Hamidreza Gohari Darabkhani

Methane Diffusion Flame Dynamics at Elevated Pressures

The chamber pressure and fuel flow rate effects on the flickering behavior of methane-air diffusion flames was studied over the pressure range of 1 to 10 bar. Photomultipliers and high-speed imaging techniques have been used to study the frequency and magnitude of the flame oscillation and the change in global flame shape. The instability behavior of the flame was observed to be sensitive to both the fuel flow rate and pressure. Particularly, it has been observed that the flame responds to the change of pressure more when the pressure is relatively low. High-speed imaging has shown that the periodical break-up of the methane flame at higher flow rates is almost symmetric. However, the methane flames at lower flow rates oscillate in a more waving manner due to the alternating lateral nature of the outer vortices. The average flame luminosity was observed to increase with pressure up to 6 bar and then starts to decrease with the further increase of pressure. The flame oscillation magnitude (L f ) and oscillation wavelength (λ) were obtained from the high-speed imaging database. It has been observed that the trends of these parameters correlate well with the standard deviation (σ) of mean pixel intensity (MPI), measured from the flame high-speed images. The increase in fuel flow rate was observed to increase the magnitude of oscillation. The dominant flickering frequency of a methane diffusion flame varies with the chamber pressure as a function of P n (f = 15.7P 0.17).

Pressure Effects on Structure and Temperature Field of Laminar Diffusion Flames

This study addresses the influence of elevated pressures up to 10 bar on the flame geometry and two-dimensional soot temperature distribution of ethylene-air laminar co-flow diffusion flame. Narrow band photography and two-colour pyrometry in the Near Infra-red (NIR) region have been used to gain a better understanding on effects of pressure on these parameters. Theoretical background, discreet considerations in the choice of two narrow band filters at 780 nm and 1064 nm and calibration of the instrument factor are also described. It has been observed that the flame properties respond very sensitively to the pressure. As the pressure is increased, the flame diameter decreased at all flame heights and soot formation dramatically increased. The flame luminosity at the flame centerline increases, first by axial position from the fuel nozzle. Then the flame became less intense with height due to cooling of the soot particles by radiative losses, leading to a smoking flame at pressures of 2 bar and above. The soot temperature results obtained by applying twocolor method in the NIR region are shown to be consistent with the pyrometry results. Soot temperature measurements show that in ethylene diffusion flame the overall temperatures decreases with increasing pressure. It is shown that the rate of temperature drop is greater for a pressure increase at lower pressures in comparison with higher pressures. The average temperature drop of about 177 K is recorded along the flame centerline for a pressure increase from atmospheric to 2 bar and also from 2 bar to 4 bar. However, at higher pressures the rate of temperature drop decreases to 1/3 of the previous temperature drop. It is found that, applying two-color pyrometry method in the NIR region, utilizing a commercial digital camera, is capable of nonintrusive measurement of two-dimensional soot temperatures with a simple and relatively high accuracy technique. The maximum recorded error of the method was found to be about 8%. It mainly occurred at the regions with the lowest concentration of soot particles.

Assessment of a common nonlinear eddy-viscosity turbulence model in capturing laminarization in mixed convection flows

ABSTRACT Laminarization is an important topic in heat transfer and turbulence modeling. Recent studies have demonstrated that several well-known turbulence models failed to provide accurate prediction when applied to mixed convection flows with significant re-laminarization effects. One of those models, a well-validated cubic nonlinear eddy-viscosity model, was observed to miss this feature entirely. This paper studies the reasons behind this failure by providing a detailed comparison with the baseline Launder–Sharma model. The difference is attributed to the method of near-wall damping. A range of tests have been conducted and two noteworthy findings are reported for the case of flow re-laminarization.

Oxy-fuel Combustion for Carbon Capture and Sequestration (CCS) from a Coal/Biomass Power Plant: Experimental and Simulation Studies

Oxy-fuel combustion is a promising and relatively new technology to facilitate CO2 capture and sequestration (CCS) for power plants utilising hydrocarbon fuels. In this research experimental oxy-combustion trials and simulation are carried out by firing pulverised coal and biomass and co-firing a mixture of them in a 100 kW retrofitted oxy-combustor at Cranfield University. The parent fuels are coal (Daw Mill) and biomass cereal co-product (CCP) and experimental work was done for 100 % coal (w/w), 100 % biomass (w/w) and a blend of coal 50 % (w/w) and biomass 50 % (w/w). The recirculation flue gas (RFG) rate was set at 52 % of the total flue gas. The maximum percentage of CO2 observed was 56.7 % wet basis (73.6 % on a dry basis) when 100 % Daw Mill coal was fired. Major and minor emission species and gas temperature profiles were obtained and analysed for different fuel mixtures. A drop in the maximum temperature of more than 200 K was observed when changing the fuel from 100 % Daw Mill coal to 100 % cereal co-product biomass. Deposits formed on the ash deposition probes were also collected and analysed using the environmental scanning electron microscopy (ESEM) with energy-dispersive X-ray (EDX) technique. The high sulphur, potassium and chlorine contents detected in the ash generated using 100 % cereal co-product biomass are expected to increase the corrosion potential of these deposits. In addition, a rate-based simulation model has been developed using Aspen Plus® and experimentally validated. It is concluded that the model provides an adequate prediction for the gas composition of the flue gas.

Sensing in sooting flames: THz time-domain spectroscopy and tomography

We present line-of-sight measurements with broadband THz radiation through a methane diffusion flame, with the motivation to develop a modality for sensing and imaging within sooty flames. We present the design details of a bespoke high-pressure burner suitable for THz line-of-sight measurements, as well as hard-field tomography imaging from 4 projections. Further we introduce the setup for THz time-domain spectroscopy (TDS) and show the results of TDS measurements of a methane-air diffusion flame at different values of peripheral air co-flow at 3, 5, 10 and 12 l/min. The time-domain waveforms of the THz electric field transmitted through the flame are processed to extract the values of the time delay for each case. The dependence of these values on the air co-flow reveals that after an initial linear increase, the optical density along the sensing path levels out after 12 l/min. This is interpreted in terms of the radial temperature distribution of the gas flow. The effect of these parameters on the correct measurement setup for flame sensing and imaging at various geometries is briefly discussed.

Improving Stability Limits of Natural Gas Buoyant Diffusion Flames With Co-Flow of Air

In this paper we report on experimental investigation of co-flow air velocity effects on the flickering behavior and stabilization mechanism of laminar natural gas diffusion flames (with more than 96% methane in the fuel composition). In this study, chemiluminescence and high speed photography along with digital image processing techniques have been used to study the change in global flame shape, the instability initiation point, the frequency and magnitude of the flame oscillation. It is found that the co-flow air is able to shift the location of the initiation point of the outer toroidal vortices created by Kevin Helmholtz types of instability. It then reaches a stage when outer toroidal vortices interact only with hot plume of gases further downstream of the visible flame. Once the toroidal structure is out of the flame zone the flickering of the flame will disappear naturally. This is in contrast with the effect of pressure which enhances formation and interaction of outer toroidal vortices with the flame due to essential changes at flow densities. It is observed that a higher co-flow rate is needed in order to suppress the flame flickering at a higher fuel flow rate. Therefore the ratio of the air velocity to the fuel velocity is a stability controlling parameter. It has been found that a non-lifted laminar diffusion flame can be stabilized with a co-flow air velocity even less than half of the fuel jet exit velocity. The oscillation frequency was observed to increase with the co-flow rate. The frequency amplitudes, however, were observed to continuously decrease as the co-flow air was increasing. The oscillation magnitude and the oscillation wavelength were observed to decrease towards zero as the co-flow air was increasing. Whereas the average oscillating flame height behavior was observed to be bimodal. It was initially enhanced by the co-flow air then starts to decrease towards the stabilized level. This height was observed to remain almost constant after stabilization, despite further increase at air flow rate.© 2011 ASME