Performance Study And Numerical Simulation Of Thermosyphon For Papermaking Process

The thermosyphon has many advantages, such as high thermo-conductivity performance and low cost, so that it has been applied in many fields. Applying thermosyphon in the papermaking industry can help improving product quality and saving energy. In this paper thermal transfer behaviors of wicked, vertical, small diameter thermosyphons were studied, featuring (i) heat transfer capacity with different fill ratio, (ii) inner and outer temperature response at start-up. At the same time, the vapor-liquid two-phase vertical flow regimes and dynamics were simulated by the Computational Fluid Dynamics (CFD) method.The thermosyphons were fabricated of glass tubes with inner diameters of Φ8 mm for thermal transfer capability and start-up behavior test, and of 04 mm for validations of CFD simulation. In the thermal transfer capability test, the characterization was made by recording the temperature gradient of a specific volume of water heated by a thermosyphon with the passage of time. In the start-up behavior of a thermosyphon test, four thermocouples were placed, at equal intervals in the thermosyphon. And another four thermocouples were placed outside at the corresponding position. The inner and outer temperature profiles were recorded during the start-up.In the numerical simulation, a 2D, planar CFD modeling using explicit Multi-Fluid VOF model within the Eulerian Multiphase model was carried out to model the interaction and interface between gas and liquid as well as fluid flow dynamics inside the tube. Real-time maps of vapor bubble generation, combination and vapor slug were derived from the simulation. Various data concerning pressure, turbulent, mass transfer rate, temperature and velocity magnitude, etc. can be obtained during the calculation.The innovative attempts made in this work include:(ⅰ) experiments and simulations were based on small diameter thermosyphons. Their heat transfer properties and hydrodynamics are different from common, big diameter thermosyphons; (ⅱ) thermometers were placed in the interior of a thermosyphon which can directly detect inner temperature distribution; (ⅲ CFD simulation based on Eulerian model with vertical evaporation and condensation was developed to gain sharp interface treatment between phases. This model has not been reported yet in publications.The present study indicates that the best thermal conductive performance is obtained with around 26% fill ratio. Experimental results showed a very good temperature uniformity of thermosyphons. Temperature differences between thermocouples at evaporator and condenser sections, inside and outside, can be obtained from the recorded data. Moreover, real-time vapor bubble generation, combination, breakup, coalescence and other fluid dynamics parameters were derived from the simulation. A good agreement was observed between CFD acquired data and experimental observations. It is evidenced that CFD is a powerful tool to model and explain the complex flow and heat transfer in a thermosyphon.