Vertical Thermal Stratification Research Articles

The impact of internal gains on thermal stratification for public buildings

The indoor temperature in public buildings varies according to its internal loads scenario. In fact, this building typology in temperate climates is generally non-air conditioned and naturally ventilated. In order to improve indoor thermal conditions with appropriate solutions, it is necessary to study the vertical temperature profile. The main objective of this study is to show the impact of thermal stratification on thermal comfort of public buildings. The thermo-airodynamic modeling using TRNSYS(C) and CONTAM(C) have been performed in this numerical simulation study. The indoor air volume is vertically subdivided into several levels to evaluate the evolution of temperature profile taking into account the different inside-outside air exchange and air coupling between sub-zones. This study is performed in the city of Morocco that represents Mediterranean climate region with a moderate temperature. The effect of several parameters, such as indoor occupation pattern, internal lighting, sub-zones air exchange and vertical thermal stratification are studied. Numerical simulation results show that thermal stratification of buildings affects the occupant’s comfort level

Modified carbon‐dioxide measurement to predict the heat release rate of fire burning in a compartment based on the three‐zone model

SummaryThe heat release rate (HRR) of fuels has been described as the single important variable of fuels in fire hazard, and the HRR experimental measurement remains a key issue in fire science. A modified carbon‐dioxide generation (CDG) method, applying a three‐zone smoke model, is developed to predict the HRR of gas, liquid, and solid fuel fires. The three‐zone smoke model with three layers is determined by the vertical thermal stratification, and their physical thermal properties are computed. The application of modified method on typical gas fuel, liquid fuel, and simple solid‐fuel fires is verified. The prediction accuracy is examined quantitatively by the cosine similarity comparison of predicted results with the experimental data. In addition, the ventilation effects on the predicted results are also explored. Results show that the application of three‐zone model improves the HRR prediction accuracy, because it can accurately capture the mixing behavior from the upper layer to the lower layer. The effect of ventilation on modified CDG method is positive as the ventilation enhances the smoke mixing and the smoke distribution in each layer is relatively uniform.

Study of marine waters hydrological characteristics near the Zmiinyi Island in 2016-2017

Purpose. Results of hydrological characteristics observations in the Zmiinyi Island coastal waters carried out by research group of the Regional Center for Integrated Monitoring and Environmental Studies (Odessa National I.I. Mechnikov University) in 2016-2017 have been summarized. Methods. Primary data on transparency, temperature and salinity of marine water have been collected using standard methods. Data processing, calculation of statistics, producing of graphs and maps have been carried out using Excel and ArcGIS software. Results. Temporal and spatial distributions of transparency, temperature and salinity of marine waters near the Zmiinyi Island for the period from April 2016 to December 2017 have been presented and analyzed. Using the results of analysis of the accumulated experimental data on transparency, temperature and salinity of sea water the peculiarities of those characteristics’ seasonal changes have been revealed. Conclusions. Impact of desalinated water masses advection from the Danube River mouth and from the North Western part of the Black Sea has been revealed. Well-visible seasonal variation was revealed in the sea water temperatures distribution near the Zmiinyi Island in 2016-2017, which reflected seasonal changes in radiation balance and heat exchange processes at the sea / atmosphere boundary, as well as the influence of water masses advection from other areas of the sea. It was shown that salinity seasonal variation in the sea water near the Zmiinyi Island, which ranged from 7.48 to 18.34 PSU, in 2016-2017 had the disturbance as the result of migration of the frontal zone of the Danube River waters distribution in the Zmiinyi Island area, as well as advection of desalinated water masses from the NWBS. Forming of stable two-layer vertical density stratification of the Zmiinyi Island coastal waters in spring-summer periods of 2016-2017 has been traced. An earlier (compared to Odessa Bay) spring vertical thermal stratification of water (May) and smoother, till late autumn, seasonal temperature decrease were revealed.

On the calculation of heat migration in thermally stratified environment of large space building with sidewall nozzle air-supply

Vertical thermal stratification is typical for partially conditioned large space building. Heat accumulation in the upper part of unoccupied zone will have significant effect on the cooling load of occupied zone. In order to accurately estimate this cooling load, calculation of the heat migration occurs in the thermally stratified environment is critical. Heat migration in this study refers to the amount of heat transferred from unoccupied zone to occupied zone (or between other adjacent zones). This paper discusses the method to calculate heat migration through convection and conduction in a large space building served by side wall nozzle air-supply system. In a scaled laboratory, three experiment cases with different exhaust air flow ratio are investigated. CFD simulations of the same cases are performed. Vertical temperature distribution and detailed cooling load results of the whole system as well as those of occupied zone are validated against experiment result. A detailed analysis on the total heat migration and its components between different zones is conducted. Inter-zonal heat transfer coefficient Cb used in a zonal BLOCK model and heat conductivity across the intersection of zones is obtained by dividing the whole space into two and four zones vertically. It is found that both Cb and thermal conductivity value across the intersection is greater than expected and affected by the local turbulent intensity.

Study on natural ventilation driven by a restricted turbulent buoyant plume in an enclosure

Vertical thermal stratification will be produced by buoyant plumes in a naturally ventilated enclosure. The buoyant plume located near the wall or corner of a ventilated enclosure will be restricted by the wall or corner during the rising process. Theoretical models are deduced and validated to quantitatively predict the thermal stratification height of an enclosure containing a buoyant plume restricted by a wall or corner. The theoretical models can describe the relationship between the thermal stratification height and the height difference between the top and bottom openings of the enclosure, the effective area of the top and bottom openings of the enclosure and the distance between the plume and the wall or the corner. Thresholds for the distance between the plume and the side boundary are defined to determine whether the plume is restricted by the side boundary. Simultaneous equations containing the enclosure dimensions are proposed to predict the thresholds. These results could be used as a reference on natural ventilation design for buildings containing restricted heat sources to make full use of the effect of thermal plumes and create a suitable indoor environment.

Study of an integrated personalized ventilation and local fan-induced active chilled beam air conditioning system in hot and humid climate

A novel integrated personalized ventilation and local fan-induced active chilled beam (PV-ACB) air conditioning system is proposed and analyzed through energy and Computational Fluid Dynamics (CFD) simulation methods. Energy performance and indoor environmental conditions of this system are investigated via Integrated Environmental Solutions (IES) and ANSYS Fluent respectively. Energy simulation results illustrate that PV-ACB system, compared with the conventional variable air volume (VAV) system, could achieve 16% energy savings at 100% peak cooling load. A maximum 42% energy savings is obtained at 25% part cooling load, indicating the superior energy saving potential under part load conditions. Vertical thermal stratification is observed in CFD simulation results, 23–24 °C at the occupant level (0–2 m) and 26–27 °C in the upper zone (2–4 m). Horizontally, temperature around occupants is 22–23 °C, lower than the temperature of 26–27 °C in other areas, demonstrating the horizontal temperature difference caused by the conditioned outdoor air from personalized ventilation. Both energy and CFD simulation results illustrate that this novel PV-ACB air conditioning system is able to achieve considerable energy savings, superior occupant thermal comfort and enhanced indoor air quality (IAQ).

Experimental study on the characteristics of secondary airflow device in a large enclosed space building

Large space building using side wall nozzle air-conditioning system may result in poor ventilation effectiveness for positions out of the reach of primary supply air. A secondary airflow device is proposed to be applied in such system to improve the uniformity of temperature and velocity distribution in the occupied zone under cooling condition. The characteristics of the secondary airflow device, including central-line velocity decay and jet trajectory model were investigated through experiments to provide fundamental data for the system design. The indoor thermal environment parameters were measured and compared between with and without the secondary airflow device system in a large space factory workshop conditioned by side wall air supply nozzles. Experimental results with the secondary airflow device showed that the vertical thermal stratification height slightly changed, with lower air temperature in unoccupied zone and higher temperature in occupied zone comparing to the cases without secondary airflow device. At occupied level, air diffusion performance index (ADPI) significantly improved, while the percentage dissatisfied (PD) due to draught decreased in the occupied zone. Air velocity uniformity at occupied level also significantly increased. The results of this study provide an exemplary reference for the design of the air distribution system in large-space buildings.

Experimental investigation of BWR Suppression Pool stratification during RCIC system operation

In Boiling Water Reactor (BWR) nuclear power plants with the Mark I containment, the condition of the Suppression Pool can be a large influence on overall plant safety. When the Reactor Core Isolation Cooling (RCIC) System is operating, steam from the reactor drives the RCIC turbine and is then exhausted to the Suppression Pool. When subcooled, the pool can readily condense the steam, warming it up in the process. However, if hot spots or thermal stratification appear, this can limit the Suppression Pool’s ability to perform its safety functions, and can be a limiting factor for RCIC System operation.In order to better understand the RCIC system and its true limits of long-term operation, an experimental model of the system was constructed at the Laboratory for Nuclear Heat Transfer Systems at Texas A&M University (TAMU). These tests provide confirmation of thermal stratification in the Suppression Pool from RCIC System operations, and show a significant degree of dependence on pressure in the airspace above the pool. In the TAMU facility, vertical thermal stratification was limited to 21°C when fully vented to atmospheric pressure, while pre-pressurization led to stratification well in excess of 60°C.

Characterisation of coastal counter-currents on the inner shelf of the Gulf of Cadiz

At the Gulf of Cadiz (GoC), poleward currents leaning along the coast alternate with coastal upwelling jets of opposite direction. Here the patterns of these coastal countercurrents (CCCs) are derived from ADCP data collected during 7 deployments at a single location on the inner shelf. The multiyear (2008–2014) time-series, constituting ~18months of hourly records, are further analysed together with wind data from several sources representing local and basin-scale conditions. During one deployment, temperature sensors were also installed near the mooring site to examine the vertical thermal stratification associated with periods of poleward flow. These observations indicate that the coastal circulation is mainly alongshore and barotropic. However, a baroclinic flow is often observed shortly at the time of flow inversion to poleward. CCCs develop all year-round and exclusively control the occurrence of warm coastal water during the upwelling season. On average, one poleward flow lasting 3days was observed every week, corresponding to CCCs during ~40% of the time without seasonal variability. Thus, the studied region is distinct from typical upwelling systems where equatorward coastal upwelling jets largely predominate. CCCs often start to develop near the bed and are frequently associated with 2-layer cross-shore flows characteristic of downwelling conditions (offshore near the bed). In general, the action of alongshore wind stress alone does not justify the development of CCCs. The coastal circulation is best correlated and shows the highest coherence with south-eastward wind in the basin that proceeds from the rotation of southward wind at the West coast of Portugal, hence suggesting a dominant control of large-scale wind conditions. In agreement, wavelet analyses indicate that CCCs are best correlated with alongshore wind occurring in a band period characteristic of the upwelling system (8–32days). Furthermore, in the absence of wind coastal currents tend to be poleward during summer. This set of observations supports that CCCs develop in response to the unbalance of an alongshore pressure gradient during the relaxation of (system-scale) upwelling-favourable winds, oriented south-eastward in the basin. The relaxation periods defined based on this wind direction show a good correspondence with the periods of poleward flow.

Evolution of a physical and biological front from upwelling to relaxation

Fronts influence the structure and function of coastal marine ecosystems. Due to the complexity and dynamic nature of coastal environments and the small scales of frontal gradient zones, frontal research is difficult. To advance this challenging research we developed a method enabling an autonomous underwater vehicle (AUV) to detect and track fronts, thereby providing high-resolution observations in the moving reference frame of the front itself. This novel method was applied to studying the evolution of a frontal zone in the coastal upwelling environment of Monterey Bay, California, through a period of variability in upwelling intensity. Through 23 frontal crossings in four days, the AUV detected the front using real-time analysis of vertical thermal stratification to identify water types and the front between them, and the vehicle tracked the front as it moved more than 10 km offshore. The physical front coincided with a biological front between strongly stratified phytoplankton-enriched water inshore of the front, and weakly stratified phytoplankton-poor water offshore of the front. While stratification remained a consistent identifier, conditions on both sides of the front changed rapidly as regional circulation responded to relaxation of upwelling winds. The offshore water type transitioned from relatively cold and saline upwelled water to relatively warm and fresh coastal transition zone water. The inshore water type exhibited an order of magnitude increase in chlorophyll concentrations and an associated increase in oxygen and decrease in nitrate. It also warmed and freshened near the front, consistent with the cross-frontal exchange that was detected in the high-resolution AUV data. AUV-observed cross-frontal exchanges beneath the surface manifestation of the front emphasize the importance of AUV synoptic water column surveys in the frontal zone.

Temperature Control of a Commercial Building With Model Predictive Control Techniques

This paper addresses the problem of thermal energy control in shopping centers through the application of model predictive control (MPC) strategies. In particular, this paper uses an existing shopping center as pilot case, which is characterized by a large common multifloor space, which, in turn, gives rise to a significant vertical thermal stratification. This paper explores the importance of MPC parameters to energy efficiency and comfort levels. In addition, it addresses some notable extensions, from economic optimization, which paves the way to real-time pricing techniques, to integration with renewables, to robustness enforcement through Kalman filters, to hybrid control with switching signals mixed with continuous signals.

Inversiones térmicas con advección cálida inferior en la Vega Media del Segura (Región de Murcia)

Anticyclonic situations fed by the maritime subtropical dorsal in winter are frequent in the South-east Spain. Under these conditions, the intrabaetic depressions and fluvial valleys are prone to thermal inversions. A clear example is the Middle Valley of the Segura River during winter months, especially when cooling at night appears to be accompanied by anticyclonic subsident air, warm advection SW flows at low atmospheric levels and cold katabatic winds proceeding from Sierra de Carrascoy. Such factors and the most temperature inversion situations registered in the study area during the period 2000-2011 are analyzed in this work. For this purpose data of meteorological stations and aerological soundings (e.g. temperature regime, vertical thermal stratification, indicators of atmospheric stability) were used.

Génesis de tormentas severas y su incidencia en la Región de Murcia

En el Sureste peninsular son constantes las situaciones de inestabilidad convectiva alimentadas en niveles altos atmosféricos por irrupciones de masas de aire ártica/polar. Bajo estas condiciones, debido al desplazamiento de los núcleos convectivos, las cuencas y valles fluviales intrabéticos son bastante proclives a los meteoros severos, tales como lluvias torrenciales, granizo o descargas eléctricas. Uno de los sistemas más perjudiciales, las supercélulas, suelen gestarse en los meses estivales o tardo-estivales, especialmente con la superficie terrestre muy calentada, acompañado además de altos índices de cizalla y helicidad. En este trabajo se analizan estos factores y las situaciones de inestabilidad convectiva más importantes registradas en la Región de Murcia (Sureste de España) en época reciente. La zona más afectada por las tormentas severas corresponde al norte de la Región, principalmente debido a vaguadas y vórtices fríos de origen ártico/polar marítimo de onda corta. Para ello se han utilizado: (i) los datos de estaciones meteorológicas en superficie y (ii) los sondeos aerológicos disponibles en el área, en particular los relativos a régimen de temperaturas, estratificación térmica vertical e indicadores de inestabilidad atmosférica. Se han obtenido correlaciones entre diferentes variables tormentosas, mostrando, por ejemplo, una buena relación la altitud con el número de días de tormenta, pero no así con las tormentas severas.

Stratification prediction model for perimeter zone UFAD diffusers based on laboratory testing with solar simulator

Underfloor air distribution (UFAD) is an air distribution strategy for providing ventilation and space conditioning in buildings. UFAD systems use the underfloor plenum beneath a raised floor to provide conditioned air through floor diffusers that creates a vertical thermal stratification under cooling operation. Thermal stratification affects energy, indoor air quality and thermal comfort performance.The purpose of this study was to characterize the influence of linear bar grilles and VAV directional diffusers on thermal stratification in perimeter zones by developing theoretically and empirically based prediction models. Forty-seven laboratory experiments were carried out in a climatic chamber equipped with a solar simulator. Linear bar grilles tend to produce less stratification than VAV directional diffusers and, in some cases with high airflow rates, may generate reverse stratification. Lowering internal blinds causes an increase in thermal stratification. Models to predict temperature stratification for the two tested diffusers have been developed.

Environmental Case Study of Water Quality of Ancient WellsMasabi Boudi and Narasimha Temple Well of Bijapur

Ancient historic wells or boudies, built by Adil Shahi kings in Bijapur are the huge sources of water in the drought place of northern Karnataka (India). They stand neglected and polluted. Masabi Boudi and Narasimha Temple well are two wells selected for the study. We have studied the water quality in terms of physico-chemical, biochemical, phytoplanktonic and bacteriological properties depthwise. Masabi Boudi exhibited a rare phenomenon of vertical inverse thermal stratification whereas in Narasimha Temple well winter thermal stratification broke in summer and monsoon. Phytoplankton occurrence was rare in Masabi Boudi. Desmids showed their pollution tolerance throughout the study in Masabi Boudi. Glucose, protein, creatinine, urea and uric acid in the water reads the utrophication, which resulted the growth of E-coli. Algal maximum was followed soon after the E-coli were in excess in both the wells. Chlorides, nitrates and nitrogenous content stated the rich nutritional status. The self purification of water takes place naturally as a quality control. There is a natural symmetry in the biological science between bacterioplankton and phytoplankton and high enrichment with organic matter provided a measure of stabilization to this eco-system. In all, Masabi Boudi shows better water quality than Narasimha Temple well.

Environmental Case Study with regards to Water quality of Ancient wells of Bijapur.

Historic wells (boudies) built by Adil Shahi dynasty in Bijapur, are huge sources of water in drought area, presently neglected and polluted.The two wells Taj boudi and Chand boudi were selected to study the depth wise water quality physio-chemical, phytoplankton, and bacteriological study. In Taj Boudi exceptional vertical inverse thermal stratification was observed and in Chand Boudi homothermal condition was noted. The input of medical wastes in Chand Boudi, led to the release of biochemical parameters like creatinine, protein, glucose, urea and uric acid with the increase of E-coli. As a result, floating meadow formed by the mud and plaster of paris (idol material) has the growth of macrophytes which are nourished by the organic biochemical compounds decomposed by the bacterial activity which acts as manure for the macrophytes. The self- purification of water takes place naturally as quality condition. Chlorides, nitrogenous content and carbon dioxide in the wells are high indicating rich nutritional status. There is natural symmetry expressed in the wells in the seasonal changes to the bacterio plankton and phyto plankton of water. Algal maxima were observed soon after which bacteria were in excess in Chand Boudi.

Will There Be a Significant Change to El Niño in the Twenty-First Century?

AbstractThe El Niño–Southern Oscillation (ENSO) response to anthropogenic climate change is assessed in the following 1° nominal resolution Community Climate System Model, version 4 (CCSM4) Coupled Model Intercomparison Project phase 5 (CMIP5) simulations: twentieth-century ensemble, preindustrial control, twenty-first-century projections, and stabilized 2100–2300 “extension runs.” ENSO variability weakens slightly with CO2; however, various significance tests reveal that changes are insignificant at all but the highest CO2 levels. Comparison with the 1850 control simulation suggests that ENSO changes may become significant on centennial time scales; the lack of signal in the twentieth- versus twenty-first-century ensembles is due to their limited duration. Changes to the mean state are consistent with previous studies: a weakening of the subtropical wind stress curl, an eastward shift of the tropical convective cells, a reduction in the zonal SST gradient, and an increase in vertical thermal stratification take place as CO2 increases. The extratropical thermocline deepens throughout the twenty-first century, with the tropical thermocline changing slowly in response. The adjustment time scale is set by the relevant ocean dynamics, and the delay in its effect on ENSO variability is not diminished by increasing ensemble size. The CCSM4 results imply that twenty-first-century simulations may simply be too short for identification of significant tropical variability response to climate change. An examination of atmospheric teleconnections, in contrast, shows that the remote influences of ENSO do respond rapidly to climate change in some regions, particularly during boreal winter. This suggests that changes to ENSO impacts may take place well before changes to oceanic tropical variability itself become significant.

Experimental Study on Indoor Thermal Stratification in Large Space by under Floor Air Distribution System (UFAD) in Summer

A ventilation method of down-supply up-return has grown popular in large space in recent years for comfort ventilation with low thermal load, especially in China, including gymnasium, factory space, and exhibition hall, etc. The undisturbed flow pattern in the space gives a gradient in temperature, and the vertical thermal stratification appears markedly in large space. The object for this paper is to understand the behaviour of an under floor air distribution system in a ventilated space. The thermal stratification characteristics in a real UFAD experimental space were measured. The effects of different supply conditions on the thermal stratification characteristics are investigated based on the experimental results. The relations between space air stratification and the control parameter is predicted. It can be indicated that there are 4 zones composing the vertical thermal stratification. And different zone has different control parameter.

Stability of stratified natural convection in a tall vertical annular cavity

The stability of buoyancy-driven convection between vertical concentric cylinders is examined under vertical thermal stratification conditions, with a uniform temperature gap between the cylinders. Analytical solutions are derived for the parallel base flow. Critical Rayleigh numbers are determined from these solutions at various Prandtl numbers, stratification levels, and inner to outer radius ratios. Nonaxisymmetric critical disturbances can be found for low radius ratios and small Prandtl numbers, but are otherwise symmetric and usually occur in the form of travelling waves opposite to gravity, except at large Prandtl numbers and for strong stratification where the direction of propagation is reversed. The effect of curvature is to favor the onset of instability at any stratification level, for moderate and large Prandtl numbers. For small Prandtl numbers, curvature has a stabilizing effect at low stratification levels and is most influential at moderate levels. Computation of the Landau coefficient reveals that the bifurcations are at times subcritical, depending on the parameter values.

Assessment of thermal stratification within stream pools as a mechanism to provide refugia for native trout in hot, arid rangelands

Native trout species, such as the redband trout (Oncorhynchus mykiss), occupy thermally harsh stream habitats in hot, arid rangeland basins of the western United States. Declines in the distribution and abundance of these species has generated interest in understanding how these cold water species survive in these systems, as well as in identifying opportunities to restore these species to their former ranges. The purpose of this study was to assess the potential for thermal stratification to provide thermal refuge for redband trout in stream pools characterized by warm intermittent flow conditions on arid rangelands. We studied vertical thermal stratification in two pools during three summers on Boles Creek located on the Modoc Plateau in extreme northeastern California. Water and air temperature data were collected on a 0.5 h time step from 15-Jun through 15-Sep during 1996, 1997, and 2000 using commercial temperature data-loggers. Water temperature was measured at the top (0.3 m below pool surface) and bottom (0.3 m above pool substrates) of each pool. Vertical thermal stratification occurred within these pools creating conditions as much as 7.6 C cooler and consistently more constant at the bottom of pools compared to pool surface waters. Thermal stratification was dependent upon air temperature with the magnitude of stratification increasing as air temperature increased. The magnitude of thermal stratification varied significantly from year to year, likely reflecting variation in annual weather conditions. The thermal regime in the study pools was often near the upper lethal limit reported for redband trout, but temperatures at the bottom of these pools did offer refuge from lethal temperatures realized near the pool surface. Temperatures at pool bottom were consistently above optimal levels published for redbands.