Flow　boiling　of　HFE-7100　in　conventional　open　microchannels　(OMs),　open　microchannels　with　parallel　micro-straight　fins　(OMs-PMSFs),　and　open　microchannels　with　micro-pin　fins　(OMs-MPFs)　were　experimentally　studied.　The　mass　flux　(G)　ranges　from　169.1　to　394.6　kg/(m2s),　the　effective　heat　flux　is　up　to　192.27　W/cm2,　the　outlet　pressure　is　from　102　kPa　to　156　kPa.　The　heat　transfer　behaviors　in　the　three　microchannels　are　analyzed.　The　enhanced　heat　transfer　mechanisms　are　explained　according　to　the　observed　flow　patterns.　In　the　nucleate　boiling　dominant　regime,　the　bubble　departure　diameters　in　the　OMs-PMSFs　and　the　OMs-MPFs　are　smaller　than　those　in　the　OMs,　therefore,　the　heat　transfer　is　enhanced.　The　local　heat　transfer　coefficient　(HTC)　initially　increases　and　then　decreases　with　both　the　heat　flux　and　the　outlet　vapor　quality　for　all　three　open　microchannels.　Additionally,　the　maximum　HTCs　of　the　OMs-PMSFs　and　the　OMs-MPFs　show　increase　of　21.92　%　and　90.24　%,　respectively,　at　G　=　394.6　kg/(m2s)　compared　to　the　maximum　HTC　of　the　OMs.　It　is　observed　through　flow　visualization　that　the　enhanced　capillary　effect　of　the　micro-nanoscale　composite　structures　can　accelerate　the　re-wettability　of　the　microchannel　surfaces,　which　reduces　the　period　of　the　bubble　growth　cycle　and　delays　the　occurrence　of　the　dry-out.　As　a　result,　the　intensified　nucleate　boiling　and　liquid　evaporation　significantly　enhance　flow　boiling　heat　transfer　in　the　OMs-PMSFs　and　the　OMs-MPFs　as　compared　to　that　in　the　OMs.　Moreover,　the　micro-nanoscale　composite　fins　located　on　the　bottom　of　the　microchannel　effectively　reduce　the　growth　rate　of　pressure　drop　under　two-phase　conditions.　©　2025