强化管内流动沸腾换热研究现状

介绍了强化管内流动沸腾换热国内外的研究现状,分析了微肋管内流动沸腾换热的影响因素,并且给出了几个实用的微肋管内沸腾换热关联式,最后对微肋管内沸腾换热的研究方向进行了讨论.     

Effect of Gravity on Film Boiling Heat Transfer and Rewetting Temperature during Quenching

This paper describes the experimental strategy developed to improve the modeling of liquid-vapor flows during the chill down of rocket engines by cryogenic fluid in microgravity. A similarity analysis is performed to determine the relevant dimensionless numbers for the design of an experiment similar to engine flows. A literature review on reduced gravity quenching experiments, and on rewetting temperature and film boiling heat transfer shows the lack of validated models for microgravity. Experimental results obtained with the quenching of a glass tube by FC72 during parabolic flight are presented. Especially the impact of gravity and subcooling on rewetting temperature and film boiling heat transfer is investigated. Results show an increase in rewetting temperature, and a decrease in film boiling heat transfer under reduced gravity in agreement with the literature. The comparison of 0 g flow pattern with corresponding tests on ground points out a behavior at 0 g closest to 1 g upflow than 1 g downflow.     

Flow Boiling Heat Transfer in Two-Phase Micro Channel Heat Sink at Low Water Mass Flux

Boiling heat transfer at water flow with low mass flux in heat sink which contained rectangular microchannels was studied. The stainless steel heat sink contained ten parallel microchannels with a size of 640 × 2050 μm in cross-section with typical wall roughness of 10–15 μm. The local flow boiling heat transfer coefficients were measured at mass velocity of 17 and 51 kg/m²s, heat flux on 30 to 150 kW/m² and vapor quality of up to 0.8 at pressure in the channels closed to atmospheric one. It was observed that Kandlikar nucleate boiling correlation is in good agreement with the experimental data at mass flow velocity of 85 kg/m²s. At smaller mass flux the Kandlikar model and Zhang, Hibiki and Mishima model demonstrate incorrect trend of heat transfer coefficients variation with vapor quality.     

Experimental Study of Subcooled Flow Boiling Heat Transfer on a Smooth Surface in Short-Term Microgravity

The flow boiling heat transfer characteristics of subcooled air-dissolved FC-72 on a smooth surface (chip S) were studied in microgravity by utilizing the drop tower facility in Beijing. The heater, with dimensions of 40 × 10 × 0.5 mm³ (length × width × thickness), was combined with two silicon chips with the dimensions of 20 × 10 × 0.5 mm³. High-speed visualization was used to supplement observation in the heat transfer and vapor-liquid two-phase flow characteristics. In the low and moderate heat fluxes region, the flow boiling of chip S at inlet velocity V =?0.5 m/s shows almost the same regulations as that in pool boiling. All the wall temperatures at different positions along the heater in microgravity are slightly lower than that in normal gravity, which indicates slight heat transfer enhancement. However, in the high heat flux region, the pool boiling of chip S shows much evident deterioration of heat transfer compared with that of flow boiling in microgravity. Moreover, the bubbles of flow boiling in microgravity become larger than that in normal gravity due to the lack of buoyancy Although the difference of the void fraction in x-y plain becomes larger with increasing heat flux under different gravity levels, it shows nearly no effect on heat transfer performance except for critical heat flux (CHF). Once the void fraction in y-z plain at the end of the heater equals 1, the vapor blanket will be formed quickly and transmit from downstream to upstream along the heater, and CHF occurs. Thus, the height of channel is an important parameter to determine CHF in microgravity at a fixed velocity. The flow boiling of chip S at inlet velocity V =?0.5 m/s shows higher CHF than that of pool boiling because of the inertia force, and the CHF under microgravity is about 78–92% of that in normal gravity.     

Influence of Quantum Turbulence on the Processes of Heat Transfer and Boiling in Superfluid Helium

We demonstrate that in a wide range of heat fluxes the dynamics of heat transfer in superfluid helium is determined by the existence of remanent quantized vortices. The vortex density dynamics determines the rise of temperature near the heater and the boiling-up of superfluid helium. It permits to understand the results of the experiments of several groups.     

青藏铁路冻土重力热管传热特性的研究

应用重力热管是解决青藏铁路建设中冻土路基冻融问题的一个较为实际的方法.本文通过分析重力热管的热阻网络,提出了重力热管的传热模型,并利用现有的热管内部传热的经验关系式,对不同倾角、不同管径以及不同冷凝管长度条件下的传热量进行模拟计算.     

Regenerated Boiling and Enhancement of Heat Transfer

It is demonstrated that, under certain process and geometric conditions, a regeneration (transformation) of boiling occurs, with the disappearance of the low-efficiency subprocess of evacuation of bubbles. As a result, prerequisites arise for considerable enhancement of heat transfer.     

Action of Vibrations on Heat and Mass Transfer in Boiling

The state of the art of the works known in the literature and concerned with the influence of vibrations of structures of plants and artificially initiated vibrations in working liquids on the processes involving boiling is analyzed. The authors discuss theoretical and experimental results on the influence of frequencies and amplitudes on the internal characteristics of boiling, namely, the initiation of a vapor phase, the superheat in boiling, the growth and separation of vapor bubbles and the frequency of their occurrence, the density of nucleation sites, and also on the value of the heat fluxes and on the heattransfer coefficient. Application of vibrational actions for intensification of heat and masstransfer processes is shown to be promising.     

The Influence of the Mass Flow Rate on the Critical Heat Flux during Subcooled Deionized Water Boiling in a Microchannel Cooling System

We have experimentally studied the influence of the mass flow rate on the critical heat flux (CHF) during the boiling of deionized water underheated relative to the saturation temperature in a microchannel cooling system. The experimental unit comprised a copper-based block with two 0.36-mm-deep microchannels of 2 mm width and 16 mm length. The mass flow rate ranged from 100 to 600 kg/(m² s) and the initial subcooling was varied from 30 to 70°C. It was found that the character of boiling crisis changed with increasing mass flow rate and the CHF significantly increased upon complete condensation of vapor in the distribution chamber of the cooling system.

     

Burnout Heat Transfer in Boiling under Conditions of Stepwise Heat Generation

The results of experimental investigations on the time of the onset of boiling crisis in nonstationary heat generation are presented. A standard working formula is suggested for determining the time of the onset of burnout heat transfer in a wide range of heat loads, subcoolings, and pressures.     

The Influence of Gravity and Confinement on Marangoni Flow and Heat Transfer Around a Bubble in a Cavity: A Numerical Study

Marangoni thermocapillary convection and its contribution to heat transfer during boiling has been the subject of some debate in the open literature. Despite extensive research efforts there still remains insufficient quantitative information regarding the impact of thermocapillary flow on the heat transfer. As a result, this paper aims to present a numerical investigation of the heat transfer enhancement due to Marangoni thermocapillary convection under both earth gravity (1-g) and zero gravity (0-g) conditions. A hemispherical bubble of fixed shape is considered atop a heated top wall of a domain with variable height. The heat transfer enhancement is quantified for Marangoni numbers in the range of 100 ≤ Ma ≤ 3,000 for channel heights of 1.5 ≤ H/R_b?≤ 7.5 which, for the 1-g cases, correspond with a Raleigh number range of 51 ≤ Ra_H?≤ 6.5 × 10⁴. For the most confined cases the flow and heat transfer were found to be very similar for the 0-g and 1-g cases. Also, the 0-g test cases were found to be very sensitive to increasing domain height whereas the 1-g simulations were far less sensitive.     

泡沫金属中池沸腾传热的研究进展

介绍了泡沫金属的结构特性,总结了泡沫金属中池沸腾的气泡生长速度、气泡直径和气泡生长现象等传热特点,以及泡沫金属的孔隙率、孔密度等参数对池沸腾传热的影响,并指出了泡沫金属中沸腾传热的研究方向。     

Heat Transfer in Superfluids: Effect of Gravity

No Heading We discuss the influence of an external field on energy transport in superfluids. He-II is not isothermal in presence of Earth gravity; instead, it supports finite temperature gradient given by a Fourier-like equation. We calculate asymptotic behavior of the effective heat resistance in the vicinity of the -transition.     

Self-Organized Superfluid States in Gravity and Heat Flow

When ⁴ He is heated from above near the superfluid transition, there can appear a self-organized region, either in normal fluid or superfluid, with the temperature gradient equal to the transition temperature gradient. When it is in a superfluid state, there can be two regimes. Regime M is realized relatively far from the superfluid transition, where thermal resistance due to vortices is described in terms of the conventional Gorter–Mellink mutual friction. In regime G vortices are densely generated, where the line density in units of ^–2 is much larger than in any other previous experiments. Such a self-organized superfluid can coexist with a normal fluid or a superfluid containing only a small number of vortices in a dynamical steady state.     

含有过冷沸腾边界台阶轴的耦合传热问题

讨论了复杂表面物体考虑导热，辐射和相变作用的传热问题，提出了一个包含导热，辐射和相变的传热控制方程组；导出了台阶轴相互射的角系列计算公式。     

Bubble Behavior and Heat Transfer in Quasi-Steady Pool Boiling in Microgravity

Pool boiling of degassed FC-72 on a plane plate heater has been studied experimentally in microgravity. A quasi-steady heating method is adopted, in which the heating voltage is controlled to increase exponentially with time. Compared with terrestrial experiments, bubble behaviors are very different, and have direct effect on heat transfer. Small, primary bubbles attached on the surface seem to be able to suppress the activation of the cavities in the neighborhoods, resulting in a slow increase of the wall temperature with the heat flux. For the high subcooling, the coalesced bubble has a smooth surface and a small size. It is difficult to cover the whole heater surface, resulting in a special region of gradual transitional boiling in which nucleate boiling and local dry area can co-exist. No turning point corresponding to the transition from nucleate boiling to film boiling can be observed. On the contrary, the surface oscillation of the coalesced bubble at low subcooling may cause more activated nucleate sites, and then the surface temperature may keep constant or even fall down with the increasing heat flux. Furthermore, an abrupt transition to film boiling can also be observed. It is shown that heat transfer coefficient and CHF increase with the subcooling or pressure in microgravity, as observed in normal gravity. But the value of CHF is quite lower in microgravity, which may be only about one third of that at the similar pressure and subcooling in terrestrial condition.     

Cryogenic Boiling and Two-Phase Flow during Pipe Chilldown in Earth and Reduced Gravity

For many industrial, medical and space technologies, cryogenic fluids play indispensable roles. An integral part of the cryogenic transport processes is the chilldown of the system components during initial applications. In this paper, we report experimental results for a chilldown process that is involved with the unsteady two-phase vapor-liquid flow and boiling heat transfer of the cryogen coupled with the transient heat conduction inside pipe walls. We have provided fundamental understanding on the physics of the two-phase flow and boiling heat transfer during cryogenic quenching through experimental observation, measurement and analysis. Based on the temperature measurement of the tube wall, the terrestrial cryogenic chilldown process is divided into three stages of film boiling, nucleate boiling and single-phase convection that bears a close similarity to the conventional pool boiling process. In earth gravity, cooling rate is non-uniform circumferentially due to a stratified flow pattern that gives rise to more cooling on the bottom wall by liquid filaments. In microgravity, there is no stratified flow and the absence of the gravitational force sends liquid filaments to the central core and replaces them by low thermal conductivity vapor that significantly reduces the heat transfer from the wall. Thus, the chilldown process is axisymmetric, but longer in microgravity.      

Heat Transfer in a Turbulent Flow of Gas in a Tube under Conditions of Resonance Oscillation of the Flow Rate

A prediction study is made into the heat transfer in a turbulent flow of gas (air) in a narrow tube with superimposed resonance oscillation. The model of turbulent transfer includes the effect of nonstationarity on the turbulent stress and heat flux. The finite difference method is used to solve the equations of motion and energy. The distribution of the flow rate and pressure along the tube is found by way of numerical solution of a set of cross section-averaged nonstationary equations of motion, continuity, and energy. The effect of the process parameters (Reynolds number, dimensionless oscillation frequency, thermal boundary conditions on the wall) on the period-average heat transfer and heat flux to the wall is analyzed.