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Mehdi Bahiraei - One of the best experts on this subject based on the ideXlab platform.

  • A comprehensive analysis for second law attributes of spiral heat exchanger operating with nanofluid using two-phase mixture model: Exergy Destruction minimization attitude
    Advanced Powder Technology, 2021
    Co-Authors: Mehdi Bahiraei, Nima Mazaheri
    Abstract:

    Abstract The present article focuses on the second law attributes of a counter-flow spiral heat exchanger working with an Al2O3–H2O nanofluid with employing the two-phase mixture model. To improve the cogency of the simulations, the turbulence modeling is performed using four-equation transition Shear Stress Transport (SST) model. The simulations are conducted for different nanoparticle volume fractions and nanofluid flow rates. It is shown that by dispersing further nanoparticles in the common fluid, the total entropy generation of the hot nanofluid significantly diminishes, whereas the cold water and the heat exchanger body exhibit higher thermal entropy generation. The overall Exergy Destruction in the heat exchanger significantly decreases by the increase of the volume fraction, while it tends to increase by the flow rate increment. For instance, an about 9.2% reduction in the overall Exergy Destruction is observed as the volume fraction increases from 0.01 to 0.04. All the conditions exhibit great second law efficiency so that the minimum second law efficiency is larger than 0.84, and increases with the raise of either the volume fraction or flow rate.

  • CFD analysis of second law characteristics for flow of a hybrid biological nanofluid under rotary motion of a twisted tape: Exergy Destruction and entropy generation analyses
    Powder Technology, 2020
    Co-Authors: Mehdi Bahiraei, Nima Mazaheri, Mohammad Rasool Daneshyar
    Abstract:

    Abstract The main goal of the current study is to evaluate the effects of a hybrid heat transfer enhancement method, i.e. simultaneous deployment of active and passive techniques, with employing an ecofriendly-produced graphene nanoplatelets nanofluid from the viewpoint of the second law of thermodynamics. The numerical simulations are performed for the tubes enhanced with the innovative rotary twisted tape. Employing the rotary twisted tape intensifies the flow mixing, disrupts the thermal boundary layer, and reduces the temperature gradients dramatically. The total entropy generation and total Exergy Destruction of nanofluid extremely diminish by the rotational speed elevation. The maximum decrement in the total entropy generation is 87.38%, which occurs by elevating the rotational speed from 0 to 900 rpm. The concentration increase considerably contributes to the Exergy Destruction decrement besides total entropy generation reduction. The lower twisted ratio exhibits the smaller thermal irreversibility, and the best second law efficiency equals to 0.932.

  • Entropy generation and Exergy Destruction for flow of a biologically functionalized graphene nanoplatelets nanofluid within tube enhanced with a novel rotary coaxial cross double-twisted tape
    International Communications in Heat and Mass Transfer, 2020
    Co-Authors: Mehdi Bahiraei, Nima Mazaheri, Hossein Moayedi
    Abstract:

    Abstract The second law performance of the ecofriendly graphene-based nanofluid inside the tubes fitted with novel rotary coaxial cross double-twisted tape (RCCDTT) is studied. Indeed, this study examines the effects of using both active and passive heat transfer enhancement methods simultaneously. Several weight fractions are examined and effects of the twisted ratio are also considered. The combination of these two techniques results in great decrement in the total entropy generation and Exergy Destruction, such that at weight fraction of 0.025% and twisted ratio of 3.5, the thermal entropy generation decreases up to 80% by rising the angular velocity from 0 to 900 rpm. Increasing the concentration results in the less irreversibility and the maximum decrement in the Exergy Destruction rate is about 24%. Moreover, the twisted ratio increment augments the entropy generation caused by heat transfer. The entropy generation due to the friction has a minute contribution and increases slightly by rising the angular velocity and weight fraction. The second law efficiency has ascending trend by the weight fraction increase and profoundly augments with increasing the angular velocity. The highest second law efficiency is attained 0.935 at concentration of 0.1%, angular velocity of 900 rpm, and twisted ratio of 2.5.

  • A novel modification on preheating process of natural gas in pressure reduction stations to improve energy consumption, Exergy Destruction and CO2 emission: Preheating based on real demand
    Energy, 2019
    Co-Authors: Mohammad Olfati, Mehdi Bahiraei, Farzad Veysi
    Abstract:

    Abstract One of the tools for optimizing energy systems is the design of the system output based on real (desired) demand. Thermodynamic performance of natural gas pressure reduction stations are functions of inlet conditions. In order to investigate the impacts of changes in inlet pressure and temperature on performance of a natural gas pressure reduction station, energy consumption and Exergy Destruction of a natural gas pressure reduction station of 10,000 SCMH are evaluated for different inlet conditions. In order to improve the station performance, a novel modification is proposed in the present research based on the real demand of preheating, wherein thermodynamic operation of the regulator is modeled and minimum pre-heating temperature of natural gas is calculated based on desirable temperature at the regulator outlet (natural gas hydrate formation temperature). Indeed, once the temperature at the heater outlet reaches the calculated minimum temperature, the heater is turned off. Compared to conventional stations, the modified station exhibits at least 33% and 15% reductions in energy consumption and Exergy Destruction, respectively. The results of investigating the performance of two sample stations also show that by implementing the proposed modification, CO2 emission can be reduced by up to 80% or even higher.

  • investigating Exergy Destruction and entropy generation for flow of a new nanofluid containing graphene silver nanocomposite in a micro heat exchanger considering viscous dissipation
    Powder Technology, 2018
    Co-Authors: Mehdi Bahiraei, Mohammad Jamshidmofid, Mohammad Amani, Ramtin Barzegarian
    Abstract:

    Abstract The second law attributes including entropy generation, Exergy Destruction and second law efficiency for flow a novel nanofluid containing graphene nanoplatelets decorated with silver nanoparticles are investigated in a micro double-pipe heat exchanger. The effects of viscous dissipation are considered in the simulation. Frictional entropy generation intensifies with increase in Reynolds number and concentration. Moreover, with the concentration increment, thermal entropy generation rate increases. The frictional and thermal Exergy Destruction rates intensify in both tube and annulus sides by increasing either Reynolds number or concentration. The results show that the friction has a more significant role than heat transfer in Exergy Destruction occurred in the heat exchanger. By increasing the Reynolds number, contribution of the nanofluid side to the Exergy Destruction increases in comparison with that of the water side. In addition, the second law efficiency decreases by increasing either Reynolds number or concentration.

Jerald A Caton - One of the best experts on this subject based on the ideXlab platform.

  • correlations of Exergy Destruction during combustion for internal combustion engines
    International Journal of Exergy, 2015
    Co-Authors: Jerald A Caton
    Abstract:

    The greatest Destruction of Exergy for internal combustion engines is a result of the combustion process. The fundamental sub–processes which contribute to the combustion irreversibility include mixing, heat transfer and chemical reactions. This work used an engine cycle simulation to provide quantitative connections between the Exergy Destruction, and eight engine operating and design parameters. For the conditions examined, the Exergy Destruction during combustion ranged between 20% and 26% of the fuel Exergy. For six engine parameters, a correlation with a high coefficient of determination (r2 = 0.99) is obtained between the Exergy Destruction during combustion and the average ratio of specific heats during combustion. This correlation includes the gas temperature dependence since the ratio of specific heats is proportional to temperature.

  • Exergy Destruction during the combustion process as functions of operating and design parameters for a spark ignition engine
    International Journal of Energy Research, 2012
    Co-Authors: Jerald A Caton
    Abstract:

    SUMMARY The second law of thermodynamics provides different perspectives compared with the first law, and provides the property Exergy. Exergy is a measure of the work potential of energy from a given thermodynamic state. Unlike energy, Exergy may be destroyed, and for reciprocating engines, the major source of this Destruction is during the combustion process. This paper provides an overview of the quantitative levels of Exergy Destruction during the combustion process as function of engine operating and design parameters, and for eight fuels. The results of this study are based on a spark-ignition, automotive engine. The amount of Exergy destroyed during the combustion process has been determined as functions of speed, load, equivalence ratio, start of combustion, combustion duration, combustion rate parameters, exhaust gas recirculation (EGR), inlet oxygen concentration, and compression ratio. In addition, design parameters that were examined included expansion ratio and the use of turbocharging. The fuels examined included isooctane (base), methane, propane, hexane, methanol, ethanol, hydrogen and carbon monoxide. For the part load base case (1400 rpm and a bmep of 325 kPa) using isooctane, the Destruction of Exergy was 20.8% of the fuel Exergy. For many of the engine operating and design parameter changes, this Destruction was relatively constant (between about 20 and 23%). The parameters that resulted in the greatest change of the Exergy Destruction were (1) equivalence ratio, (2) EGR, and (3) inlet oxygen concentration. For the base case conditions, the Exergy Destruction during the combustion process was different for the different fuels. The lowest Destruction (8.1%) was for carbon monoxide and the highest Destruction (20.8%) was for isooctane. The differences between the various fuels appear to relate to the complexity of the fuel molecule and the presence (or absence) of an oxygen atom. Copyright © 2011 John Wiley & Sons, Ltd.

  • Exergy Destruction during the combustion process as functions of operating and design parameters for a spark‐ignition engine
    International Journal of Energy Research, 2011
    Co-Authors: Jerald A Caton
    Abstract:

    SUMMARY The second law of thermodynamics provides different perspectives compared with the first law, and provides the property Exergy. Exergy is a measure of the work potential of energy from a given thermodynamic state. Unlike energy, Exergy may be destroyed, and for reciprocating engines, the major source of this Destruction is during the combustion process. This paper provides an overview of the quantitative levels of Exergy Destruction during the combustion process as function of engine operating and design parameters, and for eight fuels. The results of this study are based on a spark-ignition, automotive engine. The amount of Exergy destroyed during the combustion process has been determined as functions of speed, load, equivalence ratio, start of combustion, combustion duration, combustion rate parameters, exhaust gas recirculation (EGR), inlet oxygen concentration, and compression ratio. In addition, design parameters that were examined included expansion ratio and the use of turbocharging. The fuels examined included isooctane (base), methane, propane, hexane, methanol, ethanol, hydrogen and carbon monoxide. For the part load base case (1400 rpm and a bmep of 325 kPa) using isooctane, the Destruction of Exergy was 20.8% of the fuel Exergy. For many of the engine operating and design parameter changes, this Destruction was relatively constant (between about 20 and 23%). The parameters that resulted in the greatest change of the Exergy Destruction were (1) equivalence ratio, (2) EGR, and (3) inlet oxygen concentration. For the base case conditions, the Exergy Destruction during the combustion process was different for the different fuels. The lowest Destruction (8.1%) was for carbon monoxide and the highest Destruction (20.8%) was for isooctane. The differences between the various fuels appear to relate to the complexity of the fuel molecule and the presence (or absence) of an oxygen atom. Copyright © 2011 John Wiley & Sons, Ltd.

  • the effects of heat transfer on performance and Exergy Destruction for a turbocharged spark ignition engine
    Proceedings of the Institution of Mechanical Engineers Part D: Journal of Automobile Engineering, 2010
    Co-Authors: Vaibhav J Lawand, Jerald A Caton
    Abstract:

    The use of the second law of thermodynamics is a powerful means for assessing the performance of engines, and is the only way to determine the Destruction of Exergy. This work has examined a turbocharged, spark-ignition engine in some detail, and in particular, has quantified the effects of varying the engine cylinder heat transfer. A thermodynamic simulation was developed to investigate these effects from both the first law and second law perspectives. The engine used for this study was a 3.81, V-6, turbocharged engine with an intercooler. The engine was examined for various levels of heat transfer. The Exergy values associated with the components of the turbocharger along with the engine components were quantified as a percentage of fuel Exergy. For the base case operating conditions (full load, 2000 r/min) about 20.5 per cent of the Exergy was destroyed during the combustion process, and the turbocharger components were responsible for less than 1 per cent of the Exergy Destruction. As the heat transfer level decreased, work increased only slightly while the exhaust energy and Exergy increased much more significantly. Although engine performance was altered as the heat transfer level was changed, the Destruction of Exergy remained nearly constant.

  • Effects of exhaust gas recirculation on Exergy Destruction due to isobaric combustion for a range of conditions and fuels
    International Journal of Energy Research, 2008
    Co-Authors: Hari Shanker Sivadas, Jerald A Caton
    Abstract:

    This study was directed at examining the effects of exhaust gas recirculation (EGR) on the Exergy destroyed due to combustion in a simple constant pressure combustion system. Both cooled and adiabatic cases of EGR were studied. Higher ‘cooled EGR’ fractions lead to higher Exergy Destruction for reactant temperatures less than 2000 K. For the base case, the percentage of the reactant Exergy destroyed for 0, 20, and 40% EGR at 300 K was found to be 28, 32, and 36%, respectively. Neglecting the chemical Exergy in the products, the equivalence ratio and reactant temperature that corresponded to the lowest Exergy Destruction varied from 0.9 to 1.0 and 800–1300 K, respectively, depending on the EGR fraction. The fraction of the reactant Exergy destroyed increased with increase in the molecular mass of the fuel for the alkanes examined. The Exergy destroyed due to combustion was the least for acetylene and the highest for the alcohols. The trends stayed the same for the different EGR fractions for the eight fuels that were analyzed. For the ‘adiabatic EGR’ case, the percentage Destruction of Exergy decreased with increase in the EGR fraction with a 40% ‘adiabatic EGR’ fraction corresponding to a Destruction of Exergy of 14%. Copyright © 2008 John Wiley & Sons, Ltd.

Amitava Gupta - One of the best experts on this subject based on the ideXlab platform.

  • Optimal control strategy for minimization of Exergy Destruction in boiler superheater
    Energy Conversion and Management, 2013
    Co-Authors: Tapan K. Ray, Ranjan Ganguly, Amitava Gupta
    Abstract:

    Abstract Steam temperatures in large capacity boilers of modern electric power stations are maintained closely around the design specification by spraying water in the superheater (SH) attemperator to ensure safe and efficient operation and long plant life. Although the process of attemperation involves Exergy Destruction, and optimal controllers have previously been proposed for steam temperature control, prior studies on such controllers have not considered Exergy as an important parameter. Exergy analysis of a two-stage SH attemperator with real time operation parameters in a 500 MWe pulverized fuel fired power plant pinpoints the avenues for optimization that is beyond the scope of the traditional First-Law based analysis. Strategies to minimize Exergy Destruction by suitably varying the proportions of stage I and stage II spray flows are established. Further, a MATLAB-SIMULINK-based model is developed and optimal control strategies are devised for SH steam temperature control following a Linear Quadratic Regulator (LQR) approach. Variation of the process parameters and the Exergy Destructions during the transient operations of the attemperator under stipulated disturbances have been analyzed using the model, with different values of the controller parameters. Guidelines are formulated for the spray flow controller tuning so that the total Exergy Destructions during the system transients are minimized.

  • Second-Law Analysis in a Steam Power Plant for Minimization of Avoidable Exergy Destruction
    ASME 2010 4th International Conference on Energy Sustainability Volume 1, 2010
    Co-Authors: Tapan K. Ray, Ranjan Ganguly, Pankaj Ekbote, Amitava Gupta
    Abstract:

    Performance analysis of a 500 MWe steam turbine cycle is performed combining the thermodynamic first and second-law constraints to identify the potential avenues for significant enhancement in efficiency. The efficiency of certain plant components, e.g. condenser, feed water heaters etc., is not readily defined in the gamut of the first law, since their output do not involve any thermodynamic work. Performance criteria for such components are defined in a way which can easily be translated to the overall influence of the cycle input and output, and can be used to assess performances under different operating conditions. A performance calculation software has been developed that computes the energy and Exergy flows using thermodynamic property values with the real time operation parameters at the terminal points of each system/equipment and evaluates the relevant rational performance parameters for them. Exergy-based analysis of the turbine cycle under different strategic conditions with different degrees of superheat and reheat sprays exhibit the extent of performance deterioration of the major equipment and its impact to the overall cycle efficiency. For example, during a unit operation with attemperation flow, a traditional energy analysis alone would wrongly indicate an improved thermal performance of HP heater 5, since the feed water temperature rise across it increases. However, the actual performance degradation is reflected as an Exergy analysis indicates an increased Exergy Destruction within the HP heater 5 under reheat spray. These results corroborate to the deterioration of overall cycle efficiency and rightly assist operational optimization. The Exergy-based analysis is found to offer a more direct tool for evaluating the commercial implication of the off-design operation of an individual component of a turbine cycle. The Exergy Destruction is also translated in terms of its environmental impact, since the irretrievable loss of useful work eventually leads to thermal pollution. The technique can be effectively used by practicing engineers in order to improve efficiency by reducing the avoidable Exergy Destruction, directly assisting the saving of energy resources and decreasing environmental pollution.

  • second law analysis in a steam power plant for minimization of avoidable Exergy Destruction
    ASME 2010 4th International Conference on Energy Sustainability Volume 1, 2010
    Co-Authors: Tapan K. Ray, Ranjan Ganguly, Pankaj Ekbote, Amitava Gupta
    Abstract:

    Performance analysis of a 500 MWe steam turbine cycle is performed combining the thermodynamic first and second-law constraints to identify the potential avenues for significant enhancement in efficiency. The efficiency of certain plant components, e.g. condenser, feed water heaters etc., is not readily defined in the gamut of the first law, since their output do not involve any thermodynamic work. Performance criteria for such components are defined in a way which can easily be translated to the overall influence of the cycle input and output, and can be used to assess performances under different operating conditions. A performance calculation software has been developed that computes the energy and Exergy flows using thermodynamic property values with the real time operation parameters at the terminal points of each system/equipment and evaluates the relevant rational performance parameters for them. Exergy-based analysis of the turbine cycle under different strategic conditions with different degrees of superheat and reheat sprays exhibit the extent of performance deterioration of the major equipment and its impact to the overall cycle efficiency. For example, during a unit operation with attemperation flow, a traditional energy analysis alone would wrongly indicate an improved thermal performance of HP heater 5, since the feed water temperature rise across it increases. However, the actual performance degradation is reflected as an Exergy analysis indicates an increased Exergy Destruction within the HP heater 5 under reheat spray. These results corroborate to the deterioration of overall cycle efficiency and rightly assist operational optimization. The Exergy-based analysis is found to offer a more direct tool for evaluating the commercial implication of the off-design operation of an individual component of a turbine cycle. The Exergy Destruction is also translated in terms of its environmental impact, since the irretrievable loss of useful work eventually leads to thermal pollution. The technique can be effectively used by practicing engineers in order to improve efficiency by reducing the avoidable Exergy Destruction, directly assisting the saving of energy resources and decreasing environmental pollution.Copyright © 2010 by ASME

Ozgur Balli - One of the best experts on this subject based on the ideXlab platform.

  • advanced Exergy analyses to evaluate the performance of a military aircraft turbojet engine tje with afterburner system splitting Exergy Destruction into unavoidable avoidable and endogenous exogenous
    Applied Thermal Engineering, 2017
    Co-Authors: Ozgur Balli
    Abstract:

    Abstract A conventional and advanced Exergy analysis of a military aircraft turbojet engine is presented in this paper. In this framework, the main Exergy parameters of the engine components are introduced while the Exergy Destruction rates within the engine components are split into endogenous/exogenous and avoidable/unavoidable parts. Also, the mutual interdependencies among the components of the engine and realistic improvement potentials depending on operating conditions are acquired through the analysis. As a result of the study, the Exergy efficiency values of the engine are determined to be 39.41% at military (MIL) mode (maximum engine thrust operation without afterburner fuel combustion) and 17.90% at afterburner (AB) mode (maximum engine thrust operation with afterburner fuel combustion), respectively. The system has low improvement potential because the unavoidable Exergy Destruction rate is 93% at MIL mode and 98% at AB mode. The relationships between the components seem to be weak since the endogenous Exergy Destruction is 83% at MIL mode and 94% at AB mode. Finally, it may be concluded that the low pressure compressor, the high pressure compressor, the combustion chamber and afterburner exhaust duct of the engine should be focused on according to the results obtained.

  • Advanced Exergy Analysis of a Turbofan Engine (TFE): Splitting Exergy Destruction into Unavoidable/Avoidable and Endogenous/Exogenous
    International Journal of Turbo & Jet-Engines, 2017
    Co-Authors: Ozgur Balli
    Abstract:

    AbstractA conventional and advanced Exergy analysis of a turbofan engine is presented in this paper. In this framework, the main Exergy parameters of the engine components are introduced while the Exergy Destruction rates within the engine components are split into endogenous/exogenous and avoidable/unavoidable parts. Also, the mutual interdependencies among the components of the engine and realistic improvement potentials depending on operating conditions are acquired through the analysis. As a result of the study, the Exergy efficiency values of the engine are determined to be 25.7 % for actual condition, 27.55 % for unavoidable condition and 30.54 % for theoretical contion, repectively. The system has low improvement potential because the unavoidable Exergy Destruction rate is 90 %. The relationships between the components are relatively weak since the endogenous Exergy Destruction is 73 %. Finally, it may be concluded that the low pressure compressor, the high pressure compressor, the fan, the low pressure compressor, the high pressure compressor and the combustion chamber of the engine should be focused on according to the results obtained.

Zhichun Liu - One of the best experts on this subject based on the ideXlab platform.

  • Optimization design of slotted fins based on Exergy Destruction minimization coupled with optimization algorithm
    International Journal of Thermal Sciences, 2020
    Co-Authors: Shicheng Wang, Wei Liu, Biao Xiao, Zhichun Liu
    Abstract:

    Abstract In this paper, the influence of the slot parameters of the slotted fins on the flow and heat transfer performance and the Exergy Destruction are studied by numerical simulation, and the optimal parameters corresponding to the best overall performance and the minimum Exergy Destruction are obtained by the method of CFD coupled with optimization algorithm, respectively. In current work, the slot length and width are taken as parameters (slot length 1.2–2.1 mm, slot width 0.1–1.0 mm), 100 different cases are studied under the same boundary conditions, and the results are analyzed and optimized by the neural network and genetic algorithm. The overall performance is taken as the evaluation of the heat transfer performance and the power consumption, while Exergy Destruction caused by heat transfer and fluid flow are taken as the estimate of the loss of available energy and mechanical work. The results show that the overall performance is best when the slot length and width are 1.38 mm and 0.27 mm respectively, and the overall performance is 13.1% higher than that of flat fin. While the Exergy Destruction caused by heat transfer and fluid flow are minimized synergistically when the slot length and width are 1.37 mm and 0.12 mm, and the irreversible loss caused by heat transfer is reduced by 19.13%, at the cost of an increase of 40.6% in the irreversible loss caused by fluid flow. The difference between the parameters corresponding to best overall performance and Exergy Destruction minimization means the irreversible loss of heat transfer and fluid flow process is not minimum when the overall performance is best, and indicates the Exergy Destruction minimization can be a principle to evaluate the heat transfer and flow process. The result in this paper is of great significant to the energy utilization and the improvement of the thermal quality in waste heat recovery.

  • turbulent heat transfer optimization for solar air heater with variation method based on Exergy Destruction minimization principle
    International Journal of Heat and Mass Transfer, 2019
    Co-Authors: Hui Xiao, Junbo Wang, Zhichun Liu, Wei Liu
    Abstract:

    Abstract In this paper, a heat transfer optimization approach is brought in with focusing on Exergy Destruction to properly deal with the trade-off between irreversibility of heat transfer process and pump power consumption in turbulent flow for solar air heater. Thus, the Exergy Destruction minimization principle is mathematically extended to three dimensional turbulent flow and governing equations are derived with variation method. Successively, the optimized flow field for solar air heater is obtained at Re = 12,000 by applying SST k–ω turbulence model. Furthermore, with detail analysis of velocity and temperature distribution, it is found that the longitudinal swirl flow is generated through this optimization approach. Besides, within the scope of this study, the total heat transfer Exergy Destruction and average absorber temperature are maximally decreased by about 65% and 30 K respectively in the test section compared with the plain duct. And the maximum Nusselt number and friction factor are increased to 1.81 and 3.13 times over plain duct respectively. These results indicate that the optimization approach is effective for improving the thermal-hydraulic performance of solar air heater. Finally, the inclined vortex plate is designed to realize this kind of optimized flow field successfully. This work will promote technique developments of solar air heater.

  • evaluation of convective heat transfer in a tube based on local Exergy Destruction rate
    Science China-technological Sciences, 2016
    Co-Authors: Junbo Wang, Zhichun Liu, Wei Liu
    Abstract:

    In this study, Exergy efficiency is defined to evaluate convective heat transfer in a tube based on the local Exergy Destruction rate from the equilibrium equation of available potential. By calculating this Destruction rate, the local irreversibility of convective heat transfer can be evaluated quantitatively. The Exergy efficiency and distribution of local Exergy Destruction rate for a smooth tube, an enhanced tube into which short-width twisted tape has been inserted, and an optimized tube with Exergy Destruction minimization are analyzed by solving the governing equations through a finite volume method (FVM). For the smooth tube, the Exergy efficiency increases with increasing Reynolds number (Re) and decreases as the heat flux increases, whereas the Nusselt number (Nu) remains constant. For the enhanced tube, the Exergy efficiency increases with increasing Reynolds number and increases as the short-width rate (w) increases. An analysis of the distribution of the local Exergy Destruction rate for a smooth tube shows that Exergy Destruction in the annular region between the core flow and tube wall is the highest. Furthermore, the Exergy Destruction for the enhanced and optimized tubes is reduced compared with that of the smooth tube. When the Reynolds number varies from 500 to 1750, the Exergy efficiencies for the smooth, enhanced, and optimized tubes are in the ranges 0.367–0.485, 0.705–0.857, and 0.885–0.906, respectively. The results show that Exergy efficiency is an effective evaluation criterion for convective heat transfer and the distribution of the local Exergy Destruction rate reveals the distribution of local irreversible loss. Disturbance in the core flow can reduce Exergy Destruction, and improve the Exergy efficiency as well as heat transfer rate. Besides, optimization with Exergy Destruction minimization can provide effective guidance to improve the technology of heat transfer enhancement.

  • The application of Exergy Destruction minimization in convective heat transfer optimization
    Applied Thermal Engineering, 2015
    Co-Authors: Junbo Wang, Wei Liu, Zhichun Liu
    Abstract:

    Abstract An approach of heat transfer enhancement by reducing the irreversibility of process is discussed theoretically. The irreversibility of heat transfer process has been analysed with Exergy. The irreversibility of heat transfer process can be expressed with Exergy Destruction. Other than entropy generation minimization and entransy dissipation minimization, a new approach called Exergy Destruction minimization is proposed to optimize heat transfer process based on the analysis. By setting Exergy Destruction rate as an optimization objective and fluid power consumption as a constraint condition, a momentum equation with an additional volume force is constructed through functional variation to numerically simulate convective heat transfer in coupling with energy equation. The approach of Exergy Destruction minimization is applied in the optimization of convective heat transfer and numerical calculation is carried out to validate this approach in circular tubes. The optimum velocity field with 380% increase in Nu number and 5% increase in flow resistance f is found by the numerical calculation on optimization equations. The results disclose that longitudinal swirl flow with multi-vortexes appears in the flow field, which leads to heat transfer enhancement. The more the vortexes are, the higher Nu number can be gotten. It is found that longitudinal swirl flow with multi-vortexes is a flow pattern with good heat transfer performance in circular tubes. The results of numerical calculation can be used to guide the design for tube inserts.

  • Convective heat transfer optimization in a circular tube based on local Exergy Destruction minimization
    International Journal of Heat and Mass Transfer, 2015
    Co-Authors: Junbo Wang, Zhichun Liu, Wei Liu, Fang Yuan, Gang Chen
    Abstract:

    Abstract In this study, the equilibrium equation of available potential, which reveals the relation of available potential and local Exergy Destruction rate, is determined, and the expressions of available potential and local Exergy Destruction rate are given. To improve heat transfer enhancement and reduce increase amplitude of flow resistance, a method termed as fluid-based heat transfer enhancement is proposed relative to surface-based heat transfer enhancement. An optimal mathematical model by constructing Lagrange function with Exergy Destruction corresponding to irreversibility loss of heat transfer process and fluid power consumption to flow loss of fluid is adopted to validate this method. To obtain the optimal flow structure in a tube, the tube flow is divided into two parts: core flow and boundary flow. For reducing the irreversibility loss in the core flow, we take fluid Exergy Destruction as optimization objective with prescribed fluid power consumption. For reducing the flow resistance in the boundary flow, we take fluid power consumption as optimization objective with prescribed fluid Exergy Destruction. The optimization equations for the convective heat transfer in laminar flow are derived, which are solved numerically. The longitudinal swirling flows in the tube are found at different parameters. In the optimized flow, heat transfer is enhanced greatly while accompanied with a little increase of flow resistance. Comprehensive performance, the ratio of increases in heat transfer and flow resistance, reaches at 3.65 after optimization.