Study On Key Issues Of Microchannle Heat Exchanger Used In Residential Split-type Air Conditioner

Because of huge freon consumption of the air conditoning industry, refrigerant replacement is a hard work. China has confirmed the use of hydrocarbon refrigerant(R290) as alternatives in RAC(Room Air Conditoner) industries. However, the flammability of hydrocarbon refrigerants brings risks. To reduce the fire-risk, reducing refrigerant charge is an effective way. Microchannel heat exchanger, for its compact features, not only reduces refrigerant charge, but also improves the heat transfer performance, is used in Residential Air Conditioner widely. But as changes in the working conditions, microchannel heat exchanger also faces some challenges, such as poor condensate drainability, hard to defrost, less long-term perfromance etc. Therefore, some key issues of microchannel heat exchanger used in home air conditioning systems were studied in this paper in order to promote the application of micro-channel heat exchanger in RAC. The main contents and results are as follows:1) A model of RAC system with mcirochannel heat exchanger was established to study the effect of key parameters of microchannel heat exchanger on refirgerant charge and system performance. It was found that redusing header diameter by 48% can reduse refrigerant charge by 36.2% with no obvious perfromance degradation. Similarly, redusing hydraulic diameter of flat tube hole by 50%(with more holes to ensure the same heat transfer area) can reduse refrigerant charge by 23%. According to the model analysis results and the structure defect of traditional microchannel heat exchanger, a new mirochannel heat exchanger with low charge design was proposed. Throughthe special connection method between headers and flat tubes, the new heat exchanger has 49.5% less inner volume and 28.3% less refrigerant charge. The R290 charge of the new heat exchanger system was reduced to 190 g.2) Image segmentation method was used to study the frost growth mechanism on the louver fin surface of microchannel heat exchanger. By analysis of the density distribution through the frost layer, three stages of frost growth and their characteristics were summed up. The effect of surface condensate residual on the frosting formation is concerned, and the quantitative relationship between residual amount and heat exchanger performance(frosting time, max capacity, air pressure drop) are given. Through study on drainage performance of the micrchannel heat exchangers with different placement angles, it was found that the heat exchanger with vertical flat tubes had best drainage capacity, ensure the stability of the frosting and defrosting cycle, and increased the effective operation time by 32 %. To overcome the disadvantages of louver fin microchannel heat exchanger, a new heat exchanger with drainage design was proposed and experimentally studied in wet and frosting conditions. It was found that the perfromance of the new sample was better than that of louver fin and wavy fin samples. In wet conditon, the capacity of the new sample was 56.7% higher than the wavy fin sample. In frosting conditon, the effective operating time of the new sample system was 78% longer. This chapter researched on the frosting mechanism and fin design improvement, so as to provide theoretical support and technical solutions of microchannel heat exchanger used in residential heat pump air conditioner.3) The long-term fouling characteristic of microchannel heat exchanger was experimentally studied. By comparing the fouling process of tube-fin and microchannel heat exchanger, two fouling stages, nucleus formation and net accumulation, were found. According to these two stages, two anti- fouling principles, the reduction of fast nucleus formation region and slowingnucleation rate, were summarized. Based on these two principles, a system solution with wavy fin condenser and reverse blowing control strategy was proposed. Through 3 months’ operation, the perfromance degradation was reduced from 49.5% to 4.3%. The perfromance degradation caused by fouling were efficiently inhibit. A wind tunnel test rig was established to study air side perfromance of 7 different wavy fin samples. Using non-linear regression and F significance test method conducted correlations fitting. The correlation can predict more than 90% data within 15% error. The correlations have been applied to the design of dozens of home air conditioning and automotive air conditioning condenser, and meet the accuracy requirements.