Triangular type pyramid surfaces are made as a new type of heat transfer surface, and its sub-atmospheric pressure pool boiling performance is investigated experimentally. Pure water is cited as a working fluid with liquid height of 25 mm and saturation pressures of 15 kPa and 10 kPa. Triangular type pyramid surfaces have pyramid sizes of 2.5 mm, 5.0 mm and 7.5mm, respectively. Performance of the triangular type pyramid surfaces is compared with square type pyramid surfaces whose pyramids sizes are 2.5 mm and 5.0 mm. Heat transfer performance of the triangular type pyramid surface with size of 2.5 mm is the highest among them for both saturation pressures. Heat transfer performance for triangular type pyramid surfaces with real surface area basis is higher by about 1.5 times for size of 2.5 mm, and higher by about 1.2 times for size of 5.0 mm compared with the smooth surface at 15 kPa, and higher by about 1.7 times for size of 2.5 mm, and higher by about 1.3 times for size of 5.0 mm at 10 kPa. When comparing the heat transfer performance of triangular type pyramid surfaces with the square type pyramid surfaces, the triangular type pyramid surfaces have higher performance than the square type pyramid surfaces. Observation of bubbles generated from pyramid surfaces is also conducted for pyramid surfaces with size of 2.5 mm, as well as measurement of bubble growth rates and bubble release times. The results qualitatively show that triangular type pyramid surfaces have good bubble foaming characteristics than the square type pyramid surface.

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Heat transfer performance of aluminum porous surfaces with fine tunnel structures is investigated experimentally. We have made eight types of porous surfaces (Al: No.1~6 and Cu: No.7~8 as reference). Porous surface No.4 has similar specification with THERMOEXCEL and is used as a reference surface as well as smooth surface. Fluorine-based refrigerant HFE-7000 is used as a working fluid. Absolute pressures cited are 0.10MPa, 0.14MPa, 0.18MPa, respectively. Porous surfaces No.1 has a large number of nucleation points that is more than 300 at low superheat region of 0.1Mpa. On the other hand, it is about 10 for smooth surface and about 40 for reference surface No.2. Furthermore, we do not observe any hysteresis for No.1. Porous surface No.2 has the highest heat transfer performance among tested aluminum surfaces. The heat transfer is higher than 11 times of smooth surface and 6.5 times of reference porous surface No.4 at 80kW/m2 of 0.14MPa. Optimum fin pitch of porous surface seems to be around 300μm. When the fin height is too large compared with standard specification, heat transfer performance is significantly lowered. There is no significant difference in heat transfer performance. On the other hand, there is a notable difference in performance between copper and aluminum porous surfaces at the high heat flux region. Although heat transfer coefficient of copper porous surface increases with increment of heat flux, that of aluminum porous surface comes to an upper limit for increasing heat flux.

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