Although　many　efforts　have　been　made　to　disclose　the　energy　separation　phenomenon　based　on　theoretical　numerical　and　experimental　analysis　in　vortex　tubes,　it　is　still　difficult　to　provide　systematical　information　for　designing　vortex　tubes　due　to　the　complexity　of　the　physical　process　and　the　shortage　of　fully　generally-applicable　theory　and　methods.　The　purpose　of　current　study　was　to　search　an　effective　method　to　predict　the　energy　separation　and　flow　behavior　within　a　vortex　tube.　A　three-dimensional　computational　fluid　dynamic　model　together　with　the　experimentally　validated　turbulence　model　is　established　and　an　experimental　measurement　is　carried　out　using　an　optimal　structure　of　vortex　tube　so　as　to　validate　the　computational　model.　The　flow　characteristics　including　the　total　temperature　and　tangential　velocity　were　obtained.　The　effects　of　cold　fluid　outlet　diameter　on　the　flow　and　temperature　separation　of　a　counter-flow　vortex　tube　were　investigated　comprehensively.　The　streamlines　with　different　cold　fluid　outlet　diameters　were　presented　and　analyzed.　A　possible　explanation　for　the　cold　fluid　outlet　diameter　influences　on　the　temperature　difference　between　the　cold　fluid　outlet　and　the　hot　fluid　outlet　were　also　given.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.