Study on fluid–solid coupling heat transfer in fractal porous medium by lattice Boltzmann method

A lattice Boltzmann model is applied to simulate fluid–solid coupling heat transfer in fractal porous medium. The numerical simulation is conducted to investigate the influences of pressure drop and porosity on fluid flows and the effect of thermal conductivity ratio of solid matrix to fluid on heat transfer. The simulation results indicate that fluid flows still obey Darcy’s Law in the range of flow and pressure level in this paper, and that both velocity field and temperature evolution conform to the local structural characteristics of porous medium. The comparison of temperature results from lattice Boltzmann model against those from the finite-volume method (FVM, one of the conventional CFD methods) is also presented to demonstrate the reliability of LBM. The present results agree well with those from FVM, All these indicate the feasibility and the reliability for the lattice Boltzmann model to be used to reveal the phenomenon and rules of fluid–solid coupling heat transfer in complex porous structures.     

Radiative heat transfer in a participating medium with specular–diffuse surfaces

The results obtained by ray-tracing method can be regarded as benchmarks for its good accuracy. However, up to now, this method can be only used to solve radiative transfer within medium confined between two specular surfaces or two diffuse surfaces. This article proposes a hybrid ray-tracing method to solve the radiative transfer inside a plane-parallel absorbing–emitting–scattering medium with one specular surface and another diffuse surface (S–D surfaces). By the hybrid ray-tracing method, radiative transfer coefficients (RTCs) for S–D surfaces are deduced. Both surfaces of the medium under consideration are considered to be semitransparent or opaque. This paper examines the effects of scattering albedo, opaque surface emissivity and anisotropically scattering on steady-state heat flux and transient temperature fields. From the results it is found that the effects of anisotropic scattering is more for a bigger optical thickness medium; and keeping other optical parameters unchanged, anisotropic scattering affects transient temperature distributions so much in a small refractive index medium.     

Distribution of air–water annular flow in a header of a parallel flow heat exchanger

The air and water flow distribution are experimentally studied for a heat exchanger composed of round headers and 10 flat tubes. The effects of tube protrusion depth as well as header mass flux, and quality are investigated, and the results are compared with previous 30 channel data. The flow at the header inlet is annular. For the downward flow configuration, water flow distribution is significantly affected by tube protrusion depth. For flush-mounted geometry, significant portion of water flows through frontal part of the header. As the protrusion depth increases, more water is forced to rear part of the header. The effect of header mass flux or quality is qualitatively the same as that of the protrusion depth. For the upward flow configuration, however, significant portion of water flows through rear part of the header. The effect of protrusion depth is the same as that of the downward flow. However, the effect of header mass flux or quality is opposite to the downward flow case. Compared with the previous 30 channel configuration, the present 10 channel configuration yields better flow distribution. Possible explanation is provided from flow visualization results.     

Steady revolving flow and heat transfer of a non-Newtonian Reiner–Rivlin fluid

The steady revolving flow and heat transfer of a non-Newtonian Reiner–Rivlin fluid is studied. The momentum equation gives rise to a highly nonlinear boundary value problem. Attempt has been made to study the properties of the solution of the momentum equation analytically before proceeding for numerical solution. The effects of non-Newtonian fluid characteristic on the velocity and temperature fields have been discussed in detail and shown graphically.     

Characterization of melting and solidification in a real-scale PCM–air heat exchanger: Experimental results and empirical model

This paper describes the experimental studies carried out to test thermal cycling of a real-scale PCM–air heat exchanger at ambient temperatures. To achieve this goal an experimental setup previously designed and used for testing real-scale prototypes of PCM–air heat exchangers is modified. The PCM used is commercially available, organic, and paraffin based. The total energy exchanged during melting and solidification, as well as the time elapsed until total melting/solidification are determined from the power curves experimentally obtained. The influence of the inlet air temperature and air flow is studied, and results show that the continuous thermal cycling of the unit is a repetitive process: running experiments with similar conditions leads to the same thermal behavior, no degradation in the PCM properties is noticed. Pressure drop is measured for different air flows. Depending on the inlet air temperature, full solidification of the PCM could be achieved in less than 3 h for an 8 °C temperature difference between the inlet air and the average phase change of the PCM. Average thermal powers of up to 4.5 kW and 3.5 kW for 1 h are obtained for melting and solidification stages, respectively. An empirical model is developed from the experimental results, which could be a useful designing tool for applications that use such technology: green housing, curing and drying processes, plant production, HVAC, and free-cooling.     

Heat transfer in a reaction–diffusion system with a moving heat source

A general formulation is presented for a moving boundary problem in which heat is generated at the boundary due to an exothermic reaction involving a species which diffuses into a dispersed phase from an external medium of finite volume. The speed of the moving boundary is prescribed based on the solution of the mass diffusion problem and an analysis is presented of the thermal dynamics of the system. The set of equations describing heat transport leads to a Green’s function type problem with time dependent boundary conditions and the Galerkin finite element method is employed to develop a numerical solution. Transformations are introduced to freeze the moving boundary and partition the domain for ease of computation, and an iterative scheme is defined to satisfy the heat flux jump boundary condition and match the temperature field across the moving boundary. The numerical results are used to set the limits of applicability of an analytical perturbation solution. Essential aspects of thermal dynamics in the system are described and parametric regions resulting in a local temperature hot spot are delineated. Computed contour plots describing thermal evolution are presented for different combinations of parameter values. These may be of utility in the prediction of thermal development, for control and avoidance of hot spot formation, and in physical parameter estimation.     

Effects of heat absorption and thermal radiation on heat transfer in a fluid–particle flow past a surface in the presence of a gravity field

A continuum two-phase fluid–particle model accounting for fluid-phase heat generation or absorption and thermal radiation is developed and applied to the problem of heat transfer in a particulate suspension flow over a horizontal heated surface in the presence of a gravity field. Analytical solutions for the temperature distributions and the wall heat fluxes for both phases are obtained. Two cases of wall thermal conditions corresponding to stationary and periodic temperature distributions are considered. Numerical evaluations of the analytical solutions are performed and the results are reported graphically to elucidate special features of the solutions. The effects of heat absorption and thermal radiation are illustrated through representative results for the temperature distributions and heat fluxes of both phases for various fluid–particle suspensions. It is found that heat absorption increases the total heat transfer rate for various particulate volume fraction levels while thermal radiation decreases it.     

Heat transfer and flow characteristics of AL2O3–water nanofluid in a double tube heat exchanger

An experimental study was carried out in order to find out the effects of Al₂O₃ nanofluid with a mean diameter of 20 nm on heat transfer, pressure drop and thermal performance of a double tubes heat exchanger. The effective viscosity of nanofluid was measured in various temperatures ranging from 27 °C to 55 °C. Experiments were carried out at different Reynolds numbers ranging from 5000 to 20,000, approximately, and in various nanoparticles concentration up to 1% by volume. Results indicate that there is a good potential in promoting the thermal performance of heat exchanger by adding nanoparticles in the investigated ranges where there is not a severe pressure drop penalty. The empirical correlation was created for Nusselt number variation based on the Reynolds number and nanoparticles concentration.     

Boundary conditions at a fluid–porous interface for a convective heat transfer problem: Analysis of the jump relations

This paper presents a two-step up-scaling approach to determine the jump relations that must be imposed at the interface between a homogeneous porous domain and a free domain. We study convective heat transfer at such an interface under the assumption of local thermal equilibrium. The two-step approach has the capability of providing closed jump relations depending on intrinsic characteristic of the interface. In addition, from the resulting jump relations, it is possible to determine a particular interface location where the condition of continuity are sufficient. Thus, the use of jump or continuity conditions depend only on the interface location inside the fluid–porous transition region.     

Electrohydrodynamic capillary instability of Rivlin–Ericksen viscoelastic fluid film with mass and heat transfer

This paper presents an analysis of viscous fluids and a Rivlin–Ericksen (R-E) viscoelastic fluid interface under the influence of heat and mass transfer, while both fluids are exposed to an axial electric field. The fluids are restricted within an annular region that is enclosed by two rigid cylinders. The outer section of the annular region holds the R-E viscoelastic fluid, while the inner section is filled with the viscous fluid. To ascertain the correlation between perturbation growth and wavenumber, the theory of potential flow on viscoelastic–viscous fluids is applied, and the result is represented as a second-order polynomial. This correlation is numerically solved using the Newton–Raphson method. Variables of viscous flow, such as electric field strength, heat transfer coefficient, viscoelasticity, viscosity, and so forth, are numerically studied. With an increase in electric field strength, the perturbation growth decays and expands for the particular combinations of permittivity and conductivity ratio, showing the dual effect of the axial electric field.     

Isothermal and non-isothermal oil–water flow and viscous fingering in a porous medium

Immiscible flow of heavy oil in a porous formation by high temperature pressurized water has been numerically studied. The physical region is a square domain in the horizontal plane with low and high pressure points at the opposite corners along one of the diagonals. Water, the invading fluid, when introduced at high pressure displaces the in situ oil towards the low pressure production zone. The extent of displacement of oil by water through the porous medium in a given amount of time and the appearance of preferential flow paths ( fingers) is the subject of the present investigation. The resistance to water–oil movement arises from the viscous forces in the fluid phases and the capillary force at their interface. Based on their relative magnitudes, various forms of displacement mechanisms can be realized. As the viscosity ratio of heavy oil to water is large, viscous forces in the oil phase become dominant and constitute the major factor for controlling the flow distortions in the porous formation. A mathematical model that can treat the individual fluid pressures, capillary effects and heat transfer has been employed in the present work. A fully implicit, two-dimensional numerical model has been used to compute the pressure and temperature fields. The domain decomposition technique has been adopted in the numerical solution since the problem is computationally intensive. Naturally occurring oil-rich reservoirs to which the present study is applicable are inhomogeneous and layered. A qualitative study has been carried out to explore the effect of permeability variations on the flow patterns. Numerical calculations show that non-isothermal effects as well as layering promote the formation of viscous fingers and consequently the sweep efficiency of the high pressure water front.     

An experimental study of fuel–air mixing section on unstable combustion in a dump combustor

The main objective of this study is effect of the various fuel–air mixing section geometries on the unstable combustion. For the purpose of observing the combustion pressure oscillation and phase difference at each of the dynamic pressure results, the multi-channel dynamic pressure transducers were located on the combustor and inlet mixing section. By using an optically accessible quartz-type combustor, we were able to OH* measurements to characterize the flame structure and heat release oscillation with the use of a high-speed ICCD camera. In this study, we observed two dominant instability frequencies. Lower frequencies were measured around 240–380 Hz, which were associated with a fundamental longitudinal mode of combustor length. Higher frequencies were measured around 410–830 Hz. These were related to the secondary longitudinal mode in the combustion chamber and the secondary quarter-wave mode in the inlet mixing section. These second instability mode characteristics are coupled with the conditions of the combustor and inlet mixing section acoustic geometry. Also, these higher combustion instability characteristics include dynamic pressure oscillation of the inlet mixing section part, which was larger than the combustor section. As a result, combustion instability was strongly affected by the acoustically coupling of the combustor and inlet mixing section geometry.     

Flow and heat transfer in a micro-cylindrical gas–liquid Couette flow

Analytic solutions for the gas and liquid velocity and temperature distribution are determined for steady state one-dimensional microchannel cylindrical Couette flow between a shaft and a concentric cylinder. The solution is based on the continuum model and takes into consideration the velocity slip and temperature jump in the gaseous phase defined by the Knudsen number range of 0.001 < Kn < 0.1. The two fluids are assumed immiscible. The gas layer is adjacent to the shaft which rotates with angular velocity ω_s and is thermally insulated. The outer cylinder rotates with angular velocity ω_o and is maintained at uniform temperature. The governing parameters are identified and the effects of the Knudsen number and accommodation coefficients on the velocity and temperature profiles, reduction in the overall temperature rise due to the gas layer, the Nusselt number and shear reduction are examined. It was found that the required torque to rotate the liquid in the annular space is significantly reduced by introducing a thin gas layer adjacent to the shaft. Also, reduction in shaft temperature is enhanced through a combination of high energy accommodation coefficient and low momentum accommodation coefficients. Results also indicate that the gas layer becomes more effective in reducing the shaft temperature when the housing angular velocity is much larger than the shaft angular velocity.     

Coupling characteristic analysis and propagation direction control in hydrogen–air rotating detonation combustor with turbine

Two-dimensional numerical simulations are performed to investigate the interactions between a rotating detonation combustor (RDC) and turbine. The flowfield structure and coupling characteristics are analyzed under different equivalent ratios. Furthermore, the formation mechanism of the detonation wave direction is illustrated. Results show that although RDCs increase the average turbine work, they enhance the flowfield oscillation and maintain the quasi-periodic fluctuation in the turbine work. Compared to counter-clockwise propagation, the pressure oscillation attenuation and total pressure loss through the turbine guide vane increase when the detonation wave propagates clockwise. When the detonation wave height is low, RDCs are more prone to quenching due to the impact of reflected waves. After re-ignition, the detonation wave direction is unrelated to the initial ignition direction and is random. Introduction of deflecting wedges into RDCs enables the automatic control of the detonation wave direction but causes extra total pressure loss of at least 3.6%.     

Hydromagnetic convective diffusion of species in Darcy–Forchheimer porous medium with non-uniform heat source/sink and variable viscosity

In this paper we have analyzed the combined effects of magnetic field and convective diffusion of species through a non-Darcy porous medium over a vertical stretching sheet with temperature dependent viscosity and non-uniform heat source/sink. The boundary layer equations are transformed into ordinary differential equations using self-similarity transformation which are then solved numerically using fifth-order Runge–Kutta Fehlberg method with shooting technique for various values of the governing parameters. The effects of electric field parameter, non-uniform heat source/sink parameters and Schmidt number on concentration profiles are analyzed and discussed graphically. Favorable comparisons with previously published work on various special cases of the problem are obtained.     

Molecular dynamics simulation of heat transfer with effects of fluid–lattice interactions

The nonequilibrium molecular dynamics simulation is employed to investigate the thermal properties of fluid confined in different FCC nanochannels. The results show that fluid in different lattice channels appears diverse wetting characteristics at low temperature. Based on wall parameters, a ratio is defined to describe the fluid–lattice interaction. Wall attraction, number of absorbed particles and thermal conductivity are increased as the increase of this ratio as well as the location of particles get closer to the wall. Thermal resistance exists along with the fluid–wall interface and loses the dominant of heat transport as the system temperature gets raised. At the same time, the thermal conductivity of nanoscale experiences unconventional increase. The fluid thermal properties are influenced both by wall–fluid interaction and temperature.     

Cattaneo–Christov heat flux model and multiple slip effect on carbon nanofluid over a stretching sheet in a Darcy–Forchheimer porous medium

In several biotechnological processes, multiple slips are the most paramount, such as blood pumping from the heart to different body components, endoscopy treatment, pabulum distribution, and the heat transport phenomenon regulation. In the current research, we have studied the multiple slips, Darcy–Forchheimer, and Cattaneo–Christov heat flux model on a stretching surface exposed to magnetic carbon nanotube nanofluid. We have additionally included a heat source or sink, a chemical reaction for manipulating the heat and mass transport phenomena. The resulting governing partial differential equations have been transformed into ordinary differential equations. With the Runge–Kutta–Fehlberg fourth–fifth-order procedure, the transformed governing equations are numerically solved. Numerical solutions for different parameters for velocity, temperature, and concentration profiles (Eckert number, velocity slip, thermal slip, mass slip, etc.) are highlighted. Graphical and numerical results for the various parameters in the modeled problem have been outlined. The present numerical results are compared with the published ones for some limiting cases. The slip has been found to control the flow of the boundary layer.     

Investigation of buoyancy-driven flow and heat transfer in a trapezoidal cavity filled with water–Cu nanofluid

A numerical study is conducted to investigate the transport mechanism of free convection in a trapezoidal enclosure filled with water–Cu nanofluid. The horizontal walls of the enclosure are insulated while the inclined walls are kept at constant but different temperatures. The numerical approach is based on the finite element technique with Galerkin's weighted residual simulation. Solutions are obtained for a wide range of the aspect ratio (AR) and Prandtl number (Pr) with Rayleigh number (Ra = 10⁵) and solid volume fraction (? = 0.05). The streamlines, isotherm plots and the variation of the average Nusselt number at the left hot wall are presented and discussed. It is found that both AR and Pr affect the fluid flow and heat transfer in the enclosure. A correlation is also developed graphically for the average Nusselt number as a function of the Prandtl number as well as the cavity aspect ratio.     

Numerical simulation of heat and mass transfers radiative–convective fluid flow on a stretching porous surface with temperature-dependent thermophysical properties

This paper is focused on the analysis of heat and mass transfer radiative–convective fluid flow using quadratic multiple regression and numerical approach. The physical phenomenon is analyzed by utilizing partial differential equations (PDEs). Thermophysical properties, such as viscosity, thermal conductivity, and mass diffusivity, are varied and temperature-dependent. This study is unique because of its applications in magnetohydrodynamic power accelerators, drilling operators, and bioengineering. The governing PDEs are transformed into coupled nonlinear ordinary differential equations (ODEs). The transformed ODEs are solved numerically using the spectral homotopy analysis method. Also, a quadratic multiple regression analysis is performed on quantities of engineering interest to show the significance of the flow parameters. It is observed that the heat and mass transfer process is affected by nonlinear buoyancy impact. The Lorentz force produced by the imposed magnetic field decline the thickness of the hydrodynamic boundary layer. Findings revealed that the nonlinear convective parameter and variable thermophysical properties are greatly affected by the rate of heat and mass transfer. Previously published work was used to validate the present one, which conformed with it. The slope of linear regression through data points is adopted to show the rate of change in skin friction, Nusselt, and Sherwood numbers during the flow phenomenon.