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The disk mark on the surface of the disk. Fig. 7. Minimum required radius of the mark versus angular velocity of disk.
Source publication
An experimental technique using spin-down of a disk approaching the free-molecular flow limit is developed to measure the tangential momentum accommodation coefficient s t. The new technique uses a disk mounted on a shaft supported by gas bearings in vacuum. A differential scavenging system allows disk spin-down experiments in different gases. Repr...
Citations
... For these high demands in direct rarefied hypersonic aerodynamic measurements, a hypersonic rarefied wind tunnel (HRWT) [2,3] has been developed at Japan Aerospace Exploration Agency (JAXA). Although dependence of surface accommodation coefficients on surface materials and gas species have been studied and obtained by several methods, such as a disc-spin method [4] and a sphere-drag method [5], the study for dependence on speed ratio [6] is limited. Note that absorption effect on surface accommodation needs to be considered for high speed ratio cases [7]. ...
... The current study uses disk spindown apparatus previously described in the literature [19] to measure σ t between various gases and materials. In addition to providing estimated values for engineering applications, the goal is to show how σ t varies with molecular masses and molecular structures of gases. ...
... The measured torque is then used to obtain σ t at each ambient gas pressure. The facility can be operated with a range of materials and gases [19]. Figure 2 shows a schematic of the experimental apparatus, which was shown in previous work to provide accurate measurement of the tangential momentum accommodation for a range of gases and materials [19]. ...
... The facility can be operated with a range of materials and gases [19]. Figure 2 shows a schematic of the experimental apparatus, which was shown in previous work to provide accurate measurement of the tangential momentum accommodation for a range of gases and materials [19]. Previous experiments using this facility showed that σ t measurements were independent of disk sizes when three different aluminum disks with diameters of 0.15, 0.18, and 0.21 m were used for such measurements in the free molecular flow regime [18]. ...
A disk spindown experiment allows measurement of tangential momentum accommodation coefficients of different gases and material combinations in a consistent manner. Aluminum is tested for a range of gases, including monoatomic, diatomic, and triatomic gases. The results confirm previous experimental results that show a reduction in tangential momentum accommodation coefficients with increasing molecular masses of gases. In addition, the results reveal a previously unreported phenomenon: that the tangential momentum accommodation coefficient is also a function of the molecular structure of gases. Nomenclature A = disk face area, m 2 I = moment of inertia, kg ⋅ m 2 k = Boltzmann constant 1.38065 × 10 −23 m 2 ⋅ kg ⋅ s −2 ⋅ K −1 M = mass of the disk, kg m = mass of the gas molecule, kg n = number of disk rotations per second P = normal pressure on a surface, Pa P g = ambient gas pressure, Pa r i = shaft radius, m r o = disk radius, m s t = dimensionless tangential velocity s 3 = dimensionless normal velocity T FM = free molecular torque, N ⋅ m T g = ambient temperature, K T w = temperature of wall, K u t = random particle tangential velocity, m/s u 3 = random particle normal velocity, m/s α = incident angle, rad α d = angular deceleration of the disk, rad∕s 2 γ = specific heat ratio σ e = thermal accommodation coefficient Φ mv i = incident momentum flux, kg ⋅ m −1 ⋅ s −1 Φ mv r = reflected momentum flux, kg ⋅ m −1 ⋅ s −1 Φ mv w = reflected momentum flux under diffuse conditions, kg ⋅ m −1 ⋅ s −1 σ n = normal momentum accommodation coefficient σ t = tangential momentum accommodation coefficient τ = shear stress, Pa ω = angular velocity, rad∕s
... In the upper atmosphere, flow around a hypersonic vehicle may vary from continuum through highly rarefied. Researchers have studied continuum breakdown in both hypersonic and subsonic flows [2,[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Martin and Boyd showed that rarefied effects in a subsonic laminar boundary layer will affect the drag and heat transfer [24][25]. ...
... The facility can be operated with a range of materials and gases. Fig 1 shows a schematic of the experimental apparatus, the details of which appear elsewhere [19]. ...
... The apparatus uses an optical measurement system for instantaneous measurement of rotational speed. The instantaneous torque as a function of disk rotational speed is measured [18,19]. In the free molecular flow regime, the torque on the rotating disk is given by equation (4): where T FM is the free molecular torque, P g is the ambient gas pressure, ω is the angular velocity of the disk in rad/s, r o is the outer radius of the disk, and r i is the inner radius of the disk. ...
Gas centrifuges are widely used to isotope separation of about 154 stable isotopes. It is very important to simulate the behavior of dilute gas inside a centrifuge machine using accurate methods to increase the optimal performance of a machine. Due to the high accuracy of the molecular dynamics (MD) method in the interaction of gas molecules with the surfaces, in this paper, this method has been used for the interaction of uranium hexafluoride gas molecules with the rotor surfaces of the machine and its results have been used to simulation of the gas inside the rotor by the DSMC method. Consequently, a gas centrifuge was simulated using a new hybrid MD-DSMC method. The boundary condition used in the MD-DSMC method is the Cercignani–Lampis–Lord. In this method, the gas behavior near the scoop of the centrifuge machine was investigated using the new algorithm of the velocity reduction coefficient. The results show that for the interaction of uranium hexafluoride molecules with the composite surfaces of rotor, the accommodation coefficients take non-unit values. Also, simulation of the rarefied gas inside the rotor by the MD-DSMC method shows that for a velocity reduction coefficient equal to 0.84, the separation work value for the machine will be maximized. Finally, by comparing the separation factors obtained from MD-DSMC and DSMC methods with experimental results, it can be concluded that applying the effects of the polymeric surface of the rotor and gas conditions using the MD-DSMC method increases the accuracy of the results until 6 percent.
The dropper-type gas flow measuring apparatus is widely used in the vacuum pumping speed measurement. An innovative method based on the weighing principle was proposed in the paper. The structure and working principle of the dropper-type gas flow measuring apparatus is introduced. An electronic scale was placed under the storage tank to measure the oil mass into dropper. The corresponding formulas of gas flow are deduced and the error analysis of measuring accuracy is carried out. Comparing to the disadvantages of the original measuring method, the characteristics of new method was analyzed. A miniature measuring apparatus was set up in the laboratory, and the pumping speed curve of a K-200 diffusion pump was measured by experiment. The feasibility and convenience of the measuring apparatus were verified. This improvement can provide theoretical support and reference method for achieving automatic measurement of vacuum pumping speed and minimal gas flow.
