本實驗共測試四根半弧/圓形無蕊式熱管，分別為填充甲醇佔蒸發段容積20、35、50及65%，其蒸發段為半弧形，液體夾於兩邊，夾角的表面張力使蒸發段壁面保持潤濕，提高其有效熱傳導係數，液汽流動介面小，降低了傾斜角度對熱阻的影響。而圓形冷凝段具較大蒸汽空間與凝結面積，相對的凝結液膜變薄，使冷凝段等效熱傳導係數增加，提升整體性能。四根熱管的總長度皆為1380mm，分別為蒸發段1000mm、絕熱段80mm及冷凝段300mm。熱傳機制、填充量和傾斜角的影響皆已量得並討論。 填充率65%、50%與35%在低加熱功率下皆有啟動困難的情形發生，唯20%在低加熱功率下即可啟動，且整體性能良好，將此半弧/圓形熱管應用於太陽能熱水器上能有效解決晨昏日射量低啟動不易以及架設角度低等缺失；水槽內採自然對流循環方式，熱阻較熱管部份高出許多，未來應改善水槽水與熱管冷凝段間熱傳，可考慮將熱管插入於水槽底端以增進整體熱傳能力。 This paper presents an experimental analysis of the heat transfer performance of a semi-arc/circular wickless heat pipe. Methanol was filled as the working fluid with 20%, 35%, 50% and 65% filling ratios of the evaporating section volume. Owing to the action of surface tension drainage in the sharp corner, it keeps the evaporating of the heat pipe wet and cause the higher evaporating thermal conductivity than that of circular tube. It also can reduce the effect of thermal conductivity. Evaporating and condensing space of circular condensing section was larger hence the condensing liquid film become thinner and improved the performance. The total length of all heat pipe are 1380mm. For evaporating section is 1000mm、adiabatic section is 80mm and condensing section is 300mm. The heat transfer mechanism and effect of filling ratio and inclination angle were measured and discussed. The situation of start up difficult was occurred at filling ratio 65%, 50% and 35%. However it’s startable at 20% for low power support with good performance. The disadvantages of start up difficult at low insolation and low incline angle for solar water heater would be improve by semi-arc/circular wickless heat pipe. Natural convection heat transfer was found in water tank section and caused the higher heat resistance compared to heat pipe section. The heat resistance of water tank section was higher than heat pipe section, since the heat transfer was natural convection. Future work is necessary to improve the heat transfer performance of water tank section. It’s believed that the performance will be improved by inserting the heat pipe into bottom of water tank.