Sinan Demir

Towards a predictive scenario of a burning accident in a mining passage

To reveal the inner mechanisms of a combustion accident in a coalmine, the key stages and characteristics of premixed flame front evolution such as the flame shapes, propagation speeds, acceleration rates, run-up distances and flame-generated velocity profiles are scrutinised. The theories of globally spherical, expanding flames and of finger-flame acceleration are combined into a general analytical formulation. Two-dimensional and cylindrical mining passages are studied, with noticeably stronger acceleration found in the cylindrical geometry. The entire acceleration scenario may promote the total burning rate by up to two orders of magnitude, to a near-sonic value. Starting with gaseous combustion, the analysis is subsequently extended to gaseous-dusty environments. Specifically, combustible dust (e.g. coal), inert dust (e.g. sand), and their combination are considered, and the influence of the size and concentration of the dust particles is quantified. In particular, small particles influence flame propagation more than large ones, and flame acceleration increases with the concentration of a combustible dust, until the concentration attains a certain limit.

Effect of local variations of the laminar flame speed on the global finger-flame acceleration scenario

Numerous formulations describing the dynamics and morphology of corrugated flames, including the scenarios of flame acceleration, are based on a “geometrical consideration”, where the wrinkled-to-planar flame velocities ratio, Sw /SL , is evaluated as the scaled flame surface area, while the entire combustion chemistry is immersed into the planar flame speed SL , which is assumed to be constant. However, SL may experience noticeable spatial/temporal variations in practice, in particular, due to pressure/temperature variations as well as non-uniform distribution of the equivalence ratio and/or that of combustible or inert dust impurities. The present work initiates the systematic study of the impact of the local SL -variations on the global flame evolution scenario. The variations are assumed to be imposed externally, in a manner being a free functional of the formulation. Specifically, the linear, parabolic and hyperbolic spatial SL -distributions are incorporated into the formulations of finger flame acceleration in pipes, and they are compared to the case of constant SL . Both two-dimensional channels and cylindrical tubes are considered. The conditions promoting or moderating flame acceleration are identified, and the revisited equations for the flame shape, velocity and acceleration rate are obtained for various SL -distributions. The theoretical findings are validated by the computational simulations of the reacting flow equations, with agreement between the theory and modelling demonstrated.

Influence of gas compressibility on a burning accident in a mining passage

A recent predictive scenario of a methane/air/coal dust fire in a mining passage is extended by incorporating the effect of gas compressibility into the analysis. The compressible and incompressible formulations are compared, qualitatively and quantitatively, in both the two-dimensional planar and cylindrical-axisymmetric geometries, and a detailed parametric study accounting for coal-dust combustion is performed. It is shown that gas compression moderates flame acceleration, and its impact depends on the type of the fuel, its various thermal-chemical parameters as well as on the geometry of the problem. While the effect of gas compression is relatively minor for the lean and rich flames, providing 5–25% reduction in the burning velocity and thereby justifying the incompressible formulation in that case, such a reduction appears significant, up to 70% for near-stoichiometric methane–air combustion, and therefore it should be incorporated into a rigorous formulation. It is demonstrated that the flame tip velocity remains noticeably subsonic in all the cases considered, which is opposite to the prediction of the incompressible formulation, but qualitatively agrees with the experimental predictions from the literature.

Thermodynamic analysis and assessment of a municipal wastewater treatment system: a field study on sewage treatment

This paper is a field study on thermodynamic analysis and assessment of an actual municipal wastewater treatment facility. The system operation is described and a methodological approach based on the energy and exergy analyses is provided to assess the performance of a complex wastewater treatment system. Since the specific exergy value of the sludge is increased through treatment process, a new exergy based parameter, net exergy ratio (NExT), is defined to reflect the renewable and sustainable nature of the process. The system treats nearly 222,000 m² domestic wastewater per day by using the primary and secondary treatments. The NExT values for primary and secondary treatments are found as 2.81 and 1.31, respectively. When the sludge outputs of both processes are considered as waste, the exergetic efficiencies of subsystems in the actual municipal wastewater treatment plant (WWTP) can be obtained as 57.4% for primary treatment system (PTS) and 29.7% for secondary treatment system (STS).

Exergoeconomic assessment of a municipal primary and secondary sewage treatment

This paper presents the exergoeconomic analysis and assessment of a municipal primary and secondary sewage treatment systems. Operation of an existing municipal sewage treatment plant is described in detail and an exergoeconomic methodology based on SPECO method is provided to allocate cost flows through subcomponents of the plant. The system treats nearly 222,000m³ domestic wastewater per day by using the primary and secondary treatment systems. The exergetic efficiencies of the primary and secondary sewage treatment systems are determined to be 53.4% and 14.8%, respectively. The exergetic cost rate and the specific unit exergetic cost of the treated wastewater at the exit of the WWTP are found to be 62.05

Analysis of “Finger” Flame Acceleration as a Stage of a Methane-Air-Dust Fire in a Coal Mine

/h and 3.804 ¢/m³, respectively.

Economic Optimization of Indirect Sewage Sludge Heat Dryer Unit for Sewage Sludge Incineration Plants

To reveal the inner mechanism of gas explosion, the entire scenario of premixed flame front evolution within an accidental fire is prescribed. Specifically, “finger” flame shape, which is one of the key stages of flame evolution, is scrutinized with the situation of a methane-air explosion. A transition from a globally-spherical front to a finger-shaped one occurs when a flame starts approaching the passage walls. While this acceleration is extremely strong, it stops as soon as the flame touches the passage wall. This mechanism is Reynolds-independent; being equally relevant to micro-channels and giant tunnels. The flame speed increases by an order of magnitude during this stage. To implement dusty environments, Seshadri formulation for the planar flame [Combustion and Flame 89 (1992) 333] is employed with a non-uniform distribution of inert dust gradients, specifically, linear, parabolic and hyperbolic spatial dust distribution gradients are incorporated into the “finger” flame shape. This study systematically investigates how the noncombustible dust distributions affect fire evolution, the flame shape, and propagation velocity.Copyright

Effect of Wall Boundary Conditions on Flame Propagation in Micro-Chambers

This paper presents the economic optimization of indirect sewage sludge heat dryer for sewage sludge incineration plants. The objective function based on two-phase heat transfer, and economic relations is provided to demonstrate the optimum size for the minimum investment cost. De-watered sludge is fed into the dryer with a mass flow rate of 165 tons per day and consists of 27% dry matter. After the sludge drying process, the dryness of sludge increases up to 40%. In the indirect sludge dryer unit, thermal oil is used to heat the dryer wall and to prevent heat loss. Thermal oil is circulated in a closed cycle and gathered into an oil tank. Total cost of the sludge dryer unit changes proportional to the dryer area. The optimum dryer area is found as 32.54 m2. The corresponding minimum cost is found as

Assessment of Sewage Sludge Potential from Municipal Wastewater Treatment Plants for Sustainable Biogas and Hydrogen Productions in Turkey

35,700.Copyright

Thermoeconomic assessment of a sustainable municipal wastewater treatment system

Flame dynamics in micro-pipes have been observed to be strongly affected by the wall boundary conditions. In this respect, two mechanisms of flame acceleration are related to the momentum transferr ...