The current work studies air-water flow through a ½-inch flow restricting orifice installed in a 1-inch pipe. Investigation of two phase flow downstream the orifice and its effects on vibration of the piping structure have been carried out. Several flow regimes from bubbly to stratified-wavy flow have been analyzed to evaluate the effects of flow pattern, phase redistribution, bubble frequency, and liquid flow rate on the vibration of the structure. The liquid velocity fields have been obtained using Particle Image Velocimetry (PIV) along with post processing algorithm for phase discrimination. Proximity sensors have been used to capture the pipe response in two orthogonal directions. Also, a capacitance sensor was used to measure the two-phase void fraction. The results show that the magnitude and nature of vibrations of the piping structure is largely affected by the frequency and size of the bubbles upstream, vortex creation by pressure fluctuation downstream, liquid flow rate, and the flow pattern upstream. Slug flow and stratified flow patterns induced significant vibrations in the examined structure. The location of the transition region of slug flow on flow pattern maps, play important role in the dynamic response of the structure to the flow.

Thermal fatigue may develop in piping elbow with high temperature stratified flow. To prevent the fatigue damage by stratified flow, it is important to know the distribution of thermal stress and temperature history in a pipe. In this study, heat conduction inverse analysis method for piping elbow was developed to estimate the temperature history and thermal stress distribution on the inner surface from the outer surface temperature history. In the inverse analysis method, the inner surface temperature was estimated by using the transfer function database which interrelates the inner surface temperature with the outer surface temperature. Transfer function database was calculated by FE analysis in advance. For some patterns of the temperature history, inverse analysis simulations were made. It was found that the inner surface temperature history was estimated with high accuracy.

Thermal stratification is a common phenomenon in the surge line of Pressurized Water Reactors (PWR). The stratification temperature difference (ΔT) and cyclic behavior severities are the most prevalent during heat-up and cool-down operations of a PWR, when temperature difference between the pressurizer and the hot leg in the Reactor Coolant System (RCS) becomes the largest and inventory fluctuations in RCS are the highest. These behaviors cause cyclic damages and deformations of the pipe. In addition, USNRC recommended integrity evaluations of the piping system submitted to thermal stratified flow in USNRC Bulletin 88–11. In this reason, the importance of monitoring the fatigue induced by the thermal stratified flow in thick-wall pipes rises more and more. To raise the accuracy of solutions for the fatigue damage induced by thermal stratification, the exact global temperature profile is needed to be derived. Many studies have been performed to obtain these profiles. The purpose of this paper is to predict the global temperature profiles in the piping systems submitted to thermal-stratified flow from measured temperatures on the outside wall. It presents both the inverse transfer function method and the numerical-analysis method.

Air-water experiments were carried out in a horizontal 1 ″ pipe system to measure the magnitude of the forces induced by the multiphase flow. Forces and accelerations were measured on a number of bends and T-joint configurations for a wide range of operating conditions. Five different configurations were measured: a baseline case consisting of straight pipe only, a sharp edged bend, a large radius bend, a symmetric T-joint and a T-joint with one of the arms closed off. The gas flow was varied from a superficial velocity of 0.1 to 30 m/s and the liquid flow was varied from 0.05 to 2 m/s. This operating range ensures that the experiment encompasses all possible flow regimes. In general, the slug velocity and frequency presented a reasonable agreement with classical models. However, for high mixture velocity the measured frequency deviated from literature models. The magnitude of the measured forces was found to vary over a wide range depending on the flow regime. For slug flow conditions very high force levels were measured, up to 4 orders of magnitude higher than in single phase flow for comparable velocities. The annular flow regime resulted in the (relative) lowest forces, although the absolute amplitude is of the same order as in the case of slug flow. These results from a one inch pipe were compared to data obtained previously from similar experiments on a 6mm setup, to evaluate the scaling effects. The results for the one inch rig experiments agreed with the model proposed by Riverin, with the same scaling factor. A modification of this scaling factor is needed for the model to predict the forces measured on the 6mm rig. The validity of the theories developed based on the 6mm experiments were tested for validity at larger scales. In case of slug flow, the measured results can be described assuming a simple slug unit model. In annular and stratified flow a different model is required, since no slug unit is present. Instead, excitation force can be estimated using mixture properties. This mixture approach also describes the forces for the slug regime relatively well. Only the single phase flow is not described properly with this mixture model, as would be expected.

Temperature gradients in the thermally stratified fluid flowing through a pipe may cause undesirable excessive thermal stresses at the pipe wall in the axial, circumferential, and radial directions, which can eventually lead to damages such as deformation, support failure, thermal fatigue, cracking, etc. to the piping systems. Several nuclear power plants have so far experienced such unwelcome mechanical damages to the pressurizer surge lines, feedwater nozzle, high pressure safety injection lines, or residual heat removal lines. In this regard, to determine the transient temperature distributions in the wall of a piping system subjected to internally thermal stratification with accuracy is the essential prerequisite for the assessment of the structural integrity of the piping system subjected to internally thermal stratification. In this study, to predict the transient temperature distributions in the wall of PWR pressurizer surge line with a complex geometry of 3-dimensionally bent piping realistically, 3-dimensional transient CFD calculations involving the conjugate heat transfer analysis are performed for the actual PWR pressurizer surge line subjected to stratified internal flows either during out-surge or in-surge operation using a commercial CFD code. In addition, the wall temperature distributions obtained by taking account of the existence of wall thickness as it is are compared with those by neglecting the existence of wall thickness to identify some requirements for a realistic and conservative thermal analysis.

CFD analysis is widely adopted for determination of design characteristics of major equipment with flexible geometry and flow. However, the accuracy of CFD analysis strongly depends on the numerical model, governing equation and simulation parameters. A thermal stratification phenomenon can lead to unanticipated damages of nuclear piping because of the stresses caused by different fluid densities due to stratified flow. In this paper, systematic CFD analyses are performed for surge line which is one of primary piping system by using representative commercial code to investigate key parameters; (1) mesh size and time step effect (2) turbulence parameter effect (3) material property approximation effect (4) conjugate heat transfer effect (5) insurge and outsurge flow effect. From numerical analysis results related to the items (1) through (3), the optimum CFD model as well as reasonable input parameters was determined. With regard to the item (4), thermal difference was bigger as 82∼208% than without considering conjugate heat transfer. On the other hand, for the item (5), stratified flows were come out clearer in outsurge flow. Based on the parametric CFD analyses to simulate stratified flows, most of numerical issues were resolved while further investigation is required for the conjugate heat transfer effect.

This paper addresses three-dimensional numerical analyses of the unsteady conjugate heat transfer and thermal stress for a PWR pressurizer surge line pipe with a finite wall thickness, subjected to internally thermal stratification. A primary emphasis of the present study is placed on the investigation of the effects of surge flow direction on the determinations of the transient temperature and thermal stress distributions in the pipe wall. In the present numerical analysis, the thermally stratified flows (in-surge flow and out-surge flow) in the pipe line are simulated using the standard κ-ε turbulent model and a simple and convenient numerical method of treating the unsteady conjugate heat transfer on a non-orthogonal coordinate system is developed. The unsteady conjugate heat transfer analysis method is implemented in a finite volume thermal-hydraulic computer code based on a non-staggered grid arrangement, SIMPLEC algorithm and higher-order bounded convection scheme. The finite element method is employed for the thermal stress analysis to calculate non-dimensional stress distributions at the piping wall as a function of time. Some numerical calculations are performed for a PWR pressurizer surge line pipe model with shortened length, subjected to internally thermal stratification caused either by insurge or outsurge flow with a specified velocity, and the results are discussed in detail.

During a loss-of-coolant-accident in advanced light water reactors, outside coolant enters the cold leg by gravity to cool the core. This coolant is at a substantially lower temperature and thus is heavier than the liquid in and from the reactor. Consequently, stratified flow may occur. A stratified flow may cause condensation-induced water hammer, and will influence the coolant flow behavior. Two sets of experiments are in progress to better understand stratified flow conditions that lead to water hammer, and the density stratification behavior. The first set uses air-oil-water as the test media. Its purposes are to conduct exploratory tests and to provide instruction an apparatus for education purposes. The second set of tests will use steam and water and, later, the refrigerant R123. This paper describes the exploratory test facility, gives a brief description of the facility that will be used for the steam-water and refrigerant tests, describes the overall test plan, and finally gives some preliminary results on the intrusion of a lighter liquid into a pipe against flow.

Stratified flow is encountered in many situations. The flow of hydrocarbons transported in horizontal pipes often gets stratified. The prediction of pressure drop and liquid hold-up is essential for reservoir and pipe management and optimizing the cost of transportation of constituents. The present paper presents a simple mathematical model to predict the pressure drop, water and oil hold up and stratified layer. A good agreement with the experimental data was found. The model will be further developed and incorporated within a numerical model in order to investigate the flow field characteristics and establish correlations for a wide range of parameters.

An analytical study to investigate small fire in a ventilated room is presented. Most existing studies are either numerical or experimental and correlations for predicting characteristics of flame propagation such as temperature difference, flame (plume) length and terminal velocity are based on empirical data. A simple mathematical -from the first principles- is developed to predict these characteristics. The predictions are compared with the published experimental data and good agreement has been obtained. Such a simple model would be very useful to practicing engineers for fire control programs, designing fire detection mechanisms.