Vertical Thermal Stratification Research Articles

Analysis of nitrogen migration and transformation in the typical deep and large reservoir of the Lancang River — Evidence from nitrogen and oxygen isotopes

The environmental effect resulting from dam construction and the formation of large reservoirs has long been a topic of significant concern. While current research at a macro scale provides a well understanding regarding the evolution of nitrogen nutrient status in river-reservoir ecosystems, the vertical migration and transformation characteristics of nitrogen in reservoirs under changing runoff remain less defined. This study centers on a typical deep reservoir along the Lancang River-Nuozhadu (NZD) Reservoir in April and December. The monitoring reveals a distinct vertical stratification phenomenon in the NZD Reservoir in the vertical profile in April, which can be categorized into a mixed layer (ML) (0–5 m), a thermocline layer (TL) (5–40 m), and an isothermal layer (IL) (>40 m) based on the water temperature. However, little stratification in the profile is occurred in December. The results of nitrogen and oxygen isotopes in April indicate that the biochemical process of nitrogen in the NZD Reservoir is predominantly driven by nitrification. However, different nitrogen biochemical processes are coupled in various stratified regions in the vertical direction. In the ML, simultaneous assimilation of ammonia nitrogen by phytoplankton occurs. In the TL, there is a process of oxidation and decomposition of particulate organic nitrogen due to a significant stratification effect and an extended hydraulic retention time. In the region of minimal dissolved oxygen (30–40 m), simultaneous denitrification may take place. In December, the main process is nitrogen assimilation in reservoir surface (ML). Statistical analysis of environmental factors highlights water temperature (T) and dissolved oxygen (DO) as crucial environmental factors influencing nitrogen migration and transformation within the reservoir. This study suggests that in deep reservoirs experiencing vertical thermal stratification, the characteristics of DO and nitrogen migration and transformation exhibit significant vertical spatial heterogeneity. This phenomenon may lead to diverse ecological environmental effects. Future efforts should focus on enhancing the fine monitoring of stratified water bodies in deep and large reservoirs and analyzing the ecological environmental effects of material cycling in greater detail.

Dynamic effects of occupant motion on indoor vertical thermal stratification in the displacement ventilation system

As an inevitable disturbance source, indoor occupant motion can weaken vertical thermal stratification and reduce the ventilation efficiency of the displacement ventilation (DV) system. However, the weakening process of vertical thermal stratification during occupant motion remains unclear. In order to clarify how occupant motion weakens the indoor vertical thermal stratification, occupant motion within a stratified environment in the DV system by numerical simulation was analysed. Initially, the Q criterion number, which represents the second invariant of the velocity gradient tensor, was used to visualise the vortexes in the wake of the moving occupant. Subsequently, the indoor vertical thermal profile variation during long-term occupant motions was analysed. Finally, the indoor vertical thermal profiles under different occupant moving velocities in the DV system were explored. Our research revealed that the reverse horizontal flow generated by the collision of the wake with the walls is the main reason for the weakening of the vertical thermal stratification. The vertical thermal profile undergoes four stages during the weakening process. In the first stage, the profile shape varies, followed by a monolithic translation. Thirdly, temperature variations only occur near the floor. Finally, indoor temperature stabilises even as occupants keep moving. When the human body moves at normal speeds ranging from 1.0 m/s to 1.6 m/s, the temperature gradient can decrease by approximately 45.0 % to 75.7 %. This study provides a more comprehensive understanding of how occupant motion affects the vertical thermal stratification of the DV systems to the designers, thereby offering robust support for research and practices in relevant fields.

Theme: Thermal and Dynamic Study of Convection-Radiation Coupling in a Closed Enclosure

In this work, we present a two-dimensional study of natural convection in a square air-filled cavity whose two vertical sides are subjected to a temperature difference, while the other two horizontal ones are adiabatic. The numerical method used is that of finite elements used in the COMSOL calculation code. We first studied pure natural convection, by varying the Rayleigh number from to , then its coupling with surface radiation, by varying the emissivity of the cavity surfaces from 0 to 0.6. The results are presented in the form of isotherms, pressure, current etc. It appears from this study that for a Rayleigh number less than 100, the fluid presents a vertical thermal stratification due to heat transfer only by conduction which does not generate any convective flow. The presence of surface radiation does not modify the equations governing fluid movements but only alternates the thermal boundary conditions. The coupling of natural convection and surface radiation occurs only through thermal boundary conditions. The variation in the emissivity of the walls does not influence the temperature isotherms.

Experimental study on the vertical temperature and thermal stratification for subway station fire

Experimental study on the vertical temperature and thermal stratification for subway station fire

Inertial instability and phase error in Euler forward predictor-corrector time integration schemes: Improvement of modeling Great Lakes thermal structure and circulation using FVCOM

This study investigates the inertial stability properties and phase error of numerical time integration schemes in several widely-used ocean and atmospheric models. These schemes include the most widely used centered differencing (i.e., leapfrog scheme or the 3-time step scheme at n-1, n, n+1) and 2-time step (n, n+1) 1st-order Euler forward schemes, as well as 2nd-stage and 3rd- and 4th-stage Euler predictor-corrector (PC) schemes. Previous work has proved that the leapfrog scheme is neutrally stable with respect to the Coriolis force, with perfect inertial motion preservation, an amplification factor (AF) equal to unity, and a minor overestimation of the phase speed. The 1st-order Euler forward scheme, on the other hand, is known to be unconditionally inertially unstable since its AF is always greater than unity. In this study, it is shown that 3rd- and 4th-order predictor-corrector schemes 1) are inertially stable with weak damping if the Coriolis terms are equally split to n+1 (new value) and n (old value); and 2) introduce an artificial computational mode. The inevitable phase error associated with the Coriolis parameter is analyzed in depth for all numerical schemes. Some schemes (leapfrog and 2nd-stage PC schemes) overestimate the phase speed, while the others (1st-order Euler forward, 3rd- and 4th-stage PC schemes) underestimate it. To preserve phase speed as best as possible in a numerical model, alternating a scheme that overestimates the phase speed with a scheme that underestimates the phase speed is recommended. Considering all properties investigated, the leapfrog scheme is still highly recommended for a time integration scheme. As an example, a comparison between a leapfrog scheme and a 1st-order Euler forward scheme is presented to show that the leapfrog scheme reproduces much better vertical thermal stratification and circulation in the weakly-stratified Great Lakes.

Advanced turbulence modeling improves thermal gradient prediction during compressed hydrogen tank filling

In order to use gaseous hydrogen for mobility of light and heavy duty vehicles, the standard J2601 from the Society of Automotive Engineers (SAE) recommends that the temperature in the tank must not exceed 85 °C for safety reasons. Prior experiments reported that a vertical thermal stratification can occur during the filling of horizontal tanks under specific conditions. Thermodynamic modeling of hydrogen tank filling can predict the average gas temperature but not the onset of stratification. In a previous study, the computational fluid dynamics (CFD) software OpenFOAM was used to carry out simulations of hydrogen filling for a type IV 37 L tank. The CFD results, by comparison with experimental results, were capable to predict the rise of the thermal stratification with however an underestimation of thermal gradient magnitudes. The maximal temperature predicted at the end of the filling was 15.05 °C bellow the experimental measurements. In this work, the k − ω SST turbulence model is replaced by the k − ω SST SAS turbulence model to limit the prediction of high levels of eddy-viscosity in stagnation areas which over-diffuses the temperature. By using the same mesh as in the above mentioned study, (651 482 cells in the fluid region and 449 126 cells in solid regions), the k − ω SST SAS turbulence model is found to be more appropriate for CFD simulation of tank filling as it predicts a thermal gradient magnitude in the gas in better agreement with experimental measurements than the k − ω SST turbulence model for a similar time of simulation. The maximal temperature predicted at the end of the filling is 2.17 °C bellow the experimental measurements.

Investigating the temperature distribution of non-enclosed atriums with air infiltration in winter

Low-temperature air infiltration and vertical thermal stratification lead to an unsatisfied bottom-occupied environment of the atrium and increase the space heating load in winter. An integrated prediction model was proposed regarding the interaction between thermal stratification and air infiltration. It includes the extended Block model for predicting the vertical temperature distribution in a non-enclosed atrium, the air infiltration model for calculating the buoyancy-driven air infiltration rate, and the Velocity propagating model for describing the airflow pattern within a foyer. By applying this model, we quantitatively analysed the indoor thermal characteristics in different atrium/foyer spatial morphologies. Beyond the indoor net height, the atrium diameter is also a dominant factor affecting the flow rate of air infiltration introduced from the foyer. The lower the section aspect ratio (SAR) and the higher the bottom floor height of the atrium, the warmer the bottom-occupied environment. The foyer's buffer effect mainly promotes occupants' thermal comfort, as lengthening the foyer length increases the heating load of air infiltration into the atrium while helping reduce the horizontal temperature gradient (HTG) when occupants walk across. This study clarified the indoor environmental characteristics in such spaces in winter and explored the interrelated influencing factors. The methodology and conclusions provide insights into practical engineering.

Investigation of vertical thermal stratification and stratified air conditioning load for large space with low-sidewall supply air based on vertical temperature node model

Large space buildings often use stratified air conditioning, which has the advantages of reducing energy consumption and improving indoor air quality. The airflow pattern of low-sidewall supply air is a common technology for creating stratified air conditioning in large space. The vertical temperature dynamic distribution is a momentous feature in stratified air conditioning thermal environment, and it is also the key to attain energy saving. In order to explore the heat transfer evolution process for large space stratified air conditioning, a vertical temperature node model for predicting the thermal stratification was established based on the airflow pattern of low-sidewall supply air. In this node model, the large space indoor environment is divided into several air layers in the vertical direction. Each air layer is considered about the aspects of wall convection heat transfer, supply air heat transfer, airflow heat transfer between air layers, temperature differences heat transfer between air layers and so on. A series of heating balance equations are established, and the vertical temperature dynamic distribution results can be obtained by matrix iterative calculation. Based on the 24-h periodic heat flow, the calculated results of the node model were compared with the experimental results. Then, based on the vertical temperature node model, the heat transfer evolution process and dynamic cooling load calculation of large space stratified air conditioning with low-sidewall supply air were discussed and investigated quantitatively.

О МЕХАНИЗМЕ РЕЗКОГО ПОНИЖЕНИЯ ТЕМПЕРАТУРЫ ПОВЕРХНОСТИ В СЕВЕРО-ЗАПАДНОЙ ЧАСТИ ЧЕРНОГО МОРЯ И У ПОБЕРЕЖЬЯ КРЫМА

An analysis of high-resolution satellite data on sea surface temperature (SST) and surface wind for 2008-2021 confirmed that the areas of a dramatic SST drop exceeding several degrees per day regularly occur in the northwestern Black Sea and off the western coast of Crimea. These areas are located not only in coastal upwelling zones. Extensive areas of this type are found in the open water areas in the northwestern part of the sea and in the vicinity of the western large-scale cyclonic gyre. A reason for the formation of such areas during the period of the stable vertical thermal stratification in the upper layer is the strengthening of wind in the atmospheric surface layer and the intensification of cyclonic vorticity. This leads to the generation of upward motions, additional turbulent mixing in the upper sea layer, and, as a consequence, the SST drop within a few days after the passage of the cyclone over the sea. This mechanism is especially effective in the case of slowly moving cyclones in the presence of a blocking or a high-pressure ridge northeast of the Black Sea region.

Thermal and draught perception in fluctuating stratified thermal environments with intermittent impinging jet ventilation

This study proposes a new integrated air supply strategy of impinging jet ventilation (IJV) and intermittent airflow to improve indoor thermal comfort. Vertical thermal stratification is created by the IJV, and a fluctuating thermal environment is created by the intermittent airflow. Three pulsating periods (3 min, 5 min, and 7 min), two supply air temperatures (18 °C and 16 °C), and four air supply velocities (1.8 m/s steady, 1.5/2.1 m/s, 0.9/2.7 m/s, and 0/3.6 m/s) were designed. In both steady and fluctuating indoor environments, fifteen college students were asked to report their perceptions of thermal sensation, thermal comfort, thermal preference, ankle air movement acceptability and preference, and perceived air quality. The results showed that pulsating airflow could lower the mean air temperature above 0.6 m and raise it below 0.6 m. Furthermore, pulsating airflow could reduce overall and local thermal sensation by up to 0.21 and 0.28 scale unit, respectively, and the thermal sensation difference between the head and the foot by up to 0.23 scale unit. While in neutral thermal conditions, the pulsating period of 3 min had the best cooling effect of the 10 cases, but the corresponding percentage of mean dissatisfied ankle draught was 4.3% higher than in the steady operation case. Draught sensation is so closely related to ankle thermal sensation that extending the pulsating periods to 5 or 7 min could reduce ankle draught risk by up to 12.3%. The perceived air quality did not differ significantly between the steady and intermittent cases. The intermittent operation of IJV was shown to have superior cooling performance in order to improve indoor thermal comfort.

Seasonal hydrological dynamics govern lifestyle preference of aquatic antibiotic resistome

Antibiotic resistance genes (ARGs) are a well-known environmental concern. Yet, limited knowledge exists on the fate and transport of ARGs in deep freshwater reservoirs experiencing seasonal hydrological changes, especially in the context of particle-attached (PA) and free-living (FL) lifestyles. Here, the ARG profiles were examined using high-throughput quantitative PCR in PA and FL lifestyles during four seasons representing two hydrological phenomena (vertical mixing and thermal stratification) in the Shuikou Reservoir (SR), Southern China. The results indicated that seasonal hydrological dynamics were critical for influencing the ARGs in PA and FL and the transition of ARGs between the two lifestyles. ARG profiles both in PA and FL were likely to be shaped by horizontal gene transfer. However, they exhibited distinct responses to the physicochemical (e.g., nutrients and dissolved oxygen) changes under seasonal hydrological dynamics. The particle-association niche (PAN) index revealed 94 non-conservative ARGs (i.e., no preferences for PA and FL) and 23 and 16 conservative ARGs preferring PA and FL lifestyles, respectively. A sharp decline in conservative ARGs under stratified hydrologic suggested seasonal influence on the ARGs transition between PA and FL lifestyles. Remarkably, the conservative ARGs (in PA or FL lifestyle) were more closely related to bacterial OTUs in their preferred lifestyle than their counterparts, indicating lifestyle-dependent ARG enrichment. Altogether, these findings enhanced our understanding of the ARG lifestyles and the role of seasonal hydrological changes in governing the ARG transition between the lifestyles in a typical deep freshwater ecosystem.

Field investigation on the thermal environment and thermal comfort in shopping malls in the cold zone of China

Reducing the energy consumption of large shopping malls is beneficial to achieving China's dual carbon goals. This study aims to explore the thermal comfort demands of occupants and provide guidance for the energy-efficient design of the indoor thermal environment in shopping malls. We selected nine typical large shopping malls in the cold area of China as the research object and carried out a year-long field study, which involved the survey of physical environment parameters and subjective questionnaires. 3175 valid data sets were obtained in all seasons. The study results found that the indoor thermal environment suffers from overheating in winter and overcooling in summer. The neutral Top is 21.3 °C, 25.4 °C, 19.0 °C, and 17.4 °C in spring, summer, autumn, and winter, respectively. The upper limit of the 80% temperature acceptability range in summer was 27.5 °C. The 80% acceptability range in winter (16.4–23.2 °C) is wider than GB 50736 standard, which can be appropriately relaxed for the HVAC temperature setting in the mall. Apparent vertical thermal stratification exists in the malls and significantly affects the occupants' thermal comfort. Compared to the PMV model, the adaptive model was more applicable for thermal environment evaluation of shopping malls in transition season and summer. This study proposes indoor temperature settings under the premise of meeting human thermal comfort, which provides references for the design and operation of shopping malls in cold-climate zones.

Analysis of the Thermal and Cooling Energy Performance of the Perimeter Zones in an Office Building

Indoor thermal conditions can be highly influenced through building envelopes by outdoor conditions, especially climatic parameters. While a lot of attention has been paid to the thermal performance in core zones in buildings, other zones, such as perimeters, experience significant heat loss and gain through building envelopes. Focusing on the energy and thermal performance in perimeter zones, the present study performed an energy simulation to find the most susceptible building orientation in an office building in South Korea regarding the cooling loads during the summer. Through field measurements, the solar radiation impact on the thermal performance in the perimeter zones was practically investigated. To reduce the cooling loads in the perimeter zones, an air barrier system was utilized. As a result, the biggest amount of heat was observed in the perimeter zones facing the west façade in the winter, according to the measurements. While the highest temperature was observed at the internal surface of the windows, the temperature in the perimeter and core zones was stably maintained. The heat that occurred through the west façade was reduced by the air barrier system by removing the vertical thermal stratification using the fan-powered unit in the system.

Investigation of inter-zonal heat transfer in large space buildings based on similarity: Comparison of two stratified air-conditioning systems

Stratified air-conditioning (STRAC) has been deployed in large space buildings to achieve vertical thermal stratification with significant energy-saving potential. This research focuses on two typical STRAC systems: floor-level sidewall air-supply system (FSAS) and nozzle sidewall air-supply system (NSAS). By employing experiment and CFD methods, airflow pattern and heat transfer between the unoccupied and occupied zone under the two air supply systems are investigated in a reduced-scale laboratory. The geometric models of the prototype building, with a geometrical scaling factor of 4:1, are established according to similarity principles and studied by CFD simulation. The results demonstrate that the two scales have similar indoor thermal performances. The numerical simulation results of the prototype building provide and compare commonly used inter-zonal heat transfer coefficients of actual large space buildings with the two typical STRAC systems. For FSAS, heat conduction caused by temperature gradient dominates the inter-zonal heat transfer. In NSAS, both heat conduction due to temperature gradient and heat convection due to airflow contributes equally to this heat transfer. The inter-zonal heat transfer coefficient Cb is affected by the airflow pattern and zonal division, and closely associated with local turbulence intensity.

High-resolution temporal detection of cyanobacterial blooms in a deep and oligotrophic lake by high-frequency buoy data

Cyanobacterial blooms are increasing in magnitude, frequency, and duration worldwide. However, our knowledge of cyanobacterial blooms dynamics and driving mechanisms is still limited due to their high spatiotemporal variability. To determine the potential driving mechanisms of cyanobacterial blooms in oligotrophic lakes, we collected a high-frequency depth profile of chlorophyll fluorescence (ChlF) and synchronous water quality, hydrometeorological data in early spring 2016 in oligotrophic Lake Qiandaohu. The vertical distribution of ChlF exhibited two patterns, “aggregated” and “discrete”, using Morisita's index, and the aggregated ChlF presented subsurface chlorophyll maxima during the thermal stratification period. The ChlF concentration was positively correlated with water temperature and negatively correlated with turbidity. Significantly linear relationships were observed between ChlF vertical structure parameters (e.g., Morisita's index, subsurface chlorophyll maxima depth and thickness) and thermal stratification parameters (e.g., mixing layer depth and relative water column stability). After rainstorm floods, the ChlF pattern suddenly change from “aggregated” to “discrete” and a ChlF concentration <1 μg/L was observed for 7–11 days with a significant increase in the mixing depth layer and turbidity. The results suggest that cyanobacterial blooms are robustly associated with thermal stratification and rainstorm floods in the deep and oligotrophic lake. Thermal stratification boosts surface phytoplankton accumulation by increasing water temperature, enhancing light availability and restricting phytoplankton vertical distribution. Rainstorm floods interrupt the accumulation by disrupting thermal stratification and decreasing the available light. Furthermore, wind speed and air temperature both regulate the phytoplankton dynamics by affecting thermal stratification. Our research quantifies the cyanobacterial bloom dynamics and their relationship between environmental factors, improving our knowledge of the driving mechanisms of cyanobacterial bloom for the protection of drinking water safety and aquatic organism health in lakes.

Prediction of vertical thermal stratification of large space buildings based on Block-Gebhart model: Case studies of three typical hybrid ventilation scenarios

Although nozzle air supply is considered to be dominant when creating the thermal environment of the occupied area in a large space building, natural ventilation and air infiltration can interact with nozzle jets, resulting in a complex air distribution inside the building. This study focuses on the prediction of indoor vertical temperature distribution in large space buildings under the coupling of multiple airflows. Based on the validated regional model, Block-Gebhart (B-G) model, three auxiliary models were introduced for three types of common building ventilation scenarios, namely nozzle air supply, natural ventilation, and air infiltration. The auxiliary models were combined with the basic model to predict the indoor thermal environment of large space buildings in three hybrid ventilation scenarios. Field measurements of the vertical air temperatures of these three buildings were carried out to verify the feasibility and accuracy of the composite model. The results showed that the average deviations of air temperature in the International Gymnastics Stadium, the Ecological Demonstration Building and the Engineering Training Plant were 0.85, 0.80, and 0.32 °C, respectively. The building with the minimum deviation was the training plant, because the prediction of air temperature of this building took into account both jet entrainment and air infiltration phenomenon, so that the description of its indoor airflow pattern was the most accurate. The composite model proposed in this paper extends the application of the B-G model in hybrid ventilation scenarios and supplements the present design system of air distribution in large space buildings.

Impact of a non-enclosed atrium on the surrounding thermal environment in shopping malls

Non-enclosed atriums, which give unobstructed access to the surrounding area, are gradually becoming common in modern shopping malls. The environmental problems they cause (e.g., vertical thermal stratification and overheating) are thus affecting adjacent occupied zones more often. This study focuses on the thermal environment of occupied zones adjacent to non-enclosed atriums and investigates influencing factors. Field tests of heat source intensity, air infiltration rate, and indoor temperature distribution were conducted in a typical shopping mall with multiple non-enclosed atriums. The vertical temperature gradient index (k) was adopted to illustrate temperature distribution among different layers where the atrium gives access straight through, which varies considerably in the presence of various heat sources. According to test results, both solar radiation transmitted and heat conducted through the glass ceiling of such an atrium contribute to the thermal stratification of indoor spaces, with the temperature gradient being 50% higher than in spaces without glass ceilings. As has been confirmed, air infiltration continuously introduces heat into the indoor environment. During summer, when air-conditioning operates, outdoor air infiltration occurs at rooftop entrances (approximately 3.0 m2 opening area), with an average flow rate of over 20,000 m³/h, accompanied by heat of 94–213 W/m2. The thermal stratification caused by non-enclosed atriums has a corresponding impact on occupants’ thermal sensation in the adjacent zones, as qualitatively verified by a subjective survey. Given this significant influence of non-enclosed atriums on the surrounding thermal environment, the study suggests weakening heat intensity through atrium form, glass ceiling, and air infiltration.

Heat transfer between occupied and unoccupied zone in large space building with floor-level side wall air-supply system

The air supply terminal located at the floor level attached to side-wall is widely used in large space buildings, leading to potential energy saving as well as significant vertical thermal stratification. The cooling load calculation of such system is challenging, especially the calculation of the load gained from unoccupied zone. This paper adopts experiment and computational fluid dynamics (CFD) methods to study the heat transfer upward and downward across the stratified surface in large space building with floor-level side wall air-supply system. Five experimental cases with different heat source power and exhaust airflow ratios are performed to study their effects on the indoor thermal environment. We investigate the same cases in CFD and verify the result of vertical temperature distribution and cooling load components. As a critical parameter in evaluating the thermal stratification environment of large space building, the inter-zonal heat transfer coefficient Cb is emphatically discussed. By comparing the Cb value obtained through the two methods, the accuracy of the microscopic method is verified by the heat balance method. The results show that the Cb value is mainly affected by the zonal division and air distribution, but less prominently by exhaust airflow ratio and heat source in the occupied zone.

Emergence of steeply stratified permafrost thaw ponds changes zooplankton ecology in subarctic freshwaters

ABSTRACT Climate change and associated permafrost thaw are creating new shallow waterbodies in vast regions of the circumpolar Arctic. These thaw ponds are characterized by high concentrations of colored dissolved organic matter originating from the degrading watershed, inducing a strong vertical thermal and oxygen (O2) stratification. We investigated the zooplankton community and biomass in eight subarctic thaw ponds and evaluated how zooplankton respond to this stratification. In a subset of three ponds, we further examined how other environmental variables, including essential fatty acids (EFA) concentration and phytoplankton, bacteria, and larval phantom midge Chaoborus biomass stratify and contribute to the vertical distribution of zooplankton in this increasingly common type of arctic freshwater system. The zooplankton community was extremely abundant in all ponds (up to 3,548 ind L−1) and dominated mainly by rotifers (35–93 percent of the biomass). Most zooplankton aggregated at the interface between the shallow well-oxygenated mixed surface layer and the deeper hypoxic but algal-rich stratified layer, and their distribution was affected by a combination of O2, Chaoborus, phytoplankton, and EFA that were supplied from opposite directions. Our findings show how water column stratification deeply affects the ecology of planktonic organisms in circumpolar freshwaters and indicate Arctic zooplankton species composition is expected to deeply change with the ongoing warming and browning.

Large-Eddy Simulation of turbulent thermal flow mixing in a vertical T-Junction configuration

Turbulent mixing of warm and cold water streams within a vertical T-junction configuration of a nuclear power plant piping system causes near-wall temperature fluctuations which may lead to High-Cycle Thermal Fatigue (HCTF) failure of the wall material. To predict frequency and amplitude of such temperature fluctuations accurately the method of Large-Eddy Simulation (LES) in the numerical simulation code OpenFOAM is used. As an experimental test case, the turbulent mixing experiment of Kamide et al. [1], where a vertical branch pipe with a cold water stream is connected from below to a horizontal main pipe with a warm water stream to form a vertical T-junction configuration. From the various observed flow patterns, the temperature and velocity data of the ‘wall jet’ are chosen as a reference because it is known to have the highest potential for thermal fatigue. Incomplete mixing and an instability associated with large turbulent structures near the junction region lead to significant temperature fluctuations close to the wall before a vertical thermal stratification further downstream stabilizes the flow. The highest near-wall fluctuation amplitudes are 26% of the temperature difference between the streams. A spectral peak occurs at a Strouhal number of 0.2. The results of the simulations demonstrate, that the mean velocity and temperature profiles of the experiments are well captured, whereas the root-mean-square (RMS) temperature fluctuations deviate from the measurements in some cases, in particular when coarse grids are used. In order to find a lower limit for the required spatial resolution three different numerical meshes with a variable number of cells up to 28 × 106 are investigated. The results demonstrate that with a sufficient mesh resolution the velocity and temperature distributions as well as the spectral peak can be simulated with good agreement to the experimental data.