In this paper, through the empirical correlation between heat transfer factor J and friction factor F, the factors affecting the heat transfer performance of parallel flow heat exchanger are theoretically analyzed and studied. The main factors affecting the heat transfer performance of heat exchanger are: head-on wind speed, tooth pitch, tooth height, louver angle, louver spacing, flat tube microchannel shape and the number of flat tube microchannel holes. The results show that the heat transfer performance of the heat exchanger can be obviously improved by increasing the head-on wind speed and reducing the tooth pitch and tooth height.
Analysis of influencing factors on heat transfer performance of parallel flow heat exchanger
3. 1 Influence of head-on wind speed on air-side heat transfer coefficient
Influence of different head-on wind speeds on air-side heat transfer coefficient. When the fin height is 6mm, other parameters remain unchanged, and the head-on wind speed is 1.5m/s and 4.5m/s, their corresponding air-side heat transfer coefficients are 142w/(m2k) and 268w/(m2k) respectively. It can also be seen from Figures 2 ~ 3 that under the same conditions, the heat transfer coefficient on the air side increases with the increase of the oncoming wind speed, and the heat transfer coefficient on the air side increases rapidly in the low-speed region. However, there is a critical wind speed for condensers with different structures. When the wind speed exceeds the critical wind speed, the air side resistance will increase rapidly and the heat exchange capacity will tend to be constant. When optimizing the design of parallel flow condenser in Changsha Plastic Surgery Hospital, its structure should be reasonably selected to make its head-on wind speed lower than the critical wind speed.
3.2 Influence of tooth height on air-side heat transfer coefficient
When the head-on wind speed is 3m/s and other structural parameters are unchanged, the air-side heat transfer coefficient is 257 w/(m2k) when the tooth height is 5mm; When the tooth height increases to 8mm, the air-side heat transfer coefficient decreases to 158w/(m2k). When the parallel flow heat exchanger is under the same flat tube width, flat tube number, head-on wind speed, tooth thickness and tooth pitch, reducing the tooth height can reduce the hydraulic diameter of the air side and increase the air flow through the teeth, thus improving the heat transfer coefficient of the air side and the heat transfer capacity of the heat exchanger. However, due to the increase of heat exchange capacity, the refrigerant flow rate increases, while the number of flat tubes remains unchanged, which increases the refrigerant pressure drop and reduces the temperature difference.
3.3 Influence of tooth pitch on air-side heat transfer coefficient
In parallel flow heat exchanger, the tooth pitch not only affects the hydraulic diameter of the air side, but also affects the air flow efficiency. The results show that when the head-on wind speed is 1.5m/s and other structural parameters are unchanged, when the tooth spacing is 1.2mm, the air-side heat transfer coefficient is 109w/(m2k); when the tooth spacing is increased to 1.8mm, the air-side heat transfer coefficient will be reduced to 99. Reducing the tooth spacing can reduce the hydraulic diameter of the air side, increase the heat exchange area per unit length of the heat exchanger and improve the heat exchange coefficient of the air side, thus improving the heat exchange performance of the heat exchanger.
3.4 Influence of louver angle on air side heat transfer coefficient
If most of the airflow passes through the louver of the toothed plate, it is considered that the heat transfer between most of the airflow and the louver of the toothed plate is effective; If most of the air flow directly through the channel without louvers with toothed plates, it is considered that the heat transfer between most of the air flow and toothed plates is invalid. Therefore, the heat transfer coefficient of the air side is also closely related to the angle of the louver.
When the head-on wind speed is 3m/s, when the louver angle changes from18 to 33, the air-side heat transfer coefficient also increases from 127w/(m2k) to 177w/(m2k). Therefore, under appropriate technological conditions, in order to improve the heat transfer coefficient on the air side, the angle of the louver can be appropriately increased.
3.5 Influence of Other Factors on Performance of Parallel Flow Heat Exchanger
The louver spacing, the shape of the flat tube microchannel and the number of holes in the flat tube microchannel will all have certain effects on the performance of the parallel flow heat exchanger. When the head-on wind speed is 3m/s, when the louver spacing increases from 0.8mm to 1.3mm, the air-side heat transfer coefficient increases from 154w/(m2k) to 158w/(m2k), with little change. The results show that the change of louver spacing has little effect on the air-side heat transfer coefficient.
Different flat tube microchannel shapes affect the heat transfer performance of parallel flow heat exchanger. The heat transfer performance of triangular microchannel is worse than that of rectangular microchannel, and the pressure loss is greater than that of rectangular microchannel. Under the same flat tube width, the heat transfer performance of porous triangular microchannel is lower than that of porous rectangular microchannel. The outlet pressure of porous triangular microchannel is 2. 1 ~ 9.9 lower than that of porous rectangular microchannel, and the heat exchange capacity is low 1.7 ~ 2.5. Although the porous triangular microchannel increases the wetting perimeter of refrigerant under the same flat tube width, the effective heat transfer area does not increase, so the heat transfer performance of porous triangular microchannel is lower than that of porous rectangular microchannel. Therefore, at present, parallel flow heat exchangers mainly adopt porous rectangular microchannels. At the same time, the more the number of micro-channel holes on the flat tube, the better the heat transfer performance of the heat exchanger.
4 conclusion
① The heat transfer coefficient of air side increases with the increase of incoming wind speed. However, in order to reduce noise, the oncoming wind speed should be controlled within the critical wind speed range, and the oncoming wind speed should be controlled within a reasonable range according to different parallel flow heat exchanger structures.
② Under certain conditions, reducing the height and spacing of the teeth can reduce the hydraulic diameter of the air side and improve the heat transfer coefficient of the air side, thus improving the heat transfer performance of the heat exchanger.
③ Other factors, such as louver angle and spacing, also have some influence on the air-side heat transfer coefficient, but the influence is small. At the same time, the shape of flat tube microchannel and the number of holes in flat tube microchannel will also affect the flow and heat transfer performance of refrigerant in microchannel.