Heat Transfer Characteristics Of Oscillating Heat Pipe Heat Exchanger And Its Application In Drying System

As a new type high efficiency heat-transfer element, the oscillating-flow heat pipe (OHP) does not only boast for its good heat transfer efficiency, but also for other strengths compared with gravity heat pipe, such as being simple in structure, flexible in bending, of adaptability to different heating methods and heating positions. This new type heat transfer equipment plays an important role in heat recovery of drying system exhaust gas with moisture and low temperature, also is widely used in chemical industry, petrochemical industry, electric power industry, metallurgy industry, and etc.The domestic and overseas research of OHP heat exchanger is still in its primary stage and has not been widely applied in key energy-consuming industries. In terms of heat transfer theory and integration of the methods of experimental research, theory analysis and numerical simulation, this thesis mainly makes a research on the heat transfer performance and enhanced heat transfer approaches of OHP heat exchanger, and presents the design method for the promotion of this heat exchanging devices.The design method for OHP heat exchanger consisting of looped heat pipe elements is studied in this thesis. Based on the design calculation of traditional gravity heat exchanger and combined with the characteristics comparing with dividing wall type heat exchanger, we developed the calculating formula and design calculation method for heat transfer coefficient and temperature different of OHP heat exchanger. There are many evaluation standards of OHP heat exchanger performance, but from the perspective of heat recovery, in this thesis we only focus on one evaluation standard, that is heat recovery rate.The performance of OHP heat exchanger is evaluated by experimental method. It can be observed by separately adding the dry hot air and mixture of air and steam into the hot end from perspectives of flow way, heating position, temperature of the hot fluid, heat and humidity. The observation leads to a formula about the relation that the heat transfer coefficient changes with the moisture content. The performance of countercurrent flow is better than concurrent flow, and the heat transfer efficiency of bottom heating is higher than top heating. For the moisture air, when the air inlet temperature is higher or the moisture is decreased, the air outlet temperature is higher than the dew point, the heat recovery efficiency will be slightly increased. But when the air outlet temperature is lower than or near to the dew point, the heat recovery efficiency will be decreased significantly. Then we can conclude the calculation method of heat convection coefficient which is of practical significance to the design of OHP heat exchanger.Field synergy principle can be adopted for the design of OHP heat exchanger to evaluate how the pipe arrangement, structural and process parameters affect the heat transfer. The flow of fluid in OHP heat exchanger is similar to the fluid flow across the tube bundles.βm value (mean value of local velocity vector plus included angle of the related temperature gradient) is used to analyze the synergism between the temperature field and velocity field. When the inlet section of cold end is of the same size of the hot end, the product of velocity and heat transfer area in cold end equals to that of the hot end, and the thermal physical difference can be ignored, the uniformity of temperature difference field and the performance of heat exchanger will be the best. Actually, this reflects the synergism of equivalent weight of cold and hot fluid.Numerical simulation of 2-D and 3-D models can be created to examine how the pipe bundle arrangement and spacing affect the heat transfer, how the inlet temperature and velocity of cold/hot fluid affect the heat transfer and confirm whether the product of the velocity multiplied by the heat transfer area at the hot end equals to that at the cold end. The following conclusion can be drawn by numerical simulation:The staggered arrangement of heat pipe is superior to aligned case. And theβm of staggered arrangement is less than it of aligned case, the Nu number of staggered arrangement is bigger. For the different pipe density, we found that no matter aligned or staggered arrangement the influence of downwind space Pl to Nu is greater than windward space Pt. Choosing the reasonable downwind space Pl is very important for getting higher field synergy level. If the heat transfer area of cool and hot side is different, we can improve the uniformity of temperature difference field by changing the match of velocity and heat transfer area. When uhAh= ucAc (the heat transfer area multiply the velocity in hot side is equal to the cool side), the uniformity of temperature difference field is the best. The inlet temperature of hot fluid has little effect on the uniformity of temperature difference field. The smaller hot fluid velocity, the higher the heat recovery efficiency.The OHP heat exchanger applied in heat recovery system of lumber drying and salt drying is developed. The experiment shows that, for heat recovery of lumber drying system, the oscillating-flow heat pipe is the perfect heat transfer element. The performance OHP heat exchanger adopts uhAh=ucAc rule and staggered arrangement design is obviously better than other structural heat exchanger. As for heat recovery of salt drying system, under same conditions, the performance of OHP heat exchanger is better than traditional gravity heat pipe heat exchanger, also the OHP heat exchanger has the advantages of small occupied size and easy maintenance, therefore it should be widely promoted.