Analysis of energy transfer in industrial gas-fired radiant tube furnaces
Abstract
A thermal system mathematical model has been developed to predict heat transfer from the products of combustion in the radiant tubes to the ultimate load in the furnace. The three-dimensional thermal model for the furnace involved the integration of various submodels for the radiant tube and the furnace enclosure. For the radiant tube, mathematical models were developed to describe turbulent interdiffusion of fuel and air, combustion, flame radiation and NO$\sb{x}$ emissions from the system by solving the conservation equations of mass, momentum, chemical species and energy. The turbulent transport was modeled using a low Reynolds number k-$\epsilon$ turbulence model, and a modified weighted-sum-of-gray-gases model was employed for spectral radiative transfer calculations using the discrete ordinates approach. The reaction between fuel and air was assumed to be a single step, infinitely fast process, and the mean concentration of the species were obtained using an appropriate probability density function for the mixture fraction. The two-dimensional radiant tube model predictions were verified using available experimental data. The temperature distribution in the refractory walls and the load were obtained by calculating the incident heat fluxes from the radiant tubes via the radiosity method. The thermal models for batch and continuous furnaces were verified using experimental data obtained from industrial furnaces. Heat transfer enhancement methods were identified by performing parametric calculations using the furnace thermal models. The thermal system model revealed that radiation is the dominant mode of heat transfer from the radiant tubes to the load in the furnace. The parametric studies revealed that increasing load emissivities increased net heat transfer to the load and thereby the overall furnace efficiency. Highly reflective refractories helped reduce the temperature of the refractories with no adverse effect on the furnace efficiency. Although increasing the fuel firing rate in the radiant tubes increased the net heat transfer rate to the load, the furnace efficiency decreased due to increased loss of energy at the radiant tube exhaust. The analysis using different stock materials in the furnace indicated that different optimum firing strategies exist for different stock materials.
Degree
Ph.D.
Advisors
Ramadhyani, Purdue University.
Subject Area
Mechanical engineering
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