Annular　fire　source　is　a　common　combustion　form　in　fire　accidents.　Effects　of　Din/Dout　(the　ratio　of　inner　to　outer　diameters　of　the　floating-roof　tank)　on　the　flame　morphology　and　plume　entrainment　mechanisms　of　annular　pool　fires　were　studied　by　numerical　simulation.　Results　show,　as　Din/Dout　increases,　the　area　with　low　combustion　intensity　near　the　central　axis　of　the　pool　surface　gradually　increases.　Combined　with　the　time-series　HRR　and　the　stoichiometric　mixture　fraction　line　of　the　fire　plume,　it　reveals　that　the　combustion　of　annular　pool　fire　is　dominated　by　non-premixed　diffusion　flame.　The　pressure　near　the　pool　outlet　decreases　with　Din/Dout,　while　the　plume　turbulence　presents　an　opposite　trend.　Based　on　the　time-sequential　plume　flow　and　the　gas-phase　material　distribution　data,　the　flame　merging　mechanism　of　the　annular　pool　fires　is　revealed.　Furthermore,　based　on　the　similarity　criterion,　it　verifies　that　the　applicability　of　the　above　scaled　simulation　conclusions　could　also　be　extended　to　guide　full-scale　fires.　©　2023,　The　Author(s),　under　exclusive　licence　to　Springer-Verlag　GmbH　Germany,　part　of　Springer　Nature.