Withdrawal of Carbon Monoxide through a Hood in a Ventilated Room Consider the same engine testing laboratory as described in Example 10.1 and as sketched in Figure E10.1a. Despite the clear evidence presented in that example, the architect overrules the chief engineer, and installs the fresh air

Question

Withdrawal of Carbon Monoxide through a Hood in a Ventilated Room
Consider the same engine testing laboratory as described in Example 10.1 and as sketched in Figure E10.1a. Despite the clear evidence presented in that example, the architect overrules the chief engineer, and installs the fresh air supply in the center of the ceiling and the room exhaust on the right side of the ceiling (the configuration of Figure E10.1c). Now the chief engineer needs to select an appropriate fan for the hood exhaust.
To do: Use CFD to simulate the flow of air and CO in the room and through the hood exhaust duct for various values of fan pressure rise, [latex]δP_{fan}[/latex].

Accepted Answer

The two-dimensional approximation is again employed for simplicity and to decrease the required computational time. The computational domain is the same as that shown in Figure E10.1a. All parameters are in fact identical to the second case of Example 10.1 (fresh air supply rate, CO supply rate, pressures specified at the room exhaust and hood exhaust pressure outlets, turbulence intensities, etc.). The only change from the previous example occurs in the hood exhaust fan boundary condition. Namely, several values of δ[latex]P_{fan}[/latex]are selected, and FLUENT is run to convergence for each value of δ[latex]P_{fan}[/latex].
Streamlines and CO mol fraction contours are shown in Figure E10.2a for the case in which [latex]δP_{fan}[/latex] = 50.0 mPa. Contours of constant CO mol fraction above 500 PPM are not shown in the figure, as previously. A large counterclockwise eddy forms on the right side of the room, and a smaller clockwise eddy appears to the left of the air supply jet. The overall features of the streamlines are not much different than those shown in Figure ElO.lc, except near the workbench and hood. Whereas the streamlines flow nearly vertically past the workbench when the fan is off, there is a clear split in the flow streamlines at the edge of the hood for the case with the fan on. Some of the room air enters the hood, drawing contaminant with it into the hood. The CO level near the worker's breathing zone is around 40 PPM, significantly below that of Figure E10.1c, and just below the OSHA PEL. The air is fairly clean on the entire right half of the laboratory, with values less than 15 PPM. Higher concentrations of CO are again seen in the small counterclockwise eddy above the hood, where CO levels are around 55 to 60 PPM. A counterclockwise eddy is also seen inside the hood itself, where the CO level is much higher, around 230 to 250 PPM, but about half of the values shown in Figure E10.1c. Finally, several streamlines clearly pass through the hood exhaust fan and up the hood exhaust duct; the CO level in the air going up the hood exhaust duct is between 210 and 215 PPM. Notice that since much of the carbon monoxide is drawn through the hood, the CO level exiting the room through the ceiling exhaust has decreased to a value between 10 and 15 PPM.
CFD results are shown in Figure E10.2b for the case in which [latex]δP_{fan}[/latex] = 200. mPa. The large counterclockwise eddy fills up nearly the entire room, and a much smaller clockwise rotating eddy fills in the space above the hood. The streamlines coming from the air supply are noticeably tilted to the left, and many of them enter directly into the hood. At this larger fan pressure rise, air crosses the vertical plane from left to right at every location between the edge of the workbench and the edge of the hood above it. Nearly all of the carbon monoxide is drawn into the hood and up the hood exhaust as it should; this value of fan pressure rise is thus considered adequate for proper operation of the hood as a local ventilation system. Note that since air drawn into the hood mixes with carbon monoxide, the CO level in the air going up the hood exhaust duct is around 100 PPM, less than half of that calculated for the case with [latex]δP_{fan}[/latex] = 50. mPa. The CO level in the entire room is less than 5 PPM (which is the smallest value of constant CO mol fraction plotted in the contour plot). Finer resolution contour plots (not shown) reveal that the CO level near the worker's breathing zone is in fact less than 1 PPM.
Finally, Figure E10.2c shows how CO mol fraction at the worker's breathing zone location decreases with increasing [latex]δP_{fan}[/latex]. There appears to be a discontinuity or "bump" in Figure E10.2c around [latex]δP_{fan}[/latex] = 50 mPa. The FLUENT solution had much more difficulty converging at this value of [latex]δP_{fan}[/latex] as well. The authors conjecture that under these conditions the physical flow would tend to oscillate at the edge of the hood where the streamlines split into those entering the hood and those remaining in the room. In other words, the solution here is forced to be steady whereas the physical flow wants to be unsteady, leading to the convergence difficulties and the discontinuity in Figure E10.2c. At even higher values of [latex]δP_{fan}[/latex], namely around 1000 mPa, the strong suction provided by the hood exhaust fan leads to reverse flow from the pressure outlet at the top of the room exhaust duct. In other words, the hood fan is too strong for the room ventilation system.
Discussion: The required fan pressure rise is low, only about 200 mPa, which is much less than one inch of water column, because of the two-dimensional assumption. Although the hood exhaust duct is only 0.050 m from wall to wall, it is 1.0 m wide (into the page) in these simulations. An actual duct would most likely have a much smaller aspect ratio, or would be round in cross-section, either of which would significantly increase the velocity and pressure drop through the hood exhaust duct, and would thereby demand a more realistic value of [latex]δP_{fan}[/latex].

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