Heat Transfer and Correlations of Jet Array Impingement with Flat and Pimple-Dimpled Plate

This study compares and analyses the heat transfer between a novel jet array impingement configuration (designated as NPR) and a baseline jet orifice plate (flat) in a maximum crossflow scheme. Both jet plates feature inline arrays of 20 x 26 circular air jets that orthogonally impinge on a flat target surface consisting of 20 segments, parallel to the jet plates. The NPR plate consists of staggered semi-spherical pimples (protrusions) and dimples (imprints) with a jet-to-pimple diameter ratio (Dj,p/Dp) of 0.07 and jet length-to-pimple diameter ratio (L/Dj,p) of ~ 1 with a protrusion ratio (tp/Dj,p) of 2.78. The dimples (imprints) have a jet-to-dimple diameter ratio (Dj,d/Dd) of 0.14 with an (L/Dj,d) of 0.5 and an imprint ratio (td/Dj,d) of 1.28. The averaged jet diameter for the NPR plate is calculated based on the definition of the total effective open area of the jets, which is equal to 3.49 mm. The flat plate is designed to be compared to the NPR plate and consists of jet orifice diameters (Dj) of 3.49 mm, with a length-to-diameter ratio (L/Dj) of ~ 1. In both plate configurations, the streamwise and spanwise directions jet-to-jet spacings (X/Dj), (Y/Dj), respectively, are maintained constant at 7.16. The physical mechanisms that cause the change in heat transfer, normalized by Nusselt number, when comparing both configurations are discussed in two regions: impingement and crossflow. Turbulent flow structures and experimental heat transfer are explored over three jet-averaged Reynolds numbers (Reav,j) of 5,000, 7,000, and 9,000, and are compared to available numerical results. Jet-to-target wall ratio (Z/Dj) is varied between (2.4, 2.87, 3.25, 4, and 6) jet diameters. Subsequently, multiple regression of the logarithms is used on the results obtained from the heat transfer experiments and are correlated into a dimensionless approach. Appropriate statistical methods are also reported along with the correlations for both flat and pimple-dimple plates. Enhancement of up to 23% in the heat transfer coefficient in the NPR plate is seen in the crossflow region, where the crossflow effects are maximized. However, this convex-concaved plate yields lower globally-averaged heat transfer coefficients

Twin impinging jets inline with a low-velocity crossflow

Vertical/short take-off and landing aircrafts at their hovering phase of flight create a three dimensional flowfield between lift jet streams, the airframe surface and the ground. The flowfield surrounding the aircraft during transition from hover to wing borne flight is of particular importance. During the transitional flight phase, the jets in crossflow phenomenon represent the most relevant configuration due to the complex flowfield that is created beneath the aircraft. The wall jets created by the impingement on the ground of the individual turbulent jet flow meet at a stagnation line and form an upwards flowing “fountain” that interacts with the airframe. Sometimes the fountain can provide a beneficial lift – generating ground cushion. Although, in most of the cases the fountain flow created generates a variety of undesirable characteristics, such as, hot gas ingestion, pressure, thermal and acoustic loads, change of the lift forces, lifting losses and the fuselage skin raise. The wall jet created by the jets impingement on the ground interacting with the free stream, results in a formation of a ground vortex far upstream of the impingement jet. This resulting ground vortex shape is strongly affected and the corresponding induced suckdown effect tends to be reduced by the upload produced by the fountain. During the past three decades, the flowfield characteristics associated with this type of aircraft have been studied extensively. However, the complexity of the new VSTOL configurations with the very stringent requirements demands more investigation. The continued development of a VSTOL aircraft with an increasing reliance on computational design techniques is dependent on a better understanding of aerodynamics of the lift jets of an aircraft in ground effect. This work is dedicated to the continuation of the experimental study began during the master’s thesis, i.e., a detailed analysis of the complex flowfield of two in-line turbulent circular air jets with a low velocity crossflow impinging on a flat surface perpendicular to the geometrical jet nozzle axis. The jets exit conditions are changed along the study to provide a better understanding of the flowfield. To complete this analysis and in order to validate the experimental results a detailed numerical study is also presented, where all the features of the experimental flow are maintained. The numerical results extend the experimental study, revealing that the deflection of the rear jet is due to the competing influences of the wake, the shear layer, the downstream wall jet of the first jet and the crossflow. The first jet deflection and the location of the ground vortex depend on the velocity ratio between the jet exit and the crossflow as well as the impingement height used. Through the rear jet velocity change, it is possible to verify the quick deflection of the second jet, never reaching the ground directly, i.e., the downstream jet is entrained by the upstream jet and not by the crossflow itself. Through the impingement height change, it is possible to observe the absence of upwash fountain formation in the region between the impingement jets, as it was expected. In this region, it is unexpectedly observed the formation of a second ground vortex, something not yet reported in the literature.Durante a fase em que uma aeronave de descolagem rápida/vertical e aterragem vertical paira no ar, um campo de escoamento tridimensional é criado entre o escoamento dos jatos de elevação, a superfície inferior da aeronave e o solo. O escoamento em torno da aeronave durante a fase de transição de voo pairado para voo convencional é de particular importância. Essa fase é dominada pelos fenómenos provocados pela interação dos jatos de elevação com o escoamento cruzado, devido ao aparecimento de um escoamento complexo na parte inferior da aeronave. Os jatos de parede, criados devido ao impacto de cada um dos jatos de elevação no solo, convergem para a linha de estagnação, formando um escoamento ascendente, como um “repuxo”, que interage com a aeronave. Por vezes, este escoamento ascendente fornece benefícios contribuindo para os efeitos de elevação da aeronave. No entanto, e na maior parte dos casos, o escoamento ascendente resultante produz características indesejáveis para este tipo de aeronave, entre elas a ingestão de gases quentes nas tubeiras de admissão, aumentos de pressão, temperatura e ruído, mudanças das forças de elevação, perdas de elevação e aumento de temperatura na fuselagem. A interação do jato de parede, resultante do impacto dos jatos de elevação no solo, com o escoamento livre leva à formação de vórtice de parede a montante do jato incidente. A forma do vórtice de solo resultante é fortemente afetada pelas condições do campo de escoamento e, devido ao escoamento ascendente, o efeito induzido de suckdown tende a ser reduzido. Passadas três décadas, as características do campo de escoamento associado a este tipo de aeronave tem sido exaustivamente estudada. Mas devido à grande complexidade das novas configurações das aeronaves VSTOL juntamente com requisitos muito rigorosos é de máxima importância a continuação da investigação deste tipo de escoamentos. Com o contínuo desenvolvimento das aeronaves VSTOL e a crescente dependência de técnicas de design computacional, é imperativo o melhoramento do conhecimento da aerodinâmica inerente à aeronave, mais propriamente aos jatos de elevação, quando esta opera com efeito de solo. Este trabalho é assim dedicado à continuação do trabalho experimental iniciado no decurso da tese de mestrado, ou seja, a análise detalhada do complexo campo de escoamento originado por dois jatos circulares de ar turbulentos em linha com um escoamento cruzado de baixa velocidade, incidentes numa superfície plana perpendicular ao eixo geométrico do bocal de saída do jato. As condições de saída do jato são mudadas no decurso do trabalho, de modo a entender o comportamento do campo de escoamento. De forma a completar a análise experimental e validar os seus resultados é também efetuado um estudo numérico detalhado, mantendo-se todas as condições que foram utilizadas no estudo experimental. Os resultados numéricos validam os resultados obtidos experimentalmente e revelam que a deflexão do segundo jato é devida às influências concorrentes da esteira, da camada de corte, do jato de parede a montante resultante do primeiro jato e do escoamento cruzado. A deflexão do primeiro jato e a localização do centro do vórtice de parede é dependente da razão de velocidades entre a saída do jato e da alteração da altura de impacto e o escoamento cruzado. Através da alteração da velocidade de saída do segundo jato é possível verificar a sua rápida deflexão, nunca tocando diretamente no solo, ou seja, o jato a jusante é arrastado pelo jato a montante (primeiro jato) e não pelo escoamento cruzado, como seria de esperar. Através da alteração da altura de impacto é possível observar a ausência de escoamento ascendente na região entre jatos de impacto, como era esperado. Nesta região inesperadamente é observada a formação de um segundo vórtice de solo, algo ainda não reportado na literatura

Volumetric PIV measurement for capturing the port flow characteristics within annular gas turbine combustors

© 2020, The Author(s). Abstract: The three-dimensional flows within a full featured, unmodified annular gas turbine combustor have been investigated using a scanned stereoscopic PIV measurement technique. Volumetric measurements have been achieved by rigidly translating a stereoscopic PIV system to scan measurements around the combustor, permitting reconstruction of volumetric single-point statistics. Delivering the measurements in this way allows the measurement of larger volumes than are accessible using techniques relying upon high depth of field imaging. The shallow depth of field achieved in the stereoscopic configuration furthermore permits measurements in close proximity to highly detailed geometry. The measurements performed have then been used to assess the performance of the combustor port flows, which are central to the emissions performance and temperature/velocity profile at turbine inlet. Substantially differing performance was observed in the primary ports with circumferential position, which was found to influence the behaviour of the second secondary port jets. The measurements indicated that the interaction between the primary and secondary jets occurred due to variations in the external boundary conditions imposed by the annular passages in which the combustor is located. Graphic abstract: [Figure not available: see fulltext.]

Heat Transfer And Pressure Drop Measurements In A High-Solidity Pin-Fin Array With Variable Hole Size Incremental Impingements

Gas turbines play a very critical role in the current energy sector in both power generation and in propulsion for almost the entire commercial and military aviation industry. Higher efficiencies can be developed from gas turbines, either land based or aero-propulsion by raising both the pressure and the temperature of combustion gases which discharge into the turbine section, which is also known as the Turbine Entry Temperature (TET). Turbine blade materials simply cannot operate safely at current TETâs of 3000 °F without implementing comprehensive cooling schemes developed in the industry over the years. Normally some of the compressed air is extracted from the compressor discharge and forced into internal cooling passages including serpentine passages in blades to cool the hottest engine components to a safer range in metal temperatures. Often, a portion of air is forced out from an array of tiny holes concentrated in the leading edge of blade aimed to provide internal cooling and a thin layer of protection from hot combustion gases while the rest of the coolant is delivered internally for convection to cool component surfaces to a sustainable temperature. However, the leading edge is quite susceptible to deposition of contaminants from the combustion products which can buildup and plug film cooling discharge holes. In addition, the surface of the leading region experiences intense turbulence, and the turbulence disrupts the film cooling layer from forming stably and protecting the blade surfaces

Review and status of heat-transfer technology for internal passages of air-cooled turbine blades

Selected literature on heat-transfer and pressure losses for airflow through passages for several cooling methods generally applicable to gas turbine blades is reviewed. Some useful correlating equations are highlighted. The status of turbine-blade internal air-cooling technology for both nonrotating and rotating blades is discussed and the areas where further research is needed are indicated. The cooling methods considered include convection cooling in passages, impingement cooling at the leading edge and at the midchord, and convection cooling in passages, augmented by pin fins and the use of roughened internal walls

A Full Coverage Film Cooling Study: The Effect of an Alternating Compound Angle

This thesis is an experimental and numerical full-coverage film cooling study. The objective of this work is the quantification of local heat transfer augmentation and adiabatic film cooling effectiveness for two full-coverage film cooling geometries. Experimental data was acquired with a scientific grade CCD camera, where images are taken over the heat transfer surface, which is painted with a temperature sensitive paint. The CFD component of this study served to evaluate how well the v2-f turbulence model predicted film cooling effectiveness throughout the array, as compared with experimental data. The two staggered arrays tested are different from one another through a compound angle shift after 12 rows of holes. The compound angle shifts from ?=-45° to ?=+45° at row 13. Each geometry had 22 rows of cylindrical film cooling holes with identical axial and lateral spacing (X/D=P/D=23). Levels of laterally averaged film cooling effectiveness for the superior geometry approach 0.20, where the compound angle shift causes a decrease in film cooling effectiveness. Levels of heat transfer augmentation maintain values of nearly h/h0=1.2. There is no effect of compound angle shift on heat transfer augmentation observed. The CFD results are used to investigate the detrimental effect of the compound angle shift, while the SST k-? turbulence model shows to provide the best agreement with experimental results

Fundamental Understanding of Interactions Among Flow, Turbulence, and Heat Transfer in Jet Impingement Cooling

The flow physics of impinging jet is very complex and is not fully understood yet. The flow field in an impingement problem comprised of three different distinct regions: a free jet with a potential core, a stagnation region where the velocity goes to zero as the jet impinges onto the wall and a creation of wall jet region where the boundary layer grows radially outward after impinging. Since impingement itself is a broad topic, effort is being made in the current study to narrow down on three particular geometric configurations (a narrow wall, an array impingement configuration and a curved surface impingement configuration) that shows up in a typical gas turbine impingement problem in relation to heat transfer. Impingement problems are difficult to simulate numerically using conventional RANS models. It is worth noting that the typical RANS model contains a number of calibrated constants and these have been formulated with respect to relatively simple shear flows. As a result typically these isotropic eddy viscosity models fail in predicting the correct heat transfer value and trend in impingement problem where the flow is highly anisotropic. The common RANS-based models over predict stagnation heat transfer coefficients by as much as 300% when compared to measured values. Even the best of the models, the v^2-f model, can be inaccurate by up to 30%. Even though there is myriad number of experimental and numerical work published on single jet impingement; the knowledge gathered from these works cannot be applied to real engineering impingement cooling application as the dynamics of flow changes completely. This study underlines the lack of experimental flow physics data in published literature on multiple jet impingement and the author emphasized how important it is to have experimental data to validate CFD tools and to determine the suitability of Large Eddy Simulation (LES) in industrial application. In the open literature there is not enough study where experimental heat transfer and flow physics data are combined to explain the behavior for gas turbine impingement cooling application. Often it is hard to understand the heat transfer behavior due to lack of time accurate flow physics data hence a lot of conjecture has been made to explain the phenomena. The problem is further exacerbated for array of impingement jets where the flow is much more complex than a single round jet. The experimental flow field obtained from Particle Image Velocimetry (PIV) and heat transfer data obtained from Temperature Sensitive Paint (TSP) from this work will be analyzed to understand the relationship between flow characteristics and heat transfer for the three types of novel geometry mentioned above. There has not been any effort made on implementing LES technique on array impingement problem in the published literature. Nowadays with growing computational power and resources CFD are widely used as a design tool. To support the data gathered from the experiment, LES is carried out in narrow wall impingement cooling configuration. The results will provide more accurate information on impingement flow physics phenomena where experimental techniques are limited and the typical RANS models yield erroneous result The objective of the current study is to provide a better understanding of impingement heat transfer in relation to flow physics associated with it. As heat transfer is basically a manifestation of the flow and most of the flow in real engineering applications is turbulent, it is very important to understand the dynamics of flow physics in an impingement problem. The work emphasis the importance of understanding mean velocities, turbulence, jet shear layer instability and its importance in heat transfer application. The present work shows detailed information of flow phenomena using Particle Image Velocimetry (PIV) in a single row narrow impingement channel. Results from the RANS and LES simulations are compared with Particle Image Velocimetry (PIV) data. The accuracy of LES in predicting the flow field and heat transfer of an impingement problem is also presented the in the current work as it is validated against experimental flow field measured through PIV. Results obtained from the PIV and LES shows excellent agreement for predicting both heat transfer and flow physics data. Some of the key findings from the study highlight the shortcomings of the typical RANS models used for the impingement heat transfer problem. It was found that the stagnation point heat transfer was over predicted by as much as 48% from RANS simulations when compared to the experimental data. A lot of conjecture has been made in the past for RANS\u27 ability to predict the stagnation point heat transfer correctly. The length of the potential core for the first jet was found to be ~ 2D in RANS simulations as oppose to 1D in PIV and LES, confirm the possible underlying reason for this discrepancy. The jet shear layer thickness was underpredicted by ~ 40% in RANS simulations proving the model is not diffusive enough for a flow like jet impingement. Turbulence production due to shear stress was over predicted by ~130% and turbulence production due to normal stresses were underpredicted by ~40 % in RANS simulation very close to the target wall showing RANS models fail where both strain rate and shear stress plays a pivotal role in the dynamics of the flow. In the closing, turbulence is still one of the most difficult problems to solve accurately, as has been the case for about a century. A quote below from the famous mathematician, Horace Lamb (1849-1934) express the level of difficulty and frustration associated with understanding turbulence in fluid mechanics. I am an old man now, and when I die and go to heaven there are two matters on which I hope for enlightenment. One is quantum electrodynamics, and the other is the turbulent motion of fluids. And about the former I am rather optimistic. Source: http://scienceworld.wolfram.com/biography/Lamb.html This dissertation is expected to shed some light onto one specific example of turbulent flows

Experimental and numerical heat transfer investigation of reverse jet impingement

This project aimed to study heat transfer and flow in double wall aerofoil cooling using two primary studies: a novel jet impingement cooling geometry and a typical film cooling arrangement. Experimental testing with thermochromic liquid crystal validated numerical work using ANSYS Fluent. A novel ‘reverse’ jet impingement geometry was developed to enhance heat transfer performance, comprising of a ‘dimple’ target enclosed within a cylindrical ‘silo’. Experimental variations included Reynolds number range of 10,000 to 70,000, jet-to-target, crossflow condition, and an extended nozzle geometry. An overall enhancement of heat transfer was achieved with the novel geometry, with optimum jet to target spacing found at around 4 jet diameters, and some reduction in crossflow effects were observed. A numerical investigation validated against experimental data for a novel 'reverse' jet impingement geometry was conducted. Optimizations in jet-to-jet and jet-to-target spacing were found, but no significant optimization of inlet condition was observed. The effect of outlet condition on discharge coefficient was significant, with an optimum nozzle length of 1 jet diameter for heat transfer enhancement. Staggered and inline dimples were shown to provide similar enhancements to heat transfer, significantly compared to a traditional flat plate target. The study evaluated heat transfer and discharge coefficients in a scaled cylindrical film cooling channel with varied Reynolds number, entry sharpness, inclination, and rotation angle

Simulations of Impinging Jet with a Range of Configuration

Impinging jet technique is widely increasing across the globe due to its ability to produce high heat and mass transfer as compared to other traditional methods. Three cases of impinging jets related to cooling technologies for a gas turbine were investigated. The first case involves a single jet impinging on a flat plate. The second case has an array of jets impinging on a curved surface. The third case deals with simulating impinging jet in crossflow. The first case was used for validation and shows the effect of mesh and inlet boundary conditions. After careful observation, it was seen that at least 15 or more prism layers should be used for these types of simulation. Also, SST turbulence model gives the best output for all the cases discussed below. For the fenot [11] case, jet-to-jet spacing (P/d) of 4 was observed to give best heat transfer. Turbulence modeling and jet versus crossflow ratio was compared, and passive scalar mixing was performed and was observed that as the crossflow increases, the heat transfer gets worse

TURBULENT MIXING BETWEEN LIQUIDS OF DISPARATE VISCOSITY ANDTHEIR EFFECT ON REACTION

The process of mass and momentum diffusion in a high Schmidt number disparate viscosity jet was observed through high fidelity particle Image velocimetry (PIV) and planar laser induced fluorescence(PLIF). The experimental campaign was carried out in a pressure driven facility specifically built to produce continuous flows with the viscosity disparities observed in chemical industry. Tests for the jet in coflow configuration were performed at constant inlet momentum and geometry with varying viscosity ratio. Test cases at viscosity ratios of 1, 10, 20, and 40 were achieved by increasing the viscosity of the outer jet. Further, these hydrodynamic experiments are coupled with reactive experiments with the same hydrodynamic inlet conditions that observe the product distribution of a mixing limited test reaction. These experiments were performed with a novel inline spectroscopy approach that enabled product distribution measurements throughout the reactor and not just at the reactor outlet. Collectively, these data sets provide the means to establish effective computational models for predictive modeling of variable viscosity flows with reaction. This will help in understanding a wide range of physical phenomena, specifically those necessary to optimize production within the materials industry. The viscosity disparity was found to retard the diffusion of both the momentum and passive scalar. This decrease in diffusion was accompanied by skewed mixing behavior leading the turbulent kinetic energy and scalar mixing to occur preferably in the low viscosity the inner jet. These findings inspired a study of conditional statistics based on the distance from the interface between the low and high viscosity fluids. This analysis demonstrated that the local shear rate at this interface decreases with increasing viscosity ratio, decreasing the role that Kelvin Helmholtz vortices can have in the mixing process. The concentration variance and Reynolds stresses showed a growing deficit in the adjustment layer that increases with viscosity ratio. This causes more of the mixing to occur in the inner stream which has implications for reactive mixing applications. Finally, this thesis demonstrated the efficacy of the dynamic spectroscopic method to observe and an LES model to simulate the complex test chemistry. For the first time, the technique was demonstrated effectively on the viscosity matched case, with product distributions matching within 3% of computational studies. These techniques were extended to the variable viscosity case with viscosity ratio ranging from 1 to 170 with similar experimental and computational agreement. This demonstrates both the effectiveness of the computational method for modeling complex reactions, but also the efficacy of the inline spectroscopic method for measuring the yield.Ph.D