Bypass Inlet Research Articles

Performance investigation on the bypass ejector for a proton exchange membrane fuel cell system

The poor entrainment performance of the conventional ejector is a significant problem that makes it hard to use in proton exchange membrane fuel cell (PEMFC) systems. This study aims to evaluate the entrainment performance of a bypass ejector for a 100 kW PEMFC system. The effects of three critical geometric parameters, namely the axial position, width, and angle of the bypass inlet, on the entrainment performance are thoroughly investigated. The results demonstrate that the bypass flow exhibits a significant performance improvement in the critical mode. In contrast, the performance improvement is negligible and even negative in the subcritical mode. After careful evaluation of the entrainment performance across various stack powers, the optimal axial position, width, and angle of the bypass inlet are found to be 1.1, 2 mm, and 10°, respectively. A comparative analysis between the bypass ejector and the conventional ejector underscores a significant advantage for the former, exhibiting a remarkable 22.1 % increase in the hydrogen entrainment ratio at the stack power of 101 kW. Nevertheless, the entrainment performance of the bypass ejector diminishes when operating at low stack powers below 24 kW.

Study of the mixing characteristics of a multibypass combustor of a turbine-based combined cycle

When the multimode and multibypass turbine-based combined cycle (TBCC) engine performs mode transition, it involves switching between multiple bypasses. During the transition process, the flow field of the combustor is adjusted rapidly, and the mixing performance is affected. However, superior mixing is the premise of the superior combustion performance of the combustor. In this paper, the mixing characteristics of the TBCC triple-bypass combustor during the mode transition process are studied by numerical simulation. Different structural and aerodynamic parameters, including equivalent expansion angle (5.5°, 9.2°, 12.8°), lobe mixer width (20, 25, 30 mm), bypass ratio (0.44, 0.56, 0.66) and inlet temperature, were studied to discuss the trending of flow field, vorticity field, temperature field, and oxygen content distribution. The results show that the performance of triple-bypass combustor is worse than that of double-bypass combustor under the condition of turbo core inlet Mach number 0.3. The total pressure loss is increased by 0.3% and the thermal mixing efficiency is decreased by 12.4%. The increase in the equivalent expansion angle reduces the mixing efficiency (12.5%) and improves the total pressure recovery coefficient (1%). However, increases in lobe width, bypass ratio, and ram inlet temperature increased mixing characteristics but increased flow losses. The increase in turbo bypass inlet temperature (300–500 K) improved the total pressure recovery coefficient by 0.17% and the thermal mixing efficiency by 7.0%. Conversely, the increase of the turbo core inlet temperature (800–1200 K) not only reduces the total pressure recovery coefficient, but also reduces the mixing performance. In addition, the studies show that the mixing performance of the lobe mixer is mainly affected by the velocity difference between the primary and secondary flow. Besides the streamwise vortex and normal vortex, the normal vortex dissipation rate controlled by the velocity difference is also one of the factors affecting the mixing performance.

Swirl-Bypass Nozzle for CO2 Two-Phase Ejectors: Numerical Design Exploration

In this work, a novel ejector design concept of a swirl-bypass nozzle is proposed to improve off-design performance of CO2 two-phase ejectors. The swirl-bypass nozzle allows part of the flow to bypass into the ejector mixing chamber to generate swirl. The design of such a device is investigated using a 3D multiphase CFD model. An extensive experimental test campaign is conducted to validate the baseline homogeneous equilibrium CFD model. The model’s prediction motive mass flow rate within 2–12% error and suction mass flow rate was predicted with 3–50% error. Based on the tested ejector geometry, simulations of different ejector swirl-bypass inlets are conducted. The results show that, for the current design, total entrainment of the ejector is reduced by 2–20% with the swirl-bypass inlet. The axial position of the bypass inlet plays a primary role in the bypass inlet flow rate, and, consequently, in suction flow reduction. This is found to be due to the bypass flow blocking off the suction mass flow rate, which has a net negative impact on performance. Finally, several design improvements to improve future designs are proposed.

Numerical Study on Proppant Transport in Supercritical Carbon Dioxide under Different Fracture Shapes: Flat, Wedge-Shaped, and Bifurcated

As a new type of waterless technology, supercritical carbon dioxide fracturing has been widely studied by scholars in recent years, and the migration characteristics of the corresponding proppant in supercritical carbon dioxide still need further research. In this paper, the Eulerian–Eulerian computational fluid dynamics method was used to study the transport capacity of supercritical carbon dioxide, and the UDF method was used to simulate the physical parameters of supercritical carbon dioxide. In view of the deficiencies of previous studies, the special cases of wedge-shaped fractures and bypass fractures are considered, and the influence of large-span pressure and temperature on migration is first analyzed in plane fractures, which makes this study more complete. The results show that: (1) compared with slickwater, the proppant transport channel in supercritical carbon dioxide is 30% smaller at 305 K and 10 MPa. (2) The transport capacity of supercritical carbon dioxide increases with the increase of pressure and decreases with the increase of temperature. But when the pressure or temperature is too high, they have little effect on it. (3) In wedge-shaped fractures, the proppant stack height and length increase initially as the shrinkage rate of fracture width (the ratio of the fracture reduced width to the fracture length) increases. However, as the fracture width ratio increases, the maximum proppant stack height decreases in the later stage. (4) In bifurcated fractures, with the increase of bypass angle, the area of proppant in the bypass zone tends to decrease. The width of the bypass inlet has little effect on the proppant settlement in the bypass. This study further understands the migration law of proppant in supercritical carbon dioxide in fractures.

Effect of non-Newtonian fluid rheology on an arterial bypass graft: A numerical investigation guided by constructal design

In post-operative scenarios of arterial graft surgeries to bypass coronary artery stenosis, fluid dynamics plays a crucial role. Problems such as intimal hyperplasia have been related to fluid dynamics and wall shear stresses near the graft junction. This study focused on the question of the use of Newtonian and non-Newtonian models to represent blood in this type of problem in order to capture important flow features, as well as an analysis of the performance of geometry from the view of Constructive Theory. The objective of this study was to investigate the effects rheology on the steady-state flow and on the performance of a system consisting of an idealized version of a partially obstructed coronary artery and bypass graft. The Constructal Design Method was employed with two degrees of freedom: the ratio between bypass and artery diameters and the junction angle at the bypass inlet. The flow problem was solved numerically using the Finite Volume Method with blood modeled employing the Carreau equation for viscosity. The Computational Fluid Dynamics model associated with the Sparse Grid method generated eighteen response surfaces, each representing a severe stenosis degree of 75% for specific combinations of rheological parameters, dimensionless viscosity ratio, Carreau number and flow index at two distinct Reynolds numbers of 150 and 250. There was a considerable dependence of the pressure drop on rheological parameters. For the two Reynolds numbers studied, the Newtonian case presented the lowest value of the dimensionless pressure drop, suggesting that the choice of applying Newtonian blood may underestimate the value of pressure drop in the system by about 12.4% (Re =150) and 7.8% (Re = 250). Even so, results demonstrated that non-Newtonian rheological parameters did not influence either the shape of the response surfaces or the optimum bypass geometry, which consisted of a diameter ratio of 1 and junction angle of 30°. However, the viscosity ratio and the flow index had the greatest impact on pressure drop, recirculation zones and wall shear stress. Rheological parameters also affected the recirculation zones downstream of stenosis, where intimal hyperplasia is more prevalent. Newtonian and most non-Newtonian results had similar wall shear stresses, except for the non-Newtonian case with high viscosity ratio. In the view of Constructal Design, the geometry of best performance was independent of the rheological model. However, rheology played an important role on pressure drop and flow dynamics, allowing the prediction of recirculation zones that were not captured by a Newtonian model.

Experimental and Numerical Investigation of the Effect of Bypass Injection on Wall Temperature Distribution of a Single-Phase Mini/Micro-Channel

Temperature uniformity is an important factor in the thermal management of electronic devices. The present study investigates the effect of bypass injection on the wall temperature distribution of a single-phase mini/micro-channel using water as the coolant. Experimental studies were conducted on an 18 mm thick copper substrate heated by a cartridge heater, with the bypass inlet located midway between the channel inlet and outlet. After validating the numerical results with the experimental data, simulations were carried out on a 1 mm thin copper and silicon substrates with uniform and non-uniform heat flux conditions on the back of the substrate, for two different locations of bypass injection. Results for the thin copper and silicon substrates with uniform heat flux condition indicate that the bypass injection helps in reducing the difference between the maximum and minimum local wall temperatures by up to 60% and 76% for silicon and copper substrates, respectively. The reduction is higher for the bypass injection located midway between the channel inlet and outlet compared to the injection located at one-fourth of the channel length from the inlet. Bypass injection is found to be beneficial for the non-uniform (step change) heat flux condition, with the higher heat flux over the second half of the channel length. The experimental and numerical results for the thick copper substrate show a near-uniform wall temperature distribution even without bypass injection.

Numerical assessment of ejector performance enhancement by means of two-bypass inlets

The pursuit of high efficiency ejectors has been the target of researchers since the invention of these devices. In the present study, numerical work on an ejector with one bypass inlet (with different inlet widths) and two bypass inlets (with different bypass inlet positions) was proposed to enhance performance. The entrainment ratios when the bypass inlet and secondary inlet were under different pressures were also analyzed. In comparison with the original ejector, the ejector with one bypass inlet had better entrainment performance, and an optimal width of the bypass inlet for the highest entrainment ratio was found. In addition, the performance of the ejector with two bypass inlets was superior to that of the ejector with one bypass inlet, when the primary or induced pressure exceeded the designed value. In comparison with the original ejector, the largest increase of entrainment ratio was 48.93% for the ejector with two bypass inlets and 34.88% for the ejector with one bypass inlet. When the bypass inlet pressures differed from the induced pressure, the optimal operational scheme for high entrainment ratio was that the induced pressure should be the maximum value, and the first bypass pressure should be the minimal value.

Experimental investigation of the effect of bypass inlet on flow boiling in a mini/micro-channel

The present work reports the findings from an experimental study on the effect of bypass inlet on the performance of flow boiling of water in a copper mini/micro-channel of dimension 2.5 mm wide x 0.6 mm deep x 25 mm long. The mass fluxes considered were 430 kg/m2 s, 640 kg/m2 s and 850 kg/m2 s and the heat flux was in the range 87–548 kW/m2. The results show a significant increase in heat transfer coefficient with the increase in bypass ratio for the subcooled boiling conditions and the enhancement decreases with the increase in the exit quality. An increase in pressure drop was also noticed due to the bypass inlet. The study indicates potential for further investigation and optimization.

A survey on exergy, energy and environmental analysis of sulfur recovery unit in case of five intensified configurations

In this study, a combination of energy and exergy analysis is applied for the industrial sulfur recovery unit (SRU) in commercial simulation software. The data of energy and exergy are obtained from the simulator and Excel (for physical and chemical exergy), respectively. In doing so, five intensified SRU configurations namely: (1) hot gas bypass (2) acid gas bypass (3) inlet stream preheating (4) acid gas enrichment and (5) oxygen (air) enrichment are compared in terms of energy production, reaction furnace temperature, sulfur recovery and pollutant emission into environment. Then, irreversibility in these five arrangements is compared and all the proposed methods show higher useful exergy comparing to the design case while the preheating method was the highest. Eventually, the main contributors and sources of exergy destruction are first incinerator (about 29–36%), then reaction furnace and waste heat boiler.

Numerical assessment of ejector performance enhancement by means of combined adjustable-geometry and bypass methods

The relatively low entrainment ratio is the key problem that confines the ejector development in the industry. In this study, the adjustable-geometry ejectors (with three different spindle positions) and ejectors with bypass (with two different bypass inlet installations) were adopted to promote the entrainment ratio of an ejector. The performance of the adjustable-geometry combined with bypass was also investigated. Results showed that the adjustable-geometry ejector and ejector with bypass could improve the entrainment ratio, and both ejectors reached a peak value. Compared with the original ejector, the maximum increment was 51.47% for the adjustable-geometry ejector and 26.0% for the ejector with bypass, respectively. Meanwhile, the combined ejector increased the entrainment ratio, although the combination was only effective at higher induced pressures than that of the adjustable-geometry ejector without bypass. The maximum increment was up to 74.5% compared to that of the original ejector.

Performance improvement of the R744 two-phase ejector with an implemented suction nozzle bypass

In this study, the concept of an R744 ejector with a bypass duct of a suction nozzle was presented. The design of the geometry and bypass positioning in a mixing section, and the idea of regulation, as well as integration, with a suction nozzle duct was described. A preliminary numerical analysis of the proposed bypass geometry was also presented. The computational platform ejectorPL integrated with an extensively validated mathematical model of transcritical R744 two-phase flow was used. Motive nozzle inlet and suction nozzle inlet conditions reflecting gas cooler and evaporator conditions characteristic of large systems, such as supermarket refrigeration units, were examined. Three different separation pressures in liquid receivers were assumed for two bypass geometries and six bypass inlet positions. The bypass positioning as a function of a mixer length was presented. Uniquely positive results were obtained for the lowest pressure conditions. Namely, the increment of the suction mass flow rate was substantial and equal to 36.9% for the bypass angle of 19°. Hence, the bypass implementation resulted in distinct potential of an efficiency improvement from 22.2% to 30.4%. A higher pressure lift case did not result in any improvement of the ejector work. The influence of the bypass geometry on the overall ejector efficiency was preliminarily characterised. Finally, the pressure distribution in the bypass type ejector was described.

Computational fluid dynamics study of the end-side and sequential coronary artery bypass anastomoses in a native coronary occlusion model†

The objective of this study was to evaluate the haemodynamic patterns in each anastomosis fashion using a computational fluid dynamic study in a native coronary occlusion model. Fluid dynamic computations were carried out with ANSYS CFX (ANSYS Inc., Canonsburg, PA, USA) software. The incision lengths for parallel and diamond anastomoses were fixed at 2 mm. Native vessels were set to be totally occluded. The diameter of both the native and graft vessels was set to be 2 mm. The inlet boundary condition was set by a sample of the transient time flow measurement which was measured intraoperatively. The diamond anastomosis was observed to reduce flow to the native outlet and increase flow to the bypass outlet; the opposite was observed in the parallel anastomosis. Total energy efficiency was higher in the diamond anastomosis than the parallel anastomosis. Wall shear stress was higher in the diamond anastomosis than in the parallel anastomosis; it was the highest at the top of the outlet. A high oscillatory shear index was observed at the bypass inlet in the parallel anastomosis and at the native inlet in the diamond anastomosis. The diamond sequential anastomosis would be an effective option for multiple sequential bypasses because of the better flow to the bypass outlet than with the parallel anastomosis. However, flow competition should be kept in mind while using the diamond anastomosis for moderately stenotic vessels because of worsened flow to the native outlet. Care should be taken to ensure that the fluid dynamics patterns are optimal and prevent future native and bypass vessel disease progression.

Impact of operational and geometrical factors on ejector performance with a bypass

The mechanical energy loss during the mixing process and the momentum waste in the diffuser are the main reasons for the relative low entrainment ratio. In the present study, the ejector with bypass is adopted to utilize the momentum in the diffuser, hoping to increase the entrainment ratio. The influence of operational and bypass geometrical factors on the entrained capacity is analyzed through CFD method. The results show that there is always an optimal location (around the place where the lowest pressure is) for the maximum entrainment ratio, and it moves downstream as the motive or induced pressure increases. Meanwhile, the increment enlarges as the motive pressure increases, and the improvement would enlarge from 10.7% to 32.8% when the motive pressure increases from 0.3 MPa to 0.4 MPa. Moreover, it is still an effective method to improve ejector performance by enlarging bypass inlet area when the bypass is installed in the optimal location. The same conclusion could be drawn when the inclination angle decreases, especially when the motive pressure is greater than the design value. Compared with the ejector without bypass, the maximum improvement could be high up to 48.7% while the inlet area is 4 mm × 8 mm and angle is 15°.

Experimental investigations on combustion characteristics of a cavity pilot augmentor of the turbine-based combined cycle engine

A cavity flame holder has been a very promising novel concept to be applied to the augmentor of a turbine-based combined cycle (TBCC) engine as it offers improvements in ignition performance, altitude relight, lean blowout limit, operating range, as well as combustion efficiency compared to conventional bluff body flame holders. The present paper describes the detailed designs of a cavity pilot augmentor and provides a laboratory-scale test rig. A series of experiments has been conducted to investigate the combustion characteristics of the test rig under atmospheric pressure. Experimental results show that for the bypass inlet temperature 343–473 K and inlet Mach number 0.25–0.40, ignition equivalence ratio and ignition pressure oscillation obtained is between 0.94–1.72 and 1.1–2.4% respectively, and lean blowout equivalence is between 0.42 and 0.92, indicating good ignition and lean blowout performances; as the total fuel-to-air ratio increases, the combustion efficiency decreases in the main at both afterburner operating mode and ramjet operating mode, while the increase of the bypass inlet Mach number or temperature is beneficial to high combustion efficiency, especially as inlet temperature over 423 K, and the combustion efficiency got in present experiment is between 70% and 95%. This paper demonstrates the feasibility for the cavity pilot flame holder to be applied to a realistic augmentor of TBCC engine preliminarily and provides reference for TBCC augmentor design.

Low volume sampling device for mass spectrometry analysis of gas formation in nickel–metalhydride (NiMH) batteries

Rechargeable nickel–metalhydride (NiMH) batteries have major advantages with respect to environmental friendliness and energy density compared to other battery systems. Research on thermodynamics and reaction kinetics is required to study the behaviour of these batteries, especially under severe operating conditions such as overcharging and (over)discharging. During these processes several reactions take place resulting in the formation of oxygen and hydrogen gas. Hence, the recombination processes should be well controlled to guarantee that the partial oxygen and hydrogen pressure inside the battery are kept low. Mass spectrometry is one of the analytical techniques capable of measuring the composition of gases released inside the battery during the charge and discharge processes. However, the sample gas needs to be withdrawn from the battery during the experiment. The gas consumption must be kept to a minimum otherwise the equilibrium inside the battery will be disturbed. A bench-top quadrupole mass spectrometer with a standard capillary by-pass inlet cannot be used for this purpose as its gas consumption is in the 1–10 ml/min range. In this paper, a new gas inlet device is presented that reduces gas consumption to a value <50 μl/h. The use of a capillary by-pass splitter and a discontinuous sampling procedure allow mass spectrometry to be used as a gas analysis tool in many applications in which small amounts of sample gas are involved. Experiments with standard AA-size NiMH batteries show that hydrogen release dominates during (over)charging at increased charging rates. Beside mass spectrometry, evolved gases are also analysed using Raman spectroscopy. Although some differences are observed, the results of similar experiments show a good agreement.

Characteristics of Bypass Flow Across PVC Pipe Elbows

By-pass flow across 90 commercial PVC pipe elbows was investigated for the case of diameters ranging from 20 to 50 mm. Factors affecting bypass flow rate were clarified. The results of this study showed that bypass flow depends not only on the available pressure differential across the elbow, but also on the bypass inlet and outlet orifice size and the tubing length. An equation giving the bypass flow as a function of the main flow discharge and the ratio of the bypass orifice diameter to the elbow diameter is presented for the case of 5- and 10-mm inlet and outlet size. The bypass flow rate and main flow discharge relationship was found to be linear. The head loss caused by the entrance and exit of fluid through the bypass inlet and outlet was clarified for the case of 5- and 10-mm orifice size.

Bypass Flow Across a Bend for Chemical Injection into Microirrigation Systems

The hydraulic characteristics of a bypass injector for chemical injection into microirrigation systems were evaluated. Bypass injectors operate on the basis of the pressure differential generated between the inner and outer walls at the midpoint of a bend. A general equation to determine the pressure head differential across a bend is presented along with the bypass and main flow discharge relationship. Bypass flow depends not only on the available pressure differential across the bend, but also on the bypass tubing size and length. An equation to predict the bypass flow as a function of the bend geometric characteristics and the available pressure differential is proposed. An equation for head loss caused by the entrance and exit of fluid through the by-pass inlet and outlet was developed. Bypass flow rates were increased up to 2.5 times by using crested nipples. The effect of size, shape, and length of the crests on bypass flow rate was also measured.