Unsteady conjugate mixed convection phase change of a power law non-Newtonian fluid in a square cavity

Transient phase change of a power law non-Newtonian fluid inside an inner thin walled container caused by external mixed convection in a square cavity has been analyzed numerically. Air was chosen as external cooling fluid and modified non-Newtonian water as the phase change fluid. Fluid mechanics and conjugate convective heat transfer, described in terms of continuity, linear momentum and energy equations, were predicted by using the finite volume method. Solidification was treated in terms of a phase change function varying linearly with temperature. The effect of the external Reynolds number, for Re = 200 and 1000 on solidification was studied along the influence of the non-Newtonian power law index (n = 0.5, n = 1.0). Results for the time evolution of streamlines, isotherms and freezing curves are analyzed. The effect of the Reynolds number on streamlines of the external fluid is remarkable, principally near the region close to the internal water filled container. Differences between cooling and freezing times are found for Newtonian (n = 1.0) and non-Newtonian modified (n = 0.5) water.     

Effect of inlet and outlet location on the mixed convective cooling inside the ventilated cavity subjected to an external nanofluid

In this paper, mixed convection flow and temperature fields in a vented square cavity subjected to an external copper–water nanofluid are studied numerically. The natural convection effect is attained by heating from the constant flux heat source on the bottom wall and cooling from the injected flow. In order to investigate the effect of inlet and outlet location, four different placement configurations of the inlet and outlet ports are considered. In each of them, both the inlet and outlet ports are alternatively located either on the top or the bottom of the sides and external flow enters in to the cavity through an inlet opening in the left vertical wall and exits from another opening in the opposite wall. The remaining boundaries are considered adiabatic. The governing equations have been solved using the finite volume approach, using SIMPLE algorithm on the collocated arrangement. The study has been carried out for the Reynolds number in the range of 50 ≤ Re ≤ 1000, with Richardson numbers 0 ≤ Ri ≤ 10 and for solid volume fraction 0 ≤ ? ≤ 0.05. Results are presented in the form of streamlines, isotherms, average Nusselt number. In addition, the effects of solid volume fraction of nanofluids on the hydrodynamic and thermal characteristics have been investigated and discussed. The algorithm and the computer code have been also compared with numerical results in order to verify and validate the model.     

Mixed convection heat transfer in a ventilated cavity with hot obstacle: Effect of nanofluid and outlet port location

In the present study, the lattice Boltzmann method is implemented to investigate the effect of suspension of nanoparticles on mixed convection in a square cavity with inlet and outlet ports and hot obstacle in the center of the cavity. The effect of outlet port location is examined on heat transfer rate then the effect of nanoparticles is inspected for volume fraction of nanoparticles in the range of 0 to 0.03 at the different position of outlet port. The study was carried out for different Richardson numbers ranging from 0.1 to 10. Grashof number is assumed to be constant (10⁴) so that the Richardson number changes with Reynolds number. The isothermal boundary condition is assumed for obstacle walls and the cavity walls are adiabatic. The result is presented by isotherms, streamlines, and local and average Nusselt numbers. The maximum heat transfer rate occurs when the outlet port is located at P2 for Ri = 0.1 and P1 for Ri = 1, Ri = 10, respectively. Results show that by adding the nanoparticles to base fluid and increasing the volume concentration of nanoparticles the heat transfer rate is enhanced at different Richardson numbers and outlet port positions. But this phenomenon is not observed at Ri = 10 when the outlet port is located at P1.     

The Effect of a Magnetic Field on the Melting of Gallium in a Rectangular Cavity

The role of magnetic field and natural convection on the solid–liquid interface motion, flow, and heat transfer during melting of gallium on a vertical wall is reported in this paper. The classical geometry consisting of a rectangular cavity with uniform but different temperatures imposed at two opposite side walls, insulated top, and bottom walls is considered. The magnetic field is imposed in the horizontal direction. A numerical code is developed to solve for natural convection coupled to solid–liquid phase transition and magnetic effects. The corresponding streamlines and isotherms predicted by the numerical model serve to visualize the complicated flow and temperature field. The interplay between the conduction and convection modes of heat transfer stimulated by the combination of the buoyancy-driven flow and the Lorentz force on the fluid due to the magnetic field are studied. The results show that the increase of Rayleigh number promotes heat transfer by convection, while the increase of Hartmann number dampens the strength of circulating convective currents and the heat transfer is then mainly due to heat conduction. These results are applicable in general to electrically conducting fluids and we show that magnetic field is a vital external control parameter in solid–liquid interface motion.     

Natural convection on an open square cavity containing diagonally placed heaters and adiabatic square block and filled with hybrid nanofluid of nanodiamond - cobalt oxide/water

A finite difference based two dimensional simulations on laminar natural convection inside the open square cavity containing diagonal heaters and a central adiabatic square block is presented by vorticity – stream function approach. The enclosure is filled with hybrid nanofluid of Nanodiamond - Cobalt Oxide/Water. The top and bottom walls are considered as adiabatic and the vertical walls have diagonal heaters. The inlet port is placed on the left end of the top wall and the outlet is placed at the bottom of the right wall. The variables considered are Rayleigh number (10⁴ to 10⁶) and volumetric fraction of Nanodiamond - Cobalt Oxide (0 to 6%) particles. The results of fluid flow with single phase model are elucidated with streamlines, Isotherms and Average Nusselt number. The strength of the primary vortex depreciated with the increasing percentage of nano composites for all the Rayleigh numbers. Intensity of heat transfer is high in the right wall than the left wall.     

一种新型液包气雾化喷嘴流场特性的研究

基于CFD技术对一种新型液包气雾化喷嘴内部的气液两相流流场进行了数值模拟,分析了该喷嘴内部气液两相流的流动特性,得到气相和液相入口压力对气液两相流流动特性的影响,并结合试验数据对模拟结果进行了验证.结果表明:当气相和液相入口压力在0.1～0.5 MPa变化时,喷嘴的出口湍流强度随气相和液相入口压力的提高而增大;提高气相入口压力有利于减小雾化粒径;提高液相入口压力有利于提高雾化粒径的均匀性.     

Analysis of temperature-sensitive magnetic fluids in a porous square cavity depending on different porosity and Darcy number

Magnetic fluids are thermo-sensitive and whose flow and energy transport processes can be controlled by temperature and external magnetic field. In the natural convection of porous cavity, magnetic force is not only the driving force, the effective gravity is also a force related to the natural convection, and the effective gravity is closely depending on the porosity and permeability of porous medium. As is known, the porous medium is in solid state with high heat capacity but low heat transfer coefficient, while the magnetic fluid is a kind of fluid with high heat transfer coefficient and easy to be controlled. Combining the complementary characteristic of magnetic fluids and porous medium, we present a study for temperature-sensitive magnetic fluids in a porous square cavity. In the study, a Lattice Boltzmann method is developed to simulate the laminar convection of temperature-sensitive magnetic fluids in a porous square cavity. We present numerical results for the streamlines, isotherms, magnetization for different values of porosity and Darcy number. In addition, Nusselt numbers on heated and cooled wall and the average Nusselt numbers are also investigated.     

Numerical visualization of mass and heat transport for conjugate natural convection/heat conduction by streamline and heatline

The method of numerical visualization of mass and heat transport for convective heat transfer by streamlines and heatlines are comprehensively studied. Functions are directly defined in terms of dimensionless governing equations or variables. Some basic characteristics of the functions are illustrated in detail, knowledge of which is essential to perceive the results and the philosophy of heat and fluid flow. The consistency of the formulations is especially addressed when dealing with conjugate convection/conduction problem. The functions/lines are unified for both fluid and solid regions, and the diffusion coefficients of the function equations are invariant. The method has been used to visualize the heat and fluid flow structures for natural convection in an air (Pr=0.71) filled square cavity over a wide range of Ra=10³−10⁶, and those for conjugate natural convection/heat conduction problem where the conduction effect of solid body on heat transfer is investigated. As to exhibiting the nature of convective heat transfer, streamlines and heatlines provide a more practical and efficient means to visualize the results than the customary ways.     

Effect of water-based Al2O3 nanofluids on heat transfer and pressure drop in periodic mixed convection inside a square ventilated cavity

A numerical study of unsteady mixed convection flows through an alumina-water nanofluid in a square cavity with inlet and outlet ports due to incoming flow oscillation is performed. It is found that an oscillating velocity at the inlet port cased to creating a periodic variation in the fluid flow and temperature field in the cavity after a certain time duration. The influence of the nanoparticle on the flow and temperature fields has been plotted and discussed. The effect of the oscillation frequency is concealed in a dimensionless number which is the Strouhal number. It is observed that the heat transfer is enhanced for all the Strouhal and Richardson numbers investigated by adding the nanoparticle to the base fluid. It is also found that the performance of the nanoparticle on the enhancement of the heat transfer at higher Richardson numbers is less than that of lower Richardson numbers.     

Effects of interaction between Rayleigh and Marangoni convection in superposed fluid and porous layers

This paper is aimed at investigating the effects of combined Marangoni and Rayleigh convections in a liquid layer, underlain by a porous layer. The two-dimensional numerical model consists of a dual rectangular cavity system in which the porous cavity is located below the fluid cavity. Both cavities are saturated with the same liquid. The interaction between the Marangoni and the Rayleigh convection is investigated in detail. The porous cavity is heated at the bottom while the top liquid cavity has a free surface and heat is lost to the environment by natural convection. The role of the ratio of the liquid layer over the porous layer in determining the convection pattern was studied. Results indicate that the Marangoni convection enhances the flow in the liquid layer, which results in a reduction of the buoyancy convection in the porous layer. A large heat transfer across the liquid layer was noticeable by displaying the variation of the Nusselt number with the liquid Rayleigh number.     

Natural convection in a cavity with undulated walls filled with water-based non-Newtonian power-law CuO–water nanofluid under the influence of the external magnetic field

Natural convection heat transfer in a square cavity (with wavy or plane wall) filled with non-Newtonian power-law nanofluid has been elucidated for several input parameters like Ra spanning from 10⁵ to 10⁶, power-law index (n) from 0.6 to 1.4, and volume fraction of CuO nanoparticles (?) from to 0 to 0.12. Effect of external magnetic field on heat transfer has been illustrated by varying the Ha from 0 to 90. In the present study, our main objective is to explore the effect of nanoparticles on heat transfer enhancement in non-Newtonian power-law fluid. It is found that the addition of nanoparticles (?) to shear thinning fluid enhances the heat transfer approximately 15% when ? increases from 0 to 0.12 for Ha less than 60 at all Ra. For a shear thickening fluid, the same thing happens for all Ha at any Ra. The average surface Nusselt number for a cavity with wavy wall is less than that of a plane wall for all cases which is not true for the case of local Nusselt number.     

EFFECT OF HEATED WALL POSITION ON MIXED CONVECTION IN A CHANNEL WITH AN OPEN CAVITY

Mixed convection in an open cavity with a heated wall bounded by a horizontally insulated plate is studied numerically. Three basic heating modes are considered: (a) the heated wall is on the inflow side (assisting flow); (b) the heated wall is on the outflow side (opposing flow); and (c) the heated wall is the horizontal surface of the cavity (heating from below). Mixed convection fluid flow and heat transfer within the cavity is governed by the buoyancy parameter, Richardson number (Ri), and Reynolds number (Re). The results are reported in terms of streamlines, isotherms, wall temperature, and the velocity profiles in the cavity for Ri=0.1 and 100, Re=100 and 1000, and the ratio between the channel and cavity heights (H/D) is in the range 0.1-1.5. The present results show that the maximum temperature values decrease as the Reynolds and the Richardson numbers increase. The effect of the H/D ratio is found to play a significant role on streamline and isotherm patterns for differentheating configurations. The present investigation shows that the opposing forced flow configuration has the highest thermal performance in terms of both maximum temperature and average Nusselt number.     

Forced convection in a square cavity with inlet and outlet ports

A finite-volume-based computational study of steady laminar forced convection inside a square cavity with inlet and outlet ports is presented. Given a fixed position of the inlet port, the location of the outlet port is varied along the four walls of the cavity. The widths of the ports are equal to 5%, 15% and 25% of the side. By positioning the outlet ports at nine locations on the walls for Re = 10, 40, 100 and 500 and Pr = 5, a total of 108 cases were studied. For the shortest distance between the inlet and outlet ports along the top wall, a primary clockwise (CW) rotating vortex that covers about 75–88% of the cavity is observed. As the outlet port is lowered along the right wall, the CW primary vortex diminishes in strength, however a counter-clockwise (CCW) vortex that is present next to the top right corner grows in size. With the outlet port moving left along the bottom wall, the CW primary vortex is weakened further and the CCW vortex occupies nearly the right half of the cavity. The pressure drop varies drastically depending on Re and the position of the outlet port. If the outlet port is on the opposite or the same wall as the inlet, the pressure drop is smaller in comparison to a case where it is located on the adjacent walls. The maximum pressure drop occurs when the outlet port is on the left side of the bottom wall and the minimum is achieved where the outlet is on the middle of the right wall. Regions of high temperature gradient are consistently observed at the interface of the throughflow and next to the solid walls on both sides of the outlet port. Local Nusselt numbers are low at three corners when no outlet port is present in their vicinity, whereas intense heat transfer rate is observed on the two sides of the outlet port. Between these minima and maxima, the local Nusselt number can vary drastically depending on the flow and temperature fields. By placing the outlet port with one end at three corners, maximum overall Nusselt number of the cavity can be achieved. Minimum overall heat transfer of the cavity is achieved with the outlet port located at the middle of the walls. The case exhibiting maximum heat transfer and minimum pressure drop is observed when the outlet port is located at dimensionless wall coordinate (2 + 0.5W).     

A numerical study on the effect of a heated hollow cylinder on mixed convection in a ventilated cavity

The present work is aimed to study mixed convection heat transfer characteristics within a ventilated square cavity having a heated hollow cylinder. The heated hollow cylinder is placed at the center of the cavity. In addition, the wall of the cavity is assumed to be adiabatic. Flows are imposed through the inlet at the bottom of the left wall and exited at the top of the right wall of the cavity. The present study simulates a practical system such as air-cooled electronic equipment with a heat component or an oven with heater. Emphasis is sited on the influences of the cylinder diameter and the thermal conductivity of the cylinder in the cavity. The consequent mathematical model is governed by the coupled equations of mass, momentum and energy and solved by employing Galerkin weighted residual method of finite element formulation. A wide range of pertinent parameters such as Reynolds number, Richardson number, cylinder diameter and the solid-fluid thermal conductivity ratio are considered in the present study. Various results such as the streamlines, isotherms, heat transfer rates in terms of the average Nusselt number and average fluid temperature in the cavity are presented for different aforesaid parameters. It is observed that the cylinder diameter has significant effect on both the flow and thermal fields but the solid-fluid thermal conductivity ratio has significant effect only on the thermal field.     

The Use of Thermal Lattice Boltzmann Numerical Scheme for Particle-Laden Channel Flow with a Cavity

In this article, particle-laden flow in a channel with heated cavity has been investigated. Calculations were performed using a point force scheme for particle dynamics, while the process of fluid renewal was modeled using the double-population thermal lattice Boltzmann method. Point-particle formulation accounts for the finite-size dispersed phase and the forces acting on the particles were modeled through drag force correlations. Two-way interactions of solid-fluid calculation were considered by adding an external force term for feedback that forced particles in the evolution of fluid distribution function. The method was first validated with steady state flow in a channel with cavity in the presence and absence of a heat source. It was then applied to mixed convection flow laden with particles at various Grashof numbers. The particle dispersion characteristics were examined in detail, where the particle removal rate from cavity upon cavity aspect ratio was emphasized. The effect of the Reynolds number on particle distribution was further investigated numerically by varying the speed of inlet flow into the channel.     

Mixing with time dependent natural convection

Chaotic mixing inside a two dimensional cavity can be achieved with time dependent natural convection if the motion of a fluid is generated by imposing alternating hot and cold wall temperatures. With this set up no moving walls are required to mix the fluid inside the container. In this comment we illustrate this idea by numerically solving the governing equations of natural convection in a two dimensional square cavity with sections of its upper and lower horizontal walls cooled and heated in a periodic manner. These conditions generate a vortex of time dependent intensity that moves its center in a closed loop around the geometrical center of the container. The mixing properties of the flow are illustrated by Lagrangian tracking of a collection of points originally located in a line.     

Computational study of thermal buoyancy from two confined cylinders within a square enclosure wifth single inlet and outlet ports

This article is devoted to investigating the mixed convection arising from two equally hot circular cylinders embedded in a square cavity of adiabatic surfaces. This cavity is filled with an incompressible fluid and contains single entry and outlet orifices. The heated cylinders are assumed to be arranged side-by-side with a fixed gap within the square cavity. Based on some simplifying assumptions, the nonlinear governing equations that express the principle of conservation of mass, momentum, and energy are obtained and numerically solved using a Computational fluid dynamics package ANSYS-CFX with finite volume technique. Pertinent results showing the roles of embedded parameters such as Richardson number (Ri = 0 to 1) and Reynolds number (Re = 1 to 40) at Prandtl number (Pr = 1) on the overall fluid flow and temperature patterns are graphically depicted in the form of representative streamlines and isotherms. The values of average Nusselt number and total drag coefficient (C_D) for both representative cylinders are also computed and discussed. Generally, an increase in buoyancy force augments the effectiveness of heat transfer only of the down cylinder. Also, a rise in Re and/or Ri numbers augment the flow instability.     

Study of conjugate natural convection between vertical coaxial rectangular cylinders

Laminar natural convection between two coaxial vertical rectangular cylinders is numerically studied in this work. The outer cylinder is connected with vertical rectangular inlet and outlet pipes. The inner cylinder dissipates volumetric heat. The fluid flow and heat transfer characteristics between the cylinders are analyzed in detail for various Grashof numbers. The heat transfer rates on the individual faces of the inner cylinder are reported. The bottom face of the inner cylinder is found to associate with much higher heat rates than those of the other faces. The average Nusselt number on bottom face is more than 2.5 times of the Nusselt number averaged on all the faces. At a given elevation, local Nusselt number on the inner cylinder faces increases towards cylinder edges. The effect of thermal condition of the walls of outer cylinder, inlet and outlet on the natural convection is analyzed. The thermal condition shows strong qualitative and quantitative impact on the fluid flow and heat transfer. The variation of induced flow rate, dimensionless maximum temperature and average Nusselt numbers with Grashof number is studied. Correlations for dimensionless buoyancy-induced mass flow rate and temperature maximum are presented.     

R141b气液两相喷射器性能研究

考虑实际流体性质、混合室阻力和喉部激波现象,采用等压混合模型,根据质量守恒、动量守恒和能量守恒建立中心进气两相喷射器一维模型。以R141b为工质,研究在不同入口参数和混合室截面积变化比(混合室喉部截面积与混合室入口截面积之比)下喷射器的升压特性以及入口参数和混合室截面积变化比对喷射器出口压力和喷射系数的影响。结果表明:在一定工况下,入口主蒸汽压力每增加0.5 MPa,喷射器出口压力提高约0.002 MPa;入口引射液体压力每增加0.1 MPa,出口压力约升高0.6 MPa。相对于入口主蒸汽参数的变化,入口引射液体参数变化对喷射器的升压特性影响更大。另外,随着混合室截面变化比的增大,升压效果下降。在入口引射液体参数为0.1 MPa/299 K和0.2 MPa/321 K的条件下,混合室截面积比分别增至0.6和0.4时,出口处蒸汽不能完全凝结。研究结果适用于大部分工质,为喷射器的设计和运行提供理论指导。     

Numerical study of the inlet/outlet arrangement effect on microchannel heat sink performance

In this study, fluid flow and heat transfer in microchannel heat sinks are numerically investigated. The three-dimensional governing equations for both fluid flow and heat transfer are solved using the finite-volume scheme. The computational domain is taken as the entire heat sink including the inlet/outlet ports, inlet/outlet plenums, and microchannels. The particular focus of this study is the inlet/outlet arrangement effects on the fluid flow and heat transfer inside the heat sinks.The microchannel heat sinks with various inlet/outlet arrangements are investigated in this study. All of the geometric dimensions of these heat sinks are the same except the inlet/outlet locations. Because of the difference in inlet/outlet arrangements, the resultant flow fields and temperature distributions inside these heat sinks are also different under a given pressure drop across the heat sink. Using the averaged velocities and fluid temperatures in each channel to quantify the fluid flow and temperature maldistributions, it is found that better uniformities in velocity and temperature can be found in the heat sinks having coolant supply and collection vertically via inlet/outlet ports opened on the heat sink cover plate. Using the thermal resistance, overall heat transfer coefficient and pressure drop coefficient to quantify the heat sink performance, it is also found these heat sinks have better performance among the heat sinks studied. Based on the results from this study, it is suggested that better heat sink performance can be achieved when the coolant is supplied and collected vertically.