Reinhold Kneer

OXYCOAL-AC: Towards an integrated coal-fired power plant process with ion transport membrane-based oxygen supply

The cooperative project OXYCOAL-AC aims at the development of a zero-CO2-emission coal combustion process for power generation. The scope of the research comprises a multitude of aspects. This article focuses on membrane-based air separation modules and their design for oxycoal conditions, the specifics of coal combustion in a CO2/O2 atmosphere including related burner design as well as the cleaning of hot flue gas from oxycoal combustion.

Investigation of the backflow phenomenon in falling liquid films

The phenomenon of backflow in the capillary wave region of laminar falling liquid films is studied in detail. For the first time, the mechanism leading to the origination of the phenomenon is identified and explained. It is shown that backflow forms as the result of a separation eddy developing at the bounding wall similar to the case of classical flow separation. Results show that the adverse pressure distribution causing the separation of the flow in the capillary wave region is induced by the strong third-order deformation (i.e. change in curvature) of the liquid–gas free surface there. This deformation acts on the interfacial pressure jump, and thereby the wall pressure distribution, as a result of surface tension forces. It is shown that only the capillary waves, owing to their short wavelength and large curvature, impose a pressure distribution satisfying the conditions for flow separation. The effect of this capillary separation eddy on momentum and heat transfer is investigated from the perspective of modelling approaches for falling liquid films. The study is centred on a single case of inclined liquid film flow in the visco-capillary regime with surface waves externally excited at a single forcing frequency. Investigations are based on temporally and spatially highly resolved numerical data obtained by solving the Navier–Stokes equations for both phases. Computation of phase distribution is performed with the volume of fluid method and the effect of surface tension is modelled using the continuum surface force approach. Numerical data are compared with experimental data measured in the developed region of the flow. Laser-Doppler velocimetry is used to measure the temporal distribution of the local streamwise velocity component, and confocal chromatic imaging is employed to measure the temporal distribution of film thickness. Excellent agreement is obtained with respect to film thickness and reasonable agreement with respect to velocity.

Experimental study of flow separation in laminar falling liquid films

In a previous publication, Dietze, Leefken & Kneer ( J. Fluid Mech. , vol. 595, 2008, p. 435) showed that flow separation takes place in the capillary wave region of falling liquid films. That investigation focused on the mechanistic explanation of the phenomenon mainly on the basis of numerical data. The present publication for the first time provides clear experimental evidence of the phenomenon obtained by way of highly resolving velocity measurements in a specifically designed optical test set-up. Characteristically, the refractive index of the working fluid was matched to that of the glass test section to provide optimal access to the cross-section of the film for the employed optical velocimetry techniques, namely, laser doppler velocimetry (LDV) and particle image velocimetry (PIV). Using LDV, time traces of the streamwise velocity component were recorded in high spatial (0.025 mm) and temporal resolutions (0.4 ms) showing negative velocity values in the capillary wave region. In addition, simultaneous film thickness measurements were performed using a Confocal Chromatic Imaging (CCI) technique enabling the correlation of velocity data and wave dynamics. Further, using PIV the spatio-temporal evolution of the velocity field in the cross-section of the film was measured with high spatial (0.02 mm) and temporal (0.5 ms) resolutions yielding insight into the topology of the flow. Most importantly these results clearly show the existence of a separation eddy in the capillary wave region. Due to the high temporal resolution of the PIV measurements, enabled by the use of a high-speed camera with a repetition rate of up to 4500 Hz, the effect of wave dynamics on the velocity field in all regions of the wavy film was elucidated. All experiments were performed using a dimethylsulfoxide (DMSO)–water solution and focused on laminar vertically falling liquid films with externally excited monochromatic surface waves. Systematic variations of both the Reynolds number ( Re = 8.6–15.0) and the excitation frequency ( f = 16–24 Hz) were performed. Results show that an increase in the wavelength of large wave humps, produced either by an increase in the Reynolds number or a decrease in the excitation frequency, leads to an increase in the size of the capillary separation eddy (CSE). Thereby, the CSE is shown to grow larger than the local film thickness, assuming an open shape with streamlines ending at the free surface.

A miniature electrical capacitance tomograph

The paper describes a miniature electrical capacitance tomography system. This is based on a custom CMOS silicon integrated circuit comprising eight channels of signal conditioning electronics to source drive signals and measure voltages. Electrodes are deposited around a hole that is fabricated, using ultrasonic drilling, through a ceramic substrate and has an average diameter of 0.75 mm. The custom chip is interfaced to a host computer via a bespoke data acquisition system based on a microcontroller, field programmable logic device and wide shift register. This provides fast capture of up to 750 frames of data prior to uploading to the host computer. Data capture rates of about 6000 frames per second have been achieved for the eight-electrode sensor. This rate could be increased but at the expense of signal to noise. Captured data are uploaded to a PC, via a RS232 interface, for off-line imaging. Initial tests are reported for the static case involving 200 µm diameter rods that are placed in the sensor and for the dynamic case using the dose from an inhaler.

Investigation of the influence of multiple gasoline direct injections on macroscopic spray quantities at different boundary conditions by means of visualization techniques

Abstract The reduction of combustion emissions and fuel consumption can be achieved by controlling the fuel mixture to be burned and therefore the fuel injection. In this study a gasoline direct injection (GDI) piezo hollow cone injector is investigated by visualization measurements. Both single and double injection events are examined in a pressurized chamber. For the single injection, the influence of the ambient temperature and pressure is analysed in detail. For the double injection, the ambient conditions and the time interval between two injections are varied. For the single injection, the general shape of the spray, the recirculation eddy formed at the spray edge, the stringy structure of the hollow cone, and defined quantities such as the penetration length or the spray width are examined with respect to the influence of the ambient conditions. For the double injection, the same properties are evaluated, with the focus put not only on the influence of the ambient conditions, but also on the mutual interaction of two spray events. The experimental results show a clear impact of both the ambient conditions and the time difference between two injections. In particular, the investigation of the recirculation eddy may be helpful in determining possible positions for the spark plug, since the spark plug is preferably placed in regions where the air–fuel mixture variations are minimal.

Heat transfer and crisis phenomena with intense boiling in the falling wave liquid films

Experimental data on heat transfer with intense evaporation in the falling films of liquid nitrogen were analysed. According to data generalization, heat transfer at evaporation becomes more intense under the precrisis modes at high heat fluxes for two studied boundary conditions on the heat-releasing surface: Tw ≈ const and qw ≈ const. The relative contributions of conductive and convective components of heat transfer for different heat fluxes were estimated due to statistical treatment of the wave characteristics carried out by the capacitance probes for measurement of the local liquid film thickness. It was found out that heat transfer intensification is mainly caused by a drastic decrease in thermal resistance of the local zones with intensely evaporating residual layer between large waves. At that, the convective component of heat transfer related to wave perturbations on a free surface of a liquid film decreases significantly with a rise of heat fluxes. New data on pulsations of the local temperature of the heat-releasing surface were obtained at different points along the flow with the modes of “dry spot” formation.

Time-gated ballistic imaging using a large aperture switching beam

Ballistic imaging commonly denotes the formation of line-of-sight shadowgraphs through turbid media by suppression of multiply scattered photons. The technique relies on a femtosecond laser acting as light source for the images and as switch for an optical Kerr gate that separates ballistic photons from multiply scattered ones. The achievable image resolution is one major limitation for the investigation of small objects. In this study, practical influences on the optical Kerr gate and image quality are discussed theoretically and experimentally applying a switching beam with large aperture (D = 19 mm). It is shown how switching pulse energy and synchronization of switching and imaging pulse in the Kerr cell influence the gates transmission. Image quality of ballistic imaging and standard shadowgraphy is evaluated and compared, showing that the present ballistic imaging setup is advantageous for optical densities in the range of 8 < OD < 13. Owing to the spatial transmission characteristics of the optical Kerr gate, a rectangular aperture stop is formed, which leads to different resolution limits for vertical and horizontal structures in the object. Furthermore, it is reported how to convert the ballistic imaging setup into a schlieren-type system with an optical schlieren edge.

Applying similarity search for the investigation of the fuel injection process

We introduce a distance-based similarity model with application to the optimization of the fuel injection process. Our model allows for an automatic evaluation of huge and complex amount of experimental data originated from optical measurement techniques analyzing the fuel injection process. The goal is to enable researchers to get deeper insight into this process based on an automatically driven analysis.

Determination of the heat flux to the workpiece during dry turning by inverse methods

Reducing metal machining costs and harmful effects to the environment are the main benefits of dry machining compared to the application of cooling lubricants during machining processes. The abandonment of any lubricants is causing higher temperatures of workpiece, chip, and tool. Thus, the temperature gradient varies with time and space over the workpiece, causing dimensional deviations and profile defects due to thermal expansion and shrinking. The aim of this work is to determine the heat flux to the workpiece from experimental data. The temperatures of the workpiece were measured during an orthogonal turning process of carbon steel (AISI 1045). An analogous thermal model is used to solve the ill-posed inverse heat conduction problem by the sequential estimation method, introduced by Beck et al. (Inverse heat conduction, Wiley, New York, 1985) and for the evaluation of the results. The heat flux to the workpiece could be used in further work as a basic boundary condition in finite element simulations to find an alternative tool path, which compensate thermally induced deviations and profile defects of the workpiece. The heat flux to the workpiece is calculated for different cutting velocities, feed rates, and the influence of TiN-coatings on cemented carbide tools. Cutting parameters have been identified to minimize the heat load on the workpiece in general and therefore reduce errors due to dry machining.

Investigation of the thermal entry length in laminar wavy falling films

The use of liquid films flow offers solutions for the problems associated with the microgravity applications. The thermal entry length of laminar wavy falling films was experimentally determined under full gravity conditions by means of infrared thermography. A dependence of the entry length on the Reynolds, Prandtl, and Kapitza number as well as the ratio Pr0/PrW between the Prandtl numbers at inflow and wall temperatures was found.