Asymptotic Fouling Resistance Research Articles

Effects of Temperature on Flow Fouling of Smooth and Nonwetting Surfaces

Mineral fouling (scaling) of heat transfer surfaces is a pervasive problem in heat exchangers, chemical reactors, and other equipment in energy, environmental, and process industrial applications. Many of the applications involve dynamic flow of fluids with the impurity minerals and, furthermore, operate at elevated temperature. Strategies to mitigate fouling under these conditions are of much value in industrial applications. This paper presents a comparative study of temperature-dependent mineral fouling deposition on smooth surfaces and nonwetting superhydrophobic and lubricant-infused surfaces under dynamic flow conditions. The surfaces are represented in a unified manner using the viscosity ratio of the infused material within the porous asperities on a surface to that of the flowing fluid such that the spectrum of surfaces from superhydrophobic to smooth is captured by the range of viscosity ratios from 0 to ∞. Using a forced convection experimental setup, deposition of calcium sulfate on the surfaces is quantified in terms of asymptotic fouling resistance over a range of temperature, Reynolds number, and mineral foulant supersaturation. Through a systematic set of accelerated fouling experiments, an empirical relationship for the asymptotic fouling resistance is developed in terms of Reynolds number, foulant concentration, temperature, and surface type. The empirical model is validated with a comprehensive set of experimental data from this study as well as from the literature. Optimum nonwetting surface designs for minimizing fouling resistance compared to conventional smooth surfaces are developed as a function of temperature. The results of the study offer insight into the temperature-dependent fouling of surfaces under flow conditions and a rational design of fouling-resistant nonwetting surfaces that can be readily translated to practice.

Influence of hydrophobic coatings on fouling mechanism of combined fouling in enhanced tubes

The rate and degree of fouling in internally helical enhanced tubes increase significantly with the enhancement of heat transfer. Surface modification technology with lower cost and simpler operation to mitigate the rapid deterioration of heat transfer performance due to fouling has aroused wide attention of researchers. However, surface modification technology has not been well integrated with fouling growth mechanism in existing research. In this study, three types of coatings were coated on the inner surface of enhanced tubes with the same structure to obtain different surface characteristics. Compared with the original surface, the hydrophobicity of coated surfaces was improved, and their surface energy decreased by 18.48 %–45.32 %. Moreover, a combined fouling test was performed at 0.9 m/s for more than 1200 h. The results of fouling thermal resistance showed that the anti-fouling effect of Coating B was the most significant, while Coating C promoted the fouling growth. The correlations between surface characteristics and two important parameters (including sticking probability and deposit bond strength) determining the fouling process were studied using the Kern-Seaton model. As indicated by the data, the sticking probability in the combined fouling ranged from 3.9841 × 10-2 to 9.1679 × 10-2, and the deposit bond strength was between 6.0720 × 106N·s/m2 and 7.1364 × 106N·s/m2. Furthermore, the sticking probability was positively correlated with the surface energy, while the deposit bond strength was found to be opposite. The above result suggested that for the enhanced tubes with lower surface energy, the foulants in the fluid will be more difficult to adhere to the heat transfer surface after coming into contact with the surface; but once adhering to the heat transfer surface, they will not be easily removed. Besides, with the increase in the surface energy, the asymptotic fouling resistance of the coated enhanced tubes also increased.

Experimental study on the fouling and heat transfer characteristics of enhanced tubes used in a cooling tower water system with the actual water quality

Accelerated fouling tests using water conditions more severe than actual project have been proven that it could not truthfully reflect the fouling development. To fill the research gaps of long-term fouling tests, three sets of continuous long-term fouling tests on eight enhanced tubes and one plain tube were performed over a duration of 86–143 days to explore the deposition behavior of the combined fouling. In this study, a decoupling method was presented to compare the heat transfer ability of different test tubes in different operation conditions. Detailed mechanisms of the effects of water velocity and tube geometries on the combined fouling process for enhanced tubes were revealed. Results showed that increased water velocity causes the introduction period of the fouling process to be reduced continuously. In addition, the asymptotic fouling resistance variation curve presented a downward parabola with increased water velocity. For one enhanced tube, the asymptotic fouling resistance decreased with the increase of water velocity from 0.9 m/s to 2.4 m/s. However, the plain tube presented asymptotic fouling resistance that first increased, and then, it decreased when the water velocity rose from 0.9 m/s to 2.4 m/s. In order to evaluate the performance of enhanced tubes when considering heat transfer, fouling, and pressure drop, a comprehensive performance evaluation criteria (CPEC) was proposed, and its calculation correlation was developed with an average error of 7.06%.

Novel nonwetting solid-infused surfaces for superior fouling mitigation

Fouling is a ubiquitous issue in several environmental and energy applications. Here we introduce novel nonwetting solid-infused surfaces (SIS) with superior anti-fouling characteristics that are durable than conventional nonwetting surfaces in a dynamic flow environment. A systematic study is presented to elucidate the fouling mitigation performance of SIS in comparison to lubricant-infused surface (LIS) and conventional smooth surface. Copper tubes with SIS, LIS or smooth inner walls are fabricated and subjected to accelerated calcium sulfate fouling in a flow fouling experimental setup. Fouling on the various surface types is quantified in terms of asymptotic fouling resistance, and the fundamental morphological differences in the interactions of the foulant and the various surface types are analyzed. Based on a systematic sweep of the parameter combinations using design of experiments and Taguchi analysis, an analytical dependence of asymptotic fouling resistance on the governing parameters namely, Reynolds number, foulant concentration and temperature is derived. The analytical model is shown to predict the asymptotic fouling resistance to within 20% accuracy with a 95% confidence. In addition, for the first time, the effects of shear durability on the fouling mitigation performance of LIS vis-à-vis SIS are studied. It is shown that the novel nonwetting SIS offers a robust option for superior fouling mitigation over LIS in the long run.

Generalized Analysis of Dynamic Flow Fouling on Heat Transfer Surfaces

Several energy applications involve flow of untreated heat transfer fluids that contain trace amounts of mineral salts, such as calcium carbonate and calcium sulfate, which show reverse solubility with temperature. The gradual accretion of these salts onto the heat transfer surfaces causes fouling that degrades thermal performance over time. While fouling of bare surfaces has been extensively studied, fouling of nonwetting superhydrophobic or liquid-infused surfaces is less understood, especially under dynamic flow conditions. This study presents a systematic experimental analysis to elucidate dynamic flow fouling of calcium carbonate and calcium sulfate on smooth, superhydrophobic and liquid-infused surfaces in a generalized manner. Copper tubes with modified inner surface wettability are fabricated and subjected to forced convection at different flow rates and fouling salt concentrations. Fouling characteristics of the two salts on the different surfaces are examined through microstructural analysis, and quantified in terms of asymptotic fouling resistance that measures the total extent of fouling, and a threshold time, which correlates to the onset of significant fouling. Using Taguchi design of experiments, the study reports, for the first time, closed-form analytical relationships for the asymptotic fouling thermal resistance and threshold time on the Reynolds number, dimensionless concentration of fouling agent and dimensionless infused liquid viscosity that represents the different surface types in a unified manner. It is shown that the analytical model accurately predicts the fouling characteristics in up to about 97% of the data, based on which contour maps of optimum surface designs for minimizing fouling resistance are presented. The results provide information on designing surfaces for reduced fouling or for estimating the fouling characteristics for a given surface in applications.

Analysis of silica fouling on nonwetting surfaces.

Ground water sources used as coolant fluids in a variety of thermal systems such as heat exchangers and power plant condensers contain silica particles that accrete on heat transfer surfaces over time leading to reduction in thermal performance, a problem that is particularly exacerbated with temperature. Nonwetting superhydrophobic, lubricant-infused, and a new class of solid-infused surfaces introduced in this work are candidates for fouling mitigation, by virtue of their water repellency, but little is known about fouling of silica on the surfaces, especially under dynamic flow conditions and as a function of temperature. This article presents, for the first time, a systematic study of dynamic flow fouling of silica on nonwetting surfaces vis-à-vis conventional copper surface over a temperature range 20-50 °C. The mechanism of silica aggregate formation and its adherence to the different surfaces is elucidated by scanning electron microscope (SEM) imaging. Sigmoidal growth model is used to describe the time evolution of fouling thermal resistance and an Arrhenius model is presented for the temperature-dependent increase in the asymptotic fouling resistance on nonwetting and conventional surfaces alike. Lubricant-infused and solid-infused surfaces are shown to reduce fouling thermal resistance by up to 25% and 13%, respectively, compared to a conventional surface, whereas superhydrophobic surfaces lose their non-wettability under flow conditions, leading to an adverse increase in the fouling resistance by up to 13%. Considering the possible lubricant depletion in lubricant-infused surfaces over prolonged exposure to a flowing fluid, solid-infused surfaces present a robust alternative.

A generalized prediction model of waterside fouling for internally enhanced tubes in shell and tube condensers

• The applicability of the constant fouling resistance (factor) is verified. • The valid concentration is proposed to improve the accuracy of fouling model. • The effect of velocity on the deposit bond strength is studied experimentally. • The correlations for sticking probability and deposit bond strength are developed. • Two fouling prediction models are developed and verified. Fouling is one of the most significant problems for internally enhanced tubes installed in the shell and tube condensers. Due to the lack of long-term test data, current fouling models are developed based on accelerated particulate fouling tests that have the low precision and hence are inapplicable for predicting combined fouling in most practical cooling tower systems. In addition, the constant values of fouling resistance (factor) recommended by the Air-Conditioning, Heating, and Refrigeration Institute (AHRI) are extremely limited under different operating conditions. To overcome these challenges, this research developed and validated two fouling prediction models based on experimental long-term tests. One of the models was in the form of a ratio of asymptotic fouling resistance of the enhanced tube to that of the plain tube ( R f ∗ / R f , p ∗ ), and the other one was in the form of the asymptotic fouling resistance of the directly enhanced tube ( R f ∗ ). Both models considered water quality, water velocity, and the tube geometries as the variables with the acceptable accuracy for prediction. 1) For the water quality, the parameter of valid concentration ( C com ) of cooling water was defined in this study, which reflected the potential amount of valid components to form the fouling. 2) For the water velocity, its impacts on the two critical parameters of the fouling process: sticking probability ( P ) and deposit bond strength ( ξ ) were investigated using experimental studies. Test results showed that in enhanced tubes with the increased water velocity the sticking probability ( P ) decreased continuously while the deposit bond strength (ξ) initially increased, and then, decreased. 3) For the tube geometries, by taking the parameters of tube geometries as variables the multi-variable correlations of the sticking probability ( P ) and deposit bond strength ( ξ ) were developed. From the results the generalized fouling prediction model as a ratio of asymptotic fouling resistance ( R f ∗ / R f , p ∗ ) was recommended for the application in HVAC&R industry due to its suitability and accuracy in practical project applications.

Experimental Study of Fouling in Hybrid Cooling Tower Used in CSP Plants: Effects of the Polymer and galvanized steel tubes

Concentrating solar power (CSP) plants require cooling systems, which are typically operated using either water resources that are scarce in arid regions or dry cooling which impacts the overall efficiency of the system. To ensure a higher sustainability of solar-electric facilities, hybrid cooling is promising solution for higher system efficiency with lower water consumption. However, fouling of cooling components can contribute to several operation issues, such as the reduction of heat transfer, precipitation on heated tubes, and increasing the maintenance cost. It can also be influenced by many parameters, namely the surface proprieties, and operating conditions (such as temperature, water and air flow rates, relative humidity….). To assess the fouling process in the hybrid cooling system, a fouling test facility with a dynamic monitoring system was built its operational performances were experimentally investigated on the basis of a long-term test of 108 days. To explore the influence of the surface materials, the hybrid towers were equipped with polymer and galvanized steel tubes. Based on fouling data, test results indicated that the influence of the tubes positioning on fouling deposition was at least as significant as the influence of tube materials. Generally, the heated tubes at the bottom positions in the test facilities were subject to severe fouling for both tube’s materials. As for the polymer heated tubes, an asymptotic fouling resistance value was achieved 22 days before the galvanized ones. Finally, the cleaning method showed that the fouling deposits on polymer tubes were cleaned easily compared with the galvanized tubes.

Simulation of particle fouling characteristics with improved modeling on two different tubes

The fouling characteristics of micron-scale magnesium oxide particles on the inside wall of the circular tube and converging-diverging tube are studied in this paper. An improved method for modeling particles in tubes was developed based on the Eulerian method. The particle deposition mechanism of the model includes: diffusion deposition, turbulent deposition, thermophoretic deposition, and gravity deposition. The model was verified by comparison with experimental results from the literature. The effects of particle concentration and flow velocity were studied for two different tubes. The simulation results show that the change rules of the fouling resistance of the two tubes were similar both the asymptotic value of fouling resistance increases with the increase of the concentration, and decreases with the increase of the inlet velocity. By comparing the fouling resistance of the two tubes, it was found that the values of asymptotic fouling resistance of the converging-diverging tube were smaller than that of the circular tube at different concentrations and velocities.

Effect of Flow Velocity on the Surface Fouling (Dept.M)

The slow velocity is one of the most important parameters affecting the fouling rate on the heat transfer surfaces. An experimental study is carried out to investigate the effect of low velocity on the surface fouling for the particulate fouling type. The experiments are carried out under constant surface temperature by using the steam as a heating medium. Water with a solid particle concentration of 1 gm/lit is used as a test fluid and the test slow velocities are ranged from 0.24 to 1.42 m/s. The runs have been operated for a long time period (about 200 hrs) to attain the asymptotic fouling resistance. The obtained experimental results showed that; increasing the slow velocity decreases both the fouling resistance and the asymptotic fouling resistance. The delay time is very small and it seems to be zero. The saw tooth effect has been obtained in all runs. There is a great effect of Now velocity on the fouling resistance due 10 velocities in the range from 0.24 to 0.35 m/s where this effect is seen to be smaller when the flow velocity increases from 1,27 to 1.42 m/s.

Composite Fouling Characteristics on Ni-P-PTFE Nanocomposite Surface in Corrugated Plate Heat Exchanger

Plate heat exchanger (PHE) has been widely used in energy transport fields for its high efficiency and compact structure. However, fouling is an intractable issue which reduces heat transfer efficiency and increases pressure drop. In this study, an experimental investigation on composite fouling characteristics of Ni-P-PTFE nanocomposite coating in corrugated PHE was conducted to evaluate the anti-fouling performance of this modified surface. The nanocomposite coating was prepared based on the stainless steel 304 plates through the electroless composite plating method. Clean tests and accelerated composite fouling tests were carried out, and the heat transfer and pressure drop performance of both the uncoated and coated plates was analyzed. Moreover, micro-structure of the surfaces and fouling deposits was mapped by scanning electron microscope. Compared to uncoated plates, the Ni-P-PTFE coated plates showed lower pressure drop by up to 15.8% and lower heat transfer performance by up to 17.6% at clean state. The smooth coating decreased the friction loss of the flow, while slightly increased the heat conduction resistance. The Ni-P-PTFE coated plates showed a significant anti-fouling performance, which decreased the fouling’s weight by 20.4% and the asymptotic fouling resistance by 28.2%, meanwhile reduced the pressure drop increment by 44.8%.

Experimental study on long-term fouling of enhanced tubes: Effect of water quality

In order to investigate the impact of water quality on fouling process of enhanced tubes, two continuous fouling tests on eight enhanced tubes (with different internal and external tube geometries) and one plain tube (with a plain inside surface and an enhanced outside surface) were carried out in duration of 98 days, using cooling water with medium and low fouling potential respectively at a water velocity of ∼1.6 m/s. Compared with the medium fouling potential water, the introduction period of fouling on a plain tube in low fouling potential water test was much longer than enhanced tubes. Test results suggested that the influence of water quality on fouling development was much larger than geometry of heat transfer tubes when the water quality changed from low to medium fouling potential. For one heat transfer tube, the asymptotic fouling resistance was continuously increasing with the increase of fouling potential of water. In this study the length index () determined by the geometry of enhanced tube was introduced to improve the accuracy of previous fouling model. Finally, an improved fouling prediction model considering the effect of water quality was developed, which had a maximum error of 22.94%.

Experimental and numerical investigation on particulate fouling characteristics of vortex generators with a hole

In this study, the particulate fouling characteristics in a heat exchange channel with a rectangular wing vortex generator were investigated both experimentally and numerically. First, the fouling and flow resistance characteristics of the smooth channel, vortex generator without a hole, and vortex generator with a hole were compared experimentally. Subsequently, the fouling characteristics of the rectangular vortex generator with and without a hole were compared under turbulent conditions via numerical simulations. Finally, the effects of the hole diameter, and the lateral and vertical hole distances in the vortex generator on the particulate fouling characteristics were evaluated. The results show that both the vortex generator with a hole and without a hole could inhibit the formation of particles on the heat transfer surface. Compared with the vortex generator without a hole, the vortex generator with a hole could better inhibit particulate fouling, and it had a low flow resistance loss. The asymptotic fouling resistance decreased first and then increased with increasing hole diameter, it decreased as the lateral distance of the hole increased, and increased as the vertical distance of the hole increased.

Investigation on fouling of enhanced tubes used in a cooling tower water system based on a long-term test

In order to investigate the fouling of enhanced tubes, a continuous fouling test on eight enhanced tubes and one plain tube was conducted for 98 days, using a medium fouling potential water as the test water at a water velocity of ∼1.6 m/s. Four periods were defined for fouling development based on the test data in this work: introduction period, initial deposit period, rapid deposit period, and stabilization period. The rank of asymptotic fouling resistance of nine test tubes was as follows: Tubes 9 > 7 >6 > 5 > 1 >3 >4 >2 >8. The pressure drops of the nine tubes increased by 0.1 kPa to 1.32 kPa due to fouling deposit at the end of the fouling test. A fouling resistance prediction model was developed, which had a maximum error of 0.0645 for predicting the fouling resistance ratio.

Experimental study on the interaction between particulate fouling and precipitation fouling in the fouling process on heat transfer tubes

This study investigated the interaction between particulate fouling and precipitation fouling on two different enhanced tubes and a plain tube in a shell-and-tube condenser. Three kinds of fouling, including particulate fouling, precipitation fouling, and combined fouling, were measured continuously for more than 720 h in three individual tests, respectively. For the particulate fouling, three tubes suffered a similar fouling process. The asymptotic fouling resistance of the precipitation fouling on enhanced tubes was lower than on the plain tube. However, for the combined fouling, the enhanced tube with a larger start number experienced the greatest fouling. On the same tube, the asymptotic fouling resistances ranked from least to greatest in the following order: particulate fouling, precipitation fouling, combined fouling; and the durations of the induction period ranked as: particulate fouling, precipitation fouling, combined fouling. As for the combined fouling existent in real cooling water systems, the fouling induction period on the plain tube was longer than that of enhanced tubes. Compared with the particulate fouling (SiO2) and precipitation fouling (CaCO3), which had a high bond strength on the tube surface, the combined fouling was soft and easily removed by flow erosion or collision with big particles during the induction period. This indicated that particles mixed in the water could extend the induction period of combined fouling. However, once the combined fouling started to grow, these particles sticking on the heat transfer surface provided nucleation sites for the crystallization process and, thus, increased the fouling thermal resistance significantly. In addition, the results indicated that the variation profile of the accelerated combined fouling after scaling down is similar to the fouling process in a real system.

Study on fouling characteristics of CaCO3 at low reynolds number

To study the effects of hardness and Reynolds number on the fouling of calcium carbonate under a low Reynolds number, the experimental method and principle for determining the fouling thermal resistance were analyzed theoretically. The experimental device for the whole heat transfer was designed. By changing the concentration of calcium carbonate and the Reynolds number in the fluid, the fouling process of calcium carbonate at a low Reynolds number was studied. The experimental results show that the fouling resistance will reach a fixed value over time, that is, the asymptotic fouling resistance. In the case of a constant Reynolds number, the concentration of calcium carbonate increased exponentially, and the asymptotic fouling resistance increased approximately. When the concentration of calcium carbonate was constant, the fouling resistance growth rate increased, and the asymptotic fouling resistance increased with a decreasing Reynolds number. However, the difference in the fouling rate and the asymptotic fouling resistance under different Reynolds numbers is obvious. Combined with the experimental results, it can be found that the initiation period is mainly determined by the heat transfer surface, and the fouling thermal resistance may be negative in the initiation period. According to the experiment, a predictive model is established. © 2019 American Institute of Chemical Engineers Environ Prog, 38:e13146, 2019

Numerical simulation of CaSO4 crystallization fouling in a rectangular channel with vortex generators

Use of passive vortex generators (VGs) is an efficient heat-transfer enhancement technique, which utilizes a special type of extended surface that mainly generates vortices parallel to the flow direction to not only improve the heat transfer performance of the heat exchangers, but also prevent the formation of fouling. In order to study the growth characteristics of fouling on the rectangular channel with different VGs, three types of VGs with the same frontal area were used in the present study and the deposition process of calcium sulfate (CaSO4) fouling was studied by numerical simulation method. First, the effects of the changes in inlet velocity from 0.5 to 0.8 m/s, concentration of CaSO4 from 2.0 to 3.5 kg/m3, and surface temperature from 320 to 350 K on fouling characteristics were investigated. Then, the Performance Evaluation Criterion (PEC) was used to evaluate the heat transfer characteristics and fouling characteristics on a channel with different types of VGs. The results showed that the fouling resistance of the three types of VGs followed the same trend. The value of asymptotic fouling resistance decreased with the increase of the velocity, and increased with the increase of the concentration and surface temperature, respectively. For the rectangular channels with three different types of VGs, when the flow resistance was taken into account, the Performance Evaluation Criterion with fouling (PECf) of the half-cylinder VG was found to be the best among the three different types of VGs.

Experimental study of fouling process and antifouling effect in convective heat transfer under ultrasonic treatment

An experimental study was carried out by a dynamic fouling monitor system to investigate the fouling process in convective heat transfer by ultrasonic treatment. During the experiment, the cooling water in heat exchanger was used as working fluid with the inlet temperature of 22.5 °C and 44 °C and the initial hardness of 300 mg/L and 500 mg/L, respectively. For all cases the inlet temperature of hot water was kept at 70 °C, and the flowrates of cooling water and hot water were set at 0.77 m3/h and 0.81 m3/h, respectively. In this experimental setup, a double-tube heat exchanger was served as a test section of heat transfer, in which hot water flows inside the inner copper tube and cooling water flows in the annular gap between the two tubes, thus forming a counter-flow situation. Further, an ultrasonic device was installed for water treatment with a frequency of 20.7 kHz and power ranging from 0 to 75 W. The results showed that the fouling resistance for hard water increased evidently with increasing of water temperature and hardness. With the ultrasonic treatment, the fouling resistance decreased remarkably compared with the untreated case, and the asymptotic fouling resistance decreased monotonously with increasing of the ultrasonic power. Subsequently, the crystal morphology of calcium carbonate was observed in microscopic view and the transform of crystal from the vaterite to aragonite and calcite was analyzed based on the theory of the formation energy. It was confirmed that the ultrasonic treatment may have significant effect on the crystal shape, and more aragonites appeared with increasing powers of ultrasound.

Numerical study of fouling characteristics on two kinds of typical heat exchangers used in the waste heat recovery systems

The problem of high exhaust temperature universally exists around the industrial boilers, which causes enormous loss of energy. Different kinds of heat exchangers are widely used in the waste heat recovery systems to recover waste energy from different heat sources. However, considering the heat exchangers mainly work in dusty environment, such as in the tail of large boiler, fly ash fouling on heat exchangers is a major problem in waste heat recovery systems, which can lead to significant efficiency deterioration. Besides, the fouling may also lead to failure of the systems with serious economic consequence. The H-type finned tube heat exchanger is a new type of heat exchanger which have been widely used in waste heat recovery systems in recent years, because of their excellent anti-wear and anti-fouling performance. In this paper, a numerical model was developed to investigate the fouling characteristics of two kinds of H-type finned tube heat exchangers used in the waste heat recovery systems. Firstly, a fouling model which contains a deposition process and a removal process is developed. The deposition process is determined by the energy balance on the particles and the removal process is determined by the the local wall shear stress and the fouling thickness on the fin surface. As the flow field directly affects the particle motion, the fluid flow characteristics of the two kinds of heat exchangers are calculated, and the discrete phase model (DPM) was employed to trace the fly ash particles in the flow field during the fouling process. Since the simulation time is much smaller in magnitude than the real time in fouling process, it is impossible to simulate the whole real fouling time period (hours) with a small time step size. In order to obtain a reliable result, it is necessary to enlarge the simulation time and results to the real time scale. In this paper, a fouling time ratio is proposed 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. According to the fouling time ratio, we can enlarge the simulation time and results to the real time scale. Then the real-time fouling characteristics of the two kind of H-type finned tube heat exchangers are investigated and the effects of inlet velocity, particle diameter on the fouling results are also studied. The results show that, the fouling mainly occurred in the flow stagnation region and the recirculation region of the H-type finned tube. The net fouling mass increases with the operating time, showing an asymptotic characteristic of growth, and finally will reach a stable value, which corresponds to the asymptotic fouling resistance. The increase in inlet velocity and particle diameter would lead to a rapidly decrease in fouling results. The anti-fouling performance of double H-type finned tubes shows slightly better than that of single ones, but the advantages decreases with the increase in inlet velocity and particle diameter.

Enhanced Heat Transfer and Fouling Propensity of Diamond-Like Carbon Coated Smooth and Finned Tubes During External Nucleate Boiling

Smooth DLC (diamond-like carbon) coated surfaces can profoundly mitigate scaling during pool boiling of calcium sulphate solutions. Previous investigations though carried out mostly for the smooth surfaces rather than structured, i.e., finned tubes. This study compares experimental results of DLC coated smooth and finned tubes at clean and fouling conditions. Fouling runs were conducted during pool boiling of saturated CaSO4 solution of 1.6 g/L at 300 kW/m2. The substrate of the attempted tubes was stainless steel and finned tubes of 19 and 40 fins per inch were used. The DLC coated smooth tube showed an enhanced clean heat transfer up to 50% and reduced fouling resistance compared to the uncoated smooth tube. After a short operating time, though, the coated smooth tube reached an asymptotic fouling resistance of 0.00005 m2 K/W whereas for the uncoated smooth tube, it was 4.8 times higher. DLC coating of the finned tubes with the physical vapor deposition (PVD) technique implicated difficulties. The base surface of the finned tubes was defectively coated. The defectiveness of the coating was attributed to the limitation of the PVD for coating of structured surfaces.