O desempenho, propósito, função, objetivo e direção da termodinâmica da transferência de calor

quinta-feira, março 21, 2019

Thermodynamics of heating

Adrian Bejan

Source/Fonte: NASA


Heat transfer is a mature science, and so is thermodynamics. They are almost 200 years old having developed largely independently until the 1980s. Maturity comes from the usefulness and success of the thermal sciences. This review uses the thermodynamics of heat transfer to focus on aspects that are usually not discussed in physics: performance, purpose, function, objective and direction of evolutionary design. The article illustrates the unity of the thermal sciences discipline (heat transfer +thermodynamics + constructal law), and uses the opportunity to correct a few recent interpretations of the thermodynamics of heat transfer regarding dissipative engines and energy storage.

1. Introduction

Heat transfer happens. It is natural because temperature differences and gradients are everywhere. In the devices conceived by humans, just like in the design of the animal body, heat transfer is present because the bigger entity (device, animal, human and machine specimen) represents a design with purpose, with direction of change and evolution.

In science, ‘purpose’ is not mentioned, yet, many scientific words and concepts account for the same irrefutable aspect of reality, for example: performance, function, objective, cause, self-organization, self-optimization, cost, good, better, easy and difficult. This multitude of terms makes it difficult to address ‘performance’ in a fundamental way. Here are a few examples.

To the biologist who studies the design of the fur of a warm-blooded animal, performance is the animal's ability to thrive in a cold environment, which depends on the performance of its fur as a thermal insulation. To the manufacturer of a heat exchanger, performance is the cost of materials, labour and fuel used during manufacturing, which have everything to do with the performance of the whole heat exchanger as a facilitator of heat transfer. To the designer of a large-scale steam-turbine power plant, in addition to the total cost, performance is the overall energy conversion efficiency of the whole plant, which is a global measure to which contributes the functioning of each of the heat exchangers through which the steam flows [1–18].

All these interpretations belong together in a most useful and tutorial way if approached from the most fundamental point of view available in science: thermodynamics. Heat transfer is one kind of energy transfer between two systems (the system and its environment), and this kind has ‘performance’ that distinguishes it unmistakably from the performance of the other kind of energy transfer, which is work transfer. Performance is why heat transfer is eminently not the same as work transfer.

The objective of this article is to trace the path of the concept of performance from its fundamental place in thermodynamics to the many and diverse counterparts of performance that are recognized in other scientific undertakings. Along the way, we will discover opportunities to correct some of the erroneous interpretations of heat transfer performance that continue to propagate in the current literature.