Exergy Transfer Research Articles

Potential of hydrogen/diesel reactivity controlled compression ignition (RCCI) combustion from the perspective of the second law of thermodynamics

The potential of reactivity controlled compression ignition (RCCI) combustion fueled with hydrogen and diesel (i.e. hydrogen/diesel RCCI) was evaluated using multi-dimensional simulations embedded with a reduced chemical mechanism. In hydrogen/diesel RCCI, the premixed hydrogen is ignited by the diesel, which is directly injected into the cylinder well before the top dead center. To investigate the potential benefits of hydrogen/diesel RCCI, its combustion characteristics were compared with that of gasoline/diesel RCCI from the perspective of the second law of thermodynamics. Meanwhile, the impacts of premixed energy ratio and initial pressure on the exergy distribution for hydrogen/diesel RCCI were explored. The results show that hydrogen/diesel RCCI has an advantage over gasoline/diesel RCCI in the reduction of exergy destruction due to higher combustion temperature, shorter combustion duration, and the distinctive oxidation pathways between hydrogen and gasoline. A higher proportion of exergy output work can be achieved for hydrogen/diesel RCCI under the conditions with the same total input energy and 50% heat release (CA50) point. Moreover, a larger premixed energy ratio (i.e. larger hydrogen proportion) is helpful to elevate exergy output work and reduce exergy destruction owing to higher combustion temperature and the undergoing oxidation pathways of hydrogen with less exergy destruction. A higher initial pressure yields raised exergy destruction because of lower combustion temperature and longer combustion duration, but exergy output work is increased owing to the significantly reduced exergy transfer through heat transfer.

Industrial Scale Ammonia Pipeline Transfer System and Exergy Analysis

Industrial Scale Ammonia Pipeline Transfer System and Exergy Analysis

Numerical investigation of the air-particles heat transfer characteristics of moving bed - effect of particle size distribution

Particle vertical moving beds (PVMB) are widely used in industry, usually accompanied by gas-solid heat exchange or chemical reaction. It is worth noting that the actual industrial particles usually have a wide particle size distribution (PSD), which is an important property that can influence the hydrodynamics and heat conversion in moving bed system. Therefore, understanding the multiphase flow and thermal behavior of PVMB with multi-size particles is essential for process designing and optimizing. Based on the CFD-DEM coupling method, a mathematical model of gas-solid heat transfer for PVMB with different PSDs is established- all with the same mean particle diameter. Furthermore, the effects of 3 influential factors on the exergy transfer in a small PVMB were analyzed. Results show that an increased PSD range leads to an increased specific surface area, but this does not imply a favorable heat transfer between air and particles in PVMB, which is contrary to the findings related to the fixed bed. Increasing particle mass flow rate or particle inlet temperature will cause the cooling air and particle outlet temperature to increase approximately linearly. More interestingly, as inlet air mass flux increases, exergy recovery efficiency decreases, however with the further increase of inlet air mass flux, exergy recovery efficiency reaches a minimum and then gradually increases. Exergy recovery efficiency decreases and tends to be flat with increasing particle mass flow rate. This method is an essential procedure for modeling the heat transfer of discrete particles in industrial-scale equipment.

A Compendium of Methods for Determining the Exergy Balance Terms Applied to Reciprocating Internal Combustion Engines

Abstract Exergy analysis of the reciprocating internal combustion (IC) engines is studied by estimating various input and output energy transfer parameters concerning a dead state reference. Exergy terms such as fuel input, work output, cooling, and exhaust gas are measured and are set into the exergy balance equation to determine the amount of loss or destruction. Exergy destructions are found in many forms such as combustion (entropy generation), cylinder wall, friction, mixing, blow-by, and others. These exergy terms have been estimated by considering various factors such as engine type, fuel type, environmental condition, and others. In this article, the different methods employed in estimating these exergy terms have been reviewed. It attempts to make a compendium of these evaluation methods and segregates them under individual exergy terms with necessary descriptions. The fuel input measurement is mostly based on Gibb's free energy and the lower heating value, whereas its higher heating value is used during the fuel exergy calculation on a molar basis. The work output of the engines is estimated either from the crankshaft or by analyzing the cylinder pressure and volume. The exergy transfer with cooling medium and exhaust gas depends on the temperature of the gas. The maximum achievable engine performance is quantified by estimating the exergy efficiency. This piece of study will not only provide plenty of information on exergy evaluation methods of IC engines but will also allow future researchers to adopt the appropriate one.

Heat transfer performance and exergy analyses of MgO and ZnO nanofluids using water/ethylene glycol mixture as base fluid

Forced laminar convective heat transfer of MgO and ZnO nanofluids through flat tube automobile radiator was studied numerically using 0.25-3ć volume concentrations at Reynolds number ranges (250–1750) for both nanofluids. The results showed an increase in the convective heat transfer coefficient 13.76% and 17.1% at 3% concentration of ZnO and MgO nanofluids, respectively, relative to water/ethylene glycol mixture (50:50 by vol%.) as a base fluid. However, increasing the volume concentration caused an increase in the pumping power For same heat rejection of W/EG mixture, a reduction of about 4.15% and 3.1% in radiator surface area was obtained at 1% concentration of MgO and ZnO nanofluids, respectively. A decrease in total exergy destruction rate of 15.7% and 12.2% compared to the base fluid was obtained for 3 vol% of MgO and ZnO nanofluids, respectively, at Re = 1750.

Energy and Exergy Efficiency Analysis of Fluid Flow and Heat Transfer in Sinter Vertical Cooler

In order to fully understand the energy and exergy transfer processes in sinter vertical coolers, a simulation model of the fluid flow and heat transfer in a vertical cooler was established, and energy and exergy efficiency analyses of the gas–solid heat transfer in a vertical cooler were conducted in detail. Based on the calculation method of the whole working condition, the suitable operational parameters of the vertical cooler were obtained by setting the net exergy efficiency in the vertical cooler as the indicator function. The results show that both the quantity of sinter waste heat recovery (SWHR) and energy efficiency increased as the air flow rate (AFR) increased, and they decreased as the air inlet temperature (AIT) increased. The increase in the sinter inlet temperature (SIT) resulted in an increase in the quantity of SWHR and a decrease in energy efficiency. The air net exergy had the maximum value as the AFR increased, and it only increased monotonically as the SIT and AIT increased. The net exergy efficiency reached the maximum value as the AFR and AIT increased, and the increase in the SIT only resulted in a decrease in the net exergy efficiency. When the sinter annual production of a 360 m2 sintering machine was taken as the processing capacity of the vertical cooler, the suitable operational parameters of the vertical cooler were 190 kg/s for the AFR, and 353 K for the AIT.

Exergy-based modeling framework for hybrid and electric ground vehicles

Exergy, or availability, is a thermodynamic concept representing the useful work that can be extracted from a system evolving from a given state to a reference state. It is also a system metric, formulated from the first and the second law of thermodynamics, encompassing the interactions between subsystems and the resulting entropy generation. In this paper, an exergy-based analysis for ground vehicles is proposed. The study, a first to the authors’ knowledge, defines a comprehensive vehicle and powertrain-level modeling framework to quantify exergy transfer and destruction phenomena for the vehicle’s longitudinal dynamics and its energy storage and conversion devices (namely, electrochemical energy storage, electric motor, and ICE). To show the capabilities of the proposed model in quantifying, locating, and ranking the sources of exergy losses, two case studies based on an electric vehicle and a parallel hybrid electric vehicle are analyzed considering a real-world driving cycle. This modeling framework can serve as a tool for the future development of ground vehicles management strategies aimed at minimizing exergy losses rather than fuel consumption.

Energy and exergy study on indirect evaporative cooler used in exhaust air heat recovery

Indirect evaporative cooler (IEC) applied as an exhaust air heat recovery component for fresh air pre-cooling in air-conditioned zone, which successfully extends its application range to hot and humid areas. To explore more effective application method of the heat recovery IEC, an energy and exergy transfer model for IEC operating as a heat recovery devices was proposed in this study. The thermal performance and exergy transfer characteristics of three basic types of IECs: cross-flow, counter-flow, and parallel-flow IECs were numerically investigated. The results show that the irreversible heat transfer in the primary air channels and the irreversible mass transfer in the secondary air channels are the main causes of exergy loss. These two factors account for more than 90% of the total exergy destruction in all the three types of IECs. To alleviate this situation and improve the energy efficiency of the IEC, two two-stage modified heat recovery systems which combine IEC with sensible heat exchanger and energy recovery exchanger were proposed and numerical analyzed. The simulation results show that, for the modified systems, the exergy transfer efficiency is increased by 104.6% and 131.7%, while the heat transfer capacity is increased by 47.6% and 48.5%, respectively.

Experimental exergy analyses in aDIdiesel engine fuelled with a mixture of diesel fuel andTiO2nano‐particle

AbstractIn this study, the operation of a four‐cylinder, four‐stroke DI diesel engine powered by fuels created by blending diesel and TiO2nanoparticles was evaluated based on energy and exergy analyses. Within the study's scope, it is possible to see the engine's performance, combustion, emission, and heat transfer distribution at various engine speeds. The engine was tested by operating it with diesel and fuel blends, diesel‐2.5 ppm TiO2. According to obtained results, it was found that by adding 2.5 ppm TiO2to diesel fuel, energy and exergy efficiency increased by 4.1% and 3%. Engine power and shaft exergy improved by 3.4%, and diesel‐2.5 ppm TiO2blends decreased irreversibility by 3.8%. Diesel‐2.5 ppm TiO2blended fuel decreases the exhaust loss exergy and exhaust heat losses by 2.5% and 1.4% and also uncounted heat losses reduced by 5.7% compared with baseline diesel. Moreover, the exergy transfer with cooling water increased by 10.5% compared with baseline diesel fuel. The brake specific fuel consumption was 3% lower for diesel and 2.5 ppm TiO2nanoparticle blended fuels. The engine fueled with blended fuel revealed mitigation in the UHC, CO, and Soot emissions than those of diesel fuel. NOx emission exhibited increasing trends with the blended fuel. It is concluded that diesel blend with the addition of TiO2nanoparticles exhibits better engine performance and reduced emissions compared to diesel fuel.

Experimental investigation of the influences of Miller cycle combined with EGR on performance, energy and exergy characteristics of a four-stroke marine regulated two-stage turbocharged diesel engine

In order to explore the technical route to meet the Tier III emission regulations, the experimental study of the influence of Miller cycle combined with EGR on the performance, energy and exergy of a four-stroke marine regulated two-stage turbocharged diesel engine was achieved by redesigning the camshaft and EGR system. The results shown that Miller cycle combined with EGR increased the peak HRR of premixed combustion period, decreased the peak HRR during the main combustion period, and delayed the crank angle corresponding to the peak HRR, resulting in longer combustion duration and a later 50% combustion position. In the 75% load, the combination of medium Miller timing and medium EGR rate can not only meet the NOx emission requirements of Tier III, but also far superior to only high EGR rate in terms of fuel economy, opacity performance and COVIMEP. As the EGR rate increased, heat transfer energy to coolant, BTE and ITE dropped significantly, while exhaust energy and combustion loss increased significantly. The change trends of exergy terms with EGR rate and Miller timing were similar to their corresponding energy. It should be noted that the exhaust energy was about 2.5 times larger than the exhaust exergy while the heat transfer energy to coolant was about 8 times larger than the heat transfer exergy to coolant, which indicated that the recoverable potential of heat transfer energy to coolant is much smaller than exhaust energy.

Exergy transfer principles of microwavable materials under electromagnetic effects

Microwave technology is gaining an essential relevance for heating processes at an industrial level due to its improvements in energy savings and product quality. Nevertheless, the microwave system design and material selection during the applications are key points that play an essential role in the successful performance of the process, its implementation, and its operation. The proper function of the microwave highly depends on the design and a good selection of the materials. There are different kinds of materials for microwave applications such as transparent (not able to be heated), semi-transparent (low absorption of microwaves), and susceptor (materials with high capacity to absorb microwaves and transform them into thermal energy). This investigation shows the way each of these materials converts microwaves into heat. Both heat transfer and exergy transfer analyses are presented, focused on those materials with high interactions with microwaves (susceptors). The heat transfer studies demonstrated the way microwaves are transformed into heat, and the exergy analysis shows the quality of those transformations. Exergy transfer analysis of microwave heating systems sheds light on the efficiency of the energy transformation taking place during microwave processing. Consequently, by combining studies of microwavable materials with exergy transfer analysis, conclusions for new microwave designs can be reached, improving this promising technology's final performance. In this sense, this work provides an easy method to determine different materials' behavior under microwave effects.

Experimental investigation of the effects of Miller timing on performance, energy and exergy characteristics of two-stage turbocharged marine diesel engine

In order to analyze and improve the thermodynamic process of marine diesel engines and improve energy efficiency, the experimental study of the influence of Miller timing on the performance, energy and exergy of marine diesel engines was realized by redesigning the camshaft and adjusting the two-stage turbocharging system. The research results shown that with the increase of Miller timing, the average pumping loss decreased, the peak HRR and the maximum combustion temperature decreased, and the crank angle corresponding to the peak HRR was delayed backward, resulting in a backward delay of the 50% point, the 10–90% combustion duration was extended, and the EER, EEE and BTE first increased and then decreased. Secondly, the Miller cycle combined with two-stage turbocharging could make marine diesel engines reduced NOx emissions by more than 25% with a small effect on the BSFC. In addition, as the Miller timing increased, the proportion of exhaust energy, heat transfer energy to coolant, mechanical loss, and combustion loss all decreased, while the proportion of brake output work first increased and then decreased. The change trends of exergy terms with load and Miller timing were similar to their corresponding energy. It was worth noting that the exhaust exergy accounted for about 40% of the exhaust energy, while the heat transfer exergy to coolant accounted for only 13% of the heat transfer energy to coolant, indicating that the exhaust energy quality was higher, the utilization potential was greater, and the efficient utilization interval was concentrated in the high load area.

A new optimization concept of the recuperator based on Hampson-type miniature cryocoolers

The novel concept of “low pressure drop instead of the low temperature difference to obtain high heat flux” is proposed to improve the performance of recuperator for Hampson cryocoolers. The equivalent temperature Teqva is introduced to repaint the T-Q diagram for intuitively observing the variation of the total exergy destruction in different structures in this study. By substituting the total exergy destruction for the heat transfer exergy destruction, the flow resistance is also considered in the recuperator optimization. It is discovered that the recuperator with first-dense-then-sparse type structure reduces the average equivalent temperature greatly, leading to lower exergy destruction. Otherwise, the non-isometric structure offers the possibility to gain both larger heat flux and decreased flow resistance at the same time. With the pressure-enthalpy diagram analysis, the favourable temperature behaviour is explained. The supercritical fluids in the recuperator of Hampson cryocoolers can obtain lower enthalpy when higher temperatures and pressure are obtained at the same time. Therefore, the optimization of the recuperator should not only concentrate on improving the temperature difference but also on reducing the pressure difference. And the average equivalent temperature will be an effective parameter to evaluate the performance of the recuperator in the future study.

The transfer law of exergy in the lines (pipes) and exergy loss analysis

In order to explore the transfer law of exergy in the lines (pipes), based on the generalized expression equation of exergy and the generalized exergy transfers and conversion equation, the transfer law of exergy in the lines (pipes) is derived, and then the exergy loss of three typical energies are analyzed based on the exergy loss model, which are electric exergy loss, pressure exergy loss and heat exergy loss. This paper reveals the change mechanism of exergy transfer process, which can provide more comprehensive guidance for energy saving.

Quantitative and qualitative analysis of thermodynamic process of a bi‐fuel compressed natural gas spark ignition engine

AbstractA comparative energy and exergy analysis was experimentally performed using a bi‐fuel compressed natural gas (CNG) spark ignition engine. The experiments were conducted for gasoline and CNG fuel at 1700 rpm and different operating loads from 5 to 30 Nm. The experiments were performed under stoichiometric air‐fuel ratio and maximum brake torque ignition timing. Quantitative and qualitative analysis were conducted using the first and second law of thermodynamics, respectively. The effect of engine operating load on various energy and exergy parameters was compared for both the fuels. Output energy with respect to engine load was found higher for CNG compared to gasoline. Engine wall heat transfer was also higher in the CNG engine case due to its high combustion chamber temperature and lower burning velocity. The difference in heat transfer energy fraction between gasoline and CNG gradually increased with engine load. The exhaust energy was found maximum in the case of CNG under low operating load and reduced to a minimum at higher engine operating load. The unaccounted energy fraction was found lower for the CNG engine and reduced with respect to engine load. CNG exhibits the highest exergy efficiency (26.80%) compared to gasoline (25.50%) at 30 Nm load. On average, a 2% higher exergy efficiency was observed with the CNG engine at all operating load conditions. This indicates CNG has a higher potential to convert chemical energy present in the fuel into useful work output. CNG shows lower exergy destruction at all operating loads compared to gasoline, and it reduces significantly at higher engine loads. In all operating loads, exergy transfer due to exhaust gas and heat transfer to the wall was higher in the case of CNG.

Exergy transfer and degeneration in thermochemical cycle reactions for hydrogen production: Novel exergy- and energy level-based methods

Thermochemical cycle is a promising way for hydrogen production. Although the Gibbs free energy function has been used to assess thermochemical cycle reactions, it is difficult to explain exergy transfer and degeneration. There is no universal principle to explain the mechanism of triggering an endothermic reaction from the viewpoint of exergy. To fill the gap, novel exergy- and energy level-based analysis methods for thermochemical cycle reactions are proposed. Interrelationships of exergy and anergy in thermochemical reactions are analyzed. Energy level equations are built to provide a basic requirement that should be satisfied to trigger a thermochemical reaction. Additionally, a basic principle is proposed to explain the mechanism behind the methods of triggering an endothermic reaction. The results find that the certain exergy (or energy level) standard should be satisfied to trigger a thermochemical reaction. Increasing the exergy input beyond the exergy requirement or decreasing the exergy requirement lower than the exergy supplied by the reaction enthalpy is the basic principle for triggering an endothermic reaction. The proposed analysis methods provide new insights to analyze thermochemical cycle reactions.

Exergetic Optimization of the Heat Recovery Steam Generators by Imposing the Total Heat Transfer Area

The paper presents an original and fast method for the heat recovery steam generator (HRSG) exergetic optimization. The objective is maximizing the exergy transfer to the water / steam circuit. The proposed approach, different from the classical method that fixes the pinch point, is essentially thermodynamic but it considers also the economics by imposing the total heat transfer area of HRSG. The HRSG may have one or two steam pressures, without reheat. The input data from the gas turbine are: the mass flow rate, the temperature and the molar composition of flue gases. The results are the optimum pressures of the superheated steam. The numerical computations were realized in Delphi programming utility. The obtained results are in agreement with the recent literature.

Effect of operating parameters on gas-solid exergy transfer performance in sinter annular cooler

The exergy transfer process of sinter granular bed in an annular cooler is numerically studied. According to the principle of non-equilibrium thermodynamics, the exergy transfer coefficient and Nu expressions of the horizontal moving bed were derived by applying the second law of thermodynamics. A steady-state calculation model was established and verified by experimental data, and the influence of thermal parameters on the exergy transfer performance of the annular cooler was analyzed. The results show that increasing the cooling gas inlet velocity, cooling gas inlet temperature, sintered ore layer height or trolley moving velocity, the exergy transfer performance within the sinter granular bed would be enhanced. The cooling gas inlet speed and cooling gas inlet temperature have a significant effect on the average synthetic exergy transfer coefficient in the bed. Compared with the two factors mentioned earlier, the impact of the sinter height and the speed of the trolley on the exergy transfer process is small, but it should still attract the attention of the designer. This is of great significance for strengthening the cooling process of sintered ore, and provides a novel theoretical idea for designing similar cooling equipment.

Thermodynamic analysis of gasoline-fueled electronic fuel injection digital three-spark ignition (EFI-DTSI) engine

Numerous researchers have devised various strategies to improve the combustion efficiency of the internal combustion engine. Despite continuous improvement during past decades, there is still scope for further development in engine performance. This paper analyzes the energy and exergy distribution of a single-cylinder 199.5 cc electronic fuel injected three-spark ignited high-speed petrol engine. The engine is operated at 25%, 50%, 75%, and 100% throttle positions for different speeds of 4000–10,000 rpm with an increment of 2000 rpm. From the detailed heat balance analysis, the best results obtained are: maximum brake thermal efficiency of 34.9% corresponding to 6000 rpm at 50% throttle opening, minimum heat carried away by exhaust gas of 18.5% at 4000 rpm and 25% throttle position, minimum heat carried by cooling water as 13% for 10,000 rpm and 25% throttle, and the minimum unaccounted energy loss of 26.6% under the condition 8000 rpm and 75% throttle position. However, the best results of exergy analysis are: second law efficiency of 51.93% corresponding to 4000 rpm and 25% throttle, maximum exergy transfer for useful work as 33.7% concerning 6000 rpm and 50% throttle, minimum exergy transfer for exhaust gas as 15.7% for 6000 rpm and 25% throttle, minimum exergy transfer associated with coolant of 4.6% at 4000 rpm and 25% throttle, and minimum exergy destruction of 39.4% corresponding to 8000 rpm and 50% throttle, respectively.

Multidimensional analysis of the impact of motion variation on exergetic performance in a free piston linear hydrogen engine

Linear engine is a promising new energy conversion device which supports variable stroke (or compression ratio) operation for thermal optimization. Previous studies have shown that hydrogen fuel is well adapted to the new engine due to fast combustion. In order to further explore the energy efficiency optimization path of linear hydrogen engine (LHE), this study proposes a multidimensional simulation to investigate the effect of motion stroke variation on exergetic performance of a two-stroke LHE. Meanwhile, a comparative evaluation of exergy between the LHE and a crankshaft hydrogen engine (CHE) is also presented. The results demonstrate that, the irreversible exergy of LHE is slightly higher, but the peak thermomechanical exergy, final chemical exergy, heat transfer exergy and work transfer exergy are lower than CHE, which is attributed to its more homogeneous gas mixture, lower pressure and temperature. The 68 mm stroke achieves more complete combustion and higher temperature for LHE, which promotes the reduction of the final chemical exergy, but inevitably leads to more heat transfer exergy. Shortening the stroke keeps the heat transfer loss of LHE within an acceptable range and effectively reduce the irreversible exergy loss, thereby maximizing the work transfer exergy and increasing the engine efficiency.