Thermal Stratification Phenomenon Research Articles

Phytoplankton Communities’ Response to Thermal Stratification and Changing Environmental Conditions in a Deep-Water Reservoir: Stochastic and Deterministic Processes

Thermal stratification has become more extensive and prolonged because of global warming, and this change has had a significant impact on the distribution patterns of the phytoplankton communities. However, the response of phytoplankton community structures and assembly processes to thermal stratification is not fully understood. We predicted that the structure and assembly processes of phytoplankton communities would be affected by thermal stratification among water layers associated with environmental condition changes, reflecting certain patterns in temporal and spatial scales. Phytoplankton from Danjiangkou Reservoir were collected from October 2021 to July 2022 to verify this prediction. During the sampling period, Danjiangkou Reservoir remained thermally stratified with stability. The composition of the phytoplankton community in the surface layer significantly differed from that in both the thermocline and bottom layer. The phenomenon of thermal stratification affected the pattern of nitrogen and phosphorus distribution and, thus, the processes of the phytoplankton community structures. Deterministic processes had a greater influence on the assembly of the phytoplankton communities in the surface and bottom layers. In contrast, stochastic processes were more prevalent in the assembly of the thermocline phytoplankton community. The phytoplankton community within the thermocline layer exhibited a broader niche range than that in the surface and bottom layers, showing notable dissimilarity from that of the bottom layer. Canonical correspondence analysis (CCA) revealed that the vertical distributions of the phytoplankton communities were significantly correlated with NH4+-N, pH, and water temperature (WT). In summary, this study explained the distribution patterns of phytoplankton community structures and assembly processes in deep-water reservoirs during the stratification period. Additionally, the study explored the potential of using the distribution patterns of phytoplankton in stratified-state deep-water reservoirs under a subtropical–warm temperate climate as climate indicators in the context of global warming.

Experimental investigation on the effect of direct contact condensation regime on thermal stratification

Direct contact condensation affects the performance of a small modular reactor with pressure suppression system. Since the heat and mass transfer varies with the condensation regimes as well as steam-water interface shapes, the identification and classification of the condensation regimes are important. The experiment of steam direct contact condensation was carried out and the corresponding condensation regimes were studied. Specifically, the experiment was studied by injecting steam vertically into a wet well with subcooled water. The physical process of steam condensation phenomenon with varying undercooling degree of wet well was recorded by high-speed camera. According to the location of the steam-water interface and the shape of the steam bubble, the four condensation regimes were studied, namely chugging, external necking, spheroidal upward bubble and T-shaped upward bubble. Besides, it is found that thermal stratification occurs in the wet well at low steam mass flux, the influence of each condensation regime on the thermal stratification of wet well was studied. The experimental results show that the internal chugging and external necking cause a large downward circulation of subcooled water, which enhance the flow mixing around the discharge tube and the bottom of the wet well. Consequently, the thermal stratification phenomenon does not happen in those condensation regimes. However, the regimes like spheroidal upward bubble and T-shaped upward bubble cause the upward circulation of subcooled water in wet well, which cause the thermal stratification. The critical Richardson number (Ri) determines the condensation regimes. Furthermore, when the Ri is smaller than 1, the condensation regime is external necking. Besides, when the Ri is greater than 1, the condensation regimes are spheroidal upward bubble and T-shaped upward bubble.

Heat transfer characteristics of passive containment cooling system with equal height difference under steady-state conditions

In the nuclear energy utilization, the passive containment cooling system (PCCS) holds paramount importance, with its heat transfer performance standing as a critical aspect of nuclear safety. This study simulated the flow and heat transfer behaviors of PCCS with equal height difference by APROS software under a broad range of steady state operating conditions. At first, the flow pattern of natural circulation is categorized by phase state of flow through operational parameters. A criterion model of boundary of phase change is proposed with prediction deviation of less than ±25 %. Furthermore, the influence of various operation parameters such as temperature, non-condensable gas content, and pressure on the heat transfer behaviors of natural circulation was also revealed. A new heat transfer coefficient correlation for both single-phase and two-phase conditions is proposed with prediction deviation of ±15 %. Finally, the thermal stratification phenomenon in the containment vessel with non-condensable gas was investigated, indicating that local heat transfer characteristics in the upper region surpass those in the lower section on account of deposition of the non-condensable gas at bottom of the vessel.

Analysis of the effect of thermal stratification on the natural circulation decay heat removal of sodium-cooled fast reactor

Thermal stratification is an important phenomenon in the process of natural circulation decay heat removal in sodium-cooled fast reactors (SFR). It affects the establishment of the natural circulation and the flow rate of the natural circulation in a station black out (SBO) accident, and therefore affects the safety of the reactor. In safety evaluation, programs are often required to perform analysis for various conditions and long-term transients. However, it is difficult to meet this requirement by adopting a multi-dimensional method or one-dimensional and multi-dimensional coupling method. Therefore, this paper proposes applying a thermal stratification model to the system analysis program THACS to analyze the thermal stratification phenomenon. First, the thermal stratification model is validated with experimental data. The validated model is applied to analyze a station black out accident in the SFR. The lumped parameter model and thermal stratification model are used respectively for the hot pool. By comparing the calculation results of the two models, the effect of thermal stratification of the hot pool on key parameters such as the natural circulation flow rate of the primary circuit, natural circulation flow rate in the inter-wrapper region, average core outlet temperature, maximum core fluid temperature, and the maximum temperature of cladding was analyzed, and the effect of thermal stratification on natural circulation decay heat removal of the SFR was evaluated. This work can provide a reference for the safety analysis of the reactor and contribute to developing a more conservative safety analysis and evaluation strategy.

1G and microgravity tank self-pressurization: Research on cryogenic fluid thermal stratification

A correlation numerical model was constructed, and relevant experiments were simulated to investigate the influence of gravity level on the self-pressurization process of the cryogenic fluid of liquid nitrogen. The calculated results agreed well with the experimental data. On this basis, the influences of a normal gravity environment and a microgravity environment on the self-pressurization process of liquid nitrogen in the normal-temperature fluid Perfluoro-n-Pentane (PnP, or C5F12) tank of National Aeronautics and Space Administration (NASA) were analyzed. According to the calculation results, an explicit horizontal interface is formed between the gas and liquid phases in the tank during the 1G (G = 9.81 m/s2) self-pressurization process. The ullage pressure increases as time progresses, and obvious thermal stratification phenomena are observed in the ullage along the axial and radial directions. However, the thermal stratification of the liquid zone is not obvious. The ullage near the gas‒liquid interface shifts from counterclockwise movement to clockwise movement, while the liquid changes from clockwise movement to counterclockwise movement. In the process of microgravity self-pressurization, the gas‒liquid interface in the tank presents an irregular shape, and the ullage pressure increases continuously before 400 s and then tends to be stable. The axial thermal stratification of fluid in the tank tends to disappear as time progresses, but obvious radial thermal stratification is observed. The ullage in the tank moves up and down as a response to the Marangoni migration effect. The temperature of the liquid zone in the tank during 1G self-pressurization tends to be consistent more quickly than that during microgravity self-pressurization, and the average temperature is higher. However, the temperature reduction rate of the ullage in the tank during microgravity self-pressurization is higher than that during 1G self-pressurization. Numerical simulation results can provide references to further study the on-orbit pressure control technique of cryogenic liquid tanks.

Numerical Simulation Calculation of Thermal Discharge Water Diffusion in Coastal Nuclear Power Plants

Coastal areas have become the primary location for nuclear power plant sites due to the convenience of seawater cooling. In China, the diffusion range of thermal discharge water has always been a hot research topic as the basis for sea use areas. The main physical process of heat loss in the numerical simulation of thermal discharge from power plants is heat exchange between the water surface and air. Currently, the methods for calculating this heat loss include the heat flux method and the water surface heat exchange coefficient method. Taking a power plant in Zhejiang as the research object, the MIKE 3 temperature–salinity module (heat flux method) and MIKE ECO Model (water surface heat exchange coefficient method) were used for modeling, and the diffusion of thermal discharge water under different modules was compared and analyzed. The results show that the calculated area of the water temperature rise envelope between the two modules differed significantly under the same conditions. The surface 4 °C temperature rise area, as calculated using ECO Model, was 45.8% smaller than that of the temperature–salinity module. The MIKE 3 temperature–salinity module can simulate the heat accumulation of water under thermal buoyancy, producing a significant thermal stratification phenomenon; the ECO module does not accurately represent the heat exchange process between the water surface and the air, and it does not adequately capture the significant vertical stratification that occurs in real-world scenarios. On this basis, this study not only lays a foundation for further exploring the impact of thermal discharge water from this powerhouse on the structure of surrounding marine biological communities and dissolved oxygen content in water bodies but also provides scientific evidence for the selection of modules when simulating thermal discharge water with the MIKE model.

Coupling Computational Fluid Dynamics and EnergyPlus to Optimize Energy Consumption and Comfort in Air Column Ventilation at a Tall High-Speed Rail Station

With the rapid development of railways, the air distribution and thermal comfort within waiting halls of high-speed railway stations receive significant attention. In this research, the EnergyPlus and CFD simulation coupling method was employed to investigate three ventilation schemes (column attached ventilation (CAV), side jet ventilation (SJV), column attached with side jet ventilation (CASJV)) for the waiting hall of a high-speed railway station in Guangzhou. The research focused on analyzing the airflow characteristics, thermal comfort, and cooling energy consumption associated with each ventilation method. The results show that thermal stratification phenomena are obvious in summer waiting halls. Most of the predicted mean vote (PMV) values in the research are from −0.5 to 0.5, indicating a comfortable thermal environment. In certain areas of both the CAV and SJV, the LPD1 > 40%, which may lead to a strong sensation of a cold draft for passengers. Compared with the SJV, the CAV and CASJV save 11.89% and 9.25% in cooling energy consumption, respectively. Therefore, the CASJV is more suitable for applications in high-speed railway station waiting halls. The results of this study aim to support the application of this combination of attached ventilation and an “air column” air supply in high-speed railway stations.

Flow and heat transfer of supercritical hydrogen in a regenerative cooling channel with the arc ribs of a rocket engine

This paper reports on the heat transfer characteristics and flow performance of supercritical hydrogen in a regenerative cooling channel with arc-shaped ribs. A 3-D numerical model of hydrogen flowing inside the cooling channel has been established and the data are compared with experimental results. The effect of rib parameters such as the number of ribs n, rib angle θ, rib size (hr × e), and rib orientation (convex or concave) on the cooling channel performance has been explored. The results indicate that arc ribs effectively mitigate the heat transfer deterioration and thermal stratification phenomena by reducing the wall temperature; nevertheless, the pressure loss due to friction would considerably increase. In studied cases, the wall temperature significantly decreases when the rib angle θ increases from 0° to 90°. The results have also shown that increasing the size of square arc ribs while maintaining the same rib spacing p/hr and length of the ribbed section Lr is beneficial for the overall heat transfer performance. Compared to smooth channel, a channel with large 90° arc ribs oriented either convex or concave to the flow direction enhanced the thermohydraulic performance η by up to 45%. This enhancement in performance of channel due to arc ribs is 1.95 and 1.384 times higher than the previously known designs; V-ribs (23%) and cylindrical ribs (32.5%), respectively. The flow field has shown that the evolution and morphology of rib-induced streamwise vortex considerably improve the wall temperature reduction by intense fluid mixing. In addition, it has been shown that the direct impact of the coolant stream on the ribs, the reattachment of the separated shear layer, and the intermittent growth of the boundary layer helps to enhance heat transfer.

Performance analysis of the phase-change heat-storage tank based on numerical simulation

Phase-change heat storage system contributes to the smooth operations of building energy supply and demand. In this paper, the numerical model of the heat-storage tank with phase change material blocks is established. The charge and discharge process of the heat-storage tank is analyzed with the numerical model. The performances of the heat-storage tank with and without phase change material blocks are compared by experiment and numerical simulation. The effects of structural parameters of phase change material blocks on the heat storage and release rate are comprehensively discussed, including the number and size of phase change material blocks. The results indicate that under the experimental conditions, the charge capacity and duration of the tank with phase change material blocks are extended by 7.43% and 8.3% respectively under the condition of replacing 6.75% volume of hot water with phase change material. The thermal stratification phenomenon in the heat-storage tank without phase change material blocks is more obvious. For certain volume of the phase change material, the performance analysis of three cases with different amount and size of phase change material blocks is investigated. The results show that the phase change material blocks with number and size of 24 and 0.45 m × 0.1 m × 0.1 m reduces the charge and discharge duration by 2.47% and 3.78% compared with those of 12 and 0.9 m × 0.1 m × 0.1 m.

Transient three-dimensional numerical simulation on asymmetrical flow and thermal characteristics of CEFR sodium pool under one secondary pump shaft stuck accident

The China Experimental Fast Reactor (CEFR) primary circuit adopts a pool-type structure. The sticking of the main circulating pump shaft in the secondary circuit is an important design-based accident for CEFR, and it is necessary to investigate the three-dimensional thermal-hydraulic phenomena in the sodium pool under the accident. However, conducting relevant experiments directly is almost impossible due to the large dimensions and intricate flow paths within the reactor. In the present work, the “modular modeling and integrated coupling calculation” method is used to build the detailed model and calculate the primary circuit of CEFR in a 1:1 scale. The three-dimensional thermal-hydraulic phenomena inside the sodium pool under the accident condition of one secondary circuit pump shaft stuck are simulated. According to the calculation results, the backflow phenomenon of the fault loop pressure pipe and the intermediate heat exchanger (IHX) begins to appear around 50 s. An obvious thermal stratification phenomenon occurs in the hot pool at the beginning of the accident. As the accident progresses, the temperature difference between the hot and cold pools decreases gradually, and the thermal stratification of the hot pool gradually diminishes. The temperature difference between the inlet and outlet of the same IHX decreases from a maximum of 170 °Cto 2 °C. During the course of the accident, the peak temperatures calculated for the fuel assembly cladding and fuel pellet are 541.78 °C and 1040.17 °C, respectively, which is far below the designed temperature limit of 800 °C and 2800 °C, respectively. Moreover, comparative analysis is conducted on the simulation results of pump shaft stuck in the primary and secondary circuits. The results suggest that when the shaft of one secondary circuit pump is stuck, the average temperature of the sodium at the outlet of the core is higher and the temperature difference between the inlet of different loop pumps is greater, which places more substantial demands on the reactor component structure. It provides significant numerical reference for the safety design and assessment of CEFR.

Comparative study of CFD and 3D thermal-hydraulic system codes in predicting natural convection and thermal stratification phenomena in an experimental facility

Natural circulation phenomena have been nowadays largely revisited aiming to investigate the performances of passive safety systems in carrying-out heat removal under accidental conditions. For this purpose, assessment studies using CFD (Computational Fluid Dynamics) and also 3D thermal-hydraulic system codes are considered at different levels of the design and safety demonstration issues. However, these tools have not being extensively validated for specific natural circulation flow regimes involving flow mixing, temperature stratification, flow recirculation and instabilities.In the present study, an experimental test case based on a small-scale pool test rig experiment performed by Korea Atomic Energy Research Institute, is considered for code-to-code and code-to-experimental data comparison. The test simulation is carried out using the FLUENT and the 3D thermal-hydraulic system CATHARE-2 codes. The objective is to evaluate and compare their prediction capabilities with respect to the test conditions of the experiment. It was observed that, notwithstanding their numerical and modelling differences, similar agreement results are obtained. Nevertheless, additional investigations efforts are still needed for a better representation of the considered phenomena.

Serpentine and furrowed wavy channels-based enhancement in heat transfer for a rocket engine chamber

The thermal protection of the rocket engine combustion chamber is one of the fundamental challenges in supersonic flight. This paper investigates serpentine and furrowed wavy cooling channels with a rectangular cross-section to enhance heat transfer performance and hydrogen turbulent fluid flow characteristics. Ansys fluent was used to investigate the effects of configuration, wavy amplitude, and channel wavelength on the heat wall temperature, Nusselt number, coolant velocity, pressure drop, turbulent kinetic energy, and thermal stratification phenomena. Four different wave amplitudes and three wavelengths are compared to determine the optimal structure required to achieve the best heat transfer enhancement with acceptable pressure drop. Compared to the straight channel, the maximum temperature of the furrowed channel (Case 6) was decreased by 15.38% at an amplitude of 0.2 mm. Furthermore, the thermal performance factor is evaluated for eight furrowed wavy structures. The result is shown that the structure with an amplitude of 0.2 mm and a wavelength of 1 mm (Case 6) can demonstrate better thermal performance, which is increased by 31.55% compared to the straight channel. The wavy surfaces cause separation in diverging zones and secondary flows in converging zones, which disturbs the boundary layer, enhances particle mixing and improves convective heat transfer.

Numerical simulation for thermally stratified Powell–Eyring fluid due to the Riga plate: application of the Cattaneo–Christov theory

The present study demonstrates the behavior of Powell–Eyring fluid flow deformed by a linearly stretchable Riga plate. The analysis of heat characteristics is scrutinized with the thermal stratification phenomenon conducted through generalized Fourier’s law (i.e. the Cattaneo–Christov theory). Heat generation/absorption is assumed to disclose the features of thermal transport phenomena. The temperature of the plate is higher than the ambient fluid temperature. It is noted that the Cattaneo–Christov model is responsible to generate some extra terms of heat absorption or generation in the energy conservation law. Appropriate transformations are implemented to achieve nonlinear governing equations in the form of an ordinary differential equation. Numerical solutions are computed for the governing equations through the Range–Kutta Fehlberg technique. In summary, the modified Hartman number is responsible to grow up velocity distribution while the opposite trend is noted for the variable thickness parameter. Heat transport phenomenon can be minimized by incorporating higher thermal relaxation parameters (i.e. it can be used as a cooling phenomenon).

Research on the Thermal Stratification Phenomenon at the Secondary Side of a Steam Generator in an Isolated Loop

In normal operation, there is no significant thermal stratification phenomenon in the tube bundle area of the steam generator of the nuclear power plant. However, this phenomenon is detected in marine nuclear power plant isolation loops in the presence of feedwater leakage. Therefore, an experiment was designed to verify the presence of thermal stratification, and the influence factors were analyzed by the RELAP5/MOD3.2 program. The results show that thermal stratification is mainly influenced by the initial water level at isolation, the initial pressure at isolation, the total leakage mass, and the internal circulation of the steam generator. Leakage flow has little effect on it. This phenomenon causes the coolant temperature at the primary side of the steam generator in the isolated loop to be lower than the temperature measured in the reactor inlet pipe or outlet pipe. This study aids the operator in determining the real coolant temperature at the primary side of the steam generator in the isolated loop and prevents reactivity insertion accidents when the isolated loop is put into operation.

Impact of thermal-solutal stratifications and activation energy aspects on time-dependent polymer nanoliquid

Nanofluids are used in our daily life and have engineering applications due to their improving energy efficiency and heat transfer in different thermal systems. Nanotechnology and thermal performance have developed a great prospect for global scientists and researchers. In our research work, we have considered time-dependent Sutterby nanofluid by capturing the aspects of thermal radiation and stratification phenomenon. Additionally, features of activation energy and heat source-sink are considered. Transformation variables are used to reduce PDEs into nonlinear ordinary differential equations. To obtain the numerical results, we have used the bvp4c scheme. Through graphs and tables, we have demonstrated a comprehensive analysis for velocity, temperature, concentration profiles, skin-friction parameter, Nusselt number, and Sherwood number. It is analyzed that the temperature profile increases as the thermal biot number raises, also, the concentration profile declines when the Schmidt number raises.

Entropy analysis in thermally stratified Powell-Eyring magnesium-blood nanofluid convection past a stretching surface

Recently, researchers are very possessive to examine the treatment of diseases caused due to the deficiency of magnesium. The deficiency of magnesium in blood caused Hypomagnesaemia, which is further stimulus for various diseases such as vomiting, sleepiness, nausea, loss of appetite etc. In order to cover such deficiency, magnesium is injected in blood (base fluid) as a nanoparticle. Based on the above applications, the current research focuses to cover the deficiency of magnesium through Powell-Eyring (Magnesium-Blood) nanofluid flow deformed by the linearly stretchable sheet in the vicinity of stagnation point. Linear thermal stratification, viscous dissipation and Joule heating are incorporated to disclose the heat transport properties. Magnetic field is implemented to the nanofluid with an angle α. A system of dimensionless governing equations is achieved through appropriate transformations. Physical behaviors of temperature and velocity fields are elaborated through comparative graphs. Skin friction, entropy generation, rates of heat transfer and Bejan number is disclosed through graphs corresponding to pertinent parameters. The well-known convergence method (Homotopy analysis method) is utilized to achieve the solution of the model. Decrement occurs in temperature field for dominant melting and thermal stratification phenomenon. Entropy has same scenario in case of dominant melting while inverse behavior in term of thermal stratification. Bejan number is de-escalates with the escalation of Eckert number. Also, it is worth mentioning that the velocity outline accelerated and temperature profile decelerated with the enhancement of melting parameter. It shows that recovery rate of magnesium patients may be enhanced through medicine with enriched magnesium nanoparticles depending on the health of the patient.

Convective transportation in thermally stratified Tween-20-water/ethyl acetate fluid reservoirs constraint to Riga surface

Researchers dedicated their efforts to enhance the efficiency of industrial fluids as well as industrial machinery. Such efficiencies are greatly depended on the heat transfer progress. The main concern is how to enhance the heating/cooling phenomenon which are more suitable, environmental friendly and economically beneficiary. In the present analysis, we have focused on the characteristics of convective boundary condition in nanofluid flow deformed by stretchable Riga plate. Tween-20 is considered as nano material while ethyl acetate and water are used as base fluids. Thermal stratification and entropy generation phenomena are implemented to disclose the heat transportation. Physical behaviors of numerous parameters are discussed comprehensively for both velocity and temperature fields. Dominant stratification decays the temperature of fluid reservoirs more rapidly in ethyl acetate in comparison to water. Thermal Biot number grows the temperature field more rapidly in ethyl acetate fluid reservoir in comparison to water. Further, entropy generation reflects higher behavior for dominant stratification, Eckert number and Biot number.

Seasonal Stratification Characteristics of Vertical Profiles and Water Quality of Lake Lugu in Southwest China

According to the vertical section monitoring data of Lake Lugu water temperature (WT), electrical conductivity (EC), dissolved oxygen (DO), pH and chlorophyll-a (Chl-a) parameters in January (winter), April (spring), July (summer), and October (autumn) in 2015, the vertical stratification structure of WT and the null seasonality of water chemistry were analyzed. The relationship between the seasonal variation of WT stratification and the spatial and temporal distribution of EC, pH, DO and Chl-a was explored. The relationship between EC and WT was found for the epilimnion, thermocline and hypolimnion. The results of the study showed that: (1) The Lake Lugu water body shows obvious thermal stratification in spring, summer and autumn. In winter, the WT is close to isothermal condition in the vertical direction; in summer, the thermocline is located at 10–25 m water depth; while in autumn, the thermocline moves down to 20–30 m. (2) The Hypolimnion WT was maintained at 9.5 °C~10 °C, which is consistent with the annual mean temperature of Lake Lugu, indicating that the hypolimnion water column is stable and relatively constant, and reflects the annual mean temperature of the lake. The thermally stratified structure has some influence on the changes of EC, DO, pH and Chl-a, resulting in the obvious stratification of EC, DO and pH in the water body. (3) Especially in summer, when the temperature increased, the thermal stratification phenomenon was significant, and DO and pH peaked in thermocline, with a decreasing trend from the peak upward and downward, and the hypolimnion was in an anoxic state and the pH value was small. Although chlorophyll a remained low below thermocline and was not high overall, there was a sudden increase in the surface layer, which should be highly warned to prevent a large algal bloom or even a localized outbreak in Lake Lugu. (4) There is a simple linear function between EC and WT in both vertical section and Epilimnion, thermocline and hypolimnion, which proves that Lake Lugu is still influenced by natural climate and maintains natural water state, and is a typical warm single mixed type of lake. (5) It is suggested to strengthen water quality monitoring, grasp its change pattern and influence factors, and take scientific measures to prevent huge pressure on the closed ecological environment of Lake Lugu, and provide scientific basis for the protection of high-quality freshwater lakes in the plateau.

Experimental Analysis of Temperature-Control Curtain Regulating Outflow Temperature in a Thermal-Stratified Reservoir.

The construction of reservoir dams has changed the environment and natural properties of the river course, and deep-water reservoirs present an obvious phenomenon of thermal stratification. Low-temperature outflow water in spring and summer will have a negative impact on the downstream ecological environment. Therefore, it is necessary to take selective withdrawal measures to regulate low-temperature outflow water. The temperature-control curtain project has the advantages of low cost, convenient construction and wide application. Based on the topographic data, a laboratory test model for regulating outflow temperature by a temperature-control curtain is established. A high-power electric heating system is adopted to form a nonlinear thermal stratification. The accuracy of the test data is verified by the prototype observed water temperature. The main parameters affecting the outflow temperature are investigated, including thermal stratification, flow height above the temperature-control curtain, water level, and discharge flow. The results show the following: firstly, the outflow temperature mainly depends on the thermal stratification, decreases with the increase of water level, and increases with the increase of discharge flow; secondly, the effect of a temperature-control curtain on improving the outflow temperature is directly related to the thermal stratification in different months, and the improvement effect is better in spring and summer; finally, the improvement effect increases with the decrease of flow height above the temperature-control curtain, increases with the increase of water level, and decreases with the increase of discharge flow.

Evaluation and Evolution of the Physico-Chemical Parameters of Ocnei and Rotund Lakes Located near the “Salina Turda” Mine, Romania

The present research brings an input of information regarding the evolution of several physico-chemical parameters of two salt lakes (Lake Ocnei and Lake Rotund), part of the ”Salina Turda” resort, Cluj County, Romania, by means of on-site determinations. Measurements were carried out at six depths for each sampling point. We attempted to describe the behaviors of the two lakes under different natural conditions, in order to identify the impact of anthropogenic activities on the quality parameters of the two lakes. Our studies showed that the qualitative parameters of the water fluctuate as an effect of anthropogenic activities. A comparative analysis of the results gathered during three monitoring campaigns in 2016, 2018, and 2020 indicated that water quality was affected by anthropogenic activities such as mixing water layers which were characterized by different salinity values. The lakes tended to lose basicity, pH values varying between 9 at the surface level and 7 at −4 m. The thermal stratification phenomenon was only evident in the first year of monitoring; later on, the waters of both lakes appeared thermally homogenous down to the depth of −2 m. It was determined that the lakes had an uppermost freshwater layer, which disappeared during the bathing season because of vertical mixing. Interestingly, the two lakes showcased different behaviors at depths beyond −3 m. In addition, the infiltration of meteoric water that was polluted with nitrites and nitrates demonstrated the fact that anthropogenic activities that take place in the vicinity of the lakes generate negative effects on water quality. The presence of the heliothermal phenomenon was confirmed by the measurements made in the upper segment of the lakes. This layer of water consists of a mixture of fresh and salt water. The purpose of the research was to evaluate the water quality of the lakes, monitor its evolution during the bathing season and update the situation regarding the water quality of the two salt lakes by testing specific parameters.