Water vapor mass fraction

Since dwx,s/dz, the rate of change of the saturation water vapor mass fraction, will be negative during condensation of water, < r = g/cp. Therefore the rate of cooling of saturated air is less than for dry air. For example, for 1000 mbar and 0°C a lapse rate of 5.8°C km is calculated, or about 60% of the dry adiabatic rate. 

By replacing the mole fraction of water with the ratio of water vapor pressure (Pw) divided by the total gas pressure (PT), one can solve for the diffusive flux of water vapor. Also, by multiplying Nw by the molecular weight of water, the mass flux of water vapor is arrived at ... 

P, yj and fw designate the total pressure, the mole fraction of component i in vapor phase and the fugacity of pure water). The mass balance in the liquid phase results in four additional equations ... 

The precision of thermobarometric equations 9.130 and 9.131 (once T is known, P is also fixed by the water-vapor univariant curve) depends on the accuracy of the last term on the right, which becomes more precise as the fractional amount of gas in vapor Xg falls. Rearranging equations 9.130 and 9.131 with the introduction of mass distribution constants of the type defined in equation 9.102, Giggenbach (1980) transformed equations 9.130 and 9.131 into thermobarometric functions based on the chemistry of the fluid. 

Combustion with complete gasification occurs when an AP pyrolant is composed of l2ip(0.86) and bi3I((0.14). The mass fraction of hydrogen chloride (HCl) among the combustion products is about 0.3. It is well known that HCl molecules combine with water vapor in the atmosphere to generate a white smoke. It is for this reason that AP pyrolants act as white smoke generators in a humid atmosphere. 

AP composite propellants without aluminum particles are termed reduced-smoke propellants and are employed in tactical missiles to conceal their launch site and flight trajectory. No visible smoke is formed when the relative humidity of the atmosphere is less than about 40%. However, since high-frequency combustion oscillation tends to occur in the combustion chamber in the absence of solid particles that serve to absorb the oscillatory energy, a mass fraction of 0.01-0.05 of metallic particles is still required for the reduced-smoke propellants. These particles and/or their oxide particles generate thin smoke trails. The white smoke trail includes the white fog generated by the HCl molecules and the condensed water vapor of the humid atmosphere. 

Gillani, Leaitch, and co-workers (1995) carried out a detailed study of the fraction of accumulation mode particles (diameters from 0.17 to 2.07 /Am) that led to cloud droplet formation in continental stratiform clouds near Syracuse, New York. When the air mass was relatively clean, essentially all of the particles were activated to form cloud droplets in the cloud interior and the number concentration of cloud droplets increased linearly with the particle concentration. However, when the air mass was more polluted, the fraction of particles that were activated in the cloud interior was significantly smaller than one. This is illustrated by Fig. 14.40, which shows the variation of this fraction (F) as a function of the total particle concentration, Nun. In the most polluted air masses (as measured by large values of Nun), the fraction of particles activated was 0.28 + 0.08, whereas in the least polluted, it was as high as 0.96 + 0.05. The reason for this is likely that in the more polluted air masses, the higher number of particles provided a larger sink for water vapor, decreasing the extent of supersaturation. 

Isotopic fractionation resulting from evaporation from standing water bodies can be described in terms of equilibrium and nonequilibrium fractionation effects. Equilibrium fractionation occurs when the isotopic composition of the evaporated water or lake evaporate is in thermodynamic equilibrium with the lake water (23). Equilibrium fractionation, however, can occur only when the water vapor in the air mass above the lake is 100% saturated. The process of equilibrium isotopic fractionation is described by Raleigh fractionation. The isotopic composition of water vapor in equilibrium with liquid water at any time is given by... 

Air enters the tower with a dry-bulb temperature of 87°F and has a wet-bulb temperature (inlet condition) of 77°F. The air leaves the tower at 90°F and is saturated. Water enters the tower at a temperature of 102°F and exits at 85°F. Determine (1) the humidity of the entering air stream, (2) the mass of dry air to the tower/lb of water feed, and (3) the fraction of water vaporized in the tower. 

Figure 4. Graph of temperature, vapor fraction, and isotopic composition of water vapor and condensate as a function of elevation for an air mass with a starting T and RH of 295 K and 80 % (modified after Rowley and Garzione 2007). Note that the vapor and condensate are the isotopic composition reflected in the scale along the top of the graph with A(8lsO) normalized relative to the initial isotopic composition of the condensate. The isotopic composition of modeled condensate is not the same as precipitation as is discussed below.

Figure 7.10 Neon (solid lines) and argon (dashed) isotopic fractionations produced by diffusion-limited hydrogen escape from representative terrestrial atmospheres, as a function of its lifetime At. Three examples that differ in the amount of C02 present are shown. Each assumes 270 bars of water vapor (i.e., the mass of the present ocean). Carbon dioxide contents are 5, 10, and 20 bars. After Ozima and Zahnle (1993).

If a fluid composed of more than one component (e.g., a solution of ethanol and water, or a crude oil) partially or totally changes phase, the required heat is a combination of sensible and latent heat and must be calculated using more complex thermodynamic relationships, including vapor-liquid equilibrium calculations that reflect the changing compositions as well as mass fractions of the two phases. 

The evaporation velocity at ambient pressure and, say, 60 °C, which corresponds to a mole fraction in the sweep gas of about 30 % water vapor, is about uuq = IO-5 m s-i. This results in Kj , = 0.904 which is rather close to 1, so that the effect of the liquid phase mass transfer resistance on the selectivity of an open distillation process with a free gas-liquid interface in most cases can be ignored. If, however, kuq becomes very small (as in the pervaporation process described in the next example), Kuq might become very small and thus reduce the selectivity of the open distillation process practically down to zero. 

Condensation of mixed vapors of immiscible liquids is not well understood. The conservative approach is to assume that two condensate films are present and all the heat must be transferred through both films in series. Another approach is to use a mass fraction average thermal conductivity and calculate the heat-transfer coefficient using the viscosity of the film-forming component (the organic component for water-organic mixtures). 

Steam produced in a boiler is frequently wet — thai is, it is a mist composed of saturated water vapor and entrained liquid droplets. The quality of a wet steam is defined as the fraction of the mixture by mass that is vapor. 

Measurements of atmospheric water vapor on Mars have found D/H values —5 times that of the Earth and have been fractionated due to Jeans escape of hydrogen to space (Owen et al., 1988), and 2-5 times the terrestrial value in SNC meteorites (Watson et al., 1994). Morphological data (Carr, 1986) and modeling of hydrogen atmospheric losses (Donahue, 1995) suggest that originally there may have been the equivalent of up to 500 m of water, or —7 X 10 g H2O. The total mass of Mars is 0.11 times that of the Earth, and so both planets originally may have had similar bulk water concentrations. 

For air-water vapor mixtures at 298 K, the mass and tliemial diffusivilies arc = 2.5 X 10 mVs and a - 2.18 X 10 mVs and thus the I.ewis number is Le = 0f/D,jB = 0.872. (We simply use the or value of dry air instead of the moist air since the fraction of vapor in the air at atmospheric conditions is low.) Theif(a/Z),tg) = 0.872 - = 0.913, which is close to unity. Also, the Lewis number is relatively insensitive to variaiions in temperature. Therefore, for air-water vapor mixtures, the relation between heat and mass ttansfet coefficients can be expressed with a ood accuracy as... 

Consider, for example, the evaporation and iran.sfer of water vapor into air in an air washer, an evaporative cooler, a vs el cooling tower, or just at the free surface of a lake or river (Fig. 14- 8). Even at a temperature of 40"C, the vapor pressure at the water surface is the saturation pressure of 7.4 kPa, which corresponds to a mole fraction of 0.074 or a mass fraction of w, = 0.047 for the vapor. Tlien the mass fraction difference across the boundary layer will be, at most, A i> — 0.047 - 0 = 0.047. For the evaporation of... 

SOLUTION The liquid layer on the inner surface of a circular pipe is dried by blowing air through it, The mass transfer coefficient is to be determined. Assumptions 1 The low mass flux model and thus the analogy between heat and mass transfer is applicable since the mass fraction of vapor in the air is low (about 2 percent for saturated air at 300 K). 2 The flow is fully developed, Properties Because of lov/ mass flux conditions, v/e can use dry air properties for the mixture at the specified temperature of 300 K and l atm, for which v = 1.58 X 10 mVs (Table A-15). The mass diffusivity of water vapor in the air at 300 K is determined from Eq. 14-15 to be... 

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