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Experimental Investigation ofCombustion Processes in Advanced Fuels

for Hybrid Rocket Propulsion

Politecnico di MilanoDipartimento di Ingegneria Aerospaziale

Laura Merotto – Matteo Boiocchi

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Outline

1. The 2D 2D slabslab hybridhybrid test test facilityfacility is described, and the methods used for averageoxidizer mass flux and average regression rate measurement are presented

2. Some examples of experimental data obtained with the slab facility are presented; a discussiondiscussion ofof the the resultsresults is given, in order to provide the tools foran autonomous analysis of the experimental data

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Objectives of the lab lessons

The essential objectivesobjectives ofof thesethese lessonslessons include:

� Learning how the test facility works and how experimental data are collected and post-processed

� Analyzing the data collected on the basis of the main physicalphenomena involved in hybrid propulsion in order to propose a suitable interpretation

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2D Slab Hybrid Test Facility

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Low Pressure Slab Burner Facility

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Pressure transducer

Interchangeable sample holdersfor different geometricalconfigurations

Nitrogen inlet system

Oxygen inlet system

2D slab hybrid test facility: description (1/2)

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� The ignition system is based on a Ni-Cr wire heated through an electrical circuit

� The wire ignites a small (about 1 g) propellant charge placed at the fuel sample head-end, and the propellant combustion ignites the solid fuel

2D slab hybrid test facility: description (2/2)

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2D slab hybrid test facility: versatility

The 2D slab burner used at SPLab allows different firing test to be performed:

� Tests on differentdifferent fuelfuel formulationsformulations, in order to assess the fuel ingredients effects on the overall system performance

� Tests on differentdifferent sample sample sizesize and and geometrygeometry, in order to investigate the scale factor and the effect of sample shape and different burning area

� High-speed video-recording, for flameflame structure investigationstructure investigation

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2D Slab Hybrid Test Facility:

Data Collection and Post-Processing

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The regression rate (rf) is linked to the fuel mass flow rate during combustion throughthe burning area and the fuel density:

Regression Rate

fbff rAm ρ=&

One of the most important performance parameters to be taken into account whenstudying hybrid propulsion is the regression rate (rf), or the rate of consumption ofthe solid fuel during combustion

As a first approximation, the regression rate depends on the oxidizer mass flux with a power law relationship:

noxf aGr =

A full characterization of a fuel formulation includes the determination of the rf vs. Gox

curve

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How can these data be obtained?

Data Collection and Post Processing

� Firing tests are performed with the aim to measure the fuel average regression rate and to obtain the curve rf vs. oxidizer mass flux

� In order to obtain the rf vs. Gox curve, the Gox and the tb are needed

bfbf

bb

fif

fbff

tA

mr

t

m

t

mmm

Arm

ρ

ρ

∆=⇒

∆=−

=

=

&

&

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� The pressure trace during each firing test isrecorded and post-processed using a pressure tranducer and a dedicated software

� Pressure trace post-processing allowsdetermining the burning time as follows:

1. the differenceΔpc between maximum pressurereached during combustion (plateau) and initialpressure is measured

2. the time corresponding to 50%Δpc (50% of the pressure increase due to combustion onset) istaken as the initial time t1

3. the time corresponding to 80% Δpc (80% of the pressure decrease due to combustion extinction) istaken as the final time t2

4. This conventional choice allows avoidingthe transient phenomena to be consideredwhen measuring rf

5. t2-t1 gives the burning time and is used toobtain rf

tb measurement

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� The oxidizer mass flux (Gox) is measured dividing the oxidizer mass flow rate (Qox) by the area through which the oxidizer flows (Aflow)

Gox measurement (1/2)

oxox

pCAQ

ρ∆= 20

flow

oxox A

QG =

flow

oxox A

QG =

� The oxidizer mass flow rate (Qox) is measured using a calibrated nozzle of inlet area A0, byknowing the flow pressure drop induced by the nozzle (Δp), the oxidizer density (ρox) and the outflow coefficient C (from literature tables)

� The oxidizer density (ρox) is obtained using the ideal gas relationship at the actual pressure in the combustion chamber: T

M

Rp=ρ

� In this facility, the nozzle pressure dropΔp is measured using two connectedwater columns

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Gox measurement (2/2)flow

oxox A

QG =

� The formula for Gox

calculation is implementedin a excel worksheet

� The Gox is obtainedselecting the rowcorresponding to the Δp and the columncorresponding to the pressure measured in the combustion chamber

� Gox is an appromate value(Gox = 87 in the example –not 87.11)!!

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� The averageaverage regressionregression raterate is obtained from the burning area, the burning timeand the fuel mass burned using the equation:

bfbf

bb

fif

fbff

tA

mr

t

m

t

mmm

Arm

ρ

ρ

∆=⇒

∆=−

=

=

&

&

� The fuel mass burned during a firing test (Δm) is obtained by weighting the fuelsample before and after combustion

� The burning area (Ab) and the fuel density (ρf) are known� The burning time is obtained from the pressure trace as described in the previous

slide

Average rf measurement

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2D Slab Hybrid Test Facility:

Results Discussion

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Tested Fuel Formulations

� HTPB-based formulations (nomenclature: H-)(baseline formulations for relative comparison)

� Gel Wax and Solid Wax-based formulations (nomenclature: GW- and SW-)(with PUF strengthening, in order to overcome the wax poor mechanical properties)

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Binder Materials: HTPB

1Heat of combustion, MJ/kg

0.913Density at room temperature, g/cm3

1200Molecular mass, g/mol

liquidPhysical state at room temperature

HTPB R-45

HTPB R-45: molecular structure and main properties

Manufacturing involves:

0.43

7.67

13.04

78.86

Mass fraction % wrt total mass

HTPB

Plasticizer: DOA (C22H42O4)

Catalyst: TIN (CH3CO2CH2CH3)

Curing agent: IPDI (C12H18N2O2 )

Ingredient

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Binder Materials: Paraffin Types

Measured viscosity

at T = 343 K (Pa s)

Measured viscosity

at T = 333 K (Pa s)

Density

(g/cm3)

Chemical

Formula

Paraffin Type

0.090.890.89C24H50Solid Wax (SW)

1.1211.290.88C12H26Gel Wax (GW)

30.1Surface tension, ηn (mN/m) at T = 243 K

324Melting temperature, Tm (K)

601Specific heat at constant pressure, Cp,solid (J/mol K) at T = 300-500 K

772.5Specific heat at constant pressure, Cp,liquid (J/mol K) at T = 330 K

Solid Wax: C24H50

-536.17Enthalpy of formation at standard conditions, ΔHform,liquid (kJ/mol)

-7901.74Enthalpy of combustion at standard conditions, ΔHc,liquid (kJ/mol)

376Specific heat at constant pressure Cp,liquid (J/mol K) at T = 298 K

Gel Wax: C12H26

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Additive Materials: Magnesium Hydride Powder

98%Purity

7.65 %Hydrogen content (weight %)

Magnesium Hydride (MgH2) Properties

Decomposition temperature (K) 553

Enthalpy of formation at standard conditions (kJ/mol) ΔHf°= -75.4

Supplier ABCR (Germany)

Magnesium Hydride, magnification x 500 Magnesium Hydride, magnification x 3000

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Additive Materials: Nano-sized Aluminum Powders

Al280C, magnificationx 5000

Al280C, magnificationx 10000

MM

PC

EEW

EEW

EEW

Production

Technique

UK – Space Industry Supplier

Russia – Peter’s Research Center

Russia - SibTermoChim

Russia - SibTermoChim

Russia - SibTermoChim

Source-Supplier

uncoated

coated

coated

uncoated

uncoated

Coating

mAl

Alex280C

Alex100C

Alex100

Alex50

Type

0.130

6.70.28

NAv0.10

15.50.10

24.50.05

Surface area,

m2/g

Nominal particle

diameter, μm

Aluminum (n-Al) powders

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Pure HTPB: Single Slab vs. Double Slab

Double slab vs. single slab configuration: +70% rf at 120 kg/m2s Higher rf / Gox exponent for double slab configuration

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Wax-based Fuels

� Structural strengthening used to overcome the limit given by wax poor mechanical properties

� The melted wax is poured in the PUF structure in order to obtain a reinforced wax� This method, developed at SPLab, allows obtaining paraffin-based fuels with

suitable mechanical properties for combustion

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Regression Rate Comparison

oxygen mass flux: 120 kg/m2soperating pressure: 1-1.5 bar

rf % variation

vs. HTPB

Fuel type

+47%H-MGH5

+208%SW-Alex505

+205%SW-Alex1005

+269%SW-MGH5

+174%SW

+200%GW-Alex1005

+95%GW-MGH5

+63%GW

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Results Discussion (1/2)

high tendency to entrainmentlow tendency to entrainment

low surface tensionhigh surface tension

low viscosity at T > 60 oCoscillating viscosity at T > 60 oC

SWGW

The reasons for the observed behavior have to be ascribed to the higher / lowertendency to entrainmententrainment effecteffect, which is determined by the fuel physical properties:

best results for MgHMgH22--filled filled fuelfuel

(coarse particles, high reactivity)

best results for AlexAlex--filledfilled fuelfuel

(fine particles)

higher rf valueslower rf values

lower data dispersionhigher data dispersion

SWGW

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Results Discussion (2/2)

SW: high tendency to entrainment

• Reduced boundary layer blocking effect• Reduced particle heat adsorption• Higher particle combustion area

• Higher overall rf

• Lower particle size influence on rf

• MgH2 reactivity full exploitation

GW: low tendency to entrainment

• Higher rf with of finer particle (Alex) • MgH2 reactivity not fully exploited

• Blocking effect due to fuel pyrolisis• High particle heat adorption• Coarser particles combustion inhibition

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Beyond the State of the Art

� All results are obtained in lablab--scalescale test test rigrig; similar rf values are obtained with large motorsin open literature

� A direct comparison is not allowed, due to the scale factor problem

� Paraffin-based fuels show suitable mechanicalproperties during combustion

100 x10 x 10 slabsolid wax1.18Nakagawa 20095

D = 127-228radialsorbitol1.30Lohner 20062

50 x 10 x 4slabSW+MgH21.52this work 2010

584 x 76 x 42.8 / 44.5slabHTPB1.10Chiaverini 20004

D= 38.1 L = 410radialHTPB+Alex2.00Risha 20033

D = 96radialPE0.58Kim 20091

Fuel size (mm)GeometryFuel typerf (mm/s)Source

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Cited References

1. S. Kim, J. Lee, G. Kim, J. Cho, H. Moon, H. Sung, and J. Kim, Combustion Characteristics of the Cylindrical

Multi-port Grain for Hybrid Rocket Motor, 2009. AIAA 2009-5112.

2. K. Lohner, J. Dyer, E. Doran, Z. Dunn, and G. Zilliac, Fuel Regression Rate Characterization Using a

Laboratory Scale Nitrous Oxide Hybrid Propulsion System, 2006. AIAA 2006-4671.

3. G.A. Risha, B.J. Evans, E. Boyer, R.B. Wehrman, and K.K. Kuo, Nano-sized Aluminum- and Boron-based

Solid Fuel Characterization in a Hybrid Rocket Engine, 2003. AIAA 2003-4593.

4. M.J. Chiaverini, N. Serin, D.K. Johnson, Y.C. Lu, and G.A. Risha, Regression Rate Behavior of Hybrid

Rocket Solid Fuels, Journal of Propulsion and Power, 16(1), 2000.

5. I. Nakagawa, S. Hikone, and T. Suzuki, A Study on the Regression rate of Paraffin-based Hybrid Rocket

Fuels, 2009. AIAA 2009-4935.

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6. L.T. De Luca, Problemi energetici in propulsione aerospaziale, Prima Ed., Politecnico di Milano, 2007.

7. G.P. Sutton, Rocket Propulsion Elements, 6th Ed., John Wiley and Sons Inc., 1992.

8. N.A. Davydenko, R.G. Gollender, A.M. Gubertov , V.V. Mironov, N.N. Volkov, Hybrid Rocket Engines: The

Benefits and prospects, Aerospace Science and Technology 11 (2007) 55-60.

9. Y. Maisonneuve and G. Lengellé Hybrid Propulsion: Past, Present and Future Perspectives, ONERA, 2002.

Other References

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Exercises

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Objectives of the given exercise

Using the data given in the Excel sheet, for each of the fuels and for each of the reported experimental tests, compute:

� the average regression rate� the initial flowing area for the oxidizer flow� the oxidizer mass flux based on this area� the O/F ratio

For each data set, find the coefficients of the power curve that best fits the experimental data.

Examining the regression rate vs. oxidizer mass flux and the O/F vs. oxidizer mass flux plots, give an interpretation of the observed trends, answering the following questions:

1. Which geometrical configuration allows obtaining the best performance?2. Among the tests performed in double slab configuration, which fuel formulation

allows obtaining the best overall performance?3. Do the experimental results confirm what expected from theory? Why?4. Which is the O/F ratio effect on the ballistic performance?

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Experimental Investigation ofCombustion Processes in Advanced Fuels

for Hybrid Rocket Propulsion

Politecnico di MilanoDipartimento di Ingegneria Aerospaziale

Laura Merotto – Matteo Boiocchi