Numerical Simulation Of Ammonia Falling Film Absorption Outside Vertical Tubes With Ultrasonic Atomization

When high-frequency ultrasound is applied in the solution.The micron-level droplets produced by ultrasonic atomization are suspended in the outer space of the absorption falling film tube,which can greatly increase the gas-liquid contact area and lead to higher absorption capacity of the absorber in a restricted volume.This paper is to investigate the influence mechanism of high frequency ultrasonic atomization on the heat and mass transfer characteristics of ammonia falling film absorption process.The vertical falling film absorber was used as a research object,a method utilizing ultrasonic atomization technology in the falling film absorbers was proposed.A numerical model was established to theoretically analyze the improvement of the absorption process with ultrasonic atomizers.Through quantitative calculation and analysis,the enhancement effect of ultrasonic atomization on the absorption performance was demonstrated and the optimum ultrasonic parameters were selected.(1)The establishment of ultrasonic atomization enhanced ammonia falling film absorption model.Through the force analysis of droplets in ammonia vapor,the kinetic equation of droplets in ammonia vapor was established.Governing equations of heat and mass transfer characteristics describing the action of liquid droplets on the ammonia vapor side were established.The heat and mass transfer model with droplets on the ammonia vapor side was embedded into the falling film absorption model outside the NH3-H2 O vertical tube through the phase interface coupling relationship,and the mathematical model of the falling film absorption of ammonia solution under the action of ultrasonic atomization was established.The physical properties of ammonia water were calculated based on the Schulz equation for ammonia water system.The finite difference method was used to solve the nonlinear partial differential equations in the ultrasonic enhanced absorption model.The model was verified and analyzed based on the experimental data in the literature.The verification results show that the model in this paper has good predictive ability and accuracy.(2)The optimal ultrasonic parameters for ultrasonic enhancement of falling film absorption were calculated.The optimal installation positions of different ultrasonic atomizers,and the optimal atomization rates were obtained by calculation.The results show that the improvement effect of ultrasonic atomization on falling film absorption increases first and then decreases with the decrease of the installation height of the ultrasonic atomizer and the increase of the atomization rate.With the increase of the atomization rate,the corresponding optimal ultrasonic atomizer installation position height gradually decreases,and as the installation height of the ultrasonic atomizer increases,the corresponding optimal atomization rate gradually decreases.On this basis,the concentration field and temperature field distributions inside the falling film absorber equipped with ultrasonic atomizers and the traditional falling film absorber were further compared and analyzed,and the results show that the thermal effect generated by the ultrasonic atomization process has little effect on the temperature rise absorbed by the falling film,which is negligible compared with its effect of strengthening absorption.(3)The effects of solution parameters,absorption conditions and droplet parameters on different ultrasonic atomization were explored.The results show that the total strengthening coefficient μ is positively correlated with the initial Reynolds number,the inlet ammonia mass fraction,the inlet ammonia solution temperature,the cooling water inlet temperature and the absorption pressure.The droplet diameter is negatively correlated,μ first increases and then decreases with the increase of atomization rate.When Re is greater than 24 or the ammonia mass fraction of the inlet solution is greater than 0.28,the enhancement effect of installing two ultrasonic atomizers on the falling film tube is greater than the improvement effect of installing one ultrasonic atomizer on the falling film tube.When the droplet diameter is greater than 0.5mm,the single-tube installation of one atomizer is better than two atomizers.