Experimental study of cryogenic flow quenching of stainless steel tubes with thickness-gradient Teflon coatings

Applying a low thermal conductivity coating layer to the inner wall of the tubes is a promising approach to accelerating the chilldown process, with extensive research focusing on the impact of coating thickness on the chilldown performance. It was concluded that there exists an optimal thickness of the coating layer for shortening chilldown time. However, the optimal thickness is apparently not uniform along the axis of tube due to the variation of heat transfer characteristics at different axial positions. Therefore, the chilldown behavior of tubes with uniform coating thicknesses ranging from 24 µm to 201 µm and the tube with thickness-gradient coating layer of approximately 146 µm to 71 µm along the axial direction are investigated by conducting horizontal flow quenching experiments using liquid nitrogen at different inlet pressures. It is found that, when liquid nitrogen flows from the thick coating layer (146 µm) side to the thin coating layer (71 µm) side, the chilldown time is reduced by 12.1–16.3% compared to the opposite flow direction, and by 7.1% compared to the uniform-coated tube with similar average thickness. It can be concluded that the thicker coating layer upstream allows earlier emergence of transition boiling, thereby accelerating the film boiling process downstream. The Leidenfrost point temperature upstream is higher than that downstream due to the increased solid-liquid contacts. Additionally, the pressure drop measurement throughout the tube reveals that the onset of the transition boiling intensifies flow turbulence, and consequently enhances heat transfer of film boiling downstream.