Particle deposition model for particulate flows at high temperatures in gas turbine components

This study proposes an improved physical model to predict sand deposition at high temperature in gas turbine components. This model differs from its predecessor (Sreedharan and Tafti, 2011) by improving the sticking probability by accounting for the energy losses during particle-wall collision based on our previous work (Singh and Tafti, 2013). This model predicts the probability of sticking based on the critical viscosity approach and collision losses during a particle–wall collision. The current model is novel in the sense that it predicts the sticking probability based on the impact velocity along with the particle temperature. To test the model, deposition from a sand particle laden jet impacting on a flat coupon geometry is computed and the results obtained from the numerical model are compared with experiments (Delimont et al., 2014) conducted at Virginia Tech, on a similar geometry and flow conditions, for jet temperatures of 950 °C, 1000 °C and 1050 °C. Large Eddy Simulations (LES) are used to model the flow field and heat transfer, and sand particles are modeled using a discrete Lagrangian framework. Results quantify the impingement and deposition for 20–40 μm sand particles. The stagnation region of the target coupon is found to experience most of the impingement and deposition. For 950 °C jet temperature, around 5% of the particle impacting the coupon deposit while the deposition for 1000 °C and 1050 °C is 17% and 28%, respectively. In general, the sticking efficiencies calculated from the model show good agreement with the experiments for the temperature range considered.     

Numerical investigations of cooling holes system role in the protection of the walls of a gas turbine combustion chamber

Numerical simulations in a gas turbine Swirl stabilized combustor were conducted to investigate the effectiveness of a cooling system in the protection of combustor walls. The studied combustion chamber has a high degree of geometrical complexity related to the injection system as well as the cooling system based on a big distribution of small holes (about 3,390 holes) bored on the flame tube walls. Two cases were considered respectively the flame tube without and with its cooling system. The calculations were carried out using the industrial CFD code FLUENT 6.2. The various simulations made it possible to highlight the role of cooling holes in the protection of the flame tube walls against the high temperatures of the combustion products. In fact, the comparison between the results of the two studied cases demonstrated that the walls temperature can be reduced by about 800°C by the mean of cooling holes technique.     

Composition dependent model for the prediction of syngas ash deposition in turbine gas hotpath

An improved physical model to predict flyash deposition is developed and discussed in this paper. This model differs from its predecessor ( [Rozati et al., in press] and [Sreedharan and Tafti, 2009] by accounting for deposition of syngas ash particles below the ash softening temperature. The modified deposition model is based on the critical viscosity approach. To test this model, deposition of ash particles impacted on a flat, 45° wedge shape geometry is computed and the results obtained from the numerical model are compared to Crosby et al. (2007). Large Eddy Simulation (LES) is used to model the flow field and flyash particles are modeled using a discrete Lagrangian framework. Results quantify deposition for 4 μm particles of various ash composition samples. Most of the deposition occurs at the stagnation region of the target plate. At 1456 K, out of all the ash samples considered in this study, WY and ND ash sample show the highest capture efficiency (15%) and KL1 ash sample exhibits the lowest capture efficiency (0.02%). In general, capture efficiencies for all ash samples followed an exponential trend with temperature. Additionally, this model is also compared to results obtained from the flat plate deposition experiments conducted here at Virginia Tech using PVC particles (Wood et al., 2010). In the case of PVC particles, the sticking probability in the deposition model assumed an exponential increase in deposition rate with temperature and was calibrated with one experimental data point. The results obtained from this model for PVC particles showed excellent agreement with the experimental measurements over a range of temperatures.     

Experimental investigations on steady wake effects in a high-lift turbine cascade

This paper reports on the investigation of steady wake effects in cascades. An annular cascade rig, where two stators having the same blade pitch can be circumferentially traversed relatively to each other, is used to analyse the profile losses and the boundary layer development of the downstream stator for different circumferential positions of the upstream stator (clocking positions). Different measurement techniques are used such as three-hole pressure probes, and hot wire- and surface-mounted hot-film probes. The results show a varying pressure loss coefficient of the downstream cascade (S2) for different clocking positions of the upstream cascade (S1_SP).     

Performance of supersonic model combustors with staged injections of supercritical aviation kerosene

Supersonic model combustors using two-stage injections of supercritical kerosene were experimentally investigated in both Mach 2.5 and 3.0 model combustors with stagnation temperatures of approximately 1,750 K. Supercritical kerosene of approximately 760 K was prepared and injected in the overall equivalence ratio range of 0.5-1.46. Two pairs of integrated injector/flameholder cavity modules in tandem were used to facilitate fuel-air mixing and stable combustion. For single-stage fuel injection at an upstream location, it was found that the boundary layer separation could propagate into the isolator with increasing fuel equivalence ratio due to excessive local heat release, which in turns changed the entry airflow conditions. Moving the fuel injection to a further downstream location could alleviate the problem, while it would result in a decrease in combustion efficiency due to shorter fuel residence time. With two-stage fuel injections the overall combustor performance was shown to be improved and kerosene injections at fuel rich conditions could be reached without the upstream propagation of the boundary layer separation into the isolator. Furthermore, effects of the entry Mach number and pilot hydrogen on combustion performance were also studied.     

An analytical-empirical model to predict heat transfer coefficients in circulating fluidized bed combustors

An analytical-empirical model was developed to predict local heat transfer coefficients in circulating fluidized bed (CFB) combustors. The fluid is considered to be composed of a dispersed phase and cluster of particles. Both convection and thermal radiation are taken into account. The estimation of the parameters in the model is discussed and suggestions as well as a sensitivity analysis are given. The model is applied to predict heat transfer coefficients on membrane walls of a CFB boiler for which an experimental investigation had been carried out previously. The predictions were in good agreement with experimental results.

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Ein analytisch-empirisches Modell zur Bestimmung der Wärmeübergangskoeffizienten in zirkulierenden Wirbelschichtöfen

Zusammenfassung Ein analytisch-empirisches Modell zur Ermittlung der lokalen Wärmeübergangskoeffizienten in zirkulierenden Wirbelschichtöfen wird vorgestellt, wobei das Fluid aus einer dispersen Phase und einem Cluster von Partikeln zusammengesetzt sein soll. Sowohl Konvektion als auch Wärmestrahlung finden Berücksichtigung. Die das Modell beschreibenden Parameter werden abgeschätzt und eingehend diskutiert. Das Modell dient zur Ermittlung von Wärmeübertragungskoeffizienten an der Membranwand eines CFB-Verdampfers, für den eine experimentelle Untersuchung vorliegt. Die Übereinstimmung zwischen Theorie und Experiment ist gut.

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Nomenclature B backscatter fraction - C _pc specific heat of cluster, J/kg K - C _pg specific heat of gas, J/kg K - C _pp specific heat of particle, J/kg K - d _p particle diameter, m - e _c emissivity of the cluster - e _w emissivity of the wall surface - e _dis effective emissivity of the dispersed phase - e _p emissivity of the particle - f time fraction the wall surface is covered by the clusters - h _c total convective heat transfer coefficient, W/m²K - h _cc heat transfer coefficient due to cluster convection, W/m²K - h _cclj heat transfer coefficient due to cluster convection fromjth subsection inlth wall section, W/m²K - h _cd heat transfer coefficient due to dispersed phase convection, W/m²K - h _r total radiative heat transfer coefficient, W/m²K - h _rc heat transfer coefficient due to cluster radiation, W/m²K - h _rd heat transfer coefficient due to dispersed phase radiation, W/m²K - h _ov overall heat transfer coefficient, W/m²K - g acceleration due to gravity, m/s² - i arbitrary wall section - j arbitrary subsection - k _i number of subsections in wall sectioni - K constant in Eq. (17) - K _c effective thermal conductivity of the cluster, W/mK - K _gg effective thermal conductivity of gas, W/mK - K _gf thermal conductivity of the gas film calculated at (T _b +T _w )/2, W/mK - K _p thermal conductivity of the particles, W/mK - L _lj residence length of the cluster of thejth subsection inlth section at the wall, m - m _i mass flow rate to theith section from the core, kg/m²s - t _lj residence time of the cluster fromjth subsection inlth section, s - T _b bulk bed temperature, K - T _c cluster temperature, K - T _w wall temperature, K - U _c cluster velocity, m/s - U _g superficial gas velocity, m/s - U _t terminal settling velocity, m/s - Y volume fraction of solid in the dispersed phase Greek letters _g thickness of the gas film between the cluster and the wall surface, m - _rough r.m.s. value of surface roughness, m - _c cluster voidage - _sus average cross-sectional voidage - _dis voidage of the dispersed phase - _w voidage near the wall - dynamic viscosity, kg/ms - _c density of the cluster, kg/m³ - _dis density of the dispersed phase, kg/m³ - _g density of gas, kg/m³ - _p density of particle, kg/m³ - _sus suspension density, kg/m³ - Stefan-Boltzmann's constant, 5.67×10^–8, W/m²K⁴     

High-speed laser diagnostics for the study of flame dynamics in a lean premixed gas turbine model combustor

A series of measurements was taken on two technically premixed, swirl-stabilized methane-air flames (at overall equivalence ratios of ϕ = 0.73 and 0.83) in an optically accessible gas turbine model combustor. The primary diagnostics used were combined planar laser-induced fluorescence of the OH radical and stereoscopic particle image velocimetry (PIV) with simultaneous repetition rates of 10 kHz and a measurement duration of 0.8 s. Also measured were acoustic pulsations and OH chemiluminescence. Analysis revealed strong local periodicity in the thermoacoustically self-excited (or ‘noisy’) flame (ϕ = 0.73) in the regions of the flow corresponding to the inner shear layer and the jet-inflow. This periodicity appears to be the result of a helical precessing vortex core (PVC) present in that region of the combustor. The PVC has a precession frequency double (at 570 Hz) that of the thermo-acoustic pulsation (at 288 Hz). A comparison of the various data sets and analysis techniques applied to each flame suggests a strong coupling between the PVC and the thermo-acoustic pulsation in the noisy flame. Measurements of the stable (‘quiet’) flame (ϕ = 0.83) revealed a global fluctuation in both velocity and heat-release around 364 Hz, but no clear evidence of a PVC.     

After fogging process in water injected gas turbine systems

In gas turbine system with after fogging, water droplets are injected after compressor. After fogging could have more significant potential for enhancement of specific power production compared to inlet fogging alone, since a larger water injection rate is possible. Transient analysis of after fogging process is carried out by using a heat and mass transfer modeling on water droplet evaporation. Transient variables such as droplet diameter and air temperature are evaluated as the droplet evaporation proceeds for different values of initial droplet diameter, pressure ratio of compressor, and water injection ratio. The evaporation time for injected droplets are also estimated. Present results show that the evaporation time decreases sensitively with increasing pressure ratio or initial droplet diameter. However, the effect of water injection ratio on evaporation time is relatively insignificant unless water injection ratio is near the critical ratio.     

Secondary loss generation by gas turbine support struts

Short cylindrical struts are commonly employed to carry services across the annular flow passages of gas turbines and to provide mechanical support. Velocity variations along the span of the strut will be large and secondary flow becomes important. For bluff bodies, boundary layer separation tends to be fixed close to the maximum thickness of the strut, or any sharp edges, so that secondary flow effects have only a minor influence on wake formation. In the case of more streamlined shapes, the effect of Reynolds number and freestream turbulence level on boundary layer growth are much more significant. Moreover, the secondary flows generated by the interaction between the strut cross-section and the end-wall boundary layers may influence the position of separation, thus changing the distributions of pressure on the strut surface and in the wake. These modifications lead to large variations in the total drag force experienced by the strut. A recent wind tunnel investigation is described in which wake pressure measurements have been used to determine the additional losses produced by the secondary flow generation. Experiments have been performed on isolated struts for both circular and streamlined cross-section over a range of Reynolds number, aspect ratio and thickness-to-chord ratio. A principal finding is that the results for the streamlined struts may be reduced to a correlation which embraces the effects of cross-sectional geometry as well as the end-wall boundary layer thickness, the Reynolds and the Mach numbers.     

球面聚心气相爆轰波传播过程的数值研究

应用频散可控耗散格式对球面聚心气相爆轰波的传播过程进行了数值模拟 研究. 通过跟踪波阵面上压力和温度变化，分析这些参数在爆轰波传播过程中的演变规律， 及其与几何尺度和初始条件之间的依赖关系. 研究结果表明，仅在远场波面压力的变化近似 只依赖于r/R，对称中心附近则需要考虑初始半径$R$的影响；波面压力与初始压力的变化呈 线性关系；汇聚过程中温度升高比压力慢得多.     

Simulation-supported measurements in large circulating fluidized bed combustors

The concept of simulation-supported measurement is suggested for the elucidation of processes occurring in the combustion chambers of large-scale circulating fluidized bed combustors where the desired information cannot be obtained by direct measurements. The concept is illustrated with the example of secondary air injection where the way the air is released, the penetration depth and the evenness of air distribution over the cross-sectional area of the combustion chamber are of interest. The measured information consists of lateral profiles of oxygen concentrations measured with gas sampling probes at two ports which were located 5 and 9 m, respectively, above the level of secondary air injection. The simulation is carried out on the basis of a 3D semi-empirical fluid-mechanical model of the circulating fluidized bed which is combined with models of gas and solids mixing, fuel distribution, devolatilization and combustion of char and volatiles. The combination of the simulation with the measurements yields a clear picture of the mechanism of secondary air injection, its penetration into the combustion chamber and its effect on the local combustion processes. The results confirm the usefulness of the concept of simulation-supported measurement for this application.     

Simulation-supported measurements in large circulating fluidized bed combustors

The concept of simulation-supported measurement is suggested for the elucidation of processes occurring in the combustion chambers of large-scale circulating fluidized bed combustors where the desired information cannot be obtained by direct measurements.The concept is illustrated with the example of secondary air injection where the way the air is released,the penetration depth and the evenness of air distribution over the cross-sectional area of the combustion chamber are of interest.The measured informat...     

Fuel-oil concentration in a gas turbine burner measured with laser-induced fluorescence

We applied the technique of two-dimensional laser-induced fluorescence to measure relative local concentrations of the evaporated fuel oil in a premixed gas turbine burner. The dependence of the fuel-oil fluorescence in the burner on the air inlet temperature, on the total fuel mass flow, and on the residence time of the fuel was investigated. A Mie scattering experiment was performed in order to observe non-evaporated fuel oil. We conclude that the fuel-oil concentrations can be determined from the fluorescence intensity within an error of about 25% in regions of complete evaporation of the fuel oil. Received: 29 March 1999/Accepted: 5 January 2000     

Spatial resolution of a chemiluminescence sensor for local heat-release rate and equivalence ratio measurements in a model gas turbine combustor

The spatial resolution of a Chemiluminescence Sensor, based on focused Cassegrain optics, to detect the location of the reaction zone and heat-release rate in a model gas turbine combustor is reported. The sensor measures simultaneously the chemiluminescent intensities from OH* and CH* excited radicals in flames in order to obtain information on the local flame characteristics. The spatial resolution was evaluated by a combined theoretical and experimental study in laminar and turbulent flames and was supported by detailed chemistry calculations, including the chemiluminescent species, of unstrained one-dimensional flames. The experimental study involved simultaneous measurements of chemiluminescence with the sensor and laser-based reaction rate imaging, using the product of OH and CH₂O radicals obtained from planar laser-induced fluorescence (PLIF), and OH PLIF for the location of the reaction zone. The study quantified the influence of flame shape and dimensions and the direction of traverse of the focal region of the sensor through the flames on the spatial resolution, thereby identifying the limitations and optimising the applicability of the sensor. The sensor was used to obtain local time-dependent measurements of heat-release and equivalence ratio of a reacting mixture, based on the chemiluminescent intensity ratio of OH*/CH*, in a swirl-stabilised model gas turbine combustor and quantified the degree of air–fuel premixedness, probability of reaction and power spectra of pressure and chemiluminescent intensity fluctuations in two unsteady flames.     

Differential interferometry with adjustable spatial carrier fringes for turbine blade cascade flow investigations

 Digital evaluated differential interferometry using adjustable spatial carrier fringes was applied to flow measurements in a transonic turbine blade cascade. The interferograms were evaluated using a two-dimensional Fourier Analysis. This evaluation provided density gradient maps of the flow field in a digital form. A specially designed interferometer was used allowing adjustment of sensitivity and superposition of carrier-fringe system separately. This type of interferometer is also highly insensitive to vibration noise. For a turbine blade cooling film a comparison of this type of differential interferometry with holographic interferometry and a Schlieren visualisation is also given. As a result differential interferometry using adjustable spatial carrier fringes and digital fringe analysis gave density-gradient maps similar to Schlieren recordings but of quantitative nature, thus enabling compensation of wave front distortions. Integration of these density gradient maps resulted in density maps which were then compared to recordings done with pulsed holographic interferometry. Received: 27 December 1996/Accepted: 30 April 1997     

Dynamic stall model for wind turbine airfoils

A model is presented for aerodynamic lift of wind turbine profiles under dynamic stall. The model combines memory delay effects under attached flow with reduced lift due to flow separation under dynamic stall conditions. The model is based on a backbone curve in the form of the static lift as a function of the angle of attack. The static lift is described by two parameters, the lift at fully attached flow and the degree of attachment. A relationship between these parameters and the static lift is available from a thin plate approximation. Assuming the parameters to be known during static conditions, nonstationary effects are included by three mechanisms: a delay of the lift coefficient of fully attached flow via a second-order filter, a delay of the development of separation represented via a first-order filter, and a lift contribution due to leading edge separation also represented via a first-order filter. The latter is likely to occur during active pitch control of vibrations. It is shown that all included effects can be important when considering wind turbine blades. The proposed model is validated against test data from two load cases, one at fully attached flow conditions and one during dynamic stall conditions. The proposed model is compared with five other dynamic stall models including, among others, the Beddoes–Leishman model and the ONERA model. It is demonstrated that the proposed model performs equally well or even better than more complicated models and that the included nonstationary effects are essential for obtaining satisfactory results. Finally, the influence of camber and thickness distribution on the backbone curve are analysed. It is shown that both of these effects are adequately accounted for via the static input data.     

Spurious counts in gas volume flow measurements by means of turbine meters

Acoustical oscillations can induce a rotation of a turbine flow meter in the absence of main flow, which leads to spurious counts. A simplified model is presented which explains the occurrence of spurious counts in the limit of very thin turbine blades and high Strouhal numbers. The predicted threshold for the occurrence of spurious counts is compared to experimentally obtained data at various gas pressures in the range from 1 to 8 bar. The simplified model provides a reasonable prediction of the occurrence of spurious counts and can be used as an useful engineering tool in the prediction of the occurrence of spurious counts.     

Numerical simulation of a low-emission gas turbine combustor using KIVA-II

A numerical study was performed to investigate chemically reactive flows with sprays inside a staged turbine combustor (STC) using a modified version of the KIVA-II code. This STC consists of a fuel nozzle (FN), a rich-burn (RB) zone, a converging connecting pipe, a quick-quench (QQ) zone, a diverging connecting pipe and a lean-combustion (LC) zone. From the computational viewpoint, it is more efficient to split the STC into two subsystems, called FN/RB zone and QQ/LC zones, and the numerical solutions were obtained separately for each subsystem. This paper addresses the numerical results of the STC which is equipped with an advanced airblast fuel nozzle. The airblast nozzle has two fuel injection passages and four air flow passages. The input conditions used in this study were chosen similar to those encountered in advanced combustion systems. Preliminary results generated illustrate some of the major features of the flow and temperature fields inside the STC. Velocity, temperature and some critical species information inside the FN/RB zone are given. Formation of the co- and counter-rotating bulk flow and the sandwiched-ring-shaped temperature field, typical of the confined inclined jet-in-cross-flow, can be seen clearly in the QQ/LC zones. The calculations of the mass-weighted standard deviation and the pattern factor of temperature revealed that the mixing performance of the STC is very promising. The temperature of the fluid leaving the LC zone is very uniform. Prediction of the NO_x emission shows that there is no excessive thermal NO_x produced in the QQ/LC zones for the case studied. From the results obtained so far, it appears that the modified KIVA-II code can be used to guide the low-emission combustion experiments.