Mass flows

These early observations, followed by recent reports by several researchers indicate that greater gas fluxes in intercellular airspaces of some wetland plants are due to pressurized convective flow (Dacey, 1981, 1987 Armstrong and Armstrong, 1991 Armstrong and Beckett, 1996 Brix et al., 

Mass flow of gases in intercellular airspaces of wetland plants can be generated by several processes  

Both thermal transpiration and humidity-induced diffusion require a porous partition within the plant tissue, with pore diameters of 0.1-3 pm (Armstrong et al., 1994), which is the molecular mean free path length for Knudsen diffusion. Armstrong et al. (1991a) defined Knudsen diffusion 

FIGURE 7.14 Gas exchange due to mass flow in water lily. 

Source Sanchez (1976). Reproduced by permission of Wiley, New York. 

Source Reprinted from Painuli et al. (1988) with permission from Elsevier Science. 

Eqnation (2.11) shows how the flow increases with increasing depth of the flood-water and decreases with increasing impermeability of the compacted layer. 

The effect of percolation on transport of solutes through the soil is quantified as follows. If there is a concentration gradient of a solnte throngh the soil, from Equation (2.4) the net flnx due to mass flow and diffnsion is 

Mass flow and diffusion act together and cannot be separated. However an idea of their relative contributions to the net flux can be obtained by estimating the distance the solute would be transported if each process acted independently. If in time t mass flow transports the solute a distance 

By definition 1 dtex = 1 gram per 10,000 m. The output per bundle is then calculated from Eq. (4). 

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Reactor heat carrier. Also as pointed out in Sec. 2.6, if adiabatic operation is not possible and it is not possible to control temperature by direct heat transfer, then an inert material can be introduced to the reactor to increase its heat capacity flow rate (i.e., product of mass flow rate and specific heat capacity) and to reduce... 

Internal Mass Flows in Sequences of Simple Distillation Columns... 

For calculation of the volumetric flow rate only the cross section area of the pipe is to be known. In order to give flow under standard conditions the temperature and pressure must be measured, and for conversion to mass flow the composition or density of the gas must be determined. These process parameters are often monitored by calibrated instrumentation. 

If a system is eoupled with its enviromnent tlirough an adiabatie wall free to move without eonstraints (srieh as the stops of the seeond example above), meehanieal equilibrium, as diseussed above, requires equality of the pressure p on opposite sides of the wall. With a diathemiie wall, themial equilibrium requires that the temperature 0 of the system equal that of its surroundings. Moreover, it will be shown later that, if the wall is pemieable and pemiits exehange of matter, material equilibrium (no tendeney for mass flow) requires equality of a ehemieal potential p. 

Albert H J and Archer D G 1994 Mass-flow isoperibole calorimeters Solution Calorimetry, Experimental Thermodynamics vol IV, ed K N Marsh and PAG O Hare (Oxford Blackwell)... 

The design of countercurrent contactors is considerably simplified when the solvents A and B are not significantly miscible. The mass flows of A and B then remain constant from one stage to the next, and the material balance at any stage can be written... 

Meters can be further divided into three subgroups depending on whether fluid velocity, the volumetric flow rate, or the mass flow rate is measured. The emphasis herein is on common flow meters. Devices of a highly specialized nature, such as biomedical flow meters, are beyond the scope of this article. 

Momentum Flow Meters. Momentum flow meters operate by superimposing on a normal fluid motion a perpendicular velocity vector of known magnitude thus changing the fluid momentum. The force required to balance this change in momentum can be shown to be proportional to the fluid density and velocity, the mass-flow rate. 

Coriolis-Type Flow Meters. In CorioHs-type flow meters the fluid passes through a flow tube being electromechanically vibrated at its natural frequency. The fluid is first accelerated as it moves toward the point of peak vibration ampHtude and is then decelerated as it moves from the point of peak ampHtude. This creates a force on the inlet side of the tube in resistance to the acceleration and an opposite force on the outlet side resisting the deceleration. The result of these forces is an angular deflection or twisting of the flow tube that is directly proportional to the mass flow rate through the tube. 

The servo voltage is a function of mass-flow rate. Axial-flow angular-momentum meters are sometimes used in measuring jet engine fuel flow as the fuel energy content correlates much mote closely with mass than volume. 

The cross-sectional area of the wick is deterrnined by the required Hquid flow rate and the specific properties of capillary pressure and viscous drag. The mass flow rate is equal to the desired heat-transfer rate divided by the latent heat of vaporization of the fluid. Thus the transfer of 2260 W requires a Hquid (H2O) flow of 1 cm /s at 100°C. Because of porous character, wicks are relatively poor thermal conductors. Radial heat flow through the wick is often the dominant source of temperature loss in a heat pipe therefore, the wick thickness tends to be constrained and rarely exceeds 3 mm. 

Assessments of control, operabiHty and part load performance of MHD—steam plants are discussed elsewhere (rl44 and rl45). Analyses have shown that relatively high plant efficiency can be maintained at part load, by reduction of fuel input, mass flow, and MHD combustor pressure. In order to achieve efficient part load operation the steam temperature to the turbine must be maintained. This is accompHshed by the use of flue gas recirculation in the heat recovery furnace at load conditions less than about 75% of fiiU load. 

There are many sources of errors in the plant. The principal ones are related to sampling (qv), mass flow rates, assaying, and deviations from steady state. Collecting representative samples at every stage of the flow sheet constitutes a significant task. Numerous methods and equipment are available (10,16,17). 

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