乙烷脉动热管的CFD数值模拟研究及实验对比

为了更好地了解脉动热管内部的内在运行机制,本文在实验的基础上采用VOF模型对乙烷脉动热管的传热特性进行了数值模拟研究.结果表明:充注工质后,脉动热管内部形成了随机的气液分布;在启动过程和稳定运行过程中,温度波动的频率随着加热功率的增大而增大;在稳定运行过程中,脉动热管内的流型也在不停地变化.将模拟结果与前期的实验结果进...     

Microgravity performance of micro pulsating heat pipes

A micro pulsating heat pipe made of a thin clear Teflon tube of 1.6 mm ID was used to observe the pulsating flow inside a heat pipe under different gravity levels using parabolic flights. More vigorous pulsating flow was observed under microgravity, compared to the depressed movements under hypergravity. Two metallic micro pulsating heat pipes made of an aluminum plate with small internal channels were also tested to investigate the effect of gravity on their heat transfer characteristics. Reduced gravity experiments were performed aboard Falcon 20 aircraft flying parabolic trajectories. Under normal and hypergravity conditions, both the orientation of the pulsating heat pipe and locations of the heated and cooled sections affected the heat transfer performance. Under reduced gravity, however, the heat pipes showed better operating and heat transfer performance than that under normal and hypergravity. These experiments have for the first time confirmed that pulsating heat pipes are capable of operating under reduced gravity and thus are suitable for deployment in space applications such as satellites.     

Pulsating Turbulent Flow of Gas in a Round Pipe

An analysis is performed of the presently available results of experimental and prediction studies into pulsating turbulent flow of liquid in a narrow pipe under conditions when the compressibility is apparent. It is demonstrated that the simulation of such flows in the general case may be performed only numerically, using a model of turbulence that adequately includes the effect of oscillation on turbulent transfer. Use is made of a model of turbulence whose validity is proved by comparing the calculation and experimental results for a wide range of flows. Calculations are performed for a pulsating flow of gas in pipes with isothermal and adiabatic walls, acoustically closed at the outlet, in the frequency range corresponding to the first resonance harmonic. The predicted variations of the heat flux to the wall and of the hydraulic drag, averaged over the oscillation period, as functions of the process parameters such as the Reynolds number of the mean flow and the dimensionless oscillation frequency are discussed.     

Convective coherent structures in a Hele-Shaw cell

Convective processes related to the onset of chaotic flows in a Hele-Shaw cell have been theoretically and experimentally studied. The properties of various modifications of a specific pulsation regime, in which certain characteristics exhibit chaotization with increasing Rayleigh number while the flow pattern remains statistically reproducible and retains a regular character, have been determined for the cavities with various ratios of the dimensions of wide sides. An analogy is established that allows pulsating vortices in the flow to be treated as simple coherent structures of a convective nature.     

Flow friction and heat and mass transfer in a swirled flow

An experimental investigation has been made of flow friction and heat and mass transfer inside cylindrical tubes with exit sections without diaphragms and various degrees of swirl of one-and two-phase flows, A comparative analysis of the results obtained has been made. The most suitable operating conditions from the energy standpoint have been established for vortex heat and mass transfer equipment.     

Numerical and experimental studies on laminar forced convective heat transfer in a channel with surface‐mounted heated blocks

Abstract

The steady separated flow due to heated blocks mounted on one principal wall of a two‐dimensional channel has been numerically and experimentally studied. Numerical solutions of the Navier‐Stokes equations using the finite‐difference and the power‐law techniques have been obtained up to a Reynolds number of 2600. The effects of the Reynolds number and the block spacing on the fluid flow and heat transfer are investigated in detail. Results show that there exists two different types of flow between blocks, the D‐type and the K‐type flows. Furthermore, the Nusselt number monotonously increases or decreases along every face of the blocks. The calculated results of the reattachment length behind the second block and the local Nusselt number distribution compare well with the results obtained by the LDV and the naphthalene sublimation measurements, respectively. Heat transfer correlating equations are presented in terms of the Reynolds number and the block spacing.     

Integral methods of calculation of heat transfer and drag under conditions of turbulent pipe flow of liquid of variable properties: Steady-state and quasi-steady-state flows in a round pipe with constant density of heat flux to the wall

Integral relations are derived for the calculation of the Nusselt number and coefficients of hydraulic drag and friction drag under conditions of pipe flow of dropping liquid and gas of temperature-dependent physical properties. In the limiting case of steady-state flow of liquid of constant properties, the expression for the Nusselt number transforms to the well-known Lyon integral. The results of calculation of heat transfer and drag by an integral method are compared with more exact results obtained using the numerical solution of the set of differential equations of convective heat transfer. An inference is made about the conditions under which integral methods may be employed. An algorithm is developed for the calculation by an integral method of heat transfer and drag under conditions of quasi-steady-state pulsating flow. It is demonstrated that the flow rate oscillations superposed on the flow in the pipe enhance the effect of the variability of the properties on heat transfer, and for gas on friction drag. For a dropping liquid under conditions of pulsating flow, the friction drag depends less significantly on the variability of the properties (viscosity) than in the case of steady-state flow. The degree of manifestation of the effects identified above is the higher, the higher the oscillation amplitude and the lower the value of the Reynolds number of averaged flow.     

Transition from nucleate boiling to convective boiling in compact heat exchangers

The boiling of pure fluids has been experimentally studied in several types of compact heat exchanger channels. Plate fin and corrugated heat exchangers have been studied (seven geometries). Controlling the flow parameters (mass flux and vapour quality), the heat flux and measuring the wall temperature, have allowed characterization of the local heat transfer coefficient.

The results clearly show that the dominant mechanisms occurring could be nucleate or convective boiling. The transition between these two mechanisms depends on the flow characteristics and also on the channel geometry. Based on these measurements, an objective criteria can be established to identify the flow boiling regime. The knowledge of such a criteria is useful if we want to extend the use of compact heat exchanger to boiling of mixtures.     

单面波浪平板脉动热管的传热性能

对一种单面波浪平板脉动热管的传热性能进行了实验研究,分析在空气强制对流冷却条件下充液率、加热功率、倾角等因素对其传热性能的影响。研究结果表明,除0°倾角外,脉动热管的最佳充液率为20%～30%,倾斜角度对脉动热管传热性能的影响很小,但90°时相对最好。脉动热管在0°放置时其传热性能较差,在低充液率的情况下甚至丧失脉动效果,主要是工质回流不畅的原因,与平板脉动热管的槽道设计有很大关系。此外低加热功率时热管传热性能存在波动,有时甚至不能启动。     

Integral methods of calculation of heat transfer and drag under conditions of turbulent pipe flow of liquid of variable properties: Pulsating high-frequency flow

An integral method is suggested for approximate calculation of oscillation-period average heat transfer and drag under conditions of pulsating high-frequency flow of gas in a pipe with constant density of heat flux to the wall. It is found that the flow rate oscillation superimposed on the flow has little effect on the period average Nusselt number and coefficient of friction drag; these quantities may be calculated by the method developed for a steady-state flow of liquid of variable properties. The oscillation affects significantly the period average coefficient of hydraulic drag whose values increase with the amplitude of superimposed oscillation.     

Experimental investigation of ice slurry heat transfer in horizontal tube

Heat transfer of ice slurry flow based on ethanol–water mixture in a circular horizontal tube has been experimentally investigated. The secondary fluid was prepared by mixing ethanol and water to obtain initial alcohol concentration of 10.3% (initial freezing temperature -4.4 °C). The heat transfer tests were conducted to cover laminar and slightly turbulent flow with ice mass fraction varying from 0% to 22% depending on test performed. Measured heat transfer coefficients of ice slurry are found to be higher than those for single phase fluid, especially for laminar flow conditions and high ice mass fractions where the heat transfer is increased with a factor 2 in comparison to the single phase flow. In addition, experimentally determined heat transfer coefficients of ice slurry flow were compared to the analytical results, based on the correlation by Sieder and Tate for laminar single phase regime, by Dittus–Boelter for turbulent single phase regime and empirical correlation by Christensen and Kauffeld derived for laminar/turbulent ice slurry flow in circular horizontal tubes. It was found that the classical correlation proposed by Sieder and Tate for laminar forced convection in smooth straight circular ducts cannot be used for heat transfer prediction of ice slurry flow since it strongly underestimates measured values, while, for the turbulent flow regime the simple Dittus–Boelter relation predicts the heat transfer coefficient of ice slurry flow with high accuracy but only up to an ice mass fraction of 10% and Re_cf > 2300 regardless of imposed heat flux. For higher ice mass fractions and regardless of the flow regime, the correlation proposed by Christensen and Kauffeld gives good agreement with experimental results.     

Normal and deteriorated heat transfer upon heating of turbulent flows of heat carriers with variable physical properties in tubes

A short survey of the available knowledge on heat transfer of gaseous heat carriers with variable physical properties (gases and their mixtures, in particular, those catalytically reacting on a wall, high pressure vapor, as well as single-phase supercritical pressure heat carriers) to turbulent flows upon intense heating in tubes is presented. The problem of heat transfer deterioration under high heat loads (in particular, in the context of flow laminarization) is discussed. The existing knowledge about the nature of these phenomena and methods for their prediction are considered. The empirical relationships for estimating the temperature regime of the wall under conditions of normal and deteriorated heat transfer are presented. The consequences of the transition to new standards in properties of supercritical pressure heat carriers are shown from the viewpoint of accuracy in the engineering analysis of heat transfer by the existing methods. Ways of rehabilitation for old empirical formulas are proposed for the case of using new standards of thermophysical properties.     

Modeling the Effect of Bubbles on a Pattern and Heat Transfer in a Turbulent Polydisperse Upward Two-Phase Flow after Sudden Enlargement in a Tube

In this work, the numerical modeling of the flow pattern and heat transfer in a polydisperse bubbly turbulent flow after sudden enlargement in a tube is performed. The pattern of average and fluctuation twophase flows at small volumetric gas flow rate ratios (β ≤ 10%) is qualitatively similar to the one-phase liquid flow pattern. It is shown that small bubbles are present almost throughout the entire cross section of a tube, while great bubbles generally pass through the flow core and the shear mixing layer. The addition of air bubbles to a one-phase liquid flow appreciably intensifies heat transfer (up to two times), and these effects become stronger with an increase in the diameter of bubbles and the volumetric gas flow rate ratios gasratios.     

脉动流发生装置诱导弹性管束振动的实验研究

为了实现对换热器内弹性管束振动的有效激发和控制,提出了一种新型脉动流发生装置,搭建了脉动流诱导弹性管束振动测试实验台,测试了换热器内各排弹性管束在不同工况下的振动响应。结果表明:在脉动流发生装置诱导下,弹性管束主要有两个振动频率,一个频率基本不受入口流速的影响,另一个频率随入口流速的增加而增加;提出的脉动流发生装置,一方面使弹性管束的振动强度明显增加,另一方面在一定程度上解决了各排弹性管束振动不均的问题。     

The Effect of Three-Dimensional Deformations on Local Heat Transfer to a Nonuniformly Heated Falling Film of Liquid

An experimental investigation is performed of heat transfer under conditions of flow of a water film on a vertical surface with a heater 150×150 mm in size in the range of the Reynolds number values from 1 to 45. A map of modes of flow of the liquid film is plotted, and regions of heat transfer are identified. Data are obtained on the longitudinal coordinate dependence of the heater wall temperature and of the local heat flux on the symmetry axis of the heater. Local coefficients of heat transfer are measured. The experimental data are compared with the results of numerical calculations for a smooth film. The effect of the forming of jet flows on heat transfer to the liquid film is analyzed.     

新型绕带式液氢高压容器的传热分析和蒸发率计算

对一新型绕带式液氢高压容器的传热过程进行了分析,建立了筒体壁（包括封头）的传热模型,计算了热流稳定工况下各界面温差、筒体和封头的传热量以及容器的蒸发率。结果表明,新结构满足设计要求。     

The Effect of Hydrodynamic Unsteadiness on the Flow Structure and on the Coefficients of Heat Transfer and Skin Friction under Conditions of Turbulent Pipe Flow of Heat-Transfer Agent

Critical analysis is made of the current status of investigations of heat transfer and skin friction under conditions of unsteady-state turbulent pipe flow. This analysis is based on the experimental data on the structure of turbulent flows and heat transfer. Comparison is made of experimental and theoretical results.     

Influence of oil on evaporator heat transfer (results for R 12 and Shell Clavus 68)

Experiments are described which investigated the influence of lubricating oil on the heat transfer and the refrigerant flow in refrigeration evaporators. The particular refrigerant was R 12, and the oil used was Shell Clavus 68. Efforts were made to model actual operation as closely as possible. It was found that the oil had a significant effect on the refrigerant flow and pressure drop. Measurements made on the heat transfer coefficient indicated that the oil had different effects depending on the fluid flow regime. For annular — wavy regimes, there was no apparent effect, while for annular and film flows, the presence of oil significantly reduced the heat transfer coefficient.The dependence of heat transfer coefficient on refrigerant quality was also investigated and was found to be a maximum when the quality lay between 0.6 and 0.8. This regime corresponds to fully developed annular flow.     

Influence of oil on evaporator heat transfer (results for R 12 and Shell Clavus 68)Influence de l'huile sur le transfert de chaleur dans l'évaporateur

Experiments are described which investigated the influence of lubricating oil on the heat transfer and the refrigerant flow in refrigeration evaporators. The particular refrigerant was R 12, and the oil used was Shell Clavus 68. Efforts were made to model actual operation as closely as possible. It was found that the oil had a significant effect on the refrigerant flow and pressure drop. Measurements made on the heat transfer coefficient indicated that the oil had different effects depending on the fluid flow regime. For annular — wavy regimes, there was no apparent effect, while for annular and film flows, the presence of oil significantly reduced the heat transfer coefficient.The dependence of heat transfer coefficient on refrigerant quality was also investigated and was found to be a maximum when the quality lay between 0.6 and 0.8. This regime corresponds to fully developed annular flow.     

An experimental study of the fluid flow and heat transfer characteristics in micro-condensers with slug-bubbly flow

Experiments were conducted to study the condensation flow pattern in silicon micro-condensers using water as the medium. Slug-bubbly flow was found to be one of the dominant flows in the micro-condenser and it was a major factor in determining the heat transfer and pressure drop properties of the fluid inside the micro-condenser. The transition from the slug-bubbly flow to a mixed flow pattern was studied. A correlation was obtained to predict when the transition of the flow pattern would occur. Only slug-bubbly flow existed under low steam mass flow rate and high heat transfer rate conditions. As the steam mass flow rate increased or the heat transfer rate dropped, the mixed flow pattern would then appear. In the slug-bubbly flow regime, the heat transfer coefficient and pressure drop in the micro-condensers were investigated in detail. It was found that micro-condensers with smaller channels could exhibit higher heat transfer coefficients with the same Reynolds number. The condensation heat transfer coefficient was higher than that in the tubes with the diameter of centimeter. Pressure drops in the micro-condensers with smaller channels were higher due to the increased transition loss. At the same time, the pressure drop in the micro-condenser was found to be lower than what could be predicted using the macro-scale correlation. Increasing the heat flux would create a longer bubble–film region and fewer unit cells in the micro-condenser resulting in an increased heat transfer coefficient and a decreased pressure drop.