In　this　first　part　of　a　three-part　study,　a　three-dimensional　numerical　model　has　been　conducted　to　study　the　heat　transfer　characteristics　of　laminar　flow　in　microchannel　heat　sinks　with　different　geometric　fan　shaped　ribs　on　sidewalls　for　Reynolds　number　ranging　from　187　to　715.　The　numerical　model　considers　entrance　effect,　conjugate　heat　transfer,　viscous　heating　and　temperature-dependent　thermo-physical　properties.　The　water　and　silicon　are　used　as　fluid　and　solid　for　the　computational　domain.　The　computations　base　on　the　finite　volume　method　and　implement　the　SIMPLEC　algorithm.　The　microchannel　has　the　width　of　0.1　mm　and　depth　of　0.2　mm　in　the　constant　cross-section　region.　The　geometric　parameters　include　the　width　(0.05-0.4　mm),　height　(0.005-0.025　mm)　and　spacing　(0.2-5　mm)　of　aligned　or　offset　fan-shaped　ribs.　The　present　study　firstly　examines　the　effect　of　fan-shaped　ribs　on　velocity　contour,　temperature　distribution　and　local　heat　transfer　characteristics　for　laminar　flow　in　such　microchannel　heat　sinks.　It　is　apparent　that　the　fan-shaped　ribs　can　effectively　prevent　the　decline　of　local　heat　transfer　coefficient　along　the　flow　direction.　Secondly,　the　average　Nusselt　number　and　the　total　thermal　resistance　for　such　microchannel　heat　sinks　have　been　studied.　Results　show　that　the　height　and　spacing　of　fan-shaped　ribs　have　significant　influences　on　the　heat　transfer　characteristics,　while　the　width　of　ribs　shows　less.　For　the　microchannel　heat　sinks　with　lower　height　and　lager　spacing　of　fan-shaped　ribs,　the　aligned　arrangement　shows　higher　heat　transfer　coefficient　than　the　offset　one.　For　the　studied　Reynolds　number　range　and　geometric　parameters　of　fan-shaped　ribs,　there　are　a　6-101%　increase　in　average　Nusselt　number　and　a　3-40%　decrease　in　total　thermal　resistance　for　the　microchannel　heat　sink　with　aligned　fan-shaped　ribs,　while　a　4-103%　increase　in　average　Nusselt　number　and　a　2-42%　decrease　in　total　thermal　resistance　for　the　microchannel　heat　sink　with　offset　fan-shaped　ribs,　comparing　with　the　smooth　one.　New　heat　transfer　correlations　for　such　microchannel　heat　sinks　are　developed,　which　fit　the　present　data　with　a　mean　absolute　error　of　2.5%　for　the　ones　with　aligned　fan-shaped　ribs　and　3.8%　for　the　ones　with　offset　fan-shaped　ribs.　Crown　Copyright　(C)　2016　Published　by　Elsevier　Ltd.　All　rights　reserved.