Changqing Tian

Experiment Investigation of R134a Flow Boiling Process in Microchannel With Cavitation Structure

One cavitation structure in which the channel cross section expanded suddenly was introduced in single straight microchannel. The experiment was carried out with R-134a as the fluid medium, which was driven by a gear pump. The flow pattern was observed by a charged coupled device (CCD) camera and microscope. The average boiling heat transfer coefficient was estimated with the calculation method proposed in this paper. The experimental results show that the boiling began at the cavitation structure, and stable flow boiling was maintained. When heat power rose, the boiling became strong; then the pressure drop in the micro-channel increased and heat transfer was enhanced. The liquid percentage in two-phase flow increased and the length of boiling area became small when the liquid subcooling degree rose with the fixed heat power.

Model simplification of scroll compressor with vapor refrigerant injection

ABSTRACT Refrigerant vapor injection in scroll compressor can improve the heat pump system performance on cold regions. A suitable model is necessary to system simulation. Based on the simulation results of an accurately dynamic model introduced in this paper, a new lumped parameter model is put forward. This model simplifies the complex injection process to a simple mixing process, with discussion of injection ports scope, mean position point, mixed process, and mass flow rate. Validations and comparisons show the model is able to guarantee accurate calculation basically, with greatly reduction of calculation time. The new lumped parameter model is fairly useful for system simulation and engineering applications.

INTEGRATED STEADY-STATE AND DYNAMIC SIMULATION OF MULTI-UNIT AIR CONDITIONERS BASED ON TWO-PHASE FLUID NETWORK MODEL

An integrated simulation model is developed to investigate the steady-state and dynamic performance of multi-unit air conditioners (MUACs). It is built as two-phase fluid network which is able to describe different systems by incidence matrixes. And the submodels are embedded in the system framework, where the submodels can be either steady-state or dynamic model, distributed parameter or lumped parameter model. For this case, the numerical submodels are employed with moving-boundary models of condenser and evaporator, steady-state models of compressor and electronic expansion valve. The comparison with experimental data shows that it cannot only predict the steady-state performance, but also catch dynamic trends with high accuracy, for example, the differences of evaporating pressure < ± 4%, condensing pressure < ± 3%, compressor discharge temperature < ± 4°C, superheating degree < ± 2°C and subcooling degree < ± 1°C. Therefore, the simulation model is confirmed as an effective tool to analyze the steady and transient characteristics and optimize the design and control algorithm of MUACs.

Droplets on Micro Structured or Patterned Surfaces With Different Roughness Parameters

Surface wetting properties play important roles in boiling and condensation. In this paper, the wetting behaviours of a single droplet on micro structured or patterned surfaces with different roughness parameters are investigated theoretically and numerically. A theoretical model is proposed to study wetting transitions. Comparison between results obtained by theoretical analysis with those of experiment indicates that the proposed model can give a better prediction of wetting transition. In addition, a numerical simulation based on lattice Boltzmann method (LBM) is performed to study surface wetting properties and also the evolution of droplet shapes, dynamic contact angle and corresponding velocity fields. In the simulation, the droplet size is comparable with the scale of micro posts on the surface.Copyright

Stroke and natural frequency estimation for linear compressor using phasor algorithm

The stroke and natural frequency are key controlling quantities for linear compressor controller. This paper proposes a phasor algorithm for the linear compress stroke and natural frequency calculation. The phasor algorithm employs vectors to express the voltage, current and velocity respectively for the calculation in frequency domain. A test bench has been installed, and the feasibility of phasor algorithm is verified with the linear compressor load changing. The relative deviation of the calculated stroke can be achieved within ± 3% compared to the experimental results at 0.4 MPa, 0.5 MPa and 0.6 MPa discharge pressure respectively. The calculated results shows that the computation time of phasor algorithm is one-third shorter than that of time domain algorithm, which means it is possible to employ relatively low energy consumption and low cost hard ware. The calculation natural frequency decreases with the stroke increases at 0.4 MPa, 0.5 MPa and 0.6 MPa discharge pressure respectively, and the tested motor efficiency reaches a maximum when the calculated natural frequency equals to the operating frequency.

Determination of TDC Using Digital Correlation Method for Linear Compressor

Moving-magnet linear compressors with sensorless top dead center (TDC) control method are an efficient solution for domestic refrigerators to save energy. TDC can be determined by the inflection point of phase difference between the sinusoidal current and piston displacement (I-X phase difference). A digital correlation method is used to calculate the I-X phase difference to obtain the TDC position for control. The TDC determining system using digital correlation method is manufactured and the error is 2.29% under various discharge pressures.

Efficiency measurement of linear oscillation motor with gas load

Linear oscillation motor (LOM) is the key assembly for a linear compressor, and its efficiency has great impact on the integral compressor performance. This paper presents a novel gas load method to measure the efficiency of LOM, analyses the effects of some main factors on the LOM efficiency in theory, and shows the efficiency test results. Different load conditions can be simulated by adjusting the discharge pressure of the gas and the load force presents nonlinear periodical curve. In the experiment, we changed the syntonic springs of the tested LOM to measure the LOM efficiency on different work conditions. The results of both theoretical analysis and experiment show that the LOM efficiency is the highest when the phase angle between current and displacement is 90◦. While the springs stiffness coefficient is 54.44N/mm, the highest efficiency of the tested LOM is 85.97% on the condition that the discharge pressure is 0.25MPa. While the springs stiffness coefficient is 44.64N/mm the highest efficiency of the tested LOM is 84.67% on the condition that the discharge pressure is 0.4MPa. While the springs stiffness coefficient is 38.4N/mm the highest efficiency of the tested LOM is 78.35% on the condition that the discharge pressure is 0.5MPa.

Experimental investigation on electromagnetic transduction characteristic of linear oscillation electrical machine

The electromagnetic transduction coefficient has a strong influence on efficiency as well as the sensorless control of linear oscillation electrical machine (LOEM). This paper presents a reciprocal LOEM method for measurement of the electromagnetic transduction coefficient, to investigate the effect of the magnet length on the characteristics of the transduction coefficient. The result shows that the transduction coefficient has two regimes, a constant regime and a nonlinear regime. As the harmonic distortion begins to appear in the induced electromotive force, the electromagnetic transduction coefficient passes from the constant regime to the nonlinear regime, and the corresponding stroke is defined as critical stroke. The range of constant regime equals the observed critical stroke, and the critical stroke is 8.81 mm and 10.78 mm when the length of the permanent magnet is 11 mm and 13 mm respectively. The relationship among the geometrical dimension of yoke and permanent magnet, the critical stroke and the stroke of the LOEM are carefully investigated. Then optimal lengths for both the yoke and the permanent magnet are suggested for the LOEM design with the rated stroke, to achieve the highest efficiency and least material cost simultaneously.

Piston Stroke Design Optimization for Linear Compressor

This paper presents a method of piston stroke design optimization for linear compressor to get high efficiency.Two linear compressor prototypes were designed and developed with the same discharge volumes and different piston strokes to compare the performance. The prototypes were operated without air load to measure the friction damping coefficient firstly. Then the prototypes were operated with air load of 0.7MPa discharge pressure. The experimental result shows that the input power of the prototype with longer stroke is lower than that of the prototype with shorter stroke although the discharge volume, the friction damping coefficient and the frequency ratio of these two prototyps are almost the same. The iron and coil loss of the prototype with longer stroke is lower than that with shorter stroke, while the mechanical loss of the prototype with longer stroke is larger than that with shorter stroke. According to the results of the performance analysis, a design optimization model for piston stroke is developed to make the energy loss be the least. Through this design optimization model, acquire the optimal piston stoke of the linear compressor on the same conditions with the experiment. The optimization results agree well with the experimental results.

Self-Sencing Control Research on a Linear Compressor

Compared to conventional compressors, the linear compressor with a free piston can improve efficiency and save energy, but the displacement varies due to different load conditions, which seriously influences its performance. In this paper, based on the theoretical analysis, a self-sensing control system with a SPWM inverter and a PC software interface for a moving-magnet linear compressor is developed, and the displacement of the linear compressor which is under control is tested. The result shows the error of the control system is under 0.2mm, and the accuracy fulfils the requirements.