Air-Water Exchange in Flowing Waters

Quandt (1962) measured the values of (CfICm - 1) at various axial positions of an air-water mixture flow in a 0.25-in. X 3-in. channel and converted the raw data to the exchange mass flux, pLV(, as shown in Figure 5-23. He also measured the film velocity Vf by injecting a pulse of dye into the liquid film and recording its transport time between two photocells. Such measured data are shown in Figure 5.24. By using the measured values for Vf, the liquid film thickness t may be calculated as... 

The potential production of sulfide depends on the biofilm thickness. If the flow velocity in a pressure main is over 0.8-1 ms-1, the corresponding biofilm is rather thin, typically 100-300 pm. However, high velocities also reduce the thickness of the diffusional boundary layer and the resistance against transport of substrates and products across the biofilm/water interphase. Totally, a high flow velocity will normally reduce the potential for sulfide formation. Furthermore, the flow conditions affect the air-water exchange processes, e.g., the emission of hydrogen sulfide (cf. Chapter 4). 

Pilot sewer studies are often carried out in systems operating with recirculation. Specific care must be taken in systems where water-gas exchange processes form a part of the mass balance. Critical points are pumps and bends that may change the flow regime, air-water exchange processes, biofilm and particle structure. Figure 7.2 is a sketch of a pilot sewer used for sewer process studies (Tanaka and Hvitved-Jacobsen, 2000). 

Platinum was introduced on the activated support by a competitive cation exchange technique. An amount of 100 g of a 8 wt% Pt solution of platinumtetrammine hydroxide (Johnson Matthey) was added dropwise to a suspension of 40 g graphite in 800 ml 1 M ammonia (Merck p.a.) and stirred at ambient temperature for 24 hours. The catalyst was subsequently separated by filtration on a Millipore filter (HV 0.45m), washed with distilled water and dried in a vacuum oven at 373 K. The dried catalyst was reduced in flowing hydrogen at 573 K for 2 hours and stored under air before use. 

Before we discuss these models, we note that, in contrast to v,w, the air-phase exchange velocity, via, is not strongly affected by the flow. Thus, the following considerations are not relevant for compounds with very small Henry s law coefficients. This is no longer true when the air-water interface is broken up by bubbles and droplets. Some models attempt to incorporate the effect of air bubbles into the exchange velocity v,w (see Eq. 20-38 below), yet air bubbles also lead to a modification of Eq. 20-3 describing the overall exchange velocity, via/w. In the context of river flow, this situation will be treated in Section 24.4. 

In rivers the water-side air-water exchange velocity v,w is influenced by both turbulence produced by the wind and turbulence produced by the flow due to friction at the river bed. In this box we summarize the important concepts for flow-induced friction (see Moog and Jirka (1999a, b) for details). 

To summarize, the air-water exchange velocity increases by about a factor 2 from stretch A to B, that is, when the small eddies are replaced by large ones. A more spectacular increase (factor 20) takes place at the transition to enhanced exchange due to bubble formation (stretch C). In all cases the flow-induced air-water exchange is more important than the influence from the wind. This is in accordance with Eq. 20-34. Note that for 10 = 10 m s-1, the wind-induced turbulence would dominate in stretches A and B, but not in C. 

Air-water exchange in rivers depends on both the river flow and the wind speed. In Eq. 20-34 we have defined the critical wind speed, Hcrit, at which the influences of both forces are equal. Explain why ucnl increases with mean water depth h. 

An underground cavern of the city drinking-water supply system consists of a water reservoir (area A0 = 2000 m2, depth hK = 4 m) and an air space for maintenance above it (mean height hz = 2.5 m). The flow rate of the water is gw = 1600 m3 Ir1. The air space is exchanged in 2 hours. 

Consider a conservative chemical (no air-water exchange, no in situ reaction, no removal to the sediments) entering a chain of three lakes. A constant rate of water is flowing into lake 1 and then through lake 2 and 3 into the outlet. The mean residence... 

Winkelmann et al. (54) have studied air-water flows in a corrugated heat exchanger. Flow visualization and two-phase pressure drop measurements have been performed. The flow visualizations have shown that the flow pattern is complex and that a wavy or a film flow occurs in most cases (Figure 29). The two-phase pressure drop depends on the total flow rate and vapor quality, and Chisholm-type correlation is proposed. More work is required to characterize the flow structure in compact heat exchangers and to develop predictive methods for the frictional pressure drop and the mean void fraction. 

A microprocessor is to be programmed to control the exchanger in Prob. 10-39 by varying the water flow rate to maintain the same exit air temperature for changes in inlet water temperature. Calculate the percentage changes necessary for the water flow rate for inlet water temperatures of 60, 70, 80, and 100°C. Assume U remains constant. 

A method of determining airborne iodine has also been reported.241 Here, iodine is absorbed into 5% aqueous KI and spectrophotometrically determined at 590 nm in the form of its complex with starch. This method is selective with respect to bromine and chlorine, and the sensitivity of this method is 0.25 mg of I2 per m3 of air. The concentration of the, 31I isotope in water can be determined by a method involving isotope exchange in the starch-iodine complex.242 Flow-injection determination of ascorbic acid (0.1-40 mg/mL) has been proposed.243 Iodine is generated in the flow system as I3- ions, which are in turn exposed to starch to produce a steady signal at 350 and 580 nm. Ascorbic acid provides inversed maxima which are measured. This method is recommended for analysis of ascorbic acid in fruit juice, jam, and vitamin-C preparations. Use of the blue complex has also been reported for determination of sodium dichloro-isocyanurate in air.244 Obviously the blue reaction is applicable in the determination of amylose, amylopectin, and starch,245-252 as well as modified starches.245,253-255... 

---