Heat Transfer and Pressure Drop in a Developing Channel Flow with Streamwise Vortices

Experiments to assess the heat transfer and pressure-drop effects of delta-wing vortex generators placed at the entrance of developing channel flows are reported in this study. The experimental geometry simulates common heat exchanger configurations and tests are conducted over a velocity range important to heating, air conditioning and refrigeration. An innovative liquid-crystal thermography technique is used to determine the local and average Nusselt numbers for an isoflux channel wall, and conventional methods are used to determine the Fanning friction factor. Vortex generators with aspect ratios of A = 2 and A = 4 are studied at attack angles of a. = 20?? to 45????. The results indicate that the streamwise vortices generated by a delta wing can enhance local Nusselt numbers by more than 200% in a developing channel flow. Under some conditions, the spatially average Nusselt number nearly doubled for a heat transfer area that was 37 to 63 times the wing area. The Fanning friction factor increased by a few percent to nearly 60%, depending on the Reynolds number.Air Conditioning and Refrigeration Project 4

Turbulent heat transfer in spacer-filled channels: Experimental and computational study and selection of turbulence models

Heat transfer in spacer-filled channels of the kind used in Membrane Distillation was studied in the Reynolds number range 100–2000, encompassing both steady laminar and early-turbulent flow conditions. Experimental data, including distributions of the local heat transfer coefficient h, were obtained by Liquid Crystal Thermography and Digital Image Processing. Alternative turbulence models, both of first order (k-ε, RNG k-ε, k-ω, BSL k-ω, SST k-ω) and of second order (LRR RS, SSG RS, ω RS, BSL RS), were tested for their ability to predict measured distributions and mean values of h. The best agreement with the experimental results was provided by first-order ω-based models able to resolve the viscous/conductive sublayer, while all other models, and particularly ε-based models using wall functions, yielded disappointing predictions

Thermo-Liquid Crystal (TLC) Thermography and Astigmatism Particle Tracking Velocimetry (APTV) for the simultaneous time-resolved 3D measurements of microscopic temperature and velocity flow fields

The ever accelerating state of technology has powered an increasing interest in heat transfer solutions and process engineering innovations in the microfluidics domain, many of which require reliable temperature flow diagnostic techniques. Thermo-liquid crystal (TLC) thermography has been a widely accepted and commonly used technique for the measurement of temperature fields in macroscopic flows. Furthermore, its combination with optical velocimetry techniques, such as Particle Image Velocimetry (PIV), has provided a means to simultaneously characterize macroscopic two-dimensional (2D) temperature and velocity flow fields. Unfortunately, however, the state of technology has not allowed for this task to be carried out in microscopic flows. Low seeding density, volume illumination, and low TLC particle image quality at high magnifications present unsurpassed challenges to its application to three-dimensional flows with microscopic dimensions. In the following chapters, a combination of cutting edge technology is presented as a means to accomplish precisely this objective: to simultaneously measure time-resolved 3D temperature and velocity fields in microscopic flows where multi-camera methods and light sheets are not possible. The process of achieving this goal happened in three phases: 1. TLC thermography had to be improved and adapted to the point where the temperature of individual particles could be evaluated and tracked over time. An emulsion of TLC micro spheres (with a narrow size distribution and no encapsulation) was manufactured to improve the image quality of TLC particles and a multi-variable calibration approach, applied to a mathematically optimal variable space, was developed to improve the estimation of temperature from color information. 2. Astigmatism Particle Tracking Velocimetry (APTV) was developed to precisely locate particles in a flow volume and track their displacement in three dimensions (3D). 3. The the signal-to-noise ratio (SNR) of TLC images had to be improved enough for the application of defocusing techniques, such as APTV, which encode the third component of the particles’ position in their images’ geometry. A state-of-the-art balanced light source which combines the light spectrum of multiple light pipes was used instead of the conventional high power flash lamps and a circular polarization filter was designed to exploit the optical properties of chiral nematic polymers. This combination made it possible to boost the SNR just enough for the combination of TLC thermography and APTV to work. Finally, a proof-of-concept experiment was performed in a simple microscopic flow example, and the 3D displacement of TLC particles was tracked simultaneously with their temperature. More complex applications were not possible due to current technological hardware limitations but the capability and potential of the measurement technique was clearly demonstrated.Der ständig andauernde technische Fortschritt führt im Bereich der Mikroströmung zu einer steigenden Nachfrage von Wärmetransportlösungen und Prozessoptimierungen, die zuverlässige Messmethoden für Temperatur und Strömungsfeld erfordern. Thermoliquid crystal (TLC)-Thermographie ist eine weithin akzeptierte und häufig verwendete Technik zur Messung von Temperaturfeldern in makroskopischen Strömungen. In Kombination mit optischer Geschwindigkeitsmesstechnik, wie Particle Image Velocimetry (PIV), kann gleichzeitig sowohl die Temperatur als auch die Strömungsgeschwindigkeit für makroskopische zweidimensionale (2D) Strömungsfelder bestimmt werden. Leider erlaubt der Stand der Technik aber nicht, dass diese Methode auf mikroskopische Strömungen übertragen werden kann. Niedrige Partikeldichte, Volumenbeleuchtung und die schlechte Qualität der TLC-Partikelbilder bei großen Vergrößerungen führen zu unüberwindbaren Herausforderungen bei der Anwendung auf dreidimensionale Strömungen mit mikroskopischen Dimensionen. In den folgenden Kapiteln wird eine Kombination verschiedener Neuentwicklungen vorgestellt, die sich genau diesen Herausforderungen stellt: Das 3D-Temperaturfeld sowie das Geschwindigkeitsfeld einer Mikroströmung sollen zeitaufgelöst vermessen werden, ohne auf Mehrkamerasysteme oder Lichtschnittverfahren aus der makroskopischen Strömungsmesstechnik zurückgreifen zu können. Dieses Ziel wurde in drei Phasen erreicht: 1. TLC Thermographie musste optimiert und angepasst werden, so dass die Temperatur der einzelnen Partikel ausgewertet und über die Zeit verfolgt werden konnte. Eine Lösung von TLCMikro-Partikeln (mit einer engen Größenverteilung und ohne Verkapselung) wurde benutzt, um die Bildqualität der TLC Partikel zu verbessern. Durch eine mehrdimensionale Kalibrierung eines optimierten Parameterraums war es möglich, die Temperaturbestimmung aus der Farbinformation zu verbessern. 2. Astigmatismus Particle Tracking Velocimetry (APTV) wurde entwickelt, um die Partikel im Strömungsvolumen genau zu lokalisieren und deren Bewegung in drei Dimensionen (3D) zu folgen. 3. Das Signal-zu-Rausch-Verhältnis (SNR) der TLC-Partikelbilder musste soweit verbessert werden, dass es für defokussierende Methode, wie APTV, anwendbar ist, da sich die dritte Raumkoordinate in der Form der Partikelbilder verbirgt. Anstelle einer einzelnen konventionellen Hochleistungsblitzlampe wurden die Spektren verschiedener geregelter Lichtquellen über Lichtwellenleiter kombiniert. Mittels eines zirkularen Polarisationsfilters konnten die optischen Eigenschaften der chiralen nematischen Polymere ausgewertet werden. Diese Kombination machte es möglich, das Signalzu- Rausch-Verhältnis gerade so weit zu steigern, dass die gleichzeitige Anwendung von TLC-Thermografie und APTV möglich war. Somit konnte schließlich ein Demonstrationsexperiment am Beispiel einer einfachen mikroskopischen Strömung durchgeführt werden. Die 3D-Verschiebung der TLC-Partikel wurde dabei gleichzeitig mit ihrer Temperatur verfolgt. Komplexere Anwendungen waren aufgrund der aktuellen technologischen Hardware-Einschränkungen noch nicht möglich. Dennoch konnten die Fähigkeit und das Potenzial der Messtechnik demonstriert werden

Cfd investigation of spacer-filled channels for membrane distillation

The membrane distillation (MD) process for water desalination is affected by temperature polarization, which reduces the driving force and the efficiency of the process. To counteract this phenomenon, spacer-filled channels are used, which enhance mixing and heat transfer but also cause higher pressure drops. Therefore, in the design of MD modules, the choice of the spacer is crucial for process efficiency. In the present work, different overlapped spacers are investigated by computational fluid dynamics (CFD) and results are compared with experiments carried out with thermochromic liquid crystals (TLC). Results are reported for different flow attack angles and for Reynolds numbers (Re) ranging from ~200 to ~800. A good qualitative agreement between simulations and experiments can be observed for the areal distribution of the normalized heat transfer coefficient. Trends of the average heat transfer coefficient are reported as functions of Re for the geometries investigated, thus providing the basis for CFD-based correlations to be used in higher-scale process models

Experimental and computational investigation of heat transfer in channels filled by woven spacers

Models of woven-type spacer-filled channels were investigated by Computational Fluid Dynamics (CFD) and parallel experiments in order to characterize the performance of Membrane Distillation (MD) modules. The case of overlapped spacers was analysed in a companion paper. Experiments were based on a non-intrusive technique using Thermochromic Liquid Crystals (TLC) and digital image processing, and provided the distribution of the local convective heat transfer coefficient on a thermally active wall. CFD simulations ranged from steady-state conditions to unsteady and early turbulent flow, covering a Reynolds number interval of great practical interest in real MD applications. A specific spacer aspect ratio (pitch-to-channel height ratio of 2) and two different spacer orientations with respect to the main flow (0° and 45°) were considered. Among the existing studies on spacer-filled channels, this is one of the first dealing with woven spacers, and one of the very few in which local experimental and computational heat transfer results are compared. Results suggest a convenience in adopting the 45° orientation for applications that can be operated at very low Reynolds numbers, since convenience decreases as the Reynolds number increases

Non-encapsulated thermo-liquid crystals for digital particle tracking thermography/velocimetry in microfluidics

The ever accelerating state of technology has powered an increasing interest in heat transfer solutions and process engineering innovations in the microfluidics domain. In order to carry out such developments, reliable heat transfer diagnostic techniques are necessary. Thermo-liquid crystal (TLC) thermography, in combination with particle image velocimetry, has been a widely accepted and commonly used technique for the simultaneous measurement and characterization of temperature and velocity fields in macroscopic fluid flows for several decades. However, low seeding density, volume illumination, and low TLC particle image quality at high magnifications present unsurpassed challenges to its application to three-dimensional flows with microscopic dimensions. In this work, a measurement technique to evaluate the color response of individual non-encapsulated TLC particles is presented. A Shirasu porous glass membrane emulsification approach was used to produce the non-encapsulated TLC particles with a narrow size distribution and a multi-variable calibration procedure, making use of all three RGB and HSI color components, as well as the proper orthogonally decomposed RGB components, was used to achieve unprecedented low uncertainty levels in the temperature estimation of individual particles, opening the door to simultaneous temperature and velocity tracking using 3D velocimetry techniques. © 2012 The Author(s)

The Use of Liquid Crystal Thermography in Selected Technical and Medical Applications—Recent Development

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In accordance of the Creative Commons Attribution License all Copyrights © 2014 are reserved for SCIRP and the owner of the intellectual property Jan Stasiek et al. All Copyright © 2014 are guarded by law and by SCIRP as a guardian. Thermochromic liquid crystals (TLC) and true-colour digital image processing have been successfully used in non-intrusive technical, industrial and biomedical studies and applications. Thin coatings of TLC at surfaces are utilized to obtain detailed temperature distributions and heat transfer rates for steady or transient processes. Liquid crystals also can be used to make the temperature and velocity fields in liquids visible by the simple expedient of directly mixing the liquid crystal material into the liquid (water, glycerol, glycol, and silicone oils) in very small quantities to use as thermal and hydrodynamic tracers. In biomedical situations, e.g., skin diseases, breast cancer, blood circulation and other medical application, TLC and image processing are successfully used as an additional non-invasive diagnostic method especially useful for screening large groups of potential patients. The history of this technique is reviewed, principal methods and tools are described and some examples are presented. Also steady-state and transient liquid crystal thermography (LCT) is used to measure local heat transfer on a plate equipped with transverse vortex generators. Automated evaluation allows determining the heat transfe

Long-time experimental investigation of turbulent superstructures in Rayleigh-Bénard convection by noninvasive simultaneous measurements of temperature and velocity fields

Large-scale mean patterns in Rayleigh-Bénard convection, also referred to as turbulent superstructures, have mainly been studied by means of numerical simulations so far, but experimental investigations are still rare. However, the analysis of turbulent superstructures, which are of great importance due to their effect on the local transport of heat and momentum, require both numerical and experimental data. Therefore, within the scope of this study measurements were performed in the horizontal mid plane and in a horizontal plane closer to the top of a Rayleigh-Bénard cell with an aspect ratio of [Gamma]=l/h=25, thereby showing the initial formation of turbulent superstructures and their long-time rearrangement. The turbulent superstructures are investigated experimentally by noninvasive simultaneous measurements of temperature and velocity fields, using the color signal of thermochromic liquid crystals (TLCs) for the evaluation of the temperature and their temporal displacement for the determination of all three velocity components in the measurement planes via stereoscopic particle image velocimetry (stereo-PIV). Applying this measuring technique it is demonstrated that the time-averaging of instantaneous temperature and velocity fields uncovers the turbulent superstructures in both fields. Furthermore, the combination of the temperature and velocity data is used to characterize the local heat flux quantified by the local Nusselt number, which confirms that the turbulent superstructures strongly enhance the heat transfer in Rayleigh-Bénard convection

Characterization of Emulsified Non-encapsulated Thermochromic Liquid Crystal Micro-particles

In this paper, the process for obtaining non-encapsulated Thermochromic Liquid Crystal (TLC) micro-particles from commercial bulk material (UN R25C10W) is described. The bulk material is analyzed in terms of morphology and rheological properties (i.e. viscosity, maximum shear rate). An experimental evaluation of surface tension values and contact angle measurements is made to complement the rheological data. On the basis of the obtained thermophysical values, an emulsification procedure is proposed and non-encapsulated TLC droplets with a dimension lower than 10 \u3bcm were acquired. Further, attention has been focused on the calibration process of TLC bulk material before and after the emulsification. A relation between the local temperature value, RGB and colour intensities (HSI) is obtained by analyzing the digital images with MATLAB Image Processing Toolbox. The obtained results indicate that the commercial bulk material UN R25C10W TLC can be used to obtain stable oil-in-water emulsion by proposed emulsification procedure in this paper

Multiphase imaging of freezing particle suspensions by confocal microscopy

Ice-templating is a well-established processing route for porous ceramics. Because of the structure/properties relationships, it is essential to better understand and control the solidification microstructures. Ice-templating is based on the segregation and concentration of particles by growing ice crystals. What we understand so far of the process is based on either observations by optical or X-ray imaging techniques, or on the characterization of ice-templated materials. However, in situ observations at particle-scale are still missing. Here we show that confocal microscopy can provide multiphase imaging of ice growth and the segregation and organization of particles. We illustrate the benefits of our approach with the observation of particles and pore ice in the frozen structure, the dynamic evolution of the freeze front morphology, and the impact of PVA addition on the solidification microstructures. These results prove in particular the importance of controlling both the temperature gradient and the growth rate during ice-templating.Comment: 20 pages, 9 figure