Subcooled flow boiling characteristics and CHF in an eccentric annulus at low pressure conditions

Controlled flow boiling conditions enhance the heat transfer rate in various industrial applications such as heat exchangers, fuel rod bundles etc. In nuclear industry, the presence of mixing vanes, spacers, channel creep deformation etc., may lead to the formation of narrow and wide zones around the fuel rods. The asymmetry in the cross-sectional flow area around the fuel rod can be modelled as eccentricity which may cause an irregular coolant flow distribution affecting the heat transfer rate. In the present study, the eccentricity (e) is defined as the ratio of offset (s) to the difference in radii of inner and outer cylinders (Ro−Ri). The current study introduces a framework that involves calibrating the departure size of vapour bubble (D) in the EMF model framework which enables the accurate prediction of subcooled flow boiling characteristics and CHF power in an annulus (0.0≤e≤0.50) at low operating pressure conditions. The developed framework is validated with experimental data and a very good match is noticed. It is observed that eccentricity causes a decrease in coolant peak velocity magnitude by 12.8% in the narrow zone for e=0.50. In the subcooled region, the accumulation of vapour bubbles impedes further heat transfer to the liquid coolant, and thus the peak heated wall temperature (≈380.88K) is observed in the narrow zone for e=0.50. Eccentricity causes early coolant phase change which in turn results in early occurrence of CHF (q=522KWm−2) in the narrow zone of e=0.50. A quadratic relationship is also established to predict the CHF power in terms of eccentricity (e) within 5% variation.