Injection and Combustion Principles at Rocket Conditions Malissa Lightfoot, Stephen Danczyk and Venke Sankaran

Transcription

1 Injection and Combustion Principles at Rocket Conditions Malissa Lightfoot, Stephen Danczyk and Venke Sankaran Air Force Research Laboratory, Edwards AFB, CA AFTC/PA clearance No , 16 January

2 Main Ideas Summary For most high pressure is 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 2

3 Limited Datasets The extremes of rocket conditions limit the available data Chamber pressures of 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

4 One Simplification Scaling T and P prevent much beyond a few pressure and wall thermocouple measurements Time-Gated Ballistic Imaging - fs Laser Shadowgraphy - fs Laser Cold flow is still plagued by optical density and beam steering issues Hot-fire, Single Injector 4

5 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 ,

6 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 6

7 Regimes Premixed Turbulent Group Combustion (Diffusion, Laminar)? Premixedness Chiu, et. al., 19 th Symp (Int) on Combustion, pp ,

8 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

9 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 9

10 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 ,

11 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? 11

12 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 m i m e (x) T P M x i dp/dx q d (x) m d (x) q w (x) τ w (x) drag q s (x) τ s (x) L δ(x) m v (x) Carbon deposits(x) Species(x) Heat Source(x) Some engines use fuel in cooling passages that could be clogged by coke Kinetics of pyrolysis is critical for these problems! 12

13 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 13

14 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) 14

15 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 15