Buildings, flow patterns

Heselberg, P., S. Murakami, and C.-A. Roulet. 1996. Annex 26 Air flow patterns m large enclosures. In Ventilation of Large Spaces in Buildings. Part 3 Analysis and Prediction Techniques. LEA,... 

Figure 7.96 illustrates typical flow patterns for wind directly approaching a building face. Airflow in the undisturbed zone has a speed profile dependent on the terrain roughness and the level of atmospheric stratification. Obviously, most wind will be deflected around and over the building. Wind... 

I he flow pattern depends on the building s relative dimensions long buildings (along wind length, L > 2.5H) versus short buildings L 2.5H). 

FIGURE 7.96 Flow pattern around a rectangular building. (Reproduced from ASHRAE 1999.)... 

Wind tunnel A fan-assisted test rig used to determine the air forces and flow patterns acting on model buildings or components. 

As mentioned in Section 11.3, fluidized-bed reactors are difficult to scale. One approach is to build a cold-flow model of the process. This is a unit in which the solids are fluidized to simulate the proposed plant, but at ambient temperature and with plain air as the fluidizing gas. The objective is to determine the gas and solid flow patterns. Experiments using both adsorbed and nonadsorbed tracers can be used in this determination. The nonadsorbed tracer determines the gas-phase residence time using the methods of Chapter 15. The adsorbed tracer also measures time spent on the solid surface, from which the contact time distribution can be estimated. See Section 15.4.2. 

Figure 2.2 shows the cash flow pattern for a typical project. The cash flow is a cumulative cash flow. Consider Curve 1 in Figure 2.2. From the start of the project at Point A, cash is spent without any immediate return. The early stages of the project consist of development, design and other preliminary work, which causes the cumulative curve to dip to Point B. This is followed by the main phase of capital investment in buildings, plant and equipment, and the curve drops more steeply to Point C. Working capital is spent to commission the plant between Points C and D. Production starts at D, where revenue from sales begins. Initially, the rate of production is likely to be below design conditions until full production is achieved at E. At F, the cumulative cash flow is again zero. This is the project breakeven point. Toward the end of the projects life at G, the net rate of cash flow may decrease owing to, for example, increasing maintenance costs, a fall in the market price for the product, and so on.

Experience has shown that a concave-downward (Fig. 17-10f) gas distributor is a better arrangement than a concave-upward (Fig. 17-10-e) gas distributor, as it tends to increase the flow of gases in the outer ortion of the bed. This counteracts the normal tendency of the gas to ow into the center of the bed after it exits the gas distributor. In addition, the concave-downward type of gas distributor tends to assist the eneral solids flow pattern in the bed, which is up in the center and own near the walls. The concave-upward gas distributor tends to have a slow-moving region at the bottom near the wall. If solids are large (or if they are slightly cohesive), they can build up in this region. 

Figure 5.4. Hydrogen accident in Stockholm street 1983 (a, above), and analysis (b) of flow pattern and H2 concentration (shaded areas) after 10 s in a vertical plane halfway between the canisters on the truck and the building wall. (From A. Venet-sanos, T. Huld, P. Adams, J. Bartzis (2003).. Hazardous Mat. A105,1-25. Used by permission from Elsevier.)...

Several sophisticated techniques and data analysis methodologies have been developed to measure the RTD of industrial reactors (see, for example, Shinnar, 1987). Various different types of models have been developed to interpret RTD data and to use it further to predict the influence of non-ideal behavior on reactor performance (Wen and Fan, 1975). Most of these models use ideal reactors as the building blocks (except the axial dispersion model). Combinations of these ideal reactors with or without by-pass and recycle are used to simulate observed RTD data. To select an appropriate model for a reactor, the actual flow pattern and its dependence on reactor hardware and operating protocol must be known. In the absence of detailed quantitative models to predict the flow patterns, selection of a model is often carried out based on a qualitative understanding of flow patterns and an analysis of observed RTD data. It must be remembered that more than one model may fit the observed RTD data. A general philosophy is to select the simplest model which adequately represents the physical phenomena occurring in the actual reactor. 

Fluid mechanical studies have shown how flows around individual obstacles become significantly distorted in the presence of nearby obstacles depending on the ratio b/d of the breadth b to separation distance d from the nearest obstacle, on the ratio b/w of breadth b to the width w of the obstacle and on the relative height to width ratio H/w. When there are many obstacles, as in an urban areas or in engineering flows, these flow interactions build up into characteristic flow patterns, which we now examine. 

As an illustration of these wind speed variations. Table I presents sample calculations for the three classes of structures and two (extreme) values of Zq- We see that the urban-rural variations are the most extreme for smaller objects. Note also that in an urban area with regularly and closely spaced buildings, wind flow patterns will be highly irregular, depending on direction with respect to street orientation, for example (Figure 1). The dramatic increase in turbulence intensity in urban areas is also shown. 

Figure 1. Flow patterns around a building (a) at various wind directions (b) in relation to building heights.

Build and operate single cells and protot5 e DMFC stacks with different anode and cathode catalysts, membrane materials, flow patterns and optimized MEAs to maximize performance and demonstrate stability. 

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