Numerical study on transient coke deposition and heat transfer characteristics of hydrocarbon fuel in a mini-channel under varying wall heat flux

Abstract

Dynamic coke deposition has significant influence on flow and heat transfer characteristics of regenerative cooling using hydrocarbon fuel. In the present work, the effect of coke deposition on flow and heat transfer performance of hydrocarbon fuel, n-Decane is numerically investigated under different heating modes including the constant heat flux (CHF), the periodic heat flux (PHF) and staged heat flux (SHF). The results show that as the heating mode varies from CHF to SHF, locally high temperature zones are formed. The flow field structure is varied by dynamic heating condition and coke deposition. The reaction rate of n-Decane also varies periodically with heat flux, resulting in an irregular “S” shaped density distribution. The two main coke precursors i.e., propylene and aromatics are highly sensitive to temperature due to secondary reactions. The thickness of coke deposition increases significantly with time regardless of the heating modes, which indicates that coke deposition process is irreversible. Compared with CHF heating mode, the thickness of coke deposition increases by nearly 25% for the SHF heating mode. Coking mechanism undergoes transition from catalytic coke to lateral coke with increasing temperature. For the CHF heating mode, the heat transfer coefficient increases from 1200 W/(m2·K) to 4400 W/(m2·K) along the flow direction. The PHF and SHF heating modes render higher Nusselt number and heat transfer coefficient than the CHF heating mode as the periodically varying wall heat flux results in locally higher heat flux and smaller temperature difference between the fluid and solid zones. In addition, the thermal conductivity and flow velocity in the regenerative cooling channel are also enhanced for the PHF and SHF heating modes. However, coke deposition leads to serious heat transfer deterioration for PHF and SHF with a decrease of up to 13% in heat transfer coefficient adjacent to the outlet section of the cooling channel. The new heat transfer correlation considering chemical reaction and coke deposition effects of supercritical hydrocarbon fuel is established for different wall heating conditions.