Pool boiling of horizontal mini-tubes in unconfined and confined columns

•Void fraction models developed for confined and unconfined tube columns.•Void fraction >∼0.64 changes critical location from bottom to top of column.•Void fraction >∼0.64 adversely affects heat transfer coefficient below CHF.•Maximum power density declines from initial peak to constant ∼30 MW m−3.•Increasing tube spacing mitigated against localised dryout at high confinements. Pool boiling on confined and unconfined tube columns was investigated. A model to predict bulk void fraction in a confined tube column was developed and validated experimentally, returning a mean predictive accuracy error of 12.5%. A semi-empirical model was also developed from bubble plume images to evaluate bulk void fraction in an unconfined plume around a tube column. Within an unconfined plume, void fraction increases monotonically with column height, indicating that a column height exists beyond which the mean column heat transfer coefficient is adversely affected by dryness. Pool boiling experiments were undertaken in Novec 649 to investigate the effects of external confinement, tube spacing, and column height on heat transfer. Increasing the column height in a column showed an initial decline in power density to a constant at ∼30 MW m−3. A tube void fraction threshold was identified as changing the location of critical heat flux from the bottom tube to the top tube in a column, and to cause a deterioration in tube heat transfer coefficient at high heat fluxes.