Flow boiling heat transfer performance of R245fa in a vertical enhanced evaporator tube with sintering and electroplating composite coats

The R245fa flow boiling performance was investigated in a vertical evaporator tube of which inner wall surface is coated by a composite sintering and electroplating technology. The effects of vapor quality, heat flux, mass flux, and flow direction were compared and analyzed in the bare tube (BT) and the sintering and electroplating treated tube (SET). The results showed that the heat transfer coefficient increased and then decreased with the rise of vapor quality and heat flux. The heat transfer performance in downward flow was better than that in upward flow, while an opposite trend was observed for frictional pressure drop. With the increased of vapor quality, the frictional pressure drop increased and then decreased, but it monotonically increased with the increasing mass flux. The heat transfer enhancement mechanism was explored through flow pattern visualization. The annular flow pattern conversion line of SET was earlier than that of BT. Due to the strong wettability of SET, the thickness of liquid film was thinner in downward and the mother bubble was bigger in upward, which had a positive effect on flow boiling heat transfer. The maximum the heat transfer enhancement factor (EF) and the performance evaluation parameter (PEC) were 1.83 and 1.75, respectively.