Real-time fouling characteristics of a typical heat exchanger used in the waste heat recovery systems
Introduction
It is well known that many industrial boilers have higher exhaust temperature, which caused a lot of useful energy is wasted from the exhaust. To reduce the wasted energy from this kind of system, adding a waste heat recovery system after the boiler is one of the solution.
One of the main component of the waste heat recovery system is the waste heat recovery heat exchanger. Due to its working environment, such as in the tail of large boiler, fly ash fouling on heat exchangers is a major problem, which can lead to significant efficiency deterioration [1], [2], or the fouling may also lead to failure of the systems with serious economic consequence.
Much research have been carried out on the fouling behaviours of heat exchangers. The research situation of fouling on the heat exchanger surface can be divided into three areas: the experimental research, the empirical equation prediction and numerical simulation research. The experimental method is very important for the study of fouling on heat exchanger, but it is difficult to describe or understand the particle motion and deposition mechanism in the fouling process. So studying the fouling behaviours with numerical methods has drawn much attention in recent years. The Lagrangian method is mainly used to simulate the particle transport in the numerical research. The impact behaviours of particles hitting the heat transfer surface is important to determine the particle deposition rate and it has been the focus of a great deal of research. It was found that, the inertial impact is the dominant transport mechanism for the large particles, while for the smaller particles, especially for the sub-micron particles, the particle motion is mainly controlled by the eddy transport and thermophoresis force [3]. It is accepted that there is a critical sticking velocity, and the particle will stick when the incident velocity is less than the critical velocity and will rebound if when the incident velocity is larger [4], [5]. Bouris and Bergeles [6] considered the fouling layer as a solid surface and used the critical velocity and normal coefficient of restitution to describe the deposition and rebound behaviour. Han and He et al. [7] investigated the particle deposition characteristics on the tube bundle heat exchangers and the effects of particle diameters, flow velocities, tube shapes and arrangements were also studied. Konstandopoulos [8] found a new criterion of critical impact angle, when the incident angle of particle is greater than the critical angle, the particle will not deposit, even if the incident velocity is less than the critical sticking velocity. Kern and Seaton [9] first point out that the fouling process was the aggregated results of the deposition and removal process and proposed one of the earliest models of fouling. So it is important to consider both the particle deposition and removal mechanisms in the simulation. Soltani and Ahmadi [10], [11] theoretically analysed the particle deposition and removal mechanisms in turbulent flow and determined the removal conditions by analyse the forces and moments balance on the deposited particles. Pan et al. [12] studied the fouling behaviours of the economizer tubes considering the deposition, rebound and removal criteria in the simulation.
Due to the limitation of simulation, it is unrealistic to simulate the whole real fouling time period, which usually needs hours or days. It is necessary to make an extrapolation by some methods. García Pérez et al. [13] predicted the deposition shapes in a classical boiler bank of a Kraft Recovery Boiler with a dynamic mesh method, and the results were extrapolated to the real fouling time by multiplying an extrapolating factor. Tong and Li et al. [14] simulated the fouling process on tubes by coupling the Lattice Boltzmann Method (LBM) and the Finite Volume Method (FVM), and a time ratio was proposed to convert the simulation time to the real time.
H-type finned tubes have been widely used in waste heat recovery systems in recent years as a new type of heat exchanger, because of their excellent anti-wear and anti-fouling performance. Jin et al. [15] numerically studied the heat transfer and pressure drop characteristics of the H-type finned tube bank with 10 rows and correlations of Nu and Eu for the 10-row tube bundles were also obtained. Chen et al. [16] experimentally investigated the heat transfer and pressure drop performance of the H-type finned tube banks and the effects of geometric parameters and Reynolds number were also examined. Han and He et al. [17], [18] numerically studied the sulfuric acid deposition characteristics of H-type finned tube banks. The effects of geometric parameters and Reynolds number were analysed and a correlation of Sh number of sulfuric acid vs fin geometries for the tube bundles was obtained. However, the heat exchangers mainly operated in dusty environment, the fouling on heat exchangers may also be a major problem, and only a few studies have investigated the fouling behaviour of heat exchangers by experimental research. As mentioned above, it is difficult for experimental research to describe the deposition mechanism or predict the fouling results in the fouling process.
In our previous work, the heat transfer and fly ash deposition characteristics (without removal process) of two kinds of H-type finned tubes were investigated [19]. While in this paper, an integrated fouling model which consists of a deposition process and a removal process is first developed. Meanwhile, a fouling time ratio is also proposed to enlarge the simulation time and results to the real time scale, by considering the volume difference between the simulated deposit layer and the real deposited particles and the concentration difference of injected particles between the simulation and real situation. Then the real-time fouling characteristics of H-type finned tube heat exchanger are investigated and the effects of inlet velocity, particle diameter on the fouling resistance are studied. The comparison with experimental results is also conducted. The results of this study could be beneficial to the design of anti-fouling heat exchangers.
Section snippets
Model description
In the following sections, the geometry model of the H-type finned tubes will be first provided, then the governing equations, fouling models and numerical methods will be presented. Some simplifications and assumptions are made in present simulation, which are described as follows:
- (1)
The flow in the computational domain is considered as a three-dimensional, viscous and incompressible turbulent flow.
- (2)
The tube surface is specified with constant temperature and the wall thickness of tubes is ignored
Grid generation and independence validation
The pre-processor software GAMBIT is used to generate the computational meshes. Enhanced wall treatment is used to simulate the near-wall turbulence and the mesh sizes in the inlet and outlet zones are properly reduced in order to decrease computing amount. Different size of grid systems have been employed to ensure the results are independent of the grid number. When the grid number increases further, changes in and are less than 5% and then the numerical results are considered to be
Fluid flow characteristics
The flow field directly affects the particle motion, so it is important to solve the flow field correctly. Fig. 6(a) shows the velocity distribution of H-type finned tubes with . The lines and arrows in Fig. 6(a) represent the path lines and flow directions, and the color of the lines represent the local velocity magnitude. It can be seen that, the fluid flows around the finned tubes and form vortices shedding alternately behind the tubes with low velocity. The motion of particles are
Conclusions
In this paper, numerical simulations were carried out on the fouling characteristics of a typical waste heat recovery heat exchanger. An integrated fouling model was first developed, and the unsteady RNG turbulence model was used to calculate the fluid flow characteristics, and the discrete phase model (DPM) was employed to trace the particle motion during the fouling process. A fouling time ratio was also proposed to enlarge the simulation time and results to the real time scale. The flow
Acknowledgement
The study is supported by National Key Basic Research Program of China (973 Program) (2013CB228304).
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