The development of hydrophilic surface coatings for enhanced wetting characteristics has led to improvement in heat transfer metrics like impinging droplet vaporization time and the heat transfer coefficient. Hydrothermal synthesis, a method of developing hydrophilic surfaces, has been previously shown to produce high performing heat transfer surfaces on copper substrates [1]. Our study applied this production method to aluminum substrates, which have the advantage of being cheaper, lighter, and a more widely used for heat sinks than copper. Previous experiments have shown that water droplets on ZnO nanostructure coated surfaces, at low superheats, evaporate via thin film evaporation rather than nucleate boiling. This leads to heat transfer coefficients as much as three times higher than nucleate boiling models for the same superheat. Our nanocoated aluminum surfaces exhibit superhydrophilicity with an average droplet liquid film thickness of 20–30 microns, which can produce heat transfer coefficients of over 25 kW/m 2 K. This study discusses characterization of ZnO nanostructured aluminum surfaces to better understand the related mechanisms which lead to such high heat transfer performance. All ZnO nanostructured aluminum surfaces produced for this study exhibited superhydrophilicity, with sessile droplet contact angles of less than 5 degrees. The challenge of achieving accuracy for such low contact angles led to the development of a new wetting metric related to the droplet’s wetted area on a surface rather than the contact angle. This new metric is predicated on the the fact that heat transfer performance is directly related to this wetted area, thickens, and shape of the expanding droplet footprint. Shape irregularity of droplets on these superhydrophilic surfaces is discussed in this study, where there appears to be advantages to irregular spreading compared with surfaces that produce symmetric radial spreading. One form of irregular spreading consists of liquid droplets spreading out both on top of the surface and within the microstructure of the surface coating. The liquid within the microstructure forms films less than 5 microns thick, making local heat transfer coefficients of greater than 100 kW/m 2 K possible. SEM microscope imaging provided additional insight to the underlying mechanisms which cause these surfaces to produce such exceptional spreading as well as irregular spreading, resulting in very good heat transfer performance. Experimental work was coupled with computational analysis to model the contact line of the droplet footprint. Image processing of experimental photos helps to analyze spreading characteristics, which can be directly related to heat transfer due to film thickness at various points during spreading. Approaches used to characterize these superhydrophilic surfaces advance understanding of the connections between nanoscale structural elements and macroscale performance characteristics in heat transfer. This understanding can reveal key insights for developing even better high performance surfaces for a broad range of applications.

In the wake of utilization of novel materials in various thermal applications open cell metal foams have received attention due to their inherent properties such as large surface area to volume ratio and higher thermal conductivity. Additionally, complex tetradecahedron structure promotes mixing and makes them a good candidate for heat transfer applications. In this paper, a relative comparison has been made between the thermal-hydraulic performance of aluminum and copper metal foam heat exchangers with the same geometry under dry and wet operating conditions. Heat exchanger consisting of round tube with annular layer of metal foam have been considered. Experiments have been conducted using a closed-loop wind tunnel to measure the heat transfer performance and pressure drop. The impact of base metal (aluminum and copper) on the heat transfer rate has been evaluated at varying air flow rates and upstream relative humidity. It has been found that due to similar geometry (flow depth, face area, pore size) both aluminum and copper foam samples have comparable pressure drop under dry conditions. However, the pressure gradient was noticeably different for two samples under wet operating conditions. An obvious difference in heat transfer rate for aluminum and copper metal foam heat exchangers was observed under both dry and wet operating conditions. The findings have been explained in terms of the impact of the thermal conductivity of base metal and condensate retention.

The present work aims to investigate a honeycomb module with PCM for electronic passive cooling. The system is a honeycomb squared solid matrix filled with Phase Change Material. The PCM is paraffin wax and the honeycomb material is aluminum. Transient regime numerical simulations are created with different configurations, for different pores per unit of length (PPU) and different thermal power. A numerical model with honeycomb structure is compared with a porous medium model. The behavior of the porous medium is modeled with the Darcy-Forchheimer assumption and local Thermal Non-Equilibrium (LTNE) hypothesis is employed to simulate the heat transfer between the fluid zone and the solid zone. The results are presented in term of melting time, melt fraction evolution, temperature fields and stored energy for the charging phase.

Nucleation of hydrates requires very long induction (wait) times, often ranging from hours to days. Electronucleation, i.e. nucleation stimulated by the presence of an electric field in the precursor solution can reduce the induction time significantly. This work reveals that porous aluminum foams enable near-instantaneous electronucleation at very low voltages. Experiments with tetrahydrofuran hydrate nucleation reveal that open-cell aluminum foam electrodes can trigger nucleation in only tens of seconds. Foam-based electrodes reduce the induction time by as much as 150X, when compared to non-foam electrodes. This work also discusses two mechanisms underlying electronucleation. These include bubble generation (due to electrolysis), and the formation of metal-ion coordination compounds. These mechanisms depend on electrode material and polarity, and affect the induction time to different extents. This work also shows that foams result in more deterministic nucleation (compared to stochastic) when compared with non-foam electrodes. Overall, electronucleation can lead to a new class of technologies for active control of formation of hydrates.

The present study investigates the thermal conductivity and coefficient of thermal expansion of bimodal SiCp reinforced Aluminum matrix composites formed via powder metallurgy method. The after-effects of proportion of particulate reinforcement as size distribution and sintering parameters on the thermal properties have been explored. The Box-Behnken design for response surface methodology was adopted to recognize the significance of chosen variables on the thermal conductivity and coefficient of thermal expansion of the composite. It is witnessed that the thermal conductivity and coefficient of thermal expansion enhanced due to increase in fine SiC particulates volume fraction. It has been exhibited that the fine SiC particulates (37μm) doped Al-matrix occupied interstitial positions and developed continuous SiC-matrix network. SEMs were conducted to evaluate the microstructure architecture for MMCs.

The flue-wall deformation during the service life of carbon anode baking furnaces has a substantial impact on the carbon anode quality (i.e., thermal, electrical, and mechanical properties) used in the reduction cell of aluminum production. Deformation of the furnace flue-walls, which is one critical mode of furnace aging, leads among others to inhomogeneous baking of the anodes and consequently to a deterioration of the resulting anode quality. This paper focuses on the development of a 3D multi-physics computational model, which is able to take into account a large number of physical phenomena and parameters that play a role in the baking process while considering different levels of the flue-wall deformation. In fact, this 3D model takes into account the thermo-hydro-mechanical coupling due to coupled fluid flow and transient heat transfer, packing coke load and the thermal expansion, and enable us to analyze the influence of these parameters on the resistance to deflection of the flue-walls, and ultimately improved baking process and furnace geometry can be proposed The developed model can predict the anode temperature distribution, creation of hot spot and anode overbaking in certain area as a function of the flue wall deformation mode. By developing this tool, we can effectively predict the deformable flue wall reliability under varying operating conditions, and provide useful insights on enhancing the long-term structural integrity through furnace retrofitting or design adjustment.

This paper will discuss the characterization of an air-PCM storage design for commercial air handling unit (AHU) applications during winter. The air-PCM storage design consists of two rows of 29 aluminum flat plate containers (0.45 m × 0.35 m × 0.01 m) filled with PCM, vertically aligned leaving an air channel between each plate of 0.011 m wide. The storage device was placed within a closed air loop which conditions the air to the desired testing temperature and velocity. The PCM selected for testing was RT44HC with a melting temperature of 44 °C. This PCM was chosen for its similar properties to other PCMs having lower melting temperatures (in the range of 5 to 18°C) that could be used in actual HVAC application implementation. The system was instrumented and calibrated with Type T thermocouples and a velocity sensor. The system was tested at various inlet temperatures (55°C to 63°C for charging and 12°C to 25°C for discharging) and flow rates. The instantaneous heat transfer rates and total energy storage were calculated for each test from the data collected. The results provide a baseline value for heat transfer rates in a simple air-PCM design, to be used for model validation.

The scalability and implementation of selective emitters in passive radiative cooling applications are limited by the high fabrication costs due to the complexity of these structures. The usage of commercially available polymers in selective emitters holds potential in lowering the cost of radiative cooling solutions. In this work, we demonstrate that thin films of polydimethyl-siloxane (PDMS) on aluminum substrates act as radiative coolers by selectively emitting in the wavelength range of 8 μm to 13 μm, where the Earth’s atmosphere is highly transparent. We also show that our device can achieve passive cooling up to 12 ° C below the ambient temperature under the night sky. This suggests that PDMS, especially due to its ease of deposition, may be a viable selective emitter in passive radiative cooling applications.

Extensive studies are being carried out by several researchers on the performance prediction of aluminum heat exchangers with different fin and tube geometrical configurations mostly for Reynolds number higher than 100. In the present study, the air-side heat transfer and pressure drop characteristics of the louvered fin micro-channeled, Aluminum heat exchangers are systematically analyzed by a 3D numerical simulation for very low Reynolds number from 25 to 200. Three different heat exchanger geometries obtained for the experimental investigation purposes with constant fin pitch (14 fins per inch) but varied fin geometrical parameters (fin height, fin thickness, louver pitch, louver angle, louver length and flow depth) are numerically investigated. The performance of the heat exchangers is predicted by calculating Colburn j factor and Fanning friction f factor. The effect of fin geometrical parameters on the heat exchanger performance at the Reynolds number range specified is evaluated. The air-side performance of the studied heat exchangers for the specified Reynolds number range is compared with experimental heat exchanger performance data available in the open literature and a good agreement is observed. The present results show that at the studied range of Reynolds number the flow through the heat exchanger is fin directed rather than the louver directed and therefore the heat exchanger shows poor performance. The effect of geometrical parameters on the average heat transfer coefficient is computed and design curves are obtained which can be used to predict the heat transfer performance for a given geometry.

This research is an experimental investigation of a double-pipe heat storage unit. The inner pipe of the unit, through which a heat-transfer fluid (HTF) is supplied, is made of aluminum and has an outer helix-like fin. The annular space between the pipes is filled with a phase change material (PCM). Actually, this research presents a novel design of the heat storage unit, which, unlike traditional designs with e.g. radial (circumferential) or longitudinal fins, has a single fin which does not divide the shell volume into separated cells. Moreover, this research focuses on close-contact melting (CCM), a process which is characterized by detachment of the solid bulk from the unit envelope and its sinking towards the hot fin surface. In previous investigations, performed in our laboratory, this effect has been achieved in units with above-mentioned traditional fin configurations. It was demonstrated that CCM reduces the overall melting time, i.e. the rate of unit charging, significantly as compared with commonly encountered melting in which the fins serve just to enlarge the heat transfer area. The experimental system employed in this study includes a vertically-oriented double-pipe heat storage unit and thermostatic baths capable of providing hot or cold HTF. The unit has a transparent Perspex shell which makes visualization possible. The entire unit may be placed in a heated water tank with transparent walls. In the latter case, close-contact melting is achieved by detaching the solid phase from the envelope and thus allowing its gravity-induced motion. Regular melting is compared to CCM and advantages of the latter are demonstrated. Also demonstrated are the advantages of the novel fin, including in solidification. Possible mathematical and numerical modeling of the melting processes is discussed.

In this work, we have observed 60% reduction in total interfacial resistance by adding an intermediate metal layer nickel between gold and aluminum oxide. Two temperature model is applied to explain the change of interfacial resistance, including both lattice mismatch with diffuse mismatch model and electron-phonon coupling effect. Simulation result agrees reasonably well with experimental data. Even though interfacial resistance due to electron-phonon coupling effect for Au-aluminum oxide is much larger than that of Ni-aluminum oxide interface, lattice mismatch is still the dominant factor for interfacial resistance.

In this paper, a numerical investigation on Latent Heat Thermal Energy Storage System (LHTESS) based on a phase change material (PCM) in a metal foam is accomplished. A vertical shell and tube LHTESS made with two concentric aluminum tubes is investigated. The internal surface of the hollow cylinder is at a constant temperature above the PCM melting temperature to simulate the heat transfer from a hot fluid. The other external surfaces are assumed adiabatic. The phase change of the PCM is modeled with the enthalpy porosity theory while the metal foam is considered as a porous media that obeys to the Darcy-Forchheimer law. Local thermal non-equilibrium (LTNE) model is assumed to analyze the metal foam and some comparison are accomplished with the local thermal equilibrium model assumption. The governing equations are solved employing the Ansys-Fluent 15 code. Numerical simulations for PCM, PCM in the porous medium in LTE and in LTNE assumptions are obtained. Results as a function of time for the charging phase are carried out for different porosities and assigned pore per inch (PPI). The results show that at high porosity the LTE and LTNE models have the same melting time while at low porosity the LTNE has a larger melting time. Moreover, the presence of metal foam improves significantly the heat transfer in the LHTESS giving a very faster phase change process with respect to pure PCM, reducing the melting time more than one order of magnitude.

Combined natural convection and forced convection gets a great attention for its importance in practical applications in various modern systems such as electronic cooling, chemical vapor deposition and solar energy systems. Buoyancy force due to the heating of the lower cavity wall induces secondary flows hence the local heat transfer increases. The onset point of the secondary flows is important because it delineates the region after which the two-dimensional laminar flow becomes three-dimensional and a transition motion from laminar to turbulent flow is observed. In this paper, mixed convection in a horizontal channel partially filled with a porous medium and the lower wall heated at uniform heat flux is studied experimentally and numerically. The heated wall temperature profiles as a function of the Gr/Re 2 values are presented. Pictures of flow visualization along longitudinal, transversal and horizontal section are given. Average Nusselt numbers are evaluated. Results in terms of wall temperature profiles, local and average Nusselt numbers are presented for different Reynolds and Rayleigh number values. Some comparison between experimental and numerical results are accomplished.

Natural convection in horizontal rectangular channel without or with aluminum foam is experimentally and numerically investigated. In the case with aluminum foam the channel is partially filled. In both cases, the bottom wall of the channel is heated at a uniform heat flux and the upper wall is unheated and it is not thermally insulated to the external ambient. The experiments are performed with working fluid air. Different values of wall heat flux at lower surface are considered in order to obtain some Grashof numbers and different heated wall temperature distributions. Two different aluminum foams are considered in the experimental investigation, one from “M-pore”, with 10 and 30 pore per inch (PPI), and the other one from “ERG”, with 10, 20 and 40 PPI. The numerical simulation is carried out by a simplified two-dimensional model. It is found that the heat transfer is better when the channel is partially filled and the emissivity is low, whereas the heated wall temperature values are higher when the channel is partially filled and the heated bottom plate has high emissivity. The investigation is achieved also by flow visualization which is carried out to identify the main flow shape and development and the transition region along the channel. The visualization of results, both experimental and numerical, grants the description of secondary motions in the channel.

The catalytic dehydrogenation reaction of isobutane to isobutylene is simulated in a commercial-scale heterogenous fixed bed reactor (FBR). The porous medium method in ANSYS Fluent combined with the reaction model capability was utilized to predict the flow behavior and species transport in a bed of spherical particles. Physical and material properties of a dehydrogenating catalyst of Chromium Oxide (Cr 2 O 3 ) on Aluminum Oxide Support (Al 2 O 3 ) were employed in the model. Several reaction models were implemented using a customized User-defined Function (UDF) subroutine. Simulation results were validated against literature data for a similar process. Good agreement was observed for the conversion of alkanes to alkenes within acceptable accuracy. It is concluded that the power-law model showed the least fit for the feed conversion and product selectivity compared to the other studied reaction models.

Two time-dependent mathematical and numerical models with different levels of complexity and fidelity were developed to investigate the melting of a phase change material (PCM) configured as a number of aluminum-encased, PCM-filled slabs with embedded micro-channel aluminum tubes, and with parallel air-flow passages interposed between the slabs. Melting was first analyzed with the COMSOL Multiphysics ® finite-element model (FEM) in a 2-D domain representing a full-size slab. The melting process is simulated via the apparent heat capacity method. The model captures the effect of natural convection in the PCM melt as well as the conjugate heat transfer through the aluminum tubes. A fast-executing quasi 2-D reduced-order model (ROM) was developed for repetitive design optimization studies. The ROM relies on a time-dependent 1-D closed-form solution of the heat conduction equation in a melting PCM, coupled with variations of the air temperature and heat transfer coefficient. Consequently, the FEM results were employed to develop corrections to the ROM. The corrected ROM was then utilized to study the melting process in a multi-slab thermal storage device that is designed to freeze the PCM at night and release 500 W-h of cooling over a span of ∼10 h during the day.

Open-cell metal foam is a class of modern porous media that possesses high thermal conductivity, large accessible surface area per unit volume and high porosities (often greater than 90%). The high porosity means very low weight. The internal structure of the foam is web-like. Internal flow inside the foam is complex and includes flow reversal, destruction of boundary layers and vigorous mixing. All of these attributes make metal foam a very attractive heat transfer core for many applications. The rather complex and intrinsically random architecture of the foam is virtually impossible to capture exactly. In this paper, we present a unit cell geometrical model that was used to represent the foam structure for numerical analysis purposes. In particular, the unit cell is used to numerically study forced convection heat transfer between aluminum foam and air. The Navier-Stokes and the governing energy equation are solved directly and the temperature fields are obtained using COMSOL. The details of the modeling process are given in this paper. The results are encouraging and lend confidence to the modeling approach, which paves the way for other investigations of the foam, as well as optimization work based on the structure of the foam.

Heat pipes are used in many applications as an effective means for transferring heat from a source to a sink. The basic heat pipe typically consists of a solid metal casing within which a working fluid is sealed inside at a given pressure. The latent heat transfer via the heat pipe’s working fluid allows it to carry a larger amount of heat energy than would normally be possible with an identically dimensioned solid metal rod. Water is often used as a working fluid due to its high heat of vaporization and suitable operating range for electronics cooling. For many applications, especially space, aluminum is desired as a casing material for its high thermal conductivity, low weight, and low cost. However, water is incompatible for use with aluminum heat pipes because it forms a non-condensable gas (NCG), hydrogen, when they contact. In this work, an inorganic aqueous solution (IAS), which has thermophysical properties similar to water, has been used as the working fluid with an aluminum alloy 5052-H2 casing. The prepared thermosiphon underwent long-term lifetime testing and the results indicate no tube failure or significant NCG formation for the duration of the 9 week study. Furthermore, the data indicate that the IAS fluid not only inhibited NCG production but also led to a reduction in heat pipe thermal resistance over time. It is believed that the chemicals in IAS react with the aluminum surface to create a compact oxide layer and electrochemical reaction which prevents hydrogen generation. A secondary, hydrophilic surface coating is also generated by the fluid on top of the first oxide (passivation) layer. This hydrophilic layer is believed to be responsible for the heat transfer enhancement which was observed during testing and the reduction in ΔT (defined as T evap −T cond ) over time. Aluminum heat pipes used currently in practice utilize ammonia, or other non-water based working fluids, which have inferior latent heats of vaporization compared to water or an aqueous-based fluid such as IAS. The use of aluminum heat pipe casings in combination with a water-based fluid such as IAS has the potential to provide a significant increase in heat transport capability per device unit mass over traditional ammonia charged aluminum heat pipes.

This paper reports a numerical study on the discharge of carbon dioxide using a non-thermal dielectric barrier discharge (DBD) plasma reactor at ambient conditions. DBD plasma reactors have been used for various applications due to their ease of production, process control, operation at different conditions. The applications of DBD plasma reactors include discharge of gases. Carbon dioxide is a greenhouse gas formed as a byproduct of fossil fuel combustion. Use of DBD non-thermal plasma reactor can be a promising technology for carbon dioxide mitigation due to its operation at low temperatures, lack of need for catalysts, and flexibility in controlling the products generated. In this study, a tubular DBD non-thermal plasma reactor was modeled with different electrode materials separated by different dielectric materials. The aim was to provide guidelines for the design and material selection for optimizing DBD plasma reactors for CO 2 discharge. A parametric set of simulations was performed using a finite element solver to investigate how electrode and dielectric materials affect the discharge volume of CO 2 and power requirement of the non-thermal plasma discharge of CO 2 . The results showed that electrode material did not affect the discharge or the power requirement. However, dielectric material with higher permittivity or lower conductivity increased the gas discharge and power requirement. Among the analyzed materials, aluminum electrode and mica tube were suggested based on the simulation results for the maximum gas discharge and low power requirement.

The preferable cooling solution to the problem of thermal management of modern electronics for increasing power dissipation could be micro heat exchangers based on forced flow boiling. Nanoparticle deposition can affect nucleate boiling heat transfer coefficient via alteration of surface thermal conductivity, roughness, capillary wicking, wettability, and nucleation site density. It can also affect heat transfer by changing bubble departure diameter, bubble departure frequency, and the evaporation of the micro and macrolayer beneath the growing bubbles. In this study, flow boiling was investigated using degassed, deionized water, and 0.001 vol% aluminum oxide nanofluids in a single rectangular brass microchannel for one inlet fluid temperature of 63°C, one flow rate of Re = 100, and two heat fluxes of 130 kW/m 2 and 300 kW/m 2 . High speed images were taken periodically for water and after durations of 25, 75, and 125 minutes of nanofluid flow boiling. The change in regime timing revealed the effect of nanoparticle suspension and nanoparticle deposition on the Onset of Nucelate Boiling (ONB) and the Onset of Bubble Elongation (OBE). Single phase flows at the channel outlet were recorded and compared for different durations of nanofluid flow boiling. The addition of nanoparticles was found to stabilize bubble nucleation and growth and increase heat transfer in the thin film regions of the evaporating menisci.