Convection effect, condensation

It is discovered that in the cooling tower the water moving downward from the jets changes its direction to upward after drop formation. There is an effective heat transfer process when the drops move upward heat transfers from the outlet air to the drops through convection and condensation. 

A similar effect occurs if the air is brought into contact with a solid surface, maintained at a temperature below its dew point. Sensible heat will be transferred to the surface by convection and condensation of water vapour will take place at the same time. Both the sensible and latent heats must be conducted through the solid and removed. The simplest form is a metal tube, and the heat is carried away by refrigerant or a chilled fluid within the pipes. This coolant must be colder than the tube surface to transfer the heat inwards through the metal. 

For the realization of all aerosol forcing mechanisms in integrated systems it is necessary to improve not only ACTMs, but also NWP/MetMs. The boundary layer structure and processes, including radiation transfer, cloud development and precipitation must be improved. Convection and condensation schemes need to be adjusted to take the aerosol-cloud microphysical interactions into account, and the radiation scheme needs to be modified to include the aerosol effects. 

Neglecting convection effects in the film (i.e., assuming pure conduction in the film, which yields a linear temperature profile), an energy balance on a differential slice of condensate of width dz (Fig. 14.4) gives... 

Rohsenow [13] showed that if the condensate temperature profile was allowed to be nonlinear to account for convection effects in the condensate film, an improved correction term, i tg = itg + 0.68c,f( Ts - TJ) results. Another correction pertains to the variation of viscosity with temperature. For the assumed linear temperature profile in the condensate, Drew [14] showed that if l/pf is linear in temperature, then the condensate viscosity should be calculated at a reference temperature equal to T, - Yt(T, - Tw). 

For practical values of H and Prf, Eq. 14.33 was found to be near unity, indicating that acceleration and convection effects are negligible. Chen [34] included the effect of vapor drag on the condensate motion by using an approximate expression for the interfacial shear stress. He was able to neglect the vapor boundary layer in the process and obtained the results shown in Fig. 14.8. The influence of interfacial shear stress is negligible at Prandtl numbers of ordinary liquids (nonliquid metals, Pr< > 1). Chen [34] was able to represent his numerical results by the approximate (within 1 percent) expression ... 

The approximate analysis of Nusselt has been applied to a wide variety of geometries. Common assumptions include uniform wall temperature, saturated and quiescent vapor, no interfacial resistance at the liquid-vapor interface, no momentum and convection effects in the condensate, and no variation of properties with temperature. The resulting equations are valid for ordinary liquids (i.e., nonliquid metals) provided (cpeATIi(g) is less than about 0.2 to 0.5. 

Shigechi et al. [112] conducted a boundary layer analysis of this problem and included momentum and convection effects in the condensate film. They obtained different solutions using as a boundary condition various inclination angles of the liquid-vapor interface at the plate edge. Their maximum average Nusselt number was found to agree well with Eq. 14.98. Chiou et al. [214] included surface tension in their model and showed that heat transfer decreases in relation to Eq. 14.98 as the surface tension of the condensate increases. 

Equation 1 is valid for diffusion through a stationary gas. It has been successfully used to describe the experimental results which follow, thereby indicating that at close spacing between the condenser and evaporation surface, the air gap is diffusion-controlled with little or no convection effect. 

With air coolers, louver closure is considered a total failure (10). Upon fan failure, or a fan drive (e.g., power or steam) failure, a credit is often taken for natural convection effects. This credit is usually 20 to 30 percent of the normal duty of induced-draft condensers. Forced-draft condensers have a considerably weaker chimney effect, and the credit taken is usually 10 to 15 percent of their normal duty. The above natural draft credit may not apply if a fire occurs near the cooler. 

The temperature and humidity distribution in the outer boundary layer of frosted systems should be investigated more thoroughly in an effort to shed more light on the combined mechanisms of convection and condensation in this area. Accurate measurements in this region are extremely difficult to obtain. The presence of the measuring device in this boundary layer furnishes sites for condensate deposit, with a siibsequent unknown effect on the measurements obtained. Optical methods might lend themselves to this application. 

Sources of error in TG There are number of sources of error in TG and they can lead to inaccuracies in the recorded temperature and weight data. Some of these errors can be corrected and others are interrelated and can not be assessed separately. The list of main sources of errors are (i) buoyancy effect of sample container (ii) random fluctuations of balance mechanism (iii) electrostatic effects on balance mechanism (iv) condensation on balance suspension (v) measurement of weight by balance (vi) convection effects from furnace (vii) turbulence effects from gas flow (viii) induction effects from furnace (ix) measurement of temperature by thermocouple and (x) reaction between sample and container. 

Dry air rising in the atmosphere has to expand as the pressure in the atmosphere decreases. This pV work decreases the temperature in a regular way, known as the adiabatic lapse rate, Td, which for the Earth is of order 9.8 Kkm-1. As the temperature decreases, condensable vapours begin to form and the work required for the expansion is used up in the latent heat of condensation of the vapour. In this case, the lapse rate for a condensable vapour, the saturated adiabatic lapse rate, is different. At a specific altitude the environmental lapse rate for a given parcel of air with a given humidity reaches a temperature that is the same as the saturated adiabatic lapse rate, when water condenses and clouds form Clouds in turn affect the albedo and the effective temperature of the planet. Convection of hot, wet (containing condensable vapour) air produces weather and precipitation. This initiates the water cycle in the atmosphere. Similar calculations may be performed for all gases, and cloud layers may be predicted in all atmospheres. 

At the moment the cloud microphysics-aerosol interaction is included in HIRLAM in a very simple way in the convection schemes, where the cloud condensation nuclei have a lower concentration than over land. Enviro-HIRLAM includes the aerosol dynamics and their indirect effects on meteorology. The use of aerosol may also be prepared by making a 3D field of aerosol that has the characteristics of the currently prescribed values, then the extension to a real 3D distribution of aerosols that can interact with the microphysics is relatively straightforward. Sensitivity studies are needed to understand the relative importance of feedbacks. First experience of Enviro-HIRLAM indicates some sensitivity to effective droplet size modification in radiation and clouds. 

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