Visualization of capillary boiling

Capillary boiling has been experimentally analyzed using an optical technique, based on the extraction of gray levels at specific locations from video frames. Boiling is achieved by placing a small capillary in a large container filled with water at boiling temperature. A heating wire located inside the capillary provides the additional heat that triggers the evaporation. The phenomenon is videotaped and digitized. A small region of the filmed field is chosen and the gray level of all the digitized frames is stored in a file which is then analyzed. This constitutes in essence the possibility of using a large number of non-intrusive, fixed (Eulerian) virtual sensors. The information extracted concerns the global properties of the phenomenon such as the time between the departure of two subsequent bubbles, and more detailed properties such as the shape and volume of the departing bubbles or even the motion of liquid packets inside the capillary. Moreover, this technique permits the study of time and space correlations. It is observed that the bubbles depart from the capillary tip with a variable frequency that can be approximately grouped around two values. Clear evidence that this is due to two different mechanisms is given. The time between the departure of two bubbles is correlated with the volume of the departing bubbles. Received: 21 April 1999/Accepted: 18 July 2000     

Dynamic characteristics of microscale boiling

Microscale boiling displays some quite unusual characteristics, like the occurrence of the so-called “fictitious boiling”. This paper conducted the thermodynamic and dynamic analyses regarding the phase change and bubble nucleation in microchannels. The role of perturbations on the dynamics of bubble embryos was investigated, whence the possible causes corresponding to the depression of bubble nucleation in microscale boiling were proposed through stochastic analysis. The external perturbations produced by pressure impulses accompanied with embryo growth and collapse could markedly alter the dynamic characteristics of bubble growth. Received on 27 March 2000     

Internal characteristics of hydrogen boiling

Study of the internal characteristics of boiling is of great importance in understanding the physics of this complex process; data on internal characteristics may be used for derivation of formulas for integral boiling characteristics (heat-liberation coefficients, critical thermal flux densities). Only a limited number of works have been dedicated to study of boiling of cryogenic liquids over a wide saturation pressure range. In [1] the internal characteristics of nitrogen boiling were studied at pressures from 0.1 to 8 bar, while oxygen was studied from 0.22 to 2 bar. In [2] detachment radius and bubble-detachment frequency were determined for hydrogen boiling over the pressure range 1–11 bar. The present study is dedicated to an examination of internal characteristics of hydrogen boiling over the saturation pressure range from triple point pressure (0.072 bar) to 2.0 bar.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 4, pp. 103–108, July–August, 1976.     

Inverted annular flow boiling

The convective heat-transfer coefficients of inverted annular flow film boiling in the region immediately downstream of the rewetting front in the bottom flooding mode were calculated and were found to agree very well with the experimental values. Also, the results of the analysis reconfirmed that having more accurate heat-transfer coefficients in a confined space immediately downstream of the rewetting front substantially improves the accuracy of the estimate for rewetting velocities.     

Pool boiling of drag-reducing polymer solutions

The effect of drag-reducing polymeric additives on the critical heat flux and minimum film boiling temperature in subcooled pool boiling of water has been experimentally examined. Three water-soluble polymers, viz. a polyethylene oxide (Aldrich No. 18946-4), a polyacrylamide (Separan AP-30), and a galactomannan polysaccharide (Galactasol-211) have been examined at concentrations of 10, 50, and 100 wppm. The experiment is performed by quenching a hot brass sphere in an isothermal pool of the fluid to be examined and obtaining the corresponding boiling curve. The experiments have been conducted at atmospheric pressure with a pool temperature of 90°C. The results show that the critical heat flux increases by more than 50% while the minimum film boiling temperature increases by more than 110°C when concentrations of 100 wppm of polyox or guar gum are used. For separan solutions, the critical heat flux and minimum film boiling temperature are lower than those for water and go through a minimum at a concentration of ～10 wppm; the maximum reductions are about 73% and 34°C respectively.     

Explosive boiling of a liquid droplet

A mathematical model describing growth of an internal vapor bubble produced by homogeneous nucleation within a liquid droplet during explosive boiling is presented. Existing experimental results for explosive boiling of superheated droplets confirm the predictions of the model. The difference between the present model and the classical theories of bubble growth is discussed.     

Transportphenomena in subcooled flow boiling

Subcooled boiling has been subject of extensive research studies in the literature. So far, the approach of studying the phenomena of subcooled boiling has rather been an integral method by measuring the heat transport from the wall to the liquid and by following the growth and collapse of the bubbles. However, little is known about the heat transport at the phase interface between the surface of the bubbles and the subcooled liquid. Experimental attempts to study the transport phenomena around the bubble surface quantitatively have been performed by using the holographic interferometry, an optical method, which works in an inertialess and non-invasive way. The conventional holographic interferometry has somewhat been modified by applying the finite fringe method. With this technique interesting insights could be gained and precise quantitative data could be evaluated.     

Unsteady waves in boiling solutions

The evolution of shock waves and pulses in vapor-liquid binary media with a bubble structure is considered. A number of binary systems are calculated. It is established that in some cases the wave profiles in binary systems do not lie between the corresponding limiting profiles calculated for the boiling single-component systems. It is found that this effect is associated with the diffusional retardation of the phase transitions in binary media.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 97–101, July–August, 1990.     

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.     

Cinemicrophotographic study of boiling of water-in-oil emulsions

Cinemicrophotography is applied to the boiling of emulsions of water dispersed in oils whose boiling points are higher than that of water. The layer of emulsion is made thin enough to make possible a microscopic observation by transmitted light through the layer. Two alternative processes of bubble formation are found: a periodical bubble formation at specified sites on the heated solid surface contacting the emulsion and a casual bubble formation some distance away from the surface. The basic mechanisms of those processes are discussed.     

Stability of nucleation sites in pool boiling

A study was carried out to observe bubble nucleation and site deactivation mechanisms under equilibrium pool boiling of liquids on single sites. The experiments were conducted with benzene, ethanol, and their mixtures on single sites made of glass. These mechanisms were filmed using a cine camera as the temperature of the boiling liquid was decreased in programmed steps. The experiments showed that the deactivation of a bubble nucleation site in the surface occurred by progressive condensation of vapor within it until the volume of vapor became equal to or smaller than the volume of a spherical bubble of diameter equal to that of the site. Based upon these experimental observations, a site deactivation mechanism is proposed for heterogeneous pool boiling and stability criteria are developed for single cylindrical cavities. The effect of relevant parameters on depth-to-diameter ratio of the sites is also determined.     

Visualization of pool boiling on enhanced surfaces

This work reports experiments to visualize nucleate boiling on an enhanced tubular surface having sub-surface tunnels and surface pores. The finned copper tube had 1575 fins/m (40 fins/in.) and 0.8 mm fin height. The fins are covered by a thin foil sheet having 0.23 mm pores at 1.5 mm pore pitch along each interfin region. Data are provided for two foil cover sheets, one copper and the other a transparent plastic. The uniqueness of this work is that the visualization method allowed direct observation of the boiling process in the subsurface tunnels. Use of a high speed camera with 30 × magnification allowed detailed observation of the evaporation process in the tunnels and of the vapor bubbles emerging from the pores. The experiments were conducted for saturated and subcooled boiling in the horizontal and vertical orientations. For the vertical tube, the saturated boiling experiments showed that all of the tunnels were vapor filled except for liquid menisci in the corners. This was also true for the horizontal tube at high heat flux. For the horizontal tube at low heat flux, portions of the tunnel length was liquid filled. A large portion (70–90%) of the region was vapor filled except for liquid menisci in the corners, and the remaining part of the region had oscillating menisci. These experiments provide conclusive proof that the heat transfer mechanism in the subsurface tunnels is evaporation on the menisci in the corners.     

Difficulties in modeling dispersed-flow film boiling

Dispersed Flow Film Boiling (DFFB) is characterized by important departures from thermal and velocity equilibrium that make it suitable for modeling with two-fluid models. The fundamental limitations and difficulties imposed by the one-dimensional nature of these models are extensively discussed. The validity of the assumptions and empirical laws used to close the system of conservation equations is critically reviewed, in light of the multidimensional aspects of the problem. Modifications that could improve the physics of the models are identified.Nebelkühlung (DFFB) ist gekennzeichnet durch eine signifikante Abweichung vom thermischen Gleichgewicht und von der homogenen Strömung, wodurch eine Modellierung mit Zweifluidmodell ermöglicht wird. Die grundsätzlichen Beschränkungen und Schwierigkeiten, die sich aus der eindimensionalen Charakteristik dieser Modelle ergeben, werden eingehend diskutiert. Die Gültigkeit der Annahmen und die empirischen Gesetze, die für die Erhaltungsgleichungen verwendet werden, werden unter dem Gesichtspunkt der Multidimensionalität des Problems kritisch besprochen. Änderungen der Modelle die die physikalische Repräsentation verbessern könnten, werden aufgezeigt.Dedicated to Prof. Dr.-Ing. F. Mayinger's 60th birthday     

Explosive boiling of water in parallel micro-channels

The objective of this study is to visualize the flow pattern and to measure heat transfer coefficient during explosive boiling of water in parallel triangular micro-channels. Tests were performed in the range of inlet Reynolds number 25–60, mass flux 95–340 kg/m²s, and heat flux 80–330 kW/m².The flow visualization showed that the behavior of long vapor bubbles, occurring in a micro-channel at low Reynolds numbers, was not similar to annular flow with interposed intermitted slugs of liquid between two long vapor trains. This process may be regarded as explosive boiling with periodic wetting and dryout.In the presence of two-phase liquid–vapor flow in the micro-channel, there are pressure drop oscillations, which increase with increasing vapor quality.This study shows strong dependence of the heat transfer coefficient on the vapor quality. The time when liquid wets the heated surface decreases with increasing heat flux. Dryout occurs immediately after venting of the elongated bubble.