Bulk Jet Velocity Research Articles

Investigating the Unsteady Dynamics of a Multi-Jet Impingement Cooling Flow Using Large Eddy Simulation

Abstract We investigate the unsteady behavior of an in-line jet impingement array of nine jets at a Reynolds number of 10,000 in a narrow channel subjected to a developing cross flow of up to 25% of the bulk jet velocity. To this end, we present an improved version of a previously published large eddy simulation (LES) now with resolved turbulence at the inflow boundaries. After a careful analysis of the transient behavior and statistical convergence of the LES, we discuss the time-averaged heat transfer characteristics of the configuration compared to numerical references of similar configurations. We then show how the large-scale unsteadiness increases from jet to jet. Both space-only and spectral proper orthogonal decompositions (POD and SPOD) are used to discuss the large-scale organization of single jets and multiple jets in combination. The latter shows a qualitative change in the unsteady behavior of the temperature footprint on the impingement wall with increasing cross flow.

Spray Flame Characterization of a Dual Injector for Compact Combustion Systems

ABSTRACT The scalability of liquid fuel operated jet stabilized combustion system toward scales and mixing timescales relevant for corresponding micro gas turbine systems is impeded by the lack of suitable injection concepts. The high momentum of the combustion air jet can deteriorate the spray quality of conventional injection systems. In combustors with reduced mixing timescales (e.g. MGT and compact aero-engines) this results in elevated emissions and a prompt primary atomization is essential. For this purpose, a canonical confined jet spray burner is developed and equipped with a novel in-house dual-pressure swirl/airblast injection concept. An extensive parameter variation on the jet bulk velocity (80 , equivalence ratio (0.6 , combustion air preheat temperature (500 (K) 800) and premixing length (0.0 (mm) ≤ 48) is conducted to delineate the operational range. The spray process originating a partially premixed and a direct injection case is characterized by means of phase Doppler interferometry and shadowgraphy. The combustion process is described via OH*-chemiluminescence, flame photographs and exhaust gas measurements. The results show that the developed injection system yields Sauter mean diameters predominantly below 25 μm over the entire parameter range due to robust and prompt primary atomization. Moreover, the primary atomization process of the dual injector is sufficient under all conditions to confine secondary atomization close-to the combustor nozzle exit. The flame stabilization and reaction progress are more sensitive to the air preheat temperature than the bulk jet velocity variation as the mixture homogenization is limited by the vaporization process. Moreover, with decreasing reactivity (i.e. equivalence ratio and preheat temperature) the flame stabilization is increasingly dominated by the recirculated hot exhaust gas, indicating a transition from conventional turbulent flame propagation to auto-ignition influenced reaction progress. The resulting NOx and CO emissions approximate the values measured previously in related methane powered combustion systems. Consequently, the developed injection system is promising to expand the flexibility of compact gas turbines to liquid fuels while maintaining low emissions.

Effects of pressure on the structure of partially premixed turbulent diffusion flames: Calculations using MMC-LES

This study numerically investigates the detailed structure of twelve partially premixed turbulent diffusion flames of methane and air across a pressure range between 1 and 20 bar. The flames are computed using the multiple mapping conditioning / large eddy simulations (MMC-LES) approach. The model is first validated against available experimental data at atmospheric pressure. This is followed by simulations of a series of flames with increasing pressure while holding either the bulk jet velocity or the jet Reynolds number constant. Results show that local extinction increases as pressure increases in cases with fixed bulk jet velocity. Meanwhile, an instability induced by body forces gradually arises as pressure increases at constant Reynolds number, but can be mitigated by reducing the gravitational acceleration, and hence the body forces, proportional to the pressure increase. Consequently, flames become more stable with less localised extinction when increasing pressure at constant Reynolds number. The study accurately reflects the effects of finite-rate chemistry with increasing pressure, as evidenced in representative plots of a Damköhler number ratio. Additionally, the approach to blow-off at different pressures is examined, and trends in temperature and species are analysed. The calculations presented in this study align well with existing data at atmospheric pressure and provide reliable blind test results that await detailed comparison with measurements not yet available at high pressures.

Effects of pressure on soot production in piloted turbulent non-premixed jet flames

Laser-induced incandescence (LII) was used to quantify the soot volume fraction in five piloted turbulent non-premixed C2H4-N2 jet flames at elevated pressures. In one series of flames, the bulk jet velocity was held constant as the pressure was increased from 1 bar (Re = 10,000) to 3 bar (Re = 30,000), and then 5 bar (Re = 50,000). In the other series, a Reynolds number of 10,000 was held constant at 1 bar, 3 bar, and 5 bar. LII measurements were calibrated with laminar diffusion flames at each pressure using 2D diffuse line-of-sight attenuation. Mean and RMS soot volume fractions along the flame centerline exhibit Gaussian profiles for all of the flames. The magnitude of soot volume fraction and the spatial soot distribution are enhanced by increases in pressure. In both series of flames, the peak mean soot volume fraction scales with the pressure as p2.2. In the constant velocity series, the scaling drops to p1.5 if the volume-integrated mean soot volume fraction on a per fuel mass basis is considered instead. At 3 bar, a threefold increase in Re leads to a decrease in the mean soot volume fraction despite the decrease in soot intermittency. At 5 bar, a fivefold increase in Re results in soot intermittency near zero at the flame’s center and a slight increase in the mean soot volume fraction.

Assessment of the stabilization mechanisms of turbulent lifted jet flames at elevated pressure using combined 2-D diagnostics

The stabilization mechanisms of turbulent lifted jet flames in a co-flow have been investigated at a pressure of 7 bar. The structure of the flame base was measured with combined OH and CH2O planar laser induced fluorescence (PLIF) and the spatial distribution of equivalence ratio was imaged, simultaneously, with CH4 Raman scattering. The velocity field was also measured with particle imaging velocimetry (PIV). Different bulk jet velocities Uj and co-flow velocities Uc were examined. Data show that flames with Uc = 0.6 m/s stabilize much further away from the nozzle than those with Uc = 0.3 m/s and that their structure does not resemble that of the edge-flames found closer to the nozzle. In addition, for Uc = 0.6 m/s, the measured lift-off height decreases with increasing bulk jet velocity, which is opposite to what is typically observed for lifted flames. Statistical examination of CH4 Raman images shows that the flames with Uc = 0.6 m/s propagate through regions of the flow where the equivalence ratio is not always stoichiometric but, instead, spans the whole flammability range. This is not consistent with edge-flames and is, instead, indicative of premixed burning. This is corroborated by PIV results which show that the flame base velocity exceeds that typically reported for edge-flames. Measurements of relevant flow properties were also conducted in non-reacting jets to predict the turbulent burning velocity of these lifted flames burning in a premixed mode. For Uc = 0.6 m/s and relatively large bulk jet velocities (Uj = 10 and 15 m/s), the predicted turbulent burning velocities are sufficiently high to counter the incoming flow of reactants and, in turn, allow flame stabilization. However, for a lower bulk jet velocity of Uj = 5 m/s, the predicted turbulent burning velocity is much less, leading to blow-out. This explains why the lift-off height decreases with increasing jet velocity for methane at 7 bar and Uc = 0.6 m/s. Data also shows that increasing pressure promotes transition from edge-flames to premixed flames due to reduced laminar burning velocity and enhanced mixing.

Experimental investigation of the near field in sooting turbulent nonpremixed flames at elevated pressures

A recently commissioned high-pressure combustion duct is used to investigate a family of ten, piloted, sooting, turbulent nonpremixed flames over a range of pressures and Reynolds numbers. For all conditions, the central jet is composed of 35% ethylene and 65% nitrogen by volume. In one series of flames, the Reynolds number is kept constant while the pressure is increased, whereas in the other series, the bulk jet velocity is maintained. The maximum pressure, p, is 5 bar and the maximum Reynolds number, Re, is 50,000. A DSLR camera and 10-kHz OH-PLIF are used to characterized the near field. DSLR camera images show that the length of the blue region immediately downstream of the nozzle decreases as the pressure increases, rapidly in the constant Reynolds number series, and gradually in the constant velocity series. Analysis of OH-PLIF images in the near field show that corrugation of the flame front is fairly insensitive to changes in pressure if the Reynolds number is held constant. As the Reynolds number (and pressure) is increased, the flame front becomes more corrugated and the frequency of OH layer extinction increases. This leads to smaller “islands” of OH that are separated by shorter distances. However, despite increased local extinction, the flames remain attached even as the Reynolds number is increased to more than twice the maximum Reynolds number that is possible at atmospheric pressure.

Modeling the Emission from Turbulent Relativistic Jets in Active Galactic Nuclei

We present a numerical model developed to calculate observed fluxes of relativistic jets in active galactic nuclei. The observed flux of each turbulent eddy is dependent upon its variable Doppler boosting factor, computed as a function of the relativistic sum of the individual eddy and bulk jet velocities and our viewing angle to the jet. The total observed flux is found by integrating the radiation from the eddies over the turbulent spectrum. We consider jets that contain turbulent eddies that have either standard Kolmogorov or recently derived relativistic turbulence spectra. We also account for the time delays in receiving the emission of the eddies due to their different simulated positions in the jet, as well as due to the varying beaming directions as they turn over. We examine these theoretical light curves and compute power spectral densities (PSDs) for a range of viewing angles, bulk velocities of the jet, and turbulent velocities. These PSD slopes depend significantly on the turbulent velocity and are essentially independent of viewing angle and bulk velocity. The flux variations produced in the simulations for realistic values of the parameters tested are consistent with the types of variations observed in radio-loud AGN as, for example, recently measured with the Kepler satellite, as long as the turbulent velocities are not too high.

Stabilization of piloted turbulent flames with inhomogeneous inlets

This paper investigates the stabilization mechanism of turbulent jet flames with highly inhomogeneous inlet conditions. A modification of the standard piloted burner is employed here with the addition of a central tube (inner) carrying fuel that can slide within the existing (outer) tube carrying air. Both tubes are located within the pilot annulus and inhomogeneity is varied by translating the inner tube upstream of the jet exit plane. Two flames with identical overall air/fuel ratios, bulk jet velocities, and pilot conditions but different levels of homogeneity in the fuel/air mixture are selected for detailed investigations. Measurements are performed using Sandia’s Raman–Rayleigh–LIF line facility, and Large Eddy Simulation (LES) using the stochastic fields approach are also conducted for the same flames. Results reported here focus on the early stabilization region.The flame with inhomogeneous inlet conditions is more stable being at 57% of blow-off compared to the homogeneous counterpart, which is at 78% of blow-off. It is found that, very close to the jet exit plane, premixed combustion dominates the flame with an inhomogeneous profile. This is in contrast to the homogeneous case, which behaves like a diffusion flame. Further downstream, but still within the pilot region, partial mixing starts to occur between richer samples and hot combustion products. A comparison of the relative conditional scalar dissipation rates, χr shows that in the upstream region, and within the reactive limits, the homogeneous case has higher values of χr. Premixed combustion with higher rates of heat release and lower scalar dissipation rates in the near field are therefore key reasons for the improved stability of the flames with inhomogeneous inlets. These findings are corroborated by results from LES.

Experimental investigation into the impact of crossflow on the coherent unsteadiness within film cooling flows

Film cooling is regularly used to cool the surface of components within the turbine stage of an aero engine. This enables them to withstand the high air temperatures that are required for maximising aero engine cycle efficiency. It is known that the mixing of a film cooling flow with the main high temperature air flow through a turbine passage is an unsteady process, with coherent unsteady features occurring across a range of blowing ratios. Upon an aero engine the cooling holes on a turbine blade commonly have a crossflow at the hole inlet. Previous work has shown that crossflow at the hole inlet modifies the time-mean flowfield downstream of a cooling hole compared to the case without crossflow.The current paper investigates the impact of spanwise orientated crossflow on the coherent unsteadiness within film cooling flows. Both cylindrical and shaped cooling holes, located on a blade pressure surface, are studied. The range of blowing ratios considered is 0.7–1.8 and the crossflow velocity is up to 0.8 times the bulk jet velocity. High Speed Photography and Hot Wire Anemometry are used to observe the presence of coherent unsteadiness, both immediately downstream of the hole exit and within the cooling hole tube.The results show that the coherent unsteadiness downstream of the hole exit is persistent and its occurrence is not significantly affected by the magnitude of spanwise crossflow. Within the cooling hole tube the existence of coherent unsteadiness is presented for the first time, inside both cylindrical and shaped holes, with a Strouhal number of 0.6–1.2. The pattern of this in-hole coherent unsteadiness is seen to change with increasing the crossflow velocity.

Direct numerical simulation of round turbulent jets in crossflow

Direct numerical simulation is used to study a round turbulent jet in a laminar crossflow. The ratio of bulk jet velocity to free-stream crossflow velocity is 5.7 and the Reynolds number based on the bulk jet velocity and the jet exit diameter is 5000. The mean velocity and turbulent intensities from the simulations are compared to data from the experiments by Su & Mungal (2004) and good agreement is observed. Additional quantities, not available from experiments, are presented. Turbulent kinetic energy budgets are computed for this flow. Examination of the budgets shows that the near field is far from a state of turbulent equilibrium – especially along the jet edges. Also – in the near field – peak kinetic energy production is observed close to the leading edge, while peak dissipation is observed toward the trailing edge of the jet. The results are used to comment upon the difficulty involved in predicting this flow using RANS computations. There exist regions in this flow where the pressure transport term, neglected by some models and poorly modelled by others, is significant. And past the jet exit, the flow is not close to established canonical flows on which most models appear to be based.

Swirling turbulent non-premixed flames of methane: Flow field and compositional structure

This paper introduces a new swirl burner which has simple, well-defined boundary conditions and which stabilizes complex, turbulent, unconfined flames that are not unlike those found in practical combustors. Two flames with identical swirl numbers but differing bulk jet velocities Uj are selected for further inves- tigations. Flow field measurements reveal that a second recirculation zone may exist on the centerline of the flames further downstream of the primary recirculation zone. This is attributed to vortex breakdown. The measurements also show the presence of highly rotating collar-like flow features present between the primary and secondary recirculation zones. These regions of the flow are characterized by high tangential shear stresses uw� . The compositional structures of these methane flames are measured using the simultaneous Raman- Rayleigh laser-induced fluorescence (LIF) technique. The LIF technique is used to measure concentra- tions of OH, CO, and NO. Results are presented as scatter plots and radial Favre mean profiles of tem- perature, mixing, and composition fields. As the fuel jet velocity increases and the flame approaches blowoff, a higher proportion of fluid samples shifts away from fully burned conditions and closer to a mixing asymptote. An interesting feature of these flames is that these locally extinguished samples originate mostly from regions of the flow near the high-shear, collar-like region, which is not found in similar bluff- body flames.

Bidirectional Relativistic Jets of the Radio Galaxy 1946+708: Constraints on the Hubble Constant

We present measurements of bidirectional motions in the jets of the radio galaxy 1946+708 at z = 0.101. This is a compact symmetric object with striking S-symmetry. Sensitive 15 GHz observations reveal a compact component at the center of symmetry with a strongly inverted spectrum, which we identify as the core. From five 4.9 GHz observations spread over 4 yr we have determined the velocities of four compact jet components. If simple kinematic models can be applied, then the inclination of the source and the bulk jet velocity can be directly determined for any assumed value of the Hubble constant. Conversely, the measurements already place constraints on the Hubble constant, and we show how further observations of 1946+708 can yield an increasingly accurate determination of H0.