Wind Tunnel Modelling of Aerodynamic Baffle Arrays for Aircraft Exhaust Plume Control
Wind Tunnel Modelling of Aerodynamic Baffle Arrays for Aircraft Exhaust Plume Control
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发表时间：
2014
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通讯作者：
S. Velikov
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作者：
S. Velikov
当地空气质量是制约机场发展的因素之一。自2010年以来，欧盟国家对氮氧化物排放量实施了更严格的新规定，在大型机场的某些地面监测地点，年平均浓度限值已经超过。本研究项目调查了在飞机起飞时通过在靠近飞机起始位置的跑道末端安全区放置一系列空气动力学防风墙（“挡板”）来实际减少飞机排气羽流的可能性。本文研究了在起飞时发动机排气喷流的路径上设置一系列挡板是否会增强羽流的分散并使其与地面分离。所进行的实验研究包括用一架BAe 146-301飞机在克兰菲尔德机场进行的小尺寸风洞试验和全尺寸外场试验。最初的风洞实验研究了放置在浮力喷嘴射流路径上的实心挡板排对下游羽流发展的影响。使用流动可视化，在没有风洞流动的情况下，成功地证明了挡板排对促进羽流远离地面的浮力上升的积极作用。研究强调了挡板相对于射流源的距离对其有效性的重要性。在存在风洞气流的情况下，挡板导致下游羽流的垂直扩散增加，但未观察到羽流与地面分离。在外场试验的准备过程中，通过风洞阻力测量研究了挡板阵列的空间布置，测量采用了摩擦天平。实验集中在关键参数，如挡板倾斜角和排间距，有利于增加高度的三排挡板的配置。根据最大挡板高度和排气射流速度，雷诺数大于18000时，观察到的结果与雷诺数无关。基于风洞测量，全尺寸挡板原型在克兰菲尔德大学设计和制造，并部署在现场试验。实地试验期间的激光雷达和点采样器测量结果表明，有一次，当飞机靠近挡板时，羽流已经远离地面上升。一个积极的影响，显示在挡板下游的浓度降低。当飞机离挡板较远时，这一结果就不再出现。随后的风洞实验集中在复制现场试验在1：200的比例在克兰菲尔德的大气边界层风洞。该飞机是代表在一个单一的固定喷嘴，而射流速度和浮力的模拟使用相似参数，如弗劳德数和环境和射流密度的比例。使用火焰电离检测器方法进行平均浓度测量，释放甲烷作为示踪气体。观察到挡板的效果主要是局部的，由于其遮蔽作用，在靠近地面的地方浓度降低。一个更为持久的影响是烟羽垂直扩散的增加，导致远离地面的平均浓度增加。总的来说，小尺寸和全尺寸实验的结果是不确定的挡板阵列的能力，以升空的羽流。看来，参数，这有一个显着的影响，对抬离现象，是周围的风速和发动机起动位置。已经提出了使用亚尺寸UAV燃气涡轮机的附加现场试验，以进一步研究所获得结果的差异。
Local air quality is one of the factors constraining the development of airports. In countries of the European Union where new, stricter regulations for emissions of nitrogen oxides (NOx) have been introduced since 2010, the limits of mean annual concentrations are already exceeded at certain ground monitoring locations of large airports. This research project investigates the possibility of practically abating the aircraft exhaust plume at take-off by placing an array of aerodynamic windbreaks (‘baffles’) in the runway end safety area close to the aircraft starting position. The thesis investigates whether an array of baffles in the path of the engine exhaust jet at take-off enhances the dispersion of the plume and causes it to separate from the ground. The undertaken experimental investigation comprised sub-scale wind tunnel tests and full-scale field trials with a BAe 146-301 aircraft, performing take-off and landing cycles at Cranfield Airport. The initial wind tunnel experiments investigated the effect of a solid baffle row, placed in the path of a buoyant nozzle jet, on the development of the plume downstream. Using flow visualisation, the positive effect of the baffle row of promoting buoyant rise of the plume away from the ground was demonstrated successfully without the presence of wind tunnel flow. The investigation highlighted the importance of the distance of the baffles relative to the jet source on their effectiveness. In the presence of wind tunnel flow, the baffles caused an increased vertical spread of the plume downstream, but the plume was not observed to separate from the ground. In preparation of the field trials, the spatial arrangement of the baffle array was investigated by means of wind tunnel drag measurements, performed with a skinfriction balance. The experiments focused on key parameters such as the baffle slope angle and row spacing, favouring a configuration of three rows of baffles of increasing height. The results were observed to be independent of Reynolds number for Reynolds numbers above 18000, based on the largest baffle height and the exhaust jet velocity. Based on the wind tunnel measurements, full-scale baffle prototypes were designed and manufactured at Cranfield University and were deployed in the field trials. Lidar and point sampler measurements during the field trials suggested that the plume had risen away from the ground on one occasion when the aircraft was located close to the baffles. A positive effect was shown in terms of reduced concentrations downstream of the baffles. This result was not replicated when the aircraft was further away from the baffles. The subsequent wind tunnel experiments focused on replicating the field trials at 1:200 scale in Cranfield’s Atmospheric Boundary Layer Wind Tunnel. The aircraft was represented at sub scale with a single stationary nozzle while the jet speed and buoyancy were modelled using similarity parameters such as the Froude number and the ratio of ambient and jet density. Mean concentration measurements were performed using a Flame Ionisation Detector method releasing methane as tracer gas. The effect of the baffles was observed to be mainly local in terms of reduced concentrations close the ground due to their sheltering effect. A more prolonged effect was found to be the increase of the plume’s vertical spread resulting in an increase in mean concentrations away from the ground. Overall, the results of the sub-scale and full-scale experiments were inconclusive with regard to the ability of the baffle array to lift-off the plume. It appears that the parameters, which have a significant effect on the lift-off phenomenon, are the ambient wind speed and the engine starting position. Additional field trials with a sub-scale UAV gas turbine have been proposed to investigate further the differences in the obtained results.