1. Introduction
Sludge is the sediment generated in the process of industrial wastewater and urban sewage treatment in sewage treatment plants. With the rapid development of industry and expansion of urban areas, sludge treatment and disposal has become one of the most important environmental problems. Drying sludge is now widely recognized as a necessary and efficient means for processing it into a solid form that can be easily handled, stored, and recycled [
1,
2]. With higher demand for both drying performance and energy efficiency of sludge drying devices, it is necessary and also a big challenge to develop innovative drying technologies [
3].
Compared to other drying machines, the paddle dryer is one of the most widely used pieces of equipment in sludge drying, owing to its low energy consumption and cost [
4]. A paddle dryer is shown schematically in
Figure 1, composed of an outer shell, electric motors, rotating shaft, and an adequate number of paddles. The paddles are made of fan-like metal plates with a hollow space inside and are vertically welded on the surface of a rotating shaft, as shown in
Figure 2. The rotating shaft is also hollow, and several pipes are installed to connect the inner spaces between the paddles and rotating shaft. As the paddle dryer is working, sludge with high water content is transported into the space between the outer shell and paddles. Meanwhile, superheated vapor enters the interior of the rotating shaft and then flows into the interior of the paddles. The vapor and sludge do not touch each other. The heat in the high-temperature vapor is transferred from the internal to the external side of the paddles, evaporating the water and drying the sludge [
5]. In addition, the rotating shaft is always rotating at a constant speed to stir the sludge and enhance the heat transfer efficiency of the paddle dryer.
Considering the importance of the paddle dryer in the sludge dyring field, over the last few decades, the heat transfer behavior of the paddle dryer has been experimentally and numerically studied. Yamahata et al. [
3] carried out experimental research on the drying characteristics of sludge in paddle dryers. Through a batch drying experiment, they obtained the change curve of the sludge drying rate and moisture content. Through continuous drying experiments, the drying morphology of sludge at different positions in the dryer as well as a heat transfer model of the dryer were obtained. Compared to the experiment, the numerical simulation method has significant advantages in terms of cost and time savings, and it has been widely used in heat transfer performance studies of sludge dryers [
6,
7,
8]. For the heat transfer process in paddle dryer, a higher thermal resistance appeared between the external side of the paddle and the sludge. However, improving the condensation heat transfer coefficient inside the paddle also has a great impact on the drying performance of the paddle dryer because the paddle temperature can be increased. A previous study performed by Chen et al. [
9] indicated that the total drying rate of the dryer could increase to 1.5 times with a 15% increase in the dryer wall temperature. In addition, it is very difficult to enhance the heat transfer on the external side of the paddle since the sludge was directly contacted and the operation environment is harsh. However, there is a stable, clean and no-impingement environment inside the paddle for steam condensation, which provides a greater possibility to implement some advanced heat transfer technology.
Steam condensation heat transfer and the enhancement of heat transfer in general had been studied in detail by many researchers [
10,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20]. Taler and Ocłoń [
10] studied the contact thermal resistance of plate fin-and-tube heat exchangers using CFD, showing the feasibility of using CFD in heat transfer thermal resistance studies. Peng et al.’s study [
11] showed that the hybrid surface can effectively enhance the condensation heat transfer in special cases, such as those with low degrees of surface subcooling or with hysteresis at small or large contact angles. The maximum enhancement factor can approach up to about 1.18. Xie et al. [
12] shows that the maximum heat transfer enhancement ratio is 1.67 compared with a purely hydrophobic surface. Han et al. [
13] studied the effect of surface tension on steam condensation. The results show that the surface tension appears in the exponential term of nucleation rate in the form of a third power, and its small change will have an important impact on the non-equilibrium condensation process of wet steam. Orejon [
14] reported for the first time the occurrence of dropwise condensation on a completely hydrophilic wettability configuration without the assistance of a hydrophobic coating. These findings pave the way for the development of microstructures for enhanced condensation heat transfer. Cao et al. [
21] conducted a numerical simulation study on the condensation heat transfer taking place in vertical plate channels based on the volume of fluid (VOF) method, and it was found that the liquid film was the thinnest at the top of the plate and flowed downward along the plate, which was in agreement with the experimental phenomenon. Mohammed et al. [
22] also carried out a numerical simulation of the acetone condensation process in horizontal circular tubes based on the VOF model. In terms of the simulation study on the enhancement of condensation heat transfer, Liu et al. [
19] simulated the heat transfer of laminar film condensation on the surface of a vertical rectangular groove, and the heat flux results obtained were in good agreement with the experiment. At the same time, they found that the mechanism of the enchancing heat transfer on the surface of a rectangular groove was the thinning of the liquid film on the top of the groove. Based on the VOF model, Ke et al. [
17] numerically simulated the heat transfer of condensation on the surfaces of four microstructures with different wettability, obtained the dynamic characteristics and heat transfer characteristics of the liquid droplets, and analyzed the heat transfer mechanism enhanced by condensation based on the droplets’ movement. The above research showed that the CFD method can simulate the condensation process from the aspects of phase motion and phase distribution mechanism, and such an approach is suitable for the research of enhancing heat transfer by steam condensation inside the blades of a blade dryer.
The droplet shape, temperature, and heat flux at the wall can be obtained in detail using CFD simulation [
23]. However, since the blades of a paddle dryer are rotating, it may have an impact on the steam condensation inside the blades, so it is necessary to study the steam condensation phenomenon with rotating motion. In this paper, the applicability of advanced heat transfer enhancement technology and surface modification applied in a paddle dryer was preliminarily studied. A CFD method was used to simulate the condensation heat transfer on the inner surface of a local area of the paddle. The influence of surface wettability and rotation on condensation heat transfer and droplet behavior were studied. The present study can help us to better understand the heat transfer characteristics in a paddle dryer and also provide theoretical instruction on the optimal design of paddle dryers.
5. Conclusions
In this paper, the applicability of advanced heat transfer enhancement technology to a paddle dryer was discussed. A CFD method was used to simulate the condensation heat transfer on the inner surface of a dryer paddle. The influence of surface wettability and rotation on condensation heat transfer and droplet behavior were studied. The main conclusions in the present study were as follows:
The present CFD model could properly simulate the condensation process on a vertical surface. With a decrease in contact angle, the condensed liquid film on the surface became small liquid droplets and the filmwise condensation turned into dropwise condensation, which resulted in a significant increase in the heat transfer coefficient.
Owing to droplet movement caused by gravity, both the liquid film thickness in filmwise condensation and the liquid fraction in dropwise condensation gradually increased from the top to the bottom of the vertical surface.
The wall temperature and evaporation rate increased when dropwise condensation occurred. An approximately 5% increase in drying performance was expected to be provided by changing the wettability of the inner surface of the paddles.
When rotating, the wall temperature and the equivalent evaporation rate were almost unchanged with a change in rotational angular velocity under a filmwise condensation condition. However, the rotational motion might cause a decrease in wall temperature and the equivalent evaporation rate under a dropwise condensation condition. Rotation could result in a longer residence time for droplets, a higher probability of droplet coalescence, and larger droplet size on the surface.
Compared to the case of filmwise condensation with rotation, the equivalent evaporation rate only increased by 2.4%, which meant that changing surface wettability inside the paddle would not be an effective means to enhance the heat transfer and drying efficiency of a rotating paddle dryer. Techniques to enhance the heat transfer between sludge and surface or increase the heat transfer area would be better measures to improve the drying efficiency of a rotational paddle dryer.