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

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

Low-temperature multi-effect distillation that employs horizontal tube falling film evaporation technology is one of the most promising desalination methods with advantage of high heat transfer coefficient at low temperature, low water- spray density and small heat transfer temperature difference. Falling film evaporators have been widely used in absorb- type chillers, desalination devices and other areas. In a horizontal tube falling film evaporator, water and steam were flowed in different directions. The vertical falling film flow was driven by gravity, whereas the steam flow across the tube bundle was driven by pressure difference. Hence, the pressure drop and the consequent temperature drop of the steam are not only caused by the tube bundle but also by the falling water. In this paper, the effects of parameters such as spray density were determined. Experimental measurement of pressure drop data during the saturated steam flowing across aluminum brass tube bundles was carried out. The pressure drop coefficient was obtained from the experimental data. The rotated square-arranged tube bundle was chosen as a physical model to calculate the steam temperature difference drop on falling film evaporation. Based on the experimental data, the fitting pressure drop coefficient was employed in this calculation. The numerical codes were developed to simulate the steady-state performance of temperature difference drop in horizontal tube falling film evaporation. The iterative procedure was required based on the initial values with a close chain of equations and parameters. The solution was obtained through a step-by-step method. This quantitative temperature difference drops caused by the pressure drop can be calculated by the solution. The influences of spray density, saturated temperature, and tube column number on vapor temperature difference drop were analyzed respectively. The temperature difference drop caused by the pressure drop will decrease the temperature difference. The effect of temperature difference drop can be analyzed in two aspects. On the one hand, the temperature difference drop directly reduces the difference in heat transfer temperature, and thus, decreases heat transfer rate; on the other hand, the temperature difference drop generally increases the heat transfer coefficient under a certain heat transfer flux. When temperature difference drop is considered in a calculation, the heat transfer coefficient can be derived more accurately. Moreover, an accurate calculation of the temperature difference drop can provide a more detailed analysis on the influence factors of the heat transfer coefficient. At a certain tube arrangement and the same steam flow flux, the results showed that the temperature difference drop increases with the increase of spray density. It can be showed that the temperature difference drop at 50°C is 3.5 times of that at 70°C. However, the temperature difference drop decreases with the increase of saturated temperature. The temperature difference drop at spray density of 0.08 kg/(m s) is almost double of that at spray density of 0.02 kg/(m s). The calculations also show that the temperature difference drop is in parabolic rise with the increase of tube column number. When the temperature difference drop is set at lower than 0.3°C, the maximum tube column ( N max) can be obtained. The results showed that the N max increases with increasing saturated temperature, but decreases with increasing spray density. The N max can be employed to determine the heat transfer area for optimization design of the heat exchanger.

Heat transfer characteristics of horizontal tube falling film evaporation for desalination

Wetting behaviour of different tube materials and its influence on scale formation in multiple-effect distillers

The overall heat transfer process in a horizontal tube falling film evaporator is mainly influenced by the falling film evaporation outside horizontal tube due to the average heat transfer coefficient which is about 50% of that of the condensation inside tube. A series of experimental studies were conducted to investigate the heat transfer coefficients of the falling film evaporation outside the horizontal tube with parameters such as the spray density, the evaporation temperature, the salinity, and the tube spacing. Experiments were conducted by using Al-brass tubes with 19 mm outer diameter and 1600 mm length. The horizontal tubes are arranged vertically in the evaporator. The test tube is heated by an internal electric heater with uniform heat flux. Temperatures of the test tube surface and saturated vapor measured by thermocouples are used to calculate the heat transfer coefficients. The seawater with salinity of 1.5%, 3.0%, and 4.5% was used as experimental fluid. The spray density varied between 0.017 and 0.087 kg/(m s), and the evaporation temperature was controlled in the range of 50–70 °C. Results show that the average heat transfer coefficients of water under different salinities increase obviously with the spray density until a certain point. The average heat transfer coefficients of seawater decrease slightly with the evaporation temperature, decrease with the salinity, increase with the tube spacing, and are almost independent of the heat flux. In addition, the comparisons with 25.4 mm outer diameter tube and the circumferential distribution of local heat transfer coefficient are presented in this study.

This paper discusses about the effect of feeder height and heat flux on the heat transfer characteristics of horizontal tube falling film evaporation in the thermal regimes. In order to investigate this, a two- dimensional CFD model was developed to perform simulation and results were compared and validated with published data available in the literature. Heat transfer co-efficients in the thermal regimes were determined from the CFD simulation and the results were recorded, analyzed and validated with the mathematical models available in the literature. The novelty of the current study is to predict the commencement of the fully developed thermal region over the tube from the simulation model under varying feeder height and heat flux. An effort was also made to measure the liquid film thickness around the tube from the CFD model in the thermal regimes. It is observed that angle of thermally developing region contracts and fully developed thermal region extends with the increase of the feeder height and heat flux. It is observed from the study that increase of heat flux by 10 kW/m2 resulted in increase of heat transfer co-efficient value by 10–12% average in thermally developing region and 12–15% average in fully developed region. Thinnest liquid film thickness observed between 85 and 127°angle. Shifting of thinnest region of liquid film upward from the mid tube with the increase of the feeder height and heat flux is noted.

Effect of flame spray coating on falling film evaporation for multi effect distillation system

Review on liquid film flow and heat transfer characteristics outside horizontal tube falling film evaporator: Cfd numerical simulation

Microscopic mechanisms of heat transfer in horizontal-tube falling film evaporation

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

Flow mechanisms, heat transfer and discharge coefficient characteristics for a representative part of a turbine casing cooling system, consisting of an array of 20 impinging jets, were numerically investigated. The study focused on the influence of the jet Mach number while maintaining the Reynolds number constant at Re = 7,500. Therefore, the orifice bore diameter or the fluid density had to be varied. The objectives of the current CFD simulations have not been adressed before in literature, not only because heat transfer characteristics and pressure drop are given for impingement jet Mach numbers up to 0.72 at a constant relatively low Reynolds number, but also because fundamental understanding of physical phenomena of the flow in the cylindrical plenum and in the small sharp-edged orifices at the bottom side of the tube is provided. Increasing the Mach number by simultaneously reducing the orifice diameters led to slightly decreasing Nusselt numbers, with average deviations of the order of 14%. However, the heat transfer coefficient increased considerably with increasing Mach number. On the contrary, the variation of the Mach number by varying the density showed only a slight influence on the heat transfer coefficient. The predicted discharge coefficients increased significantly by augmenting the Mach number.

Evaporation heat transfer and pressure drop characteristics of carbon dioxide were investigated in a multi-channel micro tube. The aluminum tube has 3 square channels with a hydraulic diameter of 2mm, a wall thickness of 1.5mm, and a length of 5m. The tube was heated directly by electric current. Experiments were conducted at heat fluxes ranging 4–16 kW/m2, mass fluxes from 150 to 750 kg/m2s, evaporative temperature from 0 to 10°C, and qualities from 0 to superheated state. The heat transfer coefficient measured was in the range of 6–15kW/m2K, and the pressure drop was 3–23kPa/m. For the qualities lower than 0.5, the heat transfer coefficient was found to increase with the quality, which is assumed to be the effect of convective boiling. For the qualities higher than 0.6, sudden drop in heat transfer coefficients was sometimes observed due to local dry-out. It was found that dry-out occurred at lower quality if mass flux was smaller. The average heat transfer coefficient was found to increase with increasing heat flux, mass flux, and evaporation temperature, of which the effect of heat flux was the greatest. At given experimental conditions the pressure drop increased almost linearly with increasing quality. The total pressure drop was found to increase with increasing heat flux, mass flux, and evaporation temperature, of which the effect of mass flux was the greatest. From the experimental results simple correlations for heat transfer coefficients and pressure drop were developed.

Experiments were conducted to investigate the heat transfer and flow characteristics of the vertical upward smooth and rifled tubes from subcritical to supercritical pressure. The distributions of wall temperature and heat transfer coefficient (HTC) were obtained, and the HTC correlations and friction resistance coefficient correlations were fitted with experimental data. In addition, the influences of heat flux and type of tube on heat transfer performance were analyzed. The research shows that heat flux has different influences on the heat transfer characteristics under different pressures. The increase in heat flux improves the heat transfer characteristics in the nucleate boiling region, yet it leads to the advance in heat transfer deterioration. However, for supercritical water, the increase in heat flux reduces the heat transfer ability. In addition, using the rifled tube not only improves the heat transfer performance, but also inhibits the occurrence of heat transfer deterioration. The fitted correlations have great predictive ability for the heat transfer coefficient and friction resistance coefficient, and the average relative fitting errors are limited to 20%.

The enhanced heat transfer performance of falling film evaporation during desalination through altering smooth tubes by corrugated tubes was investigated. An experimental research on corrugated and smooth tubes was carried out using a horizontal-tube falling-film evaporation heat transfer experimental apparatus. The changes in heat transfer coefficient with the process parameters (sprinkling density, heat flux, heat transfer temperature difference, and evaporation temperature) were obtained according to the experimental data. The influence of tube type on liquid-film distribution outside the tube was also examined by numerical simulation. Results showed that within a certain scope increasing of any one variable of sprinkling density, heat flux, and evaporation temperature has a positive impact on the heat transfer coefficient. When the sprinkling density exceeded 0.178 kg/(m·s), the heat transfer coefficient tended to be stable. Compared with the simulation results, the liquid film coverage and thickness outside the tube significantly influenced heat transfer efficiency. When the heat flux exceeded 130kW/m2, the growth of the heat transfer coefficient significantly slowed down. Under the same experimental conditions, the heat transfer coefficient of corrugated tubes increased by 30% compared with that of smooth tubes. In addition, the content of non-condensable gas within the seawater affected the heat transfer coefficient significantly.

Experimental study of heat transfer and scale formation of spiral grooved tube in the falling film distilled desalination