Experimental　investigation　had　been　conducted　to　study　the　steady-state　forced　convection　heat　transfer　and　pressure　drop　characteristics　of　the　hydrodynamic　fully-developed　turbulent　dow　in　the　air-cooled　horizontal　equilateral　triangular　ducts,　which　were　fabricated　with　the　same　length　and　hydraulic　diameter.　Inner　surfaces　of　the　ducts　were　fixed　with　square　ribs　with　different　side　lengths　of　6.35,　9.525　and　12.7　mm,　respectively,　and　the　uniform　separation　between　the　centre　lines　of　two　adjacent　ribs　was　kept　constant　at　57.15　mm.　Both　the　triangular　ducts　and　the　ribs　were　fabricated　with　duralumin.　The　experiments　were　performed　with　the　hydraulic　diameter　based　Reynolds　number　ranged　from　3100　to　11300.　The　entire　inner　wall　of　the　duct　was　heated　uniformly,　while　the　outer　surface　was　thermally　insulated.　It　was　found　that　the　Darcy　friction　factor　of　the　duct　was　increasing　rather　linearly　with　the　rib　size,　and　forced　convection　could　be　enhanced　by　an　internally　ribbed　surface.　However,　the　heat　transfer　enhancement　was　not　proportional　to　the　rib　size　but　a　maximum　forced　convection　heat　transfer　augmentation　was　obtained　at　the　smallest　rib　of　6.35　mm.　Non-dimensional　expressions　for　the　determination　of　the　steady-state　heat　transfer　coefficient　and　Darcy　friction　factor　of　the　equilateral　triangular　ducts,　which　were　internally　fabricated　with　uniformly　spaced　square　ribs　of　different　sizes,　were　also　developed.