Increase In Back Pressure Research Articles

Feasibility analysis and performance characteristic investigation of thermal energy storage system based on organic Rankine cycle for engine exhaust temperature modulation

ABSTRACT The exhaust temperature of the engine has a significant impact on the conversion efficiency of the after-treatment. A thermal energy storage (TES) system with organic fluid for engine exhaust temperature modulation is established in this paper, and the performance characteristics of the TES system with the engine exhaust temperature modulation are analyzed based on the organic Rankine cycle (ORC). First, an accumulator-centered TES system model is established based on GT- SUITE, and the effects of working fluid mass, mass flow rate, supply strategy and working fluid type on the TES system are analyzed. Second, the influence of coupled ORC-TES system on exhaust emission and after-treatment efficiency is analyzed, and the cost–benefit analysis of integrated ORC-TES system is carried out. Finally, the simulation results show that with the increase in working fluid mass, the average temperature, availability, satisfaction, and comprehensive index (CI) of the TES system gradually decrease, while the standard deviation gradually increases. Simultaneously, the average temperature, availability, satisfaction, and CI of the TES system all increase with increasing working fluid mass flow rate for the same mass of working fluid. In the high-performance region (CI = 0.7 ~ 1.0), the modulating performance of exhaust temperature mainly depends on the mass of the working fluid supply. However, in the low-performance region (CI = 0 ~ 0.4), the sensitivity of mass and mass flow should be taken into account. In addition, in the high-performance region, the supply of 4 kg of working fluid at a speed of 1.0 kg/s can also be considered the best solution, and R404a is the most suitable working fluids for thermal energy storage. Under Europe Transient Cycle (ETC), the average temperature, availability, satisfaction, and CI are increased by 2.9%, 7.46%, 34.1%, and 100%, respectively, with the TES system, and the standard deviation is decreased by 53%. Compared to the ORC system, the coupled ORC-TES system leads to an increase in exhaust back pressure (EBP), but does not lead to an increase in fuel consumption due to the improved thermal efficiency of the ORC system. In addition, the integrated ORC-TES system can improve the SCR catalytic efficiency and reduce NOx emissions. From a cost–benefit analysis, the ORC-TES system is practically feasible.

Non-equilibrium condensation flow characteristics of wet steam considering condensation shock effect in the steam turbine

A non-equilibrium condensation flow occurs in steam turbines, accompanied by condensation shock. At present, there are limited studies on the influence of condensation shock on the flow and condensation characteristics of wet steam, and the unsteady influence of condensation shock on the flow field of wet steam always have been ignored. In this paper, the unsteady flow characteristics of condensation were presented by considering the condensation shock effect. First, the condensation flow models and corresponding source terms were programmed and loaded with UDS (user-defined scalar) and UDF (user-defined function), respectively, and the governing equations were discretized using a high-resolution calculation method. The calculated results agreed well with the reported experimental results. Based on the models, the non-equilibrium condensation flow characteristics with inlet and outlet pressures in Laval nozzle considering the condensation shock effect were analyzed. Finally, during the unsteady phase change process, the effects of back pressure and saturation of the inlet on the liquid phase parameters were examined considering the condensation shock effect in the Dykas cascade. The results show that a decrease in the back pressure and an increase in the inlet pressure improve the condensation shock intensity in Laval nozzle. With the increase of condensation shock intensity, the nucleation rate increases. Moreover, in Dykas cascade, with the increase of back pressure from 48.8 kPa to 73.2 kPa, the maximum wetness decreases from 4.8 % to 2.1 %, whereas with the increase of relative saturation from 0.58 to 0.88, the maximum wetness increases from 2.4 % to 3.6 %. The results obtained in this paper are of significance to accurately analyze the actual situation of non-equilibrium condensation flow of wet steam and to develop methods for reducing wet steam loss in turbine stage.

Model based design of a turbo-compound bottomed to internal combustion engine exhaust gas

Abstract The transportation sector is living a new era, where the conventional powertrains based on thermal engines are flanked by innovative ones, based on electric and hybrid systems. This revolutionizes the behaviour and the driving habits, as well as the figure of the whole propulsive system, which should integrate different energy sources on board and the energy demand for propulsion, auxiliaries, ancillary components, vehicle needs, etc. But, for heavy-duty vehicles, it is very difficult to abandon in the short and mean term the reciprocating combustion engine technology. Also, for passenger cars and light duty vehicles, the pure electric propulsion seems to put in more evidence limits not only technological. In this panorama, the development of very high efficiency engines is mandatory to fit the emissions targets, both referred to pollutant emission and CO2. In this regard, waste heat recovery into mechanical or electrical energy is one of the most promising options to reduce fuel consumption. It is of particular interest for heavy duty engines, where the operation does not suffer so much the transient phases, and hybrid powertrains, where the energy recovered can be stored in electrical form and used for all the necessities of the vehicles. In this paper, a waste heat recovery system based on an additional turbine placed in the exhaust line of a turbocharged internal combustion engine has been studied. The auxiliary turbine is designed thanks to a model-based approach. The performance map of the turbine has been calculated referring to the thermodynamic conditions of the engine exhaust gases as input parameters. The so-designed component is then integrated with an engine model, and the benefits of a turbo-compound technology bottomed to the engine were assessed. In this way, the potential power recoverable from the turbine is evaluated under design and off-design conditions. The integration with engine model allowed to estimate the side effects related to backpressure increase on the engine exhaust manifold (which leads to an overconsumption or an underrating of the engine torque), as well as the equilibrium change on the turbocharger shaft. Definitively, the final overall engine performances are assessed including the need for a bypass which, in certain engine working conditions, must exclude the recovery.

Numerical investigation of the flow characteristics inside a supersonic vapor ejector

Integrating a vapor ejector with an air-cooled absorption cooling system (ACS) requires understanding how the ejector responds to varying condenser conditions and how the geometrical parameters affect the system's performance. This study provides a numerical investigation of the flow characteristics inside supersonic vapor ejectors. The primary objectives were identifying the best nozzle design for ACS and explaining how the secondary flow responds to different back pressures. The developed model was validated against experimental data and a one-dimensional model. Despite exhibiting increased flow fluctuations, the convex nozzle achieved an entrainment ratio of 0.4. This value was 4.9 % and 7 % higher than the values obtained by the straight and the concave nozzles, respectively. In contrast, the concave nozzle exhibits better flow stability and pressure recovery, which are considered appealing for the air-cooled ACS. The straight nozzle emerged as a balanced alternative, offering moderate entrainment alongside favorable flow stability. Moreover, secondary flow behavior at different operating modes was elaborated. Secondary flow choked at back pressures between 60–70 kPa, indicating optimal entrainment. However, at 75–80 kPa, while the secondary flow was entrained, it failed to reach sonic speed due to high-pressure waves, resulting in the sub-critical condition. Further increases in back pressure to 85–90 kPa induced back-flow due to elevated local static pressure. Mach number profiles at the mixing tube entrance remained consistent under critical operation but deviated post-critical back pressure, reflecting altered flow characteristics downstream of the mixing tube. Such elaboration of flow dynamics within ejectors paves the way for innovative designs of vapor ejectors, potentially developing ACS.

Characterizing undrained behaviour of imperfectly saturated Palar sand

The occurrence of earthquake-induced soil liquefaction poses a significant threat, leading to extensive damage to building foundations and other structures, resulting in substantial economic repercussions. The seismic performance of geotechnical systems is markedly influenced by the saturation level of the soil. This study examines the impact of dynamic response on Palar sand. Cyclic triaxial tests were conducted on partially saturated fine-grained loose sand with a relative density of 35 % and a degree of saturation ranging from 65 % to 75 %. These tests were carried out at a strain rate of 0.1 % and confining pressures of 50 and 75 kPa. The study findings reveal that an increase in back pressure corresponds to a rise in the excess pore water pressure ratio of the sand. Additionally, the sand undergoes liquefaction as the number of cycles increases, and the degree of saturation decreases for different confining pressures at frequencies of 0.75 and 1 Hz. It was observed that soil liquefies more rapidly at lower strain rates with an increase in effective confining pressure. Conversely, at higher frequencies, soil liquefaction occurs in a smaller number of cycles. Comparing the effects of confining pressure and frequency, a damping ratio of 13 % and a shear modulus of 40 MPa were achieved at a frequency of 0.75 Hz and a confining pressure of 50 kPa. The shear modulus of partially saturated sand decreases with an increase in the initial degree of saturation due to specific characteristics of the Palar sand and the loading conditions.

Prediction of the non-equilibrium condensation characteristic of CO2 based on a Laval nozzle to improve carbon capture efficiency

The supersonic separator is considered as a more efficient and environmentally friendly technology within the field of decarbonization. The flow state of the vapor and the operating environmental conditions are significant factors which affects the separation efficiency. The effect of the inlet pressure (subcritical, near-critical, supercritical), temperature (supercritical), and the outlet back pressure on the CO2 non-equilibrium condensation flow characteristics is numerically calculated. The results show that when the pressure is higher or the temperature is lower, the non-equilibrium condensation process is promoted and the formation of droplets is more stable. The nucleation interval increases with the pressure increases. The peak nucleation rate increases with the temperature decreases. The condensation shock wave is weakened and the condensable area is reduced with the back pressure increases, resulting in the liquid mass fraction significantly decreases. The critical condensation back pressure reflects the ability of the vapor to resist back pressure shocks during the condensation process. The effect of inlet pressure, temperature, and expansion angle on the critical condensation back pressure is analysed. The condensation shock wave is enhanced by increasing the pressure, expansion angle, and decreasing the temperature, the vapor has a stronger ability to resist the influence of back pressure.

The preliminary design of emergency core cooling scheme and loss-of-coolant accident analysis for Tsinghua high flux reactor

This paper proposes the preliminary emergency core cooling scheme for Tsinghua High Flux Reactor. According to the thermohydraulic characteristics of high flux reactors, forced circulation needs to be maintained by emergency pumps in the early stage. When the decay power is low enough, natural circulation between core and the reactor pool is initiated to remove the residual core heat. The sources of safety injection are accumulators and reactor pool. Accumulator injection can ensure core safety in the early stage and reactor pool injection can maintain long time stable forced circulation. To avoid emptying the reactor pool, the waterproof zone needs to be built. The waterproof zone consists of reactor pool and several finite volume dry pools. All pipelines and equipment in the reactor coolant system are placed in the dry pools. Once break accident occurs, the dry pool where the break is located can collect the leaked coolant. With the increase of break back pressure, the break flow is restricted. The current scheme is modeled by Relap5 and different size breaks at four locations (namely core inlet, core outlet, primary heat exchanger inlet and main pump inlet) are assumed. According to the response characteristics of the scheme, the accident process can be divided into five stages: non-shutdown stage, high-injection stage, low-injection stage, stable forced circulation stage and natural circulation stage. Critical heat flux predicted by Sudo CHF correlations is adopted as the primary safety criterion. Through analysis, a success path is found to maintain core safety for a wide range of loss-of-coolant accident scenarios.

Effect of ash-bridge deposition in asymmetric diesel particulate filter channels on the pressure drop and particulate matter trapping characteristics

A diesel particulate filter (DPF) is commonly used to trap particulate matter (PM). The collapse and sintering of carbon and ash deposits during exhaust gas flow and DPF regeneration frequently result in the formation of ash bridges, which are buildups of ash and PM that clog the filter channels. To investigate the impact of an ash bridge formed by accumulated ash on the pressure drop and PM trapping characteristics of asymmetric DPFs, an asymmetric DPF channel with an ash bridge was constructed. The analysis employed computational fluid dynamics to examine how the ash bridge affects the pressure drop and PM trapping characteristics in an asymmetric DPF. This study reveals that the asymmetric DPF design effectively enhances the ash-holding capacity, thereby mitigating the risk of channel blockage. The presence of ash bridges in DPF channels has a more substantial impact on the pressure drop through radial congestion than through axial congestion. The application of asymmetric thin-wall DPFs can counteract the increase in exhaust backpressure caused by ash bridges. An ash bridge located in the middle of a DPF channel intensifies the uneven deposition of PM. However, implementing an asymmetric DPF structure enhances the uniformity of PM deposition.

Exhaust Gas Flow Study of Electric Turbo Compounding (ETC) to Determine the Potential Electrical Energy Recovery from Exhaust Emission

Electric turbo compounding (ETC) emerges as a pivotal energy recovery solution, seamlessly combining an advanced turbocharger with a high-speed generator to harvest electrical energy from exhaust gas dynamics. This research delves into the practical application of ETC in vehicles, employing a comprehensive approach blending simulation techniques with experimental validation. The investigative process involves meticulous data collection on vehicle muffler dimensions, subsequent device modelling based on this data, and rigorous flow simulations, followed by thorough analysis. The study's key findings highlight a noteworthy 9% increase in engine back pressure at 850 rpm, counterbalanced by a 7% reduction at 2000 rpm. Despite the integration of the ETC mechanism, electric current measurements remain consistently within the range of 1.4 to 1.6 amperes at 1200-2000 rpm. This research not only unveils the tangible effects of ETC on engine performance but also underscores its viability as an efficient energy recovery solution in the automotive sector, setting the stage for advancements in sustainable transportation.

Aerodynamic instability evolution of a multi-stage combined compressor

Unstable operating conditions such as surge could cause damage to both aerodynamic performance and structural integrity of a compression system. This paper addresses the critical issue of aerodynamic instability in compressor design, particularly focusing on an axial-centrifugal combined compressor, a widely used yet underexplored configuration. An experimental investigation was conducted on a three-stage axial and one-stage centrifugal compressor (3A1C), using two pipe systems and employing fast-responding transducers to capture the dynamic instability process from choke condition to deep surge. Results reveal that at the design speed, 3A1C enters deep surge directly, whereas at off-design speeds, it experiences rotating stall and mild surge across a wide mass flow range. Some special instability features in the combined compressor can be found in the steady state map and dynamic process. The characteristic curve of the first axial stage keeps a positive slope during the whole mass flow range at an off-design speed. The first stage could work stably on the stall characteristic curve because the centrifugal stage has stronger pressurization and plays a dominant role in global aerodynamic instability. Besides, rotating instability occurs at the first rotor tip and disappears as the back pressure increases, which is also rarely seen in a single-axial compressor. This is also related to the strong pressurization of the centrifugal stage. The findings of this paper will contribute to the understanding of aerodynamic instabilities in combined compressors.

Gas exchange optimization in aircraft engines using sustainable aviation fuel: A design of experiment and genetic algorithm approach

The poppet valves two-stroke (PV2S) aircraft engine fueled with sustainable aviation fuel is a promising option for general aviation and unmanned aerial vehicle propulsion due to its high power-to-weight ratio, uniform torque output, and flexible valve timings. However, its high-altitude gas exchange performance remains unexplored, presenting new opportunities for optimization through artificial intelligence (AI) technology. This study uses validated 1D + 3D models to evaluate the high-altitude gas exchange performance of PV2S aircraft engines. The valve timings of the PV2S engine exhibit considerable flexibility, thus the Latin hypercube design of experiments (DoE) methodology is employed to fit a response surface model. A genetic algorithm (GA) is applied to iteratively optimize valve timings for varying altitudes. The optimization process reveals that increasing the intake duration while decreasing the exhaust duration and valve overlap angles can significantly enhance high-altitude gas exchange performance. The optimal valve overlap angle emerged as 93 °CA at sea level and 82 °CA at 4000 m altitude. The effects of operating parameters, including engine speed, load, and exhaust back pressure, on the gas exchange process at varying altitudes are further investigated. The higher engine speed increases trapping efficiency but decreases the delivery ratio and charging efficiency at various altitudes. This effect is especially pronounced at elevated altitudes. The increase in exhaust back pressure will significantly reduce the delivery ratio and increase the trapping efficiency. This study demonstrates that integrating DoE with AI algorithms can enhance the high-altitude performance of aircraft engines, serving as a valuable reference for further optimization efforts.

Investigation on gas–liquid–solid three-phase flow model and flow characteristics in mining riser for deep-sea gas hydrate exploitation

In response to the problem of gas–liquid–solid three-phase flow in deep-sea hydrate extraction pipelines, a gas–liquid–solid three-phase flow model considering the dynamic decomposition of hydrates is established using continuity equations, momentum equations, and energy equations. The numerical solution of the theoretical model is achieved using the finite difference method. Comparing the theoretical model with the experimental results, the results showed that the average error of gas holdup, liquid holdup, solid phase content, gas phase velocity, liquid phase velocity, and solid phase velocity obtained from the theory and experiment are 8.24%, 0.41%, 1.88%, 5.80%, 2.81%, and 2.22%, respectively, which verified the correctness of the theoretical model. On this basis, the influences of hydrate abundance, liquid phase displacement, and wellhead backpressure on the gas–liquid–solid three-phase flow characteristics in the pipeline were investigated, and it was found that the gas holdup rate will increase with the increase in hydrate abundance, liquid phase displacement, and wellhead backpressure, with the influence of hydrate abundance being more sensitive. The liquid holdup rate increases with the increase in hydrate abundance and liquid phase displacement, but decreases first and then increases toward the wellhead position with the increase in wellhead backpressure. The solid phase content decreases with the increase in hydrate abundance, and first increases and then decreases toward the wellhead position as the liquid phase displacement and wellhead backpressure increase. The influence of gas phase velocity on the abundance of hydrates is relatively small, but it increases with the increase in liquid phase displacement. When the wellhead backpressure increases, the instantaneous increase then tends to flatten out. The influence of hydrate abundance on the liquid phase velocity is also relatively small, but it increases with the increase in liquid phase displacement and decreases with the increase in wellhead backpressure. The solid phase velocity will increase with the increase in hydrate abundance and liquid phase displacement, but it will not show significant changes with the change of wellhead backpressure. The research results can provide a theoretical basis for the safety of hydrate mining.

Distribution of relaxation times used for analyzing the electrochemical impedance spectroscopy of polymer electrolyte membrane fuel cell

In this paper, a 25 cm2 polymer electrolyte membrane single cell is prepared by ultrasonic spraying coating method. The polarization curves and Electrochemical Impedance Spectroscopies (EIS) data of this cell are collected under different conditions, such as cell temperature, humidity, back pressure and pure O2. EIS data are analyzed by Distribution of Relaxation Time (DRT), and some results are concluded: (i) with the increase of current density, the Oxygen Reduction Reaction (ORR) time constant became lower, and the ORR reaction resistance and oxygen diffusion resistance both increases. (ii) With the increase of back pressure, the DRT peak assigned to oxygen diffusion process in cathode moves to low frequencies, the peak area decreases simultaneously. (iii) the proportion of the ORR polarization resistance gradually increases from 77 % to 84 % with the cell temperature increase from 65 °C to 80 °C. (iv) With the increase of humidity, the performance of the cell is decreased. (v) The oxygen diffusion resistance is negligible in pure oxygen working condition analyzed by DRT method.

Highly integrable silicon micropumps using lateral electrostatic bending actuators

We present the design, fabrication, and characterization of an innovative silicon-based micropump with high potential for portable lab-on-chip (LoC) as well as point-of-care (PoC) applications. The actuators of the pump are electrostatic driven in-plane bending devices, which were presented earlier (Borcia et al. in Phys Rev Fluids 3(8): 084202, 2018. 10.1103/PhysRevFluids.3.084202; Uhlig et al. in Micromachines, 9(4), 2018. 10.3390/mi9040190). This paper presents the characterization results achieved with the micropump. The dielectric non-polar liquid Novec7100™ was used as a test liquid due to its adequate physical properties. When applying a periodic voltage of 130 V, a flow rate of up to 80 µL/min was detected. The counter pressure amounts up to 30 kPa and the correspondent fluidic power (volumetric flow rate times the counter pressure) was calculated to 10 µW. The pump contains passive flap valves at the inlet and outlet, which are based on a bending cantilever design. Depending on the application requirements, the micropump can be designed modularly to adjust the specific parameters by an adequate arrangement of pump base units. In this paper, the proof of principle is shown using a single base unit with different number of stacked NED-actuator beams, as well as the serial arrangement of base units. Both modular concepts target the increase of backpressure of the NED-micropump in an inherently different way compared to conventional membrane micropumps.

Simultaneous measurement and analysis of gas needle lift and injected rate for HPDI fuel injector

Achieving a sufficient understanding of the transient fuel injection process is key to improving the efficiency of direct injection engines and reducing emissions. In this research, the momentum method and pressure–volume method are applied to measure the gas injected rate, the eddy current displacement sensor is installed underneath the open-type gas needle to measure the needle lift. The dynamic characteristic of the dual-fuel injector is studied by measuring the gas injected rate and the needle lift simultaneously. To simulate to the actual high-pressure environment of the engine, the influence of back pressure on the dynamic behavior of the injector is paid more attention in the experiment. In addition, the experiment of the main-after injection strategy is investigated to analyze the influence of the interval time on the after-injection. The experimental results are as follows: Firstly, the increase of back pressure will significantly promote the needle rising stage. That is, the maximum position that the needle can reach increases under the short energizing time, while the needle reaches the maximum limit faster under the long energizing time. When the injected pressure is 30 MPa while the energizing time is 0.7 ms, the gas injected mass increases by 33.92 mg (an increase of 86.8 % compared with the gas injected mass under atmospheric pressure), and the gas injected mass increased by 20.3 mg (an increase of 11.9 % compared to atmospheric pressure). Secondly, the reflected compression wave generated by the diesel supply system will inhibit the needle rising stage, resulting in the speed change of the needle lifting process. With the increase of back pressure, the affected proportion of the needle rising stage is sharply decreased even no longer influenced by the reflected compression wave. Finally, the gas injection rate develops from fusion to separation with the interval time increasing, resulting in two different trends during injected mass growth. The increase in back pressure leads to the delay of the main injection ending and the advance of the after injection starting, which facilitates the injection fusion.

Experimental Investigation on the Aerodynamic Instability Process of a High-Speed Axial-Centrifugal Compressor

Abstract Aerodynamic instability plays an important role in compressor design and may cause performance degradation and fatigue damage. In this paper, an experimental study on the evolution of aerodynamic instability is carried out on a compressor that combines the performance benefits of an axial stage and centrifugal stage. The spatiotemporal characteristics of unsteady wall pressure were obtained using fast-responding pressure transducers over a range of operating conditions. The results show that the axial stage works on the positive slope of the performance characteristic curve from choke to stall at low-speed operating conditions, and mainly features rotating instability. Rotating stall is also observed in the impeller (IMP) and diffuser passages. At medium-speed operating conditions, the centrifugal stage suffers a high-frequency mild surge, alternating with rotating stall. With the increase in back pressure, the mild surge diminishes, and rotating stall persists. This behavior is similar to a two-regime-surge, which has been reported for centrifugal compressors. At high-speed operating conditions, the compressor directly reaches surge without other instabilities. Further analysis of the spatial pattern of the rotating stall revealed the existence of a high-pressure region near the volute tongue, resulting in obvious pressure distortion along the circumferential direction at the volute inlet. This induced the amplitude difference of stall cells in corresponding diffuser passages. The disturbance caused by stall cells propagates upstream through the blade passage, and the largest pressure disturbance induced by the stall cell propagation appears in a circumferential position 45 deg downstream of the volute tongue at the impeller inlet and the axial stage inlet.

Experimental investigation on the impact of air flow rates and back pressures on kerosene microscopic spray characteristics discharged from an air-assisted pressure-swirl atomizer

The air-assisted pressure swirl atomizer has attracted considerable interest owing to its exceptional capacity to generate fine droplets and precisely regulate spray characteristics. The primary objective of the study was to investigate the impact of auxiliary air flow rate, orifice diameter, and ambient back pressure on the kerosene spray cone angle, droplet size and distribution. The results indicate that employing a nozzle with a smaller orifice size leads to a larger spray cone angle. The spray cone angles of the other two orifice sizes exhibit an increasing trend as the auxiliary air flow rate increases. Also, increasing the auxiliary air flow rate effectively reduces the Sauter Mean Diameter (SMD). Additionally, as the auxiliary air flow rate gradually increases, the peak of the particle size distribution shifts towards smaller diameters. Furthermore, as the environmental back pressure increases, there is a noticeable decrease in the SMD. With an increase in ambient backpressure, there is a consistent shift of the peak particle size towards smaller diameters. At an elevated ambient pressure of 1.4 MPa, an increase in the axial distance from the nozzle results in a decrease in the volume percentage of the peak diameter, while simultaneously causing the particle size distribution range to broaden.

Experimental investigation of CO2 foam flooding-enhanced oil recovery in fractured low-permeability reservoirs: Core-scale to pore-scale

Fractured low-permeability reservoirs are characterized by poor reservoir physical properties, strong interlayer heterogeneity and the existence of natural and artificial fractures, resulting in significant water or gas channeling during flooding development. CO2 foam can effectively control the mobility of CO2 and ensure a more stable flooding front. In this study, taking into account the imbibition effect of the flooding process, the flooding performance under different injection methods and the influencing factors of foam pre-injection volume, back pressure and fracture complexity on CO2 foam flooding were systematically analyzed using fractured low-permeability core flooding experiments. In addition, the mechanism of CO2 foam flooding enhanced oil recovery (EOR) in low-permeability fractured reservoirs was obtained in combination with microscopic flooding experiments. The results show that the maximum recovery of 59.36 % can be obtained by soaking the core samples for 12 h with pre-injection of 0.4 PV CO2 foam followed by CO2 foam flooding, which adequately considers the imbibition effect during the flooding process. The pressure difference and oil recovery increases with increasing foam pre-injection amount, and the oil recovery with back pressure (2 MPa and 4 MPa) is significantly higher than that without back pressure; however, a further increase in back pressure has less impact on the recovery. The oil recovery rate of the fracture-free core is lower than that of the multiple fracture core during CO2 foam flooding but higher than that of the single fracture core, and the fracture complexity of the fractured cores has less impact on the final oil recovery. Microscopic experiments show that CO2 foam can effectively prevent channeling and improve the sweep efficiency. Soaking during the CO2 foam flooding process makes foam enter the matrix through the fracture, and CO2 dissolves after the foam breaks to increase the mobility of the crude oil, while the surfactant solution emulsifies the crude oil and changes the rock wettability. The unbroken foam can increase the sweep range, and the crude oil that flows into the fracture is carried by the subsequent injected foam to be recovered as foamy oil.

Temperature prediction model in multiphase flow considering phase transition in the drilling operations

The study considers gas compression properties, gas slippage, back pressure (BP), phase transition (PT), well depth, and differences in gas-liquid physical properties. A new temperature model for multiphase flow is proposed by considering phase transition in the drilling process. The mathematical model of multiphase flow is solved using the finite difference method with annulus mesh division for grid nodes, and a module for multiphase flow calculation and analysis is developed. Numerical results indicate that the temperature varies along the annulus with the variation of gas influx at the bottom of the well. During the process of controlled pressure drilling, as gas slips along the annulus to the wellhead, its volume continuously expands, leading to an increase in the gas content within the annulus, and consequently, an increase in the pressure drop caused by gas slippage. The temperature increases with the increase in BP and decreases in gas influx rate and wellbore diameter. During gas influx, the thermal conductivity coefficient for the gas-drilling mud two phases is significantly weakened, resulting in a considerable change in temperature along the annulus. In the context of MPD, the method of slightly changing the temperature along the annulus by controlling the back pressure is feasible.

EXPERIMENTAL STUDY ON FLASH BOILING SPRAY OF HIGH-PRESSURE LIQUID AMMONIA JET WITH ROUND AND ELLIPTICAL HOLE NOZZLES

Ammonia is an ideal alternative fuel for mitigating carbon emissions. High-pressure direct injection of liquid ammonia (LNH<sub>3</sub>) offers significant advantages in enhancing energy efficiency and minimizing emissions. Due to the high saturation vapor pressure, the injection of LNH<sub>3</sub> is susceptible to flash boiling. In this study, we used high-speed micro-imaging technology with backlight lighting to establish a high-pressure common-rail LNH<sub>3</sub> jet experimental platform and investigate the flash boiling spray characteristics of nozzles with round and elliptical holes. The results demonstrated that under non-flash boiling conditions, the residual LNH<sub>3</sub> in the sac chamber and nozzle can rapidly corrode the acrylic material of the nozzle, leading to deformation and failure of the nozzle structure. Under flash boiling conditions, LNH<sub>3</sub> ejected from the hole will produce spherical macroscopic spray morphology. Then, the spray gradually transitions from an elliptical profile to a conical profile as the back pressure increases. Compared to nozzles with round holes, nozzles with elliptical holes exhibit higher flow velocity, which enhances oil-gas mixing and promotes more pronounced flash boiling phenomena. Flash boiling occurs at an earlier stage with an increase in the spray cone angle, thereby improving the atomization characteristics under both flash and non-flash boiling conditions. The tail jet of nozzles with elliptical holes terminates earlier while exhibiting a higher decrease rate in the average gray value, which improves the atomization quality in the tail spray stage and meets the requirements of timing, quantification, and precise control of the fuel injection system.