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Heat Transfer Theoretical Analysis Experimental Investigations and Industrial Systems part 7

Tham khảo tài liệu 'heat transfer theoretical analysis experimental investigations and industrial systems part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 230 Heat Transfer - Theoretical Analysis Experimental Investigations and Industrial Systems Just as the case of smooth chip the bubbles generate and departure continuously from the heating surface caused by buoyancy forces in normal gravity before the release of the drop capsule Fig. 14a . However the bubble number are much larger than that for the smooth chip indicating that the micro-pin-finned surface can provide larger number of nucleation sites for enhancing boiling heat transfer performance. At about 0.12 s after entering the microgravity condition the vapour bubbles begin to coalesce with each other to form several large bubbles attaching on the chip surface Fig. 14b . Some small bubbles are in the departure state when entering the microgravity condition so we can still see them departing from the heater surface at this time. With increasing time the bubbles coalesce to form a large spherical bubble Fig. 14c . However the large bubble covering on the heater surface does not cause obvious increase of wall temperature Fig. 15 . Fig. 16. Bulk liquid supply and micro-convection caused by capillary force Wei et al. 2009 . The capillary force generated by the interface between the large bubble and the liquid of the micro-layer beneath the bubble drives plenty of fresh liquid to contact with the superheated wall for vaporization through the regular interconnected structures formed by the micropin-fins as well as improves the micro-convection heat transfer by the motion of liquid around the micro-pin-fins as shown schematically in Fig. 16. The sufficient supply of bulk liquid to the heater surface guarantees the continuous growth of the large bubble. Therefore contrary to boiling on chip S there is no deterioration of boiling heat transfer performance for the micro-pin-finned surface in microgravity and the heater surface temperature can keep almost constant in both gravity and microgravity conditions. In summary the micro-pin-fined surface structure can provide .