Luyuan Gong

Thermal analysis of heat transfer performance in a horizontal tube bundle

The thermal analysis is demonstrated for the case where steam is condensing inside a horizontal tube bundle, while simultaneously thin water films are evaporating outside the tubes generating vapor that flows across the tubes. Three experiments are carried out each coming up with a valid model to predict the impact of the corresponding parameters on the heat transfer performance or flow characteristics. Then, a collective two-dimensional model is established according to the models from those experiments to simulate the thermal and hydrodynamic performance of the evaporator. This model consists of three parts corresponding to the three models obtained from the experiments mentioned above: the water film evaporation model, the inside-tube steam model, and the intertube vapor model. The operating conditions include the water mass rate, saturation temperatures, and overall temperature difference. Calculations are carried out for water mass rate between 0.03 and 0.09 kg/m s, inlet steam temperature between 53 and 73˚C, and overall temperature difference between 1.5 and 4˚C. Results of the analysis show that the overall heat transfer coefficient and vapor temperatures have uneven distributions within the tube bundle. The variation of vapor temperature affects the distribution of heat transfer rate and the steam inlet velocity. The utilizations of the initial temperature difference with the change of tube bundle column numbers are also analyzed. It shows that the more column numbers the tube bundle includes, the more heat transfer area will lose a certain amount of initial temperature difference.

Heat transfer characteristics of horizontal tube falling film evaporation for desalination

AbstractAn experimental study on heat transfer characteristics of falling film evaporation outside horizontal tube was carried out at low evaporation temperature. The Al-brass tubes with 19 mm outer diameter and 1,600 mm effective length were adopted and arranged vertically in the evaporator. The electric heater was fixed tightly in the test tube for generating uniform heat flux. And thermocouples were installed in the surface of the test tube to measure surface temperatures. The experimental fluids are water and seawater with salinity of 1.5, 3, and 4.5%. The variations of heat transfer coefficient h were studied in detail with saturation temperament ranging from 50 to 70°C, the spray density Γ ranging between 0.017 and 0.087 kg m−1 s−1, and the heat flux q varying from 7.75 to 12.30 kW m−2. The results show that the average heat transfer coefficients of both water and seawater increase with the spray density until about 0.052 kg m−1 s−1. The heat transfer coefficient of fresh water increases with evapor...

Critical dimensions of a large-scale falling film evaporator based on temperature difference loss

Falling film evaporator applied in multi-effect evaporation system is operated under relatively small temperature difference. The thermal performance of the evaporator is sensitive to the temperature drop. In this paper, the thermal performance of the falling film evaporator was simulated based on a validated distributed parameter model. The temperature difference loss along the tube row direction and column directions were obtained in the evaporator of different geometrical dimensions. Considering the high sensitivity of the thermal performance to the thermodynamic loss, the maximum allowable temperature difference loss was defined as 20% of the apparent temperature difference of the evaporator. The concept of critical dimension of the tube bundle was proposed to prevent the local temperature difference loss from exceeding the maximum allowable temperature difference loss. The variation of the critical dimensions of the tube bundle was analyzed under different operating conditions. Results shown that the critical tube row number increased with the increment of the inlet seawater spray density but decreased with the increment of the inlet seawater salinity and the evaporation temperature. With the increment of the apparent temperature difference, the critical column number firstly exhibited an increasing trend, then reached a plateau; the critical tube column number decreased with the increment of the inlet seawater spray density and the apparent temperature difference but increased with the increment of the seawater salinity and the evaporation temperature.Falling film evaporator applied in multi-effect evaporation system is operated under relatively small temperature difference. The thermal performance of the evaporator is sensitive to the temperature drop. In this paper, the thermal performance of the falling film evaporator was simulated based on a validated distributed parameter model. The temperature difference loss along the tube row direction and column directions were obtained in the evaporator of different geometrical dimensions. Considering the high sensitivity of the thermal performance to the thermodynamic loss, the maximum allowable temperature difference loss was defined as 20% of the apparent temperature difference of the evaporator. The concept of critical dimension of the tube bundle was proposed to prevent the local temperature difference loss from exceeding the maximum allowable temperature difference loss. The variation of the critical dimensions of the tube bundle was analyzed under different operating conditions. Results shown that the...

Experimental Study of Heat Transfer Characteristics for Horizontal-Tube Falling Film Evaporation

The horizontal-tube falling film evaporation is a widely adopted technique in multiple-effect distillation (MED) desalination plant due to the higher heat transfer coefficient under quite small temperature differences. In the present study, an experimental platform for horizontal-tube falling film evaporation was set up to measure its heat transfer characteristics. Results indicate that heat transfer coefficient (h) for both fresh water and seawater are almost independent with heat flux. The h increases firstly and then decreases with growth of Re. Along the tube circumference, the h increases after decreasing. In addition, the distribution of h for fresh water and seawater at the different evaporation temperatures and Reynolds number (Re) are also provided.Copyright