Mechanical Engineering PhD Thesis Defense by Uğur Birbilen


Title: Design and Analysis of a Miniature Linear Compressor

Speaker: Uğur Birbilen

Time: March 21, 2018, 13:00

Place: Manufacturing and Automation Research Center (MARC)
Koç University
Rumelifeneri Yolu
Sariyer, Istanbul


Thesis Committee Members:

Prof. Dr. İsmail Lazoğlu (Advisor, Koç University)

Prof. Dr. Metin Muradoğlu (Koç University)

Prof. Dr. Ata Muğan (Istanbul Technical University)

Assoc. Prof. Dr. Erdem Alaca (Koç University)

Assoc. Prof. Dr. Mustafa Bakkal (Istanbul Technical University)


Linear compressors are good alternatives for the conventional rotary compressors which usually have to deal with the conversion of rotational motion to reciprocating motion before the compression step, whereas linear compressors directly generate linear motion to be used for the compression step right away. This offers many advantages over rotary compressors. While linear compressor technology is nothing new, miniature-scale linear compressor is a recent favorite. The compactness in linear compressor design combined with the smallness in scale makes this type of compressor a very interesting candidate for many practical applications, for example in the cooling of electronic equipment or in building mini refrigerators. This research deals with the detailed analysis of a prototype miniature-scale linear compressor. Basically, a linear compressor can be said to consist of two main parts: the linear actuator part which enables the reciprocating motion to the piston directly attached to it and the cylinder-valve assembly part where the interaction with the piston causes compression.

The newly developed linear actuator is moving-magnet type. Moving-magnet type compressor provides higher thrust and efficiency and lower losses. To further enhance the compactness of the design, a tubular structure is selected. An analytical model is first presented and the formulation for the magnetic flux density and the generated thrust force is shown. Then an FEM analysis is conducted, where the axisymmetric approach is used in order to have a conformity with the tubular structure. The magnetic fluxes generated inside the actuator are again calculated and the generated thrust force is demonstrated accordingly. These results are compared with the ones coming from the analytical model. The experimental setup for the linear actuator is presented and the thrust force generated is compared with the analytical results.

The cylinder-valve assembly is presented in detail. The dynamic parameters of the entire compressor assembly are presented. This is shown for both the mechanical and electromagnetic cases. The governing system dynamics equations, which includes the thrust generated by the actuator and the stiffness and damping coming from both the mechanical equipment and the compressed gas, are provided. Motor constant and excitation frequency effects are discussed. The frequency response function is shown in order to find the optimal operating point with the best system efficiency. It is shown that 80.4 % efficiency is achieved at the resonance frequency of 45 Hz and 3.3 bar pressure is generated by the new miniature linear compressor.