Flame theory - structure - its phenomena

This chapter explains about the the flame propagation - an its properties

1. Lecture 7 Flame Phenomena – Structure speed and theory By Er . T. AYISHA NAZIBA, Dr. D. RAMESH, Dr.S . PUGALENDHI, Dept. of REE, AEC & RI, TNAU, Coimbatore-03

2. FLAMES A flame (from Latin flamma ) is the visible, gaseous part of a fire It is caused by a highly exothermic reaction taking place in a thin zone Flame is a luminous zone of the rapid exothermic reaction in combustion of vapour with the formation of light and heat energy A non-luminous region is appeared just after the flame where the temperature is slightly reduced. A flame is bounded between the ignition zone and a non-luminous gaseous zone.

3. FORMATION OF FLAME If the combustible substances produce vapour during burning process, a flame is produced. The combustion of gaseous fuels in a flame need the intimate contact of fuels with an oxidant, either oxygen or air prior to the reaction. The ranges of flammability and the point at which the mixture spontaneously ignites must be known. They must be heated to the combustion temperature and the flame produced will be at a high temperature. Then the reaction take place in within a narrow zone or region in the flame. This combustion zone is called the flame front with this mixture is often several thousand degrees.

4. FLAME TYPES Depending on the amount of oxygen  available  for burning the flames can be of two types Non­luminous  or blue flame Luminous flame When the supply of oxygen is sufficiently large, the combustion is complete and fuel burns with a blue flame.  When the supply the oxygen is insufficient, the combustion is not complete and in the flame some unburnt carbon particles are formed. For example, the flame in a pressure stove. These carbon particles become hot and glow in flame. As a result, the flame emits yellow light. This type of flame is, therefore called luminous flame. For example, the flame of a kerosene lamp. This type of flame does not give much light and is called non­luminous  flame. In kerosene lamp the fuel does not undergo complete combustion due to the insufficient supply of oxygen.

5. FLAME TYPES

6. Flame theory

7. Thermal theory

8. Inner cone (dark zone): fuel rich flame Preheating region containing fuel and air Outer cone (luminous zone): reaction and heat transfer Outer diffusion flame Typical Bunsen-burner flame is a dual flame a fuel-rich premixed inner flame a diffusion outer flame: CO and H 2 from inner flame encounter ambient air TYPICAL BUNSEN-BURNER CH 4 /AIR FLAME

9. Fuel/Air Ratio Flame colour, i.e. colour of the outer cone Fuel lean Stochiometric Fuel rich Very fuel rich Deep Violet due to large concentrations of excited CH radicals Blue Green due to large concentrations of C 2 species Yellow due to carbon particles High-T burned gases usually show a reddish glow due to radiation from CO 2 and H 2 O BASIC FEATURES OF LAMINAR PREMIXED FLAMES

10. FLAME STRUCTURE In the premixed type, the laminar flame is the most simple type. The structure of the flame may be analysed by a flame in the burner. The flame consists of four distinct regions 1. Zone containing unburnt gases 2. Reaction zone 3. Incomplete combustion zone, and 4. Complete combustion zone. The idealized shape of the reaction zone of a laminar premixed flame is a cone. The height of the cone represents the flame length, and depends on the velocity at the burner outlet.

11. STRUCTURE OF A CANDLE FLAME Outer zone (blue ) Non luminous zone (Complete combustion) Middle zone (yellow ) Luminous zone (Partial combustion) Inner zone (black ) Unburnt wax vapours Hottest part Moderately hot Least hot

12. TYPES OF COMBUSTION BASED ON FLAMES Combustion with stationary flame Surface combustion / flameless combustion Submerged combustion Combustion with explosion flame Pulsating combustion Slow combustion

13. COMBUSTION WITH STATIONARY FLAME Normal combustion process as practiced in ovens/furnaces. The resultant flame front is more or less stationary in space. A stationary flame may be premixed or diffusion flame. In the premixed type, fuel and oxidant are premixed before they enter the burning zone. When the fuel & air are separately supplied to the burning zone, the flame is called a diffusion flame. In practice, a part of the total air may be premixed with the fuel and the remaining may be directly supplied to the combustion area. The premixed air is known as primary air and the rest is called secondary air. While solid, liquid and gaseous fuels can all give premixed flame, a truly diffusion flame is obtained only with gaseous fuels.

14. SURFACE COMBUSTION OR FLAMELESS COMBUSTION All refractory solid surfaces at high temperatures accelerate the rate of combustion of fuel gas and air. Some solids e.g. platinum can accelerate the process even at low temperatures. Combustion with a stationary flame is limited by a range of velocity and concentration conditions of the gas and air. Stable combustion is possible even outside this range, if the reaction proceeds in contact with solid surfaces. This is termed as surface combustion. APPLICATION Its industrial application is to achieve rapid combustion of a large quantity of fuel in a comparatively small space with the production of high temperature and high heat transfer rate. Tunnel burner is a type of gas burner operating on the surface combustion principle in which normal flames are absent.

15. SUBMERGED COMBUSTION It is a special case of application of surface combustion process in which, the burner is partly or fully submerged in a liquid and the hot combustion products bubble through it in an agitated condition. High heat transfer rate results from direct contact of hot gases with the liquid leading to high evaporation rate. Up to 95% of the potential heat of the fuel may be useful heat for the process. Submerged combustion finds application in the evaporation of severely scaling/ corrosive solution/liquids.

16. COMBUSTION WITH EXPLOSION FLAME It occurs in a homogenous mixture of fuel and air and is characterized by the flame front progressing rapidly through the mixture. The process may be either constant pressure e.g. mine explosion, or constant volume e.g. combustion in a gasoline engine. Detonation is a special type of explosion, where the extremely high reaction rate generates high velocity pressure waves (1 to 4 km/sec) and an abnormal rate of pressure rise.

17. PULSATING COMBUSTION Pulsating combustion occurs when one end of a long tube is open and the other is closed by non-return valve, and the air & fuel are introduced at the closed end. On ignition, the pressure in the system rises sharply at near-constant volume and it prevents the flow of the air and fuel momentarily; when the exhaust gases leave the tube through the open end, a fresh supply of air and fuel arrives at the hot zone and combustion is repeated in the form of pulsations whose frequency corresponds to the resonant frequency of the combustion unit. There is no stationary flame in the system. Pulsating combustion is a specific type of explosion flame.

18. SLOW COMBUSTION Slow combustion takes place at sub-flame temperatures (<400ºC) at slow but determinable rates. Slow combustion of higher hydrocarbons is useful in determining the chain-reaction rates. In a premixed system of fuel vapour and air, slow combustion proceeds at a number of points simultaneously in the whole system. No reaction zone or flame-front is visible. This process is called homogeneous combustion which is often characterized by the appearance of cool flames in succession which emit small quantities of heat and pale bluish light usually seen only in the dark. Slow combustion has no direct industrial application. However, it is indirectly useful in the study of mechanism of combustion.

19. Combustion of liquid fuels

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