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High-flux synchrotron radiation has been employed in a time-resolved manner to characterize the distinct topology features and dynamics of different cavitation regimes arising in a throttle orifice with an abrupt flow-entry contraction. Radiographs obtained though both x-ray phase-contrast and absorption imaging have been captured at 67 890 frames per second. The flow lies in the turbulent regime (Re = 35 500), while moderate (CN = 2.0) to well-established (CN = 6.0) cavitation conditions were examined encompassing the cloud and vortical cavitation regimes with pertinent transient features, such as cloud-cavity shedding. X-ray phase-contrast imaging, exploiting the shift in the x-ray wave phase during interactions with matter, offers sharp-refractive index gradients in the interface region. Hence, it is suitable for capturing fine morphological fluctuations of transient cavitation structures. Nevertheless, the technique cannot provide information on the quantity of vapor within the orifice. Such data have been obtained utilizing absorption imaging, where beam attenuation is not associated with scattering and refraction events, and hence can be explicitly correlated with the projected vapor thickness in line-of-sight measurements. A combination of the two methods is proposed as it has been found that it is capable of quantifying the vapor content arising in the complex nozzle flow while also faithfully illustrating the dynamics of the highly transient cavitation features.
I. K. Karathanassis; M. Heidari-Koochi; Q. Zhang; J. Hwang; P. Koukouvinis; M. Gavaises. X-ray phase contrast and absorption imaging for the quantification of transient cavitation in high-speed nozzle flows. Physics of Fluids 2021, 33, 032102 .
AMA StyleI. K. Karathanassis, M. Heidari-Koochi, Q. Zhang, J. Hwang, P. Koukouvinis, M. Gavaises. X-ray phase contrast and absorption imaging for the quantification of transient cavitation in high-speed nozzle flows. Physics of Fluids. 2021; 33 (3):032102.
Chicago/Turabian StyleI. K. Karathanassis; M. Heidari-Koochi; Q. Zhang; J. Hwang; P. Koukouvinis; M. Gavaises. 2021. "X-ray phase contrast and absorption imaging for the quantification of transient cavitation in high-speed nozzle flows." Physics of Fluids 33, no. 3: 032102.
A high-speed flow visualisation set-up comprising of combined diffuse backlight illumination (DBI) and schlieren imaging has been developed to illustrate the highly transient, two-phase flow arising in a real-size optical fuel injector. The different illumination nature of the two techniques, diffuse and parallel light respectively, allows for the capturing of refractive-index gradients due to the presence of both interfaces and density gradients within the orifice. Hence, the onset of cavitation and secondary-flow motion within the sac and injector hole can be concurrently visualised. Experiments were conducted utilising a diesel injector fitted with a single-hole transparent tip (ECN spray D) at injection pressures of 700–900 bar and ambient pressures in the range of 1–20 bar. High-speed DBI images obtained at 100,000 fps revealed that the orifice, due to its tapered layout, is mildly cavitating with relatively constant cavity sheets arising mainly in regions of manufacturing imperfections. Nevertheless, schlieren images obtained at the same frame rate demonstrated that a multitude of vortices with short lifetimes arise at different scales in the sac and nozzle regions during the entire duration of the injection cycle but the vortices do not necessarily result in phase change. The magnitude and exact location of coherent vortical structures have a measurable influence on the dynamics of the spray emerging downstream the injector outlet, leading to distinct differences in the variation of its cone angle depending on the injection and ambient pressures examined. Graphic abstract
I. K. Karathanassis; J. Hwang; P. Koukouvinis; L. Pickett; M. Gavaises. Combined visualisation of cavitation and vortical structures in a real-size optical diesel injector. Experiments in Fluids 2020, 62, 1 -18.
AMA StyleI. K. Karathanassis, J. Hwang, P. Koukouvinis, L. Pickett, M. Gavaises. Combined visualisation of cavitation and vortical structures in a real-size optical diesel injector. Experiments in Fluids. 2020; 62 (1):1-18.
Chicago/Turabian StyleI. K. Karathanassis; J. Hwang; P. Koukouvinis; L. Pickett; M. Gavaises. 2020. "Combined visualisation of cavitation and vortical structures in a real-size optical diesel injector." Experiments in Fluids 62, no. 1: 1-18.
Cavitating flow dynamics are investigated in an axisymmetric converging–diverging Venturi nozzle. Computational Fluid Dynamics (CFD) results are compared with those from previous experiments. New analysis performed on the quantitative results from both datasets reveals a coherent trend and shows that the simulations and experiments agree well. The CFD results have confirmed the interpretation of the high-speed images of the Venturi flow, which indicated that there are two vapor shedding mechanisms that exist under different running conditions: re-entrant jet and condensation shock. Moreover, they provide further details of the flow mechanisms that cannot be extracted from the experiments. For the first time with this cavitating Venturi nozzle, the re-entrant jet shedding mechanism is reliably achieved in CFD simulations. The condensation shock shedding mechanism is also confirmed, and details of the process are presented. These CFD results compare well with the experimental shadowgraphs, space–time plots, and time-averaged reconstructed computed tomography slices of vapor fraction.
Maxwell Brunhart; Celia Soteriou; Manolis Gavaises; Ioannis Karathanassis; Phoevos Koukouvinis; Saad Jahangir; Christian Poelma. Investigation of cavitation and vapor shedding mechanisms in a Venturi nozzle. Physics of Fluids 2020, 32, 083306 .
AMA StyleMaxwell Brunhart, Celia Soteriou, Manolis Gavaises, Ioannis Karathanassis, Phoevos Koukouvinis, Saad Jahangir, Christian Poelma. Investigation of cavitation and vapor shedding mechanisms in a Venturi nozzle. Physics of Fluids. 2020; 32 (8):083306.
Chicago/Turabian StyleMaxwell Brunhart; Celia Soteriou; Manolis Gavaises; Ioannis Karathanassis; Phoevos Koukouvinis; Saad Jahangir; Christian Poelma. 2020. "Investigation of cavitation and vapor shedding mechanisms in a Venturi nozzle." Physics of Fluids 32, no. 8: 083306.
Viscous oils flowing in the geometrically-complex hydraulic circuits of earth-moving machines are associated with extensive friction losses, thus reducing the fuel efficiency of the vehicles and increasing emissions. The present investigation examines the performance effectiveness of different hydraulic oils, in terms of secondary-flow suppression and pressure-drop reduction. The flow of two non-Newtonian oil compounds, containing poly(alkylmethacrylate) (PMA) and poly(ethylene-co-propylene) (OCP) polymers, respectively, have been comparatively investigated against a base, monograde liquid through Particle Image Velocimetry. An 180° curved-tube layout and a check-valve replica have been selected as representative examples of the hydraulic components comprising the hydraulic circuit. The flow conditions prevailing in the experimental cases are characterized by Reynolds-number values in the range 76–1385. Precursor viscosity measurements with shear rate along with a theoretical analysis conducted using the FENE and PTT models have verified the influence of viscoelasticity and/or shear-thinning on the liquid flow behavior. PIV results have demonstrated that viscoelastic effects setting in due to the OCP additives tend to reduce the magnitude of the secondary flow pattern, commonly known as a Dean-vortex system, arising in the curved geometry by as much as 15% on average compared to the base liquid. A similar flow behavior was also demonstrated in the valve replica layout with reference to the geometry-induced coherent vortical motion in the constriction region, where a vorticity decrease up to 38% was observed for the OCP sample. On the contrary, the flow behavior of the primarily shear-thinning PMA oil was found to be comparable to that of the base oil, hence not presenting significant flow-enhancement characteristics.
I.K. Karathanassis; E. Pashkovski; Milad Heidari Koochi; Hesamaldin Jadidbonab; T. Smith; Manolis Gavaises; C. Bruecker. Non-Newtonian flow of highly-viscous oils in hydraulic components. Journal of Non-Newtonian Fluid Mechanics 2019, 275, 104221 .
AMA StyleI.K. Karathanassis, E. Pashkovski, Milad Heidari Koochi, Hesamaldin Jadidbonab, T. Smith, Manolis Gavaises, C. Bruecker. Non-Newtonian flow of highly-viscous oils in hydraulic components. Journal of Non-Newtonian Fluid Mechanics. 2019; 275 ():104221.
Chicago/Turabian StyleI.K. Karathanassis; E. Pashkovski; Milad Heidari Koochi; Hesamaldin Jadidbonab; T. Smith; Manolis Gavaises; C. Bruecker. 2019. "Non-Newtonian flow of highly-viscous oils in hydraulic components." Journal of Non-Newtonian Fluid Mechanics 275, no. : 104221.
Fuel Injection Equipment (FIE) are an integral component of modern Internal Combustion Engines (ICE), since they play a crucial role in the fuel atomization process and in the formation of a fuel/air combustible mixture, consequently affecting efficiency and pollutant formation. Advancements and improvements of FIE systems are determined by the complexity of the physical mechanisms taking place; the spatial scales are in the order of millimetres, flow may become locally highly supersonic, leading to very small temporal scales of microseconds or less. The operation of these devices is highly unsteady, involving moving geometries such as needle valves. Additionally, extreme pressure changes imply that many assumptions of traditional fluid mechanics, such as incompressibility, are no longer valid. Furthermore, the description of the fuel properties becomes an issue, since fuel databases are scarce or limited to pure components, whereas actual fuels are commonly hydrocarbon mixtures. Last but not least, complicated phenomena such as phase change or transition from subcritical to transcritical/supercritical state of matter further pose complications in the understanding of the operation of these devices.
Ioannis K. Karathanassis; Foivos (Phoevos) Koukouvinis; Manolis Gavaises. Multiphase Phenomena in Diesel Fuel Injection Systems. Energy, Environment, and Sustainability 2019, 95 -126.
AMA StyleIoannis K. Karathanassis, Foivos (Phoevos) Koukouvinis, Manolis Gavaises. Multiphase Phenomena in Diesel Fuel Injection Systems. Energy, Environment, and Sustainability. 2019; ():95-126.
Chicago/Turabian StyleIoannis K. Karathanassis; Foivos (Phoevos) Koukouvinis; Manolis Gavaises. 2019. "Multiphase Phenomena in Diesel Fuel Injection Systems." Energy, Environment, and Sustainability , no. : 95-126.
The development of a dynamic, theoretical model suitable for the prediction of the long-term performance of a parabolic-trough Concentrating Photovoltaic/Thermal CPVT system is discussed in the present study. The formulation of the mathematical model and the considered geometrical and operational parameters of the system, such as the characteristics of the employed PV modules and active cooling system are described in detail. The effect of heat capacity is taken into consideration in the thermal balances and thus the model is able to capture the transient behavior of the system. Besides, the model is validated using available experimental data of a manufactured prototype CPVT system. The daily performance of system is predicted for different values of the cooling fluid flow rate and temperature under various environmental conditions. At a second stage, an exergy analysis is conducted in order to point out the effect of the characteristics of the main system sub-components on the exergetic efficiency and exergy output of the CPVT system. It was established that the system exergetic performance is primarily influenced by the optical quality of the parabolic trough and the electrical efficiency of the PV module. Increasing these two factors to achievable values, e.g. ηopt = 0.75 and ηel = 0.25, can yield an increase of the system exergetic efficiency from 12% to 24%.
I.K. Karathanassis; E. Papanicolaou; V. Belessiotis; G.C. Bergeles. Dynamic simulation and exergetic optimization of a Concentrating Photovoltaic/ Thermal (CPVT) system. Renewable Energy 2018, 135, 1035 -1047.
AMA StyleI.K. Karathanassis, E. Papanicolaou, V. Belessiotis, G.C. Bergeles. Dynamic simulation and exergetic optimization of a Concentrating Photovoltaic/ Thermal (CPVT) system. Renewable Energy. 2018; 135 ():1035-1047.
Chicago/Turabian StyleI.K. Karathanassis; E. Papanicolaou; V. Belessiotis; G.C. Bergeles. 2018. "Dynamic simulation and exergetic optimization of a Concentrating Photovoltaic/ Thermal (CPVT) system." Renewable Energy 135, no. : 1035-1047.
The effect of viscoelastic additives on the topology and dynamics of the two-phase flow arising within an axisymmetric orifice with a flow path constriction along its main axis has been investigated employing high-flux synchrotron radiation. X-ray Phase Contrast Imaging (XPCI) has been conducted to visualise the cavitating flow of different types of diesel fuel within the orifice. An additised blend containing Quaternary Ammonium Salt (QAS) additives with a concentration of 500 ppm has been comparatively examined against a pure (base) diesel compound. A high-flux, 12 keV X-ray beam has been utilised to obtain time resolved radiographs depicting the vapour extent within the orifice from two views (side and top) with reference to its main axis. Different test cases have been examined for both fuel types and for a range of flow conditions characterised by Reynolds number of 35500 and cavitation numbers (CN) lying in the range 3.0–7.7. It has been established that the behaviour of viscoelastic micelles in the regions of shear flow is not consistent depending on the cavitation regimes encountered. Namely, viscoelastic effects enhance vortical (string) cavitation, whereas hinder cloud cavitation. Furthermore, the use of additised fuel has been demonstrated to suppress the level of turbulence within the orifice.
I. K. Karathanassis; K. Trickett; P. Koukouvinis; J. Wang; R. Barbour; Manolis Gavaises. Illustrating the effect of viscoelastic additives on cavitation and turbulence with X-ray imaging. Scientific Reports 2018, 8, 1 -15.
AMA StyleI. K. Karathanassis, K. Trickett, P. Koukouvinis, J. Wang, R. Barbour, Manolis Gavaises. Illustrating the effect of viscoelastic additives on cavitation and turbulence with X-ray imaging. Scientific Reports. 2018; 8 (1):1-15.
Chicago/Turabian StyleI. K. Karathanassis; K. Trickett; P. Koukouvinis; J. Wang; R. Barbour; Manolis Gavaises. 2018. "Illustrating the effect of viscoelastic additives on cavitation and turbulence with X-ray imaging." Scientific Reports 8, no. 1: 1-15.
High-speed X-ray phase-contrast imaging of the cavitating flow developing within an axisymmetric throttle orifice has been conducted using high-flux synchrotron radiation. A white X-ray beam with energy of 6 keV was utilized to visualize the highly turbulent flow at 67 890 frames per second with an exposure time of 347 ns. The working medium employed was commercial diesel fuel at flow conditions characterized by Reynolds and cavitation numbers in the range of 18 000–35 500 and 1.6–7.7, respectively. Appropriate post-processing of the obtained side-view radiographs enabled the detailed illustration of the interface topology of the arising vortical cavity. In addition, the visualization temporal and spatial resolution allowed the correlation of the prevailing flow conditions to the periodicity of cavitation onset and collapse, to the magnitude of the underlying vortical motion, as well as to the local turbulence intensity.
Ioannis K. Karathanassis; Phoevos Koukouvinis; Efstathios Kontolatis; Zhilong Lee; Jin Wang; Nicholas Mitroglou; Manolis Gavaises. High-speed visualization of vortical cavitation using synchrotron radiation. Journal of Fluid Mechanics 2018, 838, 148 -164.
AMA StyleIoannis K. Karathanassis, Phoevos Koukouvinis, Efstathios Kontolatis, Zhilong Lee, Jin Wang, Nicholas Mitroglou, Manolis Gavaises. High-speed visualization of vortical cavitation using synchrotron radiation. Journal of Fluid Mechanics. 2018; 838 ():148-164.
Chicago/Turabian StyleIoannis K. Karathanassis; Phoevos Koukouvinis; Efstathios Kontolatis; Zhilong Lee; Jin Wang; Nicholas Mitroglou; Manolis Gavaises. 2018. "High-speed visualization of vortical cavitation using synchrotron radiation." Journal of Fluid Mechanics 838, no. : 148-164.
I.K. Karathanassis; Phoevos Koukouvinis; Manolis Gavaises. Comparative evaluation of phase-change mechanisms for the prediction of flashing flows. International Journal of Multiphase Flow 2017, 95, 257 -270.
AMA StyleI.K. Karathanassis, Phoevos Koukouvinis, Manolis Gavaises. Comparative evaluation of phase-change mechanisms for the prediction of flashing flows. International Journal of Multiphase Flow. 2017; 95 ():257-270.
Chicago/Turabian StyleI.K. Karathanassis; Phoevos Koukouvinis; Manolis Gavaises. 2017. "Comparative evaluation of phase-change mechanisms for the prediction of flashing flows." International Journal of Multiphase Flow 95, no. : 257-270.
N. Mitroglou; V. Stamboliyski; I.K. Karathanassis; K.S. Nikas; Manolis Gavaises. Cloud cavitation vortex shedding inside an injector nozzle. Experimental Thermal and Fluid Science 2017, 84, 179 -189.
AMA StyleN. Mitroglou, V. Stamboliyski, I.K. Karathanassis, K.S. Nikas, Manolis Gavaises. Cloud cavitation vortex shedding inside an injector nozzle. Experimental Thermal and Fluid Science. 2017; 84 ():179-189.
Chicago/Turabian StyleN. Mitroglou; V. Stamboliyski; I.K. Karathanassis; K.S. Nikas; Manolis Gavaises. 2017. "Cloud cavitation vortex shedding inside an injector nozzle." Experimental Thermal and Fluid Science 84, no. : 179-189.
The operation of a high-pressure, piston-plunger fuel pump oriented for use in the common rail circuit of modern diesel engines for providing fuel to the injectors is investigated in this study from a numerical perspective. Both the suction and pressurization phases of the pump stroke were simulated with the overall flow time being in the order of 12 × 10−3 s. The topology of the cavitating flow within the pump configuration was captured through the use of an equation of state implemented in the framework of a barotropic, homogeneous equilibrium model. Cavitation was found to set in within the pressure chamber as early as 0.2 × 10−3 s in the operating cycle, while the minimum liquid volume fraction detected was in the order of 60% during the second period of the valve opening. Increase in the in-cylinder pressure during the final stages of the pumping stroke leads to the collapse of the previously arisen cavitation structures and three layout locations, namely, the piston edge, the valve and valve-seat region and the outlet orifice, were identified as vulnerable to cavitation-induced erosion through the use of cavitation aggressiveness indicators.
Phoevos Koukouvinis; Ioannis K Karathanassis; Manolis Gavaises. Prediction of cavitation and induced erosion inside a high-pressure fuel pump. International Journal of Engine Research 2017, 19, 360 -373.
AMA StylePhoevos Koukouvinis, Ioannis K Karathanassis, Manolis Gavaises. Prediction of cavitation and induced erosion inside a high-pressure fuel pump. International Journal of Engine Research. 2017; 19 (3):360-373.
Chicago/Turabian StylePhoevos Koukouvinis; Ioannis K Karathanassis; Manolis Gavaises. 2017. "Prediction of cavitation and induced erosion inside a high-pressure fuel pump." International Journal of Engine Research 19, no. 3: 360-373.