Clearance Gap Research Articles

Effects of chill vent gap and sleeve-plunger tip clearance on the vacuum degree of mold cavity and mechanical properties in high vacuum diecasting

Effects of chill vent gap and sleeve-plunger tip clearance on the vacuum degree of mold cavity and mechanical properties in high vacuum diecasting

Experimental Assessment of Correlative Approaches for the Prediction of Leakage Flow through Labyrinth Seals

Simple analytical models can be employed to estimate the leakage mass flow rate from labyrinth seals, resulting in a quick procedure, well suited for the early design process. Different formulas were proposed by many authors during the past decades and most of them employ a carry-over factor and a flow coefficient to predict the mass flow rate across the clearance area. The present work aims to compare the analytical prediction with the results of an experimental campaign. The experimental results were retrieved from a dedicated test rig for both a straight-through and a stepped labyrinth seal. Hence, for each seal, the effect of the clearance size, the Reynolds number and the pressure ratio has been investigated. Starting from the experimental required inputs, six different correlations are considered, with both direct and indirect methods. The results are shown as a function of pressure ratio and clearance gap. Despite some differences in the comparison, and most of the used correlation underestimate the measured mass flow rate, some general trends and guidelines can be highlighted.

CARD5: Chronic In-vivo evaluation of the BiVACOR rotary total artificial heart

Purpose: The BiVACOR Total Artificial Heart (TAH) combines magnetic levitation and rotary blood pump technology to create a small, durable and physiologically compatible device capable of replacing the failing heart. Pulsatile outflow is achieved via cyclic changes in pump speed, while large clearance gaps and flow paths improve hemocompatibility. The TAH is designed for implantation in women and some children, yet powerful enough to support adult males undergoing exercise. In preparation for first-in-human trials, in-vivo studies were undertaken using clinical-grade devices. Methods: The BiVACOR TAH was implanted into five calves (82 – 108 kg) for 30 days before elective termination. Hemodynamics, left-right balance, hemocompatibility, and end organ function were monitored throughout the study, while evidence of significant thromboembolism was evaluated at study termination via necropsy. Results: All five calves regained all physiological function (eating, drinking, defecating, standing, and regular treadmill exercise). The TAH demonstrated reliable operation, provided suitable hemodynamics (MAP 111 ± 6 mmHg, CVP 12 ± 5 mmHg, est.QL 12 ± 1 l/min) with pulsatile outflow (up to 40 mmHg @ 60BPM) and acceptable left/right balance (est.LAP-CVP 7.6 ± 3 mmHg). Normal end organ function was maintained (CREA 1 ± 0.44 mg/dL) with excellent hemocompatibility (pfHb 4.34 ± 0.74 mg/dL, LDH 1110 ± 182 U/L), while showing no evidence of significant device related thrombi or end organ dysfunction at necropsy. Summary: The BiVACOR TAH demonstrated promising chronic physiological performance, hemocompatibility, and physiological interaction. These results underpin progression of the BiVACOR TAH to first-in-human studies. The work was supported by NIH NHLBI under award number R44HL137454.

Aerodynamic performance measurement of a novel variable geometry turbine adjustable guide vane scheme by experimental study

As the key component of new-generation variable cycle engines, the adjustable turbine guide vane's through-flow regulation capability and aerodynamic performance have a significant impact on the performance of the turbine stage and even the core engine. Conventional adjustable guide vanes control the through-flow capability by varying the installation angle and thus inevitably result in a gap clearance, which induces a new source of flow losses. This additional loss can degrade the aerodynamic performance of the variable geometry turbine considerably. Therefore, a novel adjustable guide vane scheme with a rotatable suction surface to regulate the through-flow capability is proposed in this paper and its superiority is verified in a typical small aspect-ratio vane. To this end, a high Mach number annular cascade test rig with high-resolution aerodynamic measurement systems is designed and built to determine the aerodynamic performance of this novel adjustable guide vane as well as a conventional one. The mass flow control capability, the total pressure loss coefficient, the isentropic Mach number at the blade surface, and the static pressure distribution at the endwall are measured for these two schemes, respectively. The flow details and loss mechanisms are also clarified with the aid of CFD. The results show that this novel adjustable guide vane scheme achieves a comparable through-flow control capability with respect to the conventional one, with a total pressure loss reduction of 25%-35% at the measured Mach numbers. The flow details reveal that the novel scheme avoids the large-scale leakage vortex that exists in the conventional scheme and also produces a more uniform exit airflow angle distribution for the downstream rotor.

The effect of secondary passages on cavitation and radial forces in a liquid propellant turbopump

Numerical studies on rocket pumps are computationally expensive and hence the secondary passages such as sidewall clearance gaps, wear ring gaps, axial balancing mechanism, coolant/lubricant paths, etc. in the pump are usually not considered. In this study, a liquid propellant pump with and without secondary passages are modelled and flow simulation results are compared to analyse the differences in cavitation, vortices and radial force predictions arising due to the presence/absence of the secondary flow passages. Single-phase and multiphase simulations are conducted for the design and two off-design points. It is predicted that the presence of secondary passages has a significant effect on the type of cavitation instability predicted and on the volume of cavity generated in the inducer due to which the radial forces generated also differ significantly. The presence of secondary passages predicted large fluctuations from the average radial forces generated by the inducer. These are not obtained if the leakages are not considered. This type of underprediction during the design phase might cause severe wear and tear in the bearings during actual operation. Thus, this study stresses the need for incorporating the secondary passages even at the design stages. This study also highlights the possibility of cavitation instability type manipulation using some means in the inducer which would be significant for cavitation control research in rocket pumps.

Study of leakage flow in oil-free positive displacement rotary machines using particle image velocimetry

Rotary positive displacement machines (PDM) are widely used in industry. Oil free PDM’s are used in applications where presence of oil is not allowed, but they still are not much utilised due to challenges with reliability and efficiency related to extensive thermal loads. Their efficiency is important to meet carbon footprint targets while increasing reliability will allow their wider use. The main contributor to losses is the internal leakage through clearance gaps between stationary and rotating parts driven by the differential pressure between chambers. Due to the transient nature of leakage flows in running conditions, the measurement of velocity flow field inside the clearance gap is challenging but can provide significant information about flow physics. Thus, understanding of the velocity flow field is essential for developing methods to increase efficiency without compromising reliability.The flow field in clearance gaps can be captured using Particle Image Velocimetry (PIV) for optically accessible flows. The basic PIV system consists of a light source, camera, tracer particles, and analysis algorithms. In this study the Roots blower is used as a representative of the oil-free PDM for which the velocity field is measured inside the radial clearance gap in the running condition using the PIV technique. An extensive testing campaign has been carried out at various speeds and pressure ratios of the machine. PIV measurements were taken inside, upstream, and downstream of the clearance gap. Post-processing of PIV images allows the generation of velocity fields. These pioneering measurements of the velocity field inside the clearance in the machine operating conditions reveal interesting flow physics. Moreover, the large amount of collected experimental data provides an excellent basis for validation of simulation models and further developments.

Hemocompatibility and hemodynamic comparison of two centrifugal LVADs: HVAD and HeartMate3

Mechanical circulatory support using ventricular assist devices is a common technique for treating patients suffering from advanced heart failure. The latest generation of devices is characterized by centrifugal turbopumps which employ magnetic levitation bearings to ensure a gap clearance between moving and static parts. Despite the increasing use of these devices as a destination therapy, several long-term complications still exist regarding their hemocompatibility. The blood damage associated with different pump designs has been investigated profoundly in the literature, while the hemodynamic performance has been hardly considered. This work presents a novel comparison between the two main devices of the latest generation–HVAD and HM3–from both perspectives, hemodynamic performance and blood damage. Computational fluid dynamics simulations are performed to model the considered LVADs, and computational results are compared to experimental measurements of pressure head to validate the model. Enhanced performance and hemocompatibility are detected for HM3 owing to its design incorporating more conventional blades and larger gap clearances.

Effect of gap clearance on the mechanical properties of weld-brazed lap joints

Gas metal arc brazing (GMAB) is a non-fusion joining process used in the automotive industry for joining thin-gauge Zn-coated steel sheets to other steels or dissimilar metals. Unlike torch brazing, GMAB does not rely on capillary action to form a joint. Consequently, gap clearance has not been considered important for lap joining applications. In this study, the effect of gap clearance on the mechanical properties of arc-brazed lap joints was investigated experimentally and verified using finite element modeling (FEM). The results showed that the gap clearance of lap joints played a critical role in controlling the joint strength.

Analysis of conjugate heat transfer in a roots blower and validation with infrared thermography

• Conjugate heat transfer in a Roots blower. • Gap size calibration achieved deviation of 7.5% on flow and 4.3% on power. • Discharge air temperature deviation was within 4 °C. • High speed infrared thermography based surface temperature deviation was within 6 °C. Oil-free Roots blower is a type of Positive Displacement Machine used for low pressure ratio applications. Its volumetric efficiency is dependent on the leakage of compressed gas through the clearance gaps. In the absence of internal cooling, prediction of temperature distribution and conjugate heat transfer becomes important for reliable design and operation. The interaction of gas flow and structural deformation in the blower is highly transient and one of the most challenging problems which need to be addressed for accurate performance prediction. To achieve this, the technique which includes moving and transforming the mesh is employed. The performance test data used for validation included discharge air temperature, mass flow rate, pressure, and power. The comparison between numerical and experimental results has shown good agreement of 7.5% on flow, 4.3% on power while the discharge temperature deviation was within 4°C. High speed infrared thermography of the rotor lobe and housing surface temperatures was used to evaluate the accuracy of the Conjugate Heat Transfer model. The difference between the experiment and simulation was within 6°C. The aim of this study was to develop and validate the numerical model for analysis of heat transfer between pressurized air and compressor elements and to provide temperature distribution in structural elements. By this means, the operational gap sizes can be reliably minimised during the design of a machine.

Design and simulation of non-uniform electric field using mesh electrodes for electrorheological micro valves

The refresh rate of two-dimensional tactile information for visual impaired people using electrorheological (ER) technology is directly related to the flow rate of an ER valve. The bigger the flow rate is, the faster the tactile information is refreshed. For a parallel electrodes ER valve configuration, due to the limitation of Braille standard to the width of ER valve, the gap of flow channel which is also the gap of parallel electrodes is the only adjustable parameter to benefit the flow rate and at same time enhance operating voltages which contribute a big part of costs to a two-dimensional tactile information display system. A new mesh electrodes configuration is introduced to decouple the flow rate from the gap of electrodes while the flow cross section is determined by parameters of mesh electrodes. A quantity of effective electric field strength of an ER interaction space unit is used to evaluate the anisotropy electric field governed by mesh electrodes. The influences of geometrical parameters of mesh electrodes and applied voltages on the effective electric field strength were analyzed by using COMSOL Multiphysics. The simulation results show that the effective electric field strength increases with applied voltages and wire diameters, and decreases with electrodes gap and mesh clearances. These results provide fundamental laws for designing an ER micro valve governed by non-uniform electric field to some ER applications.

Experimental and numerical analyses of the thermodynamic and mechanical performance of an oil-injected and economized 4/6 twin-screw compressor

To advance oil-injected twin-screw compressors, it is necessary to understand and model the complex physical phenomena occurring during the compression process (e.g., mass and heat transfer mechanisms) as well as analyze the mechanical behavior of the compressor (e.g., rotordynamics, bearing loads, variation of clearance gaps). To validate these models, the in-chamber compression process as well as mechanical loads need to be accurately measured. This paper presents a comparison between experimental results and numerical modeling of a 4/6 oil-injected twin-screw compressor with slide-valve part-load modulation and economization. The compressor has been equipped with high-frequency pressure sensors, load-cells at the bearings and torque sensor on the main rotor. Bearing forces are analyzed to quantify and validate the experimentally obtained axial force data. The validated model can later be used to analyze the mechanical performance of the compressor, such as bearing loss computation. This study also discusses isentropic and mechanical efficiencies computed for the various data points.

Structure clearance design in wind tunnel tests with implications for aerodynamic drag of high-speed trains

ABSTRACT The influences of vestibule diaphragm gap, wheel-rail clearance, and strut-plate gap on the aerodynamic drag of a 1/8th-scale HST model were investigated in an 8 m × 6 m wind tunnel. The Reynolds number was set to 2.2 × 106 based on train height. It was found that the variation of the vestibule diaphragm gap changed the aerodynamic drag distribution pattern of each car; the drag coefficient of the head and middle cars might change as high as 45%; however, the change in the drag coefficient of the total train was very small. The effects of the strut-plate gap on the aerodynamic drag of each car and the total train were small. The effect of the wheel-rail clearance on the drag of the head car was not significant. It was suggested that the vestibule diaphragm gap, strut-plate gap and wheel-rail clearance of the 1:8 scale high-speed train model should not be greater than 11, 15, and 7 mm, respectively.

Influence of uneven blade tip clearances on aeroacoustic characteristics of centrifugal compressors

The blade tip clearance is a crucial factor in the compressor performance and the aeroacoustics. There are usually main blades and splitter blades, two different types of blades in centrifugal compressors. Keeping almost the same work input, three different tip clearance configurations were designed in which the tip clearance gaps of main blades and splitter blades were uneven. Then experimental investigations were carried out to study the influence of uneven tip clearances on the aeroacoustic characteristics of centrifugal compressors. The results showed that larger main blade tip clearances or larger splitter blade tip clearances would induce rotating instabilities (RI) near flow instability boundary at low rotating speeds. However, the broadband noises below the blade passing frequency (BPF) in the case with uneven blade tip clearances were suppressed significantly which might be due to the incongruous interaction between the tip clearance leakage flows of main blades and splitter blades. In addition, the blade nonsynchronous vibrations (NSV) were observed in the analysis of the frequency spectrum of far-field noise in the case which had smaller main blade tip clearances and larger splitter blade tip clearances. Based on the jet core feedback theory, a physical explanation was given for NSV occurred in this case.

Oil drag loss in oil-flooded, twin-screw compressors

Rotary twin screw compressors are widely used because of their high efficiency and reliability. Their most common mode of operation is as oil-flooded machines when delivering air and gases at moderate pressures and flow rates. In order to achieve the best performance, it is essential to be able to predict the optimum amount of oil, required for the oil injection process, accurately. Analytical procedures for the design and performance estimation of twin screw compressors are well developed and widely available, but the determination of oil drag losses, in oil-flooded machines is only guesstimated. This paper describes a more detailed and accurate procedure for estimating oil drag loss, using a combined Couette-Poiseuille flow model and gives the results of studies on three sizes of machines operating over a range of pressure ratios and speeds. To this end, a parametric analysis has been developed based on a combined Couette-Poiseuille flow model and has been used to estimate the individual effects of pressure ratio, the various clearances and the oil viscosity on the total drag loss, for different sizes of the compressor. It can be seen from the results that at pressure ratios of up to 8.5, the drag loss due to the discharge axial clearance gap is nearly 2/3rd of the total, while nearly 1/3rd is due to the radial clearance. At normal operating speeds, the loss due to the interlobe clearance is insignificant, but as the pressure ratio increases, this rises more rapidly than that due to the axial and radial losses. The gain in the drag loss due to greater oil viscosity becomes more significant as the compressor size is increased. In larger machines, when clearance values are increased, the radial and axial elements of the drag loss are reduced more rapidly than that due to the interlobe loss.

The Effect of Size and Working Fluid on the Multi-Objective Design of High-Speed Centrifugal Compressors

The impact of size and working fluid on the efficiency, operating range, and axial thrust on bearings is examined for high-speed, oil-free centrifugal compressors. First, the development and validation of a reduced-order model based on scaling principles is documented. Then, the validated compressor model is used to generate design maps for stages operating with arbitrary fluid molecules, and characterized by different size. The results show that compressors operating with fluids made by heavy and complex molecules provide lower efficiency over the entire design space, if compared to their simple-molecule counterparts. However, compressors for complex-molecule fluids require lower rotational speed, and generate lower axial thrust on bearings, thus making them particularly suitable for small-scale applications. Furthermore, a decreasing value of the size parameter has a detrimental effect on the stage efficiency, as a result of manufacturing constraints. The results computed by the compressor model suggest that the efficiency penalty is more sensitive to variations of clearance gap than to surface finishing. Lastly, the reduced-order model has been used to perform a design exercise, i.e., the multi-objective optimization of the first compressor stage of the heat pump test rig being realized at Delft University of Technology. The key characteristics of the optimal compressor design has been compared to those derived from the design maps, to corroborate their validity. The optimal design has been extensively characterized by means of CFD, providing further evidence that efficient mini-compressors operating with organic fluids, and featuring pressure ratios up to five at off-design, are feasible.

A comparison of axial turbine loss models for air, sCO[formula omitted] and ORC turbines across a range of scales

Loss models are used to evaluate the aerodynamic performance of axial turbines at the preliminary design stage. The commonly used loss models were derived for air and steam turbines and have not been sufficiently investigated for turbines working with non-conventional working fluids, relevant to new power systems, such as organic fluids and supercritical CO2 (sCO2). Thus, the aim of this study is to explore the deviation between the performance predictions of different loss models, namely Dunham and Came, Kacker and Okapuu, Craig and Cox and Aungier, for non-conventional working fluids where turbines may differ in design and operation than conventional air or steam turbines. Additionally, this paper aims to investigate the effect of the turbine scale on the trends in the performance predictions of these models. Three different case-studies are defined for air, organic Rankine cycle (ORC) and sCO2 turbines and each one is evaluated at two different scales. It is found that the selected loss models resulted in varying loss predictions; particularly for predicting the losses due to the clearance gap for all small scale designs. Furthermore, large variations were found in predicting the effect of the flow regime on the turbine performance for all models.

CFD analysis of the HVAD's hemodynamic performance and blood damage with insight into gap clearance.

Mechanical circulatory support using ventricular assist devices has become commonplace in the treatment of patients suffering from advanced stages of heart failure. While blood damage generated by these devices has been evaluated in depth, their hemodynamic performance has been investigated much less. This work presents the analysis of the complete operating map of a left ventricular assist device, in terms of pressure head, power and efficiency. Further investigation into its hemocompatibility is included as well. To achieve these objectives, computational fluid dynamics simulations of a centrifugal blood pump with a wide-blade impeller were performed. Several conditions were considered by varying the rotational speed and volumetric flow rate. Regarding the device's hemocompatibility, blood damage was evaluated by means of the hemolysis index. By relating the hemocompatibility of the device to its hemodynamic performance, the results have demonstrated that the highest hemolysis occurs at low flow rates, corresponding to operating conditions of low efficiency. Both performance and hemocompatibility are affected by the gap clearance. An innovative investigation into the influence of this design parameter has yielded decreased efficiencies and increased hemolysis as the gap clearance is reduced. As a further novelty, pump operating maps were non-dimensionalized to highlight the influence of Reynolds number, which allows their application to any working condition. The pump's operating range places it in the transitional regime between laminar and turbulent, leading to enhanced efficiency for the highest Reynolds number.

Elastic Distortion Calculation of a Controlled-Clearance Piston-Cylinder Assembly up to 1500 MPa Using Finite Element Method

Abstract A controlled-clearance piston-cylinder assembly is modelled using finite element method (FEM). Elastic distortion of the piston-cylinder engagement gap is calculated by the FEM model and then is input to a fluid dynamics model to solving the pressure distribution which works as load in the FEM model. Accurate distortion-pressure solutions are obtained for measurement pressure of (100~1500) MPa by iterative calculations until a convergence of interest is reached. Results are generated for various jacket pressures maintaining balance. Dramatic rapid decent are found near the gap exit after a steady profile for both distortion and pressure distribution curves. Gap exit width can be two orders of magnitude smaller than the nominal gap clearance, and comparable to the irregularity of the piston and cylinder, introducing risks of inaccuracy or potential damage.

Shake table tests of concrete anchors for non-structural components including innovative and alternative anchorage detailing

In recent years, the growing need for reducing non-structural damage after earthquakes has stimulated a dedicated effort to develop innovative types of fasteners for anchoring non-structural components (NSCs) to reinforced concrete (RC) host-structures. To contribute to such need, and building on previous research, this paper presents the results of a series of uni-directional shake-table tests of simulated NSCs anchored to concrete via: (1) expansion, and (2) chemical anchors; post-installed into: (a) uncracked, and (b) cracked concrete. Considering different construction details, the experimental investigation focused on traditional anchorage systems, alternative solutions comprising mortar filling into the gap clearance, and a low-damage system relying on supplemental damping devices, capable of reducing the acceleration of the NSCs as well as the force of the anchorage during seismic shakings. The experimental tests provided significant evidence on the beneficial effects of a dissipative anchorage protecting both the non-structural component and the anchorage itself, even during strong earthquakes. Moreover, when construction details allow to close the fixture clearance with a mortar filling, this stiffer solution provide an additional reduction of NSCs seismic accelerations and forces. Therefore, suggestions for further improvements of the adopted low-damage solution are also proposed.

가변노즐 벽면간극이 구심터빈 성능에 미치는 영향에 관한 연구

Radial-type turbines are used in not only aviation engine but also a variety of applications such as automotive turbochargers and power generation equipment. When the turbine cannot always operate at a constant mass flow rate depending on the operating conditions, variable nozzles are used to increase the efficiency. However, a gap between the nozzle and the wall is needed in order to adjust the nozzle according to the mass flow rate. In this study, an investigation on the performance change of a turbine was carried out with various gap clearances. For this purpose, a numerical method and a prediction method using loss models are applied. Each method was compared with the existing experimental results. The comparison of the two results shows a fairly consistent result, and the total efficiency in the turbine decreases by 1.6% for every 1% increase in gap clearance.