Study On The Key Equipment In The Low Grade Heat Source Recovery Technology

In the past 40 years of reform and opening up,China has achieved remarkable achievements.However,the advancement of science and technology and social development are inevitably accompanied by a large amount of energy consumption.At present,the development of renewable energy is the best way to alleviate the shortage of China’s energy reserves and adjust China’s energy structure.Ocean Thermal Energy Conversion system is a kind of renewable energy with great potential.China has a long coastline,especially in the South China Sea region,which is very suitable for promoting Ocean Thermal Energy Conversion system technology.The working conditions of the low-grade heat source recovery technology are similar to those in the OTEC system with auxiliary heat,and the basic cycle principle is the same.Therefore,in order to further improve the power generation efficiency of the circulatory system,this paper provides an important reference for improving the efficiency of the circulatory system in practical engineering by studying the two key equipment heat exchangers and expanders in the low-grade heat source recovery technology.The main research contents of this paper are as follows,As a basic component of the plate heat exchanger,the two-dimensional grooved channel should further clarify the flow characteristics of the pulsatile flow in the channel and the heat transfer enhancement.The amplitude of the pulsatile flow is focused at different Reynolds numbers and oscillatory fractions by measuring time-varying flow rate through the electromagnetic flowmeter.Monitoring the pressure drop of the flow path by a differential pressure transmitter.In addition,the pulsatile flow patterns are visualized through aluminum dust method.The experimental results showed that the oscillatory fraction decreases with frequency of pulsatile flow,which is also proved by the flow visualization results.It is further shown that the amplitude of pulsatile flow can approach the theoretical value only when the frequency is lower than the critical frequency f_c.It is found that the steady and unstable flow states exist in a pulsatile period.The unstable flow enhances fluid mixing between mainstream and recirculation vortex,and the flow mixing increases with the frequency increment,which is the main reason of heat transfer enhancement.Two-dimensional numerical simulations are further carried out to study this problem.The numerical results showed that heat transfer performance is better at high frequency.Moreover,the heat transfer efficiency decreases with oscillatory fraction when frequency is low.Phase shift is found between the pulsatile flow rate,the pressure drop,the outlet temperature and the area-averaged wall Nusselt number.As a third-generation volumetric expander,the scroll expander has the advantages of small size,light weight,safety and reliability,and it is very suitable as a power output device for a OTEC system.In this paper,the factors affecting the efficiency of the scroll expander are explored through experiments.The results show that both the inlet temperature and the velocity have a high impact on the power of the expander.The higher the inlet temperature,the greater the output power;there is an optimum flow rate that maximizes the output power of the expander.The numerical results of the scroll expander reveal complex flow conditions within each expansion chamber.In addition,the inlet position,shape and outlet position of the expander have a great influence on the expansion output power.By comparing eight combinations,the optimal geometric combination scheme for the output of the scroll expander is obtained.