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Physical and Ballistic Characterization of Aluminum loaded Paraffin Hybrid Rocket Fuels Yash Pal, and Ravikumar Vijayaraghavan Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01636 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

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Energy & Fuels

2

Physical and Ballistic Characterization of Aluminum loaded Paraffin Hybrid Rocket Fuels

3

Yash Pal† and V. Ravi Kumar*

4

†

5

*

1

6

School of Aeronautical Sciences, Hindustan University, Chennai-603103, India

Department of Chemical Engineering, Hindustan University, Chennai-603103, India Email: [email protected]

7 8

ABSTRACT

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The physical, thermal and ballistic performance of paraffin-based fuel loaded with aluminum

10

(Al) additive was investigated. The paraffin-based fuels were prepared using varying weight

11

percentages of polyethylene (PE) as binder and Al as energetic additive. The mechanical tests

12

showed significant improvement in compression strength and elastic modulus with the addition

13

of PE and Al to pure paraffin wax. The ignition performance, combustion characteristics and

14

exothermic

15

Thermogravimetry/DerivativeThermoGravimetry/Differential

16

(TG/DTG/DSC) experiments. The addition of PE increased the ignition and binder temperature,

17

whereas the incorporation of metallic additive lowered the decomposition temperature. The heat

18

of combustion of paraffin-based fuel samples increased as the Al loading content was increased

19

from 5 wt.% to 25 wt. %. The ignition and combustion indices were calculated to evaluate the

20

ignitability and combustion reaction ability of the fuels. The rheological investigation indicated

21

that the addition of PE to paraffin had increased the melt layer viscosity whereas the effect of Al

22

powder on viscosity was small. The ballistic tests were performed under gaseous oxygen and the

23

results revealed that the regression rate decreased with increasing PE content (5 wt. % to 10

behavior

of

these

paraffin-based

fuels

1

ACS Paragon Plus Environment

were Scanning

studied

through

Calorimetry

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wt.%) in the paraffin wax. The addition of Aluminum increased the regression rate compensating

2

for the loss of regression rate due to PE addition.

3

Keywords: Paraffin wax, Ignition Temperature, regression rate, Thermogravimetric analysis

4

INTRODUCTION

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Hybrid rocket propellant combines the features of solid and liquid propellants. Hybrid rockets

6

are safe due to various factors such as distinct physical states of fuel and oxidiser, onboard restart

7

ability, fuel insensitivity to combustion instability, throttling capability, reduced environmental

8

impact and low manufacturing cost. These unique features of hybrid propulsion system make

9

them suitable for a variety of space applications such as the sounding rocket, upper stage launch

10

vehicles, suborbital and orbital human space flight 1. Besides the advantages mentioned above,

11

the solid fuels do have certain drawbacks such as low fuel regression rate and varying oxidizer-

12

to-fuel ratio during the combustion. The diffusion-flame-limited combustion in hybrid rocket

13

motor has been cited as the main cause for low regression rate and poor combustion efficiency 2.

14

In the recent years, several improvement efforts have been carried out to address the issue of

15

low regression rate. The improvement techniques include changes in the physical design such as

16

swirl oxidizer injection

17

solid fuel grain 8,9. Alternatively, chemical approaches to improve the regression rate include the

18

use of energetic additives such as metals, oxidizer crystals and metal hydrides in solid fuels

19

during the casting process 10–12. However, these regression rate improvement techniques did end

20

up with certain shortcomings such as complexity in design and involved rigorous modeling.

21

Karabeyoglu et al.13 suggested a promising technique to solve the slow regression problem of

22

traditional solid polymeric fuels. They tested paraffin-based fuels and reported the regression

23

rate increased by 3 to 4 fold as compared to that of classical polymeric fuels (Hydroxyl-

3–5

, multi-port fuel grain

6,7

and embedding mechanical devices in the

2


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terminated polybutadiene (HTPB), Polymethyl methacrylate (PMMA) etc.). The enhancement in

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the combustion of these paraffin-based fuels is due to the creation of unstable thin liquid layer on

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the fuel grain surface, which facilitated additional mass transfer by entraining liquid droplets

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from the melt layer surface14. Hence, it was suggested that the total regression rate of paraffin-

5

based fuel is a combination of the fuel vaporization rates from its surface and the entrainment of

6

fuel droplet to combustion zone. The entrainment component of fuel regression rate can be

7

defined as 14

8 m& ent ∝

9

Pα δ β

(1)

σ π µlc

10

The entrained mass loss is a function of dynamic pressure, P , and melt layer thickness δ . The

11

Eq.1 shows that the entrainment is strongly dependent on material properties such as viscosity, µ ,

12

and surface tension, σ , of the surface melt layer. The lower values of viscosity can favor a

13

positive effect on regression rate enhancement. Galfetti et al.15 found that the addition of metal

14

additives to paraffin-based fuels reduced the viscosity of the liquid melt layer. They attributed

15

this behavior to high radiative heat transfer to liquid melt layer and thereby reducing the

16

viscosity.

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The paraffin-based fuel has poor mechanical properties, which prevented its full development

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and application for a space mission. The casting of large paraffin grain that can sustain the flight

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inertial loads, radial combustion pressure, thrust and temperature loads is quite a challenging

20

task. Many researchers have attempted to improve the mechanical properties of paraffin-based

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fuel. Maruyama et al.16 performed tensile tests on ethylene vinyl acetate (EVA) polymer based

22

formulation. They observed that the maximum strength increased about 1.6 times with 20 wt. % 3


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EVA additive. Recently, Kumar et al.17 showed that the addition of 20 wt. % EVA in paraffin

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wax improved the tensile strength of about 50 %. The PE addition to paraffin wax can

3

significantly improve the mechanical properties, thermal stability and combustion efficiency.

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Kim et al.18 has observed the improvement in tensile and compressive strength when 10 wt.% PE

5

was added to paraffin wax. Furthermore, DeSain et al.19 reported that the tensile strength and

6

percentage elongation improved with the addition of 4 wt.% low-density polyethylene (LDPE) in

7

paraffin wax.

8

The combustion and mechanical characteristics are the main attributes of solid hybrid fuel,

9

besides the ease of ignition, which decides the operating conditions of the fuel. The ignition

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temperature of solid fuel in hybrid rockets imposes several requirements on ignition system

11

design. The physical interface, moisture free environment, safety, reliability, motor ignition time

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and shock output are few functional requirement of the igniter that can affect the ignition

13

temperature of solid fuels. Therefore, the knowledge of ignition temperature in solid fuel

14

combustion process is an important parameter for combustion stability and control.

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In this study, the paraffin-based solid fuels were prepared and analysis of physical, thermal

16

and ballistic performance was carried out. The rheological investigation was carried out to

17

predict the sensitivity of PE and Al additives concentration on the viscosity of paraffin-based

18

fuels. The mechanical compression tests were performed to analyze the compression strength

19

and compression modulus. The ignition behavior and combustion characteristics of these fuels

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were examined by TG/DTG/DSC technique to evaluate the possible relevant changes triggered

21

due to PE and Al addition. Lastly, we report the effects of addition of Al and PE on the

22

regression rate of paraffin-based fuels.

4


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Energy & Fuels

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EXPERIMENTAL

2

Materials

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The materials were procured from Merck Millipore Pvt. Ltd. and the physical properties are

4

listed in Table 1.

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Table 1. Properties of materials

6

Properties

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Paraffin wax Polyethylene Aluminum

Density (kg m-3)

920

918

2700

Melting temperature (°C)

67

115

660

Molecular weight (g mol-1)

380

96000

26.59

Particle size (µm)

-

-

10-20

Sample Preparation

9

Three different classes of paraffin-based formulation were prepared with pure paraffin and

10

shown in Table 2. Paraffin-based fuel sample was prepared as described in the cited

11

literature20,18. A desired quantity of the paraffin wax was heated in a beaker kept on a hot plate;

12

the PE was added to the molten paraffin and the mixture was magnetically stirred for

13

approximately 15 min at 150 °C to ensure homogeneity. The respective quantity of Al powder

14

was added to the blend and sonicated for 30 minutes to ensure homogenous distribution of

15

additives. Prior to casting of the fuel grain, the steel mould wall surface was coated with grease

16

removal agent to facilitate the removal of grain from the mould casing. A steel rod of diameter

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15 mm was inserted in the center of the mould to create a combustion port. The mixture was

18

poured in the mould rapidly and allowed for cooling and solidiﬁcation at room temperature. All 5


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fuel grains were manufactured with circular configuration with 150 mm length and 15 mm initial

2

port diameter.

3 4 5 6

Table 2. Composition of paraffin-based solid fuels

7

Fuel Sample Pure Paraffin

Compositions Paraffin 100 %

P/PE05

Paraffin 95% + Polyethylene 5%

P/PE10

Paraffin 90% + Polyethylene 10%

P/PE/Al5

Paraffin 85 % + Polyethylene 10 % + Aluminium 5 %

P/PE/Al15

Paraffin 75 % + Polyethylene 10 % + Aluminium15 %

P/PE/Al25

Paraffin 65 % + Polyethylene 10 % + Aluminium25 %

8 9

The overall quality of manufactured solid paraffin-based fuel can be evaluated using simple 21

10

density measurement. The manufactured fuel density is calculated by a gravimetric method

.

11

These calculated fuel densities are compared to the corresponding theoretical maximum density

12

(TMD). The porosity of the manufactured formulation is indicating the quality of prepared fuel

13

sample. Low porosity suggests better quality of the fuel sample.

14

Scanning Electron Microscope (SEM)

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SEM analyses were carried out in Coxem CX-200 Microscope under a 10 kV beam. The

16

fuel samples were cracked prior to scanning and cracked edge surface was selected for

17

observation. The cracked edge surface was sputtered with a layer of Au-Pd to prevent over6


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charging of the sample. The sample was mounted on a Carbon tape before analyzing for SEM.

2

The SEM was performed to understand the miscibility and phase separation behavior of paraffin

3

and PE blend.

4

Viscosity Measurement

5

The viscosity tests were performed using Brookfield DV-II Viscometer to understand the

6

rheological behaviour of prepared paraffin-based solid fuels. The DV2T Viscometer series

7

measures fluid viscosity at set shear rates. The viscosity was measured using a spindle immersed

8

in the test liquid (fuel). The viscous drag of the liquid against the spindle is measured on rotary

9

transducer. Viscosity is calculated from the measured torque based on the selected spindle RPM.

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The viscosity measurement was performed at three different temperatures (85 °C, 100oC and 125

11

°C) and different shear rates (10 to 50 s-1). The instrument was calibrated at 25 °C using Silicone

12

oil as the reference fluid.

13 14

Mechanical Properties

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The compression tests were performed to evaluate the compression strength and compression

16

modulus of paraffin-based fuels. The tests were carried out using an INSTRON (3382)-30 kN

17

setup, with a cross-head speed of 3 mm/min. The cylindrical specimen with a diameter of 13 mm

18

and a length of 25 mm were placed between two compression plates. The compressive test

19

conditions and specimen dimensions were selected similar to the specimens tested by Kim et al.

20

18

21

DSC/TG/DTG Analysis

.

22

The DSC/TG/DTG analyses were carried out on NETZSCH STA449F3 (Netzsch, Germany)

23

under dynamic N2 and O2 environments with a flow rate of 50 mLmin-1. The solid fuel samples 7


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used for DSC/TG measurements were about 2-5 mg. The samples were heated from room

2

temperature to 800 °C at a heating rate of 10 °C/min. The pyrolysis and combustion tests were

3

performed under N2 and O2 environments, respectively.

4 5

Measurement of combustion parameters

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Several methods have been proposed in the literature to determine the combustion parameters

7

such as ignition temperature and burnout temperature of the solid fuels using TG/DTG method

8

22,23

9

temperature (Tin), ignition temperature (Tig), burnout temperature (Tb) and the duration of

10

combustion of the paraffin-based solid fuels were obtained from TG/DTG/DSC curves. The

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initial decomposition temperature, Tin, is calculated on TG curve where the pyrolysis profile

12

separates from the combustion profile and this temperature correspond to initiation of mass

13

loss24. The ignition temperature, Tig, is temperature at which fuel sample starts to burn and it

14

corresponds to temperature where the rate of mass loss is 0.1 % min-1. Burnout temperature, Tb,

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represents the temperature at which fuel sample is completely oxidized. It can be correlated on

16

DTG combustion curve where the rate of mass loss corresponds to 0.1 % min-1 25,26.

. In this study, the characteristic combustion parameters such as initial decomposition

17

The DSC scans were also performed to study the melting enthalpy and exothermic heat release

18

rate of paraffin-based solid fuels. The ignitability and combustion ability of these paraffin-based

19

fuels can be expressed in terms of ignition index and combustion index, respectively. The higher

20

the value of ignition index, the greater will be the ability of solid fuel to ignite. Similarly, higher

21

combustion activity can be obtained from higher value of combustion index

8


24

. The ignition

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index (Xi) and combustion index (Xc) were calculated using following Eq.2 and Eq.3,

2

respectively:  dm    dt  max Xi =  tm × ti

3

(2)

 dm   dm       dt  max  dt avg Xc = Tig2 × Tb

4

(3)

5

dm Where  

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ability of any fuel can be characterized by evaluating the peak temperature on DTG curve. The

7

peak temperature is a point on the DTG curve, which represents the maximum rate of weight loss

8

due to rapid oxidation; the formation of carbonaceous residue is a characteristic of this process 23.

9

The combustion rate corresponds to this peak temperature is called the maximum combustion

10

rate (dm/dt). The ignition ability of the fuel will be higher, if the peak temperature is lower. The

11

time, tm , corresponds to maximum combustion rate and ti is the ignition time corresponds to

12

ignition temperature.

 dt  max

dm and  

 dt  avg

are the maximum and average combustion rate, respectively. Ignition

13 14

Fourier Transform Infrared (FTIR)

15

The FTIR spectral measurements were carried out to identify the combustion products of

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paraffin-based fuels. The burnt samples from combustion and DSC were employed for

17

spectroscopic characterization. FTIR spectra were acquired on a Perkin Elmer Spectrum1 FT-IR

18

spectrometer. 1 mg of each of the collected combustion products from burned paraffin-based fuel

9


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1

was mixed with 100 mg of fine dry potassium bromide (KBr) homogeneously to make pellet

2

disk of 7 mm diameter and 0.5 mm thick.

3

Ballistics motor setup

Page 10 of 40

4

The static fire test and ballistic performance of prepared paraffin-based solid fuel formulation

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were performed in a gaseous oxygen environment. The experimental setup used in this study is

6

shown in Figure 1. The test setup consists of gaseous oxidizer feed system, lab-scale rocket

7

motor, ignition system and data acquisition (DAQ) system. The oxidizer system consists of two

8

gaseous oxygen cylinders mounted on weighing balance. The oxidizer supply pressure was

9

regulated by pressure regulator and a solenoid valve was installed in feed line to control the flow

10

of oxidizer for the desired amount of time. The oxidizer mass flow rate was calculated from the

11

amount of mass consumed for a set burn time. The electrical power to igniter was supplied by a

12

12 V battery. A pyrogen igniter containing a small amount of solid propellant (1 g) was used to

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ignite the solid fuel and a coil of Nichrome wire was wrapped around it. The combustion

14

chamber pressure and oxidizer chamber pressure were measured with the help of two pressure

15

transducers (Omega PX603) and data was acquired with the help of Arduino board. The thrust

16

was measured using a load cell.

17 10


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Energy & Fuels

Figure 1. Schematic of experimental setup for lab scale hybrid motor

2 3

The oxygen was supplied into inlet of injector plate through a cup shaped oxidizer chamber of

4

length 33 mm and diameter 55 mm. Injector plate of stainless steel with 47 mm pitch circle

5

diameter was used to ensure axial injection of oxidizer. Injector plate consists of 17 holes with a

6

diameter of 1 mm. The length of combustion chamber was 150 mm; with an inner diameter of 42

7

mm. Schematic of static test hybrid motor combustion chamber is shown in Figure 2. A

8

systematic testing procedure was followed to maintain consistency during the test. A gap of 8

9

mm was maintained between the solid fuel grain and the nozzle end of the motor casing. The

10

spacing is maintained for the un-burnt paraffin to react with GOX before it exits the nozzle.

11 12

Figure 2. Schematic of static test hybrid motor (dimension in mm)

13 14

The oxidizer injection pressure was regulated at the desired value using a pressure regulator.

15

When the oxygen flow becomes steady in the oxidizer chamber, a pyrogen igniter initiates the 11


Energy & Fuels

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1

combustion. The igniter was placed at the port of the fuel grain at the head end of the grain in the

2

combustion chamber. After lapsing of a pre-requisite duration, the combustion was terminated by

3

cutting off the oxidizer flow supply. The thrust and the chamber pressure were measured during

4

the firing test. The other pre-and post-test measurements consisted of the oxygen mass flow rate

5

and final fuel grain weight. Figure 3 shows the firing test of the P/PE/Al15 solid fuel formulation

6

under gaseous oxidizer environment. The data reduction methodology is presented in Appendix-

7

A.”

8 9

Figure 3. Ballistic test of P/PE/Al15 fuel (a) static firing test, (b) grain casing after test

10 11

RESULTS AND DISCUSSION

12

SEM

13

Figure 4 shows the morphology of the paraffin-based solid fuels. In Figure 4a), we can

14

observe that pure paraffin exhibits a homogeneous morphology. Figure 4b) and Figure 4c)

15

represents the addition of PE to paraffin. From Figure 4b), the addition of 5 wt.% PE shows

16

complete miscibility of PE in P/PE blend and with P/PE10 formulation, the PE dispersion in

17

blend is still uniform. Figure 4b and Figure 4c show reasonably good PE distribution in the

18

paraffin matrix. As the PE concentration is increased to 30 wt.%, a phase separation between the 12


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Energy & Fuels

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PE and paraffin is visible in Figure 4d. The Similar phase separation behavior of the PE and

2

paraffin wax was also observed by Kim et al. 18. They reported that PE could disperse better in

3

the 3D net structure with low PE concentration (less than 10 wt.%). The phase separation

4

behavior at higher PE concentration can be attributed to higher molecular weight and the chain-

5

structure of the PE

6

fuels and suppress the positive effect of PE on mechanical properties.

27,18

. This phase separation can influence the mechanical properties of solid

7

13


Energy & Fuels

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1

2

14


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1 2 3

Figure 4. SEM micrographs for (a) pure paraffin, (b) P/PE05, (c) P/PE10, (d) P/PE30 Density measurement

4

Density measurements on paraffin-based solid fuels are shown in Table 3. The addition of PE

5

does not show any substantial change in the density of the P/PE mixture and this is expected due

6

to similar chemical and physical characteristics of P and PE. The addition of Al powder has

7

significantly increased the density compared to that of pure paraffin fuel. Table 3 shows the

8

lower value of fuel porosity and this indicates the quality of the blending process. It has been

9

observed that lower the porosity, higher is the blending characteristic. Although, the solid fuel

10

density does not have much impact on regression rate performance, it can be used to rate the

11

volumetric specific impulse of solid fuels. Recently, Mazzetti et al. 1 discussed a merit parameter

12

(performance/cost ratio) to understand the hybrid rocket technology economic aspect. They

13

considered solid fuel density as key parameter to evaluate the cost per unit mass.

14

Table 3. Comparison of density measurement of paraffin-based formulations 15


Energy & Fuels

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Sample Paraffin

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Experimental Standard Theoretical Density Deviation Density 3 3 (kg/m ) (kg/m ) (kg/m3) 915 22 920

Porosity (%) 0.5

P/PE05

913

18

919

0.7

P/PE10

917

22

919

0.2

P/PE/Al05

942

12

947

0.5

P/PE/Al15

948

18

952

0.4

P/PE/Al25

961

21

968

0.7

1 2

Viscosity measurement

3

In the current study, the viscosity of paraffin-based fuels is studied as a function of

4

temperature and shear rate. The viscosity has strong dependency on regression rate of solid fuels;

5

therefore, these paraffin-based formulations are tested for their rheological behavior. Figure 5

6

shows that the viscosity of each fuel formulation decreased with increasing temperature at a

7

shear rate of 35 s-1. The addition of PE to paraffin shows an increasing trend of the viscosity. It

8

can be seen from Table 4 that addition of Al powder in P/PE blend has barely affected the

9

viscosity value at low temperatures. Recently, Dermanci et al. 28 studied the effect of addition of

10

Al powder on viscosity of paraffin wax solid fuels. Al powder suspension in the formulation

11

showed limited effect on viscosity. The studies on experimental investigation on the regression

12

rate with the PE and Al is not fully understood.

16


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Energy & Fuels

1 2

Figure 5. Comparison of viscosity variation with temperature of paraffin-based formulations

3 4

Table 4. Measured viscosities of paraffin-based formulations at shear rate 35 s-1 Temperature

Viscosity (cP) (at shear rate 35 s-1)

(°C)

P

P/PE05 P/PE10 P/PE/Al5 P/PE/Al15 P/PE/Al25

85

12.52

22.84

28.55

29.08

29.86

31.1

100

9.5

17.69

25.17

26.16

27.41

29.1

125

7.67

13.91

19.8

22.03

21.8

24.3

5 6

The viscosity measurements of paraffin-based formulation at different shear rates were

7

performed. Figure 6 shows the viscosity behavior with respect to shear rate at temperature of 125

8

°C. Neat paraffin wax shows that the measured viscosity is independent of the applied shear rate

9

and represents the Newtonian behavior. Formulations with 5 wt.% PE, 10 wt. % PE and Al

10

shows a non-Newtonian behavior, where the viscosity changes with the applied shear rates. 17


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1 2

Figure 6. Behavior of paraffin-based formulations viscosity as function of shear rate at 125 °C

3 4

Due to shear thinning behavior, the formulations exhibit reduced viscosity as a function of

5

increased shear rate. Figure 7 represents the shear thinning study at a lower temperature (85 °C)

6

but the trend is not as pronounced as at 125 °C, which is presented in Figure 6. For all the Al-

7

based formulations, the viscosity increment observed is lower than 6% as compared to P/PE10

8

formulation at 125 °C, whereas at 85 °C, the viscosity increment is more than 12 %. The

9

viscosity increment is not significant as compared to addition of PE. The crystalline nature of the

10

additive and higher molecular weight of PE contributes to enhanced viscosity

18


27

. Also, PE has

Page 19 of 40

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Energy & Fuels

1

long-chain branched structure alters the flow properties and makes the fuel formulation more

2

viscous. Therefore, the addition of PE would decrease the regression rate of liquefying fuels.

3 4 5

Figure 7. Behavior of paraffin-based formulations viscosity as function of shear rate at 85 °C Compression tests

6

Figure 8 shows the compression test results for paraffin-based fuels. It can be seen that

7

addition of PE to paraffin wax increased the compression strength. The addition of 5 wt.% Al to

8

P/PE further increased the compression strength about 54 % as compared to baseline paraffin

9

sample. At 25 wt.% Al loading, the compression strength increased about 63.2 % as compared to

10

pure paraffin formulation. The compressive modulus is also found to increase as a function of PE

11

loading in paraffin.

19


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 40

1 2

Figure 8. Comparison of average compression strength of paraffin-based solid fuel formulations

3 4

It can be seen from Figure 9, that the compression modulus increased by a factor of 13% with

5

the increase in Al loading content from 5 to 25 wt.%. The results shown in Table 5 indicate that

6

the compression strength for P/PE10 increased about 41 % compared to that of pure paraffin

7

fuel. However, 5 wt.% Al addition increased the compressive strength by 6.7 % with respect to

8

P/PE10 fuel. It could be attributed to crystallinity and molecular structure of material. The

9

paraffin wax undergoes crystallinity during solidification

10

19

. The addition of PE reduces the

crystallinity of wax and makes it more uniform.

20


Page 21 of 40

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Energy & Fuels

1 2

Figure 9. Comparison of average compression modulus of paraffin-based solid fuel formulations

3

The addition of Al to P/PE blend acting as reinforcing filler resulting in higher compressive

4

29

5

strength has been observed in similar work

6

sample is stiffer than other tested samples and it can withstand more radial combustion pressure

7

loads during the operation of rocket motor. The compression tests results reported by Kim et al.

8

18

9

that poor mechanical properties of paraffin-based rocket solid fuel can be improved with addition

10

. From Table 5, it is noticeable that the P/PE10

are in good agreement with results of current study. Based on these results, it can be concluded

of PE.

11 12 21


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 40

1 2

Table 5. Results of compression test of paraffin-based solid fuel formulations *

Sample P

3

Average Compressive Strength (MPa) 3.07

*

SD (MPa) 0.02

Average Compressive Modulus (MPa) 82.3

SD (MPa) 6.1

P/PE05

3.21

0.1

119.7

7.1

P/PE10

4.33

0.19

164.1

17.5

P/PE/Al05

4.62

0.02

172.7

3.4

P/PE/Al15

4.89

0.07

179.5

14.2

P/PE/Al25

5.01

0.04

182.5

9.85

*

the average results in this table are based on 3 tests for each fuel sample

4 5

Ignition and Thermal Decomposition of paraffin-based fuels

6 7

Ignition temperatures of paraffin-based fuel formulation were evaluated from TG and DTG

8

analysis. The pyrolysis and combustion profiles of pure paraffin fuel sample are presented in

9

Figure 10. The pyrolysis TG curve and combustion TG curve diverged at 201 °C which

10

corresponds to initiation of decomposition process. The ignition of paraffin starts at 242°C after

11

the combustion reaction, when the rate of mass loss is -0.1% min-1 on DTG curve. The oxidation

12

process of paraffin completed at 517 °C.

22


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Energy & Fuels

1 2

Figure 10. Ignition temperature calculation for pure paraffin sample with the TG-DTG method

3 4

The combustion TG curves for all paraffin-based solid fuels are shown in Figure 11. The

5

decomposition initiation temperature for P/PE05 sample is around 211 oC and the ignition

6

temperature is at 250 oC. Similarly, for the P/PE10 sample, it was found to be 218 and 271 oC

7

respectively. It is clear that addition of PE to paraffin increases the decomposition and ignition

8

temperature. This is due to the higher thermal stability of P/PE blend with the increase in PE

9

content 30. The higher thermal stability results in lower regression rate 31.

10

From the combustion and pyrolysis TG profiles of paraffin sample presented in Figure 10, a

11

noticeable mass loss takes place in both pyrolysis and combustion conditions. In the presence of

12

oxygen, the paraffin ignites and burns with a significant mass loss compared to pyrolysis.

23


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 40

1

The pure paraffin sample followed single step degradation, whereas the P/PE blend degraded

2

in two steps. All the paraffin-based formulations are thermally stable up to the temperature

3

around 180 °C and show a significant char yield at temperatures higher than 530 °C. The first

4

weight loss step for all paraffin-based samples was observed at around 190 °C to 226 °C, while a

5

second weight loss at temperatures around 320 °C to 550 °C, which corresponds to the oxidation

6

of PE. Krupa et al.30 reported that the initial decomposition temperature increased as the PE

7

percentage increased for uncross-linked blends. The addition of Al powder lowered the

8

decomposition process of paraffin-based fuels, which can facilitate regression rate and

9

combustion efficiency in the hybrid rocket.

10 11

In order to check the effect of Al powder addition on thermal decomposition, ignition and

12

burnout temperature, the P/PE10 formulation was doped with Al and the loading was varied

13

from 5 wt.% to 25 wt.%. Figure 11 shows the initiation of decomposition process for P/PE/Al5

14

sample starts at around 226 °C and ignition take place at around 260°C. As the percentage of Al

15

loading increased from 5 to 15 wt.% in P/PE10 formulation, the initiation of decomposition

16

process and ignition temperature reduced to 216 °C and 260 °C respectively. It can also be

17

observed from Table 6 that the initiation of decomposition and ignition temperature further

18

reduced to lower value, 191 °C and 216 °C respectively, as the Al loading increased to 25 wt.%.

19

This can be attributed to the Al thermal conductivity, which raises the initial fuel surface

20

temperature. The heat feedback from Al particles to the fuel matrix can raise the surface initial

21

temperature and lower the initiation and ignition temperature of the formulations. Therefore,

22

higher initiation and ignition temperature associated with P/PE-based formulation can be reduced

23

with the addition of Al powder. 24


Page 25 of 40

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Energy & Fuels

1 2

The peak temperature on combustion DTG curve represents the degree of the combustibility.

3

The solid fuel with lower peak temperature can be easily ignited 32. It is reported in literature 24,33

4

that lower the burnout temperature, higher is the reactivity of fuel sample during the combustion

5

process. The peak temperatures, burnout temperatures and their corresponding times of all fuel

6

samples are given in Table 6. The burnout temperatures for paraffin, P/PE05 and P/PE10

7

samples are found to be around 517 °C, 520 °C and 530 °C, respectively. The higher burnout

8

temperature of P/PE10 sample indicates that the reactivity of PE and paraffin during the

9

combustion decreased as percentage of PE increased in the P/PE blend. On the other hand, the

10

peak temperature increased with increase in Al loading in the paraffin wax and making the

11

formulation more difficult to ignite. However, the effect of the Al concentration on the peak

12

temperature and burn out temperature are the most notable. The addition of Al has increased the

13

reactivity during combustion process and dropped the ignition temperature to lower values.

14

The ignition and combustion indices are calculated to rate the ignitability and combustion

15

reaction ability of solid fuels. The lower the value of combustion index, the lower the

16

combustion reaction ability of solid fuel 24,33. Table 6 shows that the values of ignition index are

17

in agreement with ignition temperature of the solid fuels. The similar trend can observe with

18

values of combustion index. This may be due to result of the corresponding lower maximum

19

combustion rate.

25


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 11. Thermal decomposition of the paraffin-based fuels

3 4

The results obtained from DSC analysis are summarized in Table 7. In the Figure 12, DSC

5

curve for all paraffin-based samples shows two endothermic peaks, the first peak is related to

6

solid-solid transition of the paraffin and the second peak is associated with the melting of

7

paraffin wax. The third endothermic peak is observed between 110.2 °C to 115.5 °C for the fuel

8

samples loaded with PE. This peak corresponds to melting of PE. After ignition, a sharp

9

exothermic peak is observed due to combustion process and area under the peak represents the

10

enthalpy of combustion. Furthermore, all the paraffin samples loaded with Al powder show an 26


Page 26 of 40

Page 27 of 40

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Energy & Fuels

1

endothermic peak between 654 °C and 655 °C, which is associated with melting of Al powder.

2

The melting temperatures, specific enthalpy of melting and net exothermicity of combustion

3

process for all paraffin-based samples is presented in Table 7. The total specific enthalpy of

4

melting of P/PE blend decreases with increasing the PE content in the blend. This can be

5

attributed to higher crystallinity of paraffin wax compared to PE. The solid fuels with lower

6

specific melting enthalpy can act as fast regressing fuel for hybrid rocket applications 31.

7

It can be noted from Table 7 that the total specific melting enthalpy has barely affected by the

8

Al powder addition. The exothermic heat release of these paraffin-based samples shows an

9

increasing trend with additive concentration. The addition of Al powder to P/PE blend

10

significantly increased the exothermic heat release. This indicates that the increase of Al loading

11

sped up the oxidation reaction and enhanced the exothermicity of fuel sample. DSC/TG results

12

from this study could serve as input thermodynamic parameters for the combustion modeling of

13

these paraffin-based fuels.

14 15 16 17 18 19 20 21 22 23 24 25 27


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

1

Sample

P

Page 28 of 40

Table 6. Parameters from TG/DTG measurement of paraffin-based solid fuel formulations Initial Max. Ignition Burn out Burn Max Ignitio Decomposi Peak combustio Ignition Temperatu Tempera out Combusti n tion Tempera n Time re ture Time on Time Index, Temperatu ture (°C) rate (Min) (°C) (°C) (min) (min) x10-5 re (°C) (mg/min) 201 242 296 517 97.34 0.15 42.52 54.85 6.23

Combusti on Index, x10-9 4.81

P/PE05

211

250

299

520

98.1

0.13

43.92

53.3

5.72

4.12

P/PE10

218

271

306

530

100.2

0.11

48.37

54.1

4.2

2.83

P/PE/Al5

216

260

302

535

101.5

0.12

46.1

53.9

4.82

3.32

P/PE/Al15

198

221

255

531

99.4

0.13

39.2

45

7.36

5.02

P/PE/Al25

191

216

241

526

96.8

0.14

38.6

43.4

8.35

5.72

2 3

Table 7. Parameters from DSC measurement of paraffin-based solid fuel formulations Sample

Melting Temperature, Tm1 (°C)



∆Hm1 (J/g)

∆H2 (J/g)

∆H3 (J/g)

Net Endothermicity ∆Htot ( J/g)

Net Exothermicity ∆H1 (J/g)

P

67.1

-

-

16.8

-

-

16.8

180.2

P/PE05

68.5

115.5

-

13.1

0.98

-

14.08

216

P/PE10

69.1

111.3

-

11.5

1.43

-

12.93

254.1

P/PE/Al05

69.1

110.2

655

10.9

1.23

0.45

12.58

267.5

P/PE/Al15

69.3

112.2

654

10.3

1.4

0.92

12.62

289.8

P/PE/Al25

69.2

112.1

653

8.9

1.5

1.6

12

331.1

28


Page 29 of 40

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1 2

Energy & Fuels

Figure 12. DSC curves for parafﬁn-based solid fuels

3 4

The ignition process of hybrid rocket motor can be made easier with lower ignition

5

temperature of solid fuels. The ignition temperature increased gradually with increasing PE

6

percentage in paraffin, whereas Al powder addition shows a positive effect on formulation

7

ignition. Therefore, the P/PE based solid fuels required a more powerful igniter to start the

8

ignition process during motor operation. The higher burnout temperature of solid fuel indicates

9

the higher burning duration of solid fuel

33

. This could be beneficial in term of higher specific

10

impulse, especially for long-range rocket mission. It is obvious that the regression rate

11

performance of these P/PE based fuels will be affected. 29


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

2 3

Figure 13. FTIR spectra before and after burnout test of P/PE/Al5

4 5

Figure 14. FTIR spectra before and after burnout test of P/PE/Al15 30


Page 30 of 40

Page 31 of 40

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Energy & Fuels

1 2

Figure 15. FTIR spectra before and after burnout test of P/PE/Al25

3 4

The peak near 1500cm-1 correspond to Carbonyl linkage and all indicating the combustion

5

process as shown in Figure 13- Figure 15. The post-burn samples shows this distinct peak, which

6

is missing in the pre-burn samples.

7

Comparison of Regression rate of paraffin-based fuels

8

All solid formulations were tested in gaseous oxygen environment, with oxidizer mass fluxes

9

varied from 41.43 kg/m2s to 103.1 kg/m2s, while the combustion pressure ranged from 0.35 MPa

10

to 0.94 MPa. The results of the firing tests on paraffin-based solid fuels doped with PE and Al

11

additives along with baseline neat paraffin are illustrated in Figure 16. The pure paraffin

12

formulation showed a regression rate of 1.76 mm/s at the oxidizer mass flux of 48.39 kg/m2s,

13

while at 96.51 kg/m2s, the regression rate reached 2.41 mm/s. Under the same operating 31


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 40

1

conditions, the regression rates of the investigated P/PE5 fuel varied from 0.907mm/s to 1.53

2

mm/s. The addition of 10 wt.% PE in the paraffin wax has reduced the regression rate to 0.77

3

mm/s at low oxidizer mass flux of 46.11kg/m2s and at higher oxidizer mass flux of 103.13

4

kg/m2, the regression rate is 1.52 mm/s. The regression rates of P/PE/Al based fuels were found

5

to vary from 1.13 mm/s to 2.08 mm/s under the oxidizer mass flux conditions of 44.95 kg/m2s

6

and 95.79 kg/m2s, respectively.

7 8

It is clear from the Figure 16 that the addition of PE in paraffin wax has significantly reduced

9

the regression rate with respect to pure paraffin wax over the investigated oxidizer mass flux

10

range. It is reported that the higher regression rate of pure paraffin caused by the entrainment of

11

paraffin droplets along with fuel mass transfer between the flame zone and the regressing surface

12

creates a faster fuel regression regime13-14. Karabeyoglu et al.14 suggested that the viscosity and

13

surface tension of liquid melt layer are the most important factors in enhancement of the

14

regression rate of paraffin fuels as compared to conventional polymer based fuels. The addition

15

of PE in paraffin increased the melt viscosity and thus decreased the rate of stripping of liquid

16

droplets into the combustion zone. This is corroborated by the viscosity results presented in the

17

previous sub-section. Kim et al.18 studied the paraffin droplets entrainment in the combustion

18

zone. It was observed that the paraffin fuel droplets entrained to combustion zone at higher rates

19

compared to fuel loaded with PE. Blending pure paraffin with PE leads to controlled release of

20

droplets generated from paraffin wax and can improve the combustion efficiency.

21 22

Both the PE and HTPB fuels are considered to be non-liquefying fuel since the entrainment of

23

fuel droplets from the fuel surface is almost insignificant. The regression rate of these classical 32


Page 33 of 40

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Energy & Fuels

1

polymeric fuels only depends on the vaporization amount. The mechanics of vaporization and

2

entrainment for both the fuels is very similar. Several researchers have attempted the entrainment

3

visualization experiments of PE and paraffin fuels18,31. They observed that the paraffin fuel

4

generates the wavelets/droplets on the fuel surface, which increases the regression rate whereas

5

droplets or stripping of fuel from the surface is not observed with PE. Therefore, the regression

6

rate data of PE tends to be significantly lower than paraffin fuel (3-4 times) as shown in Figure

7

16. Moreover, pure PE is more thermally stable and has higher value of viscosity compared to

8

paraffin which makes the PE to burn at slower regression rate.

9 10

The regression rates of P/PE/Al based formulations are below the value of paraffin wax, but

11

exhibit a higher value than that of P/PE10 formulation. This is related to the metal combustion

12

and higher specific surface area providing enhanced radiant heat transfer to the regressing

13

surface. As expected, the P/PE/Al25 formulation exhibits a significant higher regression rate

14

compared to P/PE10 formulation, which is facilitated by an increase in flame temperature in

15

combustion zone. In addition, the susceptibility of the liquid layer on the fuel surface to entrain

16

into combustion zone increases with decreasing viscosity and surface tension of the melt layer14.

17

The addition of Al in P/PE formulation does not affect the viscosity and hence mass entrainment

18

mechanism to combustion zone as compared to the pure P/PE fuel. The regression rates of all the

19

paraffin-based fuels are still much higher than that of traditionally used HTPB solid fuel for

20

hybrid rocket applications.

33


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 40

1 2

Figure 16. Comparison of regression rate for paraffin-based solid fuel formulations

3 4

The regression rate of pure paraffin is around 3-4 fold higher compared to polymeric HTPB

5

and PE based fuel. Whereas, the mechanical properties of pure paraffin fuel are very low to

6

sustain the various loads during the flight operation, hence restricting the full scale adaptability

7

of paraffin based fuels. A balance between the regression rate and mechanical performance of

8

paraffin-based fuel can be obtained by blending with a suitable reinforcement. Polyethylene

9

addition to paraffin wax can significantly improve the mechanical properties, thermal stability,

10

and combustion efficiency. Even though the regression rate decreased due to addition of PE, the

11

same can be compensated with the addition of Al additive.

34


Page 35 of 40

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Energy & Fuels

1

The achieved regression rate versus oxidizer mass flux trend can be approximated to the

2

following power law:

r& = aGoxn

3 4 5

(4)

Where the coefficient ‘ a ’ and exponent ‘ n ’are determined experimentally. Table 8. Comparison of regression rate exponents of paraffin-based fuel Fuel Sample

a

n

r& = aGoxn

P

0.22

0.52

0.22Gox0.52

P/PE5

0.06

0.71

r& = 0.06Gox0.71

P/PE/10

0.05

0.7

r& = 0.05Gox0.7

P/PE/Al5

0.07

0.67

r& = 0.07Gox0.67

P/PE/Al15

0.14

0.56

r& = 0.14Gox0.56

P/PE/Al25 HTPB18

0.11

0.63

r& = 0.11Gox0.63

0.061

0.50

r& = 0.061Gox0.50

6 7

It can be observed from Table 8 that the pure HTPB and paraffin displays the same

8

dependency (n=0.52-0.5) of regression rate on the oxidizer mass flux. The addition of the

9

additives in paraffin reported comparatively higher exponents (n=0.56-0.71) and hence

10

suggesting that the paraffin-based fuel tend to displays a similar regression rate behavior with

11

increasing oxygen mass flux. The value of oxidizer mass flux in the classical diffusion-limited

12

theories is reported as 0.8, which is reasonably higher than the value identified for Al-based

13

formulations in this study.

14 15

CONCLUSIONS

16

In this work, the thermal, mechanical and ballistic performance of paraffin-based solid fuel

17

was investigated using PE and Al as additives. We found that the addition of Al in P/PE blend 35


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 40

1

has shown a slight improvement in mechanical properties. Thermal analysis showed that the

2

ignition and burnout temperatures of the paraffin wax increased considerably with PE addition.

3

Whereas, the Al addition to P/PE blend led to lowering of ignition and decomposition

4

temperatures. The addition of Al increased the reactivity during combustion process and dropped

5

the peak oxidation temperature to lower values. The DSC study also showed that the addition of

6

Al significantly improved the exothermic heat release of these paraffin-based samples, which has

7

been attributed to the high thermal conductivity of Al. The large percentage of Al in P/PE blend

8

enhances the oxidation reaction and also the exothermicity of the fuel sample. The ballistic tests

9

showed that the regression rates of P/PE samples decreased as PE concentration was increased

10

from 5 to 10 wt%; whereas increasing Al doping from 5 to 25 wt. % increased the regression rate

11

by 95%. The regression rates of P/PE/Al based formulations were lower than the value of

12

paraffin wax, but exhibited a higher value than that of P/PE formulations. The theoretical

13

modeling of the combustion data followed a power law model for all the formulations.

14 15

REFERENCES

16

(1)

Mazzetti, A.; Merotto, L.; Pinarello, G. Paraffin-based hybrid rocket engines applications:

17

A review and a market perspective. Acta Astronaut. 2016, 126, 286–297 DOI:

18

10.1016/j.actaastro.2016.04.036.

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(2)

Chiaverini, M. J. Review of Solid-Fuel Regression Rate Behavior in Classical and

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Nonclassical Hybrid Rocket Motors. In Fundamentals of Hybrid Rocket Combustion and

21

Propulsion; Progress in Astronautics and Aeronautics; American Institute of Aeronautics

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and Astronautics, 2007; 37–126.

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(3)

Yuasa, S.; Shimada, S.; Imamura, T.; Tamura, T.; Yamoto, K. A technique for improving

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the performance of hybrid rocket engines. In 35th Joint Propulsion Conference and

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Exhibit; Joint Propulsion Conferences; American Institute of Aeronautics and

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Astronautics, 1999, AIAA 1999-2322. 36


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Behavior of Vortex Hybrid Rocket Engines. J. Propuls. Power 2002, 18 (3), 600–609

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DOI: 10.2514/2.5974.

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(5)

Kim, Y. J.; Sohn, C. H.; Hong, M.; Lee, S. Y. An analysis of fuel–oxidizer mixing and

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Aerosp. Sci. Technol. 2014, 37, 37–47 DOI: 10.1016/j.ast.2014.05.006.

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Kim, S; Kim, J.; Moon, H.; Sung, H.; Lee, J.; Kim, G.; Cho, J.; Park, S. Combustion

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Characteristics of the Cylindrical Multi-Port Grain for Hybrid Rocket Motor. In 45th

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AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit; Joint Propulsion

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Tian, H.; Li, X.; Zeng, P.; Yu, N.; Cai, G. Numerical and experimental studies of the

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hybrid rocket motor with multi-port fuel grain. Acta Astronaut. 2014, 96, 261–268 DOI:

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10.1016/j.actaastro.2013.12.001.

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Kumar, R.; Ramakrishna, P. A. Effect of protrusion on the enhancement of regression rate.

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Appendix-I The mass loss was used to calculate the regression rate and it is the most widely used method12, 13. The regression rate r& of the solid fuels is calculated by

r& =

d b − d ig

(A1)

2tb

9

Where db and d ig are the fuel port diameter at the strand after burnout and before the ignition

10

process respectively. The burn time, tb , is the time between the start ignition of the ignition

11

process and the extinction of the oxidizer flow. The fuel port diameter db , at the burnout

12

condition, is described by equation:

13

   2 mb  d b =  d ig +  π ρ f lf   4  

14

Where mb indicates the burnt mass of fuel, ρ f is the actual measured density of the fuel and

15

l f is the length of the fuel grain. The oxidizer mass flux rate is calculated by equation:

16

Gox =

m& ox Ap

(A2)

(A3)

17

& ox the oxidizer mass flow rate, and Ap is the combustion port cross-sectional area. The Where m

18

combustion port dimension is calculated by averaging the fuel port diameter at the strand after

19

burnout and before its ignition process:

20 21

AP =

π  db + dig   4

2

2

 

(A4)

40


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