21-06-2012, 02:32 PM
Flame Temperature
04-Flame Temperature.ppt (Size: 743 KB / Downloads: 267)
Energy Balance on a System
A simple steady-state thermal energy balance can be constructed around a constant-pressure combustion system (such as a boiler).
Next slide (Fig. 4.1)The energy flows into and out of the system.
If the gross calorific value is used, then HR should contain a latent heat term equal to the mass of water produced per kilogram of fuel multiplied by the latent heat of evaporation of water at 25℃ (hfg).
If the net calorific value is used, then the flue gas enthalpy will consist of sensible heat terms only.
In this chapter we are concerned with predicting the temperature reached within the flame, hence the net calorific value/sensible heat terms system is the more appropriate.
Adiabatic Flame Temperature
Table 4.1 shows flame temperatures for some common fuels. The higher the flame temperature, the greater should be the effectiveness of the heat exchanger section.
Specific Heats of Gases
Simple ideal gas theory predicts that the specific heat of a gas is not a function of its temperature or pressure.
While the latter implication is effectively true in practice, the specific heat of a gas does increase with temperature above about 100℃. This effect arises because molecules have vibrational (internal) energy as well as kinetic energy due to the motion of the complete molecule.