Numerical Simulation of Multiphase Flows in Spiraled Cooling Channels with High Surface Roughness

This thesis presents a numerical and analytical investigation of the regenerative cooling system of a 500 N liquid rocket engine developed by the German startup DeltaOrbit. The engine was originally designed to operate under single-phase liquid conditions; however, it is currently undergoing experimental testing and study to transition to a multiphase operating regime. The present case study considers a fully liquid flow at the inlet of the cooling circuit, which, through heat exchange with the combustion chamber, undergoes complete evaporation, reaching the outlet of the cooling channel and subsequently the combustion chamber, in a gaseous state. Beyond the multiphase nature of the system, this work also addresses the effect of the spiralized configuration of the cooling channels, which was specifically designed to enhance local heat transfer. Furthermore, considering the manufacturing method employed, the additive manufacturing, which inherently produces surfaces with high roughness levels, the impact of surface with high roughness on heat transfer performance and flow development within the cooling channels has been thoroughly investigated. The primary objective of this study is to evaluate the efficiency of the regenerative cooling system and to provide a foundation for future design modifications and further research efforts. The thesis is structured into two main phases: an initial analytical phase, during which a MATLAB code was developed to perform a one dimensional analysis of the cooling channels based on the thermoelectric analogy, followed by a numerical phase conducted using Ansys Fluent. In the analytical phase, particular attention was devoted to the identification of suitable models for the estimation of the heat transfer coefficient, as well as to the accurate prediction of the flow evolution and wall temperatures on both the cooling and hot gas sides. A comparison among all the heat transfer coefficient correlations available in the literature was conducted, followed by an investigation into the use of potential correction factors accounting for curvature and surface roughness, such as the Norris modification. In the numerical phase, significant effort was dedicated to the formulation and validation of a numerical model through the verification of a benchmark case, subsequently applied to the rocket engine cooling system. The numerical simulations were conducted using an Eulerian-Eulerian implicit mul tiphase model (EEMP) coupled with a Critical Heat Flux (CHF) boiling model. Adetailed justification for the selection of the models and sub-models adopted is provided within the thesis. Additionally, a comprehensive parametric study was performed on the Cylindrical Chamber section of the Cooling circuit, investigating the system’s response to varying boundary conditions: Pressure, sub cooling temper ature, vapor fraction and roughness. A comparison between the results obtained by both models (numerical and analytic) was performed. and also the nozzle section was studied with analytic model. All results are critically discussed, highlighting strengths and limitations of the study, and proposing possible improvements and directions for future work.