Diffuse surface reflection in free-molecular flow at a speed of 10 km/sec

An experimental investigation has been made of a diffuse-reflection scheme for conditions where the velocity of the particles bombarding a surface is 10 km/sec, and the surface is covered with an adsorbing layer. It is shown that the use of diffuse reflection is valid over a range of incident angles from θ=0 to θ?40?. For θ>40? the diffuse reflection is a crude approximation, since it does not take into account the contribution of scattered particles which are reflected in a quasispecular manner and have speed approximately equal to the incident speed.     

An aerodynamic wake in a hypersonic free-molecular gas flow

An experimental investigation was made of the form of an aerodynamic wake and the flow in it for the case where the flow is set up using a point source and is propagated in a motionless residual gas. It is shown that the geometry of the wake is determined by the mutual arrangement of the source and model, that the ground pressure is negligibly small, and that the flow in the wake is part of the oncoming flow, scattered by the residual gas. The value of the transverse velocity ratio exceeds by several orders of magnitude the value of the longitudinal ratio.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 183–186, January–February, 1976.     

Experimental determination of the speed of sound in cavitating flows

This paper presents measurements of the speed of sound in two-phase flows characterized by high void fraction. The main objective of the work is the characterization of wave propagation in cavitating flows. The experimental determination of the speed of sound is derived from measurements performed with three pressure transducers, while the void fraction is obtained from analysis of a signal obtained with an optical probe. Experiments are first conducted in air/water mixtures, for a void fraction varying in the range 0–11%, in order to discuss and validate the methods of measurement and analysis. These results are compared to existing theoretical models, and a nice agreement is obtained. Then, the methods are applied to various cavitating flows. The evolution of the speed of sound according to the void fraction α is determined for α varying in the range 0–55%. In this second configuration, the effect of the Mach number is included in the spectral analysis of the pressure transducers’ signals, in order to take into account the possible high flow compressibility. The experimental data are compared to existing theoretical models, and the results are then discussed.     

Experimental determination of the flow transport coefficients in the coupled equations of two-phase flow in porous media

The transport coefficients in the coupled equations of two-phase flow are defined if the pressure gradient in one of the two flowing fluids is equal to zero. This definition has been used in experiments with oil and water in a sandpack and the four transport coefficients have been measured over wide water saturation ranges. The values of the cross coefficients were found to be significant as they ranged from 10 to 35% of the value of the effective permeability to water and from 5 to 15% of the effective permeability to oil, respectively.     

Aerodynamic characteristics of an element of surface washed by a high-speed free-molecular flow

An analysis is presented of the dependence of local aerodynamic coefficients on the angle of attack for surfaces covered by an adsorbed layer, for a flow velocity of ≈10 km/sec. The analysis is based on comparison of coefficients calculated for diffuse reflection and experimental data on momentum transfer between the flow and the surface, and with the assumed quasispecular reflection in which the velocity of the reflected particles depends on the angle of attack. It is shown that the quasispecular case is a more accurate approximation in comparison with the diffuse case.     

Limiting self-similar motion of a solid particle in a free-molecular gas flow escaping from an orifice

A problem of motion of a single spherical solid particle in the far wake of a free-molecular gas flow escaping from an orifice is considered. It is shown that the spatial distributions of macroscopic parameters of the gas are completely determined by functions of one variable: coordinate along an arbitrary straight line normal to the axis of symmetry. Based on this property, a dimensionless equation of particle motion is derived, which has self-similar solutions: trajectories of motion and particle coordinates (traces) on the target for different initial conditions. Conditions of determining the gas flow behavior on the basis of particle traces on the target are considered.     

Effect of swirling flow on unburned ratio and nitrogen oxide concentration in a spray combustion system

The effect of swirling flow on the unburned ratio and NO concentration in exhausted gas was studied for slurry [coal-water mixture (CWM)] spray combustion with variations of swirl numbers. A numerical analysis for CWM combustion was performed for axisymmetric flow in a cylindrical geometry. First, to check the performance of three previous k-ε turbulence models modified with swirling flow, velocity components of isothermal swirling jets were measured by laser-Doppler anemometry (LDA) and compared with predicted results. The two modified models gave more reliable results than the conventional one. Next, as the swirl number could not be estimated by the angular momentum derived from the vane angle of the circular swirler, the reduction rate of the tangential momentum flux through the tube of the circular swirler was measured and calculated. Both measured and predicted results showed that when the swirl number S′ given by the vane angle was 2.0, the effective swirl number S_eff decreased by about 60% to S′. To take the results mentioned above into consideration, effects of swirl number on both the exhausted NO concentration and unburned ratio were investigated. The predicted unburned ratio showed good agreement with the experimental results. Both experimental and calculated results showed that the optimum operating conditions controlling the exhausted NO concentration and unburned ratio in this spray combustion system were obtained when the swirl number S_eff was about 0.5.     

Experimental determination of the boundaries of the region of existence of anomalous convection flow in an tilted cube

This paper presents an experimentally study of the bifurcation of steady-state air convection in a cubic cavity heated from below under controlled deviations from equilibrium heating conditions due to a slow quasisteady-state tilt of the cavity at a predetermined angle α. It is found that in the supercritical range of Rayleigh numbers Ra at a tilt of the cavity not exceeding 7°, the existence of two stable steady-state convection regimes (normal and anomalous) with circulation in opposite directions is possible. A study is made of the transformations of the temperature distribution in the middle (with respect to the planes in which heat exchangers are located) plane during transition from the anomalous flow regime to the normal regime by instantaneous rotation of the entire mass of air in the cavity around the vertical axis by an angle of 90 to 135°. It is shown that this rotation occurs when the tilt of the cavity exceeds a critical value α _cr(Ra), which was determined experimentally for Rayleigh numbers 0 < Ra < 25Racr, where Racr is the critical Rayleigh number for stability of mechanical equilibrium for heating from below.     

Basic concepts in the surface interaction of free-molecular flows or molecular beams

Summary This paper reports the Author's point of view on some old and new basic concepts on the surface interaction, as appearing from his recent scientific production in this field. Namely, there are considered the concepts of state variables in highly rarefied gases in maxwellian equilibrium; local and global stagnation temperature; energy accomodation coefficients; sitting time; free-molecule surface sources and sinks with application to the Author's re-emission model and comparison with experimental results.

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Sommario In questo lavoro viene esposto il punto di vista dell'Autore su alcuni concetti, vecchi e nuovi, basilari nello studio dell'interazione superficiale, quale appare dalla sua recente produzione scientifica in questo campo. Precisamente vengono presi in esame i concetti di variabili di stato in gas molto rarefatti in equilibrio maxwelliano; di temperatura d'arresto locale e globale; di tempo di soggiorno; di coefficienti di accomodamento; di sorgenti e pozzi superficiali di molecole libere, con applicazione al modello di riemissione dell'Autore e confronti con risultati sperimentali.

     

Experimental study of the effects of spanwise rotation on the flow in a low aspect ratio diffuser for turbomachinery applications

Two dimensional time accurate PIV measurements of the flow between pressure and suction side at different spanwise positions of a rotating channel are presented. The Reynolds and Rotation numbers are representative for the flow in radial impellers of micro gas turbines. Superposition of the 2D results at the different spanwise positions provides a quasi-3D view of the flow and illustrates the impact of Coriolis forces on the 3D flow structure. It is shown that the inlet flow is little affected by rotation. An increasing/decreasing boundary layer thickness is reported on the suction/pressure side wall halfway between the channel inlet and outlet. The turbulence intensity moves away from the suction side wall and remains close to the pressure side wall. The instantaneous measurements at mid-height of the rotating channel reveal the presence of hairpin vortices in the pressure side boundary layer and symmetric vortices near the suction side. Hairpin vortices occur in rotation in the pressure and in the suction side, for the measurement plane close to the channel bottom wall.     

The effects of flow split ratio and flow rate in manifolds

This paper reports a combined experimental and numerical investigation of three-dimensional steady turbulent flows in inlet manifolds of square cross-section. Predictions and measurements of the flows were carried out using computational fluid dynamics and laser Doppler anemometry techniques respectively. The flow structure was characterized in detail and the effects of flow split ratio and inlet flow rate were studied. These were found to cause significant variations in the size and shape of recirculation regions in the branches, and in the turbulence levels. It was then found that there is a significant difference between the flow rates through different branches. The performance of the code was assessed through a comparison between predictions and measurements. The comparison demonstrates that all important features of the flow are well represented by the predictions.     

An analytical determination of speed factor in gear designs

In practical gear design, the “speed factor” is usually employed which is defined as a ratio of the allow-able stress at required running speed of the gear system to the stress at zero speed. At present, several expressions for the speed factor are recommended by several authorities; however, those are almost all empirical and not analytical. The present paper aims to contribute to an analytical determination of the speed factor for spur gears as stated below. First, from a dynamic photoelastic test of a pair of spur gears, the following results were obtained:

1. (1)
   At a contant speed and torque and at the same point of engagement, the maximum fillet stress did not have a specific constant value but had somewhat scattered values due to the vibration of gear system under operation.     
   
   

Experimental study on the confined 3D laminar flow past a square prism with a high blockage ratio

An experimental PIV study is presented that addresses the confined 3D laminar flow behaviour past a square prism. The Reynolds number (Re), based on prism cross-section height varies between 100 and 256. The channel aspect ratio is 1/1 and the blockage ratio is 1/2.5. This geometry is representative of a passive method to enhance mixing in otherwise highly ordered laminar channel flow. It is found that the lateral walls exert a strong effect on the flow behaviour with two main consequences: (a) the onset of vortex shedding is delayed to a Re in the vicinity of 170, as opposed to the unconfined case where the critical Re is reported to be between 50 and 60 and (b) transition from the steady closed recirculation bubble regime to the vortex shedding regime is not abrupt. In particular, there is a range of Re for which the closed recirculation bubble pulsates with increasing amplitude prior to the onset of the Karman street regime. The experimental results are supported by numerical computations, and the relation of the results with the practical design of engineering systems is also discussed.     

Experimental investigation of critical pressure ratio in orifices

To determine the value of the critical pressure ratio in orifices, critical mass flow rate of air through straight-bore orifices and knife orifices was measured. The straight-bore test orifices with varying orifice diameters of 4, 7, 10 and 12 mm were installed in a 20-mm pipe. The knife or sharp-edged test orifices with orifice diameters of 10, 15 and 18 mm were installed in a 40-mm pipe. The test orifices with diameter ratio from 0.2 to 0.6 exhibited a constancy of discharge at ratios of the downstream to upstream pressures of less than 0.17, which is considerably lower than the theoretical critical pressure ratio for an ideal nozzle. An empirical expression to calculate the value of the critical pressure ratio, which includes the relevant primary parameters and which fits the data well, is suggested for engineering design purposes.     

Experimental analysis of the swirling flow in a Francis turbine draft tube: Focus on radial velocity component determination

The draft tube of a hydraulic turbine is the component where the flow exiting the runner is decelerated, thereby converting the excess of kinetic energy into static pressure. In the case of machine refurbishment of an existing power plant, most of the time only the runner and the guide vanes are currently modified. For financial and safety reasons, the spiral casing and the draft tube are seldom redesigned, even if these components present some undesirable behaviour. In some cases, the installation of an upgraded runner leads to a peculiar and undesirable efficiency drop as the discharge is increased above the best efficiency point value. It is found to be related to a corresponding sudden variation in the draft tube pressure recovery coefficient at the same discharge.The swirling flow exiting the runner is complex and highly turbulent. The radial velocity is rarely measured because a quite complicated measurement setup is needed. However, this velocity component is greatly needed in order to properly initialize the numerical simulations, and its influence is important in spite of its small magnitude. Velocity measurements downstream of the runner include radial component made at CREMHyG (Grenoble) by LDV, and PIV techniques are presented. An analytical formulation for this velocity component based on the formulation for the conical diffuser and on the three vortices structure is proposed and compared with measurements.