Madjid Birouk

EFFECT OF FUEL NOZZLE GEOMETRY ON THE STABILITY OF A TURBULENT JET METHANE FLAME

The effect of asymmetric fuel nozzles on the stability of turbulent jet methane flame discharging into still air is investigated experimentally. The emphasis of this study is however, more on the flame liftoff height and velocity, as well as reattachment and blowout velocities. Five nozzles; a pipe, a contracted circular, a triangular, a rectangular and a square, which all have an equivalent diameter of approximately 4.50 mm, are tested. The experimental results reveal that asymmetric nozzles reduces the jet flame liftoff height, and hence stabilizes the flame base closer to the nozzle compared with conventional circular nozzles. This finding correlates well with the entrainment rate of the corresponding non-reacting jets. That is, the flame liftoff height decreases as the jet entrainment increases. Furthermore, the asymmetric nozzles are found to significantly influence the blowout, liftoff and reattachment velocities. The blowout is believed to be primarily governed by the far-field local mixing rate.

Assessment of the Performances of RANS Models for Simulating Swirling Flows in a Can-Combustor

The paper presents an assessment of the performances of RANS turbulence models for simulating turbulent swirling can-combustor flows with different inlet swirl intensities (i.e. S=0.4 and S=0.81). The predictions compared against published experimental data reveal that the eddy-viscosity models can not show the central recirculation zone in the case of a weakly swirling flow. However, although they reveal the existence of this region in a strongly swirling flow, they are incapable of predicting its correct size. On the other hand, the Reynolds stress models are able to predict the cor- ner and the central recirculation zones in both flow cases. The predictions of turbulence intensities by using the realizable k-� and the SST k-� are comparable to those of the Reynolds stress closures. The shear stresses are not well predicted by all the tested models. Both the eddy-viscosity and the Reynolds stress closures show relatively less approximation errors in the weakly swirling flow.

STABILITY OF A TURBULENT JET METHANE FLAME ISSUING FROM AN ASYMMETRICAL NOZZLE WITH SUDDEN EXPANSION

The effect of quarl (i.e., a sudden expansion) on the stability of turbulent diffusion flame of methane jet issuing from asymmetric nozzles and discharging into still air environment is examined experimentally. The study covers the flame liftoff height and velocity, as well as the reattachment and blowout velocities. Five nozzles with different internal geometries but with similar equivalent exit diameter are tested. The results reveal that the jet flame liftoff height is reduced further when quarl is attached to the exit of the nozzle. Moreover, the stability range of the lifted flame is found to expand when quarl is attached to the nozzle. Quarl leads to an increased flame blowout velocity, which is the upper stability limit, and decreasing the liftoff velocity, which is the lower lifted flame stability limit. This might be of great interest for industrial applications in which a wide stability range of the lifted flame is desired.

Three‐dimensional numerical simulation of an equilateral triangular turbulent free jet

Purpose – This paper aims to describe the numerical simulation of a three‐dimensional turbulent free jet issuing from a sharp‐edged equilateral triangular orifice into still air.Design/Methodology/approach – The numerical simulation was carried out by solving the governing three‐dimensional Reynolds‐averaged Navier‐Stokes equations. Several two‐equation eddy‐viscosity models (i.e. the standard k‐e, renormalization group (RNG) k‐e, realizable k‐e, shear‐stress transport (SST) k‐ω), as well as the Reynolds stress models (i.e. the standard RSM and the SSG) were tested to simulate the flowfield. The numerical predictions were compared with experimental data in order to assess the capability and limitations of the various turbulent models examined in this work. Findings –The vena contracta effect was predicted by all the tested models. Among the eddy‐viscosity models only the realizable k‐e model showed good agreement of the near‐field jet decay. None of the eddy‐viscosity models was capable of predicting the ...

A numerical model for calculating the vaporization rate of a fuel droplet exposed to a convective turbulent airflow

Purpose – The purpose of this paper is to develop a three‐dimensional (3D) numerical model capable of predicting the vaporization rate of a liquid fuel droplet exposed to a convective turbulent airflow at ambient room temperature and atmospheric pressure conditions.Design/methodology/approach – The 3D Reynolds‐Averaged Navier‐Stokes equations, together with the mass, species, and energy conservation equations were solved in Cartesian coordinates. Closure for the turbulence stress terms for turbulent flow was accomplished by testing two different turbulence closure models; the low‐Reynolds number (LRN) k‐e and shear‐stress transport (SST). Numerical solution of the resulted set of equations was achieved by using blocked‐off technique with finite volume method.Findings – The present predictions showed good agreement with published turbulent experimental data when using the SST turbulence closure model. However, the LRN k‐e model produced poor predictions. In addition, the simple numerical approach employed ...

Droplet Evaporation in a Turbulent Environment at Elevated Pressure and Temperature Conditions

A three-dimensional numerical model is developed to assess the effect of freestream turbulence on the vaporization of n-heptane droplet, which is exposed to a freestream of nitrogen at elevated pressure and subcritical temperature conditions. The freestream pressure, temperature and turbulence intensity are varied in the range of 0.5–10 MPa, 324–502 K, and 0%–60%, respectively. Variable thermophysical properties, the unsteadiness behavior of the gas and liquid phases, as well as heat transfer by radiation are all considered. In addition, non-ideality behavior of the gas phase, solubility of the gas into the droplet and pressure dependence of the gas-phase thermophysical properties are also accounted for. The turbulence terms in the conservation equations of the gas-phase are modeled by using the shear-stress transport (SST) model. The results show that, for the temperature range (T∞ < Tc) explored in the present study, the droplet lifetime increases, and thereby the vaporization rate decreases, as pressure rises. However, the effect of pressure gradually diminishes as the ambient temperature increases and vanishes when T∞ approaches the critical temperature of n-heptane, Tc. Moreover, the effect of freestream turbulence intensity, which is found to enhance droplet heat and mass transfer, weakens as pressure increases. Finally, droplet turbulent heat and mass transfer correlations are proposed which account for all the aforementioned parameters.

Numerical Study of Sphere Drag Coefficient in Turbulent Flow at Low Reynolds Number

The drag coefficient of a sphere immersed in turbulent air flow in the Reynolds number (Re = U ∞ d/ν ∞) range up to 250 and turbulence intensity (u ∞′/U ∞) up to 60% is computed numerically. Reynolds-averaged Navier-Stokes equations (RANS) are solved in Cartesian coordinates by using a blocked-off technique. To our knowledge, the present work is the first to employ the blocked-off technique for flow over a sphere. Closure for the turbulence stress term is accomplished by testing four different turbulence closure models. The main findings of the present investigation are that the laminar numerical data compare well with numerical and experimental published work. However, different turbulence closure models produce different trends in the range of Reynolds number up to Re = 100, and this difference is demarcated by the nonagreement between the turbulent predictions and the “standard” drag coefficient results. However, the results obtained using Menters SST turbulence model show fair agreement with the well-known sphere “standard” drag over the range of test conditions explored here. Thus, the present results confirm recently published findings, which suggest that the free-stream turbulence intensity does not have a significant effect on the sphere mean drag.

Non-Premixed Turbulent Biogas Flame: Effect of The Co-Airflow Swirl Strength on the Stability Limits

This experimental study reports on the effect of swirl strength on the stability of non-premixed biogas flame. The swirl strength was varied by changing the vanes angle (25° and 50°) of the swirl generator. Zero-angle vanes swirl was also used as a reference. The results revealed that the swirl enhances significantly the biogas flame stability operating range. In addition, it was found that the biogas flame behaves differently depending on the swirl strength. The effect of low (25°-angle vanes) swirl on the flame stability was found to depend upon the co-airflow exit velocity. At low co-airflow velocity, the flame was attached and its blow-off limits are comparable to those of the zero (0°-angle vanes) swirl. However, as the co-airflow velocity increases, the low swirl attached flame lifts-off the burner and stabilizes over a wide range of co-airflow and fuel jet velocity. In contrast to low-swirl, high (50°-angle vanes) swirl generates only an attached biogas flame regardless of the co-airflow velocity. Moreover, the high swirl attached flame was found to stabilize over a wider range of flow conditions compared to that of low swirl. Particle image velocimetry (PIV) data were used to explain the stability limits and empirical correlations were proposed to describe these limits.

Vaporization and Combustion of a Soybean Biodiesel Droplet in Turbulent Environment at Elevated Ambient Pressure

This article reports experimental data on the vaporization and combustion of soybean (derived) biodiesel droplet in turbulent environment at elevated ambient pressure and temperature conditions. Test conditions consisted of varying turbulence intensity and ambient pressure while keeping ambient temperature constant at 473 K. Soybean biodiesel droplet was formed using an in-house developed injector, and suspended onto the tip of a quartz fiber in the center point of a spherical vessel. The initial diameter of the formed droplet ranged between 1.00 mm and 1.50 mm. The characterization of the turbulent field, generated by four pairs of axial fans, revealed that turbulence is essentially isotropic and homogeneous with nearly zero-mean flow within the 40-mm-diameter volume in the center of the vessel. The experimental results showed that the biodiesel droplet vaporization and burning followed the d2-law. More importantly, the biodiesel droplet vaporization rate was shown to depend on both turbulence and ambient pressure. The droplet vaporization results showed that the effect of turbulence becomes more effective with increasing ambient pressure. In addition, turbulence was found to enhance the biodiesel droplet burning rate only at elevated ambient pressure. Heat loss from the flame predominates at high levels of turbulence, which consequently causes droplet flame extinction.

A Hydrocarbon Droplet Evaporating in Turbulent Environment at Elevated Ambient Pressure and Temperature

This study presents new experimental results on the vaporization process of hydrocarbon droplet in a turbulent environment at elevated ambient pressure and temperature conditions. n-Heptane and n-decane, which provide a wide range of hydrocarbons properties, were tested. The initial droplet diameter was on the order of 1 mm, and its surrounding ambient consisted of varying turbulence intensity up to 3.10 m/s, pressure up to 16 bar, and temperature up to 150°C. The results revealed that the hydrocarbon droplet followed the d2-law throughout its entire lifetime under all ambient conditions explored here. Increasing the ambient pressure increases the droplet vaporization lifetime, whereas increasing the ambient temperature reduces the droplet lifetime. More importantly, turbulence becomes more effective as ambient pressure increases, whereas it diminishes with increasing the ambient temperature. The experimental data were used to develop a more comprehensive hydrocarbons droplet mass transfer correlation.