Pure Natural Convection Research Articles

Non-local modeling for fluid flow coupled with heat transfer by using peridynamic differential operator

The problem of a fluid flow coupled with heat transfer is encountered in many engineering applications. In this study, a non-local model is developed towards such problems by using the peridynamic differential operator. The classical governing equations are reformulated from their differential forms into their integral forms. Furthermore, the non-local model is also cast into the non-dimensional form. Finally, the developed model is applied to simulate pure heat conduction, natural convection, and mixed convection problems. The simulation results are compared with the ones from ANSYS and the published literature. The validity of the proposed model is demonstrated good agreements. The non-local model developed in this study provides an alternative approach for the numerical simulations of the fluid flow coupled with heat transfer problems.

Magnetohydrodynamic instability of mixed convection in a differentially heated vertical channel

The present paper investigates the instability of mixed convection in a differentially heated vertical layer of an electrically conducting fluid under the influence of a uniform horizontal magnetic field. The key parameters which influence the instability characteristics of the system are the Hartman number, the Reynolds number, the Grashof number and the Prandtl number. The computational results reveal that two-dimensional disturbances are more unstable than the three-dimensional disturbances. Further, influence of the Grashof number and the Hartmann number is analyzed on the basic flow. The effect of increasing the Hartman number shows stabilizing effect on the fluid flow and dissimilar is the case with an increase in the Reynolds number and the Prandtl number. In the presence of Reynolds number, Re, stationary instability disappears altogether which is contrary to the results observed in the case of pure natural convection ( $ Re\rightarrow 0$ ). The numerical results obtained under the limiting cases are shown to be in excellent agreement with the existing ones.

On the discrepancy of modelling the heat transfer for pure natural convection

In the present study, buoyancy driven flow fields in a bench scale apparatus are experimentally investigated by means of a non intrusive measurement technique, here the 2D-2C particle image velocimetry (PIV). An electrically heated copper block was used to induce a plume. As the temperature at the surface of the heating source is the main driver for the fluid dynamics, the surface temperature field was measured by thermography. The experimental results are compared to simulations with the Fire Dynamics Simulator (FDS). Simulations with a prescribed, homogeneous distributed surface heat flux and a 3D thermal heat conduction with a volumetric heat source were carried out. The impact of both approaches on the resulting flow fields showed no significant deviations, however, both do not agree with the experimental data. Whereas the surface temperatures of the heating source and its vicinity are in a good agreement for the 3D heat transfer approach. This leads to the conclusion that the modelled heat transfer mechanism does not provide valid predictions in this setup.

Numerical Study of Convection and Radiation Heat Transfer in Pipe Cable

Pipe cable is considered as an important form for underground transmission line. The maximum electrical current (ampacity) of power cable system mostly depends on the cable conductor temperature. Therefore, accurate calculation of temperature distribution in the power cable system is quite important to extract the cable ampacity. In the present paper, the fluid flow and heat transfer characteristics in the pipe cable with alternating current were numerically studied by using commercial code COMSOL MULTIPHYSICS based on finite element method (FEM). The cable core loss and eddy current loss in the cable were coupled for the heat transfer simulation, and the difference of heat transfer performances with pure natural convection model and radiation-convection model was compared and analysed in detail. Meanwhile, for the radiation-convection model, the effects caused by radiant emissivity of cable surface and pipe inner surface, as well as the cable location in the pipe, were also discussed. Firstly, it is revealed that the radiation and natural convection heat flux on the cable surface would be of the same order of magnitude, and the radiation heat transfer on the cable surface should not be ignored. Otherwise, the cable ampacity would be underestimated. Secondly, it is found that the overall heat transfer rate on the cable surface increases as the cable surface emissivity increases, and this is more remarkable to the upper cable. While the effect caused by the radiant emissivity on the pipe inner surface would be relatively small. Furthermore, it is demonstrated that, as cable location in the pipe falls, the natural convection heat transfer would be enhanced. These results would be meaningful for the ampacity prediction and optimum design for the pipe cable.

Investigations of turbulence-radiation interaction in non-Oberbeck-Boussinesq buoyancy-driven flows

This study describes a computational framework for non-Oberbeck-Boussinesq buoyancy-driven turbulent convection coupled with thermal radiation at large temperature difference. A low-Mach number (LMN) model based on the Favre-averaged (Navier-Stokes and energy) equations with the standard k-ε model is presented using unstructured finite volume method. The proposed model is used to simulate turbulent buoyancy-driven convection influenced by radiative heat transfer in a differentially heated square cavity with large temperature and density variation. The primary emphasis of the present study is to understand the non-Oberbeck-Boussinesq effects on buoyancy driven turbulent convection-radiation interaction in enclosures and its influence on overall flow dynamics and heat transfer. In this context, simulations are performed at Rayleigh numbers Ra=109,1010 and 1011 for pure natural convection and coupled convection with surface and gas radiation (τ = 0.2 and 1) at a large temperature difference. Results for Ra=1010 are discussed extensively for all the scenarios. Studies on the influence of Rayleigh number are performed for a particular case of coupled convection with gas radiation (τ=0.2). These simulations reveal that the consideration of wall and gas radiation leads to asymmetry in the flow and heat transfer. Further, the convection currents intensify, and the magnitude and extent of the turbulence effects primarily increase with the consideration of thermal radiation in the simulation. Additionally, the deviations between the proposed variable density low-Mach number model and popularly used incompressible model based on Reynolds averaged governing equations are quantified. The deviation between the two models is found to be growing with the increase in the optical thickness which invalidates the commonly used Boussinesq approximation for such flows.

Develop the nano scale method of lattice Boltzmann to predict the fluid flow and heat transfer of air in the inclined lid driven cavity with a large heat source inside, Two case studies: Pure natural convection & mixed convection

Nano scale method of lattice Boltzmann is developed to predict the fluid flow and heat transfer of air through the inclined lid driven 2-D cavity while a large heat source is considered inside it. Two case studies are supposed: first one is a pure natural convection at Grashof number from 400 to 4000000 and second one is a mixed convection at Richardson number from 0.1 to 10 at various cavity inclination angles. Using LBM to simulate the constant heat flux boundary condition along the obstacle, is presented for the first time while the buoyancy forces affect the velocity components at each inclination angle; hence the collision operator of LBM and also a way to estimate the macroscopic velocities should be modified. Results are shown in the terms of streamlines and isotherms, beside the profiles of velocity, temperature and Nusselt number. It is observed that the present model of LBM is appropriately able to simulate the supposed domain. Moreover, the effects of inclination angle are more important at higher values of Richardson number.

Heat transfer within an eccentric annulus containing heat generating fluid

In this paper, the heat transfer within an eccentric annulus containing heat-generating fluid is investigated. The inner cylinder of the annulus is heated and kept at constant temperature, while the outer cylinder is cooled and maintained at a constant temperature lower than that of the inner cylinder. The full governing equations of mass, momentum, and energy have been solved to investigate the characteristics of flow and thermal fields within the annulus. Besides considering the numerical solution in the case of natural convection at moderate Rayleigh number of 104, the study also considered the numerical and analytical solutions in case of pure heat conduction within the annulus. Moreover, the study discussed the effect of annulus radius ratio Rr, the eccentricity e, and the heat generation parameter S on the heat transfer process. The study considered the range of Rr from 1.4 to 3.2, the range of e from −0.6 to 0.6, and the range of S up to 5. The study manifests that the numerical results for the case of pure conduction are in excellent agreement with the corresponding analytical results. The results for both pure conduction and natural convection cases showed that the average dimensionless temperature of the heat generation fluid increases with the increase in S. Similarly, the heat rejected from the outer cylinder increases with the increase in S for a given Rr. On the contrary, the heat added to the annulus through the inner cylinder decreases as S increases and changes to heat rejection if S exceeds a certain critical value S∗. The value of S∗ is found to decrease with the increase in Rr for both pure conduction and natural convection cases. Moreover, the heat transfer rates at inner and outer walls of the annulus decrease for a given Rr as the inner cylinder moves upward from negative eccentricity to positive eccentricity until both start increasing again at a certain positive eccentricity.

Effect of nanoparticle dispersion and inclination angle on melting of PCM in a shell and tube heat exchanger

Abstract This paper presents an experimental numerical study on melting behavior of a phase change material in a shell and tube heat exchanger. Thus the effects of parameters including adding Nano-Enhanced Phase Change Material (NEPCM) and inclination angle are considered and investigated. Experimental studies are designed to characterize PCM/NEPCM properties. The details of the process progression are presented via temperature contours, streamlines and phase front displacement. Enthalpy porosity method is used for modeling of the phase change process in which pure conduction and natural convection are considered in the simulation. Results reveal that thermal conduction mechanism is dominated at initial and final steps of melting that slows down the process. As the process advances, the buoyancy effect becomes dominant in the melting process. By increasing mass fractions of nanoparticle, thermal conductivity coefficient increases that leads to faster melting rate at the bottom of the heat exchanger. Also results indicate that increasing inclination angle leads to faster pace of melting at higher attitudes that results in shorter total melting time. Contemporaneous adding nanoparticle and increasing inclination angle reveals that both natural convection and thermal conductivity improves that leads to better melting of NEPCM.

Characteristics of the laminar convective heat transfer of molten salt in concentric tube

In this paper, laminar convective heat transfer performance of molten salt with Reynolds number of 300–2300 and Grashof number of 8.56×104–3.95×106 in a concentric tube heat exchanger is reported. The effects of temperature on the heat transfer coefficient of molten salt are experimentally studied with the temperature of 473K–573K. Results show that the measured Nusselt number is larger than that predicted by pure forced convective heat transfer correlations and the deviations decrease as Reynolds number increases. In addition, the experimental result displays a completely different dependence on the temperature from forced convective correlations. It is presumably that the radial density gradient gives rise to buoyancy motion resulting in the formation of natural convection and then strengthen the heat transfer performance in laminar region. By comparison, it is found that the experimental result follows the trend of laminar convective heat transfer correlations derived from traditional working medium, however, the traditional correlations cannot describe the molten salt adequately. Therefore, based on the experimental results and existing correlations, an improved correlation composed of forced convection term and pure natural convection term is proposed, and shows a good agreement with the experiment data with deviations less than ±15%.

The K-type and H-type transitions of natural convection boundary layers

The K-type and H-type transitions of a natural convection boundary layer of a fluid of Prandtl number 7 adjacent to an isothermally heated vertical surface are investigated by means of three-dimensional direct numerical simulation (DNS). These two types of transitions refer to different flow features at the transitional stage from laminar to turbulence caused by two different types of perturbations. To excite the K-type transition, superimposed Tollmien–Schlichting (TS) and oblique waves of the same frequency are introduced into the boundary layer. It is found that a three-layer longitudinal vortex structure is present in the boundary layer undergoing the K-type transition. The typical aligned $\wedge$-shaped vortices characterizing the K-type transition are observed for the first time in pure natural convection boundary layers. For exciting the H-type transition, superimposed TS and oblique waves of different frequencies, with the frequency of the oblique waves being half of the frequency of the TS waves, are introduced into the boundary layer. Unlike the three-layer longitudinal vortex structure observed in the K-type transition, a double-layer longitudinal vortex structure is observed in the boundary layer undergoing the H-type transition. The successively staggered $\wedge$-shaped vortices characterizing the H-type transition are also observed in the downstream boundary layer. The staggered pattern of $\wedge$-shaped vortices is considered to be caused by temporal modulation of the TS and oblique waves. Interestingly the flow structures of both the K-type and H-type transitions observed in the natural convection boundary layer are qualitatively similar to those observed in Blasius boundary layers. However, an analysis of turbulence energy production suggests that the turbulence energy production by buoyancy rather than Reynolds stresses dominates the K-type and H-type transitions. In contrast, the turbulence energy production by Reynolds stresses is the only factor contributing to the transition in Blasius boundary layers.

Natural Convection in Redox Electrochemistry: Model, Simulations and Experiments

A natural convection model has been implemented on redox species TMPD with supporting electrolyte TBAP in acetonitrile solvent. The cross section geometry used for the two-dimensional simulations is 20 mm (W) x 10 mm (H). The working electrode is a 2 mm wide strip at the center of the cell floor and the counter electrode is the entire cell ceiling. For the three-dimensional simulations, a 3mm diameter working electrode in the semi-infinite configuration was used. A uniform magnetic field was applied in the direction perpendicular the working electrode surface. The first case we have studied has an initial concentration of 10.3 mM TMPD- and no oxidized form of TMPD. The supporting electrolyte concentration was 0.5 M. The cell was operated in the potential step mode and the applied potential was high enough for operation in the diffusion-limited regime. Simulations were run for two values of the magnetic field intensity. The electrical conductivity of the bulk solution was 0.625 S/m. The density was calculated using the formulation in our natural convection model based on electroneutrality, where non-electroactive ions migrate to the diffusion layer to neutralize the charge buildup due to the redox reaction. Cases with gravity not present, and gravity applied in the +x, -x, +y and -y directions in 2D were investigated. For the 3D simulations, gravity was applied in the direction normal to the electrode, toward and away from the electrode surface. To study the effect of pure natural convection, cases were run without applying the magnetic field. For this electrochemical system, the density changes from the initial bulk density (785 kg/m3) is very small with a maximum value of 0.0231 kg/m3, which gives a difference of 0.00294% between the highest and the lowest density. Even under this small difference, the evolution of the flow field is quite remarkable. With increasing time from the start, the effect of natural convection on the flow field becomes greater. The maximum velocity increased by an order of magnitude in ~30s from the start. At larger times, the maximum velocity tends toward an asymptotic value along with the electrode current also leveling off and reaching an asymptotic value. The behavior of mixed convection characterized by competing forces of buoyancy and Lorentz force was also investigated, where two values of the magnetic field were chosen. The natural convection-only cases show that the effect is stronger when the established flow is away from the electrode than when it is toward the electrode. This can be attributed to the electrode surface acting to provide a stable flow field configuration, confirming published results. The velocity is an order of magnitude higher when the flow is away from the electrode surface. Note that the cell we used is an order of magnitude higher than a typical microfluidic cell. Work is in progress to study the effect of geometry on natural convection where buoyancy is the only body force, and mixed convection where both buoyancy and MHD are present.Experimental results have been obtained on the ferri-ferro-KCL system. Quantitative Micro-schlieren (QMS) is an instrument suitable for measuring density gradients. A lens-based design makes packaging more compact. An LED light source is used to reduce cooling requirements. All the optics are research grade, thereby improving resolution and reducing aberrations. The electrochemical cell was mounted on a three-axis stage that is mounted on the optical rail for easy alignment. This schlieren system differs from conventional schlieren systems in its compact size (~6m X 2m vs. 1.5m x 0.5m, in footprint), its use of a more uniform intensity LED light source, variable density filter, bandpass filter, lenses instead of mirrors, a digital frame camera, and modern image processing techniques to extract quantitative information from the images. While conducting the redox electrochemistry experiments, it’d be important to ensure that the data are not corrupted by natural convection induced by temperature gradients that might be introduced by sources such as LED illumination. While it is not possible to completely eliminate this source of error, it can be minimized by keeping the LED intensity as low as possible and having the LED on only when necessary. The acquired sequence of images during a time period up to ~300s from the start show a column of high density fluid that starts from the surface and grows in length with time. At larger times (>100s) the column develops a transverse instability with a wave length of ~30 mm. Figure 1

Experimental study based on large-scale smoke propagation fire tests through a horizontal opening connecting two mechanically ventilated compartments

This work describes an experimental study of the flow through a horizontal opening (also referred to as a vent), applicable to specific situations typically encountered in nuclear installations. The configuration consisted of two rooms, which were mechanically ventilated and connected to each other by a horizontal opening, the fire being located in the lower room. The flow was governed by buoyancy due to the heat release from the fire, inertia resulting from the mechanical ventilation, and local momentum from the ceiling jet. Two flow regimes (bi-directional and uni-directional) were encountered depending on the fire power and the ventilation set-up. This study presents 17 large-scale fire tests, investigating the behaviour of the flow at the horizontal opening according to several fire scenario parameters: the fire heat release rate, the fire location, the ventilation configuration and the ventilation flow rate. This range of parameters enabled us to focus on different flow regimes, from pure natural convection (bi-directional) to forced convection (uni-directional). The new set of data obtained, based on detailed flow measurements, offers new insights for understanding the flow and developing sub-models to be used in zone codes.

A new general model for phase-change heat transfer of waxy crude oil during the ambient-induced cooling process

ABSTRACTThe thermal process during shutdown (a stoppage state of the pipeline), of which the essence is an irregular phase-change process accompanied by natural convection, non-Newtonian behavior, and sometimes turbulence, is a critical problem in crude oil transportation engineering. An accurate calculation of the thermal process during shutdown is more than necessary for the safety of crude oil pipeline; however, it faces some challenges due to the complexity of the phase change. In this study, the phase change of waxy crude oil during the cooling process is divided into four stages, which includes a pure liquid natural convection, solid/liquid dispersion natural convection, coexistence of dispersion system natural convection and porous media natural convection, and pure porous media convection, according to different heat transfer mechanisms on different stages. Based on this division, a general phase-change heat transfer model is proposed for the thermal calculation of waxy crude oil during shutdown. Compared with the previous research, this model appropriately includes the influences of non-Newtonian behavior, phase evolution as well as turbulence. With the proposed model, the temperature drop characteristic of a sample pipeline is analyzed and the influencing factors are investigated.

Higher-order and length-scale statistics of velocity and temperature fluctuations in turbulent boundary layer along a heated vertical flat plate

Time-developing direct numerical simulation (DNS) was performed to clarify the higher-order turbulent behaviors in the thermally-driven boundary layers both in air and water along a heated vertical flat plate. The predicted statistics of the heat transfer rates and the higher-order turbulent behaviors such as skewness factors, flatness factors and spatial correlation coefficients of the velocity and temperature fluctuations in the natural-convection boundary layer correspond well with those obtained from experiments for space-developing flows. The numerical results reveal that the turbulent structures of the buoyancy-driven boundary layers are mainly controlled by the fluid motions in the outer region of the boundary layer, and these large-scale structures are strongly connected with the generation of turbulence in the thermally-driven boundary layers, in accordance with the actual observations for space-developing flows. Moreover, to specify the turbulence structures of the boundary layers, the cross-correlation coefficients and the characteristic length scales are examined for the velocity and thermal fields. Consequently, it is found that with a slight increase in freestream velocity, the cross-correlation coefficient for the Reynolds shear stress and turbulent heat flux increases for opposing flow and decreases for aiding flow, and the integral scales for the velocity and temperature fields become larger for opposing flow and smaller for aiding flow compared with those for the pure natural-convection boundary layer.

Non-linear interaction of buoyancy with von Kármán’s swirling flow in mixed convection above a heated rotating disc

In this paper, a systematic computational and theoretical study of the thermo-fluid-dynamics governing the flow above a heated horizontal rotating disc is presented. The fluid flow field is much more complex here as compared to von Kármán’s original solution (which took into account only the effect of disc rotation), and the effects of non-linear interaction between buoyancy and rotation are critically analysed here by studying the separate and combined roles of disc rotation and buoyancy on the fluid dynamic and heat transfer characteristics. The self-similarity of von Kármán’s flow field is lost, and the present paper establishes, for the first time, that the flow field above a heated rotating disc is divided into three distinct fluid dynamic regions. The three regions are demarcated by the loci of V̂z=0 and V̂r=0. In region 1 (R-1), V̂r is positive and V̂z is negative (such directions of the velocity components are characteristic of von Kármán’s flow or pure forced convection). In region 2 (R-2), V̂r is negative and V̂z is positive (such directions of the velocity components are characteristic of pure natural convection near a static disc surface). In region 3 (R-3), both V̂r and V̂z are negative. The forced convection results are obtained asymptotically at a large non-dimensional radius R within the region R-1 showing the dominance of forced convection mechanism, however, the fluid retains the signature of natural convection even at large values of R in the region R-3 where there is an inward radial velocity. Similarly, although a plume forms in the central portion of the disc where the solution is dominated by the effects of buoyancy, the fluid retains a signature of the disc rotation in the helical pathlines of fluid particles rising in the plume (whereas there is no swirl velocity present in pure natural convection above a static disc). The non-linear interaction between buoyancy and rotation results in several peculiar, rather non-intuitive, flow phenomena. Examples of such peculiarity include (i) the presence of a very sensitive spot on the upper boundary such that for a small change in this initial position the fluid pathlines may face drastically different final outcomes, (ii) the presence of a small portion near the centre of the disc where the fluid supplies energy to the disc, (iii) the effect of rotation on the rate of convective heat transfer being diminished by buoyancy over certain part of the disc while being enhanced over another part. This non-linear effect on Nusselt number is quantified here in terms of a Grashof number defined for mixed convection (Grmc).

A phenomenological study on the convection heat transfer around two enclosed rotating cylinders via an immersed boundary method

In this paper, an extended immersed boundary method is used to study the convection of heat from two rotating hot cylinders in a square cold cavity containing air with Pr=0.7. The variables in this study are the rotation scenario, Rayleigh and Richardson numbers. The computations have been carried out for three different rotation scenarios and a range of Rayleigh and Richardson numbers, i.e. Ra=104,105and106 and Ri=0.1,1,10,100. The pure natural convection case, i.e. Ri→∞, has also been considered. Isotherms, streamlines, local and average Nusselt numbers on all solid boundaries are reported in each test case. It is found that the Rayleigh and Richardson numbers are the dominant variables. Rotation scenario is also important and the rotation may significantly increase the heat transfer rate from the cylinders compared to the corresponding buoyancy-induced convection case. Unstable, periodic flow patterns are observed in some test cases at Ra=106.

A Computational Model for Natural Convection and Forced Convection in Redox MHD Systems Based on Electroneutrality and Migration

We present a new non-dimensional parameter for buoyancy effects in natural convection and mixed convection problems. The TN number [1] for mixed convection in redox MHD systems compares MHD forces to buoyancy forces to determine if buoyancy will have a significant effect on system behavior. When convection is caused solely by buoyancy, the Rayleigh number is a relevant parameter to consider the effect of natural convection. However, when the system is influenced by other competing forces such as the Lorentz force, the relative magnitudes of these additional phenomena would determine if buoyancy needs to be considered. If the velocity magnitudes due to forced convection are an order of magnitude higher than those due to natural convection, it would be reasonable to assume that natural convection can then be neglected. A factor that accounts for their respective regions of influence would provide a better representation of the overall influence of natural convection. Based on these arguments, we proposed the non-dimensional group called the TN number. Density change in the present context is a consequence of the charge balance required for electroneutrality which is attained by a complex process of species transport in the vicinity of the electrode including supporting electrolyte ions. Redox electrochemical systems do not involve deposition, but electrochemical reactions occur by electron transfer at the electrode-solution interface. The effect of natural convection in redox systems are less pronounced than in electroplating. It is generally accepted that, in redox electrochemistry, the main reason for natural convection is the migration of ionic species to maintain electroneutrality, a term used to describe the tendency of a fluid volume element to be electrically neutral. To achieve electroneutrality, cations and anions in the solution rearrange themselves, and cause density gradients and natural convection. The L3 -dependence in the TN number indicates the strong dependence on geometric scaling. Micro scale and nanoscale systems might show behavior not observed in larger-scale systems. Bard and Faulkner [2] provide an insightful discussion of this topic by considering balance sheets for mass transfer. The redox reactions lead to concentration gradients of the electroactive species in the diffusion layer which result in a charge imbalance. Cations and anions of the supporting electrolyte migrate into or out of the diffusion layer dictated by their transference numbers in order to neutralize the charge imbalance caused by electron transfer. Our natural convection model is based on the charge balance sheet approach. For the redox reaction at the electrode, we can write an expression for the charge in the diffusion layer relative to the bulk solution. The supporting electrolyte ions moving in and out of the diffusion layer to compensate for the resulting charge imbalance and satisfy electroneutrality is quantitatively modeled and used to calculate the corresponding density change in the diffusion layer. The degree to which the density changes in the diffusion layer will depend on the difference in the molecular weights and the transference numbers of the anions and the cations of the supporting electrolyte. In our model, the changes in the concentrations of the anions and cations of the supporting electrolyte in the diffusion layer with reference to the bulk solution are calculated using the molecular weights and the transference numbers of the ions of the supporting electrolyte. This then enables the calculation of the solution density in the diffusion layer. Thus, we have a quantitative model for density change caused by redox electrochemical reaction. The change in density is then used to calculate the body force due to gravity (buoyancy force). This buoyancy force will be an additional term in the momentum equation similar to the Lorentz force term. This modified momentum equation is solved along with the continuity and species transport equations in a coupled manner using user defined functions (UDF) in the simulations. Results show that three regimes can be delineated by calculating the TN number: pure natural convection, mixed convection and forced convection. The above model should be used in the first two regimes to account for density gradients. Results also show that the magnitude of natural convection depends on the orientation of the gravity vector relative to the electrode surface normal. The simulations also support the L3 -dependence present in the TN number.

Analysis of the effect of eccentricity and operational parameters in PCM-filled single-pass shell and tube heat exchangers

In the present study, melting behavior of RT50 as a phase change material in a shell and tube heat exchanger is considered. Therefore the effects of parameters including a geometrical property (eccentricity) and flow specifications (mass flow rate and inlet temperature of heat transfer fluid) on different decisive parameters of the PCM are investigated. The selected parameters which are critical for every storage systems are liquid fraction, melting time and thermal storage performance. Enthalpy porosity method is used for the modeling of phase change process and pure conduction and natural convection are considered in the simulation. The differential governing equations are solve using SIMPLE algorithm in which momentum and energy equation are discretized employing QUICK differentiating scheme. The grid size and the time step were chosen after careful examination of the independency of the results to these parameters. The details of the process progression are presented via temperature contours and streamlines. The study declares that the initial stages as well as ending portion of melting process are dominated by weak mechanism of conduction. Results show that the average temperature of the PCM isn’t a function of eccentricity as long as there is solid PCM above the inner tube. Generally by increasing the eccentricity the rate of heat transfer and average temperature at the last stages of melting process increases. The Results indicated that although increasing eccentricity and inlet temperature of the heat transfer fluid influences the rate of heat transfer in the heat exchanger especially when both variations are considered simultaneously, mass flow rate does not leave such a significant impression on the rate even in eccentric cases.

Full Eulerian lattice Boltzmann model for conjugate heat transfer.

In this paper a full Eulerian lattice Boltzmann model is proposed for conjugate heat transfer. A unified governing equation with a source term for the temperature field is derived. By introducing the source term, we prove that the continuity of temperature and its normal flux at the interface is satisfied automatically. The curved interface is assumed to be zigzag lines. All physical quantities are recorded and updated on a Cartesian grid. As a result, any complicated treatment near the interface is avoided, which makes the proposed model suitable to simulate the conjugate heat transfer with complex interfaces efficiently. The present conjugate interface treatment is validated by several steady and unsteady numerical tests, including pure heat conduction, forced convection, and natural convection problems. Both flat and curved interfaces are also involved. The obtained results show good agreement with the analytical and/or finite volume results.

Effects of rotation on the trailing vortex and heat transfer from a horizontal rotating cylinder at higher Grashof number

In the present study, effects of rotation on the trailing vortex and heat transfer from a large diameter horizontal rotating cylinder in still air have been experimentally investigated whereas earlier studies considered cylinders with smaller diameters. As the rotational speed n is increased, the trailing vortex would deflect in the rotational direction, the flow state of air near the cylinder would vary from laminar to turbulent, the heat transfer would transit from pure natural convection to mixed convection, and finally to pure forced convection resulted from rotation, the trailing vortex would break down and disappear. The variation of deflection angles ω with the parameter Rer/Gr can be expressed as w=32.3Rer/Gr at lower rotational speed. The discriminant to determine the disappearance of trailing vortex can be presented as Rer2/Gr=7.51.