In order to overcome this limitation in the use of conventional exhaust hoods, Aaberg exhaust hoods (or reinforced exhaust systems) have been developed where the exhaust extracting contaminated air is reinforced by an injected supply jet flow that enhances the exhaust flow considerably in comparison to other outlets. Through a balanced combination of the injection and exhaustion flows, the airflow toward the exhaust opening, in the region where there is the largest concentration of the contaminant, may extend over a far greater distance than is possible by using an exhaust alone. [Pg.956]

The two versions of the Aaberg exhaust system, namely an axisymmetri-cal version and a workbench version, both work on the same principle. In order to illustrate the principle of the Aaberg we describe the axisymmetrical version but the full theoretical, computational, and experimental basis is presented for both systems. [Pg.956]

FIGURE 10.77 A schematic representation of the Aaberg principle. [Pg.956]

FIGURE 10.79 Typical streamlines for the flow near the exhaust hood when there is (o) only suction. (b) some exhaust Bow, and (cl a large exhaust flow. (The flow is symmetrical about X = 0.) The shaded area represents the predicted effective capture region. [Pg.958]

FIGURE 10.80 A typical Aaberg ventilator unit. v hich is suspended above the floor white smoke is released on the floor beneath the venciiator. [Pg.959]

FIGURE 10.82 The geometry and coordinate system of the ASE model. [Pg.961]

It should be noted that when there is no jet reinforcement of the flow, i.e., the exhaust hood is used in its conventional mode, then in the two-dimensional form of the Aaberg principle the fluid flow velocity due to the exhaust decays approximately inversely proportionally to the distance from the exhaust opening. However, for three-dimensional exhaust hoods the fluid velocity outside the hood decays approximately inversely as the square of the distance from the exhaust hood. Thus in the three-dimensional conventional hood operating conditions the hood has to be placed much closer to the contaminant in order to exhaust the contaminant than is the situation for the two-dimensional hood (see section on Basic Exhaust Openings). Thus for ease of operation it is even more vital to develop hoods with a larger range of operation in the three-dimensional situation in comparison with two-dimensional hoods. [Pg.961]

In addition to the momentum ratio I, numerous other geometrical aspect ratios should be investigated. [Pg.962]

Thus it is recommended the simple potential flow model be used to obtain a first estimate for the optimization of the effective capture region in any particular application. Once this has been achieved, the equipment should be built to this specification but with sufficient flexibility to adjust it to obtain the practical optimum effective capture region. [Pg.962]

Outside the jet and away from the boundaries of the workbench the flow will behave as if it is inviscid and hence potential flow is appropriate. Further, in the central region of the workbench we expect the airflow to be approximately two-dimensional, which has been confirmed by the above experimental investigations. In practice it is expected that the worker will be releasing contaminant in this region and hence the assumption of two-dimensional flow appears to be sound. Under these assumptions the nondimensional stream function F satisfies Laplace s equation, i.e.. [Pg.962]

For a free jet. [Pg.963]

For a wall jet. [Pg.963]

The mathematical model presented in the previous section has been developed under the assumptions that the flow induced by an Aaberg exhaust hood is inviscid and potential and that turbulent effects have been limited to the flow in the jet. However, the typical experimental operating conditions of an Aaberg exhaust hood lead to Reynolds numbers in the order of 10 to 10. Thus the fluid flow in the jet and in the region surrounding the exhaust inlet are very likely to be turbulent. However, in the region of practical interest, i.e., the region of the flow where there is likely to be large amounts of contaminant, the airflow created by the Aaberg exhaust hood is a convergent flow and therefore in this region we expect a low level of turbulence. [Pg.964]

Hunt and Kulmala have solved the full turbulent fluid flow for the Aaberg system using the k-e turbulent model or a variation of it as described in Chapter 13— the solution algorithm SIMPLE, the QUICK scheme, etc. Both commercial software and in-house-developed codes have been employed, and all the investigators have produced very similar findings. [Pg.964]

as described earlier, we model the exhaust opening as a finite circular opening across which the fluid flows with a constant speed. The axis of the coordinate system is a streamline which we take to be R = 0 and on the surface of the flange of the Aaberg exhaust system is also a streamline on which = 0, due to the nondimensionalization given in Eq. (10,112). Further, at the outer edge of the et, which we assume to be at d — 0—i.e., it is assumed that the jet is infinitesimally small—the boundary condition (F.q. (10.119)) is appropriate. [Pg.965]

As with the two-dimensional workbench problem, the numerical solution of this problem can be found by solving the full turbulent fluid flow equations using the methods described in Chapter 13. [Pg.966]

Plane jets could be used to create a closed volume in which a contaminant source could be placed. In some ways, these systems are similar to Aaberg exhaust hoods (Section 10.4.4). The objective is to use plane jets instead of walls around an exhaust opening to create a vortex which enhances the capture efficiency of the exhaust. [Pg.1007]

G. R. Hunt. The fluid mechanics of the Aaberg exhaust hood. Ph.D, thesis, University of Leeds, 1994. [Pg.1010]

Aaberg exhaust hoods, 955-964 air curtains, 936-944 biological safety cabinets, 984-992 [Pg.1500]

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