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.