Numerical investigation of heat transfer enhancement in shell-and-tube heat exchangers with helically coiled tube for low-temperature cold-start applications

Shell-and-helically coiled tube heat exchangers (SHCTHEs) are widely used in industry, yet limited research exists on natural convection and heat transfer enhancement strategies for such exchangers. This study presents a numerical investigation of thermal performance and associated oil natural convection behavior in a SHCTHE for a diesel engine lubricant tank under low-temperature cold-start conditions. The accuracy of numerical method was validated experimentally. For the baseline finless tube, the influence of coil pitch on heating performance was analyzed, elucidating natural convection mechanisms in low-viscosity oils at varying pitches. Subsequently, Annular coil fins were then added to optimize heat transfer. Parametric studies focused on fin geometry, height, and fin cycles, with Rayleigh and Nusselt number correlations identifying the enhancement mechanisms. Lastly, the optimal fin structure was obtained through the overall performance factor. Results showed that closely spaced smooth coils promoted the development of the thermal boundary layer, which inhibited heat transfer. As coil pitch increased from 1.05 to 1.2, the average oil temperature rose from 18.89 °C to 34.41 °C, with heat transfer power improving by 23.2 %. Further increases in coil pitch had minimal effect on the performance, with the optimal result at a coil pitch of 1.8. This increase weakened the thermal boundary layer on the tube wall, enhancing natural convection and improving heat transfer. For the tube with annular coil fins, fin shape had little effect on heat transfer, while increasing fin cycles and height improved performance by improving thermal conductivity. Additionally, excessive fin length suppressed the occurrence of natural convection in the oil. The optimal fin configuration was found to be a fin height-to-pitch ratio of 0.8 and 200 fin cycles, balancing heat transfer efficiency and material consumption.