Several experiments have reported that departure from nucleate boiling (DNB) in a pressurized water reactor (PWR) fuel assembly often takes place just upstream of a grid spacer. Since DNB inception is to be tightly related with an extremely high void fraction in the vicinity of a heated wall, it is speculated that one of the causes of DNB inception is lateral migration of bubbles toward a fuel rod due to a flow induced by the grid spacer. Therefore, experiments are conducted to investigate the motion of small bubbles and liquid velocity profiles just upstream of a grid spacer in a two by three rod bundle. Tap water and air are used for the liquid and gas phases, respectively. The bulk liquid velocity ranges from 1.0 to 4.0 m/s and experiments are conducted at atmospheric pressure and room temperature. Trajectories of single small bubbles are recorded using a high-speed video camera. Mean liquid velocities and turbulence distributions upstream of the spacer are calculated based on 10,000 instantaneous velocity distributions measured by Particle Image Velocimetry (PIV). Discussion on the effects of the gird spacer on bubble motion is made based on the difference between the measured bubble trajectories and liquid streamlines. Numerical simulations using a three-dimensional two-way bubble tracking method are also carried out. The calculated trajectories of single bubbles and liquid velocity distributions are compared with the measured data. The main conclusions obtained are as follows: (1) just upstream of the grid spacer, bubbles are apt to migrate toward the rods and accumulate in the vicinity of the rod surface, (2) a centrifugal force caused by a curved streamline just upstream of the grid spacer is the main cause of lateral migration of bubbles toward the rods, and (3) the two-way bubble tracking method gives good predictions for trajectories of small bubbles upstream of the grid spacer and for liquid velocity profiles.

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