Preface

In 2028, one hundred years will have passed since the publication of the N.N. Semenov's paper (Semenov 1928a), which laid the foundation for the thermal explosion theory. It has been fascinating to see how the approach of this study, essentially amounting to a single first-order ordinary differential equation, has been influential in establishing new research directions on the subject and attracting the attention of generations of scientists. The reasons for this, nearly one hundred years after the publication, are clear.

First, Semenov's theory captures the very physical essence of the problem, explaining it and making predictions in a succinct mathematical form. Owing to these virtues, Semenov's theory became classical. Second, since the proposed approach was very simple and involved a number of assumptions, the theory allowed for generalizations.

The present book reviews, along with the Semenov's theory itself, its major logical extensions. There is still no monograph written on the subject, and making such a monograph available has been the primary aim of the development of the present book.

Originally, the theory was practically applied to identify safe conditions for storage or chemical processing of various chemically reactive substances.

Consequently, the major generalizations have been concerned with traditional reactive systems (in particular, gas mixtures and solid energetic materials) and developed primarily in two main directions: (1) consideration of complicated heat transfer regimes between the system and its surroundings, and (2) effects of complicated chemical kinetics.

Several examples of the first stream of developments may be mentioned here.

One practical problem related to civil explosives is their application for underground works. In these circumstances, an explosive substance is being subjected to increased ambient temperature and, while being inert at the ground-level temperature, may unintentionally explode in the process of transportation into deep wells. It is well known that temperatures in the wells of up to 8?km depth may reach 350?°C, or up to 400?°C in more deeper wells.

A similar problem arises upon studying thermophysical and kinetic properties of solid substances in specifically designed laboratory experiments, applying the technique of Thermogravimetric Analysis (TGA). In these experiments, samples are heated in such a way that their temperature increases linearly (or less often, according to some other law, e.g. parabolic, exponential, etc.).

It is essential that the sample maintains integrity during heating. The onset of a thermal explosion may result in the destruction of the sample. Besides, knowledge of the conditions causing thermal explosion allows the accuracy of the experiment under different heating rates to be estimated. Therefore, the possibility of thermal explosion needs to be taken into account when designing such laboratory experiments.

The next example is the concept of conjugate thermal explosion, proposed and further developed by the author. In this problem, conjugate heat transfer between different chemically reacting materials needs to be investigated. Analysis of the problem leads to the prediction of conditions under which such type of thermal explosion occurs. These conditions are significantly different from the analogous conditions of the classical thermal explosion theory.

Examples of the second major line of developments, identified above, are modern Computational Fluid Dynamics (CFD) simulations of thermal explosion, which take into account very detailed chemical kinetic mechanisms, and are often performed for complicated regimes (e.g. turbulent flows of reactive media).

As time has passed, it has gradually become apparent that manifestations of thermal explosion are much more diverse than were assumed traditionally. Thermal explosion may occur in different media and at a very wide range of circumstances. It has become especially evident with industrial developments which occurred later in the twentieth and at the beginning of the twenty-first century. Indeed, whilst chemical processing and storage of reactive materials is still highly relevant, a number of modern technologies have emerged which, whether in traditional or new forms, inevitably use chemical energy sources. In various circumstances, these technologies are prone to thermal explosion.

These technological advancements brought up new and challenging applications of the thermal explosion theory. Their overview provided the second motivation for writing this book.

There are at least three areas of such developments.

The first one, that has emerged over the last few decades, concerns the handling of new types of chemical and biological substances. The increasing use of environmentally friendly fuels, combined with their low (compared to traditional energy sources) calorific values, results in the need to store, transport and burn them in very large quantities. This circumstance presents a significant challenge from a safety point of view. Storages of such fuels (various biosolids, wood chips, Refuse Derived Fuels (RDF), Refuse Paper and Plastic Fuels (RPF) and similar substances) have been known to be a source of fires, accompanied by casualties and very difficult to extinguish. The ignition of such fires is caused by a thermal explosion in the system. Development of a thermal explosion in such substances has peculiar features (e.g. the presence of microbiological activity that initiates chemical reactions with more substantial heat release) that are still not completely understood quantitatively.

As the application area of these fuels widens, there will be a greater need for a more reliable prediction of thermal explosion conditions, and for the development of safe fuel handling procedures.

The theory of thermal explosion is capable of predicting to what extent storage sizes and ranges of storage conditions must be limited in order to avoid catastrophic accidents.

The second area of development is related to the application of new generations of electric batteries and other innovative devices utilizing chemical energy. The most remarkable example of these recent technologies is lithium-ion electric batteries of various capacity. The accidents involving Boeing 787 and Samsung Galaxy Note 7 lithium-ion batteries are most commonly known. The cause of these accidents is still the same, that is thermal explosion developing in electric batteries in a quite specific way. The same safety concern applies to other devices such as fuel cells and microcombustors.

The development of the above technologies is expected to continue, demanding an understanding of their thermal explosion scenarios and efficient means to prevent them.

A very different and important third area where considerations of thermal explosion have recently proved to be very helpful is fire protection design. It has become apparent over the last two decades that the most severe, dangerous and most often resulting in fatalities type of compartment fire (the so-called Fire Flashover) is a very specific form of thermal explosion. The problem has become more severe in recent years with a fundamental shift of combustible load in compartments towards plastics and similar highly flammable materials. The available data suggests that the time to flashover (which is effectively the time available to occupants to leave the property) has decreased in modern compartments by several times, or perhaps even up to an order of magnitude.

Understanding the flashover mechanism from the point of view of thermal explosion theory, and the application of respective quantitative models, can play a vital role in identifying scientifically justified ways towards safer building designs.

There are also other areas where the application of methods of the thermal explosion theory will be important, for example in handling and storage of highly energetic propellants of the new generation (metalized propellants).

There are some, probably inevitable, limitations to the scope of the book. Thus, multi-phase systems are not considered (except for explosion prevention application in Chapter 9). Extension of the analysis to multi-phase case in any context, not just related to thermal explosion, results in a dramatic increase in complexity. Correspondingly, it would require a considerable increase of the volume of the book.

The author's own scientific preferences are also reflected in the scope, predominantly via noticeable attention given to applications for fire safety.

It should be noted, finally, that while the book is essentially confined to the analysis of chemical sources of energy generation, the phenomenon of thermal explosion may also be considered in a more broad sense. Some very interesting and important manifestations of identical behaviour have been found in systems where energy generation progresses due to physical mechanisms, such as in some processes related to the mechanics of polymers.

Thus, the theory of thermal explosion provides a unified and powerful framework for understanding, prediction and practical handling of phenomena in very different fields of human activity. This fact emphasizes the strength and vitality of the theory created almost one hundred years ago.

This book is organized into nine Chapters.

The first chapter is introductory. It seeks to provide very general exposition of the subject. It introduces the concept of Thermal Explosion, provides an informal description of this phenomenon in relation to other similar processes and...