Cryogenic Turbopump Research Articles

On the Performance of Hydrostatic Journal Bearings in Air, Water, and Liquid Nitrogen: Measurements and Predictions

Abstract Turbopumps in reusable and low-cost liquid rocket engines require compact and simple hydrostatic (i.e., externally pressurized) bearings to support large static and dynamic loads at high shaft speeds with no bearing DN (diameter in millimeters times rpm) life limitation. Hydrostatic bearings are one of the key mechanical components for cryogenic turbopumps to improve their reliability and reusability. Hydrostatic bearings offer significantly enhanced durability with very low friction and wear while providing accurate rotor positioning as well as large load and static stiffness characteristics even working with low-viscosity liquids. This work measures static load characteristics of hydrostatic journal bearings in air (25 °C), water (from 25 °C to 70 °C), and liquid nitrogen (−197 °C). Air, water, and liquid nitrogen are used as test fluids for the test bearing to evaluate the effects of fluid properties and temperatures on the bearing performance. A complete set of applied static load, bearing eccentricity, and bearing supply pressure for each test fluid (i.e., air, water, and liquid nitrogen) is presented for the test hydrostatic bearing. Measurements show that the applied static load on the test bearing significantly affects the recorded journal eccentricity ratio, bearing flow rate, and bearing static stiffnesses. Predictions are in good agreement with measurements. The predictable performance of hydrostatic journal bearings in air, water, and liquid nitrogen can further their application in various cryogenic turbopumps for liquid rocket engines.

Friction Characteristics of SUS440C/Si3N4 Lubricated by Polytetrafluoroethylene Under Cryogenic Environment

Abstract The friction characteristics of SUS440C-Si3N4 lubricated by polytetrafluoroethylene (PTFE) under cryogenic environment have not been researched well. The lubrication mechanisms in such friction condition need to be clarified to improve the fidelity of performance modeling for ball bearings used in liquid rocket engine turbopumps. This research experimentally investigated the friction coefficient of bearing materials via PTFE lubrication under cryogenic environment to clarify the friction characteristics of cryogenic turbopump ball bearings. These experiments revealed the dependence of the contact pressure on the friction coefficient. By several analyses, it was found that lower friction and better wear-resistance characteristics were caused by PTFE film that strongly adhered to metal fluoride formed on the surface of SUS440C due to moderate friction energy.

Influence of Rotating Speeds and Thermodynamic Effects on LOX Turbopump Performance

As a key factor of a reusable turbopump, axial thrust directly determines the reliability of the bears and seals, especially considering the influence of rotational speeds and thermodynamic effects. In this study, numerical simulations and experimental tests were conducted to explore the impact of rotational speeds and the divergences of the turbopump performance with/without the thermodynamic effects. The experimental data obtained at 15000 r/min are in good agreement with the numerical results, illustrating the validity of the numerical model. The results show that the head deviations based on the affinity laws are relatively smaller, only 0.1% difference at normal condition, yet the efficiency increases from 72.5% to 73.8%, which need to be modified through the formula correction. Notably, the axial thrusts acting on impeller are significantly affected by the rotational speeds, and the values at rated flowrate decrease 17.6% after increasing the operating condition to rated speed. The affinity laws are not applicable for obtaining the turbopump axial thrusts under different speeds. Furthermore, due to the changes of the local physical properties, the values of head coefficient and efficiency decrease 2.8% and 1.3% at rated flowrate, respectively, when the thermodynamic effects of liquid oxygen are considered. Meanwhile, the relative variations of axial thrust are between 1.46% and 3.74% within the whole flow range. Finally, an in-depth analysis of the internal flow was conducted through the velocity field and vorticity method. In conclusion, both the rotational speeds and thermodynamic effects significantly affect the performance of a cryogenic turbopump, especially the axial thrust, and the unsteady results of the rotor-stator cavity leakage flow need to be analyzed in the future.

Research on the Power Loss of High-Speed and High-Load Ball Bearing for Cryogenic Turbopump

This paper studies the lubrication characteristics of ball bearings for cryogenic turbopumps. First, the frictional coefficients between 440C and a Ag coating, 440C and solid PTFE (polytetrafluoroethylene), and 440C and a PTFE coating in LN2 (liquid nitrogen) are obtained using a ball-on-disk testing machine under a high sliding speed in the range of 0 to 8 m/s and a high contact stress in the range of 2.5 to 3.5 GPa. Dynamic and power loss models of high-speed and high-load ball bearings are established to study the key factors affecting the heat generation characteristics. In order to verify the correctness of these two theoretical models, a coupled fluid-thermal finite element model is built to evaluate the temperatures of the outer ring under different bearing speeds, which are then proved by experiments with ball bearings for cryogenic turbopumps. The results show that the power loss due to the spinning-sliding of the ball and the churning and drag of LN2 account for more than 80% of the total power loss; the spin-roll ratio of the ball on the inner raceway is a key indicator for this type of ball bearing, and the relatively small radial clearance and contact angle are suggested. Both of the proposed theoretical models have sufficient accuracy and can be used in the performance evaluation and optimization design of bearings.

Numerical Prediction of Internal Flows in He/LOx Seals for Liquid Rocket Engine Cryogenic Turbopumps

Cryogenic turbopumps are used in high-performance, lightweight liquid rocket engines for space applications. The development of bearings and shaft seals for cryogenic turbopumps requires detailed characterization of the internal flow, taking into account the effects of boiling and multi-component two-phase flow. Here, a flow network solver was developed to analyse the secondary flow circuit of a cryogenic turbopump where the propellant is mixed with high-temperature helium after bearing cooling. The network solver is based on an extension of a classic 1D homogeneous model, originally developed for a pure substance, to the case of two-phase multi-component flow. The solver is capable of predicting pressures, temperatures, flow rates, and species concentrations in a complex two-phase flow in the presence of non-condensable gases. The unsteady mass, momentum, and energy conservation equations are implemented in conjunction with the thermodynamic equations of state using a general-purpose finite volume formulation, where the pressure drop and the heat transfer are calculated using correlations. The numerical tool was validated by comparing its predictions with experimental data obtained during tests on the secondary circuit of an oxygen turbopump developed at Avio S.p.A. A number of engine operating conditions were considered (inlet helium temperature in the range of 250–280 K, helium/liquid oxygen drain in the range of 165–230 K). The predicted temperature values showed good agreement with the experimental data in most conditions.

Numerical simulation of cryogenic cavitating flow in LRE oxygen turbopump inducer

Turbopump is a critical component of modern liquid rocket engine (LRE), which employs cryogen as propellant. Its working performance is closely relevant to the reliability of LRE. The performance improvement of turbopump is limited by cryogenic cavitation condition. While the cryogenic cavitating flow characteristic is not well understood till now. A novel numerical method for cryogenic cavitation simulation was developed based on a transport-based cavitation model in the present investigation. The real thermodynamic properties of fluids and an additional energy source item which takes the heat transfer during phase change into account were imbedded into the code, and the empirical constants of the cavitation model are unnecessary to alter. It is found that a moderate reduction of released heat by vapor condensation in the energy source item provides better prediction accuracy for temperature distributions, and an optimal reduction proportion has been suggested. The cavitating flow features inside a three-bladed inducer using liquid oxygen as working fluid have been investigated in detail based on the proposed method, thermal effect reduces both the cavitation regions and the vapor volume fraction inside cavities remarkably, producing less blocking effect on the blade flow channels, which delays the head breakdown. Higher temperature shows stronger thermal effect and better cavitation performance. A classical semiempirical theoretical model is introduced to quantitatively predict the influence of thermal effect on cavitation performance. The predicted results are in consistent with the calculated results. The developed numerical model shows favorable universality to act as a reliable tool for cryogenic turbopump design.

Heat Generation in Cryogenic Turbopump Bearing

Heat Generation in Cryogenic Turbopump Bearing

Heat Generation in Cryogenic Turbopump Bearing

Heat Generation in Cryogenic Turbopump Bearing

New Insight Into Aspect Ratio's Effect on Secondary Losses of Turbine Blades

Abstract In view of improving the understanding of the loss mechanisms existing on a compact turbine driving a cryogenic engine turbo-pump for a satellite delivering rocket, a new perspective on how the aspect ratio of turbine blades affects the secondary and profile losses is presented. This perspective, originally based on published experimental data, is further developed by a series of back-to-back highly resolved computational fluid dynamics (CFD) numerical simulations, with the aim of acquiring further insight into the dynamics of the secondary vortices, and the intermediate boundary layer flow for varying blade heights. The main outcome redefines the extreme cases and the partition of losses between secondary and profile, and establishes a new critical ultra-low aspect ratio of 0.35 as a threshold distinguishing two different behaviors. The final venue of this new perspective is the possibility to further improve existing off-design turbine loss models, like those presented by Craig–Cox and Ainley–Mathieson.

Amorphous Carbon Films In Situ Formed From PTFE Transfer Layer in Solid Lubricated Cryogenic Turbopump Bearings

In order to fully evaluate the service reliability of cryogenic turbopump bearings, a newly developed solid lubricated bearing was tested using a custom ground bearing test system cooled by water. Stains with different color intensity were occasionally exhibited on the surfaces of the bearing balls after ground tests. This may pose a risk for the reliable service of the related bearings in real flight missions. Then the chemical composition and structural information of the stain layers were systematically analyzed by x-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and high-resolution transmission electron microscope (HRTEM). The results manifested that the stain layer had the characteristic of α-C film. The color intensity of stains was not caused by the structural changes, but was due to the difference of stain layers’ thickness. The α-C films in situ formed during bearings operation should result from the defluorination of polytetrafluoroethylene (PTFE) layers, which transferred from retainer to balls in the previous step. Correspondingly, FeF2 was produced on the surfaces of the tested bearing rings as a by-product. Although the tested bearings with α-C films formed on the surfaces of balls did not reach the state of final failure, the affected balls showed high surface roughness, which could be an indicator of an imminent bearing failure.

Performance comparative analysis of hydrostatic bearings lubricated with low-viscosity cryogenic fluids

Abstract The lubricant for the hydrostatic bearings in an operating cryogenic turbopump is generally liquid hydrogen or liquid oxygen. However, in experiments, these are typically replaced by water or liquid nitrogen for economical and safety reasons. In this study, the static performance characteristics of an orifice-compensated hydrostatic bearing in these four lubrication-fluid cases are calculated and compared by numerically solving the turbulent Reynolds equation. The effects of the fluid viscosity and density on the bearing performance are discussed. A combined parameter of viscosity and density is introduced to describe the effect of the fluid properties on bearing performance. Ultimately, four improved bearings based on the original bearing are proposed, one each for the four fluids, to achieve better performance.

Turbulence Modeling of Cavitating Flows in Liquid Rocket Turbopumps

An accurate prediction of the performance characteristics of cavitating cryogenic turbopump inducers is essential for an increased reliance on numerical simulations in the early turbopump design stages of liquid rocket engines (LRE). This work focuses on the sensitivities related to the choice of turbulence models on the cavitation prediction in flow setups relevant to cryogenic turbopump inducers. To isolate the influence of the turbulence closure models for Reynolds-Averaged Navier–Stokes (RANS) equations, four canonical problems are abstracted and studied individually to separately consider cavitation occurring in flows with a bluff body pressure drop, adverse pressure gradient, blade passage contraction, and rotation. The choice of turbulence model plays a significant role in the prediction of the phase distribution in the flow. It was found that the sensitivity to the closure model depends on the choice of cavitation model itself; the barotropic equation of state (BES) cavitation models are far more sensitive to the turbulence closure than the transport-based models. The sensitivity of the turbulence model is also strongly dependent on the type of flow. For bounded cavitation flows (blade passage), stark variations in the cavitation topology are observed based on the selection of the turbulence model. For unbounded problems, the spread in the results due to the choice of turbulence models is similar to noncavitating, single-phase flow cases.

A Water-Lubricated Hybrid Thrust Bearing: Measurements and Predictions of Static Load Performance

Process fluid-lubricated thrust bearings (TBs) in a turbomachine control rotor placement due to axial loads arising from pressure fields on the front shroud and back surface of impellers. To date, prediction of aerodynamic-induced thrust loads is still largely empirical. Thus, needs persist to design and operate proven TBs and to validate predictions of performance derived from often too restrictive computational tools. This paper describes a test rig for measurement of the load performance of water-lubricated hydrostatic/hydrodynamic TBs operating under conditions typical of cryogenic turbo pumps (TPs). The test rig comprises of a rigid rotor composed of a thick shaft and two end collars. A pair of flexure-pivot hydrostatic journal bearings (38 mm in diameter) supports the rotor and quill shaft connected to a drive motor. The test rig hosts two TBs (eight pockets with inner diameter equal to 41 mm and outer diameter equal to 76 mm); one is a test bearing and the other is a slave bearing, both facing the outer side of the thrust collars on the rotor. The slave TB is affixed rigidly to a bearing support. A load system delivers an axial load to the test TB through a nonrotating shaft floating on two aerostatic radial bearings. The test TB displaces to impose a load on the rotor thrust collar, and the slave TB reacts to the applied axial load. The paper presents measurements of the TB operating axial clearance, flow rate, and pocket pressure for conditions of increasing static load (max. 3600 N) and shaft speed to 17.5 krpm (tip speed 69.8 m/s) and for an increasing water supply pressure into the TBs, max. 17.2 bar (250 psig). Predictions from a bulk flow model that accounts for both fluid inertia and turbulence flow effects agree well with recorded bearing flow rates (supply and exiting through the inner diameter), pocket pressure, and ensuing film clearance due to the imposed external load. The measurements and predictions show a film clearance decreasing exponentially as the applied load increases. The bearing flow rate also decreases, and at the highest rotor speed and lowest supply pressure, the bearing is starved of lubricant on its inner diameter side, as predicted. The measured bearing flow rate and pocket pressure aid to the empirical estimation of the orifice discharge coefficient for use in the predictive tool. The test data and validation of a predictive tool give confidence to the integration of fluid film TBs in cryogenic TPs as well as in other more conventional (commercial) machinery. The USAF Upper Stage Engine Technology (USET) program funded the work during the first decade of the 21st century.

Influence of the Balls Kinematics of Axially Loaded Ball Bearings on Coulombic Frictional Dissipations

Ball bearings have been used for a long time. Nevertheless, the description of their behavior remains incomplete in spite of the large number of surveys dedicated to ball bearings. Particularly, the exact balls kinematics has still to be addressed in depth. This paper proposes a new way to calculate the balls kinematics by using a simplified quasi-static approach for dry lubricated and axially loaded ball bearings. This method does not use the classical restrictive race control assumptions. More specifically, the role played by the balls kinematics is emphasized by means of the power dissipated within contacts between balls and races. The need for a correct evaluation of the balls behavior is illustrated by using an example, viz., a ball bearing of cryogenic engine turbopump. Indeed, the dissipated power is one of the main concerns in this particular case.

액체질소를 이용한 산화제펌프의 극저온 성능시험

초록: 액체질소를작동유체로한극저온시험설비를이용하여산화제펌프의성능시험을수행하였다. 설계회전수의30~55%영역에서시험을수행하였으며, 그결과를작동유체로물을이용한상온수류시험의결과와비교/분석하였다. 수력성능에있어서는회전수에대한상사성을만족시킴으로써, 설계회전수인20000rpm에서의성능예측을가능하게했다. 펌프의극저온흡입성능에서는설계유량에서극저온임계캐비테이션수가0.012으로나타났으며, 상온수류시험의경우는0.024를보이면서, 모든시험회전수와시험유량영역에서수류시험의경우보다향상된결과를보였다. 이러한향상된극저온환경에서의흡입성능은극저온유체에서펌프의열역학적인효과로부터기인하는것으로판단된다.Abstract: Performance tests of a liquid-oxygen pump were carried out using liquid nitrogen (LN2) as a working fluidin a cryogenic turbopump test facility in Korea Aerospace Research Institute (KARI). The tests were performed at30-55% of the design rotational speed, and the results were compared with those from a water test. The experimentalresults confirmed the similarity of the hydraulic performance, which allows the prediction of the pump performance ata design rotational speed of 20,000 rpm. The overall cavitation performance of the pump in the cryogenic environmentwas better than that in the water environment for all ranges of flow rates and rotational speeds. Critical cavitationnumber at the design flow rate was determined as 0.012 from the cryogenic test, and as 0.024 from the water test.The improved cavitation performance is due to the thermodynamic effect in cryogenic fluids.

Theoretical Analysis of Thermodynamic Effect of Cavitation in Cryogenic Inducer Using Singularity Method

Vapor production in cavitation extracts the latent heat of evaporation from the surrounding liquid, which decreases the local temperature, and hence the local vapor pressure in the vicinity of cavity. This is called thermodynamic/thermal effect of cavitation and leads to the good suction performance of cryogenic turbopumps. We have already established the simple analysis of partially cavitating flow with the thermodynamic effect, where the latent heat extraction and the heat transfer between the cavity and the ambient fluid are taken into account. In the present study, we carry out the analysis for cavitating inducer and compare it with the experimental data available from literatures using Freon R‐114 and liquid nitrogen. It is found that the present analysis can simulate fairly well the thermodynamic effect of cavitation and some modification of the analysis considering the real fluid properties, that is, saturation characteristic, is favorable for more qualitative agreement.

Hydrostatic lubrication with cryogenic fluids

The paper presents a computational procedure that can be useful in assessing the main performance parameters (load, flow rate, attitude angle) of hydrostatic journal bearings, without resorting to full numerical computing. The procedure is based on the ‘lumped resistance method’, which has been extended to take into account turbulence and inertia effects. Comparison with available experimental data shows a pretty reasonable agreement, in spite of the approximations that have been introduced. The procedure has been used to design a number of bearings that have been tested in order to study the possible replacement of rolling bearings in cryogenic turbopumps.

High‐Pressure Annular Seal Leakage and Rotordynamics with Application to Turbomachinery

The paper presents theoretical investigations that deal with the steady state mass flow rate, labyrinth seal-influence coefficients, and unbalance response of a cryogenic turbopump. A numerical method for calculating the leakage flow rates, stiffness, and damping coefficients of liquid hydrogen‐ lubricated seals is presented. Rotation and seal surface roughness effects are also included for leakage and dynamic force calculation. A finite element solution of a Reynolds equation is obtained for steady state as well as dynamic pressure distributions produced by a small amplitude whirl of the seal journal center (a first order perturbation solution). Film rupture is taken into account with the boundary to the ruptured film zone determined by an iterative procedure. A brief procedure is presented for modeling of the rotor/seals system. This work extends the previous theory for plain centered seals to large eccentricities using a perturbation analysis, which includes surface roughness effects. Investigations have been carried out for seal breadth-to-diameter ratio (BDR) of 0.25 to 0.5. The performance of a convergent and three-stepped seal is estimated. Numerical data for these seals for an eccentricity ratio up to 0.8 has been obtained.

Rotordynamic Evaluation of a Roughened-Land Hybrid Bearing

Hybrid bearings represent an attractive alternative to ball bearings for use in highspeed cryogenic turbopumps. However, the internally-developed cross-coupled forces can generate instabilities responsible for a speed limitation of the machine. To reduce these forces and raise the onset speed of instability, the use of deliberately-roughened stators, already successfully tested for liquid “damper” seals, is investigated. Rotor-dynamic results are presented for a five-pocket orifice-compensated hole-pattern-land hybrid bearing tested with water at high speed and high pressure. Experimental data show a good prediction of leakage flow rate and direct damping but a significant improvement in stability compared to a conventional smooth-land hybrid bearing, resulting in an elevation of the onset speed of instability. Comparisons between measurements and predictions from a code developed by San Andres (1994) shows good predictions for flowrate and direct damping but an over prediction for the direct and cross-coupled stiffness coefficients by about 30 and 50 percent, respectively. The use of the Moody friction-factor model is thought to be mainly responsible for the poorer predictions of stiffness coefficients.

Two-Phase Flow in Floating-Ring Seals for Cryogenic Turbopumps

A flow visualization study on the two-phase flow in floating-ring seals for cryogenic fluid was carried out to identify the two-phase flow area (A2) inside the sealing clearance induced by viscous frictional heating and the pressure drop. The effect of the two-phase flow area on leakage was investigated by comparing leakage measurements (Q) and the values of A2 obtained from observed results. A2 increased with increasing rotational speed; however, the increase of A2 did not always bring about a decrease in the mass leakage rate. Moreover, the effect of the two-phase flow on the reduction of leakage was discussed by comparing the measurements of Q with the computational results (Qliquid) obtained from the incompressible flow equation. It was confirmed that the effect of the two-phase state on the reduction in leakage can be broadly classified into the following two effects: effect of the all-liquid choked flow and effect of the two-phase flow area (A2) inside the sealing clearance. In a high degree of subcooling conditions, the increase in A2 tends to depress the reduction effect on the leakage rate. Presented as a Society of Tribologists and Lubrication Engineers paper at the ASME/STLE Tribology Conference in Toronto, Ontario, Canada, October 26–28, 1998