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

1

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

2 2

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.

3 3

Limited Datasets

• The extremes of rocket conditions limit the available

data

– Chamber pressures of 40-250 atm, Reynolds number

O(105) to O(106), 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
• f data

– How, exactly, to simplify remains a contested issue

“CECE”, upper-stage sized engine

250 Klbf pintle water test photos courtesy Frank Stoddard, TRW

Distribution A–Approved for public release; distribution unlimited.

4 4

One Simplification

• T and P prevent much beyond a few pressure and

wall thermocouple measurements

• Cold flow is still plagued by optical density and

beam steering issues

“Scaling” Time-Gated Ballistic Imaging - fs Laser Shadowgraphy - fs Laser

Hot-fire, Single Injector Hot-fire, Single Injector Hot-fire, Single Injector

Distribution A–Approved for public release; distribution unlimited.

5 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. 835-842, 1998.

Distribution A–Approved for public release; distribution unlimited.

6 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

Distribution A–Approved for public release; distribution unlimited.

7 7

Regimes

Premixed Turbulent Group Combustion (Diffusion, Laminar)

?

Premixedness

Chiu, et. al., 19th Symp (Int) on Combustion, pp. 971-980, 1982.

Distribution A–Approved for public release; distribution unlimited.

8 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

9 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

Distribution A–Approved for public release; distribution unlimited.

10 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

• rder 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.

11 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
• r 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.

12 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

• Some engines use fuel in cooling passages that

could be clogged by coke

• Kinetics of pyrolysis is critical for these problems!

T∞ P∞ M∞ x∞i dP/dx

τw(x) q”w(x) mi me(x) mv(x) δ(x) q“s(x) τs(x) q“d(x) md(x) drag Species(x) Heat Source(x) L Carbon deposits(x)

Distribution A–Approved for public release; distribution unlimited.

13 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

• nes that contribute to combustion

– Kinetics models needs and applicability of existing models

Distribution A–Approved for public release; distribution unlimited.

14 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)

Distribution A–Approved for public release; distribution unlimited.

15 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(106)

– 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.