Cryogenic Turboexpander Research Articles

Simulation studies on aerostatic thrust bearings with special reference to cryogenic turboexpanders

The present paper details theoretical studies on aerostatic thrust bearings with special reference to cryogenic turboexpander rotors. A special case of 4.5K, 250W helium refrigerator has also been considered where both the turboexpanders are supported by aerostatic thrust bearings of similar dimensions. The bearings experience continuously changing ambient (discharge) pressure during the start-up and shut-down transients, thereby requiring a proper estimation of bearing characteristics for successful turboexpander operation at all times. Performance charts, in terms of load capacity, stiffness and bearing gas flow with respect to change in clearance have been presented for different bearing gas supply and discharge pressures.

Numerical study on the spontaneous condensation flow in an air cryogenic turbo-expander using equilibrium and non-equilibrium models

The difficulty of data measurement in cryogenic environments and the complicated mechanism of nucleation process have restricted the design of wet type turbo-expander for cryogenic liquid plants. In this paper, equilibrium and non-equilibrium models are used to model the spontaneous condensation flow in a cryogenic turbo-expander along the main stream passage including nozzle, impeller and diffuser. The comparison shows a distinct difference of the predicted wetness fraction distribution along the streamline between the equilibrium model and the non-equilibrium model. In non-equilibrium model, the distributions of supercooling and nucleation rate along the length of turbo-expander are given for the analysis of flow characteristics. The comparison of outlet wetness fraction with the experimental data is also provided for verification and discussion. Both the effects of the rotation on nucleation and the effects of the nucleation on flow along suction side of the impeller are investigated.

Influence of thermophysical properties of working fluid on the design of cryogenic turboexpanders using nsds diagram

Cryogenic turboexpanders are an essential part of liquefaction and refrigeration plants. The thermodynamic efficiency of these plants depends upon the efficiency of the turboexpander, which is the main cold generating component of these plants, and therefore, they should be designed for high thermodynamic efficiencies. Balje's [1] nsdschart, which is a contour of isentropic efficiencies plotted against specific speed and specific diameter, is commonly used for the preliminary design of cryogenic turboexpanders. But, these charts were developed based on calculations for a specific heat ratio (γ) of 1.4, and studies show that care should be taken while implementing the same for gases which have a higher γ of 1.67. Hence there is a need to investigate the extent of applicability of nsds diagram in designing expansion turbines for higher specific heat ratios. In this paper, Computational Fluid Dynamics (CFD) analysis of cryogenic turboexpanders was carried out using Ansys CFX®. The turboexpanders were designed based on the methodologies prescribed by Kun and Sentz [2] following the nsds diagram of Balje and Hasselgruber's technique for generating blade profile. The computational results of the two cases were analysed to investigate the applicability of Balje's nsds diagram for the design of turboexpanders for refrigeration and liquefaction cycles.

Wetness loss prediction for a wet-type cryogenic turbo-expander based on 3-D numerical simulation

With growing demand for the high-efficiency cryogenic liquid plants, it becomes more and more attractive to develop wet-type cryogenic turbo-expander for high cooling capacity. Spontaneous condensation has significant influence on the thermal efficiency of the wet-type turbo-expander. In this study, the Baumann method is used to calculate the wetness loss in the cryogenic turbo-expander. Two reference expansions, including dry and equilibrium condensation, are modeled with the help of computational fluid dynamics (CFD) method. The model is verified by the experiment measurements on an air turbo-expander. The numerical results show that a uniform value of Baumann factor is unable to accurately predict the loss for certain wetness fractions, especially leading to an underestimation at low wetness fraction. Hence, Baumann factors are determined as a function of wetness fraction based on the experimental data and CFD results. Quantitative analysis on the losses in dry and wet expansions is presented. The proposed 3-D numerical method with wetness-loss-correction Baumann factors has been demonstrated for predicting the wetness loss of the cryogenic turbo-expander.

초전도 전력 케이블 냉각 시스템 적용을 위한 극저온 터보 팽창기 설계 및 해석

The cryogenic cooling system should maintain the HTS power cable below 77 K. As the length of HTS power cables has increased, there have been many efforts to develop large capacity cryocoolers. Brayton, Joule-Thomson, and Claude refrigerators were considered for the large capacity cryocooler. Among the various cryocoolers, the Brayton refrigerator is the most competitive in terms of the HTS power cable. At present, it is thought that a 10-kW class refrigerator will be able to be used as a unit cooling system for the commercialization of HTS power cables in the near future. The Brayton refrigerator is composed of recuperative heat exchangers, a compressor, and a cryogenic turbo expander. Among the various components, the cryogenic turbo expander is the part that decreases the temperature, and it is the most significant component that is closely related with overall system efficiency. It rotates at high speed using high-pressure helium or neon gas at cryogenic temperatures. This paper describes the design of a 300-W class Brayton refrigeration cycle and the cryogenic turbo expander as a downscale model for the practical 10-kW class cycle. Flow and structural analyses are performed on the rotating impeller and nozzle to verify the efficiency and the design performance.

The measurement of thermodynamic performance in cryogenic two-phase turbo-expander

Liquid fraction measurement in cryogenic two-phase flow is a complex issue, especially for an industrial cryogenic system. In this paper, a simple thermal method is proposed for measuring the liquid fraction in cryogenic two-phase turbo-expander by an electric heating unit in experimental study. The liquid fraction of the cryogenic two-phase flow is determined through the heat balance built at the outlet of the turbo-expander (inlet of heating unit) and the outlet of the heating unit. Liquid fractions from 1.16% to 5.02% are obtained from five two-phase expansion cases. Under the same turbo-expander inlet pressure and rotating speed, five superheated expansion cases are tested to evaluate the wetness loss in two-phase expansion. The results show that the proposed method is successful in measuring the liquid fraction of cryogenic two-phase expansion for turbo-expander in an industrial air separation plant. The experimental isentropic efficiency ratio and the tested Baumann factor decrease with the increasing mean wetness. Based on prediction of Baumann rule, the cryogenic turbo-expander with low liquid fraction in two-phase expansion cases suffers from more severe wetness loss than that with the higher liquid fraction.

Study on the Dynamic Performance of the Helium Turboexpander for EAST Subsystems**supported by Joint Funds of the National Natural Science Foundation of China (No. 11176023), National Natural Science Foundation of China (No. 51306135), and partially supported by the China Postdoctoral Science Foundation (No. 2013M532040) and Special Financial Grant of China Postdoctoral Science Foundation (No. 2014T70917)

An increase of the cooling capacities in the liquid helium temperature area is required by Experimental Advanced Superconducting Tokamak (EAST) due to the extension of its subsystems in the near future. Limited by the heat exchangers, cryogenic pipes, and cryogenic valves, it is difficult to enlarge the present EAST helium system. 102 W@4.5 K level helium cryogenic systems are needed in view of feasibility and economy. A turboexpander is the key component of a helium cryogenic system. In this article, a hydrostatic gas lubricated cryogenic helium turboexpander for a 900 W@4.5 K cryogenic helium system was developed for the EAST updated subsystem by the Institute of Plasma Physics, Chinese Academy of Sciences and the Institute of Cryogenic and Refrigeration of Xi'an Jiaotong University. The main components, such as gas bearings, expansion wheel, shaft, and brake wheel, were briefly presented. The dynamic performance of the journal and thrust gas bearings was investigated numerically. The rotordynamic performance of the developed turboexpander was studied experimentally. The results show that the axial and radial load capacities supplied by the journal gas bearing and thrust gas bearing are enough to balance the axial force and radial force of the rotor. A 43% overspeed operation was achieved, which validated the reasonable design of the turboexpander.

Automatic PID Control Loops Design for Performance Improvement of Cryogenic Turboexpander

Cryogenics field involves temperature below 123 K which is much less than ambient temperature. In addition, many industrially important physical processes—from fulfilling the needs of National Thermonuclear Fusion programs, superconducting magnets to treatment of cutting tools and preservation of blood cells, require extreme low temperature. The low temperature required for liquefaction of common gases can be obtained by several processes. Liquefaction is the process of cooling or refrigerating a gas to a temperature below its critical temperature so that liquid can be formed at some suitable pressure which is below the critical pressure. Helium liquefier is used for the liquefaction process of helium gas. In general, the Helium Refrigerator/Liquefier (HRL) needs turboexpander as expansion machine to produce cooling effect which is further used for the production of liquid helium.Turboexpanders, a high speed device that is supported on gas bearings, are the most critical component in many helium refrigeration systems. A very minor fault in the operation and manufacturing or impurities in the helium gas can destroy the turboexpander. However, since the performance of expanders is dependent on a number of operating parameters and the relations between them are quite complex, the instrumentation and control system design for turboexpander needs special attention. The inefficiency of manual control leads to the need of designing automatic control loops for turboexpander. Proper design and implementation of the control loops plays an important role in the successful operation of the cryogenic turboexpander. The PID control loops has to be implemented with accurate interlocks and logic to enhance the performance of the cryogenic turboexpander. For different normal and off-normal operations, speeds will be different and hence a proper control method for critical rotational speed avoidance is must. This paper presents the design of PID control loops needed for the efficient performance of cryogenic turboexpander (Radial Inflow type) to ensure that the control systems meet the technical conditions and constraints more accurately and ensure the equipment safety.

CFD Analysis of Turbo Expander for Cryogenic Refrigeration and Liquefaction Cycles

Computational Fluid Dynamics analysis has emerged as a necessary tool for designing of turbomachinery. It helps to understand the various sources of inefficiency through investigation of flow physics of the turbine. In this paper, 3D turbulent flow analysis of a cryogenic turboexpander for small scale air separation was performed using Ansys CFX®. The turboexpander has been designed following assumptions based on meanlineblade generation procedure provided in open literature and good engineering judgement. Through analysis of flow field, modifications and further analysis required to evolve a more robust design procedure, have been suggested.

Collisionless tool orientation smoothing above blade stream surface using NURBS envelope

In five-axis machining, tool orientation above a blade stream surface may lead to tool collision and a decrease in workpiece rigidity. Hence, collisionless tool orientation smoothing (TOS) becomes an important issue. On the basis of a constant scallop height tool path, the triangular facets in the faces, vertices format are constructed from cutter contact (CC) using the Voronoi incremental algorithm. The cutter location (CL) points candidate set is represented by an oblique elliptic cone whose vertex lies at CC using NURBS envelope. Whether the CL point is above its CC is judged by the dot product between the normal vector and the point on triangulation nearest to the CL point. The curvatures at CC are obtained by fitting a moving least square (MLS) quadratic patch to the local neighborhood of a vertex and calculating eigenvectors and eigenvalues of the Hessian matrix. Triangular surface elastic energy is employed as the weight in selection from the NURBS envelope. The collision is judged by NURBS surface intersection. TOS can then be expressed by selecting a CL point for each CC point and converted into a numerical control (NC) code automatically according to the postprocessor type of the machine center. The proposed method is verified by finishing of a cryogenic turboexpander impeller of air separation equipment.

Calculation of the Critical Speed and Stability Analysis of Cryogenic Turboexpanders with Different Structures

A modularized code based on the Finite Element QZ (FEQZ) method is developed, for a better estimate of the critical speed and a more convenient method of rotor-dynamic stability analysis for a gas bearing high speed turboexpander rotor system with actual structure and application of a cryogenic turboexpander. This code is then validated by the experimental data of a gas bearing turboexpander, with a rotor diameter of 25 mm and a rated speed of 106,400 rpm. With this code, four rotors with different structures, available to the turboexpander, are parametrically analyzed by the available speed range, vibration modes and logarithmic attenuation rate. The results suggest that the rotor with a structure of two thrust collars on the system exhibits a better performance in the designed conditions.

Development of bearings for a small high speed cryogenic turboexpander

PurposeThe purpose of this paper is to examine the development of aerodynamic thrust bearings and aerodynamic journal bearings applied to a small high speed cryogenic turboexpander in Indian conditions.Design/methodology/approachAs a part of indigenous programme in the process of development, some input parameter was taken from the available literature and then dimensions were optimized and computed while taking care to minimize fabrication constraints.FindingsA series of tests were conducted to confirm the findings. Detailed study of the effect of stability and vibration of bearings was taken up. The maximum rotational speed obtained was 200,000 rpm.Practical implicationsThe outcome may help the designers, researchers and manufacturers of these components.Originality/valueAlthough manufacturers design and develop the bearings of turboexpanders for their production, no academic literature has been available for this purpose until now.

Dynamic Stability Study of Static Gas Bearing for Small Cryogenic Turbo-Expander

An experimental method is presented to analyze the dynamic stability of the gas bearing for small cryogenic turbo-expanders. The rotation imbalance response and the shape of the rotor orbit were obtained for different speeds up to 110,000 rpm, and the critical speed of the rotor-bearing system was determined by a Bode diagram. An FFT signal analytical method was applied to identify the resonance frequency, and the waterfall plot was presented. During the whole process of speeding up to the designed speed of 110,000 rpm, the rotor-bearing works stably with no whirl instability, which is validated in a waterfall plot. Also, the tested rotor-bearing model was analyzed theoretically. It was proved that the experimental results were highly consistent with those of theoretical calculations. Thus the experimental method proposed here to analyze the dynamic stability of the gas bearing is feasible.

Mathematical Analysis for Off-Design Performance of Cryogenic Turboexpander

A study has been conducted to determine the off-design performance of cryogenic turboexpander. A theoretical model to predict the losses in the components of the turboexpander along the fluid flow path has been developed. The model uses a one-dimensional solution of flow conditions through the turbine along the mean streamline. In this analysis, the changes of fluid and flow properties between different components of turboexpander have been considered. Overall, turbine geometry, pressure ratio, and mass flow rate are input information. The output includes performance and velocity diagram parameters for any number of given speeds over a range of turbine pressure ratio. The procedure allows any arbitrary combination of fluid species, inlet conditions, and expansion ratio since the fluid properties are properly taken care of in the relevant equations. The computational process is illustrated with an example.

A numerical model for the design of a mixed flow cryogenic turbine

Present day cryogenic gas turbines are in more popular as they meet the growing need for low pressure cycles. This calls for improved methods of turbine wheel design. The present study is aimed at the design of the turbine wheel of mixed flow impellers with radial entry and axial discharge. In this paper, a computer code in detail has been developed for designing such turbine wheel. To determine the principal dimensions of the turbine wheel, optimum operating speed has been taken from design charts based on Similarity principles. The algorithm developed, allows any arbitrary combination of fluid properties, inlet conditions and expansion ratio, since the fluid properties are properly taken care of in the relevant equations. The computational process is illustrated with an example. The main dimensions, thermodynamic properties at different states, velocity and angles at entry and exit of the turbine wheel were worked out. The work may help the researchers for further design and development of cryogenic turboexpander depending on their operating parameters. Keywords: Cryogenic, turboexpander, mixed flow turbine, flow angle, flow velocity

Experimental performance study of cryogenic turboexpander by using aerodynamic thrust bearing

Abstract An indigenous programme on design and development of a small high speed cryogenic turboexpander has been taken up at NIT, Rourkela. This paper aims at the development of a small high speed cryogenic turboexpander in Indian condition by using aerodynamic thrust bearings. Attention has been paid to the study of the performance of turboexpander and the effect of stability on the vibration of bearings. Series of tests were conducted to confirm the findings. The maximum rotational speed obtained was 200,000 rpm and a temperature drop of 30 °C. The outcome may help the designers, researchers and manufacturer of these components.

Comparative test on two kinds of new compliant foil bearing for small cryogenic turbo-expander

Two kinds of new compliant foil journal bearing for a small cryogenic turbo-expander used in the Brayton cycle cryocooler are presented in this paper. The foil of bearings is supported with copper wires and elastic material respectively, so the bearings are much simpler in structure than previous compliant foil bearing. The performances of the bearings are studied under the same experiment conditions. The results indicate that both bearings have better damp performance and the foil bearing supported with elastic material has preferable stability to the one supported with copper wires.

The Experimental Study of Aerodynamic Plate-Foil Journal Bearings for High Speed Cryogenic Turboexpander

Experiments have been conducted on the application of a new type of gas lubricated self-acting journal bearing—plate-foil bearing to a high speed cryogenic turboexpander. The 25.0 mm diameter bearing consists of a rigid housing and a flexible foil element; different structure parameters of the bearing are selected in the experiments. Experimental results indicate the foil bearing presented here offers better system stability performance and high durability, and it confirmed that the application of this type of plate-foil bearing to the cryogenic turboexpander is feasible. The experiment results, including the effect of the plate-foil bearing structure parameters on the rotor-bearing system stability, are described in this paper. Presented at the 54th Annual Meeting Las Vegas, Nevada May 23–27, 1999

Foil Air Bearings Cleared to Land

This article explores use of foil air bearings in land-based turbomachinery. A machine with foil air bearings is more reliable than one with rolling element bearings because it requires fewer parts to support the rotative assembly and needs no lubrication. Foil air bearings can handle severe environmental conditions such as the ingestion of sand and dust. A reversed pilot design at the cooling flow inlet prevents large particles from entering the bearing's flow path, and smaller particles are continually flushed out of the bearing by the cooling flow. Many applications of foil air/gas bearings other than air cycle machines have been built and successfully tested, but nothing appears to be currently in production. Foil bearings have strong potential in several applications. Among these are small general aviation gas turbine engines; oil-free cryogenic turboexpanders for gas separation plants; auxiliary power units for various aerospace and ground vehicles; and, taking advantage of automated manufacturing methods, automotive gas turbine engines, vapor-cycle centrifugal compressors, and commercial air/gas compressors.

Development of aerodynamic foil journal bearings for a high speed cryogenic turboexpander

The research presented in this paper is aimed at the development of aerodynamic foil journal bearings applying to a small high speed cryogenic turboexpander. A small high speed cryogenic turboexpander is designed. Attention has been paid to the study of the effect of foil stiffness on the vibration performance of bearings. From rotation tests, it is clear that, with the proper choice of foil stiffness, the foil bearing presented here can possess sufficiently high stability. The maximum rotational speed obtained is greater than 230 000 rpm. Therefore, owing to its simplicity and high performance, this type of foil journal bearing can hopefully be applied to a small high speed cryogenic turboexpander.