S. N. Medvedev

Experimental proof of the energy efficiency of the Zel’dovich thermodynamic cycle

207 In 1940, Zel’dovich advanced the idea of the possi bility of using the energy of detonation combustion [1]. According to his estimates, the thermodynamic efficiency of a cycle with detonation combustion of fuel can significantly exceed the efficiency of the Bray ton cycle with constant pressure combustion. Later, these theoretical conclusions and estimates [1] were confirmed by thermodynamic calculations and multi dimensional gas dynamics calculations with consid eration of dissipative processes. For example, thermo dynamic calculations [2] showed that the efficiency of the Zel’dovich cycle (as the detonation combustion cycle is called now) can be 20–30% higher than that of the Brayton cycle, and multidimensional gas dynam ics calculations of the process in a liquid propellant rocket engine with detonation combustion [3] pre dicted a 13–15% gain in comparison with a usual liq uid propellant rocket engine. Although the theoretical conclusions about the energy efficiency of the Zel’dovich cycle are unquestioned, there have been no direct experimental proofs of these conclusions to date. In this work, for the first time, we experimentally proved that the Zel’dovich thermodynamic cycle with continuous detonation combustion of a hydrogen– oxygen mixture in an annular chamber is more effi cient than the Brayton thermodynamic cycle with continuous combustion of the same mixture, other conditions being equal. The specific impulse of a rocket engine prototype operating in the continuous detonation mode was 6–7% higher than that in the continuous combustion mode. There are two main designs to perform detonation combustion: in detonation waves circulating continu ously in the tangential direction across an annular combustion chamber (continuous detonation cham bers [4]) and in periodic detonation waves travelling along a combustion chamber (pulse detonation cham bers [5]). At the present time, the problem of experi mental implementation of continuous and pulse deto nation combustion of various fuels (from hydrogen to aviation kerosene) with various oxidizers (air, oxygen enriched air, oxygen) has generally been solved. The issue is that there have still been no experimental proofs of the energy efficiency of the Zel’dovich cycle. For example, all the known experiments on continu ous detonation combustion in annular combustion chambers were accompanied by high pressure loss in propellant component feed systems and demonstrated low process efficiency below the ideal efficiency of continuous combustion chambers. No direct compar ison has been made between the measured thrust per formances of pulse detonation and pulse combustion jet engines. The purpose of this work was to experimentally prove the energy efficiency of the Zel’dovich cycle by directly comparing the thrust performances of a rocket engine prototype operating in the continuous detona tion and continuous combustion modes. For this pur pose, we designed and built a test stand and a rocket engine prototype, which can operate in the modes of continuous detonation and continuous combustion of a mixture of gaseous hydrogen and gaseous oxygen. The test stand consists of hydrogen and oxygen receivers 0.64 and 0.32 m3 in volume, respectively, high performance system of fast response valves, large section propellant component feed manifolds, thrust table, and precision system for measuring the thrust and the propellant component feed pressures. The maximal mass flow rate of the propellant mixture that can be reached on this stand is close to 1.5 kg/s. A rocket engine prototype is an annular combus tion chamber to which, to one end face, an injector PHYSICAL CHEMISTRY

Cyclic deflagration-to-detonation transition in the flow-type combustion chamber of a pulse-detonation burner

The possibility of realization of a rapid cyclic deflagration-to-detonation transition (DDT) with a frequency of up to 2 Hz under conditions of high-velocity flow (∼10 m/s) and separate supply of the combustible mixture components (methane and air) in a tube, 5.5 m in length and 150 mm in diameter, with an open end at a low ignition energy (∼1 J) is for the first time demonstrated. It is shown that such a tube with turbulizing obstacles of special shape and placement can ensure reliable DDT at a distance of 3–4 m from the ignition source within ΔτDDT ≤ 20 ms after ignition. The results will be used in the development of a new type of industrial burner—a pulse-detonation burner for high-rate heating and fragmentation, combining thermal and shock-wave (mechanical) impacts on the treated object.

Pulse-detonation burner unit operating on natural gas

The possibility of controlled cyclic deflagration-to-detonation transition within a length of 2.5–3.0 m in an open-end tube (94 mm in diameter) with separate continuous supply of natural gas and air was demonstrated for the first time. Based on experimental studies, a workable pulse detonation burner, a prototype of new generation of industrial burners, was developed. It can produce a combined effect on the objects blown on with combustion products—shock-wave (mechanical) and thermal.

Simulation of the autoignition and combustion of n-heptane droplets using a detailed kinetic mechanism

The autoignition and combustion of n-heptane droplets are simulated using a detailed kinetic mechanism. A mathematical model, based on first principles, contains no adjustable parameters. The burning rate constants for the combustion of droplets are calculated over a wide range of pressures, temperature, fuel-to-oxidizer equivalence ratios of the gas-droplet suspension, and droplet diameters. The calculated and measured delay times of autoignition of droplets are compared. The calculation results agree well with the available experimental data. The detonability of gas-droplet suspensions with partial pre-evaporation of fuel is estimated.

Autoignition and combustion of hydrocarbon-hydrogen-air homogeneous and heterogeneous ternary mixtures

A numerical simulation of the ignition and combustion of hydrocarbon-hydrogen-air homogeneous and heterogeneous (gas-drop) ternary mixtures for three hydrocarbon fuels (n-heptane, n-decane, and n-dodecane) is for the first time performed. The simulation is carried out based on a fully validated detailed kinetic mechanism of the oxidation of n-dodecane, which includes the mechanisms of the oxidation of n-decane, n-heptane, and hydrogen as constituent parts. It is demonstrated that the addition of hydrogen to a homogeneous or heterogeneous hydrocarbon-air mixture increases the total ignition delay time at temperatures below 1050 K, i.e., hydrogen acts as an ignition inhibitor. At low temperatures, even ternary mixtures with a very high hydrogen concentration show multistage ignition, with the temperature dependence of the ignition delay time exhibiting a negative temperature coefficient region. Conversely, the addition of hydrogen to homogeneous and heterogeneous hydrocarbon-air mixtures at temperatures above 1050 K reduces the total ignition delay time, i.e., hydrogen acts as an autoignition promoter. These effects should be kept in mind when discussing the prospects for the practical use of hydrogen-containing fuel mixtures, as well as in solving the problems of fire and explosion safety.

Mathematical modeling of the chemical inhibition of the detonation of hydrogen-air mixtures

A mathematical modeling of the chemical inhibition of the detonation of hydrogen-air mixtures is performed. It is demonstrated that a one-dimensional model of detonation based on a chain-branching mechanism of hydrogen combustion makes it possible to describe the main regularities of the effect of inhibitors on detonation. The calculation results, which are in good agreement with the available experimental data, show that inhibition causes a narrowing of the concentration limits of detonation and an increase in the critical diameter of detonation.

Detailed kinetic mechanism of the multistage oxidation and combustion of isobutane

The aim of the study is to construct a detailed kinetic mechanism of the oxidation and combustion of isobutane, capable of describing both the high-temperature process and the multistep process at low temperatures. Isobutane was chosen because it is the first member of the homologous series of isomerized alkanes, with isooctane, a higher member of the series, exhibiting a multistage autoignition in experiment. It is shown that, under certain conditions, the autoignition of isobutane occurs in three stages, typical of normal alkanes and isooctane: cool and blue flames and a hot explosion. The proposed detailed kinetic mechanism is used to calculate the ignition delay time and laminar flame speed, with the simulation results being compared with the available experimental data. A satisfactory qualitative and quantitative agreement is observed. Autoignition during compression and an increased knock resistance of isobutane with respect to autoignition in internal combustion engines is considered. The anti-knock properties of isobutane are demonstrated to be better than those of normal butane.

Oxidation and combustion mechanisms of paraffin hydrocarbons: Transfer from C1-C7 to C8H18, C9H20, and C10H22

At the present time, detailed kinetic mechanisms (DKMs) for higher hydrocarbons, which include hundreds of particles and thousands of reactions, are proposed. These DKMs have a number of undoubted advantages because they aspire to description of a wide class of phenomena. However, their application, e.g., for modeling of turbulent combustion, is difficult due to their extreme inconvenience. In addition, they are limited to a certain degree and cannot be considered to be comprehensive. As an alternative to such DKMs, we construct no maximum mechanism in this work, but an optimum mechanism of high- and low-temperature oxidation and combustion of normal paraffin hydrocarbons. This mechanism, in accordance with the previously proposed algorithm, contains only general processes that govern the reaction rate and the formation of basic intermediate and final products. A such mechanism has the status of an nonempirical DKM because all parts, including elementary reactions, have kinetic substantiation. The mechanism itself has two features: (i) Reactions of so-called double addition of oxygen (first, to the alkyl radical, then to the isomerized form of the formed peroxide radical) are lacking because the first addition is considered to be sufficient; (ii) Isomeric compounds and their derivative substances as intermediate particles are not considered, because this means of oxidation is slower than through molecules and radicals of the normal structure. The application of the algorithm results in sufficiently compact mechanisms that are important for modeling chemical processes with the participation of paraffin hydrocarbons Cn with large n. Previously, this was done for propane, n-butane, n-pentane, n-hexane, and n-heptane; in the present article, it was done for n-octane, n-nonane, and n-decane. The major feature of all the mechanisms is the appearance of stages, viz., cold and blue flames during low-temperature spontaneous ignition. The direct comparison of the calculation and experiment results is carried out.

Modeling of Low-temperature oxidation and combustion of droplets

Key features of radiation extinguishing of spherical hot flame around a single droplet with its subsequent low-temperature oxidation and combustion under microgravity conditions—a phenomenon discovered in experiments onboard the International Space Station—have been reproduced using the mathematical model of droplet combustion and detailed kinetic mechanism of n-heptane oxidation and combustion. It has been demonstrated that experimentally observed repeated temperature flashes were blue flame flashes, and their duration was determined by the hydrogen peroxide decomposition time. In addition to this phenomenon, calculations predict the existence of new modes of low-temperature oxidation and combustion of droplets without the hot flame stage. In such modes, the basic reaction is concentrated very close to the droplet surface, and fuel vapor reacts in it only partially.

Thermal tests of a pulse-detonation high-speed burner

The steady-state temperatures of the elements of a high-speed pulse-detonation burner (HSPDB) running on a natural gas-air mixture were measured in the course of long-term tests of the burner operating in the pulse-detonation mode without forced cooling at a frequency of 2 Hz. Knowledge of the steady-state temperatures is required for the development of an energy-efficient forced cooling system for the HSPDB. The experiments have shown that the maximum steady-state temperature (∼500°C) is reached after approximately 200 s of operation at internal elements of the HSPDB, more specifically, turbulizing obstacles placed in that part of the burner duct through which the detonation wave travels periodically. The HSPDB wall in this part of the burner duct is heated to 420°C within ∼1000 s. In the part of the burner duct through which the deflagration wave travels periodically, the HSPDB walls and internal elements are heated to a steady-state temperature not exceeding 330°C. The results show that the forced cooling of the HSPDB is generally required only for those parts of the burner duct through which the detonation wave passes periodically.