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 pro perties and run conditions. In this study, a simplified method is used.The average radiative heat transfer between radiative medium and solid surfaces is firstly studied by simulation with fins assumed having 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, having a error within 15%. A calculation method of average wall radiative heat transfer coefficient is further proposed by 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 p/n, 2H/D and nt/pD defined. Three cases of 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 to choose the material for fins.The micro-fins or the special treatments on the tube wall are a best choice for the fin material having a relatively small conductivity.

Determination of Radiative Heat Transfer Coefficient at High Temperatures Using a Combined Experimental-Computational Technique

The radiative heat transfer coefficient at high temperatures is determined using a combination of experimental measurement and computational modeling. In the experimental part, cement mortar specimen is heated in a laboratory furnace to 600°C and the temperature field inside is recorded using built-in K-type thermocouples connected to a data logger. The measured temperatures are then used as input parameters in the three dimensional computational modeling whose objective is to find the best correlation between the measured and calculated data via four free parameters, namely the thermal conductivity of the specimen, effective thermal conductivity of thermal insulation, and heat transfer coefficients at normal and high temperatures. The optimization procedure which is performed using the genetic algorithms provides the value of the high-temperature radiative heat transfer coefficient of 3.64 W/(m2K).