Factors affecting the cylinder displacement of a Wankel compressor in a microcooling system.
The factors that influence the performance of a Wankel compressor compressor, machine that decreases the volume of air or other gas by the application of pressure. Compressor types range from the simple hand pump and the pistonequipped compressor used to inflate tires to machines that use a rotating, bladed element to achieve mainly include the compression ratio compression ratio Degree to which the fuel mixture in an internalcombustion engine is compressed before ignition. It is defined as the volume of the combustion chamber with the piston farthest out divided by the volume with the piston in the fullcompression position ( and dimensions (shape factor, eccentric distance, and cylinder height). In this study, theoretical equations are derived to analyze the effect of these factors and carry out a transient simulation of the nonsteadystate behavior of a mesoWankel compressor using a dynamic mesh to support the theoretical equations. The simulation and theoretical results agree well with each other. Cylinder displacement gradually increases with an increasing shape factor, whereas cylinder displacement per unit crosssectional area gradually decreases. Both the cylinder displacement and cylinder displacement per unit crosssectional area increase with different eccentric distances, and this growth follows a converse trend. The optimum shape factor is approximately 6 for the Wankel compressor design. On the basis of the design parameters in this article, optimum cooling capacities of 418 and 352 W are obtained from the theoretical and simulation results, respectively. The relative deviation is about 16%. Introduction With the increasing requirement for temperature control of meso and microfacilities, the demand for microcooling systems has become increasingly urgent in recent years. Meso and microcompressors act as the core parts of a cooling system cooling system: see air conditioning; internalcombustion engine; refrigeration. cooling system Apparatus used to keep the temperature of a structure or device from exceeding limits imposed by needs of safety and efficiency. , and Wankeltype units have become prime choices for microcooling systems because of their unique advantages, including simplicity in structure, high efficiency, and low noise. Some achievements in research on the microWankel engine and microWankel compressor include prototype design and manufacturing, as well as corresponding experimental research (Heppner et al. 2007; Lee et al. 2004; Fu et al. 2001). Martinez et al. (2003) and Swanger et al. (2004) improved the sealing mode of the Wankel engine Wankel engine: see internalcombustion engine. , thereby increasing the sealing efficiency by 10% to 50%. A numerical simulation of the performance of a mesoscale Wankel compressor was conducted, and the factors affecting the miniaturization of the compressor (leakage, friction loss Friction loss refers to that portion of pressure lost by fluids while moving through a pipe, hose, or other limited space. Causes Friction loss has several causes, including: In decreasing the dimensions of a Wankel compressor to satisfy the miniaturization requirements of a microcooling system, the surfacetovolume ratio becomes larger, and the surfaces in relative motion cannot be adequately lubricated lu·bri·cate v. lu·bri·cat·ed, lu·bri·cat·ing, lu·bri·cates v.tr. 1. To apply a lubricant to. 2. To make slippery or smooth. v.intr. To act as a lubricant. as in the normal scale. Because of these effects, the performance of meso and microWankel compressors tremendously decreases, and high efficiency becomes difficult to achieve. Therefore, strategies for improving the performance of a Wankel compressor have become an important issue, especially for microand mesoWankel compressors. Many factors affect the performance of a Wankel compressor. Such factors mainly include the compression ratio and the dimensions affecting the profile and volume of the working chamber. The dimensions are the shape factor and eccentric distance, which affect the profile of the chamber, as well as the height of the cylinder. In the current work, the effects of these factors on the performance of a Wankel compressor are analyzed in detail. Related equations are also obtained. The theoretical analysis results are supported by the simulation. The variations in flow field, cylinder displacement, exhaust time, cooling capacity, and cooling capacity per unit crosssectional area of the cylinder are analyzed under the corresponding operational conditions. Finally, a Wankel compressor prototype is designed based on the optimum dimensions obtained from the theoretical and simulation analyses. Performance of a Wankel compresssor The PV diagram of the thermal process of a Wankel compressor is shown in Figure 1. The sequence 12341 represents the ideal thermal process of a Wankel compressor. Sequence 12 illustrates the compression process, in which the gas pressure in the compression chamber See: hyperbaric chamber. gradually increases with decreasing volume. The gas pressure continues to increase until it reaches the point of back pressure, after which the operation turns into an exhaust process, as shown in sequence 23. The gas pressure is kept stable during the exhaust process until the minimum volume of the exhaust chamber is achieved. At this time, the exhaust valve is shut, and the expansion process begins. The gas pressure declines with increasing clearance volume until the pressure reaches the intake process, as illustrated in sequence 34. The operation process transforms into the intake process (sequence 41), and the pressure is kept unchanged until the volume of the intake chamber reaches its maximum. [FIGURE 1 OMITTED] Cylinder displacement of a Wankel compressor The cylinder displacement of a Wankel compressor is calculated by the difference between the compression chamber volume at the end of the compression process ([V.sub.2]) and the exhaust chamber volume at the end of the exhaust process ([V.sub.3]) (Figure 1): [V.sub.d] = [V.sub.2] [V.sub.3]. (1) The compression process in the compression chamber is assumed to be a polytropic process Polytropic process A process which occurs with an interchange of both heat and work between the system and its surroundings. The nonadiabatic expansion or compression of a fluid is an example of a polytropic process. , expressed as [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII ASCII or American Standard Code for Information Interchange, a set of codes used to represent letters, numbers, a few symbols, and control characters. Originally designed for teletype operations, it has found wide application in computers. .] (2) The cylinder displacement of a Wankel compressor can be expressed as [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (3) where y is the compression ratio, [gamma] = [P.sub.2]/[P.sub.1]. A Wankel compressor consists of three working chambers, and each volume is bounded by the internal surface of the cylinder, rotor, apex seals, and two endplates. The volume of the working chambers can be described as [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (4) The Wankel compressor must have clearance between the rotor and internal surface of the cylinder because of the limitation in machining accuracy and the need for friction reduction. Considering clearance (also called offset a, as shown in Figure 2), the volume of the working chamber of a Wankcl compressor can be approximately calculated using the following equation: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (5) [FIGURE 2 OMITTED] The maximum ([V.sub.1]) and minimum volumes ([V.sub.3]) of the working chamber are expressed as [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.], (6) [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (7) Volumes [V.sub.1] and [V.sub.3] are substituted into Equation 3. The displacement of the Wankel compressor can then be obtained by [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.]. (8) The rotor and cylinder profiles are vital to the shape and volume of the working chamber. The corresponding crosssectional area of the working chamber plays a decisive role in cylinder displacement and in the performance of the Wankel compressor. The profile equations of the Wankel compressor cylinder are [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (9) The crosssectional area of the cylinder can be calculated by the column coordinate equations of a cylinder, expressed as [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (10) where [beta] = [pi]/3 The cylinder displacement per unit crosssectional area of a Wankel compressor is obtained by the ratio of cylinder displacement [V.su.d] to the crosssectional area of cylinder A: [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (11) According to according to prep. 1. As stated or indicated by; on the authority of: according to historians. 2. In keeping with: according to instructions. 3. Equations 8 and 11, the factors causing cylinder displacement in a Wankel compressor include the compression ratio ([lambda]), shape factor (K), eccentric distance (e), offset (a), and cylinder height (a). Results and discussion of the cylinder displacement of a Wankel compressor According to theoretical Equations 8 and 11, the cylinder displacement and cylinder displacement per unit crosssectional area of a Wankel compressor can be obtained using different parameters. Figures 35 show the results at different compression ratios, shape factors, and eccentric distance (the offset is assumed to be 0.5 mm). At shape factor, eccentric distance, and cylinder height values of 6, 3 (0.118 in.) and 11.5 mm (0.453 in.), respectively, the cylinder displacement and cylinder displacement per unit crosssectional area gradually decrease with increasing compression ratiothe rate at which the displacement decreases continuously drops, as shown in Figure 3. The other parameters are unchanged, except for the shape factor; the compression ratio is 3.5. The cylinder displacement constantly increases with increasing shape factor, whereas the cylinder displacement per unit crosssectional area shows an opposite trend (Figure 4). The common range of the shape factor is from 6 to 7 (Zhang and Wang 2011) in the design of a Wankel compressor. To make the compressor more compact, the cylinder displacement per unit crosssectional area should be as large as possible. Therefore, the shape factor of a Wankel compressor should have a value of around 6, especially for the meso and microWankeltype units. [FIGURE 3 OMITTED] Figure 5 shows that the cylinder displacement and cylinder displacement per unit crosssectional area continuously increase with increasing eccentric distance under the same parameters (K = 6, B = 11.5 mm [0.453 in.], and [lambdda] = 3.5). The growing rate of the cylinder displacement continuously increases with the increase in eccentric distance, but the cylinder displacement per unit crosssectional area exhibits a converse growth rate. According to the analysis above, the cylinder displacement of a Wankel compressor is primarily affected by the shape factor, eccentric distance, and cylinder height on the basis that the compression ratio is determined by the design parameters. Among these design parameters, the shape factor and eccentric distance are the most important factors that influence the dimensions and performance of a Wankel compressor. Therefore, the effect of the shape factor and eccentric distance on the performance of Wankel compressors is analyzed using computational fluid dynamics Computational fluid dynamics The numerical approximation to the solution of mathematical models of fluid flow and heat transfer. Computational fluid dynamics is one of the tools (in addition to experimental and theoretical methods) available to solve . Numerical simulation of a Wankel compressor Initial conditions of a Wankel compressor In general, the flow field in a Wankel compressor is three dimensional. The degree of importance of the threedimensional effects is related to the position of the inlet and outlet, as well as their heights. In the present investigation, the inlet and outlet are assumed to be uniformly spread along the height of a Wankel compressor (Figure 6). Therefore, the threedimensional effects can be disregarded, and twodimensional models can be used for the simulations in this study. [FIGURE 4 OMITTED] For a simplified simulation of a Wankel compressor, the following assumptions are made: 1) the threedimensional effects on the flow field are disregarded, and the twodimensional model is used for the simulation; 2) the seals apex comes into contact with the internal surface of the cylinder, indicating the leakage between the three chambers is disregarded and no fluid exchange occurs between the three chambers; 3) the friction between moving parts Moving parts are the components of a device that undergo continuous or frequent motion, most commonly rotation. "Parts" only include the mechanical components which does not include fuel, or any other gas or liquid. of the compressor parts (that is, not considering friction heat) is disregarded; and 4) all solid boundaries are assumed adiabatic. [FIGURE 5 OMITTED] The design of a Wankel compressor is based on its cooling capacity and the state parameters of intake and exhaust gases. In this study, the simulation object is a mesoWankel compressor, and the operational parameters are shown in Table 1. [FIGURE 6 OMITTED] Simulation method for a Wankel compressor The simulation of the internal flow field of the compressor is carried out using the fluid dynamic software FLUENT (2D). The governing equations for the simulation include the conservation equations of mass, momentum, and energy. A finitevolume approach and the implicitdifference method based on pressure are used to solve the governing equations. A secondorder upwind scheme is selected to discretize the governing equations, and the SIMPLE algorithm is used for pressurevelocity coupling. In general, Wankel compressors have high rotation speed, and the internal flow field is a complex turbulent flow. Aside from the governing equations, turbulence models for the calculation of Reynolds stresses In fluid dynamics, the Reynolds stresses (or, the Reynolds stress tensor) is the stress tensor in a fluid due to the random turbulent fluctuations in fluid momentum. The stress is obtained from an average (typically in some loosely defined fashion) over these fluctuations. are also added to the solution equations for the simulation. For turbulence modeling Turbulence modeling is the area of physical modeling where a simpler mathematical model than the full time dependent NavierStokes Equations is used to predict the effects of turbulence. , the RNG See RELAX NG. [kappa Kappa Used in regression analysis, Kappa represents the ratio of the dollar price change in the price of an option to a 1% change in the expected price volatility. Notes: Remember, the price of the option increases simultaneously with the volatility. ][epsilon] model is used, while the turbulent viscosity is computed from a differential equation differential equation Mathematical statement that contains one or more derivatives. It states a relationship involving the rates of change of continuously changing quantities modeled by functions. . This model is a modified representation of the turbulent flows with Reynolds number Reynolds number [for Osborne Reynolds], dimensionless quantity associated with the smoothness of flow of a fluid. It is an important quantity used in aerodynamics and hydraulics. in a large range and a case with curved streamlines in circulation regions. The modified terms in Equation e of the RNG [kappa][epsilon] model enhance the dissipation Dissipation See also Debauchery. Breitmann, Hans lax indulger. [Am. Lit.: Hans Breitmann’s Ballads] Burley, John wasteful ne’erdowell. [Br. Lit. rate in the vicinity of the stagnation Stagnation A period of little or no growth in the economy. Economic growth of less than 23% is considered stagnation. Sometimes used to describe low trading volume or inactive trading in securities. Notes: A good example of stagnation was the U.S. economy in the 1970s. region and prevent the augmentation AUGMENTATION, old English law. The name of a court erected by Henry VIII., which was invested with the power of determining suits and controversies relating to monasteries and abbey lands. of turbulent kinetic energy Turbulent Kinetic Energy (TKE) is the mean kinetic energy per unit mass associated with eddies in turbulent flow. It is a concept used to assess what contribution to buoyancy is brought by turbulence. in this region. The model of the Wankei compressor is meshed with an unstructured grid An unstructured grid is a tessellation of a part of the Euclidean plane or Euclidean space by simple shapes, such as triangles or tetrahedra, in an irregular pattern. Grids of this type may be used in finite element analysis when the input to be analyzed has an irregular shape. . The meshes near the apex seal are refined to ensure that the minimum clearance has a threelayer mesh, which is the minimum number of mesh layers required in simulating fluid flow. The time step is 0.000005 s, while the rotating speed of the compressor is 1800 rpm in the simulation processes. The operation of a Wankel compressor is an unsteady process, and the shape and volume of the working chambers constantly change as the rotor rotates. Therefore, a dynamic mesh should be used in simulating Wankel compressors; in the current study, a userdefined function is created to define the movement principle and control rotor movement. In accordance with the analysis above, the following boundary conditions are used in the simulation. Inlet and outlet boundary condition boundary condition n. Mathematics The set of conditions specified for behavior of the solution to a set of differential equations at the boundary of its domain. : the pressure boundary condition is used for the inlet and outlet, and the total and static pressures are determined based on initial conditions. Wall boundary condition: the effect of heat transfer is disregarded, and a no slip boundary condition is used for the internal surface of the cylinder and external surface of the rotor. The rotor has a rotating wall, and the rotation speed is about 600 rpm. Boundary conditions in relation to turbulence: Standard wall functions are used for the turbulence model. Turbulent kinetic energy and the turbulent kinetic energy dissipation rate can be obtained by the following empirical formula empirical formula: see formula. : [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII.] (12) This article focuses on the influence of the shape factor and eccentric distance on the performance of a Wankel compressor. First, shape factor K, which controls the shape of the cylinder and the rotor, is considered, using the same eccentric distance and the ideal design of the inlet and outlet (the intake and exhaust are controlled by the valve on the inlet and outlet) as a basis. A reasonable shape factor is obtained in designing the Wankel compressor. Second, the effect of eccentric distance is taken into account on the basis of the ideal design of the inlet and outlet, as well as the shape factor obtained in the previous step. The reasonable eccentric distance is determined using the design parameters in Table 1. Finally, a mesoWankel compressor prototype is created on the basis of the results from the abovementioned steps and the simulation carried out in this article. During the simulation, variations in some parameters of the working chamber should be monitored to ensure the reliability of the simulation results. These parameters include gas pressure, gas temperature, and gas velocity. Effect of shape factor on performance under the same cylinder heights and eccentric distances To analyze the influence of the shape factor on the performance of a Wankel compressor, the following initial conditions should be set: 1) an eccentric distance of 3 mm (0.118 in.); 2) a cylinder height of 11.5 mm (0.453 in.); 3) the ideal design of the inlet and outlet is used in the simulation, and the intake and exhaust gases are controlled by the valves set on the inlet and outlet; and 4) the shape factor is usually within the range of 6 to 7 for a Wankel machine. To increase the universality and applicability of the simulation, the shape factor value chosen is 5.5 to 7.5. [FIGURE 7 OMITTED] Figures 79 show that the opening time of the exhaust valve gradually advances with increasing shape factor, and the corresponding exhaust time continuously increases. The cylinder displacement, exhaust mass flow, and cooling capacity exhibit the same growth trend as the shape factor increases. The cooling capacity per unit crosssectional area of the cylinder decreases with an increasing shape factor, and better performance is achieved when the shape factor of a Wankel machine is in the range of 6 to 7. In accordance with the decreasing trend of the cooling capacity per unit crosssectional area of the cylinder with increasing shape factor, the value of ~6 is taken as tile optimum shape factor to make the Wankel compressor more compact, especially for meso and microWankel compressors. Effect of eccentric distance on performance under the same shape factors and cylinder heights On the basis of the analysis above, ~6 is chosen as the value of the shape factor of the Wankel compressor. In addition to the shape factor, the following initial conditions should be established to analyze the influence of the eccentric distance on the performance of the compressor: 1) a cylinder height of 11.5 mm (0.453 in.); 2) the ideal design of the inlet and outlet is used in the simulation, and the intake and exhaust gases are controlled by the valves set on the inlet and outlet; and 3) in accordance with the geometrical relationship and strength requirement of a Wankel compressor, the eccentric distance is restricted in the range of 2 to 5 mm (0.0787 to 0.197 in.) (Zhang and Wang 2011). [FIGURE 8 OMITTED] [FIGURE 9 OMITTED] As shown in Figures 1012, the exhaust time, cylinder displacement, exhaust mass flow, cooling capacity, and cooling capacity per unit cylinder crosssectional area continually increase with an increasing eccentric distance. However, their growth rates Growth Rates The compounded annualized rate of growth of a company's revenues, earnings, dividends, or other figures. Notes: Remember, historically high growth rates don't always mean a high rate of growth looking into the future. are different from one another. Among the growth rates, those of the exhaust time and cooling capacity per unit crosssectional area gradually decrease with the increase in eccentric distance. The growth rates of the other parameters continuously increase. The simulation results for the effect of the shape factor and eccentric distance on the performance of the Wankel compressor agree well with the theoretical results. The reasonable shape factor is approximately 6. In accordance with the design cooling capacity and considering a certain surplus in cooling capacity (because leakage is not considered in this article), a value of 3 mm (0.118 in.) is taken as the eccentric distance. [FIGURE 10 OMITTED] Simulation of the Wankel compressor prototype A mesoWankel compressor prototype (Figure 13) is created based on the analysis, in which shape factor and eccentric distance are 6 and 3 mm (0.118 in.), respectively. The prototype is composed of a main shaft, gear pairs, seals, rotor, cylinder, endplates, and valve plates. The cylinder and rotor are both 11.5 mm (0.453 in.) high, and the compressor has a footprint of 40 x 50 mm (1.575 x 1.969 in.) and a thickness of 24 mm (0.945 in.). [FIGURE 11 OMITTED] The simulation of the mesoWankel compressor prototype is conducted, and a variation in gas pressure in the working chambers ([V.sub.1], [V.sub.2], and [V.sub.3]) is shown in Table 2. Working chamber [V.sub.3] is taken as an example in comprehensively analyzing the pressure variation. The exhaust process of working chamber [V.sub.3] ends when the time is 0 s, at which the volume reaches its minimum and the operation turns into the expansion process. With increasing [V.sub.3] volume, the gas pressure gradually decreases, and the process turns into the intake process until the gas pressure in [V.sub.3] reaches the point of intake pressure. The intake time is 0.00855 to 0.025 s, and the volume of [V.sub.3] reaches its maximum at the end of the intake process. The compression process then begins. The gas pressure in [V.sub.3] continuously increases with the decrease in the volume of [V.sub.3] until the gas pressure reaches the point of back pressure (1.2 MPa in this article). The process of the Wankel compressor then turns into the exhaust process, which ends when the volume of [V.sub.3] reaches its minimum. The exhaust time is approximately 0.00804 s (from 0.04196 to 0.05 s) for the Wankel compressor with valves on the inlet and outlet. The corresponding cooling capacity is about 352 W. [FIGURE 12 OMITTED] [FIGURE 13 OMITTED] According to the dimensions of the mesoWankel compressor prototype, the maximum and minimum volumes can be measured and substituted into Equation 3. The cylinder displacement and corresponding cooling capacity are approximately 900 [mm.sup.3] (0.055 in. (3)) and 418 W The relative deviation is about 16% compared with the simulation results. Conclusion Through simulation and theoretical analyses, it was determined that the compression ratio and dimensions (shape factor, eccentric distance, and cylinder height) considerably affect the performance of a Wankel compressor. According to the consistency of results between the simulation and theoretical discussions, the cylinder displacement constantly increases with an increasing shape factor, whereas the cylinder displacement per unit crosssectional area gradually decreases. Both the cylinder displacement and cylinder displacement per unit crosssectional area of the cylinder increase with increasing eccentric distance and have a linear increase with cylinder height. The same changing trends apply to the cooling capacity and cooling capacity per unit crosssectional area. The optimum shape Factor is approximately 6 for the Wankel compressor, and the value of 3 mm (0.118 in.) is taken as the eccentric distance based on the design cooling capacity and considering a certain surplus of cooling capacity. The cooling capacity obtained from the theoretical and simulation results are 418 and 352 W, respectively, and the relative deviation is about 16%. DOI (Digital Object Identifier) A method of applying a persistent name to documents, publications and other resources on the Internet rather than using a URL, which can change over time. : 10.1080/10789669.2012.635948 Acknowledgment acknowledgment, in law, formal declaration or admission by a person who executed an instrument (e.g., a will or a deed) that the instrument is his. The acknowledgment is made before a court, a notary public, or any other authorized person. This work was supported by the National Natural Science Foundation of China (grant 50976067). Nomenclature nomenclature /no·men·cla·ture/ (no´menkla?cher) a classified system of names, as of anatomical structures, organisms, etc. binomial nomenclature a = offset, mm (in.) A = crosssectional area of cylinder, [mm.sup.2] ([in.sup..2]) B = height of the cylinder, mm (in.) [C.sub.[micro]] = constants of the turbulence model, the default value [c.sub.[mu]]= 0.09 is used in this simulation e = eccentric distance, mm (in.) I = turbulence intensity K = shape factor of the Wankel compressor, equal to the ratio of generation radius and eccentric distance l = turbulence length, m (in.) n = polytropic exponent exponent, in mathematics, a number, letter, or algebraic expression written above and to the right of another number, letter, or expression called the base. In the expressions x^{2} and x^{n}, the number 2 and the letter n [p.sub.1] and [p.sub.2] = gas pressures before and after compression process, Pa (psi) R = generation radius of the rotor, mm (in.) u = mean velocity, m * [s.sup.1] fit * [s.sup.1]) [V.sub.1], [V.sub.3], and [V.sub.2] = maximum chamber volume, minimum chamber volume, and volume of the compression chamber at the end of the compression process, [mm.sup.3] ([in..sup.3]) [V.sub.d] and [V.sub.ds] = cylinder displacement of cylinder, mm 3, and cylinder displacement per unit crosssectional area, mm (in.) Greek symbols [gamma] = compression ratio [epsilon] = turbulent kinetic energy dissipation rate, [m.sup.2] * [s.sup.3] ([ft.sup.2] * [s.sup.3]) [[theta Theta A measure of the rate of decline in the value of an option due to the passage of time. Theta can also be referred to as the time decay on the value of an option. If everything is held constant, then the option will lose value as time moves closer to the maturity of the option. ].sub.max] = maximum swinging angle, rad [kappa] = the turbulent kinetic energy, [m.sup.2] * [s.sup.2] ([ft.sup.2] * [s.sup.2]) [phi] = eccentric angle, rad Received April 20, 2011; accepted September 8, 2011 References Fu, K., A. Knobloch, F. Martinez, D. Walther, A.C. FernandezPello, A. Pisano, and D. Liepmann. 2001. Design and fabrication fabrication (fab´rikā´shn), n the construction or making of a restoration. of a siliconbased MEMS (MicroElectroMechanical Systems) Tiny mechanical devices that are built onto semiconductor chips and are measured in micrometers. In the research labs since the 1980s, MEMS devices began to materialize as commercial products in the mid1990s. rotary engine rotary engine, internalcombustion engine whose cycle is similar to that of a piston engine, but which produces rotary motion directly without any conversion from reciprocating motion. . ASME ASME  American Society of Mechanical Engineers International Mechanical Engineering Congress and Exposition, New York New York, state, United States New York, Middle Atlantic state of the United States. It is bordered by Vermont, Massachusetts, Connecticut, and the Atlantic Ocean (E), New Jersey and Pennsylvania (S), Lakes Erie and Ontario and the Canadian province of , November 1116, pp. 87580. Heppner, J.D., D.C. Walther, and A.P. Pisano. 2007. The design of ARCTIC: A rotary compressor thermally insulated [mu]cooler. Sensors and Actuators A 134(1):4756. Lee, C.H., K.C. Jiang, P. Jin, and RD. Prewett. 2004. Design and fabrication of a micro Wankel engine using MEMS technology. Microelectronic Engineering 73(4):52934. Martinez, F., A. Knobloch, K. Fu, and A.P. Pisano. 2003. Apex seal design for the MEMS rotary engine power system. Proceedings of ASME 2003 International Mechanical Engineering Congress and Exposition (IMECE IMECE International Mechanical Engineering Congress and Exposition ), Washington DC, November 1621, IMECE200342071. Swanger, M., D.C. Walther, A.C. Fernandezpello, and A.P. Pisano. 2004. Smallscale rotary engine power system development status. WSS04S9 for Western States Section/Combustion Institute, Davis, CA. Zhang, Y.L., and W. Wang. 2011. Effects of leakage and friction on miniaturization of Wankel compressor. Frontiers of Energy and Power Engineering in China 5(1):8392. Yilin Zhang is PhD Student. Wen Wang, PhD, is Professor. Yilin Zhang and Wen Wang * Institute of Refrigeration refrigeration, process for drawing heat from substances to lower their temperature, often for purposes of preservation. Refrigeration in its modern, portable form also depends on insulating materials that are thin yet effective. and Cryogenics cryogenics: see lowtemperature physics. cryogenics Study and use of lowtemperature phenomena. The cryogenic temperature range is from −238°F (−150°C) to absolute zero. At low temperatures, matter has unusual properties. , Shanghai Jiao Tong University Shanghai Jiao Tong University (Simplified Chinese: 上海交通大学; Traditional Chinese: 上海交通大學 800 Dongehuan Road, Shanghai 200240, China * Corresponding author email: wenwang@sjtu.edu.cn Table 1. Operational conditions of a Wankel compressor with refrigerant R134a. No. Parameter Description Value 1 Q Cooling capacity 300 W (1023.6 Btu * h1) 2 n Motor speed 1800 rpm 3 [T.sub.eva] Evaporation 5[degrees]C (41[degrees]F) temperature 4 [T.sub.con] Condensation 46[degrees]C temperature (114.8[degrees]F) 5 [P.sub.in] Intake pressure 0.35 MPa (50,763 psi) 6 [P.sub.out] Back pressure 1.20 MPa (174,045 psi) Table 2. Variation in pressure and the state of the Wankel compressor prototype. [V.sub.1] [V.sub.2] Time, s Pressure, MPa State Pressure, MPa State 0 0.457 Compression 0.35 Intake process process 0.00855 1.18 Compression 0.353 Compression process process 0.025 0.362 Expansion 1.14 Compression process process 0.04196 0.351 Compression 0.35 Intake process process 0.05 0.457 Compression 0.35 Intake process process [V.sub.3] Time, s Pressure, MPa State 0 1.2 End of exhaust process 0.00855 0.35 Beginning of intake process 0.025 0.35 End of intake process 0.04196 1.2 Beginning of exhaust process 0.05 1.2 End of exhaust process 

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