Xingsen Mu

Heat transfer performance and bundle-depth effect in horizontal-tube falling film evaporators

Abstract A set of experimental facilities were set up to measure overall heat transfer coefficient of horizontal-tube falling film evaporator with square-pitch bundle. Effect of spray density, saturation temperature, total temperature difference, and inlet steam velocity on the overall heat transfer coefficient K is studied. The tubes are made of HAL77-2A aluminum brass with an outer diameter of 25.4 mm. Fluids inside and outside the tubes are steam and fresh water respectively. The results indicate that growth of spray density and saturation temperature helps to increase the K. The K could also be increased by reducing the total temperature difference. However, the impact of the inlet steam velocity on the K is less significant. Furthermore tube bundle-depth effect and space distribution of local overall heat transfer coefficient in the evaporator are also presented. Based on this investigation, basic engineering design information will be provided to establish the governing parameters for horizontal-tub...

Experimental study of falling film evaporation heat transfer coefficient on horizontal tube

Abstract The horizontal-tube falling film evaporation is a widely adopted technique in multiple-effect distillation desalination plant. It has a high heat transfer coefficient under quite small temperature difference. In this paper, an experimental equipment for horizontal-tube falling film evaporation was set up. Experiments were carried out to show how the heat transfer coefficient is affected by different parameters including heat flux, circumference direction of tubes, spray density, evaporation temperature, and experimental fluid. Results indicate that the heat transfer coefficient decreases after a little increase with growth of spray density. The heat transfer coefficient decreases along the tube circumference, but at the bottom of the tube, it shows increasing trend. In addition, a simple comparison between seawater and fresh water in heat transfer coefficient is also provided.

Flow and heat transfer during a single drop impact on a liquid film

ABSTRACT A two-dimensional incompressible laminar computational model was established to analyze flow and heat transfer characteristics during a single liquid drop impinging onto a liquid film, with an underneath surface of relatively low temperature. Using the coupled level set and volume of fluid method, the gas–liquid interface at different time sequences can be obtained clearly. Concerning the heat transfer process, three different factors including impact velocity, film thickness, and drop diameter were discussed. Results indicate that liquid inside the film can be classified as three zones: the impact zone, the transition zone, and the static zone, specifically according to different heat flux. Average surface heat flux can be increased by increasing impact velocity, while effects of film thickness and drop diameter are minor. Corresponding mechanisms were interpreted as well. For heat flux distribution in the impact and transition zones, both film thickness and drop diameter influence the distribution greatly. With an increment in film thickness and drop diameter, heat flux in the impact zone decreases, while heat flux in the transition zone appears to be an opposite trend. Also in the transition zone, the fluctuation amplitude of the heat flux rises as the two factors are reduced.

Experimental investigation on heat transfer in horizontal-tube falling-film evaporator

AbstractAn experimental platform for horizontal-tube falling-film evaporation was set up to measure heat transfer characteristics. Experiments were carried out to show how the heat transfer coefficient (HTC) was affected by different parameters including spray Reynolds number (Re), saturation temperature, salinity, and tube arrangement. The results revealed that the HTC increased first and then decreased with growth of Re, and the HTC of seawater decreased with increasing saturation temperature. The results also showed that the HTC of rotated square pitch was higher than triangular pitch, rotated triangular pitch, and square pitch, but the heat transfer capacity per unit volume of triangular pitch was the highest. Meanwhile, the HTC decreased during the increase of salinity.

Numerical investigation on characteristics of falling film in horizontal-tube falling film evaporator

AbstractHorizontal-tube falling film technology is widely used in multi-effect desalination plant. The film characteristics of falling water film outside the tubes in horizontal-tube falling film evaporator were simulated. The numerical result was compared with the experimental result and they were proven with good agreement. The film thickness distribution at different Reynolds number and different circular angle had been investigated. Also, the effects of liquid flow rate and tube diameter on the film thickness distribution were discussed in detail. Numerical simulation result shows that, with the increase in Reynolds number, the falling film thickness at a certain circumferential angle increases with almost linear growth rate, and the thinnest film at the surface of the tube appears at the angle of about 120°. The result also indicates that the local dry out spot on the surface of the tube would occur when the fluid flow rate decreases to a certain value. In addition, the film thickness decreases with ...

Research for the adjustable performance of the thermal vapor compressor in the MED–TVC system

AbstractAs the expansion in the desalination industry, multiple-effect distillation (MED) with thermal vapour compression (TVC) system becomes more attractive than other thermal systems due to effectiveness, easier operation and maintenance, and good economic characteristics and is expected to take a considerable portion in the desalination field in the near future. The TVC is an essential part that governs the overall process in the MED–TVC system. The flow and heat transfer in the TVC is very complex due to the strong compressibility and supersonic turbulent flow of the stream. In this paper, based on gas-dynamic theory, a kind of semi-empirical calculation method of ejector, including calculation of the design point and variable operating condition of ejector, has been presented. The model gives the variation of the performance of the ejector as a function of the discharge pressure for different area values of throat and provides a foundation for the research for the performance of the adjustable eject...

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

Heat Transfer Coefficient in Falling Film Evaporators With Different Tube Arrangements

A set of experimental facilities were set up to measure overall heat transfer coefficient of horizontal-tube falling film evaporators with triangular, rotated square and square-pitch bundles. Effect of spray density, saturation temperature, total temperature difference and inlet steam velocity on the overall heat transfer coefficient K is studied respectively. The tubes are made of HAL77-2A aluminium brass with an outer diameter of 25.4 mm. Fluids inside and outside the tubes are steam and fresh water respectively. The results indicate that growth of spray density and saturation temperature helps to increase the K. The K could also be increased by reducing the total temperature difference. However, the impact of the inlet steam velocity on the K is less significant. The K in the evaporator with rotated square-pitch arrangement is supreme. Furthermore, space distribution of local overall heat transfer coefficient K in the evaporators is also discussed. Based on this investigation, basic engineering design information will be provided to establish the governing parameters for horizontal-tube falling film evaporator in the field of seawater desalination.Copyright