Numerical And Experimental Analysis Of A Linear Compressor

The paper presents the study of a new compressor for household appliances, developed through a virtual simulation system, with Simulink and Adams software. Linear compressor has a linear motor coupled to a mechanical system: a piston and a spring. The advantage of this compressor respect to a standard reciprocating compressor is due to the high reduction of energy losses and variable capacity provided. From the numerical optimized model a running prototype compressor has been produced, which obtain the high performances predicted by the virtual model. NOMENCLATURE z1 stator position b1 supporting spring damping K1 supporting spring stiffness m1 stator mass z2 piston position K2 piston spring stiffness b2 piston spring damping m2 piston mass Fel electric force Fth pressure force FT total force i current B magnetic flux density Beff effective flux density leff effective length wire V voltage source V1 motion induced tension Φ magnetic flux Leq equivalent inductance Ri iron losses Lc coil inductance Rc coil resistance Req equiv. resistance P chamber pressure V chamber volume Ω section m mass transfer γ polytrophic exponent Pin pressure before valve Pout pressure after valve Tin temperature before valve ρ pressure compression ratio ε degree of dead volume p oil pressure W oil velocity vector μ oil viscosity ρ oil density δ film thickness r radial dimension of piston z piston axial dimension p1 suction pressure h piston height u oil velocity along radius INTRODUCTION Due to the increasing demand of high efficiency compressors and the strong diffusion of electronic control devices, it is now important to develop better compressor systems. Linear compressor has been studied by several authors, because of the interesting energy consumption capabilities. The paper presents the development of a new single cylinder electrodynamics compressor from the mathematical simulation to the experimental tests. In a linear compressor the motion of the piston is directly due to the magnetic field forces while in standard compressor the rotational motion of the motor is transformed in an alternative motion by a crankshaft mechanism. The linear compressor has a simpler mechanism because the magnetic field impresses an alternative force to the piston. The first advantage is the mechanical simplicity of the system, which can also have a compact and robust structure. The second advantage is the reduction of the friction forces virtually to zero: all forces are directed along the piston motion direction, so there are lower power loses, theoretically there is no necessity of lubrication and there is no wear. The third advantage of this compressor is due to the possibility of varying the capacity by controlling the displacement of the piston. So this compressor can be used at a constant frequency of the current and can have a variable piston displacement. Anyway the possibility presents a disadvantage: controls for the system can be necessary in order evaluate piston position. MECHANICAL SYSTEM The mechanical system has been analyzed through a dynamic model developed with Adams software. The advantage of this approach is that it is possible to analyze also flexible elements so modal behavior of these elements at higher frequencies; however it can be also analyzed with a simpler two degree of freedom system (Fig. 1). Defining with the index 1 the characteristics of the stator: (z1 stator position, K1 and b1 suspension spring characteristically stiffness and damping) and with the index 2 the terms related to the piston (z2 piston position, K2 and b2 piston spring characteristically stiffness and damping) As known, the equations of this system are: ( ) ( ) ( ) ( )   = − − − − = − + + − + + T F z z K z z b z m z K z K K z b z b b z m 2 1 2 2 1 2 2 2 2 1 1 2 1 2 1 1 2 1 1 1 0 & & & & & & & & The previous differential equation system can be solved numerically. The external applied force FT depends on several terms: 1. Magnetic force 2. Pressure force applied by the gas 3. Gravity force 4. Friction force The first force (Fel) is due to the electric system and will be analyzed in the paragraph Electric system. The second force (Fth) depends on the thermodynamic cycle and on the valves dynamic. It will be analyzed in the paragraph thermodynamic system. The third term has impact on static equilibrium position, but has no relevant importance on the model. The forth term is due to friction forces along sliding parts. It will be analyzed in the paragraph friction losses. Impact forces on piston and valves have been considered in the model, using Adams features. z1 z2