Thermal plumes

In many calculation methods, the momentum contributions to plume rise are considered negligible when compared to the thermal plume rise, and hence are ignored. 

FIGURE 7.9 Influence of exhausced airflow on airflow pattern in the naturally ventilated room fo> airflow in the convective plume smaller than exhausted airflow (b) airflow in the convective pluime equal to the exhausted airflow (c) airflow In the thermal plume at the stratification level equal to the exhausted airflow (t, air temperature along the room height, t, - average room temperaoire)... 

Thermal plumes above point (Fig. 7.60) and line (Fig. 7.61) sources have been studied for many years. Among the earliest publications are those from Zeldovich and Schmidt. Analytical equations to calculate velocities, temperatures, and airflow rates in thermal plumes over point and line heat sources with given heat loads were derived based on the momentum and energy conservation equations, assuming Gaussian velocity and excessive temperature distribution in... 

FIGURE 7.59 Thermal plume above a horizontal surface. 

TABLE 7.19 Characteristics of Thermal Plumes above Point and Line Sources... 

In reality, heat sources are seldom a point, a line, or a plane vertical surface. The most common approach to account for the real source dimensions is ro use a virtual source from which the airflow rates are calcu-lared " " see Fig. 7.64. The virtual origin is located along the plume axis at a distance on the other side of the real source surface. The adjustment of the point source model to the realistic sources using the virtual stmrce method gives a reasonable estimate of the airflow rate in thermal plumes. The weakness of this method is in estimating the location ol the virtual point source. 

Calculate maximum air velocity, airflow rate, and excessive temperature (relative to the ambient air temperature equal to 20 °C) in thermal plume above the heated cube (0.66 m x 0.66 m x 0.66 m) with convective heat production = 225 W, at heights of 2.0 m and 4.0 m above the floor level. Neglect temperature gradient along the room height. Compare the results with predictions made for the same case using CFD code (Fig. 7.80). 

To calculate the thermal plume, the cube can be presented as a cylinder with a diameter equivalent To the hydraulic diameter of the top of the cube ... 

The maximum excessive temperature in the thermal plume, from Table 7.19, is... 

Kofoed, P. 1991. Thermal plumes in ventilated rooms. Ph.D. thesis. University of Aalborg, Denmark. 

Aksenov, A.A., A.V. Gudzovski, E.O. Shilkrot, and A.M. Zhivov. 1998. Thermal plumes above heat sources in rooms with a temperature stratification. In Roomvent 98. 

The use of a natural ventilation system assumes temperature stratification throughout the room height. Air close to heat sources is heated and rises as a thermal plume (Fig. 7.105). Part of this heated air is evacuated through air outlets in the upper zone, and part of it remains in the upper zone, in the so-called heat cushion. The separation level between the upper and lower zones is defined in terms of the equality of and G, which are the airflow rate in thermal plumes above heat sources and the airflow supplied to the occupied zone, respectively. It is assumed that the air temperature in the lower zone is equal to that in the occupied zone, and that the air temperature in the upper zone is equal to that of the evacuated air,... 

Typically, the share of the static pressure across the inlets, p, is selected to be between 0.1 and 0.4. This allows one to keep a low velocity of airflow through inlets so as not to disturb thermal plumes above heat sources. 

Buoyancy forces creating vertical air movement along tbe passage between two rooms located on different levels, or thermal plumes creating temperature and contaminant differences between two zones located on different levels of the same room (Fig. 7.108c). 

Thermal plumes originating from machines or high-temperature processes... 

Convection is the heat transfer in the fluid from or to a surface (Fig. 11.28) or within the fluid itself. Convective heat transport from a solid is combined with a conductive heat transfer in the solid itself. We distinguish between free and forced convection. If the fluid flow is generated internally by density differences (buoyancy forces), the heat transfer is termed free convection. Typical examples are the cold down-draft along a cold wall or the thermal plume upward along a warm vertical surface. Forced convection takes place when fluid movement is produced by applied pressure differences due to external means such as a pump. A typical example is the flow in a duct or a pipe. 

Thermal updraft The air movement that is created by a thermal plume. 

EXAMPLE 4.6 Measurements of near-field thermal plume downstream of a power plant (similitude in heat transport)... 

Field measurements of a thermal plume downstream of a thermal power plant, illustrated in Figure E4.6.1, are required to determine the impact of the heated water on the river biota. These are near field because they occur before the river is mixed across its width and depth. Since field measurements are expensive and time consuming, it is desirable to select the measurements on which to concentrate the effort. 

The discharge of warm wastewaters into a surface receiver may have many adverse effects on aquatic life. The increase in temperature results in a decrease in the oxygen concentration in water and the elimination of the most sensitive species. Temperature changes may also cause changes in the reproductive periods of fishes, growth of parasites and diseases, or even thermal shock to the animals found in the thermal plume. 

Given the opposing signs of the Clapeyron slopes of the primary phase transitions associated with these seismic discontinuities, any elevated mantle temperatures associated with thermal plumes may be expected to yield thinning of the transition zone (Figure 2), via depression of the 410 and uplift of 660 (Shen et al., 1998 Bina, 1998c Lebedev et al., 2002). Some global and... 

Courtney R. and White R. (1986) Anomalous heat flow and geoid across the Cape Verde Rise evidence for dynamic support from a thermal plume in the mantle. Geophys, J. Roy. Astr. Soc. 87, 815 - 867. 

In this schlieren image of a girl, the rise of lighter, warmer air adjacent to her body indicates that hrnnans and warm-blooded animals are suirounded by thermal plumes of rising waim air. 

Figure 8.13 LES simulation of 2-d thermal plume growing along ground surface 116 seconds after ignition. No canopy Top - temperature bottom - velocity field.

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