| In this dissertation,the propulsion performances of micro-nano space thrusters driven by electroosmosis or pressure in micro-nano channels are studied.The propulsion performance includes specific impulse,thrust,efficiency and thrust-power ratio of the thrusters,which are key factors to evaluate whether the micro-nano thrusters meet the requirements of different space missions.Through theoretical analysis and numerical calculation,the influences of soft channels and related effects,high zeta potentials and non-Newtonian fluids on the performances of space thrusters are studied,respectively.These studies are divided into the following six parts:(1)Electroosmotic thrusters in soft nanochannels for space propulsion.Space propulsion of electroosmotic thrusters(EOTs)with a soft charged nanochannel is investigated considering the Navier slip boundary and constant surface charge density on the walls of slit channels.The soft nanochannel is characterized by a wall-grafted ion-penetrable charged polyelectrolyte layer(PEL).The PoissonBoltzmann equation is solved to give the electric potential distribution based on the assumption of the Debye-Hückel linearization for the low electric potential.An analytical solution of the electroosmotic velocity through the soft channel is obtained.The thrust,specific impulse,and total input power of EOTs produced by the electroosmotic flow are presented,and then,two significant physical quantities,thruster efficiency and thrust-to-power ratio,are described.It is found that these performance curves strongly depend on the slip length,surface charge density on the walls,drag coefficient,equivalent electric double layer thickness,PEL thickness,and density ratio of the PEL to the electrolyte solution layer.By analyzing and optimizing these design parameters,the simulated EOTs can deliver the thrust from 0 to 10?N as well as the specific impulse from 40 to 100 s,and the thruster efficiency up to 87.22%is realized.If more thrust control and kinetic energy are needed for different space missions,an array composed of thousands of single EOT emitters is constructed and maintains high thruster efficiency.Moreover,during mission operation,the total potential can be simply varied to optimize the performances of thrusters at any moment.(2)Space electroosmotic thrusters in ion partitioning soft nanochannels.Space electroosmotic thrusters(EOTs)are theoretically investigated in a soft charged nanochannel with a dense polyelectrolyte layer(PEL),which is considered to be more realistic than a low-density PEL.When the PEL is dense,its permittivity is smaller than the one of the electrolyte solution layer,leading to rearrangement of ions in the channel,which is denoted as the ion partitioning effect.It is noted that fluid viscosity becomes high within the PEL owing to the hydration effect.An analytical solution for electroosmotic velocity through the channel is obtained by utilizing the Debye-Hückel linearization assumption.Based on the fluid motion,thruster performances,including thrust,specific impulse,thrust-to-power ratio,and efficiency,are calculated.The ion partitioning effect leads to enhancement of the thruster velocity,while increase of the dynamic viscosity inside the PEL reduces the flow rate of the fluid.Therefore,these performances are further impacted by the dense soft material,which are discussed in detail.Moreover,changes or improvements of the thruster performances from the dense PEL to the weak PEL are presented and compared,and distributions of various energy items are also provided in this study.There is a good result whereby the increase in electric double layer thickness promotes the development of thruster performances.Ultimately,the simulated EOTs produce thrust of about 0 to 20?N and achieve thruster efficiency of 90.40%,while maintaining an appropriate thrust-power ratio of about 1.53 m N/W by optimizing all design parameters.(3)Steric effects on electroosmotic thrusters in soft nanochannels.The influence of steric effects on the performances of space electroosmotic thrusters(EOTs)was numerically delineated in soft nanochannels for which its walls are covered with polyelectrolyte materials.The size effect of the ionic species,namely the steric effect,is neglected in many previous research studies,but it has vital influences on electrostatic potential and electroosmotic velocity,which is further introduced into the present study in order to understand and improve the exploration of nano electroosmotic thrusters with soft channels.The thrust,specific impulse,total input power,thruster efficiency and thrust-to-power ratio are computed based on finite difference methods.It is found that the thruster thrust and specific impulse increase with the steric parameter while the efficiency and thrust-to-power ratio possess opposite trends due to the enhancement of Joule heating dissipation.For real situations with the consideration of ion size,although the thruster thrust could be promoted,the efficiency is only 30-70%,and the peak values of thrust-to-power ratio fade away.(4)Steric effects on electroosmotic nano-thrusters under high zeta potentials.This paper numerically presents a study of space electroosmotic thrusters in a rigid nanochannel with high wall zeta potentials,considering effect of finite size of the ionic species.The effect,which is called as steric effect,is often neglected in many researches about micro/nano thrusters,however,it has vital influences on electric potential and flow velocity in electric double layers so that thruster performances generated by fluid motion are further affected.These performances including thrust,specific impulse,thruster efficiency and thrust-to-power ratio are described via using numerical algorithms,after obtaining the electric potential and velocity distributions under the high wall zeta potentials ranging from-25.7m V to-128.5m V.As is expected,the zeta potentials can promote development of thruster performances so as to satisfy requirement of space missions.Moreover,for the real situation with consideration of the steric effect,thruster thrust and efficiency significantly decrease to 5-30 micro Newtons and 80-90 percent,respectively,but the thrust-to-power ratio is opposite,which expends short a specific impulse about 50-110 seconds.(5)Micro pressure-driven thrusters under streaming potential effect.A mathematical model of micro pressure-driven thrusters is presented and investigated for the first time in the study,considering the influence of induced streaming potential field.The pressure drop between two ends of the microchannel ranges from 1 to 30 MPa and the height of the microchannel is set as 10?m.After obtaining the expression of the streaming potential via using the electrically neutral condition in the channel,the analytical solution for the flow velocity is acquired in full.Based on the fluid motion,thruster thrust,specific impulse and total input power are firstly generated so that thrust-to-power ratio and thruster efficiency are calculated for the pressure-driven micro-thrusters.It is found that,although the streaming potential field resists against the fluid flow,the present micro thruster driven by the pressure is able to obtain thrust about 0 to 8000?N and thrust-to-power 0 to 1400?N/W,which expends short a specific impulse about 100 to 4000 ms.It has to mentioned that excessive viscosity dissipation leads to low thruster efficiency about only 3%.Moreover,the specific impulse,thrust and thruster efficiency increase with the pressure drop,while the variation of the thrust-to-power ratio is opposite.The enhancement of the reciprocal of the EDL thickness promotes development of the thruster performances.(6)Micro electroosmotic thrusters of power-law fluids for space propulsion.Electroosmotic thrusters(EOTs),which are full of non-Newtonian power-law fluids with flow behavior index n of the effective viscosity,are theoretically investigated in a microchannel.Different values of the flow behavior index represent two kinds of non-Newtonian power law fluids,pseudoplastic fluid(n<1)and dilatant fluid(n>1),which have not yet been considered as a propellant in micro thrusters.Analytical solutions of the electric potential and flow velocity are obtained under the Debye-Hückel linearization assumption and the approximate scheme of hyperbolic sine function.Then thurster performances of power-law fluids including specific impulse,thrust,thruster efficiency and thrust-to-power ratio,are explored in details.Results show that these performance curves strongly depend on the flow behavior index and electrokinetic width.It is noted that the non-Newtonian pseudoplastic fluid is most suitable as a propeller solvent in micro electroosmotic thrusters owing to improving or optimizing deficiencies of these performances of the existing Newtonian fluid thrusters. |
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