Cold Bypass Research Articles

Dynamics and control of a turbocharged solid oxide fuel cell system

• A cold bypass valve control can comply with the fuel cell system constraints. • Four controllers were tested on the system during load variation scenarios. • The cascade controllers with feed-forward approaches are the most promising. • A proportional-integrative cannot control the system properly for quick transients. The purpose of this paper regards the design and testing of control systems for a 30-kW turbocharged solid oxide fuel cell system fuelled with biogas. The adoption of a turbocharger, instead of a micro gas turbine, for the fuel cell stack pressurisation, is an innovative solution that is expected to decrease the capital cost of such systems and to facilitate their penetration into the energy market. However, not being connected to an electric generator, the turbocharger rotational speed, and thus the air mass flow, cannot be directly controlled as in microturbines. The control of turbocharged solid oxide fuel cell systems is a novel topic, characterised by many technical challenges that have not been addressed before. To regulate the stack temperature, a cold bypass valve is included, connecting the compressor outlet to the turbine inlet. A dynamic model of this system was developed in Matlab-Simulink® to analyse the response of the turbocharged solid oxide fuel cell system to a cold bypass valve opening step change. System information obtained from this analysis was used to design and tune four controllers: a conventional proportional integral controller and three different cascade controllers. The controller performance was evaluated under two different scenarios, considering quite aggressive power ramps. The best results were obtained with a cascade controller, where the feedback loop was complemented by a feed-forward contribution based on power demand. This analysis demonstrated that such a control system effectively tracks the fuel cell maximum temperature target, complying with all the system operative constraints.

Off-design performance analysis of a turbocharged solid oxide fuel cell system

The aim of this study is to analyse the off-design performance of an innovative turbocharged solid oxide fuel cell system, fed by biogas and designed to generate 30 kW during nominal operating conditions. The layout of such a plant combines the high efficiencies of the solid oxide fuel cell with a reduced-cost option for fuel cell pressurization: fuel cells are usually pressurized by a micro gas turbine, but the alternative use of a turbocharger, a mass-produced component widely used in the automotive field, could reduce significantly the capital cost of the system. This kind of layout has been rarely investigated in literature, and a detailed performance analysis of a turbocharged SOFC system would be a valuable source of information for both academia and industry. To perform this analysis, a steady-state model has been created using a modular tool developed in Matlab®-Simulink® that includes off-design models of the system components. The model has been used to compare different control strategies. The most suitable control strategy, based on wastegate and cold bypass valves, was adopted, obtaining compliance with the operative constraints and high system efficiency. Afterward, the system steady-state operation was simulated for various electric power loads and ambient temperatures. The performance analysis focused on the effect of these two variables on the system behaviour and has provided insight on the influence on the most significant system outputs, including global efficiency, temperatures and pressures. A system efficiency increase was observed at part load, with values growing from 50.8% to 57.3%. Higher values of ambient temperature resulted in a more significant pressurization of the fuel cell, affecting positively the system efficiency: from 50.5% at 0 °C to 51.0% at 30 °C. Great attention was paid to the system constraints, to verify that the plant could operate properly in all the considered conditions. The proposed control strategy was tested and proved to be effective at achieving this objective.

Effect of hypothermic versus normothermic cardiac bypass in patients undergoing coronary artery bypass grafting surgery on coagulation: a randomized controlled trial

Objectives To assess and compare early postoperative (PO) hematological and coagulation profile of patients undergoing on-pump coronary artery bypass grafting surgery with hypothermic (HT) versus normothermic (NT) cardiopulmonary bypass. Patients and methods A total of 86 patients were divided into two equal groups: NT group included patients who received warm bypass and HT group included patients who received cold bypass. PO monitoring included changes in 2-h PO hematological and coagulation profile in relation to preoperative profile and amount of PO daily blood loss and number of transfused blood units and its relation to change in coagulation profile. Results Mean activated clotting time estimated before wound closure was significantly longer in patients of HT group. At 2-h PO, hemoglobin concentration and platelet count (PC) were significantly lower, ADP-induced platelet aggregation (IPA) was significantly decreased, whereas activated partial thromboplastin time (aPTT) and international normalized ratio (INR) were significantly increased in HT patients. Estimated activated clotting time, IPA, INR, aPTT, and PC showed a negative significant correlation with the use of HT. The amount of PO daily bleeding and number of transfused units showed a positive significant correlation with the use of HT, 2-h PO aPTT, and INR, whereas showed a negative significant correlation with 2-h PO estimated IPA and hemoglobin concentration Regression analysis and receiver operating characteristic curve analysis defined the use of HT and prolonged aPTT as significant predictors for development of PO bleeding. Conclusion On-pump coronary artery bypass grafting surgery deleteriously affects hematological and coagulation profiles of patients, and this effect was accentuated by the use of HT. Two-hour PO altered PC and function and prolonged clotting times correlated with amount of PO daily bleeding and number of transfused blood units, but prolonged aPTT is the best predictor for these events.

Design, Assembly, and Use of a Device to Eliminate Earth Faults Caused by Metallic Debris in the LHC Main Dipole Circuits

The superconducting dipole magnets of the Large Hadron Collider operate in a superfluid helium bath at 1.9 K. As a part of the magnet quench protection system, each dipole magnet is equipped with a bypass diode located in the helium bath. The connection between the superconducting magnet and the cold by-pass diode is made through a clamping system called “half-moon,” located at the lowest point of the cold-masses. This area is prone to receiving metallic debris residual from the assembly technological processes. The metallic debris might move and create an earth fault during helium flows that occur not only during the cool-down, warm-up, and flushing of the cryogenic installation but also during magnet quenches at high currents. In case of appearance, the earth fault is detected by the protection system of the circuit and as a consequence, the current is ramped down to zero. Subsequently, with the circuit current already at zero, the fault can be eliminated using a device denominated as the Earth Fault Burner. The fault elimination must follow a strict procedure as it is not fully risk-free. This paper describes the details of such earth fault elimination, including preliminary diagnostics and necessary hardware. Two examples from the LHC operation are described and discussed.

Transition to turbulence and noise radiation in heated coaxial jet flows

Laminar-turbulent transition and noise radiation of a parametrized set of subsonic coaxial jet flows with a hot primary (core) stream are investigated numerically by Large-Eddy Simulation (LES) and direct noise computation. This study extends our previous research on local linear stability of heated coaxial jet flows by analyzing the nonlinear evolution of initially laminar flows disturbed by a superposition of small-amplitude unstable eigenmodes. First, a baseline configuration is studied to shed light on the flow dynamics of coaxial jet flows. Subsequently, LESs are performed for a range of Mach and Reynolds numbers to systematically analyze the influences of the temperature and the velocity ratios between the primary and the secondary (bypass) stream. The results provide a basis for a detailed analysis of fundamental flow-acoustic phenomena in the considered heated coaxial jet flows. Increasing the primary-jet temperature leads to an increase of fluctuation levels and to an amplification of far-field noise, especially at low frequencies. Strong mixing between the cold bypass stream and the hot primary stream as well as the intermittent character of the flow field at the end of the potential core lead to a pronounced noise radiation at an aft angle of approximately 35∘. The velocity ratio strongly affects the shear-layer development and therefore also the noise generation mechanisms. Increasing the secondary-stream velocity amplifies the dominance of outer shear-layer perturbations while the disturbance growth rates in the inner shear layer decrease. Already for rmic > 40R1, where rmic is the distance from the end of the potential core and R1 is the core-jet radius, a perfect 1/rmic decay of the sound pressure amplitudes is observed. The potential-core length increases for higher secondary-stream velocities which leads to a shift of the center of the dominant acoustic radiation in the downstream direction.

Contact Resistances in the Cold Bypass Diode Leads of the Main LHC Magnets

The main superconducting dipole and (de)focusing quadrupole magnets of the LHC are equipped with cold bypass diodes to assure the passage of the current outside the magnets in case of a magnet quench. Each magnet has a parallel bus bar circuit with a diode that is connected via resistive bolted connections. Heat sinks are pressed to the wafers under a constant load to assure a good thermal and electrical contact in order to avoid overheating of the wafers. During the series production, all the diodes of the LHC have passed the reception tests in industry. However, during high current testing in the LHC, some of the diode leads showed an anomalous resistance, triggering an off-line verification of the design and assembly procedures supported by cold powering tests. This paper summarizes the measurement results obtained during the series production of the diodes before their installation in the magnets, complemented by a smaller number of specific tests performed at CERN in the past and recently.

Development of a transonic wind tunnel to investigate engine bypass flow heat exchangers

The design of a new transonic wind tunnel to reproduce bypass aero-engine flow conditions is described. This study has been driven by the interest to investigate and optimize the aero-thermal effects of an air/oil integrated surface heat exchanger placed at the inner wall of the secondary duct of a turbofan. The test section of the intermittent wind tunnel consists of a complex three-dimensional (3D) geometry. The flow evolves over the splitter, duplicated at real scale, as it does in the engine, guided by helicoidally shaped lateral walls. Numerical flow analyses have been used to guide the design of the complete wind tunnel. Zero-dimensional models have been developed to recreate the wind tunnel behaviour. Two- and three-dimensional computations have been performed to optimize the test section and the inlet guide vanes that duplicate the flow downstream of the engine fan. The reproduction of the bypass flow structure has been numerically analysed. Flow computations with and without the presence of instrumentation in the test section have been contrasted. Additionally, the influence of an aerodynamic probe on the flow has been studied numerically. This study should lead to improvements in the thermal management of future aircraft power plants, particularly by taking benefit of the cold bypass air to refrigerate the lubrication oil, without penalizing the propulsive efficiency.

First clinical report of differential perfusion during cardiac surgery using the Cardeon Cobra aortic cannula

Objective. Warm bypass offers hemodynamic advantages, whereas cold bypass provides organ protection. An aortic cannula (Cobra Cannula, Cardeon, Cupertino, CA) has been devised to allow for selective perfusion of the aortic arch and descending aorta by flow segmentation to the greater and lesser curvatures of the aorta via an endoaortic baffle. The goal of this study was to determine the ability of this device to maintain adequate flows and pressures during cardiopulmonary bypass while simultaneously achieving differential temperature control.

Room B, 10/17/2000 9: 00 AM - 11: 00 AM (PS) Post-Operative Temperature Following Cardiac Surgery: The Influence of Warm Versus Cold Bypass

<b>Room B, 10/17/2000 9: 00 AM - 11: 00 AM (PS) Post-Operative Temperature Following Cardiac Surgery: The Influence of Warm Versus Cold Bypass</b>

Cerebral oxygenation during cardiopulmonary bypass

Cerebral fractional oxygen extraction (FOE) was monitored in 30 children, using near infrared spectroscopy during cardiopulmonary bypass, to investigate the effect of hypothermia and circulatory arrest. One group of children...

Temperature during cardiopulmonary bypass for coronary artery operations does not influence postoperative cognitive function: A prospective, randomized trial

Objective: The objective was to examine the effect of temperature (28° vs 36° C) during cardiopulmonary bypass on postoperative cognitive functions in a prospective, double-blind, and randomized manner. Methods: Sixty-two patients scheduled for coronary operations were randomized to warm or cold cardiopulmonary bypass. Preoperative and postoperative (7 days) neuropsychologic evaluations were performed by an observer unaware of cardiopulmonary bypass temperature. Results: Fifty-four patients completed the study (cold bypass, n = 24; warm bypass, n = 30). Significant ( p < 0.01) postoperative deterioration for tests of psychomotor coordination and verbal memory was noted in both warm and cold groups, but no differences were observed between groups. Conclusion: Temperature during cardiopulmonary bypass for coronary operations does not influence postoperative cognitive function(J Thorac Cardiovasc Surg 1997;114:123-8)

Blood gas management and degree of cooling: Effects on cerebral metabolism before and after circulatory arrest

This study investigated the effects of different cooling strategies on cerebral metabolic response to circulatory arrest. In particular, it examined the impact of blood gas management and degree of cooling on cerebral metabolism before and after deep hypothermic circulatory arrest. Sixty-nine 1-week-old piglets (2 to 3 kg) were placed on cardiopulmonary bypass (37°C) at 100 ml/kg per minute. Animals were cooled to 18°or 14°C as follows: alpha-stat strategy to 18°C ( n = 9) or 14°C ( n = 6), pH-stat strategy to 18°C ( n = 9) or 14°C ( n = 7), or pH-stat strategy for 18 minutes followed by a switch to alpha-stat strategy for the last 5 minutes of cooling to 18°C ( n = 12) or 14°C ( n = 10). Animals underwent 60 minutes of circulatory arrest followed by rewarming with alpha-stat strategy to 36°C. Control animals were cooled with alpha-stat strategy to 18°C ( n = 10) or 14°C ( n = 3) and then maintained on cold cardiopulmonary bypass (100 ml/kg per minute) for 60 minutes. Three animals were excluded (see text). With the use of xenon 133 clearance methods, cerebral blood flow was measured at the following points: point I, cardiopulmonary bypass (37°C); point II, cardiopulmonary bypass before circulatory arrest or control flow (18° or 14°C); and point III, cardiopulmonary bypass after rewarming (36°C). Cerebral metabolic rate of oxygen consumption was calculated for each point. At point II, cerebral metabolism was more suppressed at 14°C compared with that at 18°C. At any given temperature (18° or 14°C), pH-stat strategy provided the greatest suppression of cerebral metabolism. In control animals, cerebral metabolic oxygen consumption at point III returned to baseline values after 60 minutes of cold bypass. Sixty minutes of circulatory arrest resulted in a significant reduction in cerebral metabolic oxygen consumption at point III compared with that at point I regardless of cooling temperature or blood gas strategy. The amount of cerebral metabolic recovery was significantly reduced in the pH-stat 14°C group compared with that in the pH-stat 18°C group at point III. The use of pH-stat strategy followed by a switch to alpha-stat at 14°C provided better cerebral metabolic recovery compared with either strategy used alone. The use of pH-stat strategy during initial cooling may provide the animal with maximal cerebral metabolic suppression. The cerebral acidosis produced with pH-stat cooling may worsen cerebral metabolic injury from circulatory arrest, but this effect is eliminated with the use of alpha-stat just before the period of circulatory arrest. (J T HORAC C ARDIOVASC S URG 1995;110:1649-57)

Warm versus cold coronary bypass surgery

Warm versus cold coronary bypass surgery

Effect on CNS of warm versus cold bypass

Effect on CNS of warm versus cold bypass

Theory and test of flow mixing for turbofan engines.

For certain applications of advanced turbofan engines, it is advantageous to transport various amounts of the gas-producer kinetic energy to a second stream. The reasons for and the advantages and methods of gasdynamic energy transport (mixing process) are explained with different examples. The optimum layout of fan engines with and without mixing is discussed, and the influence of partial mixing on performance is pointed out. Theoretical considerations, which show that the thrust of bypass turbojet engines may be increased by mixing the hot exhaust jet with the cold bypass flow, are compared with experimental data. Tests of three mixer configurations were conducted on an experimental rig with very accurate thrust measurements and flow profile instrumentation at different sections of the mixing chamber. Results of these experiments are discussed.

Optimization for Reaction Operation of Modified Cold Bypass Multistage Operation

“Modified cold bypass operation” which the author has tentatively named, is called alien cold bypass operation in oversea literatures.This operation specifies itself with those chemical equilibrium changes through the addition of a coolant.Optimization of this kind of operation is already reported by R. Aris in his book. However, his method was through dynamic programming.The author derived here an optimization theory for the same through analytical procedures, which saves much labor in the trial and error in design calculation.The author divided modified cold bypass operations into two divisions for the convenience of analysis.1) Pure modified cold bypass (special)2) Hybrid modified cold bypass (more general)Optimization conditions for several conceivable cases were derived. These necessary conditions are expected to be applicable in the designing of multistage reaction operation.