Flow & Ventilation Subsystem

The purpose of the flow and ventilation subsystem is to ensure adequate airflow throughout the habitat to transport oxygen, excess humidity, carbon dioxide, airborne contaminants, and particulates. Work this semester focused on developing conceptual equipment, register, and ducting layouts, utilizing 3D CAD and computational fluid dynamic (CFD) modeling to verify the key driving requirements, and providing input on lunar dust mitigation strategies.

Analysis work focused on determining a needed airflow rate through fluid simulations, sizing and locating registers, and sizing and selecting a fan that can meet the determined rate. The system has three key driving requirements:

• The HLS shall maintain a ventilation rate within the internal atmosphere such that two thirds of the atmosphere velocities are between 4.57 m/min and 36.58 m/min (0.076 - 0.61 m/s) (HLS-S-HMTA-0019)

• The HLS shall limit the levels of microbial contaminants by maintaining a continuous flow of air (HLS-S-HMTA-0051)

• The HLS shall limit the levels of lunar dust particles between 0.02 and 10 μm in size in any habitable cabin atmosphere (HLS-S-HMTA-0047)

The system is principally concerned with three key metabolic outputs in the cabin atmosphere: heat, moisture (exhaled and perspired), and carbon dioxide (CO2). These outputs will quickly move their associated parameters outside the range of environmental requirements if the ventilation system does not transport them out of the cabin and to their respective revitalization subsystems.

CFD Studies

My focus during this semester of work was on doing a set of CFD studies to verify the cabin flow requirement and validate the system that was designed. This included fan sizing, inlet and outlet register size, shape, orientation, and location, and supporting equipment and ductwork layout. These studies also sought to characterize the system’s ability to transport heat in the cabin atmosphere.

Methodology

3D Model

The habitat model developed is an intentionally low-fidelity “surrogate” model made specifically for this investigation. This is done to simplify and generalize findings in the absence of a known model geometry, and assist compute time for running each simulation. Only the habitable volume is modeled with smaller surfaces representing vent locations. Ductwork is conceptual only (not included in the Ansys simulation model) and driven by proposed vent locations.

Part of the process of developing a surrogate model involves making simplifying assumptions about the model. The assumptions utilized in the final studies of the semester are summarized below:

• Inlets defined as velocity (early studies) or mass flow (later studies)

• Outlets defined as zero pressure

• Side walls and floor assumed to be equipment cabinets with heat flux of ~100-200 W/m2

• Nothing modeled inside the cabin, including crew

• Lunar gravity: 1.625 m/s2

• Exploration atmosphere of 8.2 PSI in the cabin

• Cabin inlet air temperature: 285 K (53°F)

Taking advantage of the knowledge that Northrop Grumman had initially specified a 500 cfm fan to deliver the needed ventilation to the cabin, the equivalent mass flow rate of 0.16031 kg/s was set as a baseline.

Vents

The vents are bi-directional and angled outward by 15° on each side, as well as angled up into the clerestory portion of the habitat by 20° to resolve the airflow dead spots encountered in earlier studies. The overall vent size changed several times during the semester and is 10x30 cm (~4x12 in.) in the final iterations. This is likely the largest plausible vent size given the possible range of duct sizes, and future development may need to reduce the size of these vents, increasing outlet velocity to achieve the same airflow. Register locations also varied throughout different design studies, but eventually settled to 3 inlets and outlets mirrored in the cabin

Heat Flux

As an ancillary investigation to establishing flow rates, the simulations also make possible the addition of thermal loading into the cabin atmosphere. In this case, two of the walls and the floor in the cabin are assumed to have an associated heat flux.

The International Standard Payload Racks (ISPRs) on the International Space Station are allowed to emit up to 500 W of heat into the cabin atmosphere beyond what is removed by the cold plate system. If this comes from the front (interior) face of the rack, this translates into approximately 240 W/m2. HLS is assumed to have smaller equipment cabinets, be less densely packed with equipment, and unlikely to carry more than a handful of in situ science experiments, so this heat flux value is assumed to be a maximum of 200 W/m2 beyond what is handled by the internal thermal control system.

The final and most methodical set of study cases, study 5 in the table above, sought to address two questions: what is the minimum airflow that will satisfy the flow requirement in the cabin, and what is the greatest heat flux that the system can accommodate.

Airflow Cases

The first plot on the left and first set of path line images above show cases where equipment wall heat flux is held constant at 100 W/m2 and inlet air is always 285 K, but the airflow rate varies from 500 to 100 cfm. As the path line images show, all but the two lowest ventilation rates likely satisfy the requirement, and the 200 cfm level may. However, all but the 2-3 highest flow rates fail to adequately transport heat away from the cabin atmosphere.

Heat Flux Cases

In these cases the ventilation rate is held constant at 500 cfm while the equipment wall heat flux varies from 200 to 25 W/m2. In most cases the make-ip air is kept at 285 K, but one case explores the effect of lowering it to 280 K (44°F). We can extrapolate this effect across all heat flux cases (orange line in the plot) since results are so linear, suggesting some heat levels can be moved into the comfort zone by fine-tuning this parameter.

Conclusions

Flow requirement verified. A 500 cfm fan easily handles ascent stage ventilation and could likely go as low as 250 cfm and still meet the requirement. However, adding a descent stage to the model increases habitable volume by >50%, suggesting a ~400 cfm fan could be sufficient. This result has positive implications for interfacing with Gateway segments, such as HALO and Orion, where minimal ventilation could potentially be supplied in an emergency.

Heat transport is not really sufficient, given assumptions. Even at the highest flow rates, the system can’t handle the 200-220 W/m2 range of heat flux. At around 125 W/m2 and >400 cfm, it can get into the comfort range, especially if makeup air temperature comes down by a few degrees.