Dottorato in INGEGNERIA AERONAUTICA E SPAZIALE

Titolo della tesi: Critical aspects of modelling, design and experimental testing of LRE regenerative cooling systems

While the field of cooling for liquid propulsion systems is commonly viewed as mature, there are still numerous challenges demanding attention and a comprehensive understanding. This dissertation provides an insight of critical aspects that characterize liquid rocket engines regenerative cooling systems, on different levels: numerical modelling, system design, experimental testing. The first part of the thesis is intended to assess the limitations and predictive capabilities of simplified approaches to address complex phenomena in the context of design and development of liquid rocket engine cooling systems, to get results with reasonable accuracy in affordable times. In this framework, flow boiling and transient analyses of cooling systems during typical off-design engine operations have been investigated. The former topic intends to study two-phase flows in cooling channels from a macroscopic point of view, through the homogeneous equilibrium model, using both an in-house computational fluid dynamics solver and EcosimPro. Numerical predictions have been compared to experimental data and satisfactory agreement is obtained in the post-critical-heat-flux regime. The latter topic delves into a conjugate heat transfer approach, proposed to study the transient evolution of thermal fields during off-design operations of the cooling system, which are amongst the most critical for engine life due to the large thermal gradients that can be experienced. A simplified version of the RL10 regenerative cooling jacket is considered as a reference for the simulation of both chill-down and start-up transient of the engine. A complete overview of the strengths and the limits of the partitioned coupling approach is provided. The second part of the thesis is devoted to the investigation of design opportunities given by additive manufacturing techniques, which are becoming key technologies for the development of liquid rocket thrust chambers thanks to the reduced times and costs. Conjugate heat transfer simulations have been performed assuming different channel geometries with trapezoidal and ribbed cross-sections. Parametric analyses have been carried out and results compared to conventional rectangular shapes, comparing performance in terms of heat transfer and pressure drop. The advantages of additive manufacturing have also led a renewed interested towards aerospike nozzles, which have proven to be an efficient solution capable of yielding high performance in a wide range of chamber-to-ambient pressure ratio values with respect to conventional bell counterparts. However, despite the different design configurations proposed over the years, the thermal management has been one of the most challenging tasks to accomplish, especially for annular configurations. In the present work, a streamlined methodology is followed to find a possible design of a dual regenerative cooling system for an annular plug nozzle. The cooling system performance is then analyzed and critically discussed through conjugate heat transfer simulations. Eventually, the third part of the thesis addresses specific problems concerning the commissioning of a facility whose goal is to perform experimental investigations of heat transfer phenomena in 3D printed cooling channels.