A review of gas turbine engine with inter-stage turbine burner

Society is going through transformations at a rate that is unprecedented in human history. One such trans- formation is the energy transition, which will affect almost every facet of our society. Gas turbine engines are state of the art machines, a backbone of modern society, and used in various applications, right from power generation to propelling aircraft and ships. This paper reviews the possibilities offered by the Inter-stage Turbine Burner (ITB) configuration for both aviation and power generation with a view on sustainability and fuel flexibility. First, the thermodynamic characteristics of a Brayton-Joule cycle with ITB is elaborated, followed by dis- cussions on the design and the off-design performance characteristics of such a gas turbine architectural variation. Finally, the viability of ITB architecture in reducing emissions and enabling “ Energy Mix ” in aviation is elaborated. The paper concludes with an outlook on the technological readiness ladder that the engineering community will have to address in the future.


Introduction
Driven by a continuous increase in the world population and a strong development in the economy, a significant increase in energy demand is anticipated in the future. On the other hand, the society is committed to mitigating its environmental impact through various transformations. The energy transition is one of such transformations, which will affect the energy sector, industrial processes, and mobility, including aviation. Increasingly, the selection of energy sources and fuels will be strongly influenced by their emissions and environmental impacts.
Gas turbines play an essential role in stationary power generation and remain a critical part of the foreseeable future for multi-fold reasons. So far, a vast effort has been made to improve the gas turbine efficiency because of operational cost and greenhouse gas emissions (mainly CO 2 ). Fig. 1 illustrates the significant increase in terrestrial gas turbine efficiency over the last decades (roughly 15% for the Simple Cycle (SC) and around 20% for the Combined Cycle (CC)). The improvements have been possible mainly due to increased Overall Pressure Ratio (OPR), maximum Turbine Inlet Temperature (TIT), and turbomachinery component efficiencies enabled by a combination of technology efforts: • Improved materials with high thermal resistance; • The development of thermal barrier coatings; • Improved cooling technique; • More advanced aerodynamic design for blades; • Improved sealing technique; The emissions from gas turbines are relatively low as compared to other power plants. A complete comparison for emissions of CO 2 , SO 2 , NO x , and Particulate Matter (PM) between various power plants is presented in Fig. 2 [2]. For instance, a Gas Turbine Combined Cycle (GTCC) reduces CO 2 emissions by more than 50%, NO x emissions by approximately 80%, and eliminates sulfur dioxide (SO 2 ) compared to the power plants burning coal. The fact that gas turbines produce low emissions is due to low carbon fuels, e.g., natural gas, improved cycle efficiency, and superior combustion technology.
Furthermore, in the aviation sector, technological advancements have managed to reduce fuel consumption (and thereby the CO 2 emissions) per passenger-km by around 75%. About 50% of this reduction is due to improved gas turbine efficiency (seen from Fig. 3). Means of enabling such improvements are mainly directed at increasing the thermal and propulsive efficiency of conventional aero engines. Next to the efficiency improvement, ambitious emission reduction goals have been set by the Advisory Council for Aeronautics Research & Innovation in Europe (ACARE) to reduce CO 2 emissions significantly by 75%, NO x emissions by 90%, and the perceived noise emission by 65% by the year of 2050 relative to the technology introduced in the year of 2000 [3].
The thermal efficiency is principally a function of OPR, TIT, component efficiencies and pressure losses in the ducts. Whereas, the propulsive efficiency is a function of Fan Pressure Ratio (FPR) and BPR. Fig. 4 shows the Thrust Specific Fuel Consumption (TSFC) of aero engines related to thermal and propulsive efficiency. The current high bypass ratio engines reach thermal efficiencies in the region of 0.45 and propulsive efficiencies in the area of 0.8. With the further increase of cycle parameters, the envisaged very high bypass ratio engines in the future are expected to reach the thermal efficiency of 0.55 and the propulsive efficiency of 0.825-0.85.
Considering the theoretical limits imposed by developing material technologies, which allow a further increase of TIT and OPR, thermal efficiency of 0.6 might be possible. Nevertheless, the NO x emissions tend to increase with the increased combustion temperature, especially when the temperature in the combustor is beyond 1800K; one would expect an exponential increase of NO x . For environmental issues, it would be   necessary to limit the TIT to a value of approximately 2000 K at take-off. Therefore, the thermal efficiency improvement via OPR and TIT is expected to approach its practical limits of 0.55 imposed by NO x emissions represented by the dashed line in Fig. 4.
The other question is how much increases in propulsive efficiency can still be possible via increasing the engine BPR. As BPR increases, the overall engine diameter increases, which increases drag and weight that eventually would offset the benefits of having higher propulsive efficiency. A possible future engine configuration could then be the open rotor configuration. Thus, the open rotor's propulsive efficiency is considered a theoretical limit for propulsive efficiency of about 0.95, as indicated by the solid blue line in Fig. 4. Overall, improving the engine efficiencies to reduce fuel consumption and the consequent CO 2 emissions is facing diminishing returns.
Gas turbines with conventional Brayton-Joule cycle appear to be reaching their performance and capacity limits. As a result, novel cycles with the perspective of better performance or larger operating capabilities are under investigation [6][7][8][9]. The concept of adding heat in the turbine expansion process, known as sequential combustion (reheat) through the turbine, is one of the cycle variations.

Thermodynamic variation of reheat through turbine expansion
The world's first industrial gas turbine plant unit with a sequential combustion system dates back to the 1940s. This application was initiated to increase the cycle efficiency when the technology readiness did not facilitate high turbine operating temperature [10]. Up to now, research on reheat cycles has been conducted widely for both industrial and aero applications. The research interests are mainly divided into two streams: Constant Temperature Turbine Burner (CTTB) and Constant Pressure Turbine Burner (CPTB).

Constant temperature turbine burner
The working principle of the CTTB follows that combustion takes place in the turbine passage simultaneously while work is being extracted to drive the compressor or other auxiliary units [11][12][13][14]. By balancing heat addition and energy extraction, the operating temperature through the turbine remains constant. Such a process is considered close to a Carnot cycle (isothermal heat addition), allowing maximum thermodynamic efficiency.
Previous research mainly focused on the effects of CTTB on aeroderived gas turbine engines. One example is to apply the CTTB in turbojet studied by Andriani et al. [11,12]. The engine scheme, shown in Fig. 5, comprises an intake, a compressor, a constant pressure combustion chamber, the turbine in which the constant temperature combustion process happens, and a nozzle. The corresponding thermodynamic cycle is depicted using a well-known temperature (T)-entropy (S) diagram in Fig. 6. The CTTB engine's performance was compared to a simple turbojet and a turbojet equipped with an afterburner. It was evident that the CTTB turbojet could produce comparable Specific Thrust (ST) as the afterburner turbojet while maintaining the fuel consumption at nearly the same level as the simple turbojet engine. A further study by Sirignano et al. [15,16] showed that the CTTB could increase the ST by 20% compared to a simple turbojet at the expense of a 10% increase in the engine thrust specific fuel consumption (TSFC). In contrast, the afterburner turbojet would increase the TSFC by 50%. In these studies, the simplification in the modeling approach of not adequately considering the possible changes in turbine cooling might overestimate the efficiency gains.   Later on, Chiu et al. [14] applied a more sophisticated approach incorporating a turbine cooling model. The analysis confirmed that replacing the afterburner with the CTTB significantly increased the turbine cooling. The authors recommended thoroughly analyzing the effects of the increased turbine cooling on the cycle efficiency. Nevertheless, using the CTTB allowed a smaller and more efficient engine for a supersonic aircraft than the traditional afterburner engines.
From the previous research, we understand that the CTTB engine qualitatively produces more specific power than the simple engine and is more efficient than the afterburning engine. The absolute performance gain remains arguable due to simplifying assumptions in the analytical approach used in most research, e.g., without explicitly considering the turbine cooling. Moreover, the understanding of the CTTB gas turbine so far has been mainly from various theoretical studies, as the application of CTTB is practically challenging. Transferring heat within an expansion process in a gas turbine is not straightforward. Nevertheless, the CTTB engine cycle itself represents the limits that the reheat through turbine expansion can reach.

Constant pressure turbine burner
Unlike the CTTB, the working process of a CPTB involves a nearly constant pressure combustion process in an additional combustor located between turbine stages, for which a CPTB is also known as "Inter-stage Turbine Burner (ITB)" [17][18][19][20][21][22][23][24][25]. For illustration purposes, Fig. 7 presents a simplified CPTB engine scheme and the station numbers   Unlike the structure of CTTB in Fig. 5, the ITB (CPTB) is explicitly separated from the expansion process in the High-pressure Turbine (HPT) and the Low-pressure Turbine (LPT) stages. The ITB is more like an afterburner in such configuration, but occurring at a relatively high pressure environment.
As the ITB is separated from the turbine competent, one could accumulate multiple ITB stages infinitely to get as close to a CTTB cycle [26]. Fig. 8 attempts to demonstrate such a process. The T-S diagram in Fig. 8 a) represents a CTTB engine cycle identical to the cycle presented in Fig. 5, whereas Fig. 8 b) is the T-S diagram of a single-stage ITB engine cycle. In a single-stage ITB engine, the heat is added in the main burner (the process 3-4) and the ITB (the process 45-46), respectively. The hot gas at the ITB exit expands further through the LPT (the process  and the nozzle (the process 7-8). As the ITB stages increase (Fig. 8 c)-d)), the expansion through the turbine happens more discretely with the continuous heat addition in between. As the ITB stages increase infinitely, the gas temperature is close to a constant value, which behaves like a constant temperature expansion. Therefore, the T-S diagram's shape in Fig. 8 d) approaches the CTTB engine cycle as in Fig. 8 a). Such a similarity implies that the CTTB engine's performance is the theoretical maximum achievable for an ITB engine. In other words, an ITB engine is a concept that lies in between a CTTB engine and a conventional engine.
Despite the possibilities of adding multiple ITB stages, most of the previous research was focused on one stage ITB [11,26,28,29], possibly because it is technically more feasible. In this paper, our focus of interest is, therefore, on the features of a single-stage ITB engine configuration, if not mentioned in the following sections.

Characteristics of gas turbines with one stage ITB
In this section, the effects of an ITB on the characteristics of gas turbines are discussed. We focus on three aspects: on-design condition, off-design condition, and as an enabler to alternative fuels for aero applications, where the mass and volume of the fuel storage are critical.

On-design condition
At the on-design condition, the impacts of an ITB on gas turbine design flexibility, cycle efficiency, NO x emissions, and turbine cooling requirements are considered.

Design flexibility
Because of the two combustion chambers, the ITB engine is flexible in its design for various purposes. Fig. 9 shows two example ideas. Fig. 9 A) is the T-S diagram of a conventional engine cycle where the heat (Q) is added in a single combustion process. Based on this simple cycle, one can make two types of transformations. The first option (as shown in Fig. 9 B)) is to maintain the exit temperatures of two combustors identical to the baseline engine [30,31]. Such a design philosophy delivers high specific power, which might be of great interest to military applications or supersonic flights [20]. The high specific power feature is especially useful for having the supersonic cruise capability in modern fighter aircraft. It is noticeable that in such an engine concept, the turbine cooling air would increase substantially, which introduces an additional penalty to the engine cycle efficiency.
Alternatively, the operating strategy presented in Fig. 9 C) produces the same amount of the specific power as the baseline engine in Fig. 9 A). This way, the exit temperatures of both combustors are significantly lower than the baseline engine. Such an engine cycle has three A) The original single combustor engine (baseline); B) the baseline engine is extended with an ITB operated at the same combustor exit temperature; C) the baseline engine is extended with an ITB optimized for the same specific thrust. advantages: 1) the lower TIT is beneficial in reducing/eliminating the turbine cooling air and the associated penalty on engine performance; 2) combustion at low pressure is thermodynamically inefficient; limiting the heat addition in ITB can minimize the efficiency penalty due to ITB. 3) the thermal NO x emissions decrease [32]; In the following sections, we will elaborate on these three features.

Cycle efficiency
In ITB, the fuel is burnt at relatively low pressure, limiting the further expansion of the hot gas; hence, an ITB engine's exhaust temperature is expected to be higher than a conventional engine. The effects of such a phenomenon on the cycle efficiency differ for industrial gas turbines and aero engines.

Industrial application.
In power generation, combined-cycle gas turbines are popular due to their high thermal efficiency, lower emissions, lower ground footprint, lower maintenance, and running cost. The working principle is that after completing the first engine cycle, the exhaust flow is still hot enough such that a second heat engine can extract energy from the exhaust. In such a process, the higher exhaust gas temperature due to ITB is not detrimental to the overall efficiency. The GT24/GT26 from ABB power generation Ltd (currently GE power) is one recent industrial gas turbine featuring an ITB configuration. Fig. 10 presents the architecture and the thermodynamic cycle of the GT24/GT26, respectively. The first combustion chamber is the environmental combustor (EV combustor), and the ITB is the sequential environmental combustor (SEV combustor). The analysis by Joos [21,22] confirmed that compared to a single combustor gas turbine, the GT24/GT26 produced high specific power.
Moreover, the exhaust temperature of GT24/GT26 is ideal for combined cycle operation, which can further improve the cycle efficiency. Eventually, the achievable cycle efficiency of GT24/GT26 is around Fig. 10. a) Architecture of GT24/GT26 gas turbine [33]; b) the thermodynamic cycle of GT24/GT26 [21].

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Progress in Aerospace Sciences 121 (2020) 100695 8 58%. The EV combustor and SEV combustor system enable a significant reduction in emissions, which will be discussed later in this paper. [27,34,35], we observe that an ITB engine can have benefits at high OPR, high BPR, and increased flight speed than a conventional engine. The modern gas turbines are mostly favoring high OPR and high BPR to attain high cycle efficiency. Accordingly, we expect that the efficiency penalty associated with using ITB will be less prominent. One example is the study by Voegler [34], where the research was focused on a preliminary concept of using ITB in a single spool geared fan system, like a turboprop. Thanks to ITB's additional power, this concept attained a substantial reduction in engine fuel consumption while guaranteeing the specific thrust. Furthermore, research in Ref. [27,36] suggested optimizing the power ratio between turbine segments separated by ITB can improve an ITB engine's performance.

Aero application. From earlier research
Furthermore, in aeronautical applications, the engine operates under a broader range of ambient and operating conditions. Depending on the specific working condition, different requirements, as summarized in Fig. 11, have to be satisfied. For instance, the thermal and mechanical design mostly happens at a hot-day take-off condition or hot and high take-off condition. In these conditions, the engine operates at its maximum operating temperature and the highest spool speed. Hence, the thermal system should fulfill the maximum cooling requirement, and the mechanical design should sustain the high stress of the disks and shafts. The aerodynamic design is at the Top of the Climb (TOC) where the gas paths are sized based on the maximum corrected mass flow rate. The energy design point is then at cruise conditions, where the cycle efficiency is of great importance.
Research-oriented design philology is mostly directed at optimizing the cruise cycle efficiency while taking the take-off and TOC requirements as constraints. The engine thrust lapse rate (decreasing thrust with increasing flight speed), especially for turbojet or low BPR turbofan engines, is smaller than that of aircraft. Therefore, to meet the thrust requirement at take-off, the engine cycle at cruise is often oversized. Hence, the engine is operating at off-design at cruise. With the modern UHBR engine, such a mismatch of the thrust lapse rate between engines and aircraft reduces; nevertheless, the thrust requirement at extreme working conditions like hot-day/high altitude take-off constrains the engine cruise performance. For an ITB engine, the extra power required at non-cruise conditions can be provided by flexibly switching on the ITB. This way, the engine cycle can be optimized for maximum cruise efficiency, reducing the overall mission fuel burn.
Research in Ref. [37] addresses the effects of ITB on engine design partly. A virtual geared turbofan (GTF, baseline) engine and a GTF ITB engine at cruise (design condition) are optimized respectively within the same design space and constraints imposed by the off-design requirements, including thrust, surge margin, and flat rating at hot-day take-off. Table 1 presents the two engine designs. It is noticeable that to meet the off-design requirements, the baseline GTF engine had to be oversized, meaning the actual fuel burn during the cruise is higher than the ITB engine for the same thrust. A further mission analysis based on these two engine cycles in Ref. [37] showed that the ITB GTF engine fuel consumption is around 4% lower at cruise, 2% lower at TOC, 5% lower at take-off, and about 7% lower at the maximum static thrust condition. That is to say, using an ITB relaxes the constraints of engine design to allow a reduction in the total block fuel consumption.

NO x emissions
The technological limits are challenging the conventional approach of reducing engine emissions. For instance, increasing the operating Fig. 11. Various engine design conditions for a commercial turbofan engine [37].

Table 1
Comparison of cycle performance between a single combustor engine and an ITB engine [37]. The operating condition is on-design cruise level. The off-design performance requirements are constraints to derive the optimized cycle. pressure and temperature to increase thermal efficiency increases NO x emissions. Fig. 12 shows that for a given combustion technology level, the NO x emissions (gram per kilo newton thrust) have increased substantially with the engine OPR. Though the data is for aero engines, the effect is expected to be similar in industrial gas turbines. The low level of NO x emissions achieved with an ITB engine cycle is a combined effect of three mechanisms. First, the dual combustors' operating flexibility allows lowering the peak flame temperatures in both combustion chambers, leading to a significant decrease in NO x emissions. This feature makes ITB a means of addressing the paradox between CO 2 reduction and NO x reduction, as shown in Fig. 12. Secondly, the vitiated air that enters the ITB has less oxygen concentration reducing the NO x formation in the ITB. Thirdly, the re-burning process in the ITB helps to dissociate some of the NO x emissions formed in the primary combustor.

ITB combustor characteristics.
An excellent example of the ITB engine's low NO x feature is the sequential combustion system of GT24/ GT26. A schematic of this gas turbine engine is given in Fig. 10. Fig. 13 is a cross-section of the combustion system comprising the main burner (EV combustor) and the ITB (SEV combustor). . For further information on the simulation setup, please refer to Ref. [39].
From the figure, we observe that the single SEV combustor produces less NO x than the single EV combustor would have for the same flame temperature. The full reheat engine's NO x emissions, a summation of the two combustors' absolute values, are also lower than the single EV combustor. The possible reason is that the SEV burner worked at a reduced O 2 level and increased H 2 O in the reactants. Research in Ref. [40] showed a similar feature.

Reduction in peak flame temperature.
The turbine inlet temperature of an ITB engine reduces significantly compared to a conventional engine architecture for a given thrust requirement. This feature is beneficial in reducing thermal NO x emissions. ITB's effects on the   13. GT24/GT26 sequential combustion system [39].

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temperature reduction inside the combustor and the consequent NO x emissions have been discussed in Refs. [24,41]. Fig. 15 shows, at hot-day (ISA+15K) Sea Level Static (SLS) condition, the variation of HPT and LPT inlet temperatures concerning the interpretation of ITB energy fraction, a ratio of the amount of heat added in ITB to the amount heat added to both the main combustion chamber and the ITB, as defined in Eqn. (1). We can conclude that increasing the ITB energy fraction decreases the HPT inlet temperature substantially from about 2300 K to 1700K to deliver the same thrust while meeting the same constraints. Meanwhile, the LPT inlet temperature is increased slightly from 1400 K to 1500K.

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Where f 1 and f 2 are fuel flow rates in the first combustor and ITB, LHV is the lower heating value (J/kg) of a given fuel. Fig. 16 demonstrates the corresponding normalized reduction in NO x emissions versus the ITB energy fraction. The NO x emissions at the exit of the first combustion chamber and the ITB are presented separately. As the ITB energy fraction increases from 0 (baseline single combustor engine) to 0.35 (roughly 35% thermal energy provided by ITB), the NO x at the exit of the first combustion chamber decreases by approximately 35%. In contrast, the total NO x reduction at the exit of ITB is slightly more. There are two reasons for this variation pattern. First, the decrease of HPT inlet temperature reduces the combustion temperature in the main burner, hence reduces the NO x emission substantially. Secondly, some of the NO x from the first combustion chamber is re-burnt in the ITB, meaning a fraction of NO x formed in the first combustor dissociates in the ITB, reducing the total NO x emissions at the exit of ITB further. Such a NO x re-burning process has been observed by Perpignan et al. in Ref. [42]. What is interesting to see from Fig. 16 is that even a small fraction of ITB energy fraction, around 0.15, can reduce NO x emissions

Low NO x combustion technique in ITB.
The ITB's operational environment provides a good opportunity to use low NO x combustion techniques like Flameless Combustion (FC) for gas turbines. The FC has been widely used in industrial furnaces [43,44]. FC's favorable conditions are low O 2 concentration and high inlet temperature (above the fuel's auto-ignition temperature), as shown in Fig. 17. A large recirculation volume is required to achieve low O 2 concentration and provide sufficient residence time. This large volume requirement is challenging in a conventional aero engine. As the ITB is downstream of the first combustion chamber, the combustion in the main burner increases the concentration of flue gases such as CO 2 and H 2 O (vapour) in the inlet stream of ITB. It also reduces the O 2 concentration, thereby creating a high temperature vitiated environment at the ITB inlet. This is ideal for the FC to take place [45].
There are very few attempts made in the literature to design an ITB combustor for an aero engine. There are several constraints in designing an ITB for an aero-engine such as minimizing pressure losses and providing enough residence time to complete combustion while minimizing the volume required. Fig. 18 shows an example of an ITB design for a multi-fuel aero-engine [47,48]. One can notice that the ITB has a toroidal shape to enhance the recirculation ratio within the combustor to facilitate flameless combustion [49].
Correspondingly, Fig. 19 a) displays the flow stream, and Fig. 19 b) shows the velocity distribution within the flameless ITB in Fig. 18. The

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chemical mechanism in such a combustor is quite different from a normal combustor. Perpignan et al. [50] showed that the prompt NO x formation mechanism of flameless combustion was the dominant pathway and determined the overall NO x emissions behavior. The thermal path has a relatively low contribution, being less critical than the NNH and N 2 O-intermediate pathways.

Turbine cooling
In modern aircraft engines, a substantial portion of the core flow is extracted for cooling the HPT blades and vanes, in the order of 20% [25]. Using such a significant amount of cooling penalizes the HPT efficiency substantially [25]. Furthermore, as the engine core becomes smaller in the future UHBR engine, the engine off-design performance, such as the compressor surge margin, would become more sensitive to the amount of bleed air.
Using an ITB reduces the turbine-cooling requirement, provided if the main-burner and the ITB exit temperatures are managed well. Such an example is presented in Fig. 15. Given the same engine design space and constraints, the HPT inlet temperature reduces substantially as the ITB energy fraction increases (at the expense of a marginal increase in the LPT inlet temperature). Correspondingly, the HPT cooling requirement reduces by half while the LPT cooling is not required until 15% of the ITB energy fraction, as indicated in Fig. 20 A) and B). This turbine cooling reduction can compensate for the penalty of using an ITB on engine efficiency.

Off-design performance
The application of ITB improves the performance of a gas turbine engine under off-design conditions. The actual effects differ between the industrial and aeronautical applications.

Part-load performance of industrial gas turbines
The combined gas turbine cycle plays a vital role in the energy structure because of its high design efficiency, fuel flexibility, operational flexibility, low emissions, and low ground footprint. However, the performance of such a system drops significantly when the load decreases. Therefore, optimizing the gas turbine's performance under partload conditions is an important issue, especially with the incorporation of intermittent renewable energy sources like wind energy and solar energy. Using ITB could be a practical approach to improve part-load efficiency. An example of the operational strategy for a reheat cycle in GT24/GT26 can be seen in Fig. 21 [39]. From the load range of 0-12%, the operating temperature of the EV-combustor and the SEV-combustor   21. schematic of the operating concept of GT24/GT26 with load variation adjusted by ITB fuel flow rate [39]; EV-combustor is the primary combustor, and the SEV combustor is the ITB.
increase. Afterward, the operating temperature of the EV-burner decreases negligibly until the 100% based load is reached. This way, the efficiency of the bottoming Rankine cycle is not deteriorating as much as in a configuration with a conventional gas turbine.
In contrast, the SEV burner's operating temperature increases further sharply until up to 40% of the baseload and remains almost constant. When beyond 80% of the baseload, the SEV-burner works at a constant temperature equal to the maximum temperature of the first burner. This operating strategy allows the maximum specific power output. As a result, the gas turbine (GT)'s exhaust temperature first increases (up to 40% based load) and then is nearly constant (up to 80% of the baseload), followed by a slight reduction until 100% based load is reached.
The associated benefits are: • The variation of the ITB operations allows avoiding the start-stop cycles due to the varying loads, hence reducing the thermal stress; • The nearly constant temperature of the main burner maintains the cycle efficiency of the bottoming cycle;

Relaxation of design constraints
As mentioned in the earlier section, using ITB allows the relaxation of some design constraints imposed by the off-design requirements. The   Fig. 22. Engine flat rating at SLS condition for various ambient temperatures [37]. The increase of ITB energy fraction indicates the increase in the fuel flow rate in ITB. Fig. 23. A possible future energy scenario for long-range commercial aircraft [56].

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engine can work at its optimum condition as much as possible to minimize the mission fuel burn.
An example of ITB's effects on engine thrust rating (a process of maintaining the thrust output uniform until the ambient conditions do not allow the engine to operate) is illustrated in Fig. 22 [37]. The baseline engine and the ITB engine are designed at cruise conditions (shown in Table 1), whereas the engine thrust rating is studied at the SLS condition. Again, the ITB energy fraction, defined in Eqn. (1), is used to change the power split ratio of ITB over the total power output: ITB energy fraction of 0 is a single combustor working mode. It is noticeable that, for the same thrust rating, the achievable ambient temperature is much lower for the baseline engine with one combustion chamber than the ITB engine. A 30% power split by ITB enables the flat rating temperature of ISA+15K without affecting the engine design performance. The baseline engine has to be redesigned if the same flat rating temperature were to be achieved.

Fuel flexibility-enabler of the energy transition in aviation
The current technological limits are preventing the aviation industry from reducing its environmental footprint. Furthermore, corresponding to the increase of air traffic, the jet-fuel demand is expected to increase by about 2-3% annually despite improved aircraft fuel efficiency [51]. On the other hand, the oil reserves are depleting. The discrepancy in supply and demand will lead to a significant increase in fuel costs. Fig. 23 provides insight into future energy scenarios for short medium to long-range commercial aircraft. "Drop-in" fuels, e.g., biofuels or synthetic aviation fuels, are a part of the solutions in the short term, as they can be implemented directly into the existing airplanes without incurring significant changes to the aircraft nor the airport infrastructure. Some commercial flights have been successfully operated with biofuels [52,53]. To achieve aviation's carbon-neutral growth, alternative fuels, like Liquid Natural Gas (LNG) and Liquid Hydrogen (LH2), are the most attractive [47].
Note that Fig. 23 does not include electricity power as an energy option. With the limited power density of batteries, the fully electric flight seems only feasible for small regional aircraft. The hybrid-electric propulsion system might be an option for short and medium-range missions [54,55], with marginal fuel savings (1.8% for a mission range of 1000 km and the projected 2030 technology level [56]). Furthermore, it will not be feasible to fly a long-range mission with any form of an electric propulsion system.
As hydrogen-rich fuels, LH2 and LNG are certainly favorable from a long-term perspective. One of the challenges to use the LNG and LH2 in a conventional aircraft is the large volume required to carry the energy needed. The LNG and LH2 have to be stored in cylindrical tanks with a well-insulated system to prevent the fuel from leaking and boiling off. Therefore, the required volume to keep cryogenic fuels, especially LH2, is more than four times that of kerosene. This volume increase would cause additional aerodynamic drag, increasing energy consumption [57][58][59]. The Cryoplane project [58,[60][61][62] is a well-known example of implementing LH2 for a conventional tube wing aircraft configuration. Fig. 24 demonstrates the proposed design, where the LH2 fuel tanks are located on top or at the rear of the fuselage. The analysis reported that replacing the kerosene with LH2 increases the mission energy consumption by approximately 10% due to the increased aircraft wetted area caused by LH2 storage [63]. From the previous research, we can conclude that retrofitting the existing aircraft for pure LH2 is not feasible.
One promising solution to introduce LNG/LH2 for aviation is to use them in the form of the "energy mix" concept, namely, burning two types of fuels simultaneously. An example of such a multi-fuel aircraft    [66][67][68][69]. MFBWB is one of these radical concepts. The inherent feature of the BWB aircraft offers a large amount of space, which makes it competitive in transporting payload. In the MFBWB design, the central fuselage is reserved as the passenger compartment. Space at both sides can perfectly accommodate the cryogenic fuel tanks. The wings' internal volume is still used to store normal liquid fuels (Kerosene/Biofuels).
Such an aircraft requires a different propulsion system as two types of fuels need to be burnt simultaneously. Accordingly, a multi-fuel hybrid engine that originated from the ITB turbofan concept was proposed. A simplified engine scheme is shown in Fig. 26. The hybrid engine has many unique features, like contra-rotating fans [70][71][72], a cryogenic bleed air cooling system to cool the bleed air, and a dual combustion chamber system to reduce the emissions. Details on this engine concept are discussed in Ref. [48]. On top of all features, the energy mix concept is highlighted in this section. The duel combustion chambers of the ITB engine burn two types of fuels (cryogenic fuels and biofuels) simultaneously such that the thrust power is provided in a hybrid manner. This way, the volume required to store cryogenic fuels reduces, which provides a possibility for cryogenic fuels in aviation.
The emissions (mainly NO x , CO 2, and H 2 O) of the MFBWB are compared to the current state-of-the-art aircraft B787 for a given mission, as seen in Fig. 27 [48]. The MFBWB using LNG/kerosene or LH2/kerosene can reduce both the CO 2 and NO x emissions substantially.

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While the example mentioned above is dedicated to a long-range aircraft, the multi-fuel philosophy in aviation is not necessarily restricted to long-range aircraft. In Ref. [73], Jebbawi has initiated the study on retrofitting Airbus A320 to work on LNG. The design of the aircraft fuselage and fuel tanks are presented in Fig. 28. This study shows that an A320 type single-aisle aircraft's cargo compartment can be modified to store cryogenic fuel tanks. While the study is on LNG, it provides an idea for using LH2 in single-aisle aircraft. That is to keep some of the mission energy requirement (around 25-30%) from LH2 in the aircraft cargo compartment. The rest of the mission energy is provided by kerosene in a conventional way. An ITB engine would facilitate such a multi-fuel design. The ITB engine enables using dual fuels and can change engine emissions for various mission segments offered with a proper power management plan. The emissions of such configurations have yet to be investigated in detail.

Opportunities
• As the constraints imposed by off-design conditions restrict the conventional turbofan engines' design performance, using an ITB provides an opportunity to decouple the design and off-design conditions and allows the engine to work at its optimal cruise condition as much as possible, simultaneously meeting the off-design performance requirements. • As we will move towards a hydrogen-based society, fuel flexibility in aeronautical applications will become essential. In this context, an ITB engine allows future aviation to adapt itself to multiple energy sources, which is far superior compared to conventional aero-engine architecture. • Even though it is known that NO x re-burning occurs in the ITB, the reaction mechanisms for re-burning have to be validated and tested. Subsequently, the effect of NO x re-burning in the ITB on the total NO x emission of the gas turbine should be investigated further.

Challenges
• Low emission ITB design: The ITB is positioned downstream of the HPT, where the inlet operating temperature and the flow velocity are much higher than the inlet of the main burner. If the same combustion volume were considered, the residence time available for the flow in the ITB is low, not allowing the combustion to complete. This might increase CO and UHC emissions. Therefore, designing an ITB with low emissions would be one challenge for future research. • Incorporation of an ITB increases the engine complexity substantially with multiple fuel lines and fuel controls. • The introduction of an ITB to a normal turbofan engine will increase the engine length and weight. This negates some of the fuel savings advantages that ITB might provide. Therefore designing an ITB for an aero-engine is an extremely challenging task. Research in Ref. [74] has focused on the integration of an ITB to a jet turbine engine. The ultra-compact combustor studied in this research shows a large potential in reducing the additional weight of the ITB. The research in the AHEAD project also confirms that using ITB appropriately can minimize the resulting weight penalty. Since the weight and volume constraints for industrial gas turbines are not that stringent, incorporating ITB in land-based gas turbines is beneficial.

Conclusions
In this paper, a review of the gas turbine with an inter-stage turbine burner has been presented. The potentials of using ITB for future green aviation are summarized from three aspects: • Using an ITB relaxes some of the constraints imposed on the gas turbine design by the off-design requirements. Accordingly, the engine can work optimally at all operating conditions to reduce the total mission fuel consumption. • Using an ITB can reduce the turbine inlet temperature substantially such that the turbine cooling requirements and NO x emissions are significantly lower. • The prompt NO x formation mechanism is the dominant pathway in the ITB using flameless combustion and determines the overall NO x emissions behavior. The thermal pathway has a relatively low contribution. • The ITB engine configuration provides a perfect architecture to use non-drop in fuels such as liquefied hydrogen for future civil aviation to achieve a substantial reduction in CO 2 emissions without contradicting the reduction of NO x emissions.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence