توضیحات
Thermal phenomena, including heat transfer and heat-work conversion,
are very common and basic in human life. From day to night, from the sun
to the carbon nanowires, from cooking in a kitchen to working in
spacecraft, one can easily see the transport or conversion of heat almost
everywhere in nature. However, the understanding and application of
thermal phenomena are by no means easy. As is known, it took so long a
time for people to find that there is no perpetual motion. The controversy
between the caloric theory of heat and the thermal-motion theory also
lasted many years. With the untiring efforts from generation to generation,
the understanding of the thermal phenomena has deepened, many
technologies have been developed, and human life has been improved
accordingly.
Nowadays, we are also facing many thermal problems. For instance,
thermal design is often required to ensure that the equipment are working
under proper temperatures. In practical applications, thermal optimization
is also very necessary to obtain the best thermal performance so that the
energy utilization efficiency can be increased and the economic cost can
be decreased. When trying to solve the thermal problems, researchers have
developed many different theories and methods, the vast majority of which
can only be used in specific fields to solve specific problems.
However, in academic circles, researchers have a preference for
finding a unified theory. Even in a very narrow field, they always have a
beautiful expectation that they could use only one theory or method to solve all problems. Generally, this goal is not easy to achieve no matter
how narrow the field is. This is also true in thermal analyses and
optimizations. Up to now, we still do not have any unified theory in this
area. During the past decades, the entropy generation minimization has
been widely used and found to be effective in many cases. Sometimes, it
was even used without checking the applicability. It seems that this theory
is the unified one that we want. But, is it true? In this book, this question
can be discussed in detail, and the applicability of entropy generation
minimization is analyzed with theoretical derivations and different
numerical examples.
The optimization directions and the application preconditions of
entropy generation minimization are summarized and discussed. In thermal
engineering, when the design objectives are inconsistent with the
optimization directions of entropy generation minimization or the application
preconditions are not satisfied, it is shown that the entropy generation
minimization may not lead to the objectives. In heat transfer and heatwork conversion, this point is demonstrated clearly with different
examples, in which the entropy generation may not be the minimum, but
the maximum or an intermediate value when the objectives are achieved.
As there are many different design objectives in thermal analyses and
optimizations, only some typical ones are discussed in this book, including
the maximum heat transfer rate, the minimum average temperature of the
heated domain, the maximum heat exchanger effectiveness, the minimum
thermal resistance, the system mass minimization, the minimum economic
cost, the maximum output work, the maximum thermal efficiency, the best
thermo-economic performance, etc.
Especially, the entropy generation paradox is analyzed in detail. It is
shown clearly that the entropy generation rate in heat exchangers does not always decrease with increasing heat exchanger effectiveness or heat
transfer rate, and the entropy generation paradox may not disappear
even if one considers the effect of heat exchangers on the whole energy
system in which the heat exchangers are organs. In heat exchanger
networks, it is also shown that the entropy generation minimization does
not always give the best system performance. Therefore, entropy generation
is not a proper parameter to evaluate the heat transfer performance of heat
exchangers and heat exchanger networks.
In addition, the applicability of the dimensionless parameters of
entropy generation, the entropy resistance minimization and the exergy
destruction minimization is discussed and found to be conditional. In other
words, these parameters and methods are not omnipotent or perfect, either.
The limitations of entropy generation minimization are also pointed out
directly and discussed in detail. It is clearly revealed that the entropy
generation minimization has a definite application scope, beyond which
the theory may not be effective.
All in all, although the entropy generation minimization has been
widely used, it is not the unified theory in thermal analyses and
optimizations, and cannot be used to solve all thermal problems. Therefore,
before its application, one should pay enough attention to the applicability
to avoid the abuse of the theory.
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ترجمه ماشینی :
پدیده های حرارتی، از جمله انتقال حرارت و تبدیل کار حرارتی، در زندگی انسان بسیار رایج و اساسی هستند. از روز تا شب، از خورشید تا نانوسیمهای کربنی، از پخت و پز در آشپزخانه تا کار در فضاپیما، میتوان به راحتی انتقال یا تبدیل گرما را تقریباً در همه جای طبیعت مشاهده کرد. با این حال، درک و استفاده از پدیده های حرارتی به هیچ وجه آسان نیست. همانطور که مشخص است، زمان زیادی طول کشید تا مردم متوجه شوند که حرکت دائمی وجود ندارد. مناقشه بین تئوری کالری گرما و نظریه حرکت حرارتی نیز سالها به طول انجامید. با تلاشهای بیوقفه از نسلی به نسل دیگر، درک پدیدههای حرارتی عمیقتر شده است، \ فنآوریهای زیادی توسعه یافتهاند و زندگی انسانها بر این اساس بهبود یافته است. امروزه با مشکلات حرارتی زیادی نیز مواجه هستیم. به عنوان مثال، طراحی حرارتی اغلب مورد نیاز است تا اطمینان حاصل شود که تجهیزات در دمای مناسب کار می کنند. در کاربردهای عملی، بهینه سازی حرارتی برای به دست آوردن بهترین عملکرد حرارتی بسیار ضروری است تا بتوان بازده مصرف انرژی را افزایش داد و هزین
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