Experimental And Numerical Research On The Flow And Heat Transfer Characteristics Of Lithium Bromide Solution Bubble Absorption In Vertical Single Tube

As one of the key technologies for achieving the "double carbon" goal,absorption heat pump technology can make full use of medium/low quality energy and solar energy,and is friendly to the environment.As one of the key components of the absorption heat pump system,the absorber affects the overall efficiency of the absorption heat pump system.Bubble absorption is a kind of absorption method which can improve the heat transfer ability of absorber and has become a research hotspot.However,the size of the bubbles during the bubble absorption process is constantly changing with the shape of the bubble,and its heat transfer is also a complex and variable process.Therefore,it is vital to research the form and size of bubbles during bubble absorption,especially the heat transfer characteristics of this process.Until now,there are mainly the following key scientific issues:(1)The diameter of the bubble size affects the heat transfer characteristics of the absorber,the bubble size varies as the bubble rises,and the classic bubble size model cannot accurately predict the average size of the bubbles in the forced space in the vacuum environment.(2)The physical process of bubble absorption is the process of convective heat transfer under the condition of variable bubble shape and size in the forced space,the traditional bubble absorption heat transfer model which ignores the bubble morphology and size changes cannot accurately predict the actual heat transfer characteristics.In this research,the bubble absorption of Lithium Bromide solution was taken as the research object,and a set of experimental equipment was built to establish the bubble absorption flow and heat transfer characteristics of Lithium Bromide solution,the bubble flow morphology and the effect of steam inlet flow rate on bubble size was explored through visualization experiment,the classical bubble size model was modified.The effects of different factors on the bubble absorption heat transfer characteristics were explored experimentally,and a bubble absorption convection heat transfer model was established based on the physical process of the experiment.The flow field and temperature field in the vertical tube bubble absorber and the heat transfer characteristics of the bubble absorption process were analyzed numerically.The main research contents and conclusions are as follows.1.Visualization experiment was employed to research the effects of steam inlet flow rate on bubble shape and average size at different liquid level.The results showed that the classical bubble size model cannot accurately predict the average bubble size in the forced space under vacuum environment.Based on the experimental data,the error between the calculated values of the model and the experimental values was within ±10%.2.Non-visual experiment was employed to research the effects of cold water temperature,solution inlet temperature,solution inlet flow rate and supercharging ratio on heat transfer characteristics of Lithium Bromide solution.The results showed that the heat transfer capability increased with the decrease of cold water temperature and solution inlet temperature,and increased with the steam inlet flow rate,solution inlet flow rate and booster ratio.The average heat transfer coefficient increased by70%,when the temperature of cooling water decreased from 28℃ to 20℃.The average heat transfer coefficient increased by 51%,when the inlet temperature of the solution decreased from 39℃ to 35℃.The average heat transfer coefficient increases by 28%,when the inlet flow rate increases from 10L/h to 50L/h.The average heat transfer coefficient increases by 52%,when the supercharging ratio increases from 1.1to 1.5.3.The accuracy of the bubble absorption convection heat transfer model was verified by visual and non-visual experiments.The maximum error between the simulated and experimental was-0.5%,when the liquid level was 0mm.The maximum error between the simulated and experimental was-5.8%,when the liquid level was 500 mm.The maximum error between the simulated and experimental was-3.8%,when the liquid level was 900 mm.The maximum error between the simulated value and the experimental value of the average heat transfer flux of the solution under different flow rates was +3.6%.The model can accurately predict the process of bubble absorption convection heat transfer.4.Numerical simulation was employed to research the effects of steam inlet flow,solution inlet flow and steam temperature on velocity field and temperature field of Lithium Bromide solution bubble aborption.The bubble disturbs the solution more obviously with steam inlet flow.The convective effect becomes more obvious with solution inlet flow.Steam temperature has little effect on velocity distribution.The steam inlet flow rate and solution inlet flow rate enhance the bubble absorption heat transfer capacity by enhancing the convective heat transfer effect,and the evaporation temperature enhances the bubble absorption heat transfer capacity by increasing the heat transfer driving force.5.Numerical simulation was employed to research the effects of solution concentration,nozzle hole diameter,absorber diameter,steam temperature,cooling water flow and steam inlet flow on heat transfer characteristics of Lithium Bromide solution bubble aborption.The results showed that the bubble absorption heat transfer capacity increased with the decrease of solution concentration and nozzle hole diameter,and increased with tube diameter,steam temperature,cooling water flow and steam inlet flow.The average heat transfer coefficient increased by 27%,when the solution concentration decreased from 60% to 50%.The average heat transfer coefficient increased by 11%,when the diameter of nozzle hole decreased from 6mm to 2mm.The average heat transfer coefficient of the solution increases by 12%,when the diameter of the absorber increasesd from 20 mm to 40 mm.The average heat transfer coefficient of the solution increased by 48%,When the steam temperature increased from 20℃ to 28℃.The average heat transfer coefficient of the solution increasedd by 28%,when the cooling water flow rate increases from 50L/h to 90L/h.The average heat transfer coefficient of the solution increased by 18%,when the steam inlet flow rate increases from 200L/h to 400L/h.