Thermal sublayer

Figure 2.4 Saturated boiling liquid temperature variations within thermal sublayer, and bubble counts departing from heating surface. (From Dougall and Lippert, 1967. Copyright 1967 by American Society of Mechanical Engineers, New York. Reprinted with permission.)...

Typical mass balance methods to measure the air-sea gas transfer have one major drawback the response time is of the order of hours to days, making a parameterisation with parameters such as wind forcing, wave field, or surface chemical enrichments nearly impossible. The controlled flux technique uses heat as a proxy tracer for gases to measure the air-sea gas transfer rate locally and with a temporal resolution of less than a minute. This method offers an entirely new approach to measure the air-sea gas fluxes in conjunction with investigation of the wave field, surface chemical enrichments and the surface micro turbulence at the water surface. The principle of this technique is very simple a heat flux is forced onto the water surface and the skin-bulk temperature difference across the thermal sublayer is measured. 

Fig. 2. Naturally occurring heat fluxes at the ocean surface and transport mechanisms of heat in the thermal sublayer...

In a first realization, Jahne et al. (1989) forced a periodical heat flux density onto the water surface using a chopped heat source above the water surface. The temperature response at the water surface was detected with point measuring radiometer. In a further implementation of this technique, HauBcckcr (1996) developed the so-called passive controlled flux method that estimates the skin-bulk temperature difference under natural heat flux conditions assuming a surface renewal model. The naturally occurring heat fluxes at the ocean surface (latent, sensible and long wave radiative heat flux) cause the surface temperature to decrease or increase depending on the direction of these fluxes. The net heat flux forces a skin-bulk temperature difference AT across the thermal sublayer, commonly referred to as the cool skin of the ocean (compare Fig. 2). 

Fig. 5. Schematic of the two operational modes of the wind wave facility to estimate the skin-bulk temperature difference across the thermal sublayer...

If the air space of the wind-wave flume is flushed with dry air (relative humidity approximately 75%), a latent heat flux is established through evaporation. The surface temperature drops a few tenth of a degree ( cool skin ) and skin-bulk temperature difference is forced across the thermal sublayer. 

In the thermal conduction sublayer near solid walls, a linear law can be used. In the turbulent near-wall region, Reynolds analogy between momentum and energy transport allows to derive a logarithmic law for the mean temperature, similar to (12.5-8). In general, the thickness of the thermal conduction layer differs from the thickness of the viscous sublayer and depends on the physicochemical properties of the fluid. The thermal sublayer thickness can be estimated from the intersection between the linear law and the logarithmic law. 

Heatshield thickness and weight requirements are determined using a thermal prediction model based on measured thermophysical properties. The models typically include transient heat conduction, surface ablation, and charring in a heatshield having multiple sublayers such as bond, insulation, and substmcture. These models can then be employed for any specific heating environment to determine material thickness requirements and to identify the lightest heatshield materials. 

For turbulent flow of a fluid past a solid, it has long been known that, in the immediate neighborhood of the surface, there exists a relatively quiet zone of fluid, commonly called the Him. As one approaches the wall from the body of the flowing fluid, the flow tends to become less turbulent and develops into laminar flow immediately adjacent to the wall. The film consists of that portion of the flow which is essentially in laminar motion (the laminar sublayer) and through which heat is transferred by molecular conduction. The resistance of the laminar layer to heat flow will vaiy according to its thickness and can range from 95 percent of the total resistance for some fluids to about I percent for other fluids (liquid metals). The turbulent core and the buffer layer between the laminar sublayer and turbulent core each offer a resistance to beat transfer which is a function of the turbulence and the thermal properties of the flowing fluid. The relative temperature difference across each of the layers is dependent upon their resistance to heat flow. 

The transfer of heat and/or mass in turbulent flow occurs mainly by eddy activity, namely the motion of gross fluid elements that carry heat and/or mass. Transfer by heat conduction and/or molecular diffusion is much smaller compared to that by eddy activity. In contrast, heat and/or mass transfer across the laminar sublayer near a wall, in which no velocity component normal to the wall exists, occurs solely by conduction and/or molecular diffusion. A similar statement holds for momentum transfer. Figure 2.5 shows the temperature profile for the case of heat transfer from a metal wall to a fluid flowing along the wall in turbulent flow. The temperature gradient in the laminar sublayer is linear and steep, because heat transfer across the laminar sublayer is solely by conduction and the thermal conductivities of fluids are much smaller those of metals. The temperature gradient in the turbulent core is much smaller, as heat transfer occurs mainly by convection - that is, by... 

If the temperature gradient across the laminar sublayer and the value of thermal conductivity were known, it would be possible to calculate the rate of heat transfer by Equation 2.1. This is usually impossible, however, because the thickness ofthe laminar sublayer and the temperature distribution, such as shown in Figure 2.5, are usually immeasurable and vary with fluid velocity and other factors. Thus, a common engineering practice is the use of the film (or individual) coefficient of heat transfer, h, which is defined by Equation 2.16 and based on the difference between the temperature at the interface, and the temperature of the bulk of fluid, f], ... 

Figure 5.2 shows the temperature gradients in the case of heat transfer from fluid 1 to fluid 2 through a flat metal wall. As the thermal conductivities of metals are greater than those of fluids, the temperature gradient across the metal wall is less steep than those in the fluid laminar sublayers, through which heat must be transferred also by conduction. Under steady-state conditions, the heat flux q (kcal In m 2 or W m ) through the two laminar sublayers and the metal wall should be equal. Thus,... 

The TFC-801 membranes did not exhibit any measurable CCRO effect under equivalent test conditions. These results are consistent with our previous work on osmotic membranes. (9,) Thin-film-composite desalination membranes derived from polyamine precursors (such as TFC-801) contain a gel layer within the porous sublayer formed by thermal crosslinking of the polyamine.(10) This gel layer retards the diffusion of ethanol into the membrane from the recirculation solution. Thus the ethanol concentration inside the porous sublayer is not increased effectively by increasing the concentration of the recirculation solution. By contrast, Loeb-Sourirajan-type asymmetric membranes or composite membranes derived from monomeric precursors do not contain any gel layer. Such membranes are better suited to use in the CCRO process. 

If the temperature gradient across the laminar sublayer and the value of thermal conductivity were known, it would be possible to calculate the rate of heat transfer by Equation 2.1. This is usually impossible, however, because the thickness of the... 

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