The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Koji Okamoto - One of the best experts on this subject based on the ideXlab platform.
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Richardson number criteria for direct-contact-condensation-induced Thermal Stratification using visualization
Progress in Nuclear Energy, 2020Co-Authors: Nejdet Erkan, Koji OkamotoAbstract:Abstract This study aims to suggest a dimensionless number to determine the formation and disappearance of Thermal Stratification induced by direct contact condensation in a 1/20 scaled-down suppression pool of the Fukushima Daiichi nuclear power plant. The modified Richardson number, which represents the ratio of buoyancy force to inertia of steam, was employed to explain the Thermal Stratification in the suppression pool. Steam condensation experiments were performed at both sub-atmospheric and atmospheric pressures. A blow-down pipe with an inner diameter of 12.7 mm was used to inject and condense steam in the suppression pool. The effects of steam mass on the Thermal Stratification behaviors were examined by varying the steam mass flow rate (mass flux) from 0.50 kg/hr (1.10 kg/m2·s) to 2.50 kg/hr (5.48 kg/m2·s). The steam condensation was visualized using a high-speed camera. In the results, Thermal Stratification was successfully reproduced in this study, and it was observed that the behaviors of Thermal Stratification (vertical temperature profile) was affected by the steam mass flow rate and the subcooling temperature. The Richardson number was evaluated by measuring the size and frequency of steam bubbles. The time evolution of the Richardson number was analyzed with vertical temperature profiles of water in the suppression pool. The critical Richardson number, which determines the formation and disappearance of Thermal Stratification, was suggested to be of the order of 1.
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Thermal Stratification in a scaled down suppression pool of the fukushima daiichi nuclear power plants
Nuclear Engineering and Design, 2016Co-Authors: Nejde Erka, Shinji Takahashi, Daehu Song, Wataru Sagawa, Koji OkamotoAbstract:Abstract Thermal Stratification in the suppression pool of the Fukushima Daiichi nuclear power plants was experimentally investigated in sub-atmospheric pressure conditions using a 1/20 scale torus shaped setup. The Thermal Stratification was reproduced in the scaled-down suppression pool and the effect of the steam flow rate on different Thermal Stratification behaviors was examined for a wide range of steam flow rates. A sparger-type steam injection pipe that emulated Fukushima Daiichi Unit 3 (F1U3) was used. The steam was injected horizontally through 132 holes. The development (formation and disappearance) of Thermal Stratification was significantly affected by the steam flow rate. Interestingly, the Thermal Stratification in the suppression pool vanished when subcooling became lower than approximately 5 °C. This occurred because steam bubbles are not well condensed at low subcooling temperatures; therefore, those bubbles generate significant upward momentum, leading to mixing of the water in the suppression pool.
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relationship between Thermal Stratification and flow patterns in steam quenching suppression pool
International Journal of Heat and Fluid Flow, 2015Co-Authors: Daehun Song, Nejdet Erkan, Koji OkamotoAbstract:Abstract This study aims to examine the relationship between Thermal Stratification and flow patterns in a steam-quenching suppression pool using particle image velocimetry. Thermal Stratification was experimentally evaluated in a depressurized water pool under different steam mass flux conditions. The time evolution of the temperature profile of the suppression pool was presented with the variation of condensation regimes, and steam condensation processes were visualized using a high-speed camera. The Thermal Stratification condition was classified into full mixing, gradual Thermal Stratification, and developed Thermal Stratification. It was found that the condition was determined by the flow patterns depending on the force balance between buoyancy and momentum. The force balance affected both the condensation regime and the flow pattern, and hence, the flow pattern was changed with the condensation regime. However, the force balance had a sensitive influence on the flow in the pool; therefore, distinct flow patterns were observed even in the same condensation regime.
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dimensional analysis of Thermal Stratification in a suppression pool
International Journal of Multiphase Flow, 2014Co-Authors: Daehun Song, Nejdet Erkan, Koji OkamotoAbstract:Abstract Due to complexity of direct contact condensation (DCC), it is difficult to predict the Thermal hydraulic phenomenon in a suppression pool (SP) of LWRs. Especially, the momentum, induced at condensation interfaces, depends on several interrelated parameters such as the steam mass flux, subcooling, and the diameter of the injection nozzle. Complicated interaction of those parameters creates difficulties in developing a comprehensive analytical model, which applicable to various conditions. To investigate the criteria of Thermal Stratification created by DCC, experiments were performed using a downsized suppression pool. Time resolved temperatures were acquired by vertically aligned thermocouples. Additionally, steam bubbles were visualized by a high speed camera in order to examine bubble shapes according to the mass flux and subcooling. Both steam bubble frequency and amplitude were analyzed for different DCC regimes. Finally, Richardson number was chosen as a suitable parameter for the dimensional analysis of experimental results. Corresponding velocity at far field in synthetic jet theory was employed to calculate Richardson number. The criteria for the occurrence of the Thermal Stratification were clearly determined according to the Richardson number.
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experimental investigation into Thermal Stratification by direct condensation in a scaled suppression pool of fukushima daiichi nuclear power plant
2014 22nd International Conference on Nuclear Engineering, 2014Co-Authors: Shinji Takahashi, Nejde Erka, Daehu Song, Wataru Sagawa, Koji OkamotoAbstract:Experimental and numerical studies into Thermal Stratification by direct steam condensation in a torus type suppression pool were carried out to investigate the reactor core isolation cooling in the accidents of Fukushima Daiichi nuclear power plants. The suppression pool was manufactured to be a 1/22 scaled model of a Fukushima Daiichi nuclear power plant. Two different types of spargers were employed to simulate different units of the plants. In a sparger, 132 holes were uniformly drilled on the side of a pipe. However, the other sparger injected steam to the bottom. Flow rate was varied in a wide range to examine the effect on Thermal Stratification in the suppression pool. The experimental results showed that the sparger type influenced formation of Thermal Stratification. Moreover, steam flow rate strongly affected the onset time of Thermal Stratification, and the disappearance of the Thermal Stratification was affected by subcooling temperature. Computer simulation using a commercial software was conducted and the results show similar temperature profiles to the experimental results. Steam condensation was visualized in a vicinity of the spargers using high speed camera.Copyright © 2014 by ASME
Simon Furbo - One of the best experts on this subject based on the ideXlab platform.
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Thermal Stratification built up in hot water tank with different inlet stratifiers
Solar Energy, 2017Co-Authors: Janne Dragsted, Simon Furbo, Mark Dannemand, Federico BavaAbstract:Abstract Thermal Stratification in a water storage tank can strongly increase the Thermal performance of solar heating systems. Thermal Stratification can be built up in a storage tank during charge, if the heated water enters through an inlet stratifier. Experiments with a test tank have been carried out in order to elucidate how well Thermal Stratification is established in the tank with differently designed inlet stratifiers under different controlled laboratory conditions. The investigated inlet stratifiers are from Solvis GmbH & Co KG and EyeCular Technologies ApS. The inlet stratifier from Solvis GmbH is a rigid plastic pipe with holes for each 30 cm. The holes are designed with flaps preventing counter flow into the pipe. The inlet stratifier from EyeCular Technologies ApS is made of a flexible polymer with openings all along the side and in the full length of the stratifier. The flexibility of the stratifier prevents counterflow. The tests have shown that both types of inlet stratifiers had an ability to create Stratification in the test tank under the different test conditions. The stratifier from EyeCular Technologies ApS had a better performance at low flows of 1–2 l/min and the stratifier for Solvis GmbH & Co KG had a better performance at 4 l/min. In the intermediate charge test the stratifier from EyeCular Technologies ApS had a better performance in terms of maintaining the Thermal Stratification in the storage tank while charging with a relative low temperature.
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Thermal Stratification in a hot water tank established by heat loss from the tank
Solar Energy, 2012Co-Authors: Jianhua Fan, Simon FurboAbstract:Abstract This paper presents numerical investigations of Thermal Stratification in a vertical cylindrical hot water tank established by standby heat loss from the tank. The transient fluid flow and heat transfer in the tank during cooling caused by standby heat loss are calculated by means of validated computational fluid dynamics (CFD) models. The measured heat loss coefficient for the different parts of the tank is used as input to the CFD model. Parametric studies are carried out using the validated models to investigate the influence on Thermal Stratification of the tank by the downward flow and the corresponding upward flow in the central parts of the tank. Tank design parameters such as tank volume, height to diameter ratio and insulation and different initial conditions of the tank are investigated. It is elucidated how Thermal Stratification in the tank is influenced by the natural convection and how the heat loss from the tank sides will be distributed at different levels of the tank at different Thermal conditions. The results show that 20–55% of the side heat loss drops to layers below in the part of the tank without the presence of Thermal Stratification. A heat loss removal factor is introduced to characterize the effect of the buoyancy driven flow on exchange of heat loss between tank layers by natural convection. Based on results of the parametric studies, a generalized equation for the heat loss removal factor is obtained by regression which takes into account the influences of tank volume, height to diameter ratio, tank insulation and initial conditions of the tank. The equation is validated for a 150–500 l tank insulated with 0–7 cm mineral wool and a tank height to diameter ratio of 1–5. The equation will be implemented in an existing tank optimization and design program for calculation of Thermal performance of a hot water tank.
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Thermal Stratification in small solar domestic storage tanks caused by draw offs
Solar Energy, 2005Co-Authors: Ulrike Jordan, Simon FurboAbstract:Abstract Storage tanks with different cold water inlet devices for small Solar Domestic Hot Water (SDHW) systems are compared. The objective of the investigation is to reveal the impact of the cold water inlet device on the Thermal Stratification in two marketed tanks and to evaluate the possible enhancement in the annual system performance of small solar heating systems. Two different marketed inlet designs are compared, one connected to a small curved plate placed above the inlet tube, the other one connected to a much larger flat plate. The cold domestic water enters the stores in vertical direction from the bottom of the tanks. Temperature measurements were carried out for different operating conditions. It was shown that the Thermal Stratification inside the two tanks depends differently on the flow rate, the draw-off volume, as well as the initial temperature in the storage tank. To carry out system simulations, a multi-node storage model was used and expanded by an additional input variable to model the mixing behaviour depending on the operating conditions. The inlet device with a comparatively large plate compared to the less favourable design results in an increase of the solar fraction of about 1–3%-points in annual system simulations with a solar fraction of about 60% and fairly large domestic hot water flow rates. This corresponds to a reduction of the auxiliary energy supply of the solar heating system of about 3–7% (58–155 MJ/year) for the investigated solar domestic hot water system.
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Thermal Stratification in vertical mantle heat exchangers with application to solar domestic hot water systems
Applied Energy, 2004Co-Authors: Soren Knudsen, Simon FurboAbstract:Experimental and numerical investigations of vertical mantle heat-exchangers for solar domestic hot-water (SDHW) systems have been carried out. Two different mantle inlet positions are investigated. Experiments based on typical operation conditions are carried out to investigate how the Thermal Stratification is affected by different positions of the mantle inlet. The heat transfer between the solar collector fluid in the mantle and the domestic water in the tank is analysed by CFD-simulations. Furthermore, side-by-side laboratory tests have been carried out with SDHW systems with different mantle inlet-positions. It is shown that for a high inlet-temperature to the mantle, it is an advantage to have the inlet located at the top and for a low inlet temperature it is an advantage to have the inlet moved down. Marketed tanks have typically the mantle inlet located at the top of the mantle. The side-by-side laboratory tests indicate that it is an advantage to move the inlet down from the top.
Zilong Wang - One of the best experts on this subject based on the ideXlab platform.
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an experimental investigation on Thermal Stratification characteristics with pcms in solar water tank
Solar Energy, 2019Co-Authors: Huajie Huang, Zilong Wang, Hua Zhang, Xiuhui Huang, Hao Liang, M A GoulaAbstract:Abstract Thermal storage technology with phase-change materials (PCMs) is an important approach for improving solar energy utilisation efficiency. In this study, for analysing the Stratification of a Thermal tank with PCMs at an initial water temperature of 353.15 K and inlet water temperature of 278.15 K, a Thermal storage tank containing sodium acetate trihydrate with a phase change temperature of 325.15 K and super-cooling temperature below 278.15 K was developed. This study thoroughly investigated the effect of the positions of the PCMs on Thermal Stratification characteristics at various flow rates (0.06, 0.18, 0.3, 0.42, and 0.54 m3/h) and with increasing dimensionless time. This study further examined the fill efficiency, which was compared with the exergy efficiency, MIX number, and Richardson number to characterise the Stratification of the Thermal tank. The experimental results demonstrated that when the temperature of the water storage tank increased from 278.15 K to 353.15 K, the energies of the water tank and PCM tank were 18.81 and 19.34 MJ, respectively. At the same inlet flow rate, increasing the PCMs close to the inlet resulted in improved Thermal Stratification of the tank. With high flow rates, the cold–hot water mixing intensified and the thermocline thickness in the tank increased, thereby weakening the Thermal Stratification. Moreover, as the water-release process progressed, the cold–hot water mixing in the water tank tended to be stable, thereby forming a stable thermocline. The Thermal Stratification of the ordinary tank was superior to that of the PCM tank. However, as the PCMs were located at the bottom of the water tank, the Thermal Stratification was optimal when the inlet flow rate was higher than 0.42 m3/h.
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the Thermal Stratification characteristics affected by a novel equalizer in a dynamic hot water storage tank
Applied Thermal Engineering, 2017Co-Authors: Zilong Wang, Huajie Huang, Hua Zhang, Binlin Dou, Guanhua ZhangAbstract:Abstract The Thermal storage that uses water as a medium is one of the key technologies in the field of solar energy. With the purpose of improving the Thermal Stratification performance of a Thermal storage tank, this paper proposes a novel equalizer and compares the direct and three-layer orifice under an initial temperature of 70 °C and an inlet temperature of 10 °C. With regard to the changes in temperature with an increase of the dimensionless time, this paper analysed the impact of the inlet structure on the Thermal Stratification under different flow rates (1 L/min, 2 L/min, 3 L/min, 4 L/min, 5 L/min, and 6 L/min). The results show that the Richardson number and the Fill efficiency of the equalizer mode are larger than those of the other two modes and that the MIX number for the equalizer mode is smaller than those of the other two modes. When the flow rate was 6 L/min, the Richardson numbers for the three modes were 6, 4 × 104 and 4 × 106; the fill efficiencies were 0.53493, 0.58493 and 0.81405; and the MIX numbers were 0.53154, 0.30923 and 0.03971 (at a dimensionless time of 0.4). As the flow velocity increases, the fill efficiencies of the direct and three-layer orifice modes gradually decrease from 0.71827 and 0.74001 at 1 L/min to 0.53493 and 0.58493 at 6 L/min, respectively. Comparatively, the Fill efficiency of the equalizer mode first increases from 0.77599 at 1 L/min to 0.85471 at 3 L/min and then decreases to 0.81405 at 6 L/min. These results demonstrate that the proposed equalizer can reduce the mixing process to improve Thermal Stratification.
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experimental and numerical research of Thermal Stratification with a novel inlet in a dynamic hot water storage tank
Renewable Energy, 2017Co-Authors: Zilong Wang, Huajie Huang, Hua Zhang, Binlin Dou, Zhiyun WangAbstract:Heat storage is an important task in the use of solar energy; in particular, the use of water as the Thermal storage medium is one of key technologies in solar Thermal energy utilization. With the purpose of improving the Thermal Stratification of a heat storage tank, a novel equalizer was designed in this paper. To investigate the influence factor of Thermal Stratification in hot water storage tank, numerical analyses based on the three-dimensional (3D) unsteady Computational Fluid Dynamics (CFD) model were performed using the commercial software ANSYS. The initial and inlet temperatures were considered along with various flow rates. The performance parameters, such as the Richardson number, the MIX number and exergy, were involved in the evaluation. This study was further extended to explore the fill efficiency as the performance parameters of Thermal Stratification within a storage tank. The numerical model was validated with the experimental data; the results were determined to be in good agreement. The results demonstrate that with the growth of flow rate, the Richardson number decreases, fill efficiency and exergy increased first and later decreased, but the MIX number decreased first and later increased. When the flow rate was 3 L/min, the equalizer performs best, and the storage tank had a better Thermal Stratification. The RMS error increased first and subsequently decreases before increasing again with the growth of the flow rate. Furthermore, the MIX number reaches a minimum at the dimensionless time of 0.5 in the numerical results, whereas it is 0.4 in the experimental results. It was also observed that the contribution of the equalizer on the flow-suppressing of influent results in a decrease of mixing process between the hot and cold water, which could lead to improvement of the Thermal Stratification.
Huajie Huang - One of the best experts on this subject based on the ideXlab platform.
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an experimental investigation on Thermal Stratification characteristics with pcms in solar water tank
Solar Energy, 2019Co-Authors: Huajie Huang, Zilong Wang, Hua Zhang, Xiuhui Huang, Hao Liang, M A GoulaAbstract:Abstract Thermal storage technology with phase-change materials (PCMs) is an important approach for improving solar energy utilisation efficiency. In this study, for analysing the Stratification of a Thermal tank with PCMs at an initial water temperature of 353.15 K and inlet water temperature of 278.15 K, a Thermal storage tank containing sodium acetate trihydrate with a phase change temperature of 325.15 K and super-cooling temperature below 278.15 K was developed. This study thoroughly investigated the effect of the positions of the PCMs on Thermal Stratification characteristics at various flow rates (0.06, 0.18, 0.3, 0.42, and 0.54 m3/h) and with increasing dimensionless time. This study further examined the fill efficiency, which was compared with the exergy efficiency, MIX number, and Richardson number to characterise the Stratification of the Thermal tank. The experimental results demonstrated that when the temperature of the water storage tank increased from 278.15 K to 353.15 K, the energies of the water tank and PCM tank were 18.81 and 19.34 MJ, respectively. At the same inlet flow rate, increasing the PCMs close to the inlet resulted in improved Thermal Stratification of the tank. With high flow rates, the cold–hot water mixing intensified and the thermocline thickness in the tank increased, thereby weakening the Thermal Stratification. Moreover, as the water-release process progressed, the cold–hot water mixing in the water tank tended to be stable, thereby forming a stable thermocline. The Thermal Stratification of the ordinary tank was superior to that of the PCM tank. However, as the PCMs were located at the bottom of the water tank, the Thermal Stratification was optimal when the inlet flow rate was higher than 0.42 m3/h.
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the Thermal Stratification characteristics affected by a novel equalizer in a dynamic hot water storage tank
Applied Thermal Engineering, 2017Co-Authors: Zilong Wang, Huajie Huang, Hua Zhang, Binlin Dou, Guanhua ZhangAbstract:Abstract The Thermal storage that uses water as a medium is one of the key technologies in the field of solar energy. With the purpose of improving the Thermal Stratification performance of a Thermal storage tank, this paper proposes a novel equalizer and compares the direct and three-layer orifice under an initial temperature of 70 °C and an inlet temperature of 10 °C. With regard to the changes in temperature with an increase of the dimensionless time, this paper analysed the impact of the inlet structure on the Thermal Stratification under different flow rates (1 L/min, 2 L/min, 3 L/min, 4 L/min, 5 L/min, and 6 L/min). The results show that the Richardson number and the Fill efficiency of the equalizer mode are larger than those of the other two modes and that the MIX number for the equalizer mode is smaller than those of the other two modes. When the flow rate was 6 L/min, the Richardson numbers for the three modes were 6, 4 × 104 and 4 × 106; the fill efficiencies were 0.53493, 0.58493 and 0.81405; and the MIX numbers were 0.53154, 0.30923 and 0.03971 (at a dimensionless time of 0.4). As the flow velocity increases, the fill efficiencies of the direct and three-layer orifice modes gradually decrease from 0.71827 and 0.74001 at 1 L/min to 0.53493 and 0.58493 at 6 L/min, respectively. Comparatively, the Fill efficiency of the equalizer mode first increases from 0.77599 at 1 L/min to 0.85471 at 3 L/min and then decreases to 0.81405 at 6 L/min. These results demonstrate that the proposed equalizer can reduce the mixing process to improve Thermal Stratification.
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experimental and numerical research of Thermal Stratification with a novel inlet in a dynamic hot water storage tank
Renewable Energy, 2017Co-Authors: Zilong Wang, Huajie Huang, Hua Zhang, Binlin Dou, Zhiyun WangAbstract:Heat storage is an important task in the use of solar energy; in particular, the use of water as the Thermal storage medium is one of key technologies in solar Thermal energy utilization. With the purpose of improving the Thermal Stratification of a heat storage tank, a novel equalizer was designed in this paper. To investigate the influence factor of Thermal Stratification in hot water storage tank, numerical analyses based on the three-dimensional (3D) unsteady Computational Fluid Dynamics (CFD) model were performed using the commercial software ANSYS. The initial and inlet temperatures were considered along with various flow rates. The performance parameters, such as the Richardson number, the MIX number and exergy, were involved in the evaluation. This study was further extended to explore the fill efficiency as the performance parameters of Thermal Stratification within a storage tank. The numerical model was validated with the experimental data; the results were determined to be in good agreement. The results demonstrate that with the growth of flow rate, the Richardson number decreases, fill efficiency and exergy increased first and later decreased, but the MIX number decreased first and later increased. When the flow rate was 3 L/min, the equalizer performs best, and the storage tank had a better Thermal Stratification. The RMS error increased first and subsequently decreases before increasing again with the growth of the flow rate. Furthermore, the MIX number reaches a minimum at the dimensionless time of 0.5 in the numerical results, whereas it is 0.4 in the experimental results. It was also observed that the contribution of the equalizer on the flow-suppressing of influent results in a decrease of mixing process between the hot and cold water, which could lead to improvement of the Thermal Stratification.
John E Dec - One of the best experts on this subject based on the ideXlab platform.
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effect of engine size speed and dilution method on Thermal Stratification of premixed homogeneous charge compression ignition engines a large eddy simulation study
International Journal of Engine Research, 2020Co-Authors: Aimilios Sofianopoulos, Benjamin Lawler, Sotirios Mamalis, Mozhgan Rahimi Boldaji, John E DecAbstract:High heat release rates limit the operating range of homogeneous charge compression–ignition engines to low and medium loads. Thermal Stratification has been shown to stagger autoignition, lower he...
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smoothing hcci heat release with vaporization cooling induced Thermal Stratification using ethanol
SAE International Journal of Fuels and Lubricants, 2011Co-Authors: Magnus Sjoberg, John E DecAbstract:Ethanol and ethanol/gasoline blends are being widely considered as alternative fuels for light-duty automotive applications. At the same time, HCCI combustion has the potential to provide high efficiency and ultra-low exhaust emissions. However, the application of HCCI is typically limited to low and moderate loads because of unacceptably high heat-release rates (HRR) at higher fueling rates. This work investigates the potential of lowering the HCCI HRR at high loads by using partial fuel Stratification to increase the in-cylinder Thermal Stratification. This strategy is based on ethanol's high heat of vaporization combined with its true single-stage ignition characteristics. Using partial fuel Stratification, the strong fuel-vaporization cooling produces Thermal Stratification due to variations in the amount of fuel vaporization in different parts of the combustion chamber. The low sensitivity of the autoignition reactions to variations of the local fuel concentration allows the temperature variations to govern the combustion event. This results in a sequential autoignition event from leaner and hotter zones to richer and colder zones, lowering the overall combustion rate compared to operation with a uniform fuel/air mixture. The amount of partial fuel Stratification was varied by adjusting the fraction of fuel injected late to produce Stratification, and also by changingmore » the timing of the late injection. The experiments show that a combination of 60-70% premixed charge and injection of 30-40 % of the fuel at 80{sup o}CA before TDC is effective for smoothing the HRR. With CA50 held fixed, this increases the burn duration by 55% and reduces the maximum pressure-rise rate by 40%. Combustion stability remains high but engine-out NO{sub x} has to be monitored carefully. For operation with strong reduction of the peak HRR, ISNO{sub x} rises to around 0.20 g/kWh for an IMEP{sub g} of 440 kPa. The single-cylinder HCCI research engine was operated naturally aspirated without EGR at 1200 rpm, and had low residual level using a CR = 14 piston.« less
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effects of engine speed fueling rate and combustion phasing on the Thermal Stratification required to limit hcci knocking intensity
SAE transactions, 2005Co-Authors: Magnus Sjoberg, John E DecAbstract:Thermal Stratification has the potential to reduce pressure-rise rates and allow increased power output for HCCI engines. This paper systematically examines how the amount of Thermal Stratification of the core of the charge has to be adjusted to avoid excessive knock as the engine speed and fueling rate are increased. This is accomplished by a combination of multi-zone chemical-kinetics modeling and engine experiments, using iso-octane as the fuel. The experiments show that, for a low-residual engine configuration, the pressure traces are self-similar during changes to the engine speed when CA50 is maintained by adjusting the intake temperature. Consequently, the absolute pressure-rise rate measured as bar/ms increases proportionally with the engine speed. As a result, the knocking (ringing) intensity increases drastically with engine speed, unless counteracted by some means. This paper describes how adjustments of the Thermal width of the in-cylinder charge can be used to limit the ringing intensity to 5 MW/m2 as both engine speed and fueling are increased. If the Thermal width can be tailored without constraints, this enables smooth operation even for combinations of high speed, high load, and combustion phasing close to TDC. Since large alterations of the Thermal width of the charge are not alwaysmore » possible, combustion retard is considered to reduce the requirement on the Thermal Stratification. The results show that combustion retard carries significant potential since it amplifies the benefit of a fixed Thermal width. Therefore, the Thermal Stratification required for operation with an acceptable knocking intensity can be decreased substantially by the use of combustion retard. This enables combinations of high engine speed and high fueling rate even for operation with the naturally occurring Thermal Stratification. However, very precise control of the combustion phasing will likely be required for such operation.« less
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potential of Thermal Stratification and combustion retard for reducing pressure rise rates in hcci engines based on multi zone modeling and experiments
SAE transactions, 2005Co-Authors: Magnus Sjoberg, John E Dec, Nicholas P CernanskyAbstract:This work investigates the potential of in-cylinder Thermal Stratification for reducing the pressure-rise rate in HCCI engines, and the coupling between Thermal Stratification and combustion-phasing retard. A combination of computational and experimental results is employed. The computations were conducted using both a custom multi-zone version and the standard single-zone version of the Senkin application of the CHEMKIN III kinetics-rate code, and kinetic mechanisms for iso-octane. This study shows that the potential for extending the high-load operating limit by adjusting the Thermal Stratification is very large. With appropriate Stratification, even a stoichiometric charge can be combusted with low pressure-rise rates, giving an output of 16 bar IMEPg for naturally aspirated operation. For more typical HCCI fueling rates (Φ = 0.38 - 0.45), the optimal charge-temperature distribution is found to depend on both the amount of fuel and the combustion phasing. For combustion phasing in the range of 7 - 10°CA after TDC, a linear Thermal distribution is optimal since it produces a near-linear pressure rise. For other combustion phasings, non-linear distributions are required to achieve a linear pressure rise. Also, the total Thermal width must be greater at higher fueling rates to avoid excessive pressure-rise rates. The study also shows that increasing the natural Thermal width of the charge by 50% would allow the equivalence ratio to be increased from 0.44 to 0.60, with an associated increase of the IMEPg from 524 to 695 kPa for naturally aspirated operation. It was also found that the naturally occurring Thermal Stratification plays a major role in producing the experimentally observed benefit of combustion-timing retard for slowing the combustion rate. Reduced chemical-kinetic rates with combustion retard are found to play a lesser role.
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comparing enhanced natural Thermal Stratification against retarded combustion phasing for smoothing of hcci heat release rates
SAE transactions, 2004Co-Authors: Magnus Sjoberg, John E Dec, Aristotelis Babajimopoulos, Dennis N AssanisAbstract:Two methods for mitigating unacceptably high HCCI heat-release rates are investigated and compared in this combined experimental/CFD work. Retarding the combustion phasing by decreasing the intake temperature is found to have good potential for smoothing heat-release rates and reducing engine knock. There are at least three reasons for this: 1) lower combustion temperatures, 2) less pressure rise when the combustion is occurring during the expansion stroke, and 3) the natural Thermal Stratification increases around TDC. However, overly retarded combustion leads to unstable operation with partial-burn cycles resulting in high IMEPg variations and increased emissions. Enhanced natural Thermal Stratification by increased heat-transfer rates was explored by lowering the coolant temperature from 100 to 50°C. This strategy substantially decreased the heat-release rates and lowered the knocking intensity under certain conditions. To further exploit the effect, the heat-transfer rates were further enhanced by increasing the in-cylinder air swirl. This led to even longer combustion durations. Unfortunately, the higher heat losses associated with high air swirl decreased the IMEP g . When the fueling rate was increased to compensate, most of the improvements on the heat-release rates were lost. Overall, combustion phasing retard was found to have better potential for smoothing heat-release rates than enhancing the Thermal Stratification by the means considered in this work. However, operation with highly retarded combustion requires precise control of the ignition timing. Furthermore, it is found that the acceptable intake temperature range narrows rapidly with increasing equivalence ratio. Above a certain fueling rate a steady state operating point cannot be established by setting the intake temperature to a fixed value. This problem is caused by wall heating and the coupling between wall temperature and combustion phasing.
