Isothermal Wall Research Articles

Study of Rayleigh-Taylor instability in viscosity-stratified fluid layers

The Rayleigh-Taylor instability in viscosity varying fluid layers is studied. A heavier fluid layer is superimposed on a lighter fluid layer. Both the fluid layers are bounded between two parallel isothermal horizontal walls. Out of the two walls, one wall is isothermally heated, while the other wall is isothermally cooled. The Rayleigh-Taylor instability is studied for axisymmetric configuration. Viscosities of both the fluids are considered to be varying with temperature. The effect of viscosity ratio, temperature ratio, heating location, Prandtl number and Weber number on the instability is studied. When the viscosity of fluid layers vary with temperature, the configuration becomes more unstable compared to that for constant viscosity fluid layers. For varying viscosity fluid layers the spike undergoes large deformations. At lower viscosity ratios, mushroom shaped spike is formed with elongated skirt structure. Whereas, at higher viscosity ratios, spike with fluid column structure is formed. It is found that increasing viscosity ratio shows stabilizing effect. Increasing temperature ratio is found to show destabilizing effect with complex spike structure formation at high temperature ratios. In top wall heating configuration, formation of mushroom shaped spike and skirt of the spike occurs earlier in a thinner and elongated form compared to that of bottom wall heating configuration. Prandtl number showed insignificant effect on the instability. For the parameter values of the study, when Weber is lower than 55 the configuration is stable and does not undergo instability. Overall, surface tension has shown stabilizing effect on the instability.

Study of mixed convective–radiative fluid flow in a channel with temperature-dependent thermal conductivity

In this paper, the heat transfer equation is coupled to Navier–Stokes equations for a steady vertical channel. The objective of the present study is to investigate the electrically conducting viscous fluid with isothermal wall conductions with variable properties. The combined effects of temperature dependent thermal conductivity and temperature dependent viscosity are examined. The analysis is presented in dimensionless form and the resultant equations are obtained both analytically (Perturbation method) and numerically by fourth order R–K​ Fehlberg with shooting technique. The dimensionless parameters that influence the temperature field and the transverse velocity field are variable thermal conductivity parameter −0.5≤bk≤0.5, variable viscosity parameter −0.5≤bv≤0.5, applied electric field parameter E=−1,0,1, Hartman number 1≤M≤3, buoyancy parameter 0≤N≤1.5 and wall temperature ratio parameter −2≤m≤2.

Entropy Generation during Head-On Interaction of Premixed Flames with Inert Walls within Turbulent Boundary Layers.

The statistical behaviours of different entropy generation mechanisms in the head-on interaction of turbulent premixed flames with a chemically inert wall within turbulent boundary layers have been analysed using Direct Numerical Simulation data. The entropy generation characteristics in the case of head-on premixed flame interaction with an isothermal wall is compared to that for an adiabatic wall. It has been found that entropy generation due to chemical reaction, thermal diffusion and molecular mixing remain comparable when the flame is away from the wall for both wall boundary conditions. However, the wall boundary condition affects the entropy generation during flame-wall interaction. In the case of isothermal wall, the entropy generation due to chemical reaction vanishes because of flame quenching and the entropy generation due to thermal diffusion becomes the leading entropy generator at the wall. By contrast, the entropy generation due to thermal diffusion and molecular mixing decrease at the adiabatic wall because of the vanishing wall-normal components of the gradients of temperature and species mass/mole fractions. These differences have significant effects on the overall entropy generation rate during flame-wall interaction, which suggest that combustor wall cooling needs to be optimized from the point of view of structural integrity and thermodynamic irreversibility.

Studies on effects of wall temperature variation on heat transfer in hypersonic laminar boundary layer

The effects of wall temperature variation on heat transfer in the hypersonic laminar boundary layer are investigated using a typical blunt-headed cone under isothermal and radiation equilibrium wall boundary conditions, respectively. It is found that, under the isothermal wall boundary condition, the wall heat flux is strongly linearly related to the wall temperature for perfect gas flow, while linearly related to the wall enthalpy for chemical nonequilibrium flow. Under the radiation equilibrium wall condition, even for the same wall point with the same wall temperature, the heat flux may show significant difference from the result of the isothermal wall due to the spatial variation of wall temperature on the upstream flow filed. Consequently, using the heat flux under the isothermal condition to estimate the heat flux under the radiation equilibrium condition by linear interpolation based on the local wall temperature would introduce considerable errors. Further, the analysis of the heat transfer for the radiation equilibrium wall with different emissivity reveals that the heat flux has a strong linear relationship with the local wall temperature (for perfect gas flow) or wall enthalpy (for chemical nonequilibrium flow). Finally, a new method using the heat flux results obtained under known wall parameters to estimate the heat flux under new wall parameters based on the local wall enthalpy linear interpolation is proposed, and the numerical experiments show that the relative errors are basically below 5% for the isothermal wall and the radiation equilibrium wall cases studied in this paper.

Heat transfer characteristics of a periodically transient flow for turbulent wall-bounded dual jet

The thermal characteristics and aerodynamic behaviours of an unsteady flow for a turbulent wall-bounded dual jet are computationally studied in the present paper. It is quite important to analyse thermal characteristics of such flows, as a function of independent parameters like Reynolds number. These jets are used in different industrial applications, such as, cooling of electronic devices and heat exchange systems. The capabilities of different Reynolds-Averaged Navier-Stokes (RANS) turbulence modeling approaches, such as, renormalization groupk-ɛ(RNG), realizablek-ɛ, standardk-ω,and shear-stress transportk-ω(SST) models in predicting thermal performance of a turbulent dual jet are investigated. The comparison shows that among four turbulence models, the SSTk-ωmodel provides a good agreement with the reported experimental results. The transient RANS equations have been used to simulate the complex behaviour of a turbulent dual jet. The Reynolds number (Re) is varied from 10,000 to 40,000 for a fixed offset ratio of 2 under isothermal boundary wall conditions. The results reveal that the axial position of the merge point is independent of Re and remains almost constant as Re is further increased. It is also noticed that the velocity becomes self-similar and independent of Re after a streamwise position of X = 30. The time series of U and V velocity signals unveil sinusoidal oscillations and display peak to peak amplitude difference at the far axial location. The FFT assessment of the U velocity signal displays a solo peak dominant frequency which is the same as shown by V velocity in the FFT plot corresponding to the Strouhal number of the vortex shedding phenomenon. The average Nusselt number and the average heat flux increase with Re. A correlation has been developed for the average Nusselt number and the average heat flux. These outcomes will improve the current knowledge of various engineering and industrial applications to enhance the heat transfer in different heat exchange systems, such as, electronics cooling systems.

The effect of the altitude on the performance of a solar chimney

This study aimed to show the relationship between the altitude and the performance of a solar chimney based on simulation analysis and numerical calculation. Steady-state heat transfer from isothermal vertical walls of a chimney is modelled analytically. The paper introduces the basic concepts and governing equations for modelling heat transfer and air flow rates within the solar chimney at various altitudes. The relationship between altitude, temperature, and mass flow rates for the buoyancy-driven chimney flow is complex. A multidimensional equation comprising the inverse tangent and inverse hyperbolic tangent functions is developed. A Matlab program was developed to overcome the problem of the non-linearity of the equation. The results obtained showed that the altitude has modest influence but not substantial on the performance of the solar chimney if we assumed a constant ambient air temperature for all altitudes. But in real climatic conditions, the ambient temperature is altitude dependent. In these circumstances, the performance of the solar chimney is significantly influenced by the altitude. Therefore, microclimatic conditions should be used rather than large-scale area climatic data for solar chimney design.

Parametric study of natural convection showing effects of geometry, number and orientation of fins on a finned tube system: A numerical approach

The present study provides a thorough numerical analysis of natural convection heat transfer from a horizontal cylinder with external longitudinal fins. The Three-dimensional(3D) forms of Continuity, Navier-stokes, and the energy equations have been solved to understand the flow field and temperature distribution using the Ansys Fluent software. The purpose of the study is to investigate the effects of fin length Lf, fin thickness (t), number of fins (N) and material of fin (Aluminum & Steel) on the parameters namely, the heat transfer rate from the inner isothermal wall and effectiveness (Q*)of the system for a constant Rayleigh number in the laminar range (Ra ≤ 106). Additionally, the effect of two different orientations (A&B) of fins has been discussed. Orientation-A consists of fins starting from horizontal plane placed at equal angular spacing subsequently, whereas the fins in orientation-B originate from the vertical plane followed with equal angular spacing. Non-dimensional fin length (L*) is being varied from 0.222 to 2, non-dimensional fin thickness (t*) as 0.022 ~0.044 and the number of fins (N) between 6~24. It is observed that for a fixed number of fins there exists an optimum fin length and for a fixed fin length there exists an optimum fin thickness & an optimum number of fins. For a fixed number of fins there exist an optimum fin length and maximum effectiveness, the maximum possible effectiveness for the taken fin-tube system is 4.34 for 12 fins. The maximum effectiveness reported in the current study was 1.86 for L*=0.444, t*=0.022 and N=12. Furthermore, the optimum length for N=12 came out to be L*=1.8.

Simulation of melting and solidification process of polyethylene glycol 1500 as a PCM in rectangular, triangular, and cylindrical enclosures

Energy storage systems are critical as the world strives to reduce its reliance on fossil fuel energy for environmental and economic reasons. Because of their large latent heat storage capacity, phase change materials (PCM) are well characterized as candidates in this respect. Material selection, operational conditions, and geometrical configurations are all important elements to consider when designing an energy storage system. The simulation study's major purpose is to assess the solidification, melting time, and heat transfer processes for triangular, rectangular, and cylindrical enclosures with polyethylene glycol 1500 as the same PCM volume and fully isothermal walls. This is a critical issue in the industry because full contact of the enclosure's walls with surrounding fluids such as air, oil, and so on is unavoidable in some industrial applications (such as heating–cooling systems and air conditioning, panels used in building walls, the automotive industry, and the textile industry). The results indicated that horizontal rectangular and triangular enclosures melted the fastest because of their sharp angles and long-flat bottom walls, which are thought to have the most influence on natural convection. The results illustrated that the quantity of solidification liquid fraction for a cylindrical is higher at a given time than for other geometries.

Impacts of wall conditions on flame acceleration at the early stages of burning in channels

The role of mechanistic (free-slip or non-slip) and thermal (adiabatic or isothermal) wall conditions on flame acceleration at the early stages of burning in channels is studied by computational simulations of the reacting flow equations involving fully compressible hydrodynamics and transport properties (heat conduction, diffusion and viscosity). A qualitatively new effect is observed in the case of isothermal walls preheated to a high temperature. Specifically, additional flame segments are formed near a wall in that case, leading eventually to emergence and propagation of an intriguing octopus-like flame front.

Linear stability analysis of hypersonic boundary layers computed by a kinetic approach: a semi-infinite flat plate at $$\varvec{4.5\le \mathrm{M}_\infty \le 9}$$

Linear stability analysis is performed using a combination of two-dimensional Direct Simulation Monte Carlo (DSMC) method for the computation of the basic state and solution of the pertinent eigenvalue problem, as applied to the canonical boundary layer on a semi-infinite flat plate. Three different gases are monitored, namely nitrogen, argon and air, the latter as a mixture of 79\% Nitrogen and 21\% Oxygen at a range of free-stream Mach numbers corresponding to flight at an altitude of 55km. A neural network has been utilised to predict and smooth the raw DSMC data; the steady laminar profiles obtained are in very good agreement with those computed by (self-similar) boundary layer theory, under isothermal or adiabatic wall conditions, subject to the appropriate slip corrections computed in the DSMC method. The leading eigenmode results pertaining to the unsmoothed DSMC profiles are compared against those of the classic boundary layer theory. Small quantitative, but no significant qualitative differences between the results of the two classes of steady base flows have been found at all parameters examined. The frequencies of the leading eigenmodes at all conditions examined are practically identical, while perturbations corresponding to the DSMC profiles are found to be systematically more damped than their counterparts arising in the boundary layer at the conditions examined, when the correct velocity slip and temperature jump boundary conditions are imposed in the base flow profiles; by contrast, when the classic no-slip boundary conditions are used, less damped/more unstable profiles are obtained, which would lead the flow to earlier transition. On the other hand, the DSMC profiles smoothed by the neural network are marginally more stable than their unsmoothed counterparts.

DNS of compressible turbulent flow in convergent-divergent nozzles with isothermal wall

Compressible turbulent flow through canonical convergent-divergent nozzles with circular cross-section, isothermal walls, with and without bell-shaped divergent portion is investigated by means of Direct numerical simulations. The goal of the present study is to demonstrate the effects of flow expansion (and local flow compression) in these nozzles, which have substantial transonic regions, on the mean flow and the turbulence structure. The efficacy of the kinematic or incompressible displacement thickness as a possible lengthscale for computing nondimensional pressure gradient parameters which are indicators of self-similar regions in these flows is shown. Elongation of near-wall flow structures and the dominance of vortex stretching as local flow topology are found in regions of expansion. This is accompanied by a decrease in Reynolds stresses. Regions of local flow compression are found in the divergent portion of the bell-shaped nozzle where vortex compression is the dominant flow topology. A local increase in the streamwise Reynolds stress is found in this region.

Numerical Simulation of Solid and Porous Fins’ Impact on Heat Transfer Performance in a Differentially Heated Chamber

The development of different industrial fields, including mechanical and power engineering and electronics, demands the augmentation of heat transfer in engineering devices. Such enhancement can be achieved by adding extended heat transfer surfaces to the heated walls or heat-generating elements. This investigation is devoted to the numerical analysis of natural convective energy transport in a differentially heated chamber with isothermal vertical walls and a fin system mounted on the heated wall. The developed in-house computational code has been comprehensively validated. The Forchheimer–Brinkman extended Darcy model has been employed for the numerical simulation of transport phenomena in a porous material. The partial differential equations written, employing non-primitive variables, have been worked out by the finite difference technique. Analysis has been performed for solid and porous fins with various fin materials, amounts and heights. It has been revealed that porous fins provide a very good technique for the intensification of energy removal from heated surfaces.

Induced magnetic field and viscous dissipation on flows of two immiscible fluids in a rectangular channel

The unsteady, magneto-hydrodynamic generalized Couette flows of two immiscible fluids in a rectangular channel with isothermal walls under the influence of an inclined magnetic field and an axial electric field have been investigated. Both fluids are considered electrically conducting and the solid boundaries are electrically insulated. Approximate analytical solutions for the velocity, induced magnetic, and temperature fields have been determined using the Laplace transform method along with the numerical Stehfest's algorithm for the inversion of the Laplace transforms. Also, for the nonlinear differential equation of energy, a numerical scheme based on the finite differences has been developed. A particular case has been numerically and graphically studied to show the evolution of the fluid velocity, induced magnetic field, and viscous dissipation in both flow regions.

LATTICE BOLTZMANN METHOD INVESTIGATIONS OF NATURAL CONVECTION WITHIN A CAVITY EQUIPPED WITH A HEAT SOURCE: A PARAMETRIC STUDY

In the present work, a numerical study is carried out to investigate, with the aid of the lattice Bolkmann method (LBM), the natural convection air (Pr = 0.71) cooling of an electronic component presented as a heat source attached to the bottom wall of a rectangular cavity. The top wall is isothermal and other walls are adiabatic. In order to optimize the ability of the heat sink to reject more heat from the heat source and ensure an effective cooling of the electronic component, a series of numerical simulations are conducted by varying the cold wall position (top - right), inclination angle (0° ≤ γ ≤ 90°), the aspect ratio of the cavity (0.25 ≤ A ≤ 1.5), and the heat source position (ε = 0.25, ε = 0.35, and ε = 0.5), while the Rayleigh number is kept constant, Ra = 106. Results show that the cavity structure has a deep impact on the cooling of the electronic component. The aspect ratio A = 1.5 ensures a relatively excellent heat transfer rate when the cavity is cooled by the top wall. The heat source position within the cavity has, also, a noticeable impact on dynamic and thermal fields. Indeed, it was found that the local Nusselt number of the cold wall and heat source indicate that, by varying the position of the heat source, fluid flow and natural convection rate are more important in the right cavity compared to the left one.

Influence of the punched holes on thermohydraulic performance and flow pattern of rectangular channels with a pair of perforated vortex generators

Vortex generators (VGs) are passive elements that contribute to enhance the heat transfer and produce a low pressure loss. Rectangular channels are the basic unit of many heat exchangers. Frequently, both are combined and therefore they are investigated intensively. Regardless, the influence of punched holes on the plate surface is often ignored when the thermohydraulic performance of rectangular channels has been studied. This work deals with a numerical study about the impact on the channel thermohydraulic performance of a delta winglet pair of perforated VGs punched on the channel surface. The VGs transversal pitch and the flow pattern in the common flow area were investigated for three configurations and sixteen different arrangements. The computational domain consisted of a plate centered in the middle of the rectangular channel. The numerical model was validated by comparison with published correlations for the entry region with isothermal walls as well as inside the laminar flow regime. Secondary flow was evaluated to explain the heat transfer enhancement. A performance evaluation criterion based on the similarity of pumping power was applied. All configurations studied enhanced the heat transfer and increased the pressure loss, with values up to 12.8% and 17.5% respectively. A reverse direction of rotation of vortices was found caused by the punched holes in two configurations, a result that was not found in the literature revised. A vortices inhibition caused by the punched holes was identified, which constitutes an important result and a novelty. For Reynolds numbers higher than 120, the application of VGs punched on the channel surface is advantageous. According to the performance evaluation criterion applied, the positive impact of VGs on the surface was increased with the Reynolds number, reaching values of 1.075 for the common flow up configuration with the punched holes behind and the highest VGs transversal pitch.

Performance Assessment of Different Turbulence Models for a Dual Jet Flowing Over a Heated Sinusoidal Wavy Surface

Abstract An enhancement in heat transfer is the key objective in any thermal system where an efficient cooling is needed. This requirement becomes more important for turbulent flow. A turbulent dual jet is associated with entrainment and mixing processes in several applications. This article aims at enhancing the heat transfer rate by utilizing the wavy surface of a heated plate. Heat transfer and flow characteristics are studied using five low Reynolds-Averaged Navier–Stokes (RANS) turbulence models, namely, Yang and Shih k − ɛ (YS), Launder and Sharma k − ɛ (LS), realizable k − ɛ, renormalization group k − ɛ (RNG), and shear-stress transport k − ω (SST) models. The amplitude of the wavy surface is varied from 0.1 to 0.8 for the number of cycles fixed to 7. The Reynolds number and offset ratio are set to 15,000 and 3, respectively. An isothermal wall condition is used at the wavy wall. An experimental validation has been performed. An enhancement of 55.94% in heat transfer is achieved by the RNG k − ɛ model. Furthermore, it is noticed that the YS model fails to predict the flow separation as the amplitude of the sinusoidal wavy surface increases. However, the SST model reveals that the flow separates when the amplitude increases beyond 0.6. The thermal hydraulic performance (THP) is found to increase for the RNG model by approximately 13.9% for the maximum amplitude considered. As the profiles of the bottom walls change, various turbulence models predict different fluid flow characteristics.

Influence of thermal wall boundary condition on scalar statistics during flame-wall interaction of premixed combustion in turbulent boundary layers

The influence of thermal wall boundary conditions on the behaviour of scalar statistics during flame-wall interaction (FWI) of premixed flames within turbulent boundary layers has been investigated. Three-dimensional direct numerical simulations (DNS) of two different flow configurations for flames interacting with chemically inert isothermal as well as adiabatic walls in fully developed turbulent boundary layers have been performed. The first configuration is an oblique wall interaction (OWI) of a V-flame in a turbulent channel flow and the second configuration is representative of head-on interaction (HOI) of a planar flame in a turbulent boundary layer. The turbulence in the non-reacting conditions for these simulations is representative of a friction velocity based Reynolds number of Reτ=110 while the combustion characteristics are representative of stoichiometric methane–air mixture under atmospheric conditions. Differences in the mean behaviours of the progress variable and non-dimensional temperature have been observed for the two configurations in response to different wall boundary conditions. Mean scalar variance of the progress variable and temperature and their respective scalar dissipation rates have been investigated and it is found that the variances of the progress variable and temperature remain coupled in the case of adiabatic walls, while significant differences exist between these quantities in the vicinity of the wall under isothermal wall conditions for both flow configurations. The mean scalar dissipation rates of progress variable and temperature during FWI decrease towards the wall in both flow configurations under different thermal wall boundary conditions, but in the case of isothermal wall conditions a higher scalar dissipation rate for the non-dimensional temperature is observed when compared with the scalar dissipation rate for the progress variable. The behaviours of the mean turbulent scalar fluxes of progress variable and the non-dimensional temperature suggest that the scalar fluxes can show both gradient and countergradient type behaviour depending on the flow configuration and also on the proximity of the flame to the wall. In the case of V-flame OWI, the choice of the wall boundary condition has a significant influence on the behaviour of the scalar fluxes, whereas in the case of turbulent boundary layer HOI, no significant differences in the scalar fluxes are observed for different wall boundary conditions.

Numerical simulations and experimental investigations to study the melting behavior of beeswax in a cylindrical container at different angular positions

Phase Change Materials (PCMs) are widely used in Latent Heat Thermal Energy Storage Systems (LHTES). This work aims to study the melting behavior of low-temperature phase change materials in cylindrical containers placed at different angular positions. Simulations were performed using ANSYS FLUENT by applying the enthalpy porosity model to analyze melting of beeswax in a cylindrical glass tube subject to an isothermal wall condition. The tube was placed at angular positions of 0°, 30°, 60° and 90° to the horizontal in order to study the effect of angular position on melting behavior. The simulation results were then validated by performing experiments and capturing images at regular intervals of time to track the solid-liquid interface. Image processing using MATLAB was performed on the captured images to study the variation of melt fraction as a function of time. It is observed that the experimental results were in good agreement with the simulations. From these studies, it was observed that the total melting time increases with the angle of tilt from 0° to 90° Initially, the melting progressed similar to the analytical solutions of one-dimensional phase-change heat transfer. As the melting continued, the experimental and numerical results deviated from the analytical solutions and the rate of melting increased due to convection effects. The results obtained in this study can be used to predict the melting behavior of PCMs in a solar flat plate collector kept at different angular inclinations. This result can be kept in mind while designing a thermal energy storage system to set an appropriate angle to match the cycle time of the system.

Combined effects of local curvature and elasticity of an isothermal wall for jet impingement cooling under magnetic field effects

Combined effects of local curvature and elasticity of an isothermal wall for jet impingement cooling under magnetic field effects

Intelligent optimization of horizontal fins to improve the melting performance of phase change materials in a square cavity with isothermal vertical wall

A new combination mode of genetic algorithm (GA) and computational fluid dynamics (CFD) is proposed to solve optimization problems of fins for the enhancement of solid-liquid phase change heat transfer performance. In this optimization problem, some horizontal straight fins are installed on a vertical wall of a two-dimensional square cavity filled with PCM. The total cross-sectional area of fins is set as constraint, and the number, location and length/thickness of fins are selected as variables. The effect of natural convection on the melting characteristic of PCM is considered. The optimization results showed that fin length should be as long as possible and the fins should be arranged uniformly if the fin thickness is free. The existence of natural convection may slightly move fin downward to shorten the total melting time and increase the thermal energy storage rate. When the fin thickness is limited, the optimal number of fins depends on the total length of fins. One fin is suggested when the total length is smaller than 1.3, two fins when between 1.3 and 2.6, three fins when between 2.6 and 3.6, and four fins when greater than 3.6. Within the parameter range in this study, the thermal conductivity of fin material and fin thickness have ignorable effects on the critical values of total length, while the effects of natural convection should not be neglected. The critical length will increase with the increase of the Rayleigh number.