Average Wall Radiative Heat Transfer Characteristic of Isothermal Radiative Medium in Inner Straight Fin Tubes

Wall radiative heat transfer in inner straight fin tubes is very complex considering the coupling of heat conduction in fins and radiative heat transfer of medium with solid surfaces, influenced by a number of factors such as fin parameters, radiative properties and run conditions. The method of simulation is very inconvenient to design the heat exchanger involving the enhancement of inner straight fins on wall radiative heat transfer in tube, such as ascension pipe heat exchanger and radiant syngas cooler. In this study, a simplified method is used. The average radiative heat transfer between radiative medium and solid surfaces is first studied by simulation with fins supposed have a constant temperature. Then an approximate correlation of this radiative heat transfer coefficient is proposed using the traditional radiative heat transfer calculation method with a view coefficient, which has a little error within 15%. A calculation method of average wall radiative heat transfer coefficient is further derived based on fin theory with an average temperature of fin surface used to consider the varying of the temperature along the fin when the conductivity of fins is finite. Using the predicting method proposed, a method for design calculation of fins in tubes to optimize wall radiative heat transfer is also given with three dimensionless numbers of π/n, 2H/D, and nt/πD representing the structural properties of fins defined. Three cases of (k_s = 43 W/(m•K), ɛ_w = 0.7, T_g = 1073.15 K, T_w = 573.15 K, D = 0.66 m, κD = 1.32), (k_s = 162 W/(m•K), ɛ_w = 0.7, T_g = 1073.15 K, T_w = 573.15 K, D = 0.66 m, κD  = 1.32) and (k_s = 43 W/(m•K), ɛ_w = 0.3, T_g = 973.15 K, T_w = 673.15 K, D = 0.66 m, κD = 0.66) are analyzed in detail based on the design calculation method. It is verified that the radiative heat transfer could be enhanced twice by introducing fins. Under the same h₀, conductivity and emissivity are two important factors that should be considered to choose the material for fins. The microfins or the special treatments on the tube wall are a best choice for the fin material having a relatively small conductivity.