Discharge coefficients of ports with stepped inlets

Components of aeronautical gas turbines are increasingly being constructed from two layers, including a pressure containing skin, which is then protected by a thermal tile. Between them, pedestals and/or other heat transfer enhancing features are often employed. This results in air admission ports through the dual skin having a step feature at the inlet. Experimental data have been captured for stepped ports with a cross flow approach, which show a marked increase of 20% to 25% in discharge coefficient due to inlet step sizes typical of combustion chamber configurations. In this respect, the step behaves in a fashion comparable to ports with inlet chamfering or radiusing; the discharge coefficient is increased as a result of a reduction in the size of the vena contracta brought about by changes to the flow at inlet to the port. Radiused and chamfered ports have been the subject of previous studies, and empirical correlations exist to predict their discharge coefficient as used in many one-dimensional flow network tools. A method to predict the discharge coefficient change due to a step is suggested: converting the effect of the step into an equivalent radius to diameter ratio available in existing correlation approaches. An additional factor of eccentricity between the hole in the two skins is also considered. Eccentricity is shown to reduce discharge coefficient by up to 10% for some configurations, which is more pronounced at higher port mass flow ingestion fraction

Towards in-cylinder flow informed engine control strategies using linear stochastic estimation

Many modern I.C. engines rely on some form of active control of injection, timing and/or ignition to help combat tailpipe out emissions, increase the fuel economy and engine drivability. However, development of these strategies is often optimised to suit the average cycle at each condition; an assumption that can lead to sub-optimal performance, especially an increase in particulate (PN) emissions as I.C. engine operation, and in-particular it’s charge motion is subject to cycle-to-cycle variation (CCV). Literature shows that the locations of otherwise repeatable large-scale flow structures may vary by as much 25% of the bore dimension; this could have an impact on fuel break-up and distribution and therefore subsequent combustion performance and emissions. In the presented work, a method is presented that allows full-field flow velocity information to be estimated in real-time from only a limited number of point velocity measurements using linear stochastic estimation (LSE). Three sensor arrangements – single bisecting ‘line-of-sight’, a central cluster and a circumferential ring - which are deemed applicable to implementation in an I.C. engine are compared over all test flow conditions; with all providing useful estimations of the flow field. It is shown how with even a modest number of point measurements it is possible to achieve at least 85% correlation between estimates and original data allowing cycle characterisation to be achieved. Information gathered from this technique could provide inputs to engine control strategies to account for the CCV of the in-cylinder flow

Correcting for sub-grid filtering effects in particle image velocimetry data

Particle Image Velocimetry methodology results in a spatial averaging of the real velocity field into a set of discrete measured velocities: one for each interrogation cell. In the absence of measurement noise this filtering process results in a reduction of the measured turbulent kinetic energy and other second order statistics of the velocity field. The reduction in this energy will naturally be dependent upon the amount of turbulent energy at lengthscales smaller than can be resolved by the interrogation cells that make up the measurement grid. This paper investigates the effects of sub-grid scale filtering on the second order statistics of velocity. Several experiments are reported for which interrogation cell size to turbulent integral length scale ratios were varied. In addition, synthetic turbulent velocity fields with known spatial correlation functions are used to support experimental results and provide calibration for the estimation of the level of sub-grid filtering. It is suggested that to accurately capture all turbulent kinetic energy using PIV the interrogation cell should be at least of order 10 times smaller than the integral lengthscale of the flow. A method is then provided to estimate the level of sub-grid filtering should the interrogation cell be larger than this limit up to around the size of the integral lengthscale. With interrogation cells larger than this lengthscale then sub-grid filtering is such that second order statistics are reduced by over 50% and it should be considered unwise to rely on any second order statistics from such a scenario, corrected or otherwise

Analysis of in-cylinder engine flows and their constituents by proper orthogonal decomposition

Analysis of in-cylinder engine flows and their constituents by proper orthogonal decompositio

Cross-correlation of POD spatial modes for the separation of stochastic turbulence and coherent structures

This article describes a proper-orthogonal-decomposition (POD) based methodology proposed for the identification and separation of coherent and turbulent velocity fluctuations. Typically, POD filtering requires assumptions to be made on the cumulative energy content of coherent modes and can therefore exclude smaller, but important contributions from lower energy modes. This work introduces a suggested new metric to consider in the selection of POD modes to be included in a reconstruction of coherent and turbulent features. Cross-correlation of POD spatial modes derived from independent samples is used to identify modes descriptive of either coherent (high-correlation) or incoherent (low-correlation) features. The technique is demonstrated through application to a cylinder in cross-flow allowing appropriate analysis to be carried out on the coherent and turbulent velocity fields separately. This approach allows identification of coherent motions associated with cross-flow transport and vortex shedding, such as integral length scales. Turbulent flow characteristics may be analysed independently from the coherent motions, allowing for the extraction of properties such as turbulent length scale

Fuel gallery residence time & heat transfer experimental technique development for gas turbine fuel injectors

Two experimental techniques are presented in this paper looking at flows in complicated passages such as those of internal fuel galleries in modern fuel spray nozzles. The first relates to residence time measurement using a dye wash-out technique and the second to surface heat transfer measurement using thermochromic liquid crystals. Dye concentration can be sufficiently calibrated making this technique easily and successfully applicable to residence time measurement within the chosen geometry. This experiment highlighted areas within the geometry where fuel is likely to become trapped in a recirculation and so leading to high residence times. The liquid crystal method proved more challenging due to the susceptibility of the crystals to fail under water. One set of measurements was gathered and showed that areas of high fuel residence time also correspond to low heat transfer coefficient. Small volumes of slow or stagnant fuel will be susceptible to soaking more heat increasing the risk of coke formation

Usage of body-fitted windows in PIV image processing

Flow close to the boundaries of bodies is often difficult to measure using particle image velocimetry (PIV); factors such as glare, velocity gradients and the body itself all present challenges in obtaining good quality results. One common problem in conventional PIV algorithms is that they are based on square grids, which for most applications is not aligned with the shape of the body. This means the body will clip part of the window resulting in fewer particles and erroneously placed vectors. Being able to align the interrogation window shape to the body boundary would remove these sources of error. This study investigates the effect that applying non-square windows has, compared to conventional square windows for three test cases: a free-field, a circular body and an airfoil shape. For each of these a number of interrogation methods are tested. For the circular body, conventional and rotated square windows were tested along with a body fitted mesh. The image was also deformed into R-θ coordinates to enable conventional square window processing, before de-warping the vector field. Square, rotated square and body fitted techniques were also tested on an airfoil shape. It was found that the body-fitted and warping methods both showed significant improvements in the boundary layer over the square meshes; although the warping process added computational expense. The meshing technique was found to have little impact on the free field as there was no body or velocity gradient. Utilising this interrogation method in conjunction with developed methods of automated edge detection and more mature processing algorithms will result in better measurements close to bodies than is possible with conventional square windows. An example application of this technique on flow around a sphere is also demonstrated

Influence of asymmetric valve strategy on large-scale and turbulent in-cylinder flows

Phase-locked particle imaging velocimetry (PIV) measurements are carried out in a direct-injected spark-ignition (DISI) single cylinder optical research engine equipped with fully variable valve timing (FVVT) to assess the impact of asymmetric intake valve lift strategies on the in-cylinder flow. The engine was operated under a range of asymmetric strategies, with one valve following a full lift profile, while the second intake valve is scaled as a factor of the first, expressed as % maximum valve lift (MVL). Proper orthogonal decomposition (POD) combined with a proposed methodology allows instantaneous velocity fields to be decomposed into what are nominally demonstrated as coherent and turbulent constituent velocity fields. Analysis of the coherent fields reveals the behaviour of large scale structures within the flow, subject to cyclic variation. In the case of 40% MVL, an increase in the flow cyclic variability is observed. This is found to be as a result of a switch between a flow dominated by a counter-rotating vortex pair and a single vortex. The impact of MVL on the bulk motion is further evident by an increase in the magnitude of swirl ratio from 0.5 to -6.0 (at 75o CA). Analysis of the turbulent constituent shows how increased valve life asymmetry leads to increased turbulence during the intake stroke by over 250%. Finally, it is shown how the ensemble turbulence statistics may be misleading as stochastic fluctuations were found to be typically 66% of the total TKE calculated from the ensemble statistic in the tested conditions

LES of unsteady vortex aerodynamics in complex geometry gas-turbine fuel injectors

LES of unsteady vortex aerodynamics in complex geometry gas-turbine fuel injector

Experimental studies of the aerodynamics of spinning and stationary footballs

The accurate discrimination of the aerodynamic parameters affecting the flight of sports balls is essential in the product development process. Aerodynamic studies reported to date have been limited, primarily because of the inherent difficulty of making accurate measurements on a moving or spinning ball. Manufacturers therefore generally rely on field trials to determine ball performance, but the approach is time-consuming and subject to considerable variability. The current paper describes the development of a method for mounting stationary and spinning footballs in a wind tunnel to enable accurate force data to be obtained. The technique is applied to a number of footballs with differing constructions and the results reported. Significant differences in performance are noted for both stationary and spinning balls and the importance of the ball orientation to the flow is highlighted. To put the aerodynamic data into context the results are applied in a flight model to predict the potential differences in the behaviour of each ball in the air. The aerodynamic differences are shown to have a considerable effect on the flight path and the effect of orientation is shown to be particularly significant when a ball is rotating slowly. Though the techniques reported here are applied to a football they are equally applicable to other ball types