Browsing by Author "Nazzal, Ibrahim Thamer Nazzal"

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Turbulence and flame interaction for control of flame location in diffuser combustor
(2018-08) Nazzal, Ibrahim Thamer Nazzal; Ertunç, Özgür; Ertunç, Özgür; Başol, Altuğ; Mengüç, Mustafa Pınar; ÖZdemir, B.; Koşar, A.; Department of Mechanical Engineering; Nazzal, Ibrahim Thamer Nazzal
Achieving an appropriate flame location is desirable in many combustion system applications especial in the design of the combustion system. Given that the characteristics of turbulent flows influence flame behavior and the design of combustion systems, flame location can be controlled within desirable levels without distorting the design of the combustor geometry by selecting suitable characteristics. A feedback control method utilizes the characteristics of turbulent flows to stabilize the flame location within the desirable level. The primary objective of this study is to introduce a strategy for flame location control using characteristics of turbulence flow. Turbulence intensity and length scale are among the main parameters of turbulent flow. Therefore, the secondary goal of this study is to investigate the influence of turbulence intensity and length scale on flame location. The investigation of the dependency of flame location on turbulence is based on selecting suitable combustor geometry. For this purpose, the axisymmetric diffuser form is used to reveal the response of the flame location of a turbulent premixed flame that has been exposed to various turbulence intensities and length scales. The diffuser is selected because the flow slows down along the direction. Thus, the flame is expected to propagate towards the inlet when the flame speed increases. In this manner, the effect of turbulence can be studied without changing the thermal power. In addition, the diffuser combustor is used to avoid blow-off and flame extinction because the flow slows along the combustor. Two types of diffuser combustors are selected for this study. The first combustor is a cylindrical diffuser, while the second one is a cylindrical diffuser with a conical insert. Numerical simulations are applied to the diffuser combustor for turbulent premixed propane flames by using a coherent flame model integrated to the Reynolds-averaged Navier–Stokes flow model with k-epsilon turbulence model. Firstly, the influence of turbulence on flame location in a diffuser-type combustor is studied under steady-state conditions. Results show that the flame location moves towards the inlet of the diffuser combustor with the increase in turbulence intensity for moderate- and high-turbulence length scales. The behavior of flame location is different in the low-turbulence length scale. The flame location initially decreases with the increase in turbulence intensity and subsequently stabilizes. Furthermore, the flame area density influences the flame location with the increase in turbulence intensity and turbulence length scale. Turbulence intensity and length scale simultaneously influence the flame area density, flame shape, and flame location. Secondly, the results of the unsteady simulations indicate that turbulence intensity, length scale, and flow separation exert a significant effect on the flame location of the premixed turbulent combustion. The flame front moves toward the diffuser inlet as a result of the increase in turbulence intensity and length scale. The flame location drops to the middle of the diffuser for the high turbulence intensity. However, the effect of turbulence intensity is more visible than that of turbulence length scale within the tested range. An increase in turbulent length scale at a constant turbulence intensity causes a decrease in flame location. It is observed that the combustion and inlet turbulence cause a flow separation mainly downstream of the flame front. Consequently, the secondary flow structures influence the flame topology and location. Therefore, the flame location and shape are influenced by the flow separation and the turbulence intensity and length scale. Thirdly, a conical insert is placed in the middle of a diffuser-type combustor to eliminate the flow separation. The influence of turbulence on flame location in two diffuser-type combustors (with and without conical insert) is studied and compared. Results indicate that the flame moves towards the inlet of the diffuser with the increase in turbulence intensity and length scale in the two diffuser-type combustors. At a high-turbulence length scale, the flame rapidly drops at the inlet of the diffuser with a conical insert with the increase in turbulence intensity, whereas the flame drops to an intermediate level when the diffuser is not used with a conical insert. Moreover, a similarity was observed in the trends of the flame location at low turbulence intensities in both cases. Results show that the Taylor-scale Reynolds number is the influential parameter of flame location and not turbulence intensity and length scale. An increase in the Taylor-scale Reynolds number leads the flame location to move towards the combustor inlet. The flame drops to the inlet of the combustor at a high-turbulence Taylor-scale Reynolds number. Flow separation is observed in the diffuser without a conical insert, and flow separation is eliminated by using the conical insert. Finally, the control of the flame location in the diffuser combustor is studied under various turbulent flow characteristics. A control strategy is suggested for this purpose. Control algorithm written as a Java macro is implemented to a commercial CFD software, namely STAR CCM+. This framework is utilized to perform all the simulations for the premixed turbulent flame under unsteady-state controlled conditions. The algorithm is built to adjust the turbulent kinetic energy and turbulent dissipation rate. Feedback control is introduced to stabilize the flame location at the desired level. Results indicate that control of the turbulent kinetic energy at the inlet control the flame location within the targeted level. In addition, it is observed the flame location moved to a low level for high turbulent kinetic energy whilst it moved to the high level for low turbulent kinetic energy.