Jun Chen,Guoxiu Li*,Tao Zhang,Yinglong Liu,Rui Yang,Yang Chen

1 School of Mechanical,Electronic and Control Engineering,Beijing Jiaotong University,Beijing 100044,China

2 Beijing Institute of Control Engineering,Beijing 100190,China

3 Beijing Engineering Research Center of Efficient and Green Aerospace Propulsion Technology,Beijing 100190,China

Keywords:ADN-based thruster Catalytic bed structure Operation parameters Decomposition and combustion Thruster

ABSTRACT The decomposition and combustion characteristics of ammonium dinitramide(ADN)based non-toxic aerospace propellant are analytically studied to determine the effects of catalytic bed structure(slenderness ratio)and operation parameters(mass fraction ratio of ADN/CH3OH)on the general performance within the ADN-based thruster.In the present research,the non-equilibrium temperature model is utilized to describe the heat transfer characteristics between the fluid phase and solid phase in the fixed bed.We determined the fluid resistance characteristics in the catalytic bed by experiments involving the method of pressure-mass.We have done the simulation study based on the available results in the literature and found the complex physical and chemical processes within the ADN thruster.Furthermore, an optimized catalytic bed slenderness ratio was observed with a value of 1.75 and the mass fraction ratio of 5.73 significantly influenced the propellant performance.These results could serve as a reference to explore the combustion characteristics within the thruster and the preparation of future propellants.

1.Introduction

Environment-friendly fuels have elicited much attention in recent years due to the advantages of higher efficiency,lower pollution rate,low cost, and so on [1-4]. For example, hydroxylammonium nitrate(HAN)[5],hydrogen peroxide(H2O2)[6],hydrazinium nitroformate(HNF)[7]and ammonium dinitramide(ADN)-based are the common monopropellants. The most effective alternative to the hydrazine propellant is ADN, NH4[N (NO2)2]-based monopropellant, which is currently used in space-borne thrusters [8] has higher energy, low toxicity and the decomposition products are non-toxic.

Studies of ADN-based green monopropellant started in closely three decades before;initially,Anflo and his colleagues[9]mixed ADN with glycerol and water and named the mixture as liquid propellant(LMP)-101. Afterwards, to improve the thruster performance researchers have added different types of fuel including methanol and ethanol by considering the amount of oxidant products involved in liquid ADN-based propellant catalytic decomposition. Examples include LMP-103S and the Swedish defense agency liquid propellant (FLP)-(103, 105-107) [8,10,11]. LMP-103S propellant that consists of 60%-65%ADN,15%-20%methanol,9%-22%water,and 3%-6%ammonia may be considered as the next-generation propellant[12].In June 2010,the PRISMA(Prototype Research Instruments and Space Mission technology Advancement) satellite, which composed of two thrusters based on LMP-103S propellant,successfully achieved catalytic decomposition and the combustion performance.The demonstration on the orbital test flight was performed successfully by ADN-based liquid propellant[13-19].

Niklas Wingborg and his colleagues first characterized the ADN-based liquid propellant catalytic decomposition[20].They investigated the means of ignition and reported that the catalytic particles could be re-selected, unlike the commonly used hydrazine propellant (using Shell-405). Gr?nland and his colleagues used an engineering model and experimentally showed the performance of a new type of catalytic particles[21].They managed to produce catalytic particles with appropriate activity within 66 min which could provide strong support for the follow-up study.Afterwards,liquid ADN catalytic decomposition process was investigated by Kamal Farhat et al.[22]by using the methods of differential thermal and thermo gravimetric analyses (DTA-TGA).They demonstrated that the decomposition process occurred at a temperature lower than the decomposition temperature when no catalyst was used.Rachid et al.[23]also investigated the thermal and catalytic decomposition of liquid ADN propellant and concluded that adding catalyst could effectively reduce the initial decomposition temperature of liquid ADN propellant.

Fig.1.Schematic of the working process of the ADN thruster.

The above literatures systematically explored and carried out indepth investigation about the catalytic bed structure filled with catalytic particles;the operating parameters,such as the proportion of propellant; the properties of catalyst which had significant effect on the decomposition and combustion characteristics of the ADN-based thruster.In the present research,we have investigated spray,evaporation,heat and mass transfer in the catalytic bed,catalytic decomposition and combustion processes of the models of monopropellant thruster.Two temperature models that consider about the heat transfer between solid particles and the multinational gas temperature gap established by the ADN-based propellant decomposition and combustion reactions have been applied to investigate the heat transfer characteristics in the fixed bed.The flow characteristics of the fixed bed and the modified Ergun equation was obtained by using a new type of catalytic particles in the experiment and designed to withstand the high temperature unlike the catalyst of the hydrazine propellant used traditionally.Finally,we investigated the decomposition and combustion processes by using 3D computational fluid dynamics(CFD)model and analyzed the effects of catalytic bed structure and operating conditions of monopropellant thruster by incorporating catalytic bed slenderness ratio,mass fraction ratio of ADN/methanol in the fixed bed.

2.Numerical Stimulation,Mathematical Model,and Metho d

2.1.Physical and mathematical model

Fig.1 depicts a schematic of the ADN thruster operating process.The ADN thruster was consisted of various physical and chemical processes which could be divided into five zones:(a)the zone where the jet hit the wire barrier net;(b)multi-component evaporation zone;(c)the zone where heat and mass transfer of porous medium occurred;(d)ADN propellant catalytic decomposition and methanol combustionzone; (e) thrust production zone. The ADN-based propellant was composed of three substances: namely, ADN, methanol, and water with a mass fraction value of 0.63, 0.11, and 0.26, respectively. The mass flow rate of 0.48 g·s-1was set as an inlet boundary,and pressure was adopted as an outlet boundary conditions. The porosity and preheating temperature were 0.5 and 200°C,respectively.The structure parameters of ADN-based thruster are listed in Table 1.

Table 1 The structure parameters of ADN-based thruster

Fig.2 presents the experimental devices in which we acquired the flow characteristics of porous media in the catalytic bed and the flow resistance by using coupling pressure drop and flow rate.

The tightness of whole system was investigated at the initial stage and the outflow of the propellant was controlled by high-pressure nitrogen gas through a mass flow controller. A digital pressure gage was employed to achieve the pressure drop. The weighing method was applied to measure the water mass flow.Finally,we achieved the modified Ergun equation[24]by fitting the present experimental data,composed of viscous inertia resistances.The flow resistance in the catalytic bed filled with catalyst particles is shown in Eq.(1).(See Table 2.)

2.2.Chemical reaction kinetics model

The ADN-based propellant initially decomposed into ammonium nitrate(AN), N2O,intermediate oxidant products of O2and NO2and finally reacted with CH3OH.Fig.3 presents the schematic of chemical reactions model with 7 reactions and 11 species and justified by comparing with the previous data in the literatures[30-38].

2.3.CFD modeling computational methods

Momentum,continuity,energy,and species equations were combined and solved by ANSYS FLUENT CFD software [39,40]by using a second-order upwind scheme.The coupling equations with pressure and velocity were solved by the SIMPLE algorithm.Mesh independence was investigated by using different mesh sizes with a total mesh numbers of 34504, 50662, 73647, and 114380. Fig. 4 depicts about the mesh size with 73,647 cells to meet the demand of thruster performance. The measuring point pressure stabilized at 0.53 MPa at the combustion chamber,and the corresponding error became 11.6%,as compared with the experimental data from the previous literature[41].

Fig.2.Schematic of the catalytic bed flow resistance testing devices.

3.Results and Discussions

3.1.Effect of catalytic bed slenderness ratio on the catalytic decomposition and combustion characteristics

Fig. 5 presents the distribution of components and temperature changes with the catalytic bed slenderness ratio of the ADN thruster at different sections, including the middle area of the catalytic bed, the outlet of the catalytic bed,and the outlet of the combustion chamber.Nine different catalytic bed slenderness ratio structures were considered by setting the catalytic bed length as 17 mm,19 mm,and 21 mm with the catalytic bed diameter changing from 9 mm,and 10 mm,to 12 mm,correspondingly.The catalytic bed slenderness ratio increased from 1.42(L/D=17/12),1.58(L/D=19/12),1.7(L/D=17/10),1.75(L/D = 21/12), 1.89 (L/D = 17/9), 1.9 (L/D = 19/10), 2.1 (L/D =21/10),2.11(L/D=19/9)to 2.33(L/D=21/9).

The temperature gradually increased from the middle area of the catalytic bed,outlet of the catalytic bed to the outlet of the combustion chamber and the maximal value obtained at the combustion chamber when the slenderness ratio was 1.75 with a catalytic bed length of 21 mm and diameter of 12 mm,as shown in Fig.5(g).The concentrations of the corresponding components represented the contrary variation with gradually declining at different section. However, the concentration of O2showed a lower value of 8.74×10-4(about 22.6%lower than that at the outlet of the catalytic bed)at the middle area ofthe catalytic bed with the slenderness ratio of 1.89.Such phenomenon is mainly attributed to the distribution of components related to the temperature,which could be determined by the endothermic or exothermic chemical reactions.The decomposition reaction of AN and combustion reaction between HCOOH(with a high value of 6.32×10-4)and O2is not reacted timely, which lead to mentioning the lower concentrations of O2at the middle area of the catalytic bed.

Table 2 Main governing equations in the ADN thruster[25-29]

Fig.3.Schematic of ADN-based propellant decomposition and combustion path.

Fig.4.Steady-state combustion pressure at various mesh resolutions.

Fig.5.Distribution of components and temperature changes with the catalytic bed slenderness ratio in the ADN thruster at different sections.

Fig.5(a-f)shows that the maximum temperature can be obtained with the increase of diameter by fixing the length of 17 mm while temperature gradually changes from 1536.6 K,1554.7 K to 1532.7 K at the middle zone of the catalytic bed outlet. The corresponding value for the catalytic bed and combustion chamber outlet increased from 1616.5 K and 1646.4 K,1673.9 K and 1699.2 K to 1632.9 K and 1672.1 K,respectively.The results indicate that an extremely large or small diameter has a remarkable effect on performance. When the catalytic bed diameter increases,the interaction of the propellant and catalytic particles becomes effective.However,the flow resistance also increases,and the combustion reaction fails to effectively mix with the combustion at the same time.A small diameter leads to fast flow and fails to fully decompose.The catalytic bed length is fixed with 19 mm and 21 mm with the change in diameter;the catalytic bed slenderness ratio increases; the concentration of components decrease; thereby the temperature in the catalytic bed drops.A similar type of variations has been observed also at the outlet of the combustion chamber.The results indicate that the free space is beneficial to the combust and mix for the fuel.Besides,the heat feedback and thermal storage in the fixed bed have a noteworthy influence on the rate and degree of chemical reactions.

Fig. 6 shows the steady-state performance of the ADN-thruster,which includes the pressure drop and specific impulse at different catalytic bed slenderness ratios with the value increasing from 1.42,1.58,1.7,1.75,1.89,1.9,2.1,2.11 to 2.33.The diameter of the catalytic bed has changed from 9 mm, 10 mm to 12 mm with a fixed length of 19 mm and 21 mm,respectively;there is a noticeable increase of thrust performance.Such phenomenon is mainly attributed to more space and furnished with a larger diameter,which is beneficial to the components mixture,decomposition,and combustion.However,when the length of the catalytic bed is 17 mm,the thrust performance first increases then decreases.The results indicate that the flow resistance of the catalytic bed increases with a large value of diameter.In contrast,small value of diameter has been observed with the increase in velocity at a short reaction time.Based on the comparison and analysis,we have found an optimal matching relation between catalytic bed length and diameter with a peak pressure of 0.556 MPa and specific impulse of 217.8 s,which is higher than the standard value when the slenderness ratio is 2.11 with a catalytic bed length of 19 mm and a diameter of 9 mm.

3.2.Effect of the mass fraction ratio of ADN/CH3OH on the catalytic decomposition and combustion characteristics

Fig.7 shows the distribution of components and changes of temperature with the mass fraction ratio of ADN/CH3OH in the ADN thruster at different sections, including the middle area of the catalytic bed, the outlet of the catalytic bed,and the outlet of the combustion chamber.Five different conditions have been established: value of 2.53 with ADN/CH3OH mass fraction of 0.53/0.21, 3.625 with ADN/CH3OH mass fraction of 0.58/0.16, 5.72 with ADN/CH3OH mass fraction of 0.63/0.11,11.33 with ADN/CH3OH mass fraction of 0.68/0.06,and 73 with ADN/CH3OH mass fraction of 0.73/0.01.

Fig.6.Performance of the ADN thruster with a change in the catalytic bed slenderness ratio.

The temperature gradually increases from both the middle and outlet of the catalytic bed to the outlet of the combustion chamber and the maximal value reaches at the combustion chamber when the mass fraction ratio of ADN/CH3OH is 5.72, as shown in Fig. 7(g).However, there exists an unremarkable disparity of temperature at the middle area of the catalytic bed and the outlet of the catalytic bed,likewise, the components, such as AN, CH3OH has represented the similar regulations.Such phenomenon is mainly attributed to the high temperature zone has a forward lead which is determined by the endothermic or exothermic chemical reactions.In contrast,with the increase in mass fraction ratio(11.33)of ADN/CH3OH,the lower concentration of CH3OH fuel poorly affects the combustion process,as presented in Fig.7(a-f).The remaining components,such as HCOOH(generated by the reaction of CH3OH and NO2)and O2have been observed at different region. Furthermore, when the mass fraction ratio of ADN/CH3OH is 73, a great quantity of the remaining components, such as N2O(generated by ADN)and NO2(generated by AN),have been observed in different sections.These results indicate that the combustion processes cannot activate with a lower mass fraction (0.01) of CH3OH.However, a relatively low temperature leads to the decomposition process cannot activate timely.

When the ADN/CH3OH mass ratio increases from 2.52, 3.62, to 5.72,N2O(decomposed by AN)decreases from 2.29×10-3,1.21×10-3to 7.54×10-4at the outlet of the catalytic bed.The component HCOOH decreases from 1.13×10-4,9.61×10-5to 6.48×10-5at the outlet of the catalytic bed at a reduction rate of 46%and 35%,as compared with the result of that in ADN/CH3OH(mass ratio of 5.72 at the outlet of the combustion chamber). These results indicate that the oxidant products,such as oxygen and NO2(produced by decomposed ADN)developed by increasing the concentration of fuel CH3OH.Moreover,the temperature at different locations increases from 1486.2 K,1494.5 K,to 1573.3 K at the middle zone of the catalytic bed;raises from 1486.2 K,1573.7 K,to 1646.5 K at the outlet of the catalytic bed;increases from 1521.9 K,1607.9 K, to 1681.2 K at the outlet of the combustion chamber.However, when ADN/CH3OH increases to 11.33, especially with the ratio of 73, the temperature decreases from 1646.5 K, 1503.4 K to 810.1 K at the outlet of the catalytic bed.The decrease rate has been observed with 9%and 51.6%,as compared with the result of ADN/CH3OH(mass ratio of 5.72 at the outlet of the combustion chamber).Oxidation of the intermediate products decomposed by ADN has been developed by increasing the concentration of CH3OH.For instance,NO2increases from 1.39×10-5,3.79×10-2to 0.118 at the outlet of the catalytic bed and the concentration changes from 1.14×10-5,3.82×10-2to 0.118 at the outlet of the combustion chamber.As a combustion agent,when methanol decreases to a minimum mass fraction of 0.01,the decomposition and combustion reaction efficiency decreases and eventually leads to a degradation.

Fig.8 depicts the steady-state performance of the ADN thruster with different ADN/CH3OH mass ratios.With the increase of ADN/CH3OH mass ratio(2.52, 3.62 and 5.72), the maximal pressure gradually increases in the combustion chamber and the corresponding values are 0.521 MPa,0.538 MPa and 0.526 MPa,respectively.Meanwhile,the corresponding specific impulse has been observed by increasing 5.0%and 2.8%,as compared with the value of 206 s when the ADN/CH3OH mass ratio is 2.52.The results indicate that the increase of methanol has a significant influence on the combustion with NO2and O2by considering the catalytic decomposition products of ADN,and exhibiting excessive oxidation.However,for ADN/CH3OH,the mass ratios are 11.3 and 73 for the observed pressure of 0.525 MPa and 0.369 MPa,respectively and the corresponding specific impulses are 202.6 s and 137.2 s,respectively.Specific impulse decreases by 6.3%and 36.6%and the pressure decreases by 5.1%and 33.3%,as compared with the results of the mass ratio of 5.72 which indicate that the oxidants(decomposed by ADN)are not associated with a large number of combustion by increasing the fuel methanol. Eventually, we have found an optimal matching relation between mass fraction of ADN and methanol with a specific impulse value of 216.2 s, which is higher than the standard value when the ADN/CH3OH mass ratio is 5.72 with a value of ADN of 0.63 and CH3OH of 0.11.

Fig.7.Distribution of components and temperature changes with the mass fraction ratio of ADN/CH3OH in the ADN thruster at different sections.

4.Conclusions

We investigated numerically a new type of environment-friendly ADN-based thruster. The propellant atomization, evaporation, heat transfer in the fixed bed, and the flow characteristics by modified Ergun model have been examined through the experiment.

The complex physical and chemical processes have been observed within the ADN-based thruster.The steady-state combustion pressure could reach 0.53 MPa at the measuring point position which showed a good agreement with previous experimental data. The catalytic bed slenderness ratio and mass fraction ratio of ADN/CH3OH have been investigated.The results revealed that with a fixed diameter of 9 mm,10 mm and 12 mm,the thruster performance increased with the increase of catalytic bed length. When the catalytic bed slenderness ratio reached to 2.11,the thruster exhibited better performance than the other structures with a specific impulse of 217.8 s.Moreover,an optimal value with better performance between the oxidizing agent and the fuel has been achieved with a mass fraction ratio (5.72) of ADN/CH3OH.The present study would help the future research for the preparation of propellant and thruster characteristics.

Fig.8.Performance of the ADN thruster with a change in the ADN/CH3OH mass ratio.