Showing papers on "Linear compressor published in 2020"

Theoretical analysis of dynamic characteristics in linear compressors

Abstract: Linear compressor technology is characterized by the absence of a crank mechanism, which has gained increasing attention in vapor compression systems due to its compactness and lower friction losses in comparison to conventional reciprocating compressors. Limited work was found in the open literature related to the development and in-depth validation of a comprehensive linear compressor dynamic model that couples thermodynamic aspects with both mechanical and electrical sub-models. This paper presents a comprehensive and generalized simulation model that is used to simulate the dynamic performance of a linear compressor. The model is based on mass and energy balance equations applied to open control volumes. The overall model is composed of several sub-models including the piston dynamics, electrical motor, valve dynamics, and leakage flows. The thermodynamic model and the sub-models are integrated into a compressor overall energy balance that describes the different heat flows and losses. The linear compressor model is able to predict both transient and steady-state behaviors of the piston movement, internal pressure and temperatures as well as the overall performance. Comparisons of predicted and measured mass flow rates as a function of operating frequency are also presented within this work.

Modification of Shear Stress Transport Turbulence Model Using Helicity for Predicting Corner Separation Flow in a Linear Compressor Cascade

Abstract: Three-dimensional corner separation significantly affects compressor performance, but turbulence models struggle to predict it accurately. This paper assesses the capability of the original shear stress transport (SST) turbulence model to predict the corner separation in a linear highly loaded prescribed velocity distribution (PVD) compressor cascade. Modifications for streamline curvature, Menter’s production limiter, and the Kato-Launder production term are examined. Comparisons with experimental data show that the original SST model and the SST model with different modifications can predict the corner flow well at an incidence angle of −7 deg, where the corner separation is small. However, all the models overpredict the extent of the flow separation when the corner separation is larger, at an incidence angle of 0 deg. The SST model is then modified using the helicity to take account of the energy backscatter, which previous studies have shown to be important in the corner separation regions of compressors. A Reynolds stress model (RSM) is also used for comparison. By comparing the numerical results with experiments and RSM results, it can be concluded that sensitizing the SST model to helicity can greatly improve the predictive accuracy for simulating the corner separation flow. The accuracy is quite competitive with the RSM, whereas in terms of computational cost and robustness it is superior to the RSM.

A novel resonant frequency tracking control for linear compressor based on MRAS method

Abstract: To optimize the efficiency of the linear compressor, its operating frequency must be controlled equal to the system resonant frequency. The traditional resonant frequency tracking control algorithm relies on the steady state characteristics of the system, which suffers from slow convergence speed, low accuracy and slow system response. In order to solve these problems, a novel resonant frequency tracking control for linear compressor based on model reference adaptive system (MRAS) is proposed in this paper, and the parameter adaptive rate is derived by the Popov's hyperstability theory, so that the system resonant frequency can be directly calculated through the parameter adaptive rate. Furthermore, the traditional algorithm needs to calculate the piston stroke signal by integrating the back-EMF, which has the problem of integral drift. The algorithm proposed in this paper only needs the velocity signal, and the accuracy of the velocity calculation can be ensured by utilizing the self-adaptive band-pass filter (SABPF), thereby greatly improving the accuracy of the resonance frequency calculation. Simulation results verify the effectiveness of the proposed algorithm.

Performance analysis of linear compressor using R290 for commercial refrigerator

Abstract: Linear compressor is a promising alternative for commercial refrigerator using R290. In this paper, a design program of linear compressor is set up. The leakage mass flow rate is modeled based on small hole leakage principle. The errors of the model are within 10%. The cooling performance of linear compressor with different piston diameter for commercial referigerator using R290 is analyzed. As the linear compressor is set to have the same swept volume, small piston diameter is beneficial to the COP improvement, but has a few decrease of cooling capacity. As the linear compressor is set to have the same cooling capacity by adjusting power frequency, COP of the linear compressor with piston diameter of 28 mm and 30 mm gets a little decrease because the power frequency deviates from the inherent frequency. The optimal piston diameter of 12 CC swept volume for the commercial refrigeration is suggested to be from 30 mm to 32 mm.

Development of a new moving magnet linear compressor. Part A: Design and modeling

Abstract: This article provides a detailed design and a CAD model of a new moving magnet linear compressor for the household refrigerator. A comprehensive electromechanical analytical model is provided to quantify the dynamic parameters necessary for the estimation of a compressor's performance. Additionally, the frequency response functions of the prototype at different input power conditions are investigated in order to evaluate the effect of excitation frequency on the dynamic parameters. The developed compressor is rigorously evaluated on a test-rig to validate all the presented analytical models. This article also provides a methodology to calculate the actuator's back-emf and motor constant which performs a vital role in the analytical model developed for any linear compressor. Furthermore, performance of the proposed compressor is evaluated with R600a to validate the pressure–volume curves and frequency response functions.

Development of a new moving magnet linear compressor. Part B: Performance analysis

Abstract: This article is a continuation of the preceding article and focuses on the performance evaluation of an oil-free linear compressor prototype. The article introduces performance analysis parameters for a linear compressor along with their analytical attributes. Initially, the kinetic and kinematic analysis at resonance excitation frequency is performed with the help of a specially designed experimental setup. Furthermore, the frequency response functions of output to input parameters is presented to particularly monitor the shift in resonance frequency in the presence of refrigerant. Four different gases are used to quantify the shift and relating them with the physical properties of the refrigerant. Additionally, the pressure-volume curves for all these gases are presented and analysed. In the end, the motor and overall isentropic efficiencies of the prototype are measured at different input parameters. Additionally, the connection between the pressure output and volumetric efficiency is also discussed.

Modelling and Measurement of a Moving Magnet Linear Motor for Linear Compressor

Abstract: For the purpose of efficiency improvement, a linear motor that performs a linear reciprocating motion can be employed to directly drive the piston in a reciprocating refrigeration compressor without crankshaft mechanism. This also facilitates the modulation of cooling capacity as the stroke and frequency can be readily varied in response to heat load. A novel design of moving magnet linear motor for linear compressor was analyzed in the paper. A finite element analysis (FEA) model was built to simulate the characteristics of the linear motor. Current and displacement signals were measured from a test rig and were defined in the transient FEA model. Transient motor force was simulated with the FEA model and good agreements are shown between the results from the FEA model and interpolated shaft force from static force measurements. Major Losses, such as copper loss and core loss were also computed. Motor efficiency decreased from 0.88 to 0.83 as stroke increased from 9 mm to 12 mm, while the pressure ratio remained unchanged. Comparisons were made between the present moving magnet linear motor and moving coil linear motors. Generally, the moving magnet linear motor demonstrates higher efficiency than moving coil motors, which have significantly higher copper loss. The present moving magnet design with simple structure could be further optimized to improve motor efficiency.

Development of a spaceborne pulse tube cooler operating at 170K.

Abstract: The Infrared Focal Plane Array (IRFPA) detector needs quite low dark noise for image detection. It can be cooled to demanding low temperature by pulse tube cooler (PTC) because of low vibration and low electromagnetic interference (EMI) at cold end. A high-capacity pulse tube cooler driven by a moving-magnet linear compressor is presented in this paper. The regenerator and pulse tube are arranged in coaxial. The inertance tube and reservoir are used as passive phase shifter of the PTC. A numerical thermodynamic model is established to design and optimize overall performance of the pulse tube cold finger aimed for the best efficiency. Based on the principle of electric-mechanical-acoustic coupling field, a transient co-simulation of the PTC is proposed. The mass of the PTC is less than 12 kg without electronic controller. The oscillating linear compressor has a pair of opposite pistons to eliminate vibration and the input electric input power is 400 W at maximum. A typical cooling performance of 50 W at 170 K has been achieved with 228 W input power at reject temperature of 293 K, provided by water cooling. The specified Carnot efficiency is 15.8%. Additionally, overall cooling performances of the PTC at 150K-200 K are investigated by experiment, Not only could this PTC be used for space mission but also a promising alternative to the domestic low temperature applications.

Experimental validation and sensitivity analysis of a dynamic simulation model for linear compressors

Abstract: In comparison with other types of conventional positive displacement compressors for domestic refrigerators, oil-free linear compressors have a number of advantages due to their compactness, low friction, limited number of moving parts, and excellent performance. Limited work was found in the open literature related to comprehensive compressor modeling and model validation of commercially available linear compressors. A comprehensive and generalized simulation model of linear compressors has been recently presented by the same authors. The current paper expands on this work by using experimental data for the simulated compressors to characterize their performance over the operating envelopes and to validate the linear compressor model. The validated model was then used to quantify the major sources of losses and analyze system vibration as well as piston eccentricity, which ultimately could be used to design the next generation of linear compressors.

Influence of piston displacement profiles on the performance of a novel dual piston linear compressor

Abstract: A novel dual piston linear compressor has been proposed in this paper, which is suitable for various applications such as refrigeration and gas compression owing to capacity modulation by stroke control. A numerical model for the proposed dual piston linear compressor was built to investigate the influence of different piston displacement profiles on the performance of the linear compressor. Sub-models on the piston dynamics, cylinder gas thermodynamics, motor force and frictional force were presented using MATLAB/Simulink. The piston was controlled to oscillate with the 5 typical piston displacement profiles. From the simulation results, it is found that the displacement profile of a triangle curve shows the highest compression efficiency and highest electrical efficiency. Moreover, the piston velocity and the current in the coil of the linear motor are considered to be necessary for feedback signals in the real-time control system to adjust the motor force and to reduce the effect of the back electromagnetic force.

Performance of a linear refrigeration compressor with small clearance volume

Abstract: Linear compressors are attractive for domestic refrigerator owing to high efficiency, free piston and possible oil-free operation. Minimizing the clearance volume is an approach to increase the volumetric efficiency thus cooling capacity. This is particularly beneficial for refrigerants that require high displacement due to high friction. However, operation of linear motor with offset may deteriorate the motor efficiency due to earlier saturation. This paper experimentally compares the refrigeration performance of a linear compressor using R1234yf among using various clearance volumes and using an offset of 0 mm. Measurements with different clearances and an offset of 0 mm were carried out for compressor strokes of 11–13 mm and pressure ratios of 2.5–4.0 using R1234yf. The cooling capacity for a pressure ratio of 2.5 and a stroke of 13 mm increases by 12% as the clearance decreases from 1.07 mm to 0.4 mm. Although the motor efficiency is sacrificed, the overall coefficient of performance (CoP) increases particularly for high pressure ratio.

A sensor-less stroke detection technique for linear refrigeration compressors using artificial neural network

Abstract: Linear compressors are very attractive for domestic refrigeration due to elimination of crank mechanism, high efficiency and compactness compared with conventional compressors. The significance of stroke control in a linear compressor not only lies in avoiding the piston collision into the cylinder head but also enabling cooling capacity modulation. Predicting piston stroke without a displacement sensor reduces the cost and facilitates the stroke control especially in miniature linear compressor where there is very limited space for installing sensors. This paper reports an artificial neural network (ANN) based stroke detection approach that can be used in linear compressors and any other linear (free-piston) machines. Experimental tests were conducted in a novel linear compressor driven refrigeration system to sample and record voltage, current and displacement. Fast Fourier transform (FFT) analysis was performed on current and voltage signals to extract harmonic terms as inputs of the neural network model to predict the stroke. Six cases with different numbers of harmonic term for current and voltage were compared. Both the mean squared errors and correlation coefficients are significantly improved with the increase of harmonic terms from one to three. However, small difference is indicated between the cases with three and six terms. Best percentage error distribution was achieved in the case with six harmonic terms with the majority of percentage errors falling within ±0.7% and a maximum percentage error of 3.5%. It can be concluded that the present ANN based stroke prediction approach can be effectively adopted for linear compressors without expensive displacement sensors. This is a key step towards the commercialization of linear refrigeration compressor technologies.

A study on eccentric behavior of discharge valve in refrigerant compressor using laser sensors

Abstract: A linear compressor for household refrigerators uses a circular plate type discharge valve. The behavior of the discharge valve greatly influences the efficiency of the linear compressor. In particular, the eccentric behavior of the discharge valve may also affect the performance of the compressor. Study of this eccentric behavior has not been sufficiently reported. Therefore, in this paper, we studied the eccentric behavior of the circular plate type discharge valve of a linear compressor. For this purpose, the stiffness and natural frequency of the discharge valve were measured using a laser displacement sensor, and the eccentricity of the valve displacement due to the cylinder pressure and the valve preload were also measured. It was found that discharge valve exhibits eccentric behavior under various conditions, which can affect the linear compressor performance.

Multi-Fidelity Design Optimisation of a Solenoid-Driven Linear Compressor

Abstract: Improved management and impermeability of refrigerants is a leading solution to reverse global warming. Therefore, crank-driven reciprocating refrigerator compressors are gradually replaced by more efficient, oil-free and hermetic linear compressors. However, the design and operation of an electromagnetic actuator, fitted on the compression requirements of a reciprocating linear compressor, received limited attention. Current research mainly focuses on the optimisation of short stroke linear compressors, while long stroke compressors benefit from higher isentropic and volumetric efficiencies. Moreover, designing such a system focuses mainly on the trade-off between number of copper windings and the current required, due to the large computational cost of performing a full geometric design optimisation based on a Finite Element Method. Therefore, in this paper, a computationally-efficient, multi-objective design optimisation for six geometric design parameters has been applied on a solenoid driven linear compressor with a stroke of 44.2 mm. The proposed multi-fidelity optimisation approach takes advantage of established models for actuator optimisation in mechatronic applications, combined with analytical equations established for a solenoid actuator to increase the overall computational efficiency. This paper consists of the multi-fidelity optimisation algorithm, the analytic model and Finite Element Method of a solenoid and the optimised designs obtained for optimised power and copper volume, which dominates the actuator cost. The optimisation results illustrate a trade-off between minimising the peak power and minimising the volume of copper windings. Considering this trade-off, an intermediate design is highlighted, which requires 33.3% less power, at the expense of an increased copper volume by 5.3% as opposed to the design achieving the minimum copper volume. Despite that the effect of the number of windings on the input current remains a dominant design characteristic, adapting the geometric parameters reduces the actuator power requirements significantly as well. Finally, the multi-fidelity optimisation algorithm achieves a 74% reduction in computational cost as opposed to an entire Finite Element Method optimisation. Future work focuses on a similar optimisation approach for a permanent magnet linear actuator.

Position Sensor-less Resonant Frequency Estimation Method for Linear Compressor with Assist Springs

Abstract: Higher efficiency for a linear motor should be achievable by adding assist springs and driving the motor at the resonant frequency, even if the efficiency of the motor itself is relatively lower than that of a rotational motor. When using a linear motor with assist springs in a sealed chamber such as a compressor, it is difficult to install a position sensor in such an environment; thus, position sensor-less resonant frequency control is essential. This paper proposes a drive control method that realizes resonant frequency driving without a position sensor. The resonant frequency is estimated on the basis of the detected motor phase current, and the position is estimated using only the fundamental component of the current after removing the offset of the detected current with a high-pass filter. Experimental results show that the proposed method can successfully track the resonant frequency dynamically without a position sensor.

Free-piston Stirling cryocooler (FPSC) for low-temperature applications down to 110 K.

Abstract: This research paper describes the experiments and demonstrations of the free-piston Stirling cryocooler (FPSC) we developed. Driven with an oil-free linear compressor, the FPSC is thoroughly tested with various cold-end temperatures between 110 K and 190 K, absorbing 1.03 kW heat at cold-end temperature of 110 K with electric input of 8 kW. The resulting cooler efficiency is estimated to be 0.13 considering a compression efficiency of 0.75. The experimental results are analyzed with a pre-developed numerical model that considers not only the thermodynamics of the working fluid of the FPSC but also the mechanical behavior of the reciprocating piston and displacer by introducing a fictitious damping coefficient to the equation of motion. The numerical model is able to predict the physical variables of the FPSC within 10% deviation under various cold-end temperature conditions. Thanks to the flexibility in regulating its cold-end temperature, the FPSC can be utilized for various applications, i.e., liquefied natural gas (LNG), semiconductor etching, bio/reagent preservation applications, etc. Finally, the FPSC is integrated as a major part of a small-scale liquefier to demonstrate the feasibility of liquefaction of natural gas (NG).

Loss determination at a linear cascade under consideration of thermal effects

Abstract: Targeting higher efficiencies and lower fuel consumption of turbomachines, heat transfer and profile loss are research topics of particular interest In contrast to that, the interaction of both was, so far, rarely investigated, but gains in importance in recent research activities The profile loss of engine components can be characterised by the airfoil wakes at the blade rows utilising established measurement and evaluation methods for which an adiabatic flow is typically supposed To enable the investigation of the influence of heat transfer at the blade on the loss characteristics, a novel evaluation procedure was set up In addition to the pneumatic data, the total temperature in the airfoil wake at a linear cascade was measured by means of a five-hole probe with an integrated thermocouple For the evaluation and analysis of these data, different definitions of the loss coefficient were investigated and, finally, extended to account for thermal aspects Furthermore, established techniques to average the local wake data were applied and compared with special focus to their suitability for non-adiabatic cases Moreover, an extended version of the mixed-out average as defined by Amecke was utilised applying not only a far-reaching consideration of a temperature gradient but also the inclusion of the third spatial dimension to enable the evaluation of field traverses in addition to single wake traverses These techniques were applied to wake measurement data from a linear compressor cascade gained in a special test set-up in the high-speed cascade wind tunnel for different operating points and different blade temperatures The suitability of the new methods could be proven, and initial steps of the aerodynamic analysis of the resulting data are presented Thereby, the acquired techniques turned out as powerful methods for the evaluation of wake traverses on compressor and turbine cascades under non-adiabatic conditions

Adaptive Detached Eddy Simulation of End-Wall Flow in a Linear Compressor Cascade

Abstract: Delayed detached eddy simulations and wall-modeled eddy simulations using the adaptive DES model were performed to simulate corner separation in the Ecole Centrale de Lyon linear compressor cascade. The adaptive DES model directly uses length scale to define eddy viscosity, which makes it nature to compute the model constant CDES dynamically. The dynamic procedure adapts viscosity to local flow and grid. Delayed detached eddy simulations, with and without the dynamic procedure, were performed to demonstrate the benefit of adapting viscosity to local flow. Recycling method was adopted to generate inflow unsteady turbulent boundary layer for wall-modeled eddy simulations. The wall-modeled eddy simulation showed improvement over delayed-DES, in terms of static pressure coefficient around the blade and total pressure loss at downstream locations.

Numerical analysis of gas bearings in oil-free linear compressors:

Abstract: Recently, linear compressor technology has gained increased attention in vapor compression systems due to its compactness and lower frictional losses compared with the conventional reciprocating co...

Moving-coil linear compressor

Abstract: The utility model provides a moving coil type linear compressor which comprises a compressor shell, a stator assembly and a rotor assembly, the rotor assembly comprises a coil frame located between aninner stator and a split type outer stator and a coil wound on the coil frame, and the left end of the coil frame is connected with a piston rod of an air cylinder through a connecting frame. A permanent magnet magnetic ring is installed on an inner ring of the split type outer stator, a solenoid frame is driven to move through magnetic field force generated between the permanent magnet magneticring and the coil, and then a piston of the air cylinder is driven to compress air to do work. The damping supporting piece is of the structure that the cylindrical spring is arranged between the twocircular rings, the damping effect on the internal structure of the compressor is achieved, vibration generated by movement of the internal structure of the compressor is prevented from being transmitted to the outer shell, and the good vibration filtering effect is achieved. Certain restoring force is provided for the piston and the rotor assembly through the resonance spring assembly in the whole working process of the compressor, so that the whole compressor system achieves a more ideal working state, and stability and reliability are good.

Anti-disturbance control system and method for single-phase permanent magnet linear compressor

Abstract: The invention discloses an anti-disturbance control system and method for a single-phase permanent magnet linear compressor. The system mainly comprises an extended state observer, a TD differential tracker, a quasi-PR controller and an SPWM inverter. The stator current and the rotor displacement are used as input quantities; the system disturbance and the motor rotor speed are obtained through the extended state observer; the obtained speed signal and displacement signal are input into a tracking differentiator; resonance frequency of the rotor is obtained through the tracking differentiator,then a speed signal serves as a given signal, a current signal serves as a tracking signal, feedback control is conducted through the PR controller, control quantity is corrected in a feedforward compensation mode, and anti-disturbance resonance control over the single-phase permanent magnet linear compressor is achieved. Internal and external disturbance of the system is observed through the extended state observer, feedforward compensation is performed on the controlled quantity, the external disturbance resistance is high, the tracking resonant frequency precision is high, and static-error-free tracking can be realized.

A Novel Resonant Frequency Estimation Method Based on Recursive Least Squares Algorithm for Linear Compressor

Abstract: To better optimize the efficiency of the linear compressor, its operating frequency must be controlled equivalent to the system resonant frequency. The traditional resonant frequency tracking control algorithm relies on the steady state characteristics of the drive system, which has the disadvantages of slow convergence speed, low accuracy, and long system response time. In this paper, a resonant frequency estimation control strategy based on the recursive least squares (RLS) method is proposed. Compared with the traditional resonant frequency tracking control strategy, the proposed method can achieve more accurate resonant frequency estimation, and has a faster convergence speed. Consequently, the response speed of the control system can be greatly improved. Comprehensive simulation results verify the effectiveness of the proposed method.

Resonant Frequency Tracking Control for a Linear Compressor with Assist Springs

Abstract: This paper proposes a drive control method for a linear compressor driven by a linear motor. Generally, the efficiency of a linear motor itself is relatively lower than that of a rotational motor. However, higher efficiency should be achievable by adding assist springs and driving at the resonant frequency. We have developed a novel resonant frequency estimation method that calculates the phase difference between a supplied voltage and a mover position. Since the phase difference changes according to the drive frequency with respect to the resonant frequency, our method calculates the phase difference by using the arctangent of a Fourier coefficient, which controls the drive frequency to keep the phase difference at 90°. Experimental results show that the proposed method can successfully track to the resonant frequency dynamically even if the mechanical condition is changed.

Measurements and modelling of a novel oil-free refrigeration system

Abstract: Vapour compression refrigeration (VCR) system is one of the most commonly used refrigeration methods for domestic refrigeration, heat pump, and automobile air conditioning. The ever rapid rise in the number of VCR units causes a dramatic increase in energy demand and greenhouse emissions, which results in a significant contribution to the global warming. To mitigate the greenhouse gas emission and improve energy efficiency, high efficiency and low carbon oil-free refrigeration system with capacity modulation can be considered as a potential solution for VCR units. A novel oil-free refrigeration system (ORS) is introduced in this thesis. The performance of the ORS has been experimentally and numerically investigated. The ORS consisting of two balanced novel oil-free linear compressors, an off-the-shelf water-cooled coaxial condenser and an evaporator with an electric heater has been instrumented. A series of experiments have been carried out over a wide range of test conditions to investigate the performance of the linear compressor using R134a. At a compressor stroke of 11 mm and a pressure ratio of 2.0, the oil-free linear compressor can achieve a volumetric efficiency of 71%. The oil-free linear compressor is capable of capacity modulation in response to heat load. Part load efficiency is even higher. An analytical model of the linear compressor has been developed. The drop-in performance of R1234yf for R134a system also has been experimentally investigated using the ORS test rig. At a condenser temperature of 40 °C, the coefficient of performance (CoP) of R1234yf is 5-20% lower than R134a. The effect of the refrigerant charge and refrigerant distribution for the ORS using R134a has been experimentally evaluated. The experimental results show that an optimal refrigerant charge, which allows the system to achieve the highest efficiency, varies with operating conditions. A higher compressor stroke and a lower pressure ratio tend to have a higher optimal refrigerant charge. To provide the numerical ORS model with evaporation heat transfer and pressure drop of refrigerants, two existing correlations for heat transfer and pressure drop for R1234yf, R152a and R134a have been improved based on the experimental data. Overall, the modelling results agree well with the measurements. A comprehensive numerical system model for the ORS consisting of linear compressor model, a heat exchanger model, and a refrigerant distribution model has been proposed using MATLAB and Simulink. The model is able to predict mass flow rate, power input, pressure drop and heat transfer in heat exchangers, and CoP of the refrigeration system. Overall, the modelling results agree well with measurements. The numerical model can be used for future work on low charge ORS design using oil-free linear compressor and microchannel heat exchangers for various applications.