Applied Optical Measurements

This paper considers the many techniques that have been developed to enhance convective heat transfer. They are presented according to the mode of heat transfer. The current advanced enhancement represents 3^rd generation heat transfer technology. Many studies of Prof. Franz Mayinger are integral to this development. It is felt that this field has a bright future.

The ammonia absorption method (AAM) and transient liquid crystal thermography (LCT) are employed to measure convective mass and heat transfer accurately with high local resolution. Both optical methods use fully automated imaging techniques and evaluation. By analogy the mass transfer results are converted to heat transfer results. The LCT needs optical accessibility during the experiment, the AAM does not. Experimental set-ups, procedures and results are given. The AAM and LTC results are compared for the very complex three dimensional channel flow with heat transfer generated by wing type vortex generators at one channel wall. Global and span averaged Nusselt number differences are below the individual error estimates of 5%. Local differences can be higher and are traced to differences in boundary conditions. For local measurements the AAM is superior to the LCT because of the constant concentration/temperature boundary condition.

This article mainly provides a selection of various applications of holographic interferometry in heat and mass transfer research, investigations of compact heat exchangers, the cooling of electronical equipment, simultaneous heat and mass transfer, mixing processes and mass transfer in agitated vessels, condensation of steam bubbles and the supersonic hydrogen combustion will be presented. But also other engineering fields in which this technique is implemented, such as the non-destructive material testing, are described.

This work demonstrates the feasibility of applying optical methods, which depend on the spatial variations in the refractive index of the liquid, to characterize the transport phenomena during solidification of pure water and aqueous ammonium chloride solution in a rectangular cavity. In the first case, holographic interferometry is applied to visualize the convective flow and temperature field during a water freezing process. Experimental results indicate that the supercooling and density inversion significantly influence the water freezing process. In the second case, the solidification processes of aqueous ammonium solution are observed using the shadowgraph method. The solidification process in eutectic composition resembles that of pure substances. In a hypoeutectic solution, double-diffusive layer and remelting occur in the lower part of the flow that is stable. On the other hand, in a hypereutectic solution, double-diffusive layer and remelting occur in the upper part of the flow. The layer is ruined seriously and the flow is unstable. In the third case, a twowavelength holographic interferometry technique is applied to further investigate ]the temperature and concentration fields of a double-diffusive layered system. Merits and limitations of the interferometric method are discussed as well.

The present work collects the main results obtained in an experimental research carried out at the laboratories of the National Institute of Thermal-Fluid Dynamics of ENEA. A photographic study of the burnout in highly subcooled flow boiling has been performed, in order to get a detailed description of the flow pattern under different conditions of boiling regime: ONB (onset of nucleate boiling), subcooled flow boiling and thermal crisis. In particular, the flow visualisation is focused on the phenomena occurring on the heated wall during the thermal crisis up to the physical burnout of the heater.

Pool boiling of both water and aqueous solutions of cationic surfactant Habon G in an open tank has been investigated by infrared thermography in combination with video recording- This technique provided information on the bubble dynamics, on the variations in wall superheat over an extensive region, and on irregularities in the behavior of nucleation sites. Heat transfer coefficient was enhanced considerably by the addition of Habon G in the range of concentration of 65–1060 ppm. This effect is more pronounced when the surfactant concentration is higher but there is diminishing as surfactant concentration is increased, with an optimum concentration, probably near 530 ppm Habon G.

A vapour bubble grows when the wall superheat at a nucleation site reaches an initiating value. Heat flows into the growing bubble through its curved dome and through its region of contact with the superheated wall. There are unresolved questions about these processes and the effect of the bubble on heat transfer from the wall following detachment. This paper presents preliminary measurements of wall temperature variations by liquid crystal thermography on the back of a thin, electrically-heated, vertical plate during bubble growth and detachment in water at sub-atmospheric pressure. Some of the difficulties of applying liquid crystal thermography to boiling are described.

Heat and mass transfer in boiling is determined by thermophysical mechanisms, especially by the interrelations between the surface of the heater and the liquid, and by the interfacial phenomena between liquid and vapor. It is generally assumed that the external forces like gravity in pool and shear forces in flow boiling are the most important factors for the bubble dynamics which determines the heat transfer. In microgravity buoyancy is completely or at least mostly eliminated. Therefore, pool boiling experiments in microgravity permit the study of heat transfer, and the related bubble dynamics caused by the growing bubbles themselves and by bubble interactions. In this article measurements of heat transfer and observed bubble behavior are discussed resulting from experiments performed in microgravity.

By their very nature, compact heat exchangers allow an efficient use of material, volume and energy in thermal systems. These benefits have driven heat exchanger design toward higher compactness, and the trend toward ultra-compact designs will continue. Highly compact surfaces can be manufactured using micromachining and other modern technologies. In this paper, unresolved thermalhydraulic issues related to ultra-compact designs will be discussed and the status of the technologies required for the production of ultra-compact structured surfaces will be summarized.

This article concerns the local structure of flow and temperature fields as well as overall heat transfer coefficients and pressure drops in flow passages of relevance for heat exchangers. Results from investigations using flow visualization, laser doppler-velocimetry and IR-thermography together with more conventional overall measurements are presented. The usefulness and opportunities with the modern techniques are discussed.

Measurements with a thermograph, spectral range 2,0-5,6 (µm, reveal temperature gradients in liquid drops that condense from a gas mixture with noncondensibles present. They indicate the presence of Marangoni convection that may explain the finding that the average thermal resistance of the condensate in plastic compact heat exchangers is lower than the expected value based on pure conduction.

Histories of temperature fields of condenser plates exhibit, amongst other things, the importance of drainage by the large drops and the predicted linear decrease of the averaged interface temperature in the flow direction.

Dedicated experiments with an initially homogeneous binary drop show that Marangoni flow can also be driven by concentration gradients at the interface, sustained by refreshing from the bulk liquid. It is unclear whether this effect manifests itself in heat exchangers.

This article summarizes four methods that were specifically developed to measure transient phenomena in two-phase vapor/liquid systems. Such systems have long been of concern to Professor Franz Mayinger of the Technische Universität München. The four cases considered here are closely related to some of the phenomena that have benefited from Professor Mayinger’s pioneering research.

Two-phase flow pressure-drop type instabilities in a two-phase upflow boiling system has been studied theoretically. The drift-flux model is adopted in predicting the steady state characteristics of the boiling system, using the finite difference method. The dynamic simulation of the quasi-static pressure-drop type instability in the boiling system is presented. The thermal non-equilibrium effect between the two phases is included by assuming the temperature profile in the subcooled boiling region. By comparing the equilibrium theory, non-equilibrium theory and experiments, it is concluded that the models including the nonequilibrium effects fit better the experimental results of the steady state characteristics of the system, as well as the system stability boundary and oscillations characteristics (amplitude and period).

For measuring the local void fraction, bubble frequency, bubble diameter and bubble velocity in two-phase flows, many types of fibre optical sensors are known. They use the difference of the refractive indices of the liquid and the gas phase as measuring principle. The power of the light emitting device has been reduced from 500 W to a few mW. The sensitive diameter of the fibre optical probe has been reduced from a few millimetres to a few micrometres. Single probes, double probes and arrangements of several probes up to bundles with 500 fibres have been developed. In the past the pure signals were displayed on a pen recorder or an oscilloscope. Nowadays high sophisticated signal processing units using transputers or FFT- analyzers give additional information about the two-phase flow pattern. Looking on their historical development an overview of the various types of fibre optical sensors will be given.

Hot-film probes have been extensively used for local velocity measurements in single-phase liquid flows. In addition, they have been widely used for the measurement of the local volume fraction and phasic velocities in gas/liquid two-phase flows. In contrast, very few researchers have tried to use hot film probes for two-phase liquid/liquid and three-phase gas/liquid/liquid measurements; in particular, for the oil/water/gas flows which are of interest in the petroleum recovery industry.

This paper presents measurements of three-phase air/oil/water flows using a hotfilm anemometer. It will be shown that accurate measurements are possible using this technique.

The newly developed tomographical dual wavelength photometry enables the measurement of the local intensity of segregation at a multitude of points inside stirred vessels. This is done by injecting a mixture of an inert and a reacting dye into the vessel. The inert dye serves as a tracer for the macromixing, whereas the vanishing of the reacting dye shows the micromixing. The concentration fields of the two dyes are measured simultaneously by transluminating the vessel from three directions with superimposed laser beams of different wavelength. The light absorption by the dyes is measured with CCD-cameras and these projections of the dye construction are used for the tomographic reconstruction of the concentration fields. Low Reynolds number measurements were performed with a combination of two Rushton turbines and a combination of two Pitched Blade Impellers. The combination of the Pitched Blade Impellers yields a good axial transport but a slow micromixing. The injection in the middle between the combination of the two Rushton turbines yields a faster micromixing, but the macrotransport is limited to the region between the stirrers.

The present work reports on short-time holography as applied at the ‘Lehrstuhl A für Thermodynamik’of the ‘Technische Universität München’and possible applications of this measuring technique to the analysis of heat and mass transfer processes. The principal optical setup for the recording, reconstruction and evaluation of the holograms is explained through the example of a current research project. Short-time holography was applied to the analysis of the disintegration process of sprays and to two phase flow in an aerated stirred vessel. By using this measuring technique, new insights into two phase flow phenomena were obtained. The results led to a more detailed set of data which now serves as the basis for new approaches in modeling and numerically simulating two-phase-flow.

This article is meant as both a review and an outlook to developments of the phase Doppler Technique. The fundamental working principles are presented with reference to more recent developments in the computation of light scattering from small particles. This provides also a basis to discuss novel variations of the technique to either improve the measurement accuracy or to extend the technique to additional measurement quantities. Some first results about the measurement of nonspherical droplets are presented.

An instrument has been developed which measures the solids concentration in slurry pipelines ranging in diameter from 5-150 cm. The operating principle is based on conductance, and in contrast to the gamma densitometer, the instrument does not require a nuclear radiation source. The instrument operates online, is non-intrusive, and provides real-time output. The instrument measures local concentration around the pipe periphery, and the local values are integrated to obtain the area-average concentration. A graphical display shows the variation of concentration from the top of the pipe to the bottom as well as the area-average concentration history. The instrument has been extensively tested in a slurry transport facility at the University of Florida as well as on-line at the Swift Creek phosphate mine in the state of Florida. Excellent performance of the instrument is observed over a wide range of operating conditions.

Smoke-wire visualization provides insight into the flow structure of round jets issuing from straight tubes and impinging on concave and convex surfaces with high relative curvature values. The technique provides a crosssectional view of the jet, allowing the imaging of the initiation and growth of ring vortices in the jet shear layer and their subsequent interaction following impingement on the cylindrical surfaces. Both photographic and video records of the flow are obtained. Effects of relative curvature, nozzle-to-surface distance and Reynolds number on the impinging jet flow structure are described.

A structure of a convective torch of axisymmetric flame in combustion of a propane-air mixture was studied experimentally under the conditions of velocity perturbations on the nozzle section. The flow was visualised by a MachZehnder interferometer and also using stroboscopic photography of the flame proper luminescence. Mean temperature distribution in a torch was studied using a Chromel-Alumel thermocouple. The obtained data show the presence of torroidal coherent vortex structures moving downstream that substantially enhance combustion processes and heat transfer in a torch.

The present work reports on the application of optical measurement-techniques to combustion-processes as they are investigated at the “Lehrstuhl A für Thermodynamik ” of the “Technische Universität München”. The outstanding characteristics of the optical measurementtechniques in combustion research are described, and their potentials for the determination of important process-variables are explained with applications of various kinds, which cover the range from explosions up to transient supersonic combustion.

This paper reports measurements of velocities, temperatures, emission and particle size in two models of pulverised coal furnaces, the one combusting and the other isothermal. In the combusting flow of pulverised coal burnt in a swirlstabilised gas flame, the ability of novel optical instrumentation to measure simultaneously, with high temporal and spatial resolution, coal particle size, temperature and velocity vector using the shadow Doppler velocimeter and a two colour pyrometer is demonstrated. In the isothermal flow, CCD-based instrumentation is applied to measure the long-term Lagrangian dispersion of individual particles in a turbulent, recirculating particle-laden water analogue of the combusting flow downstream of a sudden step expansion.

Coherent Anti-Stokes Raman Spectroscopy (CARS) is frequently the method of choice for non-intrusive temperature measurements in combustion systems. The temperature determination requires a comparison of measured spectra with theoretically calculated ones. Conventional gradient-based least squares fitting of experimental data with a library of theoretical spectra usually leads to sensible results, as long as the spectrum shapes behave well and the noise level is low.

The investigation of a 1 MW coal dust burner yielded only very noisy data that showed the limits of applicability of the gradient-based approach. Therefore in this paper a novel fitting approach based on evolutionary algorithms for such spectra is presented. The applied algorithm is explained. Temperature evaluation results, both conventionally and evolutionarily determined, are given.

A nonintrusive method to measure temperature and pressure utilizing cxbroadband excitation and detection of oxygen fluorescence is presented. The required finely-tuned laser excitation source is replaced by a more rugged and less bulky Xenon flashlamp. As a tradeoff, more lines are excited and hence more fluorescence lines need to be monitored. Using a numerical fluorescence model, temperature and pressure sensitive bins of varying sizes (5,10,15,20 nm) between 200 and 300 nm are determined. By scaling the bin values with a relatively pressure insensitive bin, a pressure insensitivity parameter was found that allowed the determination of the 5 nm bin centered about 222.5 nm to be the most temperature sensitive for the temperature range 200-1500 K.

In this paper, the authors report on a combustion technique that is applicable to direct injection, internal combustion engines, both Diesel or gasoline fueled. This technique is based on porous medium (PM) combustion technology. Theoretical considerations are presented for internal combustion engines, indicating that an overall thermal efficiency improvement will exist for the PM- engine. This is explained and the general performance of the new PM-engine is demonstrated for a single cylinder, air-cooled direct injection Diesel engine. Initial results are presented and an outlook is given on how the present developments might continue in the future.

The formulation rests on three metrics: one for the process transfer rate; the second for the quantity affected by the process; and the third for the time of interest, i.e., the time window for integrating (observing) the process. These three metrics generate, in tum, a metric which scales the fractional change. Thus, a single and very simple concept provides a master key for analyzing and scaling all transfer processes of interest

All scaling groups which characterize various thermal and fluid dynamic processes may be derived from the fractional change scaling and analysis (FCSA) methodology. This is illustrated by applying the method to three hierarchical levels characterized by three scales: macro, meso and micro.

At the meso level, the FCSA generates the Mach, Froude and other dimensionless groups when applied to waves and vibrations, and the friction factors for laminar and turbulent flows when applied to diffusion- and vorticity-dominated flows. Furthermore, for turbulent flows, the method can be used to derive the Kolmogorov-5/3 equation, and to demonstrate the wave-eddy duality which is analogous to the waveparticle duality in quantum mechanics.

Thus, a single concept and a single method can be used to scale and analyze transfer processes associated with particles, waves, diffusion and vorticity.

In closure, it is shown that the equations derived in this paper are analogous to three well-known equations in quantum mechanics, i.e., to the equations of de Broglie, Compton and Planck-Einstein, and that life span of mammals is a manifestation of Noether’s theorem.