Investigation On Two-Phase Flow Process Of Steam Flowing Across Falling Film Horizontal Tube Bundle

The gas-liquid two-phase flow in tube bundle is a common phenomenon in shell-and-tube heat exchangers, which equipment are widespread used in the fields of energy, power seawater desalination, petrochemical engineering and evaporative refrigeration. Horizontal tube falling film evaporation is one of the most promising heat transfer technology with advantages such as high heat transfer coefficient at low water-spray density, low temperature, and small temperature difference. The insight into two-phase flow process for steam flowing across falling-film tube bundle is crucial for the design optimization in the relevant heat transfer equipment. Comprehensive study has been done for the heat transfer performance of the horizontal-tube falling film evaporation, while the investigation focused on the two-phase flow; especially for the steam flowing across the falling-film tube bundle is less concerned. In this thesis, a systematic and in-depth investigation was carried out for the two-phase flow of steam flowing across falling-film tube bundle using the methods of experiment, theoretical analysis and numerical simulation.The large test bench for steam flowing across falling film tube bundle has been designed. The pressure drop for steam flowing across falling-film tube bundle is obtained and illustrated in the condition of the steam mass flow rate range of 0.31-1.24 kg/(m2·s), inlet pressure range of 12-31 kPa (corresponding to the saturation temperature of 50-70 ℃)and the spray density of 0.02-0.08 kg/(m·s). This thesis has quantitatively analyzed the impact of steam mass flow rate, saturation temperature, and spray density on flow resistance. In addition, the regular triangle arrangement and rotated square arrangement have been selected to analyze the effects of tube bundle layout on flow resistance. The change rule of flow resistance with increment of liquid spray has been analyzed from the views of liquid velocity, contact area and flow pattern. A new correlation of mathematical model for predicting the flow resistance during steam flowing across the horizontal-tube bundle with falling film flow is proposed. The new term of falling film Reynolds number Rer in the new correlation can reflect the impact of the falling film flow on flow resistance. Based on extensive experimental data, each relevant constant in the correlation is fitted and expressed. The new correlation is employed and programmed to calculate the temperature loss caused by the flow resistance. The influence of steam flow rate, steam temperature, spray density and tube column number on the temperature loss has also been discussed. It indicates that the effect of inlet steam temperature on temperature loss is much larger than that on flow resistance. On the basis of change rule of temperature loss caused by the flow resistance, the relationship between the structural parameters and operating parameters has been discussed.The rotated square pitch tube bundle widely applied for falling-film evaporators is chosen as the physical model. A two-dimensional incompressible laminar flow model is established. The computational model using VOF method to track the gas-liquid interface and combing void fraction to initialize the thickness of liquid film and the droplet diameter outside horizontal tube is employed to numerically simulate the steam flow process under the interaction of liquid film and liquid droplet. The distribution of velocity and pressure in the computational zone is analyzed and illustrated. The mechanism of gas-liquid two-phase flow resistance between the tubes is presented. With different spray density, the effect of liquid film thickness, the number and size of the liquid droplet on the distribution of velocity and pressure is discussed. In the computational zone, the pressure in the bottom area is higher than that in top area, and the minimum pressure distribution is in the lower right side near the droplet. It shows that the uneven distribution of pressure will cause the deformation of liquid droplet in the falling process.The two-phase flow process for steam flowing across falling-film tube bundle has been surveyed. The variation of offset for the liquid droplet or liquid column by the impact of horizontal steam flow has been summarized. A mathematical model describing the characteristics of the droplets and the liquid column offset in form of a correlation is proposed by theoretical analysis and derivation of formula. The effect of spray density, steam flow rate, temperature and other factors on the offset distance of liquid droplets or liquid column is calculated and analyzed. The gas critical velocity is defined to deeply analyze the two-phase flow characteristics for steam flowing across a horizontal tube with different spray density. It decreases with the increment of temperature, while increases with the increasing spray density. The relationship between the gas critical velocity and other parameters such as structural parameters, operating parameters is analyzed. In the perspective of force analysis, the movement mechanism for the flow of liquid falling film under the interaction of horizontal airflow is revealed.