Superheated Surface Research Articles

3-D modeling of the dynamics and heat transfer characteristics of subcooled droplet impact on a surface with film boiling

The impact of a subcooled water and n-heptane droplet on a superheated flat surface is examined in this study based on a three-dimensional model and numerical simulation. The fluid dynamic behavior of the droplet is accounted for by a fixed-grid, finite-volume solution of the incompressible governing equations coupled with the 3-D level-set method. The heat transfer inside each phase and at the solid–vapor/liquid–vapor interface is considered in this model. The vapor flow dynamics and the heat flux across the vapor layer are solved with consideration of the kinetic discontinuity at the liquid–vapor and solid–vapor boundaries in the slip flow regime. The simulated droplet dynamics and the cooling effects of the solid surface are compared with the experimental findings reported in the literatures. The comparisons show a good agreement. Compared to the water droplet, it is found that the impact of the n-heptane droplet yields much less surface temperature drop, and the surface temperature drop mainly occurs during the droplet-spreading stage. The effects of the droplet’s initial temperature are also analyzed using the present model. It shows that the droplet subcooling degree is related closely to the thickness of the vapor layer and the heat flux at the solid surface.

High speed video recording of bubble formation with pool boiling

A general theory for boiling heat transfer shall be developed by modelling the microscopic transport phenomena in boiling on a heated surface supported by experimental studies with equally detailed informations on roughness, formation of bubbles and on heat transfer. The heat transfer measurements and the bubble formation are carried out during the same experiment. The experiments in bubble formation are obtained by a high speed video technique. The images of the video sequence are analysed manually or by special semiautomatic evaluation programs to obtain the number and location of active nucleation sites and some selected results for bubble departure diameter and frequency. Results for the bubble formation on a horizontal copper tube in boiling propane for an intermediate pressure from low to high heat fluxes are discussed, e.g. the cumulative site densities vary from 2 mm −2 at low heat fluxes to 70 mm −2 for high heat fluxes. Additionally, the fluctuation by time and in the location on the tube is recorded. Because of the large amount of data, different automatic methods to analyse the single images have to be developed. A new template-based method for the evaluation of the video sequences allows the nearly automatic identification of the individual bubble on its nucleation site, and to track its growth and movement along the heated tube surface during the following images. First results show that the bubbles sliding up the superheated tube surface grow at a similar rate as those staying on their nucleation site. Their contribution for the vapour production in model assumptions has to be revised.

The behavior of a vapor film on a highly superheated surface immersed in subcooled water

Results are given of an experimental investigation of the behavior of a vapor film on a highly superheated hemispherical surface immersed in water subcooled to the saturation temperature. The conditions of existence of vapor films on a hot surface and the characteristics of their collapse from the heating surface and transition to nucleate boiling are revealed, as well as the effects which accompany the vanishing of vapor film. The possibilities of internal triggering of fine fragmentation of hot melt and vapor explosion are discussed.

The behavior of a vapor film on a highly superheated surface immersed in subcooled water

The behavior of a vapor film on a highly superheated surface immersed in subcooled water

The behavior of a vapor film on a highly superheated surface immersed in subcooled water

The behavior of a vapor film on a highly superheated surface immersed in subcooled water

The Gas Temperature in the Surface Layers of Protoplanetary Disks

Models for the structure of protoplanetary disks have so far been based on the assumption that the gas and the dust temperature are equal. The gas temperature, an essential ingredient in the equations of hydrostatic equilibrium of the disk, is then determined from a continuum radiative transfer calculation, in which the continuum opacity is provided by the dust. It has been long debated whether this assumption still holds in the surface layers of the disk, where the dust infrared emission features are produced. In this paper we compute the temperature of the gas in the surface layers of the disk in a self-consistent manner. The gas temperature is determined from a heating-cooling balance equation in which processes such as photoelectric heating, dissociative heating, dust-gas thermal heat exchange and line cooling are included. The abundances of the dominant cooling species such as CO, C, C+ and O are determined from a chemical network based on the atomic species H, He, C, O, S, Mg, Si, Fe (Kamp & Bertoldi 2000). The underlying disk models to our calculations are the models of Dullemond, van Zadelhoff & Natta (2002). We find that in general the dust and gas temperature are equal to withing 10% for A_V >~ 0.1, which is above the location of the `super-heated surface layer' in which the dust emission features are produced (e.g. Chiang & Goldreich 1997). High above the disk surface the gas temperature exceeds the dust temperature and can can become -- in the presence of polycyclic aromatic hydrocarbons -- as high as 600 K at a radius of 100 AU. This is a region where CO has fully dissociated, but a significant fraction of hydrogen is still in molecular form. The densities are still high enough for non-negligible H_2 emission to be produced.....(see paper for full abstract)

C201 高温面上での濡れ面の可視化

The behavior of rewetting on high superheated surface, focusing on a wetted area and a contact angle during collapse of film boiling was investigated by visualization of a liquid film on the heated wall. In the experiments, a liquid jet from a nozzle was supplied on the superheated surface, made of a platinum wire, in the stable film boiling of ΔTsub=OK or 15K, then the wetted area was observed. The measured angles between a liquid-vapor interfacial line and the heated wall corresponded to dynamic contact angle were close to the dynamic advancing contact-angles in room temperature in spite of a difference of liquid temperature.

Heat transfer of impinging air and liquid nitrogen mist jet onto superheated flat surface

This experimental study performs the detailed heat transfer measurements of an impinging air–liquid nitrogen mist jet onto a superheated flat surface at atmospheric pressure with reference to the design of an instant freezing facility. A selection of experimental results illustrates the interacting effects of jet Reynolds number, mass flow ratio of air to liquid nitrogen flows and separation distance on the spatial distributions of heat transfer over the impinging surface. Mechanism associated with phase change of impacting droplets generates an enhanced and uniformly distributed heat transfer region centered on the stagnation point. A narrow oval-ring region encapsulating the enhanced core transits heat transfer from the wetting regime of complete evaporation to the non-wetting rebound regime. Stagnation heat transfer augmentation factor in the range of 1.2–2.8 times of the air-jet level is achieved. An empirical correlation based on the experimental data, which is physically consistent, has been developed to permit the evaluation of stagnation heat transfer.

2243 プール膜沸騰における固液接触直後の濡れ面挙動 膜沸騰崩壊条件に及ぼす液流量の影響

The behavior of rewetting on high superheated surface, focusing on an effect of liquid subcooling on wetted area and a contact angle during collapse of film boiling was investigated experimentally. In the experiments, a liquid jet from a nozzle was supplied on the superheated surface, made of a platinum wire, in the stable subcooled film boiling of ΔT_<sub> = 15K, and then the wetted area was observed. The experimental results showed that the propagative collapse of the vapor film occurred when the local wall temperature was below 450℃. The measured angles between the liquid-vapor interfacial line and the heated wall corresponded to dynamic contact angle were close to the dynamic advancing contact-angles in room temperature even if the liquid is subcooled.

Submm/FIR CO line emission from the disk of the Class I protostar EL 29

We present observations towards the Class I protostar EL 29 of the CO J= 6! 5a ndJ 15 transitions obtained with JCMT and ISO LWS respectively, and of five H2 rotational lines obtained with ISO SWS. The simultaneous analysis of these observations reveals the presence of a warm gas component at about 170-250 K,550 AU in size, and whose density is 10 6 cm 3 . The mass of the warm gas is8 2410 4 M. These values compare extremely well with the predictions of the temperature and mass of the flaring disk surrounding EL 29, probed by the dust continuum. We propose that the observed FIR CO emission originates in the super-heated surface layer of the disk of EL 29 and discuss its characteristics. We find that the CO abundance in the disk is large,10 4 , implying no depletion or photodissociation and we present arguments in favor of the idea that the dust has settled on the midplane disk of EL 29 and that it is thermally decoupled from the gas. We compare the characteristics of the El 29 disk with those of the disks observed in other Herbig AeBe stars using recent studies of H2 rotational line emission. The gas temperature and mass derived for the disk of EL 29 are similar to the disks of the previously studied sample. In EL 29, as in Herbig AeBe stars, the gas and dust are probably thermally decoupled. Finally, the upper limit on the H2O emission that we obtain suggests that water is not an important coolant of the disk gas, in agreement with theoretical water abundance predictions. The present study challenges previous claims that the FIR CO line emission observed in sources similar to EL 29 originates in shocks.

Spectral Energy Distributions of Passive T Tauri and Herbig Ae Disks: Grain Mineralogy, Parameter Dependences, and Comparison withInfrared Space ObservatoryLWS Observations

We improve upon the radiative, hydrostatic equilibrium models of passive circumstellar disks constructed by Chiang & Goldreich. New features include (1) an account for a range of particle sizes, (2) employment of laboratory-based optical constants of representative grain materials, and (3) numerical solution of the equations of radiative and hydrostatic equilibrium within the original two-layer (disk surface plus disk interior) approximation. We systematically explore how the spectral energy distribution (SED) of a face-on disk depends on grain size distributions, disk geometries and surface densities, and stellar photospheric temperatures. Observed SEDs of three Herbig Ae and two T Tauri stars, including spectra from the Long Wavelength Spectrometer (LWS) aboard the Infrared Space Observatory (ISO), are fitted with our models. Silicate emission bands from optically thin, superheated disk surface layers appear in nearly all systems. Water ice emission bands appear in LWS spectra of two of the coolest stars. Infrared excesses in several sources are consistent with significant vertical settling of photospheric grains. While this work furnishes further evidence that passive reprocessing of starlight by flared disks adequately explains the origin of infrared-to-millimeter wavelength excesses of young stars, we emphasize by explicit calculations how the SED alone does not provide sufficient information to constrain particle sizes and disk masses uniquely.

825 プール膜沸騰における固液接触直後の濡れ面挙動に関する研究(動力エネルギーの基盤技術II)(OS.9 動力エネルギーの基盤技術 : 動力エネルギーシステム部門企画)

The behavior of rewetting on high superheated surface, focusing on a wetted area and a contact angle during collapse of film boiling was investigated experimentally. In the experiments, a liquid jet from a nozzle was supplied on the superheated surface, made of a platinum wire, in the stable film boiling, and then the wetted area was observed. The experimental results showed that the propagative collapse of the vapor film occurred when the local wall temperature was below 300℃, i.e. The thermodynamic limit of superheat. The wetted area right after liquid-wall contact and the advancing velocity of the rewetting front increased as the initial wall superheat decreased. The measured angles between the liquid-vapor interfacial line and the heated wall corresponded to dynamic contact angle were close to the dynamic advancing contact-angles in room temperature.

Spectral Energy Distributions of Passive T Tauri Disks: Inclination

We compute spectral energy distributions (SEDs) for passive T Tauri disks viewed at arbitrary inclinations. Semianalytic models of disks in radiative and hydrostatic equilibrium are employed. Over viewing angles for which the flared disk does not occult the central star, the SED varies negligibly with inclination. For such aspects, the SED shortward of ~80 μm is particularly insensitive to orientation, since short wavelength disk emission is dominated by superheated surface layers, which are optically thin. The SED of a nearly edge-on disk is that of a class I source. The outer disk occults inner disk regions, and emission shortward of ~30 μm is dramatically extinguished. Spectral features from dust grains may appear in absorption. However, millimeter-wavelength fluxes decrease by at most a factor of 2 from face-on to edge-on orientations. We present illustrative applications of our SED models. The class I source 04108+2803B is considered a T Tauri star hidden from view by an inclined circumstellar disk. Fits to its observed SED yield model-dependent values for the disk mass of ~0.015 M_☉ and a disk inclination of ~65° relative to face-on. The class II source GM Aur represents a T Tauri star only slightly obscured by its circumstellar disk. Fitted parameters include a disk mass of ~0.050 M_☉ and an inclination of ~60°, where the viewing angle is chosen to reproduce the observed visual extinction of A_V = 0.5 mag.

Study on Mist Cooling of a Superheated Surface. Effects of Surface Position, Size and Structure on Heat Transfer Enhancement and Control.

In order to pursue the effects of surface position, size and structure on the enhancement and control of mist cooling heat transfer, detailed experiments have been conducted regarding the boiling and evaporation of liquid film formed on circular superheated surfaces with smooth, concentric grooves and radial grooves. These results indicate that a vertical surface is more efficienct in the enhancement of heat transfer than a horizontal one, a smaller surface produces higher heat flux, and the grooved surface is more effective in the enhancement and stability of heat transfer. It is also clarified that the optimum size and structure are determined from the sprayed mass flow rate and degree of superheating.

Modelling of void formation in the subcooled boiling regime in the athlet code to simulate flow instability for research reactors

The athlet thermohydraulic code was developed at the Gesellschaft für Anlagen- und Reaktorsicherheit (GRS, Society for Plant and Reactor Safety) to analyse leaks and transients for power reactors. In order to extend the code's range of application to the safety analysis of research reactors, a model was implemented permitting a description of the thermodynamic non-equilibrium effects in the subcooled boiling regime. The aim of the extension is, on one hand, to cover the thermohydraulic instability which is particularly characteristic of research reactors owing to their high power densities and low system pressures and, on the other hand, to provide a consideration of the influence of the steam formed in this boiling regime on the neutron balance. The model developed takes into consideration the competing evaporation and condensation effects in the subcooled boiling regime. It describes the bubble production rate at the superheated heating surfaces as well as the subsequent condensation of the bubbles in the subcooled core flow. The installed model is validated by the recalculation of two extensive series of experiments. In the first series the McMaster experiments on axial void distribution in the subcooled boiling regime are recalculated. The recalculation shows that the extended programme is capable of calculating the axial void distribution in the subcooled boiling regime with good agreement with the data. The second series deals with KFA experiments on thermohydraulic instability (flow excursion) in the subcooled boiling regime, comprising a broad parameter range of heat flow density, inlet temperature and channel width. Recalculation of this experimental series shows that the programme extension ensures simulation of thermohydraulic instability.

Cooling of a superheated surface with a jet mist flow

The previously developed model of dynamic and thermal interaction of dilute mist flow with hot bodies is applied to study heat exchange in the circumstance when an axisymmetric two-phase jet falls normally onto a plate, the temperature of which exceeds the boiling temperature of the dispersed liquid. Depending on their approach velocity, impinging droplets either rebound, or come in direct contact with the plate and eventually evaporate, thus providing for an essential increase in the total heat removal. Such a crisis causes the occurrence of a temperature interval in which heat transfer from the plate decreases as the plate temperature grows. The very existence and properties of the mentioned anomalous region are explained in a good agreement with experimental findings. The results are also generalized to heat transfer involving polydisperse spray jets.

Study on Mist Cooling of a Superheated Surface. Heat Transfer Characteristics in Range of Small Degree of Superheating.

In order to determine the heat transfer characteristics of mist cooling in the range of small degree of superheating, detailed experiments have been conducted on mist cooling heat transfer from a circular superheated surface. The experimental results indicate that the heat flux curve obtained is a function of parameters such as sprayed mass flow rate and degree of superheating, in relation to the boiling and evaporation behavior of the liquid film formed on the heated surface. The effects of size, velocity and temperature of spray droplets on heat flux are also clarified. The mechanism of mist cooling heat transfer is discussed and theoretical and experimental results are compared.

Heat Transfer During Liquid Contact on Superheated Surfaces

Several boiling regimes are characterized by intermittent contacts of vapor and liquid at the superheated wall surface. A microthermocouple probe was developed capable of detecting transient surface temperatures with a response time better than 1 ms. The transient temperature data were utilized to determine the time-varying heat flux under liquid contacts. The instantaneous surface heat flux was found to vary by orders of magnitude during the milliseconds of liquid residence at the hot surface. The average heat flux during liquid contact was found to range from 105 to 107 W/m2 for water at atmospheric pressure, as wall superheat was varied from 50 to 450°C.

An analysis of subcooled turbulent film boiling on a moving isothermal surface

When film boiling occurs on a moving superheated surface, the vapor layer which forms between the surface and liquid reduces the drag force and heat transfer. In this study, heat transfer associated with subcooled turbulent film boiling on a moving isothermal surface is investigated using both local similarity and integral methods. Using a two-phase boundary-layer model and assuming power-law profiles for the velocity and temperature distributions, integral forms of the mass, momentum and energy-conservation equations, together with compatibility conditions at the vapor-liquid interface, were solved for both the vapor and liquid layers. In order to assess the merit of this analytical procedure, a local similarity solution to the same problem was obtained using the modified Cebeci-Smith eddy-viscosity model with the Cebeci-Bradshaw algorithm for turbulent boundary-layer flow. Numerical results reveal the effects of relevant parameters such as plate velocity, the ratio of plate-to-free-stream velocity, and liquid subcooling.

Similarity solution for laminar film boiling over a moving isothermal surface

Forced film boiling arises when a liquid flows over a highly superheated surface or when the surface moves through a liquid which is stationary or is itself in motion. As a result of the vapor layer separating the surface and the liquid, there is a significant reduction in the drag force and heat transfer. In this study, a similarity solution of the boundary layer equations is obtained, and for a wide range of subcolling parameters and surface velocities, related numerical results are shown to be in excellent agreement with predictions based on an integral method.