Two-phase Flow Regimes Research Articles

Optimization of ethylene dimerization in a bubble column reactor based on coupling kinetic and equilibrium models

The main object of this research is modeling, sensitivity analysis, and multi-objective optimization of the ethylene dimerization process at the presence of a homogenous catalyst. Industrially, the ethylene dimerization occurs in a bubble column reactor equipped with an external cooler to prevent temperature runaway. In the first step, the considered bubble column reactor is heterogeneously simulated considering a two-phase flow regime by coupling the kinetic and equilibrium based models. To validate the accuracy of developed model, the simulation results are compared with the plant data. The simulation results show that the mass transfer from gas to liquid phase controls the ethylene dimerization rate in the considered plant. In the second step, the effects of operating pressure and temperature on the ethylene conversion and butene selectivity are investigated by the developed model. The simulation results show that pressure has an insignificant effect on the conversion and selectivity due to the high contact time of gas and liquid medium, low gas to liquid flow rate, and high mass transfer area. In the third step, a multi-objective optimization problem is programmed and the optimal temperature of reactor is calculated by the TOPSIS decision-making method.

On the mechanism of onset of significant void in subcooled flow boiling

Theoretical investigation was carried out for a possible mechanism of the onset of significant void (OSV) or net vapor generation (NVG) in subcooled flow boiling. Based on available knowledge on the two-phase flow regime transition from bubbly to slug flows, it was postulated that bubble coalescence is intensified substantially when the local void fraction near the heated wall exceeds a critical value. It was discussed that this can be a main cause of a rapid increase in the cross-sectional area-averaged void faction since the condensation rate decreases suddenly due to a sudden decrease in the interfacial area of vapor bubbles. It was demonstrated that if the vaporization rate on the heat transfer surface is evaluated using a reliable wall heat transfer correlation, the OSV conditions calculated by the present model agree with those by a widely-used empirical correlation well.

Flow regime characterization of a silicon-based vaporizing liquid microthruster

The present work discusses an experimental investigation of the flow into a silicon-based vaporizing liquid microthruster equipped with sensing capabilities and low-power on-channel secondary heaters. The sensing capabilities (resistance temperature detectors and capacitive void fraction sensors) are used to investigate the flow instability and to evaluate its performances. The device has a sandwich structure composed of a silicon substrate and a glass substrate. This last one allows optical access into the device. The main heating of the propellant is provided using a platinum resistive heater placed on the bottom of the silicon layer. By varying the electrical power supplied to the main heater at fixed mass flow rate, three flow regimes have been observed and investigated: fully liquid flow, two-phase flow, and fully vaporized flow. Their dynamics have been captured through high-speed micro-flow visualizations under rough vacuum conditions (about 29 kPa). Furthermore, the expansion of the exhaust vapor plume exiting from the micronozzle has been analyzed via Schlieren visualizations. Results highlighted the occurrence of a cyclic flow behavior during the two-phase flow regime. In contrast, the flow exhibits the presence of both liquid and vapor phases at the micronozzle exit. Once the fully vaporized flow regime is established, the two-phase flow dynamics into the inlet chamber become more stable with complete filling and more uniform distribution of the flow at the microchannels entrance. Schlieren imaging captured the increase of the exhaust plume spreading half-angle when moving from ambient condition towards rough vacuum.

The Effect of Mass Flow Rate and Heat Flux on Vertical Two-Phase Flow Regimes in a Small Diameter Tube

In recent years, there has been a rising concentration in investigating and researching the boiling of micro channel flow; since they afford high thermal effectiveness, small size, and low weight. In this paper, the boiling heat transfer was experimentally examined using water as the working fluid in small diameter tubes. Experiments for the flow model were performed using the same Pyrex glass tube test facility. The experiments of heat transfer were carried out using an internal diameter copper tube (10.922 mm). The range of other parameters was varied: mass flow rate (0.008–0.0214kg/s); heat flux (17351–80529W/m2); it has been found that the average coefficient of heat transfer depends on the form of flow pattern. The average heat transfer coefficient increases by about 37% when the flow pattern varied from slug flow to churn flow at a constant mass flow rate of 0.008kg/s. Besides, the heat transfer coefficient (h) in the two-phase flow, is 39%, which is more than that for single-phase flow, when the mass flow rate is 0.0214kg/s, and heat flux) increases from 17351W/m2 to 49105W/m2. Also, when mass flow rate rises from (0.008kg/s) to (0.0166kg/s), the pressure drop increases by about (15%) and increases by about (5%) when the mass flow rate rises from (0.0166kg/s) to (0.0214kg/s). The main regime of flow was a slug, churn, bubbly, dispersed bubbly, and confined bubbly flow.

Local Two-Phase Flow and Performance in Polymer Electrolyte Water Electrolysis Cells

Over the past decade, investigations[1-3] have been carried out to better understand the two-phase flow characteristics within a polymer electrolyte water electrolyzer (PEWE) and optimize the porous transport layer (PTL) morphology to improve the system performance and efficiency. Zhang, et al.[3] used a high-speed camera and transparent cell to study the strong influence of temperature and current density on bubble growth rate. Ito, et al. [4] showed that PTL porosity has a minor effect on cell performance while pore diameter greatly influences performance. They categorized the type of two-phase flow experienced by an electrolyzer by using liquid/gas superficial velocity plots. Furthermore, they showed that bubble detachment diameter strongly influenced the two-phase flow regime at any operating point. In a similar direction, Majasan, et al.[5] showed that longer flow path length promotes gas accumulation and results in annular flow at high current density, negatively impacting cell performance. Immerz et al.[6] also suggested that increased local ionic resistance down-the-channel may be attributed to large bubble accumulation. Therefore, the impacts of two-phase flow characteristics on mass transport as well as ohmic losses need to be better understood.Although the serpentine flow-field is very useful to study transport phenomena within a PEWE, it is rarely used as in commercial and large-scale PEWE system; the parallel channel-land configuration is a more conventional flow-field architecture. Through this work we hope to understand how architectural features in the flow field impact local performance to inform next-generation design. Furthermore, this work focuses on characterizing the two-phase flow and bubble accumulation occurring within the PEWE and understanding its relationship with performance. Initial experimental results revealed that the parallel flow-field outperforms the triple serpentine flow-field at high (50 ml/min) flowrates as seen in Figure 1a. This may be caused by the shorter channel lengths that do not exacerbate bubble accumulation near the outlet. Preliminary calculations of the superficial gas/liquid velocities have supported this conclusion (Figure 1c). However, at lower flowrates the parallel flow-field was observed to underperform and exhibit severe mass transport limitations. We believe that the fluid velocity in the channels at these low flowrates (6 ml/min) was insufficient to facilitate proper bubble detachment at the channel/PTL interface. This led to the formation of a large and spread out region of mass transport limited current density as seen in figure 1b. In conclusion, insights into local two-phase flow characteristics and their impact on performance in a PEWE cell will be described in this presentation[1] Lopata, J., et al., Effects of the Transport/Catalyst Layer Interface and Catalyst Loading on Mass and Charge Transport Phenomena in Polymer Electrolyte Membrane Water Electrolysis Devices. Journal of The Electrochemical Society, 2020. 167(6): p. 064507 DOI: 10.1149/1945-7111/ab7f87.[2] Lopata, J.S., et al., Considering Two-Phase Flow in Three-Dimensional Computational Fluid Dynamics Simulations of Proton Exchange Membrane Water Electrolysis Devices. ECS Transactions, 2020. 98(9): p. 653-662 DOI: 10.1149/09809.0653ecst.[3] Li, Y., et al., In-situ investigation of bubble dynamics and two-phase flow in proton exchange membrane electrolyzer cells. International Journal of Hydrogen Energy, 2018. 43(24): p. 11223-11233 DOI: https://doi.org/10.1016/j.ijhydene.2018.05.006.[4] Ito, H., et al., Experimental study on porous current collectors of PEM electrolyzers. International Journal of Hydrogen Energy, 2012. 37(9): p. 7418-7428 DOI: https://doi.org/10.1016/j.ijhydene.2012.01.095.[5] Majasan, J.O., et al., Two-phase flow behaviour and performance of polymer electrolyte membrane electrolysers: Electrochemical and optical characterisation. International Journal of Hydrogen Energy, 2018. 43(33): p. 15659-15672 DOI: https://doi.org/10.1016/j.ijhydene.2018.07.003.[6] Immerz, C., et al., Local Current Density and Electrochemical Impedance Measurements within 50 cm Single-Channel PEM Electrolysis Cell. Journal of The Electrochemical Society, 2018. 165(16): p. F1292-F1299 DOI: 10.1149/2.0411816jes. Figure 1

Precise Evaluation of Gas–Liquid Two-Phase Flow Pattern in a Narrow Rectangular Channel with Stereology Method

The drive to increase the efficiency of processes based on two-phase flow demands the better precision and selection of boundary conditions in the process’ control. The two-phase flow pattern affects the phenomena of momentum, heat, and mass transfer. It becomes necessary to shift from its qualitative to quantitative evaluation. The description of the stationary structure has long been used in structural studies applied to metals and alloys. The description of a gas–liquid two-phase mixture is difficult because it changes in time and space. This paper presents a study of the precise determination of two-phase flow patterns based on stereological parameters analysis. The research area is shown against the flow map proposed by other researchers. The experiment was taken in the thin clear channel with dimensions of W = 50 × H = 1200 × T = 5 mm. The test method is based on the visualization of a two-phase air–water adiabatic flow pattern in the rectangular channel where superficial air velocities ranging from 0.006 to 0.044 m/s and the superficial water velocity ranged from 0.011 to 1.111 m/s. A high-speed camera was used for visualization. Images were analyzed with the use of stereological techniques. The study included the classification of structures according to generally accepted two-phase flow regime nomenclature for upwards co-current gas–liquid flow in a vertical rectangular channel. The result of the research was the determination of the stereological parameters’ changes with reference to the two-phase mixture flow hydrodynamics. The results were presented as waveform fluctuations in the values of stereological factors such as the volume fraction VV, interfacial surface SV, number of objects NV, mean chord l′m and the free distance λ. The description of how these parameters change with changes in phase fluxes is also presented. These waveforms help to distinguish the transient flow regimes, which allow for the automatic adjustment of the process stability. The authors found templates of the stereological parameters’ dependencies for flow pattern recognition. The research demonstrates wide possibilities of stereological methods’ application for the analysis of the two-phase gas–liquid process. The stereological model of two-phase pattern control enables the identification of process disorders.

Favourably regulating two-phase flow regime of flow boiling HFE-7100 in microchannels using silicon nanowires

High performance miniaturized electronic devices require enhanced, compact and reliable thermal management system. As an efficient compact space cooling technique, flow boiling in microchannels has recently gained wide acceptance. However, weak buoyancy effects and microgravity in avionics and numerous space systems operations hinder the performance of flow boiling microchannel thermal management system due to poor bubble departure capacity and unfavorable development of flow regimes. Here we report the flow boiling silicon nanowires (SiNWs) microchannels which can favorably regulate two-phase flow regimes by enhancing explosive boiling, minimizing bubble departure diameter, and smoothing flow regime transition. Extensive experimental investigations along with high speed visualizations are performed. The experiments are performed with the dielectric fluid HFE-7100 in a forced convection loop for wide range of heat and mass fluxes. High speed flow visualizations have been employed at up to 70 k frames per second (fps) to understand the boiling mechanism in terms of bubble dynamics, flow patterns, and flow regime developments for SiNWs microchannels. These studies show that SiNWs reduce intermittent flow regimes (slug/churn), improve rewetting and maintain thin liquid film at wall. Therefore, flow boiling in SiNW microchannels is promising to thermal management owing to its high heat transfer rate with low pressure drop and negligible microgravity sensitivity.

On instantaneous pressure surges and time averaged pressure drop in intermittent regime of two-phase flow

Vigorous aeration and instantaneous pressure fluctuations associated with intermittent flow cause severe erosion-corrosion in the piping systems which lead to material degradation or wall thinning and leakages. Sometimes, this phenomenon results in catastrophic failures and hazards in transportation pipelines for industries such as petroleum, nuclear, geothermal and chemical. Present analysis is on the instantaneous pressure surges associated with the intermittent flow patterns and the two-phase frictional pressure drop. Experiments are carried out for three distinct sub-regimes of intermittent flow namely; Plug, Less Aerated Slug (LAS) and Highly Aerated Slug (HAS) flow. Instantaneous pressure fluctuation and time averaged pressure drop along the pipe length are reported for 350 combinations of inlet flow conditions (in term of ReSL and ReSG) of intermittent flow regime. Combined images of captured visual observations and measured instantaneous pressure fluctuations are demonstrated in terms of their influence on erosion and corrosion in detail. The obtained results are compared with the existing models for estimation of two-phase pressure drop proposed by earlier researchers. Large deviations between the experimental results and existing pressure models signify the need of separate correlation for two-phase frictional pressure drop for intermittent flow regime. In this direction, a unified correlation for prediction of two-phase frictional pressure drop for intermittent flow regime is proposed. The measured experimental results and obtained correlation for pressure drop will help in avoiding the erosion-corrosion in piping system. Also, database presented in this paper will aid to the effective and safe design of piping systems for petroleum, nuclear, geothermal and chemical industries.

Mesoscopic modeling impacts of liquid water saturation, and platinum distribution on gas transport resistances in a PEMFC catalyst layer

The oxygen transport resistance in catalyst layer of proton-exchange-membrane fuel cells has garnered much attention because it increases dramatically with the decreasing platinum loading. In this paper, the local transport behaviors of oxygen in electrode with different liquid water saturations, and platinum distributions are explored at mesoscopic level. The two-phase flow regime in electrode is simulated via a multicomponent multiphase flow model to reconstruct the liquid water morphologies. The effective oxygen diffusivities under wet conditions, the limiting current density, and the electrode resistance are numerically obtained before determining the local transport resistance. The results demonstrate that the effective oxygen diffusivities under wet condition decrease with the contact angle of the electrode. The limiting current density decreases with the liquid water saturation due to the blockage of ionomer surface area and the decreasing pore size distribution. The contribution of ionomer and liquid water to the electrode resistance dominates for the electrode resistance at low platinum loading. The local transport resistance of hydrophilic electrodes increases dramatically if the oxygen permeation in liquid water is excluded. More platinum particles distributed near the gas diffusion layer are beneficial for the reduction of the local transport resistance.

Forecasting the evolution of chaotic dynamics of two-phase slug flow regime

Presenting a predictive model through handling of the dynamical system is critical for modern technologies and industrial application. Here, Hankel-based dynamic mode decomposition is used to generate a predictive model for the slug flow regime. Flow dynamics are split into linear and nonlinear parts; the latter of which is responsible for intermittency phenomena. The proposed model shows the ability to predicts the time evolution of phase fraction. Forecasting of slug flow systems is achieved with no a priori knowledge of the equations of motion. The percentage of the variation between the actual states and predicted states is approximately 20%.

Visualization of refrigerant two-phase flow before and through distributor and evaluation of its performance

Abstract This paper presents an evaluation of the performance of a refrigerant distributor with visualization of the two-phase flow. The distributor is a typical conical design w/o the orifice for about 4 kW R134a system. Three factors influencing two-phase flow distribution are varied: mass flow rate, distributor inlet quality, and orientation. We looked at the uniformity of mass flow rates, evaporator inlet qualities, and capacities of each circuit. The authors built a transparent distributor following the exact geometry of an original distributor to visualize two-phase flow regimes exiting the expansion device and entering the distributor. We related flow regimes with distributor performance. A high-speed camera is used to capture two-phase flow regimes. After acquiring results for a baseline situation, the authors applied two approaches in an attempt to improve flow distribution: The first is manual adjustment of the resistance of each circuit individually to achieve uniform mass flow rates, and the second is the homogenization of the flow regime before division by adding an orifice in the distributor before separation to four channels. The average deviations of capacity for each branch are 9% for the baseline case, 3.3% for manual adjusting, and only 1.6% for the homogenization approach, stressing the importance of that simple strategy.

An Overview of Wellhead Burning: Fundamental Science to Burn Performance Prediction

ABSTRACTWhile wellhead burning has been an oil field hazard for generations, the development of capping response technologies and practices by industry experts has enabled the oil exploration community to shift its views of wellhead burning from a hazard to an oil spill response tool. This review covers some of the fundamental scientific aspects and technical issues of wellhead burning that engineers and policy makers will need to consider as this mitigation strategy is examined as a standard oil spill response tactic. For context, we examine a potential wellhead blowout scenario over a range of oil flows and examine the regimes of two-phase pipe flows, their dependence on wellbore velocities and gas-liquid ratios, and how those regimes will influence the burn efficiency with some insight from our experimental observations from two-phase spray burn testing. Among the critical findings that we present is that the worst-case discharge flow rate cannot be assumed to be the worst-case wellhead burning scenario.

Air-water two-phase flow regime and transition criteria in vertical upward narrow rectangular channels

Upward two-phase flow in two different vertical narrow gaps was investigated experimentally. For better understanding the characteristics of air-water flow in rectangular channels, the visual images were captured through high-speed cameras and the void fraction results were obtained through three impedance void meters. The flow pattern was identified through ReliefF-FCM clustering algorithm to avoid the subjectivity of human observation. The flow regime maps were plotted in two different forms, superficial velocities and void fraction and compared with the previous model in the open literature. Through the analysis of the experimental results, the air-water flow in narrow rectangular channels is characterized by the limitation of bubble distribution, coalescence and movement. The probability of bubble coalescence in narrow rectangular channels is increased compared with those in conventional channels due to the restriction of the narrower side walls. New transition criteria was obtained according to the drift-flux model and the force balance analysis which shows satisfactory agreements with existing experimental data for gaps of 1–6 mm.

Recurrence analysis of pressure signals for identification of intermittent flow sub-regimes

Recurrence analysis of pressure signals measured from the pipe wall simultaneously with flow visualization is used to depict a methodology for identifying intermittent flow sub-regimes through opaque pipes. Pressure signal alone is not sufficient to differentiate two-phase flow regimes in an opaque pipe. Hence, the phase space trajectory of pressure signals is used to develop a recurrence plot for each sub-regime. It is observed that a homogeneously dispersed type of recurrence plot is obtained for plug flow which changes to a checkerboard-like structure for the slug flow pattern. This depicts the chaotic nature of slug flow. The chaos increases with further increase in gas superficial Reynolds number when aeration in slug flow enhances, leading to highly aerated slug flow. It is established that the recurrence plot and recurrence quantification parameters change as the flow transits from plug to less aerated slug and highly aerated slug regime. Thus the recurrence analysis of pressure signals has shown potentials to be used as a technique to identify different intermittent flow sub-regimes. • Recurrence analysis for predicting intermittent flow sub-regimes through petroleum pipes. • Pressure signals are measured simultaneously with flow visualization. • Recurrence analysis of pressure signals is used to identify intermittent flow sub-regimes. • Homogeneous type of recurrence plot for plug flow while rectangular structure for slug flow. • Rectangles formed checkerboard configuration depicting the chaotic nature of slug flow.

Evaluation of sodium boiling models using KNS-37 LOF experiments

Abstract The computational codes used in the evaluation of the ESFR-SMART reactor performance and specifically their sodium boiling models are assessed using two KNS-37 LOF experiments, i.e. L22 and L29 tests, where boiling onset and two-phase flow regime up to dry-out occurred. The well-equipped KNS-37 experimental facility provided very valuable information for understanding the physical phenomena occurring in a 37-pin subassembly under LOF conditions, as well as experimental data to be used for computational tools validation. NATOF-2D, SAS-SFR, TRACE, ASTEC-Na, CATHARE-2 & CATHARE-3 and NEPTUNE_CFD codes have been used in this exercise in order to compare the various boiling models and conclude on the advantages and limitations of them based on the comparison against the experimental data. Beyond boiling onset, the various sodium two-phase flow approaches determine the ability of the code to correctly represent the rewetting and voiding phases as well as cladding dry-out onset. A simulation performed by a CFD approach (NEPTUNE_CFD code) considering liquid-vapor interfaces by an interface-tracking method is also shown and compared with the others approaches. Conclusions on each code performance are presented where the improvements needed to solve the issues encountered are included. This paper provides a first step in the process to investigate the required evaluation of the sodium two-phase flow models able to assess the safety of new SFR core designs (e.g. low void cores) under accidental conditions such as ULOF transients.

Experimental investigation of a capillary-assisted high performance evaporator

A particular architecture for a miniaturized evaporator is proposed in which liquid is spread on the heat source using a thin capillary structure connected to two manifolds. The vapor produced by the heat source is extracted from the device using a separate outlet. A prototype has been designed, manufactured and integrated in a mechanically pumped loop in order to evaluate the evaporator efficiency in terms of heat transfer coefficient and induced pressure drop. In this paper, a capillary structure consisting in parallel rectangular micro-grooves is investigated with HFE7000 used as the working fluid. Results show that the evaporator is able to extract heat flux of the order of 10 W.cm-2 with heat transfer coefficients close to 104 W.m-2.K-1 while inducing pressure losses below a few hundred Pascal. Direct visualizations of the flow in the capillary structure allowed a precise characterization of the evaporator functioning mode, in particular the existence of two distinct two-phase flow regimes depending on the flooding of the capillary structure: nucleate boiling and liquid films evaporation. In order to predict the occurrence of these operating modes, a hydraulic model of the evaporator assembly has been developed and validated thanks to dedicated experimental campaigns. In particular, this model is able to predict the separation between the nucleate boiling and evaporation mode and highlights the fact that the capillary limit is not the main restricting factor in terms of the device maximum manageable heat flux.

Research on the air–oil two-phase flow regime in an aeroengine bearing chamber based on Hilbert–Huang transform

The development of aeroengines toward a lighter and compact structure has put forward a stringent requirement on the lubrication systems, especially on the bearing chambers which contain complex air–oil two-phase fluids. Understanding the flow characteristics is of considerable significance to ensure the cooling and lubrication effect and to improve the working reliability of the aeroengine. Hilbert–Huang transform (HHT) was used to analyze the spectral characteristics of the pressure signals in the bearing chamber, in order to establish the correlation between the energy indicator k and the flow regime. The influences of the operating conditions and lubricant physical properties on the flow regime were discussed. The findings indicate that this approach can distinguish two typical flow regimes in the bearing chamber. With the increase of oil inflow and the decrease of draft speed, the flow regime changes from homogeneous flow to stratified flow, and k shifts from high-frequency band to low-frequency band. At the same lubricant mass inflow, k shifts from low-frequency band to high-frequency band with the increase of lubricant density, and the oil film on the wall becomes thinner. As the viscosity increases, the flow resistance grows and more oil accumulates on the wall. The flow regime converts from homogeneous flow to stratified flow, with k shifting from high-frequency band to low-frequency band. In addition, higher surface tension will cause more oil agglomeration, which results in less stable two-phase flow with irregular and uneven distribution of the oil on the wall.

Identification of Upward Two-Phase Flow Regime Transition Region under High-Pressure Condition Using Deep Learning

Currently, the MH21-S R&D consortium (MH21-S), supported by the Ministry of Economy, Trade, and Industry (METI), is developing the commercial production process of methane gas from methane hydrate. In order to develop a gas production method, the objective flow regime identification technique of gas-liquid two-phase flow under high-pressure conditions would be useful. Therefore, in the present research, a new method for identifying the flow regime by a convolutional neural network (CNN) using high-speed images obtained at a narrow view was proposed, and its performance was evaluated. Specifically, the upward gas-liquid two-phase flow images were recorded using a high-speed camera, and they were merged into a single image containing information from entire frames. The CNN was created and trained with these newly merged images, and it was utilized to identify flow regimes of unlabeled test images. As a result, the proposed method using the CNN model was capable of identifying the flow regime with high accuracy. Thus, the present study has shown that the CNN model with the proposed image conversion method can properly identify the upward gas-liquid two-phase flow regime. In addition, by utilizing the developed network, the bubbly-slug transition region, which has only been studied through visual observation or void fraction signals in the past, was successfully evaluated quantitatively.

Evaluation of Length Scales and Meshing Requirements for Resolving Two-Phase Flow Regime Transitions Using the Level Set Method

Abstract New criteria for fully resolving two-phase flow regime transitions using direct numerical simulation with the level set method for interface capturing are proposed. A series of flows chosen to capture small scale interface phenomena are simulated at different grid refinements. These cases include droplet deformation and breakup in a simple shear field, the thin film around a Taylor bubble, and the rise of a bubble toward a free surface. These cases cover the major small scale phenomena observed in two-phase flows: internal recirculation, interface curvature, interface snapping, flow of liquid in thin films, and drainage/snapping of thin films. The results from these simulations and their associated grid studies were used to develop new meshing requirements for simulation of two-phase flow using interface capturing methods, in particular the level set method. When applicable, the code used in this work, PHASTA, was compared to experiments in order to contribute to the ongoing validation process of the code. Results show that when the solver meets these criteria, with the exception of resolving the nanometer scale liquid film between coalescing bubbles, the code is capable of accurately simulating interface topology changes.

Flow and heat transfer for a two-phase slug flow in horizontal pipes: A mechanistic model

In view of the practical importance of gas–liquid two-phase flow in many applications, such as chemical engineering, petroleum engineering, nuclear engineering, etc., a reliable model of flow and heat transfer for two-phase flow is of practical importance in the two-phase flow analysis. Among various two-phase flow regimes, slug flow is most complicated due to the intrinsic randomness and intermittency. This paper aims at developing a novel mechanistic model of flow and heat transfer for two-phase slug flow in horizontal pipes. First, a hydrodynamic model of two-phase slug flow is developed using the concept of slug unit cell. Then, a heat transfer model is deduced based on the hydrodynamic model. The overall heat transfer coefficient is integrated by the local heat transfer coefficients of liquid slug, liquid film, and elongated bubble. The newly developed mechanistic model is well validated by the experimental results. Finally, the dependence of the heat transfer performance on the overall flow parameters, such as superficial liquid velocity and superficial gas velocity, and the local flow parameters, such as slug frequency, pressure drop, void fraction, and ratio of slug length to unit cell length, is comprehensively investigated. The heat transfer enhancement of two-phase slug flow compared with single-phase flow is mainly attributed to the turbulence increase in liquid by the injection of air and the decrease in thermal boundary layer by the frequent alternation between the liquid slug and the elongated bubble.