Flame shape transition in an impinging jet burner over triangular shape Bluff body

Transcription

1 Flame shape transition in an impinging jet burner over triangular shape Bluff body 1 N Moharana, 2 T M Muruganandam 1 M-Tech Scholar, IIT Madras, Chennai , India 2 Associate Professor, IIT Madras, Chennai , India 1 n_moharana@rediffmail.com, 2 tmmuruganandam@gmail.com Abstract:- Combustion is ubiquitous in everyday life, and an understanding of flame behaviour is important in understanding this vital energy source. Fuel efficiency is lost when a combustor or engine loses its optimum flame due to some disturbances. Flame shape transitions are a result of loss of one stabilization mechanism while the flame is stabilized by another. If the dynamics near flame shape transitions are understood, one can design a sensor which forewarns the control system about the imminent change, in order to take corrective measures. Bluff body stabilized flames have been a research topic for quite a long time, due to their relevance to a wide range of aeronautical applications they have (e.g. turbojet afterburners, nozzle mixing burners, ramjets and scram jets). Flame stability and flame shape transitions across the triangular shaped bluff body are analyzed for a LPG flame in an impinging jet burner. The present work deals with premixed combustion. The flame holding characteristics and flame shape transition of bluff body stabilized premixed flames are studied for a rectangular (size: 80 X 20) impinging jet burner with a triangular flame holder across the burner. This paper describes an experimental investigation of the dynamics of flame shape transition across triangular shaped bluff-body stabilized flame. The experiments are conducted for different bluff body apex angles ( 90 0, 60 0 and 45 0 ) and bluff body sizes (6mm and 8mm side). The flame behaviours are recorded by high resolution camera. The recorded videos are then digitized via the computer. Still images at various operating conditions are captured and analyzed. Divided flame (DF), Lifted flame (LF), and M shape flame(mf) are the three stable flame shapes observed by changing the fuel flow rate gradually while keeping air flow rate constant. The flame shape transition for different air speed (U) and equivalence ratio (Φ) are presented in graphical form. The details of the dynamics of each of the flame shapes, and the high speed visualization of each of the transitions possible in the burner are also presented. Keywords : Bluff body, Equivalence Ratio, Flame shape transitions, Impinging jet, Premixed flame. I. INTRODUCTION In modern world, one is surrounded by machines and to run a machine one has to provide energy to it. In most of the cases the energy is produced by combustion or fire. An understanding of flame behaviour is important in understanding this vital energy source. Although most of the engines are designed to have the best possible fuel efficiency, in present time, one of the most challenging problems is to achieve even more fuel efficient systems as we have limited energy sources available on our planet. Fuel efficiency is lost when a combustor or engine loses its optimum flame due to some disturbances. Bluff body flame holders are widely used in the industry to make flame stable especially in combustors with high velocity flow. Bluff bodies are used for flame stabilization in a variety of propulsion and industrial combustion system. Bluff body flame stabilizers are extensively used in practical ducted chambers, such as ramjet and turbojet afterburner. Ground-based and aero engine applications routinely incorporate bluff body stabilized flame holders for primary or secondary combustion in high speed flows. The bluff body is placed in the mid span of a high speed duct and produces a re-circulating wake structure that allows combustion to stabilize and then propagate into the free stream. Various studies/research already made on flame stability, flame holding and blowout phenomenon. Several methods of flame stabilization have been developed so far and these studies discuss thoroughly about various methods of stabilization for different flame shapes near lean conditions [1, 2]. Different bluff body apex angle (the apex angle was varied from 45 0 to ) effects on both non-reactive and reactive flow field have been discussed by Gelan Yang, Huixia Jin and Na Bai [3]. Also discussed about the effects of apex angle on recirculation region length, recirculation zone area and flame stability. The recirculation zone area increases as apex angle is increased. A bigger apex angle makes the flame shape wider, increases chamber volume heat release, and enhances the flame stability. The impact of increased reactant temperature on the dynamics of bluff body stabilized premixed flows is investigated by R.R. Erickson, M.C. Soteriou [4]. The study on blowoff dynamics of bluff body stabilized 14

2 turbulent premixed flames by Swetaprovo Chaudhuri, Stanislav Kostka, Michael W. Renfro, Baki M. Cetegen [5] revealed the flame dynamics of a bluff body stabilized turbulent premixed flame as it approaches lean blowoff. It was found that as lean blowoff is approached by reduction of equivalence ratio, flame speed decreases and the flame shape progressively changes from a conical to a columnar shape. Study carried out by Santosh J. Shanbhogue, Sajjad Husain, Tim Lieuwen [6] overviews the dynamics of bluff body stabilized flames and describes the phenomenology of the blowoff process. Key conclusions from this review are that blowoff occurs in multiple steps local extinction along the flame sheet, large scale wake disruption, and a final blowoff whose ultimate trigger is associated with wake cooling and shrinking. Some of the researchers have used chemiluminescence and schlieren imaging techniques to predict blowout proximity and have tried to re-stabilize the flame using a control system [7, 8]. Chao Y. C., Yong-Li Chang, Chih-Yung Wu, and Tsarng-Sheng Cheng [9] did experimental analysis of lift off and the blow off processes of methane-air premixed flame by using LIPF and PIV techniques. Although lots of study has been made on flame stability, flame holding and blowout phenomenon, a very limited study is made on flame mode shape transition. Very limited literature is available on mode shape transition. Fernandes et al. [10] have done experimental investigation of various flame shapes in a laminar, stagnation point stabilized burner having stagnation plate at a constant temperature. There are more than one flame shapes possible in most of the practical combustors which may result in several flame shape transitions. All the flame shape transitions need to be investigated in order to understand complete dynamics of such combustors. Transitions must be investigated individually in order to collect key information during each transition, so that one can design a sensor which forewarns the control system in order to hold the combustor stable in a desired flame shape. II. EXPERIMENTAL SETUP Fig-1 shows a schematic diagram of the experimental apparatus. A pre-existing burner is used for experimentation. The burner exit is of rectangular cross section. The size of rectangular burner tip is 80mmX20mm. The burner is made of 2mm thick sheet metal. LPG and air are separately controlled. LPG flow is measured by digital flow meter in LPM unit. The Air flow is measured by rotameter (Range : 0 to 924LPM) in LPM unit. Conventional pressure gauges (Range : 0 to 16 bar) and pressure regulators are used for controlling and monitoring of air and LPG pressure. Fig- 1. Schematic of the experimental apparatus. 15

3 for each apex angle. Adjustable fixture is used for holding the bluff body above burner (Fig-2). Fig-2. Bluff body mounted over the Burner with the help of fixture Experimentation is carried out for two sets of triangular shape bluff body of apex angle 90 0, 60 0 and 45 0 (Fig-4). One set is with 6mm side and other set is with 8mm side Fig-3. Triangular shape Bluff body. The experimental procedure is as follows: firstly, the flow rate of the fuel air mixture is adjusted to achieve the desired inlet velocity (U), and then the inlet LPG concentration (Φ) is gradually increased from lean to rich. The flow rate is kept constant while adjusting Φ. Each time Φ is adjusted, ample time is allowed for the system to become stable. The above procedures are repeated for different bluff bodies. The flame behaviours are recorded by a high resolution camera. The recorded videos are then digitized via the computer. Recorded flame shapes/configurations are analyzed. Still images at various operating conditions are captured and studied. Fig-4, Cross sections of Triangular Shaped Bluff bodies. III. RESULT AND DISCUSSION A. Flame Shape Variations The present study aims at the investigation of the effect of fuel to air ratio, flow speed and apex angle on flame shape transition of triangular (V-shape) shape bluff body. Repeated experimentation is carried out to know the flame shape transition get the stabilized flame at bottom of the triangular shape bluff body. The schematic of flame shapes at different fuel (LPG) consumption is given below (Fig-5): Fig-5, Schematic of Flame shape transition observed during experiment. 16

4 Air Speed, U = m/s, Bluff body 60 0 apex angle and 8mm size Equivalence Ratio, Φ Increasing Lean to Rich Flame shape Side view Front View Flame Type Divided flame Lifted flame M-shape flame Fig-6, Direct photos of the flame shapes for different Equivalence ratio (Φ) at U=1.041 m/s for 60 0 apex angle and 8mm size bluff body. Divided flame (DF), lifted flame (LF) and M-shape flame (MF) are the three stable flame shapes observed by changing the fuel flow rate gradually while keeping air flow rate constant (Fig-5). The direct photographs of the flame transformation processes at the critical equivalence ratio where flame transformation ( DF LF or LF MF ) occurs are recorded by high resolution camera and are shown in Fig-6. Photos of actual flame 17

5 shapes for Bluff body 60 0 apex angle and 8mm size at Air Speed, U = m/s are given in Fig-6. The divided flame moves gradually upstream (downward) with a progressive increase of Φ. At a critical Φ = 0.988, the flame undergoes a spontaneous transformation from divided flame to lifted flame. Further increasing the equivalence ratio Φ = the lifted flame changes to M-shape flame. The transformation process at U = 0.958, & m/s are similar to that of U = m/s. B. Effect of Air Speed and Bluff body Apex angle on Flame Transition Process. Flame configuration as a function of equivalence ratio (Φ) and air speed for different types of bluff body apex angle are plotted in Fig-7 and 8. It is observed that as air speed U increases it requires higher equivalence ratio Φ to achieve the flame transformation DF LF and Similar regime plots as shown in Fig-7 are made for Bluff body apex angle 90 0, 60 0 and 45 0 one with 6mm size and other with 8mm size. All above said regime plots are combined together in Fig-8 and 9. It is observed that higher equivalence ratio, Φ required with decrease in bluff body apex angle to achieve the flame transformation DF LF and LF MF for same speed and bluff body size. Table 1 and Table 2. Critical Equivalence Ratio (Φ) for different Bluff body apex angle about which transformation from Divided flame to Lifted flame to M-shape flame takes place. Table 1 Test Data Comparisons for 8mm size Bluff body Air Speed in m/s Critical Equivalence Ratio (Φ) for different Bluff body apex angle 45 Deg 60 Deg 90 Deg Table-2 Test Data Comparisons for 6mm size Bluff body Air Speed in m/s Critical Equivalence Ratio (Φ) for different Bluff body apex angle 45 Deg 60 Deg 90 Deg IV. CONCLUSION The investigation focused on behaviour of a LPG/air premixed impinging jet burner. Experiments were performed to observe various flame shapes associated with the burner at different fuel flow rates. Regime plots in average jet velocity versus equivalence ratio were obtained. For a particular bluff body, it is observed that as air speed (U) increases it requires higher critical equivalence ratio (Φ) to achieve the flame LF MF. Also with decrease in bluff body apex angle it requires higher equivalence ratio Φ to achieve the flame transformation DF LF and LF MF. Fig-7. Regime plot for LPG/air premixed impinging jet burner with Bluff body 60 0 apex angle and 8mm size. Fig-8. Flame transformation, DF LF and LF MF for 8mm bluff body size with different apex angle. Fig-9. Flame transformation, DF LF and LF MF for 6mm bluff body size with different apex angle. transformation DF LF and LF MF. At the critical equivalence ratio of flame transformation process is completely spontaneous. Experimentation is carried out for two sets of triangular shape bluff body of apex angle 90 0, 60 0 and One set is with 6mm side and other set is with 8mm side for each apex angle. Comparing regime plots of all bluff bodies it is found out that with decrease in bluff body apex angle it requires higher equivalence ratio, Φ to achieve the flame transformation DF LF and LF MF. 18

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