Mixed Convection Conditions Research Articles

Physical intuition of entropy generation in a mixed convective hybrid nanofluid flow with chemical reaction, cross‐diffusion, and transpiration

AbstractCompared with methanol (pure) and blood, methanol‐ and blood‐based nanofluids are significant with numerous characteristics, including thermophysical effects and immersion rate of CO2 in tray column absorber. Blood‐based liquids show importance in biological treatments like digestion as well as breathing systems. Mixed convective conditions and suction or injection are helpful in controlling the heating or the cooling in industrial applications. Keeping this in view, we examined the physical perception of entropy generation (EG) in a mixed convective hybrid nanofluid (HBN) flow with chemical reaction, cross‐diffusion, and transpiration in a curved surface with activation energy and thermal radiation. Transformed principal equations are determined with the assistance of a fourth‐order Runge–Kutta method with a shooting technique. We obtained solutions for two different cases, that is, for hybrid (mixture of cases) (magnetite + hematite + blood) and (magnetite + hematite + methanol). Obtained results are discussed via graphs and tables. Also, we discussed the effects of the Bejan number and the EG. We found that the EG appears more conspicuously in the case of magnetite + hematite + methanol compare to magnetite + hematite + blood case. We also found that in the magnetite + hematite + methanol case the flow and heat transfer characteristics are more pronounced compared with that in the case of magnetite + hematite + blood. This helps us to choose magnetite + hematite + blood or magnetite + hematite + methanol compositions in the manufacturing industry. Furthermore, the rate of heat transfer can be enhanced up to 16.42% in the case of methanol‐based HBN while the rate of heat transfer can be enhanced up to 19.16% in the case of blood‐based HBN.

Machine learning approach to predict the heat transfer coefficients pertaining to a radiant cooling system coupled with mixed and forced convection

Mixed convection phenomenon over radiant cooled surfaces with displacement ventilation in living environments is becoming a popular issue due to the airborne viruses and energy economy. Artificial neural networks are one of the machine learning methods that are widely evaluated as an engineering tool. In the current study, heat transfer coefficients for a radiant wall cooling system coupled with mixed and forced convection have been predicted by a machine learning approach. This approach should be noted as a first experimental investigation couple with an artificial neural network analysis in the open sources in which mixed convection systems in real sized living environments is examined. Experimentally obtained heat transfer coefficients have been used in the development of the feed forward back propagation multi-layer perceptron network structure. So as to analyze the impact of the input factors on the prediction performance, two neural network structures with dissimilar input parameters such as various temperatures, velocities, and heat transfer rates have been developed. By means of feed forward back propagation multi-layer perceptron neural network algorithms, convection, radiation, and total heat transfer coefficients have been predicted using the experimentally acquired dataset including 35 data points belonging to the mixed and forced convection conditions. Training, validation, and test data groups include 70%, 15%, and 15% of the dataset, in turn. Training algorithm has been computed via Levenberg-Marquardt one with 10 neurons in the hidden layer. The findings obtained from the computational solution have been evaluated as a result of the contrast with the target data with in the ±5% deviation band for all heat transfer coefficients. The performance factors have been computed and the estimation precision of the numerical models has been thoroughly examined.

Two-Dimensional Mixed Convection and Radiative Al2O3-Cu/H2O Hybrid Nanofluid Flow over a Vertical Exponentially Shrinking Sheet with Partial Slip Conditions

Hybrid nanofluid is considered a modern and improvised form of nanofluid which usually used to enhance the performance of heat transfer in fluid flow systems. Previous studies found hybrid nanofluid offered a wide range of applications and this opened up numerous new opportunities to further explore the unknown behaviour of hybrid nanofluid under different body geometries and physical parameters. This paper numerically studied a two-dimensional mixed convection and radiative Al2O3-Cu/H2O hybrid nanofluid flow over a vertical exponentially shrinking sheet with partial slip conditions. The main objective is to investigate the effect of mixed convection and radiation on the velocity and temperature profiles, as well as the effect of suction on reduced skin friction and reduced heat transfer with respect to solid volume fraction of copper, velocity, and thermal slips. Exponential similarity variables transformed the governing system of partial differential equations into a system of ordinary differential equations which is solved via MATLAB’s bvp4c solver. Outcomes showed that the value of the reduced heat transfer upsurges in the first solution but declines in the second solution when the velocity slip rises. The reduced heat transfer decreases in both dual solutions when thermal slip is enhanced. As the intensity of thermal slip increases, the reduced skin friction rises in the first solution and decreases in the second. As the mixed convection parameter increases, no obvious variation is noticed in the temperature distribution within the first solution, but increasing trend is observed within the second solution. An increment in the temperature distribution also observed within the dual solutions as the thermal radiation parameter increases. In summary, findings from this study are particularly useful to understand various behaviour of Al2O3-Cu/H2O hybrid nanofluid under the influence of mixed convection, radiation, and partial slip conditions when it flows over a vertical exponential shrinking sheet.

Analysis of turbulent flow and thermal structures in low-Prandtl number buoyant flows using direct numerical simulations

A direct numerical simulation (DNS) database is presented for turbulent flow and heat transfer in a vertical channel for Reynolds number Reτ = 150 and 640, Prandtl number Pr = 0.004, 0.025 and 0.71, with and without buoyancy forcing (Ri = 0 and 0.15 or 0.21). The effects of Pr on mean and turbulent flow and thermal transport in mixed convective conditions are discussed. Aiding/opposing buoyant conditions result in acceleration/deceleration of mean flow and reduction/enhancement of turbulence. Flow turbulence is highly anisotropic and dominated by the streamwise component on the aiding side, and becomes two-dimensional on the opposing side with a decrease in Pr. Temperature distributions depend on the relative role of molecular and turbulent thermal transport. The former increases with decreasing Pr and the latter with increasing Re. Buoyancy affects thermal transport through augmentation of the wall-normal turbulent heat flux, v′θ′¯, which is more pronounced for higher Pr. A priori analysis of the DNS datasets shows that a variable formulation for turbulent Prandtl number in Reynolds-averaged Navier-Stokes simulations performs well for both high- and low-Pr flows without buoyancy and in stable convective regimes, but yields relatively high error in unstable convective regimes.

Direct Numerical Simulation of natural, mixed and forced convection in liquid metals: selected results

Selected results of three Direct Numerical Simulations are presented, on relevant test cases for the thermal hydraulics of liquid–metal-cooled nuclear reactors, encompassing a wide spectrum of turbulent convection regimes. The first test case is a Rayleigh-Bénard cell at a Grashof number Gr=5×107, representative of the conditions in the unstably stratified layer of coolant in a reactor pool in both standard operating conditions and emergency situations, e.g. shutdown of the cooling system. The second case is the mixed convection in a cold-hot–cold triple jet configuration, representative of liquid streams exiting from the core into the pool, and relevant for the modeling of thermal striping and thermal fatigue phenomena on the vessel containment walls. The third case is the fully-developed flow in a vertical bare rod bundle with triangular arrangement and a pitch-to-diameter ratio P/D=1.4, in both forced and mixed convection conditions. These regimes respectively represent normal operation or decay heat removal conditions in reactor cores. The availability of these numerical databases will allows for an in-depth analysis of the turbulent flow and heat transfer in liquid metals under different convection regimes, and is also relevant for the development, calibration and validation of turbulent heat transfer models.

Characteristics of the wall temperature field in a mixed convection turbulent boundary layer

The behavior of the heated wall temperature field adjacent to a mixed convection turbulent boundary layer flow was investigated. The wall temperature field was captured using an infrared camera. The Richardson number was varied between experiments to produce a range of mixed convection conditions ranging from buoyancy dominant (RiL = 2.0) to inertia dominant (RiL = 0.01). From the 2D temperature fields recorded, the fluctuating temperature fields and associated low-order statistics were computed. A Fourier-based analysis of the fluctuating temperature field indicated the presence of multiple modes of energy exchange between heated wall and the adjacent turbulent boundary layer flow including energy input, dissipation, and a viscous-convective subrange for RiL ≤ 0.3. The Proper Orthogonal Decomposition (POD) was performed on the fluctuating temperature field. Results show cell-like patterns in low-order POD modes at all tested RiL suggesting a convective cell-like structure in the fluid region adjacent to the interface. The manifestation of these cell-like patterns was found to occur at increasing POD mode number as buoyant contributions decreased relative to inertial contributions.

О НАДЕЖНОСТИ ОЦЕНКИ ТЕПЛОВОГО РЕЖИМА ПОДЗЕМНОГО ОБЪЕКТА ХРАНЕНИЯ ОТРАБОТАВШЕГО ЯДЕРНОГО ТОПЛИВА НА БАЗЕ ЧИСЛЕННОГО МОДЕЛИРОВАНИЯ

The results of checking the reliability of a previously performed assessment of the thermal mode of an underground facility for long-term storage of spent nuclear fuel in a variant of an integrated reinforced concrete structure on three calculation grids (Coarse, Normal, Fine) are presented. The test was performed on a previously developed three-dimensional computer model in the COMSOL program. The mathematical basis of the model is following: continuity equation, Navier-Stokes equations averaged by Reynolds, standard turbulence model and general heat transfer equation. The consideration of mixed convection conditions was implemented in the "incompressible ideal gas" approximation. It was shown that when using computational grids with high resolution (Normal, Fine), some characteristics may be predicted. They are: temperature increase inside the built-in structure and at the boundary "rock mass - built-in structure" no more than 1°C; a significant change in the structure of the spatial distribution of temperature and an increase in maximum temperatures by about 1°C on the surface of the built-in structure. The duration of calculations in variants with small grids increased approximately 3.4 (Normal) and 7.7 (Fine) times, respectively

Turbulence model assessment and heat transfer phenomena inside a rectangular channel under forced and mixed convection

A very high-temperature gas-cooled reactor (VHTR) is an attractive option for hydrogen production due to its high operating temperature. The reactor cavity cooling system (RCCS), a passive safety system of the VHTR, surrounds the reactor vessel with rectangular riser channels and removes decay heat from it. Under emergency operation conditions such as a decrease in the flow rate of the RCCS, the flow condition inside the RCCS riser can become a mixed convection condition accompanying heat transfer deterioration. To design and operate the VHTR with reliable safety, accurate predictions for various convective heat transfer conditions in RCCS riser channels are required. In this study, by comparing the results of an air flow visualization experiment, the predictability of the turbulence models was assessed for the convective heat transfer and complex flow characteristics near the corner region of the rectangular duct confirmed from the experimental study. In this paper, the heat removal rate through the test section of the visualization experiment was quantified under various convective heat transfer conditions, and local temperature measurement data from the experiment were presented to describe the heat transfer deterioration phenomena inside a rectangular riser in turbulent mixed convection. Linear and nonlinear eddy viscosity models were evaluated by comparing the heat transfer coefficient, local temperature, local flow structure including secondary flow, and turbulence quantities obtained from the experiment. Furthermore, improvements to turbulence models for enhancing the prediction of mixed convection heat transfer in rectangular riser ducts were proposed.

Numerical analysis of the equipment position influence on the premises thermal regime under gas infrared emitter operation and mixed convection conditions

The analysis of mathematical modeling results on premise heating by a gas infrared emitter (GIE) during supply and exhaust ventilation operation is presented in this article. The model is presented in a one-dimensional non-stationary setting for an incompressible medium with allowances for the radiant heat flux transfer between opaque solid surfaces in the air. The air is transparent to thermal radiation. The traditional k-ε model is used for turbulence modeling. The possibility for creating comfortable conditions in the area of a horizontal surface with different heights, imitating laboratory equipment, is analyzed.

The heat supply object thermal regime under conditions of gas infrared emitter and air exchange system joint operation

The paper presents the results of experimental studies on the formation of thermal comfort conditions in large premises with the joint operation of gas infrared emitters and air exchange ventilation system. The values of temperatures and heat fluxes on the clothing surface under natural and mixed convection conditions were obtained. It has been established that the air exchange system operation in the premises reduces the temperature drop of the person clothes surface, respectively, a more favorable temperature condition is achieved in the working area.

Mixed convection in a lid-driven cavity with partially heated porous block

In this study, the lid-driven cavity problem is investigated under mixed convection conditions by using Finite Volume Method (FVM). A porous block is placed in the middle of the domain with 0.1 porosity. A heater is placed on the edge of the porous block to imitate a local porous heater. In order to investigate the effects of the heater position on the problem physics, the heater is located in 4 different directions (left, right, top bottom) and 3 different orientations (clockwise, counterclockwise and middle). Mixed convection conditions are provided by three different Richardson numbers, 0.1 ≤ Ri ≤ 10 and porous block properties are applied by employing 4 different Darcy numbers 10−7 ≤ Da ≤ 10−1. It is observed that Darcy number can be used to control the vortex formation in the domain under specific configurations. Decrease in Darcy number can increase the average Nusselt number drastically, up to 10 times increase. Heater orientation can be used to control the heat transfer effectiveness, vortex shape and size. Heater orientation can increase the average Nusselt number by 324%. It is also shown that both heater location and orientation show a non-monotonic behavior for different flow parameters.

Numerical simulation of the thermal regime of an underground spent fuel storage facility (built-in structure variant)

The results of a numerical simulation of the thermal regime of an underground facility for long-term storage of spent nuclear fuel in a built-in reinforced concrete structure are presented. Two computer models were constructed in a three-dimensional formulation in the COMSOL programme. The first model is based on the incompressible fluid approximation, while the second model is based on the "incompressible ideal gas" approximation. The mathematical basis of models: the continuity equation, Navier - Stokes equations averaged by Reynolds, the standard (k - ε) turbulence model, and the general heat transfer equation. Consideration of mixed convection conditions is implemented in the "incompressible ideal gas" approximation, where the air density is a function of temperature only. The most thermally stressful arrangement of spent fuel placement is investigated: U-Zr - defective - U-Be. The air rate is varied in the range from 21 to 0.656 m3/s. Numerical experiments were performed for up to 5 years of fuel storage. The principal difference between the non-stationary structure of the velocity fields predicted in the "incompressible ideal gas" model and the "frozen" picture of the aerodynamic parameters in the incompressible fluid model is emphasized. It is shown that the requirements for exceeding the temperature limit values are met when the object operates under conservative ventilation conditions (rate 0.656 m3/s) with a minimum of costs for organizing ventilation. The dynamics of heat flows directed into the rock mass through the base and from the surface of the built-in structure of the U-Zr fuel compartment to the air environment are analyzed. The predominance of the heat flow from the surface of the structure and the different times when the curves of the heat flow dynamics reach their maximum values are noted. The heat flow to the array reaches its maximum significantly faster than to the air.

Modelling of Applied Magnetic Field and Thermal Radiations Due to the Stretching of Cylinder

In this study, a numerical approach was adopted in order to explore the analysis of magneto fluid in the presence of thermal radiation combined with mixed convective and slip conditions. Using the similarity transformation, the axisymmetric three-dimensional boundary layer equations were reduced to a self-similar form. The shooting technique, combined with the Range–Kutta–Fehlberg method, was used to solve the resulting coupled nonlinear momentum and heat transfer equations numerically. When physically interpreting the data, some important observations were made. The novelty of the present study lies in finding help to control the rate of heat transfer and fluid velocity in any industrial manufacturing processes (such as the cooling of metallic plates). The numerical results revealed that the Nusselt number decrease for larger Prandtl number, curvature, and convective parameters. At the same time, the skin friction coefficient was enhanced with an increase in both slip velocity and convective parameter. The effect of emerging physical parameters on velocity and temperature profiles for a nonlinear stretching cylinder has been thoroughly studied and analyzed using plotted graphs and tables.

Review of research using analogy concept for thermal hydraulic and severe accident experiments

This paper introduces the experimental method of using mass transfer systems for heat transfer studies based on the analogy between heat and mass transfer. The copper electroplating system, composed of copper electrodes submerged in an aqueous solution of H2SO4 and CuSO4, is employed as the mass transfer system. The cupric ions transferred from anode to cathode simulate heat transfer. Applications of the experimental method are presented regarding the natural convection of the oxide pool in the IVR-ERVC (In-Vessel Retention and External Reactor Vessel Cooling) and the highly buoyant mixed convective flow conditions of the RCCS (Reactor Cavity Cooling System). The experimental method is extended to two phase flows using the reduced hydrogen at the cathode simulating the vapor bubbles. Some experiments simulating the CHF (Critical Heat Flux) problems are introduced. The paper aims at introducing the analogy experimental methodology as the preliminary quantitative experimental tools.

Forecast of the thermal regime of an underground storage facility for heat-generating materials under mixed convection conditions

The paper presents the results of a study based on numerical simulation methods of the thermal regime of an underground facility for long-term spent nuclear fuel storage in the version of a built-in reinforced concrete structure. A multiphysical computer model was constructed in a two-dimensional setting by means of the COMSOL software. The mathematical model was based on the continuity equations, Navier-Stokes equations and the general heat transfer equation. The conditions of mixed convection were taken into account in the ‘incompressible ideal gas’ approximation, in which the thermophysical properties of air were a function of temperature only. For two parameters of the model, the following values were taken: the air flow rates providing the velocity at the inflow boundary = 0.01, 0.03 and 0.05 m/s, and the effective heat conductivity coefficients of the material of the built-in structure = 1.0 and 2.0 W/(m×K). Numerical experiments were performed for a period of up to 5 years of fuel storage. Special emphasis was given to the fundamental difference between the non-stationary structure of the velocity fields forecasted in the model of an ‘incompressible ideal gas’ and the ‘frozen’ picture of aerodynamic parameters in the model of an incompressible fluid. An analysis was made of the dynamics of spatial temperature field distributions in different areas of the model. It was shown that the criterion temperature control requirements were met when the facility was operated under conservative ventilation conditions in terms of the air flow rate and the heat conductivity coefficient of the built-in structure material. The dynamics of heat flows directed into the rock mass through the base and from the surface of the built-in structure into the air was analyzed. The heat flow dominance from the structure surface was also noted. Finally, the influence of the effective heat conductivity coefficient of the built-in structure material and the air flow rate on the values of heat flows directed into the air and rock mass was demonstrated.

Transportation of hybrid nanoparticles in forced convective Darcy-Forchheimer flow by a rotating disk

This research article explores the effect of entropy generation through a non-linear radiative flow of viscous fluid of hybrid nanoparticles over a stretchable rotating disk. Mixed convection and slip conditions i-e velocities and thermal are examined. The examination is accomplished in aluminum oxide (Al2O3)-water and copper (Cu)-water nanofluids. Similarity transformations are utilized to reduce the governing problem into the nonlinear ordinary differential equations. Flow in the permeable medium is analyzed by assuming the Darcy-Forchheimer model. The impact of various parameters consisting of mixed convection parameter, porosity parameter, velocities as well as thermal slips parameters, stretching parameter, non-linear radiation parameter, and Reynolds number on radial velocity profile, tangential velocity profile and temperature profile are studied. Also, entropy generation and heat transfer rates have been analyzed in view of different parameters in the current study. The mathematical formulation is numerically solved by the bvp4c techniques

Geometry optimization for supercritical water heat transfer enhancement in non-uniformly heated rifled tubes

Optimization of geometric parameters for rifled tube is of great importance for safe operation of heat exchangers. In this paper, the influences of geometry, including rib height, pitch and rib shape, on the heat transfer performance of supercritical water in circumferentially non-uniformly heated rifled tubes were comprehensively studied. The differences of geometry effects on heat transfers in rifled tubes under uniform heating and non-uniform heating conditions were compared. The impact extent of geometry changes for forced convection and mixed convection heat transfer conditions were obtained by conducting simulations under relatively low q/G and high q/G conditions respectively. Finally, the mechanism of heat transfer enhancement resulting from the changes of geometric parameters was revealed. The results show that different geometric parameters have different influences on the overall heat transfer performance of supercritical water in rifled tubes. There exists an optimum value for rib height. According to the analysis, the optimum rib height is 0.45 mm for the present study. Heat transfer is enhanced with the decrease of pitch and the influence of rib shape is negligible. The changes of geometric parameters, which can improve the turbulence intensity in the boundary layer range of 30 < y+ < 100, can enhance the heat transfer performance of supercritical water in rifled tube with circumferentially non-uniform heating.

Optimisation of the Geometric Parameters of Longitudinally Finned Air Cooler Tubes Operating in Mixed Convection Conditions

The results of optimisation calculations presented in the article are related to longitudinally finned tubes of a heat pump evaporator operating under natural wind-induced flow of outdoor air conditions. The finned surface is characterised by an unusual, wavy fin shape. The article presents the methodology applied to seeking optimal geometric parameters of the finned tube in which thermal calculations were performed by modelling a mixed convection process on the finned surface using the finite volume method. In the case of maximising the heat flow with the minimum mass of the fins, the optimal solution was dominated by the minimum mass of the fins and thus geometric parameters correspond to the number of fins n = 6, fin height h = 0.065 and fin thickness s = 0.0015 m. Optimisation calculations made for maximum efficiency of the exchanger at constant mass indicated that the tube with ten fins (n = 10) with a height of h = 0.11 m and a thickness of s = 0.0018 m allowed maximum heat flow at the assumed mass of the fins in the exchanger tube model. The article proposes a simplified method of determining the optimal geometric parameters of the profile for any mass and maximum thermal efficiency.

Numerical simulations of jacket side thermal-hydraulic performance for large stirred vessels

In the present work, 3-D numerical simulations have been carried out to understand the flow patterns and heat transfer on the jacket side of a stirred vessel under mixed convection conditions. Various parameters have been varied (1 ≤ Uin ≤ 20 m/s, 30 ≤ ΔT ≤ 60 K, 30 ≤ Jg ≤ 150 mm, Di = 1 and 3 m) to understand their effect on the heat transfer and pressure drop on the jacket side. SST k-ω model has been used for the numerical simulations because of its capability to predict natural convection heat transfer and low Re forced convection heat transfer. It was found that, the heat transfer coefficient increases by 40-80% by increasing the inlet fluid velocity from 1 to 5 m/s. The pressure drop increases by 120-200% and heat transfer per unit pumping power decreases by 60-75% as we increase the velocity from 1 to 5 m/s. It has also been observed that the flow inside the jacket is 3-D, nonuniform and has dead zones and hot pockets at various locations. The effects of inlet flow impinging have also been studied by varying inlet diameter and removing inlet pipe. Impinging by inlet flow on jacket wall contributes about 10-20% to heat transfer and 10-25% to pressure drop. Due to the presence of recirculating flow zones, dead zones, and impinging effect, existing correlations are not suitable for the estimations of heat transfer in large jackets. Large differences have been observed between the 3-D CFD predictions and the widely used empirical correlations. Further, the 3-D CFD predictions have been shown to be comparable to the experimental data available in the published literature. Based on simulations performed in the present work and also validated by experimental data, a new correlation has been proposed which is expected to be useful to design engineers, particularly for those, having limited computational facility.

Optimal Conditions of Natural and Mixed Convection in a Vented Rectangular Cavity with a Sinusoidal Heated Wall Inside with a Heated Solid Block

The present investigation deals with the natural, mixed and forced convection in a vented rectangular cavity having a sinusoidal heated vertical wall with a conducting solid block placed at one of the nine positions. The objective is to analyze numerically using finite element method the effects of the following parameters: inlet, outlet positions, solid square positions, thermal coefficient λ, amplitude ratio ɛ, phase deviation ϕ and the solid square size on the thermo-convective flows. The Richardson number is varied from 0 to 40, the Reynolds and Prandtl numbers are fixed respectively at 100 and 0.71. To quantify the heat transfer of the solid block and to get closer to real conditions, we have developed a modification based on the evaluation of the Nusselt number using the average temperature in the cavity, unlike previous works which used the input temperature. As results, the sinusoidal temperature at the right wall gives higher heat transfer enhancement. The variation of the phase deviation and amplitude ratio have a slightly effect on the average fluid temperature and average Nusselt at the right wall and at the square solid.