Forced Convection Flow Research Articles

Estimation of the local instantaneous heat flux inside a pulsating heat pipe for space applications

Pulsating Heat Pipes (PHP) are promising and effective passive two-phase heat transfer devices in terms of high heat transfer capability, efficient thermal control, adaptability and low cost and therefore they have been extensively studied in the last years. Many authors have estimated the heat fluxes at the evaporator and at the condenser area only in terms of the average values based on first principle considerations. In the present study the application of an inverse analysis technique to experimental infrared temperature data is proposed to investigate the local convective heat flux for forced convection flow inside these devices along the adiabatic zone. A PHP specifically designed to be hosted on board the heat transfer host of the International Space Station was tested in microgravity during the 67th Parabolic Flight Campaign organized by the European Space Agency. The device consists of an aluminium tube closed in a loop with 14 turns in the evaporator section, 3 mm inner diameter, half filled with FC-72 fluid. The external temperatures of the device are measured in the adiabatic zone with a high-speed infrared camera (50 Hz, 1280x1024 pixels). The images are thereafter post-processed and adopted as input data for the solution of the inverse heat conduction problem in the pipe wall (Tikhonov regularisation method) to extrapolate time-varying local heat fluxes on the tube internal surface in contact with the fluid.

Development of the hydrodynamic and thermal boundary layers of forced convective laminar flow through a horizontal tube with a constant heat flux

The development of the thermal and hydrodynamic boundary layers of laminar flow through a smooth horizontal tube was investigated for forced convective conditions. The tube was heated at a constant heat flux and had an inner diameter of 4 mm and a length of 6 m. Numerical simulations were conducted using ANSYS Fluent 19.3 with a highly structured mesh and accurate temperature-dependent thermophysical properties. Laminar flow of air, water, and a mixture of 75% ethylene glycol (by weight fraction) in water was simulated, which covered a Reynolds number range of 630 to 2000 and Prandtl number range of 0.7–70. Theoretical methods were also used to obtain the relevant boundary layer growth parameters, such as velocity profiles, velocity gradient, vorticity, temperature profiles, Nusselt numbers, as well as hydrodynamic and thermal boundary layer thicknesses. Subsequently, methods were proposed to determine the development of both boundary layers in the entrance region, the locations at which the boundary layers merged, and the location at which the flow became fully developed. By analysing the velocity and temperature profiles, it was found that our conventional understanding of the merging of the boundary layers in internal tube flows had to be modified. Three distinct regions were identified in the hydrodynamic entrance region of laminar forced convective tube flow: (1) hydrodynamic inlet region, (2) shear stress developing region, and (3) vorticity adjustment region. Furthermore, three distinct regions were identified in the thermal entrance region: (1) thermal inlet region, (2) convective diffusion region, and (3) conductive diffusion region.

Turbulent forced convective flow in a conical diffuser: Hybrid and single nanofluids

Turbulent forced convective flow of hybrid and single nanofluids in a conical diffuser is investigated numerically. Simulations are conducted for various Reynolds (Re=10000-70000) and different concentrations (ϕ=0-1.5 vol%) at equal ratio of TiO2:SiO2. The impact of using theoretical and experimental correlations for dynamic viscosity and thermal conductivity on turbulent forced convection of TiO2 showed that the mean Nusselt (Nu) number is considerably reduced with the use of the experimental model. However, when the theoretical model is used, Nu varies insignificantly. Addition of TiO2 nanoparticles decreases the heat transfer inside the diffuser, whereas addition of TiO2–SiO2 nanoparticles either enhances or decreases the heat transfer rate. Compared to the pure fluid, hybrid nanofluids show a maximum enhancement of 5% and a maximum decrease of 9.7% at ϕ= 0.5 vol% and ϕ= 0.5 vol% at Re=10000, respectively. However, TiO2 nanofluids show a maximum decrease of 19% at ϕ=1.5 vol% and Re= 30000. As Re increases, the deviations between TiO2 and SiO2-TiO2 nanofluids diminish. Moreover, the Gene Expression Programming model can accurately evaluate Nu versus Re number and nanoparticle concentration.

Exergy efficiency and exergy destruction assessment of heat transfer and fluid flow through spiral passages using source-sink model

Exergy efficiency and exergy destruction of forced turbulent convective fluid flow through spiral passages using sourcesink model are assessed. Constant and temperature dependent thermophysical properties of process fluid in sink regions have been considered, while in the source regions phase change material is kept at constant temperature. Moreover, the effect of a secondary flow phenomenon due to spiral motion of the fluid flow through narrowing spiral passages has been evaluated in the sink passage domain. It was found that this phenomenon plays an important role in decreasing the exergy destruction as a result of energy exchange between source and sink in this compact type of energy carrier or storage equipment. It was found that the De (Dean Number) whose magnitude is a measure of the secondary flow, has positive impact on exergy efficiency while decreasing along the spiral passage. The fluid properties that are temperature dependent have a significant influence on the enhancement of heat transfer process, exergy efficiency and exergy destruction. It is concluded that the source-sink model could be considered from engineering point of view as a good simple model for evaluating the thermodynamic performance of this type of thermal energy process device.

Near wall temperature measurements and turbulent features in a water flow at transition Reynolds numbers in a square heated asymmetric cavity channel

• Temperature profile and fluctuations in water flow in a heated cavity channel are measured at medium Reynolds numbers. • The conductive sublayer is clearly detected with a micro-thermocouple. • Temperature turbulent intensity shows peaks within the conductive sublayer with increasing magnitude along the channel. • Autocorrelation analysis reveals two trends in and outside the boundary layer attributed respectively to random turbulence and recirculation. • A slight peak in the Nusselt number distribution is observed in the centre of the channel as the Reynolds number increases. Passive cooling configurations are attracting an increasing interest in different thermal management applications. Nevertheless, a more efficient design requires deeper understanding of heat transfer and flow mechanisms. Among these, turbulence and convection play an important role. In particular, the so-called conductive sublayer close to the wall, where significant temperature gradients are present, is critical. Despite its importance, to the best knowledge of the authors only few experimental works have measured the temperature profile in the laminar sublayer of forced-convective turbulent flows. In the present work, measurements of the temperature profiles and temperature fluctuations in the boundary layer for water forced-convective flow in a heated cavity channel (for Reynolds number in the low turbulent regime [Re ≅ 3640-4970] and for different imposed heat fluxes [q w ≅ 9500, 13700, 19350 W/m 2 ]) were carried out using a micro-thermocouple at different streamwise positions. A new method is proposed for validating the temperature measurements. A minimum point in the temperature profile towards the inlet of the channel is observed which attributed to flow recirculation in the cavity. Increasing the fluid velocity resulted in a shift of the position of the minimum point towards the heated wall. An increase in heat flux led to a narrowing of the amplitude of the region around this minimum point. Peaks of temperature turbulence intensity with increasing magnitude along the streamwise direction were observed within the laminar sublayer. A slight peak in the Nusselt number distribution is observed at the centre of the channel as the Reynolds number was increased. Among the geometrical passive cooling configurations, the one presented in this work showed superior heat transfer performance.

Numerical studies of the simultaneous development of forced convective laminar flow with heat transfer inside a microtube at a uniform temperature

Abstract Conjugate heat transfer is a complex problem because heat is transferred from a solid medium to a liquid medium through their interfaces. The steady-state laminar flow formed inside the microtubules is subjected to a constant temperature at the outer sidewall surface. These images cover a wide range of wall-to-fluid thermal conductivity ratios (ksf = 1, 2, 3, 4, and 5) and wall thickness-to-inner diameter ratios (δ/Ri = 0.25, 0.5, 0.75, 1, 1.25, and 1.5) and Reynolds numbers (Re = 200, 400, 600, 800, and 1,000). The results are processed by a Fluent program based on the finite volume method to numerically integrate the driver’s differential equations. The results show that increasing the wall-to-fluid thermal conductivity ratio ksf increases the inner wall dimensionless temperature and decreases the average Nusselt number. Conversely, an increase in the ratio of wall thickness to inner diameter results in a decrease in the dimensionless temperature of the inner wall and an increase in the average Nusselt number.

Experimental and Numerical Analysis of Forced Convection in a Horizontal Tube Partially Filled with a Porous Medium under Local Thermal Equilibrium Conditions

The objective of the present work is to analyze experimentally and numerically the laminar forced convection flow in a horizontal pipe partially filled with a porous medium under constant heat flux and to study the influence of the eccentricity of the porous medium on the results. In a numerical analysis, the governing equations are solved in three dimensions. To simplify the grid generation and the satisfaction of the boundary conditions, conformal mapping is applied to convert the cross-section of the tube in the fluid domain (space between two eccentric circles) into a rectangle, and the equations are solved in a computational domain in this domain. The Darcy–Brinkman–Forchheimer model is applied to simulate the hydrodynamic behavior of the flow in the porous region. Thermal equilibrium between solid and fluid is assumed for the energy equation. A FORTRAN program was developed to solve the equations using the finite volume method and the SIMPLE algorithm. Velocity profile, pressure drop and average Nusselt number are studied in a wide range of Darcy numbers, thickness of porous mediums and eccentricities. The results show that the eccentricity of the porous material reduces the heat transfer coefficient and the pressure drop simultaneously; of course, the reduction in the heat transfer coefficient is less noticeable when the thickness of the porous medium is smaller. For example, at RP = 0.5, when the eccentricity of the porous medium increases up to E = 0.4, the average Nusselt number decreases by 66%, and this reduction for a smaller porous thickness decreases to 11%. The maximum pressure drop reduction for Da = 10−5 and E = 0.4 is 25%.

Heat transfer enhancement for corrugated facing step channels using aluminium nitride nanofluid - numerical investigation

The present work carries out a three-dimensional numerical analysis study of Aluminium Nitride (AlN)-water hybrid nanofluid enhanced heat transfer in laminar forced convection flow heat exchanger with four different channels, flat, backward facing step, triangle and trapezoidal facing step channels. The influence of different Reynolds number (100≤ Re ≤1500) and different solid nanoparticles volume fraction (1% and 4%) on the heat transfer and fluid flow were numerically investigated. The numerical analysis was carried out by using a laminar model of ANSYS-Fluent CFD code and the governing equations were resolved using the finite volume method. The results indicate that the thermal conductivity of the nanofluids increases with the increase values of both the nanoparticles volume fractions and Reynolds number, compared with base fluids. Likewise, the pressure drop showed slightly increased due to the increased of both parameters. The use of high nanoparticles volume fractions (4% volume) nanofluid corresponded with the use of four different channel designs resulted in heat transfer augmentation about 30% when compared to that pure water for the trapezoidal channel.

Numerical simulation for nanofluid forced/free convection in a partially heated parallelogram enclosure: Effects of wall undulation and inclined Lorentz forces

AbstractThe present study possesses the mathematical computational analysis for mixed convection nanofluid (CuO–water) flow in a lid‐driven parallelogram enclosure with corrugated walls. A partially heated source is located at the bottom corrugated wall. The remaining portions of upper and lower corrugated boundaries are adiabatic. The verticals walls are kept cold. The flow dynamic established with wall undulation and inclined Lorentz forces generate complex mathematical formulation via partial differential equations. Finite element method is implemented to get numerical interpretation of these equations. The simulation is performed against various emerging parameters and the outcomes in terms of isotherms, streamlines and line graphs. The results are recorded against various Richardson numbers , Hartman number , nanoparticles volume fraction , and amplitude of the corrugated horizontal walls . It is observed that against smaller Richardson number streamlines indicate forced convection flow while at higher Richardson number flow divert toward free convection regime. Additionally, heat transportation is maximum when magnetic field is applied in perpendicular direction as well as at maximum amplitude of wall undulations.

Oscillating Flow of Power‐Law Fluids over a Sphere: Drag and Nusselt Number Behavior

AbstractThe forced convection flow and heat transfer aspects of an isothermal sphere in an oscillating stream of power‐law fluids in the range of 5 ≤ Re ≤ 120 and 0.3 ≤ n ≤ 1.5 are numerically investigated. The effects of shear‐dependent viscosity and oscillatory flow features on the temporal variation of streamlines, patterns of streaming flows, drag coefficient, and Nusselt number are examined in depth. The power‐law index modulates the transition of the steady streaming flows between two distinct streaming regimes. Overall, shear‐thinning fluids at low oscillation frequencies are observed to yield significant increase in heat transfer. Due to the phase lag in the momentum boundary layer, the drag coefficient exhibits a phase lag ranging from π/2 to 4π/5 whereas the Nusselt number lags by about 50 % of these values.

Turbulent heat flux modelling in forced convection flows using artificial neural networks

Fluid dynamics of liquid metals plays a central role in new generation liquid metal cooled nuclear reactors, for which numerical investigations require the use of an appropriate thermal turbulence model for low Prandtl number fluids. Given the limitations of traditional modelling approaches and the increasing availability of high-fidelity data for this class of fluids, we propose a machine learning strategy for the modelling of the turbulent heat flux. A comprehensive algebraic mathematical structure is derived and physical constraints are imposed to ensure attractive properties promoting stability and realizability. The closure coefficients of the model are predicted by an Artificial Neural Network (ANN) which is trained with the available Direct Numerical Simulation (DNS) data at different Prandtl numbers. The validation shows that the ANN provides an accurate representation of the heat flux in a wide range of Prandtl numbers (Pr = 0.01–0.71) and compares well with other existing thermal closures. Nevertheless, simulations with different auxiliary models to compute the inputs of the ANN revealed a certain sensitivity of the data-driven formulation on the models used in combination with it. In particular, the ANN model strongly relies on the Reynolds stress anisotropy. Such a sensitivity limits the robustness of the model to the inaccuracies of the underline momentum field and the momentum turbulence model applied in combination with it.

Thermal Performance in Convection Flow of Nanofluids Using a Deep Convolutional Neural Network

This study develops a geometry adaptive, physical field predictor for the combined forced and natural convection flow of a nanofluid in horizontal single or double-inner cylinder annular pipes with various inner cylinder sizes and placements based on deep learning. The predictor is built with a convolutional-deconvolutional structure, where the input is the annulus cross-section geometry and the output is the temperature and the Nusselt number for the nanofluid-filled annulus. Profiting from the proven ability of dealing with pixel-like data, the convolutional neural network (CNN)-based predictor enables an accurate end-to-end mapping from the geometry input and the desired nanofluid physical field. Taking the computational fluid dynamics (CFD) calculation as the basis of our approach, the obtained results show that the average accuracy of the predicted temperature field and the coefficient of determination R2 are more than 99.9% and 0.998 accurate for single-inner cylinder nanofluid-filled annulus; while for the more complex case of double-inner cylinder, the results are still very close, higher than 99.8% and 0.99, respectively. Furthermore, the predictor takes only 0.038 s for each nanofluid field prediction, four orders of magnitude faster than the numerical simulation. The high accuracy and the fast speed estimation of the proposed predictor show the great potential of this approach to perform efficient inner cylinder configuration design and optimization for nanofluid-filled annulus.

Geometrical investigation of cooling channels with two alternated isothermal blocks under forced convective turbulent flow

The present work performs a numerical analysis of the geometrical investigation of a two-dimensional channel with two alternated isothermal rectangular blocks subjected to turbulent forced convective flows. Constructal Design associated with Exhaustive Search is used to investigate the influence of the geometry of the isothermal blocks over the performance of the cooling convective flows in a multi-objective viewpoint, i.e., considering the pressure drop and heat transfer rate. The time-averaged equations of continuity, momentum, and energy conservation are solved with the finite volume method. The k–omega shear stress transport model is used for the closure of turbulence. The effect of the two proposed degrees of freedom, the height/length ratio of the two blocks (H_{1}/L_{1} and H_{2}/L_{2}), is analyzed in relation to the proposed objectives. Regarding the thermal purpose, the geometry with the highest insertion into the channel led to the best performance, while the opposite configuration led to the best fluid dynamic performance. This behavior was similar to that previously found in the literature for forced convective laminar flows. For a multi-objective perspective, the use of technique for order preference by similarity to ideal solution indicated that asymmetric blocks led to the best multi-objective performance when both performance indicators had the same weight.

Comparison of Serpentine and Interdigitated Monopolar Plates on the Performance of an Anion Exchange Membrane Fuel Cell By CFD

The continuous energy demand and the growing population have focussed the scientific research on developing alternative energy sources or green fuels.The anion exchange membrane fuel cell (AEMFC) arises as an excellent alternative because of its low-cost electrocatalyst used at the cathode, energy production, and no pollutants emissions to the environment. Nowadays, several studies have been conducted to improve the performance of the AEMFC modifying membrane materials, types of gas diffusion layers (GDL), and catalysts, among other cell components.However, the design of the flow channels by changing plate topologies and materials is still under development. About flow channels topologies, conventional geometries array (pin, straight, parallel, and serpentine) flow fields lead the flow in the direction parallel to the electrode surface, and the reactive gas flow towards the catalyst layer (CL) mainly by molecular diffusion. On the other hand, interdigitated flow fields provide convection velocity normal to the CL and forced convection flow in GDL for better mass transfer. The interdigitated flow fields could prevent water flooding and improve high current density operations performance.These flow fields have been tested in proton exchange membrane fuel cells (PEMFC) with excellent results at low current densities. Nonetheless, there is an essential balance and management of water in an AEMFC. It presents simultaneous production and consumption of water in anode and cathode, respectively, where the production is twice the consumption.Poor water management could provoke anode flooding or membrane drying. These scenarios are undesirable due to mass transport limitations, membrane polymer degradation, and cathodic channels flow can be flooded. Therefore, an appropriate design of flow field that allows the correct water balance on AEMFC is needed for optimal performance. This research deals with the computational fluid dynamic (CFD) comparison of serpentine and interdigitated flow fields on the performance of an AEMFC. For the development of this model, the Navier-Stokes and Brinkmann equations were implemented for momentum transfer. In addition, the average mixture model represents the transport of chemical species, and the Butler-Volmer equation denotes the electrochemical reaction at the CL; water balance is analyzed at low and high current densities to evaluate the scenarios above mention.Preliminary results indicate that a serpentine design is functional at the anode due to the water management is favored. An interdigitated flow field is implemented at the cathode to improve the distribution of the reactants from channels until the CL.

Fully developed forced convection H2O-CuO nanofluid flow through concentric pipes annular sector duct by using KKL model

Fully developed hydro-thermal H2 O- CuO nanofluid flow through concentric pipes annular sector duct is carried out. Koo–Kleinstreuer–Li ( KKL) model correlations are taken into account to discuss the forced convection flow under the effect of Brownian motion. For discretizing and numerical simulation of governing mathematical problem, finite volume based method, ( FVBM) and well known technique “semi implicit procedure for pressure linked equations, ( SIMPLE) revised” are utilized, whereas constant axial wall heat flux with uniform peripheral wall temperature and, constant and uniform wall temperature known as H1 and T thermally fully developed conditions respectively, are used to discuss the heat transfer rate for different contribution of “Copper Oxide ( CuO)” nanoparticles, n, in base fluid “pure water”. By adding the contribution of nanoparticles, the enhancement in fanning friction factor, fRe, and average Nusselt number, Nu, has been estimated upto 7.72% and 7.54% respectively, in H1 thermal case, whereas 7.71% and 7.16% respectively, in T thermal case.

Selection Criterion of Stable Dendritic Growth for a Ternary (Multicomponent) Melt with a Forced Convective Flow

A stable growth mode of a single dendritic crystal solidifying in an undercooled ternary (multicomponent) melt is studied with allowance for a forced convective flow. The steady-state temperature, solute concentrations and fluid velocity components are found for two- and three-dimensional problems. The stability criterion and the total undercooling balance are derived accounting for surface tension anisotropy at the solid-melt interface. The theory under consideration is compared with experimental data and phase-field modeling for Ni98Zr1Al1 alloy.

A note on the asymptotic behaviour of axisymmetric stagnation-point flow and heat transfer of a viscous fluid past a permeable stretching/shrinking cylinder

The forced convection flow near a stagnation point on a stretching/shrinking cylinder derived originally by Wang [1] and later extended by Lok and Pop [2] is further considered through obtaining the asymptotic behaviour for the relevant dimensionless parameters, namely the Prandtl number σ, the wall velocity λ, the wall transpiration rate S and the Reynolds number R. Additional numerical results are also obtained to confirm our asymptotic predictions. Critical values are seen in the numerical results for negative values of λ with an asymptotic solution valid for large negative λ gives an asymptotic expression for these critical values. It is found that multiple solutions are possible for the limiting range of shrinking and suction parameters. Besides, the asymptotic behaviour is similar for the case of large suction and large Reynolds number. Also, the asymptotic analysis shows that for the solution to continue to large shrinking parameter, the suction parameter should be positive and large.

Achievement of wall temperature uniformity by axially graded porous materials

An analysis based on local thermal non-equilibrium was made on thermally developing forced convective flows in a channel filled with axially graded porous materials so as to find a way to control its wall temperature distribution. Both analytical and numerical methods were exploited and compared to confirm the validity of the results. A collection of axially graded metal foams were considered under equal solid volume fraction with local porosity decreasing downstream following a power function of axial distance with an exponent n. The Brinkman-Forchheimer extended Darcy model was employed to investigate the axial development of the velocity field, which reveals a weak secondary flow towards the heated wall of the channel, working favorably in view of wall cooling. The wall temperature variation is found quite sensitive to the axial gradient of the local porosity. Depending on the value of the exponent n controlling the axial gradient of the porosity, the axial variation of the wall temperature changes drastically. By adjusting the exponent n, the wall temperature can stay nearly constant throughout the channel for the case of n = 1.0, or can increase from the inlet for the case of n = 1.5, attain its maximum, and then, decreases towards the exit. The results obtained in this study provides useful information for designing effective cooling systems, where control of not only the desired working temperature but also its uniformity are required such as in EV batteries and PEMFC.

Numerical investigation of viscoelastic boundary layer in forced convection flow on surface under prescribed heat flux

AbstractThe paper intends to analyze numerically the effect of forced convection with viscoelastic boundary on porous media subjected to constant heat flux on the surface. The modified Navier–Stokes equations in nondimensional forms are formulated and modeled with Darcy–Forchheimer–Brinkman to solve the equations using bvp4c with MATLAB package. The numerical results are found for local Nusselt number, shear stress, temperature distribution, and finally the velocity of the boundary layer over a horizontal plate. The analysis includes the influence of several parameters, such as Forchheimer, porous media, Nusselt number, and viscoelastic on distributions of velocity and temperature. Also, the effects comprise the friction and heat transfer coefficients. The numerical results under constant heat flux indicate an increase in shear stress and boundary layer velocity as the Darcy parameter increases and at the same time, the values of temperature and Nusselt number are decreased. A significant increase was observed for the values of Nusselt number, temperature, and shear stress under large values of the Forchheimer parameter. The numerical results show a good agreement with available previous works.

Laboratory Experiments on Ice Melting: A Need for Understanding Dynamics at the Ice-Water Interface

The ice-ocean interface is a dynamic zone characterized by the transfer of heat, salinity, and energy. Complex thermodynamics and fluid dynamics drive fascinating physics as ice is formed and lost under variable conditions. Observations and data from polar regions have shed light on the contributions that oceanic currents, meltwater plumes, subglacial hydrology, and other features of the ice-ocean boundary region can make on melting and transport. However, the complicated interaction of mechanisms related to ice loss remain difficult to discern, necessitating laboratory experiments to explore fundamental features of melting dynamics via controlled testing with rigorous measurement techniques. Here, we put forward a review of literature on laboratory experiments that explore ice loss in response to free and forced convective flows, considering melting based on laminar or turbulent flow conditions, ice geometries representing a range of idealized scenarios to those modeling glaciers found in nature, and features such as salinity and stratification. We present successful measurement techniques and highlight findings useful to understanding polar ice dynamics, and we aim to identify future directions and needs for experimental research to complement ongoing field investigations and numerical simulations to ultimately improve predictions of ice loss in our current and evolving climate.