International Journal of Exergy (23 papers in press)
Exergetic analysis of plate evaporator using hybrid nanofluids as secondary refrigerant for low temperature applications
by Jahar Sarkar, Atul Bhattad
Abstract: The brine based hybrid nanofluids are proposed as a secondary refrigerant in counter-flow corrugated plate evaporator for low temperature applications. Different types of alumina hybrid nanofluids containing different nanoparticles (copper, silver and multi walled carbon nanotube) in equal volume with the total volume concentration of 0.8% dispersed in different brines have been assessed for certain cooling load. Various exergetic performance parameters based comparison has been performed with brines and related hybrid nanofluids to short out best fluid for milk chilling, ice making and fish freezing applications. Some energy-exergy relationships for evaporator have been established as well. Exergetic performance decreases with a decrease in application temperature due to increase in energy quality. The maximum reduction in irreversibility and nondimensional exergy destruction, and maximum increase in irreversibility distribution ratio and exergetic efficiency has been obtained for propylene glycol brine based alumina- multi-walled carbon nanotube hybrid nanofluid. The study reveals that the brine based hybrid nanofluids may be the suitable substitute as a secondary refrigerant in the plate evaporator for low temperature applications.
Keywords: Hybrid nanofluid; plate heat exchanger; secondary refrigerant; brine; irreversibility; exergetic efficiency.
Modified Laplace Adomian Decomposition Method for analyzing the entropy generation of Sakiadis flow with radiation effect
by Yinwei Lin, KUEI-HAO CHANG, ChaoKuang Chen
Abstract: In the recent paper, the modified Adomian decomposition method (ADM) with shooting method is applied to solve the Sakiadis flow problem. Traditionally, Sakiadis flow problem is solve numerically. However, numerical solution could not easily differentiate and integrate because the data is discrete. In order to improve the problem, firstly, the analytic solution for the Sakiadis flow problem must be not discrete, so that the governing equations of Sakiadis flow could be differentiable and integrated. Hence, in this paper, the analytic solution can be obtained by using modified ADM with shooting method. Secondly, standard ADM only can get small interval, the analytic solution will diverge rapidly for larger interval. The present method use Modified Laplace ADM to converge the solution of the Sakiadis flow in the large interval. So this method is reliable and efficient in obtaining the analytic solution solutions that match well with those from the Runge-Kutta method, which is considered close to the exact solution. Finally, the entropy generation of the Sakiadis flow with radiation effect has been studied. The distribution of the entropy generation are discussed by discussing some parameters. For example, Pr, Nr, Nsh, Nsf, Nsx and Be number.
Keywords: Sakiadis; Adomian; ADM; Nonlinearity; Entropy generation; Irreversibility.
Exergy and Performance Analysis of a C.I Engine Fueled with HCNG Gaseous Fuel Enriched Biodiesel
by Mustafa Kaan Baltacıoğlu, Hüseyin Turan Arat, Raif Kenanoglu
Abstract: In this experimental study, reduction of diesel fuel dependency is intended to achieve more environmentally friendly and economical fuel usage. For his purpose, soybean oil based fatty acid methyl ester is produced and blended with standard diesel fuel at 25 % volumetric ratio. Beside, pure hydrogen and compressed natural gas mixture named HCNG fuel is used with 20 liter per minute volumetric flow rates. The amount of liquid fuel injected into the cylinders is reduced by 30 % via fuel pump plunger pin without structural changes on the diesel engine. Instead of the reduced liquid fuel, 20 % hydrogen with 80 % CNG gaseous fuel mixture (by volumes) are supplied by using mixing chamber before intake manifold of the test engine. Brake power, brake torque and brake specific fuel consumption and also thermal efficiencies are presented. Additionally, exergy analysis of control volume has been performed. Using HCNG fuel enriched biodiesel effects on performance and energy efficiency are examined. Therefore comparison between diesel operation and alternative fuel usage is evaluated.
Keywords: Exergy; HCNG; Biodiesel; Diesel; Engine Performance.
Exergy analysis of the process of regeneration of spent sulfuric acid by WSA technology
by Lubka Atanasova
Abstract: With the increase of the demand for sulfuric acid and the decreasing availability of the materials used for its production, technologies are being used which allow utilizing the waste gases containing sulfur dioxide like these from metallurgic plants, as well as regenerating the spent sulfuric acid from the oil refineries. In modern technologies for production of sulfuric acid, special attention is paid to the utilization of the heat generated during its production aiming to make the process more efficient. In recent years, a new technology for processing of contaminated sulfuric acid to produce pure one was implemented, called WSA process. The survey of the literature showed that an exergy estimation of the process of sulfuric acid regeneration by the WSA technology has not been made so far. The aim of the present study is to estimate the exergy efficiency of the process of sulfuric acid production by the WSA technology and compare it to the existing ones. The publicly available project data of the TOPSOE co. were used for the analysis. To determine its exergy efficiency, the exergy method of thermodynamic analysis was used. The material, energy and exergy balances of the process were made as such balances have not been published so far. From the waste heat available in the system, the consumption of deaerated water at the inlet of the installation was calculated (1204.34 kg/t), as well as the amount of superheated steam produced for transportation out of the system - 1.18 t/t sulfuric acid obtained. The internal and external exergy losses were determined and analyzed, as well as the overall exergy efficiency of the process. The total exergy losses were high - 3387.36 MJ/t where the external losses were 25.75 % and the internal ones а - 74.25 %. The exergy efficiency was found to be 52.4%. It was compared to the exergy efficiencies of technologies for production of sulfuric acid by the contact method from various initial materials. It was found that if the waste heat was not used for steam generation, the exergy efficiency would be significantly lower (33.3 46.7 %). The exergy analysis made is of practical importance and can be used by the sulfuric acid producers when choosing an efficient technology for the installation.
Keywords: exergy; exergy analysis; exergy efficiency; sulfuric acid; sulfuric acid production; wet gas sulfuric acid technology.
Analysis of entropy generation and exergy losses of iso-octane and n-butanol adiabatic constant-volume combustion process
by Hongqing Feng, Jing Zhang, Daojian Liu, Zunqing Zheng, Xiaodong Zhang
Abstract: Butanol is a promising bio-fuel. Using iso-octane as gasoline surrogate, the entropy production and exergy balance of n-butanol and iso-octane constant volume combustion process were analyzed with the CHEMKIN software by considering detailed n-butanol and iso-octane chemical reaction mechanisms. The results of exergy balance analysis showed that, during the adiabatic constant-volume combustion process, there is a trade-off between exergy destruction and exergy loss due to incomplete combustion. The exergy destruction of n-butanol is obviously higher than that of iso-octane. The results of entropy generation analysis showed that, with low initial temperature, three apparent peaks of heat release rate and entropy generation rate are observed in the iso-octane combustion process, while two apparent peaks are observed for n-butanol. With high initial temperature, two apparent peaks of heat release rate and exergy loss rate are observed both for iso-octane and n-butanol. During combustion process, the internal energy exchange between high temperature product and the unburned reactant via a series of chemical reactions, especially the reactions of small molecule intermediate products, is the major reason for the exergy destruction.
Keywords: Bio-butanol; iso-octane; constant-volume combustion; exergy balance; entropy generation.
Assessment of the work efficiency with exergy method in aging muscles and healthy and enlarged hearts
by Jale Çatak, Mustafa Özilgen, Ali Bahadir Olcay, Bayram Yılmaz
Abstract: Thermodynamic aspects of skeletal and cardiac muscle work performance are assessed with the data obtained from the literature. Since the second law muscle work efficiency decreases with declining metabolic energy conversion efficiency in the mitochondria, followed by structural failure of the muscles during aging, the thermodynamic aspects of the muscle work aging process were simulated by incorporating the decreasing second law muscle work efficiency with the exercise data obtained with the healthy young adults.
Within limits of the data analyzed here, glucose utilization ability of the cardiac muscle appears to be the most critical factor determining its work performance. The left and the right ventricles of the enlarged heart had the ability of utilizing approximately 3.5 and 2.7 times less glucose, respectively, than their healthy counterparts. The work performance and the entropy generation by the enlarged and the healthy hearts maintained the same ratios. The results presented here for the enlarged heart is case specific and may change drastically for any other unhealthy heart.
Keywords: Aging mitochondria; entropy generation; exergy loss; heart; muscle work performance; second law efficiency.
Exergy analysis and mathematical modelling of orange peels drying in a mixed mode solar tunnel dryer and under open Sun: A study for performance enhancement
by Karthikeyan A.K
Abstract: In this study, a mixed mode forced convection solar tunnel drying was tested and compared with open Sun for drying of orange peels. The drying data obtained from the experimentation were also fitted to eleven different mathematical models. Among these models, Midilli and Kucuk model for solar tunnel and open Sun drying was found to be the best explicate thin layer drying model. The energy and exergy analysis throughout the drying process was estimated by using the first and second law of thermodynamics. Energy utilization ratio of solar tunnel varied in the range between 7.03 - 36.31%. However, the average exergetic efficiency was found to be 42.01%. Thus, on an average 57.99% of available energy was wasted in the dryer. The results obtained from exergy analysis helps to determine the real potential of the dryer and also provide useful information to further enhance the system performance.
Keywords: Solar tunnel dryer Exergy analysis Energy analysis Orange peels Forced convection Thin layer modelling Performance evaluation Open Sun drying.
Computational Fluid Dynamics (CFD) based Model Development and Exergy Analysis of Naphtha Reforming Reactors
by Jawad Mustafa, Iftikhar Ahmad, Muhammad Ahsan, Manabu Kano
Abstract: In order to realize an energy-efficient naphtha reforming process, various energy analysis methods are adopted. These methods are based on the first laws of thermodynamics or a combination of the first and second law of thermodynamics. The conventional method of energy analysis based on the first law of thermodynamics is inadequate because it cannot grasp the effect of system irreversibilities. Exergy analysis which embeds the second law of thermodynamics in the conventional method overcomes this deficiency and represents the true thermodynamic efficiency of the process. In this work, a novel technique based on computational fluid dynamics (CFD) is proposed to perform exergy analysis of naphtha reforming reactors. N-heptane cut of naphtha reforming process which comprises of paraffin, aromatics and naphthene are used to conduct this analysis. Three reactors, connected in series, are used and all the three types of exergy, i.e., physical exergy, chemical exergy and mixing exergy, are examined. The results show that the physical and mixing exergy in the reactors is decreasing along the length of the reactors whereas the chemical energy increases due to the high chemical potential of products.
Keywords: Exergy analysis; Naphtha reforming reactors; Second law of thermodynamics; Chemical potential; Computational fluid dynamics (CFD); Thermodynamic efficiency.
Investigation on the exergy performance of salt gradient solar ponds with porous mediums
by Hua Wang, Liu Gang Zhang, Yan Yang Mei
Abstract: The exergy performance of solar pond with porous media added in the lower convective zone (LCZ) has been experimentally and theoretically investigated. The experiment is conducted using an independently developed laboratory solar pond. A one dimensional simulation model for temperature and exergy simulation is developed and used to numerical study. The effect of adding porous media in solar pond are investigated under three cases to help distinguish the efficiencies, which are solar pond without adding any porous media (case A), with cinder (case B) and with multi-type porous media of cinder and cobblestones (case C), respectively. The results show the case with multi-type porous media gains the highest LCZ temperature and exergy efficiency, and the case without adding any porous media has the lowest temperature and efficiency; and the maximum transient exergy efficiencies of the three cases are found to be 19.23%, 19.83% and 21.61% respectively. The daily mean exergy efficiency of case C is 2.66% higher than case A, and 1.87% higher than case B. The results indicate that adding porous media, i.e., cinder or the mixture of cinder and cobblestones increases the LCZ temperature of solar pond, and it also improves the exergy performance of solar pond.
Keywords: salt gradient solar pond; porous media; experiment; exergy.
Exergoeconomic Results of Cogeneration System with a Gas-Turbine and Absorption Cooling system
by Eduardo J.C. Cavalcanti
Abstract: In this study, results from the exergetic and exergoeconomic analyses of three systems (Cogeneration system, gas turbine and absorption inlet cooling) to provide electricity and ice are reported to choose the better system from an economic perspective. The components with worst economic performance were identified for three configurationsby exergoeconomic factor. The results revealed that the cogenerative system has a higher electricity power, though the ice production is lower. The exergetic efficiency of cogenerative system is 54.62% and the exergetic efficiency of gas turbine and absorption cooling system are 51.54% and 4.37%, respectively. The total cost rate per exergy showed that the cogeneration system is an advantage for exergoeconomic points of view. Its minimum value is 12.4 $/GJ for low air temperature. The pressure ratio affects the net power, the ice exergy, the exergy efficiency and the total cost rate per exergy.
Keywords: Gas turbine; Absorption cooling system; Exergoeconomy; Electricity generation; Ice production.
Exergy and energy analysis with economic aspects of a diesel engine running on biodiesel-diesel fuel blends
by Abdülvahap Çakmak
Abstract: In this paper, energy, exergy and economic evaluation of a single cylinder diesel engine fuelled with diesel fuel and two types of biodiesel-diesel fuel blends (i.e., B10 and B50) were conducted by evaluating experimental data. Experiments were carried out at five different engine speeds and full load conditions. Energy and exergy components of the engine were calculated and compared for each operating conditions and test fuels. Results obtained from biodiesel-diesel fuel blends were found to be better than neat diesel fuel in respect of both energy and exergy analysis. The maximum brake thermal efficiency and exergy efficiency of the test engine were found to be 40.41% and 37.83% respectively, at 2000 rpm for B10. Also, the minimum exergy destruction occurred at 2000 rpm for all test fuels. However, from the point of specific fuel costs, B10 and B50 gave quite higher economic costs in compared to diesel fuel. Since fuel price per liter of biodiesel is quite higher than diesel fuel, higher biodiesel rates in the blend dominate increase in specific fuel costs even specific fuel consumption decrease.
Keywords: biodiesel-diesel fuel blends; diesel engine; energy analysis; exergy analysis; economic evaluation; exergy destruction.
Exergy Analysis of Evacuated Tube Solar Collectors: A Review
by Gaurav Saxena
Abstract: The most recent studies show that different methodologies have been adopted to understand the concept of Exergy in general and exergy efficiency, exergy destruction, exergy losses, etc. in particular. The present paper reviews the concept of evacuated tube solar water heating systems followed by fundamental laws of thermodynamics necessary to provide concept of exergy analysis in detail. Mathematical modelling and experimental data provide the effect of mass flow rate, temperature gradient, inlet temperature, outlet temperature, collector efficiency, etc. on exergy. Finally, the exergy analysis and exergetic efficiencies along with exergy destruction sources for the evacuated tube collectors are presented.
Keywords: solar energy; evacuated tube collector (ETC); exergy analysis; exergy efficiency; solar water heating system.
Effect of pressure drop and longitudinal conduction on exergy destruction in a concentric-tube micro-fin tube heat exchanger
by Binash Imteyaz, Syed Zubair
Abstract: A numerical study is carried out to predict the heat transfer characteristics and various exergy losses in a micro-fin concentric-tube double pipe heat exchanger. The results are compared with the experimental data from the literature and are found to be in a good agreement. The study is extended to include the effect of pressure drop and longitudinal conduction on the exergy losses. Furthermore, Ethylene Glycol-Water as a hot fluid is considered to investigate the effect of higher viscosity on the frictional irreversible losses. It is found that, at lower mass flow rates, fluid friction irreversibility can be ignored for a low viscous fluid; however, there is a substantial exergy loss for a highly viscous fluid. At higher mass flow rates, fluid friction irreversibility dominates over thermal irreversibility for both the fluids investigated in this study.
Keywords: micro-fin tube heat exchanger; exergy analysis; longitudinal conduction.
Functional exergy efficiency of an air heat recovery exchanger under varying environmental temperature
by Vytautas Martinaitis, Giedre Streckiene, Darius Biekša, Giedrius Šiupšinskas, Juozas Bielskus
Abstract: The ventilation heat recovery exchanger (HRE) for low energy buildings is becoming one of the important heat transformers in the HVAC systems. The purpose of this paper is to develop the possibilities of exergy analysis for the HRE, when the reference temperature (RT) is variable. The method was prepared through consideration of the exergy flow direction changes, when variable RT was placed below, above and across the operating temperature of working fluids. By using the coenthalpy diagrams, there were obtained three functional exergy efficiencies which had greater sensitivity than universal exergy efficiency. The prepared solution allows the reliable using of the ubiquitous thermodynamic (exergy) analysis for energy chain in the HVAC systems. This methodology may be developed for other heat exchangers with different mass flows, operated in temperatures close to the environment temperature. The results could have a practical application for designing the HVAC systems and implementing their exergy optimum operation controls.
Keywords: HVAC systems; exergy analysis; functional exergy efficiency; ventilation heat recovery exchanger; coenthalpy.
Thermo-ecological performance analysis of a double-reheat Rankine cycle steam turbine system (RCSTS) with open and close feed water heaters
by Guven Gonca
Abstract: This study presents the exergetic and thermo-ecological performance optimization of a Rankine cycle steam turbine system (RCSTS) consisting of three turbines, one open feed water heater (OFWH) and two closed feed water heaters (CFWHs). In this study, different performance characteristics such as effective power, effective power density, exergy efficiency, exergy destruction and ecological coefficient of performance (ECOP) are used to investigate and optimize the system performance. The effects of condenser pressure, OFWH pressure and reheat pressures on the performance characteristics of the system have been examined by considering the irreversibilities and system properties. The results showed that the component pressures considerably affect the system performance. Therefore, they should be optimized to obtain maximum performance characteristics.
Keywords: Steam turbine; Rankine cycle; Thermo-ecological optimization; Power density; ECOP; Exergy.
Special Issue on: IEEES-8 Progress in Using Exergy in Energy and the Environment
A comparative thermodynamic analysis on different exergetic efficiency methods for a solar photovoltaic module
by Mehmet OZALP, MUTLUCAN BAYAT
Abstract: In this paper, a comparative study on different exergetic performance of a solar photovoltaic (PV) module is presented. The exergetic efficiency of the PV module was obtained as functions of environmental, operational and design parameters, and calculated in four cases. In Case I, electrical exergy, thermal exergy and exergy destructions were considered. In Case II, a novel expression on calculation of solar exergy was proposed. In Case III, chemical and physical exergy components including enthalpy and entropy concepts were discussed. In Case IV, an empirical expression depending on the power conversion efficiency of the system was used. As a result, the exergy efficiency varied in the range 9.25% to 18.32% in Case I, whereas it varied between 9.41% and 18.34% in Case II. In addition, the exergy efficiency varied from 6.58% to 19.99% in Case III, whereas it varied in the range of 9.30% to 18.89% in Case IV during November 2015.
Keywords: Exergy; exergy analysis; exergetic performance; a comparative study; different methods; exergetic efficiency calculations; thermodynamic assessment; solar photovoltaic; PV module; experimental data; environmental parameters.
A PARAMETRIC STUDY ON EXERGETIC PERFORMANCE OF A YSZ ELECTROLYTE SUPPORTED SOFC STACK
by Ugur AKBULUT, Adnan Midilli, Ibrahim Dincer
Abstract: The main purpose of this study is to perform a parametric study on exergetic performance for an electrolyte supported solid oxide fuel cell (SOFC) stack. In this regard, a typical electrolyte material, which is yttria stabilized zirconia (YSZ) is selected. The electrolyte thicknesses are accepted between 100 and 500 μm with 50 μm intervals while anode and cathode thicknesses are considered to be 50 μm each. Considering these parameters under various current densities, operating temperatures, and electrolyte thicknesses: i) variations for ohmic, activation and concentration overpotentials, ii) rational ohmic, rational activation and rational concentration overpotentials and iii) exergetic efficiency are investigated. Decreasing operating temperature and increasing thicknesses of a YSZ electrolyte decrease the exergetic efficiency and with selecting lower electrolyte thickness than 150 μm some durability and manufacture problems can occur. It is concluded that, under the selected operating conditions, it is exergetically necessary to use the YSZ electrolyte supported SOFC stack whose exergetic efficiency is higher by average ~ 30%. In order to operate this type of fuel cell in an exergetically more efficient manner, it is necessary to select the minumum YSZ electrolyte thickness that is higher than 150 μm by taking into account the selected electrolyte thicknesses in this work. Furthermore, decreasing operating temperatures from 1273 to 1073 K and increasing YSZ electrolyte thickness from 150 to 500 μm go down the exergetic efficiency.rn
Keywords: SOFC; hydrogen; yttria stabilized zirconia; exergetic efficiency; current density; rational overpotential.
An experimental investigation on exergy analysis of an ejector expansion refrigeration system
by Arif Hepbasli, Nagihan Bilir Sağ, Halil Kürşad Ersoy
Abstract: Utilization of an ejector as an expander was experimentally investigated for an expansion work recovery in a basic refrigeration cycle. Exergy analysis was used to determine the amounts and locations of irreversibilities in the elements of the ejector expansion refrigeration system. It was seen that the ejector refrigeration cycle indicated a lower exergy destruction and a higher exergy efficiency compared to the vapor compression refrigeration cycle for each working condition. It was obtained that the values for the irreversibility of the ejector refrigeration cycle were less than in the vapor compression refrigeration cycle by 5.912.6% and the exergy efficiencies of ejector refrigeration cycle were 6.714.2% more than in the vapor compression cycle.
Keywords: Irreversibility; Ejector; Exergy analysis; Throttling; Exergy efficiency; Refrigeration.
ASSESSMENT OF THERMODYNAMIC AND ENVIRONMENTAL PERFORMANCES IN SUBCOOLING PROCESS FOR DIFFERENT REFRIGERANTS
by Onder Altuntas, M. Ziya Sogut, Enver Yalcin, T.Hikmet Karakoc
Abstract: Subcooling operation in cooling processes leads to increasing of cooling performance and energy efficiency within defined limits. This study examines subcooling effects of refrigerants on coefficient of performance (COP), exergy efficiency and environmental performances based on Total Equivalent Warming Impact (TEWI) and Human Health Damage (HDD), which are considered environmental parameters, by considering different refrigerants. In the analyses, Direct Expansion (DX) system used commonly in supermarkets and the evaporator temperatures of -35
Keywords: DX systems; COP; refrigerants; exergy analyses; CO2 emission.
Second law analysis of simultaneous heat and mass transfer through non-gray gas radiation within a cylindrical annulus
by ABIR SAKLY, Akram Mazgar, Faycal Ben Nejma
Abstract: A numerical computation is performed to analyse the entropy production due to heat transfer and mass diffusion through non-gray gas radiation and forced convection within a cylindrical annulus.The inner cylinder, covered by a thin water film, is assumed to be adiabatic, while the outer one is considered to be isothermal and dry. Special attention is given to the effect of thermal radiation on entropy generation. The governing conservation equations are solved numerically by applying the classical finite volume method. The radiative part of this study is solved by means of the Ray Tracing method through S4 directions, while the radiative properties involving non-gray gas and particle behaviours are modelled by using the statistical narrow band correlated-k (SNBCK) model. It is found that the radiative entropy generated at walls presents a clear dominance compared to entropy generated due to heat conduction, gas radiation and mass diffusion. Parametric investigation is carried out to study the effects of the dry cylinder temperature, channel thickness, ambient humidity and outer wall emissivity on entropy generation. The results show that entropy production increases with dry wall temperature, annulus width and the outer wall emissivity. It is also shown that using a thin water film offers greater potential for protecting the inner wall against overheating.
Keywords: Entropy generation; heat and mass transfer; Ray Tracing; non-gray gas; SNBCK model; thermal radiation; cylindrical annulus.
Solar radiation exergy and enviroeconomic analysis for Turkey
by Yusuf Kurtgoz, Emrah Deniz, Ilker Turker
Abstract: The knowledge of useful solar energy amount is important for designing solar energy systems. In this study, solar radiation exergy values are calculated for the 81 cities in Turkey. The data are obtained from the General Directorate of Renewable Energy and the General Directorate of Meteorology. The approaches of Petela, Spanner and Jeter are used to calculate the exergy-to-energy ratios for determining the maximum utilizable solar radiation energy. The exergy-to-energy ratios, the solar radiation exergy values, the locations having the highest solar exergy potential and the monthly, seasonal and annual total solar radiation exergy maps are presented for Turkey. Finally, enviroeconomic analysis is performed and its results are presented. The available results are visualized via maps with colorbars.
Keywords: solar radiation; exergy; solar energy; solar radiation exergy map.
Enhanced exergy analysis of a waste heat powered ejector refrigeration system for different working fluids
by Abid USTAOGLU, Mustafa ALPTEKIN, Mehmet Emin AKAY, Resat SELBAS
Abstract: Conventional and enhanced exergy analyses of a waste heat powered ejector refrigeration system using exhausted gas from rotary kiln were carried out. Isentropic (R290, R141b and R142b), wet (R134a and R152a) and dry fluids (R600a) were considered in the cycle in terms of avoidable-unavoidable parts of the exergy destruction to decide improvement potentials of overall system. The results indicate a significant issue; the importance of component can change regarding to refrigerant. The ejector comes into view as an important component to improve the overall cycle performance for R134a while the generator shows the best improvement potential for R600a.
Keywords: Enhanced exergy analysis; advanced exergy analysis; conventional exergy analysis; waste heat; rotary kiln; ejector refrigeration; isentropic fluid; wet fluids; dry fluid.
Local exergy loss rate in convective heat transfer
by Chuanshan Dai, Haojie Cheng, Haiyan Lei
Abstract: The equation of local exergy loss rate in forced convective heat transfer has been derived in present work. The obtained exergy loss equation involves exergy loss due to heat transfer and pressure drop as well as exergy generation on account of viscous heat dissipation. The dimensionless form of local exergy loss rate and entropy generation different from the previous can also be obtained. Moreover, the calculation results of two convective heat transfer problems about local exergy loss and local entropy generation proposed by Bejan are given. The effect of temperature difference, Prandtl number and Eckert number on exergy loss and entropy generation has been analyzed and the comparison between exergy loss and entropy generation under various conditions has been made. Results shows that for Poiseuille flow the local exergy loss greatly differs from the entropy generation due to the presence of pressure drop. However, viscous dissipation takes an important role in exergy loss for Couette flow on account of no pressure drop.
Keywords: local exergy loss; convective heat transfer; local entropy generation.