This study describes the development and validation of Computational Fluid Dynamics (CFD) methodology for the simulation of dispersed two-phase flows. A two-fluid (Euler-Euler) methodology previously developed at Imperial College is adapted to high phase fractions. It employs averaged mass and momentum conservation equations to describe the time-dependent motion of both phases and, due to the averaging process, requires additional models for the inter-phase momentum transfer and turbulence for closure. The continuous phase turbulence is represented using a two-equation k − ε−turbulence model which contains additional terms to account for the effects of the dispersed on the continuous phase turbulence. The Reynolds stresses of the dispersed phase are calculated by relating them to those of the continuous phase through a turbulence response function. The inter-phase momentum transfer is determined from the instantaneous forces acting on the dispersed phase, comprising drag, lift and virtual mass. These forces are phase fraction dependent and in this work revised modelling is put forward in order to capture the phase fraction dependency of drag and lift. Furthermore, a correlation for the effect of the phase fraction on the turbulence response function is proposed. The revised modelling is based on an extensive survey of the existing literature. The conservation equations are discretised using the finite-volume method and solved in a solution procedure, which is loosely based on the PISO algorithm, adapted to the solution of the two-fluid model. Special techniques are employed to ensure the stability of the procedure when the phase fraction is high or changing rapidely. Finally, assessment of the methodology is made with reference to experimental data for gas-liquid bubbly flow in a sudden enlargement of a circular pipe and in a plane mixing layer. Additionally, Direct Numerical Simulations (DNS) are performed using an interface-capturing methodology in order to gain insight into the dynamics of free rising bubbles, with a view towards use in the longer term as an aid in the development of inter-phase momentum transfer models for the two-fluid methodology. The direct numerical simulation employs the mass and momentum conservation equations in their unaveraged form and the topology of the interface between the two phases is determined as part of the solution. A novel solution procedure, similar to that used for the two-fluid model, is used for the interface-capturing methodology, which allows calculation of air bubbles in water. Two situations are investigated: bubbles rising in a stagnant liquid and in a shear flow. Again, experimental data are used to verify the computational results.

Computational fluid dynamics of dispersed two-phase flows at high phase fractions

Some characteristics of air-water two-phase flow in small diameter vertical tubes

This paper explores the recent research developments in high-heat-flux thermal management. Cooling schemes such as pool boiling, detachable heat sinks, channel flow boiling, microchannel and mini-channel heat sinks, jet-impingement, and sprays, are discussed and compared relative to heat dissipation potential, reliability, and packaging concerns. It is demonstrated that, while different cooling options can be tailored to the specific needs of individual applications, system considerations always play a paramount role in determining the most suitable cooling scheme. It is also shown that extensive fundamental electronic cooling knowledge has been amassed over the past two decades. Yet there is now a growing need for hardware innovations rather than perturbations to those fundamental studies. An example of these innovations is the cooling of military avionics, where research findings from the electronic cooling literature have made possible the development of a new generation of cooling hardware which promise order of magnitude increases in heat dissipation compared to today's cutting edge avionics cooling schemes.

Assessment of high-heat-flux thermal management schemes

One-dimensional drift-flux model and constitutive equations for relative motion between phases in various two-phase flow regimes

Numerical flow simulation utilising a full multiphase model is impractical for a suspension possessing wide distributions in the particle size or density. Various approximations are usually made to simplify the computational task. In the simplest approach, the suspension is represented by a homogeneous single-phase system and the influence of the particles is taken into account in the values of the physical properties. This study concentrates on the derivation and closing of the model equations. The validity of the mixture model is also carefully analysed. Starting from the continuity and momentum equations written for each phase in a multiphase system, the field equations for the mixture are derived. The mixture equations largely resemble those for a single-phase flow but are represented in terms of the mixture density and velocity. The volume fraction for each dispersed phase is solved from a phase continuity equation. Various approaches applied in closing the mixture model equations are reviewed. An algebraic equation is derived for the velocity of a dispersed phase relative to the continuous phase. Simplifications made in calculating the relative velocity restrict the applicability of the mixture model to cases in which the particles reach the terminal velocity in a short time period compared to the characteristic time scale of the flow of the mixture. (75 refs.)

On the mixture model for multiphase flow

The validity of a fast neutron radiography quantification method, the Σ-scaling method, which was originally proposed for thermal neutron radiography was examined with Monte Carlo calculations and experiments conducted at the YAYOI fast neutron source reactor. Water and copper were selected as comparative samples for a thermal neutron radiography case and a dense object, respectively. Although different characteristics on effective macroscopic cross-sections were implied by the simulation, the Σ-scaled experimental results with the fission neutron spectrum cross-sections were well fitted to the measurements for both the water and copper samples. This indicates that the Σ-scaling method could be successfully adopted for quantitative measurements in fast neutron radiography.

An improved fast neutron radiography quantitative measurement method

To delineate the effect of Radiation Induced Surface Activity (RISA) on boiling phenomenon, surface wettability in high-temperature environment or Leidenfrost condition and critical heat flux (CHF) of oxide metals irradiated by gamma rays were investigated. When the temperature of the heating surface reaches the wetting limit temperature, water-solid contact vanishes because of a stable vapor film between the droplet and the metal surface, i.e., a Leidenfrost condition. The wetting limit temperature increased with integrated irradiation dose. The CHF of oxidized titanium was improved up to 100% after 800 kGy 60 Co gamma ray irradiated. Radiation Induced Boiling Enhancement (RIBE) phenomenon was firstly confirmed through the experiments.Copyright © 2002 by ASME

Radiation Induced Surface Activity Phenomenon: 2nd Report — Radiation Induced Boiling Enhancement

Critical Heat Flux (CHF) in a non-uniformly heated tube is a very important designing factor of actual boiling two-phase flow equipments. In this series of studies, the CHF in a non-uniformly heated tube was discussed by using the re-distribution of a liquid film caused by the wave interaction. In the present investigation, in order to estimate the influence of the entrainment of droplet under the high heat flux condition on the re-distribution of liquid film, the CHF experiment was conducted. The experimental apparatus was a similar to the forced convective boiling system described in the previous paper, i.e. the eccentric tube with 20 mm I.D., 24 mm O.D., but heated length was just only 450 mm to keep the high heat flux condition. The experimental result expressed the influence of the magnitude of the heat flux as the increasing in the entrainment. The CHF was occurred at the high quality condition, i.e. liquid film dryout and at the low quality condition, i.e. departure from nucleate boiling (DNB), respectively. This obtained CHF was evaluated by using the film flow model, and the empirical correlation of the spreading coefficient of liquid film re-distribution was proposed.

https://www.jstage.jst.go.jp/article/kikaib/77/778/77_778_1385/_pdf

CHF in a Circumferentially Non-Uniformly Heated Tube under Low Pressure and Low Mass Flux Condition (Influence of the Magnitude of Heat Flux)

Effect of Surface Wettability Caused by Radiation Induced Surface Activation on Leidenfrost Condition

Heat transfer for flow boiling of water and critical heat flux (CHF) experiments in a half-circumferentially heated round tube under low-pressure conditions were carried out. To clarify the flow patterns in the heated section, experiments in the round tube under the same conditions were also carried out, and their results were compared. The experiments were conducted with atmospheric-pressure water in test sections with inner diameter D = 6 mm, heated length L = 360 mm, inlet water subcooling ΔTin = 80 K, and mass velocity G from 0 to 2000 kg/(m2·s) for the half-circumferentially heated round tube and from 0 to 7000 kg/(m2·s) for the full-circumferentially heated tube. The experimental data demonstrated that the wall temperature near the outlet of the half-circumferentially heated tube remained almost the same until CHF. It was found that burnout occurred when the flow regime changed from churn flow to annular flow, and the liquid film on the heated wall dried out although liquid film on the unheated wall remained. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(3): 149–164, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10022

Kaichiro Mishima

Papers

An improved fast neutron radiography quantitative measurement method

Radiation Induced Surface Activity Phenomenon: 2nd Report — Radiation Induced Boiling Enhancement

CHF in a Circumferentially Non-Uniformly Heated Tube under Low Pressure and Low Mass Flux Condition (Influence of the Magnitude of Heat Flux)

Effect of Surface Wettability Caused by Radiation Induced Surface Activation on Leidenfrost Condition

Heat transfer for flow boiling of water and critical heat flux in a half‐heated round tube under low‐pressure conditions