DETONATION AS AN ALTERNATIVE TO COMBUSTION

More than a century ago after destructive explosions in coal mines, the scientists who studied their causes turned attention to the blast process propagation at a supersonic speed accompanied by chemical reactions. This process called later on "detonation" was considered harmful, destructive and uncontrolled. Its control became a main incentive for intensive development of the detonation combustion theory and conducting of a wide range of physical experiments to organize the process and determine methods of its control. With increasing knowledge, a negative attitude to detonation changed to a desire to use a "destructive" potential of this physical phenomenon to the good.

Recently foreign aerospace organizations and companies, in particular, in France, Japan, the USA and China, are taking a keen interest in studies of effects and resources of detonation combustion in different aerojet engines. In our country these problems are tackled at the subdivisions of the RAS Siberian Branch. For example, the so-called continuous detonation high-speed processes, which are very important for creation of a new type of jet engines, are studied at the Lavrentyev Institute of Hydrodynamics. Head of the Laboratory of heterogeneous systems dynamics of this institute Sergei Zhdan, Dr. Sc. (Phys. & Math.), told Yulia Alexandrova, a Nauka v Sibiri newspaper correspondent about the history of this science development and the latest achievements of scientists in this field.

He said: "Today we consider the method of fuel detonation combustion as an alternative to a traditional method, i.e. combustion in a turbulent flame. It will allow a more intensive, profitable and stable combustion of different fuels in chambers of lesser sizes, which are determined by a lateral dimension of a detonation front."

He noted that this problem had been first theoretically studied more than 70 years ago by the outstanding physicist and one of the founders of the modern theory of combustion, detonation and blast waves Academician Yakov Zeldovich. He has showed that, from the viewpoint of thermodynamics, detonation combustion of a fuel mixture, when the detonation front spreads faster than the speed of a sound, is more profitable than deflagration combustion which runs at subsonic speeds. How-

ever, despite the "revolutionary" conclusions the experimental studies of the problem started only 20 years later. They served as a basis for creation of a line of pulsating detonation aviation engines.

Their operation principle is as follows: a fuel mixture gets into the combustion chamber, then takes place its initiation, spread of the detonation wave along the chamber, and afterwards of combustion products, which carry on mechanical work. Then the cycle is repeated. A relatively small number of pulsations in a time unit (recycling frequency) is a shortage of this technology. When increased, this parameter complicates the engine design. Besides, such power plant operates noisily, and fuel feeding should be measured out for initiation of each detonation cycle. A different matter is a continuous spin (rotary) detonation process, when a possibility of permanent continuous detonation of the fuel mixture in an annular combustion chamber is realized.

According to Zhdan, the exact structure of the spin detonation phenomenon in gases was first registered in 1957 by the outstanding scientist in the fields of mechanics and physics and a designer Acad. Bogdan Voytsekhovsky, who worked at that time at the Institute of Hydrodynamics in Novosibirsk. Owing to the photorecord-er of his design he managed to take a photo and decipher heterogeneous structure of the spin detonation wave front, moving along the percussion tube in the fuel mixture. As against a plane detonation, the only lateral blast wave appears in a spin detonation, which is followed by a layer of heated gas, which has not reacted to it, and then a zone of the chemical reaction. Such wave moves along a round percussion tube with a speed of detonation spiralwise. Thereafter the scientist improved the optical shooting method and developed a spin detonation model, which explained numerous peculiarities and regularities of this phenomenon, which had great practical importance for science.

Voytsekhovsky suggested, using an analogy of spin detonation, to realize in special annular chambers continuous burn up of combustible mixture in rotating detonation waves and carried out the first successful experiments (1959). The works of the scientist and his colleagues in this field were granted the Lenin Prize (1965) and were marked with two inventions (jointly with the Institute of Chemical Physics, the USSR AS Siberian Branch).

Under Voytsekhovsky's direct guidance, Vladislav Mitrofanov (1935-2001), Dr. Sc. (Phys. & Math.), started his brilliant career of researcher and engineer and worked his way up from a laboratory assistant to the head of the laboratory of heterogeneous systems dynamics and head of the department of high-speed processes at the Institute of Hydrodynamics for more than forty years.

Mitrofanov's first work cycle is associated with the front's instability and splitting of gas detonation wave, detailed studies of the revealed earlier cellular structures, construction of theoretical schemes of lateral waves. He

was the first to produce photos of such structures inside pipes of different diameter and show independence of their formation from walls and confirmed that detonation cells were formed by counter movement of lateral waves. As a matter of fact, Mitrofanov developed and refined Voytsekhovsky's concept of a lateral wave and its role in the mechanism of detonation combustion of gases. His works received wide recognition in this country and abroad, they gave an impulse to new large-scale research and proved to be practicable for determination of critical conditions of explosion initiation and spreading in gaseous media. The above said had a direct bearing on the problems of safe transportation of large quantities of combustible substances, and elimination of emergency situations in the sphere of production, in everyday life and coal mines.

The knowledge accumulated in this promising sector served also as a basis for creation of plants for detonation spraying in the 1980s. The economic effect of their implementation at the enterprises of the Ministry of Aviation Industry only made up about 15 mln rubles per year, an enormous sum for that time. Further studies of the process gave an impulse to use detonation wave properties in other large-scale creative purposes, i.e. for stimulation of fuel combustion in jet engines.

Mitrofanov's creative work was carried on by his colleagues. At present, according to Sergei Zhdan, due to the intensive development of computer engineering, of primary concern is a computing experiment based on numerical modeling of the method of continuous spin detonation of gas and heterogeneous systems. It helps get better understanding of field test results, gives an opportunity to compare them with the theory and therefrom sets forth a valid physical interpretation.

Sergei Zhdan noted: "At this date we believe that a fundamental scientific basis of a rapid process of continuous detonation has been laid. We hope that... different purpose engines will be developed using the continuous spin detonation principle. After all, the efficiency factor is higher here than in case of combustion. That being so, the scope of work carried out by detonation products must be higher too. If you burn the same quantity of fuel, you can increase efficiency of fuel combustion. In other words, with a fixed amount of fuel aboard, the engine thrust increases or its dimensions decrease, or the vehicle flies to longer distances. That is what an attempt to increase the efficiency factor means."

Last year Sergei Zhdan in coauthorship with Fyodor Bykovsky, Dr. Sc. (Tech.), published the monograph Continuous Spin Detonation. The book sums up the results of a long-term experimental and numerical studies of detonation combustion of a wide range of fuels for further practical use in engines and power plants. The authors note that though being more profitable in terms of thermodynamics it cannot replace the traditional turbulent combustion everywhere. But where it is necessary to burn fuel quickly and intensively in small-size facilities with less heat-stressed walls, detonation combustion can find worthy application.

Yulia Alexandrova, Just work on.--"Nauka v Sibiri", No. 5, 2014

Illustrations from the site of the RAS Siberian Branch

Prepared by Marina KHALIZEVA