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

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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.

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