Air-standard Cycle Research Articles

The Basic Thermodynamics of Turbine Cooling

Analyses of gas turbine plant performance, including the effects of turbine cooling, are presented. The thermal efficiencies are determined theoretically, assuming air standard (a/s) cycles, and the reductions in efficiency due to cooling are established; it is shown that these are small, unless large cooling flows are required. The theoretical estimates of efficiency reduction are compared with calculations, assuming that real gases form the working fluid in the gas turbine cycles. It is shown from a/s analysis that there are diminishing returns on efficiency as combustion temperature is increased; for real gases there appears to be a limit on this maximum temperature for maximum thermal efficiency.

Optimizing an irreversible Diesel cycle — fine tuning of compression ratio and cut-off ratio

A simplified irreversible model has been proposed for the air standard Diesel cycle. Global thermal and friction losses have been lumped into an equivalent friction term. Optimization of the cycle has been performed for power output as well as for thermal efficiency with respect to compression ratio and cut-off ratio. The optimum values of these ratios compare well with standard values used in real Diesel engines. The cycle also demonstrates a loop-shaped power vs efficiency curve as is exhibited by real heat engines.

Thermodynamic analysis of combined open-cycle-twin-shaft gas turbine (Brayton cycle) and exhaust gas operated absorption refrigeration unit

The exhaust gases of a gas turbine carry a significant amount of thermal energy that is usually expelled to the atmosphere without taking any further part in the power generation processes. The low grade thermal energy can however be put to beneficial use. This paper explores the utilisation of the exhaust gases of an open-cycle-twin-shaft gas turbine. An air standard cycle is assumed for the gas turbine, first with the aid of thermodynamic laws the specific network and the efficiency of the cycle as a function of temperature ratio and pressure ratio of the cycle are calculated, and the realistic bounds placed on the cycle by the thermodynamic analysis is shown. Then the temperature of the exhaust gases and the heat that can be put to benefit for precooling in terms of the temperature ratio and pressure ratio of the cycle are determined. The specific network and efficiency of a precooled cycle have been calculated and compared to conventional systems. It has been concluded that the precooling has a marked effect on the specific network and efficiency at low temperature ratios. Also without increasing the maximum cycle temperature the precooled cycle can work at a higher compressor pressure ratio and at a higher temperature ratio.

Effect of combustion on the economic operation of endoreversible otto and Joule-Brayton engine

To simplify analysis of an internal combustion engine, air-standard cycles are conceived. Air is assumed to behave like an ideal gas. In practice, air-standard analysis provides useful indication of the trends that the engine is likely to follow. Air-standard Otto and Joule-Brayton cycles are bona fide assumption and cannot represent the complex combustion process occurring in the internal combustion engines. In this paper, the complex combustion process is represented by a parameter called fuel-flame temperature. The effect of combustion on the thermoeconomic performances of Otto and Joule-Brayton engines are studied. It is observed that the efficiency at maximum power is less than the Curzon-Ahlborn efficiency. The economic performance of the engine deteriorates due to combustion. The efficiency of the engine corresponds to maximum specific-power output, depends not only on the fuel-flame temperature, but also on the specific heats of the air and fuel. Ideal gas assumption of the working fluid is relaxed in this paper. Although somewhat idealized, the effect of combustion on the performance and economics of the internal combustion engines gives a reasonable design goal and better understanding of the real-heat engine.

Effect of combustion on the economic operation of endoreversible otto and Joule–Brayton engine

To simplify analysis of an internal combustion engine, air-standard cycles are conceived. Air is assumed to behave like an ideal gas. In practice, air-standard analysis provides useful indication of the trends that the engine is likely to follow. Air-standard Otto and Joule–Brayton cycles are bona fide assumption and cannot represent the complex combustion process occurring in the internal combustion engines. In this paper, the complex combustion process is represented by a parameter called fuel-flame temperature. The effect of combustion on the thermoeconomic performances of Otto and Joule–Brayton engines are studied. It is observed that the efficiency at maximum power is less than the Curzon–Ahlborn efficiency. The economic performance of the engine deteriorates due to combustion. The efficiency of the engine corresponds to maximum specific-power output, depends not only on the fuel-flame temperature, but also on the specific heats of the air and fuel. Ideal gas assumption of the working fluid is relaxed in this paper. Although somewhat idealized, the effect of combustion on the performance and economics of the internal combustion engines gives a reasonable design goal and better understanding of the real-heat engine. © 1998 John Wiley & Sons, Ltd.

Thermodynamic analysis of combined diesel engine and absorption refrigeration unit—supercharged engine

This paper describes an attempt to calculate the amount of network, efficiency and also cooling capacity available in the exhaust gases of a supercharged diesel engine (ideal cycle) that is interfaced with an absorption refrigeration unit. An air standard cycle is assumed for the diesel engine. The variations of net work and efficiency of the diesel engine as a function of cycle pressure ratio and temperature ratio are calculated.

Ideal Cycle Evaluation of Steam Augmented Gas Turbines

A wide range of air-standard Brayton and modified-Brayton power cycles are evaluated to determine their second-law efficiencies and their volume flows per unit output. A cycle with reheating is chosen for further analysis on the basis of its potential for high efficiency through exploitation of its exhaust availability (exergy) and its low volume rates. This exploitation can be had either through a conventional Rankine bottoming cycle, or through injection of the bottoming cycle steam into the Brayton turbine. The Rankine bottoming cycle is superior with respect to second-law efficiency; the cycle augmented by injected steam is superior with respect to volume flows. Examination of irreversibilities illuminates the reasons for the better efficiency of the Rankine bottoming cycle alternative.

Air-Standard Modelling for Closed-Cycle Diesel Engines

The author posits that a model constructed from ideal processes is the most desirable starting point for analysis of real power machinery, and then presents means of following this concept in the case of a closed-cycle diesel engine. The traditional air-standard limited-pressure cycle is found unsuitable for this application in that it offers only an unrealistic constant-volume cooling as the model for the processes that must occur between cylinder exhaust and cylinder intake. The present paper substitutes isentropic expansion, throttling and constant-pressure cooling as being suitable ideal models for the actual processes. Equations are presented and sample calculations are given for the cylinder-to-cylinder part of an ideal cycle representing a four-stroke naturally aspirated engine. Two alternatives are also discussed via examples: an engine with partial bypassing of untreated exhaust gas to the cylinder intake and a two-stroke engine with blower or compressor driven by an exhaust gas turbine. A closing example is given to demonstrate one way in which the analyses can be used to find the effect of external process states an engine-cycle output.

Optimum outlet temperature of solar collector for maximum work output for an Otto air-standard cycle with ideal regeneration

The optimum solar collector outlet temperature for maximizing the work output for an Otto air-standard cycle with ideal regeneration is investigated. A mathematical model for the energy balance on the solar collector along with the useful work output and the thermal efficiency of the Otto air-standard cycle with ideal regeneration is developed. The optimum solar collector outlet temperature for maximum work output is determined. The effect of radiative and convective heat losses from the solar collector, on the optimum outlet temperature is presented. The results reveal that the highest solar collector outlet temperature and, therefore, greatest Otto cycle efficiency and work output can be attained with the lowest values of radiative and convective heat losses. Moreover, high cycle work output (as a fraction of absorbed solar energy) and high efficiency of an Otto heat engine with ideal regeneration, driven by a solar collector system, can be attained with low compression ratio.

A Second-Law Analysis of Air-Standard Diesel—Turbocharger–Bottoming Cycles

It is asserted that some of the mechanical engineering literature on power cycle analysis does not make sufficient use of the second law of thermodynamics. Advantages of second-law use are listed and then demonstrated through development by equations by which analysis of an air-standard diesel cycle can incorporate constant-pressure turbocharging and a Rankine bottoming cycle. Steps towards application to real machinery are also demonstrated. Four numerical examples are included.

SECOND LAW ANALYSIS OF IDEAL DIESEL CYCLE

The present paper is an application of the second law of thermodynamics to the air standard diesel cycle. The study investigates the effects of cycle parameters (compression ratio, dimensionless initial and maximum temperatures and initial pressure) on the cycle performance (the first law and second law efficiencies, dimensionless heat added, work done, and available and unavailable energies along the cycle). The study shows that about half of the energy lost must under any case be given to the surroungings, while some portion of the other half may be regained as work.