Long-term Heat Transfer And Pressure Drop Performance Of Finned Tube Evaporator

Fined tube heat exchangers are widely used in air-conditioners operating as evaporator, a large amount of investigations have been carried out for improving the performance of evaporator, e.g. enhanced tubes are used to enhance the tube-side heat transfer performance, enhanced fins are used to enhance the air-side heat transfer performance, and optimization on refrigerant circuitry for improving the overall heat transfer performance. However, with the increase of operation duration, the heat transfer efficiency of evaporator degrades gradually, and the energy consumption increases correspondingly. More and more researchers focus their studies on the performance of evaporator after long time operations. Therefore, the long-term heat transfer and pressure drop performance becomes an important research topic. The effect on the long-term performance including fouling deposited on the fin surfaces, microorganism growth on the fin surfaces, intermittent operation and salt spray corrosion, and so on. Compare with the study on effect of fouling deposited on the fin surfaces, the effects of intermittent operation, microorganism growth and salt spray corrosion on the evaporator are limited. As a result, to give the reply to these issues is necessary and important for investigating the long-term air-side heat transfer and pressure drop performance.The purpose of this study is to investigate the heat transfer and pressure drop performance for aluminum-fin heat exchangers and copper-fin heat exchangers under wet conditions, considering the influence of intermittent operations, biofouling and salt spray corrosion. Based on the above purposes, this dissertation realized the following work:1) The fouled fin efficiency model under wet conditions was developed. The following conclusions can be obtained. When the fin is under partial wet conditions, the difference of calculation results between the clean fin efficiency model and the fouled fin efficiency model is limited. When the fin is under full wet conditions, the efficiency of clean fin is larger than that of fouled fin. Moreover, the difference of calculation results between the two models getting much larger with the increase of inlet air relative humidity.2) In order to simulate the operation condition of actual air-conditioner under intermittent operation condition, biofouled condition and corrosion condition, a method for realizing the above operation condition were developed. The method can realize a single intermittent operation (include dry/wet cycles and high/low temperature cycles) in four minutes, realize the microorganism growth on the fin surface in a short period, and realize the fin of finned tube heat exchanger corroded in a short period.3) Conduct the experiments for an aluminum-fin heat exchanger and a copper-fin heat exchanger under intermittent operation conditions. The heat transfer and pressure drop performance were measured each 300 repetitions. The effects of intermittent operation on air-side heat transfer and pressure drop performance were analyzed according to the experimental results. New models, which reflect the difference of fin material and repetitions, were developed to predict the heat transfer and pressure drop performance of finned tube heat exchanger. The average deviation is within 3.1% for heat transfer model, and 1.6% for pressure drop model.4) Conduct the experiments for three biofouled aluminum-fin heat exchangers and three biofouled copper-fin heat exchangers. The heat transfer and pressure drop performance were measured. The effects of intermittent operation on air-side heat transfer and pressure drop performance were analyzed according to the experimental results. In order to compare with the air-side performance of clean heat exchangers, the heat transfer and pressure drop performance were measured for the biofouled heat exchangers. According to the experimental results, fewer biofouling particles can enhance the air-side heat transfer performance, while a large quantity of biofouling particles deposit on the fin surface can degrade the air-side heat transfer performance. The experimental results shows that biofouling can cause the air-side heat transfer coefficient changing with the range of -16.0%~12.7%, and pressure drop changing with the range of 1.1%~43% when the air velocity is 0.5~2.0 m/s.5) Conduct the experiments for three corroded aluminum-fin heat exchangers and three corroded copper-fin heat exchangers. The heat transfer and pressure drop performance were measured. The effects of salt spray corrosion on hydrophilicity, air-side heat transfer and pressure drop performance were analyzed according to the experimental results. According to the experimental results, the static contact angle, the advancing and receding dynamic contact angle increase with the increase of salt spray corrosion hours for aluminum-fin evaporators with hydrophilic coating and copper-fin evaporators without hydrophilic coating. This results show that the hydrophilicity of fins degrade. Corrosion can cause he air-side heat transfer coefficient changing with the range of -20.5%~36.8%, and pressure drop changing with the range of 0%~21.6% when the air velocity is 0.5~2.0 m/s.At the end of this dissertation, the author presented the main weakness and further key points should be focused on in the near future.