Onur Tuncer

The effect of academic inbreeding on scientific effectiveness

In academia, the term “inbreeding” refers to a situation wherein PhDs are employed in the very same institution that trained them during their doctoral studies. Academic inbreeding has a negative perception on the account that it damages both scientific effectiveness and productivity. In this article, the effect of inbreeding on scientific effectiveness is investigated through a case study. This problem is addressed by utilizing Hirsch index as a reliable metric of an academic’s scientific productivity. Utilizing the dataset, constructed with academic performance indicators of individuals from the Mechanical and Aeronautical Engineering Departments, of the Turkish Technical Universities, we demonstrate that academic inbreeding has a negative impact on apparent scientific effectiveness through a negative binomial model. This model appears to be the most suitable one for the dataset which is a type of count data. We report chi-square statistics and likelihood ratio test for the parameter alpha. According to the chi-square statistics the model is significant as a whole. The incidence rate ratio for the variable “inbreeding” is estimated to be 0.11 and this ratio tells that, holding all the other factors constant, for the inbred faculty, the h-index is about 89% lower when compared to the non-inbred faculty. Furthermore, there exists negative and statistically significant correlation with an individual’s productivity and the percentage of inbred faculty members at the very same department. Excessive practice of inbreeding adversely affects the overall productivity. Decision makers are urged to limit this practice to a minimum in order to foster a vibrant research environment. Furthermore, it is also found that scientific productivity of an individual decreases towards the end of his scientific career.

Fully Automatic Road Network Extraction from Satellite Images

In this paper a fully automatic road detection algorithm is introduced. It comprises of pre-processing the image via a series of wavelet based filter banks and reducing the yielding data into a single image which is of the same size as the original optical grayscale satellite image, then utilizing a fuzzy inference algorithm to carry out the road detection which can then be used as an input to a geographical information system for cartographic or for other purposes that are in need. We use a trous algorithm twice with two different wavelet bases in order to filter and de-noise the satellite image. Each wavelet function resolves features at a different resolution level associated with the frequency response of the corresponding FIR filter. Resulting two images are fused together using Karhounen-Louve transform (KLT) which is based on principal component analysis (PCA). This process underlines the prominent features of the original image as well as de-noising it, since the prominent features appear in both of the wavelet transformed images while noise does not strongly correlate between scales. Next a fuzzy logic inference algorithm which is based on statistical information and on geometry is used to extract the road pixels.

Combustion in a Ramjet Combustor with Cavity Flame Holder

Combustion characteristics in a ramjet combustor with cavity flame-holder is studied numerically. Combustor follows a constant area isolator section and comprises of hydrogen fuel injected sonically upstream of the cavity. Secondary fuel injection is performed at the cavity back wall. A diverging section follows the cavity. These concepts are utilized in many designs. Simulations were performed for an entrance Mach number of 1.4. Stagnation temperature is 702 K, corresponding to a flight Mach number of 3.3. Detailed chemical kinetics is taken into account with a reaction mechanism comprising of 9 species and 25 reaction steps. Turbulence is modeled using Menters shear stress transport model, which is suitable for high speed internal flows. It is observed that flame anchors at the leading edge of the cavity, and the flame is stabilized in the cavity mode rather than the jet-wake mode. Simulation captures all the essential features of the reacting flow field.

Effects of Hydrogen Enrichment on Confined Methane Flame Behavior

Many land based power generation units presently operate on natural gas, whose major constituent is methane, and many of them would need to tackle the challenges due to a fuel switch towards synthesis gas in the near future. Operating conditions and stability of a pre-mixed gas turbine combustor is quite sensitive to the changes in the fuel composition. Behavior of a premixed confined hydrogen enriched methane flame is studied with regard to thermo-acoustic instability induced flame flashback, emissions, flammability limits and acoustics over a wide range of operating conditions. However, most emphasis is put on lean combustion, which is an industry standard method used to lower pollutant emissions by reducing adiabatic flame temperatures. Hydrogen addition extends the flammability limits and enables lower nitric oxide emissions levels to be achieved at leaner equivalence ratios. On the other hand, increased root-mean-square pressure fluctuation levels, and higher susceptibility to flashback is observed with increasing hydrogen volume fraction inside the fuel mixture. This phenomenon is mostly attributed to much higher burning speeds of hydrogen in contrast to pure methane. A semi-analytical model has been utilized to capture the flame holding and thermo-acoustically induced flame flashback dynamics for a pre-mixed gas turbine combustor. A simple linearized acoustic model, derived from the basic conservation laws, and a front-tracking algorithm based on the Markstein’s G-equation are coupled together in order to track the flame initiation front, which in turn yields in an understanding of dynamic flame holding characteristics. A limit cycle behavior in the flame front movement is observed during simulations due to a non-linearity in the feedback term that relates acoustic velocity to heat release. Sets of experiments including flashback speed measurements have been performed at varying fuel composition. Phase locked CH radical imaging measurements have also been performed in order to track the flame initiation front in time with respect to the dominant instability cycle. Computer simulations are performed to study flashback and combustor acoustics together numerically and it is observed that these are in good qualitative agreement with the experiments.Copyright

Hydrogen Enriched Confined Methane Flame Behavior and Flashback Modeling

Most gas fueled power generation units presently operate on natural gas and many of them would need to tackle the challenges due to a fuel switch towards syngas in the near future. Operating regime of a gas turbine combustor is sensitive to the changes in the fuel composition. Behavior of a premixed enclosed methane-hydrogen flame is studied with regard to thermo-acoustic instability induced flame flashback, emissions, flammability limits and acoustics over a wide range of operating conditions. Hydrogen addition extends the lean flammability limits and enables lower NOx emissions levels to be achieved. However increased RMS pressure fluctuation levels and higher susceptibility to flashback is observed with increasing hydrogen volume fraction. This is associated with higher burning speeds of hydrogen in comparison to methane. Furthermore flashback in the experimental facility is triggered by thermo-acoustic oscillations. Therefore, an analytical model has been developed to capture the flame holding and thermo-acoustically induced flashback dynamics for a pre-mixed gas turbine combustor. A simple linearized acoustic model is derived from the basic conservation laws, and a front-tracking algorithm based on the Marksteins G-equation is coupled to combustor acoustics it in order to track the flame-front which yields in an understanding of dynamic flame holding and flashback behavior. Due to the non-linear nature of the coupling between acoustic velocity and heat release a limit cycle behavior in the flame front movement is observed during simulations. Sets of experiments including phase locked CH radical imaging have been performed in order to time resolve the flame initiation front behavior. Numerical simulations are performed to study flashback and combustor acoustics together and it is found that these are in good qualitative agreement with the experiments.

SIDE AIR-JET MODULATION FOR CONTROL OF HEAT RELEASE AND PATTERN FACTOR

ABSTRACT The effect of a forced dilution air jet, introduced through the combustor shell, on the air/fuel mixing in the combustion chamber has been investigated. Thermocouple-based temperature measurements have been made at a number of forcing frequencies in the range 100–1100 Hz and blowing ratios in the range 10–15. Open-loop integral flame response to forcing has also been acquired by recording pressure and heat release spectra. A CH-radical imaging technique is used to provide spatially and temporally resolved information about the heat release behavior. The results exhibit that the mean temperature field inside the main reaction zone can be significantly altered as a consequence of air-jet modulation. The most significant effects are observed by forcing at vertical locations that are close to the dump plane. Enhancements in temperature of the order of 100–200°C and reduction in pattern factor of the order of 40% were observed, with the lowest pattern factors achieved at the lowest forcing frequency of 100 Hz.

Dynamics, NOx and Flashback Characteristics of Confined Pre-Mixed Hydrogen Enriched Methane Flames

The operating regime of a gas turbine combustor is highly sensitive to fuel composition changes. In particular, the addition of hydrogen, a major constituent of syngas, has a major effect on flame behavior due to the higher burning rates associated with hydrogen. A laboratory scale pre-mixed test rig is constructed in order to study such effects. The fuel composition is incremented with increasing hydrogen starting from 100% methane. It is observed that increased RMS pressure levels and higher susceptibility to flashback occurs with increasing hydrogen volume fraction. Furthermore, hydrogen enrichment can cause an abrupt change in the dominant acoustic mode. Phase locked hydroxyl PLIF measurements have been performed with respect to the dominant acoustic instability limit cycle. These measurements are complemented with real time heat release, emissions and flashback measurements. Particular emphasis is put on time resolving the thermoacoustic instability induced flashback cycle of the wedgeshaped flame front and the temporal events associated with flashback.Copyright

Side Air-Jet Modulation for Control of Heat Release and Pattern Factor

The effect of a forced dilution air jet introduced through the combustor shell, on the air-fuel mixing in the combustion chamber has been investigated. Thermocouple based temperature measurements have been made at a number of forcing frequencies in the range of 100–1100Hz and blowing ratios in the range of 10–15. The open-loop integral flame response to forcing has also been acquired by recording pressure and heat release spectra. A CH-radical imaging technique is used to provide spatially- and temporally-resolved information about the heat release behavior. The results exhibit that the mean temperature field inside the main reaction zone can significantly be altered as a consequence of air jet modulation. The most significant effects are observed by forcing at vertical locations that are close to the dump plane. Enhancements in temperature of the order of 100–200 degrees C, and reduction in pattern factor of the order of 10% (e.g., from 1.13 to 1.03) were observed, with the lowest pattern factors achieved at the lowest forcing frequency of 100 Hz.Copyright

Experimental and numerical investigation of a swirl-stabilized premixed methane/air flame

Purpose – The purpose of this paper is to assess the validity of Wellers b-ω flamelet model for practical swirl-stabilized combustion applications. Design/methodology/approach – Swirl-stabilized premixed flame behavior is investigated utilizing an atmospheric combustor test rig. Swirl number of the flow is 0.74 with a cold flow Reynolds number of 19,400 based on the hydraulic diameter at the inlet pipe. Operating condition corresponds to an equivalence ratio of 0.7 at a thermal load of 20.4 kW. Reacting flow was seeded with TiO2 particles, and velocity distribution at the center plane was measured utilizing particle image velocimetry. These results serve as a validation dataset for numerical simulations. An open-source computational fluid dynamics (CFD) code library (OpenFOAM) is used for numerical computations. These unsteady Reynolds averaged Navier Stokes (RANS) computations were performed at the same load condition corresponding to experimental data. Parallel numerical simulations were carried out on 128 processor cores. To resolve turbulence, Menters k-ω shear stress transport model was utilized; flame behavior, on the other hand, was described by Wellers b-ω flamelet model. A block-structured all-hexahedral mesh was used. Findings – It is observed that two counter-rotating vortices in the main recirculation zone are responsible for flame stabilization. Weak secondary recirculation zones are also present at the sides above the dump plane. Flame front location was inferred from Mie scattering images. Experimental findings show that the flame anchors both on the tip of the center body and also at the rim of the outlet pipe. Numerical simulations capture the complex interactions between the flame and the turbulent flow. These results qualitatively agree with the flame structure observed experimentally. Practical implications – Swirl-stabilized combustion systems are used in many practical applications ranging from aeroengines to land-based power generation systems. There are implications regarding the understanding of these combustion systems. Social implications – Better understanding of combustion systems contributes to better performing turbine engines and reduced emissions with implications for the entire society. Originality/value – The paper provides experimental insight into the application of a combustion model for a flame configuration of practical interest.

Exit Temperature Profile Measurement and CFD Comparisons on Small Scale Turbojet Combustor with Air Blast Atomizer Configuration

Small scale turbojet engines are used in miniature UAVs and target drones. Combustor design at this scale is a challenge due to reduced volume for atomization, mixing and subsequent combustion. Furthermore correct estimation for the engine lifetime is critical in the design phase. In terms of lifetime hot-section components are much more critical. Exit temperature profile, which is commonly quantified using radial temperature distribution factor or using overall temperature distribution factor, is critical for the lifetime determination of critical hot-section engine components such as the first stage inlet guide vane of the turbine. This study presents the viability of the CFD approach for combustor design in terms of comparisons with experimental results put a spotlight to the areas that should improve in order to develop better CFD methods and practices. Results indicate that significant variability might occur even with the slightest manifold design modification. Also injector-to-injector flow rate variability has an effect on the exit plane temperature