Research On Water Vapor Condensation Supercooling In Fired-gas Flue Gas And Measurement Of Temperature And Humidity Of Condensed Flue Gas

The natural gas-fired condensing boilers as energy-saving products recommended by the "Boiler Energy-saving Technical Supervision Regulations",whether the boiler actual operating efficiency can meet the energy-saving target demand,will be an extremely critical issue in the energy efficiency test and supervision of natural gas-fired boilers in the future.The droplets generated by the condensation of water vapor in flue gas are supercooled.The reasons for the supercooling of the condensed droplets have not been reported in the literature.So it is necessary to investigate the characteristics of the flue gas condensation heat transfer and the cause of the formation of supercooled droplets in the steady-state environment of the laboratory.Due to the presence of supercooled droplets in wet flue gas,the measurement uncertainty of smoke temperature and humidity is relatively high.Therefore,a high-precision and low-measurement uncertainty measurement device is required to characterize and monitor the water vapor condensation process in waste heat recovery unit condensation process,that is,the change of temperature and humidity of flue gas,and then to evaluate the ability of the waste heat recovery device to recover the residual heat of the flue gas.A closed condensation chamber test platform was set up in the study of the flue gas condensation heat transfer and the cause of the formation of supercooled droplets,the evolution process of the surface temperature of single droplet and the size distribution of the droplets was obtained by infrared thermography,and the effect of the application of the sound field on the drop off of the condensate droplets was explored.The experimental results show that: the heat transfer process of single condensate drops is shorter,the surface temperature of single droplet is characterized by high temperature in the central area and low temperature in the edge region in the process of condensation heat transfer,and there is a temperature gradient in the droplet;The size of the droplets produced by the condensation of vapors containing non-condensing gases exhibits a "normal distribution-bimodal distribution-exponential distribution" distribution,when non-condensable gases are introduced into the vapor,the condensation droplets infiltrate from Wenzel mode to Cassie mode;The sound field can distort the condensed droplets and accelerate their detachment from the condensing surface.Compared with the hydrophobic surface,the sound field enhanced the droplet shedding effect on thesuperhydrophobic surface is more obvious.In order to improve the performance of the measuring device,the F-P cavity fiber temperature sensor and the fiber Prague grating humidity sensor are integrated into the device based on in situ flue gas heat tracing interlayer pumping device.The experimental results on 30-k W gas-fired condensing heat-transfer test platform show that: the sensitivity of the F-P cavity fiber temperature sensor to the measurement of flue gas temperature is stronger than the thermal resistance,and the uncertainty of measuring the temperature of the flue gas is 0.1409?,and the accuracy is higher than the thermal resistance;the humidity sensitivity,linearity,and reliability of the flue gas humidity measured by fiber Prague grating humidity sensor are higher than that of the conventional moisture-sensitive capacitance meter,the measuring device of integrated optical Prague grating humidity sensor realizes the measurement of flue gas humidity at lower heating temperature;the integrated F-P cavity fiber temperature sensor and the fiber Prague grating humidity sensor measurement device measured the the counter balance efficiency of the 30-k W gas-fired condensing heat-transfer test platform as99.13%,the difference between the positive balance efficiency and counter balance efficiency of the test platform was less than 2%.