Numerical investigation of shell-side heat transfer in helically coiled tube heat exchangers: morpho-hydrodynamic effects and a data-driven multivariable Nusselt-number surrogate

Purpose This paper aims to investigate shell-side convective heat transfer in a vertically oriented helically coiled tube heat exchanger (HCTHEX) and develop a multivariable within-domain surrogate for the shell-side Nusselt number under fluid-to-fluid operation. Design/methodology/approach A steady-state conjugate Computational Fluid Dynamics (CFD) campaign was generated in ANSYS Fluent 2023 using liquid water on both sides of the exchanger. The campaign comprises 2,400 averaged operating points spanning shell-side and coil-side flow rates and dimensionless coil pitch. A final audit identified one isolated nonphysical shell-side Nusselt number record in the post-processed input data; this record was excluded without sign correction, leaving 2,399 physically admissible cases for model development. Generalized additive and bagged regression-tree ensemble models were then assessed using identical, reproducible fivefold cross-validation partitions and the better-generalizing model was refitted using all admissible cases for final deployment. Findings Shell-side Nusselt number increased overall with the hydraulic-diameter-based Reynolds number, but substantial scatter confirmed that Reynolds number alone cannot represent the response. Fivefold out-of-fold validation identified the bagged regression-tree ensemble as the better-generalizing data-driven model, with R-squared = 0.6519, root mean square error = 26.41 and mean absolute error = 17.68. The retained surrogate is recommended only for interpolation within the investigated geometry and operating envelope. Practical implications The surrogate supports interpolation, comparison and design-space screening only within the investigated Reynolds-number, pitch, tube-size, shell-size and thermal-property ranges; extrapolation to other HCTHEX geometries or flow regimes is not recommended without additional verification. Originality/value The study provides a dense conjugate CFD campaign and a data-driven shell-side Nusselt-number surrogate for a specifically defined HCTHEX geometry and operating envelope. The analysis shows that shell-side convection is strongly affected by coupled morpho-hydrodynamic effects rather than by a single Reynolds-number trend.