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Experimental investigation of fuel composition and mix-enhancer effects on the performance of paraffin-based hybrid rocket motors

15 Experimental investigations have been performed for a lab-scale hybrid rocket motor with hydrogen peroxide 16 and paraffin-based propellants. A new paraffin-based solid fuel with favorable mechanical strength, consisting of 17 50 wt% paraffin, 20 wt% PE wax, 18 wt% EVA, 10 wt% SA and 2 wt% carbon, was proposed. The thermal 18 properties of the proposed paraffin-based fuel and its components were firstly evaluated using TG/DTG 19 measurements. It was found that the decomposition rates of each of the components of this paraffin-based fuel 20 follow the order SA > pure paraffin > paraffin-based fuel blends > PE > EVA. Measurements of the regression 21 rate of this paraffin-based fuel were performed and compared with results found in the literature. This showed 22 that the regression rate of this paraffin-based fuel is much higher than that of traditional polymer solid fuels such 23 as HTPB fuels. Finally, by adding a protrusion, a cavity and swirl blades at the end and in the middle of the 24 combustion chamber, experimental investigations into the effects of a mix-enhancer on the paraffin-based hybrid 25 rocket motor were performed. These investigations are discussed in detail. 26 27


29
A hybrid rocket motor (HRM) refers to a rocket propulsion system that uses both a solid and a liquid as 30 propellants. HRMs have multiple advantages compared to solid and liquid rocket motors: they offer high safety 31 in propellant production, storage, transportation and testing processes due to the separately stored oxidizer and 32 fuel [1][2][3][4]. Moreover, by regulating the flow rate of oxidizer, variation in the mixture ratio can easily be achieved, 33 and shutdown or restart is possible by closing or opening the oxidizer valve. HRMs can offer a higher specific 34 impulse and thus greater payload capability compared to a solid rocket motor. In addition, the HRM has only 35 half the pipework system required for a liquid rocket motor, and therefore the cost and complexity of the system 36 can be greatly reduced [5]. However, although long known, this type of propulsion has never drawn much 37 interest in the past, partially due to its low regression rate with polymer fuels commonly used as combustibles, 38 such as HTPB (hydroxyl-terminated polybutadiene), etc., and its low combustion efficiency caused by the 39 intrinsic combustion property that combustion occurs only in the turbulence boundary layer as a diffusion flame 40 [6]. 41 Due to the low regression rate of traditional polymer solid fuels used in hybrid rocket motors, for high thrust 42 levels fuel grains with multiple ports are necessary, which results in a low volumetric efficiency [7,8]. As well 43 as using the multiple-ports structure, various investigations have been performed to increase the regression rate 44 of hybrid rocket motors. It has been found that using a paraffin-based hybrid rocket motor is a possible 45 alternative for higher-thrust hybrid rocket motor applications [1,5,[9][10][11][12][13]. Paraffin-based fuel (liquefiable fuel) 46 has the advantage of a high regression rate due to its low melting point. It is found that paraffin-based fuel has a 47 paraffin, hardener, tackifier, plasticizer and carbon is proposed and evaluated by considering the trade-off 87 between achieving favorable mechanical strength to meet the requirements of practical applications and retaining 88 the advantage of a high regression rate. Thermogravimetric and derivative thermogravimetric analyses were also 89 performed for each component and paraffin-based fuel blend, to evaluate their thermal properties. Firing tests 90 were then conducted with different compositions of paraffin-based fuel grains, using a lab-scale hybrid rocket 91 motor with hydrogen peroxide as oxidizer. The regression rate of the proposed paraffin-based solid fuel in the 92 present work will be measured and compared with results from the literature. Finally, the effects of 93 post-combustion chamber length and adding a mix-enhancer (protrusion, cavity or swirl blade) on the 94 combustion performance of paraffin-based hybrid rocket motors will be investigated and discussed in detail. 95

96
It is well known that paraffin-based solid fuels have the advantage of a higher regression rate than conventional 97 polymer solid fuels such as HTPB-based fuels, but pure paraffin fuels have poor mechanical strength and usually 98 cannot meet the requirements for practical applications. Additives are necessary to improve the mechanical 99 strength. However, additives usually have a lower regression rate and poor combustion properties. The type and 100 quantity of additives should be carefully chosen by considering the trade-off between retaining the high 101 regression rate of the paraffin-based fuel and satisfying the mechanical strength requirement. In the current work, 102 additives, i.e., polyethylene wax (PE), hardener (stearic acid (SA)), binder (ethylene vinyl acetate (EVA)) and 103 toner (carbon powder) have been added to pure paraffin fuels to improve the mechanical strength. 104 The pure paraffin fuel (paraffin 58) used in the present work was supplied by the Sinopec Group with a melting 105 point of around 58°C. The polyethylene wax (PE) is used because it has better mechanical strength and thermal 106 stability than pure paraffin fuels. EVA acts as wax reinforcement among the different components because of its 107 high cohesive strength and good adhesion properties. Stearic acid is used for better mechanical properties for 108 paraffin waxes and better dipping properties for wax, mainly with a steel combustion chamber. The role of 109 carbon black is to improve the radiation absorption of the fuel, ensuring that most of the radiation from the flame 110 is absorbed at the fuel surface, to avoid sloughing of the solid fuel [24,25]. A summary of the physical 111 properties of these additives is given in Table 1   The aforementioned mixtures, consisting of pure paraffin, PE wax, SA, EVA and carbon powder, were preheated 118 to 433 K and held for 30 minutes (min.), ensuring that all the components were well melted and mixed to obtain 119 a homogeneous and delicate mixture. The melted mixtures were then cast using a high-speed rotating machine. 120 In the present work, three types of paraffin-based fuels with different compositions giving good mechanical 121 strength have been tested with a lab-scale hybrid rocket motor. The composition details of these three solid fuels 122 are listed in Table 2. 123

124
The experiments were conducted using a lab-scale hybrid rocket motor with paraffin-based fuel and hydrogen 125 peroxide (90 wt %) as propellants. As illustrated in Figure 1, the whole test facility consists of three major parts: 126 pipework, controls and instrumentation. In the pipework, two fluids are used, i.e., hydrogen peroxide serving as 127 the oxidizer and N2. A series of valves, a storage tank and various handling components have the purpose of 128 delivering the H2O2 to the combustion chamber. N2 has multiple uses: its primary purpose is to pressurize the 129 hydrogen peroxide tank, but it also serves as a combustion chamber purge, quenching the combustion after a test 130 or in the event of a test abort. Finally, it is used to actuate all the ball valves in the pipework system. For safety  (Figure 1). In the present work, the hydrogen peroxide injection time is controlled to between 3 s 150 and 10 s. Table 3 shows an example of sequential control parameters for a burning time of 5 s.

154
In order to investigate the effect of additives on the performance of a hybrid rocket motor, firing experiments 155 were firstly conducted with three types of solid fuel grains consisting of various components, as listed in Table 2. 156 Paraffin-based fuel 1, consisting of 50 wt% paraffin, 20 wt% PE wax, 18 wt% EVA, 10 wt% SA and 2 wt% 157 carbon, was used to characterize the regression rate of the proposed paraffin-based fuel. More than 30 firing tests 158 were conducted in total. All tests achieved successful ignition. Reignition tests were also performed with a time 159 interval of 10-30 s, by closing and reopening the oxidizer valves. It was observed that for both the ignition and 160 the reignition tests, a high-pressure combustion regime can be quickly established after opening the oxidizer 161 valve and no obvious combustion instability was observed. Finally, in order to investigate the mix-enhancer 162 effects on the performance of the hybrid rocket motor, firing tests were performed with three different 163 geometries of combustion chamber. The details of the experimental results analysis will be given in the 164 following sections. 165

166
The fuel grains are manufactured using casting machines and it was observed that the additives EVA, PE and SA 167 can effectively improve the mechanical strength of the solid fuel. All the three paraffin-based fuels with 8 compositions as listed in Table 2 can solidify to form a fuel grain with a smooth surface, both inside and outside, 169 and with favorable mechanical strength.

201
The fuel regression rate is a key parameter which will directly affect the internal ballistics of the hybrid rocket 202 motor. The regression rate of a solid fuel in a hybrid rocket motor is influenced by the propellant combination, 203 oxidizer mass flow rates, fuel types and fuel components. For a hybrid rocket motor, the regression rate is 204 usually assumed to be governed by the oxidizer flux. This assumption is widely used in experimental research 205 due to its simple expression [27][28][29][30]. In order to characterize the regression-rate properties of the new 206 paraffin-based fuel proposed in the present work, measurements of regression rate were performed. 207 The typical expression for regression rate is used in the present work, where is the regression rate, 208 is the oxidizer mass flux and are the constant coefficients to be determined by fitting to the 209 experimental data. Since the working time in the present study was short, the measurement methodology used 210 was the average differential weight method [29]. Ten firing tests were conducted using fuel grains with different 211 initial diameters to ensure different initial flux density. The fuel grain was weighed before and after the firing 212 test. As the geometry of the fuel grain is a regular cylinder with measurable inner and outer diameter, the 213 regression rate of the fuel grain can be determined by following expression: diameter is measurable; therefore, the inner diameter after the firing test can be expressed as: 219

Eq.2 220
where is the length of the fuel grain, is the density of the solid fuel and is the weight difference before 221 and after the firing test. It should be noted that is obtained from the pressure plot. It starts from the time 222 when the combustion chamber pressure increases to 90% of the steady value, and ends at the time when the 223 combustion chamber pressure drops to 90% of the steady value after the oxidizer valve is closed [29]. As shown 224 in Figure 6, the regression rates of the paraffin-based fuel proposed in the present work are plotted against 225 oxidizer flux density with lin-log axes and compared to experimental results found in the literature. It can be 226 seen that the paraffin-based fuel used in the current work has a higher regression rate than HTPB/GOx but has 227 lower values than paraffin/GOx [12]. This suggests that the paraffin-based fuel used in the present work can Combustion chamber pressure and thrust comparisons are plotted in Figure 8 and Figure 9. It can be seen that 247 even though a combustion chamber length of L=175 mm leads to slightly higher pressure and thrust values, the 248 differences among these three cases remain very small. Throughout the experiments, the average O/F ratios for 249 these three cases were calculated as 3.28, 4.32 and 5.64 (the flow rate of the oxidant was fixed at 43 g/s and the 250 fuel flow rate is obtained by taking into account the weight difference before and after the firing test and the 251 average t). It is worth noting that although the thrust remains similar, from the point of view of the specific 252 impulse the case of L = 100 mm has a better performance. Because the total flow of oxidant and fuel is reduced, 253 the resulting specific thrust is higher. If the thrust was the same for all three cases, then the rocket can produce 254 the same amount of thrust using a lower mass flow and is thus more mass-efficient.

256
As previously mentioned, the combustion efficiency in a hybrid rocket motor is generally lower than in a solid or 257 liquid fuel rocket motor, due to the intrinsic property that combustion occurs only in the turbulence boundary 258 layer. A mix-enhancer is necessary to improve the mixing of fuel and oxidizer. In order to investigate the effect 259 of the mix-enhancer on the combustion efficiency of a paraffin-based hybrid rocket motor, experiments were 260 performed in the present work by adding swirl blades, a protrusion and a cavity in the middle or at the end of 261 fuel grains located in the combustion chamber. The details of the dimensions for each case investigated in the 262 present work are given in Figure 10. In order to exclude the effect of the length of the fuel grain and the flow 263 rate of the oxidizer, the fuel grain length was kept at L = 110 mm and the flow rate of oxidizer was kept at 43 g/s 264 for all cases studied. For better comparison among different cases, a reference case without a mix-enhancer with 265 a fuel grain length of 110 mm was also tested (case 1). 266 (1) (2) (3) (4) (5) comparison between the reference case and the cases with swirl blades is presented in Figure 11. It can be seen 276 that both cases with swirl blades, whether located at the end or in the middle of the fuel grain, have higher 277 combustion pressure than the reference case. The primary consequence of swirl blades added after or in the 278 middle of the fuel grain is that the swirl flow enhances the mixing of the reacting chemical species involved in 279 the combustion process, leading to a more effective burning process, which in turn leads to higher combustion 280 efficiency. It was observed in the present work that the combustion pressure values of cases with swirl blades 281 increased by around 20% compared to the reference case. The results in the present work are in accordance with 282 the numerical investigation of Paccagnella et al. [20]. In their work, the effect of swirl injection on the 283 combustion performance of a hybrid rocket motor was numerically investigated and it was found that swirl 284 injection can effectively improve the combustion efficiency via better mixing of fuel and oxidizer. Comparisons were then made for cases where a cavity was added between two fuel segments (case 4), a 288 protrusion added after the fuel grain (case 5) and the reference case (case 1). As shown in Figure 12, by 289 separating the fuel grain into two parts with a cavity, a higher combustion pressure is obtained, compared to the 290 other two cases. The purpose of adding a cavity between the two parts of the fuel grains is to generate a flow 291 vortex in the combustion chamber and hence to improve the mixing of fuel and oxidizer. The dimension of 292 cavity is chosen with a depth-length ratio of 1<L/D<5 to ensure a stable recirculation zone can be established in 293 the cavity [31]. It appears that the vortex helps the transport of oxidizer away from the axis to be burnt with the 294 fuel at the interface of the vortex. Therefore, the interaction surface between the fuel and the oxidizer, and hence 295 the combustion efficiency, is increased compared with cases without a large-scale vortex. The results observed in 296 the present work are in accordance with previous numerical results reported in the work of Messineo et al. [32]. 297 It should be noted that adding a protrusion (case 5) does not lead to higher combustion pressure values compared 298 to the reference case. Experiments with a protrusion were repeated several times and the same trend was 299 observed. This differs from the previous numerical investigations of Bellomo et al. [33], although similar results 300 were reported in their experimental work, which showed that adding a protrusion in the combustion chamber led 301 to a pressure drop [34]. A possible reason for the pressure drop is that the position of the diaphragm was not the location of the protrusion in the combustion chamber. An explanation can also be found in [33], indicating 304 that the use of a protrusion alone does not result in any significant improvement in either the regression or the 305 combustion efficiency, especially for paraffin-based fuels, because the combustion of paraffin-based fuel is 306 partially dominated by droplet combustion. 307

308
Experimental investigations into the effects of fuel composition and a mix-enhancer on the combustion 309 performance of a paraffin-based hybrid rocket motor have been performed using a newly proposed 310 paraffin-based fuel consisting of pure paraffin, EVA, SA, PE wax and carbon, with H2O2 as the oxidizer. The 311 inferences drawn from the above studies can be summarized as follows. 312 From the results of TG/DTG, it is observed that up to 80% of the weight of all components is decomposed 313 between temperatures of 160°C and 300°C. The decomposition rate is observed to follow the order SA > 314 pure paraffin > paraffin-based fuel (blends) > PE > EVA. Moreover, EVA has a much higher 315 decomposition temperature compared to the other components. 316 A paraffin-based fuel consisting of 50 wt% paraffin, 20 wt% PE wax, 18 wt% EVA, 10 wt% SA and 2 wt% 317 carbon has favorable properties in terms of mechanical strength and a high regression rate compared to 318 HTPB/H2O2 as propellants. 319 Adding a mix-enhancer such as a cavity between the two parts of the fuel grains or adding swirl blades in 320 the middle of or after the fuel grain, can effectively improve combustion performance. Adding a protrusion 321 alone does not result in any significant improvement in combustion efficiency. 322 Further numerical or combustion visualization diagnostics in the combustion chamber are necessary to 323 understand the physics related to flow patterns, vortex shedding, etc. when a mix-enhancer is added.