1.4 Thesis scope
2.1.3 Propellant flow circuit
Figure2.3shows a flowchart of the propellant flow circuit in the PRexperimental setup. PR (highlighted inblue) is connected to a vacuum system downstream. Upstream of PR, there is a choice to operate with either the laboratory gas supply or the propellant subsystem (highlighted inred) [43–45]. Different instruments (dashed blocks) are used to monitor the static pressure at various points in the flow circuit.
Laboratory gas supply
In the laboratory, Ar propellant is sourced from a high pressure industrial gas storage tank. A regulator sets the gas pressure to levels manageable for the mass flow controller (MFC). The experimental setup has a thermal-based MFC (MKS 2179A) calibrated for N2 at 100SCCM (standard cubic centimetres per minute) full scale, linked with a multi-channel power supply, readout, and set-point source (MKS 247D) for monitoring and controlling the MFC.
Plenum Dischargechamber Expansiontube chamberVacuum Rotary vane pump Capacitance manometer Capacitance
manometer Pirani gauge
Mass flow
controller Regulator Gas tank
Proportional valve Pressure transducer Regulator Pressure transducer Propellant canister
Figure 2.3: Propellant flow circuit. PR(blue), propellant subsystem (red). calibration of the MFC must be adjusted according to the gas correction factor GCF:
GCF(Ar) = 1.030·ρ(N2)cp(N2)
ρ(Ar)cp(Ar) = 1.441 (2.1) whereρ is the mass density andcp is the specific heat at constant pressure of the respective
gas species at T = 273.15K and p= 1atm. The nondimensional multiplicative constant is the molecular structure correction factor, which is1.030for monatomic, 1.000for diatomic, 0.941 for triatomic, and 0.880 for polyatomic gases [59]. Hence, a 100SCCM full scale MFC calibrated for N2 dispenses 144.1SCCM of Ar at full scale, with linear calibration. If
˙
m= 100SCCM of Ar is desired, the MFC must therefore be set to 69.4 %of the full scale. A feedback loop in the MFC ensures smooth dispensation of the gas at the setm˙.
Propellant subsystem
For operation on a spacecraft, the onboard propellant subsystem must be small and light- weight, with minimal power requirements. Ar or carbon dioxide (CO2) propellant is stored in a pressurised 21mL canister at 2600psig or 800psig respectively. A miniature propor- tional solenoid valve replaces the MFC for controlling them˙ intoPR. It has a fixed electrical resistance of 100Ω, and can be controlled electronically with 120mA of electrical current required to hold the valve fully opened. A miniature regulator is used to reduce the pres- sure of the incoming propellant to below the leakage rating of the proportional valve. The regulator has an adjustable range of 5psig to30psig relative to the ambient pressure, and has been experimentally verified for operation in vacuum. The design and development of thePRpropellant subsystem is extensively documented in the following theses [43–45].
Vacuum system & pressure diagnostics
For normal laboratory experiments, both the laboratory gas supply and the propellant sub- system are connected to PR by a three way ball valve, used for selecting the propellant source. Propellant flows into PR via an inlet on the side of the structure at a nominal rate of m˙ = 100SCCM, and fills the plenum top∼1Torr static pressure. The plenum pressure varies with the species of gas used, as well as the geometry of the discharge chamber which dictates the flow behaviour in PR. Since the flow in the plenum is mostly stationary, the static pressure p in the plenum is equivalent to the stagnation pressure pst, which has the
definition:
pst = 1 2ρu
2
+p (2.2)
whereρ is the mass density andu is the flow velocity. Henceforth, pst is used to represent the macroscopic plenum pressure, as distinct from the static pressurepat a particular point
in the flow.
PRis attached to a glass expansion tube100mm in length and45mm internal diameter, and mounted to a KF-40 flange on the face of a20L six-way cross vacuum chamber. When no propellant is flowing, PR and the rest of the vacuum system achieves a base pressure of
≤ 1mTorr with a rotary vane pump (Edwards XDS10) connected to the vacuum chamber. Due to the small volume of the vacuum chamber and the limited pumping speed, the static pressure in the vacuum chamber rises top0 = 0.349Torr whenm˙ = 100SCCM= 2.97mg s
−1 of Ar is flowing, or p0 = 0.321Torr with m˙ = 100SCCM= 2.083mg s
−1 of N 2.
Different instruments are used to monitor the static pressure at various points in the vacuum system. A 10Torr full scale capacitance manometer (MKS 626B) mounted to the PR structure on the opposite side of the inlet measures the static pressure in the plenum. It functions by measuring the capacitance between a metal-on-ceramic electrode disc and a radially tensioned metal diaphragm which is displaced with variations in static pressure. The static pressure in the vacuum chamber is measured using a20Torr full scale capacitance manometer (MKS 626B), as less precision is required. A Pirani gauge (Granville-Phillips 275) is also used on the vacuum chamber to monitor the static pressure above the range of the capacitance manometer up to 760Torr during evacuation or return to atmospheric pressure. Unlike a capacitance manometer which measures true physical pressure, a Pirani gauge indirectly infers static gas pressure by measuring the rate of heat loss from a filament controlled at constant temperature in the gas medium. As such, it is sensitive to the species or chemical composition of the gas, and separate calibration is required if not operating with the default N2 calibration gas. The capacitance manometers are run from a multi-channel
power supply and readout unit (MKS PR4000B), while a separate power supply and readout unit (Granville-Phillips 307) serves the Pirani gauge.
The propellant subsystem is equipped with pressure transducers placed in series between the propellant canister, regulator, and proportional valve. The propellant subsystem com- ponents are calibrated with these transducers to supply the nominalm˙ of propellant to PR. For experiments conducted with PR and the propellant subsystem wholly within a vacuum chamber, no pressure measurement instruments are used on board as the present instruments are not designed to operate in vacuum. However, there has been investigation into miniature transducers that can be placed inside of the plenum or be integrated with the propellant subsystem [43–45].