1D plane numerical model for boiling liquid expanding vapor explosion (BLEVE)

The depressurization of a vessel containing saturated or subcooled liquid may occur in a variety of industrial processes and often poses a potentially hazardous situation. A 1D plane numerical model was developed for estimating the thermodynamic and the dynamic state of the boiling liquid during a boiling liquid expanding vapor explosion (BLEVE) event. Based on the choice of the initial nucleation sites density, the model predicts, simultaneously, the bubble growth processes in the liquid at the superheat-limit state, the front velocity of the expanding liquid, and the shock wave pressure formed by the liquid expansion through the air.Conditions of shock formation were found to be normally associated with high initial temperatures that can bring the liquid to its superheat-limit state during the initial depressurization. Furthermore, the high initial temperature also induces a generation of higher vapor pressures that forces a rapid mixture expansion.Model predictions of the shock wave strengths, in terms of TNT equivalence, were compared against those obtained by simple energy models. As expected, the simple energy models over predicts the shock wave strength. However, the simple model which accounts for the expansion irreversibility, produces results which are closer to current model predictions.     

Efficiency analysis of depressurization process and pressure control strategies for liquid hydrogen storage system in microgravity

In order to investigate dynamic characteristics of pressure fluctuation and thermal efficiency of a liquid hydrogen (LH₂) storage system during depressurization process under microgravity condition, a transient CFD model of LH₂ tank is established. Based on the assumption of lumped vapor, a UDF code is developed to solve phase change and heat transfer between liquid phase and vapor one. The thermal efficiency is provided for assessing the performance of different pressure control methods. Results show that raising the injection velocity and decreasing the temperature of the injection liquid can enhance the effect of fluid mixing and shorten the depressurization time. Increasing the pressure lower limit can also improve the efficiency of depressurization process. The model can predict the tendency of pressure changes in the tank, and provide theoretical guide to design LH₂ tank and optimize its parameters for space application.     

Study of vapor concentration during boiling process of binary liquid mixtures

The object of the study is to clarify experimentally the variation of the vapor concentration of binary mixtures generated during nucleate boiling in a saturated pool under various boiling conditions. The generated vapor concentrations in the bulk vapor layer were measured for binary mixtures of water‐ethanol and ethanol‐acetone under various liquid concentrations, heat fluxes, and liquid layer heights from heated thin wires at atmospheric pressure. Two methods of measuring concentration, namely, the dew point method and the laser absorption method, were used. The dew point measurement utilized a copper heat transfer block installed in the center of the vapor layer in the boiling vessel. The commencement of condensation on the surface was observed directly via a microscope used for determining the dew point by controlling its temperature. A laser light having a wavelength of 3.39 µm was used for the laser light absorption method utilizing its absorptive property against ethanol vapor. The measurement of the concentrations of the bulk vapor was carried out for various superheats of heated wires, the depth of liquid mixture layer, and the liquid concentrations. They were almost independent of those parameters and were almost in equilibrium with bulk liquid conditions even under higher surface superheats and lower liquid heights. Also, the measured results using both methods were in close agreement. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(8): 595–605, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10060     

An investigation of vapor concentration during boiling of a liquid mixture: Measurement of temperature and concentration variations of vapor in bubbles

Variations of the vapor concentration of a water‐ethanol mixture during saturated nucleate pool boiling were studied. The temperature changes of the vapor in bubbles produced by the boiling were measured at several different positions near the boiling surface by using a thin thermocouple having high responsivity. The superheat of the bubbles was initially higher in the superheated liquid layer near the heat transfer surface and then approached the bulk temperature along the upward direction. It was clarified that the temperature and concentration differences of the bubbles from the bulk almost disappeared at a height of 3 to 5 mm from the heat transfer surface and within 20 ms after the appearance of the bubbles. Thus, the former result of the liquid‐vapor equilibrium with the bulk condition was confirmed. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(6): 475–485, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10047     

Unstable and stable flow boiling in parallel microchannels and in a single microchannel

A simultaneous visualization and measurement study have been carried out to investigate flow boiling instabilities of water in microchannels at various heat fluxes and mass fluxes. Two separate flow boiling experiments were conducted in eight parallel silicon microchannels (with flow interaction from neighboring channels at headers) and in a single microchannel (without flow interaction), respectively. These microchannels, at a length of 30 mm, had an identical trapezoidal cross-section with a hydraulic diameter of 186 μm. At a given heat flux and inlet water temperature, it was found that stable and unstable flow boiling regimes existed, depending on the mass flux. A flow boiling map, in terms of heat flux vs mass flux, showing stable flow boiling regime and unstable flow boiling regime is presented for parallel microchannels as well as for a single microchannel, respectively, at an inlet water temperature of 35 °C. In the stable flow boiling regime, isolated bubbles were generated and were pushed away by the incoming subcooled liquid. Two unstable flow boiling regimes, with long-period oscillation (more than 1 s) and short-period oscillation (less than 0.1 s) in temperature and pressure, were identified. The former was due to the expansion of vapor bubble from downstream while the latter was owing to the flow pattern transition from annular to mist flow. A comparison of results of flow boiling in parallel microchannels and in a single microchannel shows that flow interaction effects from neighboring channels at the headers are significant.     

Model for boiling explosion during rapid liquid heating

The process of rapid liquid heating with a linearly increasing boundary temperature condition has been simulated by applying the analytical solution of 1D semi-infinite heat conduction in association with the molecular theory of homogeneous nucleation boiling. A control volume having the size of a characteristic critical cluster at the liquid boundary is considered, and the corresponding energy balance equation is obtained by considering two parallel competing processes that take place inside the control volume, namely, transient external energy deposition and internal energy consumption due to bubble nucleation and subsequent growth. Depending on the instantaneous rate of external energy deposition and boiling heat consumption within the control volume, a particular state is defined as the boiling explosion condition in which bubble generation and growth cause the liquid sensible energy to decrease. The obtained results are presented in terms of the average liquid temperature rise within the control volume, maximum attainable liquid temperature before boiling explosion and the time required to achieve the condition of boiling explosion. The model is applied for the case of water heating at atmospheric pressure with initial and boundary conditions identical to those reported in the literature. Model predictions concerning boiling explosion are found to be in good agreement with the experimental observations. The boiling explosion condition as predicted by the present model is verified by comparing the heat flux across the liquid–vapor interface with the corresponding limit of maximum possible heat flux, q_max,max, at the time of boiling explosion. A comparative study between the actual heat flux and the limit of maximum heat flux, q_max,max, at the time of boiling explosion for different rates of boundary heating indicates that, with much higher boundary heating rates, it is possible to heat the liquid to a much higher temperature before theoretical instantaneous boiling explosion occurs.     

Macrolayer formation and mechanisms of nucleate boiling,critical heat flux,and transition boiling

The drying process of a macrolayer on a 15 mm diameter boiling surface was observed with high speed video in the region of nucleate and of transition boiling close to the critical heat flux (CHF). It was found that the macrolayer rests beneath a large vapor mass. It partially dries in nucleate boiling and completely dries in transition boiling at the detachment of the vapor mass. The macrolayer thickness at CHF and in transition boiling was determined on the basis of the energy balance relation proposed by Katto and Yokoya. The macrolayer thickness at low heat flux was obtained by decreasing CHF with downward-facing heating surfaces and agreed well with the correlation proposed previously by the present authors. The macrolayer thickness in transition boiling with a vertical surface also agrees fairly well with the correlation, when the heat flux at macrolayer formation, given on the nucleate boiling curve, is extrapolated to surface superheat of transition boiling and when the surface temperature at macrolayer formation is equal to a time-averaged value. © 1998 Scripta Technical, Heat Trans Jpn Res, 27(2): 155–168, 1998     

Vapor film collapse around hot liquid drops when an external pressure wave is supplied

An external pressure wave generated by a magnetic hammer is applied to single or multiple hot LiNO₃ drops which have been in stable film boiling in subcooled ethanol. The process of vapor film collapse triggered by the pressure wave is studied by photographic observations and pressure history measurements. The vapor film begins to destabilize about 0.1 ms after pressure wave arrival at the drop. About 1.0 to 1.5 ms later, the vapor film begins to collapse from the lower part. Collapse of the vapor film develops from a portion around the drop preceded by a complex destabilization process. The ripple waves generated from a portion in the initial stage are concluded to be due to a Rayleigh–Taylor instability. Generation of interface disturbance and that of tiny bubbles as well as that of hot liquid fragments are observed, though no penetration of cold liquid jets can be observed in these experimental conditions. © 2000 Scripta Technica, Heat Trans Asian Res, 29(6): 498–511, 2000     

Numerical simulation of the quenching process in liquid jet impingement

Direct numerical simulation is performed for quenching of a hot plate in liquid jet impingement. The flow and thermal characteristics associated with the quenching process, which includes film boiling in the fluid region as well as transient conduction in the solid region, are investigated by solving the conservation equations of mass, momentum and energy in the liquid, gas and solid phases. The liquid–vapor and liquid–air interfaces are tracked by the sharp-interface level-set method modified to treat the effect of phase change. The computations demonstrate that the boiling curve of wall heat flux versus temperature does not depend on the transient or steady-state heating conditions. The effects of initial solid temperature and solid properties on the quenching characteristics are quantified.     

A numerical investigation of the evaporation process of a liquid droplet impinging onto a hot substrate

A numerical investigation of the evaporation process of n-heptane and water liquid droplets impinging onto a hot substrate is presented. Three different temperatures are investigated, covering flow regimes below and above Leidenfrost temperature. The Navier–Stokes equations expressing the flow distribution of the liquid and gas phases, coupled with the Volume of Fluid Method (VOF) for tracking the liquid–gas interface, are solved numerically using the finite volume methodology. Both two-dimensional axisymmetric and fully three-dimensional domains are utilized. An evaporation model coupled with the VOF methodology predicts the vapor blanket height between the evaporating droplet and the substrate, for cases with substrate temperature above the Leidenfrost point, and the formation of vapor bubbles in the region of nucleate boiling regime. The results are compared with available experimental data indicating the outcome of the impingement and the droplet shape during the impingement process, while additional information for the droplet evaporation rate and the temperature and vapor concentration fields is provided by the computational model.     

Experiment and prediction of fire extinguishment with water mist in an enclosed room

The correlation between oxygen concentration and fire temperature when fire was extinguished with water mist was theoretically studied. The Semenov theory was applied to analyze the critical condition when fire was extinguished with water mist, from which the correlation could be obtained. The water mist experiments were carried out by varying the fire size, atomizer number, ceiling height, system pressure, and pre-burn time in an enclosed room. The oxygen concentration near the edge of the liquid pool and the fire temperature above the center of the liquid pool were measured. A comparison of the experimental data with the correlation was made under different conditions. The results showed that fire extinguishment was a stochastic process which could be affected by many factors. This theoretical model could predict the correlation between fire temperature and oxygen concentration when fire was extinguished with water mist in an enclosed room and it can also be treated as a critical condition for fire extinguishment.     

锅炉沸腾液体膨胀蒸汽爆炸_BLEVE_的小尺寸模拟试验

1前言在锅炉设计中,依照传统的设计思想,人们所专注的是在最经济的条件下使用什么材料、设计多厚,使结构在实际运行中能够保证安全,锅炉的安全与结构强度有绝对性的关联。很多人认为只要设备的强度设计足够即可安全的运行直至报废。但事实上,在长期使用下,由于设备老化和操作失     

Flow boiling of liquid nitrogen in micro-tubes: Part I – The onset of nucleate boiling, two-phase flow instability and two-phase flow pressure drop

This paper is the first portion of a two-part study concerning the flow boiling of liquid nitrogen in the micro-tubes with the diameters of 0.531, 0.834, 1.042 and 1.931 mm. The contents mainly include the onset of nucleate boiling (ONB), two-phase flow instability and two-phase flow pressure drop. At ONB, mass flux drops suddenly while pressure drop increases, and apparent wall temperature hysteresis in the range of 1.0–5.0 K occurs. Modified Thom model can predict the wall superheat and heat flux at ONB. Moreover, stable long-period (50–60 s) and large-amplitude oscillations of mass flux, pressure drop and wall temperatures are observed at ONB for the 1.042 and 1.931 mm micro-tubes. Block phenomenon at ONB is also observed in the cases of high mass flux. The regions for the oscillations, block and stable flow boiling are classified. A physical model of vapor patch coalesced at the outlet is proposed to explain the ONB oscillations and block. Vapor generation caused by the flash evaporation is so large that it should be taken into account to precisely depict the variation of mass quality along the micro-tube. The adiabatic and diabatic two-phase flow pressure drop characteristics in micro-tubes are investigated and compared with four models including homogeneous model and three classical separated flow models. Contrary to the conventional channels, homogeneous model yields better prediction than three separated flow models. It can be explained by the fact that the density ratio of liquid to vapor for nitrogen is comparatively small, and the liquid and vapor phases may mix well in micro-tube at high mass flux due to small viscosity of liquid nitrogen, which leads to a more homogeneous flow. Part II of this study will focus on the heat transfer characteristics and critical heat flux (CHF) of flow boiling of liquid nitrogen in micro-tubes.     

Visualization of a principle mechanism of critical heat flux in pool boiling

A new experimental work was made to discover a principle mechanism of the burnout in pool boiling. Here, we directly observed a liquid layer structure under a massive vapor clot and the liquid layer-related burnout phenomenon. Based on the present observations, we have made a visual model for the formation and dryout of a liquid film under its vapor environment. At the formation process, liquid is trapped in interleaved space between growing bubbles and surface and the liquid trapping continues between coalesced bubbles and surface. In the dryout process, we especially observed vapor “holes” made by spontaneous breakup of discrete nucleating bubbles inside a vapor clot. The burnout can be triggered by the evaporation of the liquid film region expanded from rims of the holes.     

Pool boiling heat transfer in binary mixtures of ammonia/water: Effect of heat of dilution and dissolution on heat transfer coefficient

Nucleate boiling heat transfer coefficients were measured on a horizontal heated wire during the pool boiling of non‐azeotropic mixtures of ammonia/water. The experiment was carried out at pressures of 0.4 and 0.7 MPa, at heat fluxes below 2.0 × 10⁶ W/m², and over a range of mass fraction. The heat transfer coefficients in the mixtures were smaller than those in single‐component substances. No existing correlation is found to predict boiling heat transfer coefficients over the range of mass fraction of interest. In the mixtures of the ammonia/water, the heats of dilution and dissolution were generated near a liquid surface while vapor with a rich concentration of ammonia was condensed and then was diffused into the bulk liquid; while in most other mixtures, little heat was generated during any dilution and dissolution. In relation to the heat generated, the effect of the heats of dilution and dissolution on pressure and temperature in a system (pressure vessel) is shown herein. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(4): 272–283, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10034     

Theoretical investigation on heat leakage distribution between vapor and liquid in liquid hydrogen tanks

As a key factor affecting thermal behaviors of liquid hydrogen (LH₂) tanks, heat leakage plays an important role in accurate prediction of pressure build-up for safe storage and transportation of LH₂. Uniform heat flux between vapor and liquid in LH₂ tanks is widely adopted as thermal boundary condition in predicting pressure build-up process. However, a distribution of heat flux between vapor and liquid was observed during the self-pressurization process in the experimental test. In light of this, an analytically theoretical model of revealing the energy exchange process among the vapor, liquid and inner wall is proposed to investigate the heat leakage distribution ratio (HDR) between vapor and liquid in LH₂ tanks. The feasibility of the model is validated by the experimental results from NASA. In the whole self-pressurization process of 25,000 s, HDR reduces from 0.803 to 0.235 under a liquid fill ratio of 90% and a total heat leakage of 71.3 W. The results show that the existence of inner wall and different thermal properties between the vapor and liquid make the heat leakage flux non-uniformly distributed into the vapor and liquid. And the geometric structure of tank, thermal properties and initial states of the vapor and liquid have a significant effect on HDR. When coupling the model with thermal multi-zone model, the relative error in pressure prediction is reduced by 61.8% against experimental results. Benefiting from the coupled model, the relative error in pressure prediction caused by the uniform heat flux boundary condition reduces from 90.16% to 8.15%. The present work establishes theoretical foundation on analyzing heat leakage distribution between the vapor and liquid for LH₂ tanks, and provides useful guidance on modifying boundary conditions in accurately predicting thermal behaviors of LH₂ tanks.     

Pressure recovery during pressure reduction experiment with large-scale liquid hydrogen tank

The world's first Liquefied Hydrogen (LH2) Carrier which will transport LH2 from Australia to Japan has been built. At Japanese port, reducing the tank pressure will be required for the safe tank operation. However, pressure reduction will cause flashing, leading to an excess of the venting capacity or liquid leakage. Hence, the purpose of this research is to clarify the pressure reduction rate and liquid behavior through experimental and numerical approaches respectively. Pressure reduction experiment under high liquid level condition was conducted by using 30 m³ tank filled with saturated LH2. The experiment showed that the pressure recovery occurred at the beginning of depressurization, and then the pressure decreased based on the equilibrium state. From numerical analysis by VOF based in-house CFD code, it was found that the pressure recovery was caused by the boiling delay, and the tank pressure followed the saturation pressure after the liquid fully stirred.     

Pool boiling heat transfer in binary mixtures of ammonia/water

Nucleate boiling heat transfer coefficients were measured during pool boiling of the mixtures of ammonia/water on a horizontal heated wire. The experiment was carried out at pressures of 0.4 and 0.7 MPa, at heat fluxes below 2000 kW/m² and over all ranges of fraction. The heat transfer coefficients in the mixtures are markedly less than those in single component substances and, in particular, are dramatically deteriorated in the vicinity of both single component substances. An applicability of existing correlations to the present experimental data is discussed. As a result, it is difficult for any existing correlation to predict the coefficients over all ranges of fraction.In the mixtures of ammonia/water, heat of dilution and of dissolution are generated near a vapor-liquid interface, while vapor with a richer concentration of ammonia is condensed and then diffused into a bulk liquid; while in most other mixtures, little heat is generated during any dilution and dissolution. The effect of the heat of dilution and of dissolution on pressure and temperature in a system (pressure vessel) is shown.     

Experimental research on hydrogen/air explosion inhibition by the ultrafine water mist

This experimental study focused on the inhibition of ultrafine water mist on hydrogen explosion inside the closed vessel. The inhibition law and mechanism were studied through changes of explosion intensity, flame propagation velocity and temperature under different mist concentrations. Results indicate that flame propagation and pressure rise inside the closed vessel were corresponding. Explosion intensity was reduced after adding mist, which was mainly manifested in the reductions of explosion pressure and flame propagation velocity. Flame was accelerated to extinguish and the inhibition effect was enhanced with increasing mist concentration. However, the explosion prussure did not present obvious reduction as the mist concentration reached a certain value. Besides, it indicates that the absoption heat effect of ultrafine water mist was an important factor on hydrogen explosion inhibition by the reductions of flame temperature and propagation velocity. The inhibition effect was mainly attributed to the combination effect of physical and chemical inhibitions.