the general notions on reciprocal radiation from several surfaces. We distinguish total influence and reciprocal radiation from finite surfaces. This subject is central in particular to the calculation of ovens. Here, we should restrict ourselves to notions, returning to specialist works for application by professionals working with ovens.
Solving a radiation problem often involves knowing and understanding the essence of the corresponding lessons. It is therefore not so easy to produce (interesting) problems that are limited to only being a paragraph long. This is why we were led to focus in Chapter 5 on the essence of exercises dedicated to radiation and coupled transfers.
In a domain where digital methods are becoming the rule for complex situations, it seems important to reserve a particular place for “monitoring” analytical approaches. In addition to their usefulness in understanding this domain, they offer the reader a precious tool for making calculations “on the back of an envelope”.
Finally, Chapter 6 introduces the reader to a digital approach for different transfer modes. The general problem of modeling is tackled here and examples of processing using ANSYS are presented.
The Appendix covers the tabulations of GO−λT functions whose practical importance emerges from the problems.
December 2020
Introduction
I.1. Preamble
Thermal energy was probably first perceived (if not identified) by humanity, through the Sun. The themes of night and day are found at the center of most ancient myths. Humanity’s greatest fear was probably that the Sun would not return again in the morning. Fire became controlled in approximately 400,000 BP. Thermal transfer was therefore a companion of Homo ergaster, long before Homo sapiens sapiens.
However, it took a few hundred thousand years before so-called “modern” science was born. Newtonian mechanics dates from three centuries ago. Paradoxically, another century and a half passed by before energy was correctly perceived by scientists, in terms of the new field of thermodynamics. Furthermore, a systematic study of heat transfer mechanisms was carried out at the end of the 19th century, and even later for the study of limit layers, the basis of convection.
Heating, lighting and operating the steam engines of the 19th century were all very prosaic concerns. Yet this is where revolutions in the history of physics began: the explosion of statistical thermodynamics driven by Boltzmann’s genius, and quantum mechanics erupted with Planck, again with Boltzmann’s invovlement.
Advances in radiation science, particularly in sensor technology, has enabled us to push back our “vision” of the universe by a considerable number of light years. To these advances we owe, in particular, the renewed interest in general relativity that quantum mechanics had slightly eclipsed, through demonstration of black holes, the physics of which may still hold further surprises for us.
Closer to home, fundamental thermal science, whether it is conduction, convection or radiation, contributes to the improvement of our daily lives. This is particularly true in the field of housing where it contributes, under pressure from environmental questions, to the evolution of new concepts such as the active house.
The physics that we describe in this way, and to which we will perhaps introduce some readers, is therefore related both to the pinnacles of knowledge and the banality of our daily lives. Modestly, we will place our ambition in this latter area.
There are numerous heat transfer textbooks in different formats: “handbooks” attempting to be exhaustive are an irreplaceable collection of correlations. High-level courses, at universities or engineering schools, are also quite exhaustive, but they remain demanding for the listener or the reader. Specialist, more empirical thematic manuals are still focused on specialists in spite of all this.
So why do we need another book?
The authors have taught at university level and in prestigious French engineering schools, and have been involved in the training of engineers on block-release courses. This last method of teaching, which has been gaining popularity in recent years, particularly in Europe, incorporates a distinctive feature from an educational point of view. Its practice has, in part, inspired this book.
The aim is to help learners who have not had high-level mathematical training in their first years following the French Baccalaureate (therefore accessible to apprentices), and pupils with more traditional profiles. At the same time, we would like to show this broad audience the very new possibilities in the field of digital processing of complex problems.
When a miner wants to detach a block of coal or precious mineral from a wall, they pick up a pneumatic drill. If we want to construct a tunnel, we must use dynamite. The same is true for physicists.
Whether they are researchers, engineers or simply teachers, scientists have two tools in their hands: a calculator and a computer (with very variable power). Since both authors are teacher researchers, they know they owe everything to the invention of the computer. From the point of view of teaching, however, each one of the two authors has remained specialist, one holding out for the calculator and “back-of-the-napkin” calculations, and the other one using digital calculations.
The revolution that digital tools has generated in the world of “science” and “technical” fields, aside from the context of our daily lives, no longer needs to be proved. We are a “has been” nowadays if we do not talk about Industry 4.0. The “digital divide” is bigger than the social divide, unless it is part of it …
Indeed, the memory of this revolution is now fading. Have students today ever had a “slide rule” in their hands? Do they even know what it is? Yet, all the physicists behind the laws of thermal science had only this tool in hand, giving three significant figures (four with good visibility and tenacity), leaving the user to find the power of ten of the result. It goes without saying that a simple calculation of a reversible adiabatic expansion became an ordeal, which played a part in degrading the already negative image of thermodynamics held by the average student.
This reminder will seem useless to some; slide rules are at best sleeping in drawers. But there is a moral to this story: no matter what type of keyboard we type on, a calculator or a computer, our head must have control over our fingers. This book has been written on the basis of this moral.
A good physicist must have a perfect understanding of the idea of an “order of magnitude”. For this, the tool is a calculator. We always do a rough sizing of a project before moving on to detailed modeling and numerical calculations.
The two authors belong to the world of engineering sciences, meaning most of their PhD students have entered the private sector. One of them, having moved into the aerospace sector, came back to see us very surprised by the recurrence of “back-of-the-napkin” calculations in his day-to-day work.
Fundamental or “basic physics” concepts are taken from a type of manual that is resolutely different from those dedicated to the numerical approach. In this case, the authors allow themselves to believe that it is no bad thing to collect them all together in a single book, for once. This is a significant difference that will surprise some and, without doubt, be criticized by others. Nevertheless, when reading this book, an “average” student will be initiated to a field that teaching models generally promised “for later on” (or never if he/she never goes beyond a certain level of education). It is also true that fully immersed in equations and complex calculations, specialist
