Low-pressure Loop Research Articles

Off-design performance optimization for steam-water dual heat source ORC systems

High-pressure steam and hot water often coexist as industrial waste heat. In this study, dual loop and single loop ORC systems are designed for 700 kPa, 4.1 kg/s steam, and 90 ℃, 122.36 kg/s hot water conditions to study the off-design performance when steam or hot water conditions change. To maximize net output power, we employ a particle swarm optimization algorithm to optimize the evaporation and condensation temperatures. The results show that within the specified hot water conditions, the evaporation and condensation temperatures of D-ORC's low-pressure loop and S-ORC increase with rising hot water inlet temperature and flow rate. The S-ORC demonstrates a higher net output power growth rate as hot water flow rate and temperature rise. Under specific steam conditions, when the steam outlet is in a gas-liquid two-phase state, D-ORC's maximum net output power is 1.7 % higher than that of the S-ORC, with little variation in optimal evaporation and condensation temperatures with respect to steam inlet pressure. At a 3.5 kg/s steam flow rate, the D-ORC's high-pressure loop becomes ineffective, whereas S-ORC efficiently adjusts heat exchange capacity under diverse steam-water conditions, Consequently, the D-ORC's average net output power is 34.2 % lower than that of the S-ORC.

Performance comparison and characterization of multi-pressure vapor compression systems at optimized conditions for simultaneous cooling and heating application

We performed numerical thermodynamic analyses of 3 multi-pressure vapor compression system configurations (cascaded, with flash tank, and with ejector) to evaluate their coefficient of performance improvements in combined cooling and heating applications. Six refrigerants were evaluated, and 25 temperature pairs of −15 °C to 25 °C cooling and 45 °C to 85 °C heating were specified. Critical parameters of the 3 configurations were varied for performance optimization. The compressor outlet temperature was constrained to prevent lubricant breakdown in actual implementation. Maximum performance improvements of 34.41 %, 39.20 %, and 46.34 % for the cascade, with flash tank, and with ejector systems, respectively, were attained. Moreover, the total number of temperature pairs exceeding the temperature limit was reduced from 19/25 to 17/25, 1/25, and 9/25 for the cascade, with flash tank, and with ejector systems, respectively. The best refrigerants to use were found to be Ammonia or R245fa as the low-pressure loop refrigerant with R245fa as the high-pressure loop refrigerant for cascade systems, Ammonia or R245fa for systems with flash tank, and R245fa for systems with ejector. This work demonstrated and quantified the ability of multi pressure configurations to increase the performance of vapor compression systems in simultaneous cooling and heating applications.

Organic Rankine Cycle based waste heat recovery modeling and control of the low pressure side using direct condensation and dedicated fans

Organic Rankine cycle based waste heat recovery has been studied over the past decade as a potential solution to reduce fuel consumption and fulfill the requirements of upcoming regulations on CO2 emissions for heavy-duty trucks. This study, focusing on a particular configuration of the system, using direct condensation and dedicated fans, presents the models of all the components in the system and validation of the evaporator and fan model according to experimental results. A special attention has been paid to the effectiveness of the fan speed and condensation pressure control to increase the net power output of the direct condensed ORC. Road cycle simulations are run over an internal Volvo cycle that is suitable for the present application, using a "base" control strategy; afterwards a sensitivity analysis and optimization, involving the low pressure loop of the system, are performed, leading to better performance in terms of net power produced by the Rankine system.

Numerical investigation on a dual loop EGR optimization of a light duty diesel engine based on water condensation analysis

Exhaust gas recirculation (EGR) is widely used in most current automotive diesel engines as a simple, economical, and highly effective NOX reduction technology. The low pressure (LP) loop EGR has higher deNOX efficiency, but also has problem of water condensation, which causes wear of the turbocharger compressor wheel, and corrosion or freeze rupture of intake components. Using a one-dimensional engine simulation, we built a real-time water condensation model based on heat transfer analysis around the intercooler, and optimized the dual loop EGR control strategy to minimize water condensation while maintaining NOX reduction performance. As a result, water condensation decreased by up to 20.54% at 100 km/h constant-speed driving, depending on ambient temperature and humidity. At 23 °C temperature / 80% humidity, which is the standard condition of worldwide harmonized light-duty test procedure (WLTP), the amount of condensed water decreased by up to 33.39%, for various engine speeds and loads. During worldwide harmonized light-duty test cycle (WLTC) driving, condensed water decreased by up to 30.56%, while fuel consumption increased by maximum of 1.13%. Since the water condensation prediction model presented in this study was constructed through the general energy analysis, it could be applied to various types of engines and operating conditions.

Investigation into Factors Causing Damage to Low-Pressure Loop Evaporating Tubes of Large-Capacity Heat-Recovery Steam Generators

The article addresses factors causing damage—in particular, flow accelerated corrosion wear (FAC) of tube walls—to the evaporator operating in the low-pressure loop of the Ep-258/310/35-15.0/3.14/0.44-540/535/263 (P-132) heat-recovery steam generator used as part of a 795-MW combined-cycle plant. Factors influencing the FAC, namely, improper water chemistry, availability of gas shunts in the steam generator, and high mixture motion velocity in the tubes in combination with the flow path configuration, are described. The damages inflicted to the tubes and their locations in the heat-recovery steam generator low-pressure loop are described. It is shown that gas shunts in the heat-recovery steam generator result in an increased heat absorption and higher velocity of medium in the boundary tubes of low-pressure evaporator tube banks, which intensifies the tube’s wear process. It has been found that the water chemistry used for the Kirishi District Power Plant (DPP) heat-recovery steam generator was not the factor that caused damage to the low-pressure loop tubes. The Kirishi DPP steam generator was compared with other similar large-capacity heat-recovery steam generators. For analyzing the low-pressure evaporator performance indicators, the loop and the processes occurring in it were modeled using the Boiler Designer software. An analysis of the results from comparison of the steam generator design and performance characteristics has shown that it is necessary to change the two-phase mixture motion velocity in the Kirishi DPP steam generator low-pressure loop by modifying the design or operating parameters. The wear of the low-pressure evaporator tube walls was mathematically modeled, the results of which confirm that erosion wear is one of the main factors causing damage to the tubes. The erosion is caused by the intense dynamic effect of two-phase, high-velocity flow jets on the tube walls. General recommendations for decreasing the wear of heat-recovery steam generator tubes are given. It has been determined that the increased wear of tubes in the low-pressure loop of the P-132 steam generator at the Kirishi DPP is caused by a combination of a few factors, such as high velocity of steam–water mixture, availability of bends, and unsatisfactory quality of aligning the tubes at their welding places.

Influences of compressor displacement ratio on dual-loop heat pump system for ventilation heat recovery

The performance of heat pump ventilation heat recovery system can be effectively improved using multiple-loop independent compression cycle. The influence of the DR change on the performance of dual-loop system was predicted through calculation to explore the influence of the change in displacement ratio (DR) on the performance of the system and the direction of further improving the performance of the system. The optimum DR in dual-loop system was calculated. The performance differences of two dual-loop heat pump systems with different DR values were compared by experiments. Results show that the optimum DR is between 0.8040 and 0.9206, and this optimum DR maximizes the COP of the system. Experimental results show that, compared with the COP of the system with the DR of 2, that of the system with the DR of 0.5 can be increased by 14.8%. Compared with the overall COP of the triplex loop system with the same displacement in each loop, that of the system with the DR of 0.5 is even higher under certain test conditions. The comparison results of COP changes are in accordance with the prediction of simulation calculation. Therefore, a reasonable increase in the displacement of the low-pressure loop in the multi-loop heat recovery heat pump system will improve the system performance.

Analysis of the Hydraulic Regimes of Vertical Waste-Heat Boilers of Combined-Cycle Gas-Turbine Units

An analysis of the hydraulic system of the natural circulation loop of the waste-heat boilers of combined-cycle gas turbine units is performed. It is demonstrated that the standard method of hydraulic calculation of steam boilers based on an evaluation of the reliability criteria of the hydraulic system and the thermal regime of the evaporator tubes is incorrect. The existence of loop-wide hydraulic persistence of flow in the low-pressure evaporator in natural circulation caused by elevated resistance of the steam-outlet tubes is demonstrated for the first time. Recommendations are given for increasing the reliability of the hydraulic system of the low-pressure evaporator loop and selecting an optimal combined water and chemical regime to prevent erosion wear of the metal of the inner surface of the evaporator tubes.

Impact of butanol isomers and EGR on the combustion characteristics and emissions of a SIDI engine at various injection timings

The impact of butanol on a SIDI engine operating with low pressure loop EGR was investigated in this study, with each of the four butanol isomers blended with gasoline by a volume fraction of 30%. Three injection timings during the intake stroke were selected, covering the representative scenarios of severe fuel impingement, adequate fuel-air mixing without obvious fuel impingement, and insufficient fuel-air mixing time prior to combustion. The results show that n-butanol, sec-butanol, and iso-butanol are capable of accelerating flame propagation and hence shorten the combustion duration, while tert-butanol would unfavorably extend the combustion duration. The addition of all the butanol isomers lead to reduced fuel consumption and reduced CO, unburnt HC and particulate number (PN) emissions, with n-butanol and sec-butanol showing more potential in high efficiency and low emission combustion at low load. EGR yields distinct influences on the PN emission at different injection timings: when injection starts at middle or late intake process, PN concentration decreases with EGR rate, which is consistent with previous studies; when injection starts early in the intake stoke, however, PN concentration would otherwise increase with the EGR rate independent of the fuel type, which could be possibly owing to the shift of the locally rich mixture in the Φ-T diagram towards the highly sooty peninsula with the cooling effect of EGR.

Numerical study on the effect of grid mixing span in 2 × 1 spacer grid

Abstract Spacer grid or Mixing Vane Grid (MVG) is recognized as an important component that could greatly improve Critical Heat Flux (CHF) performance in the fuel assembly. The temperature around the rod would be more uniform, bubbles around the rod wall would be broken off or driven to the center of the subchannel by the mixing effects produced by spacer grids. This would potentially result in a higher CHF value. However, as the grid span increases, the mixing effects will decay as the coolant travels further away from the mixing vane grid. The local thermal hydraulic conditions become severe as the heated length increases. As a result, from the decay of the mixing function, CHF usually occurs at immediate upstream of spacer grid, or the furthest distance from the MVG upstream. On the other hand, with the narrowing of grid span, the pressure drop increases with increasing hydraulic resistance. Therefore, a proper grid span is quite important in the fuel assembly design. In this paper, the effects of grid span were studied by applying Computational Fluid Dynamics (CFD) simulation. The flow field with 2 × 1 spacer grids were simulated instead of prototype full scale spacer grid. CFD was validated by the experimental data from XJTU-ANFR low pressure and low temperature test loop. Flow field with different length of grid span were modeled to investigate the fundamental mechanism on how key factors, such as “mixing performance”, are impacted by the length of grid span. In order to evaluate the mixing performance, several popular mixing indexes, including Secondary Flow (SF), and thermal non-uniformity, were introduced to evaluate the effects of grid span on the mixing performance. Effective Mixing Length (EML) was defined by considering the effect of mixing and temperature distributions. Grid Span (GS) function was proposed to take various thermal hydraulic indexes into consideration in order to address the issue of local condition dependency and to obtain proper grid span. Different grid design thermal hydraulic indexes have different weighting factors, in such, the optimal grid span functions may not be the same for different fuel assemblies.

Improvement of BSFC and effective NOx and PM reduction by high EGR rates in heavy duty diesel engine

The test engine was a turbocharged 10.5L engine with an intercooler. A performance target was set at a rated power of 300 kW (BMEP = 1.7 MPa) and peak torque of 1842 Nm (BMEP = 2.2 MPa). Emission targets were set at a level of near future and stringent regulation standards in Japan. The engine was equipped with new technologies such as a high pressure common rail system, FCD piston, a high pressure ratio VGT and an aftertreatment system. The high and low pressure loop EGR system was installed and this system with a VGT had a high performance and could increase an EGR rate in order to reduce BSNOx while maintaining the satisfied BSFC and PM performance simultaneously not only in the steady state condition but also in the transient condition.

Fifty Years of TUDRP

Many professionals in today’s drilling engineering industry can trace their careers back to a historic research facility in Tulsa. One of the oldest drilling consortia in the world, The University of Tulsa Drilling Research Project, TUDRP, is celebrating 50 years and is poised to continue its tradition for another 5 decades. TUDRP is the concept of legendary TU professor Kermit Brown who initially established the consortium. A nonprofit cooperative between industry partners and university associates, TUDRP conducts basic and applied research to advance drilling technology. Students gain experience in the industry while partners benefit from the research developed. Professor B.J. Livesay was the group’s first director and welcomed a small group of industry partners. Livesay left the program in 1972, and John Day served as interim director until J.J. Azar accepted the permanent role. Azar arrived at TU in 1965 as an assistant professor in aerospace engineering. As director, he reached out to major oil corporations, independents, contractors, and suppliers. “We were the only existing cooperative research program in petroleum engineering that specialized in drilling while also promoting academics,” Azar said. TUDRP grew to more than a dozen members with worldwide recognition, and by the late 1980s, more than 25 companies were involved. Azar led TUDRP for 22 years while teaching drilling courses to hundreds of students and helping them publish technical papers as a direct result of the research at TUDRP. Professor Stefan Miska, who initially taught at New Mexico Tech, joined TU in 1992 and succeeded Azar as TUDRP director in 1996. Miska continued to focus on Azar’s priorities while also expanding the research. The development of a flow loop system by Azar provided a location for new experiments, and Miska was eager to implement additional applications. “I wanted to do more for the drilling industry,” he said. “I wanted to develop something that would be unique and useful.” TUDRP began looking at all aspects of drilling mechanics and introduced new tools such as the Rock Mechanics Facility to better understand factors affecting the drilling rate and ultimately drilling costs. Current TUDRP resources include a low-pressure flow loop, advanced cuttings transport facility, drillpipe/coiled tubing buckling facility, full-scale testing rig, drillpipe fatigue testing facility, fluids characterization laboratory, small and large indoor flow loops, shale-fluid interaction test cell, triaxial rock mechanics testing facility, and others. The consortium team includes eight TU faculty members as well as research technicians, several consultants, and research assistants. To date, 170 projects have been completed, and 14 are in progress.

Emission reduction through internal and low-pressure loop exhaust gas recirculation configuration with negative valve overlap and late intake valve closing strategy in a compression ignition engine

An investigation was carried out to examine the feasibility of replacing the conventional high-pressure loop/low-pressure loop exhaust gas recirculation with a combination of internal and low-pressure loop exhaust gas recirculation. The main objective of this alternative exhaust gas recirculation path configuration is to extend the limits of the late intake valve closing strategy, without the concern of backpressure caused by the high-pressure loop exhaust gas recirculation. The late intake valve closing strategy improved the conventional trade-off relation between nitrogen oxides and smoke emissions. The gross indicated mean effective pressure was maintained at a similar level, as long as the intake boosting pressure kept changing with respect to the intake valve closing timing. Applying the high-pressure loop exhaust gas recirculation in the boosted conditions yielded concern of the exhaust backpressure increase. The presence of high-pressure loop exhaust gas recirculation limited further intake valve closing retardation when the negative effect of increased pumping work cancelled out the positive effect of improving the emissions’ trade-off. Replacing high-pressure loop exhaust gas recirculation with internal exhaust gas recirculation reduced the burden of such exhaust backpressure and the pumping loss. However, a simple feasibility analysis indicated that a high-efficiency turbocharger was required to make the pumping work close to zero. The internal exhaust gas recirculation strategy was able to control the nitrogen oxides emissions at a low level with much lower O2 concentration, even though the initial in-cylinder temperature was high due to hot residual gas. Retardation of intake valve closing timing and intake boosting contributed to increasing the charge density; therefore, the smoke emission reduced due to the higher air–fuel ratio value exceeding 25. The combination of internal and low pressure loop loop exhaust gas recirculation with late intake valve closing strategy exhibited an improvement on the trade-off relation between nitrogen oxides and smoke emissions, while maintaining the gross indicated mean effective pressure at a comparable level with that of the high-pressure loop exhaust gas recirculation configuration.

Hybrid EGR and turbocharging systems control for low NOX and fuel consumption in an automotive diesel engine

An experimental investigation was made on a downsized Euro 5 turbocharged diesel engine managing high/low pressure EGR systems and the variable nozzle turbine, aiming at major reductions in NOX emissions while enhancing fuel consumption in comparison with reference configuration. Three part load engine operating conditions were chosen at low and medium speed levels, applying different strategies to a number of control variables (relative air–fuel ratio, VNT opening degree/intake pressure, valves position in low pressure EGR circuit), thus exploring the whole range of high/low pressure EGR proportion. In suitable operating modes of tested conditions, NOX emissions were reduced up to 58–66%. Brake specific fuel consumption decreased around 5–9.5% at low speed/load, 1.7–3.3% in the intermediate condition while no advantages were achieved in the third working point. The activation of low pressure loop confirmed to be beneficial for turbocharger performance, rising its rotational speed thus granting for a better transient response, while increase in soot emissions ranged in a wide interval. For each condition, optimal operating modes were identified, with the relevant EGR proportion. A prevailing contribution from the long route circuit was generally requested, up to the exclusion of short route loop at higher level of speed and load.

대형디젤엔진의 저온연소 시스템 최적화에 관한 연구

Abstract : According to the regulation on the environment and fuel efficiency is becoming strict, many experiments are conducted to improve efficiency and emission in internal combustion engines. LTC (Low temperature combustion) technology is a promised solution for low emissions but there are a few barriers for the commercial engine. This paper includes optimization that applies LTC method to heavy duty diesel engine. Adequate LTC was applied to low and middle load as adaptability in heavy duty diesel engine, and optimization focused on reduction of fuel consumption was proceeded at high load. Through this research, strategy for practical use of LTC was selected, and fuel consumption has improved on the condition that satisfies the emission regulation at systematic viewpoint. Key words : Heavy duty diesel engine( 대형디젤엔진), HP-EGR(High Pressure loop EGR, 고압 배기재순환), LP-EGR (Low Pressure loop EGR, 저압 배기재순환), VGT Turbocharger (Variable Geometry Turbine Turbocharger, 가변 터빈 터보차져), LTC(Low Temperature Combustion, 저온연소)

Some matters concerned with selecting steam parameters and process-circuit solutions to optimize the parameters of steam turbine equipment and engineering design developments

The possibility and advantages of increasing steam pressure in the steam-turbine low-pressure loop for combined-cycle power plants are considered. The question about the advisability of developing and manufacturing steam turbines for being used in combined-cycle power units equipped with modern class F gas turbines for supercritical and ultrasupercritical steam parameters is raised.

A control strategy of low-pressure loop exhaust gas recirculation and NOx storage catalyst for diesel engines

Low-pressure loop exhaust gas recirculation systems are effective means of simultaneously reducing the NOx emissions and fuel consumption of diesel engines. Further lower emission levels can be achieved by adopting a system that combines low-pressure loop exhaust gas recirculation with a NOx storage and reduction catalyst. However, this combined system has to overcome the issue of combustion fluctuations resulting from changes in the air–fuel ratio due to exhaust gas recirculation from rich operating conditions. The aim of this research was to reduce combustion fluctuations by developing low-pressure loop exhaust gas recirculation control logic. In order to control the combustion fluctuations caused by low-pressure loop exhaust gas recirculation, it is necessary to estimate the recirculation time. First, recirculation delay was investigated, and a model was developed. A good correlation was found between actual measurements and the recirculation delay estimated by this model. Next, the control logic for low-pressure loop exhaust gas recirculation was studied. The recirculation gas under rich operating conditions was detected by an air–fuel ratio sensor to examine a method of controlling the exhaust gas recirculation valve in accordance with the timing for the rich gas to reach the exhaust gas recirculation valve actually. Thus, fluctuations in torque and combustion noise were improved.

Effects of H2 on the number concentration of particulate matter in diesel engines using a low-pressure loop exhaust-gas recirculation system

We investigated the effects of H2 on the number concentration of particulate matter (PM) emissions from a diesel engine fitted with a low-pressure loop exhaust gas recirculation system. We used a 2.2-L four-cylinder direct-injection diesel engine satisfying EURO V regulations, and converted this engine to include an H2 feed. The air/fuel (A/F) ratio was varied in the range of 21.9–45.5 and the brake mean effective pressure was varied in the range of 2–6 bars to control the O2 concentration and in-cylinder temperature, both of which are significant for PM emissions. The number concentration of the emitted PM was measured using a scanning mobility particle sizer. We found that the emitted PM decreased by the addition of H2 which caused the unburned gas temperature increased. Furthermore, the degree of reduction was larger as the A/F ratio, load, and H2 energy fraction increased. However, with A/F ratios of less than 21.9, the addition of H2 increased the number concentration of emitted PM which was attributed to the small O2 concentration at these A/F ratios.

Effects of tumble combined with EGR (exhaust gas recirculation) on the combustion and emissions in a spark ignition engine at part loads

The effects of tumble combined with EGR (exhaust gas recirculation) on the combustion and emissions were experimentally investigated in a spark ignition engine in this study. A low pressure loop EGR system and a variable tumble valve in intake ports were employed, by which wide range of EGR ratio and tumble ratio could be achieved. All the tests were performed at part loads, and the equivalence ratio was kept constant at 1.0. The results show that the combination of EGR and enhanced tumble motion produces a significant improvement in fuel economy (13.1–19.5%) and a reduction in the coefficient of variations of indicated mean effective pressure (CoVIMEP) due to the significantly shortened combustion duration. By optimizing the combustion process with EGR and enhanced tumble, NOx (nitrogen oxides) emissions drastically decrease, while HC (hydrocarbons) emissions slightly rise. For CO (carbon monoxide) emissions, the enhanced tumble takes a positive effect.

Observer-Based Estimation of Air-Fractions for a Diesel Engine Coupled With Aftertreatment Systems

This paper presents a control-oriented air-fraction dynamic model for a complete diesel engine and aftertreatment system, as well as an observer to estimate the air-fractions in the integrated diesel engine and aftertreatment system. To reduce engine-out emissions and further improve engine efficiency, advanced combustion modes including low temperature combustion, premixed charge compression ignition, and homogenous charge compression ignition, are under intensive investigations. With the popular configuration of dual-loop exhaust gas recirculation (EGR) systems including a high-pressure loop exhaust gas recirculation and a low-pressure loop EGR (LP-EGR), alternative combustion modes can be potentially accomplished. In addition, aftertreatment systems including diesel oxidation catalysts (DOCs), diesel particulate filters (DPFs), and selective catalytic reduction (SCR) systems, are becoming necessary to satisfy stringent emission standards. With the increasing complexity of modern diesel engines and aftertreatment systems, air-path loop control for the diesel engine systems becomes further challenging partially because LP-EGR couples the aftertreatment systems with the diesel engine air-path system. Most of the current air-fraction dynamic models and observers only consider the air-fractions through the diesel engines and air-fraction dynamics through aftertreatment systems are ignored, which, however, will introduce significant modeling errors in the situations when active DPF regenerations and post-fuel injections are implemented. The purpose of this paper is to develop a control-oriented air-fraction dynamic model for a complete diesel engine and aftertreatment system. Based on the developed model, a Luenberger-like observer is proposed and analyzed using a Lyapunov method as well as the physical meaning of system parameters. Experimental results show that the developed air-fraction model is highly accurate and the designed observer is able to make the estimated air fractions converge to the corresponding true values quickly in both steady-state and transient operations.

Study on EGR Technology Routes of Vehicle Engine - A Review

An introduction was given to the basic structure principles and testing programs of EGR system. The technical key elements which can improve the comprehensive performances of EGR system were put forward, and these elements were composed of the choice of high pressure loop (HPL) and low pressure loop (LPL) EGR system, the design of EGR valve, the design of EGR cooling system, the uniformity of EGR in every cylinder and the matching with VGT system. Finally, a sum that EGR technical requirements meet different emission regulations was established