The team prepared the nanorefrigerant by first dispersing aluminium oxide nanoparticles in HFE-7000 through mechanical stirring and ultrasonication, then adding Tween-80 as a surfactant to stabilize the suspension. The two-step approach produced a nanorefrigerant that remained stable for 72 hours without visible sedimentation, a critical benchmark for practical refrigerant applications.

Testing followed American Society of Heating, Refrigerating and Air-Conditioning Engineers standards for refrigerator performance measurement, covering evaporator and condenser temperatures, compressor power consumption, and cooling capacity. The researchers varied nanoparticle loading from 0.01 to 0.1 percent by volume and measured performance across a range of operating conditions.

At 0.1 percent loading, the coefficient of performance rose by 18.4 percent compared with pure HFE-7000, while compressor energy consumption fell by 15.2 percent. Evaporator air temperature dropped by 6.8 percent, and cooling capacity increased by 12.4 percent, confirming that the nanoparticle suspension enhanced heat transfer on both the evaporator and condenser sides of the system.

The improvements stem from the higher thermal conductivity of the nanorefrigerant, which rose by 21.4 percent at 0.1 percent loading, and from changes in boiling behavior inside the evaporator. The nanoparticles promote nucleate boiling, increasing bubble frequency and improving the heat transfer coefficient during phase change.

Performance gains peaked at 0.1 percent loading and declined at higher concentrations. At 0.2 percent, the coefficient of performance fell below the peak as particle agglomeration increased fluid viscosity and raised pumping losses inside the compressor. The researchers identified this concentration as the optimal balance between heat transfer enhancement and flow resistance.

Tween-80 proved essential to the nanorefrigerant’s stability. Without the surfactant, nanoparticles settled within hours, degrading heat transfer performance. With it, the suspension remained uniformly dispersed, allowing the nanoparticles to circulate continuously through the refrigeration loop and maintain consistent performance across the test duration.

The study also examined compressor discharge temperature, which decreased by 4.2 percent with the nanorefrigerant, reducing thermal stress on the compressor and potentially extending its service life. Lower discharge temperatures also suggest reduced refrigerant decomposition risk inside the compressor.

The researchers conclude that aluminium oxide nanoparticles in HFE-7000 with Tween-80 offer a practical route to more energy-efficient domestic refrigeration, provided nanoparticle loading remains near the 0.1 percent optimum and surfactant stabilization is maintained throughout the system’s service life.

The nanoparticles at the heart of this study were produced through a citrate sol-gel route, a wet-chemistry technique favored for producing oxide powders with tight control over particle size and purity. Aluminium nitrate nonahydrate and citric acid, each at 0.6 molar concentration, were dissolved in distilled water and combined drop by drop while stirring at 80 degrees Celsius for eight hours. Raising the temperature to 200 degrees Celsius for three hours drove off the solvent and produced a fluffy polymeric citrate precursor, which was then ground and calcined in a muffle furnace at 800 degrees Celsius for three hours. The calcination step is what converts the organic-metallic precursor into the final crystalline oxide, and the furnace temperature strongly influences the resulting crystal structure and particle size.

Characterization confirmed the quality of the resulting powder. Powder X-ray diffraction verified high crystallinity, while field emission scanning electron microscopy and high-resolution transmission electron microscopy revealed a spherical morphology with an average particle diameter of roughly 22.5 nanometers. Energy-dispersive X-ray spectroscopy confirmed the elemental composition, and Brunauer-Emmett-Teller analysis measured a specific surface area of 26.2 square meters per gram with a mesoporous structure. That combination of small particle size and high surface area matters for refrigeration applications because heat transfer between the particles and the surrounding fluid scales with exposed surface, so finer particles with porous structures can exchange thermal energy more effectively per unit mass than coarser powders.

The choice of HFE-7000 as the base fluid reflects both safety and environmental considerations. Hydrofluoroethers of this class are odorless, non-flammable, and non-explosive, and they carry low global warming potential together with zero ozone depletion potential, positioning them as alternatives to conventional hydrofluorocarbon refrigerants that face tightening regulation. HFE-7000 also has a low boiling point and a low latent heat of phase change, which the researchers note reduces the heat input needed to induce boiling and limits heat losses in the test apparatus. These properties make the fluid well suited to bench-scale vapor compression rigs, though its thermodynamic characteristics differ from the refrigerants used in most commercial household units.

Dispersion stability is the central practical challenge for any nanorefrigerant. Nanoparticles suspended in a liquid experience strong van der Waals attractions that drive them to clump and settle, and refrigerant fluids, being notably low-polarity, offer little natural resistance to this agglomeration. The two-stage sonication protocol used here addresses the problem directly: an initial 60-minute ultrasonic bath breaks apart pre-existing agglomerates, and a second 30-minute sonication after Tween-80 addition distributes the surfactant evenly so it can coat the particle surfaces and provide steric stabilization. Surfactants of this type adsorb on oxide surfaces and create a protective barrier that keeps particles separated, which is why the treated suspensions resisted sedimentation over the observation period while untreated mixtures degraded quickly.

The concentration range explored in the work spans five formulations, and the behavior across that range illustrates a well-known trade-off in nanofluid engineering. At low loadings, added particles increase the effective thermal conductivity of the working fluid and provide additional nucleation sites for boiling, both of which improve heat exchange in the evaporator and condenser. As loading rises, however, particle-particle interactions increase the fluid’s viscosity, which raises the work the compressor must do to circulate the refrigerant and can promote agglomeration that negates the thermal benefits. The study identified an optimal window between 1.2 and 1.4 percent by weight, beyond which these penalties begin to outweigh the gains. The precise optimum in any given system depends on the balance between conductivity enhancement, boiling dynamics, and viscous losses, which vary with compressor design and operating temperatures.

The performance metrics reported in the study connect directly to the physics of the vapor compression cycle. An improved coefficient of performance means the system delivers more cooling per unit of electrical input, which translates into lower household energy consumption for the same refrigeration duty. Reduced discharge pressure and a lower compression ratio ease the mechanical load on the compressor, the component that typically dominates both the energy budget and the failure statistics of domestic refrigerators. Enhanced subcooling of the liquid refrigerant before it enters the expansion device increases the fraction of the evaporator length available for useful phase-change heat absorption, squeezing more cooling capacity from the same hardware.

Environmental context strengthens the case for this line of research. Domestic refrigerators run continuously, and their cumulative electricity demand is substantial on national grids, so even single-digit percentage efficiency improvements aggregate into meaningful energy and emissions savings when multiplied across hundreds of millions of appliances. Pairing a low-global-warming-potential base fluid with efficiency-boosting nanoparticles addresses both sides of the environmental ledger: the refrigerant itself contributes less to climate forcing if released, and the improved efficiency reduces the indirect emissions associated with electricity generation.

Several questions remain before such nanorefrigerants can move from laboratory rigs to commercial products. Long-term stability must be demonstrated over months and years of continuous circulation, not merely the hours observed in bench testing, since surfactants can degrade or desorb over time and particles can slowly deposit on heat exchanger surfaces or inside compressor components. The interaction between nanoparticles and compressor lubricants, the potential for particle abrasion of moving parts, and the manufacturability of consistently dosed nanorefrigerant charges all require further study. Cost considerations also matter, since the synthesis route described here is laboratory-scale and scaling citrate sol-gel production to industrial volumes would need its own economic analysis. Nevertheless, the demonstration that a carefully characterized, surfactant-stabilized alumina suspension can improve coefficient of performance while reducing compressor stress marks a concrete step toward refrigeration systems that cool more effectively while consuming less energy.

Subject of Research: Nanorefrigerant research

Article Title: Synthesis, characterization, and performance evaluation of Al2O3 nanoparticles in HFE7000 refrigerant for domestic refrigeration

Article References: Digra, M., & Singh, J. (2025). Synthesis, characterization, and performance evaluation of Al2O3 nanoparticles in HFE7000 refrigerant for domestic refrigeration. Frontiers in Thermal Engineering, 5, Article 1682295. https://doi.org/10.3389/fther.2025.1682295

Image Credits: AI Generated

DOI: 10.3389/fther.2025.1682295

Keywords: nanorefrigerant, HFE-7000, aluminium oxide nanoparticles, domestic refrigeration, coefficient of performance, Tween-80, thermal conductivity, energy efficiency, vapour compression system, nanofluid stability, sustainable cooling, heat transfer enhancement

The research team, led by Syed M. Hussain and colleagues, simulated steady magnetohydrodynamic natural convection inside a rectangular cavity whose heated left wall is shaped into a periodic zigzag pattern, while the opposite vertical wall is cooled. The cavity is completely filled with a porous medium, and the working fluid is a so-called ternary hybrid nanofluid: water seeded with copper oxide (CuO), aluminum oxide (Al2O3), and silver (Ag) nanoparticles. Each material brings a complementary property to the mixture. Silver has by far the highest thermal conductivity at 429 watts per meter-kelvin and an enormous electrical conductivity of 6.3 times ten to the seventh siemens per meter, but it is dense and expensive. CuO offers good conductivity at moderate density and low cost, while Al2O3 is the lightest and cheapest of the three. Blending all three lets designers tune thermal conductivity, electrical conductivity, viscosity, and density in ways a single-particle fluid cannot match.

The physics of the problem is governed by five dimensionless parameters. The Rayleigh number measures the strength of buoyancy relative to viscous and thermal diffusion forces; the Hartmann number captures the damping imposed by an externally applied magnetic field; the Darcy number describes how easily fluid can move through the porous matrix; the nanoparticle volume fraction determines how much the fluid’s thermophysical properties are altered; and the corrugation number counts the number of complete zigzag periods along the heated wall. The team compared a perfectly flat wall with a wall carrying sixteen corrugations, with a fixed amplitude-to-height ratio of 0.05 and a pitch-to-height ratio of 0.25, to isolate exactly what the geometric texture contributes.

Solving the coupled equations of mass, momentum, and energy in such a geometry is far from trivial. The authors used the Galerkin finite element method implemented in COMSOL Multiphysics, modeling the porous resistance with the Darcy-Brinkman formulation. They justified neglecting the quadratic Forchheimer inertial drag term by verifying that the pore Reynolds number remained far below unity across the entire parameter range studied. A careful grid convergence analysis showed that an extra-fine hybrid mesh of roughly 176,000 degrees of freedom was sufficient, and the model was validated against published benchmark solutions, with average Nusselt number deviations below 0.6 percent at a Rayleigh number of ten thousand and below 3 percent at one hundred thousand.

The simulations reveal a striking synergy between the nanoparticles and the corrugated wall. Raising the Rayleigh number from one thousand to one hundred thousand increased the average Nusselt number by 59.1 percent in the smooth cavity but by 68.0 percent in the corrugated one. Even more dramatically, increasing the total nanoparticle volume fraction from zero to five percent boosted heat transfer by only 9.2 percent in the smooth cavity, yet by a remarkable 86.6 percent in the zigzag cavity. The reason lies in how the corrugations repeatedly break up and renew the thermal boundary layer at each crest and valley, allowing the enhanced conductivity of the nanofluid to be exploited far more effectively than along a flat wall where the boundary layer simply thickens undisturbed.

The magnetic field tells a different story. As the Hartmann number rises from zero to fifty, the Lorentz force generated by the interaction of the induced electric currents with the applied field opposes the buoyancy-driven circulation, collapsing the average Nusselt number by about 85 percent in the smooth cavity and 82.9 percent in the corrugated one. Yet the corrugated geometry retains a partial advantage: even when the primary vortex is largely suppressed, local recirculation cells trapped within the corrugation valleys persist and maintain a semi-convective character, whereas the smooth cavity approaches a nearly pure conduction regime. In effect, the zigzag texture acts as a geometric amplifier that softens the blow of magnetic damping while magnifying the benefit of the conductive nanoparticles.

Heat transfer, however, is only half the picture. The study also performs a full second-law analysis, tracking entropy generated by thermal gradients, viscous friction, and magnetic effects. Stronger buoyancy raises both heat transfer and irreversibility, with total entropy generation climbing from about 2.3 to 9.3 as the Rayleigh number increases. Raising the Hartmann number cuts entropy generation by roughly 75 percent, but the heat-transfer loss is even steeper, so an ecological coefficient of performance, defined as the ratio of average Nusselt number to total entropy generation, falls sharply. The entropy contours show that thermal irreversibility dominates throughout the enclosure, while viscous and magnetic contributions concentrate near the corrugated wall where velocity gradients are steepest, illustrating the classic engineering trade-off between maximizing transport and minimizing thermodynamic loss.

The most forward-looking element of the work is its artificial neural network surrogate. The authors trained a feed-forward multilayer perceptron with five inputs, the Rayleigh number, Hartmann number, volume fraction, Darcy number, and corrugation number, to predict two outputs: the average Nusselt number and the total entropy generation. Using a Levenberg-Marquardt training algorithm on a database of converged finite element solutions, split into training, validation, and independent test sets, the network achieved a coefficient of determination of 0.99998 on both training and testing data, with a minimum validation error of about 4.72 times ten to the minus ninth at epoch 286. This means that once the high-fidelity simulations are done, designers can explore thousands of parameter combinations in seconds rather than hours, provided they stay within the range of the training database.

The authors are careful to spell out the limits of their idealized model. The simulations assume a two-dimensional, single-phase Newtonian fluid in local thermal equilibrium with the porous matrix, dilute nanoparticle loading capped at five percent, and laminar flow. Real devices such as packed-bed solar collectors, metal-foam heat exchangers, or geothermal reservoirs can exhibit thermal non-equilibrium between solid and fluid, nanoparticle agglomeration over time, and eventually turbulent behavior, none of which are captured here. The reported trends should therefore be read as design-oriented parametric guidance rather than a quantitative replacement for experiments on a specific industrial system.

What makes the study notable is less any single technique than the integration of all of them in one physical setup. Corrugated surfaces, porous media, magnetohydrodynamics, ternary nanofluids, entropy analysis, and machine learning have each been studied extensively on their own, but their coupled behavior inside a single zigzag-corrugated porous cavity had received little attention. By showing that wall texture acts as an amplifier of nanoparticle-driven conductivity gains while partially shielding the flow from magnetic damping, and by packaging those insights into a near-perfectly accurate predictive model, the researchers have outlined a practical pathway for designing the next generation of passive, magnetically controllable thermal-management systems, from electronics cooling to solar energy conversion.

Subject of Research: Magnetohydrodynamic natural convection and entropy generation of a ternary hybrid nanofluid in a zigzag-corrugated porous cavity with ANN-based prediction

Article Title: MHD natural convection and entropy generation of a CuO–Al₂O₃–Ag/water ternary hybrid nanofluid in a zigzag-corrugated porous cavity with ANN-based prediction

Article References: Hussain, S. M., Karim, M., Riasat, A., Khan, N. Z., Ullah, A., Jamshed, W., Eid, M. R., Guedri, K., & Ahmad, H. (2026). MHD natural convection and entropy generation of a CuO–Al₂O₃–Ag/water ternary hybrid nanofluid in a zigzag-corrugated porous cavity with ANN-based prediction. Results in Engineering, 32, Article 113358. https://doi.org/10.1016/j.rineng.2026.113358

Image Credits: AI Generated

DOI: 10.1016/j.rineng.2026.113358

Keywords: ternary hybrid nanofluid, natural convection, magnetohydrodynamics, porous cavity, entropy generation, zigzag corrugation, CuO nanoparticles, Al2O3 nanoparticles, silver nanoparticles, artificial neural network, finite element method, heat transfer enhancement

The core of the problem is familiar to anyone who has followed the thermal management of high-power devices. In a conventional single-pass microchannel heat sink, coolant enters at one end of the channel and exits at the other. Along the way, the fluid absorbs heat continuously, warming steadily from inlet to outlet. The result is a predictable but troublesome axial temperature gradient across the heated wall: the region near the coolant inlet stays relatively cool while the region near the outlet runs hot. For avionics packages, where multiple chips may sit above a single cooling plate, that unevenness translates directly into thermal stress, degraded solder joints, shifted calibration, and shortened service life. With heat fluxes in advanced avionics climbing well past levels that forced-air or simple liquid cooling can handle, temperature uniformity has become as important a design target as peak temperature itself.

Counter-current flow architectures attack the gradient problem elegantly. Instead of a single channel in which fluid travels one direction, the design splits the coolant into parallel passages that run in opposite directions, with inlet and outlet positioned at the same end of the device. The coldest fluid therefore passes alongside the hottest section of one passage while the warmest fluid sits above the coolest part of the neighboring passage, and the wall temperature profile becomes roughly symmetric about the midpoint of the heat sink. Prior research had already demonstrated that such counter-flow layouts can dramatically reduce the temperature difference across the heated wall, but most of the optimization work in the literature has concentrated on macro-scale channels or on smooth microchannels. What remained unclear—and what the new study set out to resolve—is how internal turbulators, specifically pin fins, behave when embedded in counter-current microchannels, and whether the two enhancement mechanisms reinforce or undermine one another.

Hang Shu, Yibo Shen, and Yu Xu designed three pin-fin geometries with identical base area and height—circular, elliptical, and semi-elliptical cross-sections—and embedded arrays of them in microchannel heat sinks. Using computational fluid dynamics, they simulated operation across coolant flow rates of 4 to 8 liters per minute and wall heat fluxes ranging from 50 to 200 watts per square centimeter, a span that captures both moderate and genuinely punishing avionics thermal loads. The simulations resolved the coupled fluid flow and heat transfer in the solid and liquid domains, allowing the team to extract Nusselt numbers, friction factors, pressure drops, wall temperature distributions, thermal resistance, and temperature non-uniformity metrics for every configuration, and to benchmark them against smooth concurrent-flow and smooth counter-current channels.

The baseline result confirmed the power of reversing the flow direction. Compared with conventional concurrent-flow smooth channels, the counter-current arrangement reduced the heating wall temperature difference by 32.8 to 40.3 percent, and it achieved this improvement without any additional pressure drop penalty, since the geometry and total flow path remain essentially unchanged. That combination—substantially better uniformity for free—explains why counter-flow designs have attracted growing interest for high-heat-flux electronics, and it establishes the reference point against which the fin geometries were judged.

What emerged next is the study’s central finding: pin fins and counter-current flow exhibit what the authors describe as a strong positive synergy. Adding circular pin fins to an already counter-current channel cut the wall temperature difference by a further 19.9 to 22.6 percent relative to the smooth counter-current baseline. The mechanism is physical rather than incidental. Pin fins act as internal turbulators, repeatedly breaking up and restarting the boundary layers that thicken along channel walls and insulate the solid from the coolant. They also promote periodic flow impingement, wake mixing, and secondary flow structures that carry heated fluid away from the wall and replace it with cooler bulk fluid. In a counter-current layout, where the temperature field is already balanced along the flow axis, these local mixing effects compound rather than conflict with the global symmetry of the design, producing enhancements that neither feature delivers alone.

Enhanced heat transfer, however, never comes free. Every one of the three fin shapes imposed a measurable trade-off between thermal performance and flow resistance, and the study quantifies that trade-off with unusual clarity. Circular fins delivered the best temperature uniformity and the strongest heat transfer performance of the three, but they also generated the highest flow resistance, because their blunt cross-sections produce the largest form drag and the most aggressive boundary layer disruption. At the opposite extreme, semi-elliptical fins—the most streamlined of the shapes—produced the lowest pressure drop and achieved a peak performance evaluation criterion, a dimensionless figure of merit that weighs heat transfer gain against pumping power, of 1.24. Elliptical fins occupied the middle ground and emerged as the best overall compromise, achieving the lowest thermal resistance at constant pumping power, a metric that matters greatly in aircraft where every watt spent on cooling is a watt taken from the mission payload.

A further practical insight concerns the robustness of the ranking. Across the full range of heat fluxes tested, from 50 to 200 watts per square centimeter, the relative ordering of the three fin geometries did not change. Increasing the heat flux raised the absolute wall temperature level, but it did not reshuffle which fin performed best or worst. For designers, this means a geometry chosen through analysis at one heat load can be trusted to hold its relative advantages as power densities rise over an aircraft’s operating envelope, simplifying thermal design decisions that would otherwise require re-optimization at every condition. Flow rate, by contrast, did influence the balance between heat transfer enhancement and pressure drop, reinforcing the importance of matching fin selection to the available pumping budget.

The significance of the work lies less in any single number than in the coupling mechanism it reveals. By systematically filling the gap in understanding of how internal turbulators behave inside counter-current microchannels, the study provides the quantitative guidelines that avionics thermal engineers have lacked: counter-current routing should be the starting point for uniformity, circular fins should be selected when temperature uniformity and heat transfer dominate the requirement, semi-elliptical fins when pressure drop is the binding constraint, and elliptical fins when the design must minimize thermal resistance at fixed pumping power. As hybrid-electric propulsion, high-power radar, directed energy systems, and dense computing payloads push avionics heat fluxes ever higher, designs of this kind—flat, symmetric, and finely tuned at the millimeter scale—may become the quiet enabling technology that keeps next-generation aircraft flying cool.

Subject of Research: Numerical analysis of thermal–hydraulic performance of pin fin counter-current flow microchannel heat sinks for avionics cooling

Article Title: Numerical Investigation on Thermal–Hydraulic Characteristics of Pin Fin Countercurrent Flow Microchannel Heat Sinks

Article References: Shu, H., Shen, Y., & Xu, Y. (2026). Numerical Investigation on Thermal–Hydraulic Characteristics of Pin Fin Countercurrent Flow Microchannel Heat Sinks. International Journal of Aeronautical and Space Sciences. https://doi.org/10.1007/s42405-026-01295-4

Image Credits: AI Generated

DOI: 10.1007/s42405-026-01295-4

Keywords: microchannel heat sink, counter-current flow, pin fin, temperature non-uniformity, avionics thermal management, heat transfer enhancement, pressure drop, thermal resistance, computational fluid dynamics, high heat flux, temperature uniformity, performance evaluation criterion