Swirl Generator Research Articles

Thermo-hydraulic performance of tube with decaying swirl flow generators

• Investigation of experimental and numerical study for decaying swirl flow in a pipe are presented. • Pressure drop and heat transfer were evaluated and analyzed for different swirl angle. • Due to the swirl intensity, heat transfer enhances with increasing swirl helical angle. • Performance evaluation criteria is the highest at the lowest swirl angle. • Variation Local Nusselt number along pipe is higher at larger swirl angle. In this study, the influences of swirl generators fixed at the pipe inlet and having different swirl angles (0°, 22.5°, 41°and 50°) on the heat transfer and fluid flow characteristics in a pipe flow were experimentally and numerically studied under a constant heat flux condition in the range of Reynolds ( Re ) number 2400–23000. The axial type passive swirl generator aims to increase the turbulence effect and gives tangential acceleration while increasing the total flow length in the pipe. This study is novel with the aspects of; (i) the number of helical air passage channels employed in the investigated swirl generators and, (ii) 360° rotation of each helical channel at different swirl angles, i.e., lengths of considered swirl generators are different. Experiments were carried out in a smooth aluminum pipe having an inner diameter of 15 mm and a length of 675 mm with and without swirl generators for turbulent airflows. Empirical correlations for the local and mean Nusselt ( Nu ) numbers, and friction factor ( f ) were developed as a function of Re number and swirl angle as well as smooth pipe flow. The obtained experimental results for the heat transfer and friction factor at different swirl angles were compared with the smooth pipe results. The findings showed that the maximum heat transfer rate is achieved at a swirl angle of θ = 50°, and the heat transfer rate increases as the swirl angle and Re number are increased. The decaying swirl flow leads to around 1.1–1.41 times enhancement of Nu and 2.3–6.76 times increase of f compared to the smooth pipe. The maximum value of performance evaluation criteria ( PEC ), around 0.83, is achieved by the decaying swirl flow when the θ = 0° and Re = 2400. The experimental and numerical Nu and f values for swirl flow are in good agreement within the mean deviation ± 7.1% and ± 8.4%.

Thermo-hydraulic performance evaluation of heat exchanger tube with vortex generator inserts

This work is undertaken as a scientific experiment to test a new design of a turbulent generator. The current research experiments the influence of novel vortex generator inserts on heat transfers within a tube under a uniform heat flux. A Cu tube with a 45 mm inner diameter and 1350 mm length is used along with a solid disc injector (swirl generator) that comprises ten crescent holes with equal circumferential distribution angles around the disk canter. Subsequently, a swirl flow is generated by deviating the stream flow 45? causing it to spin in the direction of the axial flow. Flow directors are on 45? angles toward the axial direction for each of the crescent holes. This study is an example of flow degradation. Reynolds numbers range from 6000 to 13500. Therefore, fluid-flow is treated as a turbulent system. All experiments done with air are regarded as a power fluid and Prandtl number is fixed at about 0.71. Thermo-hydraulic performance of heat exchanger is analyzed. The average heat transfer Nusselt number is calculated and discussed. The experiment found out that Nusselt number increases with an increase in Reynolds number as well as the number of swirl generators. At four vortex generators, the maximum augmentation in heat transfer is around 4.3 times greater than the plain tube and friction factor is about 1.28 with 5 vortex generators insets. The results indicate a promising heat exchanger enhancement in the local petroleum industries.

Analysis of the three-dimensional swirling and non-swirling jet impingement using a turbulence model with cross-diffusion correction

Three-dimensional swirling impinging jets using the aerodynamic swirl generator are carried out to understand the pure effects of swirl. A shear stress transport model with cross-diffusion correction (SSTCD) is validated for the above flows with the documented experiments. Based on the good performance of the SSTCD model, a detailed flow analysis for Re = 23,000 and swirl numbers of 0 and 0.45 at nozzle-plate spacings of 2 and 6 is presented in terms of the flow structures, mean velocity field, wall shear stress and heat transfer. The results show that the strength of the swirl is reduced with increasing the nozzle-plate spacing near the wall. On the contrary, the effect of swirl on both flow fields and heat transfer is more evident at high nozzle-plate spacing, leading to a broader impact region along the impinging plate. However, downstream, the effect of swirl can be ignored for the above fields at all nozzle-plate spacings. In addition, the swirl motion contributes to high turbulence but the heat transfer rate still decreases with an increase of swirl number. Meanwhile, the uniformity of heat transfer is improved by the swirl motion for both nozzle-plate spacings, except that near the stagnation region at H/D = 2. It is also found that this improvement depends on the Reynolds number.

Effect of various multiple strip inserts and nanofluids on the thermal–hydraulic performances of parabolic trough collectors

The thermal performance of the parabolic trough collectors can be improved by the use of swirl generator inserts added to the solar receiver. These inserts help improve the energy capture from the internal receiver walls and reduce the external wall thermal losses. If nanoparticles are added to the heat transfer fluid of the solar receiver tube, then this also increases the output working fluid temperature thereby reducing the outer wall’s temperature. The main objective of the current study is to assess the effect of various straight strip shapes with and without nanofluids. Four different strip arrangements were considered; large conical-shape strips, small conical-shape strips, rectangular-shape strips and elliptical-shape strips. All of these configurations were then also tested in 6% of SiO2 nanoparticles mixed in Therminol® VP-1 (TO) forming the nanofluid. Results showed that the straight large conical strips lead to the maximum enhancement in the Nusselt number (up to 57.49% for the pure base fluid and up to 62.53% for the nanofluid). The thermal efficiency improvement for this optimal configuration with nanofluid was found to be 15.41% with a thermal loss reduction of around 26%. A similar trend was found for the thermal exergy efficiency which showed an improvement of 15.32% for this optimal configuration with the use of nanofluid.

Hydrothermal characteristics of fluid flow in a circular tube fitted with free rotating axial-turbine-type swirl generators: Design, swirl strength, and performance analyses

The effect of inserting free rotating axial-turbine-types swirl generators (ATTSGs) in a circular tube is numerically investigated for convective heat transfer enhancement. Different types of ATTSGs with different blade designs are placed inside the tube, which are rotating freely around their axis depending on the velocity of the fluid flowing in the tube. The moving reference frame (MRF) method is used to model the fluid rotation in the zones containing the ATTSGs and the source terms arising from the transformation of stationary to rotating reference frame. Free rotational speed of the ATTSGs is estimated at different Reynolds numbers by systematically analysing the net torque applied on the ATTSGs in terms of their angular velocity. The local and average convective heat transfer and pressure drop are investigated for the cases with ATTSGs and the case of a plain tube. Considerable enhancement in the convective heat transfer is observed for all the ATTSGs, with the maximum of around 75%. To investigate the efficiency of the ATTSGs in terms of heat transfer enhancement, the intensity of the vortex structures generated by the ATTSGs is represented by the Q-criterion iso-surfaces. The ATTSGs which generate stronger swirling effect at the vicinity of the heated wall cause greater heat transfer enhancement. Inserting the ATTSGs leads to an increase in the pressure drop. Performance evaluation criterion (PEC) is introduced and evaluated for different ATTSGs to identify configurations that enhance the convective heat transfer at the lowest pumping power. The highest PEC of 1.4 is reported for the ATTSG with trapezoid twisted blade design.

On the generation mechanisms of low-frequency synchronous pressure pulsations in a simplified draft-tube cone

This paper focuses on the generation mechanisms of synchronous pressure pulsations featuring a lower frequency than the rotating frequency of a vortex core which are observed in a simplified draft-tube cone through analyzing the results of experiments along with unsteady-state numerical simulations. A Venturi-tube is selected as a simplified geometry of hydro-turbine draft-tube; and a stationary-blades swirl generator is installed upstream to generate a uniform swirling flow in the Venturi cone (diffuser part). The experiments, including the dynamic pressure measurements, are conducted for several flowrates to determine the characteristics of pressure fluctuations in the Venturi cone. Two types of pressure fluctuations are identified, namely convective and synchronous. Both cover wide ranges of different frequencies, revealing that the phenomena responsible for generation of these pressure fluctuations feature a stochastic component. The synchronous pressure pulsations, which feature lower frequencies than the convective fluctuations, are identified as the product of self-excited oscillations of vortex breakdown. High-speed visualization of the vortex shape reveals that the vortex features three states: an axisymmetric vortex, a breakdown, and appearance of several helical vortices generated downstream of the breakdown location; while the location of the vortex breakdown fluctuates in time. By using the results of unsteady CFD simulation, it is confirmed that the location of the vortex breakdown fluctuates with the same frequency (Strouhal numbers 0 to 1) as the low-frequency synchronous pressure pulsations observed in the Venturi cone. Moreover, it is observed that the volume of the stagnation region produced due to the counterflow downstream of the breakdown location fluctuates with the same low frequency. Further analysis of the CFD results showed that the flow field at the location of the vortex breakdown experiences a severe shear layer formation around the center of the Venturi. This shear layer stretches the vortex center spatially on the plane perpendicular to the vortex axis. It is also observed that several intermittent helical vortices are generated downstream the breakdown location and rotate periodically in the Venturi while featuring a stochastic nature, as well.

Swirl flow heat transfer and flow characteristics in a solid and permeable pipe with exit nozzle

This paper goal is to study swirling flow heat transfer and flow characteristics in the round solid and permeable pipe with an exit conical nozzle. The experiments were carried out in the pipe with an internal diameter of 80 mm, 1200 mm long, and an exit conical nozzle ratio of 0.5, 0.63, and 0.75. Seven swirl generators with profiled blades were tested at different flow conditions. The flow experiments in the solid pipe were carried out at Red = 42 000 –170 000, while heat transfer experiments - at Red = 30 000 – 150 000. In the permeable pipe the flow experiments were conducted at Red = 70 000 –130 000, heat transfer experiments – at Red = 50 000 – 120 000 and blowing ratio B* = 0.002 – 0.050. The heat transfer coefficients, total pressure losses, and swirl flow parameters . were presented. In the pipe with exit nozzle ratio over 0.75 the flow parameters are identical to those in the open pipe with flow swirl. At the lower exit ratio (0.5) and high swirl flow rate, the exit nozzle restricts the reverse flow from outside and radically changes the axial and rotational speed distribution inside the pipe. A few novel scientific results were obtained in the pipe with an exit nozzle, among them: the powerful vortex formation in the near-axis area of pipe, «partially suppressed» turbulent heat transfer pattern near the swirl generator, identical mechanism of heat transfer enhancement in the solid and permeable pipe, common relations linking the local and total swirl flow numbers, .axial and rotational speed. A few novel equations for heat transfer and flow parameters . were presented as a result of experimental data processing.

Effect of piston and cylinder head swirl generation techniques on the performance of ceiba pentandra and nigella sativa B20 biodiesel blended diesel engine operation

Biodiesels are renewable fuels and their usage in diesel engine can decrease the dependence on fossil-based fuels which are exhaustive in nature. Further they can effectively address the alarming increase in emission levels in the atmosphere and provide savings in the foreign exchange. The present work mainly focuses on the effect of swirl techniques adopted on the performance of the modified diesel engine powered with two B20 based biodiesels derived from Ceiba Pentandra (CPNTOME B20) and Nigella Sativa (NIZSAOME B20) respectively. The swirl techniques are adopted by suitable modifications on both the piston and cylinder head surfaces respectively. Optimized number of cavities on piston and cavities and domes on the cylinder head improved the diesel engine performance powered with selected biodiesel blended fuels. Accordingly, 6 number of cavities on the piston and 2 domes and 3 cavities on the cylinder head surfaces provided higher brake thermal break efficiency (BTE), lower hydrocarbons (HC), carbon monoxide (CO), smoke emissions while nitric oxide (NOx) emissions were increased for the diesel engine powered with two B20 blends compared to other swirl combinations used. Among the two biodiesels used CPNTOME B20 showed higher engine performance compared to NIZSAOME B20. BTE of engine fuelled with diesel increased by 4.86% and 6.54% respectively when compared with B20 blends of CPNTOME and NIZSAOME. Smoke emissions of engine fuelled with diesel decreased by 8.27% and 11.43% respectively when compared with B20 blends of CPNTOME and NIZSAOME.

Unsteady phenomena at the combustor-turbine interface

The combustor-turbine interface in a gas turbine is characterised by complex, highly unsteady flows. In a combined experimental and large eddy simulation (LES) study including realistic combustor geometry, the standard model of secondary flows in the nozzle guide vanes (NGV) is found to be oversimplified. A swirl core is created in the combustion chamber which convects into the first vane passages. Four main consequences of this are identified: variation in vane loading; unsteady heat transfer on vane surfaces; unsteadiness at the leading edge horseshoe vortex, and variation in the position of the passage vortex. These phenomena occur at relatively low frequencies, from 50–300 Hz. It seems likely that these unsteady phenomena result in non-optimal film cooling, and that by reducing unsteadiness designs with greater cooling efficiency could be achieved. Measurements were performed in a high speed test facility modelling a large industrial gas turbine with can combustors, including nozzle guide vanes and combustion chambers. Vane surfaces and endwalls of a nozzle guide vane were instrumented with 384 high speed thin film heat flux gauges, to measure unsteady heat transfer. The high resolution of measurements was such to allow direct visualisation in time of large scale turbulent structures over the endwalls and vane surfaces. A matching LES simulation was carried out in a domain matching experimental conditions including upstream swirl generators and transition duct. Data reduction allowed time-varying LES data to be recorded for several cycles of the unsteady phenomena observed. The combination of LES and experimental data allows physical explanation and visualisation of flow events.

A comprehensive review on compound heat transfer enhancement using passive techniques in a heat exchanger

The heat transfer is a very interesting field for efficient, useful, eco-friendly reasons. The economic & environmental factors demand that we get the maximum possible efficiencies from thermal power plants possible only by making more efficient heat exchangers. Increasing the effectiveness of new and existing heat exchangers is a typical criterion of engineers involved with the design of heat exchangers. Augmenting process plant efficiency and finding new ways to reduce energy requirements is always an important design priority. Therefore, the heat transfer augmentation plays a substantial role in improving energy efficiency and development of high-performance thermal systems. The heat transfer augmentation techniques are classified as passive, active and compound techniques. The current article presents an extensive review of experimental and numerical studies on passive compound heat transfer augmentation techniques (PCHTAT), combining two or more of the current techniques can be employed all together to produce an enhancement larger than that produced by using only one augmentation. The compound technique arrangement are (i.e. helical-ribbed tube with double twisted tape inserts, Dimpled tube fitted with a twisted tape swirl generator, non-uniform wire coil, and twisted tape inserts. etc.) which covers the laminar and turbulent flow regions. Interactions between different enhancement methods will increase the overall heat transfer coefficient of the thermal systems than that produced by using only one augmentation technique.

Effect of the Number of Nozzles of Swirl Flow Generator Utilized in Flat Plate Solar Collector: An Entropic Analysis

The numerical model of the pipes of a flat plate solar collector (FPSC) with several nozzles has been investigated in the present study. Indeed, the effect of the number of nozzles of the swirl generator on the entropic characteristics has been evaluated. The nozzles were applied for improving the performance of FPSC. For evaluating the proposed system based on the entropy concept, the effect of injection angle and mass flow rate has been considered. The selected injection angles were 30°, 45°, 60°, and 90°. Also, the total mass flow rates entered from all of the nozzles were 0.2 kg/s, 1 kg/s, and 2 kg/s. The effect of said variables on frictional and thermal entropy generations was analyzed; then, the overall energetic-entropic performance of the system was predicted using several dimensionless parameters including NE, NS, Nu ∗ , and heat transfer improvement (HTI). Moreover, Witte-Shamsundar efficiency ( η W − S ) was applied to pinpoint the efficiency of the system. The highest value of HTI and η W − S was 1.7 and 0.9 that achieved by “single-nozzle; A90-D50-N12.5-M0.2” and “quad-nozzle; A30-D50-N12.5-M2,” respectively.

Numerical study of airflow regimes and instabilities produced by the swirl generation chamber in counter-current spray dryers

In this work a CFD numerical study is performed to evaluate the influence of the geometry of the swirl generation chamber on the flow behaviour, the swirl intensity and the unstable oscillations in the airflow pattern. The study shows four different regimes for the flow, separated by three critical swirl intensities. The first one is used to identify the formation of the vortex breakdown. The second one makes a distinction between a free and a wall governed vortex. For a free vortex the instabilities of the flow pattern decrease when the swirl intensity increases showing a negative power correlation between them. However, in a wall governed vortex the instabilities do not correlate with the swirl intensity, being uncontrollable. The third critical swirl intensity indicates the formation of a backflow region near the walls at the bottom zone of the dryer but without impacting the top cylindrical part of the chamber.

Inducing swirl flow inside the pipes of flat-plate solar collector by using multiple nozzles for enhancing thermal performance

In this numerical study, an attempt has been made to improve the thermal performance of the flat-plate solar collector (FPSC) by inducing the swirl flow inside the tube by the considered nozzles. To this end, the effect of the number of circumferential nozzles and their inclination angles was taken into the account. The considered number of nozzles was "single", ''dual'', ''triple'', and ''quad''. For each of the said cases, the inclination angle of nozzles was taken 30°, 45°, 60°, and 90° (A30, A45, A60, A90). Moreover, the mass flow rate of single-nozzle pipe was considered 0.2 kg/s, 1 kg/s, and 2 kg/s. To analyze all of the cases under identical conditions, the said mass flow rates were distributed equally among all of the nozzles (for ''dual'', ''triple'', and ''quad''). All of the characteristics were defined in a form of "A…-D…-N…-M…'' where ''A…'', "D…", "N…", and "M…" stand for angle of injection, diameter of pipe, nozzle cross-section edge, and mass flow rate, respectively. Numerical simulation (3-dimensional) of the system was performed by Finite Volume Method (FVM). The turbulence nature of flow was simulated by the k-omega SST (shear stress transport) turbulent model. Results showed that the "single-nozzle'' swirl generator had the highest thermal performance factor (TPF) so that for all cases its values were greater than unit. Mass flow rate growth increases Nu, heat extraction rate, and kinetic energy rate (KER) while drops friction factor and outlet temperature. Increment of injection angle increases outlet temperature and friction factor and reduces KER. The maximum and minimum values of TPF are 4.19 and 0.44 which belong to "single; A30-D50-N12.5-M0.2" and "quad; A90-D50-N12.5-M0.5", respectively.

Thermal Performance Enhancement Using Absorber Tube with Inner Helical Axial Fins in a Parabolic Trough Solar Collector

In the present work, a parabolic trough solar (PTC) collector with inner helical axial fins as swirl generator or turbulator is considered and analyzed numerically. The three-dimensional numerical simulations have been done by finite volume method (FVM) using a commercial CFD code, ANSYS FLUENT 18.2. The spatial discretization of mass, momentum, energy equations, and turbulence kinetic energy has been obtained by a second-order upwind scheme. To compute gradients, Green-Gauss cell-based method has been employed. This work consists of two sections where, first, four various geometries are appraised, and in the following, the selected schematic of the collector from the previous part is selected, and four various pitches of inner helical fins including 250, 500, 750 and 1000 mm are studied. All the numerical results are obtained by utilizing the FVM. Results show that the thermal performance improvement by 23.1% could be achieved by using one of the proposed innovative parabolic trough solar collectors compare to the simple one. Additionally, the minimum and maximum thermal performance improvement (compare to the case without fins) belong to the case with P = 250 mm by 14.1% and, to the case with P = 1000 mm by 21.53%, respectively.

Swirler geometry effects (dh/do ratio) on synthetic gas flames. Part 2: dynamic flame behaviour at external y altered acoustic conditions

In a combustion device, unsteady heat release causes acoustic energy to increase when acoustic damping (energy loss) is not that effective, and, as a result, thermo-acoustic flame instabilities occur. In this study, effects of the swirler dh/do ratio (at different swirl numbers) on dynamic flame behaviour of the premixed 20%CNG/30%H2/30%CO/20%CO2 mixture under externally altered acoustic boundary conditions and stability limits (flashback and blowout equivalence ratios) of such mixture were investigated in a laboratory-scale variable geometric swirl number combustor. Therefore, swirl generators with different dh/do ratios (0.3 and 0.5) and geometric swirl numbers (0.4, 0.6, 0.8, 1.0 1.2 and 1.4) were designed and manufactured. Acoustic boundary conditions in the combustion chamber were altered using loudspeakers, and flame response to these conditions was perceived using photodiodes and pressure sensors. Dynamic flame behaviour of respective mixture was evaluated using luminous intensity and pressure profiles. Results showed that the dh/do ratio has a minor impact on dynamic flame behaviour.

Swirler geometry effects (dh/do ratio) on synthetic gas flames: Part 1: Combustion and emission characteristics

Swirling flows increase combustion performance via favouring flame stability, pollutant emissions, and combustion intensity. The strength of a swirling flow is characterized by a parameter known as swirl number, which is highly related to the dh/do ratio. In this study, effects of the swirler dh/do ratio on combustion and emission characteristics of the synthetic gas flames of premixed 20%CNG/30%H2/30%CO/20%CO2 mixture were experimentally investigated in a laboratory-scale swirl stabilized combustor. For this purpose, twelve different swirl generators were designed and manufactured. dh/do ratios of these swirlers were set as 0.30 and 0.50, and the geometric swirl number was varied between the values of 0.4 and 1.4 (at 0.2 intervals). All experiments were conducted at a fuel-lean equivalence ratio (ϕ = 0.6), room temperature, and local atmospheric conditions of the city of Kayseri, Turkey. A data logger was utilized to plot axial and radial temperatures and NOx, CO, and CO2 profiles, which were exploited to assess combustion and emission performance. Results showed that the dh/do ratio had a non-monotonic effect on the behaviour of combustion and emission of the tested synthetic gas mixture. Depending on the swirl number, increments and decrements were observed in temperature and emission values.

Heat transfer measurements of a turbine endwall with engine-representative freestream turbulence and inlet swirl

ABSTRACT Flow distortions from upstream combustors generate various and significant changes to the patterns of endwall heat transfer for an aircraft engine. In this study, to model engine-representative oncoming flows into the high-pressure turbine, turbulence and swirl generators are carefully designed at the inlet of a linear turbine cascade. Detailed heat transfer characteristics over the endwall are measured by using a steady thermochromic liquid crystal (TLC) method for various Reynolds numbers. Comparisons with a baseline case that has no upstream turbulence or swirl generators show that both turbulence and inlet swirl alter the heat transfer levels and distribution patterns, but their mechanism of action differs from one another. The turbulence alters the heat transfer by thinning the inlet boundary layer, while the inlet swirl generates the changed heat transfer due to the sweep effects of the swirl on the endwall, resulting in a much higher heat transfer enhancement than that by the turbulence. Additionally, the highest enhancement for both cases is found at the low Reynolds number of 1.0 × 105 and higher Reynolds numbers yield to a reduced enhancement. Specially, compared to the turbulence case, the enhancement by the inlet swirl drops much more rapidly, indicating that the inlet swirl effects have stronger links to the Reynolds number.

Aerothermal performance of square duct enhanced by twisted tape with inclined ribs and slots

To reduce the pressure drop penalties with substantial heat transfer enhancements for the channel flows enhanced by a twisted tape insert, the conjugate inclined ribs and slots are newly devised to modify the twisted tape. The simulated flow and pressure fields generated by the inclined rib-slot element along a twisted tape are correlatively examined with the measured Nusselt numbers and Fanning friction factors. The tilted transverse ribs convert the flow re-circulation cell behind a vertical rib to the solenoidal vortex and trip the separated flows. The inclined slots enable the mass, momentum and energy exchanges between the two axial swirls along the two spiral pathways segregated by the twisted tape. These flow mechanisms induced by the twisted tape and the inclined ribs and slots act synergistically to improve the heat transfer performance with the lessened rib-induced form drags. The average Nusselt numbers are raised to 4.2–3.8 time of the Dittus-Boelter values at the expenses of the Fanning friction factor augmentations to 32.5–40.2 time of the Blasius levels at the range of Reynolds number between 10,000 and 50,000. Compared with the tubular flows enhanced by the ribbed twist tape, the heat transfer enhancements caused by the rib-slot element along a twisted tape in a square duct are similarly retained but the reductions in Fanning friction factor are substantial. The resultant aerothermal performance improvements achieved by the inclined rib-slot element lighten the prospective applications to heat exchangers involving swirl generators. To assist these applications, the empirical correlations that calculate the average Nusselt number, Fanning friction factor and relative heat transfer power of a square duct enhanced by the twisted tape with the inclined slots and ribs are devised.

3D numerical investigations of the swirling flow in a straight diffuser for the variable speed values of the rotor obtained with a magneto-rheological brake

The self-induced instabilities developed in decelerated swirling flows lead to pressure fluctuations. The swirling flows generated under variable speeds of the rotor of a swirl generator using a magneto-rheological brake are numerically and experimentally investigated. Three-dimensional turbulent flow computation is performed for several rotor speeds. Two mean velocity components numerically computed are validated against LDV data on a survey axis located downstream to the rotor for two speed values. The torque on the rotor blade is examined for several speeds. The numerical results are checked against the design values to identify the benefits and limitations of the concepts applied to the design stage. The flux of moment of momentum is modified when the rotor speed is slow down. The circulation, the total pressure and the swirl free velocity are examined on a cross section located downstream to the rotor for all investigated speeds. As a result, the changes of the hydrodynamic flow field are quantified. The unsteady pressure measured on the cone wall at four levels is acquired. The Fourier spectra associated with different self-induced instabilities developed in the cone are examined revealing the distribution of both plunging and rotating components. A low frequency plunging component in a straight diffuser is determined on a speed range of the rotor.

Influence of the elbow shape on the unsteady pressure field in decelerated swirling flows

The induced flow instabilities are developed in the draft tube cone of the hydraulic turbines leading to the pressure fluctuations with negative impact on the mechanical components lifetime. Usually, two components (rotating and plunging) are discriminated in the unsteady pressure field induced by the swirling flow in the draft tube cone. The rotating component is associated with the precession motion of the flow instabilities (e.g. vortex rope) while the plunging part corresponds to the interaction of the swirling flow with the elbow. Extensive experimental investigations were carried out on Timisoara Swirl Generator test case in order to examine the influence of the elbow shape on the unsteady pressure field. Two elbow shapes (90° S shape elbow and 90° sharp heel) are selected to investigate their influence on the unsteady pressure field. Several geometrical configurations with pipe extensions placed between the cone and the elbow are considered for each elbow shape to determine the response of the hydraulic circuit. Two unsteady pressure signals located in opposite positions are measured on four levels placed along the cone. The Fourier spectra are determined for discriminated time series identifying both rotating and plunging components. Then, the ratio between the frequencies of the plunging and the rotating components is obtained for both elbow shapes in relation to the pipe length placed between the cone and the elbow. It is clear that the shape of the elbow and the distance between the cone and the elbow lead to a significant influence on the response of the hydraulic circuit.