Turbulent Air Stream Research Articles

Turbulent convective heat and mass transfer from accelerating particles

AbstractThe rate of evaporation of acetone from single particles accelerating freely in a downward concurrent turbulent air stream was studied over a range of air velocities from 40 to 70 ft./sec. and at a constant air temperature of 410°F. The particles consisted of celite in the shape of spheres, cubes, disks and cylinders, varying in size from 0.15 to 0.40 in. Accurate particle velocity data were obtained with a new radioactive tracer technique, and high‐speed photography at two positions along the column permitted measurement of the rate of rotation and showed that the particles rotated in a random manner.Adequate prediction of the observed heat and mass transfer data could be obtained from the integration of a rate eequation previously reported for stationary particles. The concept of a new characteristic dimension, developed for the latter case, was found to be applicable to randomly rotating shapes and to account satisfactorily for the behaviour of nonspherical particles.

Photosynthesis Under Field Conditions. II. An Aerodynamic Method for Determining the Turbulent Carbon Dioxide Exchange Between the Atmosphere and a Corn Field1

SynopsisRates of exchange calculated from measurements of wind velocity and CO2 gradients in the turbulent air stream were in the same order of magnitude as those calculated by another method and the aerodynamic method does not alter the crop environment. Turbulence may be a limiting factor in supply of CO2 to an active crop on sunny days. Photosynthesis efficiency was 5% at midday and nearly 8% for total daylight period on a single day.

Turbulent heat and mass transfer from stationary particles

AbstractConvective heat and mass transfer rates for 20 shapes suspended in various orientations in a hot turbulent air stream were correlated by the equation: for turbulent intensities between 9 and 10%, using a new characteristic dimension, L”, in the Reynolds Number. This dimension, defined as the total surface area divided by the maximum perimeter perpendicular to flow, was qualitatively explained from boundary layer considerations and was found to correlate the data for different shapes available in the literature. Qualitative pictures for the relative rates of heat and mass transfer around the various shapes were also obtained.

Thermal Transfer in Turbulent Gas Streams—Effect of Turbulence on Macroscopic Transport From Spheres

Abstract The rate of thermal transport from spheres in a flowing stream is of interest in many industrial operations. Limited data are available concerning the effect of turbulence level on this transfer process. Measurements of the thermal transport from spheres to an air stream at four levels of turbulence were made at four velocities varying between 4 and 32 fps. Results are presented in terms of the Nusselt number. The experimental work is a portion of a detailed study of the local thermal and material transport from spheres in turbulent air streams. The data indicate a significant increase in the thermal transport with an increase in the level of turbulence at a constant Reynolds number. For example, the Nusselt number was increased from 53 to 62 upon an increase in the level of turbulence from 0.05 to 0.12 at a Reynolds number of 7200.

Experiments on the Influence of Flow on Sound Attenuation in Absorbing Ducts

The influence of a turbulent air stream on the sound attenuation in ducts, lined with sound absorbing materials or structures of different kinds, has been experimentally investigated. For linings consisting either of porous materials or sufficiently damped Helmholtz resonators, decrease of absorption with increasing flow velocity has been observed, together with an increase of the frequency of maximum absorption in the case of the resonators. Somewhat surprising results have been obtained for undamped or weakly damped Helmholtz resonators, where an attenuation minimum is observed above the resonant frequency, the frequency of which increases linearly with increasing flow velocity, and which—for undamped resonators—even reaches negative attenuation values. Aside from the negative attenuation, a self-excitation of the duct is observed under certain conditions, the frequency of which is not identical with the attenuation minimum.

Interaction between a turbulent air stream and a moving water surface

AbstractThe interaction between a turbulent air stream and a water film flowing parallel to it was studied. The variables explored were the liquid flow rate, the gas flow rate, and the film height. The data were correlated in terms of a liquid and a gas Reynolds number. Thinner films or films having a lower Reynolds number were more stable. For the experiments reported in this research the transition from a smooth surface to a surface possessing two‐dimensional waves occurs at hUτρliquid/µliquid of about 520. Transition from two‐dimensional waves to a “pebbled” surface occurs at a value of this parameter of about 600. Theories presented in the literature for the initiation of waves on a liquid surface do not adequately describe the experimental results.

The diffusion of smoke from a continuous elevated point-source into a turbulent atmosphere

The problem is to determine the downwind concentration of non-buoyant smoke emitted from a continuous elevated pointsource in a turbulent airstream. The velocity and eddy diffusivity coefficients are represented by related powers of the height above ground, and are independent of the position of the source. Exact solutions are obtained for the zero and second moments of the concentration distribution along lines lying in the cross-wind direction at ground level. In special cases, these moments may also be determined along lines at general height. In one such case the concentration is determined exactly (rather than just the two moments) and it is found that the cross-wind distribution always has a Gaussian form. If it is assumed that in all cases the cross-wind profile is Gaussian, a formulation for the concentration can be given purely in terms of the known zero and second moments. When the source is moving with constant velocity across the wind, the first moment as well as the zero and second moments is exactly determined, and under a similar assumption a formula for the concentration is found.

A note on rayleigh's problem for a plate of finite width

The purpose of this note is to indicate the relation between (a) the problem of evaporation from a finite area of rectangular shape into a turbulent air stream and of the boundary layer on a flat plate of finite width, and (b) the problem of the boundary layer on a semi-infinite plate and evaporation from an infinite quadrant.

THE EFFECT OF FINITE WIDTH OF AREA ON THE RATE OF EVAPORATION INTO A TURBULENT ATMOSPHERE

The aim of this paper is to obtain an estimate of the effect of finite width on the rate of evaporation from a plane, rectangular, saturated area into a fully turbulent air stream. A rigorous solution for this problem has been previously shown to involve Mathieu functions of a type which have not yet been computed; in order to obtain an estimate of the effect of finite width a simplified version of the problem is discussed. The problem considered is that of turbulent diffusion of matter fróm a plane area of infinite length and finite width, composed of infinite line sources, when the mean wind direction is parallel to the sources. The strengths of the sources are chosen to be constant along the mean wind direction and variable across wind, in such a way that the concentration of vapour is constant at all points on the area. The use of this idealized model is justified by comparing the theoretical law obtained, for dependence of rate of evaporation on width, with experimental results. The analysis leads to the theoretical result that the rate of evaporation per unit length of the area is not directly proportional to the width, as assumed by previous investigators, but to (width)α where, for example, in the particular case of adiabatic conditions α = 0.86; the theory also indicates that the index α is independent of the mean wind speed and depends only on the degree of turbulence present in the air stream. Experimental results are given which suggest that, in wind-tunnel conditions, a law of this kind expressing the dependence of rate of evaporation on width is true at all points along an evaporating area, except very near the leading edge.

Evaporation from a plane, free-liquid surface into a turbulent air stream

In the past the experimental study of evaporation, and the interpretation of the results obtained, have shown in general only meagre reference to the aerodynamics of the problem. The present treatment, which is concerned with the evaporation from plane, free-liquid surfaces of relatively small dimensions into a tangential air stream, demonstrates the importance of the type of boundary layer flow. The rates of evaporation under the influence of a turbulent boundary layer are then tested against a hydrodynamical theory due to O. G. Sutton. Sutton’s theory assumes that the turbulent transfer of any entity is determined by the momentum interchange coefficient, which is shown to involve the kinematic viscosity of the diffusing medium, and which leads to a functional form for evaporation which has been shown previously to be in good agreement with experimental data. Developed into a computable form, and tested against the present experiments on the evaporation of bromobenzene and against experiments by Elias on the analogous problem of convective heat transfer, the theory is now shown to predict the absolute rate of turbulent transfer in a satisfactory manner. An extension of the analysis to the relative rates of evaporation of various liquids, as determined in the present experiments and in recent experiments by Wade, shows that the theory specifies inadequately the variation of rate of evaporation with type of liquid. In the absence of a precise theoretical argument, an empirical generalization of Sutton’s theory is set forth, in which the turbulent interchange coefficient is modified by the molecular diffusion coefficient appropriate to the entity undergoing transfer. The range of physical characteristics covered in the present evaporation experiments, and in those performed by Wade, is sufficient to demonstrate the closer agreement provided by the generalized form of the theory. A more general test, against previous investigations for which the aerodynamic conditions can be estimated with reasonable confidence, shows that the absolute rate of evaporation may be predicted correctly in order of magnitude. In all cases considered the observed rates are in excess of the theoretical values, but it is significant that the discrepancy decreases as the experimental, conditions conform mote closely to the ideal conditions assumed in the theoretical treatment.

Surface Finish and Performance

IN order to appreciate how surface finish affects drag one must be familiar with the main characteristics of boundary layer flow. These have been described in great detail elsewhere, see, for example, Ref. 1, but a brief outline will not be out of place here. At the surface of a body moving through air there is a thin layer of air called the boundary layer in which the velocity relative to the body rapidly falls to zero as the surface of the body is approached. Because of the large velocity gradients across the boundary layer the viscous forces are appreciable there; outside the boundary layer the flow approximates closely to the ideal inviscid flow of classical hydrodynamics. The flow in the boundary layer beginning at the forward stagnation point is usually laminar for some distance, then after a transition region the flow becomes turbulent. The transition region is appreciable in extent at low Reynolds numbers and in turbulent airstreams, but at the Reynolds numbers usual in flight it is short enough to be referred to as a point. Wo now know enough about both laminar and turbulent types of flow over smooth surfaces and the associated frictional forces to be able to calculate with fair accuracy the profile and skin friction drags of a smooth aerofoil given its thickness, Reynolds number and the position of the transition points2. It is found that the skin friction in the laminar boundary layer is much smaller than the skin friction in the turbulent boundary layer; this is illustrated in Fig. 1, which shows the skin friction distribution on one side of a smooth flat plate at a Reynolds number of 107 and with the transition points at 05 c. and 25 c. Fig. 2 shows the variation of drag with the position of the transition point for the flat plate. Similar curves are obtained for aerofoils and bodies of revolution. It is evident that the further back transition occurs the less will be the drag. Fig. 3 illustrates a point that is worth emphasizing, namely, the relative importance of the drag change due to a given transition point movement increases with wing thickness and Reynolds number.

Statistical theory of turbulence IV-Diffusion in a turbulent air stream

It was pointed out in Part I that experiments on the spread of heat from a line source ( e.g ., an electrically heated wire) in a turbulent air stream may be expected to give two elements of the statistical specification of turbulence. If the spread is measured near the source the value of the mean transverse component of velocity √¯ v 2 or v ' in the notation of Part I, can be found. If the spread is examined further down-stream it should be possible to analyse the results to find the correlation function R η , which is the principal element of the representation of turbulence in the Lagrangian system. Spread of heat nearer line source Recently the spread of heat from a heated wire in a wind tunnel has been measured at points near to the source by Schubauer. The stream was made turbulent by means of grids of round bars arranged in square pattern. Their diameters were 1 /5 of the mesh length and M varied from 5 inches to 1/2 inch. The width of the heat wake was found by measuring the angle subtended at the source by the two positions where the temperature rise was half that in the centre of the wake. This angle, denoted by α, depends partly on the amount of turbulence and to a less extent on the spread of heat due to the thermal conductivity of the air. By assuming that the effect of turbulence is to communicate to the air an eddy conductivity β, which is additive to, and obeys the same law as, true thermal conductivity, a virtual angle α turb can be deduced by the relation α 2 turb = α 2 — α 2 0 ,