Water-side mass transfer velocities

Now we need the air-side and the water-side mass transfer velocities ... 

FIG U RE 12.1 Product of the mass transfer Stanton number (St = k/ut) and Schmidt number (Sc = v/D) vs plate Reynolds number (Re = u LIv) with Schmidt number as parameter (Modified from the original Higashino, M. and M.G. Stefan. 2004. Water Environmental Research 76, 292-300.) kc is the water-side mass transfer coefficient at the sediment-water interface (cmh ), V is the kinematic viscosity of water (cm h ), Z) is the diffusivity of gypsum in water (cm h ), is the friction velocity at the sediment-water interface (cmh ), L is the gypsum plate length (cm). 

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 ... 

The rise velocity in water of an air bubble 0.004 m in diameter is about 0.2 m/s. Estimate the liquid-side mass transfer coefficient k, for oxygen transfer at 23 °C,... 

The IMW scale DWT steam generator test model has been operated since 1S>91. So far, good thermal hydraulic performance was validated. Four of 10 heat transfer tubes were plugged to obtain thermal hydraulic performance at higher water side mass velocity. 

For a given values of the gas and liquid volume flow rates, the film thickness 5 and the mean film flow rate v can be found with eqs. (4.67) and (4.68). If we assume that there are no ripples, we find that for an air-water system the effect of the gas flow on the film Aicbiess and film velocity may be considerable when the gas flow rate is on the order of 30 m/s or higher. When the film flow is laminar, the liquid side mass transfer coefficient k can be estimated by... 

Both the heat and mass transfer coefficients are functions of air velocity. However, at air speeds greater than about 15 ft/s (4.5 m/s), the ratio h kgis approximately constant. The wet-bulb depression is directly proportional to the difference between the humidity at the surface and the humidity in the bulk of the air. In the wet- and dry-bulb hygrometer, the wet-bulb depression is measured by two thermometers, one of which is fitted with a fabric sleeve wetted with water. These thermometers are mounted side by side and shielded from radiation, an effect neglected in the derivation above. Air is drawn over the thermometers by means of a small fan. The derivation of the humidity from the wet-bulb depression and a psychrometric chart are discussed later. 

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. 

Fig. 16A. Liquid side controlled volumetric mass transfer coefficient for desorption of oxygen om water for plastic Ralu-Flow 25 mm [17] versus liquid supcrficM velocity .

CFaT riverine models were presented for both the water column and bed sediment. They were then simplified to focus onto the non-flow resuspension soluble fraction using the quasi-steady state assumption to isolate the key water-side and sediment-side process elements. Field evidence of soluble release based on CFaT model derived data was reviewed for three rivers. Both the traditional particle background resuspension process and more recent soluble fraction process algorithms data interpretation were covered. Numerical field calibrated resuspension velocities and soluble mass-transfer coefficients were presented. Candidate water-side and sediment-side transport processes, selected from the literature were reviewed. Those that provided the best theoretical explanation and contained laboratory and/or field data support were selected. Finally, the flux and the overall transport coefficient which captures the essential features of the framework were presented. Following this the theoretical mass-transfer coefficients were applied to a site on the Fox River below De Pere Dam. Numerical calculations were made for the transport coefficients for both individual and combined processes. 

On the air side of recuperators, heat transfer from the separating wall to the air takes place almost entirely by convection. The radiation absorbing capacity of the small amount of water vapor in the air is practically zero. The coefiicient of heat transfer by convection increases rapidly with the mass velocity (i.e., the product of Velocity x Density) of the air or gases. 

The basic concept of the experiment is illustrated in Figure 1. Tens-of-kilograms quantities of mm-sized steel b s are heated to a uniform temperature (up to 1000 C), then transferred to an intermediate container equipped with a dumping mechanism, and within a few seconds are released into a pool of saturated (atmospheric pressure) water. The pool cross section is rectangular, 40.5 cm on the side. The major experimental parameters are pool depth (15, 25 and 50 cm), particle size (1.5 and 2.4 mm), particle temperature (600 to 1000 C), pour diameter (12 and 20 cm), and particle entry velocity (corresponding to free fall from 5, 15, and 25 cm, with an initial velocity of 0.72 m/s). The initial velocity was obtained from high-speed movies and found to be independent of particle size or the particle depth in the intermediate container. From this and the measured toM mass pour rate, the particle volume fraction at the outlet of the intermediate container could also be obtained as 1.87 and 2.5% for the 2.4 and 13 mm particles, respectively. Temperature losses in the intermediate container were minor, and the actual temperature of the particulate just before being released was reported. 

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