Natural convection and radiative heat transfer from constant surface area vertical helical coils: Effect of pitch and height

In this work, numerical simulations are carried out to delineate the natural convection and surface radiation heat transfer characteristics of vertically oriented isothermal helical coils having a constant surface area. Numerical computations using the finite-volume method (FVM) are carried out in the laminar regime for the following non-dimensional parameter ranges: Rayleigh number (10⁴ ≤ Ra ≤ 10⁸), surface emissivity of the coil (0 ≤ ɛ ≤ 1), pitch to the rod-diameter of the coil (3 ≤ p/d ≤ 7.5), and coil-height to the rod-diameter (40 ≤ H/d ≤ 60). Temperature-dependent fluid properties have been implemented to obtain accurate results. The impact of Ra and ɛ on both convective and radiative heat losses are discussed in detail. At a high Ra of 10⁸, when H/d varies from 40 to 60, the mass flow rate inducted through the coil reduces from 40.6% at p/d = 3 to 11.4% at p/d = 7.5. As a result, the relative strength of convection heat loss declines with a rise in H/d. For a higher emissivity of the coil surface (ɛ = 0.9) and a lower Ra of 10⁴, heat transfer by convection contributes only 12.66% of the total heat transfer. In contrast, the contribution of radiative heat transfer is only 7.46% for a lower emissivity (ɛ= 0.1) and a higher Ra of 10⁸.