Engineering task and calculation objective
The FLAM module calculates the heat transfer in the combustion chamber of fired equipment – such as heaters with an internal helical coil, hot-oil heaters or flame-tube boilers. Starting from the fuel quantity or the standard volume flow of the fuel, it determines flame length, flame diameter and the theoretical and actual flame temperatures, and from these the heat flux densities on the cooled heating surfaces.
The total load on the internal coil is composed of three contributions: convective heat transfer, gas radiation of the flue gas (CO2 and H2O fractions) and flame radiation of the luminous flame. For the convection zone above the flame, the heat transfer coefficient is calculated in addition. The flue gas properties required for this – density, specific heat capacity, dynamic viscosity and thermal conductivity – are obtained from a coupled flue gas module (RGas).
In practice, the combustion chamber calculation is needed to limit the maximum local heat flux density (film temperature of the heat transfer fluid, coking risk with thermal oil), to fix the flue gas exit temperature for the downstream convective heating surfaces, and to size the heating surface distribution of a firing system.
Calculation workflow
- Define fuel and flue gas data: The fuel quantity or the standard volume flow of the fuel is specified; through the coupling to the RGas module (number of the RGas module), FLAM receives the flue gas mass flow, the flue gas mass per kg of fuel, and the CO2 and H2O concentrations in the flue gas.
- Determine flame geometry and temperature: Flame length and flame diameter are estimated from the firing rate. The theoretical (adiabatic) flame temperature follows from the combustion calculation; the actual flame temperature is lower because of the heat released to the cooled walls.
- Evaluate the flue gas properties: At the mean flue gas temperature, the density, specific heat capacity, dynamic viscosity and thermal conductivity of the flue gas are determined – the basis for the convective heat transfer calculation.
- Calculate the radiation contributions: Flame radiation is captured via the emissivity ratio of the luminous flame to the cooled wall, gas radiation via the emissivities of the CO2 and H2O fractions at the mean beam length of the combustion chamber. Both yield their own heat flux densities (qF and qGs).
- Form convection and total load: For the convection zone above the flame, the heat transfer coefficient is calculated and from it the convective heat flux density qkon. The heat flux density on the internal coil results as the sum qkon + qGs + qF and serves as the design value for the heating surface.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Brennstoffs | Brennstoffs | – |
| H_u | H_u | J/kg |
| Brennerleistung | Brennerleistung | W |
| Verbrennungslufttemperatur | Verbrennungslufttemperatur | °C |
| Verbrennungsluft | Verbrennungsluft | kg/kg |
| Luftzahl | Luftzahl | -- |
| Innenwendeln | Innenwendeln | m |
| Abgas-Rückführung | Abgas-Rückführung | -- |
| Rauchgas-Rückführung | Rauchgas-Rückführung | °C |
| Flammenbereich | Flammenbereich | °C |
| Wand | Wand | -- |
| f_A | f_A | -- |
| J/N | J/N | – |
| Flammendurchmesser | Flammendurchmesser | -- |
| Flammenlänge | Flammenlänge | -- |
| Flamme | Flamme | °C |
| Innenwendel | Innenwendel | m |
| Endtemperatur | Endtemperatur | °C |
| Druck | Druck | Pa |
| Brennstoffs | Brennstoffs | kg/m³ |
| RGas-Moduls | RGas-Moduls | – |
| Flammenlänge | Flammenlänge | m |
| Flammendurchmesser | Flammendurchmesser | m |
| Flammentemperatur | Flammentemperatur | °C |
Frequently asked questions
Why is the actual flame temperature significantly below the theoretical one?
The theoretical flame temperature applies to adiabatic combustion without heat release. In a real combustion chamber, the flame radiates to the cooled walls already during combustion and mixes with recirculating flue gas; in addition, CO2 and H2O dissociate at high temperatures. The actual flame temperature is therefore the governing value for the radiation calculation.
What does the emissivity ratio of the luminous flame depend on?
On the fuel and the combustion regime: oil and solid-fuel flames are highly luminous due to soot particles (high emissivities), while natural gas flames are weakly luminous and radiate mainly through the gas bands of CO2 and H2O. An emissivity ratio set too high overestimates the wall load in the flame region, one set too low underestimates it – the value should match the burner type.
Why is the maximum heat flux density on the internal coil a critical design value?
For thermal oil and hot water heaters, the allowable film temperature of the heat transfer fluid limits the local heat flux density. If it is exceeded, the thermal oil cracks or cokes on the inner tube wall, the deposit further degrades the cooling and can lead to tube failure. The combustion chamber calculation delivers exactly this peak value in the radiant section.
What is the mean flue gas temperature needed for?
It is the reference temperature for the flue gas properties and, together with the flue gas mass flow, determines the heat balance of the combustion chamber. The resulting combustion chamber exit temperature is at the same time the inlet temperature of the downstream convective heating surfaces and thus the interface to the further boiler design.