EFFECTS OF FREE STREAM VELOCITY ON TURBULENT NATURAL CONVECTION FLOW OVER A VERTICAL FORWARD-FACING STEP

Measurements of heat transfer and fluid flow of turbulent boundary-layer air flow in natural and mixed convection over an isothermal two-dimensional, vertical forward-facing step are reported. The upstream and downstream walls and the step itself were heated to a uniform and constant temperature. Air velocity and temperature distributions and their turbulent fluctuations are measured simultaneously using a two-component laser-Doppler velocimeter (LDV) and a cold wire anemometer, respectively. The present study treats buoyancy-dominated mixed convection over a vertical forward-facing step and examines the effect of a small free stream velocity on turbulent natural convection. The experiment was carried out for a step height of 22 mm, for a range of free stream air velocities 0 m/s ? u_∞ ? 0.55 m/s (corresponding to a range of Reynolds numbers of 0 ? Re\abinf{s} ? 712), and a temperature difference, ΔT, of 30°C between the heated walls and the free stream air (corresponding to a local Grashof number Gr_xi = 6.45 × 10¹⁰). It was found that the reattachment length increases while the heat transfer rate from the downstream heated wall decreases as the small free stream velocity increases.     

Experimental study on performance optimization of air source heat pump using DOE method

ASHP system is extensively applied to maintain indoor thermal environment but contributes to high building energy consumption. Better energy efficiency is possible through cooling performance improvements. This study investigates, using full-scale experiments, the cooling performance of ASHP. In the series of experiments, we vary the major influencing factors—evaporator inlet air temperature, air velocity, and compressor frequency and measured their impacts on response variables that include cooling capacity, compressor power, and the COP. The design of experiment (DOE) approach is used to plan and analyze the experiments. The results show that cooling capacity of ASHP system significantly increases with the rising evaporator inlet air temperature, air velocity, and compressor frequency. However, because of increasing fan and compressor power with rising air velocity and compressor frequency, COP dramatically decrease. Finally, the study of develop a simple predictive model for assessing the COP of ASHP. Comparing with the predicted and experimental results shows an agreement within 10% deviation, which indicates the suitability of the prediction model. Therefore, a predictive model can help system operators to set the optimal design parameters for achieving optimal COP performance of ASHP system.     

Simultaneous Measurement of Particle Size and Particle Velocity by the Spatial Filtering Technique

The objective of this study was to compare the measuring results of a fiber‐optical probe based on a modified spatial filtering technique with given size distributions of different test powders and also with particle velocity values of laser Doppler measurements. Fiber‐optical spatial filtering velocimetry was modified by fiber‐optical spot scanning in order to determine simultaneously the size and the velocity of particles. The fiber‐optical probe system can be used as an in‐line measuring device for sizing of particles in different technical applications. Spherical test particles were narrow‐sized glass beads in the range 30–100 μm and irregularly shaped test particles were limestone particles in the range 10–600 μm. Particles were dispersed by a brush disperser and the measurements were carried out at a fixed position in a free particle‐laden air stream. Owing to the measurement of chord lengths and to the influence of diffraction and divergent angle, the probe results show differences from the given test particle sizes. Owing to the particle‐probe collisions, the mean velocity determined by the probe is smaller than the laser Doppler mean velocity.     

Micro-machining of silicon wafer in air and under water

Laser ablation micro-machining tests are conducted on silicon wafer, both in air and under flowing water stream, with the use of 355 nm-X AVIA laser. Effects of laser pulse frequency, power level, scan velocity and focal plane position on the associated laser spatter deposition (in air), irradiated areas (under flowing water film) and taper are investigated. It shows that low frequency, i.e. 30–40 kHz, and high peak power result in smaller spatter and irradiated areas, and the hole taper decreases with increase in pulse frequency. Increase in the laser fluence broadens both the areas and increases the hole taper. Both areas enlarge with the increase of scanning velocity of more than 3 mm s^?1. The scan velocity has no effect on hole taper in air environment but inconsistent hole taper is obtained under flowing water stream. Furthermore, moving the focal plane position below the workpiece surface contributes relatively smaller areas of spatter deposition, irradiation and taper in comparison to zero focal plane position. Finally, the differences between laser ablation in air and under water are identified. The reduction in the spatter deposition and irradiated areas around the perimeter of the ablated hole and a smaller taper with the use of laser trepan drilling method in air and under water machining are investigated in this paper.     

Random particle methods applied to broadband fan interaction noise

Predicting broadband fan noise is key to reduce noise emissions from aircraft and wind turbines. Complete CFD simulations of broadband fan noise generation remain too expensive to be used routinely for engineering design. A more efficient approach consists in synthesizing a turbulent velocity field that captures the main features of the exact solution. This synthetic turbulence is then used in a noise source model. This paper concentrates on predicting broadband fan noise interaction (also called leading edge noise) and demonstrates that a random particle mesh method (RPM) is well suited for simulating this source mechanism. The linearized Euler equations are used to describe sound generation and propagation. In this work, the definition of the filter kernel is generalized to include non-Gaussian filters that can directly follow more realistic energy spectra such as the ones developed by Liepmann and von Kármán. The velocity correlation and energy spectrum of the turbulence are found to be well captured by the RPM. The acoustic predictions are successfully validated against Amiet’s analytical solution for a flat plate in a turbulent stream. A standard Langevin equation is used to model temporal decorrelation, but the presence of numerical issues leads to the introduction and validation of a second-order Langevin model.     

Generation of blast waves in a channel by nonideal detonation of aluminum-enriched high-density compositions

The results of experimental studies of the nonideal detonation of high-density, high-energy aluminum-ammonium perchlorate-organic fuel-HE compositions and of the blast waves it generates in a channel filled with air are presented. Aluminum-enriched compositions have high densities (up to 2 g/cm³) and high heats of explosion, nearly twice that for TNT. The studies were performed to work out scientific fundamentals of controlling nonideal detonation and to explore the possibility of creating new high-energy high-density formulations with an enhanced fugacity effect. The factors that enable controlling the nonideal detonation of such charges were determined. It was demonstrated that, at RDX contents above 15%, the detonation velocity increases linearly with the charge density while the critical detonation diameter decreases. Adjusting the density, HE content, ratio of the components makes it possible to vary the detonation velocity in high-density charges over a wide range, from 4 to 7 km/s. The experimental data were compared to the thermodynamically calculated velocity of ideal detonation. For the compositions under study, the pressure- time histories of the blast wave generated in a cylindrical tube by the expanding detonation products at different distances from the charge were measured. The results were compared to analogous data obtained under the same conditions for the detonation of the same mass of TNT (100 g). The parameters of blast waves generated by the test compositions are markedly superior to those characteristic of TNT: the pressure at the leading front of the wave and pressure impulse at a given distance from the charge were found to be 1.5–2.0 (or even more) times those observed for TNT. The TNT equivalency at pressures 30–60 atm has similar values. The TNT equivalencies in pressure and pressure impulse depend nonmonotonically on the distance from the charge, so far unclear why. It was established that the interaction between excess fuel and air oxygen during the expansion of detonation products contributes little to supporting the blast wave.     

Radiation dose and image quality in K‐edge subtraction computed tomography of lung in vivo

K‐edge subtraction computed tomography (KES‐CT) allows simultaneous imaging of both structural features and regional distribution of contrast elements inside an organ. Using this technique, regional lung ventilation and blood volume distributions can be measured experimentally in vivo. In order for this imaging technology to be applicable in humans, it is crucial to minimize exposure to ionizing radiation with little compromise in image quality. The goal of this study was to assess the changes in signal‐to‐noise ratio (SNR) of KES‐CT lung images as a function of radiation dose. The experiments were performed in anesthetized and ventilated rabbits using inhaled xenon gas in O₂ at two concentrations: 20% and 70%. Radiation dose, defined as air kerma (K_a), was measured free‐in‐air and in a 16 cm polymethyl methacrylate phantom with a cylindrical ionization chamber. The dose free‐in‐air was varied from 2.7 mGy to 8.0 Gy. SNR in the images of xenon in air spaces was above the Rose criterion (SNR > 5) when K_a was over 400 mGy with 20% xenon, and over 40 mGy with 70% xenon. Although in human thorax attenuation is higher, based on these findings it is estimated that, by optimizing the imaging sequence and reconstruction algorithms, the radiation dose could be further reduced to clinically acceptable levels.     

Lattice Boltzmann Simulation for Complex Flow in a Solar Wall

In this letter,we present a lattice Boltzmann simulation for complex flow in a solar wall system which includes porous media flow and heat transfer,specifically for solar energy utilization through an unglazed transpired solar air collector(UTC).Besides the lattice Boltzmann equation(LBE) for time evolution of particle distribution function for fluid field,we introduce an analogy,LBE for time evolution of distribution function for temperature.Both temperature fields of fluid(air) and solid(porous media) are modeled.We study the effects of fan velocity,solar radiation intensity,porosity,etc.on the thermal performance of the UTC.In general,our simulation results are in good agreement with what in literature.With the current system setting,both fan velocity and solar radiation intensity have significant effect on the thermal performance of the UTC.However,it is shown that the porosity has negligible effect on the heat collector indicating the current system setting might not be realistic.Further examinations of thermal performance in different UTC systems are ongoing.The results are expected to present in near future.     

Monitoring acoustic emission (AE) energy of abrasive particle impacts in a slurry flow loop using a statistical distribution model

Slurry erosion has been recognized as a serious problem in many industrial applications. In slurry flows, the estimation of the amount of incident kinetic energy that transmits from particles suspended in the fluid to the containment structures is a key aspect in evaluating its abrasive potential. This work represents a systematic investigation of particle impact energy measurement using acoustic emission (AE), as indicated by a sensor mounted on the outer surface of a sharp bend, in an arrangement that had been pre-calibrated using controlled single and multiple impacts. Particle size, free stream velocity, and nominal particle concentration were varied, and the amount of energy dissipated in the carbon steel bend was assessed using a slurry impingement flow loop test rig. Silica sand particles of mean particle size 225–650 μm were used for impingement on the bend with particle nominal concentrations between 1 and 5% while the free stream velocity was changed between 4.2 and 14 ms⁻¹.     

Multifilament Model of PET Melt Spinning and Prediction of As‐spun Fiber's Quality

Based on the multifilament model with cross air blowing proposed by Dutta (1987) and the assumption that the quench air temperature around the filament obeys an exponential distribution, a multifilament model suitable for the annular air blowing condition of PET staple fiber melt spinning is proposed. The quench air velocity, quench air temperature, filament velocity, filament temperature, etc. at different positions were predicted and the relation between birefringence and the important quality index of as‐spun fiber, Eys1.5 (elongation corresponding to 1.5 times the yielding stress in a stress‐strain curve) was obtained through experiment. The as‐spun fiber properties of PET staple and its variability were predicted and the effects of spinning conditions and spinneret design on as‐spun fiber properties were discussed and verified.     

Effect of a corona discharge on the gasdynamical flow rate

We have shown experimentally that the application of an electric field to an air stream blowing over a corona point has the additional effect of increasing the air velocity. This result is derived from the interaction of two factors. We have developed a model of the phenomenon which is based on electrostatic and hydrodynamic equations. The model provides an equation which relates the additional air velocity to the basic corona discharge parameters. The resulting function agrees with the experimental results.     

A survey of low velocity and coaxial jet noise with application to prediction

A review of recent published data on low velocity jet noise is given together with previously unpublished results taken from the Rolls-Royce Noise Research Programme on model rigs and full-scale engines. Noise correlations are given which show that at low jet velocities, the low frequency exhaust noise which is commonly referred to as jet noise, emitted from the fan stream of a turbofan engine is considerably lower in level than that from the (hot) centre stream. From this result, a new prediction procedure for coaxial jet noise of turbofan engines is then developed. Comparisons are given which show that this method gives good correlation with measured results from a number of full-scale turbofan engines. The importance of accurate estimation of the “ground reflection effect” is clearly demonstrated. A critical review of published jet noise data from model coaxial jets is given and the need for further extensive testing emphasized.     

Magnetohydrodynamic plane Couette flow of an elasto-viscous fluid

Summary Using the Laplace transform, an analytical solution of the Navier-Stokes equation is obtained for a two-dimensional incompressible elasto-viscous fluid past between two infinite parallel walls. It is assumed that the lower wall is moving with velocity which is a function of any given free stream velocity. As an application of the solution, two cases for the stream velocity are studied.     

Wind excited vibration of a square section cantilever beam in smooth flow

The self-excited oscillation of a flexible, square section, slender, cantilever beam in a uniform, smooth wind stream is examined experimentally and theoretically. The excitation was due to negative damping type aerodynamic forces and the amplitude was limited as forces became non-linear. Steady oscillations occurred only when a face was oriented less than 15 degrees to the free stream velocity. Theoretical solutions based on measured quasi-static air forces agreed qualitatively with the experimental results, but failed to predict some peculiar drop-off behavior observed in the experiment. Discrepancies were attributed to slight irregularities in the cross-section shape of the beam.     

An Experimental Study of Liquid Jets Interacting with Cross Airflows

The primary break‐up of liquid jets in cross flows has been studied experimentally. An open‐circuit wind tunnel was employed in which the airflow was generated by a centrifugal fan. The test section, positioned 3 m downstream of the fan, was made of clear acrylic resin to allow optical access and visualization. The working liquid used in the present experiment was an aero‐engine lubrication oil, which was injected perpendicularly into the air flow, via a nozzle placed in the top wall of the test‐section. The study of the primary break‐up mechanisms of the jet involved three parameters, the oil viscosity, and the jet and air cross flow velocities, which were varied independently. Two different break‐up regimes were observed and identified; arcade break‐up and bag break‐up. These were separated by a transition zone. Transverse and longitudinal (or streamwise) penetrations of the jet before the liquid breakup were also measured. The correlation proposed by Wu et al. to predict the jet transverse penetration before the break‐up of the liquid, as a function of the liquid/airflow momentum‐flux ratio, was found to be applicable only to liquids with low viscosity. An empirical extension to this equation has been produced.     

Soot emission reduction in oxygenated co-flow jet flames

A computational study was performed for ethylene/air non-premixed laminar co-flow jet flames using an axisymmetric CFD code to explore the effect of oxygenation on PAH and soot emissions. Oxygenated flames were established using N₂ diluted fuel stream along with O₂ enriched air stream such that the stoichiometric mixture fraction (Ζ_st) is varied but the adiabatic flame temperature is not materially changed. Simulations were carried out using a spatially and temporally accurate algorithm with detailed chemistry and transport. A detailed kinetic model involving 111 species and 784 reactions and a fairly detailed soot model were incorporated into the code. Two different approaches, one with constant flame height and other with constant inlet velocity are comprehensively examined to bring out the effects of changes in flame structure and residence time on soot emissions with respect to Z_st. With increase in Ζ_st, a drastic reduction in the formation of soot precursors (acetylene and benzene) and thus in soot emissions are observed. In the present study, oxygenated flames with Ζ_st ≥ 0.424 are considered as blue flames or completely soot free. For various oxygenated flames a C/O ratio between 0.45 and 0.6 is found to be most favorable for soot formation.     

Numerical flow visualization of a Single Expansion Ramp Nozzle with hypersonic external flow

Numerical simulation of scramjet asymmetric nozzle flow is carried out to visualize and investigate the effects of interaction between engine exhaust and hypersonic external flow. The Single Expansion Ramp Nozzle (SERN) configuration studied here consists of flat ramp and a cowl with different combinations of ramp angle and cowl geometry. UsingPARAS 3D, simulations are performed for a free stream Mach number of 6.5 that constitutes the external flow around the vehicle. Appropriate specific heats ratio has been simulated for the jet and free stream flow. External shock wave due to jet plume interaction with free stream flow, the internal barrel shock wave and the shear layer emanating from the cowl trailing edge and sidewalls are well captured. Wall static pressure distribution on the nozzle ramp for different nozzle expansion angles has been computed for both with and without side fence. Axial thrust and normal force have been evaluated by integrating the wall static pressure. Effect of cowl length variation and side fence on the SERN performance has also been studied and found to be quite significant. Based on this study, an optimum ramp angle at which the SERN generates maximum axial thrust is obtained. SERN angle of 20° was found to be optimum when the flight axis coincides with nozzle axis.     

Supersonic velocimetry in a shock tube using laser enhanced ionisation and planar laser induced fluorescence

 A novel flow-tagging technique is presented which was employed to measure gas velocities in the free stream of a shock tube. This method is based on the laser spectroscopic techniques of Laser-Enhanced Ionisation (LEI) and Laser-Induced Fluorescence (LIF). The flow in the shock tube is seeded with small amounts of sodium, and LEI is used to produce a substantial depletion of neutral sodium atom concentration in a well-defined region of the flow, by using two wavelength-resonance excitation and subsequent collisional ionisation. At a specific time delay, single-laser-pulse planar LIF is utilised to produce a two-dimensional (2-D) inverse image of the depleted tagged region downstream of the flow. By measuring the displacement of the tagged region, free stream velocities in a shock tube were determined. Large variations in the concentration of sodium seeded into the flow were observed and even in the presence of these large variations accurate free-stream velocity measurements were obtained. The experimentally determined value for velocity compares very well with the predicted velocity. Received: 25 March 1996/Revised version: 8 July 1996     

风机封闭系统内噪声模拟分析

风机系统工作时的一个突出问题是其进风口和出风口产生的噪声。由于风机流场非常复杂,以及实验成本、实验条件限制,基于计算流体力学（Computational Fluid Dynamics, CFD）的理论逐渐成为风机噪声估计的重要方法。本文拟通过对由风机及其冷却系统构成的封闭系统进行数值建模和仿真,判断出风机主要气动噪声源的位置和种类,为处于封闭系统内的风机的噪声大小预测,提供一个可供参考的信息。结果表明：风机出口腔体内部非定常压力波动强度最大。     

Flow characteristics of supersonic gas passing through a circular micro-channel under different inflow conditions

Gas flow in a micro-channel usually has a high Knudsen number. The predominant predictive tool for such a microflow is the direct simulation Monte Carlo(DSMC) method, which is used in this paper to investigate primary flow properties of supersonic gas in a circular micro-channel for different inflow conditions, such as free stream at different altitudes, with different incoming Mach numbers, and with different angles of attack. Simulation results indicate that the altitude and free stream incoming Mach number have a significant effect on the whole micro-channel flow field, whereas the angle of attack mainly affects the entrance part of micro-channel flow field. The fundamental mechanism behind the simulation results is also presented. With the increase of altitude, thr free stream would be partly prevented from entering into micro-channel.Meanwhile, the gas flow in micro-channel is decelerated, and the increase in the angle of attack also decelerates the gas flow. In contrast, gas flow in micro-channel is accelerated as free stream incoming Mach number increases. A noteworthy finding is that the rarefaction effects can become very dominant when the free stream incoming Mach number is low. In other words, a free stream with a larger incoming velocity is able to reduce the influence of the rarefaction effects on gas flow in the micro-channel.