Numerical simulation of turbulent fluid flow and heat transfer characteristics in heat exchangers fitted with porous media

Numerical simulations have been carried out to investigate the turbulent heat transfer enhancement in the pipe filled with porous media. Two-dimensional axisymmetric numerical simulations using the k–? turbulent model is used to calculate the fluid flow and heat transfer characteristics in a pipe filled with porous media. The parameters studied include the Reynolds number (Re = 5000–15,000), the Darcy number (Da = 10^?1–10^?6), and the porous radius ratio (e = 0.0–1.0). The numerical results show that the flow field can be adjusted and the thickness of boundary layer can be decreased by the inserted porous medium so that the heat transfer can be enhanced in the pipe. The local distributions of the Nusselt number along the flow direction increase with the increase of the Reynolds number and thickness of the porous layer, but increase with the decreasing Darcy number. For a porous radius ratio less than about 0.6, the effect of the Darcy number on the pressure drop is not that significant. The optimum porous radius ratio is around 0.8 for the range of the parameters investigated, which can be used to enhance heat transfer in heat exchangers.     

Three-dimensional numerical simulation of fluid flow with phase change heat transfer in an asymmetrically heated porous channel

Fluid flow with phase change heat transfer in a three-dimensional porous channel with asymmetrically heating from one side is numerically studied in this paper. The “modified” Kirchhoff method is used to deal with the spatial discontinuity in the thermal diffusion coefficient in the energy equation. The velocity and temperature fields, as well as the liquid saturation field on the heated section of the wall with different Peclet and Rayleigh numbers are investigated. The results show that the liquid flow bypasses the two-phase zone, while the vapor flows primarily to the interface between the sub-cooled liquid zone and the two-phase zone. An increase in the Peclet number decreases the two-phase region while an increase in the Rayleigh number helps to spread the heat to a larger region of the domain. The distribution of the liquid saturation on the heated section of the wall indicates that the minimum liquid saturation increases with the increase of both the Peclet and Rayleigh numbers.     

Two-dimensional laminar fluid flow and heat transfer in a channel with a built-in heated square cylinder

A numerical investigation was conducted to analyze the unsteady flow field and heat transfer characteristics in a horizontal channel with a built-in heated square cylinder. Hydrodynamic behavior and heat transfer results are obtained by the solution of the complete Navier–Stokes and energy equations using a control volume finite element method (CVFEM) adapted to the staggered grid. The Computation was made for two channel blockage ratios (β=1/4 and 1/8), different Reynolds and Richardson numbers ranging from 62 to 200 and from 0 to 0.1 respectively at Pr=0.71. The flow is found to be unstable when the Richardson number crosses the critical value of 0.13. The results are presented to show the effects of the blockage ratio, the Reynolds and the Richardson numbers on the flow pattern and the heat transfer from the square cylinder. Heat transfer correlation are obtained through forced and mixed convection.     

错列翅片紧凑式换热器湍流流动及换热性能的数值研究

采用高雷诺数κ-ε湍流模型，对中高雷诺数下紧凑式错列翅片换热器的表面换热及流动特性进行数值模拟。结果表明，该种型式的换热器具有良好的流动和换热性能，拓宽了其空调领域的应用。     

Numerical prediction of flow and heat transfer of power-law fluids in a plane channel with a built-in heated square cylinder

Structure of unsteady laminar flow and heat transfer of power-law fluids in two-dimensional horizontal plane channel with a built-in heated square cylinder is studied numerically. The governing equations are solved using a control volume finite element method (CVFEM) adapted to the staggered grid. Computations are performed over a range of Reynolds and Richardson numbers from Re = 20 to 200 and from Ri = 0 to 8, respectively at fixed Prandtl number Pr = 50 and blockage ratio value β′ = 1/8. Three different values of the power-law index (n = 0.5, 1 and 1.4) are considered in this study to show its effect on the value of the critical Reynolds number defining the transition between two different flow regimes (symmetrical and periodic flows), the variations of Strouhal number, drag and lift coefficients and the heat transfer from the square cylinder as function of Reynolds number. Heat transfer correlations are obtained through forced convection. A discussion about the buoyancy effect on the flow pattern and the heat transfer for different power-law index is also presented.     

Direct numerical simulation of wall-normal rotating turbulent channel flow with heat transfer

Direct numerical simulation of wall-normal rotating channel flow with heat transfer has been performed for the rotation number N_τ from 0 to 0.1, the Reynolds number 194 based on the friction velocity of non-rotating case and the half-height of the channel, and the Prandtl number 1. The objective of this study is to reveal the effects of rotation on the characteristics of turbulence and heat transfer. Some statistical turbulence and heat transfer quantities, including the mean velocity, temperature and their fluctuations, turbulent heat fluxes, and turbulence structures, are investigated. Based on the present calculated results, two typical rotation regimes are identified. When 0 < N_τ < 0.06, the turbulence statistics correlated with the spanwise velocity fluctuation are enhanced since the shear rate of spanwise mean flow induced by Coriolis force increases; however, the other statistics are suppressed. When N_τ > 0.06, all the turbulence statistics are suppressed significantly. To elucidate the effects of rotation on the turbulent heat transfer, the budget terms in the transport equation of turbulent heat fluxes are analyzed. Remarkable change of the direction of near-wall streak structures of the velocity and temperature fluctuations, nearly in alignment with the absolute mean flow direction, is revealed. An attempt to evaluate the mean spacing and the direction of streaky structures near the wall has been examined based on the two-point correlations of the velocity and temperature fluctuations.     

Analysis of fluid flow and heat transfer in a channel with staggered porous blocks

Fluid flow and heat transfer characteristics in a channel with staggered porous blocks were numerically studied in this paper. The Navier–Stokes and Brinkman–Forchheimer equations were used to model the fluid flow in the open and porous regions, respectively. Coupling of the pressure and velocity fields was resolved using the SIMPLER algorithm. The local thermal equilibrium model was adopted in the energy equation to evaluate the solid and fluid temperatures. The effect of Darcy number, Reynolds number, porous block height and width on the velocity field were studied. In addition, the effects of the above parameters as well as the thermal conductivity ratio between the porous blocks and the fluid on the local heat transfer were analyzed. The pressure drops across the channel for different cases were discussed. The results show that the flow behavior and its associated local heat transfer are sensitive to the variation of the above parameters. It is predicted by the present study that an increase in the thermal conductivity ratio between the porous blocks and the fluid results in significant enhancement of heat transfer at the locations of the porous blocks.     

Momentum and heat transfer in a turbulent cylinder wake behind a streamwise oscillating cylinder

The work aims to understand the effect of an oscillating circular cylinder on momentum and heat transport in the turbulent wake of a downstream identical cylinder, which is slightly heated. The oscillating amplitude, A, was 0.79d (d is the cylinder diameter) and the frequency was 0.17f_s for L/d = 2.5 and 0.24f_s for L/d = 4, where L and f_s are the cylinder centre-to-centre spacing and the frequency of vortex shedding from the downstream cylinder, respectively, at A = 0. The velocity and temperature fields were measured using a three-wire probe at 10d behind the stationary cylinder and a Reynolds number of 5920 based on d and the free-stream velocity. It is found that the upstream cylinder oscillation modifies the frequency of vortex shedding from the stationary cylinder, which is locked on with one harmonic of the oscillating frequency. This harmonic frequency is nearest to and below the natural vortex-shedding frequency. Furthermore, the wake response to the oscillation depends on L/d in terms of the cross-stream distributions of mean velocity, Reynolds stresses, temperature variance and heat fluxes; the turbulent Prandtl number decreases at L/d = 4 but increases at L/d = 2.5. The observations are linked to whether the flow regime experiences a change under the upstream cylinder oscillation.     

Numerical simulation of natural convection heat transfer from a heated square cylinder in a square cavity filled with micropolar fluid

A numerical investigation has been performed to visualize the magnetohydrodynamic natural convective heat transfer from a heated square cylinder situated within a square enclosure subjected to nonuniform temperature distributions on the left wall. The flow inside the enclosure is unsteady, incompressible, and laminar and the working fluid is micropolar fluid with constant Prandtl number (Pr = 7). The governing equations of the flow problem are the conservation of mass, energy, and linear momentum, as well as the angular momentum equations. Governing equations formulated in dimensionless velocity and pressure form has been solved by Marker and Cell method with second-order accuracy finite difference scheme. Comprehensive verification of the utilized numerical method and mathematical model has shown a good agreement with numerical data of other authors. The results are discussed in terms of the distribution of streamlines and isotherms and surface-averaged Nusselt number, for combinations of Rayleigh number, Ra (10³–10⁶), Vortex viscosity parameter, K (0–5), and Ha parameter (0–50). It has been shown that an increase in the vortex viscosity parameter leads to attenuation of the convective flow and heat transfer inside the cavity.     

Numerical study on heat transfer of turbulent channel flow over periodic grooves

A numerical investigation of turbulent forced convection in a two-dimensional channel with periodic transverse grooves on the lower channel wall is conducted. The lower wall is subjected to a uniform heat flux condition while the upper wall is insulated. To investigate turbulence model effects, computations based on a finite volume method, are carried out by utilizing four turbulence models: the standard k − ε, the Renormalized Group (RNG) k − ε, the standard k − ω, and the shear stress transport (SST) k − ω turbulence models. Parametric runs are made for Reynolds numbers ranging from 6000 to 18,000 with the groove-width to channel-height ratio (B/H) of 0.5 to 1.75 while the groove pitch ratio of 2 and the depth ratio of 0.5 are fixed throughout. The predicted results from using several turbulence models reveal that the RNG and the k − ε turbulence models generally provide better agreement with available measurements than others. Therefore, the k − ε model is selected to use in prediction of this complex flow. In addition, the results of the heat transfer coefficient, friction factor, skin friction coefficient and thermal enhancement factor are also examined. It is found that the grooved channel provides a considerable increase in heat transfer at about 158% over the smooth channel and a maximum gain of 1.33 on thermal performance factor is obtained for the case of B/H = 0.75. This indicates that the reverse/re-circulation flow in a channel with transverse grooves can improve the heat transfer rate.     

Experiments on heat transfer enhancement from a heated square cylinder in a pulsating channel flow

Experiments have been performed to investigate heat transfer enhancement from a heated square cylinder in a channel by pulsating flow. For all the experiments, the amplitude of the pulsating flow is fixed at A = 0.05. The effects of the Reynolds number based on the mean flow velocity (Re = 350 and 540), the pulsating frequency (0 Hz < f_p < 60 Hz) and the blockage ratio of the square cylinder (β = 1/10, 1/8, and 1/6) on convective heat transfer are examined. The measured Strouhal numbers of shedding vortices for non-pulsating (A = 0) steady inlet flow are compared with the previously published data, and good agreement is found. The “lock-on” phenomenon is clearly observed for a square cylinder in the present flow pulsation. When the pulsating frequency is within the lock-on regime, heat transfer from the square cylinder is substantially enhanced. In addition, the influence of the Reynolds number and the blockage ratio on the lock-on occurrence is discussed in detail.     

Endwall heat transfer and fluid flow around an inclined short cylinder

Measurements of endwall heat transfer and flow field around a short single cylinder have been performed to examine the influence of cylinder inclination at low Reynolds number (Re_D = 1.0 × 10⁴). Both ends of the cylinder are attached to the endwalls and the length-to-diameter ratio of the cylinder varies from 2.7 to 4, depending on the inclination angle. Endwall heat transfer contours (obtained from transient liquid crystals) and endwall flow visualization results consistently indicate that the interaction between the horseshoe vortices around the cylinder and the wakes shed from the cylinder varies with the inclination. Spanwise pressure gradient induced by the inclination causes: (i) skewing of the upstream main flow as it approaches the cylinder; (ii) formation of a jet-like flow immediately downstream of the cylinder, followed by its impingement onto the endwall; and (iii) skewed separation line along the cylinder span from the cylinder axis.     

Numerical investigation of turbulent flow and heat transfer in a channel with novel longitudinal vortex generators

A novel combined longitudinal vortex generator (LVG), comprising a rectangular wing mounted with an accessory rectangular wing, is developed and the turbulent flow and heat transfer characteristics are numerically analyzed. The influences of six main parameters of the combined rectangular winglet pair (CRWP) on heat transfer enhancement and fluid flow resistance characteristics in a rectangular channel are examined. The parameters include the location of accessory wing on the main wing and geometric sizes of the accessory wing. The Reynolds number range is from 2000 to 16,000. The numerical results show that in the range of the present study, the increase of the six parameters can result in the increase of heat transfer and pressure drop. Specially, the pressure drop decreases at large value of the distance of accessory wing from the channel bottom. In comparison with RWP, the CRWP generates vortices with larger area and lower core. Furthermore, the accessory wings generate vortices that swirl downward the channel bottom and disturb the boundary layer growth more effectively. Hence the heat transfer is enhanced. The numerical result for the Nusselt number of the channel agrees well with the experimental result, indicating the reliability of the present numerical predictions.     

Numerical simulation of conjugate,turbulent mixed convection heat transfer in a vertical channel with discrete heat sources

This paper presents the results of a comprehensive numerical study to analyze turbulent mixed convection in a vertical channel with a flush-mounted discrete heat source in each channel wall. The conjugate heat transfer problem is solved to study the effect of various parameters like the thermal conductivity of the wall material (k_s), the thermal conductivity of the flush-mounted discrete heat source (k_c), Reynolds number (Re_s), modified Richardson number (Ri⁎) and the aspect ratio of the channel (AR). The standard k–ε turbulence model, modified by including buoyancy effects, without wall functions, has been used for the analysis. The two-dimensional governing equations are discretised on a semi-staggered, non-uniform grid, using the finite volume method. The asymptotic computational fluid dynamics (ACFD) technique has been then applied to obtain a correlation for the non-dimensional maximum temperature θ¯_max, which can be used for a wide range of parameters.     

流体流过带折流板通道时的流动及传热数值研究

对常物性流体在通道内的周期性充分发展层流流动和换热特性进行了二维数值计算分析。所研究的通道是由两平行平板布置于中心线位置的一系列折流板构成。平行平板保持温度恒定，折流板则分成完全导热和绝热两种情况，对不同几何参数，Re数和Pr数下的流动和换热性能进行了数值研究。文章还给出了系统流函数图和局部换热系数分布情况。     

Features of convective heat transfer in heated helium channel flow

The aim of the present work is to choose an optimal method for thermohydraulic calculation of the gas flow in channels with intense heating at the flow Reynolds number below 10,000. These conditions are typical of the cooling channels of the High-Flux-Test Module of the International-Fusion-Materials-Irradiation-Facility (IFMIF/HFTM). A low Reynolds number and a high heating rate can result in partial relaminarization of the initially turbulent flow, and hence in a decrease in the heat transfer. A number of turbulence models offered by the commercial STAR-CD code were tested on the basis of the comparison of the numerical predictions with experimental data. This comparison showed that the low-Reynolds-number k-ε turbulence models predict the heat transfer characteristics close to the experimental data. The k-ε linear low Reynolds number turbulence model of Lien was applied as more appropriate for the thermohydraulic analysis of the IFMIF high flux test module.