Supersonic Combustion

The constrained and dynamic environment present in supersonic combustors poses a number of challenges such as cold start/ignition, mixing, flame stabilization, and complete combustion. In the past several decades, numerous configurations employing both passive and active methods have been explored to circumvent challenges in both mixing and combustion present in compressible environments with short residence times, low-pressures, and low-temperatures. These methods are designed to assist in distributing fuel to the core flow, increase the flow residence time, or provide a region of high energy to the flow to help initiate chemical reactions.

Our current efforts are focused on studying the combustion enhancement effects of staging a pulsed detonation (PD) and transverse jet in supersonic crossflow. The test model is shown schematically in the figure below. It consists of a flat plate with a sharp leading edge. The primary jet consists of hydrogen expanded transverse to the crossflow and to sonic conditions through a 2 mm orifice. The PD consists of a 0.75 m long pipe, which is closed at one end and open at the end which is staged downstream of the primary jet. The detonation is generated by filling the PD with a stoichiometric flow rate of hydrogen and oxygen through two separate automotive-style fuel injectors. A spark plug is then used to ignite the mixture near the closed end of the tube, with approximately 100 mJ of energy. Two fast-response piezoelectric pressure sensors mounted near the open end of the PD are used to measure the detonation speed as well as the pressure time-history during the blowdown process.

PDSchematic_cropped

 

The PD provides a high temperature and radical-rich plume of gas to the wake of the jet. The high momentum flux ratio PD exhaust alters the fluid dynamic structure of the jet wake by dispersing and spreading the fuel deeper into the core flow. This interaction has also been observed to assist in initiating chemical reactions and promoting an earlier onset of heat release.

The impact of the PD on the flow field can be seen in the figure below. The first and second columns are OH PLIF and OH* images, respectively. With no PD present, the reacting shear layer is discontinuous in its presence of OH. Furthermore, the locations of heat release are confined to small regions near the jet exit and in the wake near the floor. With the presence of the PD issuing into the wake of the primary jet, the reacting shear layer is continuous in the presence of OH. Moreover, the extent of heat release within the field of view is significantly increased and spread throughout. It is also interesting to note the structure of the PD exhaust, which resembles the structure of a highly under expanded jet in crossflow. In particular, the region upstream of the barrel shock boundary and the region downstream of the Mach disk are identified by intense OH* emission.

caseC2

In order to quantify the extent of heat release enhancement caused by the interaction of the PD with the primary jet, the OH* images of several test cases were integrated over a 250 μs period to determine the total heat release during one PD blowdown cycle. The test cases varied in the momentum flux ratio of the primary jet , which was varied between values of 0, 0.5, 2.7, and 5.0. The rate of heat release was computed by performing a cumulative sum (Cq) along the direction of the flow for each test case. The results are shown in the figure below. The dash-dot lines show the cumulative heat release for cases without a staged PD. The solid lines are of cases with a staged PD from which the cumulative sum from the case of an isolated PD (blue-dashed curve) exhausting into the crossflow has been subtracted. Therefore, comparison of corresponding curves of the same color quantifies the amount of combustion enhancement which is due to the presence of the staged PD. Thus, the cumulative heat release is shown to increase by as much as 100% with the staged PD configuration for all cases.

Cq

Further work is being carried out to better understand the three-dimensional, unsteady reacting structure of the interacting primary jet and PD plumes. Furthermore, we are investigating the physical mechanisms responsible for the observed combustion enhancement.