含不同形状内热源的圆管内纳米流体自然对流及换热数值研究

采用格子玻尔兹曼方法对有三种恒温热源(圆形、三角形、方形)参与的圆管内纳米流体(铜-水)自然对流进行数值研究.主要研究瑞利(Ra)数,纳米颗粒体积分数以及热源几何形状等控制参数对纳米流体的流动与传热的影响.结果发现纳米颗粒体积分数的增加有利于强化传热,且在Ra数较小时,平均努塞尔(Nu)数增加的幅度要优于Ra数较大的情...     

Experimental Study of CuO/Water Nanofluid Turbulent Convective Heat Transfer in Square Cross-section Duct

Ducts with a square cross-section are widely used in many industrial applications because of their high compactness, easy forming, and low pressure drop. But the thermal performance of a duct will be reduced when the circular cross-sectional shape is not used. In this study, the convective heat transfer for a CuO/water nanofluid through a square cross-section duct in the turbulent flow regime has been investigated. The Nusselt number of nanofluids for different nanoparticle concentrations, as well as various Peclet numbers, was obtained. The results show considerable enhancement in the heat transfer coefficient and Nusselt number by increasing the nanoparticle concentrations as well as the Peclet number.     

Flow and Heat Transfer Characteristics of in-Line Impinging Jets With Cross-Flow At Short Jet-To-Plate Distance

The aim of this research is to numerically and experimentally study the flow and heat transfer characteristics of in-line impinging jets in cross-flow. The jets from a row of round orifices are perpendicularly impinged on the inner surface of a rectangular wind tunnel at a short distance between the orifice plate and impinged surface (H) of 2D, where D is a diameter of the orifice. The jet velocity was fixed corresponding to Re = 13,400 for all experiments, and the cross-flow velocity was varied at three different velocity ratios (velocity ratio, jet velocity/cross-flow velocity) of 3, 5, and 7. The heat transfer characteristic was visualized using a thermochromic liquid crystal sheet, and the Nusselt number distribution was evaluated by an image processing technique. The flow pattern on the impinged surface was also visualized by an oil film technique. The numerical simulation was used to explore a flow interaction between the impinging jets and cross-flow. The results indicated that Nusselt number peak increased by the increasing cross-flow velocity for short jet-to-plate distance. For the range determined, the maximum local Nusselt number peak was obtained at VR = 3 as the consequence of high velocity and high turbulence kinetic energy of jet impingement.     

错排环布圆管翅片式换热板芯的实验研究

本文利用传热传质之间的比拟关系研究了错排环布圆管换热板芯的平均传热特性及阻力特性。实验中我们采用了三种翅片间距(Tp)、三种管排数(Nrow)以及三种管数(Ntube)组成的27种板芯结构,传质实验采用萘升华的方法来进行。然后通过三种限制条件对不同翅片间距、不同管排数和不同管数下的换热板芯的传热性能进行了比较。最后利用最小二乘法得出了具有工程指导意义的准则关联式。     

Effects of Heating Load on Flow Resistance and Convective Heat Transfer in Micro-pin-fin Heat Sinks with Different Cross-section Shapes

The flow and convective heat transfer characteristics under different heating loads in micro-pin-fins of circle, diamond and triangle are experimentally investigated with Reynolds number ranging from 0–1,000. The pressure drops, friction factors, thermal resistance and Nusselt number in micro-pin-fins with different cross-section shapes are obtained when the heating load changes from 50 to 150 W. Basing on the experimental results, the mechanisms of the influence of heating load on the resistance and heat transfer characteristics in micro-pin-fins with different cross-section shapes are detailed analysed. It is found that pressure drops in three types of micro-pin-fins all become large with the increase of the heating load, and the change of pressure drop in triangular micro-pin-fins is larger than those in the other two micro-pin-fins. At low Re, the friction factors in the three types of micro-pin-fins become large with the increase of the heating load, but this phenomenon disappears when Re>400 for the circle and diamond micro-pin-fins, and Re>250 for the triangular micro-pin-fins. The convective heat transfer in micro-pin-fins with cross-section shapes of circle, diamond and is enhanced by increasing the heating load, but the convective heat transfer coefficients and Nu in the triangular micro-pin-fins becomes slightly smaller when Re>250.     

Laminar Heat Transfer Augmentation through A Square Duct and Circular Tube Fitted with Twisted Tapes

Experimental studies on friction factor and heat transfer characteristics for the laminar flow of ethylene glycol in a square duct fitted with twisted tapes of different twist ratios under nearly uniform wall temperature conditions are reported in this article. The Nusselt numbers were found to be 5.44–7.49 and 2.46–4.87 times that of plain square duct forced convection values based on constant flow rate and constant pumping power criteria, respectively, for y = 2.66. The augmented friction factor and Nusselt number for a square duct is about 1.9 and 2.10 times higher than that for an augmented circular tube.     

Numerical and Experimental Comparative Study on Nanofluids Flow and Heat Transfer in a Ribbed Triangular Duct

Numerical and experimental investigation is carried out to study the effect of combined vortex generator and nanofluids on turbulent heat transfer and fluid flow characteristics in an equilateral triangular duct. A triangular duct provides a lower heat transfer rate and lower pressure drop compared to other duct configurations. The improvement of heat transfer of these ducts increases their importance for providing higher heat transfer and lower pressure drop. Two different types of nanoparticles, namely Al₂O₃ and SiO₂, suspended in distilled water with two particle concentrations are successfully prepared and experimentally tested. The numerical and experimental results show dramatic heat transfer enhancement by using a vortex generator and nanofluids, simultaneously accomplished with a moderate increase in the friction factor. A low deviation has been seen between the present numerical and experimental results.     

EFFECTS OF SURFACE ROUGHNESS ON FORCED CONVECTION AND FRICTION IN TRIANGULAR DUCTS

Experimental investigations had been conducted to study the forced convective heat transfer and pressure drop characteristics of the hydrodynamic fully developed turbulent flow in horizontal equilateral triangular ducts fabricated with Ike same length and hydraulic diameter but different surface roughness of 1.2, 3.0, and 11.5 μm. The experiments were performed with hydraulic diameter-based Reynolds number ranging from 7,000 to 20,000. The entire inner wall of the duct was heated uniformly, while the outer surface was thermally insulated. It was found that the variation of Stanton number (St) with friction factor (f) can be expressed by a relationship of St = C * f, where the constant (C) increases from 0.41 to 0.50 when the surface roughness is increased from 1,2 to 11.5 μm. It was also concluded that the duct with a higher surface roughness will have better heat transfer performance. Nondimensional expressions for the determination of the heat transfer coefficient and friction factor of the equilateral triangular ducts with different surface roughness were also developed.     

Heat Transfer and Pressure Drop Characteristics of Graphene Oxide/Water Nanofluid in a Circular Tube Fitted with Wire Coil Insert

In this work, heat transfer and pressure drop characteristics of graphene oxide/water nanofluid flow through a circular tube having a wire coil insert were studied. The required graphene oxide was synthesized via the Hummer method and characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (SRD), and scanning electron microscope (SEM) methods. Dispersing graphene oxide in the water, nanofluids with 0.02, 0.07, and 0.12% volume fraction were prepared. An experimental set-up was designed and made to investigate the heat transfer performance and pressure loss of nanofluids. All experiments were carried out in the constant heat flux at tube wall conditions. The volumetric flow rates of the nanofluid were adjusted at 6, 8, and 10 L/min. Thermal conductivity, specific heat, density, and viscosity as thermophysical properties of the nanofluid were calculated using graphene oxide and water properties at the average temperature via appropriate relations. These properties were applied to calculate the convective heat transfer coefficient, Nusselt number, and friction factors for each experiment. Finally, the constant and exponents of Duangthongsuk and Wongwises's correlations for Nusselt number and friction factor were corrected by experimental results. The achieved experimental data have shown good agreement with those predicted. The results have shown that 0.12 vol% of graphene oxide in the water can enhance convective heat transfer coefficient by about 77%. As a result, it can be concluded that the graphene oxide/water can be used in the heat transfer devices to achieve more efficiency.     

Experimental investigation on thermal-hydraulic characteristics of a tube equipped with modified vortex-generator inserts

The heat transfer, pressure drop, and overall performance specification of a straight circular tube fitted with vortex-generator inserts are investigated experimentally. To modify the thermal-hydraulic performance, the longitudinal spacing of winglets is varied along the flow direction. The experiments are performed in the turbulent regime (7,470 ≤ Re ≤ 18,670). Good agreement is obtained when the results are compared and validated with previous correlations proposed for the plain tube. The results show that the use of vortex-generator inserts inside the tube yields a higher heat transfer coefficient and pressure drop than the plain tube, and these parameters augment with increasing the number of winglets. The effect of variation of longitudinal spacing of winglets along the vortex-generator inserts on the heat transfer coefficient is higher that the pressure drop. It is also detected that the variation of this parameter affects each arrangement of winglets exclusively.     

FRICTION AND CONVECTION STUDIES OF R-134a IN MICROCHANNELS WITHIN THE TRANSITION AND TURBULENT FLOW REGIMES

Fluid flow and heat transfer characteristics of single-phase flows in microchannels for refrigerant R-134a were experimentally investigated. Experiments were conducted using rectangular channels micromilled in aluminum with hydraulic diameters ranging from approximately 112 to 210 w m and aspect ratios that varied from 1.0 to 1.5. Using overall temperature, flow rate, and pressure drop measurements, friction factors and convective heat transfer coefficients were experimentally determined for steady flow conditions. Effects of Reynolds number, relative roughness, and channel aspect ratio are examined in predicting friction factor and Nusselt number for the experiments. Experiment results indicated that transition from laminar to turbulent flow occurred between a Reynolds number of 2,000 and 4,000. Friction factor results were consistently lower than values predicted by macroscale correlations but exhibited the same trends with Reynolds numbers of macroscale correlations. Nusselt number results also exhibited a similar pattern of lower values obtained in the experiments than those predicted by commonly used macroscale correlations. Nusselt number results also indicated that channel size may suppress turbulent convective heat transfer and surface roughness may affect heat transfer characteristics in the turbulent regime.     

Numerical simulation of laminar forced convection of water-CuO nanofluid inside a triangular duct

In this article, distilled water and CuO particles with volume fraction of 1%, 2% and 4% are numerically studied. The steady state flow regime is considered laminar with Reynolds number of 100, and nano-particles diameters are assumed 20 nm and 80 nm. The hydraulic diameter and the length of equilateral triangular channel are 8 mm and 1000 mm, respectively. The problem is solved for two different boundary conditions; firstly, constant heat flux for all sides as a validation approach; and secondly, constant heat flux for two sides and constant temperature for one side (hot plate). Convective heat transfer coefficient, Nusselt number, pressure loss through the channel, velocity distribution in cross section and temperature distribution on walls are investigated in detail. The fluid flow is supposed to be one-phase flow. It can be observed that nano-fluid leads to a remarkable enhancement on heat transfer coefficient. Furthermore, CuO particles increase pressure loss through the channel and velocity distribution in fully developed cross section of channel, as well. The computations reveal that the size of nano-particles has no significant influence on heat transfer properties. Besides, the study shows a good agreement between provided outcomes and experimental data available in the literature.     

Heat transfer and pressure drop measurements in channels roughened by variously shaped ribs on one wall

Heat transfer and pressure drop measurements were conducted to study the thermal-hydraulics in a square, round-edged channel roughened by ribs (e/D_h = 0.0638, p/e = 10) on one wall at Reynolds numbers ranging from 5.0 × 10⁴ to 2.5 × 10⁵. Three variously shaped ribs were investigated: Transverse ribs with square cross sections, transverse ribs, and upstream directed 60° V-shaped ribs with round-edged rib front and rear surfaces. Friction factors, Nusselt number ratios, roughness functions, and the thermal performance were presented. The highest heat transfer and best thermal performance is reached by the upstream directed V-shaped ribs.     

高温吸热管内超临界CO2传热特性的数值模拟

研究超临界CO2在高温吸热管内的传热特性是将其应用于聚光太阳能热发电技术中的基础.本文对此进行了数值模拟研究,分析了流体温度、流动方向、系统压力、质量流率和热流密度对对流传热系数和Nu数的影响.结果表明:高温区(800—1050 K)的对流传热系数和Nu数受流动方向和系统压力的影响均很小,但都随着质量流率的增大以及热流密度的减小而明显增大;而随着流体温度的升高,对流传热系数近似线性增大,Nu数则近似线性减小.另外,本文研究发现在高温区可忽略浮升力对传热的影响,而由高热流密度引起的流动加速效应会明显恶化传热.最后,选取了八种管内超临界流体传热关联式与模拟结果进行对比,发现使用基于热物性修正的关联式对高温区传热数据预测的结果优于使用基于无量纲数修正的关联式得到的结果,且其中预测效果最优的关联式得到的计算结果与模拟结果之间的平均绝对相对偏差为8.1%.     

Heat Transfer Characteristics in a Double-Pipe Heat Exchanger Equipped with Coiled Circular Wires

An experimental investigation has been carried out to study the enhancement in heat transfer coefficient by inserting coiled wire around the outer surface of the inner tube of the double-pipe heat exchanger. Insulated wires, with a circular cross-section of 2 mm diameter, forming a coil of different pitches (p = 6, 12, and 20 mm), were used as turbulators. The investigation is performed for turbulent water flow in a double-pipe heat exchanger with cold water in the annulus space for both parallel and counter flows. The experiments were performed for Reynolds numbers ranging from 4,000 to 14,000. The experimental results reveal that the use of coiled circular wires leads to a considerable increase in heat transfer coefficients compared with a smooth wall tube for both parallel and counter water flows. The mean Nusselt number increases with Reynolds number and pitch. The convective heat transfer coefficient for a turbulent water flow increases for all coiled wire pitches, with the highest enhancement of about 450% for counter flow and 400% for the parallel flow. New correlations for mean relative Nusselt numbers at different coiled wire pitches are provided.     

Experimental studies on natural convection of water in a closed-loop vertical annulus

Experimental studies on heat transfer and fluid flow of water in a vertical annulus, circulating through a cold leg forming a closed loop thermo-siphon, have been carried out in this article. The annulus has a radius ratio (outer radius to inner radius) of 1.184 and aspect ratio (length to annular gap) equal to 352. The experiments were conducted for constant heat fluxes of 1, 2.5, 5, 7.5, 10, 12.5, and 15 kW/m². Transient behavior during the heat-up period of the system until the steady-state condition is attained and discussed. Variation in the heat transfer coefficient and Nusselt number along the annulus height represent the developing boundary layer at the entrance and fully developed flow in the remaining length. A large drop in the differential pressure is experienced when the liquid is circulated through the flow meters, which restrict the flow due to their very small passages. Flow restriction causes mass accumulation and rise of pressure at the exit of the annulus. It also causes a decrease in liquid head in the cooling leg. An increase in the heat flux leads to an increase in the heat transfer coefficient and Nusselt number. As a result of the data analysis correlations for the average Nusselt number, Reynolds number and circulation rate have been developed in terms of the heat flux.     

波纹内翅片管中对流换热与阻力特性的实验研究

本文研究了空气在一种波纹内翅片管内强制对流的换热与阻力特性,得出了所测参数范围内换热Nusselt数和阻力系数f随Reynolds数变化的实验关联式,并与类似波纹内翅片管的换热效果进行了比较,结果表明波纹内翅片管换热强化的程度与其结构有很大的关系。     

Stagnation Region Heat Transfer for Circular Jets Impinging on a Flat Plate

The heat transfer characteristics in a stagnation region were investigated experimentally for five circular free jets impinging into a heated flat plate. The local temperature distributions are estimated from the thermal images obtained from an infrared camera. To get a precise heat transfer data over the plate, fully developed straight pipe jets were used in this study. Mean jet Reynolds number varied from 1,000 to 45,000, jet-to-plate vertical non-dimensional distance H/D varied from 2 to 6, and the spacing distance jet-to-jet S/D varied from 2 to 8. A geometrical arrangement of one jet surrounded by four jets an in-line array was tested. The results show that the stagnation point Nusselt number is correlated to a jet Reynolds number as Nu_st∝Re^0.61. The average Nusselt number is higher at a separation distance of 2D for three cases of spacing distances, S/D = 2, 4, and 6.     

影响回热器换热的各参数讨论

从线性热声理论出发,通过求解并简化交变流动换热系数,对规则流道的换热进行较为全面的讨论,得到平板以及圆管流道回热器换热系数的简化表达式.考虑固壁温度波动后,发现对于一定结构的回热器,换热系数是六个无量纲量的函数.特别针对低温情况下,讨论了影响换热的主要因素以及增强换热的方法.     

The numerical investigation of heat transfer and pressure drop of turbulent flow in a triangular microchannel

In this presentation, the flow and heat transfer inside a microchannel with a triangular section, have been numerically simulated. In this three-dimensional simulation, the flow has been considered turbulent. In order to increase the heat transfer of the channel walls, the semi-truncated and semi-attached ribs have been placed inside the channel and the effect of forms and numbers of ribs has been studied. In this research, the base fluid is Water and the effect of volume fraction of Al₂O₃ nanoparticles on the amount of heat transfer and physics of flow have been investigated. The presented results are including of the distribution of Nusselt number in the channel, friction coefficient and Performance Evaluation Criterion of each different arrangement. The results indicate that, the ribs affect the physics of flow and their influence is absolutely related to Reynolds number of flow. Also, the investigation of the used semi-truncated and semi-attached ribs in Reynolds number indicates that, although heat transfer increases, but more pressure drop arises. Therefore, in this method, in order to improve the heat transfer from the walls of microchannel on the constant heat flux, using the pump is demanded.