Molecular Dynamics Simulation Study of Finite-Time Thermodynamics of a Heat Engine
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Date
2014-04
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Addis Ababa University
Abstract
The upper limit for e ciency of a heat engine is thermal e ciency of a reversible Carnot
cycle, also called Carnot e ciency, C. Since a heat engine working at Carnot e ciency
delivers zero power with in nite time, the notion of extracting maximum possible power
output per cycle has been introduced. As a consequence the upper limit for e ciency
at maximum power of a heat engine, called Curzon-Ahlborn e ciency, CA, is de ned.
Due to the need of providing a sustainable supply of energy and to strong concerns about
the environmental impact of the combustion of fossil fuels, performance e ciency of a
heat engine remains a major problem in thermodynamics. In this work we study the
performance analyis of a heat engine where real gas is used as a working substance of
the heat engine. The performance analysis is conducted through studying the cyclic
thermodynamic process in the working substance of an endoreversible heat engine.
We perform a classical molecular dynamics simulation study of a heat engine operating
between two heat reservoirs and performing a Carnot-like cycle in a nite time over a
wide range of process rates. The heat engine is modeled to use real gas as the working
substance where we have used Lennard-Jones potential to consider the intermolecular
interaction in the working gas. The piston speed and temperature ratio of cold and hot
heat reservoirs are used as control parameters whereas e ciency and power output per
cycle are variable quantities of the engine. The variation of these dependent variables as
a function of the independent parameters is studied for two cases; when the piston moves
with uniform speed throughout the cycle and when it moves with constant but unequal
speed on the four branches of the cycle. It is shown that the e ciency of the Carnot-like
engine increases for a slower process rate but at the cost of power output. We also compute
maximum e ciency and e ciency at maximum power of the engine model considered and
the obtained result is in a good agreement with Carnot e ciency and Curzon-Ahlborn
e ciency respectively for the later case. It is also shown that the e ciency of the engine
x
highly increases when the cycle involves instant adiabatic process. Finally we determine
optimum values for e ciency and power output per cycle and the corresponding process
rate at which these values are attained where we use uni ed optimization criterion (
criterion) to optimize between e ciency and power
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Keywords
Thermodynamics of a Heat Engine