Performance Study On A New Gas Water Heater With Outer Surface Heat Exchanger

With the rapid development of China's economy and the improvement of quality of life of citizen,storage gas water heaters are more and more accepted by users,due to comfort of hot water.The development of high-efficiency and low-emission storage gas water heater can utilize natural gas energy better and save energy better.At present,condensing technology and full premix combustion technology are mature used in tankless water heater,but heat exchanger structure and combustion type optimization in storage gas water heater are limited due to some reasons such as tank manufacturing technology and combustion chamber shape.This thesis describes an innovative storage gas water heater,using outer surface of tank as secondary heat exchanger,and combine with negative pressure premixed combustion system,having higher efficiency and lower exhaust gas emission,without needing explosion-proof blower as the premixed system,combining with the combination of theoretical analysis and numerical simulation,to study the new design of combustion,heat transfer in the system,and through the experiment to prove the design is feasible and reliable,and get the performance of new type gas water heater.Its main contents are:First of all,combustion and heat transfer process are analyzed in theory,including the flame stability,CO/NO_x generation mechanism,and negative pressure premixed combustion system introduction.For the heat exchanger,the threaded flue pipe with baffler enhance heat transfer could enhance high temperature exhaust gas turbulence,combined with the practical engineering application,based on analysis of combustion system,heat exchangers and waterway design,building up new outer surface heat exchanger model of storage gas water heater.Secondly,using software FLUENT to simulate and calculate the new model,aiming to get the clear map of the combustion and heat transfer process.Based on the simulation result of combustion flame on burner no-uniform issue,designed new double venture structures of burner inlet to improve flame distribution,and determined preliminary excess air coefficient of 1.5 is more appropriate,based on preliminary excess air number,found out the reasonable range of excess air coefficient.Through the heat transfer process simulation analysis,it's found that the heat transfer enhanced by threaded flue pipe with baffler,sharing 50%of the total heat transfer,the new design of heat transfer on the outer surface of the tank also plays a crucial role in improving the heat transfer efficiency,sharing nearly 30%of total heat transfer.In addition,CFD and FEA coupling analysis method was used to simulate the stress distribution and safety factor in the high-temperature region of the first-stage heat exchanger,optimization design by flanging added.Finally,set up a new outer surface storage gas water heater prototype,testing flue gas emissions,thermal efficiency,hot water output ratio,and the bottom cover enamel surface temperature inner tank,based on test result,founding the negative pressure premixed combustion system of excess air coefficient within a reasonable range of 1.3 to 2.0,and in this range,CO and NO_x meet the lower emission requirements.The effectiveness of threaded flue pipe with baffler and outer surface of tank as a secondary heat exchanger was also verified by the prototype thermal efficiency test.The optimization design model has 90%thermal efficiency and about 85%hot water output ratio,which is significantly higher than similar products.Combining the risk points of CFD and FEA coupling analysis,a feasible reliability test was designed and implemented to verify the reliability of the welding improvement scheme.It is proved that through the research of this thesis,a new type of gas water heater with premix combustion system and outer surface of tank as the secondary heat exchanger,can achieve good combustion performance and high thermal efficiency by optimizing the inlet structure of burner and the sandwich rib structure on the outer surface.This innovative system is worth popularizing.