Water typical transfer velocity

Transfer velocity across gaseous boundary layer typically between 0.1 and 1 cm s"1 (up to 5 cm s 1, see Fig. 20.2). Km is the nondimensional liquid/gas distribution coefficient (for air-water interface inverse nondimensional Henry s law coefficient, i.e., Jfr w) with typical values between 10-3 and 103. DA is the molecular gaseous diffusivity, typical size 0.1 cm2s . 

Figure 20.1 Schematic view of the overall air-water exchange velo-city, via/w, as a function of the air-water partition coefficient, Ku/w, calculated from Eq. 20-3 with typical single-phase transfer velocities v,a = 1 cm s"1, vM = 10 3 cm s1. The broken line shows the exchange velocity v a/w (air chosen as the reference system). The upper scale gives the Henry s Law coefficient at 25°C, Km = 24.7 (Lbar mol"1) x Ku/W.

Mathematical models for mass transfer at the NAPL-water interface often adopt the assumption that thermodynamic equilibrium is instantaneously approached when mass transfer rates at the NAPL-water interface are much faster than the advective-dispersive transport of the dissolved NAPLs away from the interface [28,36]. Therefore, the solubility concentration is often employed as an appropriate concentration boundary condition specified at the interface. Several experimental column and field studies at typical groundwater velocities in homogeneous porous media justified the above equilibrium assumption for residual NAPL dissolution [9,37-39]. 

Convection dryers are also used to heat and dry substrates. Typically, high velocity heated air is blown at the substrate from both sides so that the substrate is elevated between the nozzles. In many cases, the heated air is used for both heat and mass transfer, to volatilize any liquids on or in the substrate such as water, and then carry the vapor away from the substrate. 

Thomerson and Billings [87] describe field tests in which chlorine was released at up to 70kg min from three 1-ton containers. Typical wind velocities were about 9 m s . Relative humidity was very low, the test site being located in the Nevada desert. It is noteworthy that the temperature of the spilled liquid stabilized at about -50 C, well below the boiling point of —34°C. The wind subcooled the liquid, which was collected in a well-insulated pan, and approximately 50% of the chlorine vaporized during a test. The use of downwind water sprays in these tests reduced the concentration of chlorine in the air by an average of 31%. This was attributed to the induction of dilution air by transfer of momentum from the spray. As noted above, the spray also forced the vapor cloud lower, so that the concentration of chlorine at 1.5 m elevation was actually higher for a distance of 230 m fi om the point of release. In these tests, portable fire water monitors performed relatively poorly. 

Here, the particle Reynolds number is based on the slip velocity. If terminal velocity is used, then the above correlation gives the minimum value for the mass transfer coefficient. Minimum mass transfer coefficients further depend on the density difference between solid particles and solvent. For the typical case of water, the approximate values presented in Table 3.7 can be used (Harriot, 1962). 

The primary variable that determines whether the controlling resistance is in the liquid or gas film is the H or Henry constant. As shown in Figure 5.15, and as is apparent from equation 39, for small values of H the water phase film controls the transfer, and for high values of H the transfer is controlled by the air phase film. Gas transfer conditions that are liquid film controlled sometimes are expressed in terms of thickness, Zw, of the water film. As indicated by equation 38, this can be done from a measured value of (or K,o,) and the diffusion coefficient of the substance Zw decreases with the extent of turbulence (current velocity, wind speed, etc.). Typical values for are in the range of micrometers for seawater, a few hundred micrometers in lakes and up to 1 nun in small wind-sheltered water bodies (Brezonik, 1994). 

The liquid-side mass-transfer coefficient, ki a (s ), is related to the superficial gas velocity, Ejc (m/s), and the gassed power per unit volume of liquid, Pq/V, (kW/m ). The viscosity term j, / j, accounts for the effect of process viscosity on the mass-transfer coefQcient relative to standard conditions, typically water at 20°C ... 

Typical heat transfer results to monodisperse sprays impacting on a heated surface are shown in Fig. 18.24. The liquid flow rate is varied over a wide range, while the droplet diameter is kept almost constant [136]. The heat flux versus surface temperature trends are similar to those of conventional boiling curves (see Chap. 15 of this handbook), and the heat fluxes are very high. The available experimental data [133, 134,137-140] show that the volumetric spray flux V (m3/m2 s) is a dominant parameter affecting heat transfer. However, mean drop diameter and mean drop velocity and water temperature have been found to have an effect on heat transfer and transitions between regimes. Urbanovich et al. [141], for example, showed that heat transfer is not only a function of the volumetric spray flux but also of the pressure difference at the nozzle and the location within the spray field (Fig. 18.25). 

A series of tests were conducted in the furnace to compare CARS temperature measurements with those acquired with a suction pyrometer [84]. A suction pyrometer is an intrusive probe to measure gas temperature in a flame. The principle of the device is to insert the water cooled probe to the measurement point and draw furnace gases over a thermocouple located at the tip of the probe in an enclosure shielded from flame radiation. The gases are drawn at sufficient velocity to enhance the convective heat transfer to the thermocouple bead. Typically, the flow rate of furnace gases through the probe tip is increased until the thermocouple temperature no longer increases. At this point the thermocouple is assumed to measure the true gas temperature. The disturbance by such a measurement technique can be considerable in the near burner region where chemical reactions are occurring and there is heat release. 

---