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Termodinámica Técnica

Ciclos de Potencia de Gas

 y
 Presión Constante
 Motores térmicos
 Volumen Constante
 Isotérmico
 Diagramas PV
 CALOR
 TRABAJO
 PRIMERA LEY DE LA TERMODINAMICA
 SEGUNDA LEY DE LA TERMODINAMICA
 CICLOS  DE POTENCIA
 CICLO DE CARNOT
 CICLO OTTO Y CICLO DIESEL
 Procesos de Motores Térmicos
 utilizan
 utilizan
 Para generar    trabajo
 Siendo    el más ideal
 Usan      procesos
Ciclos de Gas Productores de Trabajo
Esquema Conceptual

Power generation is an important area of application of thermodynamics. The devices and systems used to produce a net power output are called engines and the thermodynamic cycles they operate on are called power cycles.

In gas cycles, the working fluid remains in the gaseous phase throughout the entire cycle, while in vapor cycles the working fluid exists in the vapor phase during one part of the cycle and in the liquid phase during another part.

Gas power systems include:

ü  Internal combustion engines

ü  Gas turbines

Analysis of gas power cycles

Actual gas power cycles are pretty complicated. For simpler analysis, we make several approximations, which are commonly known as the air-standard assumptions:

Ø The working fluid is air, which continuously circulates in a closed loop and always behaves as an ideal gas.

Ø All the processes that make up the cycle are internally reversible.

Ø The combustion process is replaced by a heat-addition process from an external source.

Ø The exhaust process is replaced by a heat rejection process that restores the working fluid to its initial state.

Another assumption frequently made is that air has constant specific heats (values used are those at the room temperature). This assumption is known as cold-air-standard assumption.

A heat engine

We know from experience that the work can to be converted directly and completely in to heat, but to convert heat into work requires using special devices. These devices are called heat engines.

Heat engines differ somewhat, but it is possible to characterize them using the following scheme:

A heat engine is a physical or theoretical device that converts thermal energy to mechanical output. The mechanical output is called work, and the thermal energy input is called heat. Heat engines typically run on a specific thermodynamic cycle. Heat engines can be open to the atmospheric air or sealed and closed off to the outside (open or closed cycle).

In engineering and thermodynamics, a heat engine performs the conversion of heat energy to mechanical work by exploiting the temperature gradient between a hot "source" and a cold "sink". Heat is transferred from the source, through the "working body" of the engine, to the sink, and in this process some of the heat is converted into work by exploiting the properties of a working substance (usually a gas or liquid).

The second law efficiency

The concept of efficiency is used to measure the operation of any process, system or device. The parameter most widely used in thermodynamics is generally based on the concept of energy, which is not a distinction between high-quality energy and low quality energy. A simple example is the thermal efficiency of a heat engine, which is defined as the ratio resulting from dividing the work between the heat that added to the system during the cycle. This definition gives equal weight both heat and work. While this kind of efficiency is useful for certain purposes, do not allow an accurate measure of thermodynamic behavior. Since is availability and not the energy consumed during thermodynamic processes, it is more logical to have an efficiency based on the concept of availability. We now introduce the concept of efficiency of the second law based on the definition of availability and the availability destruction.

From the general definition of efficiency, that can be expressed as:

For a heat engine the concept of "benefit" is the work done W, and the concept of "cost" corresponds to the heat added during the cycle. Qadd. Available energy (availability) in Qadd is

Consequently the efficiency   according to the second law can be written as:

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