| With the increasing tension of energy shortage, the high-efficiency heat transfer enhancement technologies has become an important approach to save energy and im-provement efficiency of energy utilization. The horizontal falling film evaporators are widely applied in many industrial processes (the desalination and fresh water treat-ment industry, the petrochemical industry, refrigeration and air-conditioning Industry and the food processing industries, etc.) due to their high heat transfer coefficients at low mass flow rates and small temperature differences. In this paper, an new approach for heat transfer enhancement by using shaped tube in falling film evaporation is pro-posed, and detailed research has been conducted both numerically and experimentally to investigate the heat transfer enhancement mechanism.Firstly, Summaries about the hydrodynamics of falling film flow, including in-tertube flow modes, space between two jets or droplets (falling film wavelength), film thickness and film breakdown, have been done. Then empirical formulas for heat transfer coefficient are discussed and the influence factors are analyzed in details. Al-so, the mathematical model of falling film flow and heat transfer is analyzed, which will be adopted in the following numerical simulation. According to the analysis above, the falling film is generally laminar flow. It can be found that the thermal re-sistance mainly exists in the near wall region and also thermal resistance through the liquid film. Therefore, it is an effective enhanced heat transfer technology to keep disturbance to the fluid in the near wall region and thin the film. Smooth shaped tube with a special curved surface could improve the falling film evaporation by enhancing the heat convection between the wall and the liquid and thinning the film thickness.Based on the VOF method, the mechanism of heat transfer enhancement caused by two shaped tubes that are oval-and drop-shaped tubes is studied numerically in details. The results show that the shaped tubes with special curved surfaces can pro-duce an effective guidance to the fluid that flow outside the tubes and accelerate the film flow by inducing a larger gravity component in the flow direction. For example, at the angular positions of30°, the dimensionless velocity of the oval-shaped tube is about30%and4%larger than that of the circular tube neat the wall and the va-por-liquid interface, respectively. The increase of the liquid velocity in the film is helpful to enhance the heat convection between the wall and the liquid. Besides, it can improve the fall of the liquid and thin the film and strengthen the wave in the film, which resulted in enhancing heat transfer from the inner liquid to the vapor-liquid in-terface through the laminar liquid film. The circumferential angle, where the minimum film thicknesses appear, of oval-and drop-shaped tubes is larger, and the value of minimum film thicknesses decrease9.8%and2.0%compared with the cir-cular tube, respectively. The dimensionless excess temperature decreases with the increases of feeder height, flow mass rate and increases with the increase of the angu-lar positions. Compared with the circular tube, the two shaped tubes have a lower dimensionless excess temperature and a thinner thermal boundary layer with a lager temperature gradient. The analysis based on field synergy principle shows that the co-sines of average synergy angle of flow field outside the circular tube, the drop-and oval-shaped tube the tubes increase in its order, which indicates that the increased ve-locity induced by the oval-shaped tube can improve the field synergy performance in the whole film, may also proves the reliability of the numerical simulation. By com-parison, the oval-shaped tube has best heat transfer ability followed by drop-shaped tube, which means tubes with a special shaped section can effectively improve heat transfer on the falling film evaportation.In order to obtain the optimal structure, the falling film flow and heat transfer characteristics of oval-shaped tubes with six different cross-sections are analyzed at a mass flow rate of0.29kg/(m·s) and a liquid feeder height of9mm. With increasing semi-axis ratio, the liquid flow velocity in the film increases and the film thickness decreases. The circumferential position of the minimum film thickness is larger at a bigger semi-axis ratio. The falling film characteristic of the oval-shaped tube with a semi-axis ratio than4is inclined to similar with that on the vertical plate, and has a fully developed and thicker thermal boundary layer leads to weaken heat transfer. Results show the oval-shaped tube with a semi-axis ratio of2to3has a smaller di-mensionless excess temperature and a lager temperature gradient, which means a better heat transfer performance. It is found that the semi-axis ratio of2.4is the best optimal structure for oval-shaped enhanced tube by fitting the numerical data, which declares that the optimization of the cross section of horizontal tubes will be helpful to the film distribution improvement and the heat transfer enhancement of the falling film flow.For studying the falling film flow characteristic, an experimental system is set up. The intertube flow modes, modes transform and the falling film wavelength are ob-served with the help of a high speed camera. The film thicknesses along the circumference under different Reynolds number and intertube spacing are measured by using a home-made conductance probe. Experimental results show that the flow modes on the oval-shaped tube are similar to the circular tube, which is the droplet, droplet-jet, jet, jet-sheet and sheet mode with the increase of film Reynolds number. The mode transition starts early (at lower Reynolds number) by using the oval-shaped tube. The primary difference is an enlargement of the zone in which jet-sheet mode exists; that is, the jet to jet-sheet transition occurs at about40%lower film Reynolds numbers than that of the circular tube. While the jet-sheet to sheet transition occurs at somewhat higher Reynolds numbers. In practice, this means that an evaporating fall-ing film will tend to stay in the advantageous jet-sheet mode down to lower mass flow rates on oval-shaped tubes than on an array of circular tubes. The falling dimension-less wavelength increases obviously with the decrease of Reynolds number below400and firstly increases then tends towards stability with increasing of intertube spacing. The study found that the falling wavelength of the oval-shaped tube is smaller than that of the circular tube by6.66%. The film thickness increases with the increase of Reynolds number and decreases with increasing the intertube spacing with a larger effect at the top of tube. The numerical results are in good agreement with the exper-imental results. The empirical correlations including the intertube spacing for the falling film wavelength and minimum film thickness were obtained for the circular and oval-shaped tube.The falling film evaporator is designed and heat transfer experiments are carried out under the conditions of single row tubes and tube bundle, respectively. The sur-face heat transfer coefficients outside the tubes are obtained by separating the overall heat transfer coefficient. Special curved surface of oval-shaped tube is experimentally proved to be an effective method for heat transfer enhancement. Compared with the circular tube, the surface heat transfer coefficients increase for the oval-shaped tubes of13.4-16.1%and11.9%-13.6%under the conditions of single row tubes and tube bundle, respectively. When water is used as the working medium, within200<Re<3500, the heat transfer rate can be raised by increasing Reynolds number, the growth rate is reduced. The study also found that the heat transfer coefficient increase with their increases of the spray liquid temperature and intertube spacing. Based on the experimental data, the prediction correlations to estimate Nusselt number for cir-cular and oval-shaped tubes has been proposed and can serve as references for the design and calculation of this kind of falling film evaporator. |
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