Simultaneous visualization of flow field and evaluation of local heat transfer by transitional impinging jets

A combined approach has been employed to characterize the flow field and local heat transfer in jet impingement configurations, featuring a mass transfer experiment and a digital visualization technique. A jet velocity range is spanned to ensure flow regime transition.The well-known heat/mass transfer analogy has been used to infer on the local heat exchange on a infinite plate. In this experiment, a naphthalene film is ablated from a disk, due to jet exposure. Automated contact measurements of the variation of film depth in the stagnation region and beyond have been performed. From the local naphthalene loss rate the local heat transfer is then inferred. Coherent structures are created both at the interface between free jet and quiescent medium and upon impingement at plate, and need to be visualized in the vicinity of stagnation. To this end a particle image velocimetry system is exploited to extract the two components velocity instantaneous information.Ablation measurements confirm the non-monotonic progress of local heat transfer for small nozzle-to-plate spacings. The visualizations evidence that local heat transfer is strongly influenced by impingement structures: the maximum heat transfer coefficient offset which can be detected is due, even for laminar or transitional jet, to large-scale toroidal vortices impacting on the plate.     

过冷沸腾汽泡顶部射流可视化实验研究

通过商速摄像井配合激光PIV技术，对过冷沸腾中微小汽泡顶部射流进行细致现察，确认过冷沸腾中顶部射流的存在，测量显示其速度场，具体分析讨论此类射流与汽泡生长的联系。     

燃烧流场测量的PIV应用初步研究

使用丹麦Dantec公司生产的二维PIV(Particle Image Velocimetry)系统对甲烷/空气的扩散燃烧热态速度场进行了测量，得到了燃烧状态下的速度场信息，解决了利用PIV进行热态测量的问题。     

利用切向进气降低压气机喘振流量的研究

采用粒子图像速度场仪 (PIV)对叶轮进口处的旋流速度场进行测定 ,研究了切向进气装置的不同结构和工作参数对压气机叶轮进口处旋流流场的影响规律 ,并对采用切向进气方式降低压气机喘振流量进行了试验研究。试验结果表明 :采用切向进气能够有效改变压气机叶轮进口处气流的预旋角 ,可以通过改变切向进气装置的结构和工作参数来改变叶轮进口处的旋流场。压气机逼喘试验结果证明 ,即使采用简单的切向进气结构也能够有效降低压气机的喘振流量     

Dynamic PIV Measurement of a Compressible Flow Issuing from an Airbag Inflator Nozzle

Among many equipment for passenger safety,the air bag system is the most fundamental and effective device foran automobile.The inflator housing is a main part of the curtain-type air bag system,which supplies high-pres-sure gases in pumping up the air bag-curtain which is increasingly being adapted in deluxe cars for protectingpassengers from the danger of side clash.However,flow information on the inflator housing is very limited.Inthis study,we measure the instantaneous velocity fields of a high-speed compressible flow issuing from the exitnozzle of an inflator housing using a dynamic PIV system.From the velocity field data measured at a highframe-rate,we evaluate the variation of the mass flow rate with time.The dynamic PIV system consists of ahigh-repetition Nd:YLF laser,a high-speed CMOS camera,and a delay generator.The flow images are taken at4000 fps with synchronization of the trigger signal for inflator ignition.From the instantaneous velocity field dataof flow ejecting from the airbag inflator housing at the initial stage,we can see a flow pattern of broken shockwave front and its downward propagation.The flow ejecting from the inflator housing is found to have very highvelocity fluctuations,with the maximum velocity at about 700 m/s.The time duration of the high-speed flow isvery short,and there is no perceptible flow after 100 ms.     

用PIV方法研究四气门汽油机缸内滚流运动

选取一台Cagiva350单缸四气门汽油机作为试验样机,重新设计了具有透明活塞顶和透明缸套的加长部分,并使之与丰田8A汽油机的基本参数接近,以二氧化钛为示踪粒子,采用激光粒子图像速度法(PIV)研究了该样机在进气和压缩冲程缸内滚流流场的变化情况.研究结果表明,滚流在进气过程中开始形成,逐渐演变成与缸径同大的大尺度旋涡,并在实验所能实测的曲轴转角范围内一直保持,在60°CABTDC时旋涡发生严重畸变,但仍未破碎为小尺度的湍流.     

PIV measurement and turbulence scale in turbulent combustion

We have investigated turbulent combustion by PIV (Particle Image Velocimetry) technique. Comparing with LDV data, the validity of PIV measurements has been confirmed. Particularly, the conditions of sampling number and spatial resolution have been shown to yield reliable data using PIV. Based on the velocity fields in cold flow and combustion, the interaction between flame and flow has been discussed. It was observed that the flow field is changed by combustion and the turbulence is reduced. In order to determine statistical quantities such as mean velocity and RMS of velocity fluctuation, a sampling number of 1000 is needed. Moreover, the velocity correlation coefficient was evaluated to obtain the integral length scale of the flow. For both cold flow and combustion, the PIV estimated scale is very close to that of LDV based on the assumption of Taylor's hypothesis. As a result, the spatial resolution in this study is about 6 times smaller than the integral length scale. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(7): 501–512, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20129     

PIVS方法测雾滴粒径的影响因素及其实验验证

在分析讨论PIVS(particle imageve locimetry and sizing)方法测量喷雾场粒子粒径的影响因素的基础上，通过建立基于标准粒子的粒子场实验模拟装置，对PIVS系统粒径测量结果进行了验证。结果表明：现有系统只适用于对大粒子(如平均粒径大干115μm)粒径的精确测量，要实现对小粒子(如粒径小于50μm)粒径的测量，仍需改善其成像系统的性能。     

Development and experimental validation of a simulation model for open joint ventilated façades

The investigation of the thermal and fluid dynamical behaviour of open joint ventilated façades is a challenging task due to the complex airflows generated inside of the naturally ventilated cavity by the existence of open joints. For this reason, the use of advanced fluid measurement and simulation techniques is highly recommended. This paper focuses in the development and experimental validation of a simulation model for these façade systems. More specifically, different turbulence and radiation models available in the commercial computational fluid dynamic codes have been tested on a three-dimensional model and the results have been compared to particle image velocimetry measurements. The correlation between experimental and numerical data has been used in order to select the simulation procedure for this type of façades. Best fittings have been found when using the RNG k-epsilon turbulence model and the Discrete Ordinate radiation model. Using the selected scheme, parametrical simulations have been performed to investigate the effect of increasing the cavity height, and correspondingly, the number of slabs. Results show that ventilation air flow inside the cavity is enhanced by incident radiation as well as by the height of the façade.     

A comparison of three turbulence models for the prediction of parallel lobed jets in perforated panel optimization

The general context of the present study is the design of high induction HVAC air diffusers by means of passive jet control. When the diffuser is a perforated panel with lobed orifices (Meslem et al. 2010), the optimization of jet induction consists in improving the orifice’s geometry, the spacing between orifices and their arrangement on the panel. In this study, the flow field of a turbulent twin cross-shaped jet is investigated numerically using the standard k-ε model, the Shear Stress Transport (SST) k-ω model and the Reynolds Stress Model (RSM). The results are compared with PIV measurements. The objective is to assess their capability and limitations to predict the significant features of twin jet flow when the flow is numerically resolved through a lobed diffuser. It is shown that the k-ε and RSM models are more appropriate for predicting potential jet core length, the change in jet centreline streamwise velocity, and flow expansion in the symmetry plane of the twin jet flow. However, these models overestimate the overall flow expansion and the jet volumetric flow rate. The SST k-ω model seems more appropriate for the prediction of such dynamic integral quantities. A high level of turbulent kinetic energy predicted by the k-ε and RSM models in the near field of jets is probably the reason for this overestimation of jet induction. The SST k-ω model would appear to be the most appropriate tool for optimizing orifice design, orifice to orifice spacing and relative orifice orientation on a perforated panel diffuser.