Heat exchange structure design and overall performance analysis of double-concentrator solar thermal thruster

Highlights

• The platelet heat exchanger has the advantages of small volume and high heat exchange efficiency.

• The empirical correlation of Nusselt number is fitted by infinitesimal method.

• The 1D simplified model of platelet is established by Simulink and verified by 3D CFD simulation with less than 8.5% error.

• The performance of nanosatellites is doubled by the platelet heat exchanger.

Abstract

In this paper, the platelet heat exchanger in the thermal propulsion system of micro satellite with liquefied gas as propellant was designed, the complete gasification and stabilization scheme of propellant was proposed. The unsteady conjugate heat transfer (CHT) characteristics between the heat exchanger and gaseous propellant were analyzed, and the cumulative data of error estimation in the numerical simulation process was provided to ensure the credibility of the results. The CHT computational fluid dynamics (CFD) simulation of the platelet 3D model under steady-state conditions was carried out to find the empirical correlation between the average Nusselt number and the average Reynolds number. A simplified unsteady 1D model of heat exchanger was established by using the loose coupling method based on quasi steady flow field. The results show that the platelet structure could heat the working medium to more than 2379 K with the heat transfer efficiency of 86%, the outlet temperature and heat transfer efficiency of the heat exchanger were maintained at 1871 K and 69% in steady state when the mass flow rate was 1.14e-6 kg/s. The 1D model could accurately reflect the real heat transfer situation to a certain extent, the simulation error was less than 8.5% compared with the 3D model, and the calculation time was greatly shortened, making the adjustment of heat transfer strategy more convenient. Intermittent heat transfer method could keep the outlet gas temperature, thrust and specific impulse of the thruster at a high level to improve the performance of the system. Taking the nanosatellite with a total mass of 7.55 kg and a propellant mass of 0.25 kg as an example, when the duration of valve open time was 1.5s, the velocity increment of the satellite reached 72.21 m/s with the specific impulse of over 2400 m/s, which was twice than that of nanosatellites without heat exchange structure.

Introduction

In recent years, human beings have higher requirements for the propulsion performance of satellites with the more complex space tasks. The chemical propulsion system can provide large thrust in an instant, but its specific impulse is only 100–400s and requires a lot of fuel [1]. Electric propulsion is the research hotspot at present, which is used in orbit transfer, formation flight and on orbit attitude maintenance. Although its specific impulse can reach more than 1000s, the typical thrust is relatively low, generally 1 μÑ400 mN, it means more time will be taken to complete the task at the mean time [[2], [3], [4]]. The 40 cm ion thruster developed by NEXT (NASA's evolutionary xenon thruster) had a thrust of only 364 mN at a power of 10.5 kW [5]. A miniature vacuum cathode arc thruster was designed with a specific impulse of 1571s and a thrust to power ratio of 16.3 μN/W [6]. A very low power cylindrical Hall thruster for nano-satellite “PROITERES-3” under development in Osaka Institute of Technology was designed with a specific impulse of 1570s and a thrust of 2.87 mN [7]. Although the technology of cold gas propulsion is mature, it does not make full use of the properties of working medium, so the thrust and specific impulse are correspondingly low, which provided the thrust of 70–1200 mN [8,9]. By contrast, the solar thermal propulsion system can obtain a large velocity increment due to its high specific impulse and moderate thrust, which can make up for the gap in space transfer tasks [[10], [11], [12]].

Traditional solar thermal thrusters are mainly installed on satellites with compressed gas as propellant [13,14]. However, with the development trend of miniaturization of spacecraft, gas propellant gradually withdraws from the stage and is replaced by liquefied gas propellant, which has the advantages of high density and high safety factor [15]. In order to improve the specific impulse of spacecraft, propellant usually needs to be discharged after gasification. Although it is feasible to use microchannel heat exchange structure to make liquid propellant phase change in the heat exchange channel to produce gas, the gasification efficiency is low with unstable thrust, which may lead to blockage in the heat exchange channel [16,17]. Thus, the gasification of propellant should be finished before entering the heat exchanger to ensure performance, which can be easily achieved by solar thermal propulsion (STP) system.

The heat exchanger is the most important structure in the solar thermal thruster, and its heat exchange efficiency has a significant influence on the performance of the whole propulsion system. Most of the early heat exchangers use spiral channel structure, which has the characteristics of simple structure and easy processing. Markopoulos P. et al. [18] designed a direct endothermic thrust chamber. In the experiment, H₂ was used as propellant, the inlet working medium mass flow was 0.25 g/s, the temperature was 333 K, the thrust chamber pressure was 0.1 MPa, and the temperature after heating reached 2264 K. Although the spiral heat exchanger could heat the working medium to the ideal temperature, the length of the spiral channel had reached 6 m. The increase of the length of the chamber meant the increase of the mass and the decrease of the performance of the thruster. Therefore, it is necessary to design a more efficient heat exchanger. The platelet structure is mainly used for transpiration cooling in rocket engine [19,20]. Xing B.Y. et al. designed a multi-layer platelets heat exchanger in STP system based on the transpiration cooling technology of platelet [21]. In their study, the number of platelets was 20 layers, the thickness of a single platelet was 1 mm, and the depth of the control channel was 0.1 mm, the heat transfer area of the propellant between the platelets was about 5–10 times larger than that of the spiral channel under the same conditions.

In fact, the metal wall temperature cannot be maintained above 2400 K all the time due to the coupling effect. The intermittent heating strategy should be adopted to keep the working medium at a high temperature, hence the need for an analysis of the unsteady heat transfer of the heat exchanger. However, the amount of calculation of 3D CFD simulation is huge, the calculation time may reach tens or even hundreds of hours according to the difference of calculation accuracy and time steps. Therefore, researchers have developed solutions with different calculation efficiency and accuracy for different problems. The transient tight coupling method can get effective results, but it consumes a lot of computing resources and is difficult to solve practical complex engineering problems [[22], [23], [24]]. The loose coupling method based on the quasi steady flow field considers that in the whole process of fluid-solid coupling heat transfer, the flow field is in several quasi steady states, and each quasi steady flow field is solved by the steady-state Navier-Stokes equations, the research results show that the algorithm can greatly improve the calculation efficiency, but the deviation of the calculation results is large, which is mainly due to the large deviation between the treatment method of completely isolating the flow field from other parts and the actual coupling relationship [[25], [26], [27]].

To solve these problems, this paper designed a new platelet structure based on the temperature requirements, and provided a scheme of propellant gasification and pressure stabilization. According to the symmetry of the structure, the platelet was simplified to 1D model. A new loose coupling algorithm for global transient tight coupling heat transfer based on quasi steady flow field was adopted, the steady-state algorithm was used to update the flow field and solve the energy equations of fluid and solid. The results of 3D CFD simulation and 1D simulation were compared and analyzed, and the propulsion performance of the satellite under different heat exchange strategies was further discussed.