Zhong Lan

Heat Transfer and Thermodynamic Performance of LiBr/H2O Absorption Heat Transformer with Vapor Absorption Inside Vertical Spiral Tubes

In this paper, an absorption heat transformer (AHT) with falling film of aqueous LiBr solution inside vertical spiral tubes is installed and tested. The variations of coefficient of performance (COP), thermal efficiency (Eth ), and the heat transfer coefficient of the absorber at different falling film flow rates, hot water flow rates, and operating temperatures are investigated experimentally. The results demonstrated that the coefficient of performance and thermal efficiency of the system decrease with the increase in the flow rate of LiBr solution, and the influence of flow rate of hot water on COP and Eth is insignificant. The available COP in the experiments is higher than 0.4. The heat and mass transfer coefficients of the absorber increase with the increase of the flow rate of LiBr solution, up to 400W/m2/K and 0.013 kg/m2/s (temperature of waste heat is 90°C). The heat transfer coefficient of the absorber increases with the increase of the temperature of waste heat, and decreases with the increase of the cooling water temperature. Meanwhile, the computer code ABSIM (Absorption Simulation) is used to simulate the AHT systems, and the simulated results are compared with the experimental data.

Dropwise Condensation Heat Transfer Model: Effect of the Liquid-Solid Surface Free Energy Difference

Dropwise condensation heat transfer performance depends not only on the condensing conditions, but also on the interfacial interaction between condensate and condensing surface material. Based on the well-established Rose’s model, a modified model of dropwise condensation heat transfer is proposed by considering the interfacial interaction between liquid and solid, and established by rebuilding the space conformation of drop distributing into time conformation. The simulation results indicate that the heat transfer coefficient increases with the surface free energy difference increasing and the contact angle hysteresis decreasing. The larger contact angle and the smaller departure drop size result in the higher heat transfer coefficient. Different interfacial effect gives rise to the different heat transfer curves. For the identical solid-liquid-vapor system, the simulation results agree very well with the present experimental data and those reported in literature. The controversy among experimental results in literature might be well understood with the concept of the present paper.Copyright

Heat and mass transfer enhancement for falling film absorption with coated distribution tubes at high temperature

Specific coating is sprayed on the external surface of bare brass tube with the designed surface distribution form. The experimental investigation of the vertical falling film absorption outside the bare brass tube and different circumferential coated tubes at high temperature is conducted. The effects of solution spray density and inlet temperature on the heat and mass transfer are studied. The result shows that the heat transfer coefficients and mass transfer coefficients at high temperature are higher than that at low temperature on different tubes; The heat transfer coefficients on the coated distribution tube are higher than that on the bare brass tube, but the mass transfer coefficients on the coated distribution tube are lower than that on the bare brass tube.

Effect of Surface-Free-Energy-Difference on Condensation Characteristic Curves of Binary Mixture of Steam and Ethanol Vapor

The condensation heat transfer process of binary mixture of steam and ethanol vapor was studied experimentally with various ethanol concentrations and surface subcooling at atmospheric pressure. The experiments demonstrate that the condensation modes alter from filmwise, to transition and finally to dropwise with respect to the operation conditions. The marangoni effect was always used to interpret this phenomenon in the published literature. In this paper, the mechanism of condensation heat transfer with the liquid-solid-surface-free-energy-difference is used to further understand this process and the conversion of condensation modes. From this viewpoint, the condensation characteristics of binary mixture of steam and ethanol vapor can be explained reasonably with the corresponding condensation modes. Base on the data of the present paper and those reported in literature, the conversion criterion of the condensation modes is proposed. The condensation state is filmwise for the surface-free-energy-difference less than 14±1 mJ·m−2 and dropwise for more than 21±1 mJ·m−2 and transition for between 14±1 mJ·m−2 and 21±1 mJ·m−2 .Copyright