Article pubs.acs.org/IECR
Numerical Simulation on H2 Rich Blowout Limit in Bluff-Body Burner Yajun Li,*,† Hongtao Zheng,† and Lin Cai‡ †
College of Power and Energy Engineering, Harbin Engineering University, Harbin 150001, Heilongjiang Province, China China Ship Development and Design Center, Wuhan, 430064, Hubei, China
‡
ABSTRACT: In order to investigate the hydrogen rich blowout limit (RBL) at different bluff-body diameters and flow velocities, the computational fluid dynamics software Fluent was used to simulate H2 combustion in the present study. The composition probability density function (C-PDF) model was adopted to simulate H2 burning in a turbulent flame by solving twodimensional Navier−Stokes equations. To verify the accuracy of the C-PDF combustion model, a piloted methane−air jet flame was simulated at first, and a good agreement between the numerical results and the published measurements was obtained. The H2 RBL formula about bluff-body diameters and flow velocities was summarized according to the numerical results. The results show that the C-PDF model is a reasonable method to capture flame ignition, flame extinction, and the H2 RBL. There should be an optimal bluff-body diameter in the burner which stabilizes the flame best. The flame will take an “M” shape with reaction fronts inside the central recirculation zone (CRZ) near the blow-off condition. When the bluff-body diameter remains unchanged, the relationship between the H2 RBL and the flow velocity is a logarithmic function. When the flow velocity remains unchanged, the relationship between the H2 RBL and the bluff-body diameter is a quadratic function. A successful ignition sequence requires typical three phases: the startup of ignition in the recirculation zone, the energy accumulation in the recirculation zone, and the flame propagation from the recirculation zone to the main stream. A complete extinction also requires three phases: the sudden decline of the temperature in the main stream, the energy dissipation from the recirculation zone to the main stream, and the complete extinction of the flame.
1. INTRODUCTION The bluff body has been widely used in engine combustors to stabilize flames, and there is a central recirculation zone (CRZ) in the wake flow zone of a bluff body.1 The CRZ can engulf high temperature gas to become a stable ignition source.2 It can also reinforce the mixing between fuel and air and improve the flame stability. The heat will diffuse to the main stream from the flame frontier during burning. If the fuel concentration is ultralean or ultrarich, the heat released from the flame frontier cannot compensate that of dissipation to the main stream, and then the flow temperature will decrease gradually, finally leading to extinction. Many studies on flame stability in a bluffbody burner have been done by both experimental and numerical methods. Shanbhogue1 found that the flame instability is dominated by the lower intensity and the convective instability of the shear layer. He put forward that blow-off will occur in multiple steps: local extinction along the flame shear, large scale wake disruption, and a final blow-off. Frolov2 formulated a flame stabilization criterion named the “Michelson criterion”; according to this criterion, a flame will be blown off from the flame holder when the Michelson number is
