Twin-screw Machine Research Articles

High-resolution simulations of gas-liquid Couette-type sealing gap flows

Abstract The two-phase flow in sealing gaps of twin-screw compressors is investigated using highly resolved coupled multiphase simulations. Oil-injected rotary-type positive displacement compressors (RPDC) such as twin-screw machines are used for the compression of refrigerants in HVAC (Heating, Ventilation, and Air Conditioning) systems. The efficiency of such machines is largely determined by the inevitable two-phase surge and gap flows. The simulation of flows in such compressors is still a challenge due to the complexity of the two-phase flow in the narrow gaps (<0.3mm) with organic working fluids and moving boundaries. Due to the rotation of the screws relative to the housing, gaps are present between the rotors and the end plates of the housing and between the tips of the rotors and the cylindrical housing walls. Oil injection can increase the efficiency of screw compressors or expanders by more than 10%, since oil enhances the sealing of the gaps where leakage occurs. On the other hand, additional momentum losses are caused by the two-phase gap flows. The oil injection, however, increases the hydraulic losses which depend on the geometry of the gap, the liquid, and the kinematics of the established multiphase flow in the gap. In this work, a highly efficient, high-resolution numerical method is used for the simulation of the gas-liquid multiphase flow in the screw compressor gap between the rotor tip and the stationary housing. The Lattice-Boltzmann method (LBM) defines the basis of the method used in this work, forming an in-house developed framework in which each fluid phase is solved by a separate solution algorithm. To capture the motion of the liquid-gas phase boundary, a level-set method is used. In the present work, a generic gap flow is considered. A Couette-type flow configuration, where the sealant flow in the gap is driven by the moving inner wall, is investigated. The Reynolds number based on the height of the channel is Re = 20000. Finally, we discuss the impact of bubble-laden sealing gaps on the momentum losses of the system.

Influence of fluid properties and model parameters with regard to stratified gas-liquid gap flows

Abstract In rotary positive displacement machines - like twin-screw compressors - clearance flows have a big impact on the quality of the energy conversion. To partially seal the gap connections and thereby reduce internal losses, an auxiliary fluid, e.g. oil, is often injected into these machines. If a chamber model method is used to simulate the thermodynamic operating behavior of such a wet-running twin-screw machine, two-phase mass flow rates through the gaps must be calculated as accurately as possible in order to achieve a high simulation accuracy. This paper presents an extended model for simulating such two-phase gap flows. The model allows the simulation of a gap flow, taking into account the possible outgassing of a gas (e.g. refrigerant) from a liquid phase (e.g. oil) along the gap. The model is used to analyse the influence of various fluid properties (e.g. solubility, density, viscosity) or boundary conditions (e.g. pressure ratio, quality at the inlet of the gap) in relation to the simulated gap mass flow rate.

One-dimensional investigations of the periodic liquid-injection in twin-screw compressors

Abstract The injection of liquid fluid (mostly oil) in twin-screw machines brings several advantages like sealing the gaps, at least partially, and absorbs some of the compression heat. To predict the machine behavior, it is essential to consider the injection process, because it determines the amount of liquid within the working chambers. This in turn determine the absorption of compression heat, the degree of sealing the gaps and the hydraulic losses. Therefore, in this study the transient liquid injection is investigated using a one-dimensional method of characteristics within the injection nozzle. The theoretical results are compared with the actual liquid mass flow rate of an oil-injected twin-screw compressor.

Investigation of Pressure Pulsation and Vibration of the Internally Geared Screw Compressor

Abstract The Internally Geared Screw Machine (IGSM) is a relatively novel and promising type of positive displacement compressor concept that takes inspiration from classic gerotor pumps. The IGSM uses two rotors rotating in the same direction about parallel axes while maintaining continuous contact between both rotors which isolates several working chambers. By using ported end plate, the IGSM can determine the timing of the fluid entering and exiting the working chamber. There are several key advantages to the IGSM over traditional twin-screw compressors including reduction of the rotor to casting leakage, elimination of the blow hole area, reduction in sliding velocity at the point of contact, and a uniform circumferential rotor temperature profile. Although there is some research regarding the geometric profiling and chamber modeling of this type of machine, there is no research currently available on the pressure pulsation and vibration characteristics of the IGSM. The purpose of this paper is to provide an initial look into the pressure pulsation and vibration characteristics of the IGSM and how they differ from that of a traditional twin-screw machine. By explicating the key differences between the IGSM and the industry standard twin-screw compressor, this paper aims to better understand an important yet underdeveloped area of IGSM design.

Optimization of malted cowpea compositions and extrusion parameters for best quality Ethiopian emmer wheat-based snack food

Childhood malnutrition is a major concern in developing countries, particularly in Ethiopia. This study aims to address this problem by assessing the nutritional properties of snacks made from locally available raw materials. The study used a twin-screw extruder machine to study the effects of different processing conditions (barrel temperatures, feed moisture content, and cowpea/emmer wheat blending ratio) on the nutritional properties of extruded products. The results showed a highly significant effect of these processing variables on the nutritional properties of snacks. Increasing cowpea in the blend boosted the protein and fiber content of the snack products. Increasing the barrel temperature from 80 to 120 °C notably decreased the protein and fat levels in the snack. The optimization of the combined interactive effects on an extruded snack made from 19.83% cowpea and 80.17% emmer wheat, cooked at 120 °C barrel temperature with 22.04% feed moisture content, produced acceptable extruded snack items. The products developed with optimized parameters contained 16.68%, 1.55%, 2.50%, 70.95%, and 364.43 kcal/g for protein, fat, fiber, carbohydrate, and gross energy, respectively. The study concludes that blending Emmer wheat and cowpea in extruded snacks can provide high protein and gross energy. This suggests a locally viable solution for addressing protein-energy malnutrition in developing regions, particularly Ethiopia.

Impact of the spindle number on the material transport and mixing during planetary roller melt granulation

In comparison to the established twin-screw machines, the application of a planetary roller granulator for continuous operation of melt granulation is a promising alternative based on the unique process concept. An initial study focused on the material transport during processing as a key driver for the overall performance. Hereby, the impact of direct process parameters on the residence time distribution was the main objective.These investigations are complemented in this study by considering the free processing volume, which is defined by the number of planetary spindles applied within a module. The impact on the processing conditions is evaluated with respect to the process setting in terms of feed rate and rotation speed.The results highlight the potential of altering the underlying transport function in planetary roller melt granulation (PRMG) via the investigated direct process and equipment parameters. The impact of the feed rate is lower in comparison, while a higher rotation speed as well as a higher free processing volume promote material mixing. Moreover, a normalization of the determined residence time distribution (RTD) model data was feasible with respect to the process settings and the number of applied planetary spindles in the processing zone. This demonstrates the key role of the free processing volume in the fundamental mechanisms of material transport and mixing during PRMG.

Effect of Coupling Agent on the Properties of Pineapple Leaf Fiber/ Polypropylene Composite

Natural fibre matrix composites have been utilized in numerous industries, including the automotive, construction, and aerospace sectors. Maleic anhydride grafted polypropylene (MAPP) was used in this study to examine the effect of the coupling agent on the properties of a composite-produced using pineapple leaf fibre. The weight ratios of pineapple leaf fibre, polypropylene, and MAPP were 20g:180g, 15g:180g:5g, and 10g:180g:10g, respectively. The materials were mixed using a co-rotating twin-screw extruder machine at 170 °C and 50 rpm. The composite was then manufactured by injection molding at 185 °C. The mechanical and physical properties of PALF composite were examined by using a tensile test, impact test, and water absorption test. Thermal Gravimetric Analysis (TGA) was used to characterize the thermal properties of PALF/PP, and PALF/PP/MAPP composite. The surface morphology and fracture surface of the composite was characterized by using Scanning Electron Microscopy (SEM). Compared to the composite containing 5 g of MAPP, the tensile strength composite containing 10 g of MAPP produced a stronger interfacial bond. According to the impact test, adding more MAPP increases the material's strength and stress transmission. The average percentage of water absorption indicates low water absorption after the addition of the coupling agent. It can be concluded that the addition of a coupling agent improved the composite's properties.

Physical characteristics of extrudate with treatment composition ratio of mixed corn grit-soybean flour and extruder barrel temperature

Yellow corn is one of the key component in the production of popular RTE snacks. However, corn’s nutritional quality is relatively low, necessitating the incorporation of additional ingredients such as soybean to enrich the protein content. This study primarily aims to explore the influence of soybean flour composition and temperature of barrel on the physical characteristics of extrudates to produce nutrition-rich snacks. The materials utilized in the experiment consisted of corn grit (mesh 24) and soybean flour (mesh 80). Extrudates were produced using SYSLG twin-screw extruder machine with a 10-15 kg/h production capacity. The study’s experimental design employed two factors: the composition factor of soybean flour (at 10%, 20%, and 30%) and the final temperature of the barrel (at 120°C, 130°C, and 140°C). The physical attributes of the extrudates were assessed based on the parameters such as expansion ratio, extrudate texture (hardness), moisture content, WAI, and WSI. The study findings revealed that increased soybean flour composition reduced the expansion properties and a more rigid material texture. On top of that, all extrudates exhibited moisture content of less than 4% (wb). The water absorption index decreased with higher soybean flour composition and barrel temperature, whereas the water solubility index showed an increasing trend. Among the tested conditions, the treatment with temperature of the barrel of 140°C and a soybean ratio of 20% demonstrated the best quality results.

Mechanical and antibacterial properties of hybrid polymers composite reinforcement for biomedical applications

This research investigates the biocompatibility, mechanical strength, and tribological properties of a hybrid composite material composed of high-density polyethylene (HDPE), hydroxyapatite (HAp), and titanium dioxide nanoparticles (Ti ). The study explores the microstructural characteristics of the composite material using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Samples of HDPE-30%HAp with varying concentrations of Ti (5, 10, 15, and 20%) were prepared and extruded using a twin-screw machine. The hybrid composite materials underwent mechanical tests (tensile, flexural, and hardness), tribological tests (friction and wear rate), and antibacterial tests (resistance to Escherichia coli and Staphylococcus aureus bacteria). The results indicate that the optimal hybrid composite sample was HDPE-30%HAP-10% Ti , which demonstrated excellent mechanical properties (maximum tensile strength of 25.93 MPa and young modulus of 480 MPa) and a low coefficient of friction (COF∼ 0.07) while achieving high wear resistance (wear rate in the order of 1m ). The study shows that the improvement in mechanical properties results in a corresponding improvement in tribological properties. The antibacterial tests revealed that the hybrid composite material exhibited resistance to E. coli and S. aureus bacteria. The findings of this study suggest that the HDPE-30%HAP-10% Ti composite is a promising material for use in biomedical applications due to its excellent biocompatibility and desirable mechanical and tribological properties. The study demonstrates the potential of reinforced hybrid composite materials in overcoming the disadvantages of monolithic and hybrid micro-composites and highlights the importance of investigating the microstructural, tribological, and mechanical strength characteristics of composite materials for biomedical applications.

Planetary roller melt granulation (PRMG) – A new continuous method for powder processing

For the continuous operation of granulation, which is a standard unit operation in powder processing to modify the particle size, the use of twin-screw machines was established as standard method for wet and melt granulation over the last decade. A main characteristic for this technology is the pronounced mechanical energy input and the coupling of this parameter to the throughput for high throughputs. This is challenging for melt granulation with respect to temperature control and the handling of temperature sensitive materials.In this respect, the use of a planetary roller granulator is a promising alternative due to the distinctive process concept. Here, planetary spindles within a roller cylinder are circulating around a motor-driven central spindle. These three main parts are interlocking due to a 45°toothing, which results in an enhanced generated surface of the processed material. In addition with the heating concept, which includes the roller cylinder and central spindle, this leads to an enhanced ratio of heated surface to process volume.For planetary roller melt granulation (PRMG), the feed rate and rotation speed as direct process parameters are suitable to adjust the mechanical and thermal energy input, while the ratio of these correlates to the product temperature. Therefore, a predictive determination of an operating point in this context is possible by the derived corresponding model. Moreover, the variation of the direct process parameters also impacts the fraction of net granulated particles during PRMG. This is an indicator for a shift in the overall process regime with respect to the fundamental process rates.

Numerical Investigation of Compression and Expansion Process of Twin-Screw Machine Using R-134a

Increasing the efficiency of twin-screw machines is beneficial for gas compression and expansion applications. We used a computation fluid dynamic approach to obtain the flow field and efficiency of a twin-screw machine that used R-134a as the working fluid. The leakage flow and sealing lines were obtained to study their geometrical effects during the compression and expansion process. The effects of the wrap angle (280°, 290°, and 300°) and pressure ratios on the compression efficiency were studied. During the compression process, the volumetric efficiency was more than 70% regardless of the wrap angle. We found that the volumetric efficiency slightly decreased when the wrap angle increased. However, the effect of the wrap angle on the isentropic efficiency was not substantial. An increase in the pressure ratio decreased the mass flow rate and increased the leakage flow. This screw machine can also be operated in an expansion process, and the simulated expansion ratio was 3:1. However, this expansion ratio contributed to an underexpanded condition, which led to a lower volumetric and isentropic efficiencies compared with the original built-in expansion ratio scenario.

Two-phase mass flow rate through restrictions in liquid-flooded twin-screw compressors or expanders

Liquid flooded twin-screw machines are commonly used as compressors for different applications and offer excellent prospects for energy conversion in lower and medium power ranges as expansion engines in Rankine cycles for example with regard to either waste heat recovery or electrical power generation from regenerative heat sources. With the aim of minimising internal leakages, lubricating moving machine parts, or reducing thermal stress and providing the benefits of wet expansion, a twin-screw machine can be operated with an auxiliary liquid or a two-phase working fluid. In this context, the present work provides a basic approach addressing the two-phase mass flow rate calculation in water-flooded twin-screw machines. Based on a generic inlet-geometry model, two-phase mass flow rates are recorded for different operating conditions and mass dryness fractions in particular. Appropriate approaches for the theoretical two-phase mass flow rate and discharge coefficient calculation considering restrictions in the flow section—such as orifices and nozzles—are applied. Finally, the two-phase discharge coefficients are compared with flow coefficients determined by means of experimental and theoretical two-phase mass flow rates revealing the limits and uncertainties of the models applied in this study.

Simulation and Experimental Validation on the Effect of Twin-Screw Pulping Technology upon Straw Pulping Performance Based on Tavares Mathematical Model

Rice straw is waste material from agriculture as a renewable biomass resource, but the black liquor produced by straw pulping causes serious pollution problems. The twin-screw pulping machine was designed by Solidworks software and the straw breakage model was created by the Discrete Element Method (DEM). The model of straw particles breakage process in the Twin-screw pulping machine was built by the Tavares model. The simulation results showed that the highest number of broken straw particles was achieved when the twin-screw spiral casing combination was negative-positive-negative-positive and the tooth groove angle arrangement of the negative spiral casing was 45°−30°−15°. The multi-factor simulation showed that the order of influence of each factor on the pulp yield was screw speed > straw moisture content > tooth groove angle. The Box-Behnken experiment showed that when screw speed was 550 r/min, tooth groove angle was 30°, straw moisture content was 65% and pulping yield achieved up to 92.5%. Twin-screw pulping performance verification experiments were conducted, and the results from the experimental measurements and simulation data from the model showed good agreement.

A two-phase approach for simulation of water-flooded twin-screw machines validated for expander applications

In the lower and medium power range, twin-screw machines offer a high potential for energy conversion in various compressor applications with liquid injection or as expanders with respect to electrical power generation from regenerative and exhaust heat sources in trilateral or wet Rankine cycle systems, for instance. Aiming high efficiencies and reliability, the design of liquid-flooded twin-screw machines as a critical system component presents particular challenges for the engineers. Hence, reliably representative simulations to guide design are mandatory. In this context, this study presents a two-phase approach for simulation of the operational behaviour of water-flooded twin-screw machines. The thermodynamic fluid state is calculated using the humid air model that considers the two-phase mixture in thermal equilibrium. Additionally, dissipative two-phase mass flow rates are predicted regarding a slip-flow model and two-phase discharge coefficients. The proposed two-phase approach including liquid distribution in the working chamber is validated for expander applications considering available experimental data of the test twin-screw expander SE 51.2 in terms of indicator diagrams, indicated power, mass flow rate, and outlet temperature.

Experimental investigations and 3D simulations by the overset grid method on twin-screw machines in both pump and turbine mode

Twin-screw pumps due to their particular capabilities, offer u nique b enefits in comparison to other types of pumps. They can also recover wasted energy from the fluid system when run in a reversed way, i.e., in turbine mode. In this study, the possibility of benefiting twin-screw machines instead of conventional control valves in order to recover energy is investigated. Flow physics and energy recovery are analyzed experimentally and by 3D flow simulations in both, pump and turbine mode. Pump and turbine characteristics under different operating points are experimentally measured for low and high viscous fluids, i.e., water and oil, respectively. It is observed that for higher viscosity the dependency of volume flow rate on the pressure difference imposed on the pump is reduced. This observation indicates the significant effect of viscosity on the gap flow. Based on a simulation method by means of overset grid technique, unsteady 3D flow simulations with a high grid quality and a high spatial resolution particularly in gap regions in terms of y + < 1 are conducted and results are validated against experiments. The profound assessment of gap flow based on the simulation results shows that for highly viscous fluids such a soil, the rotational speed of spindles as well as the direction of rotation have a significant effect on gap flow characteristics and consequently on the pump and turbine performance. From the experiments, it is clarified that with a lower rotational speed of spindles and a higher pressure difference, a higher amount of energy up to about 50% of the fluid energy in the piping system can be recovered instead of being dissipated by a conventional control valve.

Thermodynamic simulation and experimental validation of an oil-free twin-screw expander

The chamber model method is still a powerful tool for the thermodynamic simulation of screw machines and is widely used in the industry and in science. When choosing adequate submodels for the compressible flows through machine clearances and the machine ports high simulation accuracy can be reached with minimal computation resources. However, detailed modelling is required when the complex geometry of twin-screw machines is abstracted to a time-dependent zero-dimensional chamber model. In this paper a two-chamber model simulation tool is used for the thermodynamic simulation of an oil-free twin-screw expander operating with dry air as working fluid. Insights into the modelling process are presented and the importance of precise modelling of gap flows between adjacent capacities is discussed. The simulation results are finally validated against experimental data. Integral values obtained from the experiments, such as expander mass flow rate, indicated power, and effective power, are in good agreement with the numerical calculations. To gain a deeper understanding of the thermodynamics of the expander working cycle, additional investigations are carried out at zero speed. The mass flow rates measured for a variation of stationary rotor positions are compared to values calculated with the two-chamber model method.

Analysis of the flow through the blowhole of twin-screw machines with different rotor profiles using dimensionless numbers

To ensure safe operation, gaps in a screw machine are essential. Some are relatively easy to adjust, like front and housing gaps, while it is more difficult, if they depend directly on the selected rotor profile shape, such as the inter-lobe clearance and the blowhole. Consideration of mass flow rates through those clearances is crucial to predict operation conditions or design screw machines. When using chamber model simulations, it is common to calculate idealized, theoretical mass flow rates and to combine them with a flow coefficient which represents the effect of e.g. friction losses. This paper deals with the flow coefficient of the blowhole, which is formed by the two rotors and the housing cusp. Therefore, a set of dimensionless numbers is used, including physical and geometrical influences on the flow coefficient. The real mass flow rate of an ideal gas (e.g. air) through the blowhole is calculated by CFD-simulations. The dimensionless numbers are systematically varied to identify their specific influences on the gained mass flow rates.

A Review on Two-Phase Volumetric Expanders and Their Applications

The importance of volumetric expanders has been increasing in the last decades because several studies confirmed that they lead to improved energy savings, limit the environmental impact, and reduce the energy intensity of industrial and domestic applications. In particular, several applications of the two-phase volumetric expanders, in which the operating fluid consists of liquid and vapor phases, were recently proposed. Nevertheless, the contributions in the scientific literature related to the overview of the state-of-the-art aspects of this technology are rare. For this reason, the present work discussed the potentialities and drawbacks of the available typologies of volumetric expanders that process a two-phase pure working fluid by analyzing a summary of leading studies in this field to go beyond previous efforts in the literature. The analysis revealed that twin-screw machines represent the best candidates, while reciprocating piston devices seemed the least well-adapted because of their reduced tolerance to droplets and high friction losses. Flash evaporation appeared to have the most significant impact on the expander because it affects both inlet and expansion phases, thus, determining the shape of the indicated cycle and the isentropic efficiency.

Effects of Hydrolytic and Oxidizing Agents on the Properties of Low Density Polyethylene/Halloysite Nanocomposites for Biomedical Applications

Low density polyethylene (LDPE) nanocomposites reinforced with natural halloysite nanotubes (HNTs) were synthesized and evaluated as a prospective material for biomedical applications via in vitro biostability studies in this work. A twin-screw extruder machine was used to fabricate the examined LDPE and LDPE/HNTs nanocomposites (NCs), followed by in vitro treatment of the prepared NCs via immersion in oxidizing and hydrolytic chemicals for four weeks at 37°C. The materials’ in vitro mechanical characteristics were evaluated under these harsh conditions. The analysis showed that the LDPE with 3 wt% HNTs exhibited the best nanofiller dispersion characteristics. This NC also presented smoother surface degradation characteristics compared to the plain LDPE and other LDPE NCs. Furthermore, as compared to plain LDPE, the LDPE NCs showed enhanced mechanical capabilities, and these qualities were less impacted by the in vitro conditions. The introduction of 3 wt% HNTs into the LDPE gave the best in vitro mechanical characteristics. Furthermore, the existence of a better-scattered nanotube structure was thought to offer a more sinuous pathway for the propagation of H2O and the oxidants, thereby reducing their penetration across the matrix chains. Hence, the kinetics of disintegration inside the LDPE molecular chains were slower, resulting in improved biostability.

Calculation of Process and Design Parameters of Twin Screw Mixers with Minimization of Technological Capacity

The formulation of the optimization problem and its solution in the processing of adhesive compositions in twin-screw machines are considered under the condition of minimizing the technological capacity. An algorithm and a computer program for calculating the optimal technological and design parameters of twin screw mixers have been developed.