This paper provides simple analytical correlations for selected thermodynamic and fluid transport properties for the mixture dry air and water vapor. These correlations are derived from theory as well as from numerical fitting procedures and give expressions for density , viscosity , thermal conductivity k, specific heat c p , and Prandtl number Pr at a working pressure of pϭ1 bar and for a temperature range from 100°C to 200°C. The main purpose is to present a comparatively simple set of equations, as the correlations do not reflect in every case the underlying physical background. Since experimental data are scarce for the properties under investigation, it was in some cases necessary to extrapolate the available correlations to temperatures or water vapor contents where no experimental data could be found. The derived equations are compared with the pure component values for dry air and water vapor and, as far as possible, also for air-water vapor mixtures.
When applying hot-wire anemometry to velocity measurements in air, it is standard practice to neglect the effect of humidity. In this paper the influence of the thermodynamic and transport properties of humid air on hot-wire measurements is examined on the basis of the correlations for Nusselt number proposed previously by other researchers. Experimental results at controlled levels of relative humidity between 30% and 90% at 30 • C, 50 • C and 70 • C are reproduced satisfactorily by the theoretical approaches of two of these equations. A corrective term is defined to expand formulae designed for dry air to work in a humid environment. The error in velocity by omitting the influence of humidity is estimated in terms of temperature and relative humidity.
The generation of well defined trace humidity concentrations is of
interest for several applications and especially for sensor calibration
purposes. While in national standards laboratories accuracy and stability are
the most important criteria and slow generation processes are acceptable,
faster generators are needed in industry.
After reviewing several concepts for trace humidity generation, this paper
presents a fast trace humidity generator based upon a two-stage dilution and
mixing principle. The developed generator establishes a known concentration of
water vapour in air or any other carrier gas at ambient pressures and a
constant flow rate in the range of 1 l min-1. The generated humidity
volume fractions cover a range from 100 ppb to 1000 ppm, equivalent to a frost
point temperature range from about -90 to -20 °C. The layout and
construction of the saturator and the drying unit conditioning the two gas
streams to be mixed are also described. A detailed analysis of the uncertainty
range of the generated humidity concentrations yields optimized operational
parameter settings for a minimized uncertainty in the selected humidity
concentration. The generator has been characterized by means of a recently
developed laser spectrometer for trace humidity detection.
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