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 mean droplet size and size distribution are important for detailed mechanistic modeling of annular two-phase flow. A large number of experimental data indicate that the standard Weber number criterion based on the relative velocity between droplets and gas flow predicts far too large droplet sizes. Therefore, it was postulated that the majority of the droplets were generated at the time of entrainment and the size distribution was the direct reflection of the droplet entrainment mechanism based on roll-wave shearing off. A detailed model of the droplet size in annular flow was then developed based on the above assumption. The correlations for the volume mean diameter as well as the size distribution were obtained in collaboration with a large number of experimental data. A comparison with experimental data indicated that indeed the postulated mechanism has been the dominant factor in determining the drop size. Furthermore, a large number of data can be successfully correlated by the present model. These correlations can supply accurate information on droplet size in annular flow which has not been available previously.

Generation and Size Distribution of Droplet in Annular Two-Phase Flow

An evaluation of prediction methods for saturated flow boiling heat transfer in mini-channels

Correlation of critical heat flux for flow boiling of water in mini-channels

https://catatanstudi.files.wordpress.com/2009/11/2001-flow-regime-transition-criteria-for-upward-2-phase-flow-in-vertical-narrow-reactangular-channels_hibiki.pdf

Flow regime transition criteria for upward two-phase flow in vertical narrow rectangular channels

At the Kyoto University Research Reactor Institute, the world’s first injection of spallation neutrons generated by high-energy proton beams into a reactor core was successfully accomplished on March 4, 2009. By combining the fixed field alternating gradient (FFAG) accelerator with the A-core (Fig. 1) of the Kyoto University Critical Assembly (KUCA), a series of accelerator-driven system (ADS) experiments were carried out by supplying spallation neutrons to a subcritical core through the injection of 100MeV protons onto a tungsten target of 80mm diameter and 10mm thickness. In these experiments, the proton beams from the FFAG accelerator were generated at 30Hz repetition rate and 10 pA current. The neutron intensity generated at the tungsten target was around 1 10 s . The objective of these experiments was to conduct a feasibility study on ADS from the viewpoint of reactor physics, in order to develop an innovative nuclear reactor for a high-performance transmutation system with a capability of power generation or for a new neutron source for scientific research. The A-core employed in the ADS experiments was essentially a thermal neutron system composed of a highly enriched uranium fuel and a polyethylene moderator/reflector. In the fuel region, a unit cell is composed of a 93% enriched uranium fuel plate 1/1600 thick and polyethylene plates 1/400 and 1/800 thick. In these ADS experiments, three types of fuel rods designated as the normal, partial, and special fuels were employed. From the reason of the safety regulation for KUCA, the tungsten target was located not at the center of the core but outside the critical assembly, and the outside location was similar to that in previous experiments using 14MeV neutrons. As in previous ADS experiments with 14MeV neutrons, the introduction of a neutron guide and a beam duct is requisite to lead the high-energy neutrons generated from the tungsten target to the center of the core as much as possible. The detailed composition of the normal, partial, and special fuel rods, the polyethylene rod, the neutron guide, and the beam duct was described in Refs. 3–5). To obtain the information on the detector position dependence of the prompt neutron decay measurement, neutron detectors were set at three positions shown in Fig. 1: near the tungsten target (position (17, D); 1=200 BF3 detector) and around the core (positions (18, M) and (17, R); 100 He detectors). The prompt and delayed neutron behaviors (Fig. 2), which were an exponential decay behavior and a slowly decreasing behavior, respectively, were experimentally confirmed by observing the time evolution of neutron density in ADS. These behaviors clearly indicated that neutron multiplication was caused by an external source: sustainable nuclear chain reactions were induced in the subcritical core by spallation neutrons through the interaction of the tungsten target and the proton beams from the FFAG accelerator. In these kinetic experiments, subcriticality was deduced from the prompt neutron decay constant by the extrapolated area ratio method. The difference between the measured results of 0.74% k=k and 0.61% k=k at the positions (17, R) and (18, M) in Fig. 1, respectively, from the experimental evaluation of 0.77% k=k, which was deduced from the combination of both the control rod worth by the rod drop method and its calibration curve by the positive period method, was within 20%. Note that the subcritical state was attained by the full insertion of C1, C2, and C3 control rods into the core. The thermal neutron flux distribution was estimated through the horizontal measurement of the In(n, )In Atomic Energy Society of Japan Corresponding author, E-mail: pyeon@rri.kyoto-u.ac.jp Present address: SR Center, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu-shi, Shiga 527-8577, Japan Present address: Institute of Nuclear Safety System, Incorporated, 64, Sata, Mihama-cho, Mikata-gun, Fukui 919-1205, Japan Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 46, No. 12, p. 1091–1093 (2009)

https://www.tandfonline.com/doi/pdf/10.1080/18811248.2009.9711620

Kaichiro Mishima

Papers

Generation and Size Distribution of Droplet in Annular Two-Phase Flow

An evaluation of prediction methods for saturated flow boiling heat transfer in mini-channels

Correlation of critical heat flux for flow boiling of water in mini-channels

Flow regime transition criteria for upward two-phase flow in vertical narrow rectangular channels

First Injection of Spallation Neutrons Generated by High-Energy Protons into the Kyoto University Critical Assembly