所報 104

5/38

－　　－3Published PaperHeat Transfer Characteristics of Pool Boiling under Low Pressure ConditionHitoshi MATSUSHIMA and Takuya SAITOUWe experimentally examined the possibility of enhancement of boiling heat transfer under sub-atmospheric pressure. The range of pressure tested was between atmospheric pressure to 5 kPa. The heat transfer coefficient under low pressure condition decreased dramatically at the pressure of 5 kPa. This tendency was seen for all types of heat transfer surfaces and working fluids, and differences in heat transfer between smooth surface and extended surfaces almost diminished. Heat transfer performance of pyramid 2.5 mm and copper sintering surfaces was rather good at the pressure of 5 kPa, and the performance of both was almost similar except at atmospheric pressure. Sodium myristate is a promising surfactant that showed stable performance under low pressure condition. The heat transfer coefficient of a smooth surface using pure water increased with the decrease in liquid height. However, reduction in liquid height showed a tendency to change from nucleate boiling to film boiling at an early stage.Keywords: Sub-atmospheric Pressure, Nucleate Pool Boiling, Heat Transfer, Enhancement, Experiment低圧下でのプール沸騰における熱伝達特性　本研究では，大気圧から5kPaまでの低圧下におけるプール沸騰実験を行い，低圧下での沸騰熱伝達の促進の可能性について，実験的に検討した．伝熱面や作動液の種類によらず，圧力が5kPaまで下がると熱伝達率が急激に低下し，各種の拡大伝熱面や界面活性剤使用時と純水・平滑面使用時との差異が大幅に縮小した．低圧下では，ピラミッド型2.5mmと銅繊維焼結面での伝熱性能が特に良好であり，両者は大気圧下を除きほぼ同等の伝熱性能を示した．なお，圧力の低下とともにピラミッド型伝熱面の最適なピッチは小さいものから大きいもの（2.5mm）へ次第にシフトした．沸騰開始温度についてはピラミッド型，銅繊維焼結面ともに，平滑面に比べて大きく低下した．このことは，電子機器冷却の観点から考えて重要である．今回測定した作動液の中ではミリスチン酸水溶液（100ppm）が最も良好であり，低圧下でも気泡径が小さく安定した伝熱性能を示した．低圧下では，純水・平滑面での伝熱性能は作動液水位が減少するとともに単調に増加する傾向を示した．ただし，作動液水位は小さくなると膜沸騰に遷移しやすくなる傾向を示した．キーワード: 低圧, 核沸騰, 熱伝達, 促進, 実験Journal（掲載誌）Thermal Science and Engineering, Vol. 26, No. 1, 2018, pp. 39-50.