In　this　paper,　the　combined　effects　of　ribs　and　double-layer,　dome-shaped　turning　vanes　on　heat　transfer　and　pressure　drop　are　investigated　in　an　idealized　U-bend　channel.　Five　kinds　of　ribs　including　transverse　ribs,　45°ribs,　135°ribs,　V-shaped　ribs,　and　reverse　V-shaped　ribs　combined　with　one　kind　of　double-layer,　dome-shaped　turning　vanes　are　applied.　Baseline　results　are　compared　with　the　above　composite　cooling　structures.　Numerical　simulations　are　performed　by　solving　3D,　steady　Reynolds-averaged　Navier-Stokes　(RANS)　equations　with　k-ω　turbulence　model.　The　channel　aspect　ratio　is　1:2　and　its　hydraulic　diameter　is　93.13　mm,　respectively.　Based　on　the　cooling　air　inlet　velocity　and　the　channel　inlet　hydraulic　diameter,　the　inlet　Reynolds　numbers　are　ranging　from　100,000　to　440,000.　The　detailed　three-dimensional　fluid　flow,　pressure　and　heat　transfer　distributions　are　presented.　Moreover,　the　thermal　performances　of　the　U-bend　channel　are　also　evaluated　and　compared　with　different　cases.　The　results　revealed　that　combined　with　the　double-layer,　dome-shaped　turning　vanes,　the　transverse　ribs　case　has　the　best　thermal　performance　at　the　tip　wall,　and　the　reverse　V-shaped　ribs　case　is　the　best　for　the　leading　wall.　The　pressure　drop　of　the　channel　with　double-layer,　dome-shaped　turning　vanes　without　any　rib　turbulator　is　the　lowest,　and　that　of　the　channel　with　inclined　ribs　is　significantly　higher　than　that　of　the　channel　with　transverse　ribs.　The　superposition　of　the　secondary　flow　induced　by　the　ribs　and　the　Dean　vortex　induced　by　the　180°sharp　turn　has　a　marked　impact　on　the　flow　and　heat　transfer　in　the　channel.　In　the　double-layer,　dome-shaped　turning　vanes　channel,　the　mass　flow　distribution　of　the　coolant　also　affects　the　heat　transfer　on　the　tip　wall　of　the　channel,　and　the　ribs　can　adjust　the　mass　flow　distribution.　The　helical　vortex　superposed　by　the　mainstream　flow　and　the　secondary　flow　induced　by　the　ribs　represents　typical　flow　phenomenon　in　ribbed　channels.　The　flow　and　development　of　the　helical　vortex　are　the　main　factors　affecting　the　heat　transfer　on　the　leading/trailing　walls.　Copyright　©　2020　ASME