Heat Exchanger Surfaces Research Articles

Numerical Modeling of Metal Hydride-Phase Change Material Hydrogen Storage Systems with Increased Heat Exchange surface area

Coupling Metal Hydrides (MH) with Phase Change Materials (PCM) provides a promising way of storing hydrogen without using any active systems to control the MH temperature during cycling. In this study, we perform numerical assessments on the performance of cylindrical MH-PCM storage systems with varying geometrical configurations. We investigate the ab/desorption kinetics in a straight cylindrical geometry (case A), where the PCM surrounds the MH. Its cycle time and gravimetric density for the absorption of 1.04kWh are 2,200 s and 0.30%. We then investigate two improved configurations, namely the addition of fins to the external surface of the MH and within the PCM domain (case B), and the containment of the MH powder into a helix-shaped tube immersed in the PCM (case C). Number and thickness of fins are also varied to assess their impact. Compared to case A, the helix-shaped design is 49% and 40% faster in absorption and desorption, respectively, whereas the finned geometry is 28.9% and 30% faster. The gravimetric density is 0.27% and 0.62% in case B and C, respectively.

Corrosion-Resistant Polymer Composite Tubes with Enhanced Thermal Conductivity for Heat Exchangers

The heat transfer surfaces of heat exchangers are usually made of metals which may suffer from severe corrosion. When corrosive fluids are present, highly corrosion-resistant metals, graphite or ceramics are used, resulting in high costs. This study presents measured data on the thermophysical and mechanical properties of recently developed corrosion-resistant polymer composite tubes for use in heat exchangers. Extruded polymer composite tubes based on polypropylene or polyphenylene sulfide filled with graphite flakes were investigated. The anisotropic thermal conductivities of the polymer composite tubes were measured at various temperatures. The through-wall thermal conductivity of the tubes made of polypropylene filled with 50 vol.% graphite is increased by a factor of 30 compared to pure polypropylene, resulting in a thermal conductivity of 6.5 W/(m K) at 25 °C. The tubes composed of polyphenylene sulfide filled with 50 vol.% graphite have a through-wall thermal conductivity of 4.5 W/(m K) at 25 °C. The mechanical properties of the polymer composites were measured using tensile and flexural tests at different temperatures. The composite materials are more rigid and keep their mechanical properties up to a higher temperature level compared to the unfilled polymers. Surface roughness measurements show the very smooth and sealed surface of the composite tubes. The results contribute to establishing the viability of using polymer composites for heat exchanger applications with corrosive fluids.

Optimization of Triply Periodic Minimal Surface Heat Exchanger to Achieve Compactness, High Efficiency, and Low-Pressure Drop

With advancements in additive manufacturing (AM) techniques, high-quality triply periodic minimal surface (TPMS) structures can now be produced. TPMS walled heat exchangers (HX) hold significant potential for industrial applications and are receiving increasing attention. This paper explores the impact of various TPMS design variables on flow and thermal performance to optimize TPMS heat exchangers for compactness, high efficiency, and low pressure drop. The design variables examined include the type of TPMS lattice, unit cell size, wall thickness, aspect ratio, TPMS orientation, and equivalent thickness. The study reveals that the flow and heat transfer performance of TPMS structures are significantly affected by these design variables. For the Gyroid, Diamond, and SplitP lattices, performance is nearly identical when the surface-to-volume ratio is kept constant. The average velocity of the fluid in the TPMS HX should be 0.3 m/s. The corresponding Re is between 300~800. Thin wall thickness, small equivalent thickness, and flat lattice configurations can significantly reduce pressure drop while maintaining the overall heat transfer coefficient. Additionally, the angle between the flow direction and TPMS orientation can increase pressure drop. Three aluminum heat exchangers were successfully printed using an AM machine, and testing results are comparable with theoretical prediction.

Entropy generation and thermal behavior of microchannel fitted with punched curved vortex generators

Utilizing curved vortex generators (VGs) has emerged as a promising way to improve heat transfer in heat exchangers. However, research on the placement of punched rectangular curved winglet VGs on two opposing heat transfer surfaces of microchannel heat exchangers is limited. Through experiments and simulations, the effects of five perforation positions and three pitches on fluid flow and heat transfer were examined. This analysis is performed in the turbulent flow regime (2500 < Re < 3500) and the governing equations are solved by using SST k-ω model, assuming that the fluid is incompressible, has constant thermal properties, and ignores the effects of thermal radiation and gravity. Metrics such as the Nusselt number ratio, friction coefficient ratio, secondary flow intensity, thermal enhancement factor, and entropy generation are used to analyze the flow dynamics of mixed vortices. The results indicate that different perforation positions significantly affect the vortex characteristics, altering heat transfer and pressure loss. Furthermore, the pitch of the VGs plays a crucial role in governing heat transfer and frictional losses. Specifically, the heat transfer rate increased by 12.1 % to 61.1 % compared to empty tubes, accompanied by a rise in pressure loss ranging from 50.6 % to 115.1 %. To validate the findings, the principle of entropy generation was utilized. Notably, under conditions of a Re of 2500, a perforation position proximal to the VG's leading edge (θ = +12°), and a pitch of 15 mm, a remarkable thermal enhancement factor of 1.29 was achieved.

Rapid aerodynamic characterization of surface heat exchangers for turbofan aeroengines through optical techniques and additive manufacturing

Surface heat exchangers that use the bypass flow as heat sink are becoming a widely used solution to alleviate the high thermal load of modern aeroengines. Experimental characterization of such heat exchangers in full-scale engine tests is extremely expensive and time-consuming, so carrying out experiments in scaled wind tunnels that can replicate their very challenging flow conditions is highly desirable. However, intrusive instrumentation can affect the actual aerodynamics of the component due to the reduced size of the section and the typical high air velocities of turbofan engines. For this reason, a methodology to characterize a surface heat exchanger designed to work in the turbofan bypass using optical techniques is presented in this work. Additionally, the possibility of replicating the experimental conditions using additively manufactured models of exchangers would allow rapid preliminary characterization of these components. In this investigation, different non-intrusive techniques such as PIV, LDA, Schlieren, or LDV have been applied to determine the heat exchanger aerodynamic performance and vibration response, and a detailed characterization of the flow field has been carried out. Data have been cross-validated using different measurement techniques. A 3D-printed model has also been built to compare with the aluminum heat exchanger, showing almost an identical behavior in terms of velocity distribution downstream of the heat exchanger and pressure drop induced by its fins, as well as corrected frequencies, confirming thus the suitability of additive manufacturing for the aerodynamic characterization of these devices in preliminary stages.

Heat Transfer Enhancement in a 3D-Printed Compact Heat Exchanger

The study describes experimental data on thermal tests during the condensation of HFE7100 refrigerant in a compact heat exchanger. The heat exchanger was manufactured using the additive 3D printing in metal. The material is AISI 316L steel. MPCM slurry was used as the heat exchanger coolant, and water was used as the reference medium. The refrigerant was condensed on a bundle of circular tubes made of steel with an internal/external diameter of di/de = 2/3 mm, while a mixture of water and phase change materials as the coolant flowed through the channels. Few studies consider the heat exchange in condensation using phase change materials; furthermore, there is also a lack of description of heat exchange in small-sized exchangers printed from metal. Most papers deal with computer research, including flow simulations of heat exchange. The study describes the process of heat exchange enhancement using the phase transition of coolant. Experimental data for the mPCM slurry coolant flow was compared to the data of pure water flow as a reference liquid. The tests were carried out under the following thermal and flow conditions: G = 10–450 [kg m−² s−1], q = 2000–25,000 [W m−²], and ts = 30–40 [°C]. The conducted research provided many quantities describing the heat exchange in compact heat exchangers, including heat exchanger heat power, heat exchange coefficient, and heat exchange coefficients for working media. Based on these factors, the thermal performance of the heat exchanger was described. External characteristics include the value of the thermal power and the heat exchange coefficient as a function of the mass flow density of the working medium and the average logarithmic temperature difference. The performance of the heat exchanger was presented as the dependencies of the heat exchange coefficients on the mass flux density and the heat flux density on the heat exchange surface. The thickness of the refrigerant’s condensate film was also determined. Furthermore, a model was proposed to determine the heat exchange coefficient value for the condensing HFE7100 refrigerant on the outer surface of a bundle of smooth tubes inside a compact heat exchanger. According to experimental data, the calculation results were in good agreement with each other, with a range of 25%. These data can be used to design mini condensers that are widely used in practice.

Control of deposits on pipeline systems by the method of free oscillations

RELEVANCE. There are a large number of heat exchangers in operation enterprises that operate under various temperature conditions. Steam, hot water, heated products of oil refining and other industries, which may contain oxides of iron, aluminum, calcium sulfate silicates, etc., are used as a heating agent. The efficiency of the heat exchange equipment depends on the condition of the heating surfaces. Contamination of these surfaces with various deposits dramatically reduces the heat transfer coefficient, which leads to a significant increase in heat consumption. The nature of the deposits depends on the properties of the heating agent and the heated medium.THE PURPOSE. The purpose is to assess the possibility of controlling deposits on the surfaces of heat exchange equipment by the free oscillation method. The presence of deposits changes the mass of structures and, consequently, the natural frequencies of vibrations, the analysis of which can determine not only the presence and thickness of deposits, as well as their appearance.METHODS. The paper shows the results of experimental studies conducted using a hardware and software package and calculations of natural vibrations of the surfaces of heat exchange equipment in the ANSYS software package to identify their dependence on the thickness and density of deposits. A plate, a pipe, and a pressurized pipe were modeled as heat transfer surfaces.RESULTS. The results obtained experimentally and computationally coincided, and it was concluded that the free oscillation method makes it possible to determine not only the presence of deposits on heat exchange surfaces, but their thickness and appearance.CONCLUSION. The free oscillation method can detect deposits in a timely manner and monitor their condition, which in turn will, reduce overall operating costs, increase energy efficiency and extend service life.

Innovative Fixed-Bed Reactor Integrated with Heat Transfer System for Lean Methane Mixture Removal

A new type of compact, portable fixed-bed reactor integrated with a heat transfer system was developed for the removal of volatile and flammable air pollutants such as lean methane and volatile organic compounds (VOCs). The reactor may operate in catalytic or thermal combustion conditions with the purpose of achieving autothermal processes with the possibility of energy recovery. An excess heat recovery point was designed behind the reactor bed at the place where the gas temperature is the highest to enable its usage. The mathematical model is presented together with a number of simulation calculations performed for the assessment of the developed reactor. The case study in this paper was for catalytic methane oxidation at a temperature of 400 °C, a methane concentration between 0.1% and 2% by weight, a gas flow rate of 1 m3/s STP, and a heat exchange surface for the assumed plate exchanger from 10 to 200 m2. The calculations show that the thickness of the insulation is of little importance for the operation of the equipment, and a sufficient thickness was about 20–50 mm. The optimal area for the considered case is 80–100 m2. It was found that recovery of thermal energy is possible only for higher methane concentrations, above 0.3% by weight. Using an appropriate surface for the exchanger, it is possible to recover even 50% of the combustion enthalpy at a methane concentration of 0.45% by weight. For an exchanger area below 50 m2, the recoverable energy drops rapidly. It was found that the exchanger area is the most important equipment parameter under consideration.

AUTOMATION OF MEASUREMENTS OF ENGINE HEAT CONSUMPTION IN THE LUBRICATION AND COOLING SYSTEMS

The paper presents the results of an intermediate stage of the study of heat transfer processes in high-speed diesel engines of the automotive tractor type at transient load discharge-load-injection modes, which are characteristic of this type of diesel engines. In such modes, as evidenced by the results of calculations of the thermal stress state of combustion chamber parts, in particular pistons, exhaust valves of the gas distribution mechanism, significant tensile and compressive stresses are observed compared to operation at steady-state conditions. This is the reason for the occurrence of defects in the form of cracks on the heat exchange surfaces of these parts, which limits the service life of automotive tractor engines. For a more detailed study of transient processes, as the main approach, it is proposed to use the well-known method of heat balance tests of a high-speed diesel engine, but with the involvement of automated processing of measurement results in digital form at steady-state and transient modes. It is proposed to focus on heat consumption, which is a component of the heat balance in the lubrication and cooling system. Without the use of automated information processing directly at the time of transient load shedding and load shedding, it is impossible to obtain such information on cooling and lubrication systems. Recording of transient heat transfer processes makes it possible to track the influence of engine operating parameters, the dynamics of the transient process depending on its duration, the initial and final steady-state modes between which the transition is being studied. In general, this way, it is possible to assess the adaptability of cooling and lubrication systems to heat dissipation during sudden changes in load, to avoid, or at least limit, the growth of thermal stresses. The publication presents a possible variant of the schematic solution of such a system for automated processing of the results of heat balance tests with the maximum use of equipment that is mass-produced and has already been used previously in heat balance tests.

Hybrid of non-dominated sorting genetic algorithm II and invasive weed optimization for fan duct surface heat exchanger configuration optimization design

In this research, configuration optimization was conducted for FDSHX (fan duct surface heat exchanger) that is a new type of heat exchanger adopted in aero-engine heat management in recent years. Firstly, a method of Taguchi-ANFIS (Taguchi-Adaptive Neuro-Fuzzy Inference System), which can reduce training data volume utilizing orthogonal experimental matrix, was proposed to construct a rapid response mathematical model between three configuration parameters, including fin pitch, fin thickness, and oil passages number, and two design indicators, including heat transfer capacity and operating weight, based on the data prepared by an experimental validated FDSHX heat transfer capacity calculation method using heat transfer unit simulation. Secondly, a hybrid method of NSGA II-IWO (Non-dominated Sorting Genetic Algorithm II-Invasive Weed Optimization), which can simultaneously obtain the strong abilities of exploration and escaping from trap into local optima, was proposed to drive the constructed response mathematical model to enhance heat transfer through adjusting the three configuration parameters. Optimization comparison was performed between NSGA II-IWO and four classic optimization algorithms including GA (Genetic Algorithm), IWO (Invasive Weed Optimization), CA (Cultural Algorithm), and HS (Harmony Search). This research provides an integrated solution to help mechanical engineers improve the applicability and reasonableness of FDSHX design.

Analysis on heat transfer enhancement mechanism in a cross-wavy primary surface heat exchanger based on advection thermal resistance method

This paper numerically investigates the flow and heat transfer characteristics in the primary surface heat exchanger (PSHE) with cross-wavy (CW) structures. The comprehensive performance affected by hydraulic diameters is evaluated. Moreover, the airflow shuttling behavior at the mixing area of CW-type PSHE is discussed, showing rapid heat transfer enhancement. The advection thermal resistance method and local thermal resistance analysis is proposed, while the impacts of longitudinal pitch and flowrates are considered. Results show that the case with a large hydraulic diameter displays much better comprehensive performance at lower flowrates. When raising the hydraulic diameter from 1.58 mm to 15.8 mm, the heat transfer rate per unit pumping power grows by 36.1 %. However, the priority of large channel is gradually disappeared after increasing the flowrates. Meanwhile, the larger longitudinal pitch of the CW channel may result in pronounced improvement on the heat transfer performance due to the presence of airflow shutting behavior at the mixing area as well as the secondary flow near the channel boundary layers. When no airflow shuttling exists, very high advection thermal resistance region can be observed due to the formation of boundary layers. It can be recognized that the case with airflow shuttling behavior can display similar heat transfer improvement compared to that with increasingly high Reynolds numbers, yet the pressure loss is rarely increased.

Performance analysis of precooling systems for cryogenic carbon capture: A comparative study of theoretical, numerical, and experimental methods

The urgency of addressing climate change has necessitated innovative and practical approaches to reducing greenhouse gas emissions. Cryogenic Carbon Capture (CCC) technology has emerged as a promising solution for capturing CO2 from industrial emissions. This study investigates a simplified design for a precooling system within CCC technology, focusing on optimizing the efficiency and reliability of the system while minimizing energy consumption. The research examines the effects of CO2 concentration in a gas mixture (simulated flue gas, nitrogen and carbon dioxide), cooling bath temperature, and gas mixture flow rate on the performance of heat exchangers in the precooling system. Theoretical, numerical, and experimental analyses were conducted to assess the system's performance. The use of liquid nitrogen at −196 °C in the cooling bath was found unsuitable due to CO2 freezing within the pipes. Instead, a mixture of dry ice and isopropyl alcohol at −78.5 °C proved effective in maintaining consistent temperature and pressure conditions without causing CO2 blockages. Numerical simulations highlighted the importance of optimizing pipe length to balance temperature control and pressure dynamics, with a 140 cm pipe length identified as optimal for heat exchanger design. Experimental results revealed that longer pipes enhance cooling performance due to increased heat exchange surface area, while higher CO2 compositions and flow rates result in higher outlet temperatures and pressures. The study emphasizes the need for further research and simulation work to refine heat exchanger designs for industrial applications, aiming to develop systems that maximize cooling efficiency while ensuring reliable operation.

Entropy optimization in ternary hybrid nanofluid flow over a convectively heated bidirectional stretching sheet with Lorentz forces and viscous dissipation: A Cattaneo–Christov heat flux model

Bidirectional stretching sheet models can represent surfaces in heat exchangers where fluids flow under continuous deformation. Ternary hybrid nanofluids could be employed in these systems to increase heat transfer rates between fluids in heat exchangers and improve the efficiency of energy conversion processes. In this work, we explore the novel application of a ternary hybrid nanofluid (water with titanium dioxide, cobalt ferrite, and magnesium oxide nanoparticles) for enhanced heat transfer in heat exchangers modeled by a bidirectional stretching sheet. This approach offers a potential advancement over traditional nanofluids (with one or two nanoparticles). Furthermore, we present a comprehensive analysis that incorporates ohmic heating, Cattaneo–Christov heat flux, thermal radiation, viscous dissipation, and irreversibility. The governing equations are transformed into a system of nonlinear ordinary differential equations using appropriate similarity transformations and then solved using the bvp4c solver, a MATLAB built-in function. This study's findings reveal that the Eckert number and radiation parameter increase fluid temperature, while the thermal relaxation parameter leads to a reduction in the temperature of the fluid. It is detected that an increase in magnetic field parameter and volume fraction of [Formula: see text] results in a decline of the skin friction factors in both directions. It is revealed that there is a reduction in the Nusselt number with the rise in Eckert number ([Formula: see text]), and the same number declines at a rate of 0.8252 when [Formula: see text]. It is noticed that the skin friction coefficient declines at a rate of 0.62179204 (in case of x-direction) and 0.621791816 (in case of y-direction), respectively, when the values of magnetic field parameter lie between 0 and 3.5. Furthermore, it is noticed that an upsurge in thermal relaxation parameter results in a fall in the temperature of the fluid.

A computational investigation for heat and fluid transport with electro-osmosis phenomenon in a scraped surface heat exchanger

Scraped surface heat exchangers (SSHEs) have prominently shown their eminence in various food, chemical and in pharmaceutical industries when the regular processing of fluids and pertinent materials is encountered. The heat transfer analysis of Williamson fluid flow with electro osmotic effects in a narrow-gap SSHE is designed by implementing constant temperature difference between the gaps of the rotor and the stator. A fluid model is established by using lubrication approximation theory and Debye-Huckel simplifications. The computational solution is elaborated by implementing renowned BVP4C technique in MATLAB. Exact solution of Poisson-Boltzmann equation for different scenarios of SSHE is gathered, whereas solution for velocity, stream functions and temperature in various parts of SSHE are explored computationally. Impact of various crucial physical flow parameters like, Weissenberg number, mobility of the medium number, electro osmotic parameter on the velocity profile, stream function, temperature profile, and pressure gradient has been explored graphically. The numerical solution is compared with artificial neural network (ANN) and an absolute alignment of BVP4C solution is observed with ANN solution. It is reported that fluid flow is lifted with enhancing the viscoelastic behavior and exhibits dual behavior for electroosmosis phenomenon and mobility of medium parameter. Also, it is seen that temperature of the fluid is significantly declines with Weissenberg number and is surges with enhancing mobility of medium and Brinkman number. By increasing the Brinkman number conduction phenomenon is slow down and heat produced due to viscous dissipation, raises the temperature of the fluid. The viscoelastic effect could be of enormous significance in applications, like Polymer processing in certain heat exchangers, and in mixing and blending foodstuffs etc.

Electrochemical-thermal analysis of large-sized lithium-ion batteries: Influence of cell thickness and cooling strategy in charging

With the transition to electric propulsion accelerating, lithium-ion batteries (LIBs) are essential due to their high energy density and efficiency. However, higher energy requirements and rapid charging pose significant safety and performance challenges, particularly for large cells. This research uniquely investigates the thermal and electrochemical responses of large-sized prismatic LiFePO4 cells under various cell thicknesses (ranging from 11.3 mm to 90 mm), charging conditions (ranging from 0.3C to 1C), and cooling strategies. The model reveals that higher charging rates markedly increase temperature and variability within the cells. Charging a 45 mm-thick cell at a 1C rate and with natural air cooling results in a temperature rise of 24.3K and a deviation of 6.18K. In contrast, charging at 0.3C results in 5.4K and 1.17K, respectively. Despite the smaller heat exchange surface, bottom-cooling is most efficient for thick cells due to significant anisotropic heat conduction. Single-side cooling becomes more effective for thinner cells. A critical cell thickness for such transition exists at around 22.5 mm. Double-side cooling consistently outperforms single-side cooling, significantly reducing temperature deviations across various cell thicknesses. Best cooling performance is seen in the case of the thinnest cell (11.3 mm) with double-side cooling, with only 3K temperature rise and 1K deviation, at 1C charging. Additionally, improved thermal management during charging slightly increases heat generation of the cell, due to the higher resistance at lower temperatures, elevating ohmic heat generation from separators and electrodes. Polarization heat from the negative electrode is identified as the dominant heat source, driven by increased overpotential at higher temperatures due to elevated Li ion concentration at the electrode surface. The findings provide crucial insights into the thermal and electrochemical behaviors of large-sized LIBs, offering valuable guidance for the design and management of these LIB systems to enhance safety, performance, and longevity in electric vehicle applications, addressing a pressing need in the field.

Numerical simulation of non-stationary regime of a submerged combustion setup operation

Relevance. The need to evaporate large quantities of brines at potash industry enterprises. Evaporation of brines in surface evaporators is difficult due to the encrustation of heat exchange surfaces by salt deposits. Therefore, such evaporation is most expedient to be carried out in submerged combustion apparatuses, since they do not contain heat-transmitting surfaces. However, in this type of apparatus, malfunctions may occur due to uncontrolled solid phase deposition. At the moment, the dynamics of the solid phase in submerged combustion devices is poorly studied. This study is part of a scientific program aimed at a comprehensive review of the laws of motion of solid particles in submerged combustion apparatuses. Aim. To study the hydrodynamic processes in the submerged combustion setup in the time interval corresponding to the beginning of its operation; describe the patterns of solid phase motion as a function of time. Object. Laboratory setup of submerged combustion. A simplified model of the thermal mode of operation without the subsequent transition of the liquid phase to steam is analyzed. Methods. The study was conducted by numerical experiment. The hybrid finite volume method was used in simulation in combination with the technology of the finite element method. The multiphase system was considered as two coexisting subsystems: gas–liquid and liquid–solid. Results. The paper considers the final time interval of the setup operation. It is found that during the time under consideration, a stationary mode of solid particle deposition is achieved. The authors have detected liquid flow velocity oscillations, leading to fluctuations in the mass flow rate of solid particles at the bottom of the setup. It was found that the velocity at the tip of the flue gas jet escaping from the burner nozzle, as well as the pressure at the nozzle section, have a similar form of oscillation. The authors substantiated the hypothesis about the determining influence of the instability of the jet movement of flue gases on the oscillatory behavior of the entire hydrodynamic system.

Hydrothermal analysis of hybrid nanofluid flow inside a shell and double coil heat exchanger; Numerical examination

In a shell-and-coil heat exchanger, employing a double coil instead of a simple coil causes more heat transfer compared to a simple coil because the double coil improves heat exchange through enhanced fluid flow and turbulence, enabling a longer tube length in constrained locations.. In addition to reducing fouling and the requirement for maintenance, the helical shape may also provide the benefit of a self-cleaning action. Ensuring a more even flow distribution also maximizes the heat exchanger's surface area use and reduces low flow zones. Applications needing compact solutions without compromising performance especially benefit from this design's versatility, allowing customization to meet unique operational demands. In numerous industrial applications, helical coils offer substantial benefits in terms of effectiveness, upkeep, and design freedom.In the present work, heat transfer and fluid flow in a shell-and-coil tube heat exchanger equipped with a double helical coil are evaluated numerically. The employed heat transfer fluids are water-based hybrid nanofluids, including TiO2-Mgi/water and AG-HEG/Water. The obtained outcomes are compared with the results of pure water. The range of considered Reynolds numbers is 500 – 2000. This work consists of two sections. In the first part, the impact of the type of hybrid nanofluid on the hydrothermal behaviour of the proposed heat exchanger is analysed. The volume concentration of the nanoparticles here is ϕ1 = ϕ2 = 0.3. In the second section, the best hybrid nanofluid is selected according to the results of the first section; then, the impact of the nanoparticles' volume concentration on the thermal performance of the proposed heat exchanger is evaluated. Three various nanoparticle volume concentrations, including ϕ1 = ϕ2 = 0.1, ϕ1 = ϕ2 = 0.3, and ϕ1 = ϕ2 = 0.5, are considered, and the results are compared with those of the pure water case (ϕ1 = ϕ2 = 0).The obtained results show that amongst the different heat transfer fluids considered, the Water/MgO-TiO2 model shows the most outstanding value of thermal performance in all the studied Reynolds numbers. Also, the obtained numerical results show that the use of the proposed double coil tube leads to an increase in the heat transfer rate.

EXPERIMENTAL ANALYSIS OF THE INFLUENCE OF FAN SPEED ON FROST GENERATION AND ITS IMPACT ON A HEAT PUMP

The phenomenon of frost on the heat exchanger surfaces penalizes the performance of the heat pump, one of the operating parameters that influences the evaporation temperature, this is transferred to the formation of frost, and its impact is the amount of air passing through the evaporator, in that sense, an air-water heat pump has been experimented at different fan speeds and environmental conditions inside a climatic chamber. The results allow to obtain different parameters to characterize the frost formation process and to make an effective comparison between them. The lower the fan speed, the lower the air flow rate, the lower the heating time, and the higher the number of defrost cycles required for the same environmental conditions. In this way, some correlations could be developed to predict how the frosting process is affected by the fan speed in more conditions that are not tested. The aim is to determine to what level frost formation can be suppressed by achieving different fan speeds whether it is technically and economically feasible and what implications imposes on the heat pump performance.

Steerable droplet precise bouncing on a superhydrophobic surface with superhydrophilic stripes

The precise rebound of a droplet upon hitting a solid surface has garnered significant attention in recent years due to its critical applications in self-cleaning, printing industries, and the design of heat exchanger surfaces, among others. This study introduces an innovative approach that combines femtosecond laser processing with a high-temperature stearic acid modification to create surfaces that feature superhydrophilic (SHL) stripes on a superhydrophobic substrate. By controlling the offset distance between the droplet's impact point and the SHL stripe, we achieved a directional and precise rebound of the droplets. Our findings indicate that the lateral displacement of the droplet increases with the offset distance and always tilts toward the direction of the SHL stripe. This study also incorporates numerical simulations to validate the findings, shedding light on the energy conversion mechanisms at the liquid–solid interface during the impact, particularly during the retraction phase. This discovery is significant for more accurately predicting the specific landing spots of rebounding droplets.

A Screening Apparatus for Comparing the Fouling Resistance of Heat Exchanger Surfaces

AbstractFouling is a challenge in many processes, especially in heat exchangers and evaporators. Even though many treatments have been developed to mitigate fouling on metallic surfaces, no standardized method exists to compare equipment based on its antifouling performance. In this work, a screening apparatus is presented, which allows to compare samples of treated metallic surfaces based on thermal fouling resistance. A parameter screening and a ranking of three differently treated metallic surfaces using two model substances, namely, whey protein concentrate (WPC) and calcium sulfate, are demonstrated.