Injection and Combustion Principles at Rocket Conditions Malissa - PowerPoint PPT Presentation

Injection and Combustion Principles at Rocket Conditions Malissa Lightfoot, Stephen Danczyk and Venke Sankaran Air Force Research Laboratory, Edwards AFB, CA Distribution A–Approved for public release; distribution unlimited. AFTC/PA clearance No. 15013, 16 January 2015 1

Main Ideas Summary • For most “high pressure” is 10-20 atm, for us it’s 100 atm and beyond! • Challenges facing the rocket community related to high pressures include – How to collect relevant data – What are relevant governing parameters (aka, regimes): can they be used to make data collection easier – Numerous outstanding questions related, for example, to • Thermodynamics states and transitions • Turbulence interactions with droplets / dense blobs • Chemical kinetics models • Here we’ll focus on boost and second-stage engines Distribution A–Approved for public release; distribution unlimited. 2 2

Limited Datasets • The extremes of rocket conditions limit the available data – Chamber pressures of 40-250 atm, Reynolds number O(10 5 ) to O(10 6 ), propellants from -400°F to 6000°F, flow rates O(10) to O(10,000) lb/s • Therefore, it’s critical to simplify to enable collection of data – How, exactly, to simplify remains a contested issue 250 Klbf pintle water test photos courtesy Frank Stoddard, TRW “CECE”, upper-stage sized engine 3 3 Distribution A–Approved for public release; distribution unlimited.

One Simplification “Scaling” • T and P prevent much beyond a few pressure and wall thermocouple measurements Time-Gated Ballistic Shadowgraphy - Imaging - fs Laser fs Laser Hot-fire, Single Injector Hot-fire, Single Injector Hot-fire, Single Injector • Cold flow is still plagued by optical density and beam steering issues Distribution A–Approved for public release; distribution unlimited. 4 4

Does Current Scaling Work? • Can’t match all important parameters, so we focus on critical (governing) ones • Able to capture specific behaviors but does not capture full range of behaviors – So, for example, does a good job for predicting initial breakup of liquid but not secondary breakup of droplets – Rely on either merging information from different experiments or use results for qualitative / comparative assessments only Mayer, et. al., J. Propulsion and Power, Vol 14, No. 5, pp. 835-842, 1998. Distribution A–Approved for public release; distribution unlimited. 5 5

Improvements • Can we do better? – Can we define an experiment that is tractable and, preferably (somewhat) device-independent? – How do we determine which parameters are critical? Which are important but not critical? Do they change with pressure (or other conditions)? • We can also extend these ideas modeling – Can we use models at already developed and validated conditions to predict high-pressure rocket behavior? – What do we need to capture in detail and what can be simplified when producing submodels? – Do we need to develop new submodels and approaches? • To answer these questions we need to understand the impact these extreme conditions have at a fundamental level Distribution A–Approved for public release; distribution unlimited. 6 6

Regimes Premixed Turbulent Group Combustion (Diffusion, Laminar) ? Premixedness Chiu, et. al., 19 th Symp (Int) on Distribution A–Approved for public release; distribution unlimited. Combustion , pp. 971-980, 1982. 7 7

Thermodynamic State • Pressure brings us to thermodynamic state of the complex, multicomponent system with a dynamic species profile – Propellants often enter at temperatures below critical point but at pressures above the critical point • Does this matter, in terms of behavior? – If propellants transition from subcritical to supercritical, how does this transition occur • Fast/slow; how localized • What effect does this have? Does it have a major impact on combustion or is it a small effect? • How much does state matter – High Re=shear dominated surface tension even at lower pressures – Advection importance over diffusion as pressure increases 8 8

Potential Supercritical Issues • Lowered surface tension and enthalpy of vaporization • Enhanced solubility of species – Multicomponent fuel and environment • Large property excursions near critical point – And large gradients in temperature and species • Applicable Equations of State and mixing rules – Globally applicable or localized? If they change, can numerical codes handle that? • Cross-diffusion can be significant (but are often neglected) • Examining the nozzle and beyond, transition from supercritical to subcritical is important – “Condensation” or like behaviors Distribution A–Approved for public release; distribution unlimited. 9 9

“Droplet” Questions • Experimental evidence suggests that “standard” model of primary atomization-secondary atomization-reaction is too linear – All three occur simultaneously and are important simultaneously • Submodels are typically developed by assuming combustion is in the dilute or very dilute region and droplets are small compared to Kolmogorov scales – Combustion may take place in zone of primary atomization – As Re increase, dense fluid structures may be on same order or even larger than Kolmogorov scales Jenny, et. al., Prog Energy Comb Sci, Vol 38, pp. 846-887, 2012. Distribution A–Approved for public release; distribution unlimited. 10 10

Kinetics • Rocket kinetics mantra: “Mixed is Burned” – Is this sufficiently true to get the fidelity we need? • We’ve had reasonable success using simplified approaches to predict performance, but higher fidelity is needed for stability predictions – High Reynolds number can lead to areas of high strain rate, so maybe not • Worse, high strain occurs near flame holding • Also, localized extinction may play a key role in instability • If we wanted to use kinetics models for a 50, or 100 or 200 atm rocket… – Is there a mechanism(s) that we should use to account for the elevated pressures? – How accurate are those models? Validated at our conditions? Distribution A–Approved for public release; distribution unlimited. 11 11

Soot, Coking and Cooling • Soot production may be of interest for performance, exhaust signatures, and measurement interference • Fuel film cooling produces coke and deposits carbon on the wall and changes heat transfer T ∞ P ∞ M ∞ x ∞ i dP/dx q“ d (x) m d (x) drag m e (x) Species(x) q“ s (x) τ s (x) m v (x) Heat τ w (x) Source(x) q” w (x) δ (x) Carbon deposits(x) L m i • Some engines use fuel in cooling passages that could be clogged by coke • Kinetics of pyrolysis is critical for these problems! Distribution A–Approved for public release; distribution unlimited. 12 12

Summary • Rockets are an environment of extremes, including high pressures over 100 atm • We lack good datasets and fundamental knowledge in operating conditions of interest – Measurements are difficult or impossible – Would be nice to scale to more friendly conditions • Governing parameters and scaling are not known in many cases, especially for partially mixed, highly turbulent conditions of rocket engines • Numerous fundamental questions remain for these high pressures including – Issues related to thermodynamic state and state changes – Assumptions of relatively small, dilute droplets being the ones that contribute to combustion – Kinetics models needs and applicability of existing models Distribution A–Approved for public release; distribution unlimited. 13 13

Proposed Path Forward • Develop a scaling framework based on fundamental considerations of how pressure changes a system – Start with changes at the molecular level and project known and expected changes to micro- and meso-scale – Combine this information with micro- and meso-scale changes (e.g., turbulent interactions) and established understanding of atomization, mixing and combustion – Framework may take the form of regime diagrams, set of “rules” or ordered list of governing parameters • Use the scaling framework and other developed insight to create prediction of macro-scale changes – Focus on specific changes that would occur and would be readily measurable across the range of pressures with AFRL’s capabilities • Refine framework based on data from companions AFRL and AFOSR efforts (and above experiments) Distribution A–Approved for public release; distribution unlimited. 14 14

EC-2 • High-pressure combustion laboratory – Burner selection and easy change-out • Laminar, turbulent, premixed and nonpremixed availability – 136 atm max back pressure – Designed for optical access • 4 windows, 3” viewing area, offset by 90° – Wide range of propellants and dopants • E.g., CH4, H2, liq HC, O2, Air, He, Ar – Flow rates in range of g/s but ability to span Re up to O(10 6 ) – Nearly operational: final plumbing of fuel, final safety approval and check out expected to be completed in April Distribution A–Approved for public release; distribution unlimited. 15 15