0.15-0.36 mm, while the possible other peak of bimodal bubble size distributions of de-ionized water is at 0.78-0.99 mm or 0.57-0.78 mm. Bubbles are mostly either nearly/perfectly parallel to the heated surface or perpendicular to the heated surface. The results also reveal that spherical shape for bubbles smaller than 0.2-0.25 mm, rigid ellipsoid shape for intermediate bubbles with diameter greater than 0.40 mm and less than 1.0 mm -2.7 mm, and distorted large bubbles, depending strongly on the inlet temperature. The data of bubble aspect ratio appear to be quite scattering due to the complicated nature of subcooled flow boiling. However, its general variation trend with Eötvös number or the product of Eötvös number and vapor Reynolds number agrees fairly well with some well-known correlations in the literature. Moreover, the bubble shape can also be predicted well with an extrapolated boundary of the Grace diagram on the plane of gas Reynolds number versus Eötvös number.

Further, the nature of the foam rupture and evolution of slug flow is studied using the Image J as well. Geometric characteristics including Feret diameters, equivalent diameters, angles, and aspect ratios are measured and discussed. Motion characteristics including growth rates, velocities and rising trajectories of slug bubble are also measured and analyzed. It is revealed that the slug bubble grows approximately linearly. However, the bubble growth rate demonstrates large variation and crucial effects on heat transfer. The trajectories of the slug bubbles reveal zigzag motions with large radial displacement. A theory is proposed to explain the ruptures of bubble foam.

The flow instability experiments are conducted by decreasing mass flux gradually under a specific condition of inlet temperature and heat flux. The results reveal that the artificial seawater is more susceptible to the flow instability and demonstrates 12%-79% larger mass fluxes for onset of flow instability (OFI) due to the unique two-phase flow pattern of densely bubbly flow or bubble foam flow during the subcooled flow boiling of artificial seawater. After OFI, unstable bubble foam with slug bubbles resulting from foam ruptures is found in artificial seawater resulting in significant pressure drop oscillations at lower inlet temperature of 55℃, 65℃ and 75℃. The existing criteria or correlations may not predict OFI for artificial seawater. A two-phase flow instability map is established on the plane of phase change number versus subcooling number.