Compartment Venting Analysis

Enclosure Venting and Pressure Equalization

As a launch vehicle ascends, air contained within compartments and bays must be vented overboard to a decreasing atmospheric pressure. At front-facing openings, the increasing vehicle speed means hotter air entering the compartments at those points.

Sometimes the design concern is to provide adequate (but not excessive) pressure equalization paths, and sometimes the concern is to keep avionics from getting too hot or too cold (for example, below the dew point). Such thermal design concerns can be complicated by expansion cooling and compression heating of air, which can include a strong dependency on adjacent compartments. For example, compression heating of a compartment can be exaggerated when the air entering that compartment is itself being warmed by compression of an upstream compartment.

Other times the goal is simply to calculate the pressures in the bays particularly if there are multiple holes around the vehicle that air can escape or enter.  If the vehicle is at a high Mach number, the external pressure can vary significantly from front to back and top to bottom.  This can cause the differential pressure across the external to be quite large which is a particular concern if there are doors, hatches, or other moveable components that have to seal.   
Another purpose in such analyses could be to satisfy a general ventilation requirement (for example, 10 air changes per minute in each compartment) so that any gas (such as leaking fuel vapor) will be exhausted quickly.

Similar problems face scientific balloons, aircraft bays and cabins, instrument pods, and other flight vehicles.

An intentionally generic example problem of bay venting and refilling has been developed to illustrate key modeling concepts. This example covers a vehicle that ascends from sea level to 40,000 feet and then returns, simultaneously accelerating from Mach=0.1 to 1.4 and decelerating again to Mach=0.1 as it lands. The entire flight takes 6 minutes (tf = 0.1 hours).

Four bays are arranged as follows (the openings are flush and sharp-edged, whereas in the drawing they are exaggerated in order to make them more visible):

A sketch-pad style FloCAD® model is built. When limits on positive and negative (vacuum) pressure differential and caps on internal temperatures are imposed, the initial design fails to meet the requirements. Bay 3 overheated, and the pressures were too high (relative to the external static pressure on the sides of the vehicle) in several bays.

The SINDA/FLUINT Solver (an optimization and tasking module) is then set up to find new sizes for the intercompartmental openings, inlet, and exhausts (6 design variables in total) that will meet the design requirements (3 constraints) for the flight profile while minimizing the inlet size (the opening at the leading edge to Bay #1). The design that was found decreased total flow, but increased flow through Bays 1 through 3 while reducing flow to Bay 4:

   Inlet to Bay 1:       0.316 in2 (minimized)     (was 1.7 in2)
   Bay 1 to 2:           3.16 in2                  (was 0.6 in2)
   Bay 2 to 3:           1.35 in2                  (was 0.2 in2)
   Bay 3 exhaust:        0.646 in2                 (was 0.1 in2)
   Bay 2 to 4:           0.0223 in2                (was 0.1 in2)
   Bay 4 exhaust:        0.0586 in2                (was 0.02 in2)

Click here to fetch the Compartment Venting Example from our User Forum

FloCAD online training

Class times: May 2 & 4 from 10am to 2pm MT

Cost: $425

This online class will provide an introduction to fluid modeling components within FloCAD. The class will be held over a 2-day period, with daily sessions running approximately 4 hours each. The class uses a mixture of lecture, demonstrations, and self-paced tutorials to allow attendees to practice building fluid system models and interpreting results. The presentations will comprise 2 - 3 hours of each session, and the instructor will be available during the remainder of the time for questions during tutorials. Attendees must have basic working knowledge of SINDA and Thermal Desktop as these topics will not be covered but their usage is required for FloCAD.

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Thermal Desktop, RadCAD, and TD Direct in-class training

Date: April 25-28, 2017, 8:00 a.m. to 5:00 p.m., daily
Location: Lakewood, CO

CRTech will be hosting introductory training for Thermal Desktop, RadCAD and TD Direct. Lecture and hands-on tutorials introduce attendees to basic Thermal Desktop and RadCAD usage and allow practice building models and interpreting results. The class will also introduce students to SpaceClaim direct modeling CAD interface and advanced meshing tools in TD Direct.

Daily Schedule

Day 1 and 2: Introduction to SINDA and Thermal Desktop
Day 3: Introduction to RadCAD
Day 4: Introduction to TD Direct

To learn more about this class and to register, visit our Training Page.

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Using Sinaps? It is not too soon to get started with TD/FloCAD!

This webinar describes the process for converting from Sinaps to Thermal Desktop (TD) and FloCAD. This process includes using an exporter which works with Version 6.0 of the CRTech tool suite (expected to be released in May of 2017).

Come learn about the basics of TD/FloCAD, including many compelling features not available in Sinaps. The webinar will also cover how to manage the transition period, during which you may be using both programs simultaneously. This is also a chance to ask questions. 

If you missed this webinar, please contact us for the presentation material and recording.