Evaporative instability at the superheat limit

The explosive vaporization of a single bubble inside a droplet of butane heated to the limit of superheat has been investigated experimentally using short-exposure photographs and fast-response pressure measurements. An interfacial instability driven by rapid evaporation has been observed on the surface of the bubbles. It is proposed that the Landau mechanism of instability, originally described in connection with the instability of laminar flames, also applies to rapid evaporation at the superheat limit. Calculations suggest that other technically important fluids may be even more unstable when boiling at the superheat limit. The rate of evaporation after the onset of instability is estimated from the experimental measurements to be two orders of magnitude greater than would be predicted by conventional bubble-growth theories that do not account for the effects of instability. An estimate of the mean density within the bubbles during the evaporative stage indicates that it is nearly equal to the critical density of butane.     

Liquid superheat during nonequilibrium boiling

Heat and mass transfer processes in a pure liquid subject to intense heating is investigated. The temperature escalation rate in a heated pure liquid is controlled by two competing processes; the external power deposition and the rate of nuclei formation and growth in the liquid, which acts as a heat sink. A heat balance equation is developed and solved numerically to yield the liquid temperature curve and the evaporation rate up to the maximum attainable superheat point. The effect of heating rate on the liquid temperature curve is quantified.     

Shape of the vapor bubble upon explosive boiling

A relation for the shape of a vapor bubble forming during propagation of a vaporization front is proposed. Kutateladze Institute of Thermal Physics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 41, No. 2, pp. 122–123, March–April, 2000.     

Microscope activation near the wall during explosive boiling

The inception process of nucleation in explosive boiling systems is theoretically investigated. With the effect of pulse heating or sudden cooling, the temperature distribution near the surface during explosive boiling is calculated. The liquid near the wall can maintain a stable layer induced by strong attractive force, and there exists maximum supersaturation beyond this stable layer. As the surface temperature and temperature gradient are high enough, the critical distance of maximum supersaturation can be larger than the radius of critical bubble, and the homogeneous nucleation will dominate the inception boiling process. For explosive boiling induced by pulse heating, homogeneous nucleation forms after a short time; while homogeneous nucleation can dominate the initial explosive boiling induced by sudden cooling.     

Microscopic explosive boiling induced by a pulsed-laser irradiation

This paper presents an experimental study of microscopic explosive boiling introduced by a pulsed laser. The violent explosive boiling was observed in the liquid film, and the vapor bubbles together with liquid droplets were expelled from the platinum film. It is found that the apparent bubble nucleation temperature is a strong function of the heating rate. The pressure signal appears as continuous oscillation and is intensified as laser power density increases.     

Kinetic theory analysis of explosive boiling of a liquid droplet

The process of rapid phase transition from highly superheated liquid to vapor is frequently so fast and violent that it is called explosive boiling. The paper uses the kinetic theory of evaporation to study growth of an internal vapor bubble produced by homogeneous nucleation within a highly superheated liquid droplet boiling explosively in a hot medium. Evaporation/condensation coefficient is estimated by comparing the predictions of the theory with available experimental data. We show that the value of the evaporation coefficient can be very low for high reduced temperatures (0.06 for butane at 378 K), in agreement with recent molecular dynamic simulations.     

Mechanism of the initial stage of bubble growth in a liquid close to the superheat limit

Institute of Thermophysics, Russian Academy of Sciences, Novosibirsk 630090. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 36, No. 3, pp. 130–133, May–June, 1995.     

Specific features of explosive boiling of liquids on a film microheater

Explosive boiling of liquids on film heaters under the action of pulsed heat fluxes q = 10⁸–10⁹ W/m ² is considered. A technique of stroboscopic visualization of boiling stages with a time resolution of 100 nsec is used. Numerous scenarios of evolution of explosive boiling are demonstrated. Conditions of the thermal effect (magnitude of the heat flux, duration and repetition frequency of heat pulses) are found, which ensure single and repeated boiling, intermittent boiling, and boiling with formation of complicated multi-bubble structures. It is noted that homogeneous nucleation is a dominating mechanism of incipience of examined liquids for q > 10⁸ W/m ². __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 2, pp. 81–89, March–April, 2007.     

Initiation of explosive boiling of a droplet with a shock wave

The role of incident shock waves in the initiation of vapor explosions in volatile liquid hydrocarbons has been investigated. Experiments were carried out on single droplets (1–2 mm diameter) immersed in a host fluid and heated to temperatures at or near the limit of superheat. Shocks generated by spark discharge were directed at previously nonevaporating drops as well as at drops boiling stably at high pressure. Explosive boiling is triggered in previously nonevaporating drops only if the drop temperature is above a threshold temperature that is near the superheat limit. Interaction of a shock with a stably boiling drop immediately causes a transition to violent unstable boiling in which fine droplets are torn from the evaporating interface, generating a two-phase flow downstream. On the previously nonevaporating interface between the drop and the host liquid, multiple nucleation sites appear which grow rapidly and coalesce. Overpressures generated in the surrounding fluid during bubble collapse may reach values on the same level as the pressure jump across the shock wave that initiated the explosive boiling. A simple calculation is given, which suggests that shock focusing may influence the location at which unstable boiling is initiated.     

Surface geometry of a liquid explosive burning beyond the stability limit

After L. D. Landau's work [1] on the stability of normal burning of liquid explosives, many experimental studies of this phenomenon, e. g., [2, 3], were published. In this paper, on the basis of Landau's theory, we investigate the geometry of the perturbations which develop on the surface of the liquid explosive in a vessel of circular cross section and consider the influence of vessel diameter on burning stability. Results of an experimental observation of the geometry of the liquid surface are also presented for developed turbulent combustion in a circular cylindrical tube. The calculation was based on the usual assumptions that the chemical reaction of combustion proceeds in a thin layer of vapor over the plane (meniscus neglected) surface of the inviscid liquid explosive.     

Dynamics of a system exhibiting the global bifurcation of a limit cycle at infinity

This paper concerns the dynamics of a class of non-linear oscillators of the form:

$\text{x″ + x ? ?x′(1?ax}{}^2\text{?bx′}{}^2\text{) = 0}$

. The non-linear term contains two parameters a and b which may be varied to give the Rayleigh and Van der Pol differential equations as special cases.The existence and approximation of limit cycles in this system are investigated using the Poincare-Bendixson theorem and the Lindstedt perturbation method. Analysis of the system at infinity is used to study the global bifurcation through which the limit cycle is created from four saddle-saddle connections between equilibrium points at infinity. Center manifold theory is used to determine the stability of the equilibrium points at infinity. Numerical integration is used to verify the analytical results.It is shown that an arbitrarily small perturbation to the damping term of the Rayleigh equation results in points close to the stable limit cycle escaping to infinity.     

Carbon dioxide flow boiling in a single microchannel - Part I: Pressure drops

Carbon dioxide two-phase flow pressure drops have been investigated in a single horizontal stainless-steel micro-tube having a 0.529 mm inner diameter. Experiments were carried out in adiabatic conditions for four saturation temperatures of −10; −5; 0; 5 °C and mass fluxes ranging from 200 to 1400 kg/m² s, for inlet qualities up to unity. Measurements have been compared to the predictions of well-known methods. The Müller-Steinhagen and Heck correlation and the Friedel correlation gave the best fit as well as the homogeneous model when the liquid viscosity is used to represent the apparent two-phase viscosity. Further, an analysis based on the homogeneous model has not shown any clear appearance of the laminar or the transition regimes in any given range of Reynolds number. The apparent viscosity of the two-phase mixture was found larger than the liquid viscosity at low vapour qualities, namely at the lowest temperatures. Hence, a new expression to determine the equivalent viscosity was suggested as a function of the reduced pressure. Lastly, the Chisholm parameter from the Lockhart-Martinelli correlation was found lower than expected and also mainly dependent on the saturation temperature.     

Dynamics and rheology of Janus drops in a steady shear flow

The behavior and rheology of a dispersion of Janus drops (or Janus emulsion) under a steady shear flow are explored in the infinite dilution limit. To achieve analytical progress, the Janus drops are assumed to consist of a pair of fluids bounded to hemispherical domains of equal radii. At ‘freely’ suspended conditions the Janus drops undergo periodic orbits in a shear flow that are intermediate to that of a solid sphere and a disk that depend on the viscosities of the internal fluids. Non-Newtonian behavior is found for this system on account of the anisotropic hydrodynamics of the Janus drops. The viscosity of the Janus emulsion that corresponds to the minimum energy of dissipation is analogous to that derived by Taylor (1932) for a dispersion of simple drops. It is also found that an external force can induce the Janus drops to adopt a preferential orientation in a shear flow. Interestingly, a neutrally buoyant Janus drop with a displaced center of gravity can migrate lateral to the undisturbed shear flow; it is inferred that this phenomenon can lead to spatial-dependent rheology in pressure-driven flows.     

On the departure behaviors of bubble at nucleate pool boiling

Dimensionless scales of radius and time, proposed by the authors in a previous study, were used to quantitatively analyze the bubble departure radius and time during nucleate pool boiling. The results obtained from dimensional analysis were compared with experimental data reported in many studies. These experimental data are including partial nucleate pool boiling data with constant heat flux and temperature conditions acquired over the past 40 years at atmospheric and sub-atmospheric pressures, as well as data obtained at subcooled, saturated, and superheated pool temperature conditions.It was shown that the departure radius and time could be well correlated with respect to Jakob number as proposed by the previous studies. And the bubble departure behaviors well categorized between atmospheric and sub-atmospheric pressure, which is occurred from the different growth rate near the departure time partial nucleate pool boiling.For almost all obtained under atmospheric pressure, the dimensionless departure radius and time scales were about 25 and 60, respectively. For higher Jakob number, the square root of Bond number was proportional to the power of 0.7 of Jakob number, little different from the previous correlations. The dimensional departure radius and time estimated from the relationships proposed in this study were compared with measured departure scales and the results obtained with the previous correlations. And it was shown that the relationships could well predict and describe the departure behaviors of bubble during nucleate pool boiling.     

Mechanism of wave formation at the interface in explosive welding

Some of the main progress on the investigation of the mechanism of the wave formation in explosive welding at the Institute of Mechanics is summarized and others' previous works are reviewed. Our systematic experiments and analysis do not substantiate the theory of wave formation based on Karman vortex-street analogy or Helmholtz instability. On the contrary, they show that material strength insensitive to strain rate plays an important role. A simple hydro-plastic model is presented to explain the main features regarding the interfacial wave formation and to estimate the magnitude of wave length. The result is in broad agreement with experiment.     

Frequency at which vapor bubbles form during boiling

An expression is found for the frequency at which vapor bubbles form; the familiar empirical dependences follow from this expression as particular cases. The theoretical results are in satisfactory agreement with experiment.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 143–146, September–October, 1970.