Experimental investigation of the film boiling heat transfer in He II: Heat transfer coefficient

He II film boiling is of both academic and applied interests. However, information about He II film boiling is still inadequate and further study is needed from both the technical application and the scientific aspects. In the present study, a thin stainless steel foil heater (10 μm thick) is employed to induce boiling in He II. The average heater surface temperature is measured to evaluate the heat transfer performance of He II film boiling under different thermal conditions. And meanwhile, the pressure and the temperature oscillations induced by the film boiling are also measured. It is found that the pressure oscillation and the temperature oscillation highly correlate with each other, which indicates that the vapor bubble is vibrating on the heater surface during film boiling. The heat transfer coefficient of the film boiling depends on both the pressure over the heater surface and the He II bath temperature. The heat transfer coefficients of three kinds of boiling states: noisy film boiling, transition boiling and silent film boiling, are measured in the present study. The visualization of the boiling process is also carried out.     

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.     

Experimental Study of Nucleate Pool Boiling of FC-72 on Smooth Surface under Microgravity

Experiments of highly subcooled nucleate pool boiling of FC-72 with dissolved air were studied both in short-term microgravity condition utilizing the drop tower Beijing and in normal gravity conditions. The bubble behavior and heat transfer of air-dissolved FC-72 on a small scale silicon chip (10 × 10 × 0.5 mm³) were obtained at the bulk liquid subcooling of 41 K and nominal pressure of 102 kPa. The boiling heat transfer performance in low heat flux region in microgravity is similar to that in normal gravity condition, while vapor bubbles increase in size but little coalescence occurs among bubbles, and then forms a large bubble remains attached to the heater surface during the whole microgravity period. Thermocapillary convection may be an important mechanism of boiling heat transfer in this case. With further increasing in heat flux to the fully developed nucleate boiling region, the vapor bubbles number as well as their size significantly increase in microgravity. Rapid coalescence occurs among adjacent bubbles and then the coalesced large bubble can depart from the heating surface during the microgravity period. The reason of the large bubble departure is mainly attributed to the momentum effects caused by the coalescence of small bubbles with the large one. Hence, the steady-state pool boiling can still be obtained in microgravity. In the high heat flux regime near the critical heat flux, significant deterioration of heat transfer was observed, and a large coalesced bubble forms quickly and almost covers the whole heater surface, leading to the occurrence of the critical heat flux in microgravity condition.     

Partial Nucleate Pool Boiling at Low Heat Flux: Preliminary Ground Test for SOBER-SJ10

Focusing on partial nucleate pool boiling at low heat flux, SOBER-SJ10, one of 27 experiments of the program SJ-10, has been proposed to study local convection and heat transfer around an isolated growing vapor bubble during nucleate pool boiling on a well characterized flat surface in microgravity. An integrated micro heater has been developed. By using a local pulse overheating method in the experimental mode of single bubble boiling, a bubble nucleus can be excited with accurate spatial and temporal positioning on the top-side of a quartz glass substrate with a thickness of 2 mm and an effective heating area of 4.5 mm in diameter, and then grows under an approximate constant heat input provided by the main heater on the back-side of the substrate. Ten thin film micro-RTDs are used for local temperature measurements on the heating surface underneath the growing bubble. Normal pool boiling experiments can also be carried out with step-by-step increase of heating voltage. A series of ground test of the flight module of SOBER-SJ10 have been conducted. Good agreement of the measured data of single phase natural convection with the common-used empirical correlation warrants reasonable confidence in the data. It is found that the values of the incipience superheat of pool boiling at different subcooling are consistent with each others, verifying that the influence of subcooling on boiling incipience can be neglected. Pool boiling curves are also obtained, which shows great influence of subcooling on heat transfer of partial nucleate pool boiling, particularly in lower heat flux.     

Model of vapor slug growth in the channels of power engineering equipment with sodium coolant

The problem of vapor volume growth in superheated liquid is very important both in practical and in theoretical aspects as the behavior of the solitary bubble governs the general patterns and the proceeding character of the boiling process. On the basis of the analysis of experimental and theoretical works, we create the physical and the numerical model of vapor volume growth in superheated sodium in power engineering equipment channels. On that basis, we implement the program module, making it possible to calculate the sodium boiling process under superheating. We perform a numerical analysis of the dependence of the vapor volume growth rate on the superheating value, the liquid film thickness, and the coefficient of the heat transfer with the wall. The numerical modeling results are compared to experimental data on the boiling of the sodium in the column and with the data on the sodium boiling under a decrease of the mass flow rate in the contour.     

Mathematical simulation of transient processes in a circular channel with granular layer under conditions of subcooled water boiling. Formation of pressure wave

The process of formation and growth of vapor bubbles until the instant of their merging into a vapor film is investigated in view of actual roughness of the surface and of the dependence of activation time of the respective nucleation sites on the surface temperature under conditions of pulsed heat release in the heater wall. A microlayer model is employed in view of formation of dry spot under the bubble. Also solved is the problem on the formation of a pressure wave in a circular channel as a result of explosive boiling of superheated microlayer under the bubbles. The obtained results enable one to describe a number of experimental data and investigate unsteady-state wave processes occurring under subcooled liquid boiling in the presence of a granular layer.     

Regimes of Intensified Heat Transfer during Evaporation of Thin Horizontal Liquid Layers at Reduced Pressures

The influence of structures formed during the evaporation and boiling of a liquid (n-dodecane) in thin horizontal layers on heat transfer has been analyzed. Structures shaped with the shapes of funnels and craters are formed at low pressures in liquid layers with thicknesses above the capillary constant under the action of a vapor recoil force. Increasing pressure leads to the onset of bubble boiling. It is established that the formation of these structures in the regime of intense evaporation at low reduced pressures leads to an about 70% increase in the heat-transfer coefficient at analogous layer thicknesses in comparison to the case of bubble boiling.     

Bubble — bubbles — boiling

A short overview of boiling research in microgravity performed during the past two decades is subject of this presentation. The research was concentrated on pool boiling without applying any external forces. The objective of this research was to answer the questions: Is boiling an appropriate mechanism of heat transfer in space applications, and how do heat transfer and bubble dynamics behave without buoyancy, shear or electrical field forces? Is bubble dynamics itself being able to maintain heat transfer during boiling? The correlations used today to calculate heat transfer coefficients for practical applications in pool boiling are more or less based on the assumption that buoyancy detaches the bubbles from the heating surface and carry vapor with hot liquid away. With this model heat transfer would break down in microgravity. That’s why microgravity itself is an outstanding environment to study boiling in order to gain a better understanding of the complex interrelated physical mechanisms. Various carrier systems that allow simulation of microgravity could be used, such as drop tower ZARM, drop shaft JAMIC, parabolic trajectories with NASA’s aircraft KC-135, ballistic rockets TEXUS, and finally three Space Shuttle missions. As far as the possibilities of the respective mission allowed, a systematic research program [1] was followed, which was continuously adjusted and updated to new results and parameters. We discuss the hydrodynamic and thermal behavior of single bubbles, the dynamics during coalescence processes and the interaction of bubbles at the hot wall during boiling with the processes: boundary layer superheat, nucleation, bubble growth, detachment and departure. Surprising results have been obtained, that not only saturated and subcooled boiling can be maintained in microgravity, but also that at lower heat fluxes an enhancement of heat transfer compared to terrestrial was observed, while most today used empirical correlations show a strong decrease extrapolated to lower gravity values. However, it must be pointed out that also the maximum accessible heat flux, the so called “critical heat flux”, is higher than predicted by present used relations, but as far as reliable values are available, reduced by about 50 % compared to terrestrials. With the simultaneously observed bubble dynamics the heat transfer results can be interpreted and both give rise to a better physical understanding of the boiling process.     

Thermo-fluid dynamics of several film boiling modes in He II in the pressure range between atmospheric pressure and saturated vapor pressure

Four film boiling modes including the silent film boiling and the noisy film boiling were discriminated experimentally. Each mode was classified through visual observation and transient pressure and temperature measurements near the heater. It was found that in subcooled He II there were two film boiling modes, which are the strongly subcooled and weakly subcooled film boiling modes. The variation of boiling state between these two modes could be visually observed well by use of a transparent heater. All mode of film boiling is clearly mapped in diagrams as a function of pressure, temperature and heat flux. It is elucidated that the existence of He I layer influences the development of the vapor layer.     

Analytical investigation of two different absorption modes: falling film and bubble types

The objectives of this paper are to analyze a combined heat and mass transfer for an ammonia–water absorption process, and to carry out the parametric analysis to evaluate the effects of important variables such as heat and mass transfer areas on the absorption rate for two different absorption modes — falling film and bubble modes. A plate heat exchanger with an offset strip fin (OSF) in the coolant side was used to design the falling film and the bubble absorber. It was found that the local absorption rate of the bubble mode was always higher than that of the falling film model leading to about 48.7% smaller size of the heat exchanger than the falling film mode. For the falling film absorption mode, mass transfer resistance was dominant in the liquid flow while both heat and mass transfer resistances were considerable in the vapor flow. For the bubble absorption mode, mass transfer resistance was dominant in the liquid flow while heat transfer resistance was dominant in the vapor region. Heat transfer coefficients had a more significant effect on the heat exchanger size (absorption rate) in the falling film mode than in the bubble mode, while mass transfer coefficients had a more significant effect in the bubble mode than in the falling film mode.     

Bubble Dynamics in Nucleate Pool Boiling on Thin Wires in Microgravity

A temperature-controlled pool boiling (TCPB) device has been developed to study the bubble behavior and heat transfer in pool boiling phenomenon both in normal gravity and in microgravity. A thin platinum wire of 60 μm in diameter and 30 mm in length is simultaneously used as heater and thermometer. The fluid is R113 at 0.1 MPa and subcooled by 26°C nominally for all cases. Three modes of heat transfer, namely single-phase natural convection, nucleate boiling, and two-mode transition boiling, are observed in the experiment both in microgravity aboard the 22nd Chinese recoverable satellite and in normal gravity on the ground before and after the space flight. Dynamic behaviors of vapor bubbles observed in these experiments are reported and analyzed in the present paper. In the regime of fully developed nucleate boiling, the interface oscillation due to coalescence of adjacent tiny bubbles is the primary reason of the departure of bubbles in microgravity. On the contrary, in the discrete bubble regime, it’s observed that there exist three critical bubble diameters in microgravity, dividing the whole range of the observed bubbles into four regimes. Firstly, tiny bubbles are continually forming and growing on the heating surface before departing slowly from the wire when their sizes exceed some value of the order of 10⁻¹ mm. The bigger bubbles with about several millimeters in diameter stay on the wire, oscillate along the wire, and coalesce with adjacent bubbles. The biggest bubble with diameter of the order of 10 mm, which was formed immediately after the onset of boiling, stays continuously on the wire and swallows continually up adjacent small bubbles until its size exceeds another critical value. The same behavior of tiny bubbles can also be observed in normal gravity, while the others are observed only in microgravity. Considering the Marangoni effect, a mechanistic model about bubble departure is presented to reveal the mechanism underlying this phenomenon. The predictions are qualitatively consistent with the experimental observations.     

Correlation of time dependent recovery from film boiling heat transfer in He II

A correlation is presented that describes the behaviour of time dependent recovery from film boiling in He II. In a one dimensional heat transfer experiment, the recovery time from the film boiling once heat generation stops is observed to be a function of the energy applied to the heater during film boiling. This correlation has a power law dependence which can be physically understood in terms of heat capacity of the heat transfer sample and the film boiling heat transfer coefficient. A direct comparison of experimental data with the analysis is achieved by adjusting the value of the transient film boiling heat transfer coefficient. Data can be predicted to within 20% for recovery under SVP conditions. The results are somewhat less certain for data taken in subcooled He II.     

Special features of boiling of a slightly subcooled liquid

Slightly subcooled boiling is characterized by the fact that vapor bubbles that form at active sites on the heater surface grow continuously and, having attained a maximum size, separate and float. The frequency of bubble separation, which determines the rate of heat transfer, depends on the degree of subcooling of the boiling liquid. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 71, No. 6, pp. 993–995, November–December, 1998.     

The Vapor Film Evolution at Superfluid Helium Boiling in Conditions of Microgravity

The results of numerical modeling of the vapor film temporal evolution on the cylindrical heater at film boiling of the superfluid helium (HeII) in conditions of microgravity are presented. The methods of molecular-kinetic theory have been applied to describe the vapor near the interface: vapor-HeII. For this purpose the Boltzmann equation have been solved. The system of equations of conservation for the liquid has been used. Different models of heat transfer in superfluid helium have been discussed and studied.     

Pool boiling heat transfer in microgravity

A temperature-controlled pool boiling (TCPB) device has been developed to study the bubble behaviors and heat transfer in pool boiling phenomenon both in normal gravity and in microgravity. The results on heat transfer and bubble dynamic behavior in the experiments aboard the 22^nd Chinese recoverable satellite and those in normal gravity before and after the flight experiment are reported and discussed in the present paper. The onset-boiling temperature is independent, or at least, dependent much weakly on gravity. Heat transfer of nucleate boiling in microgravity is slightly enhanced, while the scale of CHF with gravity is contrary to the traditional viewpoint and can be predicted by LD-Zuber correlation. A forward-and-backward lateral motion of vapor bubbles is observed along the wire before their departure from the wire in microgravity, while three critical bubble diameters divide the observed vapor bubbles into four regions in microgravity. These distinctive bubble behavior can be interpreted by Marangoni effects.     

IR thermographic investigation of nucleate pool boiling at high heat flux

Nucleate water pool boiling at high heat flux was investigated on the 25 µm titanium and stainless-steel heaters at atmospheric pressure. A high-speed IR thermographic camera was applied to measure the rapidly changing transient temperature field, which served as input data for calculating the transient local heat flux distributions by solving a 3-dimensional (3-D) inverse heat conduction problem (IHCP). A phenomenon of hot spot was observed at the irregular active bubble site characterized by a longer waiting time and a higher activation temperature compared to a regular active nucleation site. The results show that the temperature of the hot spot can significantly exceed the temperature of the heater in its surroundings and remains present on the boiling surface even after the bubble departure. The calculations have shown a strong reduction of the local heat flux at the spot, which represented a potential for the beginning of the boiling crisis.     

The influence of three-dimensional capillary-porous coatings on heat transfer at liquid boiling

The process of heat transfer at pool boiling of liquid (Freon R21) on tubes with three-dimensional plasma-deposited capillary-porous coatings of various thicknesses has been experimentally studied. Comparative analysis of experimental data showed that the heat transfer coefficient for a heater tube with a 500-μm-thick porous coating is more than twice as large as that in liquid boiling on an otherwise similar uncoated tube. At the same time, no intensification of heat exchange in the regime of bubble boiling is observed on a tube with a 100-μm-thick porous coating.     

Study on Local Heat Transfer in the Vicinity of the Contact Line under Vapor Bubbles at Pool Boiling

The results of an experimental study of the dynamics of local heat transfer at nucleate pool boiling of liquids are presented. Experimental data on the nucleation site density and the evolution of the temperature field underneath individual vapor bubbles were obtained by high-speed infrared thermography with high spatial and temporal resolutions. Deionized water and ethanol at the saturation line under atmospheric pressure were used as working liquids. Evolution of the distribution of the local heat flux rate in the region of an individual nucleation site has been constructed based on numerical simulation. It has been shown that the maximum rate of the local heat flux is observed in the region of the liquid microlayer during the period of vapor bubble growth and reaches a value exceeding the average heat flux rate by 15–20 times. Based on the results, the thickness of the microlayer underneath the vapor bubble during the period of the bubble growth was estimated. The estimates satisfactorily agree with experimental literature data obtained with the use of laser interferometry.     

液氢空间贮存过程膜态沸腾数值模拟

为实现液氢在空间中安全高效应用,针对微重力条件下液氢膜态沸腾现象,建立了加热细丝浸没在过冷液氢池中的数值计算模型.采用VOF方法捕捉相界面,相变模型选取Lee模型,利用文献中的实验数据验证了模型的准确性.从气泡运动行为和换热特性两方面开展研究,结果发现液体过冷度和重力水平是影响换热机理的两个重要因素.在高重力水平、低液...     

Nucleate Boiling on a Single Site: Contact Angle Analysis for a Quasi-2D Growing Vapour Bubble

Heat transfer prediction under boiling condition is still unresolved. In this paper, a basic study on bubble growth is carried out. Recent works show that contact line region plays an important role for heat and mass transfer in nucleate boiling regime. Three dimension experimental set-up lead to a mirage effect which disturbs measurements. To overcome this problem, a new quasi two dimensional experimental set-up is designed. This Hele–Shaw like configuration allows measuring the contact angle and contact line displacement during the bubble growth. A noticeable behavior of the contact angle is observed, and the influence of the sub-cooling level on the bubble growth rate and the contact angle value is studied.