Experimental study on condensation heat transfer of steam on vertical titanium plates with different surface energies

Visual experiments were employed to investigate heat transfer characteristics of steam on vertical titanium plates with/without surface modifications for different surface energies. Stable dropwise condensation and filmwise condensation were achieved on two surface modification titanium plates, respectively. Dropwise and rivulet filmwise co-existing condensation form of steam was observed on unmodified titanium surfaces. With increase in the surface subcooling, the ratio of area (η) covered by drops decreased and departure diameter of droplets increased, resulting in a decrease in condensation heat transfer coefficient. Condensation heat transfer coefficient decreased sharply with the values of η decreasing when the fraction of the surface area covered by drops was greater than that covered by rivulets. Otherwise, the value of η had little effect on the heat transfer performance. Based on the experimental phenomena observed, the heat flux through the surface was proposed to express as the sum of the heat flux through the dropwise region and rivulet filmwise region. The heat flux through the whole surface was the weighted mean value of the two regions mentioned above. The model presented explains the gradual change of heat transfer coefficient for transition condensation with the ratio of area covered by drops. The simulation results agreed well with the present experimental data when the subcooling temperature is lower than 10 °C.     

The slow rotation of an axisymmetric solid submerged in a fluid with a surfactant surface layer—II. The rotating solid in a bounded fluid

This paper extends the work in Shail (1979) on the problem of an axisymmetric submerged solid rotating slowly and steadily in a fluid whose surface is covered with a surfactant film. The bulk fluid is of finite extent, and both asymptotic and numerical results (the latter in the case of a thin circular disk) are given for boundary effects on the resistive torque and surface velocity profile when the container is a right circular cylinder and the fluid is of finite depth.     

Theoretical investigation of effects of local cooling of a nozzle divergent section for controlling condensation shock in a supersonic two-phase flow of steam

In supersonic adiabatic two-phase flows of steam, under the influence of supersonic acceleration, the fluid loses its equilibrium conditions and becomes supersaturated. Following this condition and to restore the fluid to equilibrium, micro droplets of water form in the absence of any surface or foreign particles. This phenomenon is called homogeneous nucleation and the formed minute small droplets grow along the fluid flow path. The formation of these droplets and their growth causes the release of the latent heat of evaporation to the gas phase particularly in the nucleation region, and results in an increase in the flow pressure which is called the condensation shock. In this paper, and in continuation of the series of papers by the authors, in addition to analytically solving the adiabatic gas-liquid supersonic flow of steam in a convergent-divergent channel, a novel solution to controlling the undesired effects of this pressure rise (condensation shock) is presented. In the proposed method, with the help of cooling the divergent section of the nozzle, the analytical model for the 1D non-adiabatic two-phase steam flows is further developed which shows considerable decrease in the intensity of the formed condensation shock. Also the growth rate of the formed droplets due to the cooling of the steam flow has higher importance than the nucleation itself.     

Determination of heat transfer coefficients in steam turbines

A method for the determination of heat transfer coefficients is described which is particularly suited for measurements inside steam turbine cylinders and buckets. The heat transfer coefficients are derived from the temperature frequency characteristics of the walls. Correlations are made between the time-dependent fluid temperature and the corresponding signal received by a detector just below the wall's surface. The natural stochastic temperature variation within a steam turbine can be used as test signal. Measurements in a 2 Megawatt steam turbine demonstrate, that the signal's power density is sufficiently high and the attenuation low within the important frequency range of 1/s ? ω? 10/s. The experimental verification of the method in a steam tunnel showed good results.     

Interaction phenomena between liquid droplets and hot particles—Captured via high-speed camera

The interaction of evaporating droplets and hot catalyst particles plays a major role in heterogeneously catalysed reactions. The liquid feed is injected into a gas–solid flow and is mixed with the catalyst. The interaction phenomena determine the evaporation time which should be minimised to keep the reactor vessel small. First measurements with a bed of fixed hot FCC-particles (fluid catalytic cracking) and two model fluids have been conducted. The interactions of ethanol and water droplets with the hot bed surface were captured via a high-speed camera. While the ethanol droplet developed a stable steam cushion due to Leidenfrost phenomena, water showed intense interaction and steam explosions which induced repulsion and atomisation of the droplet.     

Do we observe Gerstner waves in wave tank experiments?

We investigate theoretically the effects of viscosity and surface films on small-amplitude Gerstner waves in deep water. The analysis is performed by using a Lagrangian formulation of fluid motion. For inviscid fluids with a free surface Gerstner waves of arbitrary amplitude are exact solutions of the nonlinear Lagrangian equations. These waves have a trochoidal surface shape. They possess vorticity, but have no mean wave momentum, i.e. induce no net drift in the fluid. By expanding the wave motion after the wave steepness as a small parameter, we demonstrate how Gerstner waves to second order in wave steepness change due to viscosity, leading to a mean drift near the surface and a backward drift beneath the surface layer, so that they conserve total (zero) mean wave momentum. In addition, if the surface is covered by a freely floating inextensible film, the mean drift at the surface (the film speed) increases dramatically. A comparison with experimental data for the drift of thin plastic sheets in wave tanks is made, showing that the presence of viscosity-modified Gerstner waves cannot be ruled out on the basis of these observations.     

Transversal oscillations of a fluid in a long cylindrical container with membrane or elastic plate on the free surface

We propose approximate solutions of two-dimensional hydroelastic problems that describe free oscillations of an ideal fluid in a horizontal long cylindrical container with arbitrary symmetric cross section. The free surface of the fluid is covered by a plane membrane or an elastic plate. Using specific examples, we analyze the obtained solutions and the results of computation of frequencies and forms of oscillations of the mechanical system under consideration.     

Deformation by the external flow of the surface of a magnetic fluid covering a circular cylinder

As one of the methods of controlling separation of a flow and reducing hydrodynamic resistance in flow round the body, [1] proposes covering the surface of the body with a layer of magnetic fluid maintained by a inhombgeneous field. The effectiveness of the method is studied in the example of flow rounda circular cylinder covered with a uniform layer of a magnetic fluid the surface of which is also assumed to be cylindrical and not dependent on the external flow. The resistance of the cylinder falls (if the viscosity of the magnetic fluid is not too high) and can be greatly reduced. It is of interest to determine the nature of the deformation produced in the surface of the magnetic fluid by the external flow, since the degree to which the boundary can be deformed determines whether the proposed method can be realized in practice. Moreover, the magnitude of the deformation evidently affects the resistance of the cylinder and determines the limits of the applicability of the approximation adopted in [1] for the nondeformed state of the surface.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 28–31, January–February, 1988.     

Problem of Low-frequency Localized Oscillations in a Thin Film with Growing Islands

Influence of localization waves to islands growing on a thin film is investigated. The film is modelled as a fluid layer covered by an inertial surface with the variable density of mass and surface tension. Mathematically, the problem is reduces to analysis of a system of non-linear equations describing the growth of island nuclei and wave propagation in the films. The existence of trapped modes for the corresponding frequency-domain problem is established. We show that for large time wave localization near islands gives some contribution in the increase of the velocity of island growth.     

Transport of salts and micron-sized particles entrained from a boiling water pool

The entrainment of soluble (KI, CsI) and non-soluble (Al₂O₃) substances through droplets, which are produced by disintegrating steam bubbles at the surface of a boiling water pool, is determined in a pilot-scale facility. Integral measurements are conducted at steady-state conditions in an atmosphere of either pure steam or an air–steam mixture. The ratio of the entrained liquid mass flow and the gas mass flow through the pool, the entrainment factor, is determined for air–steam ratios between 0 and 0.47 kg/kg in the gas atmosphere and at constant total pressures between 2 and 6 bar. The influence of the vertical temperature profile in the gas atmosphere on the convective velocity field is demonstrated by phase Doppler anemometry and particle image velocimetry measurements at a location 2.1 m above the pool surface. The influences of nucleation and natural convection are demonstrated during slow de-pressurization of the facility at rates below 420 Pa/s.     

The expansion of wet steam through a compressible confined vortex in a fluidic vortex diode

The pressure-flow characteristics of a Zobel-type vortex diode have been measured using a working fluid of compressible wet steam. Tests using superheated steam with inlet/outlet pressure ratios across the diode of up to 30 have shown clearly the effects of compressibility and choking on the diode characteristics. Repeating the tests using wet steam, with known dryness fractions, has shown separately the effects of wetness on the diode performance.

When the diode was installed into the pipwork in the high-resistance direction, excessive steam wetness (quality <0.93) led to a build-up of water and when this was eventually swept through to the diode the resistance was seen to fall substantially as the strong internal vortex was destroyed.     

An improved three-dimensional model for interface pressure calculations in free-surface flows

A three-dimensional method for the calculation of interface pressure in the computational modeling of free surfaces and interfaces is developed. The methodology is based on the calculation of the pressure force at the interfacial cell faces and is mainly designed for volume of fluid (VOF) interface capturing approach. The pressure forces at the interfacial cell faces are calculated according to the pressure imposed by each fluid on the portion of the cell face that is occupied by that fluid. Special formulations for the pressure in the interfacial cells are derived for different orientations of an interface. The present method, referred to as pressure calculation based on the interface location (PCIL), is applied to both static and dynamic cases. First, a three-dimensional motionless drop of liquid in an initially stagnant fluid with no gravity force is simulated as the static case and then two different small air bubbles in water are simulated as dynamic cases. A two-fluid, piecewise linear interface calculation VOF method is used for numerical simulation of the interfacial flow. For the static case, both the continuum surface force (CSF) and the continuum surface stress (CSS) methods are used for surface tension calculations. A wide range of Ohnesorge numbers and density and viscosity ratios of the two fluids are tested. It is shown that the presence of spurious currents (artificial velocities present in case of considerable capillary forces) is mainly due to the inaccurate calculation of pressure forces in the interfacial computational cells. The PCIL model reduces the spurious currents up to more than two orders of magnitude for the cases tested.

Also for the dynamic bubble rise case, it is shown that using the numerical solver employed here, without PCIL, the magnitude of spurious currents is so high that it is not possible to simulate this type of surface tension dominated flows, while using PCIL, we are able to simulate bubble rise and obtain results in close agreement with the experimental data.     

Interaction of surface and flexural-gravity waves in ice cover with a vertical wall

The problem of the interaction of surface and flexural-gravity waves with a vertical barrier is solved in a two-dimensional formulation. It is assumed that the fluid is ideal and incompressible, has infinite depth, and is partially covered with ice. The ice cover is modeled by an elastic plate of constant thickness. The eigenfrequencies and eigenmodes of oscillation of the floating elastic ice plate, the deflection and deformation of ice, and the forces acting on the wall are determined.     

Uniform helical fluid motion in a cone with a diaphragm

The hydrocyclone model (uniform helical flow in a cone) assumed by the present author in [1] only approximately reflects the specific nature of the fluid flow within this device. In real hydrocyclones a small portion of the fluid, together with the solid particles thrown toward the wall, leave through the nozzle at the cone vertex, while the major portion of the fluid leaves the hydrocyclone through a cylindrical diaphragm inserted through the center of the base to a definite depth. With the aim of clarifying the effect of the diaphragm on the flow structure in the hydrocyclone, the present study solves the problem of uniform helical motion of a fluid in a cone with diaphragm.     

Free oscillations of a fluid in a cylindrical container with arbitrary axisymmetric bottom and elastic elements on the free surface of the fluid

We consider the problem of free oscillations of an ideal fluid in a container that has the form of a right circular cylinder with arbitrary axisymmetric bottom in the case where the unperturbed free surface of the fluid is covered by an elastic membrane or plate. Using the expansion in eigenfunctions of an auxiliary spectral problem with a parameter in boundary conditions and the method of decomposition of the domain of meridional cross-section of a container, we obtain an analytic solution of the problem. Individual examples of mechanical systems are considered, for which we construct solutions by using the proposed algorithm, analyze these solutions, and compute the frequencies and forms of oscillations.     

Nonlinear interaction of surface waves in a basin covered with broken ice

The nonlinear interaction of periodic traveling waves of the first and second harmonics in a constant-depth uniform fluid covered with broken ice is considered. Uniform asymptotic expansions up to third-order values for the velocity potential of the fluid and the elevation of the basin surface are found by means of the multivariable expansion procedure. The dependence of the wave perturbations on the thickness of the ice and the interacting-harmonic characteristics is analyzed. Sevastopol. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 4, pp. 136–143, July–August, 1994.     

Phase discontinuities in water flows through a porous medium

Flow of a fluid through a porous medium is considered with allowance for heat conduction processes and phase transitions. Discontinuities in flows between both single-phase zones saturated with water and steam and single-and two-phase zones saturated with an equilibrium steam-water mixture are studied. It is shown that only the evaporation fronts are evolutionary for a convex-downward shock adiabat of the discontinuity inside the steam-water mixture. The structure of these fronts is considered and a condition supplementary to the conservation laws and necessary for the well-posed formulation of problems whose solution contains this front is found from the condition of existence of a discontinuity structure between the water (steam) and the steam-water mixture.     

A study on hydroentangling waterjets and their impact forces

We study the possibility of obtaining an intense fluctuating force during the steady-state operation of a waterjet. In this paper, characteristics of waterjets in the so-called first wind-induced breakup regime are briefly explained and the impaction between such waterjets and a smooth flat plate is discussed. We consider these waterjets to consist of three different regions: (1) a continuous portion, (2) a discrete portion (a stream of drops) and (3) a spray region. Using fluid dynamics simulation we obtain an impulsive impact force for the discrete portion of these waterjets. The peak of this impulsive impact force is found to be 3.5–4 times greater than that of the continuous portion. We validate our simulations by conducting an experiment for a stream of large low-speed drops. The impact force of these drops is in good agreement with that of simulation.     

The Effect of Cracks and a Steam Cap on Hydrothermal Eruptions

The shock-tube model for a hydrothermal eruption in a geothermal reservoir (Fullard and Lynch, Trans Porous Med, 2011) is used to simulate eruptions that have a steam phase present near the surface in the form of a steam cap or a large crack. Simulations are performed with various steam cap/crack depths and it is shown that the presence of a steam phase greatly reduces the size of an eruption. We show that a steam cap type eruption is physically unlikely because of the large pressure differences required, but conclude that rock cracking is potentially a viable initiation mechanism for a hydrothermal eruption.     

Influence of nanoparticle surface coating on pool boiling

The purpose of this work is to study the effects of nanostructured surface coatings on boiling heat transfer and CHF. Boiling experiments are performed on a 100 μm diameter platinum wire immersed in saturated water or pentane at 1 bar. Nanostructured surface coating is obtained by deposition of charged γ-Fe₂O₃ nanoparticles (average diameter of 10 nm) on the platinum wire. Two different processes are compared: vigorous boiling and electrophoresis.The deposition of nanoparticles onto the heated surface induces a significant increase of the boiling critical heat flux (CHF) related to the increase of wettability. It also induces a decrease of the heat transfer coefficient when the wire is entirely covered with nanoparticles. The critical heat flux enhancement depends on the wettability of the fluid compared with the bare heater. Different physical mechanisms are also studied to explain the evolution of the characteristic parameters of the boiling on nanostructured surfaces.