A one dimensional model for the prediction of stratification in horizontal pipes subjected to fluid temperature transient at inlet (original) (raw)
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1977
Recent studies (Taite! and Dukler, 1976) have provided a method for predicting conditions for flow regime transitions by modelling the actual physical processes taking place. This work was concerned only with steady flow conditions. Of considerable importance to the analysis of a Loss of Cooling Accident is the ability to predict the flow regime under conditions of transient flow. This report extends the steady state method of analysis to that of transient flow. It is shown that under transient conditions the transitions between flow patterns can occur at flow rates different than that which witl take place if the changes in flow rate were to take place infinitely slowly. Furthermore, for rapid changes in flow rates from one set of conditions to another, flow regimes can appear which would not be observed if the change were made slowly. The theory for flow regime changes under transient flow conditions is developed and•methods fo~ predicting the onset of transition under transients presented. Experimental confirmation of the theory is described.
INVESTIGATIONS ON FLOW REVERSAL IN STRATIFIED HORIZONTAL FLOW
The phenomena of flow reversal in stratified flows are investigated in a horizontal channel with application to the Emergency Core Cooling System (ECCS) in Pressurized Water Reactors (PWR). In case of a Loss-of-Coolant-Accident (LOCA), coolant can be injected through a secondary pipe within the feeding line of the primary circuit, the so called hot leg, counter-current to the steam flow. It is essential that the coolant reaches the reactor core to guarantee coolability. Due to high temperatures in such accident scenarios, steam is generated which escapes from the reactor core vessel through the hot leg. In case of sufficiently high steam flow rates, only a reduced amount of coolant or even no coolant will be delivered to the reactor core. The WENKA test facility at the Institute for Nuclear and Energy Technologies (IKET) at Forschungszentrum Karlsruhe is capable to investigate the fluid dynamics of two-phase flows in such scenarios. Water and air flow counter-currently in a horizont...
An experimental study of stratified–dispersed flow in horizontal pipes
International Journal of Multiphase Flow, 2014
The tracer method has been adopted to study stratified-dispersed flow in a horizontal pipe, 80 mm in diameter and 50 m long, operating at 5 Bar with nitrogen-water mixtures. The use of the tracer method in a horizontal pipe required the development of a specially designed test section, the related electronics and a data acquisition system. It has also been necessary to develop a tracer injection system, which has been designed in order to obtain uniform tracer concentration in the liquid film immediately after its injection. The main flow parameters which can be measured with the present experimental setup are the circumferential distribution of the film height, flow rate and tracer concentration, the rates of droplet entrainment and deposition and the split of the liquid phase between the wall layer and the entrained droplets. The average tracer concentration data have been interpreted with a new three-field model of the liquid phase in the stratified-dispersed flow pattern. In the present formulation, the model holds for steady, fully developed flow conditions and is based on a one-dimensional description of the flow system. The data cover a limited number of flow conditions.
An Improved Multidimensional Finite Difference Scheme for Predicting Stratified Horizontal Pipe Flow
Nuclear Technology, 1984
Numerical predictions of the three-dimensional temperature and velocity profiles of an experimental stratified horizontal pipe flow are performed. The experiment is one of a series of flow tests conducted at Argonne National Laboratory. A new accurate and stable skew-upwind differencing scheme is employed in the finite difference solution of the energy equation. The skew-upwind predictions are in excellent agreement with the experimental data as steady-state conditions are approached at the upstream test subsection. Comparisons between the conventional upwind and the skew-upwind schemes showed that the skewupwind formulation provided a significant increase in the accuracy of temperature predictions.
Numerical Flow Analysis of The Variation of Central Axial Velocity Along The Pipe Inlet
The Eurasia Proceedings of Science Technology Engineering and Mathematics, 2018
Due to no slip flow condition at the wall, the fluid enter the pipe with a smooth velocity start to develop along the flow to comply the zero velocity at the wall and maximum at the pipe center. After a certain distance where the development completed, the velocity profile becomes fully developed and no longer changes observed along the pipe flow. The region flow where the velocity profile developes is called developing flow or inlet flow and the region flow where the fully developed profile govern are called fully devloped flow. Computation of the flow properties in the fully developed region can be enabled with various empirical theories, but the complex flow styructure in pipe inlet region still has not been solved exactly. However It is quite important to know the flow behavior at the pipe inlet to compute the right pumping power especially in the fluid heating and cooling short pipe flow processes. the study performed, the steady pipe flows with Newtonian fluid were simulated numerically at low Reynolds numbers (ranged 1000 and 25000) covering the three flow regimes (laminer, transition and turbulence). High turbulence level and smooth velocity profile were assigned to the flow at pipe inlet. Turbulence flows were solved according to the time mean flow assumption. On the numerical results obtained, the variation of axial central velocity along the flow was examined for different relative roughnesses. Consequently, a numerical correlation which define the axial velocity and fit the numerical values well is proposed.
2012
The thesis presents a general one-dimensional mathematical model to simulate two-phase, gas-liquid, annular flow in horizontal as well as vertical pipes, and to mechanistically predict the transition from stratified to annular flow in horizontal pipes. The method is based on the transient one-dimensional two-fluid model whereby the two phases are considered as (i) liquid layer and (ii) a mixture of the gas and liquid droplets in which the droplet concentration in the mixture is considered as a flow variable. The model entails the introduction of a scalar transport equation for the conservation of mass of liquid droplets accounting for liquid transfer to and from the film liquid layer. The interface curvature is modelled by a double circle geometric configuration incorporating a new empirical relation for the specification of wetted angle. The droplet exchange rate between the liquid film and gas core is modelled by employing droplet entrainment and deposition rates derived from modi...
Numerical and Experimental Study to Predict the Entrance Length in Pipe Flows
Journal of Applied Fluid Mechanics, 2019
Here, a steady, incompressible and isothermal flow in the inlet region of a circular pipe were numerically and experimentally studied to predict the entrance length. The region in the upstream of fully developed pipe flow is referred to as the developing flow region, the effects of which on flow parameters are referred to as entrance effects. Entrance length shows the length of the developing flow region. The analysis of entrance flow is difficult and complicated as there are many parameters such as different pipe inserts affecting it. Earlier empirical results on the entrance region are inconclusive and inconsistent. Initially, an experimental study was performed with pipes of different roughness to validate the numerical results. Reynolds numbers used in the experiment ranged from 3000 to 25000. The entrance flow was numerically simulated in parallel to experimental pipe flows. Numerical results obtained were compared with those of the experimental study and of previous ones. Numerical and empirical data showed good agreement. Based on the numerical results, a well-defined numerical correlation was developed and proposed for the prediction of entrance lengths.
Case studies of fluid transients in subcooled pipe flow
Personality and Individual Differences - PERS INDIV DIFFER, 2003
Transient regimes may cause excessive water hammer, and possible column separation, slug flow, plug flow and fluid-structure interaction (FSI) in the system. Fluid transients may severely disturb operation of the piping system and damage the system components. This paper presents authors' case studies of typical transient regimes in industrial and laboratory piping systems. The first case study deals with the sudden load rejection of a Francis turbine in Pluna HPP (Slovenia). Then the results from pressure vessel blowdown experiments at UMSICHT's (Germany) pilot plant pipework are shown. Pump start with an air pocket in the system has been investigated by WL | Delft Hydraulics (The Netherlands). The last case study concerns impact tests in a structurally unrestrained pipe at the University of Dundee (UK). The underlying theory for these cases is briefly introduced and the measurements are compared with computed results.
Transient heat transfer at fluid flow in a thick-walled pipeline
MATEC Web of Conferences
The work aims to determine the transient temperature distribution of the medium and the pipeline wall using the finite difference method. Time courses of the temperature of the flowing medium and pipeline walls caused by a step change in temperature of the medium at the pipeline inlet, obtained by the numerical method, were compared with the courses calculated using strict analytical formulas. The numerical method of determining the transient distribution of temperature of medium and pipeline wall can be used in the analysis of heating and cooling of heating or steam pipelines with any changes in time of mass flow rate of the flowing medium or temperature of the medium at the inlet to the pipeline..