Academic literature on the topic 'Thermal micro-stratification'

Thermal microstructure profiling is an established technique for investigating turbulent mixing and stratification in lakes and oceans. However, it provides only quasi-instantaneous, 1-D snapshots. Other approaches to measuring these phenomena exist, but each has logistic and/or quality weaknesses. Hence, turbulent mixing and stratification processes remain greatly under-sampled. This paper contributes to addressing this problem by presenting a novel analysis of thermal microstructure profiles, focusing on their multi-scale stratification structure. Profiles taken in two small lakes using a Self-Contained Automated Micro-Profiler (SCAMP) were analysed. For each profile, buoyancy frequency (N), Thorpe scales (LT), and the coefficient of vertical turbulent diffusivity (KZ) were determined. To characterize the multi-scale stratification, profiles of d2T/dz2 at a spectrum of scales were calculated and the number of turning points in them counted. Plotting these counts against the scale gave pseudo-spectra, which were characterized by the index D of their power law regression lines. Scale-dependent correlations of D with N, LT and KZ were found, and suggest that this approach may be useful for providing alternative estimates of the efficiency of turbulent mixing and measures of longer-term averages of KZ than current methods provide. Testing these potential uses will require comparison of field measurements of D with time-integrated KZ values and numerical simulations.

Purpose The purpose of this paper is to consider the problem of thermal destratification facing engineers and scientists during the motion of fluids which consist of rigid and randomly oriented particles suspended in a viscous medium under the influence of Lorentz force. This paper provides an insight into the non-linear transfer of thermal radiation within the boundary layer. Design/methodology/approach Similarity transformation and parameterization of the non-linear partial differential equation are carried out. The approximate analytical solution of the governing equation which models the free convective flow of strong and weak concentration of micro-elements in a micropolar fluid over a vertical surface is presented. Findings It is observed that the velocity and temperature distribution are decreasing properties of thermal stratification parameter St. Maximum local skin friction coefficients are ascertained at an epilimnion level (St=0) when the magnitude of thermal radiation is small. Thermal stratification parameter has no significant effect on the temperature distribution in the flow near a free stream. Originality/value The relationship between stratification of temperature and the transfer of thermal energy during the problem of thermal destratification facing engineers and scientist during the motion of fluids which consist of rigid and randomly oriented particles suspended in a viscous medium under the influence of Lorentz force is unravelled in this paper.

In this work 10 years of data (1986-1996) from a fixed station located in the northern part of Valparaíso Bay (33º00’S; 71º35’W) were analysed to study the influence of coastal upwelling activity on the temporal variation of micro-phytoplankton (20-200 μm) and their relationship with oceanographic conditions. The upwelling activity at the bay was associated to semi-annual wind regime with an intensification of upwelling-favourable S-SW winds from September to March followed by a decrease and the occurrence of downwelling events from April to August. Oceanographic conditions showed the ascent of cold, nutrient-rich salty water in spring (September-November). However, during summertime under highest upwelling index, thermal stratification conditions were registered. This stratification might be associated to either the solar radiation or the presence of an upwelling shadow area in the bay. The upwelling period had the highest micro-phytoplankton abundance mainly dominated by diatoms. This period was associated with an increase in biomass and richness in the bay. Meanwhile during non-upwelling period —under homogenous conditions of temperature, salinity and nutrients— an increase in diversity (but low abundance and richness) associated to dinoflagellates and silicoflagellates was noted. Therefore, the results suggest the presence of a bi-modal regime of micro-phytoplankton in the bay in response to changes in oceanographic conditions related to local wind forcing and mixing/stratification.

Micro-scale cold-air flow along a gentle slope was analyzed using thermal infrared imaging (TIR), focusing exclusively on the lowermost 2 m above ground. Cold-air pulses were analyzed with regard to their vertical temperature stratification as well as flow characteristics, such as flow speed. Analyses on the transition zone between the near-surface very stable inversion layer and the less stable, warmer air above highlight turbulent situations and detrainment effects at the cold-air inversion top. Using thermal imaging in a high spatiotemporal resolution with up to 90 vertical data points and TIR pixels for 1.5 m cold-air depth, a high-precision cold-air flow analysis was realized.

The present work examines the effect of different magnetic nanoparticles and the heat transfer phenomena over the stretching sheet with thermal stratification and slips effect. The mixture of water (H 2 O) and ethylene glycol (C 2 H 6 O 2 ) is used as base fluid whereas the paramagnetic, diamagnetic, and ferromagnetic ferrites are taken as nanoparticles. In the presence of ferrite nanoparticles, the magnetic dipole has a significant effect in controlling the rate of heat transfer and the thermal boundary layers. By using suitable similarity transformations, the system of partial differential equations is transformed into nonlinear ordinary differential equations. The numerical solution of resulting equations is found out by using the variational finite element method. The effect of numerous emerging parameters on velocity, temperature, and micro-rotation velocity are represented graphically and analyzed numerically. It has been noticed that comparatively the diamagnetic ferrites have gained maximum thermal conductivity relative to the other nanoparticles. It was also observed that the thermal conduction of nanoparticles increases with the variation of volume fraction. Moreover, with increasing values of thermal stratification the thermal boundary layer thickness decreases and the heat transfer rate increases at the surface. Furthermore, the validation of code and the accuracy of the numerical technique has been confirmed by the assessment of current results with earlier studies.

Materials compatibility and durability of advanced salt/alkali metal slurry thermal energy storage systems has been demonstrated [1]. Applications are being evaluated for both space and terrestrial solar thermal power conversion [2]. High energy density of these thermal storage systems is achieved by colocation of heat input and extraction within the slurry mixture which is overwhelmingly phase-change salt. This paper addresses performance testing of these systems. Understanding of mechanisms of both micro and macro stratification of the slurry is necessary to fully predict system performance as a function of gravity and system geometry. If it can be shown the gravity stratification effects are secondary to a combination of: (1) liquid metal film adhesion (wetting) to the heat exchange surfaces and solidified salt particles, (2) solubility of alkali metal in the salt-rich phase, and (3) stirring produced by liquid to vapor conversion of the alkali metal, then system geometry limitations are greatly relaxed for space application. Performance testing was accomplished using a sodium heat pipe to transfer heat from the slurry canister to a gas gap calorimeter. Testing was accomplished with the heat pipe installed only in the vapor space above the alkali metal/salt slurry and with an increase heat pipe and minimum vapor space. This testing conclusively demonstrated the effectiveness of the pseudo-heat-pipe type heat transfer mechanism operating in the slurry system under terrestrial conditions.

AbstractThis paper is concerned with the effect of thermally stratification on the steady, two-dimensional mixed convection flow of a micropolar fluid past a vertical stretching permeable surface saturated in porous medium taking into account the effect of thermal radiation. The governing system of partial differential equations describing the problem are converted into a system of non-linear ordinary differential equations using similarity transformation. The resulting system of coupled nonlinear ordinary differential equations is solved numerically using the Chebyshev spectral method. The numerical results for the velocity, the micro-rotation and the temperature are displayed graphically showing the effects of various parameters like the buoyancy parameter, the radiation parameter, the stratification parameter, the permeability parameter and the suction/injection parameter. Moreover, the numerical values of the local skinfriction coefficient, the wall couple stress and the local Nusselt number for these parameters are also tabulated and discussed.

The paper presents an application concept for PV/T - Photovoltaic Thermal Technology in moderate climates (such as the Polish climate), at a micro scale, i.e. for a single family house. The paper analyses the operation of a PV/T system applied to Domestic Hot Water – DHW heating and electricity production. A mathematical model of the system operation has been developed. The paper focuses on modeling thermal and electrical efficiency of photovoltaic/thermal - PV/T modules. It also briefly presents the governing equations for the thermal energy balance of a storage tank, where thermal stratification effects take place. Some selected results of the numerical simulation of the PV/T system operation are described. Daily distribution of hourly averaged thermal and electrical efficiency of the PV/T modules without cover and with one glazing are presented. The PV/T systems do not give significant thermal energy output in winter. PV/T modules without glazing do not supply heat at all for three winter months, their highest thermal efficiency is in summer and it can be nearly 15%. In the same period glazed modules have efficiency equal to nearly 24%. However, the unglazed modules can give much more electrical energy in summer than those with glazing, and the electrical efficiency can reach the levels of 11.4% and 9.4%, respectively. In winter the difference is smaller, i.e. for unglazed the efficiency is 12.2%, and for glazed 11.2%.

Understanding hydrodynamic and biogeochemical processes in lakes is fundamentally important to the management of phytoplankton population and the improvement of water quality. Physical processes such as wind-driven surface mixing, thermal stratification and differential heating and cooling can affect the distribution of water, phytoplankton and sediments and the availability of nutrients and light. These lake processes, which are highly variable in space and time, affect phytoplankton dynamics in the field. This study aims to determine the spatial and temporal variability of phytoplankton and processes that either contribute to or override the variability in the artificially mixed Myponga Reservoir, South Australia. A sediment survey showed that sediments underlying deep water were richer in organic matter, carbon, nitrogen and phosphorus than the sediments underlying shallow water. This may lead to different nutrient release rates between the shallow and deep areas. Both sediment resuspension and anoxic sediment nutrient release were important internal sources of nutrient to support phytoplankton growth in summer when external nutrient supplies were limited by low rainfall in the catchment. An analysis of historical water temperature data revealed the development of micro-stratification at the sediment-water interface in summer, especially during a heatwave (air temperature > 40ºC for several consecutive days). Prolonged micro-stratification could potentially induce anoxic layers at the sediment surface, resulting in the release of nutrients. A risk assessment was conducted to predict the release of phosphorus from anoxic sediments and to evaluate the potential impact of cyanobacterial population (Anabaena circinalis) and the release of secondary metabolites (e.g. saxitoxin and geosmin). Spatial variability of surface mixed layer depths exists between the side-arm and main basin. A simple light model based on the relationships of surface mixed layer depth, daily light dose and phytoplankton growth rate, was developed to estimate the potential variation of phytoplankton population in the two different light habitats (the main basin and side-arm). The model showed that phytoplankton abundance in the main basin was lower than in the side-arm. However, differential heating drove a large basin-scale convection, which circulated the water between the side-arm and main basin within hours. This circulation overrode the time scale of days for the light-dependent growth effect between the two sites and hence there was no observable change in phytoplankton community structure. Although no spatial variability of phytoplankton was observed at community level, significant variations of phytoplankton cellular content and stoichiometry were detected. Higher carbon cellular content in the side-arm than in the main basin was probably due to a greater exposure to light (shallower surface mixed layer in the side-arm) for photosynthesis. In the situation where nutrients were scare, higher phosphorus cellular content was found in the side-arm than in the main basin; this was possibly due to a greater exposure to resuspended nutrients from the lake bottom (shallower water in the side-arm). There was also a strong seasonal pattern in phytoplankton cellular content and stoichiometry between summer and early winter of 2009. The carbon content of phytoplankton increased over time, while the phosphorus content decreased. After the first heavy rain event (70 mm over a four-day period) in early May, carbon cellular content decreased, while phosphorus cellular content increased. These changes in phytoplankton contents were most likely related to the bio-availability of phosphorus in water. This study reviews many complex, interactive processes driving the variability of lake physics and chemistry. The variability can yield rapid biological responses at physiological and cellular levels (e.g. Fv:Fm and cellular content), but does not necessarily appear at community levels (e.g. phytoplankton biomass, diversity). Often, conventional monitoring in lakes and reservoirs overlooks the subtle variability of phytoplankton dynamics. The relative scaling among physical, chemical and biological processes, therefore, is important to adequately describe the spatial and temporal variability in lakes and reservoirs.
Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 2011

Abstract We present a switched linear system approach for modeling the complex nonlinear dynamics associated with temperature inversion occurring in thermally stratified hot water tanks. Such tanks are commonly used for thermal energy storage, particularly in low- to medium-temperature waste heat recovery applications. By separating the influence of temperature inversion from the internal heat transfer between states in the governing differential equations, we paramaterize the nonlinearity using a vector of discrete variables. This vector is then used to define the switching between a set of linear, discrete time models. The proposed switched model is validated against a reduced-order nonlinear model of the thermal energy storage and then integrated with a fuel cell model to capture the dynamics of a micro-combined heat and power system. Simulation results demonstrate the importance that temperature inversion has on the stratification dynamics which in turn has implications for control of such systems.

Phase change microencapsules are the microsized particles made of phase change materials (paraffin wax ect.) sealed by the thin shell (polymer ect.) via the methods of microencapsulation. During last decade, due to the large amount of melting/solidifying heat, much attention have been paid on their application in environmental control, building, textiles and electronics ect. Also the novel thermal fluids by phase change microencapsules suspending in the traditional thermal fluids have shown their superior heat storage density and convective heat transfer performance, which can behave as heat storage media and heat transfer media simultaneously. However, the density difference between the phase change microencapsules and tranditional unitary fluid would lead to the unstable suspending states which seriously affect the heat storage and heat transfer performance. Binary mixtures such as alcohol-water etc have already played the important roles in the heat transfer equipments. In this paper, binary propanol-water mixtures of various proportion were formulated as the base fluids, and their stabilities were studied. The result shows that binary propanol-water mixtures with the desity of 941kg/m3 showed the best stability and no stratification was found after standing for 48 hours. The morphology and diameter distribution of the microencapsule particles were tested by the scanning electron microscope (SEM) and Malvern Nanosizer respectively, and the result show that the diameter of the particles is in the range of 10–80μm with the average value of 26.4μm. The phase change enthalpy and the effective heat capacity of phase change microencapsule suspensions with the concentration of 10–40wt% were measured by the differencial scanning calorimeter (DSC) and it was found the phase change enthalpy of the phase change microencapsule is 152.8J/g and the undercooling is only 7.3°C. The effect of concentration and temperature on the rheological behavior and viscosities of suspensions were experimentally studied by the TA DHR-G2 rheometer. The result shows that the suspensions behave as Newtonian fluids even when the concentration is as high as 40wt% and the viscosities fit well with Vand model. By the Hot Disk 2500S thermal constant analyzer (Sweden), the thermal conductivities of 0–40wt% suspensions were tested at 20–70°C and the variation was analyzed further. The concentration and expansion of MPCM particles during the phase change period were found to affect the thermal expansion coefficient of the MPCM suspensions obviously. The above experimental result and analyzation of stability and thermophysical properties will provide a complete and important data for the application in heat storage and heat transfer.