Engines and Power Cycles, Turbomachinery and Secondary Flows


Turbomachine Components

System-level analysis of jet and rocket engines, power generation cycles, heat pumps and refrigeration loops, etc. can be made using performance map-based descriptions of single- or multi-stage pumps, fans, turbines, and compressors. These elements predict flows and pressure drops, using either directly input maps (single curves or multiple curves per shaft speed) of flow versus pressure drop, or maps specified using equivalent states, reference states, head and flow coefficients, etc. Isentropic efficiencies may be specified, enabling the code to predict shaft power and hydraulic torque. Tables of flow and efficiency relationships are normally input, but options exist for parametric inputs, functional (algorithmic) descriptions, as well as links to turbomachine design software.

  • Pumps: Reference speeds or flow/head coefficients (to exploit pump similarity laws), cavitation detection and modeling (based on either NPSH or Nss) , viscosity corrections, and two-phase flow degradations. Nonmonotonic curves (with positively sloped regions) are permitted.
  • Turbines: Equivalent conditions, including equivalent speed options available. Handling of choking and truncated tables, and two-phase outlet states. Total-total, total-static, and other inlet/outlet state options. Efficiency may optionally be a function of U/C: the blade tip velocity to isentropic spouting velocity (or fluid jet velocity) ratio. Power (or equivalent power) may be specified instead of efficiency.
  • Compressors (Variable displacement): Equivalent conditions, including equivalent speed options available. Handling of choking and surge regimes. Total-total, total-static, and other inlet/outlet state options. Power (or equivalent power) may be specified instead of efficiency. Nonmonotonic curves (with positively sloped regions) are permitted.
  • Compressors (Positive displacement): Flow specified via volumetric efficiency (versus speed and/or pressure ratio) and displacement volume. Power may be specified instead of isentropic efficiency.

Engines and Cycles

Design and analysis of engine or power cycles can include single- or two-phase flow components such as boilers, condensers, regenerative heat exchangers, control valves, etc. in either steady or unsteady analyses. For systems with interconnected turbomachines (e.g., turbochargers, turbopumps, turbojets, etc.), shaft speeds can be predicted to balance torques in steady-states, or shaft/gear mechanical speeds can be solved in transients concurrent with the cycle thermohydraulics.

Secondary Flows

Extensive options exist for modeling passages within rotating machinery, including between rotating and stationary parts. Analysis of secondary coolant, leakage, or lubricating flows can exploit built-in correlations or user-supplied correlations for friction, heat transfer, and torque.

Validation Case

Advanced Liquid Oxygen Turbopump

Additional CRTech Resources

dispersed vs. coalesced front

Tuesday, June 26, 2018, 1-2pm PT, 4-5pm ET

This webinar describes flat-front modeling, including where it is useful and how it works. A flat-front assumption is a specialized two-phase flow method that is particularly useful in the priming (filling or re-filling with liquid) of gas-filled or evacuated lines. It also finds use in simulating the gas purging of liquid-filled lines, and in modeling vertical large-diameter piping.

Prerequisites: It is helpful to have a background in two-phase flow, and to have some previous experience with FloCAD Pipes.

Register here for this webinar

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 is designed to be attended in the order they were presented. 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.