Particulate Fouling and Challenges of Metal Foam Heat Exchangers

ABSTRACT

In recent years, open-cell metal foam has gained attention for utilization for exhaust gas recirculation coolers due to its large surface area and porous structure. Theoretically, the porous foam structure would have better transfer heat through conduction and convection processes. However, the exhaust gases that enter the cooler would carry particulate matter, which may deposit within the foam structure. The existing fouling studies cannot explain the underlying mechanisms of particulate deposition thoroughly within the foam structure. This study reviews the particulate fouling of heat exchangers, particularly in the exhaust gas recirculation system. Some past approaches to investigate fouling, particle transport, and deposition in the metal foam heat exchangers for many different applications are also included. In addition, this study also includes the challenges that lie ahead in implementing the metal foam heat exchangers in the industries.     

Modeling of Particulate Fouling on Heat Exchanger Surfaces: Influence of Bubbles on Iron Oxide Deposition

Studies of iron oxide deposition on Alloy-800 heat exchanger tubes have been part of a continuing research program at the University of New Brunswick (UNB); the present work formulates mechanisms for the effect of bubbles on deposition in water under boiling conditions. To supplement results from earlier deposition experiments in a fouling loop at UNB, measurements of bubble frequency and departure diameter as a function of heat flux were performed. High-speed movies of bubbling air/water systems indicated that a pumping action moved particles from adjacent areas at the surface to bubble nucleation sites. To explain the observations, the model considers deposition and concomitant removal. Deposition includes microlayer evaporation and filtration through the porous deposit. The deposit is sparse in the first stage, when the dominant process is microlayer evaporation including particle trapping and pumping, creating spots of deposit. Filtration becomes more important as the deposit thickens to a stage when microlayer evaporation becomes negligible. Chimney effects then control. Turbulence due to detaching and collapsing bubbles affects removal. In subcooled boiling, collapsing bubbles generate enough turbulence to maintain much of the deposit labile, while in bulk boiling bubble detachment from the nucleation site is dominant and a smaller portion of the deposit is labile and subject to removal. Model predictions are presented and shown to agree quite well with experimental data.     

Development of a Predictive CFD Fouling Model for Diesel Engine Exhaust Gas Systems

This article presents a numerical simulation procedure for studying soot particle deposition in diesel exhaust systems, with a particular focus on fouling layer thickness evolution. In the proposed algorithm, particle transport toward the wall, adhesion, and reentrainment of particles from the surface have been modeled, including Brownian motion and turbulent diffusion, thermophoresis, adhesion, and removal. This model has been implemented in ANSYS Fluent, which makes the inclusion of local effects possible. A cross-flow device, with a tube positioned transverse to the flow, has been simulated and tested. A comparison of the predicted fouling layer at several angular positions with the experimental observation shows acceptable agreement. This model makes it possible to predict the real depth of the fouling layer and its effects on the hydrodynamics of the flow. This model represents a valuable tool for the prediction of the main aspects of the performance of heat exchangers exposed to fouling.     

Mineral scale formation and mitigation on metals and a polymeric heat exchanger surface

An experimental set-up was built to study heat transfer fouling of different pipe materials used in heat exchangers. Fouling mitigation investigations using wood pulp fibres in suspension in the fouling liquid were also performed. The new set-up allows progressive visual observation of fouling with time together with a recorded history under the same solution conditions. On completion, the tube under investigation could be removed to obtain quantitative data on the progressive build up of the deposit as well as the composition of the deposit.The experimental technique involved a pipe test specimen being centrally located in a cylindrical tank concentric with a vertical agitator to give constant and uniform flow conditions near the tube surface. The investigation of calcium sulphate deposition on four different metal surfaces (copper, aluminium, brass and stainless steel SS 316 respectively) and a polycarbonate surface reveals that the fouling increases with time but at a decreasing rate. The deposition on a metal surface can be seen to increase with increasing thermal conductivity and decreasing total surface energy over the range of experiments. Low surface energy material such as polycarbonate causes less attraction to the floating crystals and receives less deposition in comparison to the SS surface.Bleached Kraft softwood fibres at various concentrations were added to the solution to examine their effects on fouling. The results indicate that fouling is reduced as fibre concentration increases. It was also found that the fouling on stainless steel, brass and copper surfaces were all retarded in presence of fibre in the solution.     

Influence of phosphorus on ash fouling deposition of hydrothermal carbonization sewage sludge fuel via drop tube furnace combustion experiments

Phosphorus effect on ash fouling deposition produced during combustion process of sewage sludge solid fuel is a very important factor. Previous studies have only focused on decrease of the ash melting temperature and increase of slagging and sintering by phosphorus content. Therefore, research regarding combustion fouling formation and its effect on temperature reduction of deposit surface by phosphorus content is insufficient. Ash fouling is an important factor, because ash in the combustion boiler process deposits on the surface of heat exchanger and interferes with heat exchange efficiency. In particular, temperature reduction of heat exchanger surface via fouling should be considered together with fouling deposition, because this is related to the heat exchanger efficiency. Synthetic ash, phosphorus vaporization, and drop tube furnace experiments were performed to investigate effect of phosphorus on ash fouling formation and temperature reduction of deposit surface under combustion condition. Phosphorus was highly reactive and reacted with ash minerals to produce mineral phosphate, which promoted ash fouling deposition during the combustion experiments. In contrast, the occurrence of sintering on deposited fouling resulted in formation of a large hollow structure, which alleviated the temperature reduction on the deposit surface. Phosphorus content had a substantial correlation with fouling deposition behavior and influenced reduction in the surface temperature of the heat exchanger, because it led to generating low temperature mineral phases.     

Mitigation of Soot Deposition on Modified Surfaces of Exhaust Gas Recirculation Coolers

Abstract

A common approach for lower emission of NO_x from diesel engines is to use exhaust gas recirculation (EGR) coolers where part of the exhaust gas is returned to the cylinder to reduce the combustion temperature. Nonetheless, the deposition of various species, i.e. soot particles, on surfaces deteriorates the thermal efficiency of EGR coolers. This study investigated the impact of surface treatment on particulate fouling of a rectangular EGR cooler. An experimental setup was assembled through which the uncoated and coated plates were exposed to the flow of exhaust flue gases. The cooler surfaces were coated by ceramic-based materials with resistance to high temperatures by spraying. The results showed that surface modification abated soot deposition to some extent and the deposit layer was easily flaked off with a force of 0.8 N when it was scratched with a nano-intender. Contrariwise, the deposit formed on the uncoated surface did not result in similar propensity and instead it required a larger force of 2.25 N. This implies weaker stickiness of soot deposit on the investigated coatings compared to baseline stainless steel surface. It was also found that the electron donor component of surface energy would determine the tendency of a surface to foul or not.     

乙醇掺混燃烧对柴油机油耗影响的实验研究

乙醇吸热后部分热解可产生多种气体的混合物,为了研究气体混合物掺混燃烧在柴油机上的节油效果,对柴油机系统进行了改造:乙醇通过安装在柴油机排气管上的小型高效换热器吸收烟气余热,部分热解后混合气体由进气道通入柴油机燃烧室改善燃烧.在该系统上进行了定功率-不同转速以及额定转速-不同功率下的节油试验,表明:该系统在高低功率条件下均有较好的节油和节能效果.柴油机转速为1 500 r/min时,节油率最高可达40％,节能率最高可达13.5％;在柴油机额定转速2 000 r/min时,节油率最高可达24％,节能率最高可达5.7％.结合乙醇热解的气体混合物的测量数据,得出了低功率下主要依靠乙醇蒸气,高功率下主要依靠小分子气体的节油原理.     

Fouling Problems in Exhaust Gas Recirculation Coolers in the Automotive Industry

Transportation is responsible for approximately 20% of global greenhouse gas emissions, such as CO₂, NO_x, and hydrocarbons that have not been burned completely in the engine. In particular, 55% of globally emitted NO_x, which is more harmful to the environment than CO₂, is produced by the automotive industry alone. Strict emission standards are now in place that set specific limits to the amount of pollutants that can be released into the environment. The widely used measure to reduce NO_x emissions in diesel engines is to return part of the exhaust gas to the intake of the engine. This is usually done through a heat exchanger known as an exhaust gas recirculation (EGR) cooler. However, EGR coolers are subject to severe fouling such that their thermal efficiency can drop by as much as 30% within a very short period of time. More importantly, the deposit layer is a blend of particulate matter and sticky heavy hydrocarbons that are very difficult to remove from the heat exchanger surfaces. The present study addresses this problem and provides a review on required research and development (R&D) activities to mitigate fouling of EGR coolers.     

Research on the applications of infrared technique in the diagnosis and prediction of diesel engine exhaust fault

This paper mainly introduces the basic principles,the methods and the applications of infrared technique in the diagnosis and prediction of diesel engine exhaust faults. The test-bed for monitoring diesel engine exhaust faults by thermal infrared imager has been designed. In different running conditions, the exterior surface radiation temperatures of the exhaust pipe of the 6135G-1 diesel engine have been measured by infrared imaging system. According to the principle of infrared temperature measurement, the real temperatures of the exterior surface of the exhaust pipe have been calculated. Based on the principle of heat transfer, the method of calculating the exhaust temperatures according to the exterior surface radiation temperatures of exhaust pipe measured by thermal infrared imager is built. The relationship between diesel engine exhaust temperatures and faults has been analyzed. It is shown that the application of infrared inspection and diagnosis to the identifying of diesel engine exhaust faults is feasible and effective.     

以流化床换热回收柴油机废气余热的试验研究

传统的对流换热式柴油机余热锅炉存在受热面容易污染和传热系数低的缺点,利用流化床换热可以解决这个问题。本文介绍了在３０ＫＷ柴油机上所作的试验研究。取得的结果令人满意,为工业应用提供了有用的设计参数。     

Influence of the Apex Angle of Cone-Shaped Tubes on Particulate Fouling of Heat Exchangers

A two-dimensional (2D) cone shape has been added to the normal circular tubes of heat exchangers to minimize the area of stagnation and to streamline the air flow around the heat exchanger tubes. An experimental setup has been developed to study the influence of the apex angle of the cone-shaped tubes on particulate fouling of heat exchangers. Fouling experiments have been performed in which calcium carbonate particles are injected during the experiments and the deposition of particles on the tubes of the heat exchanger is monitored. Four sets of experiments have been performed, in which normal cylindrical tubes and coned tubes with an apex angle of 60°, 90°, and 120° are examined. It was found that particulate fouling ceased if the apex angle of the cone-shaped tubes is smaller than 90°. The attached cones enhance the flow around the tubes of the heat exchanger, by minimizing the stagnation area and keeping the flow attached to the tubes starting from the tip of the attached cone until separation, such that particles that deposit on the top of the tubes of the heat exchanger can be removed by the air flow.     

Comparative study of mechanical and chemical methods for surface cleaning of a marine shell‐and‐tube heat exchanger

In this article, experimental analysis is done on shell‐and‐tube heat exchanger of a marine vessel for removal of fouling using optimum surface‐cleaning techniques. The main objective is to compare the performance of the heat exchanger before and after maintenance. Two identical deteriorated systems of heat exchangers are taken and real‐time analysis is conducted. The log data are taken before and after undergoing maintenance for the two systems. Two different cleaning techniques are used, namely, chemical cleaning and mechanical cleaning. Detailed calculations are made for the shell‐and‐tube heat exchanger. From the obtained data, comparisons are made for different parameters on the tube side such as friction factor, heat transfer coefficient and pressure drop, as well as total heat transfer rate on the shell side. From the analysis and comparison, it was found that greater heat transfer takes place for the tubes cleaned using the chemical cleaning method than for tubes cleaned by the mechanical cleaning method. Pressure drop is found to be less for chemical cleaning method than mechanical cleaning method. This indicates that the fouling effect is reduced for tubes cleaned by the chemical cleaning method, and furthermore these tubes remain corrosion‐resistant for longer periods of time.     

Exhaust emission and combustion evaluation of coconut oil-powered indirect injection diesel engine

This paper presents the results of experimental work carried out to evaluate the exhaust emissions characteristics of ordinary Malaysian coconut oil (COCO) blended with conventional diesel oil (OD) fueled in a diesel engine. This project complies with Malaysian Government strategy on biofuel research activity. The results showed that the addition of 30% COCO with OD produced higher brake power and net heat release rate with a net reduction in exhaust emissions such as HC, NOx, CO, smoke and polycyclic aromatic hydrocarbon (PAH). Above 30% COCO blends, such as 40 and 50% COCO blends, developed lower brake power and net heat release rate were noted due to the fuels lower calorific value; nevertheless, reduced emissions were still noted.     

水下工作柴油机排气降温系统设计及试验

排气背压对柴油机的动力性能和油耗有着重要的影响,排气背压过大会造成动力性能的损失和油耗的增加,因此,在为柴油机设计排气后处理装置时,要充分考虑排气背压的大小。本文针对水下工作的柴油机,设计了一套排气降温系统,可以使得该柴油机在水下工作时,其排气可以被冷却后直接排向大气,并且其动力性能不受影响,这就要求在设计过程中要使整个排气降温系统的流动阻力尽可能最小。通过试验结果表明:加装该排气降温系统后柴油机输出动力性能良好,无功率和转矩损失;特殊管壳式换热器换热效果良好,能使最高温度为503.4℃的高温烟气降低到39.4℃。本文研究的结果对小型水下动力装置排气降温系统的选择和设计有一定的参考价值。     

Experimental heat release analysis and emissions of a HSDI diesel engine fueled with ethanol–diesel fuel blends

An experimental study is conducted to evaluate the effects of using blends of ethanol with conventional diesel fuel, with 5%, 10% and 15% (by vol.) ethanol, on the combustion and emissions of a standard, fully instrumented, four-stroke, high-speed, direct injection (HSDI), ‘Hydra’ diesel engine located at the authors’ laboratory. The tests are conducted using each of the above fuel blends or neat diesel fuel, with the engine working at a speed of 2000 rpm and at four different loads. In each test, combustion chamber and fuel injection pressure diagrams are obtained using a specially developed, high-speed, data acquisition and processing system. A heat release analysis of the experimentally obtained cylinder pressure diagrams is developed and used, with the pertinent application of the energy and state equations. From the analysis results, plots of the history in the combustion chamber of the gross heat release rate and other related parameters reveal some very interesting features, which shed light on the combustion mechanism when using these blends. Moreover, for each test, volumetric fuel consumption, exhaust smokiness and exhaust regulated gas emissions are measured. The differences in the performance and exhaust emission parameters from the baseline operation of the diesel engine, i.e., when working with neat diesel fuel, are determined and compared. The heat release analysis results for the relevant combustion mechanism, combined with the widely differing physical and chemical properties of the ethanol against those for the diesel fuel, are used to aid the correct interpretation of the observed engine behavior.     

Impact of Thermal Shock on Fouling of Various Structured Tubes During Pool Boiling of CaSO4 Solutions

Crystallization fouling during nucleate pool boiling is characterized with relatively rapid formation of a hard and brittle deposit layer. In this study, the impact of thermal shock is experimentally investigated on cleaning of various tubes when subjected to fouling of CaSO₄ solutions. The heat transfer tubes included stainless-steel plain and finned tubes with fin densities of 19 and 40 fins per inch. The tubes were used during pool boiling for heat fluxes ranging from 80 to 300 kW/m². The thermal shock is applied by the sudden decrease or increase in heat flux for approximately 30 s after the fouling layer is formed. In terms of cleaning, the experimental results showed far better performance for the plain tube. During the sudden change in heat flux the whole fouling layer cracked and peeled off, receiving again almost initial clean heat transfer performance. Contrariwise, the results for the finned tubes showed that the fouling layer was only affected marginally by the thermal shock.     

Effect of heat exchanger material and fouling on thermoelectric exhaust heat recovery

Thermoelectric devices are being investigated as a means of improving fuel economy for diesel and gasoline vehicles through the conversion of wasted fuel energy, in the form of heat, to useable electricity. By capturing a small portion of the energy that is available with thermoelectric devices can reduce engine loads thus decreasing pollutant emissions, fuel consumption, and CO₂ to further reduce green house gas emissions. This study is conducted in an effort to better understand and improve the performance of thermoelectric heat recovery systems for automotive use. For this purpose an experimental investigation of thermoelectrics in contact with clean and fouled heat exchangers of different materials is performed. The thermoelectric devices are tested on a bench-scale thermoelectric heat recovery apparatus that simulates automotive exhaust. It is observed that for higher exhaust gas flowrates, thermoelectric power output increases from 2 to 3.8 W while overall system efficiency decreases from 0.95% to 0.6%. Degradation of the effectiveness of the EGR-type heat exchangers over a period of driving is also simulated by exposing the heat exchangers to diesel engine exhaust under thermophoretic conditions to form a deposit layer. For the fouled EGR-type heat exchangers, power output and system efficiency is observed to be 5-10% lower for all conditions tested.     

热泳力作用下柴油机微粒在冷却通道中沉降规律研究

对热泳力作用下柴油机微粒在冷却通道中的沉降规律进行了研究。推导建立了热泳沉降率数学模型，在此基础上分析了在热泳力作用下，微粒大小、排气流速、排气与壁面温差以及通道尺度等因素对柴油机微粒在冷却通道中热泳沉降率的影响规律。研究结果表明，热泳力对柴油机微粒具有一定的脱除作用，通过合理设计和控制冷却通道的结构和运行参数，可以有效地提高柴油机微粒在冷却通道中的沉降率。热泳沉降技术为柴油机微粒的净化以及柴油机微粒分布的改善提供一条有效的途经。     

Evaluation of heat exchanger surface coatings

Compact heat exchangers are very popular due to their effectiveness, small footprint and low cost. In order to protect heat exchangers in dirty applications, coatings can be applied to the heat transfer surfaces to extend effectiveness and minimize fouling. Coating selection is extremely important since the wrong coating can decrease unit effectiveness, cause more fouling, and/or erode the surface.An experimental investigation of coating effectiveness in compact plate heat exchangers is presented. New, cleaned and coated plate heat exchangers are considered in this study. Heat exchangers have been exposed to untreated lake water for various time periods. Transient effectiveness results compare the rate of fouling for coated and uncoated heat exchangers. Additional results compare deposit weight gain at the end of the test period and transient observations of heat transfer surface appearance. All heat exchanger combinations showed some deposit accumulation for the period considered.Results indicate that the thermal performance of the unit decreases with time, resulting in an undersized heat exchanger. For the conditions considered here, uncoated plates accumulate deposits up to 50% faster than coated plates and show a decrease in performance of up to 40%. Surface coating, exposure time, fluid velocity and concentration of particles can affect fouling.