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 model of a loop heat pipe

Since a significant portion of LHPs consists of simple tubing, they are more flexible and easier to integrate into thermal structures than their traditional linear cousins: constant conductance and variable conductance heat pipes (CCHPs, VCHPs). LHPs are also less constrained by orientation and able to transport more power. LHPs have been used successfully in many applications, and have become a proven tool for spacecraft thermal control systems.

However, LHPs are not simple, neither in the details of their evaporator and compensation chamber (CC) structures nor in their surprising range of behaviors. Furthermore, there are uncertainties in their performance that must be treated with safety factors and bracketing methods for design verification.

Fortunately, some of the authors of CRTech fluid analysis tools also happened to have been involved in the early days of LHP technology development, so it is no accident that Thermal Desktop ("TD") and FloCAD have the unique capabilities necessary to model LHPs. Some features are useful at a system level analysis (including preliminary design), and others are necessary to achieve a detailed level of simulation (transients, off-design, condenser gradients).

CRTech is offering a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and are designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts. While the first webinar presumes little knowledge of LHPs or their analysis, for the last three webinars you are presumed to have a basic knowledge TD/FloCAD two-phase modeling.

Part 1 provides an overview of LHP operation and unique characteristics
Part 2 introduces system-level modeling of LHPs using TD/FloCAD.
Part 3 covers an important aspect of getting the right answers: back-conduction and core state variability.
Part 4 covers detailed modeling of LHPs in TD/FloCAD such that transient operations such as start-up, gravity assist, and thermostatic control can be simulated.

May 31, 2018, 1-2pm (PT), 4-5pm (ET)

This webinar provides an overview of LHP design and operation, from a basic understand of components to a review of important performance considerations and limitations.

Many topics will be covered, from start-up issues to the purpose of the evaporator bayonet to capillary flow regulators to load balancing in parallel LHP units. However, we will cover these topics only in enough depth that you will be able to understand the reasons for various modeling approaches that will be covered in later webinars. In other words, this webinar will survey the various ways in which LHPs require a specialized approach to design analysis and simulation.

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Prerequisites: Basic understanding of two-phase thermodynamics and heat transfer.
Please register for Part 1 here

June 5, 2018, 8-9am (PT), 11am-noon (ET)

This webinar explains how the toolbox approach of Thermal Desktop and FloCAD can be used to design and simulate LHPs at a system level, where the focus is on predicting conductance of nominally operating LHP, including thermostatic control (variable conductance).

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Prerequisites: Basic understanding of Thermal Desktop and FloCAD operation as applied to two-phase systems. Basic familiarity with LHP components and operation (see Part 1).
Please register for Part 2 here

June 7, 2018, 8-9am (PT), 11am-noon (ET)

Modeling wick back-conduction in an LHP is critical to accurate prediction of the overall loop conductance and operating point. This prediction can't be separated from an understanding of what is happening in the wick core. This webinar presents time-honored methods of dealing with these complex topics in a relatively simple (if abstract) thermal/fluid network.Prerequisites:

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Basic understanding of Thermal Desktop and FloCAD operation as applied to LHP modeling (see Part 1 and Part 2).
Please register for Part 3 here

June 12, 2018, 1-2pm (PT), 4-5pm (ET)

This webinar explains how Thermal Desktop and FloCAD can be applied to simulate complex and transient phenomena in LHPs, including condenser design, start-up, thermostatic control, and gravity assist (evaporator below condenser). The design of an actual LHP will be used to demonstrate concepts; the implications of attaching large masses to the evaporators (cooled electronics and support structures) will become clear as a result.

This webinar is one of a four-part webinar series on LHPs and approaches to modeling them. Each webinar will last 60 minutes and is designed to be attended in the order they were presented. If you miss one in the series, please check out our video page for a recording, or contact us before the next webinar starts.

Prerequisites: Familiarity with LHP modeling approaches in TD/FloCAD (see Part 1Part 2 and Part 3).
Please register for Part 4 here