Modelling of combustion in natural draft burners for improved performance
Date
2024-02
Authors
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Journal ISSN
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Publisher
Federal University of Technology, Owerri
Abstract
Combustion in natural draft burners involves the passive induction of atmospheric air into the burner mixer when a fuel jet flows through a venturi. The interaction of the fuel jet and air stream at different velocities causes shear and Kelvin Helmholtz instability that grows into turbulence and molecular mixing of both species. Modeling of flow and combustion process in natural draft burners for improved performance are presented. It involves the response of the flow to changes in the axial coordinate of the nozzle and passive alteration of the flow conditions upstream of the origin. The alteration was by varying the nozzle streamlines and installing a trip ring at the nozzle exit. The Reynolds Averaged Navier-Stokes (RANS) equation was used to model the flow physics; the turbulence modelling was done with the Reynolds Stress Model (RSM), while the standard wall function captured the near wall flow behaviour. The implementation was undertaken using ANSYS Fluent 18.1, and the model was validated with Binder and Kian experimental work. A low-fidelity model developed and implemented with a MATLAB script offered a cheap and quick alternative for the parametric investigation of jet flow and combustion processes. The flow parameters of interest are the velocity decay, pressure gradient, turbulence, ambient air entrainment, and its mixing with the fuel jet stream. The ratio of the axial distance between the nozzle exit and throat 𝑙𝑇𝑁 and the throat diameter 𝑑𝑇, 𝑙𝑇𝑁 𝑑𝑇 defined the position of the nozzle from the throat. At 𝑙𝑇𝑁 𝑑𝑇=2.0 and 𝑙𝑇𝑁 𝑑𝑇 = 0.5 as reference, the increase in the velocity decay rate and the entrainment values were over 8% and 7%, respectively. These shows that when the nozzle was closer to the air inlet, velocity decay was faster, the entrainment was higher, and the mixing was better. The modification with the trip ring involved the installation of 0.5mm thick rings at the nozzle exit. The installed ring diameters were 4.9, 4.7, and 4.5mm, concentric with the nozzle trailing edge. The wake formed behind the trip ring interacted with the streamwise vortices and suppressed the near-stream turbulence. Flow modification with a 4.9mm ring increased the turbulence at the ejector outlet by over 26.6%. The final entrainment value was 16.7% higher, but the drop in the decay rate was 9.6%. The 4.9mm ring performed best because of the delayed flow separation and reduced pressure drag. Four different nozzle streamlines (PN1, PN2, PN3, and PN4) produced by straight lines and sine function combinations yielded different streamwise sectional area. With PN4 as the reference nozzle, the decay rate, entrainment and the maximum turbulence intensity increased by approximately 1.7, 8 𝑎𝑛𝑑 16%, respectively, in the PN3 nozzle, which has the smallest streamwise sectional area and the highest momentum difference between the streams. The relationship between the near stream decay rate and the streamwise sectional area was linear. Therefore, passive modification of the jet structure improved the burner performance substantially. Pressure drag formed behind the ring adversely affected performance. An optimized streamlined ring profile can delay flow separation and reduce pressure drag, thus recommended for further investigation
Description
This thesis is for the award of Doctor of Philosophy (PhD.) Degree in Mechanical Engineering (Energy and Power Engineering Option)
Keywords
Natural draft burners, combustion, modeling, shear layer, trip ring, velocity decay, pressure gradient, entrainment, turbulence, Department of Mechanical Engineering
Citation
Nwoye, F. C. (2024). Modelling of combustion in natural draft burners for improved performance [Unpublished Doctoral Thesis]. Federal University of Technology, Owerri, Nigeria