Theoretical And Experimental Researches On Capillary-Assisted Evaporation

For the evaporation of the working fluid with low saturated vapor pressure, to overcome the unfavorable influence of the liquid pressure on its saturated temperature, the falling film evaporating heat transfer method is usually adopted, such as the water evaporators in LiBr-H2O absorption chiller. Falling film evaporation needs liquid circulating pump, spraying and distributing equipment which are unfavorable to small scale machines. Aiming at solving these problems, this work introduces another film-wise evaporation method: capillary-assisted evaporation into small-scale and micro-scale refrigerators. Capillary-assisted evaporation uses capillary force to draw liquid to form evaporating thin liquid film and belongs to a passive liquid film formation method. It can form equally liquid film without the assistances of other equipments, the structure is simple, the running is reliable and the cost is low. The previous researches concerning capillary-assisted evaporation focuses mainly on micro-size (sub-millimeter) fields, and rare studies on normal-size fields can be found that is exactly the research object of this work.To understand deeply the heat and mass transfer mechanisms in capillary-assisted evaporation and solve the potential problems which maybe be encountered when applying to normal-size fields, this work has carried out theory analysis, experimental and application researches.Firstly, aiming at the capillary-assisted flow and evaporation inside the circumferential micro grooves, the relative theoretical research has been performed and a systematical mathematical model has been built. Through the model, the liquid capillary-assisted flow and evaporation heat transfer characteristics inside the micro grooves have been researched deeply. The results show that the curvature of the curved liquid-vapor interface inside the micro groove increases gradually with the axial liquid flow, and the total heat absorbed inside the axial cross section increases also. For any axial cross section, the heat transfer occurs mainly at the left-upper and right-upper corners. For the same dynamic contact angle and other conditions, the axial total heat absorption of water is higher than methanol. However, for the same groove geometry, the liquid entrance location, the evaporating pressure and the groove wall superheating, the dynamic contact angle of methanol for the same axial location is much lower than water, thus, the total heat absorption of methanol is higher than water. Besides, by theoretical analysis, a maximum groove bottom diameter exists and the corner flow will occur if exceeding this diameter. The maximum groove bottom diameter increases with the ratio of the groove's depth to its width, decreases with the groove wall superheating.To validate the mathematical model, an experimental research has been implemented to investigate the capillary-assisted flow and evaporation inside the circumferential micro rectangular grooves, and a single tube test setup has been built. According to the theoretical analysis, the groove depths are chosen as 0.5mm and 1.0mm respectively, and 0.2mm and 0.5mm are chosen for the groove widths. The evaporating liquids are water and methanol. The experimental results have shown that the liquid entrance inside the groove has the most outstanding effect on the capillary-assisted evaporation. For example, for the groove pinch of 0.35 mm,the groove width of 0.2 mm,the groove depth of 1.0 mm,the groove bottom diameter of 17.0 mm,the groove inclination angle of 90o,the evaporation saturated temperature of Te =5.0±0.1℃,the groove wall superheating ofΔT =4.0±0.2℃,for water, when the dimensionless liquid level decreases from 3/4 to 1/4, the capillary-assisted evaporation heat transfer coefficient increases by 136% (from 2400 W(/m~2K) to 5700 W/(m~2K)). That is equivalent to the falling film evaporation inside LiBr-water absorption chiller which has a film side heat transfer coefficient of 2800~4500 W/(m~2K). The capillary-assisted evaporation heat transfer coefficient decreases with superheating, increases with evaporating pressure.To validate the feasibilities of applying capillary-assisted low pressure evaporators into small-scale and micro-scale refrigerators, a micro-scale adsorption chiller with the design cooling output of 1.0 kW has been designed and manufactured inside which two capillary-assisted water evaporators have been used. The experimental investigation on the chiller has also been carried out. The results show that the capillary-assisted evaporators run stably. The theoretical analysis and performance test on a capillary-assisted water evaporator with the heat transfer power of 10.0 kW have indicated that the calculated capillary-assisted evaporation heat transfer coefficient by the model is a little higher than experimental value by 25.5%. Considering the measure errors of the temperature sensors and relative formulas, we can draw a conclusion that the mathematical model built can simulate the heat and mass transfer occurring in the capillary-assisted liquid flow and evaporation accurately. The systematic validation has shown that it is feasible to apply capillary-assisted low pressure evaporators into micro-scale and small-scale refrigerators.