Increasing Particle Concentration Research Articles

Multi‐object optimization study on the performance of gas cyclones based on heterogeneous condensation and turbulent agglomeration

AbstractImproving the separation efficiency of fine particles becomes more and more critical as environmental pollution aggravates. This study aims to investigate the effects of four key parameters on the performance of gas cyclones, including cyclone body height, particle concentration, initial supersaturation degree, and inlet temperature. Then, the two‐way coupling numerical model, in which is the process of heterogeneous condensation and agglomeration for insoluble fine particles, was achieved by user defined function. On this basis, the response surface analysis method and multi‐objective genetic algorithm were adopted to optimize the cyclone. The results show that when the particle concentration is less than 1000 mg/m3, the separation efficiency can reach above 95%. The initial supersaturation degree has the greatest effect on the separation efficiency and vapour consumption rate, while the cyclone body height is the most critical factor on the pressure drop. As the particle concentration increases, the separation efficiency decreases at first and then keeps almost stable. With the increase of inlet temperature, the separation efficiency is enhanced, and the pressure drop reduces. These research results can provide important guidance for the optimization and engineering application of this technology.

Stress–strain behavior and constitutive relation of rubberized plastic concrete under uniaxial and triaxial compression

AbstractPlastic concrete is a kind of flexible concrete cutoff wall material, which is used to solve the problem of poor coordinated deformation ability between ordinary concrete cutoff wall and surrounding soil. It has the characteristics of small elastic modulus and large limit strain, which greatly improves the operation condition of cutoff wall. In order to save bentonite resources and recycle waste automobile tires, this paper crushed waste automobile tires into rubber particles and mixed them into plastic concrete, through the uniaxial compression test and triaxial compression test, obtained the corresponding stress–strain curve, rubberized plastic concrete strength and deformation properties are studied and the rubber particles and the relationship between dosage. The test results show that the compressive strength of rubberized plastic concrete decreases with the increase of rubber particles content, but the deformation capacity increases with the increase of rubber particles content. In this paper, the regression analysis of rubberized plastic concrete constitutive relation and failure criterion including the influence factor of rubber particles are helpful to promote the engineering application of rubberized plastic concrete and reduce the exploitation of natural resources and environmental pollution.

Influence on Elastic Wave Propagation Behavior in Polymers Composites: An Analysis of Inflection Phenomena

Particulate polymer composites (PPCs) are widely applied under different elastic wave loading conditions in the automobile, aviation, and armor protection industries. This study investigates the elastic wave propagation behavior of a typical PPC, specifically a Cu/poly (methyl methacrylate) (PMMA) composite, with a wide range of particle contents (30-65 vol. %) and particle sizes (1-100 μm). The results demonstrate an inflection phenomenon in both the elastic wave velocity and attenuation coefficient with increasing volume content. In addition, the inflection point moves to the direction of low content with the increase in particle size. Notably, the elastic wave velocity, attenuation, and wavefront width significantly increased with the particle size. The inflection phenomenon of elastic wave propagation behavior in PPCs is demonstrated to have resulted from particle interaction using the classical scattering theory and finite element analysis. The particle interaction initially intensified and then reduced with increasing particle content. This study elucidates the underlying mechanism governing the elastic wave propagation behavior of high particle content PPCs and provides guidelines for the design and application of wave-absorbing composites.

The simulation of the magnetic particle deposition and the effect on the magnetic fluid sealing performance

Magnetic fluid is used for sealing because of its strong magnetic response. However, magnetic particles are aggregated at the pole tooth in the nonuniform magnetic field, which results in a change in sealing performance. This paper uses the coupling simulation of the magnetic-flow field and the diffusion equation to solve the magnetic particle concentration distribution with the different rotating speeds. The sealing performance of different magnetic particle concentration distributions is calculated using the multi-physical field coupling and the dynamic mesh method. The results show that the particle concentration inhomogeneity degree is influenced by rotating speed, particle size, and volume fractions. Besides, the increasing particle concentration inhomogeneity degree increases the sealing performance.

Strain controlled shear hardening behavior in suspensions of alumina spheres in a polydimethylsiloxane oligomer matrix

AbstractAs a model system for thermal interface materials (TIMs), the complex rheological behavior of polydimethylsiloxane oligomer (oDMS)/alumina sphere particles (AlS) suspensions (i.e., oDMS/AlS) is investigated systematically in this work. Aluminum particles with 1 and 10 μm were used as fillers. Here, a surface treatment by using a silane coupling agent as found to be effective in improving the fluidity of the suspensions with small sized AlS particles. Peculiar large amplitude oscillatory shear (LAOS) results were observed for the suspensions that exhibited three or four responses (Hookean regime, shear softening, shear hardening and a second shear softening) depending on the surface properties of AlS particles. For the oDMS/AlS10 neat system, the width of the Hookean regime is determined by the shear stress () instead of the shear strain (), whereas it completely disappears in the oDMS/AlS10t system due to the increase potential energy between particles. Both critical stress and strain at the onset point for strain hardening decreased with the increase of particle concentration. Frequency dependent measurements illustrates that it is the critical strain instead of the stress that controls the shear hardening behavior. Average distance of neighboring particles is assumed to play a key role in the formation of hydroclusters.

Experimental study on collapsible and structural characteristics of artificially prepared loess material

Collapsibility and structural are two of the typical characteristics of natural undisturbed loess. It is of great significance to effectively simulate the collapsibility and structural of natural loess by preparing artificially loess. However, the existing methods of artificially preparing collapsible loess are complex, and the collapsibility of the prepared samples is difficult to control. In this paper, the collapsibility mechanism of loess was re-analyzed, and on this basis, a new method for preparing artificial collapsible loess using remolded loess, industrial salt, CaO particles and gypsum powder was proposed. The basic principle is: the CaO particles have structural strength and would transfer to Ca(OH)2 after soaking, this progress can simulate the disappearance of loess structural strength; The dissolution of industrial salt can simulate the collapse of loess internal pores, the collapsibility of artificial loess can be adjusted by adjusting the percentage of industrial salt; the gypsum powder can simulate the cementation of loess as a bonding material. The shear test, consolidation test and collapsibility test of artificially prepared loess and undisturbed loess were carried out. The test results of artificial loess were compared with undisturbed loess. The results show that: the plastic limit and liquid limit of the artificially prepared loess is smaller than that of the undisturbed loess; The optimal moisture content and maximum dry density are close to that of the undisturbed loess; The collapsibility coefficient of artificial prepared samples increases first and then decreases with the increase of load level, and gradually increases with the increase of industrial salt particle content; The structural parameters of artificially prepared loess samples first increase and then decrease with the shear process, but the structural parameters of artificial prepared loess and undisturbed loess are different under different confining pressure conditions.

Influence of particle content and temperature on dry sliding wear behavior of rice husk ash reinforced AA6061 slurry cast aluminum matrix composites

AA6061 aluminum matrix composites (AMCs) reinforced rice husk ash (RHA) particles (0–8 wt %) were produced. The sliding wear was assessed using a pin-on-disc wear apparatus with a heating facility, and the temperature was varied from 40 °C to 240 °C. The incorporation of RHA particles was beneficial in improving the wear resistance and reducing the COF values over the entire range of temperatures used. The increase in temperature deteriorated the wear resistance of the composites. AA6061/(0–4 wt %) RHA AMCs showed a clear transition from mild wear to severe wear at a temperature of 200 °C. The increase in RHA particle content increased the transition wear temperature. The wear mode changed from abrasive wear at 40 °C to adhesive material flow at 240 °C.

Experimental Measurement of Enhanced and Hindered Particle Settling in Turbulent Gas‐Particle Suspensions, and Geophysical Implications

AbstractThe dynamics of geophysical dilute turbulent gas‐particles mixtures depends to a large extent on particle concentration, which in turn depends predominantly on the particle settling velocity. We experimentally investigate air‐particle mixtures contained in a vertical pipe in which the velocity of an ascending air flux matches the settling velocity of glass particles. To obtain local particle concentrations in these mixtures, we use acoustic probing and air pressure measurements and show that these independent techniques yield similar results for a range of particle sizes and particle concentrations. Moreover, we find that in suspensions of small particles (78 μm) the settling velocity increases with the local particle concentration due to the formation of particle clusters. These clusters settle with a velocity that is four times faster than the terminal settling velocity of single particles, and they double settling speeds of the suspensions. In contrast, in suspensions of larger particles (467 μm) the settling velocity decreases with increasing particle concentration. Although particle clusters are still present in this case, the settling velocity is decreased by 30%, which is captured by a hindered settling model. These results suggest an interplay between hindered settling and cluster‐induced enhanced settling, which in our experiments occur respectively at Stokes number O(100) and O(1). We discuss implications for volcanic plumes and pyroclastic currents. Our study suggests that clustering and related enhanced or hindered particle settling velocities should be considered in models of volcanic phenomena and that drag law corrections are needed for reliable predictions and hazard assessment.

Plasma Parameters and Kinetics of Reactive Ion Etching of SiO2 and Si3N4 in an HBr/Cl2/Ar Mixture

The parameters of the gas phase and the kinetics of reactive ion etching of SiO2 and Si3N4 under conditions of an induction RF (13.56 MHz) discharge with a varying HBr/Cl2 ratio is studied. The study includes plasma diagnostics using Langmuir probes, plasma modeling to find stationary concentrations of active particles, measuring velocities, and analyzing etching mechanisms in the effective interaction prob-ability approximation. It is found that the substitution of HBr by Cl2 at a constant argon content (a) is accompanied by a noticeable change in the electrical parameters of the plasma; (b) leads to a weak increase in the intensity of ion bombardment of the treated surface; and (c) causes a significant increase in the total concentration and flux density of reactive particles. It is shown that the etching rates of SiO2 and Si3N4 increase monotonically as the proportion of Cl2 increases in a mixture, while the main etching mechanism is an ion-stimulated chemical reaction. The model description of the kinetics of such a reaction in the first approximation assumes (a) the additive contribution of bromine and chlorine atoms and (b) the direct pro-portional dependence of their effective interaction probabilities on the intensity of ion bombardment. The existence of an additional channel of heterogeneous interaction with the participation of HCl molecules is proposed.

Reliability of double probe measurements in nanodusty plasmas

Nonthermal plasmas are attractive sources for nanoparticles synthesis, however, their plasma properties are notoriously difficult to assess due to the chemically reactive environment and high nanoparticle concentrations. Here, we are using a floating double probe to measure the plasma properties of a nanoparticle-forming argon:silane plasma. We demonstrate good stability of current–voltage characteristics over several minutes of operation. However, unexpectedly larger electron temperatures are measured with increasing the silane mole fraction. To test the validity of these results, we developed a zero-dimensional global model to investigate the effect of the presence of nanoparticles on the plasma properties. Using this model, we show that increasing particle concentration leads to an increasing electronegativity of the plasma, causing an increase of the reduced electric field. However, this causes only a moderate increase in mean electron energy, in contrast to the much larger increase measured by the double probe. We argue that these large electron temperatures are based on the fact that a double probe measures an ‘apparent’ electron temperature, which is defined by the negative inverse slope of the logarithm of the electron energy probability function (EEPF) at an energy corresponding to the probe’s floating potential. As the silane mole fraction is increased, the plasma becomes more electronegative and the probe’s floating potential moves closer to the plasma potential. Combined with the strong non-Maxwellian EEPF, this leads to the large apparent electron temperatures obtained by the probe. Thus, the apparent electron temperatures measured with the double probe do not follow the trends in mean electron energy.

İnce Toprak İçeriğin Zeminin Kayma Mukavemeti Parametrelerine Etkisi

The shear strength of the soil is a crucial factor in the structure's failure. Before any engineering structure is built on the soil, the shear strength characteristics must be thoroughly examined in order to reduce the calamity of structural collapse. The current study investigates the influence of varied percentages of fine material on soil shear strength. The soil sample consists primarily of sandy and clay soil. The study looked into the impact of fines on shear strength metrics (friction angle and soil cohesion). Several journal papers and articles were reviewed for this study. Based on the findings of several papers, the flow chart was created to describe the effect of the percentage of fine content in the soil and how it affects shear strength parameters. The shear strength parameters of sandy soil are significantly dependent on the percentage of fines in the mixture and the type of fine mineralogical composition. Both soil shear strength parameters increase at a substantial and apparent pace as compared to the particle content and additive increases in clay soil.

Formation damage induced by clay colloids deposition in Triassic clastic geothermal fields: Insights from an experimental approach

Injectivity decline problems are often encountered during geothermal water reinjection into the subsurface despite massive surface filtration. The reinjected water carrying only nano-sized particles at relatively low concentrations causes severe permeability impairment due to pore clogging by the transported particles. To provide a better understanding to injectivity decline problems, this study investigates the impact of clay minerals transport and deposition on formation damage of sandstone reservoirs. To identify the clays potentially responsible for clogging, a mineral characterization of natural samples, collected from a Triassic reservoir located in the Paris Basin (France), was realized using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The obtained results show that in this type of sedimentary environments, illite particles are the most abundant clay species that are more likely to be detached from the rock structure and migrate within the geothermal fluid. Illite particles were then used in core-flooding experiments to mimic their reinjection in sandstones under different physico-chemical conditions. The obtained flow curves were further explained by aggregation experiments carried out using the dynamic light scattering (DLS) technique. The results show a clogging-favored tendency promoted mainly by the increase of ionic strength and particle concentration and the decrease of flow rate.

Highly stable solid-like Pickering emulsions stabilized by kafirin-chitosan complex particles

This paper aimed to construct kafirin-chitosan complex particles through the anti-solvent co-precipitation method and explore its potential as a Pickering emulsifier. The results showed that the contact angle of prepared kafirin-chitosan particles was close to 90° and the zeta potential exceeded 50 mV at the mass ratio of 8:1. The highly charged kafirin-chitosan particles with good wettability could effectively adsorb at the oil/water interface. The droplet size of the Pickering emulsion decreased with the increase of kafirin-chitosan particle concentration. When the particle concentration was 2.0 wt% and the oil fraction was 70%, the droplet size of the Pickering emulsion was about 12.4 ± 0.8 μm. Such a small droplet size could increase the droplet-droplet interaction, thus endowing Pickering emulsion with good viscoelasticity. The microstructures indicated that the kafirin-chitosan particles thoroughly covered the polygonal oil droplets, which was conducive to improving the interface film strength of the Pickering emulsion. Consequently, the prepared Pickering emulsion had excellent rheological properties and anti-coalescence stability. The Pickering emulsion at low pH and ionic strength had good stability after 42 days of storage. Therefore, this present research provided a practical strategy to fabricate edible Pickering emulsions with good texture and stability.

Experimental Research on the Effect of Particle Parameters on Dynamic Stall Characteristics of the Wind Turbine Airfoil

The frequent appearance of sandy and dusty weather in Northwest China impacts the wind turbine. Meanwhile, the non-constant phenomena, dynamic stall speed during the wind turbine operation, will lead to large load fluctuations and unsafe operation. However, few studies have been conducted at home and abroad on the effect of particle parameters on the dynamic stall of airfoils. This paper investigates the impact of particle parameters on the airfoil dynamic stall through numerical simulation of the coupling between the continuous phase and discrete phase by using the SST k-ω turbulence model for a two-dimensional NACA 0012 airfoil. The effect of particle parameters on the airfoil dynamic stall aerodynamic performance, the impact of the flow field around the airfoil, and the particles motion were studied, respectively. The investigation shows a reduction in the aerodynamic performance of the airfoil, due to the addition of particles. The effect is more prominent under a large angle of attack and less under a small angle of attack. When the angle of attack increases, the loss rate of lift coefficient in the windy and sandy environment gradually decreases, while irregular fluctuations emerge when the angle of attack decreases, and the overall rate of change increases more significantly, compared to the stage of the increasing angle of attack. For the particle diameter under 50 μm, the larger the particle diameter, the more significant the change of lift coefficient becomes, as well as the larger the vortex volume near the airfoil’s leading edge, and a large number of particles gather at the suction surface of the airfoil. For the particle diameter of 50 μm, the lift coefficient decreases at any angle of attack of the airfoil movement to the oscillation cycle, the vortex volume decreases, and a large number of particles gather at the pressure surface of the airfoil. However, for particle diameters above 50 μm, the lift coefficient gets reduced, followed by a decrease in the vortex volume near the airfoil leading edge with the increase of particle diameter, so that plenty of particles gather on the pressure surface of that airfoil. At the stage of increasing the airfoil angle of attack, with the increase of particle concentration, there is a gradual decrease of the peak lift coefficient and stall angle of attack of the airfoil, as well as a corresponding decrease of the drag coefficient divergence angle of attack and peak value. In contrast, when the airfoil angle of attack is decreased, the airflow reattachment process obviously lags behind that of the clean air. As the particle concentration increases, the airfoil separation point occurs earlier, and the higher the concentration, the earlier the separation point. The erosion maximum airfoil erosion rate increases with the particle concentration.

Modeling the Breakup of Oil–Particle Aggregates in Turbulent Environments for Projectile Penetration

After an oil spill incident, the spilled oil slicks are observed to migrate to the shoreline area. Under the turbulent conditions, they break into small droplets and are suspended in the water column. The dispersed droplets are expected to interact with the suspended particles and form the oil-particle aggregates (OPAs), which significantly changes the transport of the oil. Instead of an earlier assumption that particles cover the oil surface, thus preventing further breakage or aggregation of OPAs, recent studies demonstrated that particles act like projectiles penetrating the oil droplets, resulting in the breakage of OPAs over a longer period of time. A model looking into the OPA breakup through two breakup mechanisms was proposed for the first time. The first method depicted the breakup of one large OPA into two daughter droplets owing to the turbulent nature, while the second method demonstrated the tear of the OPA surface layer caused by particle uprooting. The model was then calibrated by an experimental study targeting crude oil with varied viscosities, along with previous experimental investigations. Three key factors were identified accounting for the breakage of OPAs, where the increase in particle concentration in the natural environment and the increase in turbulent energy of the surrounding flows benefited the breakage of OPAs, and the increase in oil viscosity suppressed the breakage due to large resistance to shear stress. Besides these elements, the impact of the particle shape on the penetration depth was discussed. The model serves as a fundamental theory to describe the evolution of OPAs for fragmentation behavior.

Feasibility of applying an electrostatic precipitator integrated with a naturally ventilated double-skin façade in residential buildings

One of the main advantages of applying a double-skin façade (DSF) is allowing natural ventilation. As concerns about high outdoor particle concentrations increase, maintaining comfortable indoor air quality through natural ventilation in buildings with DSFs becomes difficult. An electrostatic precipitator (ESP) integrated with a DSF (EPID) has been proposed for maximizing the natural ventilation in buildings with polluted outdoor air. In this study, the particle removal performance and ozone generation of the EPID were experimentally evaluated to determine the feasibility of applying it in residential buildings. An EPID prototype was constructed, and laboratory experiments were performed to measure the single-pass particle removal efficiency and concentration of ozone, a byproduct of the ESP. The results showed that the average particle removal efficiency (77.4 ± 5.5%) at the upper ESP position is similar to that at the lower ESP position (78.5 ± 11.6%) for 0.25–1.82 μm particles at 9 kV applied voltage and 0.5 ms−1 inlet air velocity. When the inlet air velocity increases to 2.0 ms−1, the particle removal efficiency decreases to 39.7% depending on the ESP position. Moreover, the ESP position significantly influences the particle removal efficiency because it changes the inlet air velocity, especially at low applied voltages. Our results highlight the importance of determining the appropriate ESP position in the EPID for residential applications because low-voltage operation is recommended for the safety of occupants and reduction in ozone generation. In addition, at 0.5 ms−1 inlet air velocity, at which the EPID has excellent removal performance, the net ozone concentrations are 33.0 ppb and 35.7 ppb at 6 kV and 9 kV, respectively, which are all lower than the threshold (50 ppb), regardless of the ESP position. The results of this study verify the feasibility of applying the EPID and have important implications for future EPID designs and developing control strategies to improve the indoor air quality of naturally ventilated residential buildings with DSFs.

On the influence of stress state on ductile fracture of two 6000-series aluminium alloys with different particle content

Tension-torsion tests were conducted on two 6000-series aluminium alloys with different area fraction of constituent particles. The two alloys, denoted alloy A and B, have previously been characterized and found to have similar matrix material, albeit the three times higher area fraction of constituent particles in alloy B than in alloy A. Single notch tube specimens of the two alloys were subjected to fifteen proportional load paths by varying the ratio of axial force and twisting moment, probing stress states from torsion to plane-strain tension. The overall failure strain in the notch was estimated analytically based on the experimental data, whereas finite element simulations were used to determine the stress and strain fields within the notch region and to estimate the local failure strain. The experiments showed that the increased particle content led to a reduction in the local failure strain of alloy B compared with alloy A that varied from 16% to 60%, depending on the stress state, with an average reduction of 39%. While the overall trend was an increasing failure strain with decreasing stress triaxiality, significant influence of the Lode parameter was observed, and thus the increase was not monotonic. Applying a porous plasticity model, localization analyses were conducted to examine the underlying mechanisms for the complex variation of the failure strain with stress state.

Material Removal Characteristics of Spherical-Array-Focused Ultrasonic Abrasive Machining.

To improve the ultrasonic energy and realize far-field ultrasonic abrasive machining of complex surfaces, a spherical-array-focused ultrasonic abrasive machining system was established. By combining ultrasonic field simulation, detection and a single-factor experiment, the influences of the ultrasonic generator current, abrasive concentration, and particle size on the material removal properties and surface quality evolution of quartz glass were investigated. When the current was less than 0.4 A, the material removal showed plastic removal at the nanoscale. When the current was more than 0.5 A, the cavitation phenomenon formed micron-scale impact removal traces on the workpiece surface. The increase in abrasive concentration increased the impact density and material removal rate, while excessive abrasive concentration increased the impeding effect between abrasive particles and reduced the material removal rate. Moreover, the increase in abrasive particle concentration enhanced heterogeneous cavitation nucleation, promoted the removal of abrasive impact materials under the action of a cavitation jet, and inhibited the removal of direct surface cavitation. The abrasive particle size affects the heterogeneous cavitation nucleation and the acceleration of the cavitation jet on abrasive particles, which affects the material removal rate and surface quality. By controlling the energy of the focused ultrasound and abrasive parameters, the plastic or brittle domain removal of quartz glass can be achieved at the micro- and nanoscales.

A multi-scale study of the physical properties of recycled aggregate pervious concrete containing waste glass particles

ABSTRACT With the rapid expansion of industrial and civil construction, the demands for concrete and its production has increased significantly, resulting in massive wastage. An effective strategy to recycle solid waste involves its aggregation for producing concrete. In this study, through two layers of mortar and concrete, the changing rule and influence mechanism of waste-glass particles on the mechanical properties and permeability of recycled aggregate pervious concrete (RAPC) were analysed. The results indicated the efficacy of using waste-glass particles instead of natural river sand in promoting hydration and enhancing the calcium silicate hydrate (C–S–H) content, thereby improving the compactness and pore structure of concrete. Specifically, the strength of RAPC increased with an increase in waste-glass particle content, whereas the permeability exhibited a reverse trend. When the replacement rate of waste-glass particles is 100%, the compressive strength of RAPC peaks at 24.19 MPa, which is approximately 66.48% greater than that of pervious concrete manufactured using natural river sand, and its permeability coefficient is 0.63 mm s−1, which meets the requirements set for pervious concrete. Faced with limited availability of natural mineral resources, the substitution of fine aggregates with waste-glass particles is conducive to sustainable development.

Study of Particulate Fouling Inhibition Characteristics on a Novel Composite Coating

Particulate fouling is a common fouling in heat exchange equipment, it causes tube corrosion and increases flow resistance. Particulate fouling increases the hidden danger of equipment and requires high treatment costs. In this paper, a novel Ni−P−TiO2 composite coating is prepared on 316 stainless steel using electroless plating and the fouling inhibition characteristics of the novel composite coating are studied using a dynamic monitoring experimental system. The experimental results show that the fouling thermal resistance of the Ni−P−TiO2 composite coating is obviously lower than that of 316 stainless steel under the same working conditions. With the increase in cooling water velocity and inlet temperature, the surface fouling thermal resistance decreases, while, with the increase in particle concentration, the fouling thermal resistance increases. Based on DLVO theory, it is found that the surface energy of Ni−P−TiO2 composite coating is close to the best surface energy for inhibiting particulate fouling deposition, which can significantly inhibit particulate fouling deposition. Compared with the stainless-steel surface of a conventional plate heat exchanger, the Ni−P−TiO2 composite coating not only inhibits the accumulation of particulate fouling, but also reduces the adhesion strength of particulate fouling; additionally, the fouling is easier to strip off the heat exchange surface, which realizes the lasting and efficient fouling inhibition on the heat exchange surface. The research results can provide a data reference for the fouling inhibition design and daily efficient operation of heat exchangers.