Danyal Mohaddes Khorassani (Mechanical Engineering) | Institute for Flight Propulsion (Lehrstuhl für Flugantriebe) Technical University of Munich

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Prior to Departure

The internship was first suggested by a visiting student researcher from the Technical University of Munich (TUM) in my department (Mechanical Engineering) at Stanford. His department (Institute for Flight Propulsion, LFA) was to host a summer program on numerical simulation of combustion as applied to rocket propulsion, and given my background and research in combustion modeling, it was appropriate for me to participate. We devised a plan whereby I would be present at the TUM for the duration of the summer program, as well as some time before and after such that we could collaborate further. A research proposal was developed on this basis in conjunction with both the supervisors at Stanford and the TUM.

Technical Overview

I worked on a number of projects during my time at the TUM, and the following is a brief survey of these. Some of these projects were already part of my research at Stanford and I continued them at the TUM, receiving insightful inputs from the colleagues there, and others were new efforts stemming from the expertise available at the TUM.

The combustion of liquid fuels is an important practical consideration in all propulsion systems, be it in internal combustion engines, gas turbine engines, or rocket engines. My experience prior to my time at the TUM was in the former two domains, and I was therefore working on two related projects. The first project related to the ignition of a liquid fuel film after having impinged on a hot metal surface. This is a phenomenon that is common to diesel engines in coldstart situations, and is something that is designed against in the context of fire hazards for liquid fuel storage. However, I learned at the TUM that indeed this mode of combustion is commonly used in kerosene-oxygen fueled rocket engines, such as the Merlin engine used by SpaceX. Indeed, a student had recently completed their thesis at the institute on this very subject, and I learned a great deal about my own project from their work.

The second project related to the combustion of a liquid spray injected into a highly turbulent environment. Here, the modeling of the spray-gas dynamics is of crucial importance. The figure below is a visualization I generated while at the TUM, and shows a view of the numerical simulation I focused on. Through my colleagues at TUM, I was introduced to the wealth of experimental and numerical literature that exists for the same problem but as applied to rocket engines. In the rocket context, it is a spray of liquid oxygen which is injected into the combustion chamber, whereas in the gas turbine context it is a spray of liquid jet fuel. This comparison opened my eyes to a large realm of possibilities in terms of future research avenues, since many of the skills and methods I am working to develop are applicable in this exciting context as well.

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Model gas turbine combustion chamber for spray-flame analysis

A further project was one which I began at the TUM in collaboration with a colleague there, which focused on high-fidelity modeling of a liquid oxygen injector inside of a rocket engine. My host institute had expertise in experimental analysis of rocket engines, and had generated test data for an engine running on liquid oxygen and gaseous methane. Given the availability of this test data at the institute, we decided it would be appropriate and interesting to attempt to model one of the most complex phenomena which occurs in that type of engine, namely the breakup of the liquid oxygen jet, and its effect on the combustion which occurs thereafter. The correct modeling of the oxygen jet has consequences for mixing, combustion, and ultimately the thrust, efficiency, and emissions of the rocket engine.

During my stay I had the opportunity to participate in the TUM summer program for combustion modeling (formally the SFB TRR40 Summer Program) by attending lectures, interacting with researchers in my field from across Europe and around the world, and by participating in related social events. I found these experiences very rewarding, both in terms of contacts gained and in terms of technical expertise which was shared through the various participants and collaborators.

A final technical point worth mentioning is that I had the opportunity to witness some experiments during my stay. As noted above, my host institute had expertise in rocket engine experimentation, and I was lucky enough that an experimental campaign began during my stay. I witnessed a small-scale rocket engine test fired using liquid oxygen, gaseous methane, and which used film cooling on the side walls. Particularly interesting for me was that the experiment contained many of the physical aspects of combustion and sprays which pertain directly to my Mohaddes 3 doctoral research. I was thrilled to have the opportunity to witness those tests, as generally it is exceedingly difficult to gain access to view such experimental facilities.

Free Time and Leisure

I did not have an excessively large amount of time for leisurely activities during my stay. Indeed, I worked just as long and hard at the TUM as I do at Stanford, if not more due to the lack of distractions. However, I did manage to do some sight-seeing in and around the Munich area, and to make some weekend trips. My apartment was in the central district of Maxvorstadt, which gave me quite a large variety of activities in which to participate within walking distance. Apart from museums, parks, restaurants, and city-life exploration in Munich, I visited the following places: Karlsruhe (a nice, calm town), some little towns in the Pfalz region (for a wine festival), Heidelberg (for the scenery and the castle), Dachau (to visit the concentration camp), Berlin (an amazing city in every respect, I hope they’re hiring when I graduate) and Vienna (never before had I seen so much gold used in architecture).

Final Remarks

To say that this was a life-changing experience would be an understatement. I already spoke German to a C1 level and had had a few German friends prior to this summer internship. However, I gained a newfound appreciation of Germany, its culture, cuisine, lifestyle, and some of its endearing quirks. Some things that have stayed with me: Germans work hard, work diligently, and expect everyone else to do the same. Germans know their history, stains and all, and their museums often offer free admission to tell you about it. Germans (and I’m told in particular, Bavarians) take work-life balance very seriously, and are passionate about their hobbies. We say “Hi, how are you?” in North America, but we rarely mean it. Germans don’t take asking „Wie geht’s dir?“ as lightly as we do. There are subtle differences in culture that affect communication, personal relations, by extension then business and technical relations, and ultimately how we feel about one another. So much can be lost in translation when a depth of cultural understanding is not present, and as someone who is very seriously considering moving to Germany after graduation, the opportunity to have immersed myself in that culture for two months is invaluable.

I am eternally grateful to the persons and entities at Stanford, in the wider Stanford community, and indeed anywhere who participated in making it possible for me to have this experience. I hope that you continue this program long into the future.

„Die Luft der Freiheit weht“