Comparison of two Ventilation Configurations in an Engine Laboratory A 4.0 m long by 2.5 m high by 7.0 m wide laboratory is being constructed to test the performance of small lawnmower engines.Seven workbenches, each approximately 1.0 m wide and each with a hood, are to be placed side by side along

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Comparison of two Ventilation Configurations in an Engine Laboratory
A 4.0 m long by 2.5 m high by 7.0 m wide laboratory is being constructed to test the performance of small lawnmower engines. Seven workbenches, each approximately 1.0 m wide and each with a hood, are to be placed side by side along the left wall as sketched in Figure El0.1 a. (Note that "width" here refers to the distance into the page in Figure El0.1 a; a two dimensional slice through the center of the laboratory is shown.) A fan is located at the top of each hood, and the duct work consists of a sudden contraction, two sharp 90° bends, and a straight duct, 12.2 m in length which exhausts into the atmosphere. The lab is to be ventilated by a dilution ventilation system with fresh air supplied and exhausted through the ceiling; both ceiling openings span nearly the entire width of the room. The openings in the ceiling have already been cut, one in the center of the room and one on the right side of the room as shown in the diagram, but it has yet to be decided which of the two will serve as the fresh air supply and which will serve as the room exhaust. In either case, fresh air is to be supplied at a constant rate of four to five room air changes per hour. CO enters the work space at a rate of approximately 2600 mg/hr from each running engine (one engine per work station). The chief engineer is particularly concerned about the scenario in which a worker forgets to turn on the hood exhaust fan, and carbon monoxide escapes into the room, where it is inhaled by the worker. The approximate location of the worker's mouth is indicated in Figure El0.1 a. The architect thinks it would be better to have the fresh air supply located in the center of the room, close to the workers so that they stand nearly directly below the supply. The chief engineer disagrees, claiming that it would be better to have the room exhaust closer to the workers, and the air supply on the right side of the room. You are consulted to settle the dispute, and decide to use CFD to simulate how the air and CO mix in the room for the two cases.
To do: Simulate both ventilation options under conditions in which the hood fans are not turned on, and discuss which option would be the wiser choice.

Accepted Answer

Since the geometry is nearly two-dimensional, a 2-D simulation is performed using the geometry shown in Figure E10.1a as the computational domain. One can imagine a one-meter wide slice of the room, containing one worker and one hood. The two ceiling ducts extend 7.5 m above the ceiling to avoid reverse flow in the computations. Fresh air is supplied through the velocity inlet of the air supply duct at a speed of 0.060 m/s with a turbulence intensity and length scale of 10% and 0.20 m respectively. The volumetric flow rate of air into the room is thus 43.2 m³/hr. To simulate suction provided by the HVAC system, the gage pressure is specified as -2.3 ×[latex]10^{-3}[/latex] Pa at the outlet of the room exhaust duct. The pressure at the outlet of the hood exhaust duct is set to zero gage pressure (atmospheric pressure). The worst-case scenario is modeled, in which carbon monoxide enters the room right at the edge of the workbench, and the hood exhaust fan is off. In the simulation, CO is introduced through a velocity inlet into a 0.050 m duct from below the floor of the room, as sketched. The bulk speed of the air and CO entering this duct is set to 0.0060 m/s with a turbulence intensity of 20% and a turbulence length scale of 0.050 m. The mass fraction of CO entering the contaminant supply duct is specified as [latex]2.0 × 10^{-3}[/latex] at the velocity inlet.
Readers can verify that the mass flow rate of CO into the computational domain (the source) is thus S = 2600 mg/hr. The total amount of fresh air entering the room is 44.3 m³/hr, which is the sum of the volumetric flow rate of fresh air supplied through the ceiling and that supplied through the contaminant supply duct. As a first approximation, the well-mixed assumption of Chapter 5 is invoked to determine a base level steady-state value of CO mass concentration in the room,

[latex]c_{ss,co}=\frac{S_{co}}{Q_{supply}}=\frac{2600\frac{mg}{hr}}{44.3\frac{m^3}{hr}}=58.7\frac{mg}{m^3}[/latex]

In terms of mol fraction, the steady-state well-mixed level of CO in the room is 51. PPM, just above the OSHA PEL of 50 PPM for CO.

The two ventilation options are simulated using FLUENT, assuming steady-state turbulent flow with the realizable k-e turbulence model and enhanced wall treatment. The hood exhaust fan is off in both cases. Streamlines and CO mol fraction contours are shown in Figure E10.1b for the case in which fresh air is supplied through the duct on the right and room air is exhausted through the duct in the middle of the ceiling. Contours of constant CO mol fraction above 500 PPM are not shown in the figure. A large clockwise eddy forms in the middle of the room and draws CO from the workbench towards the exhaust. The CO level near the worker's breathing zone is around 30 to 40 PPM, below the OSHA PEL. Higher concentrations of CO are seen in the small counterclockwise eddy above the hood, where CO levels are around 80 to 90 PPM. A counterclockwise eddy is also seen inside the hood itself, where the CO level is much higher, around 250 to 260 PPM. Finally, the large counterclockwise eddy near the right wall of the room contains very clean air with CO levels less than 5 PPM.

CFD results are shown in Figure E10.1c for the opposite ventilation option, in which the air supply is through the middle duct and the room exhaust is through the duct at the right. A large counterclockwise eddy is seen on the right side of the room, and a smaller clockwise eddy forms on the left, drawing carbon monoxide from the workbench and mixing it with room air. The CO level near the worker's breathing zone is around 130 to 140 PPM, above the OSHA PEL. The overall CO concentrations in both the room and the hood are in fact much higher than those for the first case, although the average CO mol fraction leaving the room through the room exhaust duct is about 45 PPM, about the same as that of the first case. Note that in both cases a small amount of CO escapes through the hood exhaust duct even though the hood fan is off.
The first case, the one recommended by the chief engineer, is clearly the better option. Apparently, placing the exhaust near the worker's head allows more fresh air to flow into his or her breathing area from below. This may seem counterintuitive, especially since the worker's mouth is on nearly a direct line between the CO source and the room exhaust. It would be difficult if not impossible to predict this result without CFD.
Discussion: The two-D approximation is realistic only for the case in which all seven engines are running and all seven workers forget to turn on their hood exhaust fans (a highly unlikely scenario). If only one or two of the seven hood fans are off, the carbon monoxide would diffuse in the direction perpendicular to the page in Figure El0.1 a; CO levels in the room would then be much smaller than those calculated here. A full three-dimensional simulation is necessary for such detailed analysis. Both the streamlines and CO concentration levels would also change significantly if one were to include obstructions caused by the workers standing next to the workbench and/or furniture, equipment, etc. in the rest of the room.

Question 10.1: Comparison of two Ventilation Configurations in an Engine La...

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