Research On The Influence Of Bubble Characteristics And Heat Transfer Performance Of Direct Contact Heat Exchange

Direct contact heat exchanger has the advantages of no fouling,low thermal resistance,large heat transfer area and high heat transfer efficiency,which is an effective way to utilize energy.It is widely used in crystallization,seawater desalination,thermal energy storage,ice slurry production and waste heat recovery.Despite the great application potential,the problems of homogeneity of two-phase mixing and stagnation of dispersed phases have not been effectively solved in direct contact boiling.To address the problems faced so far,this paper investigates the heat transfer performance of different flow modes,establishes single bubble and bubble cluster heat transfer models for the dispersed phase,investigates the uniformity of bubble distribution and temperature difference field uniformity under different test conditions,and verifies the mechanism of enhanced heat transfer by the uniformity of bubble distribution from the perspective of the specific interface area for heat transfer in two-phase contact.This thesis provides a new idea to study the influence of bubble population morphological characteristics and heat transfer performance in direct contact boiling heat transfer process under different flow regimes,and provides a new way for heat exchanger design.Firstly,the direct contact heat exchange platform was built and experiments were conducted to obtain the bubble evolution by using a high-speed camera,and the bubble shape evolution trends in different flow patterns and different experimental conditions were analyzed.It is found that the bubble aspect ratio becomes smaller with the increase of the dispersed phase flow rate,and the bubble shape tends to be flattened.This trend is more obvious when the continuous phase is circulating at constant temperature;when the continuous phase is circulating,the bubbles reach the stable phase in a shorter time and the number of bubbles is smaller.Secondly,a new metric,i.e.,bubble average area,is proposed for evaluating the local bubble size in direct contact heat exchange.Based on the potential energy method,the uniformity of bubble distribution under different experimental conditions is analyzed;the relationship between the bubble average area and the temperature difference field uniformity factor is explored.The results show that the bubble population distribution is more uniform and the time to reach the uniform state is shorter when the continuous phase circulates and flows compared with the continuous phase stationary.This is due to the fact that the cyclic flow of continuous phase increases the perturbation between two phases and the shear force between bubbles,which makes the probability of inter-bubble fragmentation increase and easier to be distributed uniformly in the evaporator.The synergistic relationship between the temperature difference field uniformity factor and the average area of bubbles was negatively correlated with a negative correlation index of-0.956453.Finally,based on the above study,the heat transfer performance under different flow modes was analyzed,and the relationship between the uniformity of bubble distribution and the specific interface area of two-phase contact,the specific interface area and the volumetric heat transfer coefficient was explored.The results show that the volumetric heat transfer coefficient is significantly higher in the continuous phase thermostatic circulation flow mode.The bubble distribution uniformity is positively correlated with the two-phase contact specific interface area,and the actual contact area depends on the final shape of the bubble.And the contact area of the two phases depends largely on the final shape of the bubble and the uniformity of the distribution of the dispersed phase in the continuous phase.The specific interface area is positively correlated with the volumetric heat transfer coefficient.The linear correlation was 97.75% by linearly fitting the volumetric heat transfer coefficient and the specific interface area at 95% confidence interval.It was verified from the perspective of the specific interface area why the bubble distribution uniformity enhances heat transfer.