The　influences　of　the　heating　surface　structure　of　phase　change　chamber　on　heat　transfer　characteristics　are　studied　by　using　smooth,　equal-width　and　variable　cross-sectional　area　grooved　heating　surfaces,　and　visualization　is　made　of　the　boiling-condensation　coexisting　phase　change　phenomena　inside　the　chamber.　The　experimental　results　confirm　that　there　is　an　optimum　working　fluid　filling　ratio　for　the　phase　change　chamber　to　acquire　its　best　performance.　The　optimum　filling　ratio　for　the　smooth　and　equal-width　grooved　heating　surface　phase　change　chamber　is　found　to　be　36.0%　and　11.4%,　respectively.　The　experimental　results　also　disclose　that　the　equal-width　grooved　heating　surface　has　the　best　performance　and　the　overall　heat　transfer　coefficient　of　the　phase　change　chamber　is　augmented　by　more　than　44%　over　the　smooth　heating　surface.　Although　the　thermal　resistance　of　condensation　is　the　dominant　of　the　overall　heat　transfer　resistance,　it　is　proved　that　enhancing　boiling　is　still　an　efficient　way　due　to　the　strong　interactions　between　the　boiling　and　condensation　processes　inside　the　phase　change　chamber.　(C)　2020　Elsevier　Ltd.　All　rights　reserved.