Rotor Profile Research Articles

Improving Screw Compressor Displacement and Efficiency by Increasing the Rotor Profile Depth

Improving Screw Compressor Displacement and Efficiency by Increasing the Rotor Profile Depth

User defined nodal displacement of numerical mesh for analysis of screw machines in FLUENT

Growing demands to reduce energy consumption are driving researchers towards in-depth analysis of positive displacement machines. Twin screw compressors are amongst the most common types of positive displacement machines. These machines have inherently complex geometry due to intricate rotor profiles used. As the details of the internal flows are difficult to obtain experimentally, Computational Fluid Dynamics (CFD) offers a good alternative for evaluation of internal flow patterns. However, implementation of CFD is challenging due complex deforming geometries.In this paper, a customised grid generator SCORGTM developed by authors is used to generate numerical meshes for commercially available solver ANSYS FLUENT. FLUENT is an unstructured solver which offers flexibility of using both segregated and coupled solution algorithms. Segregated algorithms are generally faster which results in shorter product development time. Interface with FLUENT is implemented by performing User Defined Nodal Displacements (UDND) of grids generated by SCORG in a parallel framework. For this purpose, SCORG and UDND are coupled and extended to work with FLUENT’s parallel architecture. The developed code is compiled within the solver. The oil free air screw compressor with ‘N’ profile rotors and 3/5 lobe combination is modelled for 8000 RPM and 6000 RPM. Finally, the predicted performance values with FLUENT are compared to previously calculated CFX predictions and experimental results. FLUENT requires shorter solution time to obtain same accuracy of CFX.

Solving the conjugate tooth profile of screw compressor rotors using edge detection method based on Alpha Shape algorithm

While designing a screw compressor, the design of the rotor profiles is the most important because rotor profiles essentially determine the overall performance of the screw compressor. In the field of rotor design, the mutual solution of male and female rotors is characterized by shortcomings such as complex and error-prone calculation, and unsolvable interference. In this study, an edge detection method based on the Alpha Shape algorithm was proposed to avoid the complex calculation process in the traditional planar meshing theory and quickly obtain the point cloud data with singularity points removed. The results show that, with relatively high accuracy and fast processing speed, this algorithm can extract comparatively desirable edges so as to obtain data of the female (male) rotor profile based on the conjugate male (female) rotor profile. The analytical method and the experimental method were used to compare the data of the conjugate rotors, verifying the correctness of the design, which efficiently and highly accurately overcame the problems in mutual solution of male and female rotors. This method can also be applied to design similar conjugate products such as gears, worms, and milling cutters.

Research on structural parameter optimization and thermal performance of twin-screw expander

Abstract This paper analyzes rotor profile and meshing law of twin-screw expander. Elementary area is calculated by meshing sequence method, and elementary volume is solved by integration. The influence of geometric characteristics such as torsion angle and internal expansion angle of male rotor on the thermal performance of twin-screw expander is studied. The results show the following: reasonably reducing internal expansion angle and positive rotor torsion angle can increase isentropic adiabatic power of twin-screw expander. Taking isentropic adiabatic efficiency as optimization goal, sequential quadratic programming algorithm is used to optimize the structural parameters of twin-screw expander, and the influence of leakage on its functional force is analyzed. Considering leakage effect, actual shaft power of twin-screw expander is 16.7 kW, the isentropic adiabatic power under ideal conditions is 21.3 kW, and the adiabatic efficiency is 78.71%.

A NOVEL DESIGN OF THE ROOTS BLOWER

This paper reports a novel curve developed from non - circular gearing theory, which can be applied in rotor profile design of the two - lobe Roots blower. The formulas for calculating the volumetric efficiency and specific flow rate of the blower have also been established. To evaluate this type of the Roots blower, the volumetric efficiency and specific flow rate are being compared with those parameters of the two traditional designs and one recent variant.

Geometric design of novel straight claw rotors for claw vacuum pumps

The claw rotor profile is one of the key factors for improving the performance of claw vacuum pumps. In order to develop novel types of claw rotor profiles, the conjugate curves of claw rotor profiles were calculated, and therefore two types of straight claw rotors, a cusp claw rotor and a smooth claw rotor, were proposed in this study. Geometric models of the proposed claw rotors were established, and profile equations were also derived. Moreover, effects of the independent geometric parameters on shapes of the claw rotors and the performance of pumps were analyzed. The stress and deformation of two proposed claw rotors loaded by gas pressures at the end moment of the discharge process were analyzed and compared. The study results indicate that the cusp claw rotor is a special case of the smooth claw rotors, when the value of tip circular arcs equals to zero. The smooth claw rotor is superior to the cusp claw rotor in terms of the working performance and mechanical property.

Analyzing Rotor Profile’s Effects to Performance of Roots Vacuum Pump Applied the CFD Method

The paper is mainly focused on analysis characteristics of lobe pump with three different tooth profiles: Circular, Epi-cycloidal and Circular–Involute–Circular (CIC) rotor profiles. Based on the geometric analysis and mathematical model, the CIC rotor profile combined Circular–Involute–Circular is generated. In this paper, the CFD techniques are used to estimate the effects of tooth profile and lobe number to the pump performance. The simulation results obtained from FLUENT software illustrated that the rotor profile effects remarkably to the pump performance. The outlet pressure of CIC rotor profile is higher than that of the circular and epi-cycloidal rotor profiles. The numerical outcomes also showed that higher rotor’s speed is resulted in higher outlet pressure. Besides, the lobe number of rotor has effect slightly to the pump performance.

Optimization of Air-Gap Profile in Interior Permanent-Magnet Synchronous Motors for Torque Ripple Mitigation

Interior permanent-magnet synchronous motors (IPMSMs) have been widely used due to their high-efficiency and high-power densities. Minimization of torque pulsation resulting in vibration and acoustic noise is one of the important design considerations for IPMSMs. In this paper, a grid on/off search method for the rotor profile is proposed to mitigate torque pulsation. Selection of the rotor profile is due to the fact that air gap is the most sensitive parameter in electric machines wherein changes in flux densities can cause substantial differences in the distribution of forces. A layer comprised 20 partitions with a 0.1 mm thickness and 3° wide grids have been introduced to the rotor surface for each pole, and the possible geometries have been analyzed using the finite-element method in ANSYS Maxwell. An optimal design was found that has the lowest torque ripple with a higher average torque compared to the original design. Genetic algorithm has also been applied to the method to automate the coupling between Maxwell and MATLAB, thereby saving the simulation time. Complete structural analysis has been done for both of the original and optimal designs to verify the superiority and feasibility of the proposed design.

A Methodology for Generating a Variable-Pitch Rotor of Twin-Screw Vacuum Pump

The twin-screw vacuum pump is widely used in low and medium-low vacuum applications. Its core element consists of a pair of rotors rotating in opposite direction on parallel axes. Screw rotors with uniform pitch are usually finished by the form grinding method, but the rotors with variable pitch are not. In this paper, a manufacturing process for generating the tooth profile of the screw rotor with variable pitch by CNC lathe turning is proposed. The initial position of turning tool and normal profile error of the manufactured screw rotors are derived and verified numerically. Numerical examples are presented to validate the proposed method. The simulation results reveal that the proposed CNC turning process is feasible and flexible.

Design methodology of a new smooth rotor profile of the screw vacuum pump

The screw rotor and its cross-section profile primarily determine the performance and efficiency of the twin-screw vacuum pump (TSVP). However, the traditional cross-section profile (CSP) contains cusps or unsmooth points, so that screw rotor surfaces generated by this CSP contain unsmooth edges, which will lead to wear, deformation, and difficulty of coatings for corrosion protection in the unsmooth region. With the aim of solving this problem caused by cusps or unsmooth points of the CSP, we put forward a meshing model of a circular arc and an equidistant curve of an epicycloid (ECE), and then proposed a design methodology for generating a completely smooth CSP. Screw rotors generated by the proposed CSP were completely smooth, and the proposed screw rotors could correctly mesh. Moreover, a geometric model of the proposed CSP was established, and the influence of the geometric parameters on its shapes and characteristics were studied. In addition, the proposed screw rotor was compared with the traditional screw rotor in terms of stress and deformation. Results indicated that the proposed screw rotor had continuous meshing and contact lines, and small values of maximum stress and deformation. The proposed smooth screw rotor is superior to the traditional one.

A profile error compensation method in precision grinding of screw rotors

In the operation of positive displacement screw machine, the profile accuracy of the screw rotor has a significant effect on the meshing stability between male rotor and female rotor, which is the main factor for vibration, noise, sealing, and wear resistance. This sets extremely strict requirements of the profile accuracy of screw rotor. Therefore, in order to obtain precise screw rotor profile, form grinding is generally used as a finishing process in the manufacture of screw rotors. Unfortunately, the grinding wheel has to be frequently dressed due to inevitable grinding wheel wear. This will cause a substantial waste of time and thus lead to inefficiency. It is therefore desirable to be able to predict the profile error caused by grinding wheel wear in order to make it controllable. In this paper, the influence of installation angle, center distance, and grinding wheel wear on rotor profile error is investigated. A novel method has been proposed for prediction profile error of the screw rotor in precision form grinding. The proposed method has been employed to compensate profile error caused by grinding wheel wear and improve grinding efficiency. Verification experiments were conducted with several screw rotors produced and profile error measured. The comparison shows that the compensation results agree well with the numerical simulation results, showing the effectiveness of the proposed compensation method.

Design and feasibility study of roots-type power machine rotor based on numerical simulation

In this paper, a new type of low-pressure steam generator, roots-type power machine, is proposed to solve the shortcomings of medium- and low-temperature waste heat energy recovery. The rotor is systematically studied by means of theoretical analysis, numerical simulation and experimental verification. Firstly, the motion law of roots-type power machine is analyzed theoretically. According to the rotor profile characteristics, the rotor profile structure is divided and the parameter equation is established. In order to verify the feasibility of the designed rotor, the pressure field, velocity field and mass flow field inside the roots-type power machine where the rotor is located are analyzed by numerical simulation method. Finally, a test platform is built to test the power output characteristics, load characteristics and flow characteristics of the roots-type power machine. The feasibility of the rotor profile design method and the reliability of the rotor mathematical model are verified. The results show that the design of the rotor profile can meet the power generation requirements of the roots-type power machine. The rotor profile design method provides a new idea and strong theoretical guidance and basis for the design of the rotor of the subsequent roots-type power machine. Moreover, the development of the roots-type power machine will have a profound impact on the recycling and energy saving of low-temperature waste heat resources.

An analysis of manufacturing factors’ influences on the actual screw compressor rotors’ profile clearances

One of the most important parameters, which have high influence on screw compressor characteristics, are rotors’ profile clearances. They determine in general the smooth of the rotors’ gearing and influence on the screw compressor vibration level. Analyse of actual rotor profile clearances depending on the actual rotor temperature fields and on the manufacturing imperfection was done. Specially profiled milling tools were chosen as rotor profile manufacturing tools. Following four types of the manufacturing error, which have the highest influence on the accuracy and quality of rotors’ profile surface, were chosen to analyses: the centre distance deviation between the rotor and the milling tool axises (dAi), the departure of the milling tool position from their basic plate (dZf), the milling tool mounting position angle departure (dβ) and the milling tool radii departure (dRf).The actual screw compressor profile coordinates can be obtained by taken into account all factors presented above. They determine the actual rotor profile clearances on “cold” state when screw compressor is on shutdown conditions and can be measured during a compressor assembling or a compressor stripping. However, it should be noted that they are changed depending on the screw compressor working conditions. To correctly determining actual profile clearances depending on working condition also needs to take into account rotors’ thermal deformation. The presented multifaceted approach of the screw compressor actual rotor profile clearances calculation improves an accuracy compressor characteristic obtained by their mathematical model. A deep analysis of the obtained results can pointed to the rotors’ manufacturing factors, which should be improved firstly during the compressors’ mass production.

A correction method of screw rotor profile error based on parameter adjustment for grinding wheel dresser

Accuracy of grinding wheel profile that is generated by form grinding theory and formed by grinding wheel dresser is a crucial factor affecting profile accuracy of screw rotors. A correction method for screw rotor profile error based on parameter adjustment for grinding wheel dresser such as diameter and distance of diamond rollers is proposed. Influence of diameter and distance of diamond rollers on grinding wheel profile and screw rotor profile based on theory of segmented dressing method is analyzed, and the adjustment method for parameters of grinding wheel dresser is presented. The results of the analysis provide a theoretical basis for error correction in screw rotor grinding. Grinding experiments for female rotor were performed due to the character that the female rotor has smooth bottom profile where the change of profile error is easy to observe. The experimental results show that the height difference between the long and short sides of rotor profile at the bottom of the rotor is significantly reduced from 0.22 mm to 0.034 mm by adjusting diameter of diamond rollers, and the distance between the long and short sides of the actual rotor profile is almost consistent with the theoretical one by measuring the distance again and adjusting its value in the computer numerical control system. These results verify the correctness of the correction method.

Numerical study on screw machines with large helix angles

Modelling and performance calculation of screw machines with large helix angles such as a single and multiphase screw pumps by use of Computational Fluid Dynamics is challenging. The numerical procedures explained in literature are based on the 3D numerical meshes generated by series of 2D transverse cross sections which allows mesh to either follow the helix or be perpendicular to the rotor axis. This allows generating a conformal mesh. However, if the rotor helix angle is large, the cell skewness becomes prohibitively large which introduces errors in numerical simulation. The paper firstly attempts to generalize the generation method of rotor profiles with emphasis on producing a normal rack and rotors profiles. Then it introduces the method which uses series of 2D numerical meshes in the planes normal to each of the rotors and rack in order to decompose the working domain in two sub-domains. By this means it is possible to achieve 3D numerical mesh with extended capability of mesh refinement in clearances and alignment of the mesh to the main and leakage flows. However, special treatment is required to provide conformal interface between two moving meshes and with the casing. It is expected that it will greatly benefit accuracy and ease of performance calculation using a number of CFD solvers. In addition it is expected that it will allow generation of various different screw machine configurations like single screw machines or machines with conical rotors.

An innovative rotor milling method for flexible multi-functional machines

In this work we present an innovative method of rotor milling for flexible multifunctional machines. As an alternative to the disc cutters which are often employed in traditional milling machines, we propose a new tool, a finger cutter, which integrates easily with flexible multi-functional machines. This allows much smoother operation with lower required torque which is in line with the characteristics of flexible machines. This paper describes basic geometrical condition to generate the finger cutter profile from the rotor profile, the mathematical formulation, the basic tool design and grinding procedure of the cutter blades. It also discusses the machining process, productivity and accuracy of the proposed method. We believe that this method opens a new way of manufacturing large rotors with fewer production steps while maintaining required tolerances.

The influence of profile geometric parameters on characteristics of rotor-gearing compressor

Dry (without lubrication) claw compressors have a promising future because they feature both advantages of piston and rotary compressors. The rotor profiles have sophisticated form including several curves of similar type, combined so, that the line of contact is always parallel to the rotor axis. The effect of the main geometric parameters of the profile (R¯, αR and r) on the rotor-swept area value is considered. The research was carried out by the developed computer program intended for selection of the optimum variant of a ratio of profile geometrical parameters. In this paper the results of theoretical studies of influence of profile geometric parameters on integrated characteristics of rotor-geared compressor are given. Optimal relations between parameters are given, defining geometry of gas distribution elements.

Cfd simulation of a two stage twin screw compressor including leakage flows and comparison with experimental data

Computational Fluid Dynamics (CFD) is a common and validated simulation method in research and industry for the analysis of fluid systems. In the past years, it has proven to become more and more applicable for modeling the flow physics inside positive displacement (PD) machines. The working chamber and thus the discretized flow domain of PD machines are changing in time, characterized by complex thermodynamics. Compressible fluids, real-gas properties and leakage flows with trans- or supersonic characteristics are phenomena which have to be accounted for in order to properly model the behavior of the machine. As CFD methods evolve in general, but also for the application of PD machines in particular, the numerical model can replace a prototype during early stages of the product development. The desired simulation approach should be able to deliver sufficient accuracy at a feasible effort in terms of computational time and manpower to create the numerical model.This paper presents the methodology of creating the numerical model for a sample screw compressor provided by Sullair for research purposes. It is a dry running two stage twin screw compressor running with air at a rated power range between 160 and 250 kW. The two stages are gear driven by the main shaft at rotational speeds between 1180 and 2100 rev/min. Each stage features different rotor profiles, where the first stage has a 4-6, the second stage a 5-7 lobe combination. The total pressure ratio of the two stages combined is up to 10:1. To enhance the performance of the compressor, discharged air from the first stage is cooled down before entering the second stage. A specific meshing method is used to model the size-changing working chambers between rotors and casing, where only hexahedral cells are used and mesh topology is constant. The model accounts for radial and axial clearances between rotors and stator, where rotors and stator are connected with interfaces. The transient simulation results are compared to experimental measurements for torque, and flow rate. Also discharge pressure and temperature after first and second stage are compared to the experimental results. In addition, the possibilities of the simulation are exemplified by the gathering of time- and space-resolved monitor points like temperature or pressure at distinct points within the compressor. Apart from direct comparison to the experiment, also a sensitivity study regarding the change of housing clearances is presented, as leakage flow has severe impact on the compressor performance. These clearances and the resulting leakages are often not exactly known whereas they also vary because of manufacturing tolerances or deformations due to the load on rotors and stator. Here, the numerical simulation can serve as a helpful tool to estimate the sensitivity and change of machine characteristics, which is hard to determine in the scope of experiments.

Novel approach to single-screw compressors and expanders design**This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript,

Single-screw machines are currently employed as both compressors and expanders in vapor compression systems and organic Rankine cycles (ORCs), respectively. The working principle of single-screw machines is based on the simultaneous meshing of two starwheels with one central grooved rotor. The performance of the machine is heavily affected by the wear of the tooth meshing profile after several hours of running. In order to improve the wear resistance of the conventional straight-line profile, multi-column tooth flank designs have been introduced and investigated by several researchers. Cylindrical multi-column envelope profiles are able to distribute the local contact over a larger surface area reducing the wear of the tooth flank profile. Nevertheless, the manufacturability of such profiles is directly linked to the accuracy and limitations of CNC machines.In this work, an attempt is made to overcome the aforementioned limitations. To push the boundaries of possible tooth and rotor profile designs, 3D printing is utilized as manufacturing technique. Such approach allows the investigation of more complex designs that improves the sealing lines during the meshing process.A detailed geometry model based on polygon intersections is utilized to calculate the sealing lines, groove volume curves, and surface areas of the different designs. As a result of this study, a novel rotor design has been manufactured.

Influence of geometrical dimensions on the profile slippage in the hypogerotor pump

Relative sliding between two profiles in the contact point causes wearing out of the tooth profile. Aiming to evaluate influence of the geometrical dimension parameters of the pump rotor profile on the wear, in this paper, the authors established equation for determining slip coefficient from geometrical dimensions. Furthermore, the authors have investigated and evaluated the phenomenon of the profile slipping in order to find out the geometrical dimensional parameters for avoiding unequal wearing of the inner and outer rotors of the hypogerotor pump