Conduction, thermal metal walls

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... 

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,... 

There are three resistances/coefhcients that must be considered in a jacket-cooled CSTR. There is a film coefficient hin at the inside wall of the vessel, a thermal conductivity km of the metal walls and a him coefficient hout at the outside surface of the wall ... 

The submerged metal belt (Fig. ll-53b) is a special version of the metal belt to meet the peculiar handling properties of pitch in its solidification process. Although adhesive to a dry metal wall, pitch will not stick to the submerged wetted belt or rubber edge strips. Submergence helps to offset the very poor thermal conductivity through two-sided heat transfer. 

It has been assumed that the metal wall of the reactor has little heat transfer resistance but if it has appreciable thickness this must be taken into account. If Atf. is the thermal conductivity of the wall and and <4 its inner and outer diameters we may define a sort of Stanton number for the wall ... 

The reciprocal of this coefficient, I//1, is called a thermal resistance. For conduction through a solid, such as a metal wall of thickness and thermal conductivity k, the thermal resistance equals x, /k. Appropriately corrected for changes in area, the individual resistances may be added to give the overall resistance 1/U. 

Influence of Transverse Heat Conduction in Wall. The thermal resistance for heat conduction in the wall thickness direction is considered zero in all of the preceding e-NTU results. This is a good idealization for metal matrices with thin walls. For most rotary regenerators, the thermal resistance in the transverse direction is negligible except possibly for ceramic regenerators. 

J. Hone, M. Whitney, A. Zettl, Thermal conductivity of single-walled carbon nanotubes. Synthetic Metals, 103 (1-3), 2498-2499,1999. 

Solving, 811 — Tj = 5.5 K and Tj = 805.5 K. Only a small temperature drop occurs across the metal wall because of its high thermal conductivity. 

The overall heat-transfer coefficient U in an evaporator is composed of the steam-side condensing coefficient, which has a value of about 5700 W/m K (1000 btu/h ft °F) the metal wall, which has a high thermal conductivity and usually has a negligible resistance the resistance of the scale on the liquid side and the liquid film coefficient,... 

The selection of materials for these applications is often a compromise between the requirements of the process flow and the type of water. Associated with such heat exchangers are pumps, pipes, and valves to distribute the water and return it to source. The various metals commonly used in heat exchangers have quite different thermal conductivities (Table 8.9). However, the thermal conductivity of the metal wall is only one component of the resistance to heat transfer in a heat exchanger tube. In a condenser (i.e., where steam is condensing on cold tubes), for example, the resistance to heat transfer through a tube wall is made up of five main components as illustrated in Fig. 8.15 [8]. The tube wall resistance is comparatively small so that changes in thermal conductivity from the use of different metals in not necessarily very significant. 

The total heat delivered at the frost surface passes through the frost layer and metal wall of the vessel by conduction mechanisms. On the liquid oxygen side, heat transfer is evidenced by an increase in the sensible heat and by the boiling of the liquid. The temperature drop across the boiling film and the metal wall caused by the total heat flow is quite small in comparison to the temperature drop across the frost and the outside air film. The effect of the thermal resistance of the wall and the boiling film on heat transfer is small. Therefore, for our purposes this can be neglected. 

Sizing of heat exchangers assumes a certain heat-transfer efficiency between the bulk fluid and metal wall. Because biofilms more or less behave like gels on the metal surface, heat transfer can occur only by conduction through the biofilm. The thermal conductivity of biofilms is similar to that of water but much less than that of metals. On the basis of relative thermal conductivities (Table 2.37), a biofilm layer 41 p,m thick offers the same resistance to heat transfer as a titanium tube wall 1000 p,m thick. 

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