Study of a boiling bubble in uniform approaching flow at high bubble Reynolds numbers

The process of the bubble growth and detachment from a wall with uniform up°ow parallel to the wall has been studied in this thesis. Experiments have been designed and performed in such a way that the assumptions of an existing analytical model were met. The aim of this study was to validate the predictions and to quantify the hydrodynamic lift force on a boiling bubble. This kind of detailed modeling of detachment can in principle be combined with numerical modeling of °ow with heat transfer in evaporator tubes, e.g. ri°ed tubes, with the aid of commercially available packages. The ¯ndings can be used to predict boiling characteristics in complicated geometries, such as ri°ed tubes, for conditions that occur during di®erent operation regimes of conventionally ¯red power plants. In the corresponding EC-project, all major European electricity companies join e®orts to design a power plant operating under high-temperature steam conditions to achieve 5% increase of the net e±ciency. The model presented yields analytical expressions for the forces acting on the bub- ble. A fully closed solution of the added mass forces involved in motion and growth of bubbles footed on the wall was obtained. Validation and quanti¯cation experiments corresponding to this model are described in this thesis. Nucleate boiling experiments were performed using demineralized water at near- saturated °ow boiling conditions. Both the thermal boundary condition and the approaching °ow (inlet) condition have been prescribed. The bubble substrate tem- perature and approach velocity were both constant. A microscale heater and a Wheat- stone bridge were used to maintain a constant wall temperature around an arti¯cial cavity. This cavity was used as the nucleation site where bubble would appear on a vertical part of the wall. This bubble generator has been given a special shape to minimize the velocity boundary layer thickness at the location of the arti¯cial cavity. The bubble generator intrudes a pipe and positions the arti¯cial site at the center of the pipe. As a result, a (nearly) homogeneous liquid velocity pro¯le approaches the bubble at the boiling site. Extensive image processing analysis enabled the determination of bubble geometrical parameters and to make a 3D reconstruction of the bubble volume history. The main non-hydrodynamic force components in the direction perpendicular to the vertical wall were determined from the measured quantities. The sum of these forces should equal the hydrodynamic force that is independently assessed using model predictions and measured geometrical parameter histories. This comparison was made, and used to draw conclusions on how much the deformations in°uence comparison with the model predictions and how the model should be ex- tended. In addition to model validation, temperature and power measurements have been used in a heat transfer analysis. It is shown that a signi¯cant part (60-70%) of the heat needed to make a bubble of certain volume in °ow boiling experiments arises from the superheated liquid layer in front of the heating element in respect to the heat delivered through the micro layer beneath the bubble foot from the electrical heater. These ¯ndings are con¯rmed by comparing the heater area and e®ective area, and by investigating the area of the bubble in°uence. The main conclusion drawn is that the assumption of a truncated sphere shape of the bubble does not correspond to actual bubble shapes at early times of bubble growth. The bubble is °attened parallel to the wall in this stage of growth. Therefore the volume equivalent radius does not yield an accurate representation of the actual frontal area of the bubble. That is the reason why the prediction of the hydrodynamic lift force is not good for this case. In the second half of the growth time the bubble becomes elongated away from the wall. It is not growing any more, but it moves away from the wall and pushes the surrounding liquid. This results in the negative hydrodynamic force. Before the de- tachment the neck is formed and this phenomenon is not considered by the model. Deformations should be accounted for by introducing more than one geometrical pa- rameter to describe the shape. Other recommendations given in this thesis consider the improvement of the experimental set-up and using some other measuring tech- niques. A brief description of how the model can be combined with commercially available packages has been provided. Boiling is the most e±cient, yet least understood, phase change process. The work presented in this thesis leads to an increased understanding of the physical phe- nomenon of the bubble growing and detaching in °ow boiling. The obtained knowl- edge should be used to improve modeling of this process and to be applied in industry

The flux ratio of the [OIII] 5007,4959 lines in AGN: Comparison with theoretical calculations

By taking into account relativistic corrections to the magnetic dipole operator, the theoretical [OIII] 5006.843/4958.511 line intensity ratio of 2.98 is obtained. In order to check this new value using AGN spectra we present the measurements of the flux ratio of the [OIII] 4959,5007 emission lines for a sample of 62 AGN, obtained from the Sloan Digital Sky Survey (SDSS) Database and from published observations. We select only high signal-to-noise ratio spectra for which the line shapes of the [OIII] 4959,5007 lines are the same. We obtained an averaged flux ratio of 2.993 +/- 0.014, which is in a good agreement with the theoretical one.Comment: Accepted for publication in the MNRA

Influences of precipitation and temperature trend on maize yields

Maize yield for three decade period of the last century (1961-1990) were in Hungary for 15% higher than in Croatia (means 3.81 and 4.39 t/ha, respectively) and this trend was continued in the 1996-2007 period. However, amplitude of maize yields (differences among year (for the 1996-2007) in Hungary were higher (from 3.60 to 7.56 t/ha) than in Croatia (from 3.86 to 6.92 t/ha). Aim of this study was testing maize yield, precipitation and air-temperature variations in four Counties (Croatia: Vukovarsko-Srijemska =VSC and Virovitiţko-Podravska =VPC; Hungary: Békés = BC and Fejér =FC). Mean yield in VSC for 1996-2007 period was for 16% higher than in VPC. Yields in three less favorable years (LFY: 2000, 2003 and 2007) were considerably lower (means 5.22 and 4.41 t/ha, for VSC and VPC, respectively) than in three more favorable years (MFY: 1997, 2002 and 2005) years (means 7.50 and 7.00, respectively). Precipitation (means of two sites: Osijek and Virovitica) in 3-months period June-August was in LFY for 58% lower than in MFY (129 mm and 305 mm, respectively). At the same time, air-temperatures were for 2.0°C higher (22.7 and 20.7°C, respectively). Mean yield in BC for 1996-2007 period was for 21% higher than in FC. Yields in three LSY were considerably lower (means 3.78 and 3.79 t/ha, for BC and FC, respectively) than in three MFY (means 6.13 and 7.30 t/ha, respectively). Precipitation (means of two sites: Békéscsaba and Székesfehérvár) in 3-months period June-August was in LFY for 51% lower (115 mm and 235 mm, respectively) and air-temperatures were for 1.9°C higher (22.1 and 20.2°C, respectively) than in MFY. Precipitation and temperature trends for LFY and MFY in two sites of both countries were similar with emphasis that in Hungary they were negligible lower

Stark Broadening of in III Lines in Astrophysical and Laboratory Plasma

Besides the need of Stark broadening parameters for a number of problems in physics, and plasma technology, in hot star atmospheres the conditions exist where Stark widths are comparable and even larger than the thermal Doppler widths. Using the semiclassical perturbation method we investigated here the influence of collisions with charged particles for In III spectral lines. We determined a number of Stark broadening parameters important for the investigation of plasmas in the atmospheres of A-type stars and white dwarfs. Also, we have compared the obtained results with existing experimental data. The results will be included in the STARK-B database, the Virtual Atomic and Molecular Data Center and the Serbian Virtual Observatory

Numerical and Experimental Analysis of Transient Flow in Roots Blower

The performance of rotary positive displacement machines highly depends on the operational clearances. It is widely believed that computational fluid dynamics (CFD) can help understanding internal leakage flows. Developments of grid generating tools for analysis of leakage flows by CFD in rotary positive displacement machines have not yet been fully validated. Roots blower is a good representative of positive displacement machines and as such is convenient for optical access in order to analyse internal flows. The experimental investigation of flow in optical roots blower by phase-locked PIV (Particle Image Velocimetry) performed in the Centre for Compressor Technology at City, University of London provided the velocity field suitable for validation of the simulation model. This paper shows the results of the three-dimensional CFD transient simulation model of a Roots blower with the dynamic numerical grids generated by SCORG and flow solution solved in ANSYS CFX flow solver to obtain internal flow patterns. The velocity fields obtained by simulation agree qualitatively with the experimental results and show the correct main flow features in the working chamber. There are some differences in the velocity magnitude and vortex distribution. The flow field in roots blower is highly turbulent and three-dimensional. The axial clearances should be included, and the axial grids should be refined in the simulation method. The paper outlines some directions for future simulation and experimental work. The work described in this paper is a part of the large project set to evaluate characteristics of the internal flow in rotary positive displacement machines and to characterize leakage flow

CFD Modeling in Screw Compressors with complex multi rotor configurations

Computational Fluid Dynamics (CFD) of screw compressors is challenging due to the positive displacement nature of the process, existence of very fine fluid leakage paths, coexistence of working fluid and lubricant or coolant, fluid injection and most importantly the lack of methodologies available to generate meshes required for the full three dimensional transient simulations. In this paper, currently available technology of grid remeshing has been used to demonstrate the CFD simulations of complex multi rotor screw compressors like the twin screw compressor with parallel axis and single screw compressor with cylindrical main rotor and two planar gate rotors with perpendicular axis. Presently, methodology for grid generation of constant pitch twin screw machines is available through SCORG© (Kovacevic et al., 2007) but it is currently not suitable for different topologies like that of a single screw or variable pitch rotors. It is very challenging to handle the mesh deformation that happens in the compression chambers during operations of such machines. The methodology tested for this paper uses a technique called key-frame re-meshing in order to supply pre-generated grids to the CFD solver as the solution progresses. In order to evaluate accuracy of such approach, an adiabatic compression-expansion process in a reciprocating piston cylinder arrangement is studied and compared with a diffusion equation based mesh smoothing. It has been demonstrated, that although it is possible to simulate the complex configuration of screw compressors by key-frame re-meshing technique, there are many limitations with respect to the time consumed in pre-processing, demand to computational resources, accuracy of results and general difficulty to include advanced modeling features like turbulence, multispecies or multiphase flows. Hence it was concluded that customized tools for generation of CFD grids for such complex screw machines still remain to be developed

Apparatus and Method for Measuring 3-D Weld Pool Shape

A method and apparatus are provided allowing the determination of 3-D weld pool surface geometry in the presence of a welding arc during a welding operation. The method includes the steps of illuminating the weld pool surface with diffused, structured light and detecting specular reflection of that diffused, structured light from the weld pool surface. An acquired image results. By analyzing the acquired image it is possible to determine the geometry of the weld pool surface. The apparatus for performing the method includes a high intensity light source, a diffuser and a structured light forming grid. A camera is provided for specular reflection detection of the light reflecting from the weld pool surface

Experimental Investigation of the Transient Flow in Roots Blower

Rotary positive displacement machines are common method to pump flow in various process industries. Their performance highly depends on the operational clearances. It is widely believed that computational fluid dynamics (CFD) can help understanding and reducing internal leakage flows. However, Developments of grid generating tools for use of CFD in rotary positive displacement machines have not yet been fully validated. Thereby arising a need to validate these models that help in better understanding of the leakage flows. Roots blower is a good representative of positive displacement machines and as such is convenient for optical access to analyse flows in in such machines. This paper describes the setup of the experimental test rig with the optical Roots blower in the Centre for Compressor Technology at City, University of London and the first results obtained using three different flow visualization methods. These are namely i) the high-speed camera (HC), ii) the continuous time resolved PIV (CPIV) and iii) the instantaneous PIV obtained with double pulse PIV laser and double shutter camera (IPIV). Test results from these three tests are compared and discussed in the paper. The CPIV test shows the movement of the vortex and the general shape of the flow field clearly but is not sufficient to calculate velocity vectors of high-velocity particles due to the limitation of the laser and camera. The IPIV test can produce quantitative velocity vector images of the internal flow but needs improvement to look into the leakage flow. The work described in this paper is a part of the large project set to evaluate characteristics of the internal flow in rotary positive displacement machines and to characterize leakage flows. The objective is to enable further improvements in 3D CFD analysis of leakage flows in rotary positive displacement machines and ultimately lead to the improvement in the performance of rotary positive displacement machines

Tailoring antiplatelet therapy in older patients with coronary artery disease

The older population represents a unique subset of patients due to a higher rate of comorbidities and risk factors, which can lead to a higher rate of ischemic and bleeding events. As a result, older adults are mainly underrepresented or excluded from randomized trials. Although the advancement in the percutaneous coronary intervention field with the development of new technologies, techniques, and potent antiplatelet therapy led to a reduction of ischemic risk, there is still a concern regarding bleeding hazards. Apart from the global utilization of less invasive trans-radial approach and proton pump inhibitors to reduce bleeding risk, proper tailoring of antiplatelet therapy in the older person is imperative. So far, several antiplatelet drugs have been introduced in different clinical scenarios, with dual antiplatelet therapy (combination of acetylsalicylic acid and P2Y12 inhibitor) recommended after percutaneous coronary intervention. The decision on the choice of antiplatelet drug and the DAPT duration is challenging and should be based on the relationship between ischemia and bleeding with the purpose of reducing ischemic events but not at the expense of increased bleeding complications. This is particularly important in the older population, where the evidence is obscure. The main objective of this review is to summarize the available evidence on contemporary antiplatelet therapy and different approaches of de-escalation strategies in older patients after percutaneous coronary intervention.What is the context?The older population represents a unique subset of patients due to a higher rate of comorbidities, risk factors, and unfavorable prognostic features, which can lead to a higher rate of ischemic and bleeding events. They are either excluded or underrepresented in most randomized clinical trials, which is why guidelines recommendation should be taken cautiously. Thus, the decision on the choice of antiplatelet therapy and its duration after percutaneous coronary intervention in older adults is challenging and should be tailored to a particular patient to avoid bleeding complications but not at the expense of increased ischemic events.What is new?In this review, we summarize all available evidence on contemporary antiplatelet therapy and different approaches of de-escalation strategies in older patients after percutaneous coronary intervention. In particular, several recommended approaches in patients with high bleeding risk, are thoroughly discussed in this review: De-escalation strategies with discontinuation of one antiplatelet drugDe-escalation strategy with switching between P2Y12 inhibitorsDe-escalation strategy based on dose reductionFinally, based on the current knowledge on factors contributing to high bleeding risk and the aforementioned antiplatelet modification approaches, in this review, we propose antiplatelet algorithm after percutaneous coronary intervention in older adults.What is the impact?The review provides comprehensive knowledge on antiplatelet therapy in older population and may help in tailoring antiplatelet therapy in this unique subset of patients

Analysis of rolling bearing power loss models for twin screw oil injected compressor

The mechanical losses inside a screw compressor limit the performance of the compressor in terms of efficiency. These losses arise due to relative motion between elements inside the screw compressor. The estimation of mechanical losses predicted in the literature is around 10-15% of the total shaft power. One of the elements which contribute significantly to these losses is rolling element bearings. There are numerous mathematical models available which predict power losses in the rolling bearings. The objective of this paper is to study different models to predict power loss for rolling bearings and to predict the power losses for the bearings used for oil injected, twin screw compressor. A comparison between different power loss models for different operating conditions of compressor is also presented in this paper and results of analysis are compared with available experimental observations. The analysis helps to determine suitable power loss model for different operating conditions and more realistic predictions of the power losses. This allows designers for more accurate estimation of the performance of screw compressors