Natural convection water

Physical situations that involve radiation with convection are fairly common. Examples include solar radiation interacting with the earth s environment to produce complex natural convection, water environmental studies for predicting natural convection patterns in lakes, seas and oceans, and heat transfer along copper tubes in the furnace of a boiler. 

Open Tube Sections (Air Cooled) Plain or finned tubes No shell required, only end heaters similar to water units. Condensing, high level heat transfer. Transfer coefficient is low, if natural convection circulation, but is improved with forced air flow across tubes. 0.8-1.8... 

Natural convection is self-induced and is created by the density differences, which are temperature related the boiling of water in a kettle is an example of free convection. Forced convection is caused by an external force being applied by mechanical means such as a fan or pump the cooling of a warm bottle in cool flowing water is an example of forced convection. 

The fifth type of passive system is the natural convective loop, in which the collector is placed below the living space and the hot air that is created rises to provide heat where it is needed. This same principle is nsed in passive solar hot water heating systems known as thermosiphons. The storage tank is placed above the collector. Water is heated in the collector, becomes less dense, and rises (converts) into the storage tank. Colder water in the storage tank is displaced and moves down to the collector where it is heated to continue the cycle. 

Sections (air headers similar to water transfer. if natural convection ... 

For low-temperature heating systems using natural convective or radiant appliances the normal design water flow temperature to the system is 83°C (see also Table 27.8). Boost temperatures may be used on modulated-temperature systems because of the changes in heat output characteristics with varying temperatures. Additionally, comfort aspects must be borne in mind, as forced convective emitters operating on modulated temperature systems can deliver air streams at unacceptably low temperatures. 

These operate by setting the cooling air in motion over a surface of water. Either natural convection or a fan can do... 

If a beaker containing water rests on a hot plate, the water at the bottom of the beaker becomes hotter than that at the top. Since the density of the hot water is lower than that of the cold, the water in the bottom rises and heat is transferred by natural convection. In the same way air in contact with a hot plate will be heated by natural convection currents, the air near the surface being hotter and of lower density than that some distance away. In both of these cases there is no external agency providing forced convection currents, and the transfer of heat occurs at a correspondingly lower rate since the natural convection currents move rather slowly. 

Garner and Keey(52 53) dissolved pelleted spheres of organic acids in water in a low-speed water tunnel at particle Reynolds numbers between 2.3 and 255 and compared their results with other data available at Reynolds numbers up to 900. Natural convection was found to exert some influence at Reynolds numbers up to 750. At Reynolds numbers greater than 250, the results are correlated by equation 10.230 ... 

Confined boiling of water and surfactant solutions under condition of natural convection causes a heat transfer enhancement. Additive of surfactant leads to enhancement of heat transfer compared to water boiling in the same gap size however, this effect decreased with decreasing gap size. For the same gap size, CHF decreases with an increase in the channel length. CHF in surfactant solutions is significantly lower than in water. 

The relationships developed from field measurements have been made dimensionless with the assumptions that v = 1.33 x 10 m /s and AijO = 2.6 x 10 m /s to facilitate comparisons between relations and avoid dimensional problems. They are given in Table 9.2. The early measurements were to investigate the loss of water from the reservoirs of the Colorado River in the United States, and the later measurements were designed to investigate heat loss from heated water bodies. A revelation occurred in 1969, when Shulyakovskyi brought in buoyancy forces as related to natural convection to explain the heat loss from heated water at low wind velocities. This was picked up by Ryan and Harleman (1973), who realized that natural convection could explain the need for a constant term in front of the relationship for gas film coefficient, as had been found by Brady et al. (1969), Kohler (1954), Rymsha and Dochenko (1958), and Shulyakovskyi (1969). Finally, Adams et al. (1990) rectified... 

The physical reason behind the modification at low wind speed as suggested by field data remains unclear. Yet, we should not forget that at low wind speed, the instantaneous wind is not the only significant source of motion at the water surface. Water motions caused by wind do not stop as soon as the wind ceases. Furthermore, thermal processes lead to density instabilities and convective motion, even if there is absolutely no wind. In fact, natural surface water bodies are hardly ever at rest. 

The usual specific flow-rates for extraction are very small. In terms of space velocities, these are about 5 to 15 kg/h per litre of extractor volume, with superficial velocities in the range of 0.5 to 10 mm/s. With these small velocities, natural convection mass transfer is the favoured mechanism of transport. Gas densities are in the range of 500 to 800 kg/m3, and viscosities are about 5 x 10 7 kg/(m s), thus giving kinematic viscosities of about 10 9 m2/s, which is a very small value for a fluid. For example, the kinematic viscosity of water is 10"7 m2/s and that of ambient air is 2 x 10 5 m2/s. This makes free convection a principal mechanism for mass-transfer in high pressure gases. 

The first example illustrates the behavior of a binary mixture in an open-batch distillery with a stagnant sweep gas. Figure 4.14 shows the historical set-up from 1977 [19, 23-29]. The still was heated by a Bunsen burner, while the condenser was installed in the form of a watchglass cooled by evaporation of water due to natural convection. 

Figure 4.15 shows the calculated residue curves according to Eq. (51) at various temperatures in the still, while the temperature at the condenser was kept constant by evaporation of water due to natural convection. The azeotropic concentration of this mixture (xi = 62 mol%) in practical terms does not vary much with pressure and temperature. The arheotropic iso-propanol content at 50 °C is as low as 38 % and will not decrease much further at even lower still temperatures. The minimum arheotro-... 

Natural convection is the flow induced by the unequal pull of gravity on fluid elements of different densities. For example, if we inject a globule (or layer) of dense aqueous solution marked with a dye into a beaker of water, the dense globule will be observed to sink under the influence of gravity, as illustrated in Figure 4.8. That sinking motion is actually a form of bulk displacement or flow, specifically natural convective flow. 

Natural convective flows in porous media occur in a number of important practical situations, e.g., in air-saturated fibrous insulation material surrounding a heated body and about pipes buried in water-saturated soils. To illustrate how such flows can be analyzed, e.g., see [20] to [22], attention will be given in this section to flow over the outer surface of a body in a porous medium, the flow being caused purely by the buoyancy forces resulting from the temperature differences in the flow. The simplest such situation is two-dimensional flow over an isothermal vertical flat surface imbedded in a porous medium, this situation being shown schematically in Fig. 10.25. 

A heat pump system utilizes a heat exchanger buried in water-saturated soil as a heat source. The heat exchanger basically consists of a series of vertical plates with height of 30 cm and a width of 10 cm. These plates are effective Is at a uniform temperature of 5°C. The soil can be assumed to have a permeability of 10 0 nr and apparent thermal conductivity of 0.1 W/m-K. The temperature of the saturated soil far from the heat exchanger is 30°C. Assuming natural convective flow and that there is no interference between the flows over the individual plates, find the mean heat transfer rate to a plate. 

Although unagitated tank crystallizers arc quite commonly used, they are greatly limited with respect to control of desired crystal product characteristics. The only variables are the initial and final temperatures and initial concentration, which determine the product yield. Rate of cooling may be controlled to some extent by the use of insulation or a tempered water jacket. The only liquor circulation which takes place is due to natural convection currents. As crystals grow and settle to the bottom of the vessel, agglomeration and occlusion of mother liquor may take place. Serious difficulties arise from the inability to control crystal product size and quality. 

A 5-em-exicmal-diameier, lO-m-long hot water pipe at 80°C is losing heat to the surrounding air at 5°C by natural convection with a heat transfer coefficient of 25 W/m °C. Determine the rale of heat loss from the pipe by natural convection. Answer 2945 W... 

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