Heat Transfer and Pressure Drop Characteristics of Nanofluid Flows Inside Corrugated Tubes

An experimental investigation is carried out to study the heat transfer and pressure drop characteristics of multiwalled carbon nanotubes (MWCNTs)/heat transfer oil nanofluid flows inside horizontal corrugated tubes under uniform wall temperature condition. To provide the applied nanafluids, MWCNTs are dispersed in heat transfer oil with mass concentrations of 0.05, 0.1, and 0.2 wt%. The Reynolds number varies between 100 and 4,000. Three tubes with hydraulic diameters of 11.9, 13.2, and 15.5 mm are applied as the test section in the experimental setup. Tubes are corrugated four times on the cross section; that is, there are four different helices around the tube. Depths of the corrugations are chosen as 0.9, 1.1, and 1.3 mm, and pitch of corrugation is 14 mm. The acquired data confirm the increase of heat transfer rate as a result of utilizing nanofluids in comparison with the base fluid flow. However, corrugating the tubes decreases the heat transfer rate at low Reynolds numbers. The highest increase in heat transfer rate is observed for the Reynolds numbers for which the smooth tube is in the transition regime and the corrugated tube reaches the turbulent flow, that is, Reynolds number in the range of 1,000 to 3,000. Rough correlations are proposed to predict the Nusselt number and friction factor.     

开式脉动热管传热极限的试验研究

在由内径分别为1 mm和2 mm的细铜管弯曲而成的2组40弯头开式回路脉动热管试验装置上,采用R123为工作介质,定性分析了充液率及加热方式对传热极限的影响,并将试验值与Katpradit传热极限关联式的计算值进行了比较,结果表明:随着充液率的提高,传热极限先增大而后减小,存在一个最佳充液率(约50%);在3种加热方式中,垂直底部加热有助于脉动热管取得较大的传热极限值,而垂直项部加热则对应较小的传热极限值.通过对Katpradit传热极限关联式进行适当修正,得到了新的试验关联式.     

Dynamics of a Natural Circulation Loop: Analysis and Experiments

A dynamic one-dimensional model of a simple rectangular natural circulation loop with tube bundles in the two vertical legs has been simulated. A supporting experimental study using water at atmospheric pressure as the working fluid has been conducted to obtain data to verify the model. Good agreement was obtained between the predicted and measured mixed-mean fluid temperatures and surface temperatures of the tube bundle as well as the loop piping and insulation temperatures at several locations around the system. A number of different simulations and tests have been performed and are discussed. In general, at steady state there was less than 7% deviation between the model predictions and the test data, which indicates that the one-dimensional model is capable of predicting the transient temperature response of the loop structural components and of the circulating fluid.     

Heat Transfer of Supercritical CO2 Flow in Natural Convection Circulation System

Measurements are reported of heat transfer to supercritical carbon dioxide (SCD) flow in a natural convection circulation system that consists of a closed-loop circular pipe. Systematic data of heat transfer coefficients are given for various pressures and pipe diameters. Heat transfer coefficients of SCD flow are confirmed to be very much higher than those of usually encountered fluid flow and are shown to be expressed by a nondimensional correlation equation proposed in this work. Numerical model calculations are also presented for the velocity and temperature distributions in SCD flow to elucidate the exceedingly high value of heat transfer coefficient. The heat transfer enhancement of SCD is concluded to result from the high-speed flow near the pipe wall. This strong flow is shown to have velocity and temperature gradients steep enough to cause the enhancement of the rate of heat transfer in the vicinity of the pipe wall.     

An Experimental Study of Flow Condensation Heat Transfer Inside Circular and Rectangular Mini-Channels

Abstract

By using unique experimental techniques and the careful construction of an experimental apparatus, the characteristics of the local heat transfer were investigated using the condensing R134a two-phase flow in horizontal single mini-channels. The circular channels (D _h = 0.493, 0.691, and 1.067 mm) and rectangular channels (Aspect Ratio = 1.0; D _h = 0.494, 0.658, and 0.972 mm) were tested and compared. Tests were performed for a mass flux of 100, 200, 400, and 600 kg/m²s, a heat flux of 5 to 20 kW/m², and a saturation temperature of 40°C. In this study, the effect of heat flux, mass flux, vapor qualities, hydraulic diameter, and channel geometry on flow condensation were investigated, and the experimental local condensation heat transfer coefficients are shown. The experimental data of condensation Nusselt number are compared with existing correlations.     

Experimental Investigations on Natural Convection Heat Transfer Around Horizontal Triangular Ducts

Experimental investigations have been reported on steady-state natural convection from the outer surfaces of horizontal ducts with triangular cross sections in air. Two different horizontal positions are considered; in the first position, the vertex of the triangle faces up, while in the other position, the vertex faces down. Five equilateral triangular cross-section ducts have been used with cross-section side length of 0.044, 0.06, 0.08, 0.10, and 0.13 m. The ducts are heated using internal constant-heat-flux heating elements. The temperatures along the surface and peripheral directions of the duct wall are measured. Longitudinal (perimeter-averaged) heat transfer coefficients along the side of each duct are obtained for natural convection heat transfer. Total overall averaged heat transfer coefficients are also obtained. Longitudinal (perimeter-averaged) Nusselt numbers and the modified Rayleigh numbers are evaluated and correlated using different characteristic lengths. Furthermore, total overall averaged Nusselt numbers are correlated with the modified Rayleigh numbers. Moreover, a dimensionless temperature group was developed and correlated with the modified Rayleigh number. For the upward-facing case, laminar and transition regimes are obtained and characterized. However, for the downward-facing vertex case, only the transition regime is observed. The local (perimeter-averaged) or the overall total Nusselt numbers increase as the modified Rayleigh numbers increase in the transition regime. However, Nusselt numbers decrease as the modified Rayleigh numbers increase in the laminar regime.     

含尘气流中换热器管壁磨损特性的实验研究

对光滑圆管(0—T管)、高频焊螺旋翅片管(HF—T管)及镍基渗层钎焊螺旋翅片管(HF—T—NL管)的磨损特性进行了实验研究，确定了各种管子的磨损量及磨损部位，并对其抗磨损性能进行了比较。结果表明：由于镍基渗层极大地提高了管材的表面硬度，同时有效地增强了管子的抗氧化腐蚀能力，从而使镍基渗层钎焊螺旋翅片管的耐磨损性能远远优于其它两种换热管。使用镍基渗层钎焊螺旋翅片管可大大地提高换热设备在高温含尘气流中的工作寿命。图6表4参9     

An Experimental Investigation of Heat Transfer Enhancement Mechanisms in Microencapsulated Phase-Change Material Slurry Flows

This article investigates laminar heat transfer characteristic of two-phase microencapsulated phase-change material (MPCM) suspension flows within minichannels under a constant wall heat flux boundary. Capsules containing paraffin wax with phase-change temperature between 36.1°C and 38.1°C are examined and found to be well suited for electronics cooling applications using liquid cold plate technologies. In particular, it is shown that the large thermal capacity of MPCM slurries around the phase-change temperature can lead toward greater isothermality of isoflux systems, a characteristic of significant interest to telecommunication, laser and biomedical applications. The principal focus of the study is to examine heat transfer characteristics within standard tube flow geometries, quantify the heat transfer augmentation/degradation observed, and finally, elucidate the mechanisms from which these result. Through the study volume concentrations of the MPCM slurry were varied between 0% and 30.2%. High-resolution local heat transfer measurements were obtained using infrared thermography and results presented in terms of local Nusselt number versus inverse Graetz parameter. These spanned both the thermal entrance and the fully developed flow regions with inverse Graetz number ranging from 10^?3 to 10⁰. Results show that significant heat transfer enhancements are attainable via the use of MPCM slurries over conventional single-phase coolants. Overall, the study highlights mechanisms that lead to significant heat transfer enhancements in heat exchange devices employing microencapsulated phase-change material slurries.     

自然循环在冲击热负荷作用下的流动特性实验研究

冲击热负荷条件下的自然循环是工程中常见的汽液两相流动与传热过程 ,同时也是一个复杂的非稳态过程。转炉余热锅炉是一种典型的运行于冲击热负荷下的大型换热设备 ,冲击热负荷严重地影响着其水循环流动特性 ,从而影响它的寿命。在实验模拟氧气顶吹炼钢转炉余热锅炉的实际工作过程的基础上 ,研究了冲击热负荷、压力和初始速度等因素对不稳定传热和流动的影响。实验结果表明 ,自然循环在受到冲击热负荷后 ,水循环流量迅速达到最大值 ,呈现较快的响应速度 ,随后在最大流量附近脉动。循环流量的主要影响因素是冲击热负荷的强度 ,而压力和初始速度对循环流量的影响则较小。     

Experimental Study of Heat Transfer Enhancement for Fluid Flow Inside Annulus with Spiral Wires

The evaluation of heat transfer and pressure drop in a water flow inside an annulus of a double concentric-tube heat exchanger with spiral wires inserts was carried out. Three spiral wires with a constant pitch and different wire diameter were tested for a Reynolds number from 1500 to 5500 and a Prandtl number from 5 to 8. The results obtained showed that the spiral wires increased the heat transfer and the pressure drop in comparison with a fluid flow inside a smooth annulus. From the heat transfer point of view, this increase was proportional to the wire diameter but the effect decreases when the Reynolds number increases. General empirical correlations based on dimensionless parameters to calculate the convective heat transfer coefficient and friction factor were developed with an uncertainty of ±6.1% and ±7.6%, respectively, when these estimates were compared against experimental data. The empirical correlations developed were also compared with the estimates calculated by empirical correlations proposed by other researchers, resulting in a good agreement with these values. After the validation analysis, it was demonstrated that the new equations developed provide a good and reliable tool for the design of double concentric-tube heat exchangers with spiral wires inserted inside annulus.     

Condensation Heat Transfer and Pressure Gradient Inside Multiport Minichannels

Abstract

In this paper, the experimental heat transfer coefficients measured during condensation of R134a and R410A inside multiport minichannels are presented. The frictional pressure gradient was also measured during adiabatic two-phase flow. The need for experimental research on condensation inside multiport minichannels comes from the wide use of those channels in automotive air-conditioners. The perspective for the adoption of similar channels in the residential air conditioning applications also calls for experimental research on new high pressure refrigerants, such as R410A.

Experimental data are compared against models to show the accuracy of the models in the prediction of heat transfer coefficients and pressure drop inside minichannels.     

Evaporation Heat Transfer Coefficient in a Capillary Pumped Loop and Loop Heat Pipe for Different Working Fluids

Capillary pumped loop (CPL) and loop heat pipe (LHP) are passive two-phase heat transport devices. They have been gaining importance as a part of the thermal control system of spacecraft. The evaporation heat transfer coefficient at the tooth–wick interface of an LHP or CPL has a significant impact on the evaporator temperature. It is also the main parameter in sizing of a CPL or LHP. Experimentally determined evaporation heat transfer coefficients from a three-port CPL with tubular axially grooved (TAG) evaporator and a TAG LHP with acetone, R-134A, and ammonia as working fluids are presented in this paper. The influences of working fluid, hydrodynamic blocks in the core, evaporator configuration (LHP or CPL), and adverse elevation (evaporator above condenser) on the heat transfer coefficient are presented.     

Frictional and Heat Transfer Characteristics of Single-Phase Microchannel Liquid Flows

In this paper, we review the literature on flow frictional and heat transfer characteristics of single-phase liquid flows through microchannels. The work accentuates the existing discord between experimental observations of microscale transport process characteristics and the corresponding theoretical predictions on the basis of the classical paradigms. The role of microscale effects in inducing such disparity between experimental and theoretical frameworks, as indicated by various researchers, is critically discussed. Theoretical models and empirical correlations, proposed in the literature, for comprehending microscale flow and thermal characteristics are also highlighted for ready reference. In closure, aspects of microscale liquid flow and heat transfer requiring further scrutiny are identified, and possible future research directions are prescribed.     

管内在线防垢及强化传热的实验研究

以饱和ＣａＣＯ３为物料,研究加热器管内移动弹簧的防垢,强化传热性能和结构参数及操作变量的相互影响。文中对防垢和强化传热的机理进行了探讨和实验研究,给出了强化传热和防垢的理论依据。     

Stability of Swirling Flows with Heat Transfer in a Cylindrical Enclosure with CO/Counter-Rotating End Disks Under an Axial Magnetic Field

In this article, a numerical study of swirling flows with heat transfer generated by two rotating end disks (co- and counter-rotating) inside a cylindrical enclosure having an aspect ratio equal to 2, filled with a liquid metal, and submitted to a vertical temperature gradient and an axial magnetic field is studied. The governing Navier-Stokes, energy, and potential equations along with appropriate boundary conditions are solved by using the finite-volume method. The flow and temperature fields are presented by stream function and isotherms, respectively. This flow is very unstable and reveals a great richness of structures. In an oscillatory regime, results are presented for various values of the Hartmann number, Ha = 5, 10, 20 and 30, and Richardson numbers, Ri = 0, 0.5, 1, 2 and 4, in order to see their effects on the value of the critical Reynolds number, Re_cr. Stability diagrams are established according to the numerical results of this investigation. These diagrams show the dependence of Re_cr with the increase of Ha for various values of Ri. The flow between co-rotating end disks is very different from the flow between counter-rotating end disks. Finally, this study confirms the possibility of stabilization of a liquid metal flow by application of an axial magnetic field.     

Effect of Undulations on the Natural Convection Heat Transfer and Entropy Generation Inside a Porous Right-Angled Triangular Enclosure

In the present study, natural convection in a two-dimensional porous right-angled triangular enclosure with one wavy wall is studied numerically. Three cases with one, two, and three undulations on the left wall are studied in this analysis. The stream function-vorticity equations are solved using finite-difference technique and a structured nonorthogonal body-fitted mesh is used for computations. The effect of Rayleigh number (Ra = 10³–10⁶), Darcy number (Da = 10^?4–10^?2) and undulations on the heat transfer, fluid flow, and entropy generation is investigated. It is found that average Nusselt number increases with Darcy number and number of undulations present on the left wall at fixed Darcy number.     

多孔介质燃烧_换热器内燃烧和传热的数值模拟

通过建立二维数值模型研究了多孔介质燃烧-换热器内的燃烧和传热。研究系统配置对燃烧-换热器热效率和压力降的影响。结果表明,换热管的纵向距离对燃烧器内温度分布、传热速率和压力损失有显著的影响。减小换热管纵向距离,热效率和压力损失增大,而换热管的水平距离对热效率和压力损失的影响很小。另外,增大小球直径导致热效率增大和压力损失的急剧减小。数值模型的有效性通过实验进行验证。     

返料换热型循环流化床工业锅炉燃烧传热特性

试验研究了一台将全部蒸发埋管布置在回料阀中的11,t/h返料换热型循环流化床工业锅炉的燃烧传热特性.结果表明,锅炉分离器及回料阀中均存在显著的燃烧份额,并随锅炉负荷的变化而变化;回料阀埋管内工质蒸发量随锅炉负荷增加而显著增加,但其蒸发份额基本保持稳定;基于回料阀内循环灰碳平衡,对锅炉满负荷条件下的循环灰量进行了估算,并对循环灰特性及锅炉低负荷性能进行了探讨.     

Combined Effects of Slip Motion and Boundary Conditions on Enhanced Heat Transfer in Natural Convection Flows of Enclosed Nanofluids

The experimental studies dealing with natural convection of nanofluids in differentially heated enclosures demonstrate that the addition of nanoparticles to a pure base liquid is substantially detrimental, which can be ascribed to the formation of two stagnant fluid layers near the top and bottom adiabatic walls. Thus, if the horizontal walls are differentially heated instead of being perfectly insulated, the consequent development of a pair of concentration boundary layers near them may possibly imply a heat transfer enhancement. In this connection, a two-phase mixture model is employed to perform a numerical study of laminar natural convection in a square cavity containing water suspensions of alumina nanoparticles with a diameter of 33 nm and an average volume fraction in the range 0.001–0.04, assuming that Brownian diffusion and thermophoresis are the primary slip mechanisms between solid and liquid phases. The cavity is differentially heated at sides, whereas the horizontal walls are assumed to be either adiabatic or one heated and the other cooled, with a Rayleigh number in the range 4 × 10⁵–3 × 10⁶. It is found that the heating-from-below configuration is featured by periodic heat transfer, with a rate remarkably higher than that typical of the pure base liquid.     

The Influence of Core Capacitance on the Dynamic Performance of a Single-Phase Natural Circulation Loop With End Heat Exchangers

The effect of wall-core capacitance of heat exchangers on the dynamic behavior of a natural circulation loop (NCL) with end heat exchangers is studied under various excitations such as step, ramp, exponential, and sinusoidal. The transient one-dimensional conservation equations are derived for loop fluid, hot and cold fluid streams, and wall core of both heat exchangers. The solution of a set of transient partial differential equations and one integro-differential equation for loop fluid circulation rate is achieved through a finite-element technique. Imposing the excitations to the inlet temperature of hot fluid, the effects of wall-core capacitance on the responses of outlet temperatures of both hot and cold fluid streams and flow rate of loop fluid are studied. Wall-core capacitance diminishes the initial transients and delays the inception of hot and cold fluids outlet temperature profiles as well as loop fluid flow profile. Further, it has the ability to bring even unstable system behavior with reverse flows into a stable system with steady loop flow rate through quickly decaying oscillations. System responses are also greatly influenced by boundary conditions such as hot and cold fluids flow rates and their inlet temperature excitations such as step, ramp, and exponential. As flow stability is an important subject for single-phase NCLs, a stability map is constructed and compared with zero wall-core capacitance. Inclusion of wall-core capacitance in the present study reveals the important fact that the stable state operating zone widens with the wall-core capacitance.