Hot Gas Region Research Articles

Conjugate heat transfer analysis of internally cooled superalloy turbine blades with grooved channels

The present work reports a conjugate heat transfer analysis of a turbine blade exposed to high-temperature compressible gas flow and the convection cooling inside the blade. A nickel-based superalloy material CMSX-4 with better mechanical and anticorrosive properties has been introduced for blade materials, and grooved channels are proposed for heat transfer enhancement in internal convection. Each channel contains nine mini-grooves having groove-depth to channel-diameter ratio in the range of 0.08–1.12. Three prominent turbulence models, namely, k-ε, k-ω shear stress transport (SST), and γ-θ transition SST, are used to capture the flow turbulence in a transonic boundary layer flow. Simulations have been performed for actual operating conditions of turbine blades with a wall-to-gas temperature ratio of 0.84 and an inlet-to-outlet pressure ratio of 1.69. The inlet Reynolds number is 5.3 × 105 for the hot gas region, and for coolant flow, the Reynolds number varies from 16 000 to 70 000. The Mach number reaches to a maximum value of 1.14 in the external hot gas flow. Boundary layer transition and wake flow from nearby blades affect the flow in the suction side of the blade. The incorporation of scalable wall function improves the performance of the k-ε turbulence model. Compared to the smooth channel, a 25 K reduction in the average blade surface temperature and 27.3% enhancement in the Nusselt number in blade cooling are obtained for the grooved cooling channel.

Investigation of full-scale porous injector plate transpiration cooling coupled with combustion in high-thrust H2/O2 rocket engines

The high chamber pressure and large heat flux impose greater demands on the structure cooling and thermal protection of the future high-thrust H2/O2 rocket engine. To investigate the applicability of transpiration cooling on the full-scale injector plate in the high-thrust rocket engine, a numerical model is presented, in which the porous region and the mainstream hot gas region are directly coupled. The local equilibrium model and eddy dissipation model are considered. The numerical model is validated by transpiration cooling experiments for a subscale rocket engine injector plate. Then, the numerical simulation of transpiration cooling for the high-thrust H2/O2 rocket engine injector plate shows that the transpiration cooling with 13% of the total hydrogen mass flow rate can reduce the gas side temperature of the injector plate by about 150 K and reduce the thermal penetration depth to 9.2% of the plate thickness. Moreover, As the coolant mass flow rate increase from 4% to 13%, the gas side temperature of the plate decreases from 735 K to 668 K and the cooling effectiveness rises. The lower the coolant inlet temperature, the lower the plate temperature, but the higher the cooling effectiveness. Besides, the plate temperature drops with the decreases of the thermal conductivity of the porous media.

Investigation of Discharge Propagation in Localized High-Temperature SF6 Gas With Breakdown Voltage Measurement

In high-voltage gas circuit breakers (HVCBs), high-pressure gas is blown between the electrodes to cool the arc. The temperature distribution between the electrodes at the current zero point has a large gas temperature difference. This study aims to experimentally clarify the effect of the cold gas region on the breakdown voltage of gases with large temperature differences. In an experiment, a hot gas generator using arc heat was used to create a gas space with a large temperature difference between electrodes, and the breakdown voltage was evaluated. The breakdown voltage in the gas space with a cold gas region was twice that of the case, where the space between the electrodes was filled with hot gas. It was found that the discharge initiated in the hot gas region was suppressed in the cold gas region and that the breakdown voltage in a gas space with the hot and cold region was influenced by the existence of the cold gas.

LES/FGM investigation of ignition and flame structure in a gasoline partially premixed combustion engine

This paper presents a joint numerical and experimental study of the ignition process and flame structures in a gasoline partially premixed combustion (PPC) engine. The numerical simulation is based on a five-dimension Flamelet-Generated Manifold (5D-FGM) tabulation approach and large eddy simulation (LES). The spray and combustion process in an optical PPC engine fueled with a primary reference fuel (70% iso-octane, 30% n-heptane by volume) are investigated using the combustion model along with laser diagnostic experiments. Different combustion modes, as well as the dominant chemical species and elementary reactions involved in the PPC engines, are identified and visualized using Chemical Explosive Mode Analysis (CEMA). The results from the LES-FGM model agree well with the experiments regarding the onset of ignition, peak heat release rate and in-cylinder pressure. The LES-FGM model performs even better than a finite-rate chemistry model that integrates the full-set of chemical kinetic mechanism in the simulation, given that the FGM model is computationally more efficient. The results show that the ignition mode plays a dominant role in the entire combustion process. The diffusion flame mode is identified in a thin layer between the ultra fuel-lean unburned mixture and the hot burned gas region that contains combustion intermediates such as CO. The diffusion flame mode contributes to a maximum of 27% of the total heat release in the later stage of combustion, and it becomes vital for the oxidation of relatively fuel-lean mixtures.

Burst-mode 1-methylnaphthalene laser-induced fluorescence: extended calibration and measurement of temperature and fuel partial density in a rapid compression machine

In this work, tracer-based laser-induced fluorescence (LIF) with the tracer 1-methylnaphthalene is utilized to study temperature and fuel courses in a rapid compression machine (RCM) under high temperature and pressure conditions. A burst-mode Nd:YAG laser at 266 nm is applied for excitation of tracer fluorescence at a frame rate of 7.5 kHz. A high-speed intensified CMOS camera equipped with an image doubler is used for 2-color LIF (2c-LIF) thermometry. With known local temperature, the fuel partial density can be determined using the signal of the channel covering the complete LIF spectrum. Both temperature and fuel partial density are determined during the compression and expansion strokes in nitrogen and air atmospheres. For this purpose, first-time 1-MN LIF calibration measurements in air atmosphere were performed for cylinder pressures up to 2.8 MPa. This significantly extends the calibration data base generated in current calibration cells. Although the LIF signal dropped significantly due to oxygen quenching, first promising measurements of temperature and fuel partial density were conducted in the RCM at relevant equivalence ratios. The influence of the RCM driving gas pressure on the temperature course is shown for cylinder pressures up to 7.4 MPa in nitrogen atmosphere. Although the temperature and concentration fields are very homogeneous at early points in time during compression, inhomogeneities in terms of millimeter-sized hot and cold gas regions were resolved especially near top dead center (TDC) using the present approach. These structures were also visible in the fuel partial density field. These inhomogeneities are due to the heat transfer between the hot gas and the cool walls and are probably also induced by the piston movement. Especially at TDC, the minimum gas temperature is about 300 K lower than the peak temperature in the wall region of the cylinder head. These cool region temperatures are much lower than in piston engines and other RCMs reported in the literature at comparable conditions, which may due to the special design of the present layout of the machine.

Tracer-Based High-Speed Laser-Induced Fluorescence Measurements Of Temperature And Fuel Concentration Fields In A Rapid Compression Expansion Machine

Tracer planar laser-induced fluorescence (PLIF) using 1-methylnaphthalene (1-MN) is applied to study temperature and fuel courses during compression in a rapid compression expansion machine (RCEM). A burst mode Nd:YAG-laser at 266 nm is utilized for excitation of tracer fluorescence at a repetition rate of 7.5 kHz. A high-speed intensified CMOS camera equipped with an image doubler is applied for 2-color LIF (2c-LIF). The measurements were conducted in nitrogen atmosphere for which a calibration curve is presented that was generated in a flow cell. The temperature and fuel concentration fields are very homogeneous at early points in time during compression, however, inhomogeneities in terms of millimeter-sized hot and cold gas regions can be found especially near top dead center (TDC). These inhomogeneities were also visible in the fuel partial density field and are due to the distinct heat transfer between the hot gas and the cool walls, and probably also because of roll-up vortices induced by the piston movement. Especially at TDC the minimal gas temperature is about 300 K lower than the peak temperature near the cylinder head. These temperature differences are much larger than in piston engines and in other RCEMs reported in the literature at comparable conditions. This is because of the special design of the present layout of the machine. A relatively large difference to the adiabatic temperature can be overserved particularly at TDC. About 70 K lower temperatures were measured, which can be explained by these temperature inhomogeneities in the cylinder.

Internal cooling sensitivity analysis to improve the thermal performance of gas turbine blade using a developed robust conjugate heat transfer method

The heat transfer simulations of turbine blades with internal cooling are faced with so many uncertainties, of which some originate from the secondary air system, including the inlet hot gas temperature and pressure and the cooling side boundary conditions, and the blade material. The main objective of this work is to carry out a suitable sensitivity analysis on a specific novel turbine vane to improve the thermal performance of its internal cooling system and to quantify how the uncertainties on the designed/calculated values can desirably/undesirably affect the maximum blade surface temperature, which can consequently affect the gas turbine engine efficiency. Furthermore, the sensitivity analysis is carried out to find the effects of uncertainties of a number of key parameters on the resulting blade temperature distribution. To arrive at trustworthy conclusions, the conjugate heat transfer (CHT) method is used to analyze the heat and fluid flow behavior. This work suitably develops a CHT-based method/solver to perform the proposed study. This method/solver uses a segregated iterative procedure, in which the outer hot gas region is simulated using the computational fluid dynamics (CFD) solver, the flow passing through the connected internal cooling passages is calculated using a 1D correlated-based solver, and the vane conduction is predicted using a 3D finite-element solver, which are fully coupled. The results show that the cooling channel wall temperature has a direct impact on the convective coefficient magnitude; especially in lower temperature regions. As a novel contribution, this work takes into account the cooling wall temperature influence on the 1D code calculations. To implement this, an artificial neural network is suitably trained to predict better convective coefficient. The results of this developed CFD-CHT code are validated against experimental data available for a benchmark vane. Eventually, the sensitivity analysis is carried on the present specific novel turbine vane.

Distribution of the coronal temperature in Seyfert 1 galaxies

Active galactic nuclei (AGN) produce copious amounts of X-rays through the corona, the region of hot gas that lies close to the accretion disk. The temperature of the corona can be accurately determined by the cut-off signature in the X-ray spectrum. Owing to the high temperatures in the corona, observations well above 10 keV are necessary. Here, we explore the NuSTAR observations of 118 Seyfert 1 AGN selected using Gehrels/Swift. We modelled the spectrum using a single power-law with an exponential cut-off modified by neutral and ionised absorption as well as a reflection component. We found secure spectral cut-off estimates in 62 sources, while for the remaining ones, we derived only the lower limits. The resulting mean value is 103 keV, with a skewed distribution towards large energies with a large dispersion. When we consider the lower limits using survival analysis techniques, the mean cut-off energy becomes significantly larger, that is, about 200 keV. Because of various limitations (e.g., limited spectral passband, photon statistics, model degeneracies), we performed extensive simulations to explore the underlying spectral cut-off distribution. We find that an intrinsic spectral cut-off distribution, which has a Maxwell-Boltzmann shape with a mean value in the range of 160–200 keV, can reproduce the observations sufficiently well. Finally, our spectral analysis places very stringent constraints on both the photon index (Γ = 1.77 ± 0.01) as well as on the reflection component (R = 0.69 ± 0.04) of the Seyfert 1 population. From the values of the spectral cut-off and the photon-index, we deduce that the mean optical depth of the AGN corona is approximately τe = 1.82 ± 0.14 and its mean temperature is approximately kTe = 65 ± 10 keV.

A Nonlinear Ultrasonic Modulation Method for Crack Detection in Turbine Blades

In modern gas turbines, efforts are being made to improve efficiency even further. This is achieved primarily by increasing the generated pressure ratio in the compressor and by increasing the turbine inlet temperature. This leads to enormous loads on the components in the hot gas region in the turbine. As a result, non-destructive testing and structural health monitoring (SHM) processes are becoming increasingly important to gas turbine manufacturers. Initial cracks in the turbine blades must be identified before catastrophic events occur. A proven method is the linear ultrasound method. By monitoring the amplitude and phase fluctuations of the input signal, structural integrity of the components can be detected. However, closed cracks or small cracks cannot be easily detected due to a low impedance mismatch with the surrounding materials. By contrast, nonlinear ultrasound methods have shown that damages can be identified at an early stage by monitoring new signal components such as sub- and higher harmonics of the fundamental frequency in the frequency spectrum. These are generated by distortion of the elastic waveform due to damage/nonlinearity of the material. In this paper, new global nonlinear parameters were derived that result from the dual excitation of two different ultrasound frequencies. These nonlinear features were used to assess the presence of cracks as well as their qualitative sizes. The proposed approach was tested on several samples and turbine blades with artificial and real defects. The results were compared to samples without failure. Numerical simulations were conducted to investigate nonlinear elastic interaction of the stress waves with the damage regions. The results show a clear trend of nonlinear parameters changing as a function of the crack size, demonstrating the capability of the proposed approach to detect in-service cracks.

Numerical study of hydrodynamic perturbations caused by filiform spark discharge near wall

A numerical simulation of the high-speed jet caused by the decay of the thermal cavern after cylindrical 10 mm long spark discharge located on the wall in ambient air is performed. It is shown that after the shock wave retreats, two counter-flowing jets are formed in a low-density hot gas, moving inside the thermal cavern from the electrodes to the center of the area. A strong distortion of the hot gas region is observed and a cold gas suction into the heat cavern is appeared. At the next phase, the formed hot gas jets collide, and the flow is directed radially from the axis of the initial discharge channel. Finally, the hot low-density gas takes the form of half the toroid, which size increases in time. The central part of the flowfield is occupied with the cold gas. The jet velocity and momentum are simulated at variation of the discharge energy (5-500mJ). The simulation results are considered to be consistent with the experimental data previously published by the authors.

A numerical study of NOx reduction by water spray injection in gas turbine combustion chambers

An Eulerian-Lagrangian model is implemented to study the process of water spray injection inside gas turbine combustors. The validation of different parts of the model and the examination of the uncertainties introduced by different assumptions in this model are presented and discussed carefully. Then, the model is used to investigate the effect of different injection parameters; including position, direction, and mass flow rate, on the performance and emission of a gas turbine combustor at different swirl numbers. The results show that the interaction between spray droplets and the flow structures inside the liner plays the key role in the effectiveness of the injection process. The spray should be injected such that droplets are not trapped in the internal recirculating zone (IRZ) since they are pushed towards the colder flow near the liner walls. On the other hand, the droplet entrapment in the swirling vortex would be favorable since it increases both the droplet residence time inside the chamber and the proximity of droplets to the hot central gas regions. It is found that the best injection location satisfying these criteria is at the end of the primary zone rather than around the inlet nozzles and is targeted at the post-ignition region.

The Impact of the Group Environment on the O vi Circumgalactic Medium

Abstract We present a study comparing O vi λλ1031, 1037 doublet absorption found toward group galaxy environments to that of isolated galaxies. The O vi absorption in the circumgalactic medium (CGM) of isolated galaxies has been studied previously by the “Multiphase Galaxy Halos” survey, where the kinematics and absorption properties of the CGM have been investigated. We extend these studies to group environments. We define a galaxy group as having two or more galaxies with a line-of-sight velocity difference of no more than 1000 km s−1 and located within 350 kpc (projected) of a background quasar sightline. We identified a total of six galaxy groups associated with O vi absorption W r > 0.06 Å that have a median redshift of and a median impact parameter of . An additional 12 non-absorbing groups were identified with a median redshift of and a median impact parameter of . We find the average equivalent width to be smaller for group galaxies than for isolated galaxies (3σ). However, the covering fractions are consistent with both samples. We used the pixel-velocity two-point correlation function method and find that the velocity spread of O vi in the CGM of group galaxies is significantly narrower than that of isolated galaxies (10σ). We suggest that the warm/hot CGM does not exist as a superposition of halos; instead, the virial temperature of the halo is hot enough for O vi to be further ionized. The remaining O vi likely exists at the interface between hot diffuse gas and cooler regions of the CGM.

On the effect of ionic wind on structure and temperature of laminar premixed flames influenced by electric fields

The general influence of electric fields on flames has been known for many years, the underlying mechanisms, however, are not finally identified yet. The changes observable in the flame structure and the pollutant emission when a flame is exposed to an electric field are mainly attributed to the ionic wind. Yet, especially the resulting flame temperature under the influence of electric fields is still an open research topic. Thus, in the present study the temperature distributions in a laminar premixed Bunsen type flame were measured for different air–fuel-mixtures using point-wise vibrational coherent-anti-Stokes Raman spectroscopy (vibrational-CARS) when a static electric field is activated. Additionally, CH2O- and OH-PLIF (planar laser-induced fluorescence) and particle image velocimetry (PIV) were applied to identify the best-suited locations for the temperature measurements and to visualize changes in the flame structure induced by the electric field for clarifying the underlying mechanisms.The results show that the electric field leads to increased fresh gas and maximum flame temperatures and that this effect is most distinct for rich premixed flames. Both PIV/PLIF and CARS experiments confirm that the flame is constricted and pushed towards the burner due to the ionic wind, which explains the increased temperature of the fresh gas and of the exhaust gas. The increased burner and fresh gas temperatures due to the ionic wind were verified by additional thermocouple measurements. The ionic wind is maximal for the fuel-rich flame, which is explained by the lowest electric resistance due to high charge carrier concentration, and larger flame area, which reduces the gap between the flame and the positively charged electrode. Furthermore, a clear broadening of the hot exhaust gas region was measured, which increases the burn out of UHC and CO reported in the literature. Furthermore, this effect increases the residence time that could also contribute to changed pollutant emission for flames under the influence of electric fields. It can be concluded that the change of the flame temperature is mainly the result of the modified velocity field induced by the ionic wind. Consequently, the ionic wind is the dominant mechanism for many effects observed in flames when applying weak electric fields.

An Experimental Study on a New Heating Technique with Use of an Inner Hot Gas Region of Tubular Flames

An Experimental Study on a New Heating Technique with Use of an Inner Hot Gas Region of Tubular Flames

Extended warm gas in Orion KL as probed by methyl cyanide

In order to study the temperature distribution of the extended gas within the Orion Kleinmann-Low nebula, we have mapped the emission by methyl cyanide (CH3CN) in its J=6_K-5_K, J=12_K-11_K, J=13_K-12_K, and J=14_K-13_K transitions at an average angular resolution of ~10 arcsec (22 arcsec for the 6_K-5_K lines), as part of a new 2D line survey of this region using the IRAM 30m telescope. These fully sampled maps show extended emission from warm gas to the northeast of IRc2 and the distinct kinematic signatures of the hot core and compact ridge source components. We have constructed population diagrams for the four sets of K-ladder emission lines at each position in the maps and have derived rotational excitation temperatures and total beam-averaged column densities from the fitted slopes. In addition, we have fitted LVG model spectra to the observations to determine best-fit physical parameters at each map position, yielding the distribution of kinetic temperatures across the region. The resulting temperature maps reveal a region of hot (T > 350 K) material surrounding the northeastern edge of the hot core, whereas the column density distribution is more uniform and peaks near the position of IRc2. We attribute this region of hot gas to shock heating caused by the impact of outflowing material from active star formation in the region, as indicated by the presence of broad CH3CN lines. This scenario is consistent with predictions from C-shock chemical models that suggest that gas-phase methyl cyanide survives in the post-shock gas and can be somewhat enhanced due to sputtering of grain mantles in the passing shock front.

HIGH-ENTROPY POLAR REGIONS AROUND THE FIRST PROTOSTARS

We report on simulations of the formation of the first stars in the Universe, where we identify regions of hot atomic gas (fH2 < 10-6) at densities above 10-14 g/cc, heated to temperatures ranging between 3000 and 8000 K. Within this temperature range atomic hydrogen is unable to cool effectively. We describe the kinetic and thermal characteristics of these regions and investigate their origin. We find that these regions, while small in total mass fraction of the cloud, may be dynamically important over the accretion timescale for the central clump in the cloud, particularly as a chemical, rather than radiative, mechanism for clearing the polar regions of the accretion disk of material and terminating accretion along these directions. These inherently three-dimensional effects stress the need for multi-dimensional calculations of protostellar accretion for reliable predictions of the masses of the very first stars.

Passive Control of the Inlet Acoustic Boundary of a Swirled Burner at High Amplitude Combustion Instabilities

Perforated panels placed upstream of the premixing tube of a turbulent swirled burner are investigated as a passive control solution for combustion instabilities. Perforated panels backed by a cavity are widely used as acoustic liners, mostly in the hot gas region of combustion chambers to reduce pure tone noises. This paper focuses on the use of this technology in the fresh reactants zone to control the inlet acoustic reflection coefficient of the burner and to stabilize the combustion. This method is shown to be particularly efficient because high acoustic fluxes issued from the combustion region are concentrated on a small surface area inside the premixer. Theoretical results are used to design two types of perforated plates featuring similar acoustic damping properties when submitted to low amplitude pressure fluctuations (linear regime). Their behaviors nonetheless largely differ when facing large pressure fluctuation levels (nonlinear regime) typical of those encountered during self-sustained combustion oscillations. Conjectures are given to explain these differences. These two plates are then used to clamp thermoacoustic oscillations. Significant damping is only observed for the plate featuring a robust response to increasing sound levels. While developed on a laboratory scale swirled combustor, this method is more general and may be adapted to more practical configurations.

ABELL 1201: THE ANATOMY OF A COLD FRONT CLUSTER FROM COMBINED OPTICAL AND X-RAY DATA

We present a combined X-ray and optical analysis of the cold front cluster Abell 1201 using archival Chandra, data and multi-object spectroscopy taken with the 3.9m Anglo Australian and 6.5m Multiple Mirror Telescopes. This paper represents the first in a series presenting a study of a sample of cold front clusters selected from the Chandra, archives with the aim of relating cold fronts to merger activity, understanding the dynamics of mergers and their effect on the cluster constituents. The Chandra X-ray imagery of Abell 1201 reveals two conspicuous surface brightness discontinuities, which are shown to be cold fronts, and a remnant core structure. Temperature maps reveal a complex multi-phase temperature structure with regions of hot gas interspersed with fingers of cold gas. Our optical analysis is based on a sample of 321 confirmed members, whose mean redshift is z=0.1673 +/- 0.0002 and velocity dispersion is 778 +/- 36 km/s. We search for dynamical substructure and find clear evidence for multiple localized velocity substructures coincident with over-densities in the galaxy surface density. Most notably, we find structure coincident with the remnant X-ray core. Despite the clear evidence for dynamical activity, we find the peculiar velocity distribution does not deviate significantly from Gaussian. We apply two-body dynamical analyses in order to assess which of the substructures are bound, and thus dynamically important in terms of the cluster merger history. We propose that the cold fronts in Abell 1201 are a consequence of its merger with a smaller subunit, which has induced gas motions that gave rise to `sloshing' cold fronts. Abell 1201 illustrates the value of combining multi-wavelength data and multiple substructure detection techniques when attempting to ascertain the dynamical state of a cluster.

Identification of droplet burning modes in lean, partially prevaporized swirl-stabilized spray flames

Experimental and numerical investigations of single droplet burning modes in a lean, partially prevaporized swirl-stabilized spray flame are reported. In the experiment single droplet flames have been visualized by CH-PLIF and simultaneous recording of the Mie signal. Two single droplet burning modes were identified: the envelope flame is a spherical diffusion flame burning at near-stoichiometric conditions. The wake flame is a potentially lean, partially premixed flame located downstream of the droplet. The droplet burning mode is of practical relevance, since it has significant impact on NO formation due to incomplete prevaporization. The droplet burning mode is determined by the ratio of chemical and convective time scales. The convective time scale is related to the droplet slip velocity. The impact of turbulent gas phase velocity fluctuations on droplet mechanics and droplet burning is discussed, based on a previous numerical investigation. In the present study the droplet slip velocity was measured with the 3D Phase Doppler (3D-PD) technique. For the measured slip velocities and ambient conditions in the hot gas region of the spray flame, simulations of single droplet burning were performed utilizing detailed models for chemical reaction, diffusive transport and vaporization. An agreement between the droplet burning modes predicted by the simulation and the droplet burning modes observed in the experiments was found.

Partially premixed flames in stagnating turbulence: The merging of planar triple flames

The aim of this work, which takes a RANS perspective, is to consider the prospect of establishing a planar turbulent triple flame whose mean consists of two parallel premixed flame brushes separated by a nonpremixed flame brush. Experiments involving a counterflow between fuel-rich and fuel-lean turbulent streams are considered. A correlation of published experimental data is used to estimate premixed turbulent flame brush locations and brush thicknesses. Previously validated model calculations then allow an estimate to be made of the thickness of a central nonpremixed flame or mixing layer, a thickness which is shown to be strongly influenced by flame–turbulence interactions in the premixed flames. This thickness turns out to be orders-of-magnitude greater than the width of the hot burned gas region between the two premixed flames strongly suggesting that the three reacting flow regions will merge with each other. It is concluded that unlike the corresponding laminar counterflow planar turbulent triple flames will be difficult to establish in laboratory scale experiments.