Flames with prof Dr elkady mechanical engineering.pptx

Flames with prof Dr elkady mechanical engineering.pptx

1. FLAMES

2. Related Definitions Flames . Laminar Premixed Flame. Laminar Diffusion Flame . Presentation Outline

3. 1. Related Definitions Combustible mixture : ( flammable mixture) is one which is capable of propagation a flame indefinitely away from an ignition source. Flammability limit : it is the composition of the fuel-oxidant mixtures that will sustain a stable flame. There are two types of limits associated with the propagation of a laminar flame. The first is associated with the chemical reactive capability of the mixture to support a flame, and the second is associated with gas flow influences . Mixtures will not burn when composition is lower than the lower flammable limit (LFL), or above the upper flammable limit (UFL).

4. 2.Flames It is a thermal wave in which rapid exothermic chemical reaction occurs and travels with subsonic velocities ( deflagration wave .) Most flames result from highly exothermic reactions giving  flame temperatures about 2200 K although flames may be capable of burning down to about 1300 K , depending on the fuel-air ratio. Certain flames can be sustained below this temperature and are termed " cool" flames , only partial combustion occurs . Flame consists mainly of two zones: preheat zone : where little heat is released, reaction zone : where bulk of chemical energy is released.

5. Flames (cont.) Different types of flames can result from the way in which the fuel and oxidant are mixed in a burner and by their flow rates .   Premixed   gas flames can arise from fuel gas and air being mixed prior to entering the burner. If they mix after leaving the burner they are called  diffusion flames . The gas flow rate may be relatively low, in which case, the incoming gaseous flow of fuel and air is laminar, as is the flame. With high gas flows, they may be turbulent.

6. Flames (cont .) F lames can be laminar premixed, laminar diffusion, turbulent premixed or turbulent diffusion flames . The transition from laminar to turbulent flame takes place as the flow velocity increases . Flames can also be categorized into stationary flames or propagating flames , the former being the most widely used in domestic or industrial burners, the latter being involved in explosions.

7. Flames (cont.)

8. Flames (cont.) Flames can be classified to: premixed or diffusion laminar or turbulent homogeneous or heterogeneous stationary or traveling deflagration or detonation luminous or non-luminous

9. Flames (cont.) The flame velocity su is the velocity at which the unburned gases move through the combustion wave in a direction normal to the wave surface. The flame velocity depends on: Initial temperature. Pressure. Composition of the gases, ”combustible mixture”.

10. 3. Laminar Premixed Flames Bunsen burner is a good example of laminar pre-mixed flames, a type of flame widely used in gas cookers and central heating units. The type of flame that has been most studied is the laminar premixed flame of a gaseous fuel and air , because it is the simplest flame and exhibits characteristics common to many other systems.

11. 3. Laminar Premixed Flames (cont.) The Bunsen burner, however, illustrates both the premixed flame and the principle of the diffusion flame. The inner core is the reaction zone of a premixed flame, but the flame is fuel-rich, so the products of incomplete combustion burn in the outer core as a diffusion flame with the surrounding air. The exact nature of the flame is determined by the  fuel to air ratio .

12. 3. Laminar Premixed Flames (cont.) The combustion products would be represented by the following stoichiometric equation, : where –ΔH c  is the heat released by combustion, known as the heat of  combustion or calorific value  (cv).

13. 3. Laminar Premixed Flames (cont .) The region of incomplete combustion in the flame shown in figure represents only partial combustion of the fuel resulting in carbon monoxide and hydrogen which subsequently burns with the secondary air to give CO 2  and H 2 O

14. 21 3. Laminar Premixed Flames (cont .) The mixing chamber must be long enough to generate a premixed gas issuing from the Bunsen tube into the surroundings. If the velocity of the issuing flow is larger than the laminar burning velocity, a Bunsen flame cone establishes at the top of the tube.

15. 22 3. Laminar Premixed Flames (cont .) Classical device to generate a laminar premixed flame . Gaseous fuel enters into the mixing chamber , into which air is entrained. The velocity of the jet entering into the mixing chamber may be varied and the entrainment of the air and the mixing can be optimized .

16. 23 3. Laminar Premixed Flames (cont .) Fuel-rich pre-mixed inner flame Secondary diffusion flame Results when CO and H products from rich inner flame encounter ambient air

17. Michelson model

18. 3. Laminar Pre-mixed Flames (cont .) The actual visible flame differs from Michelson model in the following respects: The tip of an actual Bunsen burner is rounded instead of pointed one as given theoretically, and the base of theoretical flame is equal to the inner diameter of the tube while the actual flame overlaps the burner. The shape of the flame is determined by velocity profile, flame speed, heat loss to tube wall The flame remains stationary if : flame speed is equal to the speed of normal component of unburned gas at each location.

19. 3. Laminar Premixed Flames (cont .) Flame velocity of laminar premixed flames depends on:   fuel type fuel-oxidant mass ratio (equivalence ratio) initial temperature of combustible mixture pressure flow pattern. 6. geometry of system

20. Preheat zone : The mass element gains heat by conduction from the hotter elements down stream faster than it losses heat to cooler elements up stream. little heat evolved. Reaction zone : increased rate of chemical reaction . more energy is evolved due to chemical reaction. 3.Laminar Premixed Flames (cont.)

21. 3. Laminar Premixed Flames (cont.)   Flame propagation refers to the propagation of the reaction zone or “combustion wave ” through a combustible mixture. When the transport of heat and active species (free radicals) have initiated chemical reaction in the adjacent layer of the combustible mixture, the layer itself becomes the source of heat and radicals and is then capable of initiating reaction in the next layer.

22. 3. Laminar Premixed Flames (cont.)   A quantitative theory of flame propagation must be based on the transfer of heat and mass from the reaction zone to the unburned mixture.  The enthalpy rise across the flame due to combustion is balanced by conduction from the reaction zone.  

23. Laminar Premixed Flames (cont.)   It is possible therefore to stabilize a flame (obtain a stationary flame) at gas flow rates higher than the rate of flame propagation. This is the reason why Bunsen burners can maintain flames over a range of flow rates and fuel oxidant mixture ratios . A flat flame burner is stable only for the flow rate of the gas that exactly matches the flame velocity.

24. The Structure Of A Laminar Premixed Flame Microscopic structure ( Temperature and concentration gradients across the combustion wave .) Macroscopic structure ( flame shape .)

25. Factors Affecting The Flame Shape The shape of a flame is mainly governed by two factors : The flow pattern of the mixture or products. The quenching effect of the solid surface.

26. Factors Controlling the Rate of Flame Propagation Rate of heat transfer . (from reaction zone to the adjacent heating zone) . Diffusion of radicals or chain carriers (from the reaction zone to unburnt gases). Chemical kinetics .( of individual reactions in the mechanism).

27. Difference Between Diffusion Flames and Premixed Flames For Diffusion Flame: 1- C ombustion occurs at the interface between the fuel gas and oxidant gas . 2- The burning process depends more upon the rate of mixing than on the rate of chemical r eactions involved .

28. 4. Laminar Diffusion Flame

29. 4. Laminar Diffusion Flame In the case of laminar diffusion flames the fuel and the oxidant only meet at the burner mouth and mix by diffusion processes .  Axisymmetric diffusion flames can be obtained by the use of concentric tubes with the fuel usually entering via the inner tube. Diffusion applies strictly to molecular diffusion of chemical species In turbulent diffusion flames , turbulent convection mixes fuel and air macroscopically, then molecular mixing completes the process so that chemical reactions can take place

30. 4. Laminar Diffusion Flame (cont.) In slow burning diffusion f lames such as candle flame 1-fuel rises slowly and laminar flow ensures. 2-The mixing process occurs solely by molecular diffusion.

31. 4. Laminar Diffusion Flame (cont.) In diffusion flames the reaction occurs mainly in the maximum temperature region of the flame , but in the premixed flame the reaction occurs before the maximum temperature is reached . Diffusion flames are used more frequently in industry. Burning rate : is determined by the rate at which the fuel and oxidizer are brought together in proper proportion for the reaction.

32. Effect Of Gas Flow On Diffusion Flame Shape The laminar characteristic of the diffusion flame changes with increasing the gas flow . Break point : is defined as the point where the laminar stream changes to turbulent .

33. Concentration profiles through a laminar Diffusion flame

34. Any Questions