Jet Propulsion by Microwave Air Plasma in the Atmosphere” [AIP Adv

Comment on “Jet propulsion by microwave air plasma in the atmosphere” [AIP Adv. 10, 055002 (2020)]

Cite as: AIP Advances 10, 099101 (2020); https://doi.org/10.1063/5.0013575 Submitted: 12 May 2020 . Accepted: 01 August 2020 . Published Online: 15 September 2020

Peter L. Wright , Stephen A. Samples , Nolan M. Uchizono , and Richard E. Wirz

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AIP Advances 10, 099101 (2020); https://doi.org/10.1063/5.0013575 10, 099101

Comment on “Jet propulsion by microwave air plasma in the atmosphere” [AIP Adv. 10, 055002 (2020)]

Cite as: AIP Advances 10, 099101 (2020); doi: 10.1063/5.0013575 Submitted: 12 May 2020 • Accepted: 1 August 2020 • Published Online: 15 September 2020

Peter L. Wright,a) Stephen A. Samples,b) Nolan M. Uchizono,c) and Richard E. Wirza)

AFFILIATIONS Mechanical and Aerospace Engineering Department, University of California, Los Angeles, California 90095, USA

a)Authors to whom correspondence should be addressed: [email protected] and [email protected] b)Electronic mail: [email protected] c)Electronic mail: [email protected]

ABSTRACT In this Comment, we analyze the performance of a microwave plasma device presented by Ye et al. [AIP Adv. 10, 055002 (2020)]. The efficiency analysis, using conservation of energy, shows that the methods used by the original authors predict up to 8000% deviceefficiency. Our analytical model is based on a control volume analysis of the original authors’ experimental setup and conditions, indicating that blocking the exit of the device yields stagnation pressure rather than jet pressure. The results from this analysis are consistent with the reported experimental data, demonstrating that the measured pressure using this method is internal chamber pressure and cannot be used to estimate thrust. © 2020 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0013575., s

In a recent article, Ye et al.1 demonstrated microwave ioniza- control volume analysis, we show that the authors’ proposed thrust tion of air in a quartz tube. The authors attempted to estimate the estimation technique leads to a significant overestimation of thrust. thrust produced by the device by blocking the exhaust with a ball of To evaluate the reported measurements, it is instructive to variable weight. The authors assumed that the “threshold weight at perform a simple calculation of the total efficiency of the device. which the steel ball started to rattle” corresponded to the thrust pro- Ye et al.1 presented a set of data for the purported thrust of the duced by the device. Unfortunately, this method falsely relates the microwave device for a range of operating conditions with flows internal pressure of a blocked exit to the propulsive thrust that would between 0.7 m3/h and 1.45 m3/h, and input microwave powers be experienced in the absence of the blockage. In this Comment, between 400 W and 800 W. Forces measured using the ball method we show that, due to this inaccurate estimation method, the thrust were between 3 N and 10 N, which when scaled to account for the reported by the authors is up to 9 times the theoretical thrust levels net thrust at zero input power resulted in a net propulsion pressure possible for a microwave-driven thrust device, i.e., where 100% of between 2 kPa and 16 kPa. the input microwave power is converted to thrust. To describe the The efficiency of an electrically powered thruster, ηT , is the ratio source of this overestimate, we use a control volume analysis of the of jet power to input electrical power,2,3 which can be calculated experimental conditions described by Ye et al.,1 which captures the using effect of blocking the chamber exhaust. This analysis demonstrates 2 F that the measured pressure using this method is internal stagnation η = , (1) T ˙ pressure and cannot be used to estimate thrust. The results of the 2m˙ Win model are consistent with the reported experimental data. There- where F is thrust, m˙ is the mass flow rate, and W˙ in is the input electri- fore, through the combination of the efficiency analysis and the cal power. Equation (1) is developed through conservation of energy,

AIP Advances 10, 099101 (2020); doi: 10.1063/5.0013575 10, 099101-1 © Author(s) 2020 AIP Advances COMMENT scitation.org/journal/adv

By using the method of placing a weighted ball on the exhaust of the device, the authors are creating a partially sealed chamber out of the quartz tube and measuring the internal pressure rather than the thrust produced by the device. Modeling of this case can be performed with a control volume analysis, with the inside of the tube considered as the control volume. A diagram of the experimental configuration is shown in Fig. 3. The inlet flow rate must be equal to the outlet flow rate; both can be designated as m˙ . In lieu of any described acceleration mechanism associated with the plasma, the microwave source can be assumed to act only as a heating mechanism. By conservation of energy, the energy leaving the cylinder by transport and heat loss due to convection must be equal to the heat entering due to transport and as microwave power,

mc˙ pTatm = mc˙ pT + Qg + hAs(T − Tatm), (2)

where Tatm is the ambient temperature, cp is the specific heat, T is the temperature inside the tube, Q is the heat generated by microwave FIG. 1. Calculated efficiency of the thruster. The legend denotes the input flowrate g in m3/h. irradiation, h is the convection coefficient, and As is the exposed surface area of the cylinder. For the sake of simplicity, conductive heat loss through the ball and structure and radiative heat transfer are so it is applicable regardless of mechanisms specific to the investi- ignored. The compressed air is also assumed to enter the quartz tube gated device. The total efficiency of the device has been calculated at atmospheric temperature. Because the quartz walls of this device using the given values of pressure, flow rate, and input power in are thin, the temperature drop associated with conduction through Fig. 5 of Ye et al.1 assuming an inlet with standard temperature and this medium can be ignored. Equation (2) can then be simplified as pressure conditions. The “net propulsive pressure” has been con- Qg verted to the estimated thrust by multiplying by the area of the tube. T = Tatm + . (3) hAs + mc˙ p These values are presented in Figs. 1 and2 and reach up to nearly 80 times unity, i.e., there is 8000% as much jet power leaving the sys- The ball on the exhaust of the tube blocks flow from exiting the tube. tem as electrical power entering it. The calculated efficiency is above When the ball is not vibrating as described in the paper, flow can the maximum allowable efficiency of 100%, suggesting that the steel only exit through a thin channel formed at the interface between ball method is unreasonable for approximating thrust. If we con- the ball and the cylinder or through grooves caused by the surface strain the thruster efficiency to the theoretical maximum of 100%, roughness of the ball and tube. This channel has a high hydraulic with standard temperature and pressure air at the inlet and 600 W resistance that is assumed to be constant throughout all tests. An input power and 1.15 m3/h of flow, the anticipated thrust generated would only be 0.7 N, which is an order of magnitude less than the authors’ reported results.

FIG. 3. Control volume analysis with the weighted steel ball covering the exhaust FIG. 2. Calculated efficiency of the thruster. The legend denotes the input power of the tube. The weighted ball provides a flow restriction as air is only allowed to in W. exit the tube through a thin channel between the ball and the tube.

AIP Advances 10, 099101 (2020); doi: 10.1063/5.0013575 10, 099101-2 © Author(s) 2020 AIP Advances COMMENT scitation.org/journal/adv

Ohm’s law analogy for fluid flow (ΔP = QRhyd) can be used to deter- mine the pressure drop through this channel. The ideal gas law is used as an equation of state for determining the density of the gas in the tube, ρ, m˙ RT P − Patm = R = m˙ R , (4) ρ hyd P hyd

where Patm is the ambient pressure, Rhyd is the hydraulic resistance of the thin flow channel between the tube and the weighted ball, and R is the ideal gas constant. At this point, Eq. (3) can be substituted into Eq. (4) and simplified for P, √ 1 2 P = (Patm + P + 4mRTR˙ ). (5) 2 atm hyd

The required weight of the steel ball, Fw, can then be calculated with a force balance,

Fw = Ae(P − Patm), (6) FIG. 5. Modeling results with experimental data for force as a function of flow rate. The legend indicates the input power in W. where Ae is the exhaust area of the tube. With Eqs. (3), (5), and (6), the force required to hold the ball steady can be calculated. The results from this analysis are plotted with experimental data in Figs. 4 and5 with the following values assumed: c = 1 kJ/kg K, R = 1.2 p hyd Predicting the performance of the device in application as × 109 Pa s/m3, and h = 15 W/m2 K. Experimental data are taken 1 an air-breathing thruster without the steel ball restriction requires from Fig. 4 of Ye et al. This analysis shows that the increase in knowledge of the future device configuration and operating con- pressure is likely due to heating from the microwave source and the ditions. If the stagnation pressure measured using the ball method flow restriction caused by the ball blocking the exhaust of thetube. were to be expanded through a nozzle, then there would be thrust The microwave ionization simply acts as a heat source in this sce- produced, albeit at a significantly lower level than reported. The nario. The flow reaching the weighted ball is nearly stagnated, sothis microwave heating element may be integrated into a turbojet as a approach is measuring stagnation pressure, not thrust. The control replacement for the combustion chamber, although it is unclear that volume model can be extended to the normal operating conditions this would be more efficient than an electric motor directly driving a of the device (i.e., where the weighted ball is removed) by drasti- propeller. If the thruster employed a gas reservoir instead of a com- cally reducing the value of Rhyd. Equation (4) shows that as Rhyd pressor, then the presented device embodies a warm-gas thruster goes to zero, P approaches Patm; in this case, by the same analysis 4 1 commonly used for space applications with no nozzle. There are as used by Ye et al. [i.e., Eq. (6)], the propulsive thrust of the device several in-space propulsion technologies that rely on microwave or goes to zero. By comparing these two scenarios, it is clear that the RF power to ionize gas and produce thrust,5–8 although such devices presence of blockage at the exhaust of the device has a drastic effect typically employ additional electrostatic or electromagnetic accel- on the pressure in the quartz tube; experimental conditions are not eration mechanisms, rather than only thermal heating of neutral comparable with and without the steel ball. gas. Direct plasma acceleration of air has also been performed using dielectric-barrier discharges9 and magneto-plasma compressors.10 We suggest that future efforts to estimate thrust use proven and verified techniques. Additionally, the reported thrust values should be compared to theoretically achievable values. This work was supported by the NASA Space Tech- nology Research Fellowship, Grant Nos. 80NSSC17K0076 and 80NSSC18K1194.

DATA AVAILABILITY The data that support the findings of this study are openly available at https://doi.org/10.1063/5.0005814, Ref.1.

REFERENCES 1D. Ye, J. Li, and J. Tang, “Jet propulsion by microwave air plasma in the atmosphere,” AIP Adv. 10, 055002 (2020). 2 FIG. 4. Modeling results with experimental data for force as a function of D. M. Goebel and I. Katz, Fundamentals of Electric Propulsion: Ion and Hall microwave power. The legend indicates the input flow rate in3 m /h. Thrusters, JPL Space Science and Technology Series (John Wiley & Sons, 2008), pp. 27–30.

AIP Advances 10, 099101 (2020); doi: 10.1063/5.0013575 10, 099101-3 © Author(s) 2020 AIP Advances COMMENT scitation.org/journal/adv

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