Gequn Shu

Flame temperature theory-based model for evaluation of the flammable zones of hydrocarbon-air-CO2 mixtures

Theoretical models to evaluate the flammable zones of mixtures made up of hydrocarbon, carbon dioxide and air have been proposed in present study. A three-step reaction hypothesis for hydrocarbon combustion was introduced for predicting the upper flammability limit. The method to predict the parameters at fuel inertization point was put forward as well. Validation of these models has been conducted on existing experimental data reported in the literature, including the cases of methane, propane, propylene and isobutane, and an acceptable precision has been achieved. The average relative differences between the estimated results and experimental ones, except for the results at fuel inertization point, are less than 8.8% and 3.3% for upper and lower flammability limit, respectively. This work also indicated that these models possess practical application capacity and can provide safe prediction limits for nonflammable ranges of hydrocarbon diluted with carbon dioxide.

Simulations of a Bottoming Organic Rankine Cycle (ORC) Driven by Waste Heat in a Diesel Engine (DE)

A bottoming waste-heat-recovery (WHR) model based on the Organic Rankine Cycle (ORC) is proposed to recover waste heat from exhaust gas and jacket water of a typical diesel engine (DE). The ORC model is detailed built based upon real structural and functional parameters of each component, and is able to precisely reflect the working process of the experimental ORC system constructed in lab. The DE is firstly tested to reveal its energy balance and the features of waste heat. The bottoming ORC is then simulated based on experimental data from the DE bench test using R245fa and R601a as working fluid. Thermodynamic evaluations are done on key parameters like waste heat recovered, expansion power, pump power loss and system efficiency. Results indicate that maximum expansion power and efficiency of the ORC are up to 18.8kW and 9.6%. Influences of engine condition, fluid mass flow and evaporating pressure on system performance are analyzed and meaningful regularities are revealed. The combined system of DE and bottoming ORC (DE-ORC) is also investigated. The results showed that the integration of the bottoming ORC greatly changed energy distribution of the DE, and the DE thermal efficiency is up to 47.2%, increasing by 9.0%.

Theoretical Analysis of Engine Waste Heat Recovery by the Combined Thermo-Generator and Organic Rankine Cycle System

The combined thermo-generator and organic rankine cycle (TEG-ORC) used in exhaust heat recovery of internal combustion engine (ICE) is analyzed theoretically. Only about one third of the total energy released from fuel combustion is converted into useful work in engines, while the remaining energy goes into ambient environment, among which exhaust gas possesses high-grade thermal energy. Most of previous studies on energy recovery from engines have focused on exhaust heat recovery by ORC. However, if the heat is exchanged directly with high-temperature exhaust gas, organic working fluid would resolve with its lower decomposition temperature, and this is extremely harmful to ORC system. To avoid this phenomenon and utilize waste heat, preliminary thermoelectric modules are used to lower exhaust temperature and to generate electricity simultaneously. The heat rejected by TEG is used to preheat working fluid, and more energy is recovered to improve gross output power and thermal efficiency. A theoretical numerical model has been established in this paper to study the performances in both supercritical and subcritical combined TEG-ORC systems. The results suggest that, this model could efficiently identify the optimal performance parameters of both TEG and ORC systems. The utilization of TEG can extend the range of choosing working fluids if the temperature of waste heat source is high, so combined TEG-ORC system is suitable to recover waste heat from automotive vehicle engines, and thereby to improve the fuel economy of a passenger vehicle.

A quantitative risk-assessment system (QR-AS) evaluating operation safety of Organic Rankine Cycle using flammable mixture working fluid

Mixture of hydrocarbon and carbon dioxide shows excellent cycle performance in Organic Rankine Cycle (ORC) used for engine waste heat recovery, but the unavoidable leakage in practical application is a threat for safety due to its flammability. In this work, a quantitative risk assessment system (QR-AS) is established aiming at providing a general method of risk assessment for flammable working fluid leakage. The QR-AS covers three main aspects: analysis of concentration distribution based on CFD simulations, explosive risk assessment based on the TNT equivalent method and risk mitigation based on evaluation results. A typical case of propane/carbon dioxide mixture leaking from ORC is investigated to illustrate the application of QR-AS. According to the assessment results, proper ventilation speed, safe mixture ratio and location of gas-detecting devices have been proposed to guarantee the security in case of leakage. The results revealed that this presented QR-AS was reliable for the practical application and the evaluation results could provide valuable guidance for the design of mitigation measures to improve the safe performance of ORC system.

Axial vibration source identification of engine crankshaft based on auto-regressive and moving average model and analytic hierarchy process method

This paper presents a method to identify the root cause of the axial vibration of crankshafts for high speed diesel engines based on an auto-regressive and moving average model and the analytic hierarchy process. Through determining initial moving average variables and measuring axial/bending/torsional vibrations of a crankshaft at the free-end of a four-cylinder diesel engine, auto-regressive spectrum analysis is applied to the measured vibration signal. In an investigation of the root cause of the vibration, the hierarchy tree and judgment matrix are given to identify the main vibration root causes. The results show that the axial vibration of the crankshaft is mainly coupled and excited by the bending vibration at high speeds. But at low speeds, the axial vibration in some frequencies is coupled and excited primarily by the torsional vibration. Through investigation of the root cause of the axial vibration of the engine crankshafts, calculation accuracy of the vibration can be improved significantly.

A new model based on adiabatic flame temperature for evaluation of the upper flammable limit of alkane-air-CO2 mixtures

For security issue of alkane used in Organic Rankine Cycle, a new model to evaluate the upper flammability limits for mixtures of alkanes, carbon dioxide and air has been proposed in present study. The linear relationship was found at upper flammability limits between molar fraction of diluent in alkane-CO2 mixture and calculated adiabatic flame temperature. The prediction ability of the variable calculated adiabatic flame temperature model that incorporated the linear relationship above is greatly better than the models that adopted the fixed calculated adiabatic flame temperature at upper flammability limit. The average relative differences between results predicted by the new model and observed values are less than 3.51% for upper flammability limit evaluation. In order to enhance persuasion of the new model, the observed values of n-butane-CO2 and isopentane-CO2 mixtures measured in this study were used to confirm the validity of the new model. The predicted results indicated that the new model possesses the capacity of practical application and can adequately provide safe non-flammable ranges for alkanes diluted with carbon dioxide.

Engine Load Effects on the Energy and Exergy Performance of a Medium Cycle/Organic Rankine Cycle for Exhaust Waste Heat Recovery

The Organic Rankine Cycle (ORC) has been proved a promising technique to exploit waste heat from Internal Combustion Engines (ICEs). Waste heat recovery systems have usually been designed based on engine rated working conditions, while engines often operate under part load conditions. Hence, it is quite important to analyze the off-design performance of ORC systems under different engine loads. This paper presents an off-design Medium Cycle/Organic Rankine Cycle (MC/ORC) system model by interconnecting the component models, which allows the prediction of system off-design behavior. The sliding pressure control method is applied to balance the variation of system parameters and evaporating pressure is chosen as the operational variable. The effect of operational variable and engine load on system performance is analyzed from the aspects of energy and exergy. The results show that with the drop of engine load, the MC/ORC system can always effectively recover waste heat, whereas the maximum net power output, thermal efficiency and exergy efficiency decrease linearly. Considering the contributions of components to total exergy destruction, the proportions of the gas-oil exchanger and turbine increase, while the proportions of the evaporator and condenser decrease with the drop of engine load.

Large eddy simulation of the low temperature ignition and combustion processes on spray flame with the linear eddy model

Large eddy simulation coupled with the linear eddy model (LEM) is employed for the simulation of n-heptane spray flames to investigate the low temperature ignition and combustion process in a constant-volume combustion vessel under diesel-engine relevant conditions. Parametric studies are performed to give a comprehensive understanding of the ignition processes. The non-reacting case is firstly carried out to validate the present model by comparing the predicted results with the experimental data from the Engine Combustion Network (ECN). Good agreements are observed in terms of liquid and vapour penetration length, as well as the mixture fraction distributions at different times and different axial locations. For the reacting cases, the flame index was introduced to distinguish between the premixed and non-premixed combustion. A reaction region (RR) parameter is used to investigate the ignition and combustion characteristics, and to distinguish the different combustion stages. Results show that the two-stage combustion process can be identified in spray flames, and different ignition positions in the mixture fraction versus RR space are well described at low and high initial ambient temperatures. At an initial condition of 850 K, the first-stage ignition is initiated at the fuel-lean region, followed by the reactions in fuel-rich regions. Then high-temperature reaction occurs mainly at the places with mixture concentration around stoichiometric mixture fraction. While at an initial temperature of 1000 K, the first-stage ignition occurs at the fuel-rich region first, then it moves towards fuel-richer region. Afterwards, the high-temperature reactions move back to the stoichiometric mixture fraction region. For all of the initial temperatures considered, high-temperature ignition kernels are initiated at the regions richer than stoichiometric mixture fraction. By increasing the initial ambient temperature, the high-temperature ignition kernels move towards richer mixture regions. And after the spray flames gets quasi-steady, most heat is released at the stoichiometric mixture fraction regions. In addition, combustion mode analysis based on key intermediate species illustrates three-mode combustion processes in diesel spray flames.

Numerical investigation of diesel spray flame structures under diesel-engine relevant conditions using large eddy simulation

ABSTRACT Large eddy simulation coupled with the third-order Monotone Upstream-centered Schemes for Conservation Laws (MUSCL) differencing scheme was employed for investigating the ignition processes and flame structures of the reacting n-heptane spray over a wider range of diesel engine-relevant conditions. First, the effects of numerical schemes on the mixing and combustion processes are analyzed in detail. Comparisons of the mixture fraction profiles with experimental data from the Engine Combustion Network website show that the MUSCL gives a better prediction compared with Quasi-second-order upwind scheme. The mixing between fuel and air is much better using MUSCL scheme. As a result, ignition is initiated at fuel-leaner regions, but still richer than stoichiometric equivalence ratio. Second, the predicted ignition delay times and flame lift-off lengths are compared with experimental data under different initial conditions. The predicted results show good agreements with experimental results at different temperatures, oxygen concentrations, and densities. Finally, the effects of initial conditions on the spray flame structures are comprehensively analyzed using the temperature-equivalence ratio maps. The effects of initial parameters on the reacting spray structures and important radicals, such as OH and CO are investigated carefully. Results show that initial oxygen concentration has the greatest influence on the flame structures than ambient gas temperature and density.

Thermodynamic Analysis of a Novel Combined Power and Cooling Cycle Driven by the Exhaust Heat Form a Diesel Engine

A novel combined power and cooling cycle based on the Organic Rankine Cycle (ORC) and the Compression Refrigeration Cycle (CRC) is proposed. The cycle can be driven by the exhaust heat from a diesel engine. In this combined cycle, ORC will translate the exhaust heat into power, and drive the compressor of CRC. The prime advantage of the combined cycle is that both the ORC and CRC are trans-critical cycles, and using CO₂ as working fluid. Natural, cheap, environmentally friendly, nontoxic and good heat transfer properties are some advantages of CO₂ as working fluid. In this paper, besides the basic combined cycle (ORC-CRC), another three novel cycles: ORC-CRC with an expander (ORC-CRCE), ORC with an internal heat exchanger as heat accumulator combined with CRC (ORCI-CRC), ORCI-CRCE, are analyzed and compared. The cycle parameters, including the coefficient of performance (cop), the cooling capacity (Qro) and expansion power of CRC (We) have been analyzed and optimized as the variation of the high pressure of ORC, the high pressure and the outlet temperature of gas cooler of CRC, and temperature drop of heat source in heat accumulator of ORC. The results indicate that there is an optimal high pressure of CRC (about 8.6 MPa to 8.8 MPa) for the combined cycles, at which the combined cycles achieve the optimal performance. The results also show that both the expander and heat accumulator could improve the system performance. The higher ΔTi could improve the system performance, but also resulting the more insufficiency of waste heat recovery.