Hot Streak Research Articles

Infrared signature of serpentine nozzle with engine swirl

Serpentine nozzle is used to improve the stealth performance of military aircraft due to its ability to suppress the infrared signature. Investigation of the effect of engine swirl on the infrared signature of the serpentine nozzle is the primary objective of this study. The results showed that the engine swirl could effectively suppress the infrared signature, while inducing a small performance loss. Moreover, the engine swirl mainly contributed to the infrared signature suppression from the gas. The hot streak induced by the engine swirl affected the wall infrared signature level; however, exhibited limited effect on the total infrared signature level of the serpentine nozzle. The swirl angle significantly affected the infrared signature level of the serpentine nozzle. Compared to the no-swirl case, the average reduction of the total infrared signature level increased from 2.29% to 13.84% in the horizontal plane, and from 5.83% to 31.21% in the vertical plane with the increase in the swirl angle from 5° to 20°.

Hot Streak Characterization of High-Performance Double-Serpentine Exhaust Nozzles at Design Conditions

The characterization and mitigation of hot streak phenomena in double-serpentine exhaust nozzles have strategic, operational, and tactical implications for future airborne weapon systems. However, airframe integration increases the risk for material failure due to exposure of downstream components to high-temperature gases from the high-performance convergent-divergent nozzle. The formation of hot streaks must be understood if mitigation strategies are to be developed. To that end, this paper establishes a set of novel characterization parameters based on the temperature distribution and flow conditions at the nozzle exit. Hot streak phenomena observed for a set of serpentine exhaust nozzles with varying geometries are compared using a modified Dean number (called the White number) that accounts for arbitrary cross-sectional area and streamline curvature changes. Using these parameters and computational fluid dynamics, quantifiable differences are observed in the distribution and intensity of hot flow at the nozzle exit. The results show that hot steak phenomena are most prominent for nozzles with a lower White number. The results also provide insight into flow physics acting as primary drivers of hot streak phenomena. The effects of bulk engine swirl are considered and exhibit a significant impact on surface temperature distribution and the formation of hot streaks.

Transport of Entropy Waves Within a High Pressure Turbine Stage

This paper presents the results of an experimental study on the transport of entropy waves within a research turbine stage, representative of the key aero-thermal phenomenon featuring the combustor-turbine interaction in aero-engines. The entropy waves are injected upstream of the turbine by a dedicated entropy wave generator (EWG) and are released in axial direction; they feature circular shape with peak amplitude in the center and exhibit sinusoidal-like temporal evolution over the whole wave area. The maximum over-temperature amounts to 7% of the undisturbed flow, while the frequency is 30 Hz. The entropy waves are released in four azimuthal positions upstream of the stage, so to simulate four different burner-to-stator blade clocking. Time-resolved temperature measurements were performed with fast microthermocouples (FTC); the flow and the pressure field upstream and downstream of the stator and the rotor was measured with five-hole pneumatic probes and fast-response aerodynamic pressure probes. The entropy waves are observed to undergo a relevant attenuation throughout their transport within the stator blade row, but they remain clearly visible at the stator exit and retain their dynamic characteristics. In particular, the total temperature distribution appears severely altered by burner-stator clocking position. At the stage exit, the entropy waves loose their coherence, appearing spread in the azimuthal direction to almost cover the entire pitch in the outer part of the channel, while being more localized below midspan. Despite the severe and unsteady interaction of the entropy waves within the rotor, they retain their original dynamic character. A comparison with measurements performed by injecting steady hot streaks is finally reported, remarking both differences and affinities. As a relevant conclusion, it is experimentally shown that entropy waves can be proficiently simulated by considering a succession of hot streaks of different amplitude.

Extreme Shooters in the NBA

It is widely perceived that star shooters in basketball have hot and cold streaks. In this study, we explore a novel approach to the hot hand question by considering the possibility that the human perception of hot handedness may be more closely related to the percentage of shots made by a player in a game (relative to the expectations set by his season-long hit percentage) than to streaks of shots made throughout the game. We work with simple formulations of “hot” and “cold” that rely entirely on field goal percentages and number of shots taken, using data from the official NBA API on all field goals attempted in the 2015-2016, 2016-2017, and 2017-2018 NBA seasons by top shooters. We determine that certain players — including Klay Thompson, who is often considered the prime example of hot handed shooting — display signs of hot and cold handedness in our extreme shooting formulation of the hot hand question.

The effect of mismatching between combustor and HP vanes on the aerodynamics and heat load in a 1-1/2 stages turbine

In modern aero-engine and gas turbine, the hot streak and residual swirl caused by the combustor have strong impact on both aero and thermal performances of the high-temperature turbine. Due to different design philosophies of combustors and turbine, one combustor usually matches two or more vanes and the resultant mismatching effect is numerically investigated with unsteady simulation where one nonuniform core faces two Nozzle Guide Vane (NGV) blades. Results indicate that the nonuniformity clocking position dominates the averaged-temperature, while the swirl orientation dominates the radial distribution. The mismatching effect strongly impacts both time-averaged and instantaneous performances, in that neighboring blades of NGV have averaged-temperature difference of 10.5 K, and it also halves the base frequency and amplifies unsteady variation in the rotor passage. By analyzing both aero and thermal results from a matrix of four cases, the LE-pos configuration has the best performance. The complex transportation of the nonuniformity in the LE-pos case is detailed and the unsteady interactions between the inlet nonuniformity and the complex flow topology are analyzed. The residual swirl carries the hot streak fluctuating radially in rotor passages and redistributes the temperature on rotor surface. Results from this work highlight the necessity of considering the mismatching effect in gas turbine design optimization.

Author Impact: Evaluations, Predictions, and Challenges

Author impact evaluation and prediction play a key role in determining rewards, funding, and promotion. In this paper, we first introduce the background of the author impact evaluation and prediction. Then, we review the recent developments of the author impact evaluation, including data collection, data pre-processing, data analysis, feature selection, algorithm design, and algorithm evaluation. Third, we provide an in-depth literature review on the author impact predictive models and the common evaluation metrics. Finally, we look into the representative research issues, including author impact inflation, unified evaluation standards, academic success gene, identification of the origins of hot streaks, and higher-order academic networks analysis. This paper should help the researchers obtain a broader understanding of the author impact evaluation and prediction and provides future research directions.

Flow Field and Hot Streak Migration Through a High Pressure Cooled Vanes With Representative Lean Burn Combustor Outflow

Modern lean burn aero-engine combustors make use of relevant swirl degrees for flame stabilization. Moreover, important temperature distortions are generated, in tangential and radial directions, due to discrete fuel injection and liner cooling flows respectively. At the same time, more efficient devices are employed for liner cooling and a less intense mixing with the mainstream occurs. As a result, aggressive swirl fields, high turbulence intensities, and strong hot streaks are achieved at the turbine inlet. In order to understand combustor-turbine flow field interactions, it is mandatory to collect reliable experimental data at representative flow conditions. While the separated effects of temperature, swirl, and turbulence on the first turbine stage have been widely investigated, reduced experimental data is available when it comes to consider all these factors together.In this perspective, an annular three-sector combustor simulator with fully cooled high pressure vanes has been designed and installed at the THT Lab of University of Florence. The test rig is equipped with three axial swirlers, effusion cooled liners, and six film cooled high pressure vanes passages, for a vortex-to-vane count ratio of 1:2. The relative clocking position between swirlers and vanes has been chosen in order to have the leading edge of the central NGV aligned with the central swirler. In order to generate representative conditions, a heated mainstream passes though the axial swirlers of the combustor simulator, while the effusion cooled liners are fed by air at ambient temperature. The resulting flow field exiting from the combustor simulator and approaching the cooled vane can be considered representative of a modern Lean Burn aero engine combustor with swirl angles above ±50 deg, turbulence intensities up to about 28% and maximum-to-minimum temperature ratio of about 1.25. With the final aim of investigating the hot streaks evolution through the cooled high pressure vane, the mean aerothermal field (temperature, pressure, and velocity fields) has been evaluated by means of a five-hole probe equipped with a thermocouple and traversed upstream and downstream of the NGV cascade.

HIFIRE-5b Heat Flux and Boundary-Layer Transition

Hypersonic International Flight Research Experimentation (HIFiRE)-5 is a hypersonic flight-test experiment designed to investigate the aerothermodynamics of a three-dimensional geometry. The vehicle is a 7 deg minor-axis half-angle elliptic cone with a aspect ratio and 2.5 mm nose radius. The HIFiRE-5b achieved a Mach number of 7.7–7.9 and a freestream unit Reynolds number of during descent. Heat flux was calculated from thermocouple pairs, and boundary-layer transition locations were identified from these heating rates. A multilobed transition front was observed, with earliest transition at the centerline, on the leading edges, and part way in between. It is suspected that a different mechanism is responsible for each of these lobes. Transition along HIFiRE-5b’s leading edges occurred at roughly constant length-based Reynolds numbers, suggesting a different mechanism than the roughness-induced transition suspected for HIFIRE-5a. Transition near the planes of symmetry was found to be less sensitive to attitude variation than elsewhere on the vehicle surface. In particular, hot streaks suspected to be due to stationary crossflow vortices were very sensitive to small changes in attitude (roughly angle of attack and yaw). Data from 128 pairs of thermocouples show the overall temperature, heat flux, and boundary-layer state during flight.

Numerical Study of the Axial Gap and Hot Streak Effects on Thermal and Flow Characteristics in Two-Stage High Pressure Gas Turbine

Combined cycle power plants (CCPPs) are becoming more important as the global demand for electrical power increases. The power and efficiency of CCPPs are directly affected by the performance and thermal efficiency of the gas turbines. This study is the first unsteady numerical study that comprehensively considers axial gap (AG) in the first-stage stator and first-stage rotor (R1) and hot streaks in the combustor outlet throughout an entire two-stage turbine, as these factors affect the aerodynamic performance of the turbine. To resolve the three-dimensional unsteady-state compressible flow, an unsteady Reynolds-averaged Navier–Stokes (RANS) equation was used to calculate a k − ω SST γ turbulence model. The AG distance d was set as 80% (case 1) and 120% (case 3) for the design value case 2 (13 mm or d/Cs1 = 0.307) in a GE-E3 gas turbine model. Changes in the AG affect the overall flow field characteristics and efficiency. If AG decreases, the time-averaged maximum temperature and pressure of R1 exhibit differences of approximately 3 K and 400 Pa, respectively. In addition, the low-temperature zone around the hub and tip regions of R1 and second-stage rotor (R2) on the suction side becomes smaller owing to a secondary flow and the area-averaged surface temperature increases. The area-averaged heat flux of the blade surface increases by a maximum of 10.6% at the second-stage stator and 2.8% at R2 as the AG decreases. The total-to-total efficiencies of the overall turbine increase by 0.306% and 0.295% when the AG decreases.

Computational Study of Combustor–Turbine Interactions

The Open National Combustion Code is applied to the simulation of a realistic combustor configuration (Energy Efficient Engine) in order to investigate the unsteady flowfields inside the combustor and around the first-stage stator of a high-pressure turbine. We consider (24 deg) of the full annular Energy Efficient Engine combustor with three different geometries of the combustor exit: one without the vane and two others with the vane set at different relative positions in relation to the fuel nozzle (clocking). Although it is common to take the exit flow profiles obtained by separately simulating the combustor and then feed as the inflow profile when modeling the high-pressure turbine, our studies show that the unsteady flowfields are influenced by the presence of the vane as well as clocking. More importantly, the characteristics (e.g., distribution and strength) of the high-temperature spots (i.e., hot streaks) appearing on the vane are significantly altered. This indicates the importance of simultaneously modeling both the combustor and the high-pressure turbine to understand the mechanics of the unsteady formulation of hot streaks.

Heat transfer analyses of film-cooled HP turbine vane considering effects of swirl and hot streak

Conjugate heat transfer (CHT) analyses were conducted on the film-cooled first stage vane of GE-E3 engine to reveal the influences of inlet swirl and hot streak (HS) on vane film cooling. Two cases with only HS and four cases considering with combined HS and swirl were studied, including the effects of HS/swirl to vane clocking positions (HS/swirl aligned to passage or vane) and swirl orientations (positive/negative). The results indicate that the variations of incidence angle combined with effects of radial transportation of the swirl vortex worsen the film coolant attachment, decrease the film cooling efficiency and increase the heat load onto vane surface. However, for film holes with radial angles toward the outer endwall, negative incidence near hub endwall is beneficial to the leading edge film coolant attachment. Therefore, a negative swirl aligned to the vane leads to lowering the temperature at the leading edge and the pressure side than the other three cases with combined swirl and HS. Heat transfer coefficient (HTC) on the suction surface (SS) is increased due to upwash and downwash of boundary layer fluids caused by swirl. The temperature on SS is therefore increased because the heat energy transferred from fluid to solid is increased.

Hot streaks in artistic, cultural, and scientific careers.

The hot streak-loosely defined as 'winning begets more winnings'-highlights a specific period during which an individual's performance is substantially better than his or her typical performance. Although hot streaks have been widely debated in sports1,2, gambling3-5 and financial markets6,7 over the past several decades, little is known about whether they apply to individual careers. Here, building on rich literature on the lifecycle of creativity8-22, we collected large-scale career histories of individual artists, film directors and scientists, tracing the artworks, films and scientific publications they produced. We find that, across all three domains, hit works within a career show a high degree of temporal regularity, with each career being characterized by bursts of high-impact works occurring in sequence. We demonstrate that these observations can be explained by a simple hot-streak model, allowing us to probe quantitatively the hot streak phenomenon governing individual careers. We find this phenomemon to be remarkably universal across diverse domains: hot streaks are ubiquitous yet usually unique across different careers. The hot streak emerges randomly within an individual's sequence of works, is temporally localized, and is not associated with any detectable change in productivity. We show that, because works produced during hot streaks garner substantially more impact, the uncovered hot streaks fundamentally drive the collective impact of an individual, and ignoring this leads us to systematically overestimate or underestimate the future impact of a career. These results not only deepen our quantitative understanding of patterns that govern individual ingenuity and success, but also may have implications for identifying and nurturing individuals whose work will have lasting impact.

The Interactions of Turbine Hot Streak and Leakage Flow from the Inter-Platform Gap

ABSTRACTThe paper presents the effects of the leakage flow from the inter-platform gap on the migration of hot streak in a first stage turbine by three-dimensional unsteady Reynolds-Averaged Navier-Stokes simulations. Five circumferential positions of the hot streak are considered. The comparisons between the results with/without slot leakage show significant differences. The leakage changes the hot streak in the vane passage significantly and it protects the vane suction surface trailing from the hot gas. The leakage also changes the secondary flow and results in forming a new couple of vortices in the vane passage. In general, rotor passage, the hot gas usually gathers in the rotor hub and the pressure surface. In the present study, the leakage coolant from upstream slot is entrained to the unsteady rotor secondary flows and transported toward the rotor hub and pressure surface effectively. The cooling effect is related to the relative circumferential positions between the hot streaks and slot. When the hot streak is positioned at the slot suction side, the time-averaged temperature reduction on the blade leading edge can be more than twice of that for the hot streak at the slot pressure side.

Analysis of Oscillation Effects on HotStreak Migration Characteristics in Turbine Vane Passage

ABSTRACTThe high pressure turbine is subjected to the inlet hot gas resulting from the combustor, which is generally non-uniform. The strong non-uniform temperature at the turbine inlet is known as “hotstreak.” Although many studies have been performed to investigate the hotstreak migration in the turbine vane passage, the HS is generally modeled to be steady and there are relatively few studies on the unsteady oscillating hot streak (OHS). The present study performs numerical investigations on the oscillation effects on the HS migration characteristics. The OHS is modeled to be oscillating in the circumferential direction, with varying oscillation magnitude and frequency. The steady hot streak (SHS) is modeled to be positioned at the guide vanes. Comparisons between the SHS and the OHS are conducted in detail regarding the flow and heat transfer characteristics. Both of the two cases of HS are based on the NASA experiment report and the No. 4313 case in the experimental design is selected. It is found that the change of HS from steady to oscillating leads to significant differences of transient periodic fluctuating characteristics of heat load and pressure on the guide vane. The HS areas are increased significantly and the mean temperature is decreased correspondingly. The introduction of oscillation effects is found to merit a better prediction of the real HS migration features in the real turbine passage.

Modelling strategies for the prediction of hot streak generation in lean burn aeroengine combustors

The accurate prediction of the fluid dynamic conditions at the exit of gas turbine combustors are of paramount importance in the aero-thermal design of the aero-engine. In fact, both the heat loads and the aerodynamic performance of the high pressure turbine (HPT) are substantially affected by the entry conditions, such as velocity components, temperature and turbulence intensity.The problem is particularly serious in new generation devices based on a lean burn concept. Compared to standard Rich–Quench–Lean (RQL) scheme, the absence of dilution jets and the use of highly swirled flows for flame stabilization make the control of combustor exit temperature distribution a complex task. Therefore, the high-fidelity prediction of the hot streak formation within the combustor, as well as its propagation through the HPT, are becoming key aspects.In this work, different strategies for turbulence modelling are tested, mainly focusing on scale-resolving approaches such as Large-Eddy Simulation (LES) and Scale Adaptive Simulations (SAS), which are becoming increasingly popular with the availability of more powerful computational resources. At the same time, classical eddy-viscosity models based on RANS approach are considered, as they still represent the standard simulation strategy in the industrial framework, when a fast response is required in a preliminary design phase.The different methodologies are benchmarked on an experimental test article representative of an aeronautical lean burn combustor cooled by means of effusion. The benchmark performed at engine-relevant conditions allowed to draw interesting conclusions for the purposes of the aero-thermal simulation. SAS proved to be a valid alternative to LES, returning on the whole the same level of accuracy. As expected, the disagreement obtained with RANS was significant. The sensitivity to the turbulent Prandtl number was investigated to provide an insight on its reliability in compensating the underestimation in turbulence mixing.

Analysis of transient radiation-convection coupled effects on the HP turbine blade heat load with hot streak inlet

Hot Streak (HS) and thermal radiation are the two important factors affecting the heat transfer process in High Pressure (HP) turbine stage, while the previous investigations were limited in studying either of them independently. In order to investigate the combined effects of the HS and radiation, the present study is aimed at unsteady convection-radiation coupled simulations with regards to HS and uniform inlet conditions. Three dominant radiative factors were analyzed in details. Compared with the uniform cases, it is found that there are two high temperature regions in the rotor passages of HS cases. The maximum temperature increase of the vane surface is about 40 K caused by radiation. As the HS changes the temperature distribution, the radiative heat flux is redistributed and a portion of radiative heat flux transfer from the stator passage to the rotor passage. The turbine inlet radiation mainly affects on the guide vane, whereas the gas radiation has a more significant influence on the rotor blade heat load.

The Effect of Hot Streaks on a High Pressure Turbine Vane Cascade with Showerhead Film Cooling

Hot streak migration in a linear vane cascade with showerhead film cooling was experimentally and numerically investigated at isentropic exit Mach number of Ma2is = 0.40, with an inlet turbulence intensity level of Tu1 = 9%. Two tangential positions of the hot streak center were taken into account: 0% of pitch (hot streak is aligned with the vane leading edge) and 45% of pitch. After demonstrating that computations correctly predict hot streak attenuation through the vane passage with no showerhead blowing, the numerical method was used to investigate hot streak interaction with showerhead film cooling, at blowing ratio of BR = 3.0, corresponding to a coolant-to-mainstream mass flow ratio of MFR = 1.15%. The effects of mixing and coolant interaction on the hot streak reduction were interpreted under the light of the superposition principle, whose accuracy was within 12% on the leading edge region, in the central section of the vane span.

Full annulus numerical study of hot streaks propagation in a hydrogen-rich syngas-fired heavy duty axial turbine

This paper presents a study of the effect of fuel composition on hot streaks propagation in a high-pressure turbine using a full annulus unsteady computational fluid dynamics analysis of the first two stages. Hot streaks result from the inherent non-uniformities of temperature profiles at the exit of the combustion chamber. Variations in composition arise from current challenges requiring gas turbines to adapt to fuel variations driven by the need to reduce CO2 emissions through the use of synthetic hydrogen-rich fuels (syngas) typically generated from the gasification of coal or solid waste. Syngas containing 80% hydrogen has been used in this study in a heavy duty gas turbine modified to accommodate the low calorific value fuel. Calculations were conducted on the baseline gas turbine originally designed for natural gas for the comparative study. Applying combustor representative hot streak profiles, analyses were performed for different hot streak distributions and locations. Analysis of results focused on the segregation of cold and hot fluid patterns and the effects of hot streaks on secondary flows and temperature re-distributions up to the second turbine stage. The hot flow pattern is affected by the fuel composition, resulting in more concentrated thermal wake shapes for syngas when compared to the reference natural gas fuel. In effect, the interaction with the secondary flow leads to more intense flow turning of the pressure side leg of the horseshoe vortex in the first rotor passage. The higher temperature levels in the case of syngas, in combination with the effect of the enhanced secondary flow, result in higher radial spread of the hot fluid that tends to migrate towards the blade hub and tip with the effects being obvious further downstream the first turbine stage.

Hot Streak Evolution in an Axial HP Turbine Stage

This paper presents the results of an experimental study on the evolution of hot streaks generated by gas turbine burners in an un-cooled high-pressure turbine stage. The prescribed hot streaks were directed streamwise and characterized by a 20% over-temperature with respect to the main flow at the stage inlet. The hot streak was injected in four different circumferential positions with respect to the stator blade. Detailed temperature and aerodynamic measurements upstream and downstream of the stage, as well as in-between the blade rows, were performed. Measurements showed a severe temperature attenuation of the hot streaks within the stator cascade; some influence on the aerodynamic field was found, especially on the vorticity field, while the temperature pattern resulted in severe alteration depending on the injection position. Downstream of the rotor, the jet spread over the pitch above the midspan and was more concentrated at the hub. Rotor secondary flows were also enhanced by hot streaks.

Computational Simulation of Deposition in a Cooled High-Pressure Turbine Stage With Hot Streaks

Ash particle deposition in a high-pressure turbine stage was numerically investigated using steady Reynolds-averaged Navier-Stokes (RANS) and unsteady Reynolds-averaged Navie-Stokes (URANS) methods. An inlet temperature profile consisting of Gaussian nonuniformities (hot streaks) was imposed on the vanes, with vane cooling simulated using a constant vane wall temperature. The steady case utilized a mixing plane at the vane–rotor interface, while a sliding mesh was used for the unsteady case. Corrected speed and mass flow were matched to an experiment involving the same geometry, so that the flow solution could be validated against measurements. Particles ranging from 1 to 65 μm were introduced into the vane domain, and tracked using an Eulerian–Lagrangian tracking model. A novel particle rebound and deposition model was employed to determine particles' stick/bounce behavior upon impact with a surface. Predicted impact and capture distributions for different diameters were compared between the steady and unsteady methods, highlighting effects from the circumferential averaging of the mixing plane. The mixing plane simulation was found to generally under predict impact and capture efficiencies compared with the unsteady calculation, as well as under predict particle temperature upon impact with the blade surface. Quantitative impact and capture efficiency trends with the Stokes number are discussed for both the vane and blade, with companion qualitative distributions for the different Stokes regimes.