A.E. Bergles

Bio: A.E. Bergles is an academic researcher from Iowa State University. The author has contributed to research in topics: Two-phase flow & Boiling. The author has an hindex of 1, co-authored 1 publications receiving 672 citations.

Review of two-phase flow instability

Abstract: Boiling flow in a water-cooled reactor, an evaporator, or an electronic cooling system is susceptible to thermal-hydrodynamic instabilities, which may cause flow oscillations of constant amplitude or diverging amplitude. These oscillations could induce boiling crisis, disturb control systems, or cause mechanical damage. This paper identifies the causes and mechanisms of these instabilities. Based on their mechanisms, various types of instabilities are classified and tabulated. The parametric effects on flow instability, observed experimentally, are systematically presented. Various analytical techniques for predicting the instability threshold are reviewed in terms of their applicability and accuracy.

Theories of immiscible and structured mixtures

Abstract: A survey is given of continuum theories that have been developed to model mixtures of immiscible constituents, such as bubbly liquids, fluid-particle mixtures, fluid saturated porous media, and composite materials. It is emphasized that the immiscibility of such mixtures has important consequences concerning the forms of the constitutive equations, and that it can also result in the mixtures exhibiting microstructural effects. Brief descriptions are first given of the classical theory of mixtures—in which the constituents are not endowed with microstructure—and of theories of single continua with microstructure. The simplest theories of immiscible mixtures, in which the volume fractions of the constituents are introduced as additional kinematical variables, are then discussed, including postulated theories as well as theories that have been derived through various averaging procedures. Theories of mixtures having greater microstructural content are described next. A concluding chapter discusses mixture theories that have been developed explicitly to model the behavior of composite materials.

A Review of two-phase flow dynamic instabilities in tube boiling systems

Abstract: The earliest research in the field of two-phase flow was conducted by Lorentz (1909) The studies on the analysis of two-phase flow instabilities by Ledinegg (1938) created considerable interest concerning the phenomenon of thermally induced flow instability in two-phase flow systems The objective of this review is to sum up the experimental and theoretical work carried out by various investigators over a period of several years, demonstrating and explaining three main instability modes of two-phase flow dynamic instabilities, namely, density-wave type, pressure-drop type and thermal oscillations, encountered in various boiling flow channel systems The typical experimental investigations of these instabilities in tube boiling systems are indicated and the most popular models to predict the two-phase flow dynamic instabilities, namely the homogenous flow model and the drift-flux models are clarified with the solution examples and the validation of the model results with experimental findings are also provided

Closed loop pulsating heat pipes: Part A: parametric experimental investigations

Abstract: Closed loop pulsating heat pipes (CLPHPs) are complex heat transfer devices having a strong thermo-hydrodynamic coupling governing the thermal performance. In this paper, a wide range of pulsating heat pipes is experimentally studied thereby providing vital information on the parameter dependency of their thermal performance. The influence characterization has been done for the variation of internal diameter, number of turns, working fluid and inclination angle (from vertical bottom heat mode to horizontal orientation mode) of the device. CLPHPs are made of copper tubes of internal diameters 2.0 and 1.0 mm, heated by constant temperature water bath and cooled by constant temperature water–ethylene glycol mixture (50% each by volume). The number of turns in the evaporator is varied from 5 to 23. The working fluids employed are water, ethanol and R-123. The results indicate a strong influence of gravity and number of turns on the performance. The thermophysical properties of working fluids affect the performance which also strongly depends on the boundary conditions of PHP operation. Part B of this paper, which deals with development of semi-empirical correlations to fit the data reported here coupled with some critical visualization results, will appear separately.

Effects of inlet/outlet configurations on flow boiling instability in parallel microchannels

Abstract: A simultaneous visualization and measurement study has been carried out to investigate effects of inlet/outlet configurations on flow boiling instabilities in parallel microchannels, having a length of 30 mm and a hydraulic diameter of 186 μm. Three types of inlet/outlet configurations were investigated. Fluid flow entering to and exiting from the microchannels with the Type-A connection was restricted because the inlet and outlet conduits were perpendicular to the microchannels. The fluid flow had no restriction in entering to and existing from the microchannels with the Type-B connection. In the Type-C connection, fluid flow was restricted in entering each microchannel but was not restricted in exiting from the microchannels. It is found that amplitudes of temperature and pressure oscillations in the Type-B connection are much smaller than those in the Type-A connection under the same heat flux and mass flux conditions. On the other hand, nearly steady flow boiling exists in the parallel microchannels with the Type-C connection under the experimental conditions. Therefore, this configuration is recommended for high-heat-flux microchannel applications. As predicted, the stability threshold is determined by the minimum in the pressure-drop-versus-flow-rate curve. The pressure drop and heat transfer coefficient versus vapor quality for flow boiling in microchannels with the Type-C connection are presented. It is found that experimental data of pressure drop are higher and heat transfer coefficients are lower for boiling flow at high vapor quality in microchannels than those predicted from correlation equations for boiling flow in macrochannels, due to local dryout.

Two-phase flow instabilities: A review

Abstract: An updated review of two-phase flow instabilities including experimental and analytical results regarding density-wave and pressure-drop oscillations, as well as Ledinegg excursions, is presented. The latest findings about the main mechanisms involved in the occurrence of these phenomena are introduced. This work complements previous reviews, putting all two-phase flow instabilities in the same context and updating the information including coherently the data accumulated in recent years. The review is concluded with a discussion of the current research state and recommendations for future works.