- Email: [email protected]
Numerical simulation of molecular motion around laser microengine blades
Mathematics and Computers in Simulation 55 (2001) 223–230
Numerical simulation of molecular motion around laser microengine blades Masahiro Ota∗ , Tomohiko Nakao, Moriyoshi Sakamoto Department of Mechanical Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachi-oji, Tokyo 192-0397, Japan
Abstract Gas molecular motions around a laser opto-microengine are numerically simulated by direct simulation Monte Carlo (DSMC) method. We propose an opto-microengine that acts as a micro-actuator in which optical energy is supplied by a laser beam. The opto-microengine is rotated by molecular gas dynamics effects. The effects of the molecular weights of gases on laser opto-microengine performance are revealed by experiments. Helium, argon and xenon gases are used as environmental gases for operation of the opto-microengine. The numerical results show that pressure differences between front and back surfaces of the rotor blade depend on the molecular weights of the gases and accommodation coefficients. © 2001 IMACS. Published by Elsevier Science B.V. All rights reserved. Keywords: Rarefied gas dynamics; Molecular gas dynamics; Direct simulation Monte Carlo method; Thermal force; Radiometric force; Laser heating; Twisted torque; Radio thermometer; Laser opto-microengine; Micromachine
1. Introduction In this research project, an opto-microengine, which uses the molecular gas dynamics effects, is proposed as a new power source [1,2]. One of the targets of the opto-microengine research is the establishment of a method of supplying energy to the microengine. Two forces, a radiometric force and a thermal force are assumed to be the sources of opto-microengine actuation. The radiometric force is generated in a free molecular flow in which the gas molecules hardly collide with each other in space [1,2]. On the other hand, when the collision of gas molecules themselves cannot be disregarded compared with the collision of gas molecules with a solid surface, a thermal force is generated [1,2]. Both forces act from a high temperature degree side to a low temperature degree side of a solid surface with a temperature gradient put on molecular gas environment. In the case of the opto-microengine, because visible light is irradiated on a blade surface, which spread carbon black powder, there is a temperature difference between the front and back surfaces of the blade that originates in the difference in the radiation energy absorption rate. ∗ Corresponding author. Tel.: +81-426-77-2715; fax: +81-426-77-2701. E-mail address: [email protected] (M. Ota).
0378-4754/01/$20.00 © 2001 IMACS. Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 4 7 5 4 ( 0 0 ) 0 0 2 6 5 - 2
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Fig. 1. Schematic drawing of device for measuring rotational rates of rotor.
One basic characteristic of the opto-microengine has been clarified [1] that the opto-microengine generates the highest rotational rate and the maximum torque at Knudsen numbers (Kn) of 0.5 and 0.2, respectively. Kn is dimensionless and represents the ratio of the mean free path length of a gas molecule to the length of the rotor blade. However, an experiment on the basic characteristics of the opto-microengine can so far be conducted under an environment in which air is decompressed. Thus, the possibility of a further improvement in performance is expected when other gases are used because it is thought that the characteristic of the opto-microengine depends on the molecular weight and the molecular diameter of the gas that exists in operation environment. Three kinds of gas species were used: helium (He), argon (Ar), and xenon (Xe). We also investigated the pressure difference change on the blade surface, which originates in the difference in the molecular weight of the gas, according to the direct simulation Monte Carlo (DSMC) method.
2. Laser opto-microengines The engine is composed of a vacuum chamber, a micro-rotor and a laser power supply. The schematic drawing of the engine and the device for measuring a rotational rate of the rotor is shown in Fig. 1. The micro-rotor has four blades made of rectangular aluminum plates. The blade length of the rotor is 1 mm. The tip of the blade is far from the axis of the rotor by 1.25 mm. The thickness of the blade is 0.1 mm, only each one-side surface of the blades are coated with carbon-black powder. The four blades are connected with a bearing like a cap made of a Pyrex glass. In this study an argon ion laser beam with 514.5 m wavelength is used for heating. The gas pressure in the vacuum chamber is reduced down to a certain gas pressure by a vacuum pump system. At the pressure, the irradiation of the laser beam from the outside of the chamber heats a carbon-black coated surfaces of the rotor. Immediately after the irradiation, the rotor begins to be rotated or to be twisted.
3. Direct simulation Monte Carlo method The DSMC technique applied to molecular gas dynamics requires the solution of a transient problem wherein an initial condition is an appropriate molecular distribution and a desired final state is a steady
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state. Since the steady state solution is the desired result, the initial condition should be used to detect the onset of the steady state and should also be chosen to permit the most rapid approach to the steady state. A steady-state detection algorithm can be divided into three consecutive stages. In the first stage, a transient flow is simulated for a period of time equal to that which is required for example, for incoming molecules to transverse a calculation domain at a free stream velocity, and a sonic velocity with a stagnation condition at inlet boundaries. The transient simulation continues, in the second stages, until the monotonic drift in the number of gas molecules within the domain ceases. The number of incoming gas molecules from the inlet boundaries subtracted the number of outgoing gas molecules to outlet boundaries is the number of gas molecules within the calculation domain. The accurate sampling of intermolecular collisions is essential to the DSMC simulations. Traditionally, the time counter technique has been used to simulate a collision processes efficiently. However, it seems that this technique introduces significant statistical errors as a result of an insufficient collision number per time interval. Therefore, in this paper, no time counter method is applied. The global time step dt, over which the collisions and molecular motion are decoupled, is given by the following Eq. (1) dt =
(dZ)min (dV )max
(1)
Here (dZ)min is the smallest characteristic cell size and (dV)max the largest characteristic velocity within the domain. This relationship ensures that variations in the solution occurring over the dimensions of a cell are resolved. To improve the accuracy during steady-state sampling further, dt described by Eq. (1) is reduced by a factor of 10 upon reaching the steady state. The steady flows are evaluated with a time-averaging technique. Finite computer storage imposes limitations on both the total number of simulated molecules and the number inversely proportional to the square root of the sample size. If the simulated flow becomes steady after a long simulation time, the required sample size may increase along with the time required for the individual samples to become independent.
4. Numerical simulation for rarefied gas flows around laser opto-microengine blades by DSMC method The flow model for numerical simulation is shown in Fig. 2. Four blades are installed at rest in a computational domain. The domain is enclosed by a boundary in a quasi-circle. The boundary has a diffused boundary condition for gas molecules impinging against the boundary. The molecules are reflected from the boundary with the same temperatures as the boundary, but directions and speeds of the molecules are dependent with uniform random numbers. The temperatures of the enclosed boundaries are 300 K. The front surfaces are named as marked with H, temperature 600 K. On the other hand, the back surfaces are C, 300 K. Then, a numerical simulation, using the DSMC method was done. The rotor that has four static blades is physically modeled in a round calculation area that is divided into 3520 cells. As for the modeled rotor, the length of each blade is 1 mm and it becomes the size representative of the flow. The physical condition of the surrounding boundary wall and rotor blades was set as the diffuse reflection. In setting the temperature on the blade side, the high-temperature side is 600 K and the low-temperature side is 300 K.
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Fig. 2. Flow model of DSMC simulation for laser opto-microengine.
The calculation number is assumed to be 70,400 pieces in the entire calculation area. Kn is assumed to be 0.1 and the number of samples is 13,200. Fig. 3 shows the pressure difference distribution of the gas on the front and back surfaces of the rotor blade. The horizontal axis is dimensionless with ‘0’ and ‘1’ as the outer and inner edge of the blade, respectively, according to length L of the blade. A macroscopically generated pressure difference of the gas that acted on the front and back surfaces of the rotor blade may be responsible for the working torque. The momentum accommodation coefficients of He, Ar and Xe are 0.2, 0.9, and 1.0, respectively. There is a tendency that the pressure differences increase with decreasing the molecular weight of a gas and the pressure difference for He in Fig. 3 is the greatest. When the pressure difference of He is compared with that of Xe, a difference of about two times, which is similar to the experimental result, is confirmed. It is shown that the accommodation coefficient of He is about 20% that of Xe. According to the experimental
Fig. 3. Pressure difference distribution of gas on the back and front surfaces of the rotor blade by DSMC method (He, Ar, and Xe).
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Fig. 4. Velocity vector diagram around rotor (gas: helium, Kn = 0.1).
data by a molecular beam, the momentum and thermal accommodation coefficients of He are smaller than those of other gas species [4–6]. That is, it is thought that the accommodation coefficient of He, being the smallest, causes a very low efficiency of energy exchange on the surface of the blade. The velocity distributions of macroscopic gas flows are shown in Figs. 4–6. The gas species are helium in Fig. 4, argon in Fig. 5 and xenon in Fig. 6, respectively. The momentum accommodation coefficients on the blade surfaces are 1.0, all in Figs. 4–6. The Kn in Figs. 4–6 are 0.1. Maximum velocities are dependent on the molecular weight of the gas species. With increasing the
Fig. 5. Velocity vector diagram around rotor (gas: argon, Kn = 0.1).
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Fig. 6. Velocity vector diagram around rotor (gas: xenon, Kn = 0.1).
molecular weight the maximum velocities decrease. However, a large vortex is happed around the blades and moreover, small vortexes are created in the regions between the blades.
5. Characteristic dependence of laser opto-microengine on molecular gas species The schematic drawing of the experimental set-up for measuring a twisted torque is shown in Fig. 7. The rotor is suspended from a ceiling of the vacuum chamber of a Pyrex glass cylinder with a diameter 100 m. The deviation angle of a reflected He–Ne laser beam from the rotor surface between laser irradiation and without irradiation was measured and then the twisted torque was evaluated by an elastic theory. On the other hand, in case of rotational rate measurement as shown in Fig. 1, pulsed irradiated light that originates in shading by the rotor blade during rotation, is detected as a frequency by an optical photodiode and
Fig. 7. Experimental set-up for measuring twisted torque.
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Fig. 8. Dependence of twisted torque on Kn.
is converted into the number of rotations. The irradiation light output during measurement was about 31 mW. Gas pressure was used as a parameter in the experiment. Fig. 8 shows the results of torque measurement in which pressure is converted into Kn. The tendency that the torque increases as the molecular weight of the gas decreases is observed. Of note is that a maximum torque within the range of Kn = 0.1–0.3 is generated regardless of the gas used. That is, it can be said that the tendency of change in torque is irrelevant to the kind of gas used and shown only in Kn. Next, the results of rotational rate measurement are shown in Fig. 9. The horizontal axis shows Kn and the vertical axis shows the rotational rate per unit time. The highest rotational rate is generated within a certain range of Kn regardless of the gas used is similar to the results of torque measurement. In addition, the point, at which the rotor generates the highest rotational rate in the presence of He, which has the lowest molecular weight of the gases used, is similar to that in the torque measurement. However, the
Fig. 9. Dependence of rotational rate on Kn.
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rotor generates the highest rotational rate in the Kn range of 0.3–0.5. This range differs somewhat from the range of Kn in which the maximum torque is generated. As described previously although the radiometric force and the thermal force are considered to be the sources of opto-microengine actuation, the molecular weight of the gas is of no concern because the radiometric force depends only on the pressure and the temperature of the gas. On the other hand, thermal force is proportional to a gas constant, a temperature gradient and the square of a viscosity [3]. Because the gas constant of He is larger by a factor of 32 compared to the gas constant of Xe, He has a possibility to supplement the viscosity of about 80% enough compared with Xe. Therefore, it is expected that the opto-microengine that uses He demonstrates 20 times or higher output compared to the case using Xe. However, according to the experimental results of maximum torque measurement and rotational rate measurement, the values of the torque and the rotational rate of the rotor when He is used are about twice those when Xe is used. The reason for this is that the accommodation coefficient on the blade surface differs depending on the kind of surrounding gas molecule. 6. Summary and view in the future Experiment and simulation using the DSMC method clarified the influence of the difference in gas molecular weight on opto-microengine characteristics. In the experiment, when the rotor was set up in a gas environment in which He was decompressed, the maximum torque and the highest rotational rate were generated. Moreover, the highest rotational rate of 22 rps was generated by Ar+ laser light irradiation with an output of 31 mW. It was clarified that the rotational rate of the opto-microengine is related to the output of the irradiation light. Therefore, further increase in rotational rate can be expected by increasing the light irradiation output. However, an improvement of the metallic support part of the rotor is needed, because friction resistance of the support part increases with increasing rotational rate and miniaturization of the rotor. One of the features of the opto-microengine is non-contact energy supply by light irradiation. This feature is very attractive besides the power source or the use of an optical switch and an optical shutter. References [1] Masahiro Ota, Kenji Kasahara, Moriyoshi Sakamoto, Opto-microengines rotated by rarefied gas dynamic effects, Trans. Jpn Soc. Mech. Eng. B61-582 (1995–1996) 2190–2196. [2] Masahiro Ota, Noriyuki Kawata, Direct simulation of gas flows around rarefied gas dynamics engines for a micro-machine, in: Rarefied Gas Dynamics, Oxford University Press, Oxford, 1995, pp. 722–728. [3] Masahiro Ota, K. Kasahara, M. Sakamoto, Micromachine-actuator rotated by rarefied gas effects, Solid Thin Films 281/282 (1996) 651–653. [4] Masahiro Ota, M. Ishiguro, M. Sakamoto, On selective optical absorber film for rotor of opto-microengine, ASME, HTD 336 (1996) 193–226. [5] Masahiro Ota, T. Nakao, M. Sakamoto, Manipulation for fine solid particles by gradient radiation pressure of a laser beam, ASME, FEDSM98-5107, 1998. [6] Masahiro Ota, Tomohiko Nakao, Moriyoshi Sakamoto, Characteristics of laser opto-microengine rotated by effects of molecular gas dynamics, Trans. Jpn. Soc. Mech. Eng. 65 (634) (1999) (in Japanese).
Numerical simulation of molecular motion around laser microengine blades Masahiro Ota∗ , Tomohiko Nakao, Moriyoshi Sakamoto Department of Mechanical Engineering, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachi-oji, Tokyo 192-0397, Japan
Abstract Gas molecular motions around a laser opto-microengine are numerically simulated by direct simulation Monte Carlo (DSMC) method. We propose an opto-microengine that acts as a micro-actuator in which optical energy is supplied by a laser beam. The opto-microengine is rotated by molecular gas dynamics effects. The effects of the molecular weights of gases on laser opto-microengine performance are revealed by experiments. Helium, argon and xenon gases are used as environmental gases for operation of the opto-microengine. The numerical results show that pressure differences between front and back surfaces of the rotor blade depend on the molecular weights of the gases and accommodation coefficients. © 2001 IMACS. Published by Elsevier Science B.V. All rights reserved. Keywords: Rarefied gas dynamics; Molecular gas dynamics; Direct simulation Monte Carlo method; Thermal force; Radiometric force; Laser heating; Twisted torque; Radio thermometer; Laser opto-microengine; Micromachine
1. Introduction In this research project, an opto-microengine, which uses the molecular gas dynamics effects, is proposed as a new power source [1,2]. One of the targets of the opto-microengine research is the establishment of a method of supplying energy to the microengine. Two forces, a radiometric force and a thermal force are assumed to be the sources of opto-microengine actuation. The radiometric force is generated in a free molecular flow in which the gas molecules hardly collide with each other in space [1,2]. On the other hand, when the collision of gas molecules themselves cannot be disregarded compared with the collision of gas molecules with a solid surface, a thermal force is generated [1,2]. Both forces act from a high temperature degree side to a low temperature degree side of a solid surface with a temperature gradient put on molecular gas environment. In the case of the opto-microengine, because visible light is irradiated on a blade surface, which spread carbon black powder, there is a temperature difference between the front and back surfaces of the blade that originates in the difference in the radiation energy absorption rate. ∗ Corresponding author. Tel.: +81-426-77-2715; fax: +81-426-77-2701. E-mail address: [email protected] (M. Ota).
0378-4754/01/$20.00 © 2001 IMACS. Published by Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 4 7 5 4 ( 0 0 ) 0 0 2 6 5 - 2
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M. Ota et al. / Mathematics and Computers in Simulation 55 (2001) 223–230
Fig. 1. Schematic drawing of device for measuring rotational rates of rotor.
One basic characteristic of the opto-microengine has been clarified [1] that the opto-microengine generates the highest rotational rate and the maximum torque at Knudsen numbers (Kn) of 0.5 and 0.2, respectively. Kn is dimensionless and represents the ratio of the mean free path length of a gas molecule to the length of the rotor blade. However, an experiment on the basic characteristics of the opto-microengine can so far be conducted under an environment in which air is decompressed. Thus, the possibility of a further improvement in performance is expected when other gases are used because it is thought that the characteristic of the opto-microengine depends on the molecular weight and the molecular diameter of the gas that exists in operation environment. Three kinds of gas species were used: helium (He), argon (Ar), and xenon (Xe). We also investigated the pressure difference change on the blade surface, which originates in the difference in the molecular weight of the gas, according to the direct simulation Monte Carlo (DSMC) method.
2. Laser opto-microengines The engine is composed of a vacuum chamber, a micro-rotor and a laser power supply. The schematic drawing of the engine and the device for measuring a rotational rate of the rotor is shown in Fig. 1. The micro-rotor has four blades made of rectangular aluminum plates. The blade length of the rotor is 1 mm. The tip of the blade is far from the axis of the rotor by 1.25 mm. The thickness of the blade is 0.1 mm, only each one-side surface of the blades are coated with carbon-black powder. The four blades are connected with a bearing like a cap made of a Pyrex glass. In this study an argon ion laser beam with 514.5 m wavelength is used for heating. The gas pressure in the vacuum chamber is reduced down to a certain gas pressure by a vacuum pump system. At the pressure, the irradiation of the laser beam from the outside of the chamber heats a carbon-black coated surfaces of the rotor. Immediately after the irradiation, the rotor begins to be rotated or to be twisted.
3. Direct simulation Monte Carlo method The DSMC technique applied to molecular gas dynamics requires the solution of a transient problem wherein an initial condition is an appropriate molecular distribution and a desired final state is a steady
M. Ota et al. / Mathematics and Computers in Simulation 55 (2001) 223–230
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state. Since the steady state solution is the desired result, the initial condition should be used to detect the onset of the steady state and should also be chosen to permit the most rapid approach to the steady state. A steady-state detection algorithm can be divided into three consecutive stages. In the first stage, a transient flow is simulated for a period of time equal to that which is required for example, for incoming molecules to transverse a calculation domain at a free stream velocity, and a sonic velocity with a stagnation condition at inlet boundaries. The transient simulation continues, in the second stages, until the monotonic drift in the number of gas molecules within the domain ceases. The number of incoming gas molecules from the inlet boundaries subtracted the number of outgoing gas molecules to outlet boundaries is the number of gas molecules within the calculation domain. The accurate sampling of intermolecular collisions is essential to the DSMC simulations. Traditionally, the time counter technique has been used to simulate a collision processes efficiently. However, it seems that this technique introduces significant statistical errors as a result of an insufficient collision number per time interval. Therefore, in this paper, no time counter method is applied. The global time step dt, over which the collisions and molecular motion are decoupled, is given by the following Eq. (1) dt =
(dZ)min (dV )max
(1)
Here (dZ)min is the smallest characteristic cell size and (dV)max the largest characteristic velocity within the domain. This relationship ensures that variations in the solution occurring over the dimensions of a cell are resolved. To improve the accuracy during steady-state sampling further, dt described by Eq. (1) is reduced by a factor of 10 upon reaching the steady state. The steady flows are evaluated with a time-averaging technique. Finite computer storage imposes limitations on both the total number of simulated molecules and the number inversely proportional to the square root of the sample size. If the simulated flow becomes steady after a long simulation time, the required sample size may increase along with the time required for the individual samples to become independent.
4. Numerical simulation for rarefied gas flows around laser opto-microengine blades by DSMC method The flow model for numerical simulation is shown in Fig. 2. Four blades are installed at rest in a computational domain. The domain is enclosed by a boundary in a quasi-circle. The boundary has a diffused boundary condition for gas molecules impinging against the boundary. The molecules are reflected from the boundary with the same temperatures as the boundary, but directions and speeds of the molecules are dependent with uniform random numbers. The temperatures of the enclosed boundaries are 300 K. The front surfaces are named as marked with H, temperature 600 K. On the other hand, the back surfaces are C, 300 K. Then, a numerical simulation, using the DSMC method was done. The rotor that has four static blades is physically modeled in a round calculation area that is divided into 3520 cells. As for the modeled rotor, the length of each blade is 1 mm and it becomes the size representative of the flow. The physical condition of the surrounding boundary wall and rotor blades was set as the diffuse reflection. In setting the temperature on the blade side, the high-temperature side is 600 K and the low-temperature side is 300 K.
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Fig. 2. Flow model of DSMC simulation for laser opto-microengine.
The calculation number is assumed to be 70,400 pieces in the entire calculation area. Kn is assumed to be 0.1 and the number of samples is 13,200. Fig. 3 shows the pressure difference distribution of the gas on the front and back surfaces of the rotor blade. The horizontal axis is dimensionless with ‘0’ and ‘1’ as the outer and inner edge of the blade, respectively, according to length L of the blade. A macroscopically generated pressure difference of the gas that acted on the front and back surfaces of the rotor blade may be responsible for the working torque. The momentum accommodation coefficients of He, Ar and Xe are 0.2, 0.9, and 1.0, respectively. There is a tendency that the pressure differences increase with decreasing the molecular weight of a gas and the pressure difference for He in Fig. 3 is the greatest. When the pressure difference of He is compared with that of Xe, a difference of about two times, which is similar to the experimental result, is confirmed. It is shown that the accommodation coefficient of He is about 20% that of Xe. According to the experimental
Fig. 3. Pressure difference distribution of gas on the back and front surfaces of the rotor blade by DSMC method (He, Ar, and Xe).
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Fig. 4. Velocity vector diagram around rotor (gas: helium, Kn = 0.1).
data by a molecular beam, the momentum and thermal accommodation coefficients of He are smaller than those of other gas species [4–6]. That is, it is thought that the accommodation coefficient of He, being the smallest, causes a very low efficiency of energy exchange on the surface of the blade. The velocity distributions of macroscopic gas flows are shown in Figs. 4–6. The gas species are helium in Fig. 4, argon in Fig. 5 and xenon in Fig. 6, respectively. The momentum accommodation coefficients on the blade surfaces are 1.0, all in Figs. 4–6. The Kn in Figs. 4–6 are 0.1. Maximum velocities are dependent on the molecular weight of the gas species. With increasing the
Fig. 5. Velocity vector diagram around rotor (gas: argon, Kn = 0.1).
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Fig. 6. Velocity vector diagram around rotor (gas: xenon, Kn = 0.1).
molecular weight the maximum velocities decrease. However, a large vortex is happed around the blades and moreover, small vortexes are created in the regions between the blades.
5. Characteristic dependence of laser opto-microengine on molecular gas species The schematic drawing of the experimental set-up for measuring a twisted torque is shown in Fig. 7. The rotor is suspended from a ceiling of the vacuum chamber of a Pyrex glass cylinder with a diameter 100 m. The deviation angle of a reflected He–Ne laser beam from the rotor surface between laser irradiation and without irradiation was measured and then the twisted torque was evaluated by an elastic theory. On the other hand, in case of rotational rate measurement as shown in Fig. 1, pulsed irradiated light that originates in shading by the rotor blade during rotation, is detected as a frequency by an optical photodiode and
Fig. 7. Experimental set-up for measuring twisted torque.
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Fig. 8. Dependence of twisted torque on Kn.
is converted into the number of rotations. The irradiation light output during measurement was about 31 mW. Gas pressure was used as a parameter in the experiment. Fig. 8 shows the results of torque measurement in which pressure is converted into Kn. The tendency that the torque increases as the molecular weight of the gas decreases is observed. Of note is that a maximum torque within the range of Kn = 0.1–0.3 is generated regardless of the gas used. That is, it can be said that the tendency of change in torque is irrelevant to the kind of gas used and shown only in Kn. Next, the results of rotational rate measurement are shown in Fig. 9. The horizontal axis shows Kn and the vertical axis shows the rotational rate per unit time. The highest rotational rate is generated within a certain range of Kn regardless of the gas used is similar to the results of torque measurement. In addition, the point, at which the rotor generates the highest rotational rate in the presence of He, which has the lowest molecular weight of the gases used, is similar to that in the torque measurement. However, the
Fig. 9. Dependence of rotational rate on Kn.
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rotor generates the highest rotational rate in the Kn range of 0.3–0.5. This range differs somewhat from the range of Kn in which the maximum torque is generated. As described previously although the radiometric force and the thermal force are considered to be the sources of opto-microengine actuation, the molecular weight of the gas is of no concern because the radiometric force depends only on the pressure and the temperature of the gas. On the other hand, thermal force is proportional to a gas constant, a temperature gradient and the square of a viscosity [3]. Because the gas constant of He is larger by a factor of 32 compared to the gas constant of Xe, He has a possibility to supplement the viscosity of about 80% enough compared with Xe. Therefore, it is expected that the opto-microengine that uses He demonstrates 20 times or higher output compared to the case using Xe. However, according to the experimental results of maximum torque measurement and rotational rate measurement, the values of the torque and the rotational rate of the rotor when He is used are about twice those when Xe is used. The reason for this is that the accommodation coefficient on the blade surface differs depending on the kind of surrounding gas molecule. 6. Summary and view in the future Experiment and simulation using the DSMC method clarified the influence of the difference in gas molecular weight on opto-microengine characteristics. In the experiment, when the rotor was set up in a gas environment in which He was decompressed, the maximum torque and the highest rotational rate were generated. Moreover, the highest rotational rate of 22 rps was generated by Ar+ laser light irradiation with an output of 31 mW. It was clarified that the rotational rate of the opto-microengine is related to the output of the irradiation light. Therefore, further increase in rotational rate can be expected by increasing the light irradiation output. However, an improvement of the metallic support part of the rotor is needed, because friction resistance of the support part increases with increasing rotational rate and miniaturization of the rotor. One of the features of the opto-microengine is non-contact energy supply by light irradiation. This feature is very attractive besides the power source or the use of an optical switch and an optical shutter. References [1] Masahiro Ota, Kenji Kasahara, Moriyoshi Sakamoto, Opto-microengines rotated by rarefied gas dynamic effects, Trans. Jpn Soc. Mech. Eng. B61-582 (1995–1996) 2190–2196. [2] Masahiro Ota, Noriyuki Kawata, Direct simulation of gas flows around rarefied gas dynamics engines for a micro-machine, in: Rarefied Gas Dynamics, Oxford University Press, Oxford, 1995, pp. 722–728. [3] Masahiro Ota, K. Kasahara, M. Sakamoto, Micromachine-actuator rotated by rarefied gas effects, Solid Thin Films 281/282 (1996) 651–653. [4] Masahiro Ota, M. Ishiguro, M. Sakamoto, On selective optical absorber film for rotor of opto-microengine, ASME, HTD 336 (1996) 193–226. [5] Masahiro Ota, T. Nakao, M. Sakamoto, Manipulation for fine solid particles by gradient radiation pressure of a laser beam, ASME, FEDSM98-5107, 1998. [6] Masahiro Ota, Tomohiko Nakao, Moriyoshi Sakamoto, Characteristics of laser opto-microengine rotated by effects of molecular gas dynamics, Trans. Jpn. Soc. Mech. Eng. 65 (634) (1999) (in Japanese).