Numerical investigation of unsteady heat transfer from a vertical helical coil

Abstract

A comprehensive investigation was conducted utilizing three-dimensional transient Computational Fluid Dynamics (CFD) to analyze conjugate heat transfer, with a specific focus on determining the cooling time of a hot vertical helical coil. The heat transfer characteristics of the helical coil are assessed numerically with respect to parameters within certain ranges such as Rayleigh number 104≤Ra≤108, surface emissivity 0≤ε≤1, and geometrical parameters of the coil, such as coil-to-rod diameter ratio 8≤D/d≤24, and pitch-to-rod diameter ratio 3≤p/d≤7.5. Furthermore, a lumped heat capacitance model is also proposed, which significantly reduces computational time while maintaining accuracy by utilizing the steady-state correlation of Nusselt number vs. Rayleigh number (Nu vs. Ra) and other pertinent geometrical parameters of the coil. Comparative analysis between the lumped heat capacitance model and transient simulations reveals a remarkable correspondence, with deviations not exceeding 2%. Validation of the computational model is conducted against experimental data, showing good agreement. Results indicate that coils with higher pitch-to-rod diameter ratio (p/d) and coil-to-rod diameter ratio (D/d) dissipate heat more quickly, while higher initial Rayleigh numbers lead to faster cooling rates. Temperature distribution and Nusselt number variations during the cooling process are analyzed and compared between steady and unsteady behavior. The study provides valuable insights into the cooling behavior of helical coils, crucial for industrial applications requiring efficient heat transfer.