Heat Transfer and Pressure Drop Characteristics in Turbulent Flow Through a Tube

An experimental investigation has been carried out for turbulent flow through a tube with perforated strip inserts. Strips were of mild steels with circular holes of different diameters. Flow varies, with ranging Reynolds numbers from 15,000 to 47,000. Air velocity, tube wall temperatures, and pressure drops were measured for a plain and strip-inserted tube. The heat transfer coefficient and friction factor were found to be 2.80 times and 1.8 times, respectively, that of the plain tube. The heat transfer performance was evaluated and found to be 2.3 times that of the plain tube based on constant blower power.     

粒化仓内颗粒群流动换热的数值模拟

干式离心粒化余热回收工艺处理液态高炉熔渣时,通过粒化仓和余热回收装置分别实现高温渣液的粒化初冷和二次冷却.本文建立了粒化仓内风冷作用下熔渣颗粒群的流动换热模型,通过离散相模型和Realizable k-ε模型分别追踪颗粒群轨迹和处理湍流流动.模拟得到了颗粒群的运行轨迹、温度和速度演化以及温降等.进一步讨论了颗粒初始温度和粒径的影响,结果表明,增大渣粒初始温度和减小粒径都有利于颗粒群整体温降和平均冷却速率的提高,对仓内停留时间影响较小.但增大初始温度时应同时注意出口温度,以防实际会出现出仓颗粒群的黏结.     

Wall Shear Stress and Heat Transfer of Downward Bubbly Flow at Low Flow Rates of Liquid and Gas

The effect of suppression of turbulence in a downward bubbly flow and its impact on the wall shear stress and heat transfer are discussed. Measurements were carried out for Reynolds numbers Re = 5000–10000, which were calculated from the velocity of the liquid phase and with the gas volumetric flow rate ratio β = 0–0.05. Data on the size of bubbles detaching from the edges of an array of capillaries in a liquid flow are given. The influence of the disperse phase dimensions on the wall shear stress and heat transfer is discussed. It is shown that change in the size of the dispersed phase can lead to both intensification and deterioration of heat transfer as compared with a single-phase flow at constant flow rates of liquid and gas at the channel inlet. The cause of the heat transfer deterioration is “laminarization” of the flow in the near-wall region. An analysis of the spectral power of signals is given.     

Experiments on Arc Perturbation in Turbulent Gas Flow

Electric arcs burning in a strong, turbulent, axial nitrogen gas flow are studied. Time-resolving (framing) picture series of the arc and its deformations are taken through the quartz tube channel wall. Individual perturbations like kinks exist under strong, turbulent flow conditions. They move with constant acceleration in the direction of the cold gas flow. The temperature of the arc core is disturbed at the position of these disturbances, too. This shows up most distinctly, if the decay of the arc after short-circuiting is studied, as has been done additionally.     

Predictions of Conjugate Heat Transfer in Turbulent Channel Flow Using Advanced Wall-Modeled Large Eddy Simulation Techniques

In this paper, advanced wall-modeled large eddy simulation (LES) techniques are used to predict conjugate heat transfer processes in turbulent channel flow. Thereby, the thermal energy transfer process involves an interaction of conduction within a solid body and convection from the solid surface by fluid motion. The approaches comprise a two-layer RANS–LES approach (zonal LES), a hybrid RANS–LES representative, the so-called improved delayed detached eddy simulation method (IDDES) and a non-equilibrium wall function model (WFLES), respectively. The results obtained are evaluated in comparison with direct numerical simulation (DNS) data and wall-resolved LES including thermal cases of large Reynolds numbers where DNS data are not available in the literature. It turns out that zonal LES, IDDES and WFLES are able to predict heat and fluid flow statistics along with wall shear stresses and Nusselt numbers accurately and that are physically consistent. Furthermore, it is found that IDDES, WFLES and zonal LES exhibit significantly lower computational costs than wall-resolved LES. Since IDDES and especially zonal LES require considerable extra work to generate numerical grids, this study indicates in particular that WFLES offers a promising near-wall modeling strategy for LES of conjugated heat transfer problems. Finally, an entropy generation analysis using the various models showed that the viscous entropy production is zero inside the solid region, peaks at the solid–fluid interface and decreases rapidly with increasing wall distance within the fluid region. Except inside the solid region, where steep temperature gradients lead to high (thermal) entropy generation rates, a similar behavior is monitored for the entropy generation by heat transfer process.     

A Model of Heat and Mass Transfer in Gas Phase in Axial and Turbulent Dispersed Annular Flows

The problem of simulation of heat and mass transfer in the gas phase of film devices in the regime of weak and strong phase interaction has been solved. The Deissler three-layer model of turbulent boundary layer was used. Expressions have been derived for calculating the average Nusselt and Sherwood numbers. Examples of calculations for various conditions of phase interaction and comparison with experimental data are shown.     

Heat Transfer in Electric Arc Chamber of Linear Plasmatron

Classification and performance capability of electric are linear plasmatrons are presented. The results of investigation into convective and radiative heat transfer in the discharge channel of a plasmatron are reported. The radiative heat transfer dominates both in the structure of heat losses and in the formation of discharge volt-ampere characteristic at the initial region of the flow. Relations to calculate heat losses by radiation are given. Radiative convective interaction at the arc boundary is shown to play a substantial role for high-current discharges stabilized by axial vortex gas flow. It is shown that the calculation of convective heat transfer at the turbulent region of the flow may be carried out by flow averaged parameters.     

高超声速可压缩流中粗糙壁热流研究

用计算流体力学（CFD）数值模拟和理论方法对高超声速可压缩湍流中粗糙壁面热增量进行研究. 着重考虑粗糙单元密度和粗糙单元形状对粗糙表面热流的影响. 结果表明： ①粗糙单元密度变化时, CFD数值方法计算所得的粗糙单元等效热流随着粗糙单元密度的降低而增加, 理论方法预测结果的规律随方法不同而不同;②在相同粗糙单元密度和高度时,如果粗糙单元形状改变,CFD计算结果也随之发生改变. 理论预测方法所得结果不发生变化.     

Transient Heat Transfer for Forced Convection Flow of Helium Gas Over a Horizontal Plate

Transient heat transfer coefficients for helium gas flowing over a horizontal plate (ribbon) were measured under wide experimental conditions. The platinum plate with a thickness of 0.1 mm was used as the test heater and heated by electric current. The heat generation rate was exponentially increased with a function of Q₀ exp(t/τ). The gas flow velocities ranged from 4 to 10 m/s, the gas temperatures ranged from 290 to 353 K, and the periods of heat generation rate, τ, ranged from 50 ms to 17 s. The surface superheat and heat flux increase exponentially as the heat generation rate increases with the exponential function. It was clarified that the heat transfer coefficient approaches the quasi-steady-state one for the period τ longer than about 1 s, and it becomes higher for the period shorter than around 1 s. Empirical correlation for transient heat transfer was also obtained based on the experimental data.     

Flow Boiling Heat Transfer in Micro-Channel Heat Sinks under Sub-Atmospheric Pressures

A closed-loop two-phase mini-channel-based heat sink driven by a micro-gear pump was developed in this work. Using water as an example, experiments were performed in two micro-channel heat sinks under the conditions of initial pressure of P_in = 34–113 kPa, mass velocity of G = 19–468 kg/m₂s, outlet quality of x_e,out = ?14–66%, and heat flux of q″ = 0–230 W/cm², which covered single-phase flow, subcooled flow boiling, and saturated flow boiling regions. The results showed distinctive differences between the subcooled and saturated boiling regime and revealed that the influence of the system pressure. The experimental data were also compared to a boiling mechanism demarcation map and assessed against some empirical correlations, which suggests some uniqueness of the current heat sink associated with flow boiling at the mesoscale.     

Experiments to Explore the Mechanisms of Heat Transfer in Nanocrystalline Alumina/Water Nanofluid under Laminar and Turbulent Flow Conditions

Abstract

Heat transfer characteristics of water-based nanocrystalline alumina (Al₂O₃) nanofluids flowing through a uniformly heated tube under a fully developed laminar and turbulent flow regime is investigated experimentally in the present work to explore the heat transfer mechanism in nanofluids. In a laminar flow, the increase in Nusselt number was attributed to the thermophysical properties of the nanofluid. The movement of nanoparticles, along with the turbulent eddies in the turbulent core region and diffusion mechanism, such as thermophoresis, in the laminar sublayer are believed to be the reasons for enhanced heat transfer in turbulent region. The compatibility of Al₂O₃/water nanofluids was also examined by monitoring its color.     

Constructal Optimizations of Line-to-Line Vascular Channels with Turbulent Convection Heat Transfer

The multi-scale line-to-line vascular channels (LVCs) widely exist in nature because of their excellent transmission characteristics. In this paper, models of LVCs with turbulent convection heat transfer are established. Based on constructal theory and the entropy generation minimization principle, the constructal optimizations of LVCs with any order are conducted by taking the angles at bifurcations as the optimization variables. The heat flux on the channel wall per unit length is fixed and uniform. The areas occupied by vasculature and the total volumes of channels are fixed. The analytical expressions of the optimal angles, dimensionless total entropy generation rate and entropy generation number (EGN) of LVCs with any order versus dimensionless mass flow rate are obtained, respectively. The results indicate that the dimensionless total entropy generation rate of LVCs with any order can be significantly decreased by optimizing the angles of LVCs, which is significantly more when the order of LVCs is higher. As the dimensionless mass flow rate increases, the optimal angles of LVCs with any order remain unchanged first, then the optimal angles at the entrance (root) increase, and the other optimal angles decrease continuously and finally tend to the respective stable values. The optimal angles of LVCs continue to increase from the entrance to the outlet (crown), i.e., the LVCs with a certain order gradually spread out from the root to the crown. The dimensionless total entropy generation rate and EGN of LVCs first decrease and then increase with the growth of the dimensionless mass flow rate. There is optimal dimensionless mass flow rate, making the dimensionless total entropy generation rate and the EGN reach their respective minimums. The results obtained herein can provide some new theoretical guidelines of thermal design and management for the practical applications of LVCs.     

Experimental Investigation of Heat Transfer in Two-phase Flow Boiling

In this article, an experimental investigation is performed to measure the boiling heat transfer coefficient of water flow in a microchannel with a hydraulic diameter of 500 μm. Experimental tests are conducted with heat fluxes ranging from 100 to 400 kW/m², vapor quality from 0 to 0.2, and mass fluxes of 200, 400, and 600 kg/m²s. Also, this study has modified the liquid Froude number to present a flow pattern transition toward an annular flow. Experimental results show that the flow boiling heat transfer coefficient is not dependent on mass flux and vapor quality but on heat flux to a certain degree. The measured heat transfer coefficient is compared with a few available correlations proposed for macroscales, and it is found that previous correlations have overestimated the flow boiling heat transfer coefficient for the test conditions considered in this work. This article proposes a new correlation model regarding the boiling heat transfer coefficient in mini- and microchannels using boiling number, Reynolds number, and modified Froude number.     

微尺度水平管内沸腾换热特性研究

对不同质量分数下非共沸混合工质(R134a/R32)在微尺度管道内的流动沸腾换热特性进行了比较和分析,阐述了热流密度、质量流量和质量干度对换热的影响。结果表明:热流密度对换热的影响随着质量流量的增加而愈加明显;在质量分数为75%/25%和65%/35%时,换热系数随着质量流量的增大而增大;而质量分数为85%/15%时,换热系数随质量流量的变化先增加后减小;随着质量干度的增加,换热系数在各质量分数下基本上都呈上升趋势。     

Flow and Heat Transfer in Rough Micro Steel Tubes

Abstract

This article investigates the flow and heat transfer characteristics in micro steel tubes with inner diameters of 168 μm, 399 μm and relative roughness of 3.5% and 2.7%, respectively, by measuring the friction factors and the Nusselt number from laminar state to transitional state. Experiments show that the experimental Nusselt numbers are less than those predicted by the classical laminar correlation due to the effect of the variation of the thermophysical properties with temperature when Reynolds number is low. As the Reynolds number is higher than 800, the experimental Nusselt number are 25–50% higher than the predictions of the classical laminar and transitional correlations due to the effects of the roughness and the entrance length. The transition from laminar to turbulent flow occurs at the Reynolds number of 1,100–1,500.     

Hall Effects in MHD Couette Flow with Heat Transfer

An exact analysis of a generalized MHD Couette flow, on taking into account the Hall current, is presented. Expressions for the axial and transverse components of velocity, the skin friction, are derived. These are shown in graphs. The exact solution to the energy equation, assuming the linear variation of the temperature along the plate, is also derived and this is shown graphically. The effects of different parameters are discussed.     

Numerical Simulation Study on Flow Laws and Heat Transfer of Gas Hydrate in the Spiral Flow Pipeline with Long Twisted Band

The natural gas hydrate plugging problems in the mixed pipeline are becoming more and more serious. The hydrate plugging has gradually become an important problem to ensure the safety of pipeline operation. The deposition and heat transfer characteristics of natural gas hydrate particles in the spiral flow pipeline have been studied. The DPM model (discrete phase model) was used to simulate the motion of solid particles, which was used to simulate the complex spiral flow characteristics of hydrate in the pipeline with a long twisted band. The deposition and heat transfer characteristics of gas hydrate particles in the spiral flow pipeline were studied. The velocity distribution, pressure drop distribution, heat transfer characteristics, and particle settling characteristics in the pipeline were investigated. The numerical results showed that compared with the straight flow without a long twisted band, two obvious eddies are formed in the flow field with a long twisted band, and the velocities are maximum at the center of the vortices. Along the direction of the pipeline, the two vortices move toward the pipe wall from near the twisted band, which can effectively carry the hydrate particles deposited on the wall. With the same Reynolds number, the twisted rate was greater, the spiral strength was weaker, the tangential velocity was smaller, and the pressure drop was smaller. Therefore, the pressure loss can be reduced as much as possible with effect of the spiral flow. In a straight light flow, the Nusselt number is in a parabolic shape with the opening downwards. At the center of the pipe, the Nusselt number gradually decreased toward the pipe wall at the maximum, and at the near wall, the attenuation gradient of the Nu number was large. For spiral flow, the curve presented by the Nusselt number was a trough at the center of the pipe and a peak at 1/2 of the pipe diameter. With the reduction of twist rate, the Nusselt number becomes larger. Therefore, the spiral flow can make the temperature distribution more even and prevent the large temperature difference, resulting in the mass formation of hydrate particles in the pipeline wall. Spiral flow has a good carrying effect. Under the same condition, the spiral flow carried hydrate particles at a distance about 3–4 times farther than that of the straight flow.     

Perturbed Arcs in Turbulent Axial Gas Flow II. Fluctuating Local Properties

As a continuation of the research described in Pt. I, local quantities are studied experimentally by electrostatic wall probing, time resolving shadowgraphy and radial luminosity profile scanning of arcs in turbulent axial gas flow. Turbulency of the gas coolant and thermal sheath properties are discussed as well as the consequencies of the gathered knowledge to theoretical modelling.     

Perturbed Arcs in Turbulent Axial Gas Flow IV. Thermal Reignition Experiments

By means of the short-circuiting of electric arcs burning in turbulent axial gas coolant flow and their reaction on a (recovering) voltage ramp the reignition in circuit breakers is modelled. Strong stochasticity of thermal reignition is demonstrated even under relatively weak conditions. Channels or nozzles showing flow detachment are further deteriorated by that. Corresponding low-current test experiments are recommended for in-scale circuit breaker development tests.     

Pulse–Periodic Diffuse Discharge with Self-Ionization in a Gas Flow

Technical Physics - A pulse–periodic diffuse discharge self-initiated in an atmospheric-pressure air flow passing through an annular gap with a strongly nonuniform electric field has been...