# Potentials for Pressure Gain Combustion in Advanced Gas Turbine Cycles
## 1 Introduction  
Different improvements are available for conventional gas turbines. The main objectives of these improvements is to increase efficiency or specific work and thus reduce fuel consumption and emissions. [PERSON] [1] carried out an exergy analysis, which identified the conventional isobaric combustion process and the exhaust heat as two dominate loss sources of gas turbines. The conventional isobaric combustion can be substituted by pressure gain combustion (PGC), a technology that has the potential to substantially improve the thermal efficiency of gas turbines [2; 3]. The benefit of this type of combustion is associated with the reduced production of entropy during the combustion process compared to conventional constant pressure combustion. In addition, pressure gain combustion can also reduce exhaust heat losses by increasing turbine inlet pressure and thus the turbine expansion ratio, which lowers the exhaust temperature. A second option that reduces exhaust heat losses is a recuperated engine [4]. Heat is transferred from the gas turbine exhaust back into the main flow upstream the combustor. As a result, less heat has to be added into the combustor and fuel is saved. A third cycle improvement is intercooling, which increases the specific work of the gas turbine [4; 5]. The compression process is divided into multiple segments. In between each instance, heat is removed. This lowers the power requirement of the downstream compressor(s) since compressor work is directly proportional to inlet temperature. A combined intercooled and recuperated gas turbine improves specific work and thermal efficiency [6]. The combination of pressure gain combustion and advancedcycles can create synergies. This article discusses the potentials of such an engine and identifies the thermodynamic design to achieve optimum thermal efficiency. Gas turbine design studies are performed using steady-state gas turbine performance codes. However, modeling pressure gain combustion in steady-state gas turbine simulation is difficult because PGC leads to an intermittent process [7]. In addition, there are many approaches to pressure gain, so the underlying physics change from device to device. A comprehensive review was given by [PERSON] [3]. In the past, gas turbines with PGC were approximated by the Humphrey cycle, the Zeldovich, von Neumann, and Doring (ZND) cycle [2], and the Fickett-Jacobs (FJ) cycle [8]. Besides the isentropic component behavior, the assumption of idealistic isochoric (Humphrey) or detonative combustion given by the Chapman-Jouguet condition (ZND and FJ) overestimates overall performance. This is because neither model correctly captures the non-uniformity of the exhaust gas flow [7; 9]. According to the same authors, more realistically, PGC converts the usual Brayton cycle to something approaching an Atkinson cycle. Therefore, constant volume combustion is only the ideal case for realizations such as pulsed detonation combustion [10] or shockless explosion combustion [11]. When modeling PGC in steady-state gas turbine performance simulations, the principal question arises as to which representative pressure at the combustor outlet should be used to describe this inherently unsteady process. The possibilities for obtaining a representative outlet pressure are manifold: Detailed CFD simulations of single pressure gain combustion tubes were published in [12; 13]. The data provide information on the pressure profile at the outlet over time. However, these data must be condensed to a single representative value for steady-state simulation. Several authors provided guidelines as to which averaging technique is correct for the application [7; 14]. The disadvantage of this approach lies in the obtained information, which is ultimately summarized in a representative scalar value. Therefore, additional efforts are not justified and imply a level of accuracy outside the scope of steady-state gas turbine simulations. Suitable PGC models for this type of study were presented by multiple authors [15; 16; 17; 18; 19].