Non-isentropic Compression Research Articles

Characterisation of practical air conditioning cycle by computer simulation

In this paper, characterisation of a practical cycle of the air conditioning unit is predicted by developing a computer simulation algorithm. Environment-friendly refrigerant R 404A was used. The refrigerant R404A properties of the mathematical model were analysed. Superheating, sub-cooling and non-isentropic compression processes were assumed. The simulated air conditioning cycle performance parameters, such as mass flow rate, refrigeration capacity, heat rejection rate, coefficient of performance and compressor discharge temperature, were carried out under various condensing and evaporating temperatures. Experimental results found in the literature for the performance parameters were introduced and compared to those obtained by the simulation technique. The experimental and the simulated results show good conformity.

Thermodynamic Mechanism of Self-Heat Recuperative Heat Circulation System with Non-Isentropic Compression and Expansion for a Continuous Heating and Cooling Gas Cycle Process

Self-heat recuperation drastically reduces the energy consumption of thermal processes by adding work to circulate the whole process heat without heat addition. However, its theoretical foundations have not been fully established yet. This paper aims to elucidate the thermodynamic mechanism of self-heat recuperation with non-isentropic compression and expansion for a thermal gas cycle in terms of exergy analysis. The exergy analysis was performed with a modularization method using the module expression flow, the temperature–entropy, and the energy conversion diagram. The exergy analysis led to the significant conclusions that self-heat recuperation minimizes the energy consumption of gas thermal processes and that key information can be easily obtained by simply focusing on the exergy destruction without using complex process simulations. Heat circulation is driven by adding the work input to provide the minimum work required for heat transfer using the compressor, and theoretically all the excess work can be recovered to offset part of the work input using the expander. In practice, the irreversibility of adiabatic compression and expansion destroys part of the excess work, leading to a decrease in the excess work and an equivalent increase in the waste heat. The net work input is equal to the minimum work required for heat circulation and is converted into waste heat, whose anergy is transformed from the exergy destruction due to heat transfer, non-isentropic compression, and non-isentropic expansion. The minimum work required for heat circulation can be quantified by calculating the waste heat via the total exergy destruction. Thus, self-heat recuperation is the most energy-efficient solution to the design and retrofit of energy-saving gas thermal processes and is promising for process intensification. A case demonstration with numerical results on the heat circulation of air is presented to facilitate an intuitive understanding of the thermodynamic mechanism.

Comparative performance evaluations of various optimization functions for irreversible Otto cycles

A comparative performance analysis for irreversible Otto cycles has been carried out with consideration of overall and effective power loss for various optimization functions available in the literature. Basically, maximum power, maximum power density, ecological coefficient of performance and the ecological function are taken into consideration, and their optimal operating conditions were compared with respect to each other in terms of various engine parameters. The irreversible cycle model operating between two isothermal high and low temperature heat reservoirs is considered and combined with various irreversibilities for power loss analysis. In the irreversible model, non-isentropic compression and expansion, heat leak through the cylinder head and power loss due to friction are taken into account. Effects of engine volumetric efficiency, engine speed, friction and extreme cycle temperature on the overall performance for the respective optimal operating conditions have been investigated. It is seen that the overall power loss increases for increasing engine speed, decreasing volumetric efficiency, increasing friction coefficient and decreasing high temperature of the cycle. It is also observed that the maximum power conditions have higher overall power loss but lower internal irreversibility losses among the considered optimization functions.

Thermodynamic analysis of air-compression refrigerating machine based on the exergy cost theory

On the basis of the exergy cost theory, a procedure has been developed for carrying out an in-depth thermodynamic analysis of air-compression refrigerating machines and thermal pumps taking into consideration nonequivalence of exergy losses in various links of thermotransformation process and their effect on consumption of exergy supplied to the system. A thermoeconomic model of a single-stage air-compression refrigerating machine was proposed which takes into consideration structural and topological features of the process scheme and interrelation between its elements. For an equivalent mapping of structural and topological features of the refrigerating machine scheme, a parametric flow graph was developed. The procedure uses a matrix form of recording exergy balances which is the most convenient for representation of the thermoeconomic models. In order to obtain reference operating mode of the refrigerating machine, the principle of thermodynamic process idealizing was used. Application of the structural theory of thermoeconomics has made it possible to establish the portion of endogenous and exogenous destruction of exergy in the elements of the refrigerating machine. Numerical implementation of the proposed procedure has made it possible to reveal influence of internal irreversibility in the cycle determined by non-isentropic compression processes in the compressor and expansion in the expander. A significant effect of efficiency of the expansion process in the expander on the exergy efficiency of the air-compression refrigerating machine was established. Influence of the change of ambient temperature on the character of consumption of exergy by each element of the refrigeration machine was estimated. It has been revealed that even a slight change of the ambient temperature significantly affects consumption of “fuel” by each element of the system, while the dependence is linear. Generalized dependences of exergy indicators on variable factors have been obtained which enabled finding of unfavorable operating conditions for equipment with increased energy consumption. It was established that the efficiency factor of the turboexpander has the greatest influence on the change of consumption of exergy of “fuel” in the system.

Thermodynamic analysis on optimum performance of scramjet engine at high Mach numbers

In order to predict the maximum performance of scramjet engine at flight conditions with high freestream Mach numbers, a thermodynamic model of Brayton cycle was utilized to analyze the effects of inlet pressure ratio, fuel equivalence ratio and the upper limit of gas temperature to the specific thrust and the fuel impulse of the scramjet considering the characteristics of non-isentropic compression in the inlet. The results show that both the inlet efficiency and the temperature limit in the combustor have remarkable effects on the overall engine performances. Different with the ideal Brayton cycles assuming isentropic compression without upper limit of gas temperature, both the maximum specific thrust and the maximum fuel impulse of a scramjet present non-monotonic trends against the fuel equivalence ratio in this study. Considering the empirical design efficiencies of inlet, there is a wide range of fuel equivalence ratios in which the fuel impulses remain at high values. Moreover, the maximum specific thrust can also be achieved with a fuel equivalence ratio near this range. Therefore, it is possible to achieve an overall high performance in a scramjet at high Mach numbers.

Performance of irreversible Meletis–Georgiou vane rotary engine cycle with variable specific heats of working fluid

An irreversible cycle model of Meletis–Georgiou (MG) engine consisting of an isochoric heating branch, isochoric and isobaric cooling branches, two non-isentropic compression and two non-isentropic expansion branches and with heat transfer loss, internal irreversibility and the linear relation between specific heat of the working fluid and its temperature is established by using the theory of finite time thermodynamics. The analytical relations of the work output versus compression ratio, the efficiency versus compression ratio, as well as the work output versus efficiency are obtained by using numerical examples. The results show that the work output versus the efficiency characteristic of the irreversible MG cycle is a loop-shaped curve which is consistent with the general heat engine performance, and the cycle model considering the internal irreversibility and linear relation between specific heats of the working fluid and its temperature is closer to practice than the endo-reversible model with constant specific heats of the working fluid.

Effect of regeneration on the thermo-ecological performance analysis and optimization of irreversible air refrigerators

A thermo-ecological performance analysis of an irreversible regenerative air refrigerator cycle exchanging heat with thermal reservoirs is presented. In the analysis, the external irreversibility effects due to heat transfer across finite temperature differences and the heat leak loss between the external heat reservoirs while the internal irreversibilities are due to the non-isentropic compression and expansion processes and the regenerative loss are taken into account. The effects of regeneration and heat sources temperature ratio are given special emphasis and investigated in detail. A comparative performance analysis considering the objective functions of an ecological coefficient of performance, exergetic efficiency and coefficient of performance is also carried out. The maximum of the objective functions and the corresponding optimal conditions have been derived analytically. The obtained results may provide a general theoretical tool for the thermo-ecological design of regenerative air refrigerators.

Energy and exergy analyses of a two-stage vapour compression refrigeration system

In this paper exergy analysis of two-stage vapour compression refrigeration (VCR) system has been carried out with an objective to evaluate optimum inter-stage temperature (pressure) for refrigerants HCFC22, R410A and R717. A thermodynamic model based on the principles of mass, energy and exergy balances is developed for this purpose. The computed results illustrate the effects of evaporation and condensation temperatures, isentropic efficiencies of compressors, sub-cooling of refrigerant and superheating of suction vapour on optimum inter-stage saturation temperature (pressure). The optimum inter-stage saturation temperatures (pressures) for HCFC22 and R410A are proximate to arithmetic mean of evaporation and condensation temperatures (AMT) when assuming superheating of suction vapour and non-isentropic compression processes in low-pressure and high-pressure compressors. The optimum inter-stage saturation temperatures (pressures) for HCFC22 and R410A are near to geometric mean of evaporation and condensation temperatures (GMT) when it is assumed that cycle involves the effects of sub-cooling, superheating of suction vapour and non-isentropic compression of the suction vapour. The optimum inter-stage saturation temperature (pressure) for R717 is close to GMT irrespective of sub-cooling, superheating of suction vapour and non-isentropic compression in the cycle. The efficiency defects, computed corresponding to optimum inter-stage temperature in condenser is higher in comparison to the other components. Finally, it is deduced that R717 is a better alternative refrigerant to HCFC22 than R410A in two-stage VCR system. Copyright © 2009 John Wiley & Sons, Ltd.

Parametric optimization of a solar-driven Braysson heat engine with variable heat capacity of the working fluid and radiation–convection heat losses

An irreversible solar-driven Braysson heat engine system is presented, in which the temperature-dependent heat capacity of the working fluid, the radiation–convection heat losses of the solar collector and the irreversibilities resulting from heat transfer and non-isentropic compression and expansion processes are taken into account. Based on the thermodynamic analysis method and the optimal control theory, the mathematical expression of the overall efficiency of the system is derived and the maximum overall efficiency is calculated, and the operating temperatures of the solar collector and the cyclic working fluid and the ratio of heat-transfer areas of the heat engine are optimized. By using numerical optimization technology, the influences of the variable heat capacity of the working fluid, the radiation–convection heat losses of the solar collector and the multi-irreversibilities on the performance characteristics of the solar-driven heat engine system are investigated and evaluated in detail. Moreover, it is expounded that the optimal performance and important parametric bounds of the irreversible solar-driven Braysson heat engine with the constant heat capacity of the working fluid and the irreversible solar-driven Carnot heat engine can be deduced from the conclusions in the present paper.

Performance analysis and parametric optimum criteria of an irreversible Bose–Otto engine

An irreversible cycle model of a Bose–Otto engine is established, in which finite time thermodynamic processes and the irreversibility result from the nonisentropic compression and expansion processes are taken into account. Based on the model, expressions for the power output and efficiency of the Bose–Otto engine are derived. On the basis of the thermodynamic properties of ideal Bose gas, the effects of the irreversibility and the compression ratio of the two isochoric processes on the performance of the Bose–Otto engine are revealed and some important performance parameters are optimized. Furthermore, some optimal operating regions including those for the power output, efficiency, and the temperatures of the cyclic working substance at two important state points are determined and evaluated. Finally, several special cases are discussed in detail.

Exergy analysis of a compression-absorption system for heating and cooling applications

An exergy analysis of a single-stage compression–absorption system with R22-E181 as the working fluid pair is carried out. Theoretical results obtained have been compared with those obtained from the experiment. Results show that the heat of mixing of the refrigerant vapour and solution at absorber and desorber contributes a significant amount to the internal entropy generation rates. A significant part of internal entropy generation rate is also due to non-isentropic compression of refrigerant vapour at higher absorber pressure. The exergetic efficiency of the system increases with the increase in absorber pressure due to reduction in internal irreversibilities. Higher value of weak solution concentration along with the increase in solution concentration difference results in higher exergetic efficiency of the system. Thus, a compression–absorption system performs better when operated at higher absorber pressure, and an improved system performance can also be achieved with higher value of weak solution concentration with higher possible solution concentration difference. Copyright © 2008 John Wiley & Sons, Ltd.

Optimal Allocation of Heat Exchanger Inventory of a Two-Stage Vapor Compression Cycle for Maximum Coefficient of Performance

The purpose of this study is to investigate how the heat exchanger inventory allocation plays a role in maximizing the thermal performance of a two-stage refrigeration system with two evaporators. First, the system is modeled as a Carnot refrigerator and a particular heat transfer parameter is kept constant as the heat exchanger allocation parameter is allowed to vary. The value of the heat exchanger allocation parameter corresponding to the maximum coefficient of performance (COP) is noted. The results are compared to those of a non-Carnot refrigerator with isentropic and nonisentropic compression. It is found that the Carnot refrigerator can be used to predict the value of the heat exchanger allocation parameter where the maximum COP occurs for a non-Carnot refrigerator. In order to improve the accuracy of that prediction, the predicted value of the heat exchanger allocation parameter has to be inputted into the set of equations used for the non-Carnot refrigerator. This study is useful in designing a low-cost, high-performance refrigeration system.

Optimum performance analysis of an irreversible Diesel heat engine affected by variable heat capacities of working fluid

An irreversible cycle model of the Diesel heat engine is established in which the temperature dependent heat capacities of the working fluid, the irreversibilities resulting from non-isentropic compression and expansion processes and heat leak losses through the cylinder wall are taken into account. The adiabatic equation of ideal gases with temperature dependent heat capacity is strictly deduced without using the additional approximation condition in the relevant literature and is used to analyze the performance of the Diesel heat engine. Expressions for the work output and efficiency of the cycle are derived by introducing the pressure ratio and the compression and expansion efficiencies. The performance characteristic curves of the Diesel heat engine are presented for a set of given parameters. The optimum criteria of some important parameters such as the work output, efficiency, pressure ratio and temperatures of the working fluid are obtained. Moreover, the influence of the compression and expansion efficiencies, variable heat capacities, heat leak and other parameters on the performance of the cycle is discussed in detail. The results obtained may provide a theoretical basis for both optimal design and operation of real Diesel heat engines.

Optimum Criteria on the Important Parameters of an Irreversible Otto Heat Engine With the Temperature-Dependent Heat Capacities of the Working Fluid

An irreversible cycle model of the Otto heat engine is established, in which the temperature-dependent heat capacities of the working fluid, the irreversibilities resulting from the nonisentropic compression and expansion processes, and heat leak losses through the cylinder wall are taken into account. The adiabatic equation of ideal gases with the temperature-dependent heat capacity is strictly deduced without using the additional approximation condition in the relevant literature and used to analyze the performance of the Otto heat engine. Expressions for the work output and efficiency of the cycle are derived by introducing the compression ratio of two isochoric processes. The performance characteristic curves of the Otto heat engine are presented for a set of given parameters. The optimum criteria of some important parameters such as the work output, efficiency, compression ratio, and temperatures of the working fluid are given. Moreover, the influence of the compression and expansion efficiencies, the variable heat capacities, the heat leak, and other parameters on the performance of the cycle is discussed in detail. The results obtained are novel and general, from which some relevant conclusions in literature may be directly derived. This work may provide a significant guidance for the performance improvement and optimal design of the Otto heat engine.

Heat-engineering units for the generation of dense high-temperature gas

Various methods of gas compression are reviewed, which are intended for heating the gas. Special attention is given to units of nonisentropic compression of the mechanical heat-engineering type, in which the gas is transferred from chamber to chamber in order to increase the temperature. The units are subdivided into classes or systems, namely, those of impulse, pulse-periodic, and continuous operation. Methods are discussed of mathematical description of processes occurring in units of this type, including analytical investigation of simplified qualitative models and numerical simulation in more complex cases close to reality.

A new thermoeconomic approach and parametric study of an irreversible regenerative Brayton refrigeration cycle

The detailed parametric study of an irreversible regenerative Brayton refrigerator cycle using the new thermoeconomic approach is presented in this paper. The external irreversibility is due to finite temperature difference between the cycle and the external reservoirs while the internal irreversibilities are due to the nonisentropic compression and expansion processes and the regenerative loss. The thermoeconomic objective function defined as the cooling load per unit cost is optimized with respect to the state point temperatures for a typical set of operating conditions. The power input and cooling load are found to be decreasing functions of the expansion outlet temperature ( T 1), while it is the reverse in the case of COP. On the other hand, there are optimal values of the temperature T 1, cooling load, power input and COP at which the cycle attains the maximum objective function for a typical set of operating parameters. Again, the objective function, COP and cooling load further enhance, while the power input goes down, as the various values of the effectiveness or efficiency components are increased.

Performance analysis and parametric optimum design of an irreversible Diesel heat engine

An irreversible cycle model of a Diesel heat engine is established and used to investigate the influence of multi-irreversibilities, which mainly result from the non-isentropic compression and expansion processes, finite rate heat transfer processes and heat leak loss through the cylinder wall, on the performance of the cycle. Expressions for the power output and efficiency of the cycle are derived by introducing the pressure ratio and the compression and expansion efficiencies. The characteristic curves of the power output and efficiency, varying with the pressure ratio, and the power output versus efficiency are obtained. It is found from the curves that there are optimal values of the pressure ratio at which the power output and efficiency attain, respectively, their maxima. Moreover, the effects of the various irreversibilities and the highest temperature of the working substance on the performance of the cycle are discussed in detail. The optimum criteria of some important parameters, such as the power output, efficiency and pressure ratio, are obtained, and consequently, the optimal operating region of an irreversible Diesel heat engine is determined.

PERFORMANCE EVALUATION OF AN IRREVERSIBLE REGENERATIVE MODIFIED BRAYTON HEAT ENGINE BASED ON THE THERMOECONOMIC CRITERION

The irreversible cycle model of a modified regenerative Brayton heat engine has been established for the finite heat capacities of external reservoirs. The external irreversibility is due to finite temperature differences between the heat engine and the external reservoirs, and the internal irreversibilities are due to the nonisentropic compression and expansion processes in the compressor and turbine, respectively, and the regenerative heat loss. The thermoeconomic function, which is defined as the power output divided by the total cost plus the running and maintenance costs of the system, has been optimized with respect to the cycle temperatures, and the optimum performance parameters are calculated for a typical set of operating conditions. The authors find that the effect of compressor efficiency is more than that of the turbine efficiency on all the performance parameters. They also find that the effect of isothermal side heat capacitance rate is more pronounced than that of all the other parameters on the performance of the cycle. There is seen to be an optimal relation between the various heat capacitance rates in different reservoirs and within the heat engine for which the cycle attains the optimum performance.

Optimization criteria for the important parameters of an irreversible Otto heat-engine

An irreversible cycle model of an Otto heat-engine is established, in which the main irreversibilities result from the non-isentropic compression and expansion processes; finite-time processes and heat loss through the cylinder wall are taken into account. The power output and efficiency of the cycle are derived. The curves of the power output and efficiency varying with the compression ratio of two isochoric processes are presented. It is found from the curves that there are optimal values of the compression ratio at which the power output and efficiency attain their maxima. Moreover, the maximum power-output and efficiency and the corresponding relevant parameters are calculated, and consequently, the optimization criteria of some important parameters such as the power output, efficiency, compression ratio, and temperatures of the working substance are obtained.

Thermal exergy optimisation for an irreversible cogeneration power plant

The optimal design model, based on exergy analysis, of an irreversible cogeneration cycle has been presented using finite-time thermodynamics. The external irreversibility is due to the thermal resistance between the system and the heat reservoirs while the internal irreversibility is due to the non-isentropic compression and expansion process within the system. The internal irreversibility of the irreversible cogeneration cycle is characterised by a single parameter representing the ratio of relevant entropy differences. The presence of this irreversibility parameter shows that a cogeneration power plant with internal irreversibilities delivers less exergy output and has a lower efficiency than those of the reversible and endoreversible cycles. The model presented here is of more use when compared to those obtained by earlier researchers.