Foam drilling has grown tremendously in the last few years. Foam is often used as a circulating fluid in underbalanced drilling operations (i.e. to keep the pressure of the drilling fluid less than the pressure of the formation fluid). Foam drilling has been shown to provide significant advantages, such as higher penetration rate, effective cuttings transport, less formation damage, etc. However, although, a few studies have been conducted on vertical wellbores, there is little detailed published information related to its use in horizontal wellbores. In this study, the cuttings behavior in horizontal foam drilling was numerically investigated. First of all, the experimental results of foam rheology and particle settling test were shown. Experimental results showed that the foam behaves like power law fluid and foam volumetric quality has a major impact on the settling phenomena. The next, a three layer model was developed to predict and interpret the cutting transport behavior in horizontal foam drilling. This model consists of a three-layer configuration, i.e. stationary bed of drilled cuttings at the bottom; a moving bed above it; and a heterogeneous suspension layer at the top. This model is based on conservation laws of mass and linear momentum as well as on constitutive relations that describe the interactions between each phase and the walls. The model was solved numerically to predict the cuttings bed height, pressure drop at different foam flow rates under several conditions. Furthermore, numerical simulations were carried out in order to investigate some factors affecting the cutting transport in horizontal foam drilling. The results of numerical simulation are presented.

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Young's equation describes the interfacial equilibrium condition of a liquid droplet on a smooth solid surface. Young's equation is generally discussed from a thermodynamic perspective and derived by minimizing the total free energy of a system while keeping the intensive parameters in the total free energy constant (i.e., the variation of the total free energy is zero). In our previous study, we derived a modified Young's equation by using thermodynamics based on a new perspective. This equation was derived by considering the virtual work variation in the horizontal directions of the three phase contact line. However, in the conventional thermodynamic approach, the Young-Laplace equation is derived from the virtual work variation on the droplet surface. This point was not considered in our previous derivation. In this study, we give the complete derivation of the interfacial mechanical balance conditions of a liquid droplet on a smooth solid surface. We then derive both the modified Young's equation and the Young-Laplace equation from the virtual work variation of the droplet. Moreover, we consider the relationship between both equations.

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Critical Heat Flux (CHF) in a non-uniformly heated tube is a very important factor in designing boiling two-phase system, especially, a small capacity boiler with tube-nested combustor operated under low-pressure and low-mass-flux condition. However, the influence of the non-uniformly heat flux on the CHF has not been clearly understood so far. In this series of investigation, the non-uniform heat flux distribution was realized by using the Joule heating of eccentric tube. The experimental investigation on the CHF was conducted by using a forced convective boiling system with a test tube of dimensions of 20 mm I.D., 24 mm O.D. and 1800 mm in heated length, and the results were compared with the previous results of 900 mm in heated length. On the basis of these experimental results, the correlation of spreading coefficient was proposed. The new calculation model with this correlation and the treatment of the droplet has been confirmed with the experimental results.

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In the case of loss of the residual heat removal system under mid-loop operation during shutdown of the pressurized water reactor (PWR) plant, steam generated in a reactor core and water condensed in a steam generator (SG) form a countercurrent flow in a hot leg. In this study, to improve a countercurrent flow model in a transient analysis code, experiments were conducted using a scale-down model of the PWR hot leg. Flow patterns and countercurrent flow limitation (CCFL) characteristics were measured. A rectangular duct, whose height was about 1/5 of the hot leg diameter, was used to simulate the hot leg. Air and water at atmospheric pressure and room temperature were used for the gas and liquid phases. Water levels in the hot leg were also measured to investigate void fraction distributions. CCFL characteristics were well correlated with the Wallis-type correlation, and agreed well with the flow pattern transition from wavy to wavy-mist flows.

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In the previous study, flow patterns under countercurrent air-water flow were observed in a rectangular channel simulating a PWR (pressurized water reactor) hot leg and close interrelations between flow patterns and countercurrent flow limitation (CCFL) were indicated. In this study, simple calculations of CCFL characteristics based on a momentum equation and numerical simulations using the thermal hydraulic calculation code FLUENT6.3.26 were conducted. The CCFL characteristics by the simple calculations agreed with the measured results but the constant m in the Wallis correlation was underestimated in the region of large air flow rates. In the numerical calculations, flow patterns in the simulated hot leg were not reproduced by 2D calculations due to the limitation of capability to predict the flow pattern transition from stratified flow to wavy-mist flow but were successfully reproduced by 3D calculations because of the interrelation between CCFL characteristics at the inlet and outlet of the simulated hot leg. Falling water flow rates predicted by 3D calculations agreed with the data in the region of large air flow rates.

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