Experimental Investigation of Stability and Structure of Vertical LPG Inverse Diffusion Flames Issuing from an Elliptic Burner

Abstract

The present paper experimentally examines the influence of the progressive variations of the central air jet velocity in a concentric circular-elliptical inverse diffusion flame (IDF) on the visual and thermal structure and stability of the developed flames. All experiments are conducted at a fixed fuel flow rate (liquefied petroleum gas) throughput that emerges from the annular elliptic passage having an aspect ratio of 2:1. The visual images are aided via a digital camera and shadowgraphs, while the thermal structure (axial and radial temperature profiles) is acquired using a bare fine wire (125 µm) thermocouple; rendering radiation loss insignificant. The visual images and shadowgraphs clearly indicate the existence of four regimes: (i) an annular partially premixed region at the burner rim, (ii) an inner central premixed blue flame, (iii) an outer luminous yellowish post combustion zone and (iv) the flame tip buoyant zone. The progressive increases of the central average air velocity ( ̅ = 7.4 m/s up to 31.3 m/s) result in shortening the visible flame length and narrowing the flame width. These are coupled with changing the flame appearance from a yellowish diffusion flame with a sooty core regime to an intense central premixed flame surrounded by a soot ring to blue flames exhibiting intense central radiation regime associated with soot oxidation. At extremely high air velocity > 21 m/s, locals flame extinction and re-ignition occurs at the boundaries of the ellipse minor axis and further increase of ̅ causes complete extinction of the main flame. These findings are very much supported by the mean gas temperature measurements that indicate steep temperature rise associated with the formation of a central premixed combustion within the flame core which is followed by the diffusion mode of combustion. A plateau of the axial temperature profiles is observed in the transition zone between the two regions whereby the rise in temperature due to soot oxidation is balanced by the radiation loss.

Numerical Study of Jet Inverse Diffusion Flames Issuing from an Elliptical Burner

“…The current study aims to construct a computational model that can simulate the inverse diffusion flame (IDF) emanating from an elliptical burner and elucidate the complex physical and chemical processes involved in such flames. The model is validated by comparing it with previous experimental data on an elliptical burner [20]. The steady-state RANS simulations were performed using ANSYS CFX 19 software.…”

“…4. This refinement process was based on the prior experimental results reported in [20]. To resolve the exceedingly high shear layers between the air jet and the entrained fuel, the grid was densified in the flame core by a cell size of 0.1 mm, which increased axially and radially with a maximum element dimension of 4 mm in both radial directions and 7.5 mm in the axial direction.…”

“…Both air and fuel are supplied at room temperature and atmospheric pressure. The air jet bulk velocity is kept at three values, i.e., 13.26, 15.58, and 17.90 m/s, while the fuel is kept constant at 0.026 m/s to match the experimental boundary conditions provided in [20]. Fig.…”

Experimental Investigation of Flame Appearance and Temperature Distribution of a Double Elliptical Inverse Diffusion Jet Flame

“…Specifically, a low-power 20 mW compact continuous-wave 405 nm laser diode was used to illuminate the flames, and the resulting shadowgraph was displayed on a distant fluorescent screen. This method has been previously employed to image non-premixed flames emitted by a concentric elliptic burner using an ultraviolet laser (Attia et al 2018;Mahgoub, Hussien, and Emara 2021). The same digital camera used for flame imaging was also used to capture the shadowgraph images.…”

“…The properties of swirl-stabilized IDF including flame appearance, heat transport, and emissions, are discussed in Shah 2019, 2018;Zhen, Leung, and Cheung 2011). Additionally, experimental and numerical studies have been conducted to better understand IDF characteristics such as flame appearance, flame length, and centerline temperature distribution (Stelzner et al 2013;Patel and Shah 2017;Elbaz et al 2019;Mahgoub, Hussien, and Emara 2021;Çengel 2003).…”