Numerical studies on heat transfer enhancement by hollow-cross disk for cracking coils

A novel comprehensive vortex generator, hollow-cross disk (HCD) which comprises a special sinusoidal wave structure, has been proposed to modify the configuration for cracking coil. The effects of HCD on the turbulent flow patterns, heat transfer characteristics and micro-mixing efficiency of the flow are investigated by CFD modelling in the present work. The numerical results demonstrate that in the test range of flow rates, the vorticial flow induced by HCD couples with CVPs and hairpin-like vortex, efficiently enhancing the convective heat transfer locally and globally. In contrast, although the global swirling flow induced by a twisted-tape can enhance the turbulence intensity, larger pressure drop taking place in the coil leads to relatively poor comprehensive heat transfer performance and lower micro-mixing efficiency. Moreover, the mechanisms of heat transfer enhancement for the cases with inserts have been investigated by the synergy regulation and the synergy between the temperature gradient and velocity has been discussed. The results indicate that better synergy occurring in the vicinity of coil surface is a key factor to enhance heat transfer for the cracking process. The obtained results can be used as a guidance for the design and optimization of the inserts for the cracking process.