Experimental study on liquid nitrogen pipe chilldown performance under pulse flow conditions and adopting micro-fin pipe

Abstract

• Pulse flow approach saves liquid mass in LH 2 chilldown but has no significant effect in LN 2 case. • Pulse LN 2 flow causes time cost increase but no definite effect in liquid mass. • Micro-fin pipe simultaneously reaches purposes of time saving and liquid saving. • Extending valve closing period could not produce significant liquid saving in LN 2 condition. High-efficient cold energy utilization of cryogenic liquid is beneficial to propellant mass saving in transfer pipe chilldown operation, and a pulse liquid flow approach has been proposed to reach this purpose. However, the liquid inlet pattern influence on the overall performance of chilldown process is still unclear. In the present study, a cryogenic chilldown test platform, using liquid nitrogen (LN 2 ) as the working fluid and covering the inlet Reynolds number (Re) range of 5000–45000, is constructed to investigate the pulse flow effect on cryogenic chilldown behaviors. Different pipe sections, including two straight pipes and three micro-fin pipes, are tested under different inlet conditions, and the data on chilldown time and liquid consumption are significantly analyzed. The test results show that a pulse flow inlet produces periodic fluctuation of heat flux during the chilldown progress, and a longer time is required to reach the chilldown purpose when compared to a continuous liquid inflow situation. To attain chilldown time saving, both of the approaches by increasing inlet Re and adopting micro-fin pipe could yield satisfactory results. Compared to the straight pipe case with wall thickness of 2 mm, the micro-fin pipe with fin interval of 50 mm (Δ = 50 mm) saves time cost by 40–45 %. However, the approach of pulse liquid flow produces a significant increase in chilldown time. In the view of liquid consumption, the pulse LN 2 flow approach could not produce a definite effect in reducing liquid mass, which is different from the liquid hydrogen (LH 2 ) chilldown situation. By contrast, the micro-fin pipe yields an excellent effect in saving liquid mass, with the maximum saving of 65 % when compared to a conventional continuous liquid case. Therefore, the micro-fin pipe could simultaneously reach the purposes of chilldown time reduction and liquid consumption saving. The chilldown thermal efficiency is also analyzed in the present studies, and the results demonstrate that there is no apparent increase in thermal efficiency by adopting a pulse flow approach, but the micro-fin pipe gives rise to a significant increase in thermal efficiency. When adopting a pulse flow operation with a longer valve off period, a longer chilldown time is needed but the liquid saving effect is negligible.