Liquid Oxygen Turbopump

Turbomachinery System Validation Case: Advanced Liquid Oxygen Turbopump

A new advanced-cycle rocket engine system has recently undergone testing for the United States Government. The system uses liquid hydrogen and liquid oxygen propellants. This engine system represents a fundamental advance in the state-of-the-art in both system and component-level technologies.

A modeling task was recently undertaken in an attempt to validate SINDA/FLUINT Version 5.0 against the Oxidizer Turbopump of the above-mentioned rocket engine system. A model of the Oxidizer Turbopump and its components was constructed in Thermal Desktop. This model is relatively detailed and includes inducer and impeller pumping elements, fluid-operated bearings, hot-gas turbine drive system, and the internal axial thrust control system. Data for this model was obtained from design reports and engineering drawings. Since the purpose of this modeling exercise was validation of a new computational tool, no attempt was made to tune or otherwise adjust the model to fit experimental data. Formulation of the model elements was limited to “best practices” available. The hardware being simulated is under ITAR export control, so no detailed description can be provided in published form.

Steady State (Primary and Secondary Flows, Axial Thrust Control)

When the model was compared to “flange-to-flange” experimental data (both pump and turbine through-flow under varying circumstances), differences between the model and experimental data were negligible (usually less than 1%).

A more demanding validation test is that of modeling the turbopump internal (or “secondary”) flows and axial thrust balance.

The model was used to simulate a number of steady-state operating points for which data existed. For flow through various bearings and seals (as well as the axial thrust control system), the differences between the model and data were typically less than 10%. It should be borne in mind that this constitutes 10% of a mass flow that is itself approximately 10% of the total pump through-flow. It could therefore be stated the differences between model and data for these secondary flows is in a range comparable to that of the “error bars” of the flow data.

Rotor Start-up Transient (Primary Flows, Torques)

A final validation with experiment consisted of comparison of model results with that of a transient run of the Oxidizer Turbopump. The period simulated began during the initial acceleration of the turbopump, and lasted until steady-state conditions were achieved—approximately 15 seconds. Data from the turbopump test was compared with simulation data at 4 points that represent characteristic “peaks” and “valleys” of the test. In particular, data on shaft speed, pump discharge pressure, and turbine exit temperature were compared to model predictions. The largest difference between model and test data at any given point was 13%. The quantity in question was the turbine exit temperature during the period of highest system acceleration. At all other times (and for all other parameters) the differences between model and data were between 1.5% and 9.5%, with 4% to 5% being a representative average.

Conclusion

SINDA/FLUINT V5.0 was used to simulate the behavior of an advanced Liquid Oxygen Turbopump. The model was built to simulate all of the major pump internal flows that affect efficiency and axial thrust. Validation with both steady-state and transient experimental data was conducted. The correlation with steady-state experimental data was relatively good, with negligible errors between model and data for flange-to-flange operating characteristics. Internal secondary flows and their influence on axial thrust were also modeled. The maximum error in steady-state axial thrust potential between model and data was approximately 4%. The correlation with data from a transient hot-fire test of the turbopump was also good. The maximum error between model and data for this test was 13% for the turbine exit temperature. This occurred when system acceleration was near its peak. The average differences between model and data for all other times of the transient was 4% to 5% for flange-to-flange interface parameters.

For More Information

The hardware being modeled in this exercise (and therefore the model) is under ITAR export control. US Government employees wishing to view the data may refer to SBIR Contract FA9300-06-M-3011. For non-government personnel, additional information is available via the contact list below.

Dave Mohr
D&E Propulsion and Power
Phone: 1-321-267-6296

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.