An experimental and theoretical investigation of the flows induced by buoyant diffusion flames

Xianchuan Zhou, Purdue University

Abstract

The instantaneous and the mean velocity fields of the flow induced by a variety of buoyant diffusion flames are studied using a Laser Doppler Velocimeter (LDV) and Particle Imaging Velocimetry (PIV). The instantaneous entrainment flow patterns become much more irregular and unstable with an increase in fire size. A floor flush with the burner surface not only makes the entrainment velocities more horizontal, but also makes the instantaneous flow pattern more irregular. The air entrainments are calculated using the mean velocity field, and it is found that existing differences in entrainment correlations are mainly caused by differences in the definition of the entrainment boundary. A kinematic model of the flow field induced by the fires is used to understand the physical phenomena. The flow field is decomposed into an irrotational velocity field, caused by the net energy increase, and an incompressible velocity field, driven by the vorticity. The governing equations are rewritten in terms of a potential function and a stream function. The source terms in the governing equations, i.e., the thermal expansion source term and the vorticity distribution, are estimated using correlations of buoyant flame structure. The boundary conditions are approximated using asymptotic solutions. The governing equations are discretized using a control volume method, and the resulting system of algebraic equations is solved by iteration, accelerated by a multigrid scheme. Qualitative agreement is found between the predictions and the measurements of the entrainment velocity field, but large arbitrary adjustments to the source terms are necessary to obtain quantitative agreement because of the inaccuracy of source term descriptions. Techniques are developed to experimentally estimate the thermal expansion source term and the vorticity distribution. The thermal expansion source distribution is obtained by measuring mixture fraction distributions and applying the laminar flamelet state relationships, computed using the OPPDIF code. The vorticity distribution is obtained by finite difference analysis of the mean velocity field measured with PIV when using a Nd:YAG laser. The predicted velocity field based on a realistic evaluation of the source terms agrees reasonably well with the measured velocity field. The use of a computational model with appropriate boundary conditions is highly recommended over the empirical engineering correlations.

Degree

Ph.D.

Advisors

Gore, Purdue University.

Subject Area

Mechanical engineering|Environmental engineering

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