Unidirectional flow

To create unidirectional low-turbulence flow, air is supplied with a low velocity supply diffusers and exhaust openings have large surfaces (e.g., filter mats). Airflow can be either vertical (air supplied from the ceiling and exhausted through the floor or vice versa (Fig. 7.1 la)), or horizontal (air supplied through 

One of the unidirectional flow system modifications is air supply through diffusers located above the occupietl zone. The supply air temperature is lower than the desired room air temperature in the occupied zone, and air velocity is lower compared to a mixing-type air supply, bur higher than for a thennal displacement ventilation. Polluted air of the occupied zone is suppressed by an tiverlying air cushion that displaces the contaminated air toward floor-level exhausts (Fig. . 12). 

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FIGURE 7.12 Unidirectional flow system with air supply through diffusers located above the occupied zone. ... 

Davis, J.A, 1977, The unidirectional flow ventilation systems. Heating, Piping and. Air Conditioning, March, pp. 63-67. 

FIGURE 10.61 Dispersion of smoke In a hortzonul unidirectional flow bench when operator s hands are HI the upper position (undisturbed smoke dispersion). 

FIGURE 10.62 Dispersion of smoke in a horizontal unidirectional flow bench when operators hands arc in motion. 

This partial differential equation is most conveniently solved by the use of the Laplace transform of temperature with respect to time. As an illustration of the method of solution, the problem of the unidirectional flow of heat in a continuous medium will be considered. The basic differential equation for the X-direction is ... 

Considering the particular problem of the unidirectional flow of heat through a body with plane parallel faces a distance l apart, the heat flow is normal to these faces and the temperature of the body is initially constant throughout. The temperature scale will be so chosen that this uniform initial temperature is zero. At time, t = 0, one face (at x — 0) will be brought into contact with a source at a constant temperature 9 and the other face (at x = () will be assumed to be perfectly insulated thermally. 

Numerical methods have been developed by replacing the differential equation by a finite difference equation. Thus in a problem of unidirectional flow of heat ... 

Washout experiments can be used to measure the residence time distribution in continuous-flow systems. A good step change must be made at the reactor inlet. The concentration of tracer molecules leaving the system must be accurately measured at the outlet. If the tracer has a background concentration, it is subtracted from the experimental measurements. The flow properties of the tracer molecules must be similar to those of the reactant molecules. It is usually possible to meet these requirements in practice. The major theoretical requirement is that the inlet and outlet streams have unidirectional flows so that molecules that once enter the system stay in until they exit, never to return. Systems with unidirectional inlet and outlet streams are closed in the sense of the axial dispersion model i.e., Di = D ut = 0- See Sections 9.3.1 and 15.2.2. Most systems of chemical engineering importance are closed to a reasonable approximation. 

Ultrasonic methods can also be applied to velocity measurements based on measurement of the Doppler shift in the frequency of an ultrasonic wave scattered from a moving particle. The angle between the velocity vector and the direction of ultrasound propagation must be known, which practically limits the appHcation of the technique to the measurement of unidirectional flows. However, this Hmitation may be overcome again by the use of an array of transducers [11]. 

The greatest risk of contamination of a pharmaceutical product comes from its immediate environment. Additional protection from particulate and microbial contamination is therefore essential in both the filling area of the clean room and in the aseptic unit. This can be provided by a protective work station supplied with a unidirectional flow of filtered sterile air. Such a facility is known as a laminar airflow unit in which the displacement of air is either horizontal (i.e. from back to front) or vertical (i.e. from top to bottom) with a minimum homogenous airflow rate of 0.45 ms" at the working position. Thus, airborne contamination is not added to the work space and any generated by manipulations within that area is swept away by the laminar air currents. 

If this model is further simplified by considering unidirectional flow, the number of equations is reduced to four (Wallis, 1969). Another example is Bankoff s variable-density, single-fluid model for two-phase flow (Bankoff, 1960). Since it is based on an intimate mixture, both mechanical equilibrium (i.e., same velocity) and thermal equilibrium (same temperature) between the two phases must logically be assumed (Boure, 1975). 

Measurement of biofilm activity can be performed based on laboratory reactor experiments or with a technique combining biofilm growth taking place in a sewer followed by measurements in laboratory scale (Raunkjaer et al., 1997 Bjerre et al., 1998). Huisman et al. (1999) developed a sewer in situ biofilm respiration chamber. It includes a DO sensor and a chamber that can be pressed onto the sewer wall. It is designed to achieve an even and unidirectional flow distribution over the entire measurement area. Pure oxygen is injected for oxygenation. 

Control Performance of Thermally Coupled Distillation Sequences with Unidirectional Flows... 

In this work a set of two modifications of the Petlyuk system that use unidirectional flows of both interconnecting streams are considered to analyze their control behavior. The aim of the study is to show if such design modifications... 

Table 1 shows the values of the minimum energy consumption obtained for the two systems with unidirectional flows and compares them to the one... 

When a set point change from 0.986 to 0.99 for the heavy component (C) was considered, the responses shown in Figure 5 were obtained. The three options show good d mamic responses, with relatively low values of settling times, lower than 0.1 hours. The systems with unidirectional flows show some overshooting, while the Petlyuk system offers a more sluggish response. 

Mead, K. S., M. B. Wiley, M. A. R. Koehl, and J. R. Koseff. Fine-scale patterns of odor encounter by the antennules of mantis shrimp tracking turbulent plumes in wave-affected and unidirectional flow. J. Exp. Biol. 206, 181-193 (2003). 

The permeability can be determined experimentally in several different ways (e.g., in a radial flow [19] or unidirectional flow experiment [20]). The experiments can also be done with either an advancing flow front (wetting flow) or a fully saturated reinforcement under steady-state conditions. There is some debate in the scientific community whether the... 

In the preceding example and point injection the flow front will develop as a circular front, starting at the inlet, until it meets the closest side. From then on the front will tend to move unidirectionally in both directions toward the far sides (if there is no leakage at the sides). A reasonable estimate of the fill time is somewhere between the time to fill radially and the time to fill unidirectionally to the far side (flow distance 1.5 m). For the radial flow case L in Equation. 12.10 is 3 m (the diameter of a circle touching the wall farthest away). With the assumed alternative with unidirectional flow from the center toward both shorter sides, L in Equation. 12.10 is 1.5 m. It is also necessary to estimate an effective inlet radius, in this case an inlet radius of 5 mm is chosen yielding e = 3.3 10-3 that was used in the formula in Table 12.2. The constant C in Equation 12.10 is 0.5 for the unidirectional flow case and 0.65 for the radial flow case. The upper and lower limit for the fill time in this case are then ... 

The flow instability can best be understood by looking at a case with unidirectional flow (see Fig. 12.7). There will always be some nonuniformity between the sides. This will result in the flow front moving faster on one side of the core than it does on the other. The net force on the core from the resin will be higher on the side where the flow front has moved the farthest and as a result the core will be pushed away from this side. The displacement of the core will increase the permeability more and the flow front will move even farther ahead on the fast side, and so on. The process will reach an equilibrium when the reaction force from the reinforcement becomes large enough to balance the fluid pressure on the other side of the core. 

These test procedures are performed to determine average air flow velocity and uniformity of velocity within a cleanroom, clean zone, or unidirectional flow work zone, as well as to determine air supply volume uniformity. 

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