Composite wall

Fig. 1. Sketch of heat flow through (a) a plane wall where the arrow indicates the direction of heat flow and (b) a series of composite walls, (c) The...

The main fluids of interest with plastics are oxygen and water vapour (for packaging applications) and CO2 (for carbonated drinks applications). Fig. 1.13 and Fig. 1.14 illustrate the type of behaviour exhibited by a range of plastics. In some cases it is necessary to use multiple layers of plastics because no single plastic offers the combination of price, permeation resistance, printability, etc. required for the application. When multi-layers are used, an overall permeation constant for the composite wall may be obtained from... 

Solution Although the composite wall is curved, the r/h) value is large and so it can be analysed using the method illustrated for laminates. In this case, each ply is isotropic and so the properties do not vary with 6. It is thus necessary to get Q for each ply relative to the centre line of the wall thickness... 

Heat transfer based resistance theory for composite wall. The heat transfer overall coefficient is calculated. 

Figure 9.1. Heat transfer through a composite wall...

Figure 9.6 shows a composite wall made up of three materials with thermal conductivities k, k2, and k2, with thicknesses as shown and with the temperatures T1, T2, T2, and jT4 at the faces. Applying equation 9.12 to each section in turn, and noting that the same quantity of heat Q must pass through each area A ... 

Compressor ami tank testing "bunker" l Sr 1 m thick composite walls 3 meters sand 40 tons gliding door InterkirfiHedwith N> during experiments Permeation High pressure cycling testing... 

Environmental barrier coatings are a type of laminar composite. As with heat transfer, diffusion in laminar composites can be modeled as steady state diffnsion throngh a composite wall, as iUnstrated in Fignre 4.56. Here, hydrogen gas is in contact with solid material A at pressnre Pi and in contact with solid B at pressnre P2. At steady state, the molar flux of hydrogen throngh both walls mnst be the same (i.e., Jh ax = Bj) and Fick s Law [Eq. (4.4)] in the x direction becomes... 

The concentrations in the solid phases, Cj and c, are determined by the solnbilities and diffusivities of hydrogen in A and B, and so they are not eqnal. The thermodynamic activity of hydrogen has a single valne at the interface, however. (Refer to Section 3.0.1 for a description of thermodynamic activity.) Hence, the treatment of diffusion flux in a composite wall is simplified by considering activity gradients rather than concentration gradients. If the dissolution of hydrogen gas in the solid follows the reaction... 

Figure 4,56 Schematic representation of concentration profiles of hydrogen diffusing through a composite wall. Reprinted, by permission, from D. R. Gaskell, An Introduction to Transport Phenomena in Materials Engineering, p. 498. Copyright 1992 by Macmillan Publishing.

Consider the diffusion of hydrogen through a composite wall comprising Ni and Pd of identical thicknesses at 400°C. The hydrogen gas pressure in contact with the Ni is 1.0 atm, and that in contact with the Pd is 0.1 atm. The following data on each metal are provided ... 

Combine your information to calculate the flux of hydrogen (in kg/m s) through the composite wall if both metals have a thickness of 1 mm. 

A Case of a Composite Wall Optimum Insulation Thickness for a Steam Line... 

Theeuwes, F, and Ayer, A. D. Osmotic devices having composite walls, U.S. Patent 4,077,407, 1978. 

Example Calculation of Heat Loss through a Composite Wall. A furnace wall is constructed of a 3-cm thick, flat steel plate, with a firebrick insulation 30-cm thick on the inside, and rock wool insulation 6-cm thick on the outside. The inside surface temperature of the firebrick insulation is 700°C. If the temperature of the outer surface of the rock wool insulation is 50°C, what is the heat flux through the wall ... 

Flfl. 2-1 One-dimensional heat transfer through a composite wall and electrical analog. 

A composite wall is formed of a 2.5-cm copper plate, a 3.2-mm layer of asbestos, and a 5-cm layer of fiber glass. The wall is subjected to an overall temperature difference of 560°C. Calculate the heat flow per unit area through the composite structure. 

Find the heat transfer per unit area through the composite wall sketched. Assume one-dimensional heat flow. 

Related Calculations. The method described for calculating the overall heat-transfer coefficient is also used to calculate the overall resistance to conduction of heat through a composite wall containing materials in series that have different thicknesses and thermal conductivities. For this case, each individual heat-transfer coefficient is equal to the thermal conductivity of a particular material divided by its thickness. The amount of heat transferred by conduction can then be determined from the formula... 

Example 2.4 Estimation of heat flow through a composite wall with constant thermal conductivities A pipe with an outside diameter of 10 cm and a length of 110 m is carrying hot fluid. The pipe is insulated with 0.5 cm thick silica foam and 10 cm thick fiberglass. The pipe wall is at 120°C and the outside surface of the fiberglass is at 30°C. Estimate the heat flow in the radial direction of the pipe. The thermal conductivities of silica foam and fiberglass are 0.055 and 0.0485 W/(m K), respectively. 

Conduction with Resistances in Series A steady-state temperature profile in a planar composite wall, with three constant thermal conductivities and no source terms, is shown in Fig. 5-3a. The corresponding thermal circuit is given in Fig. 5-3b. The rate of heat transfer through each of the layers is the same. The total resistance is the sum of the individual resistances shown in Fig. 5-3b ... 

FIG. 5-3 Steady-state temperature profile in a composite wall with constant thermal conductivities k kg, and kc and no energy sources in the wall. The thermal circuit is shown in (b). The total resistance is the sum of the three resistances shown. 

In practice we often encounter plane walls that consist of several layers of different materials. The tbermal resistance concept can still be used to detennine the rate of steady heat transfer through such composite walls. As you may have already guessed, this is done by simply notiifg that the conduction resistance of each wall i.s IJkA connected in series, and using the electrical analogy. That is, by dividing the temperature difference between two surfaces at known temperatures by the total thermal resistance between them. 

Consider a plane wail that consists of two layers (such as a brick wall with a layer of insulation). The rate of steady heat transfer through this two-layer composite wall can be expressed as (Fig. 3-9)... 

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