Search

A study was conducted to simulate the temperature profiles during readout in a typical, commercial thermo-luminescence dosimeter (TLD) chip and to optimize the readout conditions. The study makes use of a previously developed numerical model which calculates the crystal's temperature profile evolution inside a TLD crystal compound. The calculated profiles were implemented in the Randall-Wilkins equation to obtain the estimated glow curve. A number of jet temperature profiles were investigated in order to optimize the readout process.

The current numerical study focuses on the feasibility of furnishing thermal comfort in a structure with windows, at a mid-storey in a multi-storey building in winter. The performance of phase-change paraffin wax (PCM), as a heat source in winter, is investigated. The wax melts at 27 °C. It is stored under the floor tile as a 20 mm thick layer. At night the wax is heated up to its complete melting, using off-peak electricity. At day-time the latent heat is released into the space above the floor and the wax solidifies. The simulations are performed relating to temperature variations outside and inside, coupled with heat conduction and accumulation in walls, radiation between the surfaces, natural convection of air inside, and solidification or melting of the wax for 24 h and several days. The process is analyzed with and without fins and at two power inputs in the structure with windows, and compared to the windowless structure. The electrical power required to preserve thermal comfort in the structure with windows is 20% higher. All the simulations were conducted 3-dimensionally for structures in a subtropical climate.

Semi-clathrates of tetrabutylammonium bromide (TBAB) are investigated for their potential application in the CO2 capture context based on hydrate technology. The three-phase lines of semi-clathrates of CO2-TBAB-H2O and N2-TBAB-H2O are established simultaneously with their structure using in situ Raman scattering performed at high pressure. The preferred crystal phase obtained at ambient pressure from solutions of 5 and 40 wt % TBAB initial concentrations is shown to change upon enclathration of CO2 or N2, or by applying a higher pressure on the system. Deep in the stability field, metastable hydrate phases are occurring at the onset of the formation and correspond to the ones expected under ambient pressure conditions. Depending on the pressure, they progressively transformed into the most stable ones when approaching equilibrium and dissociation points. Besides, it is shown that a 5 wt % TBAB original solution forms preferentially a mixed structure of both type B and type A at low gas pressure with CO2 as the guest gas. A new structure is spectroscopically characterized at pressures higher than ∼2 MPa CO2. Type A is demonstrated to be stable at 5 wt % initial TBAB concentration with N2 as the guest molecule and pressure between 8 and 12 MPa. These structural data address new insights on the relationship between the hydrophilic-anion and hydrophobic-cation intercalation with a guest gas producing hydrophobic interaction in a distorted water lattice.

In the present study, the effects of close-contact melting (CCM) are studied for geometries suitable for latent heat thermal energy storage. Specifically, a vertical double pipe concentric storage unit with circumferentially finned inner tube is explored experimentally. It is demonstrated that CCM enhances significantly the heat transfer rate, shortening the melting time by almost 2.5 times in that specific laboratory-scale device. These results show that it is favorable to supply heat to the outer shell of a latent-heat storage unit in order to initiate close-contact melting and achieve higher heat transfer rates. In order to analyze the process more closely, a single-cell vertical enclosure containing a phase-change material (PCM) is explored. A numerical model, which combines an enthalpy method with CCM modeling, is developed and validated experimentally. The solid bulk of the PCM is allowed to sink, thus enabling close-contact melting on the non-isothermal fin surface. The fins are thus much more important than just extended surfaces for heat transfer enhancement. The agreement between the numerical predictions and experimental findings is very good both in terms of the total melting time and instant melting patterns. The validated numerical model is further used for a detailed study, in which time dependent melt fractions and heat transfer rates are obtained for various temperature conditions. In all cases, the findings are compared with a simplified analytical model which accounts for the close-contact melting only, revealing effects not predicted by common CCM modeling approaches in the literature. The analytical model yields theoretical expressions for the time-dependent melt fraction, heat transfer rate and molten layer thickness in a dimensionless form. For the conditions of the present study, the dimensional analysis indicates that the melt fraction depends on the Fourier and Stefan numbers combined as FoSte 3/4 only, whereas the Nusselt number and the normalized layer thickness both depend also on the same additional group, Ste 1/4 . Based on these findings, the numerically calculated melt fractions, Nusselt numbers and layer thicknesses are generalized completely, showing a remarkable agreement.

An experimental and numerical study was conducted to elucidate local heat transfer processes under multiple microscale jet impingements using a dielectric coolant, HFE-7000. The micro device used in this experiment was made of a 400-μm thick silicon wafer, a 210-μm thick vinyl sticker, and a 1-mm thick Pyrex substrate. Fourteen jet orifices were etched using deep reactive ion etching (DRIE) on the silicon wafer, and four 100-nm thick resistance temperature detectors (RTDs) and a heater were fabricated from titanium on the Pyrex substrate. The double-sided vinyl sticker was used to bond the layers and to form a micro fluidic channel having dimensions of 1.9 mm × 14.8 mm × 210 μm. Jet Reynolds numbers in the experimental study ranged from 162 to 4057 and nominal heat fluxes ranged from 10 W/cm2 to 80 W/cm2. A three-dimensional numerical model was developed to predict the jets hydrodynamics and the convection heat transfer coefficients using a turbulent flow model for the turbulent range of the flow. Good agreement was found between the numerical predictions and experiments. The numerical results also provided valuable insight into the flow patterns formed due to multiple jet interactions. Average Nusselt number and pressure drop coefficient values found in the present study correspond well with the existing correlations pertinent to jet array impingement.

An authoratative analysis of researchers' theories on the Sumerian culture, language, and historical origins. (JB)

The present work deals with a combined device in which the heat may be both absorbed by a phase-change material (PCM) and dissipated to the ambient air. Accordingly, the heat sink, made of aluminum 6061, has compartments where the PCM (eicosane C 20 H 42 ) is stored and fins exposed to the air. The experiments are performed in both room- and elevated-temperature environment. A simplified thermal model, based on a two-dimensional enthalpy formulation for the PCM, is developed for a conservative estimation of temperature evolution. The results, presented in terms of base temperature (experimental and numerical) and PCM melt fraction (predicted), show a fairly good agreement between the experiments and simulations. Also shown are some findings obtained using a more complex model, which accounts for convection in the melt. A dimensional analysis yields the melt fraction and Nusselt number as functions of the Fourier and modified Stefan numbers. It also addresses the relative contributions of latent/sensible accumulation and heat transfer to the air. The dimensionless curves for latent-heat-based accumulation rate are rather similar in various cases. On the other hand, it is found that the share of sensible-heat-based accumulation rate tends to increase when the heat input increases.

The current numerical study focuses on the feasibility of furnishing thermal comfort in a structure, by using paraffin wax stored on a plate below the ceiling in a multi-storey building. The method is aimed to reduce energy demands at the increasing thermal loads. In summer, in daytime, walls of the building are exposed to the ambient thermal load, and heat transferred inside is absorbed by the melting wax. The study is numerical. It relates to temperature variations outside and inside, coupled with heat conduction and accumulation in walls, with radiation between the surfaces, with natural convection of air inside and melting of the wax at the ceiling. Fins spacing on the storage plate, visualization of the melting process, and its parametric investigation provide an insight into the physical phenomena. Temperature and flow fields were investigated for 3 mm and 12 mm thick layers of wax. At the specified conditions of the present study a 3 mm layer provides thermal comfort for most of the day, while a 6 mm layer may suffice for the entire day. Fluent 6.3 software was used in the computations.

GABA and glycine are the primary fast inhibitory neurotransmitters in the mammalian spinal cord, but they differ in their regulatory functions, balancing neuronal excitation in the locomotor circuitry in the mammalian spinal cord. This review focuses on the unique role of GABAergic transmission during the assembly of the locomotor circuitry, from early embryonic stages when GABA(A) receptor-activated membrane depolarizations increase network excitation, to the period of early postnatal development, when GABAergic inhibition plays a primary role in coordinating the patterns of locomotor-like motor activity. To gain insight into the mechanisms that underlie the dominant contribution of GABAergic transmission to network activity during that period, we examined the morphological and electrophysiological properties of a subpopulation of GABAergic commissural interneurons that fit well with their putative function as integrated components of the rhythm-coordinating networks in the mouse spinal cord.

The present study deals with single bubble growth on an uneven wall. A model problem is defined and solved using a three-dimensional numerical simulation. The wall has the shape of a triangular cavity and feature vortices. The equations solved in the present study are based on macro region modelling of the bubble alone and describe its growth from the initial state to detachment from the surface and consequent motion. The model includes a simultaneous solution of conservation equations for the liquid and gaseous phases, in conjunction with three-dimensional interface tracking. The latter is achieved using the level-set method. The numerical modeling includes the multi-grid method. The complete three-dimensional model is discretized using an original in-house numerical code realized in MATLAB. Different cases of bubble growth on the triangular cavity walls are investigated. The main conclusion from the calculations is that the bubble shape and its growth rate strongly depend on its location and on the channel orientation. New features, not possible for flat walls and special for this case, are revealed and discussed. It is demonstrated that under certain conditions, the bubble is obstructed by the surface geometry. It is also shown how a growing bubble affects the flow field inside a cavity, interacting with the vortex structure.

This research is an experimental investigation of a double-pipe heat storage unit. The inner pipe of the unit, through which a heat-transfer fluid (HTF) is supplied, is made of aluminum and has an outer helix-like fin. The annular space between the pipes is filled with a phase change material (PCM). Actually, this research presents a novel design of the heat storage unit, which, unlike traditional designs with e.g. radial (circumferential) or longitudinal fins, has a single fin which does not divide the shell volume into separated cells. Moreover, this research focuses on close-contact melting (CCM), a process which is characterized by detachment of the solid bulk from the unit envelope and its sinking towards the hot fin surface. In previous investigations, performed in our laboratory, this effect has been achieved in units with above-mentioned traditional fin configurations. It was demonstrated that CCM reduces the overall melting time, i.e. the rate of unit charging, significantly as compared with commonly encountered melting in which the fins serve just to enlarge the heat transfer area. The experimental system employed in this study includes a vertically-oriented double-pipe heat storage unit and thermostatic baths capable of providing hot or cold HTF. The unit has a transparent Perspex shell which makes visualization possible. The entire unit may be placed in a heated water tank with transparent walls. In the latter case, close-contact melting is achieved by detaching the solid phase from the envelope and thus allowing its gravity-induced motion. Regular melting is compared to CCM and advantages of the latter are demonstrated. Also demonstrated are the advantages of the novel fin, including in solidification. Possible mathematical and numerical modeling of the melting processes is discussed.

Experimental and simulation studies were performed to reveal local heat transfer coefficients under jet impinging in micro domain with Nitrogen gas. The experimental device was made of a 500 μm thick Pyrex and 400 μm thick silicon wafers. On the Pyrex wafer, four 100 nm thick resistance temperature detector (RTD) thermistors and a heater were fabricated from titanium. Jet orifices were etched by deep reactive ion etching (DRIE) on a silicon wafer, which was attached to the Pyrex wafer through a vinyl sticker (250 μm thick). A 1.9 mm × 14.8 mm × 250 μm micro channel was formed by laser drilling into the sticker. Varying flow rates of Nitrogen gas and heat fluxes of the heater, temperatures of the four thermistors were collected and local heat transfer coefficients were inferred enabling to divulge the jet impinging cooling characteristics. Initial simulations were used to complement experiments and to obtain detailed flow patterns of the jet, temperature distribution on the heater area, and fluid temperature distribution.

The present study deals with a new numerical approach for solid-liquid phase-change modeling. The new model is based on the enthalpy method and takes into account natural convection in the melt which can be coupled to the solid bulk sinking motion by the force balance on the solid bulk. A basic configuration is investigated, namely a two-dimensional rectangular cavity with a constant wall temperature. The effect of rather low values of the Archimedes and Rayleigh numbers on the sinking motion of the solid bulk, the melting rate and melting patterns, is explored. It is found that the density difference, between the solid and liquid phases, for the studied case does not affect considerably the results. However, it is shown that by natural convection alone the solid sinking motion is established, the melting rate is enhanced and the melting patterns are completely different in comparison with the first studied case.