Xiaorong Xiang

Thermal management of lubricating oil under the extreme heat loads of high‐supersonic flight remains a critical challenge for air turbo rocket (ATR) engines, where conventional fuel-cooled heat exchangers approach their heat sink limit. Liquid nitrogen (LN₂), a cryogenic fluid available on board, is a promising alternative coolant; however, its coupled phase-change behavior with high-viscosity lubricating oil in helical tube heat exchanger (HTHE) has not been systematically investigated. To address this gap, an experimental and numerical study was conducted, with a CFD model incorporating temperature-dependent properties and LN₂ phase change validated within ±20% against experimental measurements. Results show that while helical-induced Dean vortices enhance overall heat transfer, the oil-side Nusselt number exhibits a non-monotonic dependence on LN₂ flow rate, governed by competition between thermal enhancement and near-wall viscosity-driven flow degradation. The wall-to-bulk viscosity ratio (μw/μb) is identified as the critical parameter dictating this behavior. New generalized correlations for oil-side Nusselt number and Darcy friction factor were developed for Re = 1000–12,000, Pr = 65–120, and μw/μb = 1–2, with prediction errors within ±10%. This work provides practical design guidance and reliable predictive tools for advanced thermal management systems in aero-engine lubrication operating under extreme thermal and flow conditions.

Segmented annular seals in aircraft engine bearing chambers are susceptible to transient detachment, impact, and wear under extreme operating conditions, which limits their lifespan and reliability. In this work, a transient numerical model coupling discontinuous film flow with the segments is developed, and a convergent micro-texture is proposed to improve stability. This research elucidates the transient failure mechanisms, leakage behavior, and hydrodynamic lift-off characteristics, achieving non-contact stable operation of the seal. Under rotor disturbances, the motion of an individual segment comprises both overall rotor-following motion and oscillatory motion. Periodic detachment and impact primarily result from insufficient radial clamping force. The convergent micro-texture significantly enhances the gas film reaction force by approximately 82%, enabling complete lift-off at 8,000 r/min. Under stable operating conditions, the leakage rate increases with rotational speed, rotor amplitude, and pressure differential. When detachment occurs, the leakage rate of the conventional seal decreases with increasing pressure differential, whereas that of the hydrodynamic seal initially decreases and then increases slightly. The leakage rate of the hydrodynamic seal remains significantly lower than that of the conventional seal. These findings provide a theoretical foundation for the design and application of segmented annular seals under extreme operating conditions.

Trailing-edge shock and wake losses are crucial factors that constrain the aerodynamic performance of a highly-loaded transonic turbine. Pressure-side cutback cooling configurations can control loss while providing effective thermal protection for the trailing-edge, but the correlation between cutback configurations and aerodynamic loss remains to be established. Especially for the high-pressure blade of a vaneless counter-rotating turbine with a design outlet Mach number (Maout) exceeding 1.5, the demand for robust loss control across a wide range of supersonic flow conditions poses a significant challenge to the cutback design. In this study, tapered lands and concave lips are applied to the cutback structure. The influence mechanisms of the cutback geometry on the shock and wake losses with Maout ranging from 1.05 to 1.65 are revealed. For the tapered land, it decreases the wake loss under all studied conditions compared with the base cutback structure (arranged with straight lands). However, its influence on the shock loss depends on the width of the flow passage formed by the wake and the suction-side of an adjacent blade, which varies with Maout. The tapered land significantly weakens the shock loss under high Maout conditions, whereas under low Maout conditions, a λ-shock is generated and its intensity is increased by the tapered land. To avoid the adverse effect of the tapered land, the concave lip design is proposed. The shock induced by the concave lip can counteract the expansion wave induced by the tapered land, thus effectively reducing the flow Mach number and the λ-shock loss. Using response surface methodology, the optimal parameters of the cutback arranged with tapered lands and concave lips are determined. Under the premise of ensuring film cooling effectiveness, the optimized structure can reduce the energy loss coefficient relative to the base structure under various mainstream and coolant conditions, with an average reduction exceeding 10%.

Trailing-edge shock and wake losses are crucial factors that constrain the aerodynamic performance of a highly-loaded transonic turbine. Pressure-side cutback cooling configurations can control loss while providing effective thermal protection for the trailing-edge, but the correlation between cutback configurations and aerodynamic loss remains to be established. Especially for the high-pressure blade of a vaneless counter-rotating turbine with a design outlet Mach number (Maout) exceeding 1.5, the demand for robust loss control across a wide range of supersonic flow conditions poses a significant challenge to the cutback design. In this study, tapered lands and concave lips are applied to the cutback structure. The influence mechanisms of the cutback geometry on the shock and wake losses with Maout ranging from 1.05 to 1.65 are revealed. For the tapered land, it decreases the wake loss under all studied conditions compared with the base cutback structure (arranged with straight lands). However, its influence on the shock loss depends on the width of the flow passage formed by the wake and the suction-side of an adjacent blade, which varies with Maout. The tapered land significantly weakens the shock loss under high Maout conditions, whereas under low Maout conditions, a λ-shock is generated and its intensity is increased by the tapered land. To avoid the adverse effect of the tapered land, the concave lip design is proposed. The shock induced by the concave lip can counteract the expansion wave induced by the tapered land, thus effectively reducing the flow Mach number and the λ-shock loss. Using response surface methodology, the optimal parameters of the cutback arranged with tapered lands and concave lips are determined. Under the premise of ensuring film cooling effectiveness, the optimized structure can reduce the energy loss coefficient relative to the base structure under various mainstream and coolant conditions, with an average reduction exceeding 10%.

A physical model for the precooler in a SABRE engine was established, and both the influencing degrees and physical mechanisms of inlet conditions and geometric parameters on the flow-heat transfer characteristics of the precooler unit were explored through parametric analysis and sensitivity analysis, respectively. The results demonstrated that the air massflow rate exerted the greatest influence on the total pressure loss coefficient of the precooler unit due to its significant impact on airflow velocity and thermal loading. The inlet total temperature had a dominant effect on heat transfer rate, owing to its substantial influence on air viscosity and thermal loading. Furthermore, both alone and in combination with air massflow rate, inlet total pressure exhibited a greater impact on total pressure loss of precooler unit compared to inlet total temperature and its coupling effects with air massflow rate. In contrast, both air massflow rate and its coupling effects with inlet total temperature significantly affected the heat transfer rate. On the other hand, tube spacing had the most significant impact on both the total pressure loss coefficient and heat transfer rate due to its substantial influence on air throughflow area and acceleration between microtubes. Moreover, the number of tube rows played a predominant role in determining the heat transfer rate of the precooler unit as it caused significant changes in the contact area between the hot air and the microtubes, as well as airflow velocity. However, only two geometric interaction terms (corresponding to interactions between the number of tube rows and tube spacing, as well as between tube spacing and tube diameter) significantly affected the total pressure loss coefficient, and no interaction term was found to be significant for influencing the heat transfer rate. Eventually, two optimal schemes involving inlet conditions and geometric parameters were established to enhance the flow-heat transfer characteristics of the precooler unit.