Shape Optimization of a Dimpled Channel to Enhance Heat Transfer Using a Weighted-Average Surrogate Model

Shape optimization of a rectangular channel with the opposite walls roughened by staggered arrays of dimples has been performed not only to enhance turbulent heat transfer but also to reduce friction loss. The dimpled channel shape is defined by three geometric design variables, and the design points within design space are selected using Latin hypercube sampling. The shape of the channel is optimized with three-dimensional (3-D) Reynolds-averaged Navier–Stokes analysis and surrogate approximation methods. A weighted-sum method for multi-objective optimization is applied to integrate multiple objectives related to heat transfer and friction loss into a single objective. A weighted-average surrogate model is employed for this optimization. By the optimization, the objective function value is improved largely and heat transfer rate is increased much higher than pressure loss increase due to shape deformation. The optimum design results in lower channel height, wider dimple spacing, and deeper dimple. The flow mechanism shows the heat transfer rate is increased mainly in the rear portion of the dimple.     

Numerical Optimization of Turbulent Flow and Heat Transfer Characteristics in a Ribbed Channel

By using an optimal method coupled with the numerical simulation of the response surface methodology and genetic algorithm, the geometric configuration for two-dimensional ribbed channels is optimized in this paper. The parameters studied are the height of rib, the thickness of rib, and the pitch. The objective of optimization is to maximize the performance factor of the ribbed channel. The results show that the optimal method works for an optimized design of a two-dimensional ribbed channels. The ribs are demonstrated such that they can significantly affect the heat transfer rate and the friction factor. For the in-line ribbed channel, the performance factor increased by 1.1–1.5. In a staggered ribbed channel, the performance factor reached 2.681.     

Numerical Simulation of Flow and Heat Transfer in a Dimpled Channel with a Fish-Scale Pit

A new dimple configuration (fish-scale pit) is proposed here. Numerical simulations of steady compressible flow were performed to determine the turbulent flow behavior and heat transfer capability in a dimpled channel with the fish-scale pit or spherical pit. The results show that the wall heat transfer coefficient of the surface with the fish-scale pit is higher than that with the spherical pit, and the surface with the fish-scale pit is better in terms of reducing friction drag. This indicates the prospective applications of surfaces with the fish-scale pits, which can enhance wall heat-transfer and reduce skin-friction drag.     

Wall Shape Effects on Convection Heat Transfer Over a Corrugated Channel

This study investigates the effects of the wall waviness on forced convection and its fluid flow in a channel bound by two wavy walls. The lattice Boltzmann method based on the boundary fitting method is used to simulate flow and thermal fields in the corrugated channel. The problem is investigated for different Reynolds numbers (50 to 150), wall amplitudes (0 to 0.35), number of wall wavelength (2 to 8), and phase difference of the walls (0 to 270) when the Prandtl number is equal to 0.71 for air flow. The study represents the significant effects of wavy walls on flow and thermal fields in a two‐dimensional channel. It is found when the phase difference between the channel walls has a value equal to 90°; the best heat transfer rate can be achieved in comparison with other geometrical conditions and the flow is likely to be periodically unsteady at lower Reynolds numbers.     

Large Eddy Simulation of Turbulent Heat Transfer in a Channel With a Square Cylinder

Three-dimensional turbulent channel flow and heat transfer in the presence of a square cylinder were investigated numerically for a Reynolds number of 3000. Turbulent flow was simulated using large eddy simulation in which the eddy viscosity was evaluated by a subgrid-scale model of structure function and selective structure function. The attached wall boundary layer of the channel starts to separate if an insulated cylinder is placed in the channel, and a subsequent recirculation zone appears downstream of the cylinder. Nusselt number distribution along the floor of the channel showed an increase in the downstream part of the cylinder with a relative maximum slightly downstream of the reattachment point. The position of the cylinder in cross-stream direction affects the heat transfer from the channel wall. This quantity increases with decreasing the distance between the cylinder and the channel wall. A correlation was developed for the mean total Nusselt number over the floor of the channel with the position of the cylinder in the channel.     

Turbulent Opposing Mixed Convection Heat Transfer in a Vertical Flat Channel with One-Side Heating

The paper presents experimental data on turbulent local heat transfer in a flat vertical channel with one-side heating for opposing mixed convection in wide ranges of airflow parameters (Re = 4 × 10³–4 × 10⁴; Gr_q = 1.7 × 10⁸–1.4 × 10¹⁰; pressures p = 0.2, 0.4, 0.6 MPa). Analysis has been performed by application of different buoyancy parameters. General correlation was suggested to predict the heat transfer rate. The experimental data were compared with the available correlations for vertical tubes.     

Heat Transfer Characteristics and Reynolds Stress Budget of Spatially Advancing Turbulent Flow in a Curved Channel

Direct numerical simulation was performed for the spatially advancing turbulent flow and heat transfer in a two-dimensional curved channel equating the radius ratio to 0.92 or 0.8. The frictional Reynolds number was fixed at 150, whereas the Prandtl number was set at 0.71. According to the numerical result, the remarkable enhancement of heat transfer occurred on the outer wall, suggesting the organized vortex activated the heat transfer. The budgets of Reynolds stresses clarified that the onset and growth of the organized flow was assisted by the direct energy transfer from the mean flow.     

Heat Transfer Enhancement in a High Aspect Ratio Channel with System Optimization

Outdoor digital information display (DID) installed near public areas has become a very popular medium of communication. Cooling performance of an outdoor DID system has become a critical issue from increasing system temperature due to heat generation at the back panel, in addition to irradiation from the sunlight. Gap distance between the heating plate and glass could control the main heat transfer rate considering its positive or negative effect on the pressure and temperature gradient at the wall. Additional reduction of maximum surface temperature can be achieved using tapered geometry with increased inlet gap distance, as well as guide vane installation at the system exit to lower pressure loss. Full system optimization has also been performed, including correlation for tapered geometry and guide vane effect. With system optimization, we can find the optimal gap clearance of parallel or tapered geometry for best performance.     

Simulation of Swirling Turbulent Heat Transfer in a Vortex Heat Exchanger

ABSTRACT

This article presents a numerical simulation of swirling turbulent flow and heat transfer in a novel vortex heat exchanger. A new algebraic Reynolds stress/heat flux model (ASM/AFM) is applied to the simulation. The computation is performed under different air flow rates for both swirling and nonswirling flows. The calculated mean heat transfer coefficients on both inner and outer walls of the annular duct are compared with the measured data. They are generally improved over the results predicted by the new ASM/k–? model. The effects of swirl on enhancing heat transfer in the annular duct are illustrated. The heat transfer performance of the vortex heat exchanger under different air flow rates is obtained.     

Heat Transfer and Fluid Flow on Dimpled Surface With Bleed Flow

This study investigates the effects of bleed flow on heat transfer and fluid flow on a dimpled surface in a rectangular channel. The heat transfer on a dimpled surface with bleed flow is compared with that on a dimpled surface without bleed flow. The height of the channel is 15.0 mm. The dimples are arrayed in staggered on the bottom surface of the channel with a pitch of 15.0 mm. The dimple depth is 3.75 mm and the dimple footprint diameter is 13.0 mm. The bleed hole is installed on the inner surface of the dimple and the diameter of the hole is 1.3 mm. The tests were conducted with varying Reynolds numbers from 1000 to 10,000 and 0.5% of total mass flow is flowing out through a bleed hole. A numerical method was employed to determine the detailed heat transfer coefficients. Commercial computational fluid dynamics software, ANSYS CFX 13.0, is adopted and the Shear Stress Transport model is set to turbulent model. As a result, the overall heat transfer rate on dimpled surface with bleed flow is 10–20% higher than that without bleed flow.     

Three-Dimensional Numerical Study of Fluid and Heat Transfer Characteristics of Dimpled Fin Surfaces

In the present study, a code based on the nonorthogonal curvilinear coordinates is developed with a collocated grid system generated by the two-boundary method. After validation of the code, it is used to compare simulated results for a fin-and-tube surface with coupled and decoupled solution methods. The results of the coupled method are more agreeable with the test data. Simulation for dimpled and reference plain plate fin-and-tube surfaces are then conducted by the coupled method within a range of inlet velocity from 1.0 m/s to 5 m/s. Results show that at identical pumping power the dimpled fin can enhance heat transfer by 13.8–30.3%. The results show that relative to the reference plain plate fin-and-tube surface, heat transfer rates and pressure drops of the dimpled fin increase by 13.8%–30.3% and 31.6%–56.5% for identical flow rate constraint. For identical pumping power constraint and identical pressure drop constraint, the heat transfer rates increase by 11.0%–25.3% and 9.2%–22.0%, respectively. By analyzing the predicted flow and temperature fields it is found that the dimples in the fin surface can improve the synergy between velocity and fluid temperature gradient.     

Turbulent Heat Transfer in the Entrance Region of a Tube

Correlation of the data for heat transfer between a fluid in turbulent flow and the entrance region of a tube is made for the entrance shapes normally used in heat exchangers. Equations representing the variation of the “average heat transfer” with tube length, Reynolds number, and Prandtl number are suggested.     

Numerical Study of Heat Transfer of a Porous-Block-Mounted Heat Source Subjected to Pulsating Channel Flow

A numerical analysis was conducted to investigate the convective characteristics of pulsating flow through a channel with a porous-block-mounted heat source. Comprehensive time-dependent flow and temperature data are calculated and averaged over a pulsation cycle in a periodic steady state. The impacts of the Darcy number, pulsating frequency and amplitude, and porous blockage ratio are documented in detail. The results indicate that the periodic alteration in the structure of recirculation flows, caused by both porous block and flow pulsation, has a direct impact on the flow behavior in the vicinity of the porous block and on the heat transfer rate from the heater.     

LES of a Turbulent Slot Impinging Jet to Predict Fluid Flow and Heat Transfer

In this article, large eddy simulation (LES) is performed for a turbulent slot jet impingement heat transfer at a Reynolds number of 13,500 and a nozzle to plate spacing of 10. Various aspects of predicting a turbulent jet impinging flow in an optimum domain size and grid resolution for LES have been assessed. Two inflow conditions, one without any fluctuations and the other with fluctuations generated by the spectral synthesizer, were tested and comparisons of various mean flow, turbulence, and heat transfer data showed that LES without any inflow fluctuations provides good agreement with the corresponding experimental and numerical results reported in the literature. Further, various important dynamical flow structures have been visualized from the instantaneous computed data. Finally, mean flow and turbulence statistics have been presented in the wall jet region close to the stagnation point, which could be useful as data for validation of RANS-based turbulence models.     

球突翅片的传热流动特性及等效热阻数值分析

为了提高翅片管换热器的传热系数和减小压降,提出了一种球突型翅片,通过数值模拟研究其传热与流动性能,同时应用(火积)耗散理论对其传热的不可逆性进行分析。计算结果表明:与平片相比,其传热能提高26.21%～39.53%,而阻力系数仅提高16.62%～27.04%,同时综合性能增加16.54%～32.56%;这说明该翅片具有高传热系数低压降的特点,是一种性能优良的翅片。通过(火积)耗散分析可以看出:球突翅片的等效热阻减小,其传热的不可逆性减弱。     

Mixed Convection Heat Transfer in a Grooved Channel with Injection

Improved heat transfer is very important for the energy efficiency, higher performance, and size reduction of a system. In this context, the paper presents an efficient way for improving mixed convection heat transfer in a grooved channel by dividing total flow into main flow and injection under the assisting flow configuration. The influences of the pertinent injection parameters (e.g., position, size, and injection flow) are investigated systematically for Richardson number 0.1–10 and Reynolds number 50–200 using an in-house CFD code. The results reveal that performance with injection is always superior and heat transfer enhancement is found to increase from 50 to 218% depending upon injection, Richardson number, and Reynolds number.     

Recent Trends in Computation of Turbulent Jet Impingement Heat Transfer

A review of the current status of computation of turbulent impinging jet heat transfer is presented. It starts with a brief introduction to flow and heat transfer characteristics of jet impinging flows considering the simplest jet impinging geometry: normal impingement of a single jet into a flat surface. Subsequently, a review of recent computational studies related to the same geometry is presented. The effects of different subgrid scale models, boundary conditions, numerical schemes, grid distribution, and size of the computational domain adopted in various large eddy simulations of this flow configuration are reviewed in detail. A review of direct numerical simulation of the same geometry is also presented. Further, some recent attempts in Reynolds-averaged Navier–Stokes modeling of impinging flows are also reviewed. A review of computation of other complex impinging flows is also presented. The review concludes with a listing of some important findings and future directions in the computation of impinging flows.     

Analysis of Thermal Nonequilibrium Inverse Heat Transfer in a Porous Channel

This article is concerned with the one-dimensional transient inverse heat conduction between two parallel plates filled with a porous medium under a nonthermal equilibrium condition between the solid and the fluid phases. The estimation of transient heat flux using the conjugate gradient method (CGM) along with the differential adjoint equations has been carried out under nonthermal equilibrium conditions between two phases. Derivation of the adjoint differential equations in the case of nonthermal equilibrium and calculation of gradient function from coupled adjoint equations are presented in detail here. The transient wall heat flux imposed on the porous boundary is estimated using the aforementioned method, and results show that sensor locations and existing error in the measured data have important effects on the calculated heat flux. Nonetheless, accurate heat flux estimation is quite achievable.     

Turbulent Heat Transfer Analysis of a Three-Dimensional Array of Perforated Fins Due to Changes in Perforation Sizes

The quasi stationary-state solution of the two-dimensional Rosenthal equation for a moving heat source using the meshless element free Galerkin method is studied in this article. Node-based moving least square approximants are used to approximate the temperature field. Essential boundary conditions are enforced by using Lagrange multipliers. A Gaussian surface heat source is used for the modeling of the moving heat source. The results obtained for a two-dimensional model are compared with the results of the finite-element method.     

Heat Transfer Coefficients for Turbulent Flow in Concentric Annular Ducts

On the basis of a large number of experimental data from the literature, correlations were developed for the heat transfer coefficient for turbulent flow in concentric annular ducts. A proven correlation for heat transfer in circular tubes was extended by factors that take into consideration the effect of the diameter ratio of the annulus and the different boundary conditions for heating or cooling.