Unsteady Bulk Boiling in a Tall and Slender Water Pool

摘要

Bulk-boiling processes in large pools are often observed to be extremely non-uniform in time and space, eventually leading to local 'geyser'-like eruptions in turn with relatively quiet, non-boiling temporal phases. For the investigation of such phenomena we have built a tall and flat experimental facility (pool) with transparent walls to allow optical observation of the bulk boiling process at atmospheric pressure. It is heated at its bottom so it can be kept at almost constant temperature conditions for several hours. After heat-up of the entire pool to almost saturation temperature the unsteady boiling process begins randomly in time and space with the appearance of discrete eruptions, associated with the growth of a large number of vapor bubbles. Optical image processing is applied to visualize and track individual bubbles, as well as for the determination of their geometrical parameters. The bubble growth due to the decrease in static pressure is initiated well above the heated surface, i.e. in the 'bulk' region of the pool. The mechanism, which initiates an eruption of bubbles, is identified as a growth-shrinking oscillation of initially small bubbles in the region of upwards rising higher-temperature plumes due to thermal instability and fluid turbulence. It is modeled by a new empirical formula for the number-density of micro-bubbles. A CFD model for the bulk-boiling process has been developed and implemented into the CFX-12 code using the two-fluid formulation of the liquid-vapor two-phase flow. The new formula is applied in the numerical model and the experimentally observed growth and shrinking oscillation is simulated in good qualitative agreement.