Thermal Mixing Characteristics Research Articles

Transient and Dynamic Simulation of the Fluid Flow through Five-Way Electric Coolant Control Valve of a 100 kW Fuel Cell Vehicle by CFD with Moving Grid Technique

In order to maintain the performance of a fuel cell vehicle, it is essential to maintain a constant temperature of the stack. Therefore, it is very important to distribute the optimal coolant flow rate to each major component under very diverse and rapidly changing dynamic operating conditions. The part responsible for this is a five-way electric coolant valve. Therefore, this study aims to investigate transient dynamic flow characteristics of the fluid flow through a five-way electric coolant valve (PCCV: Penta-Control Coolant Valve). To achieve this goal, this paper attempts a three-dimensional dynamic simulation of the fluid flow through the valve using a commercial CFD solver with moving mesh technique to consider flow inertia and dynamic flow in the opening and closing stages of the ball valve rotating motion. The dynamic flow characteristics and the thermal mixing inside the PCCV ball valve during the opening and closing stages are analyzed. It was found that the discrepancies between dynamic and steady-state simulations are remarkable when fluxes with different levels of enthalpy and momentum flow into the PCCV, leading to strong flow interference and flow inertia, while the discrepancies are relatively small at low rotation speed and weak flow interference. Subsequently, the effect of the dynamic flow characteristics of the valve on the dynamic thermal mixing characteristics at two different ball valve rotation speeds and rotation directions are investigated. It was found that the dynamic flow and thermal mixing characteristics inside the PCCV are greatly affected by the rotation speed, rotation direction, and degree of flow interference between fluxes. It also helps design better coolant control strategies and improves the FCEV thermal management system.

Numerical investigation of thermal-mixing characteristics at vertically oriented T-junction pipelines

When hot and cold fluids are mixed at pipeline tees, temperature fluctuations are generated and can lead to thermal fatigue damage in the piping materials. At the University of Stuttgart in Germany, experimental investigations have been performed for understanding the flow mixing mechanism causing thermal fatigue. It was found that the inlet flow rate is one of the decisive factors on the mixing behaviors at a horizontal T-junction.In this study, the influence of the inlet flow rate on the flow mixing behaviors at the vertical T-junction is investigated using the large-eddy simulation (LES) method. The simulations are validated with the previous experiments and provide a good overview of the mixing flow. The LES method shows the advantage of illustrating the collision of two flow streams at the T-junction, which leads to different behaviors of the mixing flow. Generally, the temperature fluctuation in the mixing flow as well as the stability of thermal stratification decreases along the flow direction due to the dissipation of the turbulent kinetic energy in the mixing flow. The decrease in the main flow rate shows the impact of the increase in thermal stratification stability and the reverse flow distance. The decrease in the branch flow leads to a slight increase of the thermal stratification stability downstream of the T-junction and significant turbulent penetration in the branch pipe. The maximum temperature fluctuation at the pipeline inner surface is located at the joint position of the pipelines with a value higher than 30 K. Moreover, the flow collision at the T-junction leads to significant secondary flow in the mixing region, which can be influenced by the variations in the inlet flow rate. In particular, the secondary flow due to cold fluid presents a phenomenon of counter-rotating vortex pair (CRVP), and generates a temperature fluctuation with a frequency of approximately 1.1 Hz, which is recorded by the monitor points at both horizontal sides of the pipeline. In comparison with the previous work at the horizontal T-junction, the complete flow collision at the vertical T-junction is the reason for the totally different mixing phenomenon.

Thermal Mixing and Flow Characteristics Study in a T-Junction

The objective of the current paper is to investigate the turbulent flow characteristics, thermal mixing behavior, and backflow features of coolant fluid in a T-junction using three-dimensional numerical simulations. The impact of critical flow parameters and different branch pipe axial and transverse incident angles on thermal mixing performance are explored as a function of momentum ratio. The investigation identifies the impact of jet flow, deflection flow, and wall jet flow patterns. The results reveal that the upside backflow of T-junction is more significant for a lower momentum ratio (<0.32) and decreases with higher momentum ratio (0.32 < momentum ratio < 2.7). The efficiency of thermal mixing is considerably improved by changing the incident angle and momentum ratio. Thermal mixing length decreases by ∼39%, 28%, and 21% as compared to the reference case. The current findings will help researchers to understand flow behavior and thermal mixing in a T-junction. It is also useful in high-temperature engineering applications for the evaluation of thermal cracking and safety management.

Future projections of thermal regimes and mixing characteristics in a monomictic reservoir under climate change

Stratified structures are basic physical characteristics of lakes and reservoirs, which play important roles in vertical convection and mixing processes. Global warming is expected to alter the thermal regimes and stratified characteristics of lakes and reservoirs, leading to potential environmental risks and ecological problems. In this study, we used a two-dimensional hydrodynamic model to explore the response characteristics of the thermal regime to climate change in the Xiluodu Reservoir. The study is conducted based on four representative concentration pathways (RCP 2.6, RCP 4.5, RCP 6.0, and RCP 8.5). The results show that the projected surface water temperature of the reservoir will significantly increase (0.13 °C/10a-0.34 °C/10a) from 2022 to 2099, and the warming rate of the water in the epilimnion and thermocline will be greater than that in the hypolimnion, leading to enhanced vertical stratification in the reservoir. The stratification stability index (SI) will increase by 14.4 % under RCP 8.5 compared to that under RCP 2.6. The main responses of the reservoir to climate change are earlier start date and later end date of stratification, which may increase the duration of hypoxia in the hypolimnion and promote the succession of planktonic algal communities. The withdrawal water temperature in the reservoir will significantly increase under the climate warming scenarios (0.03 °C/10a-0.22 °C/10a). Furthermore, our research results indicate that selective withdrawal is still needed to mitigate the adverse effects of cold water from the reservoir during the fish spawning period under the strongest warming scenario, and can also be an effective adaptive management strategy to regulate thermal stability in the reservoir. This study provides a scientific and theoretical reference for the sustainable management of similar deep monomictic reservoirs under climate change.

Flow and thermal mixing characteristics of a multi-jet bypass flue with main flue baffles

The decrease of inlet flue temperature of denitrification reaction unit is a major challenge that restricts the low load operation of thermal power units. In this study, a new bypass flue heating technology based on multi-pipe and multipoint was proposed. The effect of different baffle openings (RA), directions and position distance (Db) on the mixing characteristics of multiple jets was quantitatively investigated by the experimental system. Combined with the simulation, the changes of the baffle on the multi-jet flow and mixing are revealed. The results show that the mixing temperature increases with increasing RA, but high RA leads to a significant increase in the thermal mixing temperature inhomogeneity (ζT). As RA increases from 0.4 to 0.7, ζT rises from the minimum 0.368 to the maximum 0.559. The mixing temperature and ζT of the baffle in the +x direction are higher than those in the −x direction. When Db increases from 70 mm to 130 mm, the maximum heating effect decreases from 25.97 °C to 18.89 °C, and ζT decreases gradually at y/Dh = 1.25. The findings provide guidance for bypass flue modification and low load denitrification in engineering.

A case study of thermal mixing behavior of hot and cold fluid in T-junction with/without mixing jets

Three-dimensional numerical simulations are performed to study the turbulent mixing of hot and cold fluids at a T-junction with unique branch pipe extended mixing jets. The major aim of the current research is to compare the performance of two distinct designs for thermal mixing and flow characteristics of a T-junction. We investigate the effects of the momentum ratio and temperature difference of mixing fluids on temperature gradients for proposed configurations of T-junctions. The finding demonstrated that a T-junction with jets appears to mix hot and cold fluids more homogeneously. The addition of the mixing jets significantly lessens the internal wall's temperature gradients and improves the efficacy of thermal mixing. The thermal mixing efficiency for T-junctions with mixing jets increased by 25%, 42%, and 50%, respectively. At the same momentum ratio, the length of the thermal mixing in the T-junction with mixing jets is dramatically reduced by 77%, 79%, and 83%. The stress distribution is also determined using thermo-mechanical coupled analysis. The result shows the reduction in peak stresses substantially with the mixing jet T-junction. The current study will help the researcher to evaluate safety issues in high-temperature applications and better understand thermal mixing, flow characteristics, and thermal cracks.

Mixing performance of preheated flue gas entering selective catalytic reactor under low loads

The reduced inlet flue gas temperature of the selective catalytic reduction (SCR) denitrification device is a significant problem during the low-load operation of boilers. In this study, an experimental system for turbulent thermal mixing of high and low-temperature flue gases was built to study the thermal mixing method in bypass flue gas technology and the effect of the temperature difference and flow rate variation of high and low-temperature flue gases at low load on the thermal mixing characteristics. The experiments evaluated the thermal mixing characteristics by measuring the temperature field distribution, calculating the inhomogeneity parameters ζT, and investigate the multi-jet distribution characteristics. The investigation was carried out more accurately by combining the simulation results. The results show that the tube wall jet combined with the bottom jet method provides the highest warming effect and possesses the lowest mixing non-uniformity parameter. At the location which is 1.25 times the hydraulic diameter of the duct away from the jet pipe, the temperature difference between the mainstream and the jet drops from 270 °C to 180 °C, causing ζT to drop from 0.46 to 0.37. Decreasing the jet velocity and increasing the main flow velocity promote a decrease in ζT, indicating that the ratio of jet velocity to main flow velocity and the velocity ratio UJUm are crucial factors affecting the thermal mixing characteristics, which directly affects the heat transfer between hot and cold fluids. The ζT decreases from 0.52 to 0.42 as UJUm changes from 3.58 to 2.15. This study provides a reference for ensuring the safe and stable operation of SCR denitrification system during the low-load operation of coal-fired boilers.

Thermal mixing characteristics of T-junction incident along central axis based on Large-Eddy simulations

A large eddy simulation model was used to explore the temperature mixing characteristics of a T-junction in which a branch pipe extends into the main and the branch fluid is injected along the central axis of the main pipe. The influence of the incident velocity of the branch tube on the temperature fluctuation of the fluid mixing is considered. In the main mixing region, the dimensionless time-averaged temperature（T∗¯）is positively related to the dimensionless root mean square（Trms∗）, while the connecting pipe region shows the opposite trend. With the increase of the velocity ratio, the influence of the branch pipe structure on the main flow field gradually decreases. The rebound vortex accelerates the rupture of the vortex ring, and the higher injection speed of the branch pipe leads to an increase in the vortex strength and the range of mixing area, which enhances the mixing of the fluid and makes the temperature distribution of the flow field more uniform. Finally, through the power spectral density analysis of temperature fluctuations, it is not only found that the high-energy vortex ring falls off to the near-wall region, dominating the temperature fluctuation in this region, but also this type of T-junction greatly reduces the risk of thermal fatigue failure.

Thermal Mixing Characteristics of T-Junction Incident Along Central Axis Based on Large-Eddy Simulations

Thermal Mixing Characteristics of T-Junction Incident Along Central Axis Based on Large-Eddy Simulations

Simulation Analysis of Thermal Mixing Characteristics of Fluids Flowing Through a Converging T-junction

Temperature fluctuation occurs while mixing of hot and cold fluids in a T-junction due to incomplete thermal mixing. This temperature fluctuation can produce thermal fatigue at the weld area of the T-junction. The present study aims to numerically investigate the thermal mixing characteristics of hot and cold fluids in a T-junction. The realizable k-ε turbulence model is used with natural gas as the working fluid. Temperature distribution, mixing quality, and intensity of temperature fluctuation are evaluated and compared along with the mixing outlet. The inlet temperature difference and branch to main pipe flowrate ratio have a direct influence on thermal mixing. The higher temperature difference can reduce the thermal mixing performance. Thermal mixing increases with the increase of branch to main pipe flowrate. The intensity of temperature fluctuation is found within a short distance from the intersecting point of the two inlets. With the increase of distance along with the mixing outlet, the frequency of temperature fluctuation decreases, and thermal mixing increases.

Effect of the Reynolds and Richardson numbers on thermal mixing characteristics

This paper presents a detailed numerical study regarding the influence of Reynolds and Richardson numbers in T-junctions with circular cross-section. The numerical investigation is based on the wall-resolved Large-Eddy Simulation (LES), which has been experimentally validated. The Reynolds number in the main pipe covers a range from Re=78400 to Re=235200, which is high enough to approach the engineering applications. This also belongs to the primary contribution of the current study. Furthermore, the temperature difference between the main pipe and branch pipe (Tm−Tb) is 180 K and the mass flow rate ratio (m˙m/m˙b) is 3. The primary focus is on thermal mixing characteristics in the downstream as well as on the evolution of the flow by consideration of gravity and buoyancy effects. Additionally, our study considers the thermal field and its corresponding fluctuations in the near-wall region. In addition, this paper also includes simulations with different T-junction configurations therefore we can examine the influence of the entrance orientation of the heavier cold branch pipe fluid on the mixing process and heat transfer. The analysis of our results indicates that gravity effects can be neglected in cases with increased Reynolds number and simultaneously decreased Richardson number. The area with high fluctuation intensities becomes larger by the higher Reynolds number and lower Richardson number of the mixing flow therefore they shows a higher potential for High Cycle Thermal Fatigue (HCTF). Furthermore, the highest near-wall thermal fluctuations are 40% of the temperature difference between the mixing flows.

Effect of temperature difference on the thermal mixing phenomenon in a T-junction under inflow pulsation

The effects of inlet temperature difference between the main and branch fluids on the flow and thermal mixing characteristics in the T-junction under inflow pulsation are numerically investigated by large eddy simulation (LES) turbulent model. The inlet velocities of both the main and branch pipes are assumed to be sinusoidal distribution, which is similar to the rolling condition in the ocean. Thermal mixing between hot and cold fluids with temperature differences of 15 K, 30 K and 45 K is carried out in the present study. The flow and mixing characteristics, including qualitative and quantitative analyses of temperature and velocity are studied. The results show that the reverse flow occurs in the unsteady thermal mixing processes due to the effect of inertia force, and the area of reverse flow decreases as the inlet temperature difference increases. Increasing inlet temperature difference induces a more stable thermal stratification flow with smaller thermal mixing length and lower temperature gradient. The maximum root mean square (RMS) of temperature fluctuation slightly decreases and shifts to the upper of main pipe as the inlet temperature difference increases. In addition, the spectral analysis of temperature fluctuation performed by Wavelet Transform (WT) indicates that the number of frequency peaks decreases as the temperature difference increases. Meanwhile, the main frequencies of wavelet power spectrum densities (WPSDs) for the lower points (points 1 ~ 3) decrease, while those for the upper points (points 5 ~ 7) increase as the temperature difference increases.

Thermal mixing enhancement of liquid metal film-flow by various obstacles under vertical magnetic field

Heat removal techniques of various liquid divertor concepts have been widely studied. In this study, thermal mixing characteristics of the liquid metal film-flow with locally heated on the surface under the vertical magnetic field were experimentally investigated by using various types of obstacles as a vortex generator. Especially, the efficiency of heat transport was investigated by the thermocouples installed on the bottom-wall. The velocity distributions were measured using the electric potential probe rake on the downstream of the channel. A delta-wing, a hemisphere or a cubic obstacle was used as a vortex generator installed at the center of the bottom-wall. The experiments were conducted for laminar flow region where Re = 487–1583 and in the range of N (=Ha2/Re) = 0–149 in the vertical magnetic field in the acrylic rectangular duct. GaInSn alloy was used as a working fluid. According to the comparison of heat flux distributions on the bottom-wall, the entire distributions moved towards the upstream with increasing of the strength of the vertical magnetic field for all cases. This tendency means the heat removal from the free-surface of liquid metal film-flow to the bottom-wall can be shortly achieved in case of the relatively high vertical magnetic field.

Numerical research on thermal mixing characteristics in a 45-degree T-junction for two-phase stratified flow during the emergency core cooling safety injection

The thermal mixing and direct contact condensation are the most significant phenomena appeared during the Emergency Core Cooling (ECC) system safety injection process. The three-dimensional thermal mixing characteristics accompanied by complicated direct contact condensation are investigated using CFD method. Firstly, the feasibility of two-phase CFD models are validated by the experimental data obtained from the XJTU-ECC experimental apparatus. Results show that the established CFD models could predict the two-phase mixing process correctly and the errors between numerical results and experimental data are smaller than 30%. Then the two-phase thermal mixing features, including temperature fields, velocity fields and volume fraction, are achieved in detail. Moreover, the local steam condensation characteristics are studied carefully and results show that the condensation rate is proved closely related to the coolant thermodynamic ratio between the main pipe and branch pipe. Finally, an empirical correlation is proposed to predict the condensation rate in case that the thermodynamic ratio is lower than 0.55. This work provides a fundamental guideline for the ECC system structure optimization and fatigue aging in the advanced nuclear power plants.

Advanced flow pattern for describing tangential flow oscillation in thermal-mixing pipe flow at a horizontal T-Junction

T-Junction experiments have been performed on the Fluid-Structure-Interaction (FSI) test facility at the University of Stuttgart to investigate the thermal fatigue effect in thermal mixing processes. The previous experimental works have been performed with different working conditions to discuss the influence of flow parameters (e.g. flow rate, flow temperature) on the mixing characteristics. The experiments have been performed with continuous and periodical mixing processes. 48 high-performance micro-thermocouples are installed in six measurement cross-sections close to T-junction to capture the near-wall fluid temperature. The temperature measurement data are analyzed and discussed with the viewpoint of their relevance for thermal fatigue. An unexpected mixing phenomenon, which is defined as the additional low frequencies, has been detected in the results of frequency analysis on the temperature data. Such kind of frequency with the value about 2.66 Hz can be found in most of our experiments. This similarity of the additional low frequency from the experimental work is summarized in this paper and compared with each other. With a small-scale experiment, this phenomenon is identified as harmonic oscillation of thermal stratification in pipe tangential direction, which has never been found in any of the previous T-junction investigations, nor in any literature on fluid mechanics. According to this phenomenon, novel flow patterns are introduced to categorize the mixing characteristics at horizontal T-junction. Among the patterns, the deflecting jet has the lowest possibility to initiate thermal fatigue in the mixing process at horizontal T-junction. With the results of the experimental and numerical investigations, the flow patterns of the thermal mixing processes at the FSI test facility can be summarized in the mixing envelope diagram, which presents the thermal mixing characteristics as the property of the specific horizontal mixing tee. Furthermore, once the mixing envelope has been drawn for a specific mixing tee, it can be used as operation guideline to reduce or avoid thermal fatigue close to the mixing tee.

Buoyancy driven turbulent plume induced by protruding heat source in vented enclosure

The buoyancy driven thermal plume behavior inside a partial square enclosure with protruding isothermal heat source block is numerically investigated. The turbulent flow is modeled by using unsteady Favre-averaged Navier-Stokes (UFANS) equation with buoyancy modified Lam Bremhorst low Reynolds number k−ϵ model. A Simplified Marker and Cell algorithm (SMAC) is used to solve the governing equations and the computations are performed using an in-house code based on Finite difference method. The heat transfer characteristics and the bidirectional exchange across the vent are analyzed for different Grashof number 108 ≤ Gr ≤ 1011. Further, the thermal plume patterns are investigated by mounting the heat source block at five different locations δ = 0.25 H, 0.35 H, 0.5 H, 0.65 H and 0.75 H. A significant enhancement in the net mass flow rate and average Nusselt number is observed by increasing the Grashof number. It is found that the heat source location has strong influence on the vent bidirectional exchange rate, thermal mixing and heat transfer characteristics inside the enclosure. The present model is validated with the experimental and numerical benchmark results available in literature.

Experimental study and large Eddy simulation of a coaxial jet with perforated obstacles to control thermal mixing characteristics

ABSTRACTThermal mixing phenomena of a coaxial jet with perforated obstacles are analyzed both experimentally and numerically. Perforated obstacles are used in front of a coaxial jet that has different temperatures of the fluid to control the thermal mixing behavior. An experimental set-up was constructed to perform several cases and all cases are simulated by using large eddy simulation (LES) turbulence model. Results of the study presented that inserting perforated obstacle affects the mixing performance positively and the best mixing performance is obtained in the case that the obstacle has the highest permeability. Experimental and computational results were compared and a good agreement was obtained.

Experimental investigation on the thermal mixing characteristics at a 90° T-Junction with varied temperature differences

At the Fluid-Structure-Interaction (FSI) facility at the University of Stuttgart, thermal mixing experiments have been conducted to investigate thermal fatigue (a.k.a. thermal stripping) effects close to a weld connection in the piping material downstream of a 90° T-junction. For the mixing conditions close to the reality in nuclear power plants, have been performed with a cold flow of 20°C in the branch pipe (DN40) and a hot flow of max. 280°C in the main pipe (DN80) at 75bars. A geometric weld seam model is installed in the mixing zone downstream the T-junction to represent the rimmed root of a weld seam.The T-junction experiments in this work are performed with either temporally cyclical or continuous mixing processes. By a cyclical cold flow injection, an Artificially Induced TEmperature Change (AIPTEC) is created in the mixing region downstream of the T-junction. The experiments are conducted with different flow temperatures in the main pipe of 240°C, 200°C and 160°C, while the flow temperature in the branch pipe line was held constant at 20°C. Micro-thermocouples are installed in pipe modules in the mixing region downstream the T-junction and the flow inlet regions upstream of the T-junction to capture the temperature in the near-wall fluid with a sampling rate of 100Hz. The circumferential temperature distribution in the mixing region has been reconstructed from the near-wall temperature data. The thermocouple measurement data are processed by spectral analysis and presented as power-spectrum density (PSD) diagrams. The results of the analyses are summarized in the form of spectrum patterns. The spectrum patterns show information necessary for a characterization of the thermal mixing in relation to thermal fatigue. A pipe inner wall area with high potential for material damage due to thermal fatigue could be identified in the spectrum patterns. The results show that the change of the temperature difference has a significant influence on the stability of the thermal stratification in the mixing flow, as well as the phenomenon of reverse flow. It therefore has a significant influence on the thermal fatigue in downstream of the T-junction.

Experimental and computational analysis of thermal mixing characteristics of a coaxial jet

In this study, experimental and numerical analyses have been performed to investigate the mixing behavior of hot and cold fluids for a coaxial jet. Thermal mixing phenomenon is an important issue for many industrial applications, since thermal stress occurs when fluids do not mix completely. Several test cases were considered to investigate the mixing quality and frequency of temperature fluctuations. Computational study was performed using Large Eddy Simulation (LES) turbulence model, since this model is proven in this type calculations. A commercial CFD code, ANSYS-Fluent is used for numerical calculations. The study is performed for governing parameters as ratio of mass flow rate of hot (annular) jet to cold (central) jet and temperature difference between hot and cold jet. It is found that there is a good agreement between experimental and numerical studies with respect to spectral analyses. The dominant frequency of temperature fluctuation is found as 5Hz as compatible with literature. Thermal mixing efficiency increases with increasing temperature difference between hot and cold jet. Also, thermal mixing performance getting better with enhancing flow rate of hot jet in the first half of the channel and the best mixing observed at ṁh/ṁc=2 along second half of the channel.

Experimentally investigating the thermal mixing and thermal stripping characteristics in a T-junction

Through T-junction experiments, this paper investigates thermal mixing and thermal stripping characteristics. Different flowrates in the branch/main pipe and different branch injection directions are considered. The flow mixing pattern in a T-junction and the reverse phenomenon upstream a T-junction can be revealed in the measured temperature distributions and be observed from the blue dye distribution by the high-speed camera. The thermal stripping in a T-junction can be expressed using the non-dimensional power spectrum density (PSD∗) versus the Strouhal (St) number since the degradation of thermal fatigue on the pipe wall depends strongly on the frequency of fluid temperature fluctuation. The frequency spectrum can be obtained from the time history data by way of the fast Fourier transform (FFT) analysis. Based on the measured results, the velocity ratio (Vr) can be used to indicate the thermal mixing characteristics in a T-junction; however, the momentum ratio (MR) is suitable to discriminate the flow pattern and to reveal the reverse flow phenomenon from the branch injection. Detailed comparison of the present data and the measured results from the previous work reveals that the reverse flow upstream a T-junction can occur if MR is less than about 0.05, demonstrating that this conclusion has generality. In addition, according to the comparison of PSD∗ versus St number for x/D=1 and 9 under various Vr values, the piping degradation due to thermal fatigue for a T-junction is severe near its intersection and is stronger for the higher Vr values.