Numerical Simulation and Experimental Investigation of Conjugate Heat Transfer between a Turbulent Hot Air Jet Impinging on a Cookie-shaped Object

ABSTRACT: Numerical simulation of the flow field and conjugate heat transfer for a turbulent jet impinging on the surface of a model cookie were carried out at different nozzle-to-plate spacings. Numerical predictions were compared with the experimental results on average and the local heat-transfer coefficient. Results indicated good agreement between the numerical and experimental results over the range of jet velocities, that is, 10 to 40 m/s, which corresponds to Reynold's number range of 7500 to 32000 based on the jet orifice diameter. Numerical simulation also confirmed that the surface heat-transfer coefficient was independent of the thermo-physical properties of the cookie. The local heat-transfer coefficient on the top surface of the cookie was highest at the center of the impinging jet.     

Impingement Drying of Paper

ABSTRACT

The complete drying history of paper dried under an array of multiple round jets of air was determined. The drying rate – paper moisture content relations, determined as a continuous function by on–line measurements for about 80 sheets, permitted examination of the following parameters: basis weight 20 to 50 g/m2, which covers the tissue to communication paper range; initial moisture content 1.5 to 3 kg water/kg fibre; nozzle to paper spacing of 5, 7.6 & 8.5 nozzle diameters: nozzle plate open area ratio from 1.4 to 3.1%; jet Reynolds number in the range of 450 to 11.100.

Three methods of quantifying the complete drying rate curves were tested. Use of the common assumption of drying rate linear with moisture content over the falling rate period gave inadequate representation. Secondly, the general method of Churchill for any transport process that has a transition between two asymptotic relations was applied for the fmt time to the drymg of paper. This approach gives     

Determination of Local Heat-Transfer Coefficients Around a Circular Cylinder Under an Impinging Air Jet

Heat transfer coefficients around a model food shaped as a circular cylinder placed on a flat surface and impinged by a slot air jet has been determined using an inverse heat transfer method. The determination was based on time-temperature data measured with a thermocouple in the cylinder and in the air jet. The cylinder was rotated around its horizontal axis to determine the heat-transfer coefficients at different locations around the cylinder. A sensitivity analysis using Monte Carlo simulations was also performed. The local heat-transfer coefficients determined, were compared to computational fluid dynamics (CFD) simulations using the k-ω SST and RSM models. The heat-transfer coefficients determined from temperature measurements was larger than predicted by the CFD simulations. The heat-transfer rates were in better agreement on the upper part of the cylinder, including the decrease along the cylinder due to flow separation, than on the lower part close to the wake recirculation area. The SST model predicted in general a slightly higher heat-transfer rate on the upper part of the cylinder and slightly lower on the lower part of the cylinder, as compared to the RSM model.     

采用激光诱导荧光法测量油膜厚度的研究

激光诱导荧光(Laser Induced Fluorescence,LIF)技术是一种先进的测量微米级油膜厚度的方法,可用于研究燃油附壁现象.通过标定试验,找到了把荧光信号转化为油膜厚度的方法,同时还分析了背景光以及激光脉冲照射对标定试验的影响.另外,为了保证喷油器、激光器以及示波器的同步工作,设计了一个控制器.在此基础上对石英平板上的油膜厚度进行了测量,得到了油膜厚度在空间与时间上的分布曲线,验证了采用激光诱导荧光法测量油膜厚度的可行性和有效性.     

CrN/NbN coatings deposited by HIPIMS: A preliminary study of erosion-corrosion performance

Nanoscale CrN/NbN multilayer PVD coatings have exhibited resistance to erosion-corrosion. However growth defects (under dense structures and droplets) in the coating produced by some deposition technologies reduce the ability to offer combined erosion-corrosion resistance. In this work a novel High Power Impulse Magnetron Sputtering (HIPIMS) technique has been utilised to pretreat substrates and deposit dense nanoscale CrN/NbN PVD coatings (HIPIMS-HIPIMS technique). This new technique, rich with metal ion plasma, deposits very dense structures and offers virtually defect free coatings (free of droplets as observed in cathodic arc technique and under-dense structures observed in standard dc sputtering). Plasma diagnostic studies revealed a high metal ion-to-gas ion ratio (Cr:Ar) of 3:1 for HIPIMS pretreatment conditions with the detection of 14% Cr²⁺ and 1% Cr³⁺ ions and J_s of 155 mAcm^− 2. For deposition conditions the metal ion-to-gas ratio was approximately 1:4 which is significantly higher compared to DC at 1:30. Characterisation results revealed a high adhesion of L_C 80 N, high hardness of 34 GPa and Young's modulus of 381 GPa. Low friction coefficient (0.46) and dry sliding wear coefficient, K_C (1.22 × 10^− 15 m³Nm^− 1) were recorded. The effect of deposition technique (droplet defect and intergranular void free coatings) on erosion-corrosion resistance of CrN/NbN coatings has been evaluated by subjecting the coatings to a slurry impingement (Na₂CO₃ + NaHCO₃ buffer solution with Al₂O₃ particles of size 500-700 µm) at 90° impact angle with a velocity of 4 ms^− 1. Experiments have been carried at − 1000 mV, + 300 mV and + 700 mV representing 3 different corrosion conditions.     

自由表面二维平面射流冲击传热的理论分析

对二维平面射流冲击平板时的层流流动（和传热）给出了速度和温度边界层的综合解析,用积分方法得到了平面射流冲击传热的驻点区和部分壁面射流区的换热关系式,并将等热流壁面的分析结果与实验数据进行比较,结果基本吻合,     

Self‐sustained oscillations in opposed impinging jets in an enclosure

The flow of jets in confining enclosures has significant application in many engineering processes. In particular, the impingement of axisymmetric jets in a confined space has been examined using flow visualization, laser Doppler anemometry, and numerical simulations. Several flow regions were found; stable steady, regular oscillatory, and irregular oscillatory. Initially, a steady flow field existed for all arrangements for Re_d < ?90 (based on the nozzle diameter d, the fluid kinematic viscosity v and the volumetric flow rate Q through the nozzle (Q = πd²/4U_avg)) but subsequent increments in the fluid velocity caused a regularly oscillating flow field to emerge. The onset of the oscillations and the upper limit of finite oscillations were found to be a function of the Re_d, and the nozzle diameter to chamber dimension ratio. Steady numerical simulations predicted the steady flow field well and good agreement was obtained in unsteady simulations of the oscillating flow field. The oscillating flow field is considered to be a class of self‐sustaining oscillations where instabilities in the jet shear layer are amplified because of feed back from pressure disturbances in the impingement region.     

阵列射流冲击冷却流场与温度场的数值模拟

采用数值模拟方法对冲击冷却的流动和传热过程进行了三维数值研究。特别研究了在冲击孔叉排方式下,相邻孔间距、冲击距离以及射流入口雷诺数对冲击表面冷却流动传热特性的影响规律。     

凝固组织数值模拟中晶粒接触因素的修正系数

通过数值计算与实验相结合的方法,对凝固组织数值模拟中晶粒接触因素修正系数的已有形式进行了分析、比较。在此基础上,提出了当固相体积分数(fs）大于π/6后采用[1-(fs-π/6）]^4作为反映晶粒接触因素的修正系数是一种更合理的形式,经实验验证,在该形式下的模拟结果更接近于实际。     

凝固过程微观数值模拟中的碰撞因子

通过数值计算与实验相结合的方法,对凝固过程微观数值模拟中碰撞因子的已有形式进行了分析、比较。在此基础上,提出了在固相体积分数（ｆｓ）大于π／６后采用「１－（ｆｓ－π／６）」＾４作为反映晶粒接触因素的碰撞因子是一种更合理的形式。经实验验证,在该形式下的模拟结果更接近于实际。