Analysis of coolant flow and heat transfer in highly rough channels for LRE
Creators: name:  B. LATINI 
name:   M. FIORE 
name:   F. NASUTI 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: PROPULSION PHYSICS
LIQUID ROCKET ENGINE
CHAMBER COOLING

description: The design of modern rocket thrust chambers of liquid rocket engines relies on additive layer manufacturing (ALM) techniques, which are replacing other traditional techniques because of their advantages in terms of production costs and times. A consequence of ALM processes is the rough as-built surface roughness, which leads to friction and heat transfer increase, although heat transfer enhancement is less than the friction one because of different physical phenomena [1]. Dealing with cooling channels of tiny cross section (Dh 3 mm ), they are characterized by high relative surface roughness as a result of the manufacturing process. Despite the negative aspect of having high roughness because of the corresponding high friction losses, the acceptance of a high roughness design can be interesting as a mean for increasing heat transfer and thus cooling efficiency, in particular when considering methane as a possible fuel and replacement of hydrogen [2]. Such designs constitute a challenge for the heat exchange performance prediction by engineering or numerical simulations, because of the lack or data and modeling of the phenomenon. Particularly, in the field of RANS solver, the most common approaches to account for wall roughness are based on the equivalent sand grain approach. This approach links the real roughness to an idealized equivalent sand grain roughness, with reference to Nikuradse\'s experiments. The roughness effect is mimicked by increasing the turbulent eddy viscosity in the wall region to obtain higher skin friction and heat transfer levels. However, it preserves the Reynolds analogy with the consequence of a high overestimation of heat transfer capabilities when high roughness are considered. To respond to the request of a reliable prediction model, the present study proposes the extension and calibration of a roughness correction aiming to improve RANS model predictions, more specifically by developing a thermal correction able to take into account the role of high roughness. The revised roughness model is developed in the framework of the Spalart-Allmaras turbulence closure [3]. Because of the lack of experimental data, the proposed model is based on the correlation of Dipprey and Sabersky for heat transfer on fully rough surfaces [4]. The thermal correction is based on the turbulent Prandtl number increase in the wall region, with the purpose of decreasing heat transfer to the wall. To calibrate the model, RANS simulations are carried out for a uniformly heated axisymmetric channel, whose operating conditions are changed to analyze a wide range of roughness levels and Reynolds numbers. Moreover, different Prandtl number fluids have been considered. The implementation of the proposed correction allows to predict realistic values of the convective heat transfer coefficient, in agreement with the experimental results reported in the literature.