Two-phase heat transfer in deep throttling of liquid rocket engines

The operational flexibility demanded by modern propulsion systems requires liquid rocket engines with deep-throttling capabilities. These off-design operating conditions often trigger two-phase flow within the regenerative cooling system, where the continuous evolution of boiling regimes makes heat transfer predictions particularly challenging. To provide reliable yet efficient simulations in support of liquid rocket engine design, a one-dimensional two-phase heat transfer model is proposed for system-level analyses. The accuracy of the model is validated against experimental data for cryogenic fluids, focusing on wall temperature prediction and critical heat flux position, benchmarking the results against standard empirical correlations. Results demonstrate the excellent overall accuracy of the present approach across a wide range of conditions. Finally, using the EcosimPro system analysis software, the model is applied to the system-level simulation of an expander cycle engine to assess the impact of two-phase phenomena on global performance and operability. Methane-fueled and hydrogen-fueled configurations are compared, investigating the coupling between heat transfer modeling and engine power balance in deep-throttling regimes down to 5% of the nominal thrust.