Colin P. Hale

Pressure gradient and holdup in horizontal two-phase gas- liquid flows with low liquid loading

Abstract Pressure gradient and holdup data are presented for air–water and air–oil flows in a horizontal, 0.079 m diameter pipe. Addition of a very small liquid flow was found to result in a considerable increase in the pressure gradient compared with single phase gas flow. The pressure gradient and the holdup data were compared with predictions of the ‘apparent rough surface’ (ARS) and the ‘double-circle’ models. The ARS model generally gave better predictions for the holdup over the experimental range. Both models predicted the pressure gradient for air–water flows at high gas flowrates reasonably well. However, the predictions of both methods were unsatisfactory for air–water experiments at low gas flowrates and for air–oil experiments. Overall the ARS model was judged to be the more robust.

Experimental Investigation and Modeling of the Effects of Rising Gas Bubbles in a Closed Pipe

Summary: Transient multiphase flow in the wellbore causes problems in well-test interpretation when the well is shut in at surface and the pressure is measured downhole. Pressure-buildup data recorded during a test can be dominated by transient wellbore effects (i.e., phase change, flow reversal, and re-entry of the denser phase into the producing zone), making it difficult to distinguish between true reservoir features and transient wellbore artifacts (Gringarten et al. 2000). This paper is a follow-up to paper SPE 96587 (Ali et al. 2005), which presented experimental results of phase redistribution effects on pressure-buildup data. Though the results of the experiments were revealing, they are complex because they reflect the real well situation. To obtain results in which the phase redistribution in the well is studied independently of the interaction with the reservoir, a further set of experiments was carried out. In these experiments, the tube (simulating the well) was isolated at both the top and the bottom at the same time. The pressure distribution was measured during the transient following shut-in and for the steady-state final condition, in which there was a liquid-filled zone at the bottom of the test section and a gas-filled zone at the top. A substantial number of tests were conducted in the bubbly-flow region and could therefore be analyzed by a simple 1D model for bubbly flow. The results of the comparison between the model and the experimental data are presented in this paper.

FLOW AND HEAT TRANSFER IN PRESSURIZED WATER REACTOR REFLOOD

In a pressurized water rector (PWR), following a postulated large break loss of coolant accident (LBLOCA) an important stage in the recovery of the core is the reflood process in which the core is reflooded (rewetted) by water injected from the emergency core cooling system (ECCS). This review summarises the studies which have been carried out on the reflood process. Despite all the work that has been done, it is concluded that there is still a need for improved basic understanding of the detailed processes occurring, and in particular the nature of the fluid/hot solid interactions in the close vicinity of the rewetting front.

Rewetting Processes During Top/Bottom Re-Flooding of Heated Vertical Surfaces

Rewetting of the heated fuel rods is one of the most important phenomena to be considered in analysis of the design basis loss of coolant accident (LOCA) in light water reactors. The rewetting phenomenon is a complex and violent one with the rewetting front moving rather slowly over the heated surface. For water temperature close to saturation, the rate of progression of rewetting front is independent of flow rate of the water approaching the rewetting front; this is an indication of the fact that the rewetting process is governed by events local to the rewetting front [1]. This paper describes an experimental study on the rewetting of heated vertical surfaces during top/bottom reflooding. Through an infrared transparent substrate fixed in the surface, processes occurring locally at the quench front have been studied by using a fast response thermal imaging system (Cedip Titanium 560M). The existence of a cyclic bursting phenomenon at the quench front has been observed. Multiple events of this type gradually remove heat from the metal, allowing the rewetting front to progress slowly over the surface. These intermittent contacts occur over a short axial length. Temperature measurements indicate that the metal surface temperature at the rewetting front is close to the homogeneous nucleation temperature.Copyright

Mitigating flow assurance challenges in deepwater fields using active heating methods

Flow assurance challenges, mainly of hydrates and wax depositions, are amongst the key issues that must be resolved and mitigated to ensure that hydrocarbons can be efficiently and economically transported from well to processing facilities. As wells step further away from shore into deeper water, the flow assurance challenges are increasing tremendously due to prevalence of higher pressure and lower temperature conditions. Thus, the development of cutting edge technologies to cater for the ever increasing demand in exploring the hostile and technologically challenging deepwater fields is a matter of great urgency. One of the effective solutions to prevent the formation of wax or hydrates is to use active heating methods. This paper describes an overview of the available active heating methods and mechanisms which are being implemented as thermal management systems for flowline in deepwater fields. It also discusses the thermal performance calculation models available to aid the design and modelling of such systems. Some comparative studies are carried out to determine the advantages and disadvantages of each of the methods to establish a general reference source on the technology that provides the most significant economic impact without compromising the reliability and efficiency of the overall system. Active heating systems have been used in several projects in the North Sea, Gulf of Mexico and Offshore West Africa. This paper also summarizes these projects and their operating experience from open literature. In general, due to their operational flexibility and high efficiency through control of the pipeline temperature above the hydrate formation and wax deposition temperatures, active heating seems to be the most practical, economical and viable solutions in managing flow assurance issues; especially for the development of deepwater fields.

Numerical and Experimental Investigation of the Behaviour of Non-Contacting Droplets During the Reflood Phase After a LOCA

During the reflood phase, following a Large Loss of Coolant Accident (LOCA) in a Pressurised Water Reactor (PWR), a flow of vapour containing small saturated droplets (of order 1mm diameter) is responsible for the precursory cooling before the quenching of the rods by the liquid water. The main mechanism for this cooling process is convective heat transfer to the vapour, with the vapour being cooled by the evaporation of the entrained saturated droplets. If the fuel rod temperature exceeds the Leidenfrost [1] value, the droplets do not wet it, but rather bounce off from it due to the formation of a vapour film between the droplet and the metal. Secondary cooling of the rods is provided by this process. Both the hydrodynamics of these impacts and the droplet-vapour-wall heat transfer mechanisms affect the degree of this secondary cooling. We investigate here the heat transfer attributable to such droplets in typical reflood conditions by a combination of new experimental observations, numerical simulations and correlations based on earlier studies [2], [3], [4]. Using an infrared technique we obtain spatial temperature measurements of the area below a non-contacting droplet [5]. At the same time we observe the hydrodynamic behaviour of the droplet by means of a high speed optical camera. Combining our experimental results with an analytically-computed droplet-wall interaction rate we estimate the cooling by those droplets in typical reflood conditions. These measurements are used for the validation of numerical simulations which are conducted using the CFD code TransAT© , to support its application to cases beyond the present reach of the experimental technique.Copyright

Visual and Thermal Studies of Single Tube Reflood Under Typical PWR LB-LOCA Conditions

The common approach to safety in a nuclear power plant is to design the system to respond safely to a large postulated accident, the so-called design basis accident. Accidents more severe than the design basis accident (“severe accidents”) are assessed but the system is not designed to withstand them; they are considered too unlikely to require specific design actions. For the pressurised-water reactor (PWR), the design basis accident (DBA) is the Large Break Loss-of-Coolant Accident (LB-LOCA), in which it is assumed that one of the large inlet coolant pipes from the circulating pump to the reactor vessel is completely broken and moves apart to allow free discharge of the primary coolant from both broken ends. For this type of break total coolant loss will occur in 100s or less. Although the reactor is by this time sub-critical so that little power is produced from fission, a large amount of decay power exists and causes the fuel rod claddings to have a temperature in the region of 600–800 °C. This paper describes experimental investigations designed and performed in order to provide detailed information about the macroscopic behaviour of the steam-water flow occurring during the reflood phase following a PWR LB-LOCA. Specifically, a bottom-up rewetting process was studied, in which water droplets may be entrained in the vapour flow and contribute to cooling of the hot fuel pin before it is quenched. In these experiments the test section is initially preheated to temperatures up to 600 °C and then quenched by introducing water at the bottom of the tube at atmospheric pressure. During the course of this transient process axial temperature and heat flux profiles will be recorded, extending the existing databank of cases for code validation. Simultaneously, an axial viewing technique will be applied to observe the quench front, and any pre-cursory droplet production, occurring during these singletube reflood experiments. As part of the preliminary validation of this novel technique, a series of air-water vertical upflow conditions have been examined. The results of these preliminary studies provide detailed visualisation of typical entrainment processes likely to be encountered during single-tube reflood.Copyright