Onset Of Significant Void Research Articles

A comparison of predictive models for the onset of significant void at low pressures in forced-convection subcooled boiling

The predictive models for the onset of significant void (OSV) in forced-convection subcooled boiling are reviewed and compared with extensive data. Three analytical models and seven empirical correlations are considered in this paper. These models and correlations are put onto a common basis and are compared, again on a common basis, with a variety of data. The evaluation of their range of validity and applicability under various operating conditions are discussed. The results show that the correlations of Saha-Zuber (1974) seems to be the best model to predict OSV in vertical subcooled boiling flow.

Prediction of the Local Pressure Gradient during Partially Developed Subcooled Boiling in Research Reactors of High Power Density

A model is presented for predicting the pressure gradient in partially developed subcooled boiling of water for velocities from 15 to 30 m/s and inlet peaked, nonuniform axial flux profiles with channel average flux values of 6 MW/m2. The partially and fully developed boiling regions are considered separately, however; the same general modeling technique is used for both. Several correlations for the void fraction at onset of significant void are considered, and their effect on the channel pressure drop is evaluated. The effect of nonuniform axial heat flux on the channel pressure drop is also evaluated. The model is compared with pressure drop data from the thermal-hydraulic test loop at Oak Ridge National Laboratory and is found to agree with the data within 24%.

Scaling and correlation of subcooled void growth for circular and finned annular geometries at low pressure

Void growth measurements in subcooled flow boiling for two finned annuli and two circular annuli were obtained for flow rates between 0.02 and 0.2 kg −1 (mass fluxes between 80 and 1400 kg m −2s), for outlet pressures of 105, 200 and 300 kPa, and for inlet temperatures of 30 to 90°C. A parametric study of the influence of outlet pressure, velocity, geometry, subcooling, and power on void growth is presented, new scaling parameters for void growth are introduced, and a non-dimensional linear correlation based on the Nusselt number and the onset of nucleate boiling is developed. It is found that a flow rate of 0.02 kgs −1, the onset of significant void is independent of input parameters. Scaling of void growth showed that the region before the onset of significant void is important and that void fraction increases linearly in this region.

Subcooled void growth mechanisms and prediction at low pressure and low velocity

Measurements were made of void growth and wall temperatures for two circular annular geometries and forced-convective subcooled nucleate boiling conditions, with flow rates between 0.02 and 0.20 kg/s, pressures of 1.05, 2 and 3 bar, and inlet temperatures between 30 and 90°C. Void growth results at low pressure show that the highly subcooled void region is important, and void fraction at the onset of significant void may be as high as 10%. Observations of bubble ebullition and detachment mechanisms obtained from a high speed photographic study were compared to assumptions generally used in void growth modeling. The photographic results show that bubbles do not travel far downstream after nucleating, that there is no region of attached void and that bubbles slide along the wall before being ejected into the flow after the onset of nucleate boiling. Unlike findings for high pressure systems reported in the literature, the onset of significant void was found to be independent of (i) bubble detachment, (ii) the location where the bubble is first ejected from a bubble layer and (iii) the transition from partial nucleate boiling to fully developed boiling. A phenomenological void growth model is presented which accounts for the vapor volume inside a heated channel at atmospheric pressure, and includes the bubble ebullition cycle, formulated on the basis of information obtained from the high speed photographic study.

Prediction of the Onset of Significant Void in Flow Boiling of Water

Prediction of the Onset of Significant Void in Flow Boiling of Water

The effect of flow direction on void growth at low velocities and low pressure

Subcooled void fraction experimental data is presented for upward and downward flow in an annulus for a velocity range of 0.07 to 0.46 m/ s, and pressure of 155 kPa. The buoyancy effect causes the onset of significant void, OSV, to occur at higher subcooling for downward flow than for upward flow. This effect is significant close to the bubble rise velocity (0.23 m/ s) and decreases at higher velocities. Analysis of void growth for both flow directions shows that bubble detachment is not the only critical parameter affecting OSV at low velocities and low pressures.

The onset of significant void in up-flow boiling of water at low pressure and velocities

Results of experiments to determine the point of onset of significant void (OSV) for up-flow boiling of water at low pressure ( $ ̃ 150 kPa) and low velocities (<0.5m s −1) show that the sub-cooling at OSV increases as liquid inlet velocity increases. According to the Saha and Zuber OSV correlation there should be no effect of velocity under the experimental conditions since the Peclet number is less than 70 000. At higher Peclet numbers, empirical evidence and existing correlations and models show that the subcooling at OSV decreases as liquid velocity increases. A model, based on the findings of Winterton on bubble departure, has been developed which predicts the observed trend of sub-cooling at OSV with velocity in these experiments as well as the opposite trend observed by others at higher velocities.