竖直三维内肋管管内变压器油自然对流换热特性

利用田口法对具有不同肋结构参数的竖直三维内肋管管内变压器油自然对流换热特性进行了实验研究,获得了肋高、肋宽、轴向间距和周向间距等肋结构参数对管内自然对流换热性能的影响规律,提出了肋结构参数最佳组合。结果表明,变压器顶层油温从40℃变化至100℃,肋高和肋宽对换热性能的影响占比基本不变,周向间距的影响占比逐渐减小,轴向间距的影响占比逐渐增大;相比光管,三维内肋管的自然对流强化换热性能随顶层油温的增大而增加,变压器油自然对流Nu最大约为光管的1.75倍。拟合了竖直三维内肋管管内变压器油自然对流换热计算关联式,其与实验数据的最大偏差为±16%。     

The heat transfer and pressure drop characteristics of the finned tube banks in forced convection (effects of fin height on heat transfer characteristics)

In recent years the requirement for the reduction of energy consumption has been increasing to solve the problems of global warming and the shortage of petroleum resources. For example, in the power generation field, as thermal power generation occupies 60% of the power generation demand, considerable improvement of thermal efficiency is required. This paper describes the heat transfer characteristics of finned tube banks used for the heat exchanger in thermal power generation that were clarified by testing serrated finned tube banks with different fin heights for improved higher heat transfer and conventional spiral finned tube banks with different fin height. Then an equation to predict the heat transfer coefficient which is necessary for the design of the heat exchanger was proposed. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(3): 194–208, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20111     

Effect of fins on forced convection heat transfer around a tube

Effect of fins on heat transfer around a tube was investigated experimentally. A test tube of 30 mm diameter was installed in a test section of an open‐type wind tunnel as a single tube, or as a center tube in a single tube row and in a tube bundle of staggered layout. Fins made of paper were put on the test tube having certain fin spacing. It was clarified from the experiment that the local heat transfer coefficient around the tube degrades with decreasing fin spacing, especially on the downstream side of the tube, and the minimum fin spacing where the effect of the fin begins to appear is the largest for the single tube and the smallest for the tube bundle. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 445–454, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10098     

Heat transfer performance in 3D internally finned heat pipe

An experimental study of heat transfer performance in 3D internally finned steel-water heat pipe was carried out in this project. All the main parameters that can significantly influence the heat transfer performance of heat pipe, such as working temperature, heat flux, inclination angle, working fluid fill ratio (defined by the evaporation volume), have been examined. Within the experimental conditions (working temperature 40 °C–95 °C, heat flux 5.0 kw/m²–40 kw/m², inclination angle 2–90°), the evaporation and condensation heat transfer coefficients in 3D internally finned heat pipe are found to be increased by 50–100% and 100–200%, respectively, as compared to the smooth gravity-assisted heat pipe under the same conditions. Therefore, it is concluded that the special structures of 3D-fins on the inner wall can significantly reduce the internal thermal resistance of heat pipe and then greatly enhance its heat transfer performance.     

径向错列翅片管内凝结对流换热数值模拟分析

采用数值模拟方法,对径向错列翅片管内含不凝结气体水蒸气的凝结对流换热及阻力特性进行了综合分析。将编写的自定义函数(UDF)导入ANSYS FLUENT软件,对新型强化管传热性能和阻力性能进行了数值模拟,并根据管长方向壁面上蒸汽质量分数的变化情况,讨论分析了凝结过程中翅片管传热性能的变化规律。分析结果表明:与光管相比,内翅片管的强化传热效果随翅数增多、翅片换热接触面积增大而更加显著;另一方面,翅片管的流动阻力相应增大,对管路换热产生不良影响。在所研究翅型范围内16翅y=2x~2型翅片管综合强化换热效果更优;此外随着换热过程的持续,蒸汽凝结逐渐放缓;入口速度增大导致水蒸气凝结不充分,对换热效果的提升有一定制约。     

Heat transfer augmentation in 3D internally finned and microfinned helical tube

Experiments are performed to investigate the single-phase flow and flow-boiling heat transfer augmentation in 3D internally finned and micro-finned helical tubes. The tests for single-phase flow heat transfer augmentation are carried out in helical tubes with a curvature of 0.0663 and a length of 1.15 m, and the examined range of the Reynolds number varies from 1000 to 8500. Within the applied range of Reynolds number, compared with the smooth helical tube, the average heat transfer augmentation ratio for the two finned tubes is 71% and 103%, but associated with a flow resistance increase of 90% and 140%, respectively. A higher fin height gives a higher heat transfer rate and a larger friction flow resistance. The tests for flow-boiling heat transfer are carried out in 3D internally micro-finned helical tube with a curvature of 0.0605 and a length of 0.668 m. Compared with that in the smooth helical tube, the boiling heat transfer coefficient in the 3D internally micro-finned helical tube is increased by 40-120% under varied mass flow rate and wall heat flux conditions, meanwhile, the flow resistance is increased by 18-119%, respectively.     

Numerical study of the intensified heat transfer of an internally longitudinal ridged finned tube under pulsating flow

The turbulent pulsating flow and heat transfer in an internally longitudinal protuberant finned tube was numerically investigated by solving unsteady three‐dimensional elliptical Navier–Stokes equations. The realized k–? turbulent model was adopted. The dynamic behaviors of velocity field, average Nusselt number, and friction number of the internally longitudinal protuberant finned tube were numerically analyzed in a pulsating period, and it was further investigated by changing the frequency of the pulsating flow. It was found that the intensity of heat transfer enhancement increases with an increase of pulsating frequency, while the pressure drop will be increased simultaneously, the intensification of heat transfer in internally longitudinal protuberant finned tubes are gradually better than the pressure drop with an increase of pulsating frequency. © 2009 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20253     

The heat transfer and pressure drop characteristics of finned tube banks in forced convection (effects of fin height on pressure drop characteristics)

In recent years the requirement for the reduction of energy consumption has been increasing to solve the problems of global warming and the shortage of petroleum resources. For example, in the power generation field, as thermal power generation occupied 60% of the power generation demand, considerable improvement of the thermal efficiency is required. This paper describes the pressure drop characteristics of finned tube banks used for heat exchangers in thermal power generation that were clarified by testing serrated finned tube banks with different fin heights for improved heat transfer and conventional spiral finned tube banks with different fin heights, and an equation to predict pressure drop which is necessary for the heat exchanger design is proposed. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(3): 179–193, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20112     

Study of combined effect of natural convection and radiation heat transfer from annular finned vertical cylinder

The present numerical study reports the combined effect of natural convection and radiation heat from a vertical cylinder with annular fins. The study involves simulation for laminar as well as turbulent regimes. For the present study, Rayleigh's number is varied in the range ${10}^8-{10}^{12}$, emissivity in the range $0.2-0.8$, and the fin spacing ratio (s/d) in the range $0.1-10$. The radiation heat transfer has been found to share a considerable amount in the total heat transfer of the system for the laminar regime, but in the turbulent regime, its effect is minimal and can be neglected. When the fin spacing ratio is reduced, the total heat transfer increases for both the turbulent and laminar flow conditions. But the radiation heat increases with a reduction in fin spacing ratio for laminar and in case of turbulent flow radiation heat rate reduces with a reduction in s/d ratio. For the range of Rayleigh numbers considered in the present study, the Nusselt number increases with the increment of the fin spacing ratio. Thus, it can be concluded that there is a remarkable enhancement in the heat transfer rate in laminar cases with the fins. For turbulent cases, the fin efficiency lies between 40% and 50%.     

Heat transfer and pressure drop characteristics of finned tube banks in forced convection (comparison of the heat transfer characteristics between spiral fin and serrated fin)

In recent years the requirements for the reduction of energy consumption have been increasing to solve the problems of global warming and the shortage of petroleum resources. For example in the power generation field, as thermal power generation occupied 60% of the power generation demand, an improvement in thermal efficiency is greatly needed. This paper describes the clarification of heat transfer characteristics of finned tube banks used for a heat exchanger in thermal power generation by testing serrated finned tubes banks for a heat transfer improvement and conventional spiral finned tube banks under the same test conditions. The equations to predict the heat transfer coefficient necessary to design the heat exchanger are proposed. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(2): 120–133, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20043     

Natural convection heat transfer in horizontal and vertical closed narrow enclosures with heated rectangular finned base plate

Natural convection heat transfer and fluid flow characteristics in horizontal and vertical narrow enclosures with heated rectangular finned base plate have been experimentally investigated at a wide range of Rayleigh number (Ra) for different fin spacings and fin lengths. Quantitative comparisons of finned surface effectiveness (ε) and heat transfer rate between horizontal and vertical enclosures have been reported. In comparison with enclosure of a bare base plate, insertion of heat conducting fins always enhances heat transfer rate. Optimization of fin-array geometry has been addressed. The results gave an optimum fin spacing at which Nusselt number (Nu_H) and finned surface effectiveness (ε) are maximum. It has been found that: (1) increasing fin length increases Nu_H and ε; (2) increasing Ra increasesNu_H for any fin-array geometries and (3) for any fin-array geometry and at Ra > 10,000, increasing Ra decreases ε while for fin-array geometries of large fin spacing and at Ra < 10,000, increasing Ra increases ε. Useful design guidelines have been suggested. Correlations of Nu_H have been developed for horizontal and vertical enclosures. Correlations predictions have been compared with previous data and good agreement was found.     

Experimental study of forced convection heat transfer from a cam shaped tube in cross flows

An experimental investigation has been conducted to clarify forced convection heat transfer characteristic and flow behavior of an isothermal cam shaped tube in cross flow. The range of angle of attack and Reynolds number based on an equivalent circular tube are within 0° < α < 180° and 1.5 × 10⁴ < Re_eq < 2.7 × 10⁴, respectively.The results show that the mean heat transfer coefficient is a maximum at about α = 90° over the whole range of the Reynolds numbers. It is found that thermal hydraulic performance of the cam shaped tube is larger than that of a circular tube with the same surface area except for α = 90° and 120°. Furthermore, the effect of the diameter of the cam shaped tube upon the thermal hydraulic performance is discussed.     

A numerical study of combined forced and natural convection heat transfer in a vertical cylinder bundle

Combined forced and natural convection in a bundle of vertical cylinders have been studied numerically with the aid of Riemann geometry. The cylinders heated uniformly are arranged in an equilateral triangular pattern of three bundle spacings S/D = 1.1, 1.5, and 1.9. Numerical calculations are made of the mixed convection of air (Pr = 0.7) in a subchannel of the bundle at ratios of modified Grashof number to Reynolds number Gr*/Re from 1 to 10⁵. The results show that the reverse flows appear along the center line in the subchannel with increasing Gr*/Re. Local Nusselt numbers for the thermally fully‐developed region are well correlated with Gr*/Re. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(7): 639–649, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10118     

Experimental and numerical study of forced convection heat transfer in different internally ribbed tubes configuration using TiO2 nanofluid

TiO₂/water nanofluid is used together with a ribbed tube for heat transfer augmentation. This paper presents an experimental and numerical investigation to study the influence of the ribs' pitch distance and ribbed tube configuration on heat transfer using TiO ₂ nanofluid in a turbulent regime with Reynolds numbers of 5000‐40 000. Meanwhile, the fluid properties are assumed to be constant with temperature under uniform heat flux. The average nanoparticle size is 50 nm and volume fractions of 0% to 1% are adopted. The study is accomplished by using the finite volume method, and its objective involves finding a low friction factor and high heat transfer enhancement in the presence of TiO ₂/water nanofluids. In comparison with the plain tube, a helical ribbed tube provides higher performance evaluation criteria (about 2.0%), while circumferentially ribbed tube provides 1.9% and longitudinal ribbed tube provides 1.88%. The helical ribbed tubes with a 5.89 mm pitch distance gave higher turbulent kinetic energy due to a stronger swirl intensity, resulting in a thinner thermal boundary layer and a higher Nusselt number with uniform distribution. The nonlinear models of friction factor and Nusselt number have been predicted with a maximum deviation of ±3% and ±2%, respectively.     

Effect of fins on forced convection heat transfer around a tube in an aligned‐arranged tube bundle

The effect of fins on heat transfer around a tube in an aligned‐arranged tube bundle was investigated experimentally, and the obtained results were compared for three arrangements, i.e., single tube, single tube row, and staggered‐arrangement. It was found from the experiment that the effect of fins begins to appear in an aligned‐arrangement with larger fin spacing than in a staggered‐arrangement. The degradation in the local heat transfer coefficient due to fins can be recognized not only on the rear region of the tube, as observed in other arrangements, but also on the frontal region. As a result of this phenomenon, the degradation in the average heat transfer coefficient in an aligned‐arrangement becomes larger than in other arrangements with the same fin spacing. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 555–563, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20091     

Evaluating the performance of forced convection heat transfer enhancement

Two methods for assessing thermal performance were evaluated for four kinds of forced convective heat transfer augmentations. On method uses the first law of thermodynamics, i.e., the heat transfer improvement at (1) constant Reynolds number, (2) constant pressure loss, and (3) constant pumping power. The other method uses the second law of thermodynamics, i.e., the entropy generation. The first method restricts the effective region and the second method supplies the condition for achieving the minimum entropy generation rate. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(2): 142–154, 1998     

Thermal boundary condition effects on forced convection heat transfer

We propose a numerical solution of an adjoint problem of forced convection heat transfer to evaluate the mean heat transfer characteristics under arbitrary thermal boundary conditions. Using the numerical solution of the adjoint problem under the Dirichlet condition, which can be computed by slightly modifying a conventional heat transfer code, we obtain an influence function of local surface temperature on total heat transfer. As a result, the total heat transfer for arbitrary surface temperature distributions can be calculated by the influence function. Similarly, using the numerical solution of the adjoint problem under the Neumann condition, we can also obtain an influence function of the local heat flux on the mean surface temperature. The influence functions for a circular cylinder and for an in-line square rod array are presented to illustrate the capability of this method. © 1999 Scripta Technica, Heat Trans Asian Res, 28(3): 227–238, 1999     

Natural convection in an internally finned phase change material heat sink for the thermal management of photovoltaics

Elevated operating temperatures reduce the solar to electrical conversion efficiency of building integrated photovoltaic devices (BIPV). Phase change materials (PCM) can be used to passively limit this temperature rise although their effectiveness is limited by their low thermal conductivities and crystallisation segregation during solidification. This paper presents an experimental evaluation of the effects of convection and crystalline segregation in a PCM as a function of efficiency of heat transfer within the finned PV/PCM system. The thermal performances of bulk PCM with crystallisation segregation for different internal fin arrangements are presented. It is noted that the addition of internal fins improves the temperature control of the PV in a PV/PCM system.     

Combined forced and natural convection heat transfer for upward flow in a uniformly heated,vertical pipe

For predicting the fully developed upward flow in a uniformly heated, vertical pipe by taking account of the buoyancy force, the k-ε models of turbulence for low Reynolds number flows were adopted. The regime map for forced, mixed and natural convections as well as for laminar and turbulent flows was plotted from the numerical predictions. At the same time, experiments were carried out at Reynolds numbers of 3000 and 5000, with the Grashof number varying over a wide range, by using pressurized nitrogen gas as a test fluid. In agreement with the prediction, buoyancy-induced impairment of heat transfer was correctly measured in the mixed convection regime. Furthermore, from hot-wire measurements, complete laminarization was demonstrated in the mixed-convection region at a Reynolds number of 3000.