Heat And Mass Transfer Enhancement And Thermodynamic Analysis Of High Temperature Absorption Heat Transformer

Increasingly tense as the energy supply, it is important to improve the energy efficiency in order to reduce the cost of industrial production and environmental pollution. The absorption heat transformer is a device that can increase the temperature of waste heat. It is playing increasingly important role in improving energy efficiency.In the absorption heat transformer (AHT), the absorber is the heat output section. It is the key component of AHT. The heat exchange tubes performance inside the absorber determines the efficiency of the absorber and the whole system. The falling film design has become the most popular style due to its high efficiency, easy operation, simple structure, and other advantages. Scholars over the world wide are trying to improve the lithium bromide falling film efficiency in the absorber. Now, the most mainstream way is to improve the heat exchange efficiency of the tubes.PTFE coating is sprayed on the bare copper tube surface with the designed surface distribution form. The coating material changes the hydrodynamic properties of parts of the tube surface. The hydrophobic property of the coating makes the liquid film re-mixed when the liquid film falling through the hydrophobic and hydrophilic interface. The disturbance of the liquid film can be promoted, so as to achieve the purpose of strengthening heat and mass transfer.Three different specifications of spiral PTFE coated enhanced tubes was prepared in this paper. High-speed video camera was used to observe the falling film characteristics and the minimum spray density. It is found that in a small spray density, the fluctuations of the falling film on spiral coated tube surface is very obvious, and the minimum value of the spray density is also in line with theoretical calculations law. The state more than the minimum spray density, the capacitance sensor was used to measure the film thickness and the amplitude and frequency of the falling film fluctuations. The result shows that the falling film fluctuation frequency is related to the spacing of the coating strips, the smaller the spacing, the greater the fluctuation frequency; falling film fluctuation is related to the width of the coating and the spray density as well, reducing the coating width and increasing the spray destiny can increase the liquid film fluctuations; the thickness of the falling film is related to coating area, the larger the area of hydrophobic coating, the thinner the film thicknessIn order to determine the heat and mass transfer efficiency, an experimental investigation of the spiral hydrophobic coated tubes is conducted at a high temperature. The result shows that the spiral hydrophobic coated tubes have a better performance in heat transfer. It proves that the hydrophobic coating really makes the falling film fluctuate. In the end. a high temperature absorption heat transformer is analyzed with second law of thermodynamics. The changes of COP, ECOP and total exergy destruction of the system at different operating temperature, effectiveness of solution heat exchanger, flow ratio and GTL were studies in this paper. The result shows the COP and ECOP increase with the generator and evaporator temperature and decrease with the condenser temperature and absorption temperature. The GTL of the system should not be over42℃for a good performance.