In China, Ding Guoliang of Shanghai Jiaotong University and others have made a simulation study on CO2 automobile air conditioning. horse y？ The optimal configuration of expander in transcritical two-stage compressed CO2 refrigeration system is studied. Yang made thermodynamic analysis and comparison on three different cycle forms of transcritical two-stage compressed CO2 refrigeration system with expander, and obtained the best configuration form of expander in the two-stage compressed CO2 refrigeration system.
Research Status of CO2 Expansion Mechanism
1) piston expander
From 65438 to 0994, Professor Heyl P and Dr Gaga of Dresden University in Germany began to develop a transcritical CO2 cycle expander. Hey? The first generation of free piston expansion compressor developed by Professor P and Dr Gaga H (1999) adopts double-acting symmetrical structure, with two expansion cylinders and two compression cylinders. The test results on the CO2 refrigeration experimental platform show that the COP of the system can be increased by 30% compared with the system with throttle valve. Nickl(2002) introduced the second generation free piston expansion compressor in his published paper. By adding a double-arm rocker arm, the motion speed of expander piston and compressor piston is different, thus solving the problem that the first generation expander piston and compressor piston must run synchronously, reducing the efficiency loss, and improving the system performance by 10% compared with the first generation.
The third-generation free piston expansion compressor developed by Nickl et al. (2003) once again adopts the full-pressure expansion principle of the first generation, but through three-stage expansion, the recovery of expansion work is improved and the efficiency loss is reduced. Gaga et al. (2004) successfully carried out the principle experiment on the prototype of the third generation expansion compressor. The experiment proves that the control mechanism of the expander is completely feasible, and it is also verified that the lubricating oil carried by CO2 can meet the lubrication requirements of the machine without additional lubrication system. Nickl(2005) gave the P-V diagram of the prototype, and estimated that the isentropic efficiency of the expander reached 65%-70%, and that of the compressor exceeded 90%.
Li et al. (2000) made a thermodynamic analysis of different expansion devices in CO2 cycle system, and proposed to adopt scroll expander and piston expander to reduce throttling loss. BaekS(2002) transformed a commercial four-stroke two-cylinder engine into a piston expander, and the opening and closing of the intake and exhaust ports were controlled by a fast solenoid valve. The experimental results show that the isentropic efficiency of the expander is about 65,438+00%, and the COP of the CO2 refrigeration system can be increased by 7%-65,438+00%. BaekS(2005) established a detailed mathematical model for the piston expander, and analyzed the prototype through this model.
2) reel expander
Preissner(200 1) and HuffJ(2003) transformed two semi-closed R 134a scroll compressors into CO2 expanders. The disk height of prototype I is reduced to 1.7mm, while the disk height of prototype II remains unchanged at14 mm. However, due to large internal leakage, the maximum isentropic efficiency and volumetric efficiency of prototype I are only 28% and 40%. For prototype II, the influence of internal leakage is weakened due to the large working volume of the expander, and its performance is higher than that of prototype I, with the maximum isentropic efficiency and volumetric efficiency of 42% and 68% respectively. Yan Ni D(2004) also studied the CO2 scroll expander theoretically, put forward the design scheme and work recovery mode of the CO2 scroll expander, and predicted that its leakage loss was about 20%, friction loss was about 65,438+05%, and the total efficiency could reach about 72%.
3) Rolling rotor expander
Wei Dong, Cha Shitong, Guan and others of Tianjin University have successively developed and studied the CO2 rotary expander. Weidong developed the first generation D3ER 1.0 rolling piston expander. The preliminary experiment shows that the expander prototype can run normally. On the basis of the first generation, Chastone developed the second generation D3ER2.0 rolling piston expander. By adding needle bearings, the internal friction of the expander was reduced, and the generator and the expander were combined to prevent external leakage. Li Minxia further improved the D3ER2.0 expander into a new sliding plate rolling piston expander, model D3ER2. 1, and changed the line seal to face seal, which can reduce the leakage by 50% theoretically. In addition, Li Minxia also designed and developed D3ESW 1.0 swing rotor expander, which combined the rolling piston with the sliding plate to reduce the internal leakage of the expander. The prototype test results show that the efficiency of D3ER2. 1 and D3ESW 1 is 33%-44% and 35%-47% higher than that of D3ER2.0, respectively. On the basis of predecessors' research, a swing rotor expansion compressor was designed and developed, and the efficiency of the expander and compressor in the prototype was tested to be 30%-50% and 60%-80% respectively.
4) Other expanders
Stosic of City University of London (2002) theoretically studied the carbon dioxide twin-screw expansion compressor. The rotors of the expander and the compressor are connected by a shaft and placed in two independent cavities, thus avoiding the internal leakage of the working medium. Through this configuration, the axial load of the expansion compressor can be completely offset, and the radial load is less than 20%.
Fukuta(2003) studied the vane expander. The simulation results of the established mathematical model show that leakage is the main factor affecting the performance of the vane expander, and the influence of heat transfer is relatively small. The model predicts that the total efficiency of vane expander is 20%-40%, and it will increase with the increase of rotating speed. The prototype of CO2 vane pump modified from vane pump has a total efficiency of 43% under the working conditions of inlet pressure 9.65438±0 MPa, temperature 40℃ and outlet pressure 4.65438±0 MPa. Fukuta(2006) developed a prototype of sliding vane expansion compressor, in which the compressor part is used as the secondary compressor of CO2 cycle. The experimental results show that the performance of the compressor is mainly affected by the pressure difference and speed before and after the compressor.
MIEE, England? Driver company improved the ordinary vane expansion compressor and applied for a patent.
5) Other expansion equipment
In 2006, Li established the isobaric mixing model of the ejector, and further established the two-phase flow ejector and the corresponding CO2 cycle system model. The calculation results show that the isentropic efficiency of the main nozzle is 95%, while that of the auxiliary nozzle is very low, only 26%.
Tdell(2006) studied the CO2 impact expander. At present, the efficiency of this expander is very low. The isentropic efficiency of the nozzle is only 60%, and the recoverable work only accounts for about 20%-30% of the isentropic expansion work.
Research status of carbon dioxide compressor
1) piston compressor
During 1998, Süβ and Kurse studied the open CO2 piston compressor produced by Bock Company and the swash plate CO2 compressor produced by Danfoss A/S Company.
Doreen Company demonstrated the semi-closed CO2 piston compressor developed at IKK Expo 1998, including single-stage compressor and two-stage compressor. The University of Zurich in Switzerland has developed a semi-closed small oil-free piston CO2 compressor for domestic water heaters.
Nesk et al. studied semi-closed two-stage CO2 piston compression. The experimental results show that when the rotating speed is 1450 r/min, the maximum efficiency and isentropic efficiency reach 0.8 and 0.6, respectively, and its performance is better than that of single-stage compression at low temperature.
The piston CO2 compressor was developed by Denso Corporation of Japan in cooperation with Shizuoka University. The prototype was tested and compared with the theoretical calculation results. It is found that the sealing effect of piston ring is very good, but the leakage prevention through the valve has a great influence on the efficiency of compressor with relatively small working volume.
Chen Jiangping of Shanghai Jiaotong University and Shanghai Yichu General Motors Co., Ltd. jointly developed the swash plate CO2 compressor for vehicles and conducted a series of research.
2) Rolling piston compressor and oscillating piston compressor
Japan Sanyo Company has developed a totally enclosed CO2 two-stage rolling piston compressor. This gas path design makes the pressure in the casing be the first-class exhaust pressure, about 5-6MPa, which reduces the leakage between the compressor working chamber and the casing chamber and is beneficial to improving the efficiency of the compressor. It is reported that the highest adiabatic efficiency can reach above 0.8 at the working frequency of 50-80 Hz.
Daikin Company of Japan has designed and developed an oscillating rotor CO2 compressor. According to the research of Daikin Company of Japan, the design strength of CO2 oscillating rotor compressor is equivalent to that of R4 10A compressor because of its small eccentricity, although its working pressure difference is large.
Hubacher and Groll tested a fully enclosed two-stage carbon dioxide rotary compressor. The results show that the pressure ratio is in the range of 1.5-5 and the volumetric efficiency is 0.78-0.9. Dreiman and Bunch developed a totally enclosed CO2 rotary compressor. Yokoyama et al. developed a rolling rotor CO2 compressor for two-stage compression and interstage gas supply in heat pump system, and conducted experimental research. Under the conditions of high pressure ratio and low speed, the two-stage compression CO2 compressor is superior to the single stage in efficiency and heat production.
In China, since 2004, Qing 'an Refrigeration has carried out detailed research on rolling rotor CO2 compressor. The main work focuses on the overall structure design of compressor, reliability design of bearing system, oil supply system design, static and dynamic strength design of parts, wear-resistant design of key parts, research and analysis of oil content in compressor operation, lubricating oil evaluation, material selection of parts, motor design, traction control scheme research of concentrated winding DC motor, controller design and manufacturing technology. A prototype was developed in 2008, and its volumetric efficiency reached 0.75%-0.9 1%, and it passed the reliability evaluation experiment.
3) scroll compressor
Denso Corporation of Japan developed CO2 scroll compressor for CO2 heat pump water heater.
On the basis of 4 10A scroll compressor, Panasonic Corporation of Japan redesigned the scroll and shell, and developed a prototype of CO2 scroll compressor. The experimental results of the prototype show that the volumetric efficiency and adiabatic efficiency of the compressor increase with the increase of rotating speed. In the working frequency range of 34.6-48.2 Hz, the volumetric efficiency is 0.72-0.86, and the isentropic efficiency is 0.43-0.47. Mitsubishi Heavy Industries of Japan has also developed a scroll compressor for CO2 heat pump water heaters, and the adiabatic efficiency of the compressor can reach 0.76. Yano and Nakao also developed large-capacity carbon dioxide scroll compressors.
4) Sliding vane compressor
The University of Maryland in the United States and Shizuoka University in Japan have cooperated in the theoretical research of CO2 vane compressor, including the feasibility, the analysis of key parameters such as temperature and pressure in the compression chamber, the estimation of volumetric efficiency and indicating efficiency, and the stress of the vane. It is found that leakage loss is the main factor affecting compressor efficiency. In addition, the two-stage compression vane CO2 compressor and vane expansion compressor are analyzed.
5) Screw compressor
Japan Maycom Company has developed a CO2 single-stage screw compressor, and the designed unit is used for both cooling and heating. The CO2 discharged from the compressor is first used for heating hot water and then used for refrigeration after throttling. City University of England has developed a screw expansion compressor for CO2.
Research status of carbon dioxide heat exchanger
From 65438 to 0998, Pattersen of NTNU, Norway developed a compact heat exchanger for CO2 system, in which multiple plates were used to form heat transfer tubes, and these plates were extruded into microchannels.
Schonfeld and Kraus made theoretical calculation and experimental study on the heat transfer of supercritical fluid, and found that the calculated results were higher than the experimental values, which indicated that supercritical fluid could not be accurately calculated by conventional convective heat transfer methods. Dang and Hiara have also done the above work, compared several correlations, and established a new correlation on the basis of Pilta equation. The error between the calculated results and the experimental results is 20%. Hihara and Tanaka of the University of Tokyo have done a lot of experiments on the boiling of CO2 fluid under high pressure. Because the fluid in the evaporator involves two-phase heat transfer, the flow pattern of the fluid has a great influence on heat transfer. Pattersen of NTNU, Norway, made an experimental study on the low-pressure boiling flow pattern of CO2 fluid in microchannels, gave the flow pattern diagram and measured the pressure drop of CO2 evaporation flow. Grol and Kim studied theoretically and experimentally the influence of dryness of CO2 fluid on heat transfer coefficient of horizontal tube. When CO2 fluid completely changes into steam, the heat transfer coefficient decreases rapidly and the heat transfer effect is poor. Choi conducted an experimental study on evaporation and heat transfer of CO2 fluid in a vertical tube. It is found that the heat transfer coefficient increases with the increase of dryness in low dryness region, and decreases rapidly when dryness exceeds a certain value. Kim et al. made theoretical and experimental research on CO2 multi-layer microchannel evaporator, and the theoretical model was in good agreement with the experiment. Kulkarmi et al. studied how to eliminate the unevenness of dryness in each channel of CO2 microchannel heat exchanger.