Effective thermal conductivity of a heat generating rod bundle dissipating heat by natural convection and radiation

► Transport processes in isothermal hexagonal sheath with 19 heat generating rods is studied. ► Correlation is given to predict the maximum temperature considering all transport processes. ► Effective thermal conductivity of rod bundle can be obtained using max temperature. ► Data on the critical Rayleigh numbers for p/ d ratios of 1.1–2.0 is presented. ► Radiative heat transfer contributes to heat dissipation of 38–65% of total heat. A numerical study of conjugate natural convection and surface radiation in a horizontal hexagonal sheath housing 19 solid heat generating rods with cladding and argon as the fill gas, is performed. The natural convection in the sheath is driven by the volumetric heat generation in the solid rods. The problem is solved using the FLUENT CFD code. A correlation is obtained to predict the maximum temperature in the rod bundle for different pitch-to-diameter ratios and heat generating rates. The effective thermal conductivity is related to the heat generation rate, maximum temperature and the sheath temperature. Results are presented for the dimensionless maximum temperature, Rayleigh number and the contribution of radiation with changing emissivity, total wattage and the pitch-to-diameter ratio. In the simulation of a larger system that contains a rod bundle, the effective thermal conductivity facilitates simplified modelling of the rod bundle by treating it as a solid of effective thermal conductivity. The parametric studies revealed that the contribution of radiation can be 38–65% of the total heat generation, for the parameter ranges chosen. Data for critical Rayleigh number above which natural convection comes into effect is also presented.