THE FIRST LAW of thermodynamics can be mathematically expressed in a variety of ways. All these expressions, however, are easily viewed as rearrangements of the statement that energy can neither be created nor destroyed but only converted from one form to another. In contrast, there is no single universally agreed upon statement of the second law of thermodynamics. Kline [1] indicates that many seemingly different statements have been accepted as the second law, all of which, however, can be shown to be equivalent after a careful and sometimes subtle application of logic. This multiplicity of apparently disparate statements can lead to confusion in understanding the second law. In this chapter, we examine several statements of the second law and discuss the consequences of each.

IN THIS CHAPTER, we apply the basic conservation principles and other key concepts to analyze a number of important devices. Here we investigate typical components of more complex systems; these components include nozzles, diffusers, throttles, pumps, compressors, fans, turbines, and heat exchangers. In Chapter 9, we will combine these simple devices in more complex systems, which include steam power plants, jet engines, other power and propulsion cycles, heat pumps, refrigeration cycles, and air conditioning and humidification systems.

IN THIS CHAPTER, we apply the fundamental principle of energy conservation to both closed and open thermodynamic systems. In our analyses of closed (fixed-mass) systems, we will express energy conservation for incremental and finite changes in state and at an instant. In dealing with open systems, we again follow a hierarchical development by starting with simple, steady-state, steady-flow cases and then adding detail and complexity to arrive at more general statements of energy conservation. We apply the energy conservation principle to analyses of steady-flow devices, linking our theoretical developments to practical applications.illustrates the key role that this chapter plays in our study.

Modern kinetic mechanisms are intricate and can comprise tens of substances and hundreds of reactions [46, 95, 96]. For example, paper [97] deals with low-temperature decomposition of hydrocarbons to analyze the combustion mechanism totaling 340 substances and 3400 reactions. The authors of [98] exploited the combustion mechanism including 120 substances and 721 reactions for simulation of n-decane ignition. In the calculation of the oxidation of hydrocarbons described in [99], a mechanism comprising 71 substances and 417 reactions was analyzed.

In numerical analyses aimed at developing high-efficiency combustion chambers for various engines and thermal power systems, it is necessary to have an adequate understanding of hydrodynamic and chemical processes related to flowing, mixing, and combustion of two-phase fuels and oxidizers. The occurrence of such processes is described by the availability of zones differing in type, space, and time scale of these processes in the working volume.

The model of the combustion in the flame front is commonly used for the simulation of operating parameters and emission characteristics of combustion chambers of different combustion systems as one of the main simulation fragments in models of premixed flames. The typical scheme of combustion in the flame front was described in the Section 1.1. Combustion in the flame front predetermines to a considerable extent the further afterburning processes and parameters of reacting flows in the combustion unit and combustion products emission. In accordance with the generally accepted definition, the flame front is identified as a thin layer separating an unburned fresh mixture of the reactants from the combustion products wherein maximum gradients of concentrations of the reactants and reaction products are observed (Figure 4.1). Once the fresh mixture is ignited, a resulting premixed flame propagates in the x direction, consuming the unburned mixture. The chemical interaction in the flame front under conditions of intensive self-acceleration of the processes caused by the transfer of both heat and active catalyzing centers from the products of reactions to the unburned fresh mixture.

Combustion processes (that is, conversion of chemical energy of propellant components into thermal energy of combustion products) are typical for various engineering systems. Working volumes wherein these processes can occur may be represented by combustion chambers of liquid-propellant rocket engines (LPRE), solid-propellant rocket engines (SPRE), air-breathing engines (ABE) steam-gas generators, magnetohydrodynamic generators (MHD generators), boiler furnaces of thermal electric power stations, and cylinders of internal combustion engines (ICEs) [1]. Besides, further conversion of combustion products with chemical conversions can proceed also in aircraft and rocket engine nozzles, ICE exhaust systems, LPRE gas ducts, etc.

The model of the combustion in the flame front is commonly used for the simulation of operating parameters and emission characteristics of combustion chambers of different combustion systems as one of the main simulation fragments in models of premixed flames. The typical scheme of combustion in the flame front was described in the Section 1.1. Combustion in the flame front predetermines to a considerable extent the further afterburning processes and parameters of reacting flows in the combustion unit and combustion products emission. In accordance with the generally accepted definition, the flame front is identified as a thin layer separating an unburned fresh mixture of the reactants from the combustion products wherein maximum gradients of concentrations of the reactants and reaction products are observed (Figure 4.1). Once the fresh mixture is ignited, a resulting premixed flame propagates in the x direction, consuming the unburned mixture. The chemical interaction in the flame front under conditions of intensive self-acceleration of the processes caused by the transfer of both heat and active catalyzing centers from the products of reactions to the unburned fresh mixture.

Evaporation and combustion of dispersed propellants in a high-temperature reacting flow are typical for the most diverse propulsion and power generation systems such as internal combustion engines (ICE), combustors of air breathing engines (ABE), combustion chambers of liquid-propellant rocket engines (LPRE), liquid gas generators (LGG), and steam-gas generators (SGG), combustion chambers of furnaces, etc. Liquid-propellant atomization, spray formation, and droplet evaporation processes are seen to bear strong influence on the efficiency of the combustion process and, hence, the operating and ecological parameters of these combustion systems.

Feeding high-pressure gas into the gas space of the tanks filled with liquids and even solids aims to maintain this gas space at a preselected pressure history bounded by tanks’ structural requirements or required propellant supply pressures, to prevent propellant pump cavitation, to avoid uncontrolled chemical reactions in gas space, etc. The pressurization process and corresponding pressurization systems are used in diverse technical devices. These include apparatus for chemical technology, oil and ore tankers, aircraft fuel tanks, and propellant tanks of LPREs. Processes related to high-pressure and often high-temperature gas feeding are extremely diversified because of the complex flow patterns of gas in the free space of the tanks, possible heat exchange with structural elements and the propellant, mass exchange caused by the evaporation of liquids, the chemical reactions in gas, and liquid phases.