"The Main III"

I don't like to speak in extremely technical terms, but sometimes it becomes necessary. In order to control the quality of the flame in the burner, it is necessary to take care of the "Flame Moment" of two other control parameters.

Specific Impulse

Another of the control parameters of the main burner is: Specific Impulse.

This has a concept similar to the “Flame Moment”, but it is calculated through the product of the mass flow rate of the primary air by the speed of that air in the burner nozzle. It expresses the effect of flame stabilization due to a more energetic mixture between the fuel and the oxidant. The calculation is performed separately for each of the constituent airs of the primary air.

 Where:

I = Relative impulse of each of the airs that make up the primary air - axial, tangential, transport and central.

Qt = Mass flow rate of each of the airs that make up the primary air - axial, tangential, transport and central (kg/s) - calculated according to the measurement of gas flow rate.

Vax = Axial speed of each of the airs that make up the primary air - axial, tangential, transport and central (m/s).

The calculation of the velocity of each of the components of the primary air must be carried out as explained during the evaluation of the flame moment. It is important to highlight that the tangential air enters the oven with a certain tormentation due to the presence of a slight angular displacement that gives the air the appearance of a rose in the burner's mouth. Then he has two components, one being axial and the other radial.

Since it is only possible to measure the total velocity, the axial component of that velocity must be calculated. Other airs only have the axial component.

Vt = Total tangential air velocity

Vax = Axial velocity of the tangential air

Vtg = Tangential velocity of the tangential air

? = Angular offset

The total relative impulse will then be the sum of the relative impulse of each of the components, considering the axial component of the tangential air.

Where:

Itotal = Total impulse of primary air

In order to better evaluate the flame, it would be useful to use the so-called "Specific Impulse", which consists of a dimensionless number calculated from the ratio between the relative impulse of the primary air and the relative impulse of the secondary air.

That number expresses the balance between the effect of primary air in the formation of a rigid flame and the effect of secondary air to break that stiffness.

However, the determination of the relative impulse of the secondary air is difficult to evaluate because to calculate it it is necessary to use the temperature of the secondary air, which is only obtained through a balance in the cooler.

Then, instead of using the relative impulse of the secondary air, the thermal power of the burner is used, since it is proportional to the flow rate of the secondary air, this because the more fuel we use, the more air will be needed to effect the combustion.

Where:

Iesp = Specific impulse [N.h / GJ]

Itotal = Total impulse of primary air [N]

Pot = Burner thermal power [GJ / h]

Where:

Q comb = Mass flow rate of fuel in the combustion zone

PCI = Lower calorific value of fuel [GJ / kg]

Swirl

The third control parameter of the main burner is the Swirl, parallel to the specific impulse, the Swirl or rotational impulse is calculated, which represents the amount of movement that the burner imposes on the flame to cause its rotation. It is calculated through the following equations:

Where:

Irot = Rotational impulse [N.m]

Qradial = Tangential air mass flow rate [kg / s]

Vtangential = total tangential air velocity [m / s]

? = Angular offset

Rg = Rotation radius, relationship between the external and internal radii of the angular displacement [m]

Where:

R1 = Internal radius of angular displacement

R2 = External radius of angular displacement

? = Angular offset

The swirl is a relationship between the rotational impulse and the total relative impulse of the burner, it represents the opening or width of the flame.

Where:

Sw = Swirl [dimensionless]

Irot = Rotational impulse [N.m]

Itotal = Total relative impulse [N]

Keeping these parameters correctly adjusted allows a stable, strong and precise flame, the positive results are qualitative and quantitative. The optimization area professionals can help you with this adjustment. If you need specific help, hire a specialist in combustion and burners.

(For calculation purposes, the references of measures used in this article are those of "International Measurement System")

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