Study On Alternate Heat And Mass Transfer Absorption Performances Of Staggered Tube Bundle With M-W Corrugated Mesh Guiders

Absorption refrigeration can not only use the solar energy and industrial waste heat and other low grade heat source but also satisfies the requirement of the sustainable development of low carbon environmental protection. In the field of air-conditioning absorption refrigeration, lithium bromide absorption refrigeration is the best choice because of its high performance of coefficient. In a lithium bromide absorption chiller, absorber is the largest and most important component, its heat and mass transfer characteristics have significant influence on the whole machine features. So further promoting lithium bromide falling film absorber structure of absorption refrigeration system and revealing its heat and mass transfer mechanism have not only important academic value but broad application prospects. This dissertation concentrates on alternating heat and mass transfer structure of staggered tube bundle with M-W mesh guiders which is a new absorption structure scheme with compound enhancement. The theory modeling, the CFD simulation, experimental system installation and commissioning were conducted, and the experimental data and simulation results comparison analysis were carried out respectively.So-called staggered tube bundle with M-W mesh guiders scheme refers to that the longitudinal corrugated stainless steel mesh guiders, with corrugation height and pitch equal respectively to the vertical tube spacing and the horizontal center distance of the tube bundle, were inserted into the staggered tube bundle. The corrugated mesh layers shaped like M and W alternatively and formed the absorption core structure for the lithium bromide solution flows alternately on the surfaces of the horizontal tubes, with heat and mass transfer coupling absorption process, and then on the mesh guiders, with adiabatic mass transfer absorption process. Therefore, this scheme is a combination of internal cold absorption and adiabatic absorption with alternately heat and mass transfer enhancement. The mesh packing not only has good mixing effect, but also can form a very thin dual-side exposed liquid solution film the on its surface.Theoretical modeling was performed that the whole surface of an absorber, which is divided respectively into tube falling film absorption zone, mesh guiders layer in adiabatic absorption zone and cooling water tube flow zone and free fall of droplets or liquid column flow zone between bare horizontal tubes. By integrating these sub-models the overall heat and mass transfer models of the new absorber and its counterpart were established. The models were built and solved with MATLAB software. Firstly verification of the current models was performed with satisfactory agreements with the ones of Kyung et al., by calculation of the thickness, velocity, temperature and concentration distribution of liquid film on a single horizontal tube and on a column of tubes. Then by studying the liquid film flows on single horizontal tube surface, on the mesh guiders and on alternate structure of a column of horizontal tubes with mesh guiders as well as on the single column of horizontal tubes, the local heat and mass transfer coefficients, the influence of the different tube diameter and tube spacing on the performance of absorber were calculated and analyzed. The results show that the porosity of 0.9 mesh guiders layer is of best absorption feature; and the heat and mass transfer performance of the scheme of a single column of horizontal tubes with mesh guiders is better than that of the single column of horizontal bare tubes.CFD simulation was performed using GAMBIT software to built up 3D geometric models respectively for single horizontal tube, mesh guider, horizontal tube bundle, horizontal tube bundle with mesh guider inserts, and using FLUENT software with VOF method of CFD to solve the models respectively. The liquid film at outside of horizontal tube is in laminar state, the CFD simulation results of liquid film velocity, temperature and concentration are in conformity with those of in the literature. The results show that the VOF model for calculation of falling film absorption on the outside horizontal tube is feasible. The CFD simulation results show that the liquid film in the center of the mesh net hole forms concave dual-side liquid film, the nephograms of concentration and temperature distributions show the opposite trends that higher the temperature lower the concentration and vice versa, the differences of temperature and the concentration of liquid film at inside and outside surfaces are less than 2℃ and 1% respectively. The CFD simulation results of liquid film absorption performances on tube bundles of two schemes with M-W mesh guiders and without any mesh layer show that the M-W mesh guiders structure enlarges the tube surface wetting area thus makes full use of the cooling capacity of tube; the solution temperature and concentration fluctuate more or less with gradual reduction trends along the absorption process. The average local heat transfer coefficient on the tubes of the scheme with M-W mesh guiders is higher than that of the scheme without any mesh screen. It verified that the M-W mesh guiders can significantly improve the heat and mass transfer efficiency of the absorber.The experimental research was conducted begin with that the applicant participated in the design and set up the falling film absorption test rig, which is a closed experiment system with single pressure absorption and desorption and with parameter control, measurement and data acquisition functions. Based on the experiment system, the impacts of different parameters on the performance of the heat and mass transfer of the absorber were investigated, and the experimental results were compared with the calculation results of numerical model and CFD model respectively. The results show that the average heat and mass transfer coefficients increase with the increase of the sprinkle density, concentration of solution, absorption pressure and cooling water flow rate, and the decrease of temperatures of both solution and cooling water. Under the same working condition, the heat and mass transfer coefficients of the alternating absorber of staggered tube bundle with M-W mesh guiders are increased by 15%-25% and 48%-94% respectively compared with those of horizontal bare tube bundle absorber. Both CFD simulation data and the results of numerical model calculation can fit well with the experimental data, the deviation is about 10%, showing that the established model can reflect well the heat and mass transfer performance of the absorber.