Transition to a periodic flow induced by a thin fin on the sidewall of a differentially heated cavity

The transition to a periodic flow induced by a thin fin on the sidewall of a differentially heated cavity is numerically investigated. The numerical results are compared with a previously reported experiment. It is demonstrated that the transient flow obtained numerically shows features consistent with the experimental flow. Based on the present numerical results, the temporal development and spatial structures of the thermal flow around the fin are described, and the separation of the thermal flow above the fin is discussed. It is found that the presence of the fin changes the flow regime and results in the transition of the thermal flow to a periodic flow. The present numerical results also indicate that the unstable temperature configuration above the fin results in intermittent plumes at the leeward side of the fin, which in turn induce strong oscillations of the downstream boundary layer flow. It is demonstrated that the oscillations of the boundary layer flow significantly enhance the heat transfer through the finned sidewall (by up to 23%).     

Natural Convection Heat Transfer in a Cubic Cavity Submitted to Time-Periodic Sidewall Temperature

The laminar unsteady natural convection in a cubic cavity is comprehensively studied here using a high accuracy temporal-spatial pseudospectral method. In this study, the cavity is filled with air and one of its sidewalls is submitted to sinusoidally varying temperature, while constant lower temperature is imposed on the opposing sidewall and other sidewalls are adiabatic. Computations are performed to explore the effects of several influential factors on the fluid flow patterns and heat transfer performances within the cavity, including Rayleigh number and the amplitude and period of pulsating sidewall temperature. Numerical results reveal that the heat transfer enhancement is complexly determined by the above influential factors, and the heat transfer resonance is observed in the case of a large Rayleigh number and amplitude of pulsating sidewall temperature. The three-dimensional effects on fluid flow patterns and heat transfer are discussed. Finally, the backward heat transfer is quantitatively studied.     

Numerical simulation of fluid flow and heat transfer in a microchannel heat sink with offset fan-shaped reentrant cavities in sidewall

The paper is focused on the investigation of fluid flow and heat transfer characteristics in a microchannel heat sink with offset fan-shaped reentrant cavities in sidewall. In contrast to the new microchannel heat sink, the corresponding conventional rectangular microchannel heat sink is chosen. The computational fluid dynamics is used to simulate the flow and heat transfer in the heat sinks. The steady, laminar flow and heat transfer equations are solved in a finite-volume method. The SIMPLEX method is used for the computations. The effects of flow rate and heat flux on pressure drop and heat transfer are presented. The results indicate that the microchannel heat sink with offset fan-shaped reentrant cavities in sidewall improved heat transfer performance with an acceptable pressure drop. The fluid flow and heat transfer mechanism of the new microchannel heat sink can attribute to the interaction of the increased heat transfer surface area, the redeveloping of the hydraulic and thermal boundary layers, the jet and throttling effects and the slipping over the reentrant cavities. The increased heat transfer surface area and the periodic thermal developing flow are responsible for the significant heat transfer enhancement. The jet and throttling effects enhance heat transfer, simultaneously increasing pressure drop. The slipping over the reentrant cavities reduces pressure drop, but drastically decreases heat transfer.     

Convective instability of a vertical thermal boundary layer in a differentially heated cavity

The convective instability of a vertical thermal boundary layer adjacent to the sidewall of a water-filled differentially heated cavity over a range of Rayleigh numbers (5 × 10⁷–3.44 × 10⁹) is investigated using direct stability analysis. The results show that the dominant frequency of the convective instability changes as perturbations travel downstream due to the presence of the horizontal boundaries, which is different from that of the vertical thermal boundary layer adjacent to an infinite or semi-infinite thermal wall. The features of the convective instability of the vertical thermal boundary layer adjacent to the sidewall are described, and the dependence of the dominant frequency on the Rayleigh number is obtained. Furthermore, the dependence of the flow rate and heat transfer through the cavity on the Rayleigh number is quantified by numerical results.     

Natural Convection Heat Transfer of Copper–Water Nanofluid in a Square Cavity With Time-Periodic Boundary Temperature

This paper presents a numerical study of natural convective heat transfer of copper–water nanofluid in a square enclosure where the temperature of the left vertical sidewall is sinusoidally oscillated with a constant average temperature, the right sidewall is cooled at a relatively low temperature, and the other walls are kept adiabatic. The influence of pertinent parameters such as Rayleigh number, solid volume fraction of copper nanoparticles, and dimensionless time period on the heat transfer characteristics is studied. The results show that the heat transfer rate increases using copper nanoparticles.     

Comparative Study of the Flow and Thermal Performance of Liquid-Cooling Parallel-Flow and Counter-Flow Double-Layer Wavy Microchannel Heat Sinks

Applications of microchannel heat sinks for dissipating heat loads have received great attention. Wavy channels are recognized to be an alternative cooling technology to enhance the heat transfer, and are successfully applied in heat exchangers. In this article, three kinds of liquid-cooling double-layer microchannel heat sinks, such as a rectangular straight microchannel heat sink, a parallel-flow wavy microchannel heat sink, and a counter-flow double-layer wavy microchannel heat sink, have been designed and the corresponding laminar flow and heat transfer have been investigated numerically. The effects of the wave amplitude and volumetric flow ratio on heat transfer, pressure drop, and thermal resistance are also observed. Results show that the counter-flow double-layer wavy microchannel heat sink is superior at a larger flow rate, and a more uniform temperature rise is achieved. For a slightly larger flow rate, the parallel flow layout shows better performance. In addition to the overall thermal resistance, other criteria for evaluation of the overall thermal performance, e.g., (Nu/Nu₀)/(f/f₀) and (Nu/Nu₀)/(f/f₀)^1/3, are applied and similar results are obtained.     

Transition of natural convection boundary layers—a revisit by Bicoherence analysis

Previous studies have revealed that heat transfer through a convective thermal boundary layer can be significantly enhanced by perturbing the thermal boundary layer to advance linear to nonlinear transition. It has also been demonstrated that the enhancement of heat transfer is mostly achieved in the nonlinear regime. In this study, the transition of the thermal boundary layer adjacent to an isothermally heated vertical surface is revisited by means of Bicoherence analysis, which is a statistical approach for identifying and quantifying quadratic wave interactions. The streamwise evolution of Bicoherence spectra suggests that the thermal boundary layer can be classified into three regimes: a linear flow regime, a transitional flow regime and a nonlinear flow regime. The positions of the transition from the transitional to nonlinear regimes in the thermal boundary layer at various Rayleigh numbers, perturbation frequencies and perturbation amplitudes are determined using Bicoherence analysis. It is found that in the nonlinear flow regime, the number of resonance groups fluctuates, which indicates the occurrence of coupling and decoupling of harmonics in the boundary layer. This process may be the mechanism responsible for the resonance induced enhancement of heat transfer.     

Numerical investigation on side heat transfer enhancement in 300 kA aluminum reduction cell

Industrial test and numerical simulation were synchronously applied to analyze the side heat transfer process and enhance heat transfer in aluminum reduction cell. The 3D slice finite element model of aluminum reduction cell was developed, with which the sidewall temperature field of the cell was computed by using software ANSYS. The main influencing factors on heat dissipation were analyzed and some effective measures were proposed to enhance sidewall heat transfer. The results show that the shell temperature of the test cell and the common cell is respectively 312°C and 318°C and the ledge thickness is 16 cm and 15 cm when side coefficient of heat transfer between the shell and the surroundings is 70 W/(m²·K). With the increase of the side coefficient of heat transfer between the shell and the surroundings, the temperature of the shell decreases but the thickness of the side ledge increases when the electrolytic temperature, the ambient temperature, the coefficient of heat transfer between molten bath and ledge, the eutectic temperature and the thermo-resistance of the side lining are constant.     

Numerical investigation on side heat transfer enhancement in 300 kA aluminum reduction cell

Industrial test and numerical simulation were synchronously applied to analyze the side heat transfer process and enhance heat transfer in aluminum reduction cell. The 3D slice finite element model of aluminum reduction cell was developed, with which the sidewall temperature field of the cell was computed by using software ANSYS. The main influencing factors on heat dissipation were analyzed and some effective measures were proposed to enhance sidewall heat transfer. The results show that the shell temperature of the test cell and the common cell is respectively 312°C and 318°C and the ledge thickness is 16 cm and 15 cm when side coefficient of heat transfer between the shell and the surroundings is 70 W/(m²K). With the increase of the side coefficient of heat transfer between the shell and the surroundings, the temperature of the shell decreases but the thickness of the side ledge increases when the electrolytic temperature, the ambient temperature, the coefficient of heat transfer between molten bath and ledge, the eutectic temperature and the thermo-resistance of the side lining are constant.     

Simultaneously estimating the contact heat and mass transfer coefficients in a double-layer hollow cylinder with interface resistance

Based on the conjugate gradient method, this study presents a means of solving the inverse boundary value problem of coupled heat and moisture transport in a double-layer hollow cylinder. While knowing the temperature and moisture history at the measuring positions, the unknown time-dependent contact heat and mass transfer coefficients can be simultaneously determined. It is assumed that no prior information is available on the functional form of the unknown coefficients. The accuracy of this inverse heat and moisture transport problem is examined by using the simulated exact and inexact temperature and moisture measurements in the numerical experiments. Results show that excellent estimation on the time-dependent contact heat and mass transfer coefficients can be simultaneously obtained with any arbitrary initial guesses.     

Three-dimensional convective flow adjacent to backward-facing step - effects of step height

Three-dimensional simulations are presented for incompressible laminar forced convection flow adjacent to backward-facing step in rectangular duct and the effects of step height on the flow and heat transfer characteristics are investigated. Reynolds number, duct's width, and duct's height downstream from the step are kept constant at Re=343, W=0.08 m, and H=0.02 m, respectively. The selection of the values for these parameters is motivated by the fact that measurements are available for this geometry and they can be used to validate the flow simulation code. Uniform and constant heat flux is specified at the stepped wall downstream from the step, while other walls are treated as adiabatic. The size of the primary recirculation region and the maximum that develops in the Nusselt number distribution increase as the step height increases. The “jet-like” flow that develops near the sidewall within the separating shear layer impinges on the stepped wall causing a minimum to develop in the reattachment length and a maximum to develop in the Nusselt number near the sidewall. The maximum Nusselt number, in the spanwise distribution, develops generally in the same region where the reattachment length is minimum. The maximum in the friction coefficient distribution on the stepped wall increases with increasing step height inside the primary recirculation flow region, but that trend is reversed downstream from reattachment. The three-dimensional behavior and sidewall effects increase with increasing step height.     

Plume separation from an adiabatic horizontal thin fin placed at different heights on the sidewall of a differentially heated cavity

The separation of plumes from an adiabatic horizontal thin fin attached to a sidewall of a differentially heated cavity at quasi-steady stage is experimentally and numerically studied at three Rayleigh numbers (0.92 × 10⁹, 1.84 × 10⁹ and 3.68 × 10⁹) and over a range of fin positions. Regular plume separation is observed over the present range of parameters during the quasi-steady stage. Both experimental and numerical results reveal that the plume separation frequency increases with the Rayleigh number and decreases with the fin height measured from the leading edge. A higher Rayleigh number leads to a more unstable flow above the horizontal thin fin which in turn leads to a higher plume separation frequency. The decrease of the plume separation frequency with the increasing fin height is mainly due to the reduction of the adverse temperature gradient in the unstable layer above the thin fin as a result of the cavity-wide temperature stratification. It is further revealed that the heat transfer through the sidewall is improved by the presence of the thin fin. An optimum fin height for maximum heat transfer enhancement has been identified for the case with a Rayleigh number of 0.92 × 10⁹. For the other two higher Rayleigh number cases considered in this study, the heat transfer through the sidewall monotonically decreases with the fin height.     

Heat transfer from immersed vertical tube in a fluidized bed of group A particles near the transition to the turbulent fluidization flow regime

Experiments were performed in a 0.29 m ID fluidization column to investigate heat transfer from a vertical tube immersed in a bed of 70 μm FCC particles in the range of superficial velocities close to the transition to the turbulent fluidization regime. The results show that the transition is a gradual process and that the changing hydrodynamics affect the heat transfer. The highest heat transfer coefficients were found in the range of superficial gas velocities where the transition to turbulent regime occurred. Radial profiles of heat transfer coefficient were almost flat in the turbulent fluidization regime and changed very little with increasing superficial gas velocity.     

Bifurcated three-dimensional forced convection in plane symmetric sudden expansion

Simulations of bifurcated three-dimensional laminar forced convection in horizontal duct with plane symmetric sudden expansion are presented to illustrate the effects of flow bifurcations on temperature and heat transfer distributions. The stable bifurcated flow that develops in this symmetric geometry leads to non-symmetric temperature and heat transfer distributions in the transverse direction, but symmetric distributions with respect to the center width of the duct in the spanwise directions for the Reynolds number of 400-800. A strong downwash develops at the corner of the step and a smaller reverse flow region develops adjacent to the lower stepped wall than the one that develops adjacent to the upper stepped wall. The downwash and the “jet-like” flow that develop near the sidewall create a strong swirling spanwise flow in the primary recirculating flow regions downstream from the sudden expansion. The magnitude of maximum Nusselt number that develops on the lower stepped walls is higher than the one that develops on the upper stepped wall. The locations of these maximum Nusselt numbers on the stepped walls are near the sidewalls and are upstream of the “jet-like” flow impingement regions. Results reveal that the locations where the streamwise component of wall shear stress is zero on the stepped walls do not coincide with the outer edge of the recirculation flow region near the sidewalls. Velocity, temperature, Nusselt number, and friction coefficient distributions are presented.     

基于红外测温的双盘式浮顶储油罐散热损失分布规律研究

采用红外热像仪、表面温度计等对双盘式浮顶储油罐的表面温度场进行测试。结果表明:罐顶表面温度呈轴对称分布,径向温度梯度远高于周向,且距离罐中心越远,表面温度越高。油蒸汽挥发导致浮顶和罐壁间的一二次密封处散热损失明显升高,使其成为罐顶表面温度最高的区域。浮舱隔板、桁架和椽子等结构形成了热桥,使局部位置的表面温度升高,增大了罐顶的散热损失。罐壁周向表面温度梯度低于轴向,并且受油温影响较大,在罐壁保温结构的结合部位、局部保温结构破损位置的表面温度较高,散热损失较大。基于表面温度法,结合环境温度和风速测试结果,采用强迫对流换热关联式计算得到储罐不同部位的散热损失。结果表明:对于双盘式浮顶储油罐,罐顶散热损失最大,约占储罐总散热损失的67%,罐壁散热损失约占25%,罐底散热损失约占8%。     

Natural Convection Heat Transfer and Entropy Generation in a Porous Trapezoidal Enclosure Saturated with Power-Law Non-Newtonian Fluids

Abstract

In the present study, natural convection heat transfer and its associated entropy generation in a porous trapezoidal enclosure saturated with a power-law non-Newtonian fluid has been numerically investigated. Horizontal walls of the enclosure are assumed to be adiabatic while the side walls are considered to be kept at a constant temperature. A continuum-based approach is adapted here to model the fluid flow through porous media and the Darcy’s law is modified to account for non-Newtonian rheological behavior of the fluid. The obtained governing equations are discretized using the finite volume method and a detailed parametric study is undertaken to account for the effects of various relevant parameters of the problem on the heat transfer and entropy generation rates. It was shown that the impact of the power-law index on both entropy generation and heat transfer significantly intensifies in a convection-dominated flow regime inside the enclosure, especially for a shear thinning liquid. Moreover, heat transfer rate and entropy generation increase as the sidewall angle is elevated.     

Numerical investigation of thermal runaway propagation in a Li-ion battery module using the heat pipe cooling system

It is a promising cooling strategy to use the heat pipe for the Li-ion battery module, which can maintain the temperature of the battery module properly and prevent high temperature, triggering the thermal runaway among adjacent batteries. In this study, the thermal runaway model is simulated through the internal short circuit, which couples with Volume of Fluid (VOF) model of the heat pipe cooling and solves in ANSYS FLUENT to realize the heat and mass transfer between batteries and heat pipes. A user-defined function (UDF) code including mass source and energy source is used to calculate the heat and mass transfer in VOF model during the thermal runaway process. Numerical simulations are adopted to probe thermal runaway processes of a single battery under different operation conditions and the thermal runaway propagation from a battery to adjacent batteries. It is concluded that the heat pipe cooling system cannot prevent the thermal runaway of a single battery, but it can prevent the thermal runaway propagation from a battery to adjacent batteries.     

Towards a unifying heat transfer correlation for the entire boiling curve

The mechanistic understanding of boiling processes is still inadequate. Major physical effects determining the heat transfer in high heat flux nucleate and transition boiling regions have not yet been captured adequately. Thus, existing design correlations are often valid only for one of the boiling regimes. In this paper, the wetting structure close to the boiling surface is identified using the experimental data from an optical probe, obtained during pool boiling of FC-72 on a horizontal surface, together with a mathematical model for the interfacial geometry based on two-phase flow averaging theory. In the same framework, a unifying correlation to describe the heat flux along the entire boiling curve is presented. The suggested correlation is based on the same physical quantities regardless of the boiling regime; it employs only a single fitting parameter in its most simple form. Alternative correlations are compared to the suggested correlation and their relative merit is assessed by statistical model discrimination techniques. The results suggest that transfer phenomena associated with the interfacial evolution, in particular the volumetric presence of interface close to the heater surface, together with the superheat, play an important role for the overall boiling heat transfer mechanism.     

An analytic study of boiling heat transfer from a fin

Analytic expressions for the one-dimensional temperature distribution in a pin fin or a straight fin of rectangular profile are derived if various types of boiling occur simultaneously at adjacent locations on such a fin's surface. The heat transfer coefficients for the transition and nucleate boiling are taken as being the power functions of the wall superheat and that for film boiling as being constant. The number of cases analysed is 66. Some of the results obtained are compared with those of experiments carried out elsewhere. A quite reasonable degree of agreement is found between the theory and the experiment carried out in practice.     

CFD ANALYSIS OF DOUBLE-LAYER MICROCHANNEL CONJUGATE PARALLEL LIQUID FLOWS WITH ELECTRIC DOUBLE-LAYER EFFECTS

This work deals with a code development using a finite-volume scheme on a double-layer microchannel conjugate heat transfer problem consisting of the simultaneous determination of the temperature field in both the solid substrate and the fluid, with streaming potential as the driving force. The concept of electric double layer (EDL) was introduced to explain the microscale deviation. Governing equations were derived for fully developed rectangular microchannels' pressure-driven flows. Toward a realistic modeling of the problems, we conducted a conjugate analysis that solves both the solid and liquid regions. An additional source term resulting from the EDL effects was introduced in the conventional momentum equation, thereby modifying the flow and heat transfer characteristics. Analysis concerning the comparison of the double-layer configuration versus the conventional single-layer microchannel heat sink were included. The computed results reveal significant deviations in the velocity and temperature profiles under EDL effects, in particular, when the hydraulic diameter of the channel is less than 40 u m.