Three-dimensional coking simulation of endothermic hydrocarbon fuels in rectangular cooling channels

Abstract

Coking deposition is a critical phenomenon for endothermal hydrocarbon fuel cooling that can significantly affect the performance of a regenerative cooling system. Because of the complicated interactions between fluid flow, heat transfer, fuel cracking, and precursor coking kinetics, previous numerical studies have been limited to simplified two-dimensional circular channels, which cannot reveal the actual spatial distribution with consideration of buoyancy effect in rectangular cooling channels. This work proposes a novel framework for shrinking motion with an O-type hybrid mesh, permitting the direct three-dimensional simulation of coke deposition in complex channels and the visualization of both the axial and circumferential deposition distributions. The concept is tested in a rectangular channel using n-decane as an example, combining a detailed pyrolysis kinetic model with the MC-II coking model, and predictive results have been obtained. Results indicate two locations with heavy deposition rates. The buoyancy effect is weakened due to the acceleration resulting from the reduced cross-sectional area of the channel by the coke layer. The coupling of the flow and pyrolysis is discussed in terms of the dimensionless Damköhler number. The maximum temperature after coking can be 138 K higher than the initial. However, the conversion of n-decane at outlet is decreased due to the reduced flow residence time. The decreased total heat sink per temperature increment and the higher pressure drop are also the penalties from coking. The new framework for the direct three-dimensional simulations of coking is significant for the comprehensive investigation of the efficiency of regenerative cooling.