Benefits and Cost-effectiveness Analysis of Exhaust Energy Recovery System Using Low and High Boiling Temperature Working Fluids in Rankine Cycle

AbstractIn this paper, six attactive working fluids, including low boiling refrigerants such as R123, R141b and R245fa (Group L) and high boiling substances such as cyclohexane, ethanal and water (Group H), are applied on Rankine cycle, in order to examine the potential of these two categories of working fluids in high temperature exhaust energy recovery system (EERs) from a gasoline engine. The influences of engine speed at full load and evaporating pressure on the EERs performances are analyzed. The results reveal that water in Group H and R141b in Group L contribute the peak improvement in system benefits, while fluids in Group H show better cost-effectiveness. The EERs performances would be influenced strongly by evaporating pressure at high engine speed, while it also requires high pressure to enhance the performances at low speed. Besides, when the evaporating pressure is low, selection of working fluid should be emphasized

Effect of pressure wave disturbance on auto-ignition mode transition and knocking intensity under enclosed conditions

Pressure wave propagation behavior is an essential feature for the combustion under enclosed conditions, e.g. internal combustion engines. Previous work by Pan et al. (2016) and Yu et al. (2015) showed that pressure wave disturbance not only affects hot-spot formation and knocking origin, but also induces detonation wave through a coupling mechanism between pressure wave and flame front. On this basis, this study further investigates the role of pressure wave disturbance in auto-ignition mode and knocking intensity by means of detailed numerical simulations with stoichiometric H2/air mixture. Firstly, the pressure waves with different levels in strength have been obtained by adjusting ignition temperature of hot ignition kernel. It shows that as ignition temperature is raised at each initial temperature, pressure wave strength is decreased monotonously, with declining compression ratio and temperature rise caused by pressure wave disturbance. Secondly, three auto-ignition modes have been observed with the variations of pressure wave strength, i.e. detonation, mixed mode and supersonic deflagration. As the weakness of pressure wave strength, there is an auto-ignition mode transition from detonation to supersonic deflagration, accompanied by rapid decreases in pressure peak, obvious pre-flame partial reaction and significant increases in auto-ignition reaction front speed. These observations are still maintained at elevated initial pressure conditions. Finally, such auto-ignition modes and knocking intensity for the detailed computations are summarized in the non-dimensional Bradley's diagram. The results show that both auto-ignition mode and initial thermodynamic state can affect knocking intensity, and the modifications in knocking intensity by pressure wave disturbance are mainly through auto-ignition mode transition. This is qualitatively consistent with the distribution of combustion regimes in Bradley's diagram, even though some deviations do exist because the diagram is constructed on basis of initially non-reactive flows

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

© 2017 Elsevier B.V. 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-CO 2 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-CO 2 and isopentane-CO 2 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

Part-Load Performance Prediction and Operation Strategy Design of Organic Rankine Cycles with a Medium Cycle Used for Recovering Waste Heat from Gaseous Fuel Engines

The Organic Rankine Cycle (ORC) is regarded as a suitable way to recover waste heat from gaseous fuel internal combustion engines. As waste heat recovery systems (WHRS) have always been designed based on rated working conditions, while engines often work under part-load conditions, it is quite significant to analyze the part-load performance and corresponding operation strategy of ORC systems. This paper presents a dynamic model of ORC with a medium cycle used for a large gaseous fuel engine and analyzes the effect of adjustable parameters on the system performance, giving effective control directions under various conditions. The results indicate that the intermediary fluid mass flow rate has nearly no effect on the output power and thermal efficiency of the ORC, while the mass flow rate of working fluid has a great effect on them. In order to get a better system performance under different working conditions, the system should be operated with the working fluid mass flow rate as large as possible, but with a slight degree of superheating. Then, with the control of constant superheat degree at the end of the heating process, the performance of the combined system that consists of ORC and the engine at steady state under seven typical working conditions is also analyzed. The results indicate that the energy-saving effect of WHRS becomes worse and worse as the working condition decreases. Especially at 40% working condition the WHRS nearly has no energy-saving effect anymore

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

CFD Simulation of a Supercritical CO2 Rolling Rotor Expander for Waste Heat Recovery System of Engines

The supercritical CO2 power cycle system for waste heat recovery (WHR) of internal combustion engine (ICE) has widely been concerned as a research hotspot. And the expander is a key component in the supercritical CO2 power system. Rolling rotor expander has the following advantages: compact size, light weight, less moving parts, high stability and long service life, which qualify it a very suitable choice for engine’s waste heat recovery system. For a self-designed rolling rotor expander using supercritical CO2 as its working fluid, FLUENT software was used to simulate its internal flow field in this study, obtaining the changes of the internal pressure field and turbulent kinetic energy. The causes of local vortex in the expansion process were analyzed. Under different working conditions of the expander, the change of internal pressure and the distribution of P-V curve were observed, and the work capacity under different inlet pressure was analyzed. Results show that, the work capacity increases if the intake pressure increases within a certain range. However, if the inlet pressure keeps increasing and exceeds a reasonable limit, the local vortex in the suction process is enhanced and the pressure loss is increased and the degree of the turbulence is strengthened, which causes a certain impact on the expansion process and expander service life. The rotating speed has a great influence on the operation of the expander. These results provide guidance for the design and optimization of the supercritical CO2 expander in the future

A review of researches on thermal exhaust heat recovery with Rankine cycle

Internal combustion (IC) engines are the major source of motive power in the world, a fact that is expected to continue well into this century. To increase the total efficiency and reduce CO2 emissions, recently exhaust heat recovery (EHR) based on thermoelectric (TE) and thermal fluid systems have been explored widely and a number of new technologies have been developed in the past decade. In this paper, relevant researches are reviewed for providing an insight into possible system designs, thermodynamic principles to achieve high efficiency, and selection of working fluids to maintain necessary system performance. From a number of researches, it has been found the Rankine cycle (RC) has been the most favourite basic working cycle for thermodynamic EHR systems. Based on the cycle, various different system configurations have been investigated. Accepting a certain design and manufacture cost, a system based on heavy duty vehicle application can increase the total powertrain efficiency by up to 30% (based on NEDC driving condition). To achieve the highest possible system efficiency, design of systemic structure and selections for both the expander and the working fluid (medium) are critical.Internal combustion (IC) engine Exhaust heat recovery (EHR) Rankine cycle (RC) Working fluid (medium)