Radiation burner a radiation burner is a type of burner
author：Martin 2019-12-17
Radiant burner a radiant burner is a burner that, by taking certain measures, allows the combustible mixture to burn vigorously on the surface of the burner without the appearance of a luminous flame.The burner itself is in a hot state, and most of the chemical energy of the fuel is exchanged with the outside world in the form of radiation through the wall of the burner.
Burner principle
In a radiant burner, most of the energy carried by the fuel is converted to radiant energy.Radiant heat is related to the temperature of the radiant surface and the material.The radiation burner is a full premixed low pressure burner.When the medium on the burner surface reaches a certain temperature, the gas radiates infrared ray outward.When the object receives the infrared ray irradiation, it absorbs part of it, reflects part of it, and the absorbed part is converted into the energy of the object itself, which makes the thermal motion of the object molecules become more intense, achieving the effect of heating.
Early studies
At the beginning of this century, Bone et al. observed that gas-air mixture flowed out of small holes in fire-resistant tiles and burned fiercely on the surface of tiles, making the tiles hot but without obvious flame.Bohn called this "surface combustion," which he attributed to the catalytic action of refractory materials.Subsequent studies have shown that no catalysis occurs, that thermal conductivity is more important than surface activity, and that the mixed air passing through the pores in the refractory accelerates combustion due to preheating.No clear evidence of surface activity has been found in the study of multi-channel refractory bricks, but Coles and Bagge, based on their experimental studies on porous media burners with very small pores, believe that the mechanism of this phenomenon is a combination of gas combustion and surface-catalyzed combustion.
Classification of radiant burners
1.Porous media burner
The mixture flows from the porous ceramic plate and burns on its surface to produce a uniform radiant surface.Figure 1 (a)
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The surface temperature of the model shown is about 860 ° c and can be higher when different materials are used.The luminescent wall design developed from this model (FIG. 2b) consists largely of porous firebrick blocks.It USES a standard specification of porous firebrick with holes drilled 50mm from the brick face.The mixture from the forced air supply mixer enters the forced air supply chamber, reaches the back of the refractory brick, and flows out to the surface for uniform combustion through holes in the refractory brick.During the mixture preheating process, the refractory bricks are cooled so that only a thin layer can reach high temperatures.In this way, the heat loss of the wall is minimized and the frequent heating and cooling of the burner structure is reduced.The burners typically have a thermal strength of 250 kw/m 2, up to 470 kw/m 2.The surface temperature is generally limited to 1100℃.[3]
2.A burner burning in a block passage
The most common is the schwank burner (FIG. 1c), where the mixture is fed by an atmospheric ejector and burns on the surface through a hole about 1.4 mm in diameter at a temperature of about 850 ° c.The radiant surface is composed of ceramic panels each 65 by 45 mm in size.When artificial gas is used, the position of the flame root is about 1.5 mm below the plate surface, and the flame surface is in the balance between the flow rate and combustion rate, which is restricted by the channel diameter and pre-heat of mixture.After the conversion of natural gas, the flame surface is no longer lower than the plate surface, reducing the pre-heat to the mixture and thus increasing the deignition tendency.The solution to this problem is to install a metal mesh about 5mm above the block surface, which reflects the heat back and increases the preheating of the mixture, thus stabilizing the flame.[3]
3.Cup type radiant burner, flat flame burner
These burners differ from each other in that the mixture does not flow through channels in the radiant block
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Out, its radiant surface is a cup-shaped or cone-shaped refractory block, which is fed with mixture from its center, or with gas and air separately mixed by nozzle mixing.The design often includes a flame-stabilizing deflector, which distributes the mixture evenly on the fire-resistant surface to form a thin layer for combustion.[3]
4. Radiant tube burner
One alternative to the muffle furnace, where the combustion product needs to be separated from the heated workpiece, is to use an internal-combustion radiant tube.Since most of the heat is transferred by radiation, it is also a type of radiant burner.
5.Mesh burner
In low temperature applications, mesh burners are often used instead of porous plate burners.A Swiss model USES a double-layer mesh, with the inner layer kept cool to prevent tempering.Surface temperature up to 900℃, but only for low temperature environment to avoid the burner overheating.Figure 2(d) shows the application of this burner in the overhead radiant radiator.It includes an atmospheric WBC burner, which is used to heat a truncated tapered mesh cover made of ceramic fibers to a temperature of up to 1200 ° c.The temperature of the cylindrical stainless steel mesh cover outside is 850℃, forming a radiant surface.[3]
6.Catalytic burner
On the surface of the catalytic material, the gas can be burned at a very low temperature (450℃ for natural gas) without producing a visible flame, but with the same calorific value as normal high temperature combustion.In continental Europe, LPG smokeless catalytic furnaces were widely used for domestic and commercial purposes, while in the Soviet union and the United States, LPG and gas catalytic burners were used for non-domestic heating applications.There are no such combustion devices on the UK market, but the Watson institute is still studying their availability in heating.
In principle, a catalytic burner consists of a porous plate filled with a catalytic material through which pure gas passes and oxygen from the outside passes in the opposite direction, resulting in a combustion reaction on the surface of the catalytic material.Heat is radiated from the surface and combustion products are pushed into the atmosphere by air currents.
Performance of radiant burner
The most convenient way to express the performance of a radiant burner is to find out its radiation efficiency, that is, the radiation transport
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Output/heat input.Coles and bargh have calculated the ideal radiation efficiency at various surface temperatures at low calorific values by using the relation between the radiation output and the flue gas heat output equal to the heat input.Their calculations assume that the radiator is a black body and that there is no other heat loss (such as the heat loss of the burner wall).Figure 3 shows the radiation efficiency of the ideal radiant heater changing with the surface temperature.