Investigation Of Vertical Falling Film Flow And Process Of Heat And Mass Transfer In Scrubbing-Cooling Chamber

Based on the scrubbing-cooling chamber,the scrubbing-cooling place of high temperature crude syngas in OMB coal gasification technology,the study aims to optimize its structure and improve the simultaneous heat and mass transfer efficiency and industrial operating stability.The experiment method and simulation method have been cooperated to investigate the falling film flow and heat and mass transfer between gas and liquid phases in the scrubbing-cooling chamber.(1)The falling liquid film thickness and velocity inside the scrubbing-cooling tube with opposed scrubbing-cooling ring has been measured by a Ultrasonic Doppler Velocimetry.The results show that the mean film thickness is nonuniform circumferentially in the tube.The film thickness is larger at 0° and smaller at other circumferencial positions.The mean film thickness becomes uniform circumferentially along with the increase of flow distance.When the liquid Reynolds number increases,the global liquid film thickness becomes larger and the circumferencial distribution becomes nonuniform.When Re1?1.53×104,the film at 0° breaks because the local film is too thick.The gas Reynolds number has little influence on the mean film thickness.The liquid film in the tube is violently turbulent and the wave characteristic of liquid film surface is complex.The relative standard deviation becomes smaller along with the increase of liquid Reynolds number in middle and upper section,but the variation tendency changes in lower section.The probability density function curve is right-skewed and its kurtosis is larger than 3 compared with the curve of Gaussian distribution.The kurtosis decreases when the liquid Reynolds number increases.The large wave frequency located in the thin liquid film area is sensitive to the liquid Reynolds number.The share of gas phase decreases the large wave numbers on liquid film surface.The results of liquid film mean velocity measurement show that the mean film velocity is also nonuniform circumferentially in the tube.The velocity is largest at 0°.The global velocity of liquid film increases with the increase of liquid Reynolds number.There is a turning point of liquid Reynolds number of liquid film velocity variation tendency.When the liquid Reynolds number is less than the turning point,the liquid film mean velocity increases with the increase of liquid Reynolds number,otherwise the liquid film mean velocity decreases.When the outlet width of the scrubbing-cooling ring is 3 mm,the turning point of Reynolds number equals to 1.73×104.The increase of outlet slot width of the ring increases the turning point of Reynolds number.(2)A 3D cold model of laboratory scale scrubbing-cooling chamber has been established based on experiment research.The cold model successfully predicts the break of local liquid film at 0° and the simulation result agrees with experiment result well.The circumferencial nonuniform of liquid film is caused by the structure of the scrubbing-cooling ring.The circumferencial velocity component of liquid film in the outlet of the ring points to 0°,which results in thick and easily broken liquid film.When the outlet width of the scrubbing-cooling ring is 3 mm,the break of local liquid film occurs when the Reynolds number equals to about 1.27×104.Furthermore,a hot model coupling with heat and mass transfer has been established.The circumferencial nonuniform of liquid film also has influence on heat and mass transfer inside the tube.The circumferencial temperature distribution is nonuniform.The break of local liquid film at 0° makes the liquid film thinner at other circumferencial positions.The tube wall at 45° is easier to expose to syngas,which may damage the tube by heat because the cooling water can not protect the tube and go against the regular operation.(3)The liquid film formed by a rotational scrubbing-cooling ring has been researched using experiment and simulation methods.The liquid film contains circumferencial velocity component which results in the rotational flow and it increases when the liquid Reynolds number increases.When Re1=2.04×104,the liquid film flows circumferencially across near 360°.The rotational flow decreases the circumferencial difference of temperature field inside the tube.Compared with the result of normal scrubbing-cooling ring,the local liquid film doesn't break because of the rotational flow of liquid film,which increases the global liquid film thickness and circumferencial distribution uniform and the coverage rate of cooling water is increased when the flow rate is small.The rotational scrubbing-cooling ring distributes cooling water better than the normal ring.(4)3D models of the fluid flow and heat and mass transfer process in an industrial scale scrubbing-cooling chamber have been established based on the above research.The liquid film in the industrial scale scrubbing-cooling tube is still circumferencially nonuniform.The local film at 45° is thick and easily breaks into droplets,and it is thin at other circumferencial positions.The tube wall at 0° is easier to expose to syngas,which is against the operation.When the syngas inlet velocity is small,the temperature descending section is short and the water evaporation rate is large.The mole fraction of water vapor in the syngas gets large rapidly.When the syngas inlet velocity increases,the water evaporation rate becomes small and the temperature descending section lengthens.When the syngas inlet velocity increases from 1 to 5 m/s,the syngas temperature at outlet also increases by 145 K.The whole process thermal load increases and the cooling effect of syngas becomes worse.The inlet cooling water velocity and syngas temperature has little influence on the operation of scrubbing-cooling chamber,but the change of inlet syngas velocity has a distinct influence.