Mechanical Engineering PhD Thesis Defense bu Armin Bijanzad








Title: Development of a Novel Linear Compressor and Thermo-Mechanical Analysis


Speaker: Armin Bijanzad


Time: September 26, 2017, 11:00


Place: Manufacturing and Automation Research Center-MARC (ENG B203)

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

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

Prof. Dr. Metin Muradoğlu (Koç University)

Asst. Prof. Arif Karabeyoğlu (Koç University)

Prof. Dr. Ata Muğan (Istanbul Technical University)

Assoc. Prof. Özgen Akalın (Istanbul Technical University)



Compressors in the refrigeration cycle are the main mechanism that increases the gas medium pressure by reducing the volume and maintain a refrigeration flow in the cycle. The refrigerant mass flow correlates the cooling capacity and heat absorption from the environment. Therefore, mainly they are referred as the heart of the refrigerators. The conventional reciprocating compressors utilize a crank-driven mechanism to convert the rotational motion of the electric motor armature to the linear reciprocating motion. Therefore, fixed boundary constraints of refrigerant mass flow and fixed cooling capacity results in constant performance of the compressor. Additionally, the conversion of rotational motion to the linear motion causes an energy loss and friction loss zones in the corresponding bearings resulting in lower compressor efficiency. However, new developed linear compressors are directly using the linear electromagnetic force and springs to generate an oscillation. Additionally, harmonic oscillation in the resonance excitation reduces the input power of the compressor immensely. In this thesis, a novel linear compressor was developed for refrigeration cycle for the first time in Türkiye.


Two types of linear compressors named as solenoid based and moving magnet linear compressors were developed. First, two prototypes of solenoid based linear compressor with different electromagnetic force generating capabilities were manufactured for testing. The stator coil was excited with the pulse width modulated signal which caused linear oscillations in the armature using helical spring. Dynamic characteristics of the linear compressor were studied analytically considering the nonlinearity of the gas and electromagnetic force. The system dynamic models were validated with the finite element simulation as well as a specially designed experimental setup. Frequency response functions of stroke to current as well as pressure to current ratios were generated to evaluate the effect of excitation frequency on the compressor performance. The higher efficiency of the solenoid actuator was achieved when excited at the natural frequency. Secondly, the oil-free moving magnet linear compressor prototype for household refrigerators was proposed. The design and components of the moving magnet prototype including the detailed CAD model of the compressor are provided. Additionally, the dynamic characteristics of the moving assembly are investigated analytically and validated experimentally. With and without gas harmonic oscillations of the moving assembly are demonstrated using the system dynamics and electrical circuit models. Moreover, the effect of nonlinear gas force and different gas properties on the output parameters with respect to the power input are investigated for four different gases. Moreover, three gas force modeling approaches are presented and compared with the instantaneous experimental gas force. Furthermore, the frequency response functions of the stroke to the input power as well as the discharge line pressure to the input power are generated. The motor and isentropic efficiencies of the prototype are measured for different power inputs. Finally, the behavior of the suction valve with different valve stiffness is experimentally evaluated to improve the linear compressor’s performance.