Thermal Performance Analysis of A Furnace P M V Subbarao Professor Mechanical Engineering Department Test for Cooling Capacity of Furnace Surface….

Further Geometrical Details of A Furnace

Determination of Furnace Size  = 30 to 50O  > 30O  = 50 to 55O E = 0.8 to 1.6 m d = 0.25 b to 0.33 b

Heat Transfer in A Furnace The flame transfers its heat energy to the water walls in the furnace by Radiation. Convective Heat Transfer < 5%. Only Radiation Heat Transfer is Considered for Performance Analysis!

Simplified Approach Thermal efficiency factor, y. Emitted Radiation heat flux of flame: Emitted Radiation = Available Heat Heat flux absorbed by walls : Thermal efficiency factor, y. The rate of heat absorption

Coal fired furnace Furnace Exit Two functions of coal fired furnace: Release of chemical energy by combustion of fuel Transfer of heat from flame to water walls Combustion space surrounded by water walls Furnace Exit Structure of water walls* Hot Exhaust gases Burner Flame Heat Radiation & Convection

Burner arrangement & flame shapes An array of burner installed on walls or at corners of furnace Fuel and combustion air projected from the each burner create a complex shape of flame. Intense mixing of fuel and air stream at the centre Tangential fired furnace* Down fired furnace Opposed wall fired furnace

Classification of Radiation Surface radiation Gas radiation Surface phenomena Volumetric phenomena Constant radiation intensity Variable intensity Basic radiation quantity- emissive power intensity Simple analysis Complex analysis

Types of Radiation from Flames Tri-atomic gases - CO2, H2O and SO2 Soot particles Coke particles Ash particles

Radiation inside furnace Types of radiation: Surface and volumetric radiation Characterization of participating media: usually, the radiant energy is scattered, absorbed and emitted by tiny suspended particles or gases like CO2 and water vapor, such media are called participating media. Gas radiation involved Absorption: attenuation of intensity  absorption coefficient  Emission: augmentation of intensity emission coefficient Scattering  scattering coefficient  Radiant heat transfer occur from the source (Flame) to sink (water walls) in a furnace

Gas radiation-Governing equation Assumptions: All six boundaries are diffuse and gray Absorbing, emitting, non scattering gray medium Same absorption coefficient at all points Thermophysical properties e.g. density, specific heat, thermal conductivity and optical property like extinction coefficient are constant. Absorption coefficient = emission coefficient Face 5 Face 4 Face 1 Face 2 x y z L W Face 6 Face 3 South North East West H w n e s μ η ξ Co-ordinate system for cubic enclosure Governing equation for participating media (RTE): Where; S is line of sight distance in the direction of propagation of the radiant intensity I

Basic models for RTE in gas radiation Optically Thin Self-absorbing Optically Thick Directional Averaging Differential Approximation Energy Hybrid DTM Ray Tracing Radiation Element 2-Flux 4-Flux Multiflux DOM Moment Modified- Moment PN - Approx. Zone MCM Numerical (FD, FV)