Heat and Mass Transfer of NH_3-H_2O Falling-Film Absorption on Horizontal Round Tube Banks

摘要

The absorption process has been confirmed as the most important process in absorption refrigeration machines in terms of improving their total efficiency. For this reason, absorber structures in general and heat and mass transfers in absorber in particular have attracted the interest of many researchers in this field. Commonly, the falling film absorber structure is the liquid mixture flows over tubes in a film mode. Mathematical model is developed for the falling film flowing on horizontal round tubes absorber derived from the mathematical model of the test volume element. The two-dimensional numerical simulation is written to solve partial differential equations predicting absorption efficiency. For evaluating the parameters which affect the coupled heat-mass transfer as NH_3-H_2O diluted solution flowing over horizontal round tubes absorb NH_3 vapor to become the higher concentration solution. The fields of velocity, temperature, concentration and thickness of the falling film solution varied by the input conditions of diluted solution and cooling water temperature flowing in the tube represented for a test volume element of the tube. The correlations which give the heat transfer coefficient and mass transfer coefficient in the absorption process in range of solution concentration ω = 28% ÷ 31%, solution mass flow rate per unit tube length Γ = 0.001 ÷ 0.015 kgm~(-1)s~(-1), coolant temperature t_(water) = 28℃ ÷ 38℃ are set as two functions. The accuracy of numerical model and experiments are compared by the inlet, outlet the tube bundle of cooling water temperatures and absorber heat load. The absorber heat load deviation of the computing program Q_(a_cumpute) and experimental result Q_(a_meas) is 4.3%. The absorber heat load deviation of simulation result Q_(a_sim) and experimental result Q_(a_meas) is 12.3%. The overall heat transfer coefficient k used for simulation result of absorber heat load was taken from the relationship of the heat transfer coefficient k = f(C; Г; T) = f(0.308; 0.008; 306.3) = 0.863 kWm~(-2)K~(-1)). The results were also evaluated with other similar studies by other authors. Based on these simulations, the theoretical studies were done for absorption refrigeration system in order to narrow the working area where the experiments later focused on. The results of this study will be the basis for subsequent application research of falling film absorbers.