Control of confined nonpremixed flames using a microjet Ashok Sinha a , Ranjan Ganguly a,b , Ishwar K. Puri a, * a Department of Engineering Science and Mechanics, Virginia Polytechnic Institute and State University, 223 Norris Hall (0219), Blacksburg, VA 24061, USA b Department of Power Engineering, Jadavpur University, Calcutta 700032, India Received 14 April 2004; accepted 2 November 2004 Available online 19 December 2004 Abstract Industrial burners, such as those used in materials processing furnaces, require precise control over the flame length, width, over- all shape and other physical flame attributes. The mechanism used to control the flame topology should be relatively simple, safe, and devoid of an emissions penalty. We have explored the feasibility of hydrodynamic control of confined nonpremixed flames by injecting air through a high-momentum microjet. An innovative strategy for the control of flame shape and luminosity is dem- onstrated based on a high-momentum coaxial microjet injected along the center of a confined nonpremixed flame burning in a coflowing oxidizer stream. The introduction of the microjet shortens a nonpremixed flame and reduces the amplitude of the buoy- ancy-induced flickering. For a microjet-assisted flame, the flame length is more sensitive to the fuel flowrate than for laminar or turbulent nonpremixed flames. This provides greater flexibility for the dynamic control of their flame lengths. Measurements of NO x and CO emissions show that the method is robust. Effective flame control without an emissions penalty is possible over a large range of microjet velocities that significantly alter the flame shape. Since the influence of the microjet is pri- marily of a hydrodynamic nature, inert microjet fluids like recirculated exhaust gas can also be used in practical devices. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Nonpremixed flame; Flame control; Jet entrainment 1. Introduction Industrial burners, such as those used in materials processing furnaces, require precise control over the flame length, shape and other physical flame attributes. For instance Hanus and Hubo (1999) studied variable flame length burners required in the steel industry for the flame strengthening of steel. These burners allow control over the furnace heat transfer characteristics by varying the flame length and the heat release profile, as in Pesenti and Meunier (2000). The active control of flame length and shape is also desirable in other energy- intensive applications, such as glass and other melting furnaces, and in welding torches. Ideally, a control tech- nique should not require sophisticated instrumentation or significant modifications to the burner hardware. There are potential pitfalls to altering the flame struc- ture, such as an increase in undesirable flame emissions and a loss of flame stability. Therefore, the mechanism used to control the flame topology should be relatively simple (e.g., as compared to Lawton and Weinberg (1969) who applied an external electric field or Hertz- berg (1977) who used acoustic forcing), safe, and devoid of an emissions penalty. Thring and Newby (1953) and Becker et al. (1981) carried out several investigations characterizing the con- ditions that alter the flame shape and spread in both confined and nonconfined flames. Hydrodynamic effects at the burner exit are known to change the flame topology. Apart from flame shape and size, flame 0142-727X/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.ijheatfluidflow.2004.11.001 * Corresponding author. Tel.: +1 540 231 2343; fax: +1 540 231 4574. E-mail address: ikpuri@vt.edu (I.K. Puri). www.elsevier.com/locate/ijhff International Journal of Heat and Fluid Flow 26 (2005) 431–439