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Impact of Flow Velocity on Surface Particulate Fouling - Theoretical Approach

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Impact of Flow Velocity on Surface Particulate Fouling - Theoretical Approach Journal of American Science 2012;8(9) http://www.jofamericanscience.org Impact of Flow velocity on Surface Particulate Fouling - Theoretical Approach Mostafa M. Awad Mech. Power Eng. Dept., Faculty of Engineering, Mansoura University, Egypt [email protected] Abstract: The objective of this research is to study the effect of flow velocity on surface fouling. A new theoretical approach showing the effect of flow velocity on the particulate fouling has been developed. This approach is based on the basic fouling deposition and removal processes. The present results show that, the flow velocity has a strong effect on both the fouling rate and the asymptotic fouling factor; where the flow velocity affects both the deposition and removal processes. Increasing flow velocity results in decreasing both of the fouling rate and asymptotic values. Comparing the obtained theoretical results with available experimental ones showed good agreement between them. The developed model can be used as a very useful tool in the design and operation of the heat transfer equipment by controlling the parameters affecting fouling processes. [Mostafa M. Awad. Impact of Flow velocity on Surface Particulate Fouling - Theoretical Approach. J Am Sci 2012;8(9):442-449]. (ISSN: 1545-1003). http://www.jofamericanscience.org. 62 Keywords: Surface fouling, flow velocity, particle sticking, mass transfer. 1. Introduction increasing the flow velocity. They also found that the Fouling of heat transfer surface is defined as the asymptotic fouling factor is very sensitive to the flow accumulation of unwanted material on the heat transfer velocity especially at low flow velocities where this surface. This accumulation deteriorates the ability of sensitivity decreases with increasing flow velocity. the surface to transfer heat besides to the increase of There are many parameters which affecting the the pressure drop through the heat transfer apparatus. fouling process, and according to many investigators Therefore fouling tends to decrease the heat exchangers the most important ones are the flow velocity and the performance and can lead to operation failure. surface temperature. Many efforts have been made by Transport and capture of particles on heat transfer numerous researchers to study the affecting parameters surfaces are functions of the size of particles, the on the fouling phenomenon in heat exchangers. They chemical and physical properties of the transported have focused on various methods to predict fouling particles and operating conditions. Fouling can lead to behavior more accurately which in turn helps prevent increase capital and maintenance costs and major fouling problems. Lee and Cho [8] experimentally production and energy losses in many energy-intensive studied the velocity effect on electronic anti fouling industries. Many investigators have studied the fouling technology to mitigate mineral fouling in enhanced- phenomenon theoretically and experimentally. Kern tube heat exchanger. They concluded that the and Seaton [1, 2] and Kern [3], made the groundwork characteristics of the fouling mechanism are very of the fouling studies. Recently, Yang et al., [4] sensitive to fluid velocity in a heat exchanger, surface constructed a model for the fouling induction period. geometry, particle concentration, bulk temperature, and They found that the shorter induction periods are tube material. Abd-Elhady et al., [9] concluded that as dealing with higher surface temperature. Nesta and the flow speed in the heat exchanger increases, the Bennett [5] introduced a new design of heat thickness and the surface area of the fouling layer exchangers. They concluded that minimize wall deposited over the heat exchanger tube are reduced. temperature and maximize the flow velocity tends to There is a limiting flow speed above which fouling is minimize fouling; they also found that the heat avoided. This limiting speed is related to the critical exchanger material has a pronounced effect on fouling flow velocity required to roll a particle resting on a flat particularly when biological fouling is a concern. surface. Li [10] concluded that the asymptotic fouling Subbarao et al., [6] studied particulate fouling of glass factor increases as the particle concentration increases particles under high temperatures. They concluded that and Reynolds number decreases. the fouling layer formation is strongly dependent on the During the past years, many achievements have gas phase temperature and gas phase velocity and once been obtained on deposition and removal models to the deposit has formed, increasing gas velocity hasn't predict particulate fouling on heating surfaces under any effect on removal of particles from the deposited inertial impaction. Thornton and Ning [11], and layer. Mostafa et al., [7], experimentally studied the Konstandopoulos [12] studied deposition criteria for effect of flow velocity on both of the fouling resistance particle inertial collision with the tube wall. Feng et al., and the asymptotic values. They found that the fouling [13] researched the effect of influence parameters on resistance and the asymptotic values are decreased by particle-wall inertial collision deposition. Abd-Elhady http://www.americanscience.org 442 [email protected] Journal of American Science 2012;8(9) http://www.jofamericanscience.org et al., [14] proposed that inertial impact speed is the process and removal process. Therefore the fouling rate main parameter of collision deposition for particles is given by with a powdery layer. Van et al., [15] developed a two- Accumulation rate = deposition rate – removal rate, or body collision deposition mechanism for particle dm f impaction with a powdery layer. Huang et al., [16] d r (1) developed a numerical model for the deposition rate d using macro probability statistics. 2.1. Deposition Rate (φd) Fouling removal is another important process of In the present model, only the colloidal particles the fouling growth. Rodriguez et al., [17] reported that i.e. dp < 50 µm will be considered, where for large fouling removal is mostly depended upon gas flow particles, dp ≥ 50 µm, there is another type of fouling velocity. Abd-Elhady et al., [18] found that fouling mechanism which is known as sedimentation fouling removal is related to the impact speed or the contact (i.e. deposition occurs under the effect of gravity). time of the incident particles. Polley et al., [19] Sedimentation fouling can be often prevented in heat concluded that fouling removal rate is proportional to transfer equipment by pre-filtration. the 0.8 power of the Reynolds number. Previous The increasing rate of the fouling layer thickness (xf) is research in fouling mechanism has respectively focused given by on the process of particulate deposition or fouling dx f SN removal. (2) d A Fouling growth on heating surfaces is d f s determined by the difference between the deposition Where the stickability (S) is given by the Arrhenius and removal of particles on and from the fouling layer. equation as Particulate fouling is mainly influenced by S kexp E R T 1 (3) physicochemical properties and transport mechanisms s g s of suspended particles, such as particulate size, Where ks is constant, known as sticking coefficient transport forces arising from the gradients of density, As shown in Fig. (1), the particle flow rate toward the temperature and velocity in the flow field. An surface (N) is represented as; integrated fouling model was developed by Pan et al., NCM [20] by considering the combined suspended particles (4) deposition and the fouling removal processes. k1 () Cs C b M Some examples of using FLUENT code for Where k1 is constant, for steady flow conditions and predicting fouling phenomena occurring at heating constant fluid properties, the constant │k1│ = │kD│ surfaces can be found in publications [21, 22]. where kD is the mass transfer coefficient. Particulate fouling of convective heat-transfer surfaces The particles concentration at the surface (Cs) is given is usually assessed by empirical correlations. by Nevertheless, constant progress in numerical non stick particles calculation methods allows for predicting fouling Cs phenomena occurring at heating surfaces, Wacławiak flowrate, M (5) and Kalisz [23]. Much more research needs to be NS(1 ) carried out on the effect of velocity because its effect is much more complex than that of surface temperature, M Yang et al., [4]. From Eqns. (4) & (5), the particles mass flux toward Up to now, there is a lack in literatures which the surface is given as theoretically investigates the effect of flow velocity on the surface fouling. This lack of theoretical studies was NS(1 ) the motivation of the present work to improve our N k1 M Cb understanding of this problem. M (6) k C M 2. Theoretical Approach 1 b From the previous theoretical studies [1-3, 25- 1k1 (1 S ) 38], it is known that the particulate fouling process is consisting of two sub-processes which are deposition From Eqns. (2) & (6); http://www.americanscience.org 443 [email protected] Journal of American Science 2012;8(9) http://www.jofamericanscience.org d() x SN f Concentration d A distribution d f s N C . b k C S M M 1 b fA s 1 k1 (1 S ) but Cs xf M u F Therefore Fouling layer Heat transfer surface dx f k C S uF Fig. (1) The deposition process 1 b d A1 k 1 S d f s 1 And the deposition rate is given as dx f d f d d (7) k C S uF 1 b As 1 k1 1 S Velocity distribution 2.2. Removal Rate (φ ) u r . As shown in Figs. (2, 3) the decreasing rate in the M fouling layer thickness due to removal process dx/ d is f r proportional to the shear stress (τ), the fouling layer τ thickness (x ), and to the inverse of deposit strength (ψ), therefore f xf dx f 1 x Fouling layer Heat transfer surface d f r Fig. (2) The removal process dx f 1 k x d 2 f r Where k2 is constant and the deposit strength (ψ) is represented by the weaker force of the adhesion or cohesion forces.
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