| Micro-channel evaporator with parallel flow with its low cost, light weight, compact structure,good performance of heat transfer and many other advantages, had a very broad applicationprospected in the field of civil and military refrigeration. The characteristic of the micro-channelevaporator was that refrigerant flow arranged in many parallel flat tubes, also called parallel flow typeevaporator, and related to the flow distribution. The flow of the micro-channel evaporator withparallel flow and heat transfer performance of work needed to be further improved. Based on theabove issues, flow and heat transfer performance of micro-channel evaporator was studied andstructure optimization work was done.Based on the hypothesis of continuum, the mechanism of micro-channel heat exchanger wasdiscussed. Using numerical simulation method, combined with the existing experimental setup anddata, the influence of wall thickness on the micro-channel heat transfer was studied. The heat transferlaw in the micro-channel was proved to be the same as the conventional channel.The numerical simulation method was used to study the structure of deflectors and flowdistribution in micro-channel evaporator with24flat tubes: a deflector structure with12holes wasdevised; the flow distribution uniformity and pressure drop rule of a variety with structures and openarea of deflectors in different position were studied, and the optimum opening area of deflectors werefounded. The micro-channel evaporator with27flat tubes also was studied by the same method.The next experiments were processed on the micro-channel evaporator with24flat tubes. Inorder to verify the structure and clarify the flow distribution law of deflector, flow distribution testplatform was built; as water as the working fluid, flow distribution characteristics of deflectors wasstudied. The results show that the flow distribution uniformity and stability of the deflector with12holes are always the best, and as the entrance Reynolds number increased, the flow distributionuniformity is improved slightly. The deflector with reasonable structure can improve the fluid flowdistribution uniformity in micro-channel evaporator.Effects of deflectors with different structures on the flow and heat transfer performances of theevaporator were experimental investigated and compared. The results show that the structure ofdeflector affects the flow and heat transfer performances of the evaporator remarkably. With the sameopen area in the deflector, the number and location of holes in deflectors produce no evident effect onthe resistance coefficient of the evaporator. The performance of deflector with12holes is proved to bethe best.Structure optimized design and performance tests of micro-channel evaporator were done: in the condition of flow distribution of refrigerant being not decreased, the increase of circulation area candecrease the pressure drop and improve cooling capacity.Flow distribution characteristics of the three deflectors with same structure but different openingarea (45.6mm2，150.7mm2，339mm2) were experimentally studied with water as the working fluid.The results show that the flow distribution stability of deflector with the opening area of150.7mm2isthe best. Compared with the condition of without deflector, the flow distribution uniformity of thedeflector with12holes is improved more than doubled.Then effects of the deflector opening area on the performance of evaporator were investigatedwith above three deflectors. R134a was the working fluid. The results show that with the samedeflector structure, increasing the open area of holes in the deflector can reduce the resistancecoefficient of the evaporator, but the cooling capacity of evaporator is first increased and thendecreased. There is an optimum open area of holes in deflector of the micro-channel evaporator, withwhich the negative impact of pressure drop and mal-distribution of refrigerant on the cooling capacityof evaporator is the minimum, so the cooling capacity can get to the maximum. For the evaporator, theoptimum open area of holes in the first deflector is about150.7mm2. |
PDF Full Text Request