Kinematic Viscosity Of Air Research Articles

Particle Filter Based on Thermophoretic Deposition from Natural Convection Flow

We present an analysis of particle migration in a natural convection flow between parallel plates and within the annulus of concentric tubes. The flow channel is vertically oriented with one surface maintained at a higher temperature than the other. Particle migration is dominated by advection in the vertical direction and thermophoresis in the horizontal direction. From scale analysis it is demonstrated that particles are completely removed from air flowing through the channel if its length exceeds Lc = (b4g/24Kv2), where b is the width of the channel, g is the acceleration of gravity, K is a thermophoretic coefficient of order 0.5, and v is the kinematic viscosity of air. Precise predictions of particle removal efficiency as a function of system parameters are obtained by numerical solution of the governing equations. Based on the model results, it appears feasible to develop a practical filter for removing smoke particles from a smoldering cigarette in an ashtray by using natural convection in combinat...

A new expression for the production rate of sea water droplets on the sea surface

A new set of empirical formulas for the production rate and the number concentration of sea-water droplets on the sea surface are proposed, synthesizing past observation data of sea-salt particles in the sea and water droplets in wind-wave tanks. A new levelz c is introduced as the effective wind-sea surface where seawater droplets are produced. The new formulas are expressed in linear functions in logarithmic scales ofu*2/vσ p , a parameter to describe overall conditions of airsea boundary processes, whereu * is the friction velocity of air,v the kinematic viscosity of air and σ p the peak angular frequency of wind-wave part of wave spectra. A model of coexistence of spray droplets and suspended particles near the sea surface is proposed. As for the independent parameter, a comparison between the uses ofu*2/vσ p and ofu * 3 which was the traditional way of parameterization excluding wave measure, shows that the advantage of usingu*2/vσ p is statistically significant with a confidence limit 89% in F-test.

Use of a graded diffusion battery in measuring the activity size distributions of thoron progeny

This paper describes the performance of a graded diffusion battery in measuring the activity size distribution of 212Pb. The diffusion battery consisted of five stainless steel screens with 24, 35, 50, 200 and 635 mesh numbers and a backup filter. Calibration of the screens indicated that the collection of ultrafine particles on these screen follows the diffusion battery theory if the flow rate is kept at a level at which the flow Reynolds number (Re= d w U/ ν, where d w is the wire diameter, U is the face velocity and ν is the kinematic viscosity of air) is less than 1. The activity size distribution of 212Pb in an aging chamber was measured in ultra-pure air with dry (< 10% r.h.) and humid (40–50% r.h.) conditions. The particle size of 212Pb (about 1 nm) did not change significantly in the humid air when the mean residence time was between 0.33 and 8.3 min. However, in the dry air, the thoron progeny grew with aging time from about 1 to 3 nm. By adjusting the relative humidity and the aging time, 212Pb of activity median diameters from 1 to 3 nm could be produced. This allowed us to study the effect of the particle size of ‘unattached’ thoron progeny on the deposition efficiency in the upper respiratory tract.

A new thermal rating approach: The real time thermal rating system for strategic overhead conductor transmission lines -- Part II: Steady state thermal rating program

The elements of the steady state thermal rating program are described which includes how the convective heat transfer from overhead conductors is affected by wind turbulence, wind direction, velocity profile (a function of height above ground, temperature lapse rate, and type of terrain), conductor surface type (stranded or smooth), conductor pitch, and the proximity of conductors in a bundle configuration. Equations are developed for the thermal conductivity and the kinematic viscosity of air. It is also shown how the conductor temperature may be solved directly by formulating a biquadratic equation rather than utilizing an iterative procedure.

Thermal and economic analysis of the overlapped-glass plate solar-air heater

The heat transfer analysis of the overlapped-glass plate air heater was based on the quasi-steady state performance and was carried out in two steps. In the first step, the glass plates were assumed to absorb the short-wave solar radiation, but the long-wave emission from the plates was ignored. In the second step, the approximate glass temperatures obtained from the first-step solution were used in the exact determination of the long-wave radiation exchange between the glass plates; therefore, improved results were obtained. The computer programs in Fortran IV were run on IBM 7044 and 360 O/S computers; convective film coefficients, Reynolds numbers and boundary-layer thicknesses were determined along the glass plates throughout the whole collector. Further refinement was attained by considering axial heat conduction within the glass plates and the change in the kinematic viscosity of air along the glass plates, due to temperature variation. The air and glass plate temperatures were calculated by taking into account the uneven distribution of the incoming air stream between the glass plates, obtained from a water table analogy. The model was constructed and tested under clear-sky conditions at the Middle East Technical University, Ankara, Turkey. The air and glass temperatures were measured on each plate using copper-constantan thermocouples. The total air-flow rate was measured using a Venturi-Herschell tube. The solar radiation intensity was measured using a Kipp and Zonen actinometer and a silicon solar cell pyranometer. The cost analysis of the collector was carried out for various glass plate lengths, spacings, interest rates, three different levels of total sunshine hours, ten years of expected life, and cost figures based upon the local market prices and salary rates reported in Turkey.

Breaking of wind waves and the sea surface wind stress

In the conventional treatment of the coefficient of sea surface wind stress by plotting it against 10-m wind speed, there are inevitable discrepancies among results of various investigators. The reason is considered to lie primarily in the fact that the state of the sea surface or of waves is disregarded, which may have great influence on the sea surface wind stress. Former concepts concerning the conditions which control the sea surface wind stress are discussed, and it is shown that a more universal expression may be obtained by plotting the coefficient against a kind of roughness Reynolds number:Re 2 *=u*H/ν, whereu* is the friction velocity of air, ν the kinematic viscosity of air, andH the characteristic wave height.H is used here to treat some data in wind-wave tunnels, as a tentative variable, one step towards a more rigorous approach to the problem. This variableRe 2 *, orRe 4 *=u *w /L/v w =2πgu *w /v w n 1, where the subscript ω represents values for water,L andn 1 the characteristic wave length and frequency, respectively, is also the condition describing the air entrainment or the breaking of wind waves. In this case, these Reynolds numbers are interpreted as the quantity describing the intensity of turbulence of the water surface itself. It is shown, using data from our wind-wave tunnel experiments, that the breaking commences asRe 2 * reaches 1×103, or asRe 4 * reaches 3×103. Simultaneously, the stress-coefficient begins to increase sharply at this value ofRe 2 *. This phenomenon is understood as an increased momentum transfer from the air to the water through “boundary penetration of turbulence” caused by the breaking of wind waves. Further, it is suggested that there is a possibility that this excess momentum transfer does not increase wave momentum, but reinforces drift current.

Triple velocity correlations in isotropic turbulence

AbstractThe triple velocity correlation, in turbulence produced by inserting a square-mesh grid near the beginning of the working section of a wind tunnel, has been measured for mesh Reynolds numbers of RM = 5300, 21,200 and 42,400 (RM = UM/ν, where U is the mean wind speed in the working section of the tunnel and M is the centre to centre spacing of the rods making up the grid; ν is the kinematic viscosity of air). At the lowest Reynolds number the correlation has been measured at distances downstream of the grid varying from 20 to 120M. This range covers practically all of the initial period of the decay of turbulence, where the turbulent intensity varies as t−1.

Societies and Academies

Societies and Academies