Dissertationen zum Thema „Cavitation nozzle“

Master of Science
Department of Mechanical and Nuclear Engineering
B. Terry Beck
Mohammad H. Hosni
Cavitation is the formation of vapor phase from the liquid phase by reduction in its absolute pressure below the saturation pressure. Unlike boiling, where the temperature of the liquid is increased to cause vaporization, the reduction in the pressure alone can cause the liquid to turn into vapor. Cavitation is undesirable in many engineering applications as it is associated with reduction in efficiency and is known to cause damage to pump and propeller components. However, the endothermic nature of cavitation could be utilized to create a region of low temperature that could be utilized to develop a new refrigeration cycle. The work presented in this thesis is part of ongoing research into the potential cooling capacity of cavitation phenomena, where the cavitation in a converging-diverging nozzle is being investigated. Due to the constricting nature of the throat of the converging-diverging nozzle, the liquid velocity at the throat is increased, obeying the continuity law. With an increase in velocity, a reduction in absolute pressure is accompanied at the throat of the nozzle according to the Bernoulli’s principle. The local absolute pressure at the throat can go lower than the saturation vapor pressure, thereby causing the fluid to cavitate. The effect of water temperature on the flowrates, the onset of cavitation within the nozzle, and the resulting length of the cavitation region within the nozzle are the subject of this thesis. Experimental results and analysis are presented which also show that near the onset of cavitation, the flowrate can go beyond the choked flowrate, causing the local pressure in the throat to go well below zero for an extended amount of time in the metastable state, before nucleating (cavitating) into a stable state. Flow visualization using a high speed digital camera under different operating conditions was aimed at investigating the region of cavitation onset, which appears to be associated with boundary layer separation just downstream of the nozzle throat. In order to delay the boundary layer separation point in the downstream section of the nozzle, the diffuser region of the nozzle was modified to enable two flow paths, where one path would suck the flow near the inner walls of the nozzle and the other would allow the bulk of the flow to pass through. This was achieved with the use of inserts. Various inserts were tested in an attempt to capture the effect of inserts on the cavitation phenomena. Their effect on the flowrates, length of two phase region, and cavitation onset are presented in this thesis.

This thesis deals with the issue of cavitation and its effects. In this context, it describes the mechanism of origin and implosion of cavities and cavitation regimes. It lists various types of hydrodynamic cavitation. It presents the Rayleight-Plesset equation and describes micro jet. It also highlights cavitation erosion and the effects of cavitation on some types of materials. It deals with three types of cavitation resistance testing, namely cavitation tunnels, a vibrating cavitation system, supported by the ASTM G32 standard, and last but not least, cavitation nozzles, which follow the ASTM G134-17 standard. In correlation with cavitation nozzles, it frames its four basic parameters, which are stand of distance, the cavitation number, the speed of sound and the geometry of the nozzle. At the end of the theoretical part it characterizes the construction of test bench. The practical part is focused on performing the experiment. It first presents the procedure for carrying out the experiment and then evaluates this experiment. Part of the evaluation is the visual observation of selected samples of AlCu4Mg1Mn1 material and the monitoring of cavitation erosion on specific samples. First, these data are processed in the form of graphs and tables. It uses a microscope as a tool for detailed observation of samples. The conclusion of the practical part is devoted to the evaluation of the experiment.

Master of Science
Department of Mechanical and Nuclear Engineering
B. Terry Beck
Mohammad H. Hosni
Cavitation is the change of a liquid to a two-phase mixture of liquid and vapor, similar to boiling. However, boiling generates a vapor by increasing the liquid temperature while cavitation generates vapor through a decrease in pressure. Both processes are endothermic, removing heat from the surroundings. Both the phase change and heat absorption associated with cavitation provide many engineering applications, including contributing to a new type of refrigeration cycle under development. Cavitation can occur at or below the vapor pressure; conditions that delay cavitation and allow for a metastable liquid are not well understood. A converging-diverging nozzle was designed and fabricated to create a low pressure region at the nozzle throat. The converging section of the nozzle increased the water velocity and decreased the pressure, according to Bernoulli’s principle. A cavitation front was formed slightly past the nozzle throat. The cavitation location suggested that the water was metastable near the nozzle throat. Flow through the system was controlled by changing the nozzle inlet and outlet pressures. The flowrate of water was measured while the outlet pressure was lowered. The flowrate increased as the outlet pressure dropped until cavitation occurred. Once cavitation initiated, the flow became choked and remained constant and independent of the nozzle outlet pressure. High-speed imagery was used to visualize the flow throughout the nozzle and the formation and collapse of cavitation in the nozzle’s diverging section. High-speed video taken from 1,000 to 35,000 frames per second captured the formation of the cavitation front and revealed regions of recirculating flow near the nozzle wall in the diverging section. Particle Image Velocimetry (PIV) was used to measure the velocity vector field throughout the nozzle to characterize flow patterns within the nozzle. PIV showed that the velocity profile in the converging section and throat region were nearly uniform at each axial position in the nozzle. In the diverging section, PIV showed a transient, high-velocity central jet surrounded by large areas of recirculation and eddy formation. The single-phase experimental results, prior to cavitation onset, were supplemented by Computational Fluid Dynamics (CFD) simulations of the velocity distribution using Fluent software.

Cavitation is a phenomenon that occurs in liquids when the pressure drops below the vapor pressure of the liquid. Previous research has verified that cavitation bubble collapse is a dynamic and destructive process. An understanding of the behavior of cavitation is necessary to implement this destructive mechanism from an axisymmetric jet for underwater material removal. This work investigates the influence of jet pressure and nozzle diameter on the behavior of a cloud of cavitation bubbles generated by a submerged high-pressure water jet. First, this investigation is put into context with a condensed historical background of cavitation research. Second, a description of the cavitation-generating apparatus is given. Next, the experimental methods used to explore the behavior of the cavitation clouds are explained. Finally, the results of the investigation, including propagation distance, cloud width and area, pulsation frequency, and cloud front velocity are presented. Among the results is a discussion of the significant experimental factors affecting the behavior of the cavitation clouds. It is shown that the Reynolds number, specifically the diameter of the nozzle, has a significant effect on the measurements. In some cases the jet pressure, and subsequent jet velocity, had a less significant effect than was expected. Overall, this research describes the cavitation cloud formed when a submerged high-speed water jet discharges.

In Diesel engines, the internal flow characteristics in the fuel injection nozzles, such as the turbulence level and distribution, the cavitation pattern and the velocity profile affect significantly the air-fuel mixture in the spray and subsequently the combustion process. Since the possibility to observe experimentally and measure the flow inside real size Diesel injectors is very limited, Computational Fluid Dynamics (CFD) calculations are generally used to obtain the relevant information. The work presented within this thesis is focused on the study of cavitation in real size automotive injectors by using a commercial CFD code. It is divided in three major phases, each corresponding to a different complementary objective. The first objective of the current work is to assess the ability of the cavitation model included in the CFD code to predict cavitating flow conditions. For this, the model is validated for an injector-like study case defined in the literature, and for which experimental data is available in different operating conditions, before and after the start of cavitation. Preliminary studies are performed to analyze the effects on the solution obtained of various numerical parameters of the cavitation model itself and of the solver, and to determine the adequate setup of the model. It may be concluded that overall the cavitation model is able to predict the onset and development of cavitation accurately. Indeed, there is satisfactory agreement between the experimental data of injection rate and choked flow conditions and the corresponding numerical solution.This study serves as the basis for the physical and numerical understanding of the problem. Next, using the model configuration obtained from the previous study, unsteady flow calculations are performed for real-size single and multi-hole sac type Diesel injectors, each one with two types of nozzles, tapered and cylindrical. The objective is to validate the model with real automotive cases and to ununderstand in what way some physical factors, such as geometry, operating conditions and needle position affect the inception of cavitation and its development in the nozzle holes. These calculations are made at full needle lift and for various values of injection pressure and back-pressure. The results obtained for injection rate, momentum flux and effective injection velocity at the exit of the nozzles are compared with available CMT-Motores Térmicos in-house experimental data. Also, the cavitation pattern inside the nozzle and its effect on the internal nozzle flow is analyzed. The model predicts with reasonable accuracy the effects of geometry and operating conditions.
Patouna, S. (2012). A CFD STUDY OF CAVITATION IN REAL SIZE DIESEL INJECTORS [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/14723
Palancia

Efforts to improve diesel fuel sprays have led to a significant increase in fuel injection pressures and a reduction in nozzle-hole diameters. Under these conditions, the likelihood for the internal nozzle flow to cavitate is increased, which potentially affects spray breakup and atomisation, but also increases the risk of causing cavitation damage to the injector. This thesis describes the study of cavitating flow phenomena in various single and multi-hole optical nozzle geometries. It includes the design and development of a high-pressure optical fuel injector test facility with which the cavitating flows were observed. Experiments were undertaken using real-scale optical diesel injector nozzles at fuel injection pressures up to 2050 bar, observing for the first time the characteristics of the internal nozzle-flow under realistic fuel injection conditions. High-speed video and high resolution photography, using laser illumination sources, were used to capture the cavitating flow in the nozzle-holes and sac volume of the optical nozzles, which contained holes ranging in size from 110 micrometers to 300 micrometers. Geometric cavitation in the nozzle-holes and string cavitation formation in the nozzle-holes and sac volume were both observed using transient and steady-state injection conditions; injecting into gaseous and liquid back pressures up to 150 bar. Results obtained have shown that cavitation strings observed at realistic fuel injection pressures exhibit the same physical characteristics as those observed at lower pressures. The formation of string cavitation was observed in the 300 micrometers multi-hole nozzle geometries, exhibiting a mutual dependence on nozzle flow-rate and the geometry of the nozzle-holes. Pressure changes, caused by localised turbulent perturbations in the sac volume and transient fuel injection characteristics, independently affected the geometric and string cavitation formation in each of the holes. String cavitation formation of was shown to occur when free-stream vapour was entrained into the low pressure core of a sufficiently intense coherent vortex. Hole diameters less than or equal to 160 micrometers were found to suppress string cavitation formation, with this effect a result of the reduced nozzle flow rate and vortex intensity. Using different hole spacing geometries, it was demonstrated that the formation of cavitation strings in a particular geometry became independent of fuel injection and back pressure once a threshold pressure drop across the nozzle had been reached.

Master of Science
Department of Mechanical and Nuclear Engineering
Steven Eckels
Both cavitating and flashing flows are important phenomena in fluid flow. Cavitating flow, a common consideration in valves, orifices, and metering devices, is also a concern in loss of coolant accidents for liquid water in power plants when saturation pressures are below atmospheric pressure. Flashing flow is a common consideration for devices such as relief and expansion valves and fluid injectors as well as for loss of coolant accidents in which the coolant’s saturation pressure is above atmospheric. Of the two phenomena, flashing flow has received greater interest due to its applicability to safety concerns, though cavitating flow is perhaps of greater interest in terms of energy efficiency. It is possible for cavitating and flashing flow to actually become sonic. That is, the local velocity of a fluid can exceed the local speed of sound due to the unique properties of two-phase mixtures. When a flow becomes sonic, it is possible for the flow to accelerate and impose additional energy losses that would not otherwise occur. Models of this aspect of two-phase flow are not well developed, typically only being presented for the case of constant area ducts. In this paper two models for cavitating sonic flow are developed and described by applying the integral forms of the mass, momentum, and energy equations to a control volume of variable cross-sectional area. These models, based on the homogeneous equilibrium model (HEM) and separated flow model, are then applied to experimental data taken by the author with R-134a as the fluid of interest. Experimental data were taken with four instrumented converging-diverging nozzles of various geometries using a custom testing rig that allowed for precise control and measurement of flow parameters such as mass flow, temperature, and pressure. The resultant data from the models are then examined, focusing on the resultant velocities, Mach numbers, quality, and shear stresses.

The master´s thesis deals with the flow induced by rotation of cavitating fluid in converging-diverging nozzle, which simulates the vortex rope in impeller of water turbines. Measurement is performed on an experimental circuit in laboratory. Results from experimental measurements are compared with CFD simulation of single and two-phase flow. The main focus is to compare the difference of hydraulic losses and shapes of cavitating structures identified in the experiment and in the simulation.

Master of Science
Department of Mechanical and Nuclear Engineering
Mohammad H. Hosni
A traditional cooling/refrigeration cycle has four main system components which are an evaporator, a compressor, a condenser, and an expansion valve. This type of cycle requires use of refrigerants which have been found to be harmful to the environment, including causing damage to the atmospheric ozone layer. The main objective of the project was to investigate a water-based non-vapor compression cooling system. Water as a working fluid has the advantages of being inexpensive and environmentally safe for use, as compared to commercially available chemical refrigerants. The water-based cooling system investigated employed cavitation phenomena in converging-diverging glass nozzles. Cavitation is an important phenomenon in fluids, and is common occurring in many devices such as pumps, refrigeration expansion valves, and capillary tubes. It occurs when the static pressure of the fluid falls below the vapor pressure, into a metastable liquid state. Cavitation can be in the form of traveling bubble cavitation, vortex cavitation, cloud cavitation, or attached wall cavitation. In this thesis, the focus was first on visualizing cavitation for water flowing through converging- diverging glass nozzles. These nozzles had throat diameters between 2 mm and 4 mm. Two systems were used: (1) a continuous flow system, where water was driven by a centrifugal pump, and (2) a transient blow down system, where water flow was initiated using a suction pump. A high-speed camera was used to record videos and images of the associated cavitation phenomena. A thermal infrared camera was used in an attempt to measure temperature drop in the nozzle while the system was running The second part of this thesis focused on the understanding of the fundamental thermodynamics phenomena and on the development of practical knowledge relevant to the cavitation process. Two equations of state were used in the analysis, the van der walls equation of state, and the Peng Robinson equation of state. Equations of state were used to predict the transition from vapor to liquid. At a given temperature, the equations were solved for a pressure value corresponding to saturated liquid and saturated vapor specific volume values. Then, the equations were used to determine the spinodal liquid and vapor lines, which represent the metastabillity limits for the liquid and vapor. The characteristic equations of state, combined with implementation of the Law of Corresponding States and thermodynamic theory, were used to estimate the temperature reduction available for refrigeration.

The main goal of this thesis is the examination of hydraulic characteristics for different cavity nozzles, influence of liquid rotation and visualization of cavitating flow. Thesis is divided into two parts, theoretical and practical. Theoretical part deals with the description of cavity - creation, development and its extinction. This part also contains description of vortex flow and basic vertex models. Practical part compares nozzles performance experimentally. The aim of experiment was to measure hydraulic characteristics and their comparison. Jet performance was judged using visualizations and measured data was processed in Microsoft Excel and Parametr.

Šiame darbe tiriama švarių paviršių paruošimo sūkuriniu pulsuojančiu srautu galimybė. Darbo tikslas: išsiaiškinti švarių paviršių paruošimo galimybę, naudojant vandens srautą, kuriame turbulencijos lėtėjimo dėka sukeliama kavitacija. Darbas sudarytas iš keturių dalių. Pradžioje apibūdinami švarūs paviršiai, jų klasifikacija. Toliau apžvelgiami švarių paviršių paruošimo metodai, problemos. Po to, skaitmeninės simuliacijos būdu, tiriamos sūkurinį pulsuojantį srautą generuojančių purkštukų konstrukcijos. Eksperimentinėje dalyje pateikiami bandymo rezultatai, kuomet iš aliumininės plokštelės šalinamos abrazyvo liekanos, įstrigusios paviršiuje šlifavimo metu. Darbo pabaigoje pateikiamos išvados. Darbo apimtis – 51 psl. teksto be priedų, 36 iliustracijos, 3 lentelės, 13 bibliografinių šaltinių. Atskirai pridedami darbo priedai.
This study investigates the preparation of clean surfaces with pulsating vortex flow option. The aim of work: to determine the possibility of clean surface preparation using a water flow with generated cavitation. The work consists of four parts. At the start of the work characterized clean surfaces and their classification. The following provides an overview of clean surface preparation methods, problems. After that, the digital simulation method investigated vortex-generating jets pulsating flow structures. In the experimental part are presented the test results, when the aluminum plate is disposed abrasive residues trapped on the surface of the grinding time. At the end of the work there are given conclusions. Work size - 51 text pages without appendixes, 36 figures, 3 tables, 13 bibliographical sources.

Vegetable oil methyl esters obtained by transesterification of vegetable oils are considered to be suitable alternative fuels for diesel engines. However, higher viscosity, surface tension and boiling temperatures of biodiesels may adversely affect spray characteristics as compared to those of diesel. Thus, spray characteristics of Jatropha Methyl Ester (JME) are studied by comparing them to those of diesel in a high-pressure chamber with optical access to simulate the actual in-cylinder conditions. Also, the effect of inner-nozzle cavitation on JME and diesel sprays is studied by utilizing two nozzles, one with sharp entry-radius and the other with larger entry-radius. Finally, spray characteristics of surrogate fuels such as n-dodecane and n-hexadecane are also studied. The first part of the work concerning precise measurements of inner-nozzle geometry revealed that one of the nozzles has a hole diameter of 190-µm and entry-radius of around 70-µm, while the other has a hole diameter of 208-µm and entry-radius of around 10-µm. Injection rate-shape and coefficient of discharge for JME and diesel flow through the two nozzles were then measured. It was observed that while the coefficients of discharge (Cd) are almost identical for JME and diesel, the nozzle with entry radius of 10-µm exhibited around 20% lower Cd than that of the entry-radius of 70-µm. This observation coupled with insight from complementary CFD simulations of inner-nozzle flow showed that the lower Cd of the nozzle with entry-radius of 10-µm could be attributed to inner-nozzle cavitation. The second part of the work involved measurement of non-evaporating spray characteristics including spray-tip penetration, spray-cone angle and droplet size measurement under realistic operating conditions using techniques such as Shadowgraphy and Particle/Droplet Imaging Analysis (PDIA). The non-evaporating spray of the fuels are studied by injecting them using a common-rail fuel injection system into the high-pressure chamber maintained at room temperature. Experimental results show that JME is associated with a slightly faster spray-tip penetration and narrow spray-cone angle indicating inferior spray atomization which is confirmed by around 5% larger droplet sizes. Slower spray-tip penetration, wider spray-cone angle and around 5% smaller droplet sizes are observed for the spray from the cavitating nozzle. Thus, the inner nozzle cavitation is observed to improve the atomization of diesel and JME sprays. The differences in spray characteristics of JME and diesel reduce as the injection pressure increases. The spray-tip penetrations of both surrogates are observed to almost match that of diesel. The third part of the work involved measurements of evaporating spray liquid length, vapour penetration and spread angle for JME, diesel and surrogates at conditions of 50 bar chamber pressure and 900 K temperature. It is observed that the JME exhibits around 16% longer liquid length than that of diesel. The liquid length of n-dodecane is significantly lower than that of diesel and liquid length of n-hexadecane is around 20% higher than that of n-dodecane mimicking the trend of JME and diesel. The liquid length of n-hexadecane is very close to that of diesel at all the three test conditions. Interestingly, the vapour penetration and spread angle for all the fuels is observed to be almost identical. As the cold spray and evaporating spray characteristics of n-hexadecane match well with those of diesel, n-hexadecane can be chosen as a pure component surrogate for diesel. Finally, an analytical model for predicting the spray vapour penetration is assessed with the experimentally-observed trends of penetration and spray spread angle. The model indicated that the effect of fuel density variation is compensated by the corresponding variation in injection velocity for a given injection pressure to result in a similar vapour penetration. Overall, the present work, in addition to studying the effect of fuel physical properties and cavitation on sprays, has generated a comprehensive experimental database on non-evaporating and evaporating sprays of biodiesel, diesel, and pure component surrogates, which would aid significantly in validation of CFD simulations.

Thesis (Ph. D.)--University of Notre Dame, 2008.
Thesis directed by Patrick F. Dunn for the Department of Aerospace and Mechanical Engineering. "May 2008." Includes bibliographical references (leaves 88-89).

Two problems are considered in this thesis: the nonlinear dynamics of a cloud of cavitation bubbles, and bubbly cavitating flows in a converging-diverging nozzle. The focus of the first problem is to explore the characteristics of the growth and collapse of a spherical cloud of bubbles. The prototypical problem solved considers a finite cloud of nuclei that is exposed to a decrease in the ambient pressure which causes the cloud to cavitate. A subsequent pressure recovery then causes the cloud to collapse. This is typical of the transient behaviour exhibited by a bubble cloud as it passes a body or the blade of a ship propeller. The simulations employ the fully nonlinear, non-barotropic, homogeneous two-phase flow equations coupled with the Rayleigh-Plesset equation for the dynamics of individual bubbles. A Lagrangian integral method is developed to solve this set of equations. The computational results confirm the idea put forward by Morch and his co-workers (Morch [1980], [1981], [1982]; Hanson et al. [1981]) who speculated that the collapse of the cloud involved the formation of a shock wave on the surface of the cloud and that inward propagation and geometric focusing of this shock would lead to very large localized pressure pulses. The effects of varying the bubble population density, the cavitation number, and the ratio of the cloud size to the bubble size are examined. The theoretical results are shown to provide a satisfactory explanation for dynamic structures and acoustic signature observed in recently conducted experiments of cloud cavitation at California Institute of Technology (Reisman and Brennen [1996]; Brennen et al. [1996]). It is concluded that the formation and focusing of bubbly shock waves are responsible for the severe noise and damage potential in cloud cavitation. The second problem investigates the nonlinear behavior of a bubbly cavitating flow, both steady and unsteady, through a converging-diverging nozzle. Two different flow regimes are found from steady state solutions: quasi-steady and quasi-unsteady. The former is characterized by the large spatial fluctuations in the downstream of the flow. Bifurcation occurs as the flow transitions from one regime to the other. An analytical expression for the critical bubble size at bifurcation is obtained. Finally, unsteady solutions in a period of consecutive times are presented. These solutions are characterized by the downstream spatial fluctuations coupled with large pressure pulses changing in both magnitude and location with time. The characteristics of these pulses are similar to the shock pulses of Part I and are produced by the local violent collapse of the bubbles in the flow.

Cavitation is the rapid growth and subsequent collapse of air bubbles in a liquid. This phenomenon occurs in many fluid systems such as marine propulsors, artificial heart valves and in the nozzles of fuel injectors. Cavitation is often seen as a nuisance as it causes unnecessary damage to these structures as the collapse of bubbles in the system can cause surface erosion and damage. In fuel injectors however, some cavitation is necessary to improve the atomization of the spray leading into the combustion chamber. It is evident that modeling cavitation for fast and accurate computation of this phenomenon is vital in the design of fluid systems in which it occurs. Preferred computational models are Eulerian in nature and utilize the Volume of Fluid (VOF) method to develop a class of models known as Homogeneous Mixture Methods (HMM). These models define source terms which govern the "mass transfer” between liquid and vaporous regions within the domain. Current cavitation models base their source terms on approximated bubble dynamics theory often neglecting salient attributes such as nuclei, viscosity and surface tension. Moreover, these models are often beset with several ad-hoc parameters which tend to be heuristically defined in their use. The focus of the work herein is to assess the current state of homogeneous mixture method cavitation models and provide improvements to include previously absent physical characteristics. The current work uses the open-source CFD tool box OpenFOAM due to its availability for solver customization, development and extension. An assessment of the trends associated with current cavitation models is conducted to understand the present deficiencies in cavitation modeling. Next a method for including a heterogeneous nuclei distribution is described and results from its implementation presented. The method can be used to extend existing cavitation models which currently only allow for a homogeneous distribution of nuclei whereas in practice nuclei range considerably in both size and concentration. The current work outlines modifications to the algorithm of the solver to allow for heterogeneous nuclei distribution in the cavitation model. In order to more faithfully adhere to the bubble dynamics which govern cavitation, the Kinzel cavitation model, which includes inertial, thermodynamic and surface tension effects is implemented into the OpenFOAM framework. The results show an improved inception criteria for cavitation which is dependent on nuclei size and concentration. A crucial relationship between the critical pressures needed for cavitation and the size of nuclei within the cavitating liquid, not seen in previous cavitation models, is demonstrated. Finally, efforts related to simulation of various bubble dynamics using the volume of fluid method are presented. Their potential use in further cavitation model development is discussed.
2020-09-28T00:00:00Z

Cavitation occurs when the liquid pressure drops below a critical threshold causing rapid bubble growth and violent collapse. The presence of cavitation inside fuel injector nozzles has been linked not only to damage associated with cavity collapse near the walls but has been found to enhanced fuel spray atomization. Proper fuel atomization increases engine performance while reducing fuel emissions. The majority of laboratory experimental studies found in the literature are highly reliant on optical access to the working fuel, requiring transparent material, specific geometry, and relatively slow flows to enable even minimally time-resolved optical images with ex-pensive high-speed cameras. While such studies are appropriate for gaining intuition into the types and spatial locations of cavitation phenomena possible in nozzle flows at high Re, there is a need for techniques which can not only be reliably employed at idealized laboratory conditions, but also can be deployed to study real steel fuel injectors while also yielding quantitative information. The objective of this work is to develop alternative non-intrusive acoustic and vibration methods to experimentally study cavitation phenomena in fuel injectors. First, a study was conducted utilizing a combination of optical and acoustic techniques to determine onset and activity of cavitation in small scaled nozzles. Experiments are conducted with acrylic nozzles of various geometry. Unfocused single element transducers are used for acoustic sensing, while digital imaging is used for optical study. Cavitation onset thresholds and development are studied as functions of flow rate and nozzle geometry. Substantial agreement between optical and acoustic methods was observed for both onset and development regimes of cavitation in nozzles. A second study was conducted utilizing laser Doppler vibrometry to measure the vibration response of a commercial fuel injector. An attempt was made to use injector flexural oscillations to determine the void fraction for different fuel injector conditions. Experiments were performed at the Oak Ridge National Laboratory on a commercial grade field injector using cyclopentane fuel with varying injection pressure and fuel temperature as control parameters. Frequency shift and mode shape are measured and correlated with cavitation-inducing control parameters. Analysis suggests that observed frequency shifts may allow inference of dynamic void fraction during cavitation in nozzles.
2020-07-02T00:00:00Z

Two problems are considered in this thesis: the modeling of heat and mass diffusion effects on the dynamics of spherical bubbles, and the computation of unsteady, bubbly cavitating flows in nozzles. The goal of Part I is to develop a reduced-order model that is able to accurately and efficiently capture the effect of heat and mass transfer on the dynamics of bubbles. Detailed computations of forced and oscillating bubbles including heat and mass diffusion show that the assumptions of polytropic behavior, constant vapor pressure, and an effective liquid viscosity do not accurately account for diffusive damping and thus do not accurately capture bubble dynamics. While the full bubble computations are readily performed for single bubbles, they are too expensive to implement into continuum models of complex bubbly flows where the radial diffusion equations would have to be solved at each grid point. Therefore reduced-order models that accurately capture diffusive effects are needed.

We first develop a full bubble computation, where the full set of radial conservation equations are solved in the bubble interior and surrounding liquid. This provides insight as to which equations, or terms in equations, may be able to be neglected while still accurately capturing the bubble dynamics. Motivated by results of the full computations, we use constant heat and mass transfer coefficients to model the transfer at the bubble wall. In the resulting reduced-order model the heat and mass diffusion equations are each replaced by a single ordinary differential equation. The model is therefore efficient enough to implement into continuum computations. Comparisons of the reduced-order model to the full computations over a wide range of parameters indicate agreement that is superior to existing models.

In Part II we investigate the effects of unsteady bubble dynamics on cavitating flow through a converging-diverging nozzle. A continuum model that couples the Rayleigh-Plesset equation with the continuity and momentum equations is used to formulate unsteady, quasi-one-dimensional partial differential equations. Flow regimes studied include those where steady state solutions exist, and those where steady state solutions diverge at the so-called flashing instability. These latter flows consist of unsteady bubbly shock waves traveling downstream in the diverging section of the nozzle. An approximate analytical expression is developed to predict the critical back pressure for choked flow. The results agree with previous barotropic models for those flows where bubble dynamics are not important, but show that in many instances the neglect of bubble dynamics cannot be justified. Finally the computations show reasonable agreement with an experiment that measures the spatial variation of pressure, velocity and void fraction for steady shock free flows, and good agreement with an experiment that measures the throat pressure and shock position for flows with bubbly shocks. In the model, damping of the bubble radial motion is restricted to a simple "effective" viscosity to account for diffusive effects. However, many features of the nozzle flow are shown to be independent of the specific damping mechanism. This is confirmed by the implementation of the more sophisticated diffusive modeling developed in Part I.