Density Difference Induces Flow

I d better phone Professor Peterson to apologize. I just now remembered that we did learn about this concept that density difference between two columns of fluid causes flow. Professor Peterson taught us the idea in the context of draft in a fired heater. Cold combustion air flows through the burners and is heated by the burning fuel. The hot flue gas flows up the stack. The difference in density between the less dense hot flue gas and the more dense cold air creates a pressure imbalance called draft. Just like the fish tank story. 

However, I can t call Professor Peterson. He s dead. I wouldn t call him anyway. I know what he would say Lieberman, the analogy between the air lift pump and draft in a fired heater is obvious to the perceptive mind, which apparently excludes you.  

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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 crystallization of the D-enantiomer is therefore considered to be induced by crystal growth on the surface of the seed crystal and at the same time initial breeding may play a role that causes small crystals near the seed crystal. The propagation of nucleation in distance from the seed may be caused by convective flow of the solution due to density difference during the crystal growth. 

Mechanism, Vapor enters the downcomer with the froth that flows over the outlet weir. Additional vapor is entrained into the liquid due to the impact of the falling liquid on the liquid surface in the downcomer, in the same manner as a waterfall induces air entrainment into the pool below it. Inside the downcomer, vapor disengages from the liquid due to its higher buoyancy. The driving force for vapor disengagement is the density difference between the liquid and the vapor. 

Description The unique reactor design uses a simple vertical U-shaped leg connected to a horizontal gas-liquid separation vessel. Reactant gases are fed to the bottom of the U where they dissolve and combine under sufficient static pressure to prevent boiling in the reaction zone. Above this zone, the heat of reaction produces vapor bubbles that flow upwards into the horizontal vessel. A natural circulation of EDC is induced by the density difference in the two legs of the U. ... 

This is presented schematically in Fig. 6.3, which also shows that the kinetics of these processes is described by the transport rate of A from the wall to the adjacent media. Using Fig. 6.3, we can establish that two elementary processes are presented in this system. The first is the flow induced by the concentration gradient and the second is the mass transfer sustained by the processes on the surface (a chemical reaction in the case of the metal placket immersed in a specifically formulated liquid and the transport through the porosity in the case of the drying wall). The case presented here corresponds to the situation when, in respect of the bulk density, the fluid density begins to decrease near the wall. This generates the displacement of the media and the specific ascension force, which is equivalent to the density difference. This phenomenon depends on the concentration difference in fluid A Aca=(cap - c ). From Fig. 6.3 we can write a list of process variables ... 

Convection is called forced convection if Ihe fluid is forced to flow over the surface by external means such as a fan, pump, or the wind. In contrast, convection is called natural (or free) convection if the fluid motion is caused by buoyancy forces that are induced by density differences due to the variation of temperature in the fluid (Fig. 1 33). For example, in the absence of a fan, heat transfer from the surface of the hot block in Fig. 1-32 is by natural convection since any motion in the air in this case is due to the rise of Ihe warmer (and thus lighter) air near the surface and the fall of the cooler (and thus heavier) air to fill its place. Heat transfer between the block and the surrounding air is by conduction if the temperature difference between Ihe air and the block is not large enough to overcome the resistance of air to movement and thus to initiate natural convection currents. 

The free thermal convection of groundwater, as described here, is attributable to temperature-induced density differences of the groundwater only if it is assumed that there are no other driving forces for groundwater flow. This assumption is unlikely to be met in sedimentary basins. According to Bethke (1989) there has been little work to determine the extent to which free convection persists in the presence of other groundwater flow systems. 

Convection. The transfer of heat by the mixing or movements of fluids or fluids with a solid. Mixing may occur as a result of density difference alone, as in natural convection. Alternatively, mechanically induced agitation may produce forced convection, as in turbulent flow in a heat exchanger tube, or to the heat transfer fluid in the jacket of an agitated vessel. The rate of heat transfer is ... 

Circulation evaporators, however, can operate over a wide range of concentration between feed and thick liquor in a single unit, and are well adapted to singie-efiect evaporation. They may operate either with natural circulation, with the flow through the tubes induced by density differences, or with forced circulation, with flow provided by a pump. 

Convection, sometimes identified as a separate mode of heat transfer, relates to the transfer of heat from a bounding surface to a fluid in motion, or to the heat transfer across a flow plane within the interior of the flowing fluid. If the fluid motion is induced by a pump, a blower, a fan, or some similar device, the process is called forced convection. If the fluid motion occurs as a result of the density difference produced by the temperature difference, the process is called free or natural convection. 

In the spirit of our restriction to the laminar regime, we shall only briefly touch on natural convection—that is, flows produced by buoyancy forces acting on fluids in which there are density differences. A common example is buoyant motion in a gravitational field where the density difference arises from heat exchange. Even in weakly buoyant motions generated by small density differences, turbulence is ubiquitous. We shall, however, consider convection induced by surface tension gradients. 

Microfluidics handles and analyzes fluids in structures of micrometer scale. At the microscale, different forces become dominant over those experienced in everyday life [161], Inertia means nothing on these small sizes the viscosity rears its head and becomes a very important player. The random and chaotic behavior of flows is reduced to much more smooth (laminar) flow in the smaller device. Typically, a fluid can be defined as a material that deforms continuously under shear stress. In other words, a fluid flows without three-dimensional structure. Three important parameters characterizing a fluid are its density, p, the pressure, P, and its viscosity, r. Since the pressure in a fluid is dependent only on the depth, pressure difference of a few pm to a few hundred pm in a microsystem can be neglected. However, any pressure difference induced externally at the openings of a microsystem is transmitted to every point in the fluid. Generally, the effects that become dominant in microfluidics include laminar flow, diffusion, fluidic resistance, surface area to volume ratio, and surface tension [162]. 

The two-fluid model allows the phases to have thermal nonequilibrium as well as unequal velocities. In this model, each phase or component is treated as a separate fluid with its own velocity, temperature, and pressure. Thus, each phase has three independent set of governing balance equations for mass momentum and energy. The velocity difference as in the separated flow is induced by density differences and the temperature differences between the phases is fundamentally induced by the time lag of energy transfer between the phases at the interface as thermal equilibrium is reached. The two-fluid model... 

Li et al. 48) studied how 1.0 pm particles of polystyrene and poly(methyl methacrylate) interacted when they were melt processed at 180 °C. They observed by confocal microscopy on a hot stage that there was a preferential motion for particles, which they attributed to a buoyancy-driven flow because of the 10% density difference between the polymers. Jang et al. had made a similar observation 49), Li et al. did not consider possible surface-tension induced convection or that droplets could migrate in a temperature gradient, as has been observed by Balasubramaniam et al. for the thermocapillary migration of bubbles 50),... 

The fluid dynamic structure within the boundary layers adjacent to natural solid surfaces such as soil, sediment, snow, and ice, is complex. Typically, the flows fields have both laminar and turbulent regions. The flows magnitudes and directions respond according to the angle of incidence to the surface and the overall shape of the object as well as thermal-induced fluid density differences (i.e., stratification) and so on. All these factors operate into shaping the mass transfer boundary layer which controls the chemical flux. Ironically, the traditional approach to handling such complex flow situations has been to use a simple flux equation. The so-called convective mass flux equation is... 

In Section 2 several nuclear reactor designs as well as natural circulation applications have been described.The single phase natural circulation flow is driven by a gravity head induced by coolant density differences the mass flow is established according to the balance between driving head and flow resistance losses. Because the (one component) density is a ftinction of the temperature there is a functional interaction between heat exchange and natural circulation flow. 

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