Use of Natural Instability For Enhancement of Flow Mixing in Annuli

A technique has been proposed to increase flow mixing in annuli by means of vortex generation. Corrugations in the form of axisymmetric ribs are placed at the walls of annulus to modulate the axial flow which can potentially induce vortex instabilities. Unlike other vortex generation methods which suffer from relatively high pressure losses, this technique is expected to cause less pressure drop. Spectral algorithm based on Fourier and Chebyshev expansions has been used to study the stationary state and its stability. Due to the irregularities of the boundaries, the immersed boundary conditions (IBC) method is used to enforce the flow boundary conditions. The effect of geometric and flow parameters on pressure losses and stability have been thoroughly investigated. Characteristics of vortex mode and travelling wave instabilities as well as region of dominance in each case are also determined. Moreover, it has been shown that effect of arbitrary ribs can be accurately captured using reduced geometry model

Effects of Surface Topographies on Heat and Fluid Flows

Responses of annular and planar flows to the introduction of grooves on the bounding surfaces have been analyzed. The required spectral algorithms based on the Fourier and Chebyshev expansions have been developed. The difficulties associated with the irregularities of the physical domain have been overcome using either the immersed boundary conditions (IBC) concept or the domain transformation method (DT). Steady flows in annuli bounded by walls with longitudinal grooves have been studied. Analysis of pressure losses showed that the groove-induced changes can be represented as a superposition of a pressure drop due to a change in the average position of the bounding cylinders and a pressure drop due to the flow modulations induced by the shape of the grooves. The former effect can be evaluated analytically while the latter requires explicit computations. It has been shown that the reduced-order model is an effective tool for extraction of features of the groove geometry that lead to flow modulations relevant to drag generation. It has been shown that the presence of the longitudinal grooves may lead to a reduction of the pressure loss in spite of an increase of the wetted surface area. The form of the optimal grooves from the point of view of the maximization of the drag reduction has been determined. When mixing augmentation is not available, heat can be transported across micro-channels by conduction only. A method to increase this heat flow has been proposed. The method relies on the use of grooves parallel to the flow direction. It has been shown that it is possible to find grooves that can increase the heat flow and, at the same time, can decrease the pressure losses. The optimal groove shape that maximizes the overall system performance has been determined. Since it has been assumed that the flow must be laminar, it is of interest to determine the maximum Reynolds number for which this assumption remains valid. The stability characteristics of flow in a grooved channel have been studied. Only disturbances corresponding to the travelling waves in the limit of zero groove amplitude have been found. It has been shown that disturbances corresponding to the two-dimensional waves in a smooth channel play the critical role in the grooved channel. The highly three-dimensional disturbance flow topology at the onset of the instability has been described. It has been demonstrated that the presence of the grooves leads to flow stabilization for the groove wave numbers 4.22 and flow destabilization for larger . The stabilizing/destabilizing effects increase with the groove amplitude. Variations of the critical Reynolds number over the whole range of the groove wave numbers and the groove amplitudes of interest have been determined. Special attention has been paid to the effects of the long wavelength, drag reducing grooves. It has been shown that such grooves lead to a small increase of the critical Reynolds number compared with the smooth channel

Fabrication of smart glass electrochromic device using rf magnetron sputtering

Electrochromic device is an important functional device to control the amount of light through a glass. It usually used in sunlight control window glazing for buildings and automobile. The important feature of electrochromic glass is the ability to response toward the apply voltage in shortest time, and endurance to maintain in color shape after apply voltage. In this thesis, the oxygen gas percentage is optimized during the fabrication of tungsten trioxide (WO3) as an electrochromic glass for window glazing application by using RF magnetron sputtering. The oxygen flow rate for the deposition is varied from 10sccm -22sccm which is 25%, 27%, 30%, and 35% of oxygen flow. The structures of WO3 were investigated using X-Ray diffraction, Field effect scanning electron microscopy (Fe-Sem) and Atomic force microscopy (AFM). The electrochromic properties were characterized by a cyclic voltammogram and UV-Vis absorption spectra. The results show that nanocrystalline film with particle size of 51.54nm was deposited at 27% oxygen flow rate has the largest charge capacity and coloration efficiency among the others. The time respond taken for complete coloration at 4V is 2second. This result is a starting point for future work such as optimizing the film thickness or doping by other metals

Application of optical methods to the study of convective heat transfer in rib-roughened channels

The non-invasive liquid-crystal and schlieren methods have been applied to the study of convective heat transfer in rib-roughened channels. The importance of investigating heat transfer from rib-roughened surfaces and of using non-invasive tools to gain information on thermal fields for similar complex geometries is stressed in Chapter 1. Chapter 2 is devoted to a description of main important published papers related to this project. Firstly, studies concerning heat transfer from rib-roughened surfaces in forced and free convection are listed and discussed in detail. Moreover, a literature survey on optical methods in heat transfer is presented, with special attention to the methods (liquid-crystal thermography and schlieren) applied in this study. Experiments performed by using liquid-crystal thermography are presented and discussed in Chapter 3. Three configuration geometries have been investigated: a smooth channel (having flat plates) and two different ribbed channels. The investigated heat transfer mechanism was forced convection. These experiments were performed at City University, London. Chapter 4 is devoted to experiments performed by using the schlieren optical technique. Again, the experimental study included a preliminary activity on smooth channels, followed by tests performed for rib-roughened channels. The heat transfer mechanism was natural convection. These experiments, which constitute the main body of the project, were conducted at the Dipartimento di Termoenergetica e Condizionamento Ambientale, Universita di Genova, Italy. Finally, most important conclusions are drawn in Chapter 5. Details of relationships between the recorded optical data and the thermal field (or heat transfer coefficients) are reported in Appendix Al and A2 for the liquid-crystal thermography and the schlieren method, respectively

Characteristics of forced convection heat transfer to R125 at supercritical state in a horizontal tube

In the last 100 years, the energy use has risen significantly in various sectors. Up to 42% of the worldwide energy is used in industry. However, of this share 50% could be recovered as residual heat (i.e. waste heat). Therefore, there is a huge potential for waste heat recovery from industry. However, the temperature of this heat is usually lower than 200°C and cannot be used in the classical thermal processes for electricity production. A promising technology that can convert low-to-medium temperature heat (90-250°C) into electricity is the organic Rankine cycle (ORC). The ORC is a thermodynamic power cycle that resembles the classical Clausius-Rankine cycle but instead of water it uses an alternative working fluid (e.g. refrigerants). The ORC consists of four basic components: a pump, a heat exchanger (an evaporator or a vapour generator), a condenser and an expander. The working fluid is at first pressurized and transported to the evaporator. In the evaporator the working fluid is heated at constant pressure to superheated or saturated vapour. Then there is an expansion process in the expander/turbine to extract the mechanical work. By dissipating the heat, the working fluid is condensed to saturated liquid in the condenser. The condensed liquid is pumped again to the desired pressure with which the cycle closes and the process repeats again. Even though the ORC is well known technology there is still room for improving the efficiency and the performance. One possible way to achieve this is to ensure supercritical heat transfer in the vapour generator. The heat addition in the heat exchanger occurs at a near constant pressure which is above the critical pressure of the working fluid. This means that the two-phase region of the saturation curve is omitted and the heat addition is accompanied by a temperature increase of the working fluid. The benefit lays in a reduced temperature difference between the heat source and the working fluid temperature profile. The supercritical vapour expands in the turbine/expander and generates mechanical work. The working fluid is cooled down in the condenser up to saturated liquid. Then the working fluid is pressurized with the pump and the cycle is closed. Hence, operating with supercritical pressure can lead to improved cycle efficiency. This thermodynamic cycle, with supercritical heat addition and subcritical heat rejection is called a transcritical cycle. Research activities on heat transfer to fluids at supercritical pressures started in the early 20th century. In these early works the focus was mainly on invesxix 1 tigating supercritical heat transfer of water and CO2. Furthermore, the scope of the earlier experimental investigations was limited to vertical flow directions (upward and downward flow) in small tube diameters. There are many heat transfer correlations available in literature. However, their use for practical applications is limited because they are mainly validated with the specific data that they were derived for. Furthermore, there is little information in literature available about supercritical heat transfer to refrigerants circulating in horizontal flow and in large tube diameters. Additionally, the buoyancy has a different effect in horizontal and vertical flow direction. Therefore, experimental investigation of forced convection heat transfer to refrigerants at supercritical pressure is necessary. Particularly for this thesis, a new test facility was built. The aim was performing heat transfer measurements to supercritical refrigerants under organic Rankine cycle conditions. More specifically, this means heat transfer measurements at low temperatures (heat fluxes) of the heating fluid (90-125°C). On a component level, the test facility ’iSCORe’ is similar to an organic Rankine cycle but instead of an expander and expansion valve was used. Furthermore, the configuration of the test section was a horizontal tube-in-tube heat exchanger with a counter-current flow. The working fluid was flowing in the central tube while the heating fluid in the annulus. The central tube has an outer diameter of 28 mm and inner diameter of 24 mm. Moreover, the test facility was equipped with a number of pressure and temperature sensors needed for control and measurements purposes. A number of measurements were performed by varying the inlet parameters (mass flux, heat flux, pressures). The mass flux was in the range of 400-650 kg=(m2s), the heat flux was between 14-28 kW=m2 and the pressure of the working fluid was (1.05-1.15)pcr. A real challenge when working with fluids at supercritical state is closing the heat balance over the test section. In certain operating conditions the deviation of the energy balance can reach up to 20%. This is especially noticeable when the fluid is near the pseudocritical point. Moreover, the reliability of the test facility was verified by repeating several different measurements. Based on the experimental results the supercritical heat transfer is strongly affected by the mass fluxes and the heat fluxes. Higher mass flux and lower heat flux lead to enhanced heat transfer. However, there was one deviation noticed on this trend. The heat transfer shows enhancement at lower mass flux when the fluid is close to the pseudocritical temperature. This could be due to the rapid changes of the thermophysical properties. Furthermore, at pressure closer to the critical pressure of the working fluid enhanced heat transfer is observed. Moreover, it was determined that the buoyancy effect cannot be neglected in horizontal flow. Furthermore, the results from the measurements were compared with heat transfer correlations from literature. Even though the heat transfer correlations have a correction factor in order to account for the drastic property changes, they do not show good agreement with the experimental results in the entire range. Therefore, for deriving general heat transfer correlation that will be xx Summary applicable for wider operating conditions it is important the heat transfer correlation to be validated with extensive experimental data. In conclusion, first set of measurements for supercritical R125 was obtained. The reproducibility tests prove good operation of the test facility. There were proposed suggestions for practical improvements to the test facility and to lower the uncertainties in the measurements

A Review of Recent Passive Heat Transfer Enhancement Methods

[EN] Improvements in miniaturization and boosting the thermal performance of energy conservation systems call for innovative techniques to enhance heat transfer. Heat transfer enhancement methods have attracted a great deal of attention in the industrial sector due to their ability to provide energy savings, encourage the proper use of energy sources, and increase the economic efficiency of thermal systems. These methods are categorized into active, passive, and compound techniques. This article reviews recent passive heat transfer enhancement techniques, since they are reliable, cost-effective, and they do not require any extra power to promote the energy conversion systems' thermal efficiency when compared to the active methods. In the passive approaches, various components are applied to the heat transfer/working fluid flow path to improve the heat transfer rate. The passive heat transfer enhancement methods studied in this article include inserts (twisted tapes, conical strips, baffles, winglets), extended surfaces (fins), porous materials, coil/helical/spiral tubes, rough surfaces (corrugated/ribbed surfaces), and nanofluids (mono and hybrid nanofluids).Ajarostaghi, SSM.; Zaboli, M.; Javadi, H.; Badenes Badenes, B.; Urchueguía Schölzel, JF. (2022). A Review of Recent Passive Heat Transfer Enhancement Methods. Energies. 15(3):1-55. https://doi.org/10.3390/en1503098615515