Microlayering

Because of their hydrophobic nature, siUcones entering the aquatic environment should be significantly absorbed by sediment or migrate to the air—water interface. SiUcones have been measured in the aqueous surface microlayer at two estuarian locations and found to be comparable to levels measured in bulk (505). Volatile surface siloxanes become airborne by evaporation, and higher molecular weight species are dispersed as aerosols. 

Zhao YH, Masuoka T, Tsuruta T (2002) Unified theoretical prediction of fully developed nucleate boiling and critical heat flux based on a dynamic microlayer model. Int J Heat Mass Transfer 45 3189-3197... 

Hardy, J. T., Apts, C. W., Crecelius, E. A. and Fell-ingham, G. W. (1985). The sea-surface microlayer fate and residence times of atmospheric metals. Limnol. Oceanog. 30, 93-101. 

Maguire RJ, Kuntz KW, Hale EJ. 1983. Chlorinated hydrocarbons in the surface microlayer of the Niagara River. J Great Lakes Res 9 281-286. 

Heussner S, Cherry RD, Heyraud M (1990) Po-210 and Pb-210 in sediment trap particles on a Mediterranean continental margin. Cont. Shelf Res 10 989-100 Heyraud M, Cherry RD (1983) Correlation of Po-210 and Pb-210 enrichments in the sea-surface microlayer with neuston biomass. Cont Shelf Res 1 283-293 Honeyman BD, Santschi PH (1989)The role of particles and colloids in the transport of radionuclides and trace metals in the oceans. In Environmental particles. Buffle J, van Leewen HP (eds) Lewis Publishers, Boca Raton, p 379-423... 

Diisopropyl methylphosphonate is slightly soluble in water (0.1—0.3 g/L at 25°) and has been demonstrated in laboratory studies to quickly diffuse between the surface microlayer into the water column after deposition as aerosols on fresh water (Van Voris et al. 1987). The solubility of diisopropyl methylphosphonate was 80 g/L (8%) and remained in solution even when the temperature was lowered to freezing (Bucci et al. 1997). In addition, there was no significant loss of diisopropyl methylphosphonate from the water column to the atmosphere, in either the presence or absence of a light wind over the water surface. Human exposure resulting from the vaporization of diisopropyl methylphosphonate from surface water is considered insignificant (EPA 1989). 

Evaporation-of-microlayer theory. A later hypothesis for the mechanism of nucleate boiling considers the vaporization of a micro layer of water underneath the bubble. This was first suggested by Moore and Mesler (1961), who measured... 

Figure 2.13 A newly formed vapor bubble illustrating an evaporating liquid microlayer. 

Microlayer evaporative mechanism (Fig. 2.19c) Section 2.2.5.5 gives the model for this mechanism. A theoretical heat transfer rate is possible through evaporation of a fluid into a receiver ... 

Estimation of microlayer evaporation The model, incorporating the evaporation from a microlayer surf ace underneath a bubble attached to the heater surf ace, was used by Hendricks and Sharp (1964). With water as the fluid, at somewhat subcooled conditions, the heat transfer rates were as high as 500,000 Btu/hr ft2, or... 

Figure 2.24 Comparison of experimental results of bubble period with predictions of a model involving different mechanisms (a) nucleate boiling only (b) nucleate boiling and natural convection (c) nucleate boiling, natural convection, and microlayer evaporation. (From Judd, 1989. Copyright 1989 by American Society of Mechanical Engineers, New York. Reprinted with permission.)...

A boiling heat transfer model incorporating nucleate boiling, natural convection, and microlayer evaporation was formulated as... 

Judd (1989) interpreted experimental results of Ibrahim and Judd (1985), in which the bubble period first increased and then decreased as subcooling varied over the range 0 < (7 t - Tm) < 15°C (27°F), by means of a comprehensive model incorporating the contributions of nucleate boiling, natural convection, and microlayer evaporation components. The mechanism responsible for the nucleation of bubbles at exactly the frequency required at each level of subcooling is the subject of their continuing research. 

Readers should note that in Section 2.2.5.5, the term microlayer is used for the liquid sublayer beneath a single bubble. The macrolayer here includes a liquid sublayer and vapor stems. It is the same layer as shown in Figure 5.21. 

The basic mechanism of dryout almost invariably involves the rupture of a residual thin liquid film, either as a microlayer underneath the bubbles or as a thin annular layer in a high-quality burnout scenario. Bankoff (1994), in his brief review of significant progress in understanding the behavior of such thin films, discussed some significant questions that still remain to be answered. 

Heat transport by continuous evaporation at the root of the bubble and condensation at the top of the bubble, while the bubble is still attached to the wall, q"b2 (microlayer evaporation)... 

An improved CHF model for low-quality flow The Weisman-Pei model was later improved by employing a mechanistic CHF model developed by Lee and Mu-dawwar (1988) based on the Helmholtz instability at the microlayer-vapor interface as a trigger condition for microlayer dryout (Fig. 5.21). The CHF can be expressed by the following equation due to the energy conservation of the microlayer (Lin et al. 1989) ... 

If the local liquid enthalpy flowing into the microlayer is assumed to be independent of bulk subcooling, it can be approximated by the saturated liquid enthalpy, //L sat, near the dryout point ... 

Figure 5.21 Schematic of the onset of microlayer dryout. (From Lee and Mudawwar, 1988. Copyright 1988 by Elsevier Science Ltd., Kidlington, UK. Reprinted with permission.)...

Dwyer, O. E., and C. J. Hsu, 1975, Liquid Microlayer Thickness in Nucleate Boiling on a Heated Surface, Lett. Heat Mass Transfer 2 119. (2)... 

Dzakowic, G. S., 1967, An Analytical Study of Microlayer Evaporation and Related Bubble Growth Effects in Nucleate Boiling, Ph.D. thesis, University of Tennessee, Knoxville, TN. (2)... 

Fath, H. S.,andR. L. Judd, 1978, Influence of System Pressure on Microlayer Evaporation Heat Transfer, Trans. ASME, J. Heat Transfer 100 49 55. (2)... 

Jawurek, H. H., 1969, Simultaneous Determination of Microlayer Geometry and Bubble Growth in Nucleate Boiling, Int. J. Heat Mass Transfer 12 843. (2)... 

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