Double-curved pipes are widely employed as essential components of subsea pipeline systems. Considering the layout flexibility and application diversity, there are various spatial structures for the double-curved combinations. However, few studies have compared the flow characteristics in different double-curved pipes. The dissipations of the corresponding downstream flow have not been thoroughly investigated, which are crucial for the measurement accuracy and flow assurance. In this paper, the turbulent flow in double-curved pipes with different spatial structures (i.e., Z-, U-, and spatial Z- type) was numerically studied by employing the ω-Reynolds stress model. The major purpose was to develop an in-depth knowledge on the secondary flow characteristics in different double-curved pipes and quantify the dissipations of the downstream flow. The effects of the spatial angle and interval distance of the two curves on the flow fields are taken into consideration, and the swirl intensity S_i is introduced to evaluate the secondary flow dissipation. It is found that the secondary flows in the Z- and U-type structures are in opposite directions when the interval distance is short (3D), and the secondary flow in the spatial Z-type exhibits an oblique symmetric form. Only in the Z-type pipe with a short interval distance the secondary flow exhibits an exponential dissipation, and the fully developed flow is easier to achieve than the other cases. However, as the interval distance increases, the directions of the secondary flow in the U- and Z-type structures are the same, and the flow dissipations in all the structures return to the exponential types. The obtained dissipation rates for the secondary flow downstream of Z-, U-, and spatial Z-pipes with the 9D interval distance were 0.40, 0.25, and 0.20, respectively. The results are expected to guide the design of pipeline layouts and provide a reference for the arrangements of flowmeters in a complex subsea pipeline system.

A series of experiments are conducted to investigate the transition mechanisms and characteristics of six typical gas–liquid flow patterns in a horizontal–vertical elbow using electrical capacitance tomography and high-speed camera. The dominant modes and corresponding time coefficients are obtained by performing proper orthogonal decomposition on the pulsating gas holdup (GHU) distribution data to explore their physical mechanisms and correlations. Reduced-order descriptions for different flow patterns are discussed. The results show that after passing through the elbow, the horizontal slug or bubble flow turns into vertical bubble flow due to the small gas volume content and the mixing effect of secondary flow, accompanied by a swirl-switching phenomenon. A slug flow forms at the elbow outlet when there is a stratified flow comes from the horizontal pipe, and changes in flow conditions will affect the generation frequency and stability of Taylor bubbles. The horizontal annular or mist flow with high gas volume content will be transformed into churn flow in the vertical pipe. The modal decomposition analysis indicates that, for all the investigated conditions in the present study, mode 1 represents the mean distribution of GHU fluctuations, and there is a pair of modes representing the dominant swirling features. For the slug and churn flows, mode 2 characterizes the features of gas slug or large bubbles, the time coefficient of which is highly connected with that of mode 1. Meanwhile, it is also shown that the obtained low-dimensional descriptions of different flow patterns using the dominant modes are able to reconstruct most of the GHU distribution features in gas–liquid flows with the reconstructive loss less than 3%.

Blind tees, as important junctions, are widely used in offshore oil and gas transportation systems to improve mixing flow conditions and measurement accuracies in curved pipes. Despite the significance of blind tees, their unsteady flow characteristics and mixing mechanisms in turbulent flow regimes are not clearly established. Therefore, in this study, Unsteady Reynolds-Averaged Navier–Stokes (URANS) simulations, coupled with Explicit Algebraic Reynolds Stress Model (EARSM), are employed to explore the complex turbulent flow characteristics within blind-tee pipes. Firstly, the statistical flow features are investigated based on the time-averaged results, and the swirl dissipation analysis reveals an intense dissipative process occurring within blind tees, surpassing conventional elbows in swirling intensity. Then, the instantaneous flow characteristics are investigated through time and frequency domain analysis, uncovering the oscillatory patterns and elucidating the mechanisms behind unsteady secondary flow motions. In a 2D-length blind tee, a nondimensional dominant frequency of oscillation (St_bt = 0.0361) is identified, highlighting the significant correlation between dominant frequencies inside and downstream of the plugged section, which emphasizes the critical role of the plugged structure in these unsteady motions. Finally, a power spectra analysis is conducted to explore the influence of blind-tee structures, indicating that the blind-tee length of l_bt = 2D enhances the flow-mixing conditions by amplifying the oscillation intensities of secondary flow motions.

Building external wall insulation materials are widely used in the construction industry, which is one of the main materials for building energy saving. In order to study the combustion characteristics and fire spread behaviour of typical exterior wall materials at different pressures. Four environmental pressures of 40KPa, 60KPa, 80KPa, and 101KPa were set in an experimental cabin, measuring the quality, temperature, heat radiation, and heat flow of rigid polyurethane foam (RPU) through experiments. The experimental results are as follows: the masses loss curve of RPU combustion approximately obeyed the quadratic curve, which was similar to the general solid fuel combustion law; the mass change was the most dramatic at 101KPa, the lower the pressure, the slower the mass change; as the pressure decreased, the flame temperature increased slightly, and the spread of the fire slowed down; as the pressure decreased, the peak value of the thermal radiation curve decreased, and the curve tends became flat, but the horizontal axis time increased, indicating that the total heat release remained the same.