Natural Convection and Radiative Heat Transfer From Constant Surface Area Vertical Helical Coils: Effect of Pitch and Height

Abstract

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 finitevolume method are carried out in the laminar regime for the following nondimensional parameter ranges: Rayleigh number (104 ≤ Ra ≤ 108), surface emissivity of the coil (0 ≤ ɛ ≤ 1), pitch to the roddiameter of the coil (3 ≤ p/d ≤ 7.5), and coilheight to the roddiameter (40 ≤ H/d ≤ 60). Temperaturedependent fluid properties have been implemented to obtain accurate results. The impact of Ra and ɛ on both convective and radiative heat losses is discussed in detail. At a high Ra of 108, when H/d varies from 40 to 60, the mass flowrate 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 of 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 of the coil surface of 0.1 and a higher Ra of 108.