Mixed Convective Air Flow Research Articles

Measurement of the turbulent heat fluxes in mixed convection using combined stereoscopic PIV and PIT

The results of simultaneous measurements of velocity and temperature fields in a turbulent mixed convection airflow are analyzed and discussed. To access local temperature and velocity fields in airflows, we present a combination of stereoscopic particle image velocimetry and particle image thermometry. The obtained flow fields make it possible to determine the local convective heat fluxes, thus giving insight into the dynamics of plumes and Taylor–Görtler-like vortices. The evaluated mean local heat fluxes further reveal that the main convection roll transports a substantial amount of heat along the cooling plate and back to the heated bottom plate. Yet, the associated mean turbulent heat fluxes remain positive as they are dominated by the correlation of the temperature and the vertical velocity component. More specifically, a statistical analysis of the local heat flux distribution reveals that Taylor–Görtler-like vortices lead to more skewed distributions of the turbulent convective heat fluxes than plumes.Graphical abstract

On the fast fluid dynamics and fractional step methods to predict the coupled indoor temperature and velocity fields

The two-way coupling of temperature and flow fields in the indoor environment makes the conventional CFD simulation computationally intensive, especially when the transient evolution of temperature and velocity fields is of interest in areas such as unsteady ventilation. This study, by applying different splitting CFD methods (fast fluid dynamics (FFD) and fractional step (FS) method) to solve the momentum equation to decouple velocity and pressure fields, further explored different strategies to tackle the coupled temperature equation, and validated the different sequences of operation based on different combinations of temperature/velocity solution schemes. Each operational sequence of different combined numerical schemes was validated on a mixed convection airflow benchmark case with a heated floor. The results showed that different operational sequences using the numerical schemes which considered the strong coupling effect of temperature and flow fields could predict the final steady-state result well, while for the transient simulation that need to track the physical flow/thermal fields variation process precisely, the operational sequence which introduce the effect of buoyancy into volumetric flux to solve pressure Poisson equation showed better accuracy and owned the capability of predicting the same transient evolution process as conventional CFD method. Based on the comprehensive comparison, several promising numerical schemes under proper operational sequence were recommended and the highest computing efficiency was provided by sequence D of RIPC + Iterative which saved 68% computing time compared to the conventional CFD method.

3D Numerical study of the effect of aspect ratio on mixed convection air flow in upward solar air heater

A 3D Numerical study of mixed convection air flow in upward solar air heater with large spanwise aspect ratio (A = 10 to 40) was performed using CFD commercial code Fluent 14.5 (ANSYS). The main objective of this study is to investigate the channel height's effect (aspect ratio) on flow pattern and heat transfer in upward solar air heater in the particular case of low Re and high aspect ratio. The bottom plate (absorber) was submitted to Constant Heat Flux (CHF) in the range of 200 to 1000 W/m2 and Reynolds number was varied from 50 to 1000. Our results are in concordance with most of authors conclusions about Poiseuille–Rayleigh–Benard flows. In mixed convection, increasing heat flux enhances heat transfer unlike forced convection flows. Simulation results of flow visualizations and Nusselt number calculations have shown that depending on Ri*, the velocity and temperature distributions in SAH vary greatly with the channel's height. The obtained results were different from previous studies. Indeed, our investigation of channel's height was achieved for the same heat flux but different Grashof numbers. For low channel's heights (high aspect ratio), increasing heat flux has not a significant effect but for higher channel's heights, an augmentation of heat flux enhances buoyancy effects in the flow and causes high turbulence. Also, increasing Reynolds number in low channel's heights (high A), can enhance substantially heat transfer. For higher channel's heights (low A), increasing Reynolds number decreases Ri* and thus buoyancy forces. Heat transfer is reduced and so Nusselt number. The obtained results may be very useful for engineers in designing and testing solar collectors.

Steady and unsteady flow regimes in two-dimensional mixed convective flow of air past a heated square cylinder

The present work involves a numerical investigation of bifurcations/transitions between unsteady to steady as well as steady to unsteady flow states found to exist in two-dimensional mixed convective laminar flow of air past a heated square cylinder. The unsteady flow governing equations, subjected to Oberbeck-Boussinesq approximation, are marched forward in time using a SMAC type pressure correction scheme on a structured body-fitted grid. A finite difference type discretization is employed to discretize the equations in space. Numerical simulations are carried out in the parametric space of Richardson number (Ri) and free-stream orientation (α) with respect to gravity for fixed values of Reynolds number (Re) in the range [20, 120] in steps of 20. Computations are carried out for 0 ≤ α ≤ 90° and 0 ≤ Ri ≤ 1.6. At Re = 20, only steady flows are observed for any (α, Ri) setting within the ranges considered. However for 40 ≤ Re ≤ 120, at a fixed (α, Re) with progressively increasing values of Ri, transitions from both unsteady to steady as well as steady to unsteady are observed. Stuart-Landau theory is employed to analyze the bifurcations and to estimate the critical Ri corresponding to the bifurcations/transitions. The critical Ri as a function of α (or neutral-curve) is obtained at different Re in the range 40 ≤ Re ≤ 120.

Performance of characteristic numerical boundary conditions for mixed convective flows past a heated square cylinder using a non-Boussinesq approach

This work represents an investigation of the performance of characteristic numerical boundary conditions on the unconfined 2D mixed convective flow of air past a heated square cylinder governed by a non-Boussinesq flow model. The working fluid (air) is taken to be thermally perfect with variable thermo-physical and transport properties. The fully compressible Navier-Stokes equations are transformed using body-fitted coordinates and solved using particle velocity upwind (PVU-M+) scheme on a structured O-type grid. To investigate the performance of characteristic numerical boundary conditions, the gravity aligned upward flow past a heated square cylinder at (α = 0°, M = 0.1 and 0.2, Re = 100, Fr = 0.5 and 1.0, Pr = 0.71, γ = 1.4, and ε = 0.07 and 1.0) is considered. The overheat ratio (ε) is defined as (Tw – T∞)/T∞, where Tw and T∞ are the uniform cylinder and free stream temperatures, respectively. In order to determine the proper choice of boundary conditions to be imposed on the far boundary, algebraic characteristic boundary conditions of Euler equations (AECBC) are considered in a local 1D sense normal to the far boundary. In this work, two sets of algebraic characteristic boundary conditions are considered. In one set (BC Set I), AECBC is used over the entire far boundary. While in the other set (BC Set II), AECBC is used over a portion of far boundary, that is characterized by normal flow velocity magnitude in excess of a threshold value, and entropy preserving extrapolating conditions for pressure and density over the remaining portion of the far boundary. It is shown that BC Set I generates spurious wave reflections and numerical noise from the far boundary, whereas BC Set II eliminates these numerical artifacts and captures the flow physics far more accurately.

Forced, natural and mixed convection benchmark studies for indoor thermal environments

In this study, some well-known experimental studies related to forced, natural, and mixed convections were used for validation of k-ε and k-ω turbulence models. For this purpose ANSYS-Fluent 16.0 is used. International Energy Agency IEA Annex20 room, a tall differentially heated rectangular cavity, and a mixed convective air flow within a square chamber with a heated bottom wall were considered for forced, natural, and mixed convection respectively. Standard, RNG and Realizable models of k-ε group, and Standard, SST and BSL models of k-ω group with enhanced wall treatment for near wall modeling were tested by comparing the velocity and temperature distributions with available measurement values of employed geometries. In total, the results of Standard and RNG k-ε models are in good agreement with experimental measurements. Although the performance of k-ω group models is well in natural convection, some results of these models do not agree well with test data in forced and mixed convection cases.

Simulation of a novel hybrid solar photovoltaic/wind system to maintain the cell surface temperature and to generate electricity

A hybrid solar photovoltaic/wind system is proposed and investigated theoretically. The hybrid system is based on attaching a converging inclined duct beneath the photovoltaic (PV) panels and directed upward after the end of the panels. A wind turbine is attached at the exit of the converging duct. The converging duct will capture wind currents that at its inlet and enhances these current by buoyancy effect created by the rejected heat from the panels. The mixed convection air flow is used in cooling the PV panels and in generating electricity by driving the wind turbine at the duct exit. A mathematical model is proposed to describe the system hydrodynamic and thermal behavior. In addition to the mixed convection case, the pure free convection case, when there is no wind speed, has been tested. The design of the wind duct capturing system is not included in this study, which should be carefully manufactured to eliminate the reversed flow. The simulation results show that the integration of both systems not only enhances the performance of PV cell due to the effective cooling but also generates more electric power from the inserted turbine. At low wind speeds, it is found that the ducting system helps more in cooling the panels rather than driving the wind turbine. At these low wind speeds, the buoyancy effect may have a significant effect. However, at high wind speeds, the ducting system acts in both cooling the panels and driving the turbine, and at these high speeds, the buoyancy effect is insignificant.

Simulación numérica de la convección mixta en un canal vertical aletado

A numerical simulation for mixed convective airflow through a vertical finned channel was carried out by means of a MATLAB® code. Steady state and two dimensional conditions are considered for solving the problem. The discretization technique used was finite volume and the SIMPLE (Semi-Implicit Pressure Linked Equations) algorithm was used to link the pressure and velocity fields inside the computational domain. The simulation was carried out for Re = 50, 100 and 150. The heat transfer problem was solved for a temperature difference between the hot wall and the fluid temperature at the entrance of 50K and 70K. These combinations resulted on Richardson of 0.46, 1.04 and 4.16 corresponding to forced, mixed and natural convection, respectively. As a result, the fluid flow structures, the flow velocity and temperature fields as well as the recirculation zones adjacent to the fines are shown. Local Nusselt number and bulk temperature through the channel are presented. The Nusselt number and the recirculation zones increase when the Reynolds number is augmented.

SIMULACIÓN NUMÉRICA DE LA CONVECCIÓN MIXTA EN UN CANAL HORIZONTAL ALETADO

Se realizo la simulacion numerica para flujo de aire con transferencia de calor por conveccion mixta en estado permanente y regimen laminar en dos dimensiones a traves de un canal horizontal con 4 aletas, mediante un codigo numerico en la plataforma MATLAB®. Se utilizo la tecnica de los volumenes finitos para la discretizacion de las ecuaciones que gobiernan el fenomeno y el algoritmo SIMPLE (Semi-Implicit Pressure Linked Equations) para acoplar la distribucion de presiones y el campo de velocidades dentro del dominio computacional. El analisis de flujo se dio para un parametro de Re=50, 100 y 200; y una diferencia de temperaturas entre la pared mas caliente y la temperatura del fluido a la entrada de 50 y 70. Como resultado se presentan las estructuras de flujo, las distribuciones de velocidad y temperatura, se muestran las zonas de recirculacion adyacentes a las aletas. Se presentan las graficas del Nusselt local encontrando que este aumenta con el incremento del Reynolds. Asimismo, se encuentra que las zonas de recirculacion crecen en tamano a medida que se aumenta el Reynolds.

Numerical Simulation of Mixed Convection Airflow Under a Dome-Shaped Roof

Mixed convection airflow under a ventilated dome-shaped roof is studied numerically using the Low-Reynolds-Number (LRN) k−ω model for turbulence modeling. The axisymmetric two-dimensional governing equations are discretized using control volume approach. The SIMPLER algorithm is used for coupling between pressure and velocity components. Air is used as the working fluid and computational results are presented in the form of streamline and temperature patterns. The local and average Nusselt numbers over the heated and cooled boundaries for a wide range of Reynolds numbers (Re = 1 × 102−5 × 104) and Grashof numbers (Gr = 1 × 103−1 × 1010) which include both laminar and turbulent flow regimes are presented as well. The results are categorized into two primary flow patterns based on the inflow Reynolds number values. The influence of Reynolds number, Grashof number, and inflow non-dimensional temperature on the flow filed and thermal patterns is shown and discussed. The results show that in the natural convection dominated regime, by increasing Grashof number most of the air motion is restricted beneath the hot dome. Meanwhile, much of the core region is occupied with a low velocity fluid which are the characteristics of a stable thermal stratification. In the forced convection dominated flow regime, two primary patterns were recognized. In the first pattern a simple flow is formed, while in the second pattern a strong clockwise shear vortex under the dome hot surface is created. Thermal communication between the hot and cold surfaces is attributed to the presence of this shear vortex.

Numerical analysis of turbulent mixed convection air flow in inclined plane channel with k-ɛ type turbulence model

Numerical study on turbulent mixed convection in inclined plane channels, from 15° to 90° (vertical), was carried out to examine the effect of inclination on fluid flow and heat transfer distributions. The turbulent air flows upward or downward into the duct with one wall heated from bottom. Calculation results with several kinds of k-ɛ type turbulence models were used to compare the experimental data with those in literatures to determine suitable model. The dependents of Nusselt number on the inclination angle of both the buoyancy-aided and buoyancy-opposed flow are discussed.

Turbulent mixed convection flow over a forward-facing step—the effect of step heights

Measurements of heat transfer and fluid flow of turbulent mixed convection boundary-layer air flow over an isothermal two-dimensional, vertical forward-facing step are reported. The upstream and downstream walls and the step itself were heated to a uniform and constant temperature. Air velocity and temperature distributions and their turbulent fluctuations are measured simultaneously by using, respectively, a two-component laser-Doppler velocimeter (LDV) and a cold wire anemometer. The present study examines the effect of forward-facing step heights on turbulent mixed convection flow along a vertical flat plate. The experiment was carried out for step heights of 0, 11, and 22 mm, at a free stream air velocity, u ∞ , of 0.55 m⋅s −1, and a temperature difference, Δ T, of 30 °C between the heated walls and the free stream air (corresponding to a local Grashof number Gr x i = 6.45 × 10 10 ). It was found that the turbulence intensity of the streamwise and transverse velocity fluctuations and the intensity of temperature fluctuations downstream of the step increase as the step height increases. Also, it was found that both the reattachment length and the heat transfer rate from the downstream heated wall increase with increasing step height.

Stabilization and elimination of transient unstable mixed convective vortex flow of air in a bottom heated horizontal flat duct by top plate heating

An experiment combining flow visualization and temperature measurement is carried out here to study the possible stabilization and elimination of the buoyancy driven unstable longitudinal, transverse and mixed vortex flow in mixed convection of air in a bottom heated horizontal flat duct by the top plate heating. The top plate temperature is varied systematically to examine its effects on the spatial and temporal flow structures in the duct. How the top plate temperature and the Reynolds and Rayleigh numbers of the flow affect the vortex flow characteristics is investigated in detail. Specifically the experiment is conducted for the Reynolds number varying from 1 to 50, Rayleigh number from 4000 to 8000 and the non-dimensional top plate temperature from 0 to 1 at an interval of 1/8, covering a wide range of the buoyancy-to-inertia ratio. The results indicate that the top plate heating substantially stabilizes and for some cases even eliminates the longitudinal, transverse, mixed longitudinal and transverse, and irregular vortex flows induced by the buoyancy associated with the heated bottom plate of the duct. At the high top plate temperature even the entire irregular vortex flow can be eliminated and the flow becomes unidirectional in the duct. Obviously the transient velocity and temperature oscillations in the flow are completely suppressed.

Structure of mixed convective longitudinal vortex air flow driven by a heated circular plate embedded in the bottom of a horizontal flat duct

In this study experimental flow visualization combined with transient temperature measurement are conducted to investigate the structure of the buoyancy driven longitudinal vortex rolls in low Reynolds number mixed convective air flow through a horizontal flat duct with an isothermally heated circular disk embedded in the bottom plate of the duct for the Reynolds number ranging from 15.1 to 99.2 and Rayleigh number from 3506 to 29,493. How the circular geometry of the heated surface affects the longitudinal vortex flow characteristics is investigated in detail. The results indicate that the longitudinal vortex rolls (L-rolls) in the duct core are induced at more upstream locations than those near the duct sides, which is completely opposite to those induced in a duct with a uniformly heated bottom. Besides, the thermals driven by the circular heated surface are not evenly spaced in the spanwise direction and are slightly asymmetric. It is of interest to note that at a given Rayleigh number Ra the thermals are unstable at high Reynolds numbers, suggesting the existence of the inertia driven instability. Thus the L-rolls evolved from these thermals are also unstable with the presence of nonperiodic generation and disappearance of new L-rolls. But at slightly lower Re the thermals and L-rolls are steady and regular. The vortex flow becomes unstable and irregular for a further reduction in the Reynolds number, which obviously results from the buoyancy driven instability. The simultaneous presence of these two instability mechanisms explains the appearance of the reverse steady–unsteady transition in the vortex flow. Based on the present data, a flow regime map is given to delineate various L-roll patterns driven by the circular heated plate. In addition, the boundaries separating these patterns are empirically correlated. Empirical correlations for the onset points of the L-rolls are also provided.

Stationary transverse rolls and U-rolls in limiting low Reynolds number mixed convective air flow near the convective threshold in a horizontal flat duct

Combined experimental flow visualization and temperature measurement are carried out in the present study to explore the buoyancy driven vortex flow patterns in a limiting low Reynolds number mixed convective air flow through a bottom heated horizontal flat duct. In Particular, attention is paid to the flow approaching the natural convection limit ( Re=0) for Re=1.0 and 2.0 with the Rayleigh number near the critical level for the onset of convection for 1200⩽ Ra⩽4000. Results from the flow visualization have revealed two unfamiliar vortex flow patterns which were not seen in our earlier study [Int. J. Heat Mass Transfer 44 (4) (2001) 705]. One is characterized by the stable stationary transverse rolls in the duct entry and stable longitudinal rolls in the downstream. Another is in the form of U-rolls. The relations of these two patterns with those reported in the literature from analytical, numerical and experimental studies are discussed. Moreover, stable longitudinal rolls along with nonperiodic traversing waves, and mixed longitudinal and transverse rolls as well as irregular cells which appear in the higher Reynolds number for 3.0⩽ Re⩽5.0 are also noted here. The temporal and spatial characteristics of the unfamiliar vortex flows are inspected in detail. In addition, the flow formation processes leading to the two unfamiliar vortex flow structures are also examined carefully. During the flow formation we noted merging of longitudinal and transverse rolls to form U-rolls, splitting of rolls into cells and the reverse process of cell integration into rolls, aside from the generation of the longitudinal and transverse rolls. Finally, a flow regime map is provided to delineate various vortex flow structures observed in this study and in the previous study (cf. the above-mentioned reference) driven by the slightly supercritical and subcritical buoyancies for 1.0⩽ Re⩽5.0.

Vortex flow patterns near critical state for onset of convection in air flow through a bottom heated horizontal flat duct

In this paper, an experiment is carried out to investigate the buoyancy driven vortex flow at slightly subcritical and supercritical Rayleigh numbers in a mixed convective air flow through a bottom heated horizontal flat duct. Particular attention is paid to the flow at a very low Reynolds number for 3.0≤ Re≤5.0. Results from the flow visualization have revealed two new vortex flow patterns in addition to those often seen in literature such as longitudinal rolls (L rolls), moving transverse rolls (T rolls), and mixed longitudinal/transverse rolls (M rolls). The newly observed vortex flow patterns include the stable longitudinal rolls near the duct sides along with nonperiodic traversing transverse waves in the duct core, and the mixed longitudinal and transverse rolls as well as irregular cells. Moreover, steady longitudinal rolls and nonperiodic traversing transverse waves are noted even at subcritical Rayleigh numbers. A flow regime map delineating various vortex flow patterns is given. Temporal characteristics of the flow are also inspected. Furthermore, a correlation is given to estimate the local onset locations of the longitudinal rolls. Meanwhile, the oscillation frequency and convection speeds of the transverse rolls are correlated from the present data. Finally, many complicate processes during the vortex flow formation are noted, such as the generation of the L and T rolls and transverse waves, splitting of rolls into cells and the reverse process of cell integration into rolls, aside from the moving and bending of T rolls.

Large Eddy Simulation of Natural and Mixed Convection Airflow Indoors with Two Simple Filtered Dynamic Subgrid Scale Models

Large eddy simulation (LES) with a dynamic subgrid-scale model (DSM) is a powerful tool to predict indoor airflow. Some DSM models need to average the model coefficient over a homogeneous direction or need to use an additional parameter. The others are too complicated. This study proposes a simple filtered dynamic subgrid-scale model (FDSM) for indoor flow without a homogeneous direction. The predicted air velocity, air temperature, and turbulence distributions agree reasonably well with the experimental data. The results show that the FDSM can be used to simulate indoor airflow.

Mixed Convection Transport from a Module on the Bottom Surface of Three Dimensional Channel

Conjugate heat transfer from a heat generating module () bonded through the module support on the floor of a parallel-plate channel(20mm high, 400mm wide, and 800mm long) to mixed convective air flow(0.20.9m/s) is studied experimentally. The input power to the module is changed in a range 1.04.5W, the floor thickness 0.25mm, and the thermal resistance of module support, Rc:=0.06, 1.03 and 82.0K/W. Thermal conductance(Uc) of the board and convective thermal conductance() from the module were derived, and the effect of V; Rc and t on Uc was investigated. It is found that the conjugate conductance (Uc) and the conductive heat transfer ratio (/Q) depend on the thermal resistance of the module support, the air velocity and the board thickness. The change of the module support resistance and the board thickness helps to elucidate the relative significance of heat transfer paths through the module support, the board, and from the board surface to the air. Additional information is investigated about the dependence of the heat transfer rate on the mixed convection parameter

Stabilization of Buoyancy-Driven Unstable Vortex Flow in Mixed Convection of Air in a Rectangular Duct by Tapering Its Top Plate

Abstract Stabilization of the buoyancy-driven unstable mixed convective vortex air flow in a bottom heated rectangular duct by tapering its top plate is investigated experimentally. Specifically, the duct is tapered so that its aspect ratio at the duct inlet is 4 and gradually raised to 12 at the exit of the duct. In the study the secondary flow in the duct is visualized and the steady and transient thermal characteristics of the flow are examined by measuring the spanwise distributions of the time-average temperature. The effects of the Reynolds and Grashof numbers on the vortex flow structure are studied in detail. Moreover, the spanwise-averaged Nusselt numbers for the horizontal rectangular and tapering ducts are also measured and compared. Furthermore, the time records of the air temperature are obtained to further detect the temporal stability of the flow. Over the ranges of the Re and Gr investigated for 5⩽Re⩽102 and 1.0×104⩽Gr⩽1.7×105, the vortex flow induced in the rectangular duct exhibits temporal transition from a steady laminar to time periodic and then to chaotic state at increasing buoyancy-to-inertia ratio. Substantial change in the spatial structure of the vortex flow is also noted to accompany this temporal transition. The results for the tapering duct indicate that more vortex rolls can be induced due to the increase in the aspect ratio of the duct with the axial distance. But the vortex rolls are weaker and are completely stabilized by the tapering of the top plate. [S0022-1481(00)70301-X]

Observation and Computation of Vortex and/or Reverse Flow Development in Mixed Convection of Air in a Slightly Inclined Rectangular Duct

Combined flow visualization and conjugated numerical heat transfer analysis were carried out to study the axial evolution of the buoyancy induced secondary vortex and reverse flow in a mixed convective air flow through a bottom heated, slightly inclined rectangular duct. Results were obtained for the Grashof number Gr ranging from 1.6 × 103 to 2.8 × 105, inclined angle φ from −20 deg to 26 deg and the Reynolds number Re below 102 covering the steady and time dependent flows. For the buoyancy-opposing case, at a certain critical buoyancy-to-inertia ratio depending on the Re and φ both the experimental and numerical results clearly showed the generation of the longitudinal vortex rolls in the entry half of the duct and a slender reverse flow zone was induced near the exit end of the duct. At a higher buoyancy-to-inertia ratio the stronger reverse flow moves upstream and is in a time periodic snaking motion which is considered to result from the Kelvin-Helmholtz instability associated with the two counter flow streams, namely, the downstream moving longitudinal vortex rolls and the upstream moving reverse flow. Through the viscous shearing effects the strong snaking reverse flow induces a number of eddies moving along it and the longitudinal rolls are pushed towards the duct sides. This strong interaction between the vortex flow and reverse flow leads to an earlier transition to turbulence. A correlation equation was proposed for the penetration length of the reverse flow. However, for buoyancy-assisting flow no reverse flow is induced and the longitudinal vortex rolls prevail for the buoyancy-to-inertia ratio up to 2.8 × 105. Significant conjugated heat transfer effects were noted from the numerical results.