Aspect ratio effect on natural convection in a square enclosure with a sinusoidal active thermal wall using a thermal non-equilibrium model

ABSTRACT

A numerical investigation of the aspect ratio effect on natural convection in a square enclosure is carried out by adopting the local thermal non-equilibrium model. The top and bottom walls of the enclosure are adiabatic, the left vertical wall is partially heated and cooled by the sinusoidal thermal boundary condition, and the right vertical wall is maintained at uniform thermal boundary condition. The results show the value of periodicity parameter increasing. The streamlines vary in different patterns, rotating clockwise and counterclockwise simultaneously when N > 1, and the number of clockwise and counterclockwise rotating cells increases with the increase of N and equals the value of N. The sinusoidal local Nusselt number profiles are observed and the wave amplitude of local Nusselt number decreases with the increase of aspect ratio, and the absolute values of average Nusselt number at left wall of porous cavity reach maximum when Ar = 1. The absolute value of solid-to-fluid temperature differences decreases as the inter-phase heat transfer coefficient (H) increases and it increases as the value of aspect ratio increases. The total heat transfer of porous cavity can be enhanced by increasing the aspect ratio and the thermal conductivity ratio.     

Numerical simulation of condensation for R410A at varying saturation temperatures in mini/micro tubes

ABSTRACT

Heat transfer and pressure drop characteristics of condensation for R410A inside horizontal tubes (d_h = 0.25, 1, and 2 mm) at saturation temperatures Tsat = 310, 320, and 330 K are investigated numerically. The results indicate that local heat transfer coefficients and pressure drop gradients increase with increasing mass flux and vapor quality and with decreasing tube diameter and saturation temperature. Liquid film thickness also increases with increasing saturation temperature because of the lower surface tension at higher saturation temperature. When gravity dominates the condensation process, a vortex with its core lying at the bottom of the tube is found in the vapor phase region. For the annular flow regime, stream traces point from the symmetry plan to the liquid–vapor interface, where the vapor phase becomes the liquid phase. Numerical heat transfer coefficients and pressure drop gradients are compared to available empirical correlations. Two new models for heat transfer coefficients and frictional pressure drop gradients are developed based on the numerical work.     

Numerical analysis on heat transfer enhancement by longitudinal vortex based on field synergy principle

Three-dimensional numerical simulation results are presented for a fin-and-tube heat transfer surface with vortex generators. The effects of the Reynolds number (from 800 to 2 000) and the attack angle (30° and 45°) of a delta winglet vortex generator are examined. The numerical results are analyzed on the basis of the field synergy principle to explain the inherent mechanism of heat transfer enhancement by longitudinal vortex. The secondary flow generated by the vortex generators causes the reduction of the intersection angle between the velocity and fluid temperature gradients. In addition, the computational evaluations indicate that the heat transfer enhancement of delta winglet pairs for an aligned tube bank fin-and-tube surface is more significant than that for a staggered tube bank fin-and-tube surface. The heat transfer enhancement of the delta winglet pairs with an attack angle of 45° is larger than that with an angle of 30°. The delta winglet pair with an attack angle of 45° leads to an increase in pressure drop, while the delta winglet pair with the 30° angle results in a slight decrease. The heat transfer enhancement under identical pumping power condition for the attack angle of 30° is larger than that for the attack angle of 45° either for staggered or for aligned tube bank arrangement. Translated from Journal of Xi’an Jiao Tong University, 2006, 40(7): 757–761 [译自: 西安交通大学学报]     

Numerical analysis of the effect of boundary layer thickness on vortex structures and heat transfer in the wake behind a hill

This study presents a three‐dimensional numerical analysis of the effect of boundary layer thickness on vortex structures and heat transfer behind a hill mounted in a laminar boundary layer. When the thickness of the velocity boundary layer is comparable to the hill height, a hairpin vortex is formed symmetrically to the center of the spanwise direction in the wake. A secondary vortex is formed between the legs, and horn‐shaped secondary vortices appear under the concave parts of the hairpin vortex. When the boundary layer thickness increases, the legs and horn‐shaped secondary vortices move toward the center of the spanwise direction, and thus heat transport and heat transfer increase there. At this time, high‐turbulence areas generated locally move toward the center of the spanwise direction with an increase in the boundary layer thickness. With a further increase in the boundary layer thickness, steady streamwise vortices are formed downstream of the hill, but the heat transfer decreases. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20261     

Convective Transport Around a Rotating Square Cylinder at Moderate Reynolds Numbers

Two-dimensional numerical simulation is performed to analyze the thermofluidic transport around a rotating square cylinder in an unconfined medium. The convective transport originates as a consequence of the interaction between a uniform free-stream flow and the flow evolving due to the rotation of the sharp-edged body. A finite volume-based method and a body-fitted grid system along with the moving boundaries are used to obtain the numerical solution of the incompressible Navier–Stokes and energy equations. The Reynolds number based on the free-stream flow is considered in the range 10 ≤ Re ≤ 200, and the dimensionless rotational speed of the cylinder is kept 0 ≤ Ω ≤ 5. Depending on the Reynolds number and the rotational speed of the cylinder, the transport characteristics change. For the range 10 ≤ Re < 50, the flow remains steady irrespective of the rotational speed. In the range 50 ≤ Re ≤ 200, regular low-frequency Kármán vortex shedding (VS) is observed up to a critical rate of rotation (Ω_cr ). Beyond Ω_cr , the global convective transport shows a steady nature. The rotating circular cylinder also shows likewise degeneration of Kármán VS at some critical rotational speed. However, the heat transfer behavior varies significantly with a rotating circular cylinder. Such thermofluidic transport around a spinning square in an unconfined free-stream flow is reported for the first time.     

Performance Improvement of Thermosyphon Heat Exchangers by Using Two Kinds of Working Fluids

In this study, the concept of introducing two-fluid thermosyphons is examined. Calculations were performed for both low and high temperature ranges with parallel and counter-flow arrangements. For lower-temperature application, 125°C &gt; T _hi &gt; 75°C, use of ammonia in some rows and water in the rest of the thermosyphon can slightly improve the associated heat transfer performance for balanced counter-flow arrangement. However, for balanced parallel-flow arrangement in both lowand high-temperature applications, the concept of using two-fluid thermosyphons may not be feasible. The use of two-fluid thermosyphons is especially advantageous for high-temperature applications. For instance, in the range of 375°C &gt; T _hi &gt; 350°C, the two-fluid thermosyphons (Dowtherm A-water) shows a 15-99% increase of heat transfer performance relative to Dowtherm A alone.     

Unsteady Forced Convection Heat Transfer Over a Semicircular Cylinder at Low Reynolds Numbers

An unsteady two-dimensional numerical simulation is performed to investigate the laminar forced convection heat transfer for flow past a semicircular cylinder in an unconfined medium. The Reynolds number considered in this study ranges from 50 to 150 with a fixed Prandtl number (Pr = 0.71). Two different configurations of the semicircular cylinder are considered; one when the curved surface facing the flow and the other when the flat surface facing the flow. Fictitious confining boundaries are chosen on the lateral sides of the computational domain that makes the blockage ratio B = 5% in order to make the problem computationally feasible. A finite volume-based technique is used for the numerical computation. The flow and heat transfer characteristics are analyzed with the streamline and isotherm patterns at various Reynolds numbers. The dimensionless frequency of vortex shedding (Strouhal number), drag coefficient, and Nusselt numbers are presented and discussed. Substantial differences in the global flow and heat transfer quantities are observed for the two different configurations of the obstacle chosen in the study. It is observed that the heat transfer rate is enhanced substantially when the curved surface is facing the flow in comparison to the case when the flat surface is facing the flow.     

Numerical investigation and analysis of heat transfer enhancement in channel by longitudinal vortex based on field synergy principle

3-D numerical simulations were presented for laminar flow and heat transfer characteristics in a rectangular channel with vortex generators. The effects of Reynolds number (from 800 to 3 000), the attack angle of vortex generator (from 15° to 90°) and the shape of vortex generator were examined. The numerical results were analyzed based on the field synergy principle. It is found that the inherent mechanism of the heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, that is, the second flow generated by vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. The longitudinal vortex improves the field synergy of the large downstream region of longitudinal vortex generator (LVG) and the region near (LVG); however, transverse vortex only improves the synergy of the region near vortex generator. Thus, longitudinal vortex can enhance the integral heat transfer of the flow field, while transverse vortex can only enhance the local heat transfer. The synergy angle decreases with the increase of Reynolds number for the channel with LVG to differ from the result obtained from the plain channel, and the triangle winglet performs better than the rectanglar one under the same surface area condition. __________ Translated from Journal of Xi’an Jiaotong University, 2006, 40(9): 996–1000 [译自: 西安交通大学学报]     

Momentum and Heat Transfer from a Semi-Circular Cylinder to Power-Law Fluids in the Vortex Shedding Regime

Flow and heat transfer from a semi-circular cylinder to power-law fluids has been studied in the laminar vortex shedding regime for the range of conditions as follows: Reynolds number, 40 ≤ Re ≤ 140; Prandtl number, 0.7 ≤ Pr ≤ 50; and power-law index, 0.2 ≤ n ≤ 1.8. Extensive results are presented in terms of the streamline, vorticity and isotherm profiles, drag and lift coefficients, Strouhal number, and Nusselt number. The influence of Reynolds number, Prandtl number, and power-law index are seen to be more prominent in shear-thinning fluids (n < 1) than that in shear-thickening (n > 1) fluids. The shear-thinning fluid behavior enhances the rate of heat transfer, whereas shear-thickening fluid behavior impedes it.     

Heat transfer augmentation in a plate‐fin heat exchanger using a rectangular winglet

This study presents numerical computation results on laminar convection heat transfer in a plate‐fin heat exchanger, with triangular fins between the plates of a plate‐fin heat exchanger. The rectangular winglet type vortex generator is mounted on these triangular fins. The performance of the vortex generator is evaluated for varying angles of attack of the winglet i.e., 20, 26, and 37° and Reynolds number 100, 150, and 200. The computations are also performed by varying the geometrical size and location of the winglet. The complete Navier–Stokes equation and the energy equation are solved by the (Marker and Cell) MAC algorithm using the staggered grid arrangement. The constant wall temperature thermal boundary conditions are considered. Air is taken as the working fluid. The heat transfer enhancement is seen by introducing the vortex generator. Numerical results show that the average Nusselt number increases with an increase in the angle of attack and Reynolds number. For the same area of the LVG, the increase in length of the LVG brings more heat transfer enhancement than increasing the height. The increase in heat transfer comes with a moderate pressure drop penalty. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com ). DOI 10.1002/htj.20318     

Evaporative Cooling Heat Transfer of Water From Hierarchically Porous Aluminum Coating

A study of evaporative cooling of water was conducted using dual-scale hierarchically porous aluminum coating. The coating was created by brazing aluminum powders to a flat aluminum plate. The effects of particle size and thickness on evaporative heat transfer were investigated using average aluminum particle diameters of 27, 70, and 114 µm and average coating thicknesses of 560, 720, and 1200 µm. Constant ambient temperature of 24°C and relative humidity of 50% were provided throughout the study. Evaporative cooling tests on the coated surfaces were compared to the plain surface. Tested dual-scale porous coatings enhanced evaporative heat transfer significantly, compared to that of the plain surface, due to the effective wicking of water to the entire heated area. With particle size increase, both the wickability and dryout heat flux were significantly increased. The dryout heat flux with the particle size of 114 µm was 3.2 times higher than that with the particle size of 27 µm. At the fixed particle size of 70 µm the dryout heat flux increased as thickness increased, which resulted in the maximum dryout heat flux of 10.6 kW/m² and the maximum heat transfer coefficient of 251 W/m²K at the coating thickness of 1200 µm.     

Mixed Convection Heat Transfer from Tandem Square Cylinders in a Vertical Channel at Low Reynolds Numbers

The fluid flow and heat transfer characteristics around two isothermal square cylinders arranged in a tandem configuration with respect to the incoming flow within an insulated vertical channel at low Reynolds number range (1 ≤ Re ≤ 30) are estimated in this article. Spacing between the cylinders (S) is fixed at four widths of the cylinder dimension (d) and, the blockage parameter (B) is set to 0.25. The buoyancy-aided/opposed convection is examined for the Richardson number (Ri) ranges from ?1 to 1 with a fixed Prandtl number (Pr) of 0.7. The transient numerical simulation for this two-dimensional, incompressible, laminar flow and heat transfer problem is carried out by a finite volume code based on the PISO algorithm in a collocated grid system. The results suggest that the flow remains steady for the entire range of parameters chosen in this study. The representative streamlines, vorticity, and isotherm patterns are presented to interpret the flow and thermal transport visualization. Additionally, the time average drag coefficient (C _D ) as well as time and surface average Nusselt number (Nu) for the upstream and downstream cylinders are determined to elucidate the effects of Re and Ri on flow and heat transfer phenomena.     

Heat Transfer to Power-Law Fluids from a Heated Square Cylinder

Forced-convection heat transfer to power-law fluids from a heated square cylinder has been investigated numerically for the range of conditions 1 ≤ Re ≤ 45, 0.5 ≤ n ≤ 2.0 and 1 ≤ Pr ≤ 100 (the maximum Peclet number being 4,000). In this range of Reynolds number, the flow is known to be steady and two-dimensional. The variation of the local Nusselt number on the individual surfaces of the square cylinder and the representative isotherm plots, for both the constant-temperature and uniform-heat-flux boundary conditions prescribed on the surface of the square obstacle, are presented to elucidate the role of Reynolds number, Prandtl number, and power-law index on the heat transfer characteristics. Using the present numerical data, appropriate predictive correlations are obtained for estimating the value of the mean heat transfer coefficient in a new application.     

Development and Testing of an Energy Storage Material/Phase Change Material Enhanced Heat Sink

A concept of using energy storage material (ESM) or phase change material (PCM) to enhance the heat transfer dissipation by a conventional compact fin-based heat sink is demonstrated. An actual design is developed, fabricated, and tested to demonstrate the heat transfer enhancement. The heat sink is light weight (made with Aluminum) and miniature in size with a total fin length of 26 mm. Test results demonstrated that under a high peak load (4.4 W/cm²) and low duty power cycle (30/55 on/off ratio with a period of 85 sec), the peak temperature at the heating surface with the ESM/PCM heat sink is 5°C lower than that of a conventional heat sink. At the tip of the fin, the peak temperature with the ESM/PCM heat sink is 3°C lower than that of a conventional heat sink. When the external heat transfer coefficient increases (with increase air velocity), the impact of the PCM/ESM on the heat transfer performance is less. A numerical model, based on COMSOL, is developed to provide a theoretical understanding of the experimental observation.     

Numerical modeling of microchannel reactor with porous surface microstructure based on fractal geometry

A microchannel reactor with porous surface for hydrogen production can enhance fluid flow and heat transfer characteristics. To improve the fluid flow and heat transfer characteristics of a microreactor with a porous surface, a numerical model is proposed based on fractal geometry. The porous surface in the microreactor is fabricated using a layered powder sintering and dissolution method with NaCl particles, in which two sizes of NaCl particles (180–280 μm and 280–450 μm) are utilized. For the construction of the porous surface, these two types of fabricated surfaces are measured and the fractal dimensions are characterized as 1.905 and 1.849, respectively. Subsequently, a numerical model based on fractal geometry for a microchannel reactor with porous surface is developed to study the fluid flow and heat transfer characteristics. This is followed by the microchannel reactor fabrication and experimental testing. Both model calculation and experimental results demonstrate that a microreactor with a porous surface can enhance the heat transfer performances compared with that with a non-porous surface, and that a microchannel reactor fabricated with larger NaCl particles (280–450 μm) has better heat transfer characteristics compared with a microreactor with small NaCl particles (180–280 μm). Thus, the developed numerical model based on fractal geometry can be used to accurately predict the fluid flow and heat transfer characteristics of the microreactor for hydrogen production.     

Investigation on laminar convection heat transfer in fin-and-tube heat exchanger in aligned arrangement with longitudinal vortex generator from the viewpoint of field synergy principle

3-D numerical simulation results are presented for laminar flow heat transfer of the fin-and-tube surface with vortex generators. The effects of Reynolds number (from 800 to 2000), the attack angle (30° and 45°) of delta winglet vortex generator are examined. The numerical results are analyzed from the viewpoint of field synergy principle. It is found that the inherent mechanism of heat transfer enhancement by longitudinal vortex can be explained by the field synergy principle, the second flow generated by the vortex generators results in the reduction of the intersection angle between the velocity and fluid temperature gradient. In addition, the heat transfer enhancement of delta winglet with the attack angle of 45° is larger than that of 30°, while the delta winglet with the attack angle of 45° results in an increase of the pressure drop, however, the delta winglet with the attack angle of 30° results in a slight decrease.     

3D Numerical study on the flow topology and heat transfer characteristics of turbulent forced convection in spirally corrugated tube

ABSTRACT

This paper presents a numerical analysis on flow configurations and heat transfer characteristics of turbulent forced convection in spirally corrugated tubes. The influences of corrugation depth (DR = 0.02–0.16), pitch ratio (PR = 0.10–1.00), and Reynolds number (Re = 5,000–20,000) on flow structure and heat transfer characteristics are described. Comparisons between the full length and periodic domains are also reported. The results show that spirally corrugated tubes induced vortex flows which helped to increase heat transfer due to enhanced fluid mixing. The maximum thermal enhancement factor of 1.16 was obtained by using the spirally corrugated tube with DR = 0.06, PR = 0.25 at Re = 5,000.     

Influence of packed honeycomb ceramic on heat extraction rate of packed bed embedded heat exchanger and heat transfer modes in heat transfer process

Under the condition that the transient oxidation heat extraction process of coal mine ventilation air methane (VAM) is equivalent to a series of steady state process, the steady state heat extraction experiment platform is built. The influence of the honeycomb ceramic packed in heat extraction zone and its two-side space on heat extraction rate and heat transfer modes is investigated. The experimental results show that the honeycomb ceramic packed in heat extraction zone two-side space can always strengthen heat extract ion of heat exchanger by increasing gas physical flow velocity in bed and radiation heat exchanging area and disturbing heat exchanger leeward side flow field. The contradictory dual characteristic of the influence of the honeycomb ceramic packed in heat extraction zone on heat exchanger heat extraction rate determines that the honeycomb ceramic has no great influence on heat extraction rate and doesn't always strengthen heat exchanger heat extraction. Contribution of heat transfer modes on packed bed embedded heat exchanger heat extraction is investigated using the method of coating heat exchanger outer surface silver; the experimental result shows that 55% contribution of packed bed embedded heat exchanger heat extraction rate is from radiation when gas mass flow rate is 0.15 kg·s^− 1·m^− 2 and its temperature is 1113 k; with the gas temperature being increased further, radiation will become the main way of packed bed embedded heat exchanger heat extraction.     

电子元件散热器翅片自然对流散热性能研究

针对室外电子设备在自然对流条件下的散热问题,设计开发了一种新型散热器翅片结构——翅片开孔式涡流发生器,易于加工且直接与翅片连接,不存在焊接的问题。利用数值模拟的方法探究了涡流发生器的布置方式与攻角对散热器散热的影响。结果表明:翅片开设三角孔,在翅片间空气流动过程中起到了明显的作用,有效降低了三角形涡流发生器造成的流动阻力,增加了空气的扰动;当涡流发生器间距为40 mm、攻角为15°时,散热器翅片高温热源区域的散热效果最好。     

A numerical study on the fluid flow and heat transfer in the confined jet flow in the presence of magnetic field

The present study numerically investigates two-dimensional fluid flow and heat transfer in the confined jet flow in the presence of applied magnetic field. For the purpose of controlling vortex shedding and heat transfer, numerical simulations to calculate the fluid flow and heat transfer in the confined jet are performed for different Reynolds numbers in the absence and presence of magnetic fields and for different Prandtl numbers of 0.02 (liquid metal), 0.7 (air) and 7 (water) in the range of 0 N 0.05, where N is the Stuart number (interaction parameter) which is the ratio of electromagnetic force to inertia force. The present study reports the detailed information of flow and thermal quantities in the channel at different Stuart numbers. As the intensity of applied magnetic fields increases, the vortex shedding formed in the channel becomes weaker and the oscillating amplitude of impinging jet decreases. The flow and thermal fields become the steady state if the Stuart number is greater than the critical value. Thus the pressure coefficients and Nusselt number at the stagnation point representing the fluid flow and heat transfer characteristics also vary as a function of Stuart number.