Suction Port Research Articles

The role of miscible PAG lubricants in ammonia refrigeration systems reduction and compactness

Today’s society is going through an expanding use of natural refrigerants, earlier present only in big industrial facilities. This has made necessary the charge reduction and compactness improvement of the equipment, to fit the safety measures and standards requirements. The refrigerant charge and the complexity of an ammonia refrigeration system are mainly because of flooded evaporators use and the need for an efficient oil separation system, since conventional lubricants and ammonia are immiscible. The use of ammonia miscible lubricants can address the oil separation problem. A Poly-Alkylene-Glycol (PAG) can be designed and formulated to be miscible with ammonia, and the plant operated like a conventional halocarbon refrigerant one. The circulating lubricant will be recovered through the compressor suction port of the compressor. This will help in the operation of Direct Expansion (DX) evaporators, reducing to a testimonial size the oil separation circuit. This article is a translation of the article by Muñoz-Alonso M, Dixon L, Seeton CJ, Karnaz J. The role of miscible PAG lubricants in ammonia refrigeration systems reduction and compactness. In: Proceedings of the 9th IIR Conference on the Ammonia and CO2 Refrigeration Technologies. Ohrid: IIF/IIR, 2021. DOI: 10.18462/iir.nh3-co2.2021.0018 Published with the permission of the copyright holder.

Optimization of hydraulic structure and well flushing parameters of a truncated cone drill bit for drilling shaft sinking

ABSTRACT Based on a research method combining numerical simulations and model testing, a numerical model and test platform for gas lift reverse circulation multiphase coupling in well washing were developed. The hydraulic structure and well washing parameters of a φ5 m truncated cone bit used in the Beifeng well at Taohutu Coal Mine were optimized, and the distribution law of the well washing flow field was determined. The findings revealed that: (1) with a bit taper of 40 and a length-diameter ratio of 0.32 for the truncated cone, the hydraulic structure was optimal. Compared with the original bit structure, the axial speed of mud at the slag suction port increased by 15.9%, and the average sliding speed of mud at the inclined rock surface increased by 63.2%. (2) The multiphase fluid in the slag discharge pipe flowed successively in the form of “low-speed stable two-phase flow” and “high-speed disordered three-phase flow,” while the multiphase fluid in the horizontal and inclined bottom holes flowed predominantly in radial and tangential patterns, respectively. (3) The optimal parameter combination for efficient well washing with the truncated cone bit included a bit rotation speed of 8 r/min, a gas injection rate of 5600 m3/h, an air duct sinking ratio of 0.79, and a mud viscosity of 176 mPa·s, with bit rotation speed being the most significant factor affecting effective washing efficiency. These results could provide a valuable reference for improving the efficiency of drilling and well flushing in Jurassic strata of western China.

Blocked force characterization and transposition at secondary connections of a compressor

Applications in sustainable energy systems, like heat pumps, are encountering emerging challenges, among which acoustic comfort stands out as a critical factor in shaping the end-user experience. Early integration of vibroacoustic into each industrial design phase ensures the prevention and prediction of potential noise issues, thus avoid costly design modification loops. BDR Thermea and CETIM, in a collaborative project, have implemented new methodologies for designing acoustically optimized products, highlighting the importance of considering vibroacoustic design at each design stage of new products developments. In this study, structure-borne noise characterization methods are applied to a heat pump compressor using the ISO 21955 and ISO 20270 standards. The forces transmitted at its suction and discharge ports were studied through component based TPA for the intrinsic characterization of the compressor. Once obtained, these forces were transposed to a new receiving structure. The results allowed a comparison of the outcomes from both approaches and highlighted the importance of considering the influence of the stiffness of the connections at the compressor ports.

Odor emission pattern of the waste storage workshop of kitchen waste treatment plant and control strategy study with CFD simulation.

Odor emission has become a great issue for kitchen waste management plants. Among all, unorganized emission source such as waste storage tank is the key cause. It is necessary to understand the odor emission characteristics and provide a proper control solution. In this study, a typical kitchen waste treatment plant located in Guangdong Province of China was selected to investigate the odor emission characteristics. According to the survey, the main complaint due to odor emission is on waste storage workshop. Hence, its odor emission has been investigated in this study. The gas samples were collected from the workshop in different season. According to the results, the odor emission during summer is the worst. In total, 105 odorous gases were detected from the waste storage workshop. The main odorous gases can be categorized into sulfur compounds, oxygen-containing organic compounds and terpenes. In specific, ethanol, acetic acid, methylmercaptan, α-pinene, methioether and limonene were the major odorous pollutants. Based on grey correlation, principal component analysis (PCA) and step-up regression analysis, methylmercaptan contributes the most to the odor concentration. It suggests that the odor emission control should pay more attention on methylmercaptan. The Computational Fluid Dynamics (CFD) stimulation was employed to investigate the odor distribution with applying air blowing as a curtain to separate the inside and outside atmosphere or suction to vacuum the inside air to prevent the odor emission. It was found that it could efficiently prevent odor emission by setting a 45° inclined air suction port at the top of the entrance gate. The study provides a theoretical basis on odor control for the waste storage workshop of kitchen waste management plants.

The Effectiveness of the Gnana Laryngeal Mask Airway II (GLA-II) With Novel Suction Tubing in Gastrointestinal (GI) Cases.

The Gnana laryngeal mask airway II (GLA-II) is a supraglottic airway device similar to the classic laryngealmask airway, except it has an additional suction port. This suction port allows for the removal of secretions and saliva. A previous version of the Gnana laryngeal airway 4 was made of silicone, while this newer version is made of polyvinyl chloride (PVC), which is more affordable. This study aimed to demonstrate the effectiveness and tolerability of this PVC-designed GLA-II and evaluate its ability to suction secretions. The prospective cohort study included 100 gastrointestinal (GI) cases to determine the effectiveness and toleration of the GLA-II. The American Society of Anesthesiologists (ASA) class 1-3 patients were evaluated with a Mallampati airway score for GI-related procedures. After anesthesia induction with propofol, the GLA-II was inserted, and the time for successful insertion was recorded. All cases were completed within 62 minutes. During this time, the secretion volumes were also measured. One hundred patients were included in the study: 52% were males and 48% were females. Thirty-four patients were scored as ASA class 1 or 2, while 66 were scored as ASA class 3. The GLA-II insertion was successful on the first attempt in 92 patients, and a second attempt was necessary for six patients. It was unsuccessful in two patients. The average time for successful insertion was 28.3 ± 4.3 seconds. The average amount of saliva suctioned was 9.3 ± 2.6 mL. There were no intraoperative or postoperative complications during these cases. The PVC GLA-II device is distinguished by its ability to allow suctioning during placement. With an adequate epiglottic seal, it can be safely and successfully inserted in a short period of time. More research should be conducted to explore the use of GLA-II devices in other settings, such as emergencies and life-saving scenarios.

Investigation on the pressure pulsation characteristics in a twin-screw multiphase pump

A three-dimensional gas–liquid two-phase transient numerical model for a twin-screw multiphase pump based on dynamic grid technology was established and validated with experiments. The pump's simulated pressure distributions, velocity fields, and pressure pulsations were analyzed to reveal the mechanisms of pressure transmission and pressure pulsation characteristics. The results indicated that the flow rate of the pump fluctuated twice due to the discharge of the male and female rotors in one cycle. As the inlet gas volume fractions increased, the flow rate decreased, but the pressure pulsations increased. At the engaging positions of the two rotor tips, a sudden pressure drop happened due to the combined effect of both tooth-tip and tooth-flank leakage. When the discharge port opened, the backflow happened; the flow rate and the pressure in the discharge chamber decreased, but the pressure in the working and suction chambers increased. When the suction port closed, a slight compression of the fluid in the low-pressure working chamber occurred, causing a pressure increase in the working chamber. The working chambers inhaled and discharged once in one cycle, so the first harmonic of pressure pulsations at the suction and discharge chamber was two times the running speed. The transient flow due to the simultaneous closing and opening of the suction and discharge ports at both sides of the male and female rotors generated a harmonic of four times the running speed. This study would help to improve the operational stability of twin-screw multiphase pumps.

CFD–DEM method is used to study the multi-phase coupling slag discharge flow field of gas-lift reverse circulation in drilling shaft sinking

To elucidate the distribution law of the multiphase coupling slag discharge flow field in gas-lift reverse circulation during drilling shaft sinking, a numerical analysis model of gas–liquid–solid multiphase coupling slag discharge was established by CFD–DEM (Coupling of computational fluid dynamics and discrete element method) method, taking the drilling of North Wind well in Taohutu Coal Mine as an example. This model presented the distribution of the multiphase flow field in the slag discharge pipe and at the bottom hole, and was validated through experimentation and theoretical analysis. Finally, the impact of factors, including bit rotation speed, gas injection rate, air duct submergence ratio, and mud viscosity on the slag discharge flow field was clarified. The results indicated that the migration of rock slag at the bottom of the well was characterized by “slip, convergence, suspension, adsorption, and lifting”. The slag flow in the discharge pipe exhibited the states of “high density, low flow rate” and “low density, high flow rate”, respectively. The multiphase fluid flow patterns in the well bottom and slag discharge pipe were horizontal and axial flows, respectively. The model test of the gas lift reversed circulation slag discharge and the theoretical model of the bottom hole fluid velocity distribution confirmed the accuracy of the multiphase coupling slag discharge flow field distribution model. The rotation speed of the drill bit had the most significant impact on the bottom hole flow field. Increasing the rotation speed of the drill bit can significantly enhance the tangential velocity of the bottom hole fluid, increase the pressure difference between the bottom hole and annular mud column, and improve the adsorption capacity of the slag suction port. These findings can provide valuable insights for gas lift reverse circulation well washing in western drilling shaft sinking.

A Prototype 3D Printed Suction Port Adapter for a Wireless Otoendoscope

ABSTRACTObjective: To design and fabricate a suction port adapter to use various sizes of suction cannulaswith a wireless otoendoscope enabling ear cleaning under endoscopic guidance demonstrated using an ear examination simulator. Methods:Design: Instrument InnovationSetting: Tertiary Private Training HospitalPatient: Ear Examination Simulator Results: The fabricated suction port adapters were able to hold the wireless otoendoscope and suction cannulas together, allowing simultaneous inspection of the ear canal and suctioning of ear canal debris using the Ear Examination Simulator Conclusion: Our prototype 3D-printed suction port adapters for a wireless otoendoscope mayimprove ear cleaning by enhancing the accuracy of suctioning debris and decreasing durationsince they hold the suction cannulas in place under endoscopic guidance. They may aid ENTphysicians in easier visualization and simultaneous ear cleaning of patients and improve earcleaning techniques and times, especially among less experienced physicians, but actual clinicaltrials are needed to confirm this.

Analysis of muck migration behavior and suction port extending forward performance in the working chamber of the slurry shield machine under clay stratum

This study aimed to solve the problem of clogging in the working chamber of a large-diameter slurry shield passing through the clay stratum. Numerical simulation and experimental methods were employed to investigate the migration behavior of the muck in the original working chamber structure. The impact of increasing the percentage of scouring flow and implementing the suction port extending forward method on the muck discharge performance was also investigated. The results indicated that raising the scouring flow percentage could not improve the muck discharge performance under the original structure. However, extending the suction port to 500 mm from the slurry gate resulted in a 75.19% increase in the muck discharge rate and an 84.70% reduction in muck accumulation at the bottom of the chamber. Moreover, closing the scouring pipelines V25 and V7 after extending the suction port forward allowed for more efficient use of the limited slurry flow. Finally, an optimized suction port extending forward method was proposed and successfully applied to alleviate the clogging issue.

Numerical Investigation of the Influence of Different Suction and Discharge Ports on Scroll Expander Performance

Background: With the rapid consumption of non-renewable energy such as coal, oil and natural gas, the growing demand for environmental protection, the re-utilization of low-grade waste heat energy has become an important approach to improve energy utilization efficiency. As a new technology, organic Rankine cycle (ORC) power generation technology can make full use of and convert heat waste. Objective: Both the suction and discharge pressures of the scroll expander have a certain influence on the output and motion characteristics of the orbiting scroll. By studying the position and arrangement of the suction and discharge ports of the expander, a theoretical basis can be provided for the design of these ports. Methods: For the scroll expander using working fluid R134a, establishing the geometrical and three-dimensional models of the suction and discharge ports of the scroll expander with different positions and structures, based on the Computational Fluid Dynamics (CFD) method. Results/Discussion: Through comprehensive comparison, it was found that the structure of the original suction pipe outperformed any of the other structures; the fluid flow in the original discharge pipe was more complicated, and the simplified model of the commonly used scroll mechanical discharge pipe had the optimal performance. Conclusion: Compared with the original prototype SEI2, the suction port area is increased, and the suction port pulsation intensity coefficient and the suction pressure loss coefficient of the prototype SEI4 are reduced by 34.833% and 5.264% respectively, which can make the suction process of the expander more stable. Since the unilateral discharge ports Outlet3 and Outlet5 are located in the moving and static regions, respectively, there is a difference in the perturbation of the outlet fluid by the movable scroll, so that the gas pulsation intensity at Outlet3 is nearly double that of Outlet5.

Performance analysis of a two-stage evaporation heat pump drying system for graded cooling/dehumidification

Single-stage evaporation heat pumps are widely used in drying fields, but they suffer from relatively low efficiency. To enhance the performance of conventional heat pump drying systems with single-stage evaporating, a two-stage evaporation heat pump drying (TSE-HPD) system is proposed in this paper for graded cooling/dehumidification. In this innovative system, the quasi dual-stage compression, matching a double temperature level evaporation process, achieves graded cooling and dehumidification of air. Its performance is investigated using energy and exergy analysis methods and compared with that of a single-stage compression heat pump drying (SSC-PHD) system. The results indicate that the TSE-HPD system exhibits 15.5 %–19.3 % improvement in the coefficient of performance (COP) and 16.3 %–18.9 % enhancement in the specific moisture extraction rate (SMER) compared to those of the SSC-PHD system when the condensing temperature varies between 50 °C and 70 °C. The parametric analysis reveals that the highest COP, SMER, and exergy efficiency (ηex) occur when the optimum first-stage evaporating temperatures are 16 °C, 25 °C, and 34 °C, respectively, under three standard operating conditions. At this time, the first-stage evaporator handles 36 %–40 % of the cooling/dehumidification tasks, the refrigerant mass flow ratio at the intermediate-pressure suction port of the compressor ranges from 47 %–58 %, and the optimum first-stage and second-stage compression ratios are 1.49–1.52 and 2.45–2.61, respectively. The research results provide guidance and reference for the determination of matching design parameters and efficient operation control parameters of drying systems.

Continuous ultrasonic ozone coupling technology-assisted control of ceramic membrane fouling coupled enhanced multiphase mixing to treat dye wastewater and CFD flow field simulation

In this study, ozone catalysts (hydrogenation-modified red mud, HM-RM) successfully prepared by hydrogenation-modification of industrial hazardous solid waste red mud (RM) as a raw material in accordance with the viewpoint of treating waste with waste and using waste. Meanwhile, as for the common phenomenon of membrane fouling, uneven distribution of multiphase solid catalysts and ozone in liquids, the addition of ultrasound can not only disperse materials, but also play a role in online cleaning of ceramic membranes and catalysts. The optimum treatment conditions for Rhodamine B (RhB) solution with volume of 2 L and concentration of 40 mg/L were catalyst concentration of 0.4 mg/L, reaction temperature of 45 °C, ultrasonic time of 1 h, ultrasonic intensity of 600 W, removal rate of RhB was up to 90 %. In addition, the computational fluid dynamics (CFD) simulation method was used to investigate the fluid flow between the two gas-liquid phases and the effect of the negative pressure of the membrane pump on the fluid by the analysis of flow, pressure and ozone flux of the ceramic membrane(CM) reaction apparatus. The CFD simulation results showed that at the inlet gas-liquid flow rate of 3 m/s and the negative pressure of 20,000 Pa, the maximum flow rates of CM-1 were 3 m/s, 0.752 m/s for CM-2, and 0.228 m/s for CM-3, respectively. Vortices, which are beneficial to solid-liquid mixing and gas-liquid mass transfer, formed between the suction port CM-1 of CM-1 and the inlets of CM-2 and CM-3. This discovery is consistent with relevant experimental research results. Significantly higher concentrations of both •OH and dissolved ozone were observed in the US/HM-RM/O3 system compared to other systems, indicating the significant improvement in ozone utilization rate through the application of ultrasound. The superiority of the US/HM-RM/O3 device was demonstrated. The real dye effluent was tested under optimum operating conditions and the results showed that COD and TOC were reduced by 81.34 % and 60.23 % respectively after 180 min of treatment. The above research can provide technical support for the treatment of dye wastewater using Ultrasound-enhanced ozone oxidation ceramic membranes.

Integrated design and hydraulic model optimization of model horizontal main pump based on CFD fluid dynamics analysis

There are few studies and reports related to horizontal main pumps for nuclear power plants. The installation of vertical main pumps for small reactors requires the design of special elbows on the pressure vessel. Horizontal main pump, as a new main pump structure, can reduce the diameter of the main pressure vessel and simplify the structure of the pressure vessel. Compared with the vertical main pump, the horizontal main pump needs to control its axial dimension to improve its stiffness, which makes it impossible to set a long straight pipe section at the inlet, which has an impact on the pump performance. On the other hand, the compression of the axial dimension of the pump casing makes the overflow area of the press-out chamber small, resulting in large hydraulic losses. In this paper, in order to improve the efficiency of the pump and reduce the influence of non-uniform incoming flow, the optimization design is carried out with the SST k-ω model, and it is shown that: 1. the suction port of the curved impeller is easier for the fluid to enter into the impeller in the axial direction; and 2. the axially symmetric worm gear produces more vortex flow losses.

Dynamic Characteristic and Performance Analysis of Claw Vacuum Pumps with Multi-Claw Rotors

The number of lobes of multi-claw rotors determines the performance of claw vacuum pumps including the pumping speed, dynamic characteristics, and operating conditions. In order to reveal the effects of the number of lobes on rotor performance, in this study, a geometric model of multi-claw rotors was established. The structure and geometric performance of multi-claw rotors including suction port and discharge port, pumping speed, and volumetric utilization ratio were compared. Furthermore, a dynamic characteristic model of claw rotors was established, and, therefore, axial gas forces, radial gas forces, and gas resistance torques of multi-claw rotors were obtained. The results indicate that double-claw rotors increased 34.67 % and 4.59 % in terms of pumping speed and volumetric utilization ratio compared to single-claw rotors, and triple-claw rotors showed similar increases as double-claw rotors. The triple-claw rotors have a 49.39 % and 31.38 % reduction in compression duration compared to the double-claw rotors and single-claw rotors. The gas resistance torques of the multi-claw right rotors are consistently more than that of the left rotors, which demonstrates that the multi-claw right rotors are more suitable to serve as the driving rotors. The research contents of this paper are of great theoretical significance for the selection of rotor lobes and the design of claw vacuum pumps with prominent dynamic characteristics.

Plastic bronchitis caused by Haemophilus influenzae.

Plastic bronchitis is characterized by sputum obstructing the bronchus and causing atelectasis. Bronchoscopic removal of the clogged bronchial cast is typically performed, but small-diameter bronchoscopes with narrow suction ports cannot achieve adequate suction. Suction using a bronchoscope as a guide sheath may be effective for treating plastic bronchitis in children.

Differences in residual volume above different tracheostomy tube cuffs depending on tube structure, tube tilt angle, and liquid viscosity

IntroductionProper management of aspirated material above the tracheostomy tube cuff is crucial to prevent complications, such as aspiration pneumonia. This study aimed to thoroughly examine the effects of aspirated liquid viscosity, suction port positioning, and tube tilt angle on residual volume above the cuff (RVAC).MethodsFive types of tracheostomy tubes (approximately 9 mm outer diameter) were placed through a transparent cylinder with an inner diameter of 18 mm. The cuff was inflated to completely seal the interior of the cylinder. Four liquids with different viscosities were poured onto the cuff, and the liquid above the cuff was suctioned from the side port. The cylinder was angled at 90° and 20°, and each test was performed thrice to determine the average RVAC.ResultsAfter side-port suctioning, some liquid residue was observed on the cuff of all tracheostomy tubes. The RVAC increased with higher liquid viscosity. The tubes with a longer distance from the suction port opening to the cuff top exhibited more RVAC. Moreover, the RVAC was almost the same regardless of the cylinder angle for tubes with a suction port on the lateral side. However, tubes with backside ports showed a decreased RVAC with cylinder tilt.ConclusionsThis study underscores the persistence of residual material on cuffed tracheostomy tubes even with regular subglottic secretion drainage. This emphasizes the need for specialized tracheostomy tube development aimed at reducing post-suction RVAC. Improved designs can potentially minimize complications associated with residue accumulation.

Numerical modeling of flashing in TFC expanders for the efficient exploitation of low-grade heat

This paper presents a novel semi-empirical thermodynamic model for the simulation of two-phase expansion in twin-screw expanders. The model utilizes a limited number of unknown parameters, the values of which are calculated through a calibration-optimization process that exploits numerical experiments available in the literature. Two-phase expansion simulations are performed for a wide variety of operating conditions, revealing that the vapor quality at the suction port of the expander is the main factor affecting its efficiency. Increased vapor qualities at the onset of expansion lead to an increase in the expander’s efficiency because a better matching between the WF’s volume ratio and the expander’s built-in volume ratio can be achieved. The thermodynamic model is applied to assess the performance of a TFC engine operating under varying heat duties and temperatures of the heat source, indicating that a detailed analysis is necessary to conclude whether the TFC or the ORC is the optimal solution for a given application.

Omnidirectional (Flexible) Ureteral Access Sheath: Safety, Efficacy, and Initial Experience Report.

Background: Recently a novel omnidirectional (OD) ureteral access sheath (UAS) has been developed. By retrospectively reviewing and comparing the flexible ureteroscopic lithotripsy (FURL) cases in our institution with either a conventional Cook UAS or an OD UAS in the past year, we shared our experience of the safety, efficacy, and relevant issues on the usage of OD UAS. Materials and Methods: The medical history and surgery details of 199 patients with kidney stones or ureterojunctional stones who underwent FURL in Xinhua Hospital, including 61 Cook UAS and 138 OD UAS, were reviewed and compared. The maximal deflection angle was measured by steering four different types of ureteroscopes to bend the OD UAS in different states. Result: The deflection angle of OD UAS was ∼110° to 130° free load, and 90° to 130° when loaded with different instruments. The stone burden and position were similar in two groups. Given a similar prestent ratio and operation time, the OD UAS group achieved a higher single-session stone-free rate (SFR) (63.9% vs 94.2%, p < 0.0001) at 1-month follow-up evaluated by a CT scan. Conclusion: OD UAS is a novel device with high safety and efficacy. The unique flexible design allows it to bend with the ureteroscope and enter renal calices and be set close to the stone. Combined with the suction port, OD UAS contributes greatly to dealing with large-burden kidney stones, shortens operation time, and improves single-session SFR.

Design and Parameter Optimization of a Negative-Pressure Peanut Fruit-Soil Separating Device

This study proposes a negative-pressure fruit-soil separating device for peanuts cultivated in hilly and mountainous areas after combined harvesting, and the mechanism of the movement of the material in the process of material screening, fruit-soil separating, and pneumatic conveying of the device was analyzed. In addition, a four-factor, three-level Box-Behnken regression design test was used to explore the optimum operating parameters of the peanut fruit-soil separation device. The results showed that the best fruit-soil separation effect was achieved when the wind speed of the blower was 13.58 m/s, the height of the suction nozzle from the screen surface was 27 mm, the length of the suction port was 64 mm, and the feeding rate was 600 kg/h. Validation tests demonstrated that the impurity rate of 0.16% and the peanut pod loss rate of 0.2% exceeded the industry standard, indicating superior performance.

Effects of pump parameters on the oil absorption characteristics of bidirectional involute internal gear pumps

The symmetrical oil suction and discharge chambers and the reversing rotation condition of bidirectional involute internal gear pumps result in low oil absorption capacity and flow pulsation due to cavitation. However, efforts to address these issues suffer from an incomplete understanding of the effects of oil properties, suction chamber volume, suction pressure, and rotational speed on the oil absorption capacity and cavitation characteristics of these pumps. The present work addresses this issue by conducting detailed computational fluid dynamics (CFD) simulations. The results of analysis demonstrate that the degree of cavitation caused by changes in the mass fraction of dissolved gas in the oil is positively correlated with flow pulsation, while the degree of cavitation caused by changes in the suction port size, suction pressure, and rotational speed is negatively correlated with flow pulsation. The degree of cavitation in the oil suction cavity initially increases as the mass fraction of dissolved gas in the oil increases, and then decreases. The rotational speed exhibits a critical value, where the oil absorption capacity increases with increasing rotational speed below the critical value, and decreases with increasing rotational speed above the critical value. The results of this research are expected to be of great significance for guiding improvements in the design of bidirectional involute internal gear pumps and the determination of critical working conditions.