Constant volume combustion (CVC) in gas turbines is a promising way to achieve a step change in the efficiency of such systems. The most widely investigated technique to implement CVC in gas turbine systems is pulsed detonation combustion (PDC). Unfortunately, the PDC is associated with several disadvantages, such as sharp pressure transitions, entropy generation due to shock waves, and exergy losses due to kinetic energy. This work proposes a new way to implement CVC in a gas turbine combustion system: shockless explosion combustion (SEC). This technique utilizes acoustic waves inside the combustor to fill and purge the combustion tube. The combustion itself is controlled via the ignition delay time of the fuel-air mixture. By adjusting the ignition delay in a way such that the entire fuel-air volume undergoes homogeneous auto-ignition, no shock waves occur. Accordingly, the losses associated with a detonation wave are not present in the proposed system. Instead, a smooth pressure rise is created due to the heat release of the homogeneous combustion. The current paper explains the SEC process in detail, and presents the identified challenges. Solutions to these challenges and the numerical and experimental approach are presented subsequently alongside with first preliminary results of the numerical studies.

Since lean premixed combustion allows for fuel-efficiency and low emissions, it is nowadays state of the art in stationary gas turbines. In the long term, it is also a promising approach for aero engines, when safety issues like lean blowout (LBO) and flame flashback in the premixer can be overcome. While for the use of hydrogen the LBO limits are extended, the flashback propensity is increased. Thus, axial air injection is applied in order to eliminate flashback in a swirl-stabilized combustor burning premixed hydrogen. Axial injection constitutes a nonswirling jet on the central axis of the radial swirl generator which influences the vortex breakdown (VB) position. In the present work, changes in the flow field and their impact on flashback limits of a model combustor are evaluated. First, a parametric study is conducted under isothermal test conditions in a water tunnel employing particle image velocimetry (PIV). The varied parameters are the amount of axially injected air and swirl number. Subsequently, flashback safety is evaluated in the presence of axial air injection in an atmospheric combustor test rig and a stability map is recorded. The flame structure is measured using high-speed OH* chemiluminescence imaging. Simultaneous high-speed PIV measurements of the reacting flow provide insight in the time-resolved reacting flow field and indicate the flame location by evaluating the Mie scattering of the raw PIV images by means of the qualitative light sheet (QLS) technique. The isothermal tests identify the potential of axial air injection to overcome the axial velocity deficits at the nozzle outlet, which is considered crucial in order to provide flashback safety. This effect of axial air injection is shown to prevail in the presence of a flame. Generally, flashback safety is shown to benefit from an elevated amount of axial air injection and a lower swirl number. Note that the latter also leads to increased NOx emissions, while axial air injection does not. Additionally, fuel momentum is indicated to positively influence flashback resistance, although based on a different mechanism, an explanation of which is suggested. In summary, flashback-proof operation of the burner with a high amount of axial air injection is achieved on the whole operating range of the test rig at inlet temperatures of 620 K and up to stoichiometric conditions while maintaining single digit NOx emissions below a flame temperature of 2000 K.

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Abstract. This experimental load control study presents results of an active trailing edge flap feedforward controller for wind turbine applications. The controller input is derived from pressure based lift estimation methods that rely either on a quasi-steady method, based on a three-hole probe, or on an unsteady method that is based on three selected surface pressure ports. Furthermore, a standard feedback controller, based on force balance measurements, is compared to the feedforward control. A Clark-Y airfoil is employed for the wing that is equipped with a trailing edge flap of x/c = 30 % chordwise extension. Inflow disturbances are created by a two-dimensional active grid. The Reynolds number is Re = 290,000 and reduced frequencies of k = 0.07 up to k = 0.32 are analyzed. Within the first part of the paper, the lift estimation methods are compared. The surface pressure based method shows generally more accurate results whereas the three-hole probe estimate overpredicts the lift amplitudes with increasing frequencies. Nonetheless, employing the latter as input to the feedforward controller is more promising as a beneficial phase lead is introduced by this method. A successful load alleviation was achieved up to reduced frequencies of k = 0.192.

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In the context of lean premixed combustion, the prevention of upstream flame propagation in the premixing zone, referred to as flashback (FB), is a crucial challenge related to the application of hydrogen as a fuel for gas turbines. The location of flame anchoring and its impact on FB tendencies in a technically premixed, swirl-stabilized hydrogen burner are investigated experimentally at atmospheric pressure conditions using planar laser-induced fluorescence of hydroxyl radicals (OH-PLIF). The inlet conditions are systematically varied with respect to equivalence ratio (ϕ=0.2−1.0), bulk air velocity u0 = 30–90 m/s, and burner preheat temperature ranging from 300 K to 700 K. The burner is mounted in an atmospheric combustion test rig, firing at a power of up to 220 kW into a 105 mm diameter quartz cylinder, which provides optical access to the flame region. The experiments were performed using an in-house burner design that previously proved to be highly resistant against FB occurrence by applying the axial air injection strategy. Axial air injection constitutes a nonswirling air jet on the central axis of the radial swirl generator. While a high rate of axial air injection yields excellent FB resistance, reduced rates of air injection are utilized to trigger FB, which allowed to investigate the near FB flame behavior. Results show that both, fuel momentum of hydrogen and axial air injection, alter the isothermal flow field as they cause a downstream shift of vortex breakdown and, thus, the axial flame front location. Such a shift is proven beneficial for FB resistance from the recorded FB limits. This effect was quantified by applying an edge detection algorithm to the OH-PLIF images, in order to extract the location of maximum flame front probability xF. By these means, it was revealed that for hydrogen xF is shifted downstream with increasing equivalence ratio due to the added momentum of the fuel flow, superseding any parallel augmentation in the turbulent flame speed. The parameter xF is identified to be governed by J, the momentum ratio between fuel and air flow, over a wide range of inlet conditions. These results contribute to the understanding of the sensitivity of FB to changes in the flow field, stemming from geometry changes or specific fuel properties.