Void distribution and bubble motion in bubbly flows in a 4×4 rod bundle. Part I: Experiments

Abstract

Lack of local void fraction data in a rod bundle makes it difficult to validate a numerical method for predicting gas-liquid two-phase flow in the bundle. Distributions of local void fraction and bubble velocity in each subchannel in a 4 × 4 rod bundle were, therefore, measured using a double-sensor conductivity probe. Liquid velocity in the subchannel was also measured using laser Doppler velocimetry (LDV) to obtain relative velocity between bubbles and the liquid phase. The size and pitch of rods were 10 and 12.5 mm, respectively. Air and water at atmospheric pressure and room temperature were used for the gas and liquid phases, respectively. The volume fluxes of gas and liquid phases ranged from 0.06 to 0.15 m/s and from 0.9 to 1.5 m/s, respectively. Experimental results showed that the distributions of void fraction in inner and side subchannels depend not only on lift force acting on bubbles but also on geometrical constraints on bubble dynamics, i.e. the effects of rod walls on bubble shape and rise velocity. The relative velocity between bubbles and the liquid phase in the subchannel forms a non-uniform distribution over the cross-section, and the relative velocity becomes smaller as bubbles approach the wall due to the wall effects.

Void distribution and bubble motion in bubbly flows in a 4×4 rod bundle. Part II: numerical simulation

“…This drag model was applied to single bubbles rising through stagnant water in a two by two rod bundle, and the estimation agreed well with their experimental data [14]. The velocity reduction in the near wall region observed in our recent experiments [9] might therefore be predicted by making use of this drag model.…”

“…Fluxes of n G through the boundary between the shaded and white regions are forced to be zero, which ensures that n G = 0 in the white (near wall) region throughout a simulation. Since bubbles near the rod wall were apt to be spherical [9], the minimum distance between the wall and the bubble center is set at d/2. Bubbles in the gap region took distorted prolate shapes and the maximum aspect ratio was 1.35, where the aspect ratio E is the ratio of the maximum vertical dimension to the maximum horizontal dimension of a bubble.…”

“…The volume fraction in a cell, IJK, required for θ I JK n Gm,LMN f m,LMN⇒I JK (9) where θ is the computational cell volume, the subscripts I, J and K are the cell indexes in the x, y and z directions, and L, M and N are the indexes of cells surrounding the cell IJK. The subscript LMN ⇒ IJK denotes the contribution from the cell LMN to IJK.…”

“…Distributions of liquid and gas velocities and void fraction of air-water bubbly flows obtained in the previous experimental study [9] were used for comparison with predictions.…”

“…Although multidimensional simulations of flows in a rod bundle have been carried out [1][2][3][4][5][6][7][8], their validation is still a difficult task due to the lack of detailed experimental data such as distributions of void fraction and velocities. In our previous study [9], we therefore carried out experiments on air-water bubbly flows in a four by four rod bundle to obtain experimental data applicable to the validation of the numerical methods. The experiments showed that the presence of rod walls restricts the motion of bubbles near the walls and affects the bubble shape.…”